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1. 5 612 779 Sheet 2 of 8 18 1997 0 5 Patent 8 Bld 08 2 Di4 o1 a JIWSNVHL b LINN OL v J ndoaH SNO H 5 7 Svid XL 5 612 779 Sheet 3 of 8 Mar 18 1997 U S Patent N ONS 31 A YOLVYVdNOO EUERE e oae aX 5 big AlddNsS H3MOd AH H3MOd 5 H3MOd O e 0 5 Patent Mar 18 1997 Sheet 4 of 8 5 612 779 C 98 eo i til 196 Ed 188 ORRECTION E Fig 6 180 186 7 U S Patent Mar 18 1997 Sheet 5 of 5 612 779 178 256 258 HOLD OFF ookan TO FROM Fig 4 Fig 6 216 246 RUN CLAMP O N TO Fig 6 260 CAL DITHER O P TO Fig 6 U S Patent Mar 18 1997 Sheet 6 of 8 5 612 779 292 5 H O dd coms H1 0 NER MODE H gt 294 278 4 258 HOLD OFF 5 260 PASAN P CAL DITHER r PASIAIN DETE 254 PAA AIN Pee T ___ PANAM PA2 AIN 8 p oNPRMICAL 156 Pavan 154 MODE PAVAN DISPLAY 9 ro d 274 E li 152 5 OUT CLK 527 0 6 EFE oe on BE RP UEM T IN SS DIES 250 Ep at om
2. US005612779A United States Patent Patent Number 5 612 779 Dunne 45 Date of Patent Mar 18 1997 54 AUTOMATIC NOISE THRESHOLD 5 046 839 9 1991 Krangle DETERMINING CIRCUIT AND METHOD 5 157 403 10 1992 Urkowitz s 342 111 FOR A LASER RANGE FINDER 5 221 956 6 1993 Patterson et al 5 359 404 10 1994 Dunne 2 356 5 75 Inventor Jeremy Dunne Littleton Colo OTHER PUBLICATIONS 73 Assignee Laser Technology Inc Englewood Pro Laser IL Infrared Lidar System User Manual P N Colo 006 0535 00 Kustom Signals Inc Chanute Kansas right 1991 pp 1 37 21 Appl No 375 810 ProLaser II Traffic Safety Lidar Lidar System Functions Lidar System Specifications Brochure Kustom Signals 22 Filed Jan 19 1995 Inc Lenexa KS Feb 1994 pp 1 4 6 Marksman LTI 2020 Laser Speed Detection System 531 Int 9016 3 08 6015 13 08 Operator s Manual Laser Technology Inc Englewood 52 US L 356 5 01 356 5 05 342 203 Colorado copyright 1994 pp 1 45 342 134 367 127 58 Field of Search ss 356 5 01 5 08 Primary Examiner Stephen C Buczinski 356 28 342 134 137 202 204 367 127 Attorney Agent or Firm William J Kubida Holland amp Hart 56 References Cited 57 ABSTRACT U S PATENT DOCUMENTS An automatic noise threshold circuit and method automati 3 325 750 6 1967 O
3. average ten pulses to establish the CAL average and ten pulses to establish the minimum precision or rough FLIGHT range to the target Another group of ten through thirty laser pulse FLIGHT 4s may be also averaged in order to obtain a higher precision distance to a target as indicated by a precision flag which may be displayed on the LCD display 32 within the laser range finder 10 eyepiece Nevertheless the actual values derived in these time expansions will of course vary with time temperature and aging and affects the gain of the transistors the leakages as well as the value of the resistances and capacitances Initially the exact values of these effects are completely unknown but through the use of the zero and calibration functions above described the zero problem has been eliminated and a crystal reference calibration has been provided for the entire flight time without having to resort to a complicated counter circuitry 5 612 779 13 Another aspect of the precision timing section 34 is the automatic set noise control and invertor 168 which provide in conjunction with other circuit elements a hardware hold off function Upon firing of the laser and receipt of the reference signal REF on line 94 at the CLK input of flip flop 158 a certain time must elapse as determined by the time constant of resistor 164 and capacitor 166 before the D input goes high Until that time all noise pulses and or early la
4. FIGS 7A 7B and 7C are individual graphic representa tions of the voltages V and V of certain of the precision timing section circuit nodes during the zero calibration and laser firing phases of operation from which the values and Lasere are derived to enable rapid and accurate calculation of the distance to an object from the laser range finder and FIG 8 is a final detailed schematic diagram of the automatic noise threshold section of the laser range finder of FIG 1 illustrating the various components thereof as well as the signals coupling the same to the laser receive section and CPU DESCRIPTION OF A PREFERRED EMBODIMENT With reference now to FIG 1 a logic block diagram of a laser range finder 10 in accordance with the present inven tion is shown The laser range finder 10 includes in pertinent part a main power supply unit PSU 12 as operatively controlled by a trigger switch 14 The main power supply unit 12 is coupled to a high voltage power supply unit 16 for supplying operating power in conjunction with the main power supply unit 12 to a laser transmit section 18 15 20 25 30 35 45 50 55 60 65 4 The laser transmit section 18 activates a laser emitting diode 20 for directing a laser signal toward an object in the operation of the laser range finder 10 The laser transmit section 18 also supplies a FIRE signal to the central processing unit
5. Hern et al cally sets an operating threshold for a signal receiving 3 644 740 2 1972 Dobratz et al section of a laser pulse transmitting device such that a 3 652 161 3 1972 Ross constant noise pulse firing rate is output from a detector to 3 959 641 5 1976 Miller et al provide maximum return signal sensitivity and enable detec 4 214 242 7 1980 Colin tion of the weakest possible laser pulse in order to obtain eos Pio al maximum performance out of a laser range finder The 4 527 89 4 7 1985 al circuit sets the noise pulse rate at that point at which in 4 560 599 2 1986 Bolkow et al conjunction with a firmware based process the actual return 4 571 085 2 1986 Anderson signals from the target can be discrimintated from the 4 620 788 11 1986 Giger accompanying noise 4 699 508 10 1987 Bolkow et al 4 770 526 9 1988 et al 40 Claims 8 Drawing Sheets OSCILI 30 34 CPU CLOCK aliti 10 28 PRECISION TIMING 258 258 LCD i CPU DISPLAY SECTION AUTOMATIC 32 NOISE THRESHOLD SECTION MAIN TRANSMIT se POWER POWER SECTION SUPPLY SUPPLY TRIGGER UNIT UNIT 22 LASER RECEIVE 24 SECTION 5 612 779 Sheet 1 of 8 Mar 18 1997 U S Patent AV1dSIG 82 NOILOAS uasvi NOLLO3S LINSNVYLL uasvi NOILO3S QIOHSSYHL 3SION Ou VWO4nv NOILO3S fido 0135 ONIWIL 193140 Xu NOISIO3Hd 30019 YOLVTTIOSO
6. are two control lines from the microcomputer 28 such that when held low or high adjust the noise rate to obtain the maximum range to different reflectivity targets If both lines 308 and 310 are taken high V4 will drop to compensate to maintain a constant threshold noise Similarly potentiometer 332 pro vides an adjustment such that the threshold point may be set together with the level of Typically the V point might be set equal to 0 5 1 0 1 5 and 2 0 volts as desirable choices for the average noise firing rates As such since resistor 338 is approximately twice the value of resistor 340 four voltage combinations are obtained roughly equally spaced in voltage by half a volt Potentiometer 332 is used to set the first voltage level to 0 5 or the last one to 2 0 while the intervals are determined by the logic control lines 308 and 310 set NSET1 and NSET2 Obviously this approach could be extended four combinations provides adequate resolution in the particular implementation of the laser range finder 10 described and shown When both lines 308 and 310 are high there is a current injected into the node comprising the V inreshoia line 102 and to compensate for that must drop 50 less current flows through resistor 326 and vice versa will follow these values depending on the permutations of logic high and low signals on the lines 308 and 310 Resistor 330 is used just to set where this whole block resides while potentiome
7. CPU section 28 as will be more fully described hereinafter The main power supply unit 12 also supplies operating power to a laser receive section 22 which further has as an input a signal generated by a laser receiving diode 24 as the laser signal emitted from the laser emitting diode 20 is reflected from an object back thereto The laser receive section 22 receives V from an automatic noise threshold section 36 and a supplies an RX OUT signal precision timing section 34 both of which will be described in more detail hereinafter The CPU section 28 receives as one input a signal from a mode switch 26 by means of which an operator can change the operating mode and functional operation of the laser range finder 10 An oscillator 30 supplies a clocking signal to the CPU section 28 as well as to the precision timing section 34 The CPU section 28 provides an output indica tive of the distance from the laser range finder 10 to an object as sighted through a viewing scope thereof on an insight liquid crystal display LCD 32 The precision timing section 34 provides a number of signals to the CPU section 28 including a TIMER and RX DETECT signals as shown and receives a R N CLAMP signal back therefrom The CPU section 28 provides a number of signals to the precision timing section 34 includ ing a HOLD OFF NORM CAL RESET and a CAL DITHER signal The automatic noise threshold section 36 also receives a number of inpu
8. drops to a low value Initially will be higher and the input of comparator 134 shown in FIG 3 will be higher than the input forcing a logic low on the output as the starting state As the level of on node 320 falls the voltage level on the pin of comparator 134 starts approaching the level of the signal on the positive pin of the transimpedance amplifier 116 When it approaches the noise zone noise pulses start appearing As soon as noise pulses start appearing a charge appears on node 320 so V charges When V1 and V3 match the feedback point is reached the charging stops Basically the voltage on the threshold is set at such a point that the noise firing rate maintains V at that voltage which is necessary to maintain V ihreshota Because very small changes in make very large change in the noise firing rate typically a ten millivolt change in V 4 Will change the voltage V at 10 15 20 25 30 40 45 50 55 60 65 16 node 320 by about a volt What is produced then is a fairly high gain feedback loop such that V preshora will track very closely the noise firing rate and will stabilize very accurately and rapidly This further provides the capability to adjust the noise firing rate by controlling the bias and forcing to compensate The voltage V at node 320 then represents the noise firing rate NSETI line 308 and NSET2 line 310
9. electrical reference level until said first and third electrical reference levels become equal again re establishing said first and second electrical ref erence levels Epis mE said second electrical reference evel again changing said second electrical reference level at said second rate that is higher than said first rate for a time period that is related to said actual flight time of said pulse toward said target in order to establish a fourth electrical reference level allowing said fourth electrical reference level to change at said first rate relative to said first electrical reference level storing a third time period T3 that is representative of a time that expires between said step of once again unclamping said second electrical reference level until said first and fourth electrical reference levels become equal and computing a range to said target as a function of the quantity T3 T1 T2 T1 30 The method of claim 29 wherein said steps of estab lishing re establishing and again re establishing are carried out by clamping a voltage on a capacitor 31 The method of claim 30 wherein said steps of initially unclamping again unclamping and once again unclamping are carried out by a transistor switch 32 The method of claim 30 wherein said steps of allowing said second electrical reference level to change allowing said third electrical reference level to change and allowing said fourth electrical reference lev
10. illustrating the significant functional aspects thereof inclusive of a laser signal transmitting and receiving section central processing unit and the precision timing and automatic noise threshold sections thereof FIG 2 is a detailed schematic diagram of the laser transmit section of FIG 1 illustrating inter alia the laser signal producing diode and the associated driving and ref erence signal producing circuitry FIG 3 is an additional detailed schematic diagram of the laser receive section of FIG 1 illustrating inter alia the laser signal recciving diode transimpedance amplifier and the precision comparator for establishing the and RX Out signals for the precision timing and automatic noise threshold circuits FIGS 4 and 5 are further detailed schematic diagrams of the precision timing section of the laser range finder of FIG 1 illustrating the circuit nodes for establishing the voltages V and V during the zero calibration CAL and laser firing phases of operation FIG 6 is an additional detailed schematic diagram of the central processing unit portion of the laser range finder of FIG 1 illustrating the CPU associated oscillator and the in sight liquid crystal display LCD for display ing measured distances to an operator of the laser range finder in addition to the various signals for operative asso ciation with the precision timing and automatic noise thresh old sections thereof
11. start and end of the calibration pulse is gated via transmit gate 204 to actually turn the transistor 210 on and off in order to function as a current source typically sourcing 10 milliamps of current It should be noted that prior to turning transistor 210 on transistor 214 must first be turned off and when the system is in the reset state ready to start the whole measurement sequence transistor 210 is off Transistor 212 which is the current sink in the system is always on and typically sinks on the order of 10 microamps of current In the reset condition transistor 214 is on and that clamps the voltage at the top plate of capacitor 244 to a voltage level designated as V1 at node 228 A voltage V2 is defined as the voltage at node 232 at the input of comparator 236 It should also be noted that a metal oxide semiconductor field effect transistor MOSFET may be utilized for transistor 214 and would exhibit a much lower offset than the bipolar device shown However due to the lower cost of bipolar transistors and the fact that any offset cancels during the processing of the signal a bipolar transistor is entirely adequate for this purpose When transistor 214 is on the voltage on the positive plate of capacitor 244 is clamped to voltage V1 plus a fixed offset due to the transistor 210 which is small and typically 5 612 779 9 on the order of 50 millivolts During the zero calibration function transistor 214 is
12. to a pulse previously transmitted from the signal transmitting device Further included are means responsive to the central processing section for placing up to a preselected number of the possible signal pulse values in a stack until a predeter mined number of them coincide within a specified precision The value of one or more of the predetermined number of the possible signal values is then considered to be represen tative of the actual return signal The predetermined number of the possible signal pulse values may further be averaged to represent the actual return signal to a greater degree of precision In a more particular embodiment the signal transmitting device is a laser range finder and the pulse value of the possible signal pulses corresponds to a possible flight time of the pulses transmitted from the laser range finder to a target The means for determining the desired signal to noise ratio may comprise a detector coupled to an output of the signal receiving section for producing a substantially con stant noise pulse firing rate output and an operational amplifier coupled to an output of the detector for providing a threshold signal to the receiving section in response thereto The threshold signal may be supplied to a summing node with at least one noise level setting signal from the central processor section for further determining the actual threshold signal depending upon the target characteristics Also disclosed is a meth
13. to the zero calibration value to about 4500 For example during the zero calibration mode the count value in the microcomputer 270 might be 150 but there is no way of knowing just how close the count actually is to 149 to 151 By utilizing the CAL DITHER signal to force the count over a couple of count boundaries for example 150 150 150 151 151 152 the resolution of the counter may be effectively raised by a factor of two without having to utilize additional fine counters In the embodiment shown the resultant resolution is sufficient to maintain calibration to plus or minus one yard over a range of one thousand yards or less Although implementations may vary the CAL DITHER signal may be held high for five out of ten pulses and low for the remainder to provide the foregoing resolu 10 15 35 40 45 50 55 60 65 10 tion enhancement Due to the fact that the actual laser flight time varies due to noise in the laser pulses and variability in target aiming there is general enough scatter in the measured laser flight time such that it covers more than one clock boundary and so will automatically average to a higher resolution through the use of the precision timing section 34 without invoking the CAL DITHER function in the laser mode of operation With reference additionally now to FIGS 7A 7B and 7C the operation of the precision timing section 34 is shown in the zero calibration fixed pulse width calibr
14. turned on by holding the RUN CLAMP signal on line 260 high thereby applying a positive current to its base through resistor 248 To initiate the zero calibration the TIMER signal on line 250 is asserted and supplied to the microcomputer 270 of the CPU section 28 Utilizing the ST6240 unit shown in FIG 6 when the microcomputer TIMER pin is held high the device is counting Conversely the microcomputer stops counting when the pin is allowed to go low In operation the output comparator 236 determines whether or not the voltage at the top plate of capacitor 244 is greater or less than V2 and its output determines whether the TIMER pin on the micro computer 270 is high or low In the normal reset condition the output of the comparator 236 is high which means the timer is active In sequence the microcomputer 270 initiates the TIMER function and then turns off transistor 214 by lowering the control signal RUN CLAMP on line 260 to unclamp capacitor 244 Capacitor 244 then starts discharg ing towards zero due to the current being drained out of it via transistor 212 at a rate of about ten microamps When it has discharged such that the charge removed drops the voltage V1 at node 228 to the level of V2 the output of the comparator 236 changes state to stop the TIMER function In the particular embodiment shown V1 is typically on the order of 1 0 volts and V2 is about 0 9 volts The micro computer 270 of the CPU section 28 now has a count
15. 519 d 8 Fig 5 308 B 298 306 5 NSET 1 C 302 300 amp 310 E SH 96 o OL Ls ADLOW B O PB2 AIN 304 Y BOreingcrx OUT 80 196 if BATTER o 05 POWER CORRECTION 284 TRIGGER 286 Fig 4 o gt POWER CONTRO 2 FROM 1 pem S Fig 6 5 612 779 Sheet 7 of 8 Mar 18 1997 U S Patent AWIL INIL 1v9 OF 3iogyaz 3 HON XVN NIW xu 1 eh lt ERN S b S SAIL gt Yh ouaz 01 81 49 EL 81 01 3 1 ougz 5 612 779 Sheet 8 of 8 Mar 18 1997 U S Patent 001 1NO BI4 Bly OL Bid wous NOILO3143H 3 ore ZIE Z LASN 5 9 L LISN 98 gee 0000 ax L g n MOE 9 Big WOHJ 926 8 big 026 P bce p Bl4 Od 915 PLE ool 5 612 779 1 AUTOMATIC NOISE THRESHOLD DETERMINING CIRCUIT AND METHOD FOR A LASER RANGE FINDER CROSS REFERENCE TO RELATED APPLICATIONS The present invention is related to those disclosed and claimed in U S Patent applications Ser No 08 375 945 for Laser Range Finder Having Selectable Target Acquisition Characteristics and Range Measuring Precision and Ser No 08 375 941 for Self Calibrating Precision Timing Cir cuit and Metho
16. T to in the zero calibration mode except for the step due to the charging of capacitor 244 As a consequence the value of ZERO equals minus T and the value of CAL value equals the time due to the value not due to the ZERO value which is T minus T minus the 2 value or minus T In essence then very small flight times are effectively disregarded and the value of CAL is known Therefore with the zero calibration function and the addition of a known calibrated pulse width the time delay at zero is known together with the time delay for the known pulse width providing the origin and scale for determining dis tance with a constant linear discharge of capacitor 244 With particular reference additionally to FIG 7C the operation of the precision timing section 34 is shown in the laser measurement mode of operation The laser measure ment operation is essentially the same as the fixed pulse width calibration mode except that the NORMAL CAL signal on line 154 to the shift register 160 is held high and the RESET signal on line 156 is taken high at time T to enable the flip flops 158 162 to trigger At time the timer is started and at T at precisely the same relationship T minus equals T minus T equals T minus the clamp 15 20 25 30 45 50 55 60 65 12 is released There is normally a fifty microsecond wait and then the laser pulse is fire
17. adjusting a noise threshold of said laser light receiver further comprises a voltage source selectively connectable to said summing node to desensitize said receiver 24 The automatic noise threshold circuit of claim 23 wherein said voltage source selectively connectable to said summing node is operated in response to said programmed microcomputer 25 A method for automatically adjusting a noise thresh old in a laser range finder comprising generating an electrical signal in response to a received light signal from a laser light receiver said electrical signal containing signal and noise light pulses and adjusting a noise threshold of said laser light receiver to a level at which said laser light receiver produces a noise light pulse output having a constant pulse firing Tate 26 The method of claim 25 wherein said step of adjusting a noise threshold of said laser light receiver to a level at which said laser light receiver produces a noise light pulse output having a constant pulse firing rate comprises charg ing a capacitor circuit with said noise light pulse output to produce a threshold feedback voltage that is related to a number of said noise light pulses in said electrical signal 27 The method of claim 25 wherein said step of adjusting a noise threshold of said laser light receiver to a level at which said receiver produces a noise light pulse output having a constant pulse firing rate comprises adjusting said noise thres
18. aged time between them to maximize the sensitivity of the laser range finder 10 should be of the order of two microseconds or so As an example if a 5096 voltage were desired and the high state was occurring for 50 nanoseconds while the low state average was occurring for one micro second a 20 1 ratio would be produced Nevertheless the optimum ratio has been determined empirically to be about 150 1 as previously described and is related to average pulse widths typically on the order of 30 nanoseconds in length and pulse repetition rates on the order of 4 microseconds with a typical voltage level of 1 5 volts Op amp 318 is configured as a unity gain buffer although it need not be unity gain with a voltage V at its input pin on node 320 The input is high impedance and the output is low impedance in order to drive external circuitry The voltage that is derived at the output of the op amp 318 is then fed into a resistor network comprising resistor 338 resistor 340 resistor 342 and resistor 330 A summing node of the resistor network on line 102 goes to the threshold control to provide the signal V esnoiq to the laser receive section 22 shown in FIG 3 Resistor 330 is connected to the center tap of a potentiometer 332 so that the DC voltage on the other end of resistor 330 can be controlled In combination the circuit comprises a feedback network such that if there are no noise pulses then V is zero and V nreshota aNd
19. ain oscillator 30 as applied to the CLK input of the shift register 160 The signal applied to the CLK input of the shift register 160 directly tracks the main oscillator 30 and the serial data input to the shift register 160 is a logic line 154 from the CPU section 28 designated NORM CAL When the NORM CAL signal is high the precision timing section 34 is in its normal mode of opera tion and when it drops to a logic low state the fixed pulse width calibration function is initiated Thereafter typically about fifty microseconds later at time T the NORM CAL signal on line 154 will be dropped low It should be noted that during both the zero and the fixed pulse width calibra tion modes the logic reset signal RESET on line 156 is held low its active state In the logic low state the two flip flops 158 162 determine whether the input signal comes from shift register 160 which generates the fixed pulse width or whether it comes from the REF and RX OUT signals an relates to an actual laser flight time The RESET signal is generally held low at all times during the fixed pulse width calibration process so that any noise on the RX OUT receive line 100 will not accidently clock flip flop 162 and therefore trigger the precision timing section 34 resulting in an indeterminate time period measurement invalidating the calibration The reset state for the Q outputs of flip flops 158 162 is low but is high for the Q outputs Therefore the Q output
20. ation and laser measurement function modes of operation respectively In its normal state the voltage on the top plate of capacitor 244 is clamped at V1 and at a time To the precision timing section 34 will initiate the TIMER by changing the output state of comparator 236 to the logic high state After a very short fixed number of instructions later shown as 1 the clamp transistor 214 will be turned off and the voltage on capacitor 244 will begin discharging slowly until that volt age crosses V2 at time T4 when the output of comparator 236 will change state In essence during the zero calibration process transistor 210 is never turned on thereby determin ing the timing conditions of what would effectively be a zero flight time Therefore if there is no charge current applied to capacitor 244 T T zero is the time that would be in the microcomputer 270 and the timer in whatever units they operate which is usually dependent on the CPU section 28 crystal frequency In the embodiment shown the microcom puter 270 utilizes an 8 MHz crystal and the internal timer has a 1 5 microsecond resolution resulting in a count of about 150 During the fixed pulse width calibration process shown particularly in FIG 7B at time T once again the micro computer 270 stops the TIMER and a short time later at it releases the clamp At a known pulse width is applied to the base terminal of transistor 210 which is precisely derived from the m
21. bability of a noise pulse appearing over a longer flight 10 15 20 25 40 45 50 55 60 65 18 range and therefore a quick acquisition on a bright white target can be achieved Thus by depressing the mode switch 126 different modes of operation of the laser range finder 10 can be selected As an example one mode might be utilized to find the range to reflective road signs out to a distance of 1000 yards or more Alternatively aiming the laser range finder 10 at something like wet black tree bark might reduce the maximum range to only 350 400 yards and so a different operational mode might be selected which would otherwise require a relatively long time to hit the road sign if ever because there would always be a noise pulse in the way The mode switch 126 allows the setting of these variables to maximize the range of the laser range finder 10 depending on the target quality and a visual indication of the target quality selected may be provided to the operator on the insight LCD display 32 wherein the first mode would correspond to the brightest target or most reflective target and the Nth mode would correspond to the least reflective target While there have been described above the principles of the invention in conjunction with specific apparatus it is to be clearly understood that the foregoing description is made only by way of example and not as a limitation on the scope of the invention What is claime
22. connected to receive an output of said laser light receiver and a capacitor connected to said diode to be charged by said output said capacitor operating to produce a threshold feedback voltage related to a number of noise pulses in said output 20 The automatic noise threshold circuit of claim 19 wherein said circuit for automatically adjusting a noise threshold of said laser light receiver further comprises a threshold comparator having a first input to which said electrical signal is connected and having a second input to which said threshold feedback voltage is connected said threshold comparator being operative to produce an output when a said electrical signal on said first input is higher than a said threshold feedback voltage on said second input 21 The automatic noise threshold circuit of claim 18 wherein said circuit for automatically adjusting a noise threshold of said laser light receiver comprises circuitry for adjusting said noise threshold in response to a programmed microcomputer 22 automatic noise threshold circuit of claim 19 wherein said circuit for automatically adjusting a noise threshold of said laser light receiver comprises a summing node receiving said threshold feedback voltage and cir cuitry connected to said summing node for adjusting said threshold feedback voltage in response to a programmed microcomputer 23 The automatic noise threshold circuit of claim 22 wherein said circuit for automatically
23. ction 28 shown in FIG 1 An additional diode 82 has its anode connected to the collector of transistor 72 and its cathode coupled to circuit ground 60 through resistor 86 A capacitor 84 couples the cathode of diode 82 to the common connected collector of transistor 54 and base of transistor 56 The common con nected collector of transistor 54 and base of transistor 56 are coupled through a voltage divider network comprising resis tor 88 and resistor 90 to circuit ground A resistor 92 coupled 5 612 779 5 between resistor 88 and resistor 90 provides REF signal line 94 for application to the precision timing section 34 shown in FIG 1 With reference additionally now to FIG 3 the laser receive section 22 is shown in more detail The output of signal of laser receiving section 22 is the signal RX OUT This RX OUT signal on conductor 100 is applied to FIG 8 s showing of automatic noise threshold section 36 and to FIG 4 s showing of precision timing section 34 Conductor 102 from FIG 8 s showing of automatic noise threshold section 36 provides the threshold signal Vthreshold to laser receiving section 22 and more specifically to the input of comparator 134 A source of 50 volts providing a receive RX BIAS signal is input to the laser receive section 22 from the HV power supply unit 16 on supply line 104 A low pass filter network 106 comprising resistors 108 and 112 in conjunction with capacitors 110 and 114 co
24. d for a Laser Range Finder all filed con currently herewith and assigned to the assignee of the present invention Laser Technology Inc Englewood Colo the disclosures of which are hereby specifically incorporated by this reference BACKGROUND OF THE INVENTION The present invention relates in general to the field of distance or range measuring equipment More particularly the present invention relates to a laser based range finder which may be inexpensively produced yet provides highly accurate precision range measurements of up to 1000 yards or more with a resolution of less than 1 yard The laser range finder herein disclosed has a number of user selectable target acquisition and enhanced precision measurement modes which may be viewed on an in sight display during aiming and operation of the instrument Extremely efficient self calibrating precision timing and automatic noise threshold circuits incorporated in the design provide a compact low cost highly accurate and reliable ranging instrument for a multitude of uses Laser based distance and range measuring equipment have been used for a number of years to provide extremely accurate distance measurements to a remote target or object A representative instrument is the Criterion 100 laser range finder developed and marketed by Laser Technology Inc assignee of the present invention Although a highly accurate and reliable device its great distance ranging capability and
25. d is 1 An automatic noise threshold system for operative association with central processing and signal receiving sections of a signal pulse transmitting device for discrimi nating between an actual return signal pulse and noise that is associated with said actual return signal pulse said system comprising determining means responsive to said central processing section for determining a desired signal to noise ratio for a series of said actual return signal pulses and said associated noise received through said signal receiving section each one of said return signal pulses and associated noise having a representative pulse value with respect to a signal pulse that was previously transmitted from said signal pulse transmitting device and that corresponds thereto and stack placing means responsive to said central processing section for placing up to a preselected number of said representative pulse values in a stack until a predeter mined number of said representative pulse values coin cide within a specified precision wherein said prede termined number of said representative pulse values that coincide within said specific precision are consid ered to be representative of said actual return signal pulse 2 The automatic noise threshold system of claim 1 wherein said signal transmitting device is a laser range finder 3 The automatic noise threshold system of claim 1 wherein each of said representative pulse values correspond
26. d means for again re establishing are carried out by clamping a voltage on a capacitor 37 The apparatus of claim 36 wherein said means for initially unclamping said means for again unclamping and said means for once again unclamping are carried out by a transistor switch 38 The apparatus of claim 36 wherein said means for allowing said second electrical reference level to change said means for allowing said third electrical reference level to change and said means for allowing said fourth electrical reference level to change are carried out by removing charge from said capacitor as determined by a resistor that is switched in parallel with said capacitor 39 The apparatus of claim 35 wherein said first time period T1 said second time period T2 and said third time period are determined by a clock reference source 40 The apparatus of claim 39 wherein said clock refer ence source comprises a crystal oscillator k k
27. d to the anode of diode 316 which has its cathode connected to the input of operational amplifier 318 forming V3 node 320 node 320 is coupled to circuit ground through the parallel combination of resistor 322 and capacitor 324 The output of OpAmp 318 is coupled back to the input thereof as well as to line 102 through resistor 326 for supplying the signal to the laser receive section 22 shown in FIG 1 Line 102 is connected through resistor 20 30 40 45 50 55 60 65 14 330 to the center tap of potentiometer 332 which has one terminal thereof connected to a source of 5 volts through resistor 334 and another terminal thereof coupled to circuit ground through resistor 336 Lines 308 and 310 from the microcomputer 270 shown in FIG 6 are connected through resistors 338 and 340 respectively to line 102 Additionally line 312 from micro computer 270 is connected to line 102 through resistor 342 as shown In operation the automatic noise threshold section 36 in conjunction with the CPU section 28 shown in FIG 6 provides a simply implemented yet highly effective thresh old adjustment to the laser receive section 22 shown in FIG 3 As shown in FIG 3 the laser receiving diode 24 utilizes a high voltage source of about 50 volts supplied via a noise filtering network comprising low pass filter network 106 to bias it The diode 24 responds with an output current p
28. d when the microcomputer 270 asserts the FIRE signal on line 80 to initiate the firing sequence Upon firing the laser emitting diode 20 the laser transmit section sends the REF signal on line 94 to the CLK input of flip flop 158 of the precision timing section 34 This opens the transmit gate 204 which tums on the current source transistor 210 which in turn charges capacitor 244 at a known rate When the reflected laser pulse comes is detected by the laser receiving diode 24 of the laser receive section 22 shown in FIG 3 the RX OUT signal on line 100 is directed to the CLK input of flip flop 162 The output signal of flip flop 162 is inverted by invertor 202 which turns off the transmission gate 204 so that the current source transistor 210 is on for the flight time duration of the laser pulse to charge capacitor 244 to a level determined by the timer during that flight time The charge applied to the capacitor 244 may be anything from just a few millivolts essentially zero distance and flight time to up to two volts maximum range and flight distance depending on the distance to the target Time T represents the firing of the laser as indicated by the REF signal and T represents the receipt of the reflected laser signal as indicated by the RX OUT signal Transistor 210 is turned at and turned off at As a consequence V1 will equal V2 at anytime between minimum distance when T and are essentia
29. diode 316 capacitor 324 and resistor 322 In operation when the threshold pin of the comparator 134 FIG 3 is at a considerably higher voltage than the input pin no noise pulses will appear at the output due to the inherent amplifier and optically generated noise As the voltages on the threshold and input pins are brought closer together noise pulses will appear at the output and when the voltage levels are nearly coincident a great deal of noise can be seen In essence then the automatic noise threshold section 36 sets the noise pulse rate at that point at which given the right firmware algorithm one can still acquire the target and not be blinded by the noise The higher the noise that can be tolerated and the closer the voltage levels at the threshold and input pins of the comparator 134 the weaker the laser pulse that can be detected The auto matic noise threshold section 36 automatically adjusts that threshold level to maintain constant noise pulse firing rate As shown in FIG 8 this is accomplished by monitoring the digital logic receive signal RX OUT on line 100 that goes to the receive flip flop 162 shown in FIG 4 The detector monitors line 100 for the presence of noise pulses via a detector comprising the aforementioned resistor 314 diode 316 capacitor 324 and resistor 322 The value of resistor 322 is typically considerably greater than that of 5 612 779 15 314 on the order of a 150 1 ratio The peak ampli
30. e the flight time is expanded so that the slower clock in the CPU section 28 can then count it accurately The microcomputer 270 utilized in the CPU section 28 has a 1 5 microsecond resolution and because the incoming flight time has been expanded by a factor of 1 000 on the input side to the precision timing section 34 it is the equivalent of a 1 5 nanosecond resolution which corresponds to a measurement resolution for the laser range finder 10 of on the order of nine inches Therefore given that the laser range finder 10 is intended to be a one yard instrument with a nine inch resolution sufficient resolution is provided to be able to measure distances up to a thousand yards to a one yard accuracy The precision timing section 34 of the laser range finder 10 has three distinct modes of operation including a zero calibration fixed pulse width calibration and laser measure ment function as will be more fully described hereinafter The portion of the precision timing section 34 comprising transistors 210 214 and 212 shown in FIG 5 is the essence of the integrating flight time expander Transistor 210 functions as a current switch which is turned on for the duration of the laser flight time in the laser mode of operation and is also turned on for the duration of whatever calibration pulse is placed into it during the calibrate mode In the latter instance a calibration pulse is supplied by the Shift register 160 via flip flop 158 and the
31. ected signal pulses with respect to a transmission of at least one of said series of transmitted signal pulses 15 The method of claim 11 wherein said step of com paring is carried out by the steps of placing said assigned pulse values in a stack and comparing each of said assigned pulse values placed in said stack with others of said assigned pulse values previously placed in said stack 16 The method of claim 11 wherein said comparing step and said continuing said comparing step are carried out by means of a microcomputer 17 The method of claim 11 wherein said determining step includes averaging said assigned pulse values of said predeter mined number of said assigned pulse values to deter mine said actual return signal 18 An automatic noise threshold circuit for use in a laser range finder comprising a laser light receiver for generating an electrical signal in response to a received light signal said received light signal containing both signal and noise light pulses and a circuit for automatically adjusting a noise threshold of said laser light receiver to a level at which said laser 15 20 25 30 35 40 45 55 60 65 20 light receiver produces an output from said noise light pulses having a constant pulse firing rate 19 The automatic noise threshold circuit of claim 18 wherein said circuit for automatically adjusting a noise threshold of said laser light receiver comprises a diode
32. eedback loop comprising the detected average noise firing rate forming a feedback loop that controls the threshold Use of this circuit has resulted in an addition of almost 5066 to the range of the laser range finder 10 compared to attempting to manually set the thresh old By setting the noise firing rate noise pulses are being produced deliberately all the time The only way to take advantage of that fact is by implementing a firmware algo rithm in the microcomputer 270 that discriminates between 5 612 779 17 noise pulses and laser return pulses The algorithm operates as follows During the laser firing process on the first pulse that fires the algorithm gets a laser pulse and it places it in a stack of pulses For example the stack may have locations designated 0 through 9 to enable 10 pulses to be maintained in the stack The values of the are saved corrected for power return the microcomputer 270 deter mines the power level of the return signal and corrects the flight time for power return and placed in one of the locations in the stack Upon receipt of the next pulse the microcomputer 270 will then compare the next pulse with the remaining locations in the stack Initially most of the locations will be empty and there will be no match If no match is found the microcomputer 270 puts the pulse in the stack and carries on merely placing pulses in the stack Then when it gets to the top it goes back and o
33. el to change are carried out by removing charge from said capacitor as determined by a resistor that is switched in parallel with said capacitor 33 The method of claim 29 wherein said first time period T1 said second time period T2 and said third time period T3 are determined by a clock reference source 34 The method of claim 33 wherein said clock reference source comprises a crystal oscillator 35 Apparatus for determining a range to a target said range being based upon an actual flight time of a pulse that is directed toward said target said method comprising the sequential steps of means for establishing first and second electrical refer ence levels 20 30 35 45 50 55 60 65 22 means for initially unclamping said second electrical ref erence level means for allowing said second electrical reference level to change at a first rate relative to said first electrical reference level means for storing a first time period T1 that is represen tative of a time that expires between said step of initially unclamping said second electrical reference level until said first and second electrical reference levels become equal means for re establishing said first and second electrical reference levels means for again unclamping said second electrical refer ence level means for changing said second electrical reference level at a second rate that is higher than said first rate for a predetermined t
34. evel the laser range finder 10 must work Having found a match the average of the match values may then be used to compare all subsequent pulses rather than needing to place them in a stack and only pulses that match up with that initial match average will contribute to the measurement If a certain number of pulses elapse before another matching pulse is received it may be assumed that an accidental lock on to noise has been achieved and the process restarts By adjusting the various parameters a trade off can be made between the time it takes to get a measure ment to how far into the noise the laser range finder 10 must work Because the noise rate can set to whatever is desired by means of the automatic noise threshold section 36 it is possible to optimize the algorithm to provide the optimum acquisition characteristics against time and against range The higher the value of the more noise is coming out of the receiver and the more sensitive the laser receive section 22 is running The probability of a noise pulse showing up is proportional to the flight time so given a very black target the maximum range will be less but the maximum flight time is also less so a higher noise rate can be tolerated Therefore running at a higher gain will provide the best range to a black target On the other hand if the target is very reflective a high gain is not required so the noise rate can be lowered which then provides the same pro
35. f claim 9 wherein said noise level setting signal from said central processing section selectably alters said desired signal to noise ratio to be an alternative signal to noise ratio that is selected by a user of said signal transmitting device 11 A method for discriminating between an actual return reflected signal and associated noise in a signal receiving section of a signal transmitting device the method compris ing the steps of transmitting a series of signal pulses to a target receiving a number of reflected signal pulses from said target said reflected signal pulses including both noise and actual return reflected signal pulses assigning a pulse value for each of said reflected signal pulses with respect to said series of signal pulses transmitted to said target comparing each of said assigned pulse values with other ones of said assigned pulse values continuing to perform said comparing step until a prede termined number of said assigned pulse values coincide within a specified precision and determining said actual return signal to be represented by said predetermined number of said assigned pulse values 12 The method of claim 11 wherein said step of trans mitting is carried out by a laser transmitter 13 The method of claim 11 wherein said step of receiving is carried out by a laser receiver 14 The method of claim 11 whercin said step of assigning is carried by measuring a receipt time of said refl
36. hold in response to a programmed microcom puter 28 The automatic noise threshold circuit of claim 25 further comprising selectively applying a desensitizing volt age to said noise threshold 29 A method for determining a range to a target said range being based upon an actual flight time of a pulse that 5 612 779 21 is directed toward said target said method comprising the sequential steps of establishing first and second electrical reference levels initially unclamping said second electrical reference level allowing said second electrical reference level to change at a first rate relative to said first electrical reference level storing a first time period T1 that is representative of a time that expires between said step of initially unclamping said second electrical reference level until said first and second electrical reference levels become equal re establishing said first and second electrical reference levels again unclamping said second electrical reference level changing said second electrical reference level at a second rate that is higher than said first rate for a predetermined time period in order to establish a third electrical reference level allowing said third electrical reference level to change at said first rate relative to said first electrical reference level storing a second time period T2 that is representative of atime that expires between said step of again unclamp ing said second
37. ime period in order to establish a third electrical reference level means for allowing said third electrical reference level to change at said first rate relative to said first electrical reference level means for storing a second time period T2 that is repre sentative of a time that expires between said step of again unclamping said second electrical reference level until said first and third electrical reference levels become equal means for again re establishing said first and second electrical reference levels means for once again unclamping said second electrical reference level means for again changing said second electrical reference level at said second rate that is higher than said first rate for a time period that is related to said actual flight time of said pulse toward said target in order to establish a fourth electrical reference level means for allowing said fourth electrical reference level to change at said first rate relative to said first electrical reference level means for storing a third time period T3 that is represen tative of a time that expires between said step of once again unclamping said second electrical refer ence level until said first and fourth electrical reference levels become equal and means for computing a range to said target as a function of the quantity T3 T1 T2 T1 36 The apparatus of claim 35 wherein said means for of establishing said means for re establishing and sai
38. inherent complexity translates to a cost and form factor most suitable only for certain specific applica tions A need therefore exists for a laser based range finder of perhaps more limited range which can be economically manufactured as a rugged compact unit to provide accurate distance measurement capabilities in other less stringent types of applications SUMMARY OF THE INVENTION Herein disclosed is a precise yet accurate and reliable laser range finder which may be economically produced and is adapted to individual portable use in a unit potentially weighing less than a pound with an on board battery based power supply Moreover the compact instrument herein provided has a number of user selectable target acquisition operational modes which may be invoked depending on the distance type and reflectivity of the target being sighted Through the use of an in sight display distance or range information can be shown while the user may also view and select the instrument s mode of operation through succes sive actuations of a push button mode switch while simul taneously sighting the target object A precision mode of operation may also be invoked in which an even more precise measurement to an object may be achieved follow ing an initial measurement together with the visual indica tion of a precision flag on the in sight display 20 25 30 40 50 55 65 2 A highly precise range measurement is made possible thr
39. intervening obstructions This is accomplished by not allowing flip flop 162 to trigger until a set timer period has elapsed Transistor 174 is the switching device utilized to allow setting of an extension to the hold off range and gate 180 is used to determine the receive pulse width in conjunction with the discharge rate of capacitor 194 This allows the microcomputer 270 which has a built in analog to digital A D convertor to determine the residual voltage on capacitor 194 and therefore derive a measure of the pulse width which is a measure of the return signal power and thus use an internal lookup table to correct for that power variation and get a higher range accuracy When the logic reset signal RESET on line 156 is low transistor 190 clamps capacitor 194 to the 5 volt rail During the laser measurement routine the transistor 190 is turned off When a pulse subsequently arrives that bit turns on transistor 200 and the voltage in capacitor 194 will be discharged via resistor 198 for the duration of that pulse The charge on capacitor 194 is then digitized by the processor to determine the effect of incoming power With reference additionally now to FIG 8 the automatic noise threshold section 36 of the laser range finder 10 is shown The automatic noise threshold section 36 receives the RX OUT signal from the laser receive section 22 shown in FIG 1 on line 100 for input thereto through resistor 314 Resistor 314 is connecte
40. lly coincident and maximum range of the laser range finder 10 Times through represent the range of times depending on the distance to the target when the value of V1 is discharged below the level of V2 and the comparator 236 output changes state stopping the timer The actual laser flight time LASER or FLIGHT ryme then equals T or minus minus ZERO or minus T 3 The time has to be greater than T and is greater than or equal to There is no theoretical limit on the lower range of the laser range finder 10 and flight time and distance can be measured down to zero due to its linearity The only factors in the near zero range are the time it takes transistor 210 to turn on the propagation time of the laser beam and the various circuit gates but since the time for each of these factors is the same during calibration as during flight time they essentially cancel out The precision timing section 34 can be effectively utilized down to on the order of ten nanoseconds and still remain perfectly linear RANGE to a target is then a constant times the quantity FLIGHT over 2 rpg For each of the values ZERO 4 and FLIGHT values are accumulated and are expressed in time units that derive from the very accurate crystal oscil lator 30 Typically ten pulses may be utilized to establish the
41. od for discriminating between an actual return signal and associated noise in a signal receiving section of a signal transmitting device The method com prises the steps of transmitting a series of signal pulses to a target and receiving a number of possible reflected signal pulses therefrom with the possible reflected signal pulses including both noise and actual signal pulses representa tive pulse value is assigned for each of the possible reflected signal pulses with respect to the series of signal pulses transmitted to the target and each of the representative pulse values is compared with other ones of the representative pulse values Each of the representative pulse values are compared until any predetermined number of the represen 5 612 779 3 tative pulse values coincide within a specified precision and thc actual return signal is determined to be represented by the predetermined number of the representative pulse values or they arc averaged to producc a greater precision value DETAILED DESCRIPTION OF THE DRAWINGS The foregoing and other features and objects of the present invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of a preferred embodiment taken in conjunction with the accom panying drawings wherein FIG 1 is a simplified logic block diagram of a laser range finder in accordance with the present invention
42. ollector terminal of transistor 174 is coupled through capacitor 170 to the input of the invertor 168 coupled to the Q output of flip flop 158 Transistor 174 has its based coupled to circuit ground through resistor 176 and receives a HOLD OFF signal on node 178 received from the CPU section 28 The flip flop 158 receives an input to its CLK terminal on line 94 comprising the REF output signal from the laser transmit section 18 shown in FIG 1 Its data D input is coupled to a source of 5 volts and the Q1 output of the shift register 160 is provided to the active low set 5 input as shown The Q output of flip flop 158 is supplied as one input to a transmit gate 204 having its other input coupled to the output of an invertor comprising an additional NAND Schmitt trigger 202 Invertor 202 has one input connected to a source of 5 volts and another input connected to the Q output of flip flop 162 Flip flop 162 has its S input coupled to the Q7 output of shift register 160 and its D input connected to the output of invertor 168 The Q output of flip flop 162 is supplied on line 184 to comprise a RX DETECT signal for input to the CPU section 28 shown in FIG 1 The flip flop 162 has its CLK input connected to line 100 for receiving the RX OUT signal from the laser receive section 22 shown in FIG 1 which is also supplied as one input to NAND Schmitt trigger 180 The other input of NAND Schmitt trigger 180 is connected to line 184 th
43. ough the use of a novel and efficient timing circuit which makes use of the instrument s internal central processing unit crystal oscillator A likewise unique automatic noise threshold determining circuit allows for instrument opera tion with a low signal to noise ratio to optimize sensitivity and performance in conjunction with a processor based pulse discrimination procedure which nevertheless assures accurate range measurements The unit herein disclosed can be utilized in a multitude of endeavors including such recreational activities as golf where it can be utilized to very accurately determine the distance to a flag or pin as well as to trees and other natural objects The principles of the invention are further appli cable to the design of a laser based tape measure where ranges can be precisely measured with resolutions of on the order of an inch or less Specifically disclosed herein is an automatic noise thresh old system for operative association with central processing and signal receiving sections of a signal transmitting device for discriminating between an actual return signal and associated noise The system includes means responsive to the central processing section for determining a desired signal to noise ratio for a series of possible signal pulses including both noise and actual signal pulses received through the signal receiving section The possible signal pulses each have a representative pulse value with respect
44. p meaning that the fixed width pulse feeding the current switching circuit at the output of the transmit gate 204 is precisely six clock cycles The time difference between the Q1 and Q7 outputs of the shift register 160 is exactly 750 nanoseconds when utilizing an 8 MHz oscillator 30 applied to its CLK input The invertor 202 adds an additional delay of about 10 nanoseconds for a total of delay of about 760 nanoseconds which varies only slightly with temperature perhaps one or two nanoseconds yet still provides sufficient precision for measurements of less than one yard resolution Transistor 210 is then turned on for a period of time between T and to enable the capacitor 244 to charge very rapidly and then discharge at the same rate as has been previously shown with respect to FIG 7A As V1 reaches the level of V2 the TIMER signal goes low at Time The fifty microsecond delay between the unclamping at and Tg is to allow the clamp transistor 214 to turn off fully since it is a relatively inexpensive bipolar device If a MOSFET were used instead its turn off would be virtually instanta neous and the additional delay it introduced would not be a problem because the microcomputer 270 could not issue the next instruction quickly enough Utilizing a bipolar device approximately 20 microseconds are required for the dis charge to become linear and the slope of the discharge curve between and T is then identical to the slope from
45. r 134 is supplied through a resistor 148 to line 100 to provide the RX OUT signal while the output of the precision comparator 134 is supplied through resistor 150 to line 102 to provide the signal With reference additionally now to FIG 4 a portion of the precision timing section 34 shown in FIG 1 is illustrated A CPU clock CLK signal is input to the precision timing section 34 on line 152 to the CLK input of a serial in parallel out shift register 160 from the oscillator 30 as previously shown in FIG 1 An additional input to the shift register 160 is received on line 154 comprising a NORM CAL signal from the CPU section 28 to the data set B DSB input thereof The active low clear CLR input and DSA input are held high as shown An additional input to the precision timing section 34 is received from the CPU section 28 shown in FIG 1 on line 156 comprising a RESET signal for input to the reset R inputs of D type flip flop 158 and flip flop 162 The Q output of flip flop 158 is supplied as one input to an invertor comprising a portion of a NAND Schmitt trigger 168 through a low pass filter comprising resistor 164 and capacitor 166 as shown The remaining input to the invertor 168 is connected to a source of 5 volts A resistor 172 couples a source of 5 volts to the collector of transistor 174 having its emitter coupled to circuit ground 25 30 45 55 60 65 6 The c
46. roportional to the incoming laser light which is generally a short duration laser pulse producing a short current pulse which is amplified by transistors 118 120 122 124 com prising the active circuit elements of a transimpedance amplifier 116 The transimpedance amplifier 116 produces an output voltage pulse proportional to the incoming laser pulse impinging on the laser receiving diode 24 The output of the transimpedance amplifier 116 is capacitively coupled to the input of comparator 134 which is a high speed comparator When the laser pulse input to the input crosses a threshold determined by the voltage on the threshold pin a positive output pulse is produced maximize performance the threshold of the compara tor 134 has to be set for maximum sensitivity in order detect the weakest possible laser pulse to get the maximum per formance out of the laser range finder 10 Conventional approaches include using digital controls or a potentiometer to adjust the threshold However these approaches have the down side that over time and temperature changes gain of the receiver will change with the background noise generated by the background light rendering a fixed thresh old as less than an ideal solution The automatic noise threshold section 36 of FIG 8 discloses a circuit that automatically sets a threshold such that a constant noise pulse firing rate is output from the detector comprising resistor 314
47. rough resistor 182 and coupled to circuit ground through capacitor 186 The output of Schmitt trigger 180 is supplied to the base electrode of transistor 200 which has its collector terminal coupled to circuit ground Line 196 comprising an analog to digital A D POWER CORRECTION signal 15 supplied to the emitter terminal of transistor 200 through resistor 198 as well as to the collector terminal of transistor 190 which is coupled to circuit ground through capacitor 194 The RESET signal on line 156 is supplied to the base terminal of transistor 190 through resistor 188 A source of 5 volts is connected to the emitter of transistor 190 as well as through resistor 192 to the base of transistor 190 to provide an operating bias Referring additionally now to FIG 5 the remaining portion of the precision timing section 34 shown in block form in FIG 1 is illustrated The HOLD OFF signal output from CPU section 28 to the precision timing section 34 is supplied on line 258 through resistor 256 to node 178 for input to the base of transistor 174 shown in FIG 4 The output of transmit gate 204 appearing on node 206 is supplied through resistor 208 to the base terminal of tran sistor 210 A source of 5 volts is supplied to the emitter terminal of transistor 210 through the series connection of resistor 216 and resistor 222 The node intermediate resistors 216 and 222 is coupled to circuit ground through the parallel combination of capacitor
48. rred embodiment comprise a ST6240 device An 8 megahertz MHz crystal 274 forms a portion of the oscillator 30 for providing an oscillator OSCIN and oscillator out OSCOUT signal to the microcomputer 270 as well as supplying a CPU CLK signal on line 152 for input to the precision timing section 34 as previously described The VDD input of microcomputer 270 is coupled to a source of 5 volts and the RESET input thereof is held high through pull up resistor 276 which is coupled to circuit ground through capacitor 278 Output from the microcom puter 270 is taken on a display bus 280 comprising the communication lines COM 1 COM 4 and 516 528 lines for input to the LCD display 32 An A D LOW BATTERY signal a TRIGGER signal and a POWER CONTROL signal are input to the microcomputer 270 on lines 284 286 and 288 respectively The A D LOW BATTERY signal on line 284 is also supplied to the input of comparator 296 which is coupled to circuit ground through capacitor 304 The input of comparator 296 is coupled to a source of 5 volts through resistor 298 which is also coupled to circuit ground through the parallel com bination of resistor 300 and capacitor 302 The output of comparator 296 appearing on line 306 provides a SHUT DOWN signal for the laser range finder 10 in the event the onboard battery voltage drops below a predetermined limit The microcomputer 270 supplies the HOLD OFF signal on line 258
49. s to a flight time of a said signal pulse previously transmitted from said signal pulse transmitting device 4 The automatic noise threshold system of claim 1 wherein said preselected number of said representative pulse values placed in said stack is ten 5 The automatic noise threshold system of claim 1 wherein said predetermined number of said representative pulse values placed in said stack is two 6 The automatic noise threshold system of claim 1 wherein said predetermined number of said pulse values that coincide within said specific precision are averaged to represent said actual return signal pulse 7 The automatic noise threshold system of claim 1 wherein said determining means for determining said 5 612 779 19 desired signal to noise ratio comprises a detector coupled to an output of said signal receiving section for producing a substantially constant noise pulse firing rate output from said detector 8 The automatic noise threshold system of claim 7 wherein said detector further comprises an operational amplifier coupled to an output of said detector for providing a threshold signal to said signal receiving section 9 The automatic noise threshold system of claim 8 wherein said threshold signal is supplied to a summing node with at least one noise level setting signal from said central processing section for determining said threshold signal as an output of said summing node 10 The automatic noise threshold system o
50. s 218 and 222 as well as to the output of comparator 236 through resistor 246 to provide a TIMER signal on line 250 for input to the CPU section 28 as will be more fully described hereinafter The source of 5 volts is also connected to the base terminal of transistor 210 through the series connection of resistors 216 and 224 A V node 228 at the common connected base of transistor 212 and emitter of transistor 214 is coupled through a source of 5 volts through resistor 216 and resistor 226 Node 228 is connected through resistor 230 to V node 232 which in turn is connected to circuit ground through resistor 240 A capacitor 238 couples V node 228 to circuit ground node 232 is connected to the input of comparator 236 node 228 is connected to line 254 from the CPU section 28 shown in FIG 1 to receive the CAL DITHER signal through resistor 252 5 612 779 7 The collector terminal of transistor 210 is coupled to the collector terminals of transistors 212 and 214 as well as to the terminal of comparator 236 which in turn is coupled 10 circuit ground through capacitor 244 A RUN CLAMP signal output from the CPU section 28 shown in FIG 1 is furnished on line 260 through resistor 248 for input to the base terminal of transistor 214 With reference additionally now to FIG 6 the CPU section 28 is shown in greater detail The CPU section 28 comprises in pertinent part a microcomputer 270 which may in a prefe
51. s can not be directly driven with the reset circuit and must be driven off the Q outputs in both cases which introduces a small fixed offset delay which must be accounted for later As soon as the NORM CAL signal on line 154 is dropped low which occurs approximately 50 microseconds after the clamp has been released the low 5 612 779 signal propagates through the shift register 160 precisely with the main oscillator 30 clock The Q0 output of the shift register 160 is the first to be triggered but is not used because it is used to synchronize with the incoming signal The Q1 is then the first output of the shift register 160 to be utilized and on every positive edge of the clock the zero signal that is applied into the serial input will propagate one state of the shift register 160 from Q zero to Q7 Therefore the Q1 output will go low first and as soon as that output goes low the set line input S forces the output of flip flop 158 to go high since the Q output of flip flop 162 is in the low state As a result logic level ones appear at the two inputs of the transmit gate 204 which turns on the current switch tran sistor 210 Exactly six clocks later the same thing happens with flip flop 162 which has its S input coupled to the Q7 output of the shift register 160 As the Q output of flip flop 162 goes high the output of the invertor 202 goes low and the transmit gate 204 will be turned off At this point the count pulse will sto
52. ser pulses on the clock line are ignored The purpose for this function is that when the laser fires it generates unintended ground bounce and noise that may prematurely trigger the receive flip flop 162 rather than the real laser return signal RX OUT For that reason a hold off period is provided corresponding to the minimum range of the laser range finder 10 and as an example considering a minimum range of about twenty yards the holdoff time is approxi mately 60 nanoseconds With a lower sensitivity laser range finder 10 utilized at shorter ranges the function can be eliminated and it is clearly most useful with a high sensi tivity receiver where the noise from the firing circuit deter mines an effective minimum range Transistor 174 provides an additional function and allows the microcomputer 270 to extend the hold off range by asserting thc HOLD OFF signal on line 258 In this manner the minimum range of the laser range finder 10 may be extended out to for example sixty or eighty yards whatever is the desirable setting This microcomputer 270 hold off function may be implemented by the mode switch 126 and would allow shooting through branches twigs precipitation or other partial obstructions By extending the hold off range out beyond such partial obstructions there is insufficient back scatter from the obstructions to trigger the precision timing section 34 and the measurement will be made to the desired target instead of the
53. ter 332 is used to establish the initial set point Since the noise characteristics from unit to unit will vary somewhat potentiometer 332 enables the setting of the initial device characteristics Resistor 342 is of a considerably lower value than resis tors 338 and 340 and its value is chosen such that when the REFLECT 10 MODE signal on line 312 is asserted by being taken high V4 will drop to zero and will stay there because it cannot go below zero At this point the feedback loop is saturated and no longer effective so V is no longer is stabilized In operation line 312 will be pulled high by a considerable voltage on the order of 0 4 volts such that it completely desensitizes the laser receive section 22 so the laser range finder 10 will then only respond to a retro reflector In this mode of operation the receiver is detuned and its non cooperative range drops from 500 yards down to about 30 or 40 yards such that the laser range finder 10 only latches onto a retro reflector or survey prism comprising a high grade reflector that returns the laser energy back to the source Possible applications also include determining the distance to a particular golf hole where a laser reflector is attached to the pin and the signal might otherwise be actually returned from trees behind or in front of the green in a more sensitive mode of operation The essence of the automatic noise threshold section 36 is as previously described a f
54. the RUN CLAMP signal on line 260 the CAL DITHER signal on line 254 the RESET signal on line 156 and the NORM CAL signal on line 154 for input to the precision timing section 34 as has been previously described The microcomputer 270 receives as outputs from the precision timing section 34 the RX DETECT signal on line 184 and the TIMER signal on line 250 Additional inputs to the microcomputer 270 are the FIRE signal on line 80 from the laser transmit section 18 shown in FIG 1 as well as the A D POWER CORRECTION signal on line 196 from the precision timing section 34 as shown in FIG 4 A MODE input signal on line 294 is received from the mode switch 126 which is otherwise held to 5 volts through resistor 292 Microcomputer 270 supplies an NSET1 and NSET2 signal on lines 308 and 310 respectively as well REFLECTION MODE signal on line 312 for input to the automatic noise threshold section 36 as shown in FIG 1 In overall operation a reference signal REF on line 94 is generated by the laser transmit section 18 shown in FIG 2 when the laser range finder 10 is fired by placing a current pulse through the laser emitting diode 20 in response to manual actuation of the trigger switch 14 The REF signal on line 94 is derived from the current placed through the laser emitting diode 20 and not from the light pulse itself and is sufficiently precise for accurately indicating the time of the laser firing The REF signal is ultimately inp
55. ts from the CPU section 28 including a number of noise set NSET signals and REFLECTION MODE signal to operatively control its function With reference additionally now to FIG 2 the laser transmit section 18 is shown in more detail The laser transmit section 18 receives a transmit TX BIAS signal on supply line 50 of approximately 110 to 140 volts for application through resistor 52 to the emitter of transistor 54 The emitter of transistor 54 is coupled to its base by means of a resistor 58 which also couples the collector of transistor 56 to resistor 52 The emitter of transistor 56 is connected to circuit ground on ground line 60 A capacitor 62 couples the emitter of transistor 54 to the cathode of the laser emitting diode 20 which has its anode also connected to circuit ground 60 An additional diode 64 is coupled in parallel with the laser emitting diode 20 having its anode connected to the cathode of the laser emitting diode 20 and its cathode connected to circuit ground 60 A resistor 66 is placed in parallel with the laser emitting diode 20 and the diode 64 source of 5 volts is also received by the laser transmit section 18 on supply line 68 through resistor 70 Resistor 70 is coupled to the emitter of transistor 72 as well as to circuit ground 60 through a capacitor 74 A resistor 76 couples the emitter of transistor 72 to its base which is coupled through resistor 78 to line 80 for supplying a FIRE signal to the CPU se
56. tude of the noise pulses is typically at or near the logic threshold except for very narrow pulses where the comparator will not reach full amplitude however the width of these pulses is going to vary randomly because it depends on the noise signal that is being detected Moreover thc spacing of the noise pulses will also vary at a random rate but for any given threshold setting there will be a fixed average rate The average rate is dependent on the threshold Therefore during the time the pulse is high capacitor 324 charges via resistor 314 and diode 316 at a rate determined by the high on the logic pulse resistor 314 and whatever voltage is still existing on capaci tor 324 Initially capacitor 324 is charged as follows Once the noise pulse terminates the logic line goes back to zero There is a residual voltage on capacitor 324 diode 316 will be reverse biased and the discharge path is now via resistor 322 As previously described the value for resistor 322 is chosen to provide a relatively longer time constant by a factor of 150 When another pulse comes in capacitor 324 will charge a bit more What will then happen is quite rapidly within few milliseconds the voltage across capacitor 324 stabilizes at a rate that is proportional to the average firing rate The reason for having a large ratio between resistor 314 and resistor 322 is because the noise pulses typically may average 50 nanoseconds wide and the aver
57. uples the supply line 104 to circuit ground 60 to provide a bias signal to the cathode of the laser receiving diode 24 The laser receiving diode 24 has its anode connected to the base of transistor 118 which in conjunction with transistors 120 122 and 124 comprises a transimpedance amplifier 116 providing an output on node 126 which is capacitively coupled to the input of a precision comparator 134 A source of 5 volts is input to the laser receive section 22 from the main power supply unit 12 shown in FIG 1 for input to the transim pedance amplifier 116 through a low pass filter comprising resistor 130 and capacitor 132 The 45 volt RX supply voltage is also coupled to the V input of the precision comparator 134 through resistor 136 and is coupled to circuit ground through capacitor 138 The input of the precision comparator 134 is connected between the plus 5 volt RX voltage source and circuit ground 60 through the node intermediate resistor 142 and resistor 144 The precision comparator 134 which may in a preferred embodiment comprise a MAX 913 low power precision transistor transistor logic TTL comparator available from Maxim Integrated Products Inc Sunnyvale Calif has its LE and ground GND inputs connected to circuit ground 60 as shown capacitor 146 couples the output of the precision comparator 134 to circuit ground 60 as shown The O output of the precision comparato
58. ut to the CLK input terminal of flip flop 158 which has its output coupled to the transmit gate 204 which then turns on the current switch comprising transistor 210 and starts charging the capacitor 244 When the receive pulse RX OUT line 100 comes back from the laser receive section 22 10 15 20 25 30 35 40 45 50 55 60 65 8 shown in FIG 3 it triggers the flip flop 162 at its CLK input Flip flop 162 has its Q output coupled to the input of invertor 202 which then shuts the transmit gate 204 off stopping the current pulse At this point a constant current sink discharges capacitor 244 In this manner capacitor 244 is charged up with a relatively large current on the order of 10 milliamps and later discharged with a small current on the order of 10 microamps applied over the entire flight time of the laser pulse from its firing from the laser emitting diode 20 to its reflection from a target back to the laser receiving diode 24 Because the laser range finder 10 is intended for a shorter maximum range than other laser based range finding instruments the use of this technique does not require a separate counting oscillator followed by an inter polation operation and the entire flight time is essentially stretched by a factor of 1000 and then the stretched result is counted By charging capacitor 244 at a fast rate and then discharging it and then monitoring the time it takes to discharg
59. value that relates to the mount of time it takes for capacitor 244 to discharge from V1 down to V2 This process is repeated several times and the result is averaged Typically ten iterations may be performed with the results accumulated and an average time computed As shown particularly with respect to FIG 5 the CAL DITHER signal on line 254 is applied to the base terminal of transistor 212 and is utilized during both the zero cali bration and fixed pulse width calibration times and incor porates a relatively high value resistor 252 The CAL DITHER signal allows for the introduction of a deliberately controlled change in the discharge current in order that the resultant count will vary slightly such that when the total counts are averaged together a finer resolution is produced than would be the case merely using a fixed current to get the same count value An adjustment of one part in about a thousand is provided during the zero calibration and fixed pulse width calibration modes because the finite resolution of the microcomputer 270 timer otherwise provides discreet timing intervals of 1 5 nanoseconds which would only provide distance measurement resolution of approximately one yard operation the zero calibration count in the microcomputer 270 will typically be about 150 while in the fixed pulse width calibration mode it will be on the order of 900 The flight time count during the laser mode of operation can be anything from close
60. verwrites the base so history of N number of pulses is developed in the stack Any time a new pulse comes in it compares the entire stack for a match If N 10 it searches the preceding ten pulses for a match The reason for doing that is since a high noise firing rate has been deliberately set to get maximum sensitivity many noise pulses are going to have shown up but the noise pulses will be of random occurrence and the chance of a precision match is very low Because the tolerance can be set as any other firmware parameter a default value will be typically loaded that has been determined empirically As an example a tolerance of a few nanoseconds may be set for a match to be assumed to be a real target and not a noise pulse Utilizing the algorithm the process continues trying to lock on the target until a match is achieved The match need only be two pulses within the preset tolerance providing very acceptable results or if higher sensitivity were desired a match of three through N may be specified depending on the reli ability needed to guarantee a real target and not a noise pulse In an exemplary operation the first pulse pulse 0 could be the real target followed by eight noise pulses and as long as the ninth pulse is again the real target the distance to the target can be accurately determined The stack can be increased in size up to whatever memory limit is available in the system depending on how far into the noise l

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