MultiPoint Calibration Primer
Contents
1. SINT SENSOR CORPORATION Application Note Multipoint Calibration Primer Introduction PNI Corporation s Reference Magnetic Sensor Suites are based on patented technology that delivers breakthrough cost effective magnetic sensing performance The sensors change inductance with different applied magnetic fields This variable inductance property is used in a patented temperature and noise stabilized oscillator counter circuit to detect field variations PNI s 2 axis RM2000 Reference Magnetic Sensor Suite is comprised of two Sen X Y magneto inductive sensors and the 3D MagIC ASIC controller The 3 axis RM3000 Suite adds the z axis Sen Z sensor The Sen XY and Sen Z sensors employ a single solenoid winding and consume roughly an order of magnitude less power than conventional fluxgate or magneto resistive technologies The sensor coil serves as the inductive element in a very simple low power LR relaxation oscillator with its effective inductance being influenced by the magnetic field component parallel to the coil axis The frequency of the oscillation changes with the magnetic field The 3D MagIC outputs a digital signal that can be fed directly into a microprocessor This eliminates the need for signal conditioning or an analog digital interface between the sensor and a microprocessor The simplicity of the sensor circuit combined with the lack of signal conditioning makes the magneto inductive sensor easier and less expensive to use in2. Y values when the backlight is turned on take the raw sensor readings subtract the standard calibration offsets and then subtract the backlight offsets Thus the X and Y values used to calculate heading would be e X Xraw Xoffset Xbacklight_offset e Y Yraw Yoffset Ybacklight_offset If the backlight is off then do not subtract the Xbacklight_offset and Ybacklight_offset when performing the heading calculations PNI Sensor Corporation Doc 1001766 r02 Multipoint Calibration Primer September 2010 Page 8 of 8
3. a graph of the points X Y should show a circle with the center in the origin The following code demonstrates how the raw X Y point is corrected using the calibration values to get a perfect circle at the origin Compute Heading float pi atan 1 4 compute PI i nt Xval Yval i nt angl e headi ng Get RawXY get coordi nates Xval Xraw Xoff set subtract out the offsets Yval Yraw Yoff set if Xrange gt Yrange Yval Yval Xrange Yrange perform gain matching else Xval Xval Yrange Xrange perform gain matching angle atan Yval Xval 180 pi compute the angle if Xal gt 0 amp amp Yval gt 0 heading angle quadrant X Y 0 to 90 else if Xval lt 0 amp amp Yval gt 0 heading 180 angle quadrant X Y 91 to 180 else if Xal lt 0 amp amp Yval lt 0 heading 180 angle quadrant X Y 181 to 270 else if Xal gt 0 amp amp Yval lt 0 heading 360 angle quadrant X Y 271 to 359 PNI Sensor Corporation Doc 1001766 r02 Multipoint Calibration Primer September 2010 Page 5 of 8 Figure 4 shows the plot of the corrected X Y points using the calibration values The offsets move the center of the circle back to the origin and the ranges are used to match the gain so the graph becomes a centered circle instead of the original off center ellipse Figure 4 PNI Sensor Corporation Doc 1001766 r02 Multipoint Cali
4. your product design than fluxgate or magneto resistive sensors This primer covers the basics of how to determine heading calibrate out hard iron distortions and perform sensor gain matching for a 2 axis compass built with the RM2000 Reference Magnetic Sensor Suite More detailed information regarding how to implement the RM2000 Sensor Suite and the underlying technology can be found on PNI s website Please refer to the following e Manuals RM2000 RM3000 Reference Magnetic Sensor Suite Manual e White Papers Magneto Inductive Technology Overview Compass Coordinates X Figure 1 In Figure 1 the X axis points to magnetic north while Y axis points to magnetic east With this arrangement the heading 1s the clockwise angle from the X axis We calculate the angle to the X axis X or X and not necessarily the heading as follows angle atan Y X The heading calculation depends on the sign of the X and Y values the quadrant and is calculated as follows X Value Y Value Formula Quadrant T XoY 0 90 lt 0 Heading 180 angle X Y 181 270 gt 0 Heading 360 angle X Y 271 359 Doc 1001766 r02 PNI Sensor Corporation Page 2 of 8 Multipoint Calibration Primer September 2010 Sensor Orientation For the heading computation to be valid the following must be true e The sensors are at right angles with respect to each other an
5. bration Primer September 2010 Page 6 of 8 Notes Declination Declination is the angular difference between magnetic north and geographic north headings There is a difference between the two directions because Earth s magnetic north pole is not in the same location as the geographic north pole A compass measures Earth s magnetic field so it points to magnetic north Also the difference between magnetic north and geographic north varies from place to place Fortunately there are many ways to obtain declination Most hiking topographic maps provide the declination value Declination also can be found at web pages such as the National Geophysical Data Center s site http www ngdc noaa gov geomag Declination is defined as east if magnetic north falls to the east of geographic north and west if magnetic north falls to the west of geographic north A positive declination is east and a negative declination is west For instance the NGDC website listed above shows that for Mountain View CA 37N 122W the declination is 15d 25 7m First it is positive so it is eastward declination The d stands for degrees the m stands for minutes 60 minutes 1 degree so the declination is 15 4 east Once declination is known the compass heading can be corrected so that heading can be determined relative to geographic north Add the declination value to the compass measurement to get the geographic heading For example if the d
6. d lie perpendicular with respect to the Earth s gravity e By convention the positive end of the X axis points north and the positive end of the Y axis points east See Figure 2 e Prior to calibration the compass must be installed into the host system in its intended operating configuration and in a horizontally level position Front Figure 2 Determine Calibration Values As the title of this document implies multiple X Y points must be acquired to properly calibrate the compass The sensing circuit is composed of the Reference Magnetic Sensors the 3D MaglIC external bias resistors an external clock resistor and external capacitors Refer to the RM3000 RM2000 User Manual for the recommended circuit configuration Only one sensor can be measured at a time The user sends a command byte to the 3D MagIC through the SPI port specifying the sensor axis to be measured The 3D MagIC will return the result of a complete forward reverse measurement of the sensor in a 24 bit 2 s complement format with a range from 8388608 to 8388608 PNI Sensor Corporation Doc 1001766 r02 Multipoint Calibration Primer September 2010 Page 3 of 8 The sample code below shows how the acquisition phase of the calibration is done Acquisition Code i nt Xnrax Xm n Ymax i nt Xr aw Yraw Xmax Ymax 8388608 Xni n Ymi n 8388608 for 3 Get RawXY lf Xraw gt Xmx xmax Xraw lf Xraw lt Xnin X
7. eclination is 15 E add 15 to the compass measurement If the declination is 10 W subtract 10 from the compass measurement EL Backlight Disturbances When an electro luminescent EL or other backlight is used in a compass system turning the backlight on may cause a change in the heading reading This is because the backlight system usually draws enough current during operation to cause the voltage level of a battery based power supply to drop and this drop may affect the zero offset of the ASIC Additionally if there is backlight support circuitry such as inductors or other current loops then the magnetic fields generated by these components can directly affect the sensor readings An algorithmic solution to this issue can be implemented during the user calibration procedure First obtain the standard calibration values Xoffset and Yoffset with the backlight turned off Next turn on the backlight for 1 2 second and take a single magnetic measurement Xbacklight_on and Ybacklight_on and another measurement PNI Sensor Corporation Doc 1001766 r02 Multipoint Calibration Primer September 2010 Page 7 of 8 with the compass orientation unchanged but the backlight off Xbacklight_off and Ybacklight_off Calculate the backlight offsets Xbacklight_offset and Ybacklight_offset as follows e Xbacklight_offset Xbacklight_on Xbacklight_off e Ybacklight_offset Ybacklight_on Ybacklight_off To calculate the corrected X and
8. ni n Xraw If Yraw gt Ymx Ynax Yraw If Yraw lt Ynrin Ymi n Yraw a Nx NNS w x NS i where you store maxi mum and m ni mum val ues for X amp Y values you get fromthe 3D Magl C The nunbers here wll be 24 bit si gned val ues Positive 0x000000 to 0x7FFFFF Negative 0x800000 to OxFFFFFF start wth owest possi ble value start wth highest possi ble value acquire a X Y point now sort it The above code results in an infinite loop it should be modified so the loop stops when enough points are acquired Figure 3 below shows a possible graph of acquired points PNI Sensor Corporation Multipoint Calibration Primer September 2010 Figure 3 Doc 1001766 r02 Page 4 of 8 The graph in Figure 3 is elliptical with the major axis along the Y axis The elliptical nature is due to the Y values having a greater gain than the X values The center of the ellipse is also displaced The next section of code demonstrates how to compute the calibration values based on the maximum and minimum X and Y values acquired These values will later be used to correct the raw X and Y values acquired when computing the heading Calibration Values i nt Xrange Yrange i nt Xoffset Yoffset Xoffset gt gt 1 calculate offsets Yoff set Ymax Ymn gt gt 1 Xr ange i calculate the range Yr ange Compute Heading To compute the correct heading
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