Controlling the Motor – Programming the PWM
Contents
1. But after a very short time the PWM turns on again and the motor turns on again Hence the motor sees the average voltage of the PWM You can see how PWM controls the speed of the motor Higher the duty cycle the longer the PWM remains high the higher the average voltage and faster the motor turns The most important observation about the motor driver is the difference between pins 8 and 16 Pin 8 drives the motor refer to figure 11 so you should hook this up to the battery Please please be very cautious when connecting pins 8 and 16 PLEASE DO NOT SWITCH THEM Doing so will destroy the board If in doubt PLEASE ASK THE TA IN THE LAB 5 Motor code The code to be added to the previous program in this chapter is shown below Note that this assumes one motor is connected to PORT8 0 and PORTS 1 while the other is connected to PORTS 2 and PORTS 3 PLEASE NOTE YOU ALSO NEED TO HAVE A DUTY CYCLE GREATER THAN 80 FOR THE MOTORS TO TURN MODIFY YOUR PWM CODE ACCORDINGLY void main void Initialize MOTOR CONTROL DP8 0x03 set direction P8 0x02 Turn the motor END MOTOR CONTROL PWM CODE GOES HERE for 6 Summary This chapter showed you how to make the robot walk As you read through this chapter notice some subtle design problems you encountered 38 1 Taking care not to blow the board the motor driver 2 Understanding PWM generation2. cycle of 096 that is the signal is always low A value equal to approximately PP0 2 gives a waveform of duty cycle 50 high for one half the period and low for the other Hence the duty cycle depends on the value in PWO and is given by PWO 1 required duty cycle 100 0 PPO Eq 4 The required duty cycle is a float between 0 and 100 To summarize the formulae for setting PPO and PWO are PPO floor 1 0 counter resolution seconds x desired frequency Hz Eq 5 PWO 1 required duty cycle 100 0 PPO Eq 6 Please do not memorize the formulae above Rather try to understand how they came about As a good exercise try the following in the order of increasing difficulty 1 What is the PPO value for a frequency of 45 KHz with a counter resolution of 50 ns 2 What is the PPO value for a frequency of 100 Hz 3 What is the PPO value for a frequency of 1 MHz The answers are given below not in the same order as the questions Fill in the correct question number in the parenthesis next to the answer Figure 5 Eq 5 and Eq 6 are all you need for answering the questions Once again we are only concerned with mode 0 in figure 5 3125 444 20 My solutions are explained on the next page 3 Although you could specify decimal values for the duty cycle the value in PWO is still an integer So you cannot get the exact decimal duty cycle you want 28 Solution 2 3125 1 44
3. obtain 19 53 KHz Since we are talking about bit resolution what is 1023 in binary You will see it is 1111111111 in other words it has 10 bits Hence the formula for setting PPO is PPO floor 1 0 counter resolution seconds x desired frequency Hz Eq 3 The bit resolution concept is not very important I just mentioned it so that you won t be scratching your head when you read 10 bit PWM resolution The table is divided into bit resolutions just for convenience The most important concepts are shown in Eq 1 and Eq 2 Thus all you have to remember is that your period will depend on the product of your value in the PPO register and your counter resolution In our example we said it takes 50 ns for a count However looking at figure 5 we see that we can make the C167 take 3 2 us to count These are the two possible counter resolutions How do we tell the C167 which one to use Well we are going to be controlling counter resolution See that word control This should turn on some light bulbs in your head control registers Whoa hold on there Before we go on to control registers we should talk about how we specify the duty cycle The C167 keeps track of the count in the PTO register see figure 4 27 e Specifying PWM Duty cycle Figure 4 again comes to our rescue PWO or Pulse Width register O is used to specify the duty cycle A value equal to or greater than PPO results in a waveform that has a duty
4. 21 V on the top left is the trigger level 33 4 Supplying enough current The SN754410 In order to supply enough current you will be using a chip called a motor driver The part number for this chip is the SN754410 It is manufactured by Texas Instruments Figure 9 shows you how to mount this chip onto the KitCON 167 UU E J 26 v Xd ih Ye Figure 9 The white rectangle encloses my motor driver connections You can unscrew the plastic standoffs enclosed by the blue rectangles and tilt the board up You can insert the motor driver in the IC sockets underneath the KitCON DO NOT insert the motor driver in the area enclosed by the red rectangle Figure 10 shows an approximate model of this chip and figure 11 shows the wire wrap connections that you need to make between this chip and the KitCON Notice from figure 9 that I inserted a paper showing the different pins of the motor driver This is always a good idea while wire wrapping This extra work makes sure you don t get the pins mixed up Wire wrapping the motor driver is probably the most tedious boring and hardest part of the project PLEASE PLEASE PLEASE PLEASE WIRE WRAP NEATLY AND CORRECTLY Doing so will save you a ton of time when debugging Better yet your project might require no debugging at all Don t tell me I didn t warn you if the wire wrap is bad By the way my wire wrap in figure 9 is horrible 34 MOTOR DRIVER CHIP SN754410 DIAGRAM TO ILLUSTRATE
5. 4 3 20 1 What is the PPO value for a frequency of 45 KHz with a counter resolution of 50 ns PPO 444 Plugging in the counter resolution and the frequency in Eq 5 gives you the answer 2 What is the PPO value for a frequency of 100 Hz PPO 3125 Here we have not been specified the counter resolution We can determine it from figure 5 In order to get a frequency of 100 Hz you can t do it using the 50 ns counter resolution The maximum integer that can be stored in PPO is 65535 because this will take up all 16 bits This corresponds to a frequency of 305 2 Hz only with the 50 ns counter resolution Can we go lower than this Yes if we use the 3 2 us counter resolution Hence using 3 2 us and 100 Hz in Eq 5 we get PPO 3125 3 What is the PPO value for a frequency of 1 MHz Look at figure 5 What is the highest frequency you can get Remember PPO can take on any value between 0 and 65535 A value of 0 in PPO gives you a frequency of 0 in PPO What about a value of 1 in PPO Remember how the PWM works the counter counts till the PPO value incrementing every counter resolution and resetting back to 0 when the counter reaches PPO Well if PPO had a value of 1 then every 50 ns the counter would reset This corresponds to a frequency of 20 MHz Hence the maximum frequency you can get is 20 MHz which is the frequency of your on board clock So you should be able to do it with the 50 ns resolution Plugging in 50 ns and 1
6. Chapter Five Controlling the Motor Programming the PWM 1 A Quick Introduction to Motors The CalBOT is going to move on treads A pair of motors drives these treads You may wonder why we don t make a walking robot If you look up research literature you will find that legged robotics is still an exotic research subject The reasons for these are many and complicated A good starting point is the legged robotics lab at MIT www ai mit edu For the CalBOT we will be satisfied to use just a motor Figure 1 shows how a motor works Figure 1 Motor turns in one direction You supply power to the motor as shown in figure 1 and the motor turns If you want the motor turn in the other direction simply reverse the connections So to drive the motor from your KitCON you simple hook it up to one of the ports and write a 1 or 0 as needed correct WRONG If you do this you will end up blowing the board The reason the motor draws current when it is running As a simple exercise measure the current the motor draws by connecting your motor to the desktop power supply You will see that it is much higher than the maximum current the board can supply approximately 300 mA refer to the KitCON hardware manual Even if you could drive the motor you cannot control how fast your motor is turning Your digital board can output only 5 V or 0 V so you can just turn the motor on or off Hmmm so you want to accomplish three things 1 Con
7. M Channel 0 1 Burst Mode Control Bit 0 Channel 0 and channel 1 work independently in their respective standard mode 1 Outputs of channels 0 and 1 are ANDed to POUTO in burst mode PSx PWM Channel x Single Shot Mode Control Bit 0 Channel x works in respective standard mode 1 Channel x operates in single shot mode Figure 7 PWM control register 1 30 Figures 6 and 7 show the two PWM control registers They are extremely simple to understand First we enable the output and choose the edge aligned PWM mode for the PWM module we want I am using modules 0 and 1 so I set PVMCONI accordingly PWMCONI 0x0003 Next we choose the counter resolution and start the timer Suppose I want PWM module 0 to have a counter resolution of 3 2 us and PWM module 1 to have a counter resolution of 3 2 us The value in PWMCONDO is PWMCONO 0x0033 But where does the output go to If you read the first page of the PWM module chapter in the C167 hardware manual you will see PWMO through PWM3 are mapped to PORT7 0 through PORT7 3 Hence the PWM outputs go to port 7 in the C167 Thus we need to set DP7 0 and DP7 1 toa 1 DP7 0x03 Use P7 0 and P7 1 as output P7 20x00 Initialize P7 PORT7 0 and PORTT7 1 can be accessed using the KitCON connector Hence all we have to do now is connect a logic analyzer input to PORT7 0 and PORT7 1 We can then see the PWM waveforms 3 PWM Code The code for turning on PWM module 0 and 1 and setting the cor
8. MHz in Eq 5 gives you the answer Incidentally you can t get a frequency of 1 MHz using the 3 2 us counter resolution You can only go till a frequency of 3 125 KHz corresponding to a value of 1 in PPO when the counter resolution is 3 2 us All we have left to do is to understand how to control the counter resolution start the PWM and send it to the outside world Here is where the control registers come in 29 f PWM Control Registers PWMCONO PWM Control Register 0 SFR FF30 98 Reset value 0000 14 13 12 11 10 rw nw nw rw nw Bit Function PTRx PWM Timer x Run Control Bit 0 Timer PTx is disconnected from its input clock Ts Timer PTx is running PTIx PWM Timer x Input Clock Selection 0 Timer PTx clocked with CLKcp 1 Timer PTx clocked with CLKcp 64 PIEx PWM Channel x Interrupt Enable Flag 0 Interrupt from channel x disabled NE Interrupt from channel x enabled PIRx PWM Channel x Interrupt Request Flag 0 No interrupt request from channel x 15 Channel x interrupt pending must be reset via software Figure 6 PWM control register 0 PWMCON 1 PWM Control Register 1 SFR FF32 99 Reset value 00004 15 14 13 11 Bit Function PENx PWM Channel x Output Enable Bit 0 Channel x output signal disabled generate interrupt only 1 Channel x output signal enabled PMx PWM Channel x Mode Control Bit 0 Channel x operates in mode 0 i e edge aligned PWM 1 Channel x operates in mode 1 i e center aligned PWM PBO1 PW
9. OPERATION PINS ARE NOT IN CORRECT ORDER Pins 4 5 GND Goes to Board GND Pin 1 1 2 EN Pin2 1A Pin3 1 Y Pin 6 1Y Pin 7 1A AIL GND S SHOWN GO TO PINS PINS 4 5 12 and 13 Pin 8 VCC2 From Battery Directly Figure 10 The motor driver dissected Pin 16 VCC1 Pins 13 12 GND Goes to Board GND Pin 15 4 A Pin 14 4Y Pin 11 3Y Pin 10 3A Pin 9 34 EN 35 WIRE WRAP GUIDE Vcc from the Board P7 0 that is KitCON pin 1 PWM Signal 1 1 2 149 or 150 P8 0 2 P8 2 Motor 1 3 Motor 2 BATTERY Gnd 4 BATTERY Gnd 5 Motor 1 amp Motor 2 P8 1 T PS 3 Battery a PWM Signal 2 P71 Figure 11 Wire wrap guide P7_0 P7_1 P8_0 P8_1 P8_2 and P8_3 refer to PORT7_0 PORT7_1 PORTS 0 PORT8_1 PORT8_2 and PORTS 3 respectively We can understand how the motor driver works using figure 12 Figure 12 is just figure 11 zoomed in on the left hand side along with port numbers from figure 11 By the way the four ground pins are connected to the die heat sink plate and are meant to be thermal grounds but only one electrical ground is needed Actually it is better to connect this to the battery ground not the kitcon ground Thanks to Konrad Aeschanbach for this entry 36 HIGH CURRENT FROM THE BATTERY n 1 1 2 EN Fin 2 14 Fin 6 1 Y Pin 7 14 All GND S SHOWN GO TO PINS PINS 4 5 12 and 13 Figure 12 Motor
10. PWM 8 bit PWM Counter resol 20 MHz 1 50 ns 16 bitPWM Resolution 305 2 Hz 152 6 Hz 12 bit PWM 14 bit PWM Resolution Resolution 4 88 KHz 1 22 KHz 244 KHz 610 4 Hz 10 bit PWM Resolution 19 53 KHz 9 77 KHz Mode Resolution 20 MHz 64 3 2 us 477 Hz 238 Hz 76 29 Hz 19 07 Hz 38 15 Hz 9 54 Hz 305 2 Hz 152 6 Hz Figure 5 Different PWM frequencies 26 d Specifying PWM Frequency In figure 5 you can ignore the row under PWM Mode 1 As I said earlier we will be using mode 0 only In order to arrive at the frequency value the C167 uses the onboard clock The concept is extremely simple we use the periodic clock signal to count up from 0 Since the on board clock runs at 20 MHz the period is 50 ns In other words we count once every 50 ns Once we reach the value we have in the PPO register we reset the counter and start again Suppose we have a value of 1023 in the PPO register Hence the time it takes to count to 1023 is 1023 counts x 50 ns count 0 512 us Eq 1 However once we reach 1023 the counter is reset back to 0 So we have a periodic reset every 0 512 us In other words the frequency of the reset is 1 0 512 us 19 53 KHz Eq 2 Wait a minute we see the value of 19 53 KHz in the first row under the column 10 bit PWM resolution in figure 5 What does 10 bit PWM resolution mean Well we set a decimal value of 1023 in PPO to
11. driver operation This side controls one motor the motor is connected to pins 3 and 6 of the motor driver Pins 2 and 7 of the motor driver control the direction of the motor I have connected pins 2 and 7 to PORTS 0 P8 0 and PORTS 1 P8 1 We see pins 2 and 7 to be connected to two switches S2 and S3 respectively don t worry about those inverters These switches can be thought of as NMOS POWER FETs Since the gate of a FET is insulated the current to run the motor has to come from the drain Looking at figure 12 again the drains of the FETs are connected through a common green wire to the drain of another FET S1 The gate of this FET is the enable input and it is connected to the PWM Hence when the PWM is high this enable FET is on and the left motor turns Thus if you want to turn the left motor 5A POWER FET is a special FET that can handle large amounts of current to drive the motor Nevertheless the SN754410 does not use POWER FETs it uses BJTs Bipolar Junction Transistors If you want to know more about them take EECS 105 DO NOT RUN YOUR MOTOR WITH PWM FREQUENCIES GREATER THAN 300 Hz 37 1 Set the direction port for PORT8 0 and PORTS 1 to output DP8 0x03 2 Write to PORT8_0 and PORTS 1 to turn the motor P8 0x02 0x01 will make it go the other direction 3 Turn on PWM connected to pin 1 of the motor driver Once the PWM goes low in the cycle the enable FET goes off and the motor starts coasting
12. in the C167 Just think about what you learned from this chapter how PWM works in the C167 Before reading this chapter you probably had no clue as to what PWM is and how it is generated in the C167 This is common in engineering 99 of the time you will be asked to do something which you have no idea of about What is the best way of doing such a thing Well your formal education is only a part of the solution Most of the time your experience and practical training will play the crucial role Before going on to connect the touch sensor to the CalBOT it will be a good idea to make sure your motor driver works and the CalBOT at least moves forward Remember the proverb You eat this elephant of life one bite at a time 39
13. ng the CalBOT all four PWM modules in the C167 function the same way You will be using only two of the four in this manual I use module 0 and 1 If you want to use modules 2 and 3 or any combination of two out of the four just use the correct data registers and set the control registers accordingly Wait a second what is this setting the control registers business Well I am going to describe the control and data registers in detail and this should clear things up I am going to be describing module 0 only since all the other modules function the same way c Edge aligned PWM The PWM module in the C167 can operate in one of four different modes edge aligned center aligned single shot and burst mode We will only be concerned with edge aligned PWM refer to figure 4 for this mode also called as mode 0 PLEASE NOTE Appendix B also has figures 4 8 for easy reference i Throughout this section refer to The C167 User s Manual pp 15 1 to 15 13 25 PTx Count Value PWx Pulse Width 0 PWx 1 PWx 2 PWx 4 PWx 6 PWx 7 PWx 8 Latch Shadow Register Interrupt Request MCA01949 Figure 4 PWM Mode 0 Figure 4 shows what you have to do in order to get a square wave out of the PWM We first set the period we want using PPO Pulse Period register with the 0 standing for the module number either 0 1 2 or 3 How do you specify the frequency you want Figure 5 gives you the answer Inp Clk fcpu x
14. rect frequency 100 Hz for motor operation is shown below I have added the function InitPWM to our program from chapter 4 to correctly setup PWM module 0 and 1 Header files include reg167 h Global variables and defines Initialization functions void InitLEDs void DP2 OxFFFF Turns on all LEDs P2 0x0000 3l f Kk kk ke ke e e e e e x A x ADDED IN CHAPTER 5 XCkck ck ck ck ko kok ok ke ke x x x x f void InitPWM void DP7 0x03 Use P7 0 and P7 1 as output P7 0x00 PWMCON1 0x0003 PPO 3125 gives us a frequency of approximately 100 Hz PWO 0 32 PPO 68 duty cycle PP1 3125 PW1 0 50 PP1 50 duty cycle PWMCONO 0x0033 choose correct resolution and start PWM KK KK A A AA ee I ae ee f void Initialize void InitLEDs InitPWM ADDED IN CHAPTER 5 IEN 1 Enable Interrupts Utility functions void kludgyWait void Float i Approx 1 2 sec delay for i O i lt 2500 1i 0 1 void main void Initialize for Program 1 PWM code Compile the above program in Keil download it to the board and run it Figure 8 shows one of the PWM signals on a portable oscilloscope Please make sure you can get a PWM signal before proceeding to the next section where we interface the board to the motors 32 Figure 8 Scope output The 43 4
15. trol the speed of the motor 2 Supply enough current for the motor 3 Change the direction of the motor so you can move backwards and turn You control the speed of the motor using PWM supply enough current for it by using the motor driver IC and make it move backwards or turn when the touch sensor is hit The first two concepts are covered in this chapter the touch sensor is the subject of chapter 6 23 2 Pulse Width Modulation First some terminology associated with signals frequency period and duty cycle The first important concept is the frequency of the signal which is the number of cycles second The unit of frequency is cycles sec or Hz The following signal has a frequency of 2 cycles second or 2 Hz 0 LZ V 3 1 1 1 time secs 2 cycles Figure 2 A signal with a frequency of 2 cycles second or 2 Hz The next concept is the period It is the reciprocal of frequency It is simply the number of seconds to complete one cycle The period of the signal in figure 2 is 0 5 seconds Lastly the duty cycle is defined as the percentage of the period for which the signal is high For example a signal that has a 50 duty cycle is high for one half of the period and low for the other half The signal in figure 2 has a 50 duty cycle a What is PWM As the name implies a PWMP f is a square wave in which we choose how long the signal stays logic high and how long the signal stays logic low A PWM voltage waveform is ill
16. ustrated in figure 3 The effective voltage delivered is just the peak voltage times the fraction of the time the voltage is on that is the average voltage The greater this on time the higher the average voltage The more voltage you supply to a motor the faster it turns Hence if we can control the width of the time the signal stays logic high we can control the speed of the motor High Vcc Average GND Figure 3 PWM signal It has a duty cycle of about 40 that is it is high for 40 the period 24 b PWM using the C167 As I mentioned in chapter 4 the C167 has two other types of registers other than ports to control a module These two registers are the CONTROL REGISTERS and the DATA REGISTERS Just like the ports of chapter 4 these registers also take on 16 bit integer values However they have special meaning to the C167 In the case of your C167 s PWM module you have the following Module Number Control Register s Data Register s 0 PWMCONO PWMCONI PPO PWO PWMCONO PWMCONI PP1 PWI PWMCONO PWMCONI PP2 PW2 PWMCONO PWMCONI PP3 PW3 Table 1 The control and data registers for the PWM module If you look at the section on the PWM in the hardware manual it seems complicated However the most important concept to understand is how the control and data registers in table 1 work together to produce a square wave You don t have to worry about all four modules For the purposes of buildi
Download Pdf Manuals
Related Search