Chapter 8 ARCB as PWM Controller to Drive Servo Motor

Transcription

1 Chapter 8 ARCB as PWM Controller to Drive Servo Motor 8.1 Introduction PWM is one of the powerful techniques used in control systems today. They are not only employed in wide range of control applications which includes: speed control, power control, measurement and communication. In the presented work a PWM (pulse width modulation) controller is designed using robot control panel. This definitely proves the universality of the robot control panel as PWM controller which can be used in many embedded robots. PWM Basics Many embedded systems are built to have PWM as a hardware function since PWM is a cheap way to do digital to analog conversion. In PWM for example, a base frequency of say 1,000Hz and each cycle output a single pulse whose width varies from 0% of the period to 100% of the period. The base frequency of the PWM is f = 1/T so that the frequency content of a PWM signal will have frequencies at f and higher due to the base frequency. The signal information is transmitted by the duty cycle and will be, generally, a much lower frequency signal. Thus a relatively simple low pass filter can eliminate the frequencies at the base frequency and above and recover an analog signal from the PWM. The Philips LPC2138 has 6 channels of pulse width modulation. There are 7 registers to accommodate the PWM with register 0 being used to set the base frequency. Thus, since there is only one base frequency register all 6 channels must have the same base frequency. 110

2 PWM on the LPC2138 can be single edge or double edged. Single edge PWM is as shown in Figure-8.1. In double edge PWM the ending point of the pulse and the starting point of the pulse is a variable and can be set each cycle. In single edge PWM only the ending edge of the PWM is a variable and the starting edge is always set at the base frequency. PWM is activated with the help of square wave whose duty cycle is changed to get a varying voltage output as a result of average value waveform. Consider a square wave shown in Figure-8.1. T on is the time for which the output is high and T off is time for which output is low. Let T be the time period of the wave such that, T = T on + T off Duty cycle of a square wave is defined as The output voltage varies with duty cycle as Vout = D x Vin = x Vin 111

3 So it can be seen from the final equation the output voltage can be directly varied by varying the T on value. If T on is 0, Vout is also 0. If T on is T then Vout is Vin or say maximum. Volts T/2 T on T T off Time Figure-8.1: Single Edge Pulse Width Modulation. Single edge PWM requires one register (plus the base register) so that one can have 6 single edge channels of PWM on the LPC2138. For double edged PWM one need a base register plus a starting edge register plus an ending edge register so that only 3 channels of double edged PWM is available on the LPC2138. One can mix single and double edged channels as long as one doesn t run out of the 6 available registers. The Figure-8.2 below shows the LPC2138 PWM block diagram. 112

4 Figure-8.2: PWM Block Diagram Note that each PWM channel has its own timer/counter so that it does not use Timer0 or Timer1. 113

5 8.2 PWM Programming in ARM7 A PWM block, like a Timer block, has a Timer Counter and an associated Prescale Register along with Match Registers. These work exactly the same way as in the case of Timers. Match Registers 1 to 6 (except 0) are pinned on LPC213x i.e. the corresponding outputs are diverted to actual Pins on LPC213x MCU. The PWM function must be selected for these Pins to get the PWM output. These pins are given in Table-8.1. Output: PWM1 PWM2 PWM3 PWM4 PWM5 PWM6 Pin Name: P0.0 P0.7 P0.1 P0.8 P0.21 P0.9 Table-8.1: PWM Pins. Note that PWM1 output corresponds to PWM Match Register 1 i.e PWMMR1, PWM2 output corresponds to PWMMR2, and so on. Also Match Register 0 i.e PWMMR0 is NOT pinned because it is used to generate the PWM Period. Also the PWM function must be selected for the pins mentioned above using appropriate PINSEL registers (PINSEL0 for PWM1,2,3,4,6 and PINSEL1 for PWM5). In LPC2138 there are 7 match registers inside the PWM block. Generally the first Match register PWMMR0 is used to generate PWM period and hence we are left with 6 Match Registers PWMMR1 to PWMMR6 to generate 6 Single Edge PWM signals or 3 Double Edge PWM signals. Double edge PWM uses 2 match registers hence one can get only 3 double edge outputs. The way this works in case of Single Edge PWM is as follows (and similarly for Double Edge): 1. Consider that our PWM Period duration is 6 milliseconds and TC increments every 1 millisecond using appropriate prescale value. 2. Now we set the match value in PWMMR0 as 6, this period will be same for all PWM outputs. 114

6 3. Then configure PWM block such that when TC reaches value in PWMMR0 it is reset and a new Period begins. 4. Now let s assume 2 PWM signals of Pulse widths 2ms and 4ms are requred. 5. So use PWMMR1 and PWMMR2 to get the 2 outputs. For that set PWMMR1 = 2 and PWMMR2 = Then, Everytime a new period starts the Pin corresponding to PWMMR1 and PWMMR2 will be set High by default. 7. Whenever the value in TC reaches PWMMR1 and PWMMR2 its output will be set to low respectively. Their outputs will remain low until the next Period starts after which their outputs again become high, hence giving Single Edge PWM. This can be shown in the timming diagram Figure-8.3. PWMMR1=2 PWMMR2=4 Timer Counter T=0ms 2ms 4ms 6ms 12ms Timer Counter Reset at PWMMR0 match Figure-8.3: Timming Diagram Single Edge PWM. Rules for Single Edge Controlled PWM Outputs 1. All single edge controlled PWM outputs go high at the beginning of a PWM cycle unless their match value is equal to Each PWM output will go low when its match value is reached. If no match occurs (i.e. the match value is greater than the PWM rate), the PWM output remains continuously high. 115

7 Rules for Double Edge Controlled PWM Outputs Five rules are used to determine the next value of a PWM output when a new cycle is about to begin: 1. The match values for the next PWM cycle are used at the end of a PWM cycle (a time point which is coincident with the beginning of the next PWM cycle), except as noted in rule A match value equal to 0 or the current PWM rate (the same as the Match channel 0 value) has the same effect, except as noted in rule 3. For example, a request for a falling edge at the beginning of the PWM cycle has the same effect as a request for a falling edge at the end of a PWM cycle. 3. When match values are changing, if one of the "old" match values is equal to the PWM rate, it is used again once if the neither of the new match values are equal to 0 or the PWM rate, nor there was no old match value equal to If both a set and a clear of a PWM output are requested at the same time, clear takes precedence. This can occur when the set and clear match values are the same as in, or when the set or clear value equals 0 and the other value equals the PWM rate. 5. If a match value is out of range (i.e. greater than the PWM rate value), no match event occurs and that match channel has no effect on the output. This means that the PWM output will remain always in one state, allowing always low, always high, or "no change" outputs. The last 6 Match Registers i.e PWMMR1 to PWMMR6 have an associated PIN from which we get the PWM outputs. The PWM block controls the output of these pins depending on the value in its corresponding Match Register. For Single Edged PWM these Pins are set to High by default when a new Period starts i.e when TC is reset as per the PWM Rules given above. 116

8 Note that the width of the Pulse is stored in the Match Register. One can change the width of the pulse by changing the value in the corresponding Match register using a special register called Latch Enable Register which is used to control the way Match Registers get updated. Some of the important register in the PWM block is discusses below. 1) PWMTCR: PWM Timer Control Register This register is used to control the Timer Counter inside the PWM block. Only Bits: 0, 1 & 3 are used rest are reserverd. Bit 0: This bit is used to Enable/Disable Counting. When 1 both PWM Timer counter and PWM Prescale counter are enabled. When 0 both are disabled. Bit 1: This bit is used to reset both Timer and Prescale counter inside the PWM block. When set to 1 it will reset both of them (at next edge of PCLK). Bit 3: This is used to enable the PWM mode i.e the PWM outputs. Other Bits: Reserved. 2) PWMPR: PWM Prescale Register PWMPR is used to control the resolution of the PWM outputs. The Timer Counter(TC) will increment every PWMPR+1 Peripheral Clock Cycles (PCLK). 3) PWMMR0 PWMMR6: Match Registers These are the seven Match registers as explained above which contain Pulse Width Values i.e the Number of PWMTC Ticks. 4) PWMMCR: PWM Match Control Registers The PWM Match Control Register is used to specify what operations can be done when the value in a particular Match register equals the value in TC. For each Match Register we have 3 options: Either generate an Interrupt, or Reset the TC, or Stop, which stops the counters and disables PWM. Hence 117

9 this register is divided into group of 3 bits. The first 3 bits are for Match Register 0 i.e PWMMR0, next 3 for PWMMR1, and so on: Bit 0: Interrupt on PWMMR0 Match If set to 1 then it will generate an Interrupt else disable if set to 0. Bit 1: Reset on PWMMR0 Match If set to 1 it will reset the Timer Counter i.e PWMTC else disabled if set to 0. Bit 2: Stop on PWMMR0 Match If this bit is set 1 then both PWMTC and PWMPC will be stopped and will also make Bit 0 in PWMTCR to 0 which in turn will disable the Counters. Similarly {Bits 3,4,5} for PWMMR1, {Bits 6,7,8} for PWMMR2, {Bits 9,10,11} for PWMMR3,{Bits 12,13,14} for PWMMR4,{Bits 15,16,17} for PWMMR5, {Bits 18,19,20} for PWMMR6. 5) PWMIR: PWM Interrupt Register If an interrupt is generated by any of the Match Register then the corresponding bit in PWMIR will be set high. Writing a 1 to the corresponding location will clear that interrupt. Here: Bits 0,1,2,3 are for PWMMR0, PWMMR1, PWMMR2, PWMMR3 respectively and Bits 8,9,10 are for PWMMR4, PWMMR5, PWMMR6 respectively. Other bits are reserved. 6) PWMLER: Latch Enable Register The PWM Latch Enable Register is used to control the way Match Registers are updated when PWM generation is active. When PWM mode is active and we apply new values to the Match Registers the new values won t get applied immediately. Instead what happens is that the value is written to a Shadow Register. It can be thought of as a duplicate Match Register. Each Match Register has a corresponding Shadow Register. The value in this Shadow Register is transferred to the actual Match Register when: 118

10 1) PWMTC is reset (i.e at the beginning of the next period). 2) And the corresponding Bit in PWMLER is 1. Hence only when these 2 conditions are satisfied the value is copied to Match Register. Bit x in PWMLER corresponds to match Register x. I.e Bit 0 is for PWMMR0, Bit 1 for PWMMR1, and so on. Using PWMLER will be covered in the examples section. 7) PWMPCR: PWM Control Register This register is used for Selecting between Single Edged & Double Edged outputs and also to Enable/Disable the 6 PWM outputs which go to their corresponding Pins. Bits 2 to 6 are used to select between Single or Double Edge mode for PWM 2,3,4,5,6 outputs. a) Bit 2 : If set to 1 then PWM2(i.e the one corresponding to PWMMR2) output is double edged else if set 0 then its Single Edged. b) Similarly {Bits 3,4,5,6} for PWM3, PWM4, PWM5, PWM6 respectively. Bits 9 to 14 are used to Enable/Disable PWM outputs. a) Bit 9: If set to 1 then PWM1 output is enabled, else disabled if set to 0. b) Similarly {Bit 10,11,12,13,14} for PWM2, PWM3, PWM4, PWM5, PWM6 respectively. 119

11 Configuring and Initializing PWM Configuring PWM is very much similar to Configuring Timer except, additionally, one need to enable the outputs and select PWM functions for the corresponding PIN on which output will be available. But first we need to do some basic Math Calculations for defining the Period time, the resolution using a prescale value and then Pulse Widths. For this First we need to define the resolution of out PWM signal. Here the PWM resolution is the minimum increment that can use to increase or decrease the pulse width. Smaller the increment more fine will be the resolution. This resolution is defined using an appropriate Prescale Value. The calculation for Prescale is the same as the Timer given below: PWM Prescale (PWMPR) Calculations: The delay or time required for Y clock cycles of PCLK at X MHz is given by: Taking PR into consideration one get Y = PR+1. Now, consider that PCLK is running at 60Mhz then X=60. Hence if we use Y=60 i.e PR=59 then we get a delay of exact 1 micro-second(s). So with PR=59 and 60Mhz our formula reduces to: Similarly when, Y=60000 i.e. PR = the delay will be 1 milli-second(s): 120

12 This delay which is defined using Prescale will be the delay required for TC to increment by 1 i.e TC will increment every PR+1 Peripheral Clock Cycles (PCLK). Now, after working out the resolution, one can now Set and Initialize the PWM device as per the following steps: 1. Select the PWM function for the PIN on which you need the PWM output using PINSEL0/1 register. 2. Select Single Edge or Double Edge Mode using PWMPCR. By default it s Single Edge Mode. 3. Assign the Calculated value to PR. 4. Set the Value for PWM Period in PWMMR0. 5. Set the Values for other Match Registers i.e the Pulse Widths. 6. Set appropriate bit values in PWMMCR. For e.g. resetting PWMTC for PWMMR0 match and optionally generate interrupts if required. 7. Set Latch Enable Bits for the Match Registers that is to be used. 8. Then Enable PWM outputs using PWMPCR. 9. Now Reset PWM Timer using PWMTCR. Finally, enable Timer, Counter and PWM Mode using PWMTCR. 121

13 8.3 DC Servo Motor and Its Working Principle A servo motor is an electromechanical device in which the electrical input determines the position of motor armature. It is actually an assembly of four things: a normal DC motor, a gear reduction unit, a position-sensing device and a control circuit. Servo motors are used extensively in robotics industry and radio-controlled cars. DC servo motors are working in a closed loop control systems where the programmed positions of motion and velocity feedback controllers are required. A simplest method to control the rotation speed of a DC motor is to control its driving voltage. The higher the voltage is the higher speed the motor tries to reach. Servos are controlled by a pulse of variable width. The sent signal of this input pulse is characterized by a minimum pulse, maximum, and a repetition rate as shown in Figure-8.1. Given the rotation constraints of the servo, neutral is defined to be the position where the servo has exactly the same amount of potential rotation in a clockwise direction as it is in a counter clockwise direction.the servo should detect a pulse every 20 ms. The length of the pulse will determine how far the motor turns. For example, a 1.5 ms pulse will make the motor turn to a 90 degree position (neutral position). The position pulse must be repeated to instruct the servo to stay in position. In many applications, a simple voltage regulation would cause lots of power loss on control circuit, thus, a pulse width modulation method (PWM) is used in many DC motor controlling applications. In the basic PWM method, the operating power to the motors is turned ON and OFF to modulate the current to the motor. The PWM control method uses the widths of pulses in a pulse train to control the speed of the motor. In the presented work Parallex standard servo is used and controlled using PWM controller based on robot control panel. The specifications of 122

14 Parallax standart servo is given below in Table-8.2. Figure-8.4 shows the different pulse length and Parallax standard servo position. Power Requirements Communication Dimentions Operating Temperature Range Wieght 4 to 6V VDC, maximum current draw is 140 +/- 50mA at 6 VDC when operating in no load conditions, 15mA when in static state PWM, ms high pulse, 20ms intervals 2.2x0.8x1.6 inch excluding servo horn -10 to +50 C 44g Table-8.2: Parallax Standard Servo Specifications. Figure-8.4: PWM pulse length changes to control servo position. 123

15 The mathematical model of DC servo motor can be simplified by means of the equivalent circuit as shown in the Figure-8.5. Figure-8.5: DC Servo Motor Equivalent Circuit. The electrical part represented by armature and the mechanical part by T and J. As the field excitation is constant, the armature controller only depends on armature voltage. The mechanical equations describing this system can be written with the assumptions that the loss is included in load torque and neglecting viscous friction constant as given below, With, E b = Kω and T e = Ki Where i = armeture current, V a = armeture voltage, R a = armeture resistance, L a = armeture inductance, K = torque and back electromagnetic constant, ω = rotor angular speed, T e = electromagnetic torque, T = total load torque, J = rotor inertia. The control input is the armature voltage V a and the total load torque T is the disturbing input. The two state variables are armature current i and angular 124

16 speed ω. Then the previous equations lead to the state space model of DC motor: The two state variables excited the angular speed in order to perform a speed regulator related with the angular speed. Therefore, ω is considered as the output of the system and V a is the input. Taking into account only these two system variables, the transfer function of the DC motor is: The two time constants are defined as: Electrical time constant, And electro mechanical time constant, Therefore, 125

17 8.4 Experimental Setup PWM1 output from Pin P0.0 is used to control a RC Servo Motor. Four tactile switches are also used to bring the Servo at 4 different positions. P0.1, P0.2, P0.3, P0.4 will be used to take the input from the switches. One end of the tactile switches will be connected to each of these Pins and other end will be connected to ground. External 5V-6V supply source to drive the servo. Servos can work with 3.3v voltage levels for input PWM signal; hence we just need to connect the MCU PWM Output to Servo PWM Input wire. Figure-8.6 shows the schametic diagram of PWM controller using ARCB. When the user will press P1.16 the Pulse width will be 1ms, and similarly when the user presses P1.17, P1.18, and P1.19 the Pulse Width will change to 1.25ms, 1.5ms and 1.75ms respectively and hence Servo will change its position correspondingly. SW4 SW3 ARCB l P1.19 P1.18 P1.17 P1.16 P0.0 VCC GND GND SW2 SW1 PWM IN Servo Motor VCC GND Power Supply Figure-8.6: PWM Controller Schametic. 126

18 8.5 Software Implementation The software is written in the Embedded C language using the Keil µvision IDE. Program for PWM initialization, PLL intialization and PWM generation on P0.0 pin of robot control panel is self explanatory and is given as below: #include <lpc213x.h> #define PLOCK 0x #define PWMPRESCALE 60 void initpwm(void); void initclocks(void); int main(void) { initclocks(); //Initialize CPU and Peripheral 60Mhz initpwm(); //Initialize PWM while(1) { if(!((io1pin) & (1<<16)) ) // Check P1.16 { PWMMR1 = 1000; PWMLER = (1<<1); //Update Latch Enable bit for PWMMR1 } else if(!((io1pin) & (1<<17)) ) // Check P1.17 { PWMMR1 = 1250; 127

19 PWMLER = (1<<1); } else if(!((io1pin) & (1<<18)) ) // Check P1.18 { PWMMR1 = 1500; PWMLER = (1<<1); } else if(!((io1pin) & (1<<19)) ) // Check P1.19 { PWMMR1 = 1750; PWMLER = (1<<1); } } } void initpwm(void) { PINSEL0 = (1<<1); // Select PWM1 output for Pin0.0 PWMPCR = 0x0; //Select Single Edge PWM PWMPR = PWMPRESCALE-1; // 1 micro-second resolution PWMMR0 = 20000; // 20ms = 20k us - period duration PWMMR1 = 1000; // 1ms - pulse duration i.e width PWMMCR = (1<<1); // Reset PWMTC on PWMMR0 match PWMLER = (1<<1) (1<<0); // update MR0 and MR1 128

20 PWMPCR = (1<<9); // enable PWM output PWMTCR = (1<<1); //Reset PWM TC & PR PWMTCR = (1<<0) (1<<3); // enable counters and PWM Mode } void initclocks(void) { PLLCON = 0x01; // PPLE=1 & PPLC=0 so it will be enabled PLLCFG = 0x24; // set the multipler to 5 (i.e actually 4) // i.e 12x5 = 60 Mhz (M - 1 = 4) // Set P=2 since we want FCCO in range PLLFEED = 0xAA; PLLFEED = 0x55; while(!( PLLSTAT & PLOCK )); PLLCON = 0x03; PLLFEED = 0xAA; PLLFEED = 0x55; } VPBDIV = 0x01; // PCLK is same as CCLK i.e 60Mhz 129

21 8.6 Results and Conclusion The simulation is used to validate the results obtained from Keil µvision4 logic analyzer during debug session. Figure-8.7 shows the different PWM signals generated in the logic analyzer screen which is the required pulse to drive the DC servo motor. (a) Pulse width 1ms, P1.16 pressed. (b) Pulse width 1.25ms, P1.17 pressed 130

22 (c) Pulse width 1.50ms, P1.18 pressed. (d) Pulse width 1.75ms, P1.19 pressed Figure-8.7: PWM Signals on PWM1 The actual setup for PWM controller using LPC2138 based robot control panel to drive servo motor is shown in Figure

23 Figure-8.8: PWM Controller Actual Experimental Setup. This chapter provides the design and development of PWM controller based on ARCB to run DC servo motor. The design can be extended upto six PWM channels. Simulation results on the Keil IDE have proven its correctness and usefullness. 132