20 "Million Miles" Raspberry Pi Car - Motor Control Learning (Control Speed)

lb8820265 · Published on 2021-12-20 22:41

20 "Million Miles" Raspberry Pi Car - Motor Control Learning (Control Speed) [Copy link]

Earlier we introduced how to obtain the motor speed of the Raspberry Pi and how to use the Raspberry Pi timer. By combining the two and using the control algorithm, we can also control the motor speed.

PID Control Algorithm

Speed control uses the PID algorithm, which is introduced a lot on the Internet. The schematic diagram is shown below.

In practice, PI control can be used, that is, the speed error is multiplied by the Kp coefficient, and then the accumulated speed error is multiplied by the Ki coefficient, and then the two are added together to give the motor control value. The selection of the values of these two coefficients is the difficulty of control.

Coding

#include <stdio.h>
#include <wiringPi.h>
#include <iostream>
#include "Emakefun_MotorShield.h"
#define SPEED_INTEGRAL_MAX 1
float Speed_Kp=255,Speed_Ki=150;
float Set_Speet=0.5;
int A_pin=21;
int B_pin=22;
float Encoder_Integral=0;
long Cnt;
long Cha;
long now,pre_time;
float Speed=0;
float Set_PWM_float=0;
uint8_t Set_PWM=0;
Emakefun_DCMotor *DCmotor1 ;
//电机速度PI控制函数
float Speed_PI(float Now_Speed,float Set_Speet)
{
static float fP;
fP=Set_Speet-Now_Speed;
Encoder_Integral+=fP;
if(Encoder_Integral>SPEED_INTEGRAL_MAX)  Encoder_Integral=SPEED_INTEGRAL_MAX; 
return fP*Speed_Kp+Encoder_Integral*Speed_Ki; 
}
//外部中断回调函数，用来计数电机速度
void Interrupt (void) {
Cnt++;
}
//定时器线程函数
PI_THREAD (timer)
{
for (;;)
{
   now=micros();
 Cha=(now-pre_time);
   Speed=(float)Cnt/Cha*50;
   Cnt=0;
   pre_time= now;
   Set_PWM_float=Speed_PI(Speed, Set_Speet);
   if(Set_PWM_float>255)Set_PWM_float=255;
   if(Set_PWM_float<0)Set_PWM_float=0;
   Set_PWM=(uint8_t)Set_PWM_float;
   DCmotor1->setSpeed(Set_PWM);
  printf ("%ld,%f,%f,%f\n",Cha,Speed,Set_PWM_float,Encoder_Integral);
   delay(100);
}
}
int main () {
  Emakefun_MotorShield Pwm = Emakefun_MotorShield();
  Pwm.begin(50);
 DCmotor1 = Pwm.getMotor(1);
 DCmotor1->setSpeed(0);
 DCmotor1->run(FORWARD);
 wiringPiSetup () ;
 wiringPiISR (A_pin, INT_EDGE_BOTH, &Interrupt) ;
 wiringPiISR (B_pin, INT_EDGE_BOTH, &Interrupt) ;
 piThreadCreate (timer) ;
   while(1) {
   delay(1000);
}
}

Parameter debugging

The parameters in the code are obtained through calculation and debugging, which are introduced below.

Normalize the maximum speed: first let the motor run at full speed, and calculate the maximum speed of the motor at full speed to be about 0.02 (this value has no actual speed meaning), multiply it by 50 to make the maximum speed become 1.

Kp parameter determination: the motor control value is 0 to 255, and the maximum speed is 1, so Kp is directly determined to be 255.

Ki parameter determination: The Ki parameter should not be adjusted continuously. It should respond quickly but not be too large to cause oscillation. After continuous adjustment, 150 is selected.

Determination of integral limit parameters: If the integral is not limited to the maximum value, the value of the integral will be infinitely large, which is obviously not acceptable. The output after the code is run is as shown in the figure below.

The first column of the output is time, the second column is speed, the third column is control amount, and the fourth column is integral value. The ideal situation is that the integral control output is zero after running. Of course, it is also possible to have a value. By giving the wheel some resistance, this value will not exceed 1, so the integral limit parameter is set to 1.

Operation effect

After running, the Raspberry Pi will control the wheels to rotate at the set speed. From the speed printed out, the speed has been maintained well, but the control amount changes dramatically, and the sound can also tell that the wheel speed is fast and slow. However, I can't measure the actual speed of the wheel. I can only know the effect after all the codes are written and actually run on the ground.

question

In the project, the driver library file and the application file are placed in one folder, which wastes space and is not conducive to reuse. If you want to put the driver library in another file, how do you set the header file include path?

Source code

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