Robot Tank Car Starter Kit Lesson 7: Use encoder to synchronize motor speed

Byamber

Robot Tank Car Starter Kit Lesson 7: Use encoder to synchronize motor speed

Objective:

In our previous lesson, we know that the motor speed is powered and controlled by PWM(Pulse Width Modulation) current from L298N. However, due to manufacturing inaccuracy, left and right motors might run at different speed even if they powered with same PWM signal. This will cause the robot car offtrack straight line.

To solve the problem, we need collect the speed data from left and right motors and use the data to adjust PWM current and synchronize the speed of both side motors.

In this lesson, we will use build-in Hall Encoder in the motor to collect speed data and send the data to Arduino, then use the data to synchronize motor speed and make car running straight.

The wire connection of Hall encoder(speed sensor).

Black wire(VM): Power for Motor

Red wire (GND): GND for Motor

White wire(V):Power for hall sensor (5V)

Yellow wire(G):GND for hall sensor

Orange wire (S1):Signal for the 1st hall sensor

Green(S2):Signal for the 2st hall sensor

motor

Connect the wires to the ESP8266 wifi board as per the following diagram:

encoder motor(left)-esp8266 encoder motor(right)-esp8266

Motor(Left)
Encoder Esp8266 uart wifi shield
V 5V
G GND
S1 D2
S2 D4
Motor(Right)
Encoder Esp8266 uart wifi shield
V 5V
G GND
S1 D3
S2 D7

Explanation:

Unlike other sensor, Hall encoder has two signal pin S1 and S2 which should work together to get speed data. Arduino also needs two digital pins to input data from S1 and S2.  Our Arduino sample code will use an algorithm called PID to synchronize the speed of left and right motor. The detail of PID  algorithm is beyond the scope of this lesson. If you are interested in this algorithm, you can google it for detail.

Software Installation:

Step 1: Install latest Arduino IDE (If you have Arduino IDE version after 1.1.16, please skip this step)
Download Arduino IDE from https://www.arduino.cc/en/Main/Software?setlang=en , then install the software.

Step 2:Download the libraries from

http://www.kookye.com/download/car/datascope.zip
http://www.kookye.com/download/car/MsTimer2.zip
http://www.kookye.com/download/car/PinChangeInt.zip

open arduino IDE  -> click sketch -> Include Libraries->Add .zip Libraries -> choose zip file “datascope.zip”, “MsTimer2.zip” and “PinChangeInt.zip” in turns -> Upload three .zip files

Step 3:Download Lesson 7 sample code from http://www.kookye.com/download/car/tank_robot_lesson7.zip, unzip the download zip file tank_robot_lesson2_encoder.zip, you will see a folder called tank_robot_lesson7_encoder.

Step 4: Connect UNO R3 board to PC with USB cable, Open Arduino IDE -> click file -> click Open -> choose code “tank_robot_lesson2_encoder.ino” in tank_robot_lesson2_encoder folder, load the code into arduino.

open

Step 5: Choose corresponding board and port for your project,upload the sketch to the board.

port

Understand the Code:

Step 1: Define the pinout of left and right motor

const byte encoder0pinA = 2;//A pin -> the interrupt pin 0
const byte encoder0pinB = 4;//B pin -> the digital pin 4
const byte rencodPinA = 3;  //rencodPinA -> the interrupt pin 1
const byte rencodPinB = 7;  //rencodPinB -> the digital pin 11

Step 2:Set the sampling period to be 10ms. When up to 10ms, enter to the interrupt instructure control

 MsTimer2::set(10, control);  //use timer2 to set the 10ms timer interrupt
 MsTimer2::start();          //enable interrupt

Step 3: Initialization of encoder pinout.

[编码器引脚初始化,左右电机编码的A相输出分别使用arduino外部中断0和外部中断1,使用跳变沿(上升沿和下降沿)]

  l_direction = true;//default -> Forward  
  r_direction = true;//default -> Forward  
  pinMode(encoder0pinB,INPUT);  
  pinMode(rencodPinB,INPUT);  
  attachInterrupt(0, lwheelSpeed, CHANGE);
  attachInterrupt(1, rwheelSpeed, CHANGE);

Step 4: The instructure of encoder interrupt service.

[编码器中断服务函数。当有跳变沿到来时候,会执行中断服务函数,对脉冲加1或减1]

void lwheelSpeed()
{
  int Lstate = digitalRead(encoder0pinA);
  if((encoder0PinALast == LOW) && Lstate==HIGH)
  {
    int val = digitalRead(encoder0pinB);
    if(val == LOW && l_direction)
    {
      l_direction = false; //Reverse
    }
    else if(val == HIGH && !l_direction)
    {
      l_direction = true;  //Forward
    }
  }
  encoder0PinALast = Lstate;
 
  if(!l_direction)  duration++;
  else  duration--;
}

void rwheelSpeed()
{
  int Lstate = digitalRead(rencodPinA);
  if((encoder0PinALast1 == LOW) && Lstate==HIGH)
  {
    int val = digitalRead(rencodPinB);
    if(val == LOW && r_direction)
    {
      r_direction = false; //Reverse
    }
    else if(val == HIGH && !r_direction)
    {
      r_direction = true;  //Forward
    }
  }
  encoder0PinALast1 = Lstate;
 
  if(!r_direction)  duration1++;
  else  duration1--;
}

Step 5: 上面的函数就是增量式PI算法的C语言实现,左右电机脉冲相减作为误差,误差的累积作为积分项,误差为0表示右边电机速度和左边电机速度一致;大于0说明右边电机比左边电机慢;小于0表示右边电机速度比左边电机快。通过PI算法会得到一个误差补给power,将这个power作用与右边电机上,就能达到调节速度的目的。

int PID_controller(int master,int slave)
{ 
  static float power,error,integralerror,lasterror;
  if(master < 0) master = -master;
  if(slave < 0) slave = -slave;
  error = master - slave;
  integralerror += error;
  power += Kp *(error-lasterror) + Ki * error;
  lasterror = error;
  return power;
}

Step 6: 10ms到达时候会执行定时器中断服务函数,在定时器中断服务函数中,将PI得到的误差补给作用到右边电机上。

void control()
{
   sei();//enable global interrupts
   if(++i >=4)//20ms
   {
   master_pulse = duration ,duration = 0;
   slave_pulse = duration1, duration1 = 0;
   pwm = PID_controller(master_pulse,slave_pulse);
   i = 0;
   }
   int newpower1 = motorspeed+pwm;
   constrain(newpower1,0,255);
   analogWrite(ENB,newpower1),analogWrite(ENA,motorspeed);
   cli();//disenable global interrupts
}
const float Kp =20;
const float Ki =1;

Hardware Installation:

Step 1: Install ESP8266 Expansion Board on UNO R3 board.
lesson2-HW-1
Step 2: Move the wire connected to digit ports(D5,D6,D8,D9,D10,D12) in UNO R3 board to its counterpart digit pin in ESP8266 wifi Board.

L298N-ESP8266
Step 3: Turn the switch of esp8266 to "1" and "2" position, as the following photo shows.

lesson2-HW-2
(If you have finished the above steps on lesson one, please skip these step.)

Step 4: Testing. Mark a red label on both side tracks and turn on the battery switch so as to  see if it synchronizes. You can adjust Kp and Ki parameter to make the speed synchronized.(Note:The car can not go straight if the smooth ground created the least friction to slip)

About the Author

amber administrator

Comments Are Closed!!!