In this post we will see how we can achieve very precise position control of simple DC motor. We have use Arduino and PID calculation to achieve precise position control of simple DC motor.
Such small project are very much fun to do, you can learn many concept like PID, Close loop system & encoder basic by building this project.
First of all I want to let you brief about what is PID, because this will help you to understand the things further.
PID stands for proportional derivative and integral calculation
PID is the part of programming, on which a complex calculation carried out by controller and manipulate the Output as per the feedback from the process.
There is 4 key values of PID
- PV (process value) or feedback
- SV (set value)
- Error
- Output
Basic PID working is as follow, I am not going into complex arithmetic an calculus formulas because I want to keep it simple.
PID try to keep PV as near as possible to SV. Error = SV-PV Error is difference between Set value and process value and Output is directly prepositional to Error hence in other words we can say greater the difference between SV and PV greater the Output, if Error is positive output is positive if error is negative output is negative. As PV reach closer and closer to SV the Error get smaller and smaller so on Output also get smaller and smaller, so in this way we get a clean and precise output. Here is a real life example.
Suppose I tell you to drive a break less car exactly 1 km on straight road because you don’t have break so the throttle is all you can use to control the car so you get in car press the throttle and accelerate for a while then release throttle to decelerate and gradually stop at the 1km point. this is all what you have done we can call it PID control. PID knows how to manipulate output before reaching the SP
Kp, Ki, Kd are the parameters by which we can fine tune the PID controller those parameter can control over shoot undershoot oscillation.
Suppose a micro-controller drive the car without PID, it will start the car and continue to accelerate until reach the 1km point.
and then release the throttle due momentum car continue to move and gradually stop way beyond to our set point of 1km.
then again micro-controller reverse the car and accelerate as he came back to 1km point release the throttle again same thing happen it continue to move in back due to momentum and such cycle repeat again and again.
you can imagine now it is almost impossible for a controller to stop the car at fixed point with out PID
Below is the visual to understand how PID smooth motion looks.
A double shaft simple DC gear motor is connected with shaft of encoder on one side and on other side a pointer is connected this pointer points the angle marked on protractor, encoder is connected with arduino on interrupt pins and DC motor drive by L293D motor IC, a HC-05 module is use to connect our system with android device
When we send angle setpoint from android device arduino receive the data and run the Motor meanwhile encoder sends real time position feedback to arduino as per predefined calculation when encoder pulse matched with requirement it means pointer reach the desire position arduinuo stop the DC motor at such potion. all the process is controlled by PID for smooth and clean motion.
For example here we have used a optical encoder which give 1600 transition pulse for 360 degree revolution, so if we want to rotate pointer to 90 degree so the 1600/360 x 90 = 400 pulse from encoder tells us it moves 90 degree.
I have used a simple BO motor a optical encoder is connected to its shaft a 360 degree protractor is used as a scale HC-05 bt module is used to connect arduino and android device and a android mobile phone to run android app to send angle set-point to arduino
Below are the link of product in case if you need to buy them
Arduino
Dc motor
Protractor
L293d IC
Optical encoder
I 3d printed some parts to mound motor and encoder, I also 3D printed pointer you can find the 3D file on the link below. https://www.thingiverse.com/thing:3221695
I have used here a 400 ppr 2 phase incremental optical encoder this encoder can give reference of 1600 translation per evolution means 800 transition per phase, we need to connect pullup resistor connected to phase A and phase B because we cannot leave pin floating to avoid any disturbance, in our case we are using arduino which have inbuilt pull up resistor facility just need to activated from code.
pinMode(encoderPin1, INPUT_PULLUP);
pinMode(encoderPin2, INPUT_PULLUP);
Do the wiring as shown in the image above, encoder green and white wire must be connected to the interrupt pin of arduino which is digital pin 2 and 3 Here I have used a L293D H bridge motor control IC is used to control DC motor it can work on voltage range of 5V to 36V it can handle 1A per channel and peak of 2A per channel HC-05 BT module must be connected to serial pin(TX, RX) pin 0 & 1 on arduino. I have used 9V battery to supply power to motor.
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Before begin towards coding you must need to install a PID library an excellent library by br3ttb this library is very easy to use and upto the point.
#include <PID_v1.h>
#define MotEnable 6 //Motor Enamble pin Runs on PWM signal
#define MotFwd 4 // Motor Forward pin
#define MotRev 7 // Motor Reverse pin
String readString; //This while store the user input data
int User_Input = 0; // This while convert input string into integer
int encoderPin1 = 2; //Encoder Output 'A' must connected with intreput pin of arduino.
int encoderPin2 = 3; //Encoder Otput 'B' must connected with intreput pin of arduino.
volatile int lastEncoded = 0; // Here updated value of encoder store.
volatile long encoderValue = 0; // Raw encoder value
int PPR = 1600; // Encoder Pulse per revolution.
int angle = 360; // Maximum degree of motion.
int REV = 0; // Set point REQUIRED ENCODER VALUE
int lastMSB = 0;
int lastLSB = 0;
double kp = 5 , ki = 1 , kd = 0.01; // modify for optimal performance
double input = 0, output = 0, setpoint = 0;
PID myPID(&input, &output, &setpoint, kp, ki, kd, DIRECT);
void setup() {
pinMode(MotEnable, OUTPUT);
pinMode(MotFwd, OUTPUT);
pinMode(MotRev, OUTPUT);
Serial.begin(9600); //initialize serial comunication
pinMode(encoderPin1, INPUT_PULLUP);
pinMode(encoderPin2, INPUT_PULLUP);
digitalWrite(encoderPin1, HIGH); //turn pullup resistor on
digitalWrite(encoderPin2, HIGH); //turn pullup resistor on
//call updateEncoder() when any high/low changed seen
//on interrupt 0 (pin 2), or interrupt 1 (pin 3)
attachInterrupt(0, updateEncoder, CHANGE);
attachInterrupt(1, updateEncoder, CHANGE);
TCCR1B = TCCR1B & 0b11111000 | 1; // set 31KHz PWM to prevent motor noise
myPID.SetMode(AUTOMATIC); //set PID in Auto mode
myPID.SetSampleTime(1); // refresh rate of PID controller
myPID.SetOutputLimits(-125, 125); // this is the MAX PWM value to move motor, here change in value reflect change in speed of motor.
}
void loop() {
while (Serial.available()) { //Check if the serial data is available.
delay(3); // a small delay
char c = Serial.read(); // storing input data
readString += c; // accumulate each of the characters in readString
}
if (readString.length() >0) { //Verify that the variable contains information
Serial.println(readString.toInt()); //printing the input data in integer form
User_Input = readString.toInt(); // here input data is store in integer form
}
REV = map (User_Input, 0, 360, 0, 1600); // mapping degree into pulse
Serial.print("this is REV - ");
Serial.println(REV); // printing REV value
setpoint = REV; //PID while work to achive this value consider as SET value
input = encoderValue ; // data from encoder consider as a Process value
Serial.print("encoderValue - ");
Serial.println(encoderValue);
myPID.Compute(); // calculate new output
pwmOut(output);
}
void pwmOut(int out) {
if (out > 0) { // if REV > encoderValue motor move in forward direction.
analogWrite(MotEnable, out); // Enabling motor enable pin to reach the desire angle
forward(); // calling motor to move forward
}
else {
analogWrite(MotEnable, abs(out)); // if REV < encoderValue motor move in forward direction.
reverse(); // calling motor to move reverse
}
readString=""; // Cleaning User input, ready for new Input
}
void updateEncoder(){
int MSB = digitalRead(encoderPin1); //MSB = most significant bit
int LSB = digitalRead(encoderPin2); //LSB = least significant bit
int encoded = (MSB << 1) |LSB; //converting the 2 pin value to single number
int sum = (lastEncoded << 2) | encoded; //adding it to the previous encoded value
if(sum == 0b1101 || sum == 0b0100 || sum == 0b0010 || sum == 0b1011) encoderValue ++;
if(sum == 0b1110 || sum == 0b0111 || sum == 0b0001 || sum == 0b1000) encoderValue --;
lastEncoded = encoded; //store this value for next time
}
void forward () {
digitalWrite(MotFwd, HIGH);
digitalWrite(MotRev, LOW);
}
void reverse () {
digitalWrite(MotFwd, LOW);
digitalWrite(MotRev, HIGH);
}
void finish () {
digitalWrite(MotFwd, LOW);
digitalWrite(MotRev, LOW);
}