PID servo throttle control

Other Hardware Robotics
1

Hello All,

While I'm not exactly new to using an Arduino, I'd still put my level of experience slightly above novice. I apologize in advance for my ignorance in what may seem obvious to you. Any and all suggestions are greatly appreciated.

As the subject title states, I am trying to control the opening and closing of a throttle via a hobby servo but also implementing PID control. I'm using the <PID_v1.h> library. Currently, I'm using a hall sensor to calculate the RPM of a fan that I can control with a potentiometer (Image attached). In a different setup I observed that with the magnets attached to the fan, max RPM was approximately 2500 RPM which is my reason for choosing that number in the code when mapping the input. I have the RPM of the fan as Input, a Setpoint currently specified at 1500. I believe the Output should be the angle of the servo which will ultimately dictate RPM by throttle control. When I run the code below the servo is not responding and I'm getting an output value of 0.

You'll notice in the code that I commented out the variable "Output" in the loop function as I felt it was circumventing the PID control (not sure if I'm correcting for thinking this) as it would work flawlessly regardless of the gain values. I'm still learning as I go but I'm concerned I'm no longer going in the right direction. Once again, any help is appreciated. Thank you.

#include <PID_v1.h>
#include <Servo.h>

Servo myServo;

volatile byte half_rev;

unsigned int rpm;
unsigned long previousTime;

double Setpoint;    // will be the desired throttle opening for a desired rpm value
double Input;       // rpm reading from hall sensor
double Output;      // throttle
//int Out = (int) Output;

//PID parameters
double Kp = 10, Ki = 0, Kd = 0;

//create PID instance
PID myPID(&Input,&Output, &Setpoint, Kp, Ki, Kd, REVERSE);

void setup() 
{
  Serial.begin(9600);
  //Hardcode the rpm value
  Setpoint = 1500;
  
  //Turn the PID on
  myPID.SetMode(AUTOMATIC);
  
  //Adjust PID values
  //myPID.SetTunings(Kp, Ki, Kd);

  //Set the output limits
  myPID.SetOutputLimits(0,180);
  
  //Set servo motor to pin 9
  myServo.attach(9);

  attachInterrupt(0, rpm_calc, RISING);
  half_rev = 0;
  previousTime = 0;
}

void loop() 
{
  if (half_rev >= 20) // RPM is updated every 20 counts of a rising edge. For a better RPM resolution increase but for faster update decrease.
  {
    rpm = 30*1000/(millis() - previousTime)*half_rev; // NOTE: this works for two interrupts per full rotation.
    previousTime = millis();
    half_rev = 0;
  }
  //Read the value of the calculated rpm from the hall sensor. Motor input: 0 to ~2500. 
  Input = map(rpm, 0, 2500, 0, 255);
  //Output = map(Input, 0, 255, 180, 0);
  
  myPID.Compute();    //PID calculation
  
  //Write the output as calculated by the PID function
  myServo.write(Output); // Servo motor is set to digital pin 9 this is a pwm pin
  //analogWrite(9,Output); // Servo motor is set to digital pin 9 this is a pwm pin
  //digitalWrite(servoPin,Output);
  
  //Send data by serial for plotting
  /*Serial.print(Input);
  Serial.print(" ");
  Serial.println(Output);
  Serial.print(" ");
  Serial.println(Setpoint);*/
  
  Serial.print("RPM =\t");
  Serial.print(rpm);
  Serial.print("\tPWM =\t");
  Serial.print(Input);
  Serial.print("\tOutput =\t");
  Serial.println(Output);
}

void rpm_calc()//This function is called whenever a magnet/interrupt is detected by the arduino
{
  half_rev++;
}

IMG_0047.JPG
IMG_0047.JPG2592×1936 1.12 MB

2

You are accepting the default parameters. What are they?

  //Adjust PID values
  //myPID.SetTunings(Kp, Ki, Kd);

In order to understand what the program is doing and to get it working correctly, you MUST print out and inspect all values of the relevant variables (like rpm and Setpoint, Input and Output, in order to make sure they are sensible.

3

I'm slightly confused. I was under the impression that the SetTunings() function would adjust my gains during run-time. I thought by commenting out that line I was no longer accepting the default values but instead setting my own. Am I wrong for assuming this. But to answer your question, my values were as follows: Kp = 10, Ki = 0, Kd = 0 but the thing is regardless of what value I entered nothing happens.

4

Sorry, you are correct. The initial K values are set when you instantiate the PID.

Nevertheless, to debug this you need to print out all the variables, in order to see what is actually happening. I'm suspect that the Output would be large and negative, if you had not included this line:

  //Set the output limits
  myPID.SetOutputLimits(0,180);

which sets anything less than zero to be zero.

Hint: PID is very, very simple in action, and in your case (with Ki=Kd=0) can be coded in one line (with ambiguity of sign):

Output=Kp*(Input-SetPoint);
5

Cross posted