PID help needed

Projects Programming
1

Hi. My project aims to implement a PID temperature control to run a heating element, using a thermistor (NTC 100k) for sensing in A0. The heating element is controled by Arduino's pin 13 using a D4184 MOSFET module to command its connection to a power supply. I'm using two libraries for the code: ArduPID and SmoothThermistor. The problem is that I can't get a control loop going, the heating element temperature just shoots straight up.

  • I have tried other code as well, replacing the "SmoothThermistor" library by various Steinhart-Hart calculation codes I found on the internet. But same outcome as before.
  • I'm aware that there are other PID libraries as well, but the one I'm using seems the most user-friendly of them all.
  • I've seen posts on this forum about PID "troubles" but didn't help.

This problem arises from the PID k values? Tried many. Or is something else going on? I can't get around this. Please help.

Thanks for your time.

Libraries:

Thermistor wiring (100k Thermistor and 100k Resistor):

Thermistor
Thermistor735×560 271 KB

Code:

#define THERMISTOR_PIN A0
#define DIGITAL_CONTROL_PIN 13

#include <SmoothThermistor.h>
//SmoothThermistor smoothThermistor(THERMISTOR_PIN, ADC_SIZE_10_BIT, THERMISTOR, RESISTOR, BETA COEF, NOMINAL RESISTANCE, SAMPLES);
  SmoothThermistor smoothThermistor(THERMISTOR_PIN, ADC_SIZE_10_BIT, 100000,     100000,   3950,      25,                 10);

#include "ArduPID.h"
ArduPID myController;
double PID_setpoint = 80;
double PID_input, PID_output;
double p = 0.9;
double i = 0.3;
double d = 0.1;

void setup(void) {

	Serial.begin(9600);
	myController.begin(&PID_input, &PID_output, &PID_setpoint, p, i, d);
}

void loop(void) {

	PID_input= analogRead(THERMISTOR_PIN);
	myController.compute();
	analogWrite(DIGITAL_CONTROL_PIN, PID_output);

	// Serial Plotter
	Serial.print(setpoint);
	Serial.print(",");
	Serial.println(smoothThermistor.temperature());
}
2

Try:

int input = analogRead(THERMISTOR_PIN); // I changed my mind... 0 - 1023 analog input

float input = analogRead(THERMISTOR_PIN); oops... sorry for the edit.

3

Actually

float input = smoothThermistor.temperature();

is probably what's needed.

4

Thanks for your help.

Already tried input = smoothThermistor.temperature(); to no avail.

6

input = THERMISTOR_PIN; Was a mistake I made tidying up the code to copy it here. Fixed it:

input = analogRead(THERMISTOR_PIN);

Thanks

7

Your input variable is PID_input but you never update the value so why do you think the output will change?

The same is true for PID_output which never gets applied to your- output. Also, Pin 13 does not support PWM so analogWrite() does not work on that pin.

8

cedarlakeinstruments, double is used as the ArduPID library requests it

9

Oh you are godsend. Right, its not a PWM a pin. Thank you.

10

original example:

#include "ArduPID.h"

ArduPID myController;

double **input**;
double output;

// Arbitrary setpoint and gains - adjust these as fit for your project:
double setpoint = 512;
double p = 10;
double i = 1;
double d = 0.5;

void setup()
{
  Serial.begin(115200);
  myController.begin(&**input**, &output, &setpoint, p, i, d);

  // myController.reverse()               // Uncomment if controller output is "reversed"
  // myController.setSampleTime(10);      // OPTIONAL - will ensure at least 10ms have past between successful compute() calls
  myController.setOutputLimits(0, 255);
  myController.setBias(255.0 / 2.0);
  myController.setWindUpLimits(-10, 10); // Groth bounds for the integral term to prevent integral wind-up
  
  myController.start();
  // myController.reset();               // Used for resetting the I and D terms - only use this if you know what you're doing
  // myController.stop();                // Turn off the PID controller (compute() will not do anything until start() is called)
}

void loop()
{
  **input** = analogRead(A0); // Replace with sensor feedback

  myController.compute();
  myController.debug(&Serial, "myController", PRINT_INPUT    | // Can include or comment out any of these terms to print
                                              PRINT_OUTPUT   | // in the Serial plotter
                                              PRINT_SETPOINT |
                                              PRINT_BIAS     |
                                              PRINT_P        |
                                              PRINT_I        |
                                              PRINT_D);
  
  analogWrite(3, output); // Replace with plant control signal
}
11

Thanks for your feedback. You are right pointing out that mistake, I made a clumsy job pasting the code here (fixed it). The code used by my Arduino didn't have that error.

12

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