• Arduino Uno
• PTC Heating Plate (110VAC, 140W, 220℃)
• CPC40055ST AC Power Switch
• MAX6675 Type Thermocouple Temperature Sensor Module
• Thermocouple Type-K 0-600°C

• A series 330Ω resistor was connected from UNO pin 3 to pin 3 of the SSR (LED+)
• SSR Pin 4 (LED-) was connected to GND
• The PTC black wire was connected to AC Neutral
• SSR load pin 2 connected to the PTC red wire
• SSR load pin 1 connected to 120VAC power
• To connect the MAX6675 board, follow instructions here

This setup uses a stainless steel type-K thermocouple and the older MAX6675 sensor module. The thermocouple was attached to the aluminum PTC heater with a clip. The surface area in contact with the aluminum wasn't ideal, as the threaded sleeve and threaded mounting hole wasn't used. Also, this thermocouple has much greater mass than the glass bead type thermocouple, so this added to the delay for thermal transfer. The MAX6675 sensor module has greatly reduced resolution and increased noise compared to the MAX31856 module.

Overall, this setup was good for testing the capability of sTune. It was found that myPID.SetSampleTimeUs(outputSpan * 1000 * 0.2); worked best for ideal response, wheras the multiplier used for the MAX31856 / glass bead type-K setup was 0.4. The lower 0.2 value was most likely required due to the increased thermal inertia.

Note that the ZN_PID tuning method was used. Other tuning methods are available if different response characteristics are required.

• Get_All_Tunings
• sTune_QuickPID

sTune tuner = sTune(&Input, &Output, tuner.ZN_PID, tuner.directIP, tuner.printOFF);

In this example, sTune is initialized usin the ZN_PID tuning method. The inflection point directIP test is run and the serial print results is turned off to allow the plotter function to plot the complete sTune test and PID run.

// user settings
uint32_t settleTimeSec = 10;
uint32_t testTimeSec = 500;  // runPid interval = testTimeSec / samples
const uint16_t samples = 500;
const float inputSpan = 200;
const float outputSpan = 1000;
float outputStart = 0;
float outputStep = 50;
float tempLimit = 150;
uint8_t debounce = 1;

• settleTimeSec is used to provide additional settling time prior to starting the test.
• testTimeSec at minimum, should be set to one time constant (the time it takes for the input to reach 63% of the max value after a step change of the controller output.
• samples is the number of samples used to perform the test. To get an accurate representation of the reaction curve, the suggested range is 200-500.
• testTimeSec / samples determines the sample period
• inputSpan represents the maximum operating range of input
• outputSpan represents the maximum operating range of output. This also specifies the softPWM windowSize, the PID output max limit, the PID sample time and is also used in gain calculations in various tuning methods.
• outputStart is the initial control output value which is used for the settleTimeSec duration and sample 0
• outputStep is the stepped output value used for sample 1 to test completion
• tempLimit is used to specify the emergency stop value at which the test aborts (reset) and the output is set to zero
• debounce controls several features in the softPWM() function.

// variables
float Input, Output, Setpoint = 80, Kp, Ki, Kd;

• These variables are linked to sTune and QuickPID

void loop() {
  float optimumOutput = tuner.softPwm(relayPin, Input, Output, Setpoint, outputSpan, debounce);

  switch (tuner.Run()) {
    case tuner.sample: // active once per sample during test
      Input = module.readCelsius();
      tuner.plotter(Input, Output, Setpoint, 0.5f, 3); // output scale 0.5, plot every 3rd sample
      break;

    case tuner.tunings: // active just once when sTune is done
      tuner.GetAutoTunings(&Kp, &Ki, &Kd); // sketch variables updated by sTune
      myPID.SetOutputLimits(0, outputSpan * 0.1);
      myPID.SetSampleTimeUs((outputSpan - 1) * 1000);
      debounce = 0; // ssr mode
      Output = outputStep;
      myPID.SetMode(myPID.Control::automatic); // the PID is turned on
      myPID.SetProportionalMode(myPID.pMode::pOnMeas);
      myPID.SetAntiWindupMode(myPID.iAwMode::iAwClamp);
      myPID.SetTunings(Kp, Ki, Kd); // update PID with the new tunings
      break;

    case tuner.runPid: // active once per sample after tunings
      Input = module.readCelsius();
      myPID.Compute();
      tuner.plotter(Input, optimumOutput, Setpoint, 0.5f, 3);
      break;
  }
}

In the loop(), the softPWM function runs continuously and drives a time proportioned output on relayPin 3. The function returns the internaloptimumOutput signal used for SSRs. This allows plotting and diagnostic use.

The switch case controlled by sTune has 3 main states - sample, tunings and runPid.

• Active once per sample during sTune inflection or full test. Here, the Input reading is updated and the plotter function is run

• active just once when sTune is done

• tuner.GetAutoTuningsupdates the Kp, Ki and Kd sketch variables with the new gains

• myPID.SetOutputLimits(0, outputSpan * 0.1); Here, the max limit is set to 10% of the outputSpan because this is the minimum output value needed to drive the temperature of the PTC heater to maximum.

• myPID.SetSampleTimeUs((outputSpan - 1) * 1000);

  The PID sample time is regulated at 1 second in the runPid case.

• debounce = 0; When debounce is set to 0, software PWM uses SSR optimum cycle mode

• The PID is set to automatic, pOnMeas and iAwClamp. Note that pOnError mode isn't used because the output won't react in advance as the input approaches setpoint. Also,iAwCondition mode isn't used as it counteracts the beneficial effect of the optimumOutput software PWM mode.

This case is active once per sample period (1-sec intervals) after tunings. Here, the temperature is read, PID compute is run and the plotter function is run.