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  • Author Author: Jan Cumps
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PID temperature controller for the EasyL1105 MSPM0 board - Pt. 3: PWM

Jan Cumps
PID temperature controller for the EasyL1105 MSPM0 board - Pt. 3: PWM

  designed a development kit for the recent Texas Instruments MSPM0 microcontroller series. 
This 4 part blog series documents the steps to design a PID temperature controller. Part 3: add PWM to generate a PID controlled output.

image
(post that introduces the kit)

Goal of this 3nd post

not a goal of this post: have the PID regulation working 100%.

Set up PWM SysConfig

The code uses timer TIMG1, channel 0, to drive PA26. There's no interrupt involved. The duty cycle gets adjusted in the regulation loop.

At this point, I have set the PWM period count to 65535, in an attempt to have the output range identical (but 32768 offset) to the input. 

image

image

Code

note: this design doesn't regulate perfectly yet. At this point in the blog series, all modules are in place and they are tied to the PID. But finetuning PID parameters, input, output, ADC and PWM settings is for post 4.

void perform_pwm() {
    DL_TimerG_setCaptureCompareValue(PWM_0_INST, i32_Output_PID + 32768,
        DL_TIMER_CC_0_INDEX); // update ccr0 value  
}


int main(void) {
    SYSCFG_DL_init();

    // /* timer 5 interrupt ticks per second */ 
    // /* Enable Timer0 NVIC */
    NVIC_EnableIRQ(TIMER_0_INST_INT_IRQN);

    NVIC_EnableIRQ(ADC12_0_INST_INT_IRQN);
    gCheckADC = false;

	/* Initialize the parameters of PID */
	Initialize_PID_Parameter();

    /* Start PWM */
    DL_TimerG_startCounter(PWM_0_INST);

    /* Start Timer counting */
    DL_TimerG_startCounter(TIMER_0_INST);    

    while (1) {
        if (perform) {
            perform = false;
            DL_GPIO_setPins(GPIO_GRP_LEDS_PORT,
                GPIO_GRP_LEDS_PIN_LED_GREEN_PIN);

            perform_adc();
            
            perform_pid();

            perform_pwm();

            DL_GPIO_clearPins(GPIO_GRP_LEDS_PORT,
                GPIO_GRP_LEDS_PIN_LED_GREEN_PIN);
        }
    }
}


void perform_pid() {
    /* Execute PID control in every TM0 interrupt. */
    i32_Output_PID = PID(&PID_Var, i32_Target_Command, gAdcResult);
}

If you compare the perform_pid() with the one from the previous post, you 'll see that it now uses ADC as feedback, and output to drive the PWM.

Demo circuit

I used the same (random) low pass filter as in  MSP432 and TI-RTOS: PID Library Part 2 - Real World Example , to turn the PWM into a DC signal.

image

The filter sits between PWM out  (P26) and ADC in (PA25).

ccs project for EasyL1105: pid_EasyL1105_20251002_02.zip

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  • Good news!

    I added the PWM code and was getting a lot of overshoot which I couldn't seem to correct, plus it seemed to flip between extremes. I tried tuning things by experimenting, but I didn't get far. Eventually I asked chat GPT for help, and it proposed doing something interesting to the PID, namely, only enable the "I" portion when it's within a few degrees of target. Don't know why. Also it made quite a few modifications, to sort out some inconsistencies I had concerning the ranges of values I was expecting to use for the inputs and the PWM range I wanted. I'll tidy up the code and post it, since currently there are two PIDs in parallel, the one where I'd been experimenting, and the one proposed by ChatGPT, and it's confusing until I delete the old one.

    Here's a run of 11 minutes, the initial set temperature was 45 degC, and then I moved the potentiometer to 55 degC. The vertical axis shows the temperature (blue is potentiometer, orange is thermistor), and the horizontal axis shows sample number, but basically it is about 11 minutes 20 seconds across the chart.

    As can be seen from the orange line, there is some oscillation, but it's not more than about 1.5 degrees C, and then it does dampen down. You can see it takes several minutes to dampen down fully, but doesn't seem too bad.

    The little glitches in the ADC measurements only seem to be from the thermistor measurement, not the potentiometer, so I'm not sure what to make of that. Maybe it's just the bad prototyping, and might disappear if it were done properly on a PCB.

    image

    In summary, all the little modules of code all fitted together, and  I think it's all functioning, but I still need to test at a much higher temperature like 150 degC. Plus I need to refresh myself on Excel, and make a more decent chart with time in minutes on the x-axis.

    I think we managed a lot.. within a week or two, a whole load of integrated peripherals on the MSPM0 chip were used to pull this project together!

    I'm really looking forward to assembling this on a PCB.

  • in reply to shabaz

    > The little glitches in the ADC measurements only seem to be from the thermistor measurement

    If the thermistor needs buffering, that's possible. The output of the internal general purpose opamp (GPAMP) can be routed to an ADC channel.
    And if you're looking for a pure buffer, no opamp feedback resistor needed. That can be set in firmware.

    image

Comment
  • in reply to shabaz

    > The little glitches in the ADC measurements only seem to be from the thermistor measurement

    If the thermistor needs buffering, that's possible. The output of the internal general purpose opamp (GPAMP) can be routed to an ADC channel.
    And if you're looking for a pure buffer, no opamp feedback resistor needed. That can be set in firmware.

    image

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