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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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  • After a lot of tweaking, it's still not perfect, but I think acceptable.

    For the results in the chart below, I initially set a temperature of 70 degrees, and experimented with adding an aluminium thermal mass on top briefly to force the temperature down by a few degrees, and then removing it to see the response back to the set temperature.  I kept the mass on top at one stage (labeled Kept Mass), to see the temperature recover with the mass present too.

    Then, I reduced the set temperature by two degrees to 68 degC, then increased it back to 70 degrees, then set it to 150 degC, then again did the thermal mass test, then finally turned the set temperature down to 100 degC.

    From the chart, what can be determined is that there is overshoot, I struggled to tame that, and didn't succeed. The overshoot is usually under 1.5 degC, but under certain conditions (such as deliberately cooling the system by about 6-8 deg C by temporarily adding a thermal mass and then removing it), when it tries to recover, the overshoot can reach about 2.5 degC.

    The damping occurs reasonably well, the error is under 0.5 degC often within the first oscillation, before it dampens out. Maybe it would be better for the system to slowly approach the temperature so that it doesn't overshoot or undershoot at all.Or maybe I'm being too picky. Apparently 3D printer hot end control systems overshoot too, and even 5 degC is considered acceptable for that use-case.

    The undershoot shown in the chart below (when lowering the set temperature) is better than the overshoot; usually under 1 degC.

    The PID code is complicated now, it kicks in the 'I' portion only within a certain range for instance. I relied heavily on Chat GPT to handle that.

    In summary, although it functions, I'm sure performance could be improved a lot, by control system experts. But, for the actual end application (heating punches for leather), and probably a whole load of other applications, I think the current performance may be acceptable (it took an entire day of tests and tweaks to get it to this state: (

    I hope the control system doesn't have any unexpected behavior under certain conditions, but I'll find out over time, as it gets used. I need to move on to the PCB, to get the project into at least a usable state.

    image

  • in reply to shabaz

    Video of the annoying flicker! The display is very bright, and the large temperature digits are readable from at least a couple of metres away, which is really nice, but cosmetically it's ruined by the flicker.

    I know of some methods to significantly reduce this, but all of them will no longer be general-purpose (currently I can place arbitrary text on the display), but if sticking to numbers only, it should be possible to pre-render and store in Flash, and that would fit in the MSPM0. Anyway, that's a mini-project in itself, for now, I'll leave the flicker as-is : (

Comment
  • in reply to shabaz

    Video of the annoying flicker! The display is very bright, and the large temperature digits are readable from at least a couple of metres away, which is really nice, but cosmetically it's ruined by the flicker.

    I know of some methods to significantly reduce this, but all of them will no longer be general-purpose (currently I can place arbitrary text on the display), but if sticking to numbers only, it should be possible to pre-render and store in Flash, and that would fit in the MSPM0. Anyway, that's a mini-project in itself, for now, I'll leave the flicker as-is : (

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