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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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Parents

  • This is the second time I've seen PID referenced and I am confused.

    PID was a Process IDentifier assigned by an operating system. Now it is a Proportional-Integral-Derivative (PID) controller, a type of control system commonly used in automation.

    Can someone explain this new coin of PID?

Comment

  • This is the second time I've seen PID referenced and I am confused.

    PID was a Process IDentifier assigned by an operating system. Now it is a Proportional-Integral-Derivative (PID) controller, a type of control system commonly used in automation.

    Can someone explain this new coin of PID?

Children
  • in reply to colporteur

    It's a Control Technique algorithm. A function that will try to regulate a process' output, in relation to a desired setting. First used in 1911.

    It requires that you give it feedback on the current output (e.g.: a sensor value).

    The algorithm uses 3 tuning parameters, P, I and D, to try and make a smooth regulation.

  • in reply to colporteur

    As it says at the bottom: "element14 is the first online community specifically for engineers." and since myself, Shabaz, and Jan are engineers, we quite naturally fall into using the engineering shorthand without being aware that we're, perhaps, excluding others. But you're right that, generally, it's useful to write an acronym out in full the first time it's used in a piece, to help people who aren't familiar with the material.

    I've got a copy of Dorf's Modern Control Systems from 1974, and he doesn't use the acronym 'PID' anywhere that I can see, so it is possibly more recent than the computer use (does that go back to mainframes in the 50s and 60s?), but you're probably still talking forty years or so, so hardly 'new' (by the 1990s it was in common use).

  • in reply to jc2048

    When I studied Control Theory (1985-86), we learned PID on a hydraulic system - no electronics involved. The controller was a device with a few regulation points and meters like you'd see on the utility water counter in your house. I tried to find a photo of a similar device but google isn't helping me today.

    >  it's useful to write an acronym out in full the first time it's used in a piece

    My excuse is that I did that the first time I talked about PID on e14 (in 2017 Slight smile): MSP432 and TI-RTOS: PID Library Part 1 - Intro 

  • in reply to Jan Cumps

    You're lucky you did something practical to focus you on why you were learning the material. For me it was dry lectures at university (around 1977). The Dorf book must have been the set text for the course. I don't remember any of it at all.

    I ought to reread Dorf. Flicking through it, he covers the frequency domain stuff - Bode charts - quite nicely and there's a good treatment of using Nyquist charts to assess stability too. Perhaps I might understand some of it this time.

  • in reply to Jan Cumps

      I have only ever heard of PID referencing control systems (40-ish years).  In the 1980's and 90's, we typically only were able to adjust the Proportional - also called Gain.  There may have been another term that I can't think of right now.  Integral and Derivative were set in firmware.  As technology advanced, we were allowed to adjust the I & D.  I can't count the number of systems I fixed by just turning the P down to 30% to 35%.