Simple code for stepper code -> jittery movement

[cristian-spiescu]

Hello,

I have an M5Stack Core (w/ ESP32) a board w/ stepper motor drivers. Their corresponding library (src) works, but unfortunately it doesn't allow precise positioning. It allows me only setting the speed/pulse frequency (in Hz). This freq is fed into a PWM that controls the pin.

I tried to write a small piece of code that removes the PWM and commands the pulses in the loop() function:

# in setup(), I deactivate the PWM
  stepmotor_driver_0.pwm_y.deinit()

# in loop()
  # e.g. freq = 10 Hz => period = 1 / freq = 0.1s = 100ms
  # for percentageOn = 0.5 => 50 ms on, 50 ms off
  # for percentageOn = 0.1 => 10 ms on, 90 ms off
  if lastState:
    # currently on
    timeToWait = 1 / freq * 1000 * percentageOn
  else:
    # currently off
    timeToWait = 1 / freq * 1000 * (1 - percentageOn) 

  if not lastMove or now - lastMove > timeToWait:
    # print(lastMove, not lastMove or now - lastMove, 1 / freq / 2)
    if lastState:
      pin.off()
      # print("off")
    else:
      pin.on()
      # print("on")
    
    lastState = not lastState
    lastMove = now

This works, but I find that the move/sound is quite jittery/unhealthy (compared w/ their lib). I made UI buttons to adjust the freq and percentageOn. The "sound" of the motor does change, but still it is not smooth.

Any hints about what I'm doing wrong?

Thanks in advance!

[ricksorensen]

Did you check the implementations here?
https://awesome-micropython.com/#stepper

The whole code is not shown (e.g. what is settting now?) In your loop the python infrastructure such as garbage collection likely is affwecting the timing of the pulses giving jitter and noise, whereas the PWM is much more consistent on pulses.

[cristian-spiescu]

Thanks for the link. I didn't know about it. I did found AccelStepper (which is mentioned in the link); I had the intention to give it a try.

I'll try some of the libs there.

Nevertheless, I'm curious what I'm doing wrong in my code. I looked at a lot of libs for Arduino. Besides the fancy acceleration & co mechanism, I didn't spot any other "tricks".

[mendenm]

@cristian-spiescu The timing stability of compiled Arduino code is very different from that of interpreted python code. With compiled code, and interrupt-driven timing, you can get timing jitter that is probably 3 orders of magnitude better than that of interpreted python code.

[cristian-spiescu]

Thanks guys!

1/ As a curiosity, I adjusted the code to work w/ microseconds instead of milliseconds. The jitter reduced, but it was still there.

2/ My attempt to keep the pin ON during an amount of time (i.e. to simulate a duty cycle of the PWM) seems to be in vain, and even hurting. I saw that people do: pin.on(), and then right away pin.off(), then wait. And repeat

3/ I switched to using a timer. And the following piece of code works like a charm:

pin = Pin(16, Pin.OUT)
timer = Timer(-1)
timer.init(freq=freq,callback=_timer_callback)

def _timer_callback(t):
  global pin
  pin.on()
  pin.off()

It's funny 🙂. I develop computers as day job, and not microcontrollers. In a previous microcontroller home project (w/ ESP8266), my reflex was to use timers + callback and avoid the loop() function. Because the code is cleaner, async oriented, etc. But I fell into a deep pit hole. Sometimes my timer callbacks were not triggering. And this because the mechanics of starting/stopping the timers was in some corner cases fast enough; the ESP8266 didn't fulfill it's contract to subscribe/unsubscribe, and guess what: it didn't report an error. It was failing silently.
Hence I switched to a loop() approach which worked OK. I.e. timers 👎, loop() 👍.

In this case it seems to be the opposite: timers 👍, loop() 👎.

4/ I tested the first lib = micropython-stepper in the list as recommended in the previous post. It works well; has a good API; is async. I think this is what I'll use for my project.

5/ I'm amazed how sensible the stepper movement is. I mean, when the framework of my M5Stack core does some house keeping tasks (e.g. bothering me w/ this kind of error message E (1310882) mqtt_client: Error create mqtt task), I hear a minor hesitation in the movement of the stepper.