[Dprglist] motion profiling with stepper motors

[markus]

Jose, Ron, thanks for your feedback those are great implementation pointers.

I did implement both approaches but have trouble with the wall clock approach (update the frequency periodically regardless if the previous update has ever been applied). Accelerating is fine the issue is with deceleration and coming to a halt at a specific point.

In the linear world the distance where I have to start constant deceleration is defined by:
  S = V^2 / (2 * a)

	V ... current velocity
	A ... constant deceleration

And again this works fine as long as the update frequency is faster than the step frequency, but once the step frequency drops below the update frequency I can't reduce the speed fast enough (because the timer only updates on overflow).

What happens is that the stepper is still turning quite fast by the time the robot reaches its target position.

Does anybody know how the math works here?

Any pointers appreciated, thanks,
Markus



On Tue, 20 Mar 2018 16:36:02 -0400
Ron Grant <deltagraph at aol.com> wrote:

> Markus,
> I am not a stepper expert by any means...
> 
> >> 1ms later we could/should set the frequency to 10Hz but that's not
> >> possible because the period of one 5Hz cycle is 200ms.  
> You mean 10ms later... right?
> 
> I think you might be having trouble due to (as you realize) trying to
> vary a speed where number of steps per update interval is very low
> (less than one in your case).
> 
> Wonder if you could be helped by using micro-stepping to increase
> apparent stepper resolution? OR are you already micro-stepping? If
> not, your 5Hz cycle might become with 16X microstep 80 Hz giving you
> ability to control slower rates more gracefully even if stepper does
> not perfectly track the micro step positions  -- fancy controllers
> and encoders help do that as far as I know.
> 
> Maybe there is an elegant solution to calculate just what your
> interval needs to be.
> 
> My first thought would be to linearly ramp your expected pulse timing
> every 10ms then force a step if need be. That is at time zero you set
> 100 ms as the width but at update 1 (10ms later) your goal is 50 ms
> pulse width (with 5Hz/10ms accel. rate), so you force a step and
> reset timer. At update 2 your goal would be 25ms but only 10 ms
> elapsed so no intervention...
> 
> Just for fun (whether right or wrong way to do things) :
> 
> My preferred way to handle steppers is with a 10 KHz interrupt that
> can decide what the step frequency is going to be with 1Hz
> resolution. Grainy when step rates are very low, but seems to work
> well.  10 KHz might be too much for slower clock speed / 8 bit MCU,
> not using any fancy timer stuff (just periodic interrupt)
> 
> code for part of 10KHz interrupt routine - for left motor below
> Note TimeL is 32 bit int
> 
> using step and direction driver, e.g. something like DRV8825 
> not show is update that calculated StepRateL(and R) for differential
> drive robot at say 100 Hz.
> 
> TimeL += abs(StepRateL);
>  if (TimeL>10000) 
>  {
>     TimeL -= 10000;
>     StepMotorDirL = (StepRateL>0);     // set DIR pin     REVERSE L
>      if (StepRateL<0) MotorL--;  else MotorL++;   // virtual encoder
> count (volatile long int) StepMotorStepL =
> 1;                                      // manually generate a step
> pulse wait_us(4); StepMotorStepL = 0;   
>  }
> 
> Of course you could use a timer to implement step rate, but take care
> in adjusting rates where you might shorten an interval to a count
> that the counter is beyond...  
> 
> Ron Grant
> 
>  
> 
> -----Original Message-----
> From: markus <markus at bibi.ca>
> To: DPRG <dprglist at lists.dprg.org>
> Sent: Tue, Mar 20, 2018 12:53 pm
> Subject: [Dprglist] motion profiling with stepper motors
> 
> I started playing around with motion profiling and thought using a
> stepper was easier for experimenting. I've implemented "constant
> acceleration" and started to wonder what the correct implementation
> is when the stepper frequency is below the update frequency.
> 
> Let's say motion profiling updates the frequency every 10ms and the
> acceleration limit is set to 5Hz/10ms. So at the initial update the
> frequency is set to 5Hz - 1ms later we could/should set the frequency
> to 10Hz but that's not possible because the period of one 5Hz cycle
> is 200ms.
> 
> So the question I have is what is the correct value to update the
> timer at the end of those 200ms?
>  -  10Hz
>  - 100Hz
> 
> Both seem wrong, if I set it to 10Hz it's not really "constant
> accelleration" at low speeds from a wall clock perspective. And if I
> set it to 100Hz I might exceed the "constant accelleration" from the
> motors perspective.
> 
> Comments, thoughts?
> Markus
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