Re: [Xenomai-help] Jitter while generating PWMs with GPIO from RTDM driver

[Wolfgang Grandegger <wg@domain.hid>]

On 04/23/2012 03:25 PM, Andrey Nechypurenko wrote:
>>> Following my tests with PWM generation using GPIO in user space [1],
>>> I've made the RTDM module [2] to further reduce the jitter. As a
>>> result, jitter was improved, but still under heavy system load the
>>> servo motor I am trying to control starts shaking. Now, I fill stuck
>>> and hope to get some help here.
>>
>> What jitters in micro-seconds are you speaking about? What are you
>> requirements? I have not found that information yet in this thread.
> 
> The short answer is - I do not know exactly :-) .
> 
> The long answer is - my goal is to keep servo motor stable under the
> high system load. I was unable to find any concrete numbers regarding
> the tolerance level of the no-name servo motor. So I am just
> experimenting. My recent observations, presented in the blog I've
> mentioned in the original post, suggest that 20-30micro-seconds jitter
> will force my concrete servo to start the movements.

You can run Xenomai's latency test under load to get an idea what you
can achieve on that system... and it will be more than 20-30us. Also try
the -t1 and t2 options.

> I was unable to further reduce jitter, so maybe this number is
> actually lower. I will try to implement what Gilles suggested (using
> timer api) and see if I can a) further reduce jitter; and b) if this
> jitter will be small enough to prevent servo from shaking.

The timer anticipation trick Gilles suggested would work fine, I think,
whithout costing too much (as your period is just 20ms).

Wolfgang.