> On 09 May 2017, at 04:30, Brian Schousek wrote:
>
> I'm having good luck achieving microsecond resolution pulse trains using DMA
> on a Raspberry Pi at near real-time rates, and I'm confident that with some
> optimization this will be workable for step generation with low cpu usage. I
> currently have this step generation hacked into a Machinekit configuration
> which monitors the trajectory planner position pins. When a loop in Python
> detects a change in axis.0.motor-pos-cmd, for example, it queries
> axis.0.joint-vel-cmd and creates a stream of n steps at the proper frequency
> where n is the number of steps that will fit into my millisecond servo loop.
Python is probably not the right vehicle for realtime tasks (That’s what I
always understood).
I’m not really sure I understand what you’re trying to accomplish here. Are you
developing a hardware driver? Are you making a planner?
>
> The DMA library I am using is capable of synchronizing sequential waveforms,
> so I would like to be able to look ahead in the trajectory planner queue a
> step or two and construct the waveforms in advance where possible, which
> should help solve some latency issues. I have a few questions towards
> achieving this:
>
> 1) Is there a better mechanism to detect a new trajectory step than
> monitoring changes in the position-commanded HAL pin?
You could write a component where you add these samples of axis.0.motor-pos-cmd
and axis.0.joint-vel-cmd to a ringbuffer. (see delayline component where you
can add a number of different type of pins to a ringbuffer). Inspect the
ringbuffer samples to detect these changes and calculate your special stuff.
Perhaps it’s easier to create a 2nd ringbuffer (than modifying the rinbuffer
samples) for playout.
> 2) Is there visibility into the trajectory planning queue somewhere?
see links below
> 3) Perhaps these are not exposed in the current Python bindings; if not, is
> there a good example of accessing it via C calls?
I’m not deep enough into that, good starting point I guess would be these:
https://youtu.be/Bg38894LPPg
https://youtu.be/ev_RI4xgzF8
Bas
> 4) Are there any other tips on source code areas I should investigate as
> resources in putting this together?
>
> Brian
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