It seems the servo thread would be the highest priority code, so why not
disable interrupts, or make it the highest priority interrupt.
Brian
On Jul 21, 2011 1:08 PM, "Peter C. Wallace" <[email protected]> wrote:
> On Thu, 21 Jul 2011, Jon Elson wrote:
>
>> Date: Thu, 21 Jul 2011 12:01:29 -0500
>> From: Jon Elson <[email protected]>
>> Reply-To: EMC developers <[email protected]>
>> To: EMC developers <[email protected]>
>> Subject: Re: [Emc-developers] Preempt-RT ... where to put the patches ?
>>
>> Dave wrote:
>>> So basically you could factor out the jitter entirely?
>>>
>>> That sounds like a really good idea! :-)
>>>
>> Well, you can't, really. You can only reduce the likelihood of the
>> jitter affecting the calculations.
>> The problem is that a thread can be interrupted at any point, without
>> any way to tell exactly
>> where the interrupt caused a delay. So, if you sample the timer here,
>> and then sample the encoders
>> THERE, you have no way to know for sure what the time difference between
>> those two events
>> is. If you disable interrupts between those two events, then you have
>> bounds on the time
>> difference, but we probably don't want to do that.
>
>
> For hardware velocity based systems (hardware stepgens, velocity mode
servos
> etc) There is no need for a base thread, and therefore no reason not to
> disable interrupts for the entire servo thread invocation. This woulbe be
a
> good thing.
>
>
>>
>> I don't know enough about the kind and duration of interrupts that might
>> preempt the servo thread to
>> make much of an evaluation, but the important thing is that those
>> interrupts can happen at ANY point
>> in the code, and trying to correct for it by taking one time sample at
>> one point in the code could even
>> make things worse! This sort of thing could be studied with a little
>> test rig. You hook up a
>> constant frequency source to an encoder counter, and use the CPU timer
>> to attempt the correction
>> as described previously. With a perfectly steady velocity, then any
>> velocity jitter improvement will be
>> due to the correction scheme.
>>
>>
>> Jon
>>
>>
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>
> Peter Wallace
> Mesa Electronics
>
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