On 7/21/2011 11:53 AM, Peter C. Wallace wrote: > On Thu, 21 Jul 2011, Steve Stallings wrote: > > >> Date: Thu, 21 Jul 2011 10:46:13 -0500 >> From: Steve Stallings<[email protected]> >> Reply-To: EMC developers<[email protected]> >> To: 'EMC developers'<[email protected]> >> Subject: Re: [Emc-developers] Preempt-RT ... where to put the patches ? >> >> Peter, >> >> Good point about interrupt jitter mattering less if >> the actual sample time does not jitter. Unfortunately >> the current RTAI setup requires that the interrupt >> comes from the host interrupt timers. If we could >> get to a system model where the hardware could >> sample the data at the same time that it also >> requests an interrupt, then the actual interrupt >> latency would be much less critical. The PC system >> design does not bring out the interrupt request >> anywhere, so syncing with it in hardware is not >> possible, nor can RTAI (as best I know) accept an >> interrupt from external hardware as the system timer. >> > > What I was thinking about is living with the jitter, that is changing the math > so that jitter is compensated for. This mostly means that any time the > constant thread time is used in a PID/stepgen/motion calculations, the actual > thread actuation time is used (jitter and all). That is if we can measure the > jitter (we have a low overhead high resolution timer to read) we can > compensate for it. > > For servo systems (or pseudo servo systems like hardware stepgens) jitter > should really only cause second order errors ( commanded accell errors not > velocity errors) > > As it is (without actual thread time factored in) even in an unaccelerated > portion of a motion profile, a 10% jitter (say the D510s 100 Usec at a 1 KHz > thread time) will cause the PID loop to see a 10 % velocity error (when in > fact there may be no error at all). > > > So basically you could factor out the jitter entirely?
That sounds like a really good idea! :-) Dave ------------------------------------------------------------------------------ 5 Ways to Improve & Secure Unified Communications Unified Communications promises greater efficiencies for business. UC can improve internal communications as well as offer faster, more efficient ways to interact with customers and streamline customer service. Learn more! http://www.accelacomm.com/jaw/sfnl/114/51426253/ _______________________________________________ Emc-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-developers
