Have you calculated the sampling/control intervals yet? If so, what are the fastest ones or most critical ones that must absolutely be deterministic?
On Thu, Feb 25, 2010 at 9:12 PM, I <kirk...@pdx.edu> wrote: > Quoting Josh Triplett <j...@joshtriplett.org>: > >> >>> http://www.osadl.org/?id=99 >>> >> >> Yeah, a huge amount of -rt has made it into mainline Linux. >> >> The question remains, what do we actually need? >> > > This is a good question, and IMHO the answer is fairly straight forward. > > -We need the ability to run one or two critical control algorithms at > regular intervals. > > -We have to be able to guarantee that they will run on-time if they are > responsible for sampling data. > > -We must guarantee that they will be completed in one control cycle. > > Since the sampling is done by the sensor nodes, and the BPF / LQG / NN / > whatever estimator will determine the state and decimate the data, we only > need to 1) be as 'on-time' as the control cycle and 2) keep up with the > data. > > Our control loop could run like this: > Get data inputs (speed, position, etc.) from BPF. > Run control algo (LQR, MPC, PID, whatever) > Send outputs to the latest values. > Yield until next control cycle. > > As long as this happens within the control cycle period, we win. If it > doesn't, the math that makes it work breaks down and we 'could' crash. To > make this guarantee, industry practice is to build run time metrics that > measure processor load (instant, min, max, running average) based on idle > time. As you build the control system, you watch the metrics. I don't let > systems into 1st production with more than 70% loading. The last one I did > shipped at less than 20% loading (fairly simple system). > > If the control task runs over (takes more than one cycle to complete) we > want to log the event so we can make sure we don't see it, and recover > gracefully if we ever do. > > That's really all that's needed for real time control. > > Thoughts? Challenges? > > > > > > > _______________________________________________ > psas-avionics mailing list > psas-avionics@lists.psas.pdx.edu > http://lists.psas.pdx.edu/mailman/listinfo/psas-avionics >
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