DSPRelated.com
Forums

Re: Control Loop Stability and Sampling

Started by Tim Wescott April 7, 2006
Steve Minshull wrote:

> Tim Wescott wrote: > >> Steve Minshull wrote: >>
-- snip --
>> > Hi Tim....Thanks for thinking about this one. I'll try to explain a > little more. > > THe model is presently in continous time but Space Vector PWM doesn't > really exist in continous domain. It works by sampling a DQ voltage > demand and working out a switching vector and dwelling times. My model > effectively calculates a new vector each PWM Time. So my PWM frequency > is 20kHz and effectively my Space Vector Control is also working at > 20kHz. THe PI part of the control operates in continous time. i.e the > transfer function is kp+(ki/s). Ultimately I'm going to a DSP and so > the PI will be discrete therefore I think I should probably go to a > fully discrete model too. > > THe components are indeed a series L and a shunt C (Bruhless ac motor > Phase) so I'll have a go at adding differential and see what happens. > > Just had a thought....Maybe my problem is the mixed continous/discrete > controller....the continous integral part will ramp up/down during PWM > period when it cannot effect the output. > > When I use an "ideal" converter (sinousoidal voltage source) The system > behaves well. It is when I model the converter at a power electronics > level (ideal switches) i get problems with the torque ripple. > > I'm going to try cross posting this to comp.dsp > > Steve Minshull
A question: What loop are you closing here? At that bandwidth I would hope it's the current, to set the torque. A comment: If it worked at the 'ideal' converter stage, but not when you changed both the controller and the drive stage, perhaps you should back out of half of your changes and just use a sampled-time controller with an 'ideal' sinusoidal drive. Even something as wheezy as putting a sample-and-hold block between the output of your controller and your sine wave drive should be enough to either validate your sampled-time controller or find your problem. If it works with a sampled controller and a perfect drive, then you can reduce your question to asking why the PWM control doesn't work like the ideal drive. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/