Reconstruction

On the other side of the coin from sampling, do you ever use any kind of reconstruction filter in a control loop? I have never, ever done so; if
I'm concerned about the edges of my drive samples it means I'm sampling too slow and I need to correct the problem. The only time that I could see doing it would be if I were driving some otherwise well-behaved plant that were somehow sensitive, but that seems to be a rare thing.
------------------------------------------- Tim Wescott Wescott Design Services http://www.wescottdesign.com
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Tim Wescott wrote:

A little RC to round the corners sets my mind at ease. I squirm if I can hear the DAC on the radio.
Jerry
--
Engineering is the art of making what you want from things you can get.

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wrote:

Most actuators provide enough roll-off to filter the output from the DAC, but I suppose if the control software didn't implement smooth switching between control modes that might cause a transition from Max to Min output, then a low pass filter might be needed to prevent the excitation of high frequencies in the system.
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In sampled-data theory (which has similarities with discrete-time digital systems), the sample-and-hold device (the DAC in a digital system) is itself a low-pass filter which does the reconstruction of the signal to produce a continuous-time signal from a discrete-time (sampled) signal. Of course, the reconstructed signal then still has a stair-case like appearance; but this is not usually a problem because, as you say, the appropriate choice of sampling frequency as a function of the plant's frequency bandwidth means that the controlled system itself provides further low-pass filtering; so that the stair-case like output signal becomes an smoothed response in the actual controlled system. Kelvin B. Hales Kelvin Hales Associates Limited Consulting Process Control Engineers Web: www.khace.com
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I haven't used it; but I would think the design considerations would be covered in articles/books on microstepping of stepping motors. The engineering theory must consider both the time and space resolution in order to do the design. Never had to do microstepping myself though, so I can say for sure.
Rrogers
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