Pole Placement: physical or software? - Page 1
Forum section: electronics / beginners

author: Wrydog
author_rank: Contributor
timestamp: April 11, 2017, 10:19:13 pm
content: Obviously the transfer function of what-have-you has poles and zeros that correspond to real R, L, & C components. When reading about "pole placement" in feedback/control literature, it's not quite clear to me if that simply means altering the components on the board or if it can also be done in the control software.Can pole placement be done in software?Thanks.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 11, 2017, 10:35:51 pm
content: Only if part of the filtering is done digitally in software. In principal you could cancel the hardware poles and then put others wherever, but unless your sampling rate is infinite and the noise is zero the cancellation won't be perfect. It is also a silly thing to actually do. Zeroes can't theoretically be cancelled, but they can be approximately cancelled with the same kind of limitations as poles. Again, usually silly.The obvious exception to the silliness comment is when the pole, or zero, is not the result of some installed arrangements of Rs, Ls and Cs, but is a physical characteristic of the system. A mechanical or rotational resonance for example. There are some interesting and useful things that can be done by cancelling or partially cancelling these poles and or zeroes.

author: MrAl
author_rank: Super Contributor
timestamp: April 12, 2017, 10:22:34 am
content: Quote from: Wrydog on April 11, 2017, 10:19:13 pmHi,That's actually part of digital control theory. Within that frame, you can use a transform to take the transfer function and convert it into a difference equation and that's basically your digital version to be done in software.One way to do this is to transform your 's' domain transfer function into the 'z' domain, then into a difference equation. Check out the Z Transform.I have an example somewhere that shows the response using the original transfer function and then the response after being transformed into the difference equation. The responses are very similar. The sampling period has to be appropriate for the job though.Found it...excitation was a unit step input.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 12, 2017, 03:47:04 pm
content: I should clarify. When I say it is silly to digitally cancel circuit characteristics from discrete R, L and C I mean that usually you should just change those values rather than try pasting compensation over them. There are always exceptions, but correcting the discrete components digitally is somewhat like a situation I encountered where there were seven inverting amplifiers in series. They had been added sequentially to correct an overall system phase that had changed for other reasons. No one ever went back a looked to see if removing a stage would be a better solution to the problem. Some of the additions may have been forgivable - there were spare op-amps in a package, and they already were hooked up so that the unused op-amps would have a defined state, so it was easy and no extra parts or real estate were consumed. But when an extra package was added someone was really asleep at the switch.