Re: Filtering a tachometer generator
- From: "Korenje" <korenje@xxxxxxxxxxx>
- Date: 8 Jan 2006 22:36:46 -0800
Jerry Avins wrote:
> In ancient times, DC servo motors usually had small generators attached
> (today, we use encoders) whose output is proportional to rpm. These
> antediluvian devices suffer from commutator ripple, which can have nasty
> effects on feedback loops. For servos that don't run over very wide
> ranges of speed, the ripple is easily dealt with. Commutators almost
> always have an odd number of segments, so the number of cycles of ripple
> per turn is double the number of segments. A 17-segment commutator's
> ripple frequency if 34 times the angular frequency of the motor, and
> it's easily filtered without excessive delay /at any one speed/.
>
> There's a rub. The filter needed to smooth the ripple at 1 rpm
> introduces enough delay at 1000 rpm to have a noticeable -- possibly
> unacceptable -- effect on stability. The simple solution lets the speed
> wobblulate at low speeds and hope nobody notices or wonders where the
> sound comes from.
>
> An adaptive filter seems to be in order. Just changing coefficients in a
> fixed structure doesn't seem to be the best way. We want a lowpass
> filter whose cutoff and group delay track the motor speed. Moreover, a
> prompt (close to minimum phase) filter preserves stability best. Is
> there a known structure for this, or is an invention (or motor
> replacement) in order?
>
> Jerry
> --
> Engineering is the art of making what you want from things you can get.
> ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Jerry,
Two solutions come to mind, but for both I am assuming that you have a
micro in your loop.
1. If you are sampling your tacho output, use the tacho spikes to
trigger your AD (with some delay), so that you always sample tacho
output x us/ms after spike. I have used this technique to overcome some
switching noise, but mine had a limited frequency range. An upgraded
version of this would be that you sample in the middele between two
spikes, this way you get rid of the ripple also.
2. filter heavily so that your system works well at low speeds. At
higher speed switch to counting nr. of spikes in fixed time. You'll
need some DC blocking filter for the second approach.
Hope any of this helps
Mitja
.
- References:
- Filtering a tachometer generator
- From: Jerry Avins
- Filtering a tachometer generator
- Prev by Date: Re: Doubt regarding Plagiarism and copyright infringement
- Next by Date: Re: OT Rant
- Previous by thread: Re: Filtering a tachometer generator
- Next by thread: Re: Timing recovery and Carrier recovery
- Index(es):
Relevant Pages
|
Loading
