PWM feedback

[[email protected]]

This is more of a practical question. I understand feedback and how it works and can design controllers. However, these have been for dc (non PWM) motors using tachos with a dc output.

What I want to know is how you achieve this with digital tachos which give out pulses.

I am thinking that the set point is still dc as before (or software based) and you just count the pulses from the tacho in a given time and this gives an error signal which you then generate PWM from - am I right? the rest is usual lag-lead compensators etc.

Another approach I suppose would be to have a square wave as the set-point and use the pulses from the tacho and create a phase-detector and PLL approach.

 

[Lasse Langwadt Christensen]

Den onsdag den 18. december 2013 21.42.31 UTC+1 skrev [email protected]:

This is more of a practical question. I understand feedback and how it works and can design controllers. However, these have been for dc (non PWM) motors using tachos with a dc output.



What I want to know is how you achieve this with digital tachos which give out pulses.



I am thinking that the set point is still dc as before (or software based) and you just count the pulses from the tacho in a given time and this gives an error signal which you then generate PWM from - am I right? the rest is usual lag-lead compensators etc.

unless you have a huge number of lines I'd measure the period of the pulses

Another approach I suppose would be to have a square wave as the set-point and use the pulses from the tacho and create a phase-detector and PLL approach.

measuring the period of the pulses and comparing to what you want the period
to be is sorta the same

-Lasse

 

[dp]

On Wed, 18 Dec 2013 12:42:31 -0800 (PST), [email protected] wrote:

...

Counting in a fixed window has a +-1 count ambiguity, which may cause
problems.

Usually instead of measuring frequency (counting pulses in a window)
measuring period (counting clock pulses with the sensor pulses being
the window) solves this issue if existing, one just uses a fast enough
clock.

Dimiter

 

[[email protected]]

You get pretty much the same effect by accepting your +/- 1 count

ambiguity (really, it's a +/- 1/2 count ambiguity), then low-pass

filtering on software. Then you're not defeating the purpose of having a

Low pass filtering = phase lag and less phase margin. So it will have to be outside the closing unity gain frequency I assume.

 

[[email protected]]

Yup. I'm not touting it as a panacea, I'm just pointing out that its not

necessarily all that different to do it in the digital realm vs. analog.



(Some, but not a lot, unless you really don't have many lines in your

encoder).



--



Tim Wescott

Wescott Design Services

http://www.wescottdesign.com

Yes but you are going to have to filter it hard to get dc out of PWM. Won't you just get a sine wave with a weak filter.

 

[josephkk]

This is more of a practical question. I understand feedback and how it works and can design controllers. However, these have been for dc (non PWM) motors using tachos with a dc output.

What I want to know is how you achieve this with digital tachos which give out pulses.

I am thinking that the set point is still dc as before (or software based) and you just count the pulses from the tacho in a given time andthis gives an error signal which you then generate PWM from - am I right? the rest is usual lag-lead compensators etc.

Another approach I suppose would be to have a square wave as the set-point and use the pulses from the tacho and create a phase-detector and PLL approach.

Back in the day ('bout 40 years ago) there were some really good frequency
to voltage converter ICs. The principles are still valid. Most digital
versions are translations of the method, pretty similar to count them up
over some fixed repeating period.

?-)

 

[Maynard A. Philbrook Jr.]

Back in the day ('bout 40 years ago) there were some really good frequency
to voltage converter ICs. The principles are still valid. Most digital
versions are translations of the method, pretty similar to count them up
over some fixed repeating period.

?-)

And they still exist in many venders!

Jamie