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PWM feedback

Started by Unknown December 18, 2013
On Friday, December 20, 2013 7:10:33 AM UTC+13, Tim Wescott wrote:
> On Wed, 18 Dec 2013 16:25:12 -0800, gyansorova wrote: > > > > > On Thursday, December 19, 2013 12:15:20 PM UTC+13, Tim Wescott wrote: > > >> On Wed, 18 Dec 2013 13:14:07 -0800, John Larkin wrote: > > >> > > >> > > >> > > >> > On Wed, 18 Dec 2013 12:42:31 -0800 (PST), gyansorova@gmail.com wrote: > > >> > > >> > > >> > > > >> >>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. > > >> > > >> > > >> >> > > >> > > >> >> > > >> > Counting in a fixed window has a +-1 count ambiguity, which may cause > > >> > > >> > problems. > > >> > > >> > > >> > > > >> > You could use an analog tach circuit to convert the pulses to > > >> > filtered > > >> > > >> > DC and then pretend it was a DC tach. > > >> > > >> > > >> > > >> 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. > > > > 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.
On Wed, 18 Dec 2013 12:42:31 -0800 (PST), gyansorova@gmail.com wrote:

>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.=20
> >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. 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. ?-)
In article <hlfbb9ljob4tj5ml7m4ladr7gda6haid8m@4ax.com>, 
joseph_barrett@sbcglobal.net says...
> > On Wed, 18 Dec 2013 12:42:31 -0800 (PST), gyansorova@gmail.com wrote: > > >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. > > 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