Low Pass Filter for DC motor Speed

Without knowing what you're getting from your FVC, and what you want to actually do, about the only response you'll get to this is "yes".

Presumably you are using a frequency to voltage converter, and presumably you don't like how bumpy it's output is. But you don't say _how_ bumpy it is, _why_ this is a problem, or _what_ you want it to do instead.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

It's very bumpy 4.5 to 7.5V at every frequency except my state state frequency of 5 volts. It's a problem because my set point to my controller is a DC value and and I'm thinking my controller will not like a bumpy input? I have yet to test it on my controller? When I use p-spice miro sim it workes fine.

Not nearly enough information for me to understand your problem.

Sorry for being so vague, but my lab has no internet connection! And i'm try to get some info before I go back. The signal to my low pass filter is a 0 to 10 volt square wave with a frequency of about 20 hertz.. I want an DC voltage from this so I can feed it into my controller. I want a steady DC voltage at all frequencies?

I was told to use a low pass filter but it only works at one frequency

20 hertz. Somebody recommended cascading some low pass filters but i don't understand that because it would seem i would have to have a filter for every frequency which just dosen't make sense to me?

Depending on your controller the bumpy input may be fine or it may be a disaster. If it's a PID controller the derivative stage will see the ripple as 'derivative' and amplify the hell out of it, possibly saturating your output stage.

The only way you can use a traditional V/F converter in this application is if the frequency it's converting is much higher than the intended passband of the motor control -- this usually means that you are generating the frequency from a multi-line encoder wheel, rather than a single pulse per revolution device.

If you _are_ just getting one pulse per revolution, you can make a V/F converter with a nice smooth output by locking onto the pulse train with a 4046 PLL chip, and taking it's VFO input as your V/F output. It'll only have ripple for as long as the PLL isn't perfectly locked; once things settle out it'll be as smooth as you please.

Or you could just run that pulse into a capture input on the counter of a microprocessor, and do all your control in software...

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

And you want the voltage to measure/track your frequency value?

So, why not gate a current source to an integrator, which produces a ramp. At high frequencies, the ramp will terminate sooner (lower voltage), and at low frequencies it will terminate later (higher voltage). A sample/hold stage can hold the last-cycle-termination-voltage until another cycle is complete. Reset the ramp, of course, during the dead time.

Using the square wave as the gate signal, there will be some dependence as well on the duty cycle of the input signal... but no matter how the square wave is generated, either the (+) or (-) phase has to have the right timing dependence to make a feedback signal this way.

One might want to clean up the signal with monostable(s), and use a negative current source if appropriate.