Calculating velocity from position in a noisy enviroment

A more typical filter would be the alpha beta (gamma) filter

MfG JRD

That's what we used to use to filter sensor values back when I worked on fire control firmware for a DoD contractor.

Not all hardware people believe that. ;)

It's as simple an an IIR filter because it _is_ an IIR filter -- it's just done with a formulation to make it easier to apply to problems like this.

OP: If you do this, pay attention to how you could use your drive command for your velocity update.

No, but it can mask a lot of problems until after you've fielded systems :-(.

1b: Use a different position sensor, such as an encoder, an RVDT, or a resolver.

2: Filter more. I assume that you've already tried this, and can't find a good tradeoff between how heavy a filter you need for noise suppression vs. how light a filter you need for good control, but I thought I'd toss out the suggestion.

3: If you need snappy positioning but can get away without the worlds best disturbance rejection, consider using feed forward.

4: Change your amplifier. If you're driving the motor with a current amplifier then try changing it to a voltage amplifier -- this has the effect of making the motor more of a constant-velocity device which adds some nice damping, at the cost of not being able to directly limit current to the motor.

5: Ask questions like this on sci.engr.control. There's folks there with lots of experience; if you make sure to explain your circumstance they'll help you out as much as they can.

Since the OP can (and it seems to me, should) prepare a process/system model for his case, the Kalman filter seems better in this case. (And more work to do.)

Jon

Kalman filters work great when you can get an exact model of the system and the noise, and the noise is Gaussian.

If you can't then by the time you get done with all the approximations the "Kalman" filter for such a simple system will look an awful lot like an alpha-beta filter -- so why not go there from the get-go?

Alpha-beta appears inferior to me if the OP can work out a model. If not, then it all boils down to an ad-hoc, static arrangement anyway and I agree.

Jon

Is the A/D under software control?

Regards,

-- Walter Banks Byte Craft Limited

Hi Walter,

The A/D converter is built into the microcontroller so I guess that yes it is under software control.

Which processor a/d ? Where I am coming from is it is possible in some cases to build some filtering into the analog conversion from the pot. This makes the problem both earlier ad likely have a better frequency response than more complex processing after the conversion.

Feel free to contact me off line

Regards,

-- Walter Banks Byte Craft Limited

Often pot noise like this is in the position domain, not the time domain

-- i.e. the mapping between position and pot voltage has hair on it, not the mapping between the pot's nominal voltage and the actual voltage being read by the ADC.

That having been said, something like sampling fast, filtering and decimating before the control loop can work wonders if the noise is temporal and not positional.