FPGA velocity estimation and VFD noise

[pewt]

Hi, I'm using an NI FPGA (a 7411, I believe) with an RT unit and VFD to get very low lag, high resolution velocity estimation from a 3600 pulse/rev quadrature encoder (mounted on the VFD driven motor). When the VFD is off and I turn the encoder by hand I get very good tracking and low noise. With the VFD enabled, however, the system is very noisy and usually has about a 1-5% error. The whole system needs to have as little lag as possible (<.01ms).

I'm looking for ways of reducing noise in the system programmatically on the FPGA or of enhancing the noisy signal to make the system more sturdy.

[srm@viewpointusa.com]

is the encoder a differential encoder?  A and A!, B and B!?  differential encoders are much more noise immune that single ended encoders.

[pewt]

Yeah its differential (with a Z channel as well). I've got a line receiver in place, plus an isolator circuit. The VFD is very noisy. With it inhibited my signal is pristine, but with it on I get both periodic bursts of noise (about 2.5s apart) and continuous lower amplitude noise as well.

[S1ack]

Can you change the switching frequency on the VFD?

Is wiring to motor in conduit? Is conduit metal? If so is it grounded?

Failing that, many cable manufactures now offer armored cable designed just for VFD applications they may reduce radiated emmisions compared standard thre phase cables.

[pewt]

I haven't tried varying the switching frequency on the VFD- will give that a shot soon. 

The motor is using conduit cable and almost all wiring is shielded and grounded. 

[pewt]

by switching frequency do you mean RPM? I vary the RPM from 1-3500RPM

[S1ack]

No I mean the PWM frequency.

Typically 2,4,6,8 KHZ. With the higher rates usually inparting a de-rating of the VFD power.

The radiated ./ conduted emmissions can be effected by the switching frequency.

[pewt]

gotcha: the VFD I'm using (an emerson unidrive sp) does allow change the PWM switching frequency. I'm currently at 3Khz (the lowest setting). I'll trying kicking it up a bit.