Measuring VFD frequency with 7i76e

My VFD outputs a frequency signal as a square wave. I think it's the VFD's output frequency, rather than anything measured from the spindle motor itself, but I'd like to use it to display spindle speed in LinuxCNC (if that's possible).

There's an encoder input on the 7i76e, but this isn't really that. Is there a way to use one of the inputs to measure frequency?

Yes it is, in several ways if you have encoder inputs.
But I doubt it does output the frequency, although that is easy to implement.
I also think that you can use a normal input on the mesa board, but that would have to be confirmed by PCW.
Usually they do 400Hz so maybe that would be possible to count from a normal input, or you can put the 7i76 in mode 2 and have another 2 encoder inputs that are usually used for MPG encoders. That would still leave the existing spindle encoder input for what it is intended.

JetForMe wrote: My VFD outputs a frequency signal as a square wave. I think it's the VFD's output frequency, rather than anything measured from the spindle motor itself, but I'd like to use it to display spindle speed in LinuxCNC (if that's possible).

There's an encoder input on the 7i76e, but this isn't really that. Is there a way to use one of the inputs to measure frequency?

Its probably possible you could use the encoder input for this as thats what we plasma guys use to decode the arc voltage from the Meas THCAD voltage to frequency converter.

JetForMe wrote: There's an encoder input on the 7i76e, but this isn't really that. Is there a way to use one of the inputs to measure frequency?

The encoders can count a single channel in "counter mode", this is true of the Mesa mpg and real encoders and the software encoder module.
For 400Hz the software counter would be fine, with 1kHz sampling rate.

I have a high frequency VFD (1500 Hz max). I don't think I'll be running it higher than 800 Hz.

at 800Hz I would probably suggest using a hardware rather than software counter.

You might see if your VFD will output analog. I believe my Siemens will for both current and frequency if I remember correctly. The 7i76e will read analog in 8 bit resolution scaled to 36v. I would like to display load on the GUI in this way.

Ideally I get the values via the MODBUS interface, but this seemed like it might be easier.

I had hoped my VFD could measure actual speed of the spindle from back EMF; I don't know if it uses that to drive the spindle (like one would for a brushless DC motor) or not.

Is your spindle synchronous? If so, your VFD absolutely knows RPM based on the frequency it is outputting. If it is inductive, then it will have some slip but will be within 80%. Most VFD's can "guess" based on a slip curve. I have no idea what kind of spindle takes 800Hz, but I'd be a little surprised if it wasn't synchronous.

There are spindles that can do 60000 RPM, seen and worked with two types of them, small but they chew through titanium and zirconium nicely. They do require 1000 Hz to work.