Using velocity amps with no motor tachs

Hi,
I am retrofitting a mill to emc2, the old control is a 3 axis fanuc 6M which uses velocity amps, although the servo motors do not have tachs.
Is it possible for emc2 to send a tach signal to the amps (6v/1000rpm).
I currently plan to use a mesa m5i20 and two 7i33 cards, so that gives me DACs for XYZA & spindle, so I have 3 DACs available to use for an analog velocity signal for the fanuc amps.
If this is not possible, is there another way to generate velocity signals from the encoder signals.

Thanks for any advice.
Dave

Well maybe...

I guess you could connect the encoder velocity signal from the HM2 encoder modules to the spare DAC channels in HAL

Here are some possible bumps going down that road:

1. Velocity feedback needs to be fast so you may need a higher than normal servo thread rate (maybe 2 or 4 KHz) for stability

2. There was a recently fixed bug in HM2s velocity estimation so you would need the development version of EMC to get a good velocity signal
(the release version has big velocity glitches on reversals)

3. The 7I33s only output +- 10V so you would be limited to ~1600 RPM (and the servos would lose velocity control and probably fault-out
if asked to run faster than ~1600 RPM)

4. Whether the velocity estimation is good enough to replace a tachometer is unknown

Sounds like a fun experiment. I would definitely try this with bare motors, expecting runaways.

Hi Dave

I could offer two chanel encoder to tacho-signal converter, which ditributes the encoder signal to the CNC system.

Best regards
Radoslav

Radoslav,

I would be interested in such a converter, can you give me some details.
I've not yet started this project, so having some options now will be good.

Dave

Dave

Contact me at: This email address is being protected from spambots. You need JavaScript enabled to view it. to send you the Manual

Best regards
Radoslav