Spindle Analog Signal Issue

[michaelwright51]

Hi,
I'm having issues driving a DYN4 spindle motor using the analog output on an V4 Acorn. I benchtested the analog out signal using both the manual process and the updated automatic test and both pass (a little on the high side). When I hook up the motor it will run slowly (around 200rpm) with the spindle turned off. The voltage across the H8 terminals is around 0.1V disconnected and 0.6V when connected to the motor. Is the normal and is there a way to calibrate the spindle speed? I'd prefer it if I didn't need to hookup the servo enable.
Thanks,
Michael

[tblough]

You are delving in experimental waters using a servo motor as a spindle drive. Other have attempted this with varying results. I suggest you search the forums here and check out the other attempts.

The Acorn and other Centroid controls are designed to output 0-10V to control variable frequency drives for spindles. 0V = 0rpm, 10V = maximum spindle speed setting. Forward and reverse are controlled by relay outputs. It is theoretically possible to change the DAC output offset and curve shape by editing the PLC program.

The DAC output can be affected by the load applied to the terminals which appears to be what you are experiencing. The DAC is not set up to source large amounts of current so if your input resistance is fairly low, you can get some strange behavior.

Tech bulletin 152 details setting up the spindle speed control.
https://www.centroidcnc.com/dealersuppo ... ds/152.pdf

[michaelwright51]

From the specifications it looks like the Acorn is rated for up to 1mA on the analog output and the DYN4 has a max of 0.1mA draw so they should be compatible. I also found a wiring guide for this application on the DMM website and it doesn't look like a enable servo signal was required:
http://dmm-tech.com/Files/schematics/DY ... pindle.pdf
Any idea if there is any compensation in the voltage if I hook up the encoder?

[tblough]

Spindle encoder input has no effect on the analog output.

[cncsnw]

I'd prefer it if I didn't need to hookup the servo enable.

You need to hook up the enable, to a spindle-run output ("SpindleEnableOut").

You cannot trust the analog signal alone to ensure the spindle remains off when it is supposed to be off, even in the best of conditions.

You are probably also getting interference with the analog signal. If possible, look at the signal with an oscilloscope. If that is not possible, try connected a 0.1 uFd capacitor across the analog terminals (from analog out to analog common).

[michaelwright51]

No oscilloscope but I hooked up a 0.033uFd capacitor (closest I have) across the terminals and it slowed the rotation down to about 60rpm. Any idea what could be causing the interference? I tried a different power supply with the same results and everything other than the Acorn and the DYN4 is disconnected.

[Muzzer]

Might also be helpful to fit one or more of those clamp on ferrite beads. But clearly you have a noise issue, so might be time to stand back and check if you have followed recommended wiring practice, such as screened cable, grounded screens, routing the motor connections apart from the control signals etc.

Again, I don't understand this current interest in using position controllers (servos) for spindle speed control. That's what VFDs are for. There's nothing magic about a servo motor, as (in my experience at least) the laws of physics still apply when it comes to torque generation in an electric machine. True, permanent magnet machines achieve a slightly higher torque/power density compared to induction motors but it's pretty a small effect (~20%) but that's not generally a major issue for us. On the other hand, using a servo drive for spindle speed control often seems to be less than straightforward, as we keep seeing.

[Richards]

I experimented quite a bit with a DMM DYN4 driver and a 750W servo. My hope was to use it with a Taig Mill so that I could do some rigid tapping. I had the same issue with "creeping" that you're describing. To solve the problem, I added a PLC to the mix using the Acorn's SpinFWD and SpinREV. You can see how that was done on the "Using DMM 750W servo as spindle motor" thread. The PLC solved the "creeping" problem. (I also used two DPDT relays in place of the PLC to simulate the PLC's logic. The two relays also worked.) On the test bench, everything worked; however, I wasn't satisfied with the DMM servo on the mill. I only used 120VDC, which limited the servo to 3,000 RPM. I geared the motor 3:1 to get 9,000 RPM maximum. After minimal testing on the mill, I went back to the stock Taig motor. The stock motor works.

[michaelwright51]

Not sure why but the DB25 cable I was using for the IO on the DYN4 was generating most of the interference. Will have to look at options for replacing it. Down to about 0.15V at idle on the analog signal which is only slightly above the bench test voltages I was seeing. Hooked up the servo enable and the direction signals to the Acorn output board and everything seems to be operating well.

You need to hook up the enable, to a spindle-run output ("SpindleEnableOut").

You cannot trust the analog signal alone to ensure the spindle remains off when it is supposed to be off, even in the best of conditions.

Totally agree, I don't trust servo motors and I'm planning on disabling the high power for tool changes.

Again, I don't understand this current interest in using position controllers (servos) for spindle speed control. That's what VFDs are for.

I looked but couldn't find a small factor AC motor (that wasn't a servo) and NEMA 56C was far too big for this application.

On the other hand, using a servo drive for spindle speed control often seems to be less than straightforward, as we keep seeing.

I'm guessing that ~0.15V is still a source of error in spindle speeds but would probably not be enough to see any movement on a VFD which might explain the difference.
Thanks for the helps everyone.

[martyscncgarage]

No oscilloscope but I hooked up a 0.033uFd capacitor (closest I have) across the terminals and it slowed the rotation down to about 60rpm. Any idea what could be causing the interference? I tried a different power supply with the same results and everything other than the Acorn and the DYN4 is disconnected.

You have to hook up an enable as Marc (cncsnw) suggested. Acorn's analog output is never at Zero.
Also, all the encoder output on Dyn4 gives you is the ability to see the encoder as a spindle encoder. Acorn does not attempt to maintain speed. The encoder is used for things like RPM display, rigid tapping etc.

I have experimented with Dyn4 as a spindle drive and using its AC servo as spindle motor. I spent way too much time trying to make it work, and I have yet to see someone successfully implement an AC servo as a spindle motor, work as intended, and successfully do rigid tapping. Most people try this combination because of the work that may be involved in properly fitting an encoder belted at 1:1 to the spindle.

A good VFD (with a braking resistor) setup properly with an encoder and Centroid's rigid tapping guide works pretty well.
Marty