Hardware Selection Help - DC Servo Amplifier

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29 Jun 2020 17:59 - 29 Jun 2020 18:00 #173115 by TheBigLebowski
I am hoping that somebody can help me with selecting a DC Servo amplifier that would fit the goals of my project.
I plan to build this setup on a bench as a learning exercise, I am not sure that I will add it to a machine.

I want to do 2 driven axis

Servo type will be a low power brushed permanent magnet dc with a rotary encoder on it. Probably 80vdc or less.

This is the tricky part, I would prefer to close the loop directly at linux cnc using PID. This means the rotary encoder should ideally be going directly to the mesa, rather than a cascading setup. (loop closed at amplifier w/ amp feeding back linuxcnc) I am not certain if this is even possible, so feel free to correct me if I am wrong.

My understanding is this makes step/dir amps a no go, so I should be looking for either PWM amp or analog input amp.

Any input is appreciated.

Note this is a Repost, I accidentally deleted my first post and the thread disappeared.
Last edit: 29 Jun 2020 18:00 by TheBigLebowski.

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29 Jun 2020 18:22 #173119 by tommylight
Have a look at www.mesanet.com/ under the motion control on the left side menu, plenty to choose from, but most of them use 50 pin header, although i am sure they can be made to work with 25 pin cards, but that should be better left to PCW to confirm.

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29 Jun 2020 19:55 #173131 by TheBigLebowski
I stared right at those for five minutes and didn't see what I was looking for.

After taking a second look it appears that the 7i40h might be what I am looking for.

Is there any good reason to use something like the 4i27 instead of an anything I/O running softdmc? It seams like the anything I/O is far more flexible.

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29 Jun 2020 21:47 #173143 by jmelson
Another option is the Pico Systems PWM controller and 2 PWM servo amplifiers.
These can easily do 80 V and up to 20 A peak. The loop is controlled by LinuxCNC PID, so you can use the Calibration menu and Halscope to tune the servo response.
The PWM controller sends a 50 KHz PWM signal and direction to the servo amp.

I have been making this setup since about 2003.

Jon

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30 Jun 2020 02:02 #173163 by TheBigLebowski
Jon,

I really like the Pico amplifier set up, but I do have a question that maybe you can answer about it. Would it be able to support a dual loop feedback setup with more than 2 axis? This is something I would like to experiment with at some point.

I think 2 axis dual loop would be fine because you could dedicate the 2 "unused" axis on the board to reading just a linear encoders.

My concern with more than 2 axis would be that because the encoder counter and PWM Controller run through separate parallel ports there could be latency issues. I don't know if this is a valid concern or not.

I think I am going to order up one of the pico controllers and a pair of amps regardless, just wondering if you had experimented with this.

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30 Jun 2020 02:19 #173164 by jmelson
OK, by dual loop, you mean a linear scale plus a shaft encoder (mostly for velocity feedback) on the same axis? Yes, it could do this, the PWM controller has 4 encoder counters, so one PWM controller could handle two axes with 2 encoders/axis.

Multiple PWM controllers can be run on the same parallel port, and they exchange synchronizing signals on that parallel port cable, so the encoders are all sampled within 1 us of each other, even on different boards. We have only tested this up to 8 channels (2 boards).

I have not done the dual feedback myself, but it is all done in HAL, so really doesn't have any specific connection to my boards.

If you later order a second controller board, just let me know you need the "daisy chain" cable, and I will provide it to connect the 2nd board to the 1st.

Jon

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30 Jun 2020 23:10 #173239 by andypugh
There is also the Gecko G320X, though that does expect to use step-dir and internal feedback from the encoder.

However, if you are using dual feedback you could take one encoder to the G320 and one to LinuxCNC.

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