Confused about servo drives

I'm starting a new retrofit. I have used steppers in the past and want to try and have a closed loop all the way back to linuxcnc. To do that I thought that getting a 7i77 card which takes encoder input would be a good idea. The problem is I can't figure out what kind of drive to use. Since the encoders are going back to the 7i77 I can't plug them into a normal servo drive. Also I imagine having two devices trying to act on encoder data would not be a great idea. I would think that the +-10V out of the 7i77 would go into a dumb drive like a VFD. Is that true? something like the automation direct GS2? Or is there something I'm missing? I've seen the 8i20 board, and think that would maybe be useable, but it takes DC and that just throws more problems into my design. I could also use a granite devices argon in +-10V mode but that seems wasteful. My motors are 2kW. If I'm correct and I'm supposed to use a VFD where does the PID tuning get done?

Thanks

Most drives that require an encoder feedback also provide an encoder pass through to the control. (That is what you'd connect to your 7i77.) If not it is usually possible to just wire the encoders to both the drive and the control.

I like the granite devices argon and it doesn't seem to have pass thru. I guess I can look for other alternatives or wire the encoders to both. It seems kind of weird to me that both the drive and the 7i77 would be able to use the encoder data without fighting about positioning. Who's in charge? Is it even possible to just use a VFD to drive axis servos? or is that just crazy talk?

joshua.stephens wrote:

I'm starting a new retrofit. I have used steppers in the past and want to try and have a closed loop all the way back to linuxcnc. To do that I thought that getting a 7i77 card which takes encoder input would be a good idea. The problem is I can't figure out what kind of drive to use. Since the encoders are going back to the 7i77 I can't plug them into a normal servo drive. Also I imagine having two devices trying to act on encoder data would not be a great idea. I would think that the +-10V out of the 7i77 would go into a dumb drive like a VFD. Is that true? something like the automation direct GS2? Or is there something I'm missing? I've seen the 8i20 board, and think that would maybe be useable, but it takes DC and that just throws more problems into my design. I could also use a granite devices argon in +-10V mode but that seems wasteful. My motors are 2kW. If I'm correct and I'm supposed to use a VFD where does the PID tuning get done?

Thanks

One possibility is to use the Pico Systems Universal PWM controller and PWM servo drives. These use digital signals from the controller to the drive. Depending on whether the motors are brush-type or brushless, we have different servo amps for that type. What kind of machine are you doing that needs 2 KW axis servos? Unless the machine weighs several tons, such large motors are probably not needed.

See pico-systems.com/osc2.5/catalog/product_...ath=3&products_id=19 for the controller and
pico-systems.com/osc2.5/catalog/product_...ath=3&products_id=26 for the brush-motor servo
amp. pico-systems.com/osc2.5/catalog/product_...ath=3&products_id=20 is the brushless
servo amp.

Jon

A few tons anyways. It's a Fadal VMC15xt. I'm just replacing the existing DC motors with AC servos of equivalent torque. The pico brushless drive looks like it could work, but like the 8i20 it seems to want a pretty beefy DC power source. I don't see anyone selling power supplies that can feed these things.

Hello.

You can use the Argons with a 7i77.
The encoder will arrive at the 7i77, and at the same place where you connect it, you can place another cable up to the Argon. Both will see the encoder.
If the Argon will be set to velocity mode (if you use the 7i77) the Argon will use the encoder feedback just to try to achieve the velocity that linuxcnc is commanding, while the encoder back to linuxcnc will inform linuxcnc about the position and linuxcnc, makes the calculations to send the correct analog velocity to the drive.

We are using and documentation a complete set up, as detailed as possible.

Just look were wiki.eusurplus,com

I can make very good prices on the Argons, drop me a message if interested (www.eusurplus.com)

joshua.stephens wrote:

A few tons anyways. It's a Fadal VMC15xt. I'm just replacing the existing DC motors with AC servos of equivalent torque. The pico brushless drive looks like it could work, but like the 8i20 it seems to want a pretty beefy DC power source. I don't see anyone selling power supplies that can feed these things.

You can make your own power supply, or possibly use components in the existing control to make a power supply.
It is pretty simple, a transformer, rectifier and capacitor. I can advise on some parts if you get to that stage.

Jon

Thanks, that's very informative. Intuitively I would have thought having them both connected would cause them to fight, but if you say it won't then that's great.

emcPT wrote:

You can use the Argons with a 7i77.
The encoder will arrive at the 7i77, and at the same place where you connect it, you can place another cable up to the Argon. Both will see the encoder.
If the Argon will be set to velocity mode (if you use the 7i77) the Argon will use the encoder feedback just to try to achieve the velocity that linuxcnc is commanding, while the encoder back to linuxcnc will inform linuxcnc about the position and linuxcnc, makes the calculations to send the correct analog velocity to the drive.

andypugh wrote:

I made my own PSU which simply rectifies mains voltage into some large capacitors. It would have been better to use an isolation transformer (1:1) but I didn't bother.

I thought about it. I'm pretty sure I understand the theory well enough, but everything I ran into online was people flipping out about not using an isolation transformer. It seems I'd need a 1:1(or thereabouts) transformer able to handle up to 3x10A at 220V or so. I could downgrade a little since all axes shouldn't be going full tilt all at the same time I guess.