[Sierevello]

I thought I read somewhere that it is possible to drive a brushed dc servo motor with a brushless servo drive?

The servo motors are 140v SEM and right now are driven by Servo Dynamics 1525-10 drives on an old Anilam 3 axis mill.

This mill uses tachs on the motors for feedback.

The ones on the mill now are aging and starting to not hold position regardless of tuning.

I looked at AMC 25A20's. Which come brushed and brushless, but I can get the brushless cheaper and I need 3 of them.

Thanks for any help you can give.

-Steve

[Jim Dawson]

Yes, it's possible, depending on the drive. But if DC drives are available that would be my first choice.

But are you sure the existing drives are the problem? I would be looking at the encoders first. Depends on the exact system that you have, but on my Anilam machine the encoders were driven by a small belt off of the lead screws. A very poor design, and the encoders were drifting due to belt slippage because of incorrect belts. The tach feedback is only for internal speed control by the drives, not for positioning.

[Sierevello]

Jim... no encoders on the Anilam Crusader M milling Machine. It uses glass scale feedback.

Thanks,
Steve

[Jim Dawson]

That's interesting, my Crusader M had rotary encoders. But glass scales can get dirty over time also, or just fail. I had to occasionally clean the DRO glass scales on my old mill. I'm not convinced that the SD drives are the problem, I'm still using mine with the new controls and they work fine.

[Al_The_Man]

I have used the Advanced Motion Controls BLDC drives for DC motor, you just set it for 60° commutation instead of 120°.
Many others will also do this, no tach used anymore, especially when used in the torque mode rather than velocity like the older tach drives.
Max.

[Sierevello]

Al,

So with the BLDC drive I do not need to hook the tachs to the servo drives? How does the drive receive the feedback on motor speed?


Thanks,
Steve

[Sierevello]

Jim,

What is the best way to clean the old scales? Don't they have a seal inside to keep you from being able to get to the glass?

Mine are pretty well protected, but I know since I have had the mill (6 years) they have never been cleaned. I doubt they were ever cleaned by the previous owner.

Mine will not seem to home properly. If I set X0,Y0,Z0 it will drift -.0005 to -.001 on the X and Y and will not return to X0Y0, no matter how many times I hit X0Y0 start into the control.

Thanks,
Steve

[Jim Dawson]

I like alcohol wipes to clean the scales, available from your local pharmacy or the pharmacy aisle in most other stores. Depending on the scales, normally you can remove the aluminum cover to get to the glass. Clean the sensor head also, normally it's spring loaded and will lift a bit.

To correct the drift, you may need to adjust the balance or signal gain pot on the SD boards. I tried to attach the manual for the drives, but it's too big (9 meg) I guess. Here is a Drop Box link to it https://www.dropbox.com/sh/er9q0a35l...3qB5hcs9a?dl=0

[Al_The_Man]

Al,

So with the BLDC drive I do not need to hook the tachs to the servo drives? How does the drive receive the feedback on motor speed?
Thanks,
Steve

Should be no need to use a DC tach on any drive now, especially if it can be configured for Torque (current) mode.
The drives I referred to, the encoder inputs direct to the controller so the drive is just a simple amplifier and receives a command from the controller accordingly.
Al. .

[Sierevello]

Al,

The Anilam doesn't have an encoder. It uses glass scales for position feedback.

Should I take out the tach and install an encoder?

Thanks for the help!

-Steve

[Sierevello]

Thanks for the manuals Jim.

-Steve

[Al_The_Man]

Al,

The Anilam doesn't have an encoder. It uses glass scales for position feedback.

Should I take out the tach and install an encoder?

Thanks for the help!

-Steve

As Jim said A tach is not used for.or in place of an encoder, the tach was used on the older DC velocity drives, I have never commissioned an Anilam machine but it is my understanding it is fairly flexible as to drives etc.
Maybe run it by Anilam, see if the system can take AMC drives etc.?
Al.

[Bdrangdo]

I may well be wrong but I suspect the older Anilam controls will not work with torque control, it is my understanding that the PID algorithms were not designed for dual use. I do know for a fact that early Fagor 8055m controls will not work correctly and are impossible to tune with torque control and I believe this could be true of most early controls. Just my two pennyworth.

[Al_The_Man]

There is a Galil Motion video tutorial by Dr Jacob Tal on the subject of using an encoder on the motor and a linear scale on the final positioning, the PID loop has to be divided between the encoder and scale to avoid any oscillation etc when any backlash exists.
Al.

[Sierevello]

I called AMC and they said that the B25A20 will take Tach feedback and can run a brushed motor. However, I didn’t see the post about torque mode prior to calling. Can’t I just run the drive in velocity mode?

Thanks for everyone’s help on this.

-Steve

[Al_The_Man]

. Can’t I just run the drive in velocity mode?

-Steve

You can if your wish.
Al.

[Sierevello]

Al, is there any downside to running in velocity mode?

Thanks,
Steve

[Al_The_Man]

Since using strictly torque (current) mode it seems tighter control and requires less tuning, no need to tune the drive (inner loop) and then the PID (outer loop).
This is why you rarely see DC motors sold with a tach for some time now.
Al.

[Al_The_Man]

Al, is there any downside to running in velocity mode?

Are your drives currently operated in ±10vdc control or step/dir?
Al.

[Sierevello]

Al,
+/- 10v DC.


Thanks,
Steve