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Author Topic: Question for anyone with 'enable/disable' capable servo drives  (Read 4006 times)

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Offline simpson36

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A couple of guys on here said they have servo drives with an 'enable' pin that kills the power side of the drive and yet keeps on tracking the encoder.

My question is; what does the drive do when it is re-enabled after a disable if the axis was moved during the disable?

If the axis was moved, does the drive consider the new position to be OK, or does it try to immediately return the axis to the pre-disable position?

Offline KTM

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Re: Question for anyone with 'enable/disable' capable servo drives
« Reply #1 on: March 18, 2010, 06:45:03 AM »
Mach3 runs open loop with no encoder feedback for positioning, so if you kill the power to the servos while Mach is sending out the pulse & direction signal, Mach will not know the exact position and will have to be recalibrated.

If you use an encoder break out board to check the position in Mach, the you can transfer the encoder position DRO data to the axis position DRO.

Offline spunk

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Re: Question for anyone with 'enable/disable' capable servo drives
« Reply #2 on: March 18, 2010, 06:48:24 AM »
KTM, you are right but not answering his question  ;)

but i too would like to know the answer

Offline KTM

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Re: Question for anyone with 'enable/disable' capable servo drives
« Reply #3 on: March 18, 2010, 07:04:28 AM »
"My question is; what does the drive do when it is re-enabled after a disable if the axis was moved during the disable?"

A: It will start from the wrong position

"If the axis was moved, does the drive consider the new position to be OK, or does it try to immediately return the axis to the pre-disable position?"

A: It will assume that the position on the axis DRO is correct.

Offline spunk

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Re: Question for anyone with 'enable/disable' capable servo drives
« Reply #4 on: March 18, 2010, 08:24:11 AM »
am i right if i say you talk about the communication between mach -3 and the servo controllers.

i think the original question is about the servo controller on its own, ie will the hard/software on the controller do anything if the encoders are changed during disable and the drive is then put back to enable. lets assume mach3 isn't connected at that moment so no extra pulses can be transmitted...

simpson please correct me if i'm wrong...

Offline Hood

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Re: Question for anyone with 'enable/disable' capable servo drives
« Reply #5 on: March 18, 2010, 09:44:37 AM »
am i right if i say you talk about the communication between mach -3 and the servo controllers.

i think the original question is about the servo controller on its own, ie will the hard/software on the controller do anything if the encoders are changed during disable and the drive is then put back to enable. lets assume mach3 isn't connected at that moment so no extra pulses can be transmitted...

simpson please correct me if i'm wrong...

No the drive will only move on a command from the controller.
If you wanted to go back to the original position you would have to have the encoder output from the drive going back to Mach and read that info on an enable and if different from where Mach thinks it is then have it move with a command. You would have to be carefull though as you may get an unexpected axis movement and in a lot of situations that would not be a good thing.

If you are using a drive as an indexing drive for something like a tool turret or spindle orientation then if disabled and moved the drive will keep a not of the encoder and when you command a further index it will move to the correct position but not until a move has been commanded. Or at least that is my experience from my drives.

Hood

Offline simpson36

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Re: Question for anyone with 'enable/disable' capable servo drives
« Reply #6 on: March 18, 2010, 12:06:30 PM »
Quote
If you are using a drive as an indexing drive for something like a tool turret or spindle orientation then if disabled and moved the drive will keep a not of the encoder and when you command a further index it will move to the correct position but not until a move has been commanded. Or at least that is my experience from my drives.
Hood
Spunk: you are more or less correct in your intepretation of the question. Hood has provided a partial answer for at least one behavior.

Mach is irrelevant, I think, for the very reason correctly pointed out by KTM, i.e. open loop operation. With a true closed loop setup, there is no issue, I would think.

If my understanding is correct, Mach only output's pulses, not positions, so it it up to the drive to determine and maintain the current absolute coordinate.

If you try to move an active servo drive, it sees the encoder moving and applies power to move back to where it thinks it should be . . by reducing the error. In the case of a drive which is 'disabled' (i.e. no motor power) and yet continuing to track the encoder, this error could become great. The concern in implimenting such a feature in an open loop system like Mach is the behavior to expect when the drive powers up and does what it does; which is to reduce the following error to zero, which as Hood pointed out could be a good thing or a bad thing depending on the situation.

So far we have Hood's drive keeping track of the error, but not acting on that error when re-enabled until new pulses are recieved form the CNC. This seems like a sensible solution. Alternately, if the drive does an immediate full power move reduce the error to zero, it could be nasty.

So are there any other behavior that anyone can share? Some drives fault and continue to track the encoder also. So if it knows it has been moved during a fault or while disabled, the reponse to being re-started (but not reset) would be whatever the firmware says to do. I have no experience with drives that have this capability, hence the question.