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No need, just use an encoder, better in every way

PCW wrote:
Great! Who has a donkey that deficates encoders?

Edit: and it isn't better in every way unless its negative logic. Otherwise a failure of the device leads to a runaway.

hmm no need to beat us up for our advice.

First your machine should have a feedback device already somewhere (unless the drive is a step and direction servo).
The easiest would be to use it.

If the linear scale is the feedback device you can have tuning issues since the scale is not directly connected
to the motor.

next easiest would be to add a rotary encoder. which should work better then the linear scale.

The hardest would be trying to make a tach (a velocity sensor) act as an encoder (a position sensor)

And it is possible to have the PID control shutdown the motor if the encoder breaks
Peter has talked of this before. I believe it does it by counting how long the PID is saturated at maximum output
and if too long ( the servo is not keeping track ) it sets the output to zero.

You give very little detail of your situation to recommend much more.
Hope that helps

PS if rereading your post - I think you are stuck on trying to get the analog tach signal into linuxcnc via the 7i29.
What everyone is saying is you don't want the analog signal!

cmorley wrote:
The advice was taken in the nature it was given. 'Better in every way' is a remarkably bold assertion.

This is my next area of inquiry. How can I find out more about Servo-Loop fault conditions in EMC2?
Who is Peter?

The information on PID control shutdown is new and helpful to me. I hope someone can follow up with more information about this.

What additional information can I provide. I have ABB/IsoFlux 544 series Servomotors with integrated 0.02V/rpm tachometers. My goal is to drive these motors with 7i29 H-Bridges using EMC2 for control. My machine is also equiped with Heidenhain glass scales and EXE-602E resolvers, that I had thought to use in a secondary feedback loop.

Not quite. I am stuck on using the existing analog tach signal with EMC2. Period.

Agreed. But its what I have now to start with.

That both tachometer and encoder feedback are positive logic an therefore subject to runaway in a wire fault condition is troublesome to me. This is also a concern of mine in using the Heidenhain scales in a feedback loop, because they are not robust. I need to learn how EMC2 bounds feedback following errors.

Anyway does it really matter to EMC2 if the primary servo feedback is encoder derived (Radians) or tacho derived (dRadians/dT)?

Thanks for letting me think out loud.
_jC
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EXE 602E is not a resolver. it is an Interpolation/shaping electronics for the scales, but maybe thats what you meant.
When I read 'resolver' I think of a feedback device sometimes found on ac servo motors.
So I am assuming that the original system used servo drives that used tach feedback and then used the scales as the positional feedback?
Or are you piecing this together your self?

Peter is the owner of Mesa electronics - he is PCW on this forum.

here is the docs for linuxcnc:
www.linuxcnc.org/docs/2.5/html/
look there and you will find info on the PID component ( and everything else )
There is a pin on it that counts how long the PID has been saturated. You could add logic to this to set an estop after a certain amount of time.
I think that is what Peter was talking of (it's been a while since that conversation)
Of course Linuxcnc will also fault if there is a (positional) following error beyond (user selectable) specifications

As it is linuxcnc expects positional feedback not velocity feedback.

You have been asking for the easiest way to make this work - and the easiest way is to add a rotary encoder or use the scales.
If you are determined to use the analog velocity signal and have linuxcnc convert it to a positional signal be prepared to buy more hardware,
do some programming and probably still not be happy with the results. What I am saying is while I think (not that I'm an expert) it might be possible it is certainly not practical.

Thinking out loud is a great way to flush out ideas!

cmorley wrote:Yes.

Also, yes
Is it EMC2 that expects this, or the 5i2x's FPGA?
Don't have emIt will be interesting to see what happens in the drive belt failure error condition. PID tuning with only the scales must be very difficult.
I am determined to complete this conversion, and stand ready to take the necessary measures to do so.

Yeah, it helps greatly, and I thank this forum for the oportunity.
Rgds
_jC
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