Where is the loop if using Yaskawa servo drives?

31 Jul 2010 21:30 #3537 by jimmyservo
Hello again everyone.
If there are any servo experts out there, I would like to know HOW something works:
Using servo drives that have their own built in auto-tuning, where is the control loop?
These drives receive the encoder signals and redirect them to the EMC2 interface. So with all the usual INI entries as:

P = 50
I = 0
D = 0
FF0 = 0
FF1 = 0
FF2 = 0

The motors move OK, but I would like to know HOW. Like what is an optimal value for the P term? Is HALscope the right tool for this?
If I use values in the other terms will this make a second nested loop? Is that better?
What about other INI entries like DEFAULT_ACCELERATION, etc?

Here are my machine specs:
Interface: Pico Systems PPMC (ENC, DAC, DIO) +/- 10V analog output
Motors: Yaskawa SGM-0xA31x series (100, 200, 400 watt)
Drives: Yaskawa SGDA-0xAS series +/- 10V analog input Velocity mode

I noticed some other EMC2 people are using Yaskawa servo drives that accept step/direction inputs (AP series instead of AS).
Gecko also makes such a drive. So is the control loop in the drive here as well?
If this has already been covered before I couldn't find it. There is a lot more stepper info out there than servo.
Anyway, there is a lot more smoke than light in my shop right now! B)

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01 Aug 2010 17:40 #3549 by robh

EMC can close the loop if you wish (which has many benafits) or you can let drive try and close the loop for you, just pass the encoder back to emc

i would close the loop on emc with just PID as you have talked about.
PID values do vary machine to machine so its very hard to say what values will and will not work for you.
halscope is your best friend in this area. monitoring the PID error value etc.

some systems where there is an autotune you may have to reduce the values a little so EMC can have more control over the motor and drive.
also check to make sure there is no smart retune or autotune while running function i have seen some drives with this, this can make things quite fun when the drive thinks it can retune the motor for a new load etc.

hope this helps some how.

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06 Dec 2010 16:12 #5896 by tom3p
there's 2 loops
the Yaskawa loop runs INSIDE the EMC2 loop

the Yask maintains velocity and passes changes in the position to the cnc (EMC2)
EMC2 maintains position and maintains the expected new position at the next time slice by asking the Yask a new velocity

this inner outer loop is common ion cnc's, esp those using newer amplifiers that have thier own PID loops

the PID parameters for the YASK are for itself, for velocity
the PID parameters for EMC are for maintaiining position ( yes they also are for velocity, but the goal is position )

this loop inside a loop is 2 guys driving the car
but , if they cooperate, and are able to able to respond to the expectations of the requestor
then, there is no fighting or lag or beating
so, get good response form the non-cnc loop with the stock Yaskawa tuning program before adding the external CNC loop


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11 Dec 2010 23:47 #5978 by jimmyservo
Thanks Tom3P!
That is exactly the case with my Yaskawa / EMC2 servo system. If fact this common setup is called a " P / PI Cascaded Loop with Velocity Feed Forward"
(I've done some reading in the months since I asked the question!) EMC2 uses only P and FF1and the Yaskawa Servo Amps use only P and I. All other values are left at zero. I found it is also important that the responsiveness of the outer (position) loop not exceed that of the inner (velocity) loop. By using Calibration and HAL scope I was able to tune the system very well.
EMC2 continues to amaze me.

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