[joeavaerage]

Hi,
the voltage loop is still enacted, but is within the code and therefore the signal presented to the output amplifier has included within it the velocity correction.
In earlier times the voltage loop was closed by the amplifier. So there were two loops, the encoder went back to the controller and the tacho went to the amplifier.

More recently the encoder goes back to the controller and it performs both position loop and velocity loop feedback, and the output amplifier is simplified.

Craig

[joeavaerage]

Hi,
a couple of diagrams may help.

The first one is an open loop controller (like Mach) driving a closed loop servo with just the rotary encoder as feedback. For example my Delta B2 servos
are like this on my machine. I have C5 ground ballscrews so expect zero to negligible backlash, which is just as well as this setup cannot correct for backlash nor
non-linearity of the screw.

The second is of an open loop controller but driving a dual loop servo, for example Delta A2. This set up can accommodate backlash and non-linearity.

The last is of a fully closed loop controller (like Galil) with feedback from a linear scale. This set up can accommodate backlash and non-linearity. Note also that that a modern AC servo
can be used in torque mode, and so there would be an additional feedback path from the rotary encoder to the drive, which I've labeled a transconductance amplifier...but can of course
be somewhat more sophisticated than an amplifier, but the general principle applies.

[suntravel]

Sorry, I was assuming that closed loop to a motor controller has the encoder on the motor. Closed loop to the controller uses an encoder on the axis.

Cheers
Peter


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You get the best percision if you have closed loop with linear scales on the axis, that will automatic compensate for backlash and a not perfect pitch in the ball screws. But it is allways best to have as little backlash as possible.

Uwe