RE: Relinear encoder source: Now Limbo Rock

From: "Carol & Jerry Jankura" <jerry.jankura@...>

Peter:

>I've worked with some of the CompuMotor servo systems. The equivalent of
>"missing steps" with a servo motor is having the difference between actual
>shaft angle and the "computed" shaft angle be larger than some allowable
>error. When this occurred, the motor was shut down and allowed to "free
>wheel" until the controller was reset. One line from the controller back to
>the computer was set to logic zero to alert the computer of the problem.
>Your software had to detect this line and take appropriate action. With
>CompuMotor controllers, this was a "wired-or" connection so several motor
>controller failures could be checked by monitoring one line.

Diference between commanded and actual position is called following error.
In servo systems, this number is normally computed at the point where the
control loop between motor amp commands and feedback system (resolver,
encoder, scale) is closed. This will be a different place depending on
what system you are using.

In the servo gecko drive amps, this is in the drive itself. If the encoder
gets more than a few steps behind the step signals from the computer, the
gecko trips out. I think that this condition is indicated on screw five.
There are a number of commercial servo amps that are driven by digital
signals that will behave roughly the same.

In the servo EMC system and many proprietary controls the loop is closed
in the PC or control computer itself. The motor drive amp is just that,
it amplifies the command signal and sends it to the motor. There will be
a fault line for overvoltage and overcurrent, but this kind of amp does not
signal any difference between commanded and actual position.

When the motion loop is closed in the computer, as it is in the EMC,
following error is settable in the ini file or by machine parameters and can
be a ramp proportional to speed. It can also be different for each axis.
In this case the PC signals a following error problem to the rest of the
system and changes its mode to machine off or e-stop.

One major difference between stepper and servo systems with respect to
following error is that in a servo system, the greater the following error
the greater will be the command signal to the motor amp in an attempt to
reduce following error.

HTH

Ray