they wire the joint position command to the set-point
input of a PID loop.
so the pid loop tries to bring the actual position to the command position.
this explains why the motor resists forced motion.
thanks, that was really helpful
Larry
On 2/12/07, Anders Wallin <[EMAIL PROTECTED]> wrote:
> hi
> I'd like to know how the position control works in emc. the pid params
> I see are only for axis speed control, what I can't understand is how
> emc holds the motor at a particlar position and apply opposing force if
> tried to rotate. I was expecting to see pid params for position loop,
> but cannot find in any ini file.
> tia
> Larry
Hi Larry,
Look at it this way: EMC only outputs the desired position for each
joint (=axis in most cases) each servo period.
That's a number that gets updated in real-time.
What you do with that number in HAL (the Hardware Abstraction Layer) is
up to you (the machine integrator in emc parlance)
Some people use steppers driven from the parallel port, so they wire the
position command to a step generator, and the output of the step
generator to the parallel port driver.
Others use servos, so they wire the joint position command to the
set-point input of a PID loop. Then wire the motor encoder to the
feedback input of the PID, and wire the output of the PID to a hardware
driver that actually commands the motor.
Some (old?) servos use velocity feedback, but the velocity loop would
always have to be surrounded by an outer position loop when used for
positioning an axis.
Hope this helps, do ask more if anything is unclear.
Anders
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