Re: Digest Number 240

Bill Wainwright wrote:

>
> Jon
>
> Unless you Are turning the motors at a very slow rate the motors with the
> step and direction to servo control move very smoothly just like a servo
> system. There is no velocity feedback so the motors move in discrete
> increments but my machine is set up with a resolution of .0001 inches
> so it is fairly smooth. At higher speeds the mechanical time constant
> of the motor tends to average the jerkiness out. The electronics consist
> of two counters that keep track of the commanded position and the real
> position and then outputs an analog signal equal to the difference.

Well, I like to have plenty of resolution. One reason is that someday, EMC
may have machine error compensation, where the computer compensates for
leadscrew pitch and other errors. This needs plenty of resolution to work
well. I am using resolutions of .000050" on the X and Y, and .000025" on
the Z axes. This would require a step output system to send a huge number
of steps per second for rapid feed. I have a power supply limitation right
now that limits me to about 105 IPM. So, I can go from 105 IPM down
to .01 IPM before the stick/slip friction causes the motion to become jerky.

Also, I can force the table inches out of position (with the servo gain turned
low) and when I let go, it will go back to the commanded position. It can
keep position even when an E-stop occurs at high speed.

Jon