Re: [CAD_CAM_EDM_DRO] Control Box Isolation--Geckos

Tom Eldredge wrote:

> Todd, and those following this thread,
>
> I am using servo motors on our plasma cutter that run on 18 volts. I know
> that is the lower limit for the Geckos. I wonder if that could be related
> to why I am having such jerky motion? We are using 32 pitch rack and 22
> teeth pinions for drive. The jerkiness is in about .60 (real roughly) jerks
> as the servo motors move.

Not likely to be the supply voltage. I assume you have the jerkiness when the
plasma machine is off?

If so, then it is a servo loop compensation problem. If there is any backlash
between the servo motors and the encoders, you will have vibrations and hunting
behavior. Backlash between the motor and the table motion can also cause
jerking, as the system bumps from one side of the gears to the other.

If you can disconnect the gearing, so the servo motor is relieved from the
table load, see if it still jerks. If not, that confirms my hypothesis.

One possible way that the 18 V supply could cause the problem is that the
motor resistance is so high that 18 V cannot produce the acceleration required
of the servo. So, error accumulates until the servo drive is at 95% (limit)
duty cycle, and then the servo overshoots the desired position. So, raising
the voltage might help, but it is a bit of a long shot. If you have a scope, you
might try to see if the servo is going to max duty cycle.

But, more likely, the mass of the table is lagging behind the required position
or velocity needed, and the servo overshoots. If so, then increasing voltage
will effectively increase gain, and it will get WORSE. If it does get worse,
that tells you what the problem is. Without tachs and linear compensation
of the servo loops, I don't know how to fix it from here. Maybe Mariss
has some suggestions for compensation of the system.

Jon