What are some common anti-dithering techniques that I can try out?
I have a DMC-30012 driving an Ametek/MCG IB34000 servo with a 2000 count quadrature encoder, which is coupled to a Wiesel linear positioning system. In order to get the smooth movement at low speeds (i.e. SP 50 to SP 100) required for the application (positioning a liquid curtain from a slot die) I have to have the Proportional gain up around 600, which seems to worsen the dithering.
I saw a suggestion somewhere about dynamically lowering the torque limit to below the point of overcoming the static friction/initial moment of the mechanical system, but that seems a bit of a hack. Is that a commonly used technique? Are there other ways of going about this without moving to sin/cos encoder?
Cheers,
Ken
P.S. We're not using the integral term, would adding some integration help or make the problem worse here? The machine is currently in another city so I'm not able to try anything out right now
MattK on 05/01/2014 - 11:16
Hi K.C.
I'm curious as to the speed requirements of your system. Are you always running between SP50 and SP100? If so there are a couple of things you can do to alleviate the dithering problem.
Ultimately, as you've pointed out, it comes down to an encoder resolution problem. At such low speeds the PID filter has no feedback information to work with since the actual encoder counts only change every few samples.
A higher resolution encoder would be the best solution, if you want to look at sin/cos encoders you'll need to consider switching to the DMC-31012.
You can also look into the Velocity Feedforward (VF) command:
http://www.galilmc.com/support/manuals/com3xxxx/index.html#velocity_fee…
This command will offset the motor command line with a voltage that is proportional to the motor reference position, helping to smooth out the motor motion.