Re: [CAD_CAM_EDM_DRO] Re: Big servo motors drive recommendations

Mark Vaughan wrote:

>
> Consider for the minute you have just started the motor and the error has
> gone quite big. Reducing acceleration would reduce this. But the servo has
> sent a command of lets say 50% on time to the Fets in the bridge stage. Big
> motors have very low resistance, mine are about 1 ohm so we are looking at a
> target current of 80 amps here. Ooops, the fets go bang or the overload
> trips.

My servo amps have "live" current limit, so that any time the
instantaneous current exceeds the limit setting, the transistors
are switched off until the next PWM cycle. This is totally
separate from the fault trip limit, which is set higher, and
requires a manual reset. Also, the output filters slow down the
rate of the rising current, so the PID as well as the current
limiting has enough time to react to it.

A proper PID contol loop should never go from zero to 50% duty
cycle in one servo cycle. The D term should slow down the rate
at which the duty cycle command rises to a level which allows
stable response. Within a couple one millisecond servo loops
the motor/encoder should begin to respond (move) and so the duty
cycle should hold at a percentage that maintains the acceleration.

Something is wrong with the servo loop tuning in your case. I
know I could make this work well with EMC and my servo amps.
EMC has the advantage of not only very flexible P, I and D
terms, but FF1 and FF2 (Feed-Forwards for velocity and
acceleration) as well.

Jon