Re: [CAD_CAM_EDM_DRO] Runaway servo systems and hexapods

At 01:21 PM 12/7/2004, Jon Elson wrote:

>Wow, that's doing it the hard way! What you want is a servo amp that will
>fault on overcurrent, not just sit there ramming peak current through the
>motor until it smokes. When the one amp faults, it signals the whole system
>to go to E-stop.
>
>Jon

Jon,

I wanted to prevent un-commanded motion before the tool is driven into the
bed or smashed into the home position. It didn't seem adequate to depend
on the servo drives tripping against the current limits. I also wasn't
convinced that in the event of a drive failure that the current limit
circuit would still operate. A shorted drive transistor can result in full
voltage being applied to the servo motor, perhaps in a way that prevents
the over-current limit circuit from stopping it.

Maybe this is a problem specific to hexapods, but the actuator motors
operate over a wide range of torque required. Depending on the servo gain
parameters, the driven motor torque could be below the current limit
threshold, but still move the head a long way at high speed. I didn't want
to accept un-commanded motion of any speed with any amount of force.

Redundant encoders and independent tracking of position error catches
errors early, before they are a threat to either the machine or the
operator. Bring the six motion fault signals into the E-stop function, and
causing E-stop to rapidly shut down power to the motors, seemed like a good
precaution.

-- Carl