Re: [CAD_CAM_EDM_DRO] Runaway servo systems and hexapods

Carl Mikkelsen wrote:

>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.
>
>

That's why my servo amps have a double-level of safety circuits in
them. The current
LIMIT circuit is totally separate from the overcurrent FAULT circuit. A
fault not
only shuts off the amplifier, it also signals another device to cut
power to the servo amps
and connect a braking resistor.

>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.
>
>

Yup, the non-linear kinematics sure make this more complicated.

Jon