Re: [CAD_CAM_EDM_DRO] Runaway servo systems and hexapods

Carl Mikkelsen wrote:

>With a hexapod, it is difficult to create a limit switch at the end
>of travel that will work. The effective limit of motion for each of
>the six legs depends on where in the working envelope the tool is
>position and with what tilts. A "home" switch, which is a limit at
>the minimum extension is easy, but the other end is not easy, and
>may be impossible.
>
>My solution was to add redundant encoders, and monitor them with a
>PLD (programmable logic device, in this case an Atmel 1504). I'm
>using the Rutex drives, which like the Gecko drives are commanded
>with step and direction pulses. Using a PLD, I accumulate
>step/direction pulses which are going to the drives, and compare
>them with the lower resolution redundant encoders. If they ever are
>more than 1/2 turn off, the PLD signals an emergency stop, which
>kills the motor DC transformer and shunts a low value resistor
>across the power supply.
>
>

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