Re: [CAD_CAM_EDM_DRO] Re: servos, steppers, limit switches

vrsculptor wrote:

>Brian,
>The diagram is correct on limits and E-stop. Servo systems can run
>away a max speed if the encoder cable becomes disconnected or an
>output transistor fails. On a big machine this is heart stopping. The
>Estop/limit switch at least powers down the motors before hitting a
>hard stop. If you don't do a power down Estop the servo would
>continue to drive against the stop. On a small mini-mill without a
>limit I broke my coupling. On a big machine you could break iron.
>Don't trust software to stop a run away machine.
>
>

Actually, limit switches are useless in a servo runaway. They don't trip
until the the machine is almost at the limits of mechanical travel. The
best practice is to have the system set up so a full-on (100 % PWM)
condition will cause an overcurrent trip in the servo amplifier. The
e-stop circuits should be wired such that an overcurrent fault in a servo
amp causes the servo amps to be powered down. This way, a failed
encoder or blown transistor in the amp will cause an IMMEDIATE
shutdown, well before the machine has moved (at max acceleration)
to the stop.

Jon