Re: [CAD_CAM_EDM_DRO] Re: Safety -- servo vs. stepper

Leslie M. Watts wrote:

>Torsten,
>
>Normally the limit switch actuates a power relay. In my case
>this is a double throw that also applies a braking resistor.
>There is a description on my engineering page.
>
>A caution is in order about suddenly breaking the DC line
>to the servos. They have some inductance, so such a break
>will create a very high voltage spike (L di/dt). Sometimes
>this happens in normal commutation (in DC brush motors) but
>completely unhooking could damage the servo amplifier.
>
>

The best solution is something that connects the braking resistor
BEFORE you cut out the bulk DC supply. I have a circuit that does
that. And, since it has withstood a number of e-stops due to a runaway,
I am pretty sure it works. I have a drawing on my web pages :
http://jelinux.pico-systems.com/EMC.html
see the 2nd schematic on this page. The drawing has a couple of extra
relay poles, because it also manages enabling and disabling the servo
amps.

>So as a further protection I have 100v Transorb devices across the
>servo amp output. The system has worked well.
>
>Another way is to remove power to the amp... but it may or may not result in
>a regenerative breaking action.
>
>A third way is to remove mains voltage from the supply. This prevents any
>voltage spike, but a means must be made to rapidly discharge the filter
>capacitor. I think Jon Elson has a diagram of this somewhere.
>
>

This doesn't discharge the bulk DC supply. It goes between the DC supply
and the servo amps. It slowly (in the millisecond range) charges the
internal
capacitors in the servo amps to prevent tripping the overcurrent logic in my
servo amps, or blowing fuses with the inrush current. After the 1 sec
delay,
a power contactor connects the bulk supply directly and enables the amps.
When the relays are turned off, the smaller relay releases first,
connecting the
braking resistor (R1) to ground. Then, the contactor releases, cutting the
connection between the servo amps and the bulk supply. This all takes place
in 10 milliseconds or so.

Jon