I'm kind of a CNC newby, working on the design of a CNC plasma cutter. Most of the hardware is figured out, prototype stuff works okay. I'm struggling with understanding what to do about Emergency Stop (E-Stop) design. In my case, I'm using closed loop feedback encoders with brushed motors (Rutex driver boards, LinuxCNC control software, PC with parallel port input/output).. but my questions are more of a general nature.
I've got a couple of ideas in my head, not quite sure what the best practices on good design are. In general, the E-stop shuts down things that move immediately, but keeps retains power to the control system (to allow troubleshooting, records of errors, etc..)
There are a number of ways to design this thing. I see five different ways to stop a rotating 'motor'.
1) Send a signal to the control system, tell it to stop all motors now. Hope the motors stop when the control system tells them to do so.
2) Cut power to the DC voltage supply that feeds (thru the driver boards) to the motors.
3) Isolate individual motors (with a single pole relay per motor) between the driver board and the motor. Let the motor freewheel to a stop.
4) Swap individual motor voltage input with a circular loop.. In essence short out the motor, to kill inductive / inertial loads. This would require one Double Pole / Double Throw Relay per motor.
5) Swap individual motor voltage input/output with a high amperage resistor bank. This requires one Double Pole / Double Throw relay per motor. This would kill inductive / 'residual inertial' loads.
Obviously, these are ranked from easiest to implement to more difficult.
#1 only offers a limited safety net, but its easy to do. (Obviously this implies that a CNC control system includes an E-Stop routine... I'm guessing most of 'em do.) I'd do number 1 in every case as a redundant stop. I don't think I'd do this as a stand along option.
#2 requires only one additional relay (SP/ST).
#3 requires three SP relays on traditional X,Y,Z axis system
#4 requires three DP/DT relays
#5 requires three DP/DT relays, three resistor banks.
I haven't had a run-away condition yet, not sure that I really want to create one just to test the E-Stop system.
Any ideas here? What are best practices? Other comments?
Many thanks in advance,
Zip
Metro Detroit, Michigan