Integrate a safety relay with drive card?

Hi all,

I was just wondering what would be the best way to integrate a safety relay with a Stepper Motors card attached to the PLC. The stepper motor card is attached to the rack so will I have to get the PLC to go into stop mode so that no signals are send to the card (ie if safety door opened etc) or will I need to cut power to the drive card on the PLC or cut power between the card and the stepper motor. Is there any adverse affects of constantly shutting power down to the card? Appreciate any feed back. Thanks

Unless you use a socalled "safety PLC" (and maybe not even then), you must NOT use the PLC for a safety stop function.

I guess that you have to put the stop in between the PLC and the Stepper motor.

Dont know if it will affect the stepper motor.

edit: How much force does the stepper motor have. Maybe a safety stop is not required. Have you made a risk assesment for the machine ?

The stepper motors don't have that much force and I've no risk assement done as it's only a demo rig that were setting up.

The reason that I enquired about stopping the PLC is that on some models there is a RUN button on the input card ie Mitsubishi FX I thought I could integrate PLC stop using this.

Thanks for the reply.

Hi Wolfi,

Tricky question in fact but all depends on machines and driven loads when you come with "safety" issues.

Generally speaking (beyond your demo rig) : examine your equipments until you determine 1) which "stop mode" is acceptable regarding your safety level for operators, machinery, process, control organizations 2) what is reasonable from technical and financial points of view :

- In case the load could move by itself when power is off and there is a safety concern, consider among other solutions to add some mechanical braking/blocking systems (i.e brake + safety electro-magnets as for lifts).
- In case the load can't move, you could cut power to the drive and eventually open the circuit between motor and drive BUT NEVER OPEN OR CLOSE this circuit when the power is on : your drive would be damaged or destroyed (power stage). If you planned to insert a relay and open it by some emergency stop circuit : FORGET IT ! OR DO IT IN SEQUENCE : disable your output -> delay ~30 ms (see manuals) -> open the relay -> power off... and always close this relay before your power is on.

Is there any adverse affects of constantly shutting power down to the card?

Click to expand...

An obvious adverse effect is that you could (or will, depending on your machine and stepping mode) lose your actual step/micro-step position each time you power on/off/on. But unless you directly open/close a relay between drive and motor... I don't figure out what could happen to your board.

- Depending on your drive, motion system, and safety level, see what can be done in basic systems with 1 ) an enable/disable input only (power stage off) : the motor shaft becomes free 2) a relay inserted in the step line.

Anyway in almost all these conditions you will lose your position and an init sequence should be required. Always think to reduce the current when the motor remains stopped and powered for a long time.

Hope this helps.
Laurent

Laurent and Jesper, just to say thanks for your replies.