EMO Relay circuit x 2

[hobbyelectronics]

Hello,

I am building a project that uses two stepper motor drivers (Proper external driver modules)
Each have an Enable input.

I need to have a Emergency automation off switch to stop both steppers should the need arise.
I want to do this using two small 24V signal relays (DPDT).
If the EMO button is pushed the 24V coil voltage will be lost and both the enable signals will be broken by the relay contacts.
One spare set of contacts will be used to give a signal to the controller so it knows the EMO has been pushed.

My question here is, what is the best way to drive both relays. Either the coils in parallel or series?
Or, as I have 4 sets of contacts the first relay can drive the second relay.

So:

One 24V EMO signal driving one relay. One set of contacts drives a second relay and one set of contacts drives a signal to the controller.
The second relay will drive the two enable signals. One set of contacts in series with the stepper A enable signal and one set for Stepper B enable signal.

Dos this sound reasonable or are their better ways of doing this.

I was trying to keep the EMO loop as simple and mechanical as possible, so it does not rely on firmware or other ICs / Devices etc in the EMO control loop. Hopefully making it safe??

Thanks

[SteveThackery]

The reliability of the emergency stop feature is paramount here. In particular, you want it to be fail-safe.

I wouldn't use relays if I could avoid it, because they are another thing to go wrong. However, if you must, then....

Putting the relay coils in series means that if one coil goes open-circuit (rare but not impossible) then both relays will release. Also, you want the relays to be operated in normal use, and release when the emergency button is pressed. I would use a simple circuit: power feed -> relay 1 coil -> relay 2 coil -> emergency break switch (latching off) -> ground.

You do not want one set of contacts operating the other relay - there is a small but significant risk of failure giving only a partial shutdown.

Now the contacts. On each relay, use one set to break the Enable signal to its associated motor. Use the other set to signal back to the controller that shutdown has happened. You have a simple decision to make here: do you want to signal that either of the relays has released, or both have released?

If only one relay has released, you obviously have a fault condition because their coils are in series. This can happen, though: relays can stick operated, relay contacts can stick together. If only one relay has released, you will get only a partial shutdown. You must decide whether to report that back, or only report a full (successful) shutdown. If the former, wire the "reporting" contacts in parallel (like an OR gate), if the latter, put them in series (an AND gate).

With this sort of application, concentrate on keeping it simple, and think about the ways it could go wrong.

Finally, I'm wondering if you should be thinking about using bigger relays and actually cutting the power to the motors, rather than relying on the Enable signal.

HOWEVER, why use two relays? I expect you can safely connect the two Enable lines together and feed both from one set of relay contacts. Please check that possibility. And finally, why use any relays? See if you can use the emergency switch itself to directly control the commoned Enable lines, or better still to cut the power to the whole shebang. You can feed a wire back from the power connection to the motors to your control unit: the absence of a voltage on that wire means the system has been shut down.

[hobbyelectronics]

Thanks for your detailed reply.

I have thought about many configurations. The motor enable signal (3.3V from the MCU gets converted to 5V to drive the enable inputs of the stepper drivers.
Both enable signals are controlled in firmware, so ideally, I would like to keep them separate.
The EMO button is some distance from the controller (Maybe up to 4+ meters away), so I wasn't happy with having the 5V signal going up to the emo button.
EMO circuits always seems to be 24V (dc or ac).

My board has some industrial style fully isolated inputs (up to 60V, typically 24V) using a MAX22191 and a ACPL-064L-560E Opto-coupler.
I did think about using these to monitor the 24V EMO signal, and maybe use this opto input into the MCU and have a interrupt to turn off the two two stepper enable signals.
But I thought maybe a couple of small signal relays would give a more reliable safety circuit, without any firmware / other devices adding potential failure modes.

Or the output of the Opto (Push-pull type), could drive the enable signals, maybe, keeping the emo/enable circuit out of the firmware.

What you said all makes sense, just trying to decide what is the best option.

Having two relays (4 sets of contacts in total) with one relay driving the other, does mean that the first relay contacts would be used up (One driving the 2nd relay and one signalling the MCU) the second 2 sets of contacts would disable the two enable signals.
If the second relay go stuck on, at least the MCU would know and turn off the enables lines. But if the 1st relay stuck on the whole safty system would fail. The MCU wouldn't know and the second relay wouldn't isolate the enables.

This does put me off using relays now 

Maybe, I should have a mains contactor and have the 24V dc signal linked to the EMO, so it just powers down everything! Just need a bigger box! But didn't really want to do that. I only need an automation off, not a full on EMO

Perhaps, using a 5V signal from the EMO to the enable signal is best. No relays, just a EMO switch. Two contacts, driving both Enables inputs of the stepper.

Uhmm. Too many options, too many questions

Sorry for all the waffle....











 

[themadhippy]

The reliability of the emergency stop feature is paramount here. In particular, you want it to be fail-safe.

I wouldn't use relays if I could avoid it, because they are another thing to go wrong

Meanwhile  relays /contactors are used in there 1000's  millions for just this very thing,hit the emergency stop button relay drops out machine turns off

[SteveThackery]

The reliability of the emergency stop feature is paramount here. In particular, you want it to be fail-safe.

I wouldn't use relays if I could avoid it, because they are another thing to go wrong

Meanwhile  relays /contactors are used in there 1000's  millions for just this very thing,hit the emergency stop button relay drops out machine turns off

Yes, but these are not your average relay, they are proper contactors which latch and are specced to switch power lines. The OP was talking about using DPDT relays (and even considering implementing relay logic) to control two Enable lines.   I wouldn't advise that.  Best to use the simplest architecture for reliability.

But yes, I agree: if you can get away with switching the power off instead of the Enable lines, then a mains contactor would be the obvious way to go.

If it has to be the Enable lines, then I would consider the points I raised above.

[hobbyelectronics]

Would it be fair to say that it is not considered acceptable to monitor the EMO switch (Automation off only, not full power) as a digital input and then simply turn off the stepper controllers in firmware?
I have isolated 24V inputs (MAX22191+ACPL064 Opto+ TVS) I could use for this.

If I used this input from the 24V EMO switch and kept it simple in firmware, would this as a solution be ok?
Of course it would work, but compared to using 2x relays, would that be considered ok. At least I do not need to worry about relays failing closed, or sticking.

I only ask, as my CNC machine using a 'proper' controller does it in a similar way. A 24V digital input from the EMO switch and then firmware to inhibit the motors / axis.

Thanks