It’s important to understand that in the SureServo 2 drive, STO disables the output
stage to the motor so the motor has no power which means it can’t produce any torque.
It’s the fastest and safest way to remove power from the motor. For example, if you
turn off power to the drive in an attempt to power down the motor – which you should
never do by the way – the drive will continue to try and execute whatever command it’s
running until the power is completely gone. That takes time and can cause all kinds of
havoc inside the drive and unpredictable motor behavior. So if you need to disable the motor
as fast and safe as possible, then STO is the option you want because it will immediately
shut down power to the motor completely independent of anything that is going on inside the drive
and it won’t harm the drive or send the motor off doing weird things. Hence the name:
Safe Torque Off. But, there is a downside. Once STO is activated,
the drive no longer has control over the motor and the motor is left to free spin down. If
it has a lot of inertia or the load has external forces keeping the shaft spinning such as
gravity pulling on the load, that could take a while. Which could be dangerous in some
applications. So, it’s up to you to decide if STO is appropriate for your application.
The drive comes with this connector pre-wired to bypass STO which is great if you are bench
testing or don’t plan to use STO in your application. The connector is pre-wired like
this where Pins 1 and 2 simply provide the signaling needed to turn on the opto-isolators
which tells the drive everything is ok. To use STO, remove the jumper wires from the
CN10 connector and wire the connector to a safety relay. I’m using this dual-channel
safety relay from AutomationDirect.com. Dual channel means you can have dual safety inputs
like this where one switch provides two contacts to the safety relay for redundancy.
Those contacts can come from any safety device like a safety limit switch, a safety pull
cable, a trap key system, interlock switches, etc. We’ll just use a simple dual contact
ESTOP pushbutton for this video. This is the part number of the one I am using but it only
comes with one contact, so the extra contact part number is this.
Let’s take a minute to understand how this safety relay works, it will give you a better
appreciation for why safety relays are so important.
When the ESTOP button is wired like this and configured for cross fault detection, the
safety relay sends a known voltage pulse out S11 and looks for that on S12. It also sends
a different voltage pulse out S21 and expects to see that on S22. So, if it doesn’t get
one of those pulses it knows that contact is open and if the wrong pulse is received
it knows it is mis-wired. So the safety relay is continuously monitoring
that the safety wiring is correct and that it is operating correctly. That ensures it
is actually going to work when you need it. As long as there is power to the safety relay,
the ESTOP is not active and the drive shorts these two pins to tell the relay everything
is ok, then these contacts will be closed, which allows power to flow into these opto-
isolators, which allows power to go out to the motor.
If the safety relay loses power or either of these ESTOP contacts opens, then these
guys open and the drive cuts off power to the motor.
And remember, the drive totally cuts power to the motor leaving it to free spin down
by itself. There are no options for the drive to provide a controlled stop when using STO.
When the drive sees STO is activated, it shorts these two pins to tell the safety relay it
got the message and did something about it. The safety relay won’t allow these relays
to close and pass power again until it gets that feedback.
When the ESTOP switch is restored and it sees that the drive has acknowledged that it got
the STO signal the relay resets automatically if configured for automatic reset. That closes
these contacts. And when the drive sees the STO condition go away, it automatically clears
the acknowledgment so the relay will need to see another acknowledgment when the next
STO occurs. The drive won’t allow the motor to run until
it sees the alarm reset toggle after the STO condition is cleared. That’s important,
a proper ESTOP requires two actions to clear it.
Hopefully, you can see why these safety relays are so important. They continuously verify
all the safety circuitry is functional to ensure that it will work when you need it.
And they monitor the status of the STO function and won’t let you reset the safety circuit
until the drive acknowledges it got the STO request.
Let’s do an example. I’m using the exact same hardware as the
Quick Start video except this STO jumper is replaced with the safety relay and I added
a 4-and-a-half-pound steel pully to give us some inertia.
This is my STO ESTOP button that will trigger the Safety Relay and this is the normally
open alarm reset button which defaults to digital input 5.
The top LED tells us the relay has power. The bottom two are lit which tells us both
of these are closed and passing power. If either of these LEDs is off, something is
wrong. I factory reset the drive, so you know exactly
where I am starting from, switched to Speed Mode and power cycled the drive. Please don’t
forget to power cycle the drive after a mode change – if things don’t work, it’s
usually because you forgot to do this. I know, because I do it all the time and it drives
me nuts trying to figure out why I can’t get the motor to spin.
We need to set up the safety relay to match all of this. There are two switches
inside the cover which has a little setup guide on the inside. Switch 1 tells the relay
we have wired it to detect cross faults. Switch 2 configures how the relay will restart when the ESTOP button is released.
We're using the up position to tell the relay to automatically restart when it sees the ESTOP is released and the drive is shorted to 2 pins.
Let me put that cover back and I’ll enable the motor and increase the speed. If I hit
the STO ESTOP, sure enough, we see the relays LEDs turn off, the STO alarm appears in the
display and the motor is just left to free spin its inertia down on its own. Again, once
STO is activated, the drive no longer has control of the motor.
Remember, when the drive saw the STO condition, it automatically reported that back to the
relay by shorting these pins. So when I release the ESTOP the relay automatically resets itself
because we configured to automatically look for those shorted pins. And that releases
the STO condition. The drive sees that the STO condition has been released so it automatically
releases the feedback so the relay is ready for another cycle.
I then hit the drive's alarm reset button to tell the drive it’s ok to apply power to
the motor. Perfect. And again, STO kills the power to the motor quickly
and safely completely independent of anything else that is going on inside the drive, so
it works no matter what mode the drive is in.
Well, that’s it. And while I did a lot of explaining in this
video, all we really did to get STO working was wire up a safety relay and set the two
switches on the safety relay telling it how we wired it up. Not bad at all.
Hopefully, you now have a better understanding of how STO works and why you would want to
use it. It’s not for every application, but when you do need it, how cool is it that
the SureServo2 drive has it built-in, makes it super easy to use, and has that configurable
feedback signal so you can use it with just about any safety relay or any safety application?
You just add a safety relay. No need for external contactors or other devices. That saves you
time, space in your cabinet, and money. Click here to learn more about the SureServo2
System. Click here to subscribe to our YouTube channel so you will be notified when we publish
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