Safe Torque Off (STO)

A driver safety function that cuts power to a motor in hardware, so it stops producing torque.

STO is a drive safety function defined in IEC 61800-5-2: once triggered, the driver uses a hardware circuit to block the switching signals to the inverter, so the motor stops producing torque and coasts to a stop under its own inertia and friction (corresponding to an IEC 60204-1 Category 0 stop). It's typically implemented with a dual-channel hardware input that doesn't depend on software, allowing it to reach a fairly high functional-safety rating (SIL/PL). Importantly, STO is not the same as cutting main power, and it doesn't actively brake or hold up a load against gravity, so vertical joints still need a holding brake alongside it; when the motor needs to decelerate first and then cut torque, the related SS1 function is used instead. A robot's emergency-stop button is typically wired directly to the STO terminals on each driver.

ExampleWhen an emergency stop is triggered on a collaborative arm, the controller asserts STO on every joint's driver while simultaneously engaging the holding brakes, preventing the arm from dropping under gravity.

Also called

STO

Related

Emergency Stop · Functional Safety · Holding Brake · Servo Drive / Motor Driver · Protective Stop

Sources

IEC 61800-5-2 (IEC Webstore)
IEC 61508 - Wikipedia