Safe Torque Off (STO)

Safe torque off (STO) is a restart lock safety feature that prevents a system restart. There are separate STO signals for each axis. The STO input provides direct electrical control of the power stage of the AKD2G drive. It bypasses the processor and will disable the power stage regardless of software or other hardware signals.

Brakes are controlled by firmware. If a brake needs to be applied when STO drops without using a controlled stop, then an external safety relay must be added in-line with the motor brake.

STO is controlled by two digital inputs per axis (AXIS#.SAFE.STO.A and AXIS#.SAFE.STO.B) which releases the power output stage of the drive as long as a 24 V signal is applied to these inputs. If one of the STO inputs goes open-circuit, then power will no longer be supplied to the motor on that axis, and the drive will lose all torque and coast to a stop. If one of the STO digital inputs does not have 24V applied to it and you attempt to enable the drive (using the hardware and software enables) the drive will generate a “Safe Torque Off” fault.

For full STO information refer to the FS1 Functional Safety section: STO (Safe Torque Off).

For full STO information refer to STO (Safe Torque Off).

STO Fault

If you see this fault you need to apply 24V to the STO input and then clear the fault (DRV.CLRFAULTS ) before you can enable the drive. The STO will not generate a fault until you try to enable the drive. You can read the current faults using DRV.FAULTS.

-->DRV.FAULTS
2-9000: Safe torque off.
-->

To disable this fault from occurring, AXIS#.SAFE.STO.REPORTFAULT can be set to 0. In this case only a warning will be generated when the STO input loses 24V.

Reading STO state

The current state of the overall STO can be read using the AXIS#.SAFE.STO.ACTIVE parameter. Individual STO signals can be read with AXIS#.SAFE.STO.A and AXIS#.SAFE.STO.B. WorkBench also shows you the state of the STO inputs for each axis in the status bar at the bottom of the window.