Safe Torque Off (STO)
Safe Torque Off (STO) function is supported on ANCA Motion’s AMD2000 Servo Drives and safely removes the torque generating power being supplied to a motor.
STO operates by disabling the internal PWM signals without having to shut down the drive or remove the input power. This not only eliminates the additional cost for external safety components, but also reduces the machines recovery time by only shutting down the drive’s motor power output. Another advantage of using drive integrated STO function, is a faster response time when compared with traditional contactor-based solutions.
ANCA Motion has implemented their STO functionality by monitoring two redundant control inputs. Once both control inputs are de-energized, the STO function is activated by cutting off the motor power immediately. Sophisticated diagnostics have also been implemented to achieve the desired dangerous failure detection coverage. Any detected fault in STO results in both STO’s activation and the opening of a faulty relay which can be monitored by the rest of the safety system.
STO can be used anywhere in a control system where reliable motor power removal is required or it can form the base for other advanced safety functions.
The AMD2000 Servo Drives STO functionality provides a SIL Claim Level 3 safety function according to IEC 61800-5-2:2007, or an equivalent performance level of PL=e, Category 4, according to ISO 13849-1:2006. The PFH (dangerous failures probability per hour) of ANCA Motion’s STO function is 6.84x10-9 which leaves sufficient safety margin at system integration level.