STO function – standard function for all ACS ACX series inverters

Safe Torque Off or STO is a basic drive integrated safety feature available in most mid to large size digital servo drives AMC manufactures. STO is essential to making it safe to clear blockages, repair damage, or perform other maintenance in your servo motor driven system without fear of unexpected motor restart.

In the early development of servo motors and motion control systems, manufacturers struggled to create a safe ‘stop’ function that could act upon a motor without corrupting data or leaving the system liable to mechanical errors.

Safe Torque Off resolved many of these issues by allowing servo and stepper motors to remove power without causing damage to the drive or surrounding parts of the motor. That’s why STO can be found in industry applications ranging from conveyor systems to robotics.

The STO function ensures that no torque can act upon the motor by blocking electrical signals from the power devices to the motor. As an emergency stop function, STO can reduce torque generating energy immediately and allow the motor to come to a dead stop using natural inertia and friction from the load. The motor in this case would be considered ‘free running.’

How does a Safe Torque Off work?STO remains the trusted safety function of most servo manufacturers because it can prevent motor shaft rotation from restarting unexpectedly during emergency procedures. The only way to reactivate a servo motor after the use of the STO function would be to manually reactive it from the controller after STO is disabled.

Once STO is enabled, all drive gate pulses are immediately stopped, preventing the drive from feeding torque to the motor. Depending on friction and natural inertia, the motor may coast for a bit before coming to a complete stop.

In this state, equipment handlers can move and operate equipment by hand without interference from the motor. It’s important to note that STO will only occur if natural inertia permits. There are many ways to achieve STO from emergency stop functions to gate interlocking systems, but mostly your STO circuitry will contain their own proximity switches to assist in its function.