Safe Torque Off (STO)

STO (Safe Torque Off) is a mechanism that prevents the drive from restarting unexpectedly. The Safe Torque Off function safely clears the output of the drive so it is reliably torque-free. STO has a wide range of use in machines/systems with moving axes (e.g. handling, conveyor technology).


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 much 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.’

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.

How it Works

Once STO is enabled, A physical disconnect is caused between the servo drive's motor phase terminals and the power devices. This immediately stops all gate pulses, 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. In addition, STO does not isolate the drive from electrical input. Its function is merely to assist in providing access to moving parts without fear of the motor restarting. Gaining access to electrical parts and circuitry in the drive would require locally isolating the drive manually from the main power supply.

STO vs. Inhibit

While the result of STO is essentially the same as a servo drive inhibit function (stopping the drive from powering the motor), but their methods and reliability are very different.

The inhibit function relies on the drive's logic to prevent the power devices from sending pulses to the motor terminals. This is usually activated by connecting the inhibit input pin(s) to ground. However, this means even a short logic glitch or poor grounding could allow the motor to start up again unexpectedly.

With safe torque off, the drive is debilitated on a hardware level. Upon STO activation, the power devices and the motor phase terminals are physically disconnected. Until STO is manually cleared, it's virtually impossible for the motor to start up again.

So while STO and inhibit are meant to achieve the same thing, STO is safer and more reliable.


  • Enables safe working when the machine is being serviced
  • Drive-integrated STO eliminates separate components (electromechanical switches) and the effort required to wire and service them

ADVANCED Motion Controls' Capabilities

  • Available standard on most DigiFlex® Performance drives and more models in the future
  • STO can be specified as an option for custom drives
  • Meets SIL 3 per IEC 61800-5-2

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