Low‑voltage servo drive STO function - Textile Machinery Section - Stepper Motor AGV Motor Servo Motor Servo Hub Motor Hollysys Motor

What is STO?

       STO stands for “Safe Torque Off”, which translates to ”Safety Torque Shutdown“ in Chinese. This function is a fundamental and commonly implemented safety feature integrated within drive systems.
       STO specifically works by using safety input signals to forcibly disable the internal power transistor drive signals of the servo drive through circuitry, thereby cutting off motor current and safely shutting down motor torque output.
       When STO is activated, the servo drive disables the servo ready output signal, placing the system in a safe state.

Why is STO Required?

STO is an embedded safety circuit function that simplifies safety management for motion equipment like robots and serves as a critical component of comprehensive safety systems.
For example, when machine or personal safety is compromised, pressing the emergency stop button cuts power to the drive, stopping motor torque output and preventing unintended machine movement. However, power cycling the drive requires time, particularly for high-power drives where repeated power cycling significantly impacts production efficiency.
STO enables torque output shutdown without disconnecting drive power, eliminating the need for frequent power cycling, allowing rapid emergency stop alarms and quick system resets.

STO Features in Low-Voltage Servo Drives

Hollysys Motor's self-developed low-voltage servo drives with STO functionality meet international safety standards including IEC 61508, IEC62061, and ISO13849.

Key features include:

Safety suppression pulses cut torque output to prevent unintended motor startup while maintaining electrical connection between drive and motor;
DC bus remains energized for rapid system restart when required;
Dual power supply design with internal detection/verification ensures high safety levels and reliability;
Reduces peripheral equipment requirements, simplifies wiring, and saves space.

Product Overview Supporting STO Functionality

During low-voltage servo motor operation, disconnecting STO+ from STO1 or STO2 will immediately engage the motor brake (if equipped) and stop rotation. For motors without brakes, rotation will continue by inertia until coming to rest.

STO Interface Connection Diagram

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