Safe Torque Off and Safe Brake Control in the S6-A Servo Drive

Modern industrial automation demands precision and safety. The KEB S6 servo drive delivers on both fronts, providing advanced functionality such as Safe Torque Off (STO) and Safe Brake Control (SBC).

These features deliver safe and efficient operation for a variety of applications, making the S6 an indispensable tool for engineers and designers prioritizing functional safety in motion control systems.

KEB has a wide array of functional safety products, making it an industry leader in safety for motion control applications. One such product is the S6 servo drive, particularly the S6-A control version, which offers flexible safety and communication capabilities.

This post will detail Module 1 of the S6-A, which provides integrated safety functions like STO and SBC.

S6-A Module 1

The S6-A is the ideal fit for applications with more robust communication and safety requirements. Module 1 of the S6-A comes standard with STO and SBC. These two safety functions allow for the safe control of both the motor shaft and brake.

Certified to SIL3 for IEC 61508 and Performance Level e under ISO 13849, these safety capabilities ensure compliance with rigorous international standards.

Additionally, these functions have been certified by TÜV Rheinland in a 3^rd party safety audit.

Safe Torque Off

Safe Torque Off (STO) is an integrated safety function that prevents the drive from applying torque to the motor shaft. It can be used for a Category 0 stop, as defined by EN 60204-1, and complies with IEC 61800-5-2 functional safety standards.

How does STO work?

The S6 drive, known for its high-speed motor control, has two independent 24V STO inputs. Both of these inputs are required for the drive to apply torque to the motor shaft. If either input is disrupted, the drive enters a stop category zero state, disabling torque output and allowing the motor to coast to a stop.

Each of the two inputs controls three of the six output IGBTs, providing redundancy and reducing the chance of common-mode failure. The likelihood of failure for both inputs is extremely low (2.3×10⁻⁷, as defined by the IEC 61508 standard).

In the rare event of simultaneous failure, a DC voltage may be applied, causing the motor to “jump” to the next pole pair. This potential rotational jerk should be accounted for during safety evaluations.

The STO inputs also support Output Signal Switching Device (OSSD) signals, which send 24V offset pulses as a diagnostic “heartbeat.” This ensures the drive can detect miswiring, short circuits, or damaged signal wires, further enhancing safety.

What are the benefits of STO?

STO is a highly effective way to ensure no torque at the motor shaft during emergency stops or other unwanted situations. The redundant dual inputs ensure the drive does not output torque even if a single contactor fails to open or becomes stuck.

Depending on the risk analysis, STO can potentially replace traditional power contactors.

This offers significant benefits:

1. Reduced cost
2. Smaller panel footprint
3. Fewer wear components needing replacement

Finally, the drive can enter the “Safe Torque Off” state without removing incoming power. This allows the drive to restart quickly, without the delays of power cycling or capacitor discharge, making it ideal for applications requiring frequent and safe stops.

Safe Brake Control

Safe Brake Control in the S6 is designed to manage an external power-off brake safely. This function is typically used to control the motor holding brake.

How does SBC work?

The S6 safety module terminal strip includes dual control inputs for SBC. The brake is controlled through the integrated 24V (2A) power supply within the drive. Both inputs must be active for the brake to function, and their switching capabilities are tested hourly to ensure proper operation.

 
In addition to the inputs, the 24V output is short-circuit-proof and actively monitored. If the voltage is outside the acceptable range (24V±10%) or fails to switch correctly, the system enters a fault state to prevent unsafe operation.

What are the benefits of SBC?

SBC provides a simple and safe way to control a 24V motor brake. Integrated into the S6 drive, SBC eliminates the need for traditional safety relays and external power supplies, which would traditionally control the brake. Because the 24V brake power supply is integrated into the drive, the traditional safe power supply can also be replaced. This reduces the number of components, simplifies wiring, and lowers costs.

The Safe Torque Off and Safe Brake Control feature integrated into Module 1 of the S6-A make it a leader in functional safety for servo applications. These features allow for safe and efficient operation while reducing system complexity and costs.

Whether you need STO, SBC, or both, the S6 Servo Drive delivers advanced safety and performance in one compact package.