How to Bridge or Connect Servo Drive STO Safety Input
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Discover the technical details and safety implications of bridging or connecting a servo drive’s STO (Safe Torque Off) safety input. This guide covers the principles, dual-channel operation, and essential precautions for industrial automation.
Practical notes for CNC router, automation and industrial motion systems.
Understanding Servo Drive STO Safety Input
In industrial automation, Safe Torque Off (STO) is a fundamental safety function integrated into servo drives. Its primary purpose is to prevent the motor from generating torque unexpectedly, thereby halting hazardous movements and ensuring personnel safety. STO achieves this by safely disconnecting the motor’s power supply, eliminating energy flow to the motor windings and preventing rotation or movement. This function is critical when safety events are triggered, such as emergency stops (E-Stop), safety gate interlocks, or light curtains.
The term “bridging” or “connecting” STO inputs typically refers to disabling this safety function by applying a continuous voltage signal (commonly 24V DC) to the drive’s STO terminals. This signals the drive that the STO function is “active” or in a “safe” state, allowing normal motor operation. However, this action bypasses the drive’s internal safety mechanism. Therefore, bridging STO inputs should only be performed by authorized and trained personnel after a thorough risk assessment and in compliance with relevant safety standards. This procedure is usually considered for specific scenarios like commissioning, testing, or when an external, higher-level safety system already provides the STO functionality.
Principle of Operation and Technical Data
Servo drives typically employ a dual-channel (dual-channel) STO configuration. This means two independent signal paths are used, ensuring that if one channel fails, the other can still maintain the safety function. The drive permits motor torque generation only when both channels are simultaneously and correctly activated (usually with 24V DC). If one or both channels are interrupted (fall to 0V DC), the drive safely disconnects power to the motor. This dual-channel design enhances system fault tolerance and enables the achievement of high Safety Integrity Levels (SIL) or Performance Levels (PL).
Bridging STO inputs involves supplying the required voltage (typically 24V DC) continuously to these dual channels from an external source. This is often done by running two separate wires from the positive terminal of a power supply to the drive’s STO1 and STO2 terminals (or similarly named connections). Some drives might feature an internal bridging option or a specific terminal block, but this is uncommon and always requires consulting the manufacturer’s manual. This process effectively keeps the drive’s internal safety logic in a “continuously safe” state, preventing the STO function from ever interrupting motor torque. This approach might be chosen for simple, non-safety-critical movements or when the overall risk assessment for the machine deems STO unnecessary. However, it does not eliminate potential hazards; instead, it shifts the responsibility to external safety systems or operational procedures.
Technically, STO inputs are often isolated using optocouplers, providing electrical isolation between the control and power circuits, protecting the drive’s internal electronics, and enhancing signal integrity. The voltage and current requirements for the input signals vary by drive model and manufacturer, but typically fall within the 24V DC and a few milliamps range. The response time is also critical; the time it takes for STO to safely disable torque is usually under 10 milliseconds, ensuring a rapid and reliable safety response.
| Parameter | Value/Description |
|---|---|
| STO Input Type | Dual Channel, P-switching or N-switching |
| Nominal Input Voltage | 24V DC (tolerance ±20%, varies by manufacturer) |
| Input Current (Per Channel) | 5-15 mA typical (depends on drive model) |
| Response Time (STO activation) | < 10 ms (time to disable motor torque) |
| Safety Level | SIL 2 / SIL 3 (IEC 62061) or PL d / PL e (EN ISO 13849-1) |
| Isolation Type | Optocoupler isolated inputs |
| Cable Cross-section | 0.5 – 1.5 mm² (shielded cable recommended) |
| External Bridging Method | Continuous 24V DC applied to both STO inputs |
Critical Considerations in Practice
- Risk Analysis and Assessment: Before bridging STO inputs, conduct a comprehensive risk analysis for the machine or system. This analysis must identify all potential safety risks associated with disabling the STO function and evaluate whether alternative safety measures (e.g., mechanical interlocks, external safety relays, other safety functions) are sufficient to reduce these risks to an acceptable level. Safety levels (SIL/PL) must be recalculated and documented.
- Strict Adherence to Manufacturer Instructions: STO input configurations and bridging methods can differ significantly between servo drive models and manufacturers. Carefully review the drive’s user manual, wiring diagrams, and safety instructions. Never use a bridging method not approved or specified by the manufacturer, as incorrect connections can damage the drive or create severe safety hazards.
- Documentation of Temporary or Permanent Solutions: Whether the STO bridging is temporary (e.g., during commissioning or testing) or permanent, it must be thoroughly documented. This documentation should detail why the action was taken, how it was implemented, and what compensatory safety measures are in place. This record should be included in the machine’s safety file for future reference during audits or maintenance.
- External Power Supply and Cable Quality: The external power supply used to provide continuous voltage to the STO inputs must be reliable and stable. Voltage fluctuations or interruptions could cause the drive to erroneously enter an STO fault state. Use appropriately sized, shielded cables suitable for industrial environments to maintain signal integrity and prevent electromagnetic interference (EMI).
- Software Settings and Parameterization: Some servo drives require specific software parameters to be set for STO function activation or deactivation. Ensure that these parameters are correctly configured in the drive’s software, in addition to the physical bridging. Incorrect software settings can lead to STO faults even with physical bridging.
- Personnel Competence and Training: All operations involving safety functions like STO must be performed only by authorized, trained personnel knowledgeable in safety standards. Staff must be aware of the risks and understand the potential consequences of their actions.
By carefully considering these points, you can ensure that any modification to the STO safety input is performed safely and responsibly. For expert guidance on integrating servo drives and ensuring machine safety, request a quote on WhatsApp today.
Related product categories: Genel · Elektronik · Mekanik
