Understanding Safe Torque Off (STO) in Industrial Robotics

Safe Torque Off (STO) is a critical safety function defined by IEC 61800-5-2. It ensures that torque-producing energy is removed from the motor, preventing unexpected movement during maintenance or emergency stops. For robot controllers, STO is not optional—it's a fundamental requirement for compliance with ISO 10218-1 and ISO 13849-1. When sourcing controllers from China, verifying STO functionality is essential to ensure operator safety and regulatory compliance.

Key STO Requirements for Robot Controllers

Before purchasing a controller, evaluate its STO implementation against these criteria:

• Redundant channels: STO must have two independent channels to meet Performance Level d (PLd) or higher. Check that the controller's safety circuit uses dual paths for fault detection.
• Response time: The STO function should de-energize the motor within 100 ms or less. Verify this with the manufacturer's datasheet or request test reports.
• Fault reaction: In case of a single fault, the controller must maintain a safe state. Look for controllers with built-in diagnostics that signal STO activation.
• Certification: Ensure the controller has a valid certificate from a recognized body (e.g., TÜV, SGS) for ISO 13849-1 and IEC 61800-5-2. This is non-negotiable for global compliance.

Controller Compliance Standards: What to Check

Compliance goes beyond STO. A compliant robot controller must meet multiple standards:

• ISO 10218-1: Specifies safety requirements for industrial robots. The controller must support safe stop functions, including STO, safe stop 1 (SS1), and safe stop 2 (SS2).
• ISO 13849-1: Defines performance levels for safety-related parts of control systems. Aim for PLd or PLe, depending on your risk assessment.
• IEC 62061: Covers safety integrity levels (SIL) for electrical systems. Check that the controller's safety functions meet SIL2 or higher.
• EMC directives: Controllers must comply with electromagnetic compatibility standards (e.g., IEC 61000-6-2) to avoid interference with safety signals.

When reviewing a controller's compliance, request the Declaration of Conformity and test reports. Many Chinese suppliers can provide these, but verify their authenticity with the issuing body.

Cost Considerations for STO-Enabled Controllers

STO adds cost, but it's a worthwhile investment. Here's what to budget for:

• Base controller price: Expect a 10-20% premium for controllers with certified STO compared to basic models. This is due to redundant hardware and safety certification costs.
• Integration costs: You may need additional safety relays or PLCs to interface with the STO inputs. Factor these into your total system cost.
• Certification fees: If you're integrating the controller into a robot system, you'll need to certify the entire system, which can cost thousands of dollars. Choosing a pre-certified controller reduces this burden.
• Long-term savings: STO reduces downtime during maintenance—no need to power down the entire system. This can save significant operational costs over time.

Sourcing STO-Compliant Controllers from China: Tips

China is a major hub for robot components, but quality varies. Use these tips to source reliable STO-compliant controllers:

• Verify certifications: Ask for PDF copies of certificates and cross-check with the certifying body's database. Be wary of suppliers who cannot provide documentation.
• Request test data: Ask for STO response time and fault injection test results. A reputable supplier will share this data.
• Conduct a factory audit: If possible, visit the factory or hire a third-party inspector to verify production processes and quality control.
• Order samples: Always test a sample unit in your own environment before bulk ordering. Check STO activation with a real motor and safety PLC.
• Check for CE/UL marks: While not mandatory for all markets, these marks indicate that the controller has been tested to international standards.
• Consider after-sales support: Ensure the supplier offers technical support and spare parts availability. STO-related issues require prompt resolution.

Common Pitfalls to Avoid

When buying STO-compliant controllers, avoid these mistakes:

• Assuming all STO is the same: STO implementations vary. Some controllers only offer basic STO without diagnostics, which may not meet your safety level.
• Ignoring environmental ratings: Controllers for dusty or humid environments need higher IP ratings. Check that the controller's enclosure is suitable for your application.
• Overlooking firmware updates: Safety functions can be affected by firmware. Ensure the supplier provides update support and version control.
• Not planning for integration: STO requires proper wiring and configuration. Ensure your team has the expertise or budget for integration services.

Future Trends in STO and Controller Safety

STO is evolving with Industry 4.0. Look for controllers that offer:

• Integrated safety functions: Some controllers now include SS1, SS2, and safe limited speed (SLS) in one unit, reducing hardware complexity.
• Diagnostic capabilities: Advanced controllers provide real-time safety status via EtherCAT or PROFIsafe, enabling predictive maintenance.
• Modular safety: Modular controllers allow you to add safety I/O as needed, making them cost-effective for small batches.

Bottom Line

STO is a non-negotiable safety feature for robot controllers. When sourcing from China, prioritize certified STO functionality, verify compliance with international standards, and budget for integration and certification. By following the tips above, you can source reliable, cost-effective controllers that ensure operator safety and regulatory compliance. Always test samples and maintain open communication with your supplier to avoid costly mistakes.