Understanding the Safety Architecture

In a modern robot cell, the safety circuit must reliably stop hazardous motion when a person enters the danger zone. Two key elements are the Safe Torque Off (STO) function on the robot controller or drive, and the light curtain that detects intrusion. STO removes power to the motor windings without relying on mechanical contactors, while the light curtain provides a non-contact, configurable detection zone. Designing the circuit correctly ensures compliance with ISO 13849-1 and IEC 62061, and avoids costly downtime or retrofits.

STO: The Core of Safe Motion

STO is a safety function that prevents the drive from generating torque. It is typically implemented via dual-channel inputs on the drive or robot controller. When the STO signal is de-asserted, the drive’s output stage is disabled, and the motor coasts to a stop (or is held by a brake). Key specifications to verify:

• Safety integrity level (SIL): STO inputs are usually rated SIL 3 / PL e. Confirm the rating matches your risk assessment.
• Input type: Sourcing or sinking? Most industrial STO inputs are 24 V DC, dual-channel, with test pulses for diagnostics. Ensure compatibility with your safety relay outputs.
• Response time: The time from STO assertion to torque removal. This affects the minimum distance calculation for light curtain placement.
• Diagnostic coverage: Look for inputs that detect cross-connections and short circuits. This reduces the required proof test interval.

When sourcing STO-equipped controllers or drives from Chinese suppliers, request the safety certificate (TÜV, UL) and the safety manual. Many suppliers offer STO as standard, but documentation quality varies. Verify that the STO function is independently certified, not just self-declared.

Light Curtain Selection and Placement

Light curtains detect body parts entering the cell. Selection depends on resolution, range, and environmental conditions:

• Resolution: 14 mm for finger detection, 30 mm for hand detection, 50–70 mm for body detection. Higher resolution requires more beams and higher cost.
• Protected height: Must cover the entire opening. Consider the minimum distance calculation (ISO 13855) based on machine stopping time and approach speed.
• Safety output: Typically dual-channel PNP or relay. Must interface with the safety relay or directly to the STO inputs if the curtain has OSSD outputs.
• Environmental rating: IP65 or higher for washdown areas; IP67 for harsh environments. Chinese suppliers offer good IP-rated models at competitive prices.
• Muting and blanking: Fixed blanking for material pass-through; muting for temporary interruption. Ensure these functions are certified and do not compromise safety.

Mount the light curtain so that the danger zone cannot be reached without passing through the beams. Calculate the safety distance using the formula: S = K × (T1 + T2) + C, where K is approach speed (usually 2000 mm/s for walking), T1 is the light curtain response time, T2 is the machine stopping time (including STO response), and C is a constant for resolution.

Integrating STO with Light Curtains

The safety circuit typically follows this chain: light curtain OSSD outputs → safety relay (or safety PLC) → STO inputs on the robot controller/drive. The safety relay provides the necessary logic (e.g., start/restart interlock, EDM monitoring) and ensures that a single fault does not lead to loss of safety. Key design points:

• Dual-channel wiring: Route both OSSD channels separately to the safety relay. Do not combine them.
• Cross-monitoring: The safety relay should detect discrepancies between channels and lock out if a fault occurs.
• Manual reset: Provide a reset button outside the cell. The reset must not be accessible from inside the danger zone.
• External device monitoring (EDM): If using contactors for additional power removal, monitor their auxiliary contacts to detect welded contacts.
• STO wiring: Use shielded cable for STO inputs to avoid noise. Keep cable length within manufacturer’s specification (often <30 m).

Some advanced robot controllers have integrated safety inputs that can accept light curtain OSSD directly, eliminating the need for a separate safety relay. This simplifies wiring but requires that the controller’s safety functions are certified for the application. Verify the controller’s safety manual for allowed configurations.

Cost Considerations and Sourcing from China

Safety components are often the most expensive part of a robot cell. Chinese manufacturers now produce light curtains and safety relays that meet international standards, often at 30–50% lower cost than European brands. However, not all are equal. When sourcing:

• Certifications: Demand CE, UL, and TÜV certificates. Check that the certificate covers the specific model.
• Documentation: Request the safety manual, wiring diagrams, and MTTFd data. Incomplete documentation is a red flag.
• Sample testing: Order a sample and test the response time, OSSD output, and fault detection. Verify that the light curtain does not false-trip under ambient light.
• Supplier audit: For volume orders, audit the factory for quality control and traceability. Many reputable Chinese suppliers welcome audits.
• Lead time: Standard models ship in 2–4 weeks; custom lengths may take longer. Plan accordingly.

For STO-equipped controllers, the cost is embedded in the drive. When comparing Chinese vs. European brands, consider the total cost of ownership: documentation, support, and certification may add hidden costs. However, for many applications, Chinese STO drives offer a viable, cost-effective alternative.

Common Pitfalls to Avoid

• Incorrect safety distance: Underestimating stopping time leads to inadequate protection. Always measure actual stopping time with a stop-time measurement device.
• Series connection of safety devices: Connecting multiple light curtains in series can reduce the safety level. Use a safety PLC or relay with multiple inputs instead.
• Ignoring restart interlock: Without it, the robot may restart automatically when the light curtain is cleared, creating a hazard.
• Poor grounding: Safety circuits are sensitive to noise. Ensure proper grounding and shielding.
• Mismatched STO logic: Some STO inputs require a high signal to enable, others low. Verify compatibility with your safety relay outputs.

Bottom line

Designing a robot cell safety circuit with STO and light curtains requires careful selection, correct wiring, and verified documentation. Prioritize certified components, calculate safety distances accurately, and test the complete circuit before deployment. When sourcing from Chinese factories, insist on certificates and samples, and consider the total cost of ownership. A well-designed safety circuit not only protects operators but also minimizes downtime and liability.