Safety Light Curtain Product

Overview

Light curtains (also called safety scanners or beam barriers) are optical safety sensors that create an invisible infrared fence across machinery access points. When a worker's hand, arm, or body interrupts the beam, the sensor immediately signals the machine controller to stop all hazardous motion. Light curtains are the primary guarding technology for injection molding machines, hydraulic presses, automated assembly lines, and robotic work cells where frequent tool changes or manual intervention is required.

This 20-beam model spans 3000 mm (10 feet) vertically, creating a fine-grain mesh that detects an arm or hand within 65 milliseconds. Dual safety-rated output relays (OSSD: Output Short-Circuit Protective Device) provide PLd category 3 safety performance. Reflective muting tape sensors allow authorized robots to pass through without triggering an alarm—critical for fully automated lines where robot loading happens within the light curtain zone.

Optical Architecture

The Emitter Unit contains a linear LED Bar Array array of 20 infrared LEDs spaced 2 mm apart. Each LED emits at 870 nm wavelength, pulsed at 20 kHz with 20% duty cycle (2 µs on, 8 µs off). This pulsed modulation allows the Receiver Unit to filter out ambient sunlight and indoor lighting, which are continuous in spectrum; only the 20 kHz IR signal passes through the Receiver Filter bandpass filter (±20 nm window around 870 nm).

A Emitter Lens lens array collimates the LED output into parallel beams, each diverging <1 degree. At 5 meters distance, each beam footprint is approximately 100 mm diameter—small enough to maintain millimeter-level positional information.

The Receiver Unit unit contains a 20-element Photodiode Array matched to the LED array. Each photodiode has an integrated Receiver Amplifier transimpedance amplifier that converts picoampere photocurrent into voltage. A Receiver Decoder FPGA-based decoder at 1 MHz sampling rate synchronously demodulates the 20 kHz received signal, extracting the information from each beam independently.

Beam-to-Shutdown Signal Path

When a human hand interrupts a beam, the light intensity at that photodiode drops. The receiver's decoder identifies which beam(s) are blocked within one 20 kHz modulation cycle (~50 µs). The decoder then signals the Safety Processor safety controller.

The processor, based on a certified safety-rated ARM Cortex-M3 MCU, evaluates the signal:

• Single beam blocked: Minor interruption, but position is known.
• Multiple adjacent beams blocked: Larger object (arm, hand) has entered the zone.

If blockage duration exceeds the configured response threshold (~50 ms typical), the processor immediately opens both OSSD Driver OSSD output channels. Each OSSD drives an independent Relay solenoid. When the relays de-energize, a dual-channel safety relay module downstream shuts off the machine's hydraulic pilot solenoid or CNC servo enable signal.

Total response time: <65 ms from beam interrupt to machine stop command. This is safe for most machinery; the ISO 13849-1 standard PLd requirement is typically 200 ms or less.

Dual-Channel Safety Architecture

Each OSSD relay output is independently wired back to the machine controller via a separate safety relay module (not shown in this BOM). The controller monitors both channels continuously:

• Normal operation: Both OSSD relays energized, contacts closed.
• Beam interrupted: Both OSSDs de-energize immediately; if either fails to open, the safety relay detects the asymmetry and triggers an alarm.

This dual-channel architecture prevents single-point-of-failure scenarios. If one OSSD relay sticks closed due to arcing or mechanical wear, the other OSSD opening is still sufficient to stop the machine, AND the asymmetry triggers a fault alert so the curtain can be serviced.

An independent Watchdog Timer timer circuit monitors the MCU clock. If the processor hangs or crashes, the watchdog immediately forces both OSSDs open, pulling the machine to safety. This watchdog is implemented as a separate digital circuit with no shared components—a true independent safety channel.

Muting Mode for Automated Production

In fully automated lines (e.g., robot-tended injection molding), a robot arm must pass through the light curtain zone to load parts. Without muting, the robot would trigger false alarms. The Muting Module solves this by incorporating two Muting Proximity Sensor reflective-mode proximity sensors positioned on the machine frame flanking the light curtain.

The robot arm is fitted with 20 mm wide strips of reflective tape. As the arm approaches the curtain, the reflective tape passes the first muting sensor, triggering the processor to enter muting mode. During muting, the processor monitors the beam pattern but does NOT open the OSSDs if beams are interrupted—it logs the interruption time instead. After 400 ms of muting (typical robot dwell time), or when the second sensor detects the arm exiting, the processor exits muting mode and resumes normal beam monitoring.

Muting is safety-critical: if a robot fails mid-motion, the curtain cannot remain muted indefinitely. The processor has a maximum muting duration (e.g., 3 seconds); if exceeded, it automatically exits muting and triggers an alarm. Additionally, muting is only allowed when the machine's main controller broadcasts a "robot mode" enable signal. If the controller loses power or the robot fails, muting is disabled.

Response Time and Minimum Object Size

Response time is defined as the delay from when an object blocks a beam to when the OSSD outputs open. At <65 ms, this is fast enough to stop most industrial machinery within safe stopping distance. The ISO 13849-1 standard defines response time requirements based on machine hazard zone speed and distance; for a press with 500 mm/s table speed, 65 ms reaction time allows the table to travel only 33 mm—well within safe margins.

Minimum object size (smallest thing the curtain can detect) depends on range and beam spacing. At 5 meters, the minimum is a 14 mm sphere (pencil-tip sized object). This is important: the light curtain cannot detect a thin wire or cable passing through; if critical wiring must cross the curtain zone, it must be routed outside the beam pattern.

Alignment and Commissioning

Upon installation, the Emitter Unit and Receiver Unit must be aligned parallel within ±1 mm over their full length. Misalignment causes beam degradation; excessive misalignment may cause beams to fail detection. The Mounting Bracket Set system provides Emitter Post Clamp and Receiver Post Clamp post clamps with ±5 degree tilt adjustment and a Pivot Assembly fine-adjust base with locking screws.

A Alignment Jig is provided—a mechanical tool with an internal laser reference line. The jig is mounted to the emitter unit; it projects a red laser beam that must hit the receiver's center lens. If alignment is within tolerance, the laser spot stays centered; if misaligned, the spot drifts and shows correction direction.

After physical alignment, the processor is powered up and performs an automatic signal strength check. Each of the 20 beams is evaluated for signal-to-noise ratio. If all beams exceed the SNR threshold (typically 5:1), the processor logs a "Ready" status. If any beam is below threshold, the processor forces the OSSDs to latch open and displays an error code—indicating that alignment or cleanliness needs attention.

Maintenance and Diagnostics

The processor incorporates a self-diagnostic system that runs continuously. Every 100 ms, it checks:

• OSSD relay continuity (coil resistance and contact continuity)
• Receiver signal quality for each beam
• Watchdog timer responsiveness
• Muting sensor feedback signal presence

If any fault is detected, the processor logs the fault code to its internal flash memory (timestamp + fault type) and immediately latches both OSSDs open. The curtain enters safe state until manually reset by an authorized technician.

The Cable Harness shielded cable connecting emitter and receiver is twisted-pair to reject EMI from nearby welding equipment or servo motors. Cable lengths up to 100 meters are supported; longer runs require signal repeaters (not supplied).

Standards and Certification

Light curtains fall under ISO 13855 (Safety of Machinery—Opto-electronic Protective Devices) and ISO 13849-1 (Safety-Related Parts of Control Systems). This model achieves PLd category 3 performance, meaning it tolerates one random hardware failure without loss of safety function. SIL 2 per IEC 61508 is equivalent to PLd for machinery safety.

Certification is typically by notified body (e.g., TÜV, DEKRA in Europe; UL, NFPA in North America). Third-party testing verifies that response time, beam detection, muting logic, and watchdog function all meet published standards under temperature extremes, humidity, and electromagnetic interference.