A press or robot cycle starts while an operator is reaching in. The guard door was bypassed, or the light curtain was mounted too far away. Machine safety is not a sign on the door; it is a physical interlock that stops motion when a person is near. Light curtains and two-hand controls are point-of-use protection, and they must be installed to the right distance and arrangement.
How a light curtain works
A light curtain sends a parallel beam of infrared light from a transmitter to a receiver across the danger zone. When an object or hand breaks the beam, the safety output stops the machine. It replaces a physical gate, allowing material in and out without opening a door. But it only protects if mounted so the operator cannot reach the hazard before the machine stops.
Safety distance
The curtain must be mounted at a distance from the hazard based on the machine’s stopping time. If the machine takes 500 ms to stop after the signal, the curtain must be far enough back that a hand cannot reach the die or blade in that time. Mount it too close and the stop signal comes too late. Calculate the distance, measure the actual stop time, and verify it after installation.
Resolution and finger vs hand detection
Beam spacing sets what the curtain detects. A coarse curtain detects a hand or arm; a fine curtain detects fingers. Use finger-resolution curtains near pinch points, and hand-resolution where arms are the concern. A coarse curtain near a die that catches fingers will not stop early enough. Match resolution to the hazard.
Two-hand control
A two-hand control requires the operator to press two buttons at the same time to run a cycle. Both hands must stay on the buttons through the dangerous motion, keeping them out of the point of operation. It is used on presses and where a curtain is impractical. The buttons must be guarded so they cannot be tied down or operated with one hand and an elbow.
Mutual arrangement and spacing
Two-hand buttons must be far enough apart that one hand cannot press both, and guarded against actuation by an elbow or arm. They must require simultaneous release and re-press for each cycle, not a held run. A two-hand station that can be bypassed with a wedge is not protection. The arrangement is part of the safety design.
Bypasses and overrides
Safety devices are sometimes muted or bypassed for setup or automatic ejection. Bypasses must be key-controlled and limited to specific modes, not left active in production. A curtain permanently muted “because it was annoying” is a serious hazard. Muting should be visible and logged. Treat any bypass as a temporary exception.
Testing and validation
Test the curtain and two-hand controls regularly. Break the beam with a test rod and confirm the machine stops. Check the stop time after maintenance. Safety devices fail silently; a curtain that was never tested may not work. Include safety checks in preventive maintenance, not only commissioning.
A worked installation
A press stops in 400 ms after the safety output opens. The approach speed of a hand is taken as 2 m/s. The minimum safety distance is roughly stop time times approach speed, plus a depth increment for resolution. Mounting the curtain 1 m away gives margin. After installation, measuring the actual stop time shows it is 350 ms, so the distance is safe. If the press brake pads wear and stop time grows, the distance must be rechecked. Safety distance is not a one-time number.
Why stop time changes
Clutches, brakes, and hydraulic valves wear over time, so the machine takes longer to stop. A curtain mounted for a new press may be too close after years of wear. Measure stop time annually or after brake service. A safety distance that was correct is now dangerous. This is the most common hidden safety failure.
Light curtain mounting details
Align the transmitter and receiver so beams are not blocked by fixed structure. Mount them so the operator cannot reach around or under the curtain. Use mirrors where geometry requires, but each mirror adds alignment sensitivity. Keep the curtain clear of weld spatter and coolant. A curtain covered in oil stops seeing. Protect it with guards where needed.
Two-hand control categories
Safety-rated two-hand controls use redundant circuits and monitoring. A single button fault should stop the machine. Use safety relays or a safety controller, not a plain PLC input. Cheap two-hand stations wired to ordinary logic can fail dangerously. The safety category must match the risk. This is not a place to save cost.
Muting and blanking
Muting allows material to pass through the curtain without stopping, for example on a conveyor. It should be active only during the material passage, with sensors confirming the object, not a permanent switch. Fixed blanking disables selected beams for a known object; it must be configured and monitored. Misuse of muting defeats the purpose.
Operator behavior
Even good devices get bypassed if they slow production. Design the cell so the curtain does not nuisance-stop. If the curtain trips on every normal load, operators will mute it. Set the resolution and mounting to match the real workflow. Safety that fights production gets defeated. Make the safe way the easy way.
Common mistakes
Mounting the curtain too close, using coarse resolution near fingers, wiring two-hand buttons to ordinary logic, leaving muting active, never testing the stop function, and ignoring worn brakes are recurring errors. Safety devices are engineered components. Calculate distance, measure stop time, test regularly, and keep bypasses controlled. The point is to stop the machine before a hand reaches the hazard.
Guarding vs presence sensing
A fixed guard door is the most reliable protection when access is rare. A light curtain is for frequent access. Choose the simplest method that works. If the operator rarely enters the cell, an interlocked gate is better than a curtain. If material passes through automatically, a curtain with muting suits the flow. Match the guarding to the task, not to a generic standard.
Safety controllers and redundancy
Modern safety systems use safety PLCs or relays that monitor the curtain and two-hand circuit redundantly. A single weld or relay fault triggers a safe stop. Wire safety devices to safety inputs, not ordinary inputs. The cost of a safety relay is trivial compared with an injury. Do not shortcut the safety circuit.
Documentation and audit
Record the safety distance calculation, stop time measurement, device model, and test results. When the machine is modified, revalidate the guarding. A safety file that shows the reasoning is what an auditor expects. Without it, the installation is just a curtain that may or may not be correct.
Training operators
A safety device works only if operators understand it. Train staff on why the curtain is at that distance, why muting is keyed, and what to do if it trips. An operator who thinks the curtain is a nuisance will bypass it. Explain the stop-time logic. Safety culture and hardware together keep people safe. Good hardware without training still gets defeated.
Finally, remember that safety guarding is a calculation plus a test, not a product purchase. Mount the curtain at the measured safety distance, wire the two-hand circuit redundantly, and verify the stop function on schedule. When brakes wear or the machine changes, recalculate. A point-of-use device that is correct today is dangerous if left untested. Safety is maintained, not installed once.
Safety guarding is a calculation plus a test, not a product purchase. Mount the curtain at the measured safety distance, wire the two-hand circuit redundantly, verify the stop function on schedule. When brakes wear, recalculate. A point-of-use device correct today is dangerous if left untested. Safety is maintained, not installed once.
Safety guarding is a calculation plus a test. Mount the curtain at the measured distance, wire the two-hand circuit redundantly, verify the stop function on schedule. When brakes wear, recalculate. Safety is maintained, not installed once.
Safety guarding is a calculation plus a test. Mount the curtain at measured distance, wire the two-hand circuit redundantly, verify stop function on schedule. When brakes wear, recalculate. Safety is maintained, not installed once.
Safety guarding is a calculation plus a test. Mount the curtain at measured distance, wire two-hand circuits redundantly, verify stop function. When brakes wear, recalculate. Safety is maintained, not installed once.
Safety guarding is a calculation plus a test. Mount curtains at measured distance, verify stop function. When brakes wear, recalculate. Safety is maintained, not installed once.
Safety guarding is a calculation plus a test. Mount curtains at measured distance. When brakes wear, recalculate. Safety is maintained.
Bottom line
Use light curtains where access is frequent, and two-hand controls where the operator must stay clear. Mount curtains at the calculated safety distance, choose the right resolution, and arrange two-hand buttons so they cannot be defeated. Test them regularly and keep bypasses keyed and temporary. Safety devices only work if installed to the hazard, tested, and not bypassed. A machine that relies on operator caution instead of interlocks is asking for an injury.
