Industrial Sensors, Machine Safety & Automation Products

How to Select the Right Device: A Working Engineer's Guide

The order in which you make decisions matters more than the catalog you buy from. Everything below follows the same sequence a risk assessment does: identify the hazard and the way people reach it, decide what class of protective device that demands, calculate the geometry, and only then pick a model number. Specific ratings, ranges and response times differ per series, so where a number would decide your choice we link to the product page that carries it rather than quoting a figure here.

1. Start from the hazard type, not the product catalog

Machine hazards fall into three access patterns, and each one has a protective device that fits it naturally. A point-of-operation hazard is one the operator deliberately reaches into every cycle — the die area of a press, the blade line of a shear, the nip of a roller. The hands cross a definable vertical plane, so a light curtain placed in that plane is the right tool, and the whole design problem becomes resolution and mounting distance. An area hazard is a floor or volume a person can stand in or walk through, such as a robot cell or the travelling path of an AGV; no single plane describes it, so a safety laser scanner with a configured protective field is the fit. A perimeter hazard is one already fenced off, where the only route in is a door or a gap — here an interlock on the door, plus a curtain or muted opening where material has to pass, is the economical answer.

Most real cells contain all three. A stamping line typically ends up with a curtain at the load opening, interlocks on the maintenance doors, a scanner or mat covering the walk-in area behind the press, and safety edges on any moving guard door. Read our light curtain vs scanner comparison for robot cells and the by-machine scenario hub if you are laying out a cell from scratch.

2. Deciding between Type 2 and Type 4

Type is not a quality grade you pick by budget — it comes out of the risk assessment. IEC 61496-1 classifies electro-sensitive protective equipment by how thoroughly the device detects its own faults; Type 4 carries the more demanding fault-detection requirement, and it is what is normally specified where a failure of the protective device could allow a severe injury the operator has no chance of avoiding. Type 2 remains legitimate for lower-severity applications where the assessment supports it, and it is often the sensible choice for guarding that supplements a physical barrier rather than replacing one.

The Type decision travels together with the required performance level of the entire safety function — the sensor, the logic and the final switching element all count. A Type 4 curtain wired into a single non-monitored relay does not deliver a Type 4 safety function. Work through Type 2 vs Type 4 explained, performance level PL vs SIL and when Type 4 / PL e / SIL 3 is mandatory before you shortlist hardware. EN ISO 13849-1 PL e / Cat. 4 and IEC 62061 SIL 3 are quoted throughout this site as functional-safety design and technical references for integration; the certified scope of any individual model is subject to its valid certificate and the models that certificate covers.

3. Resolution and mounting distance are one decision, not two

The minimum distance between a light curtain and the hazard is calculated under ISO 13855 from the approach speed constant, the total stopping performance of the machine including the response times of the sensor and the safety relay, and an intrusion term that grows as detection capability gets coarser. The practical consequence: a finger-resolution curtain can sit closer to the hazard than a hand-resolution one, and on a machine with no spare depth that is often what decides the model. Calculate the distance first, then choose the resolution and the protective height that fit — reversing the order is how retrofits end up non-compliant after the brackets are already drilled.

Useful references while you size: the ISO 13855 calculation guide, the safety distance calculator, effective protective height and specifications and selection for light curtains. For conveyor openings where material must pass but people must not, read muting vs blanking before assuming either is allowed on your machine.

4. Where curtain, scanner and interlock stop being interchangeable

A light curtain detects an interruption in a plane. It cannot tell you that someone has walked past it and is now standing inside the guarded zone, which is why a curtain alone is not enough on any cell a person can step through and stand behind — you either add a scanner or a mat covering the standing area, or you design the layout so that standing behind the plane is impossible.

A safety laser scanner covers area, but it sees in one plane at a set height, and it has a blind zone directly around the head and behind mounting obstructions. It is also more sensitive to environment than a curtain: dust, steam, fog, highly reflective floors and strong sunlight all affect it, which is why mounting height and tilt are part of the safety design and not an installation detail. See what makes a laser scanner safety-rated and choosing an industrial LiDAR scanner. The distinction that catches people out most often: a navigation or obstacle-detection LiDAR and a safety-rated scanner can look identical and cost very differently, and only the safety-rated one may be used in a protective function.

A door interlock is the cheapest and most robust option whenever the hazard is genuinely enclosed — but only if the door can be closed and the machine can stop before anyone reaches the hazard. When the machine coasts, you need a guard-locking (solenoid) switch that holds the door shut until motion has stopped, not a plain interlock. Coded magnetic and non-contact types earn their place on doors that flex, get washed down, or are opened dozens of times a shift, because there is no mechanical actuator to wear or misalign. For double-leaf and sliding guard doors, see the guard door bolt and locking switch case. Where the residual risk is a crushing line rather than an opening — a moving door edge, an AGV bumper — a safety edge or mat is the correct device; see safety mats, edges and bumpers.

On press brakes specifically, none of the above substitutes for a beam-following laser guard, because the protected zone moves with the upper tool. On mechanical power presses, two-hand control and a curtain solve different parts of the same problem and are often combined — two-hand control vs light curtains explains where each stops working, and the press brake laser guard installation manual covers the mounting.

5. Choosing a detection sensor by what you are actually sensing

For non-safety detection the target decides the technology, and the question to ask first is what the object is made of. If it is metal and close, an inductive proximity switch is the most robust answer available — no optics to foul, tolerant of coolant and swarf, and unaffected by colour or surface finish. Its limits are that it only sees metal and only at short range, and that a flush (shielded) body sensing into metal gives a shorter range than a non-flush one. Where weld spatter and magnetic fields are present, look at factor-1 weld-immune types; for the size decision start with M8 vs M12 in practice and the proximity switch selection and troubleshooting guide.

If the target is not metal — plastic, liquid, powder, grain — or you need to sense through a container wall, a capacitive switch is the counterpart, with the trade-off that it is more sensitive to build-up, humidity and temperature drift, so plan for adjustment. If the object is far away, non-metallic, or must be detected without any mechanical approach, use a photoelectric sensor and pick the detection mode by the mounting you can achieve: through-beam for maximum reliability and range in dusty air, retro-reflective when only one side can be wired, diffuse when nothing can be mounted opposite, and background suppression whenever a wall, roller or machine frame sits close behind the target — that geometry, not intensity, is what makes BGS immune to a white background behind a black part.

When the target is small, thin, dark or distant, move from an LED to a laser sensor: the collimated spot is what gives you a small, well-defined detection area and a long working range. Distinguish a laser switch (yes/no output for a PLC input) from a laser distance or displacement sensor (an analogue or serial value for thickness, height, sag or position) — they are different products for different jobs. For printed registration marks on moving web, use a color-mark sensor selected by print contrast rather than by distance, and expect to need a multi-colour or RGB model when the mark and the background are close in luminance. For transparent film and glass, where the returned signal is very weak, a vision-style learning sensor is the practical choice. Where there is no room for a sensor body at all — tight jigs, dense arrays, hot zones — a fiber-optic head with the amplifier mounted outside solves the packaging problem; the head sets the geometry and the amplifier sets the sensitivity.

Two environment checks apply to every optical choice. First, sealing: welding spatter, oil mist, flour and washdown all attack the front window, so match the IP rating to the process — see sealing levels and window protection. Second, output type: confirm PNP or NPN, and 2-wire versus 3-wire, against your PLC input card before ordering, using the PNP vs NPN wiring guide. Getting this wrong is the most common reason a perfectly good sensor appears dead on arrival.

6. Matching relays, I/O modules and the switching element

A safety sensor does not make a safety function on its own. A certified curtain or scanner puts out a pair of OSSD signals, and a verified safety relay or safety controller has to cross-monitor that pair and switch the machine's power circuit through the required safety-rated output architecture. Reset logic and external device monitoring are model-specific functions, not features of every safety relay: DQSRN is a safety relay module, but it has no manual/automatic reset function and no EDM feedback. Select the logic device against the complete safety-function design, including required inputs, outputs, diagnostics and contactor monitoring. A single-channel, non-monitored general-purpose relay cannot replace a safety relay in that protective circuit.

The delay, optocoupler, transistor and Modbus/RS485 I/O modules in this catalog are automation building blocks with no safety rating. They are the right answer for sequencing, signal conversion, level shifting and remote I/O, and the wrong answer anywhere inside a protective circuit. Where several protective devices and a fieldbus are involved, or where the machine already has a safety controller, the choice between a relay and a programmable safety solution is worth making deliberately — safety relay vs safety PLC covers where the crossover sits. For the wiring itself, see connecting curtains to PLCs and safety relays and OSSD and EDM wiring mistakes. If commissioning throws up false trips or a curtain that will not align, start with the alignment and troubleshooting walkthrough.

On the press automation side the pairing logic is similar but mechanical: the coil width, thickness, material and weight fix the decoiler and leveller, while the part length and strokes per minute decide whether an air feeder is sufficient or an NC servo feeder is required — see air feeder vs NC servo feeder and electric or pneumatic feeding. Sizing has to be quoted against your real coil and press data, so send the figures rather than picking a model from a card.

Further reference: safety standards hub, glossary of safety sensor terms, installation & wiring hub, sensor FAQ, brand replacement & compatibility, PDF & 3D downloads and certificates.