Accent D31C Photoelectric Switches: Range, Sensing Modes and Ordering

When the sensing distance you need is beyond what an inductive switch can reach, or the target is not metal, the answer is usually photoelectric. The D31C range covers that requirement with 72 variants in M18 and M30 bodies, and the single most important decision is not the model number — it is which of the three sensing modes you choose.

Get the mode right and a photoelectric sensor runs for years. Get it wrong and it becomes the device the maintenance team resents.

The three sensing modes

Through-beam puts the emitter and the receiver in separate housings, facing each other. The target is detected when it breaks the beam. This is the most reliable mode by a wide margin: the receiver sees a strong, direct signal, so dust, fog, mist and a dirty lens all have to degrade it a long way before detection fails. It is also the only mode that reliably detects shiny, dark, transparent and irregularly shaped objects, because it does not depend on anything being reflected back.

The cost is installation. Two housings, two mounting brackets, and cable to both sides.

Retro-reflective puts the emitter and receiver in one housing and a reflector opposite. The beam travels out, bounces off the reflector and returns. Detection occurs when the target interrupts it. You get most of the range of a through-beam arrangement with only one electrical connection to make — the reflector needs mounting but not wiring.

The limitation is shiny targets. A polished or reflective object can bounce enough light back to look like the reflector, and the sensor misses it.

Diffuse reflective puts everything in one housing with no reflector at all. The beam reflects off the target itself. Simplest to install, shortest range, and most sensitive to the target surface — a matt black object reflects very little and is detected at a fraction of the distance a white one is. Background objects within range can also trigger it.

The D31C range

Diffuse reflective

• M18 — 100 mm and 300 mm
• M30 — 500 mm, 1000 mm, 2500 mm and 3000 mm

Retro-reflective

• M18 — 1000 mm, 2000 mm, 3000 mm and 5000 mm
• M30 — 10000 mm, 15000 mm and 20000 mm

Through-beam

• M18 — 1000 mm, 3000 mm, 5000 mm, 10000 mm and 15000 mm

Every variant is available in all four output combinations, three-wire, 200 mA, short-circuit protected.

Reading the ordering code

A typical part number:

D31C-18TB-15000-PL-2S

• D31C — photoelectric switch series
• 18 — M18 body (or 30 for M30)
• TB — sensing mode: TB through-beam, RR retro-reflective, DR diffuse reflective
• 15000 — sensing range in millimetres
• P — output type: P for PNP, N for NPN
• L — output logic: L for light-on, D for dark-on
• 2S — short-circuit protected

So D31C-30RR-20000-ND-2S is an M30 retro-reflective unit with 20 metre range, NPN output, dark-on, short-circuit protected.

Light-on or dark-on

This catches people out because the intuitive answer is often the wrong one.

Light-on switches the output when the receiver sees light. Dark-on switches when it does not.

For a through-beam or retro-reflective sensor detecting an object passing through the beam, dark-on gives you an output when the object is present — which is usually what the control logic wants. For a diffuse sensor, light-on gives you an output when the object is present, because the object is what returns the light.

There is also a fail-safe consideration. With through-beam set to dark-on, a failed emitter, a cut cable or a power loss all produce the same state as object present. If that state stops the machine, the failure is safe. If it starts something, it is not. Think this through for the specific function rather than accepting the default.

Choosing the mode for your application

• Transparent objects — through-beam, with sensitivity adjusted. Glass and clear film are close to invisible to diffuse sensors.
• Shiny or polished metal — through-beam. Retro-reflective will miss it.
• Dark or matt targets — through-beam or retro-reflective. Diffuse ranges collapse on dark surfaces.
• Dusty environments — through-beam, and specify more range than the geometry needs so there is margin as the lenses foul.
• Long distances — retro-reflective in M30 reaches 20 metres, the longest in the range.
• Limited access on one side — retro-reflective or diffuse, since only one side needs wiring.
• Short-range presence detection with easy access — diffuse, the simplest and cheapest to install.

Specifying with margin

The quoted range is measured under clean conditions with a standard target. In a working plant, lenses accumulate dust and reflectors get knocked. Specify a sensor whose rated range comfortably exceeds the actual distance, so there is headroom to lose before detection becomes marginal.

A 3 metre gap served by a 3 metre sensor will work at commissioning and become intermittent by the second monsoon. A 5 metre sensor on the same gap will not.

Ordering

Send us the distance, the target — material, colour, surface finish, size, whether it is transparent — the environment, whether both sides of the gap are accessible, and the output your controller expects. We will specify the mode and return the part number. Our photoelectric switch range is manufactured in Mumbai, and non-standard cable lengths are available on request.

If the distance is short and the target is metal, an inductive switch from the D30C range will usually be cheaper and more robust — worth checking before defaulting to photoelectric.