Photoelectric Sensor with Background Suppression

A photoelectric sensor detects an object by putting light across the space where the object will be and watching for the light that comes back or fails to arrive. On a production line that makes it the sensor for part-present checking, counting, end-of-travel detection and guard positions, and it is usually the simplest way to answer a yes-or-no question about position.

The sensors on this page are rectangular units with a red LED source, in three sensing forms: diffuse with background suppression, through-beam, and retro-reflective. Ranges run from 100 mm on the smallest diffuse model to 15 m on the through-beam pair. Outputs are NPN or PNP open collector, the response time is 1 ms or less, and the housing is sealed to IP67, which is what allows one to be mounted on a machine rather than inside an enclosure.

Background suppression is the feature that decides whether the sensor can be used on a mixed line. It sets the switching point by where the object is rather than by how much light it returns, so a black part and a white part are detected at the same setting and a machine frame behind them does not trigger the sensor.

How background suppression works

A diffuse sensor without background suppression measures how much light comes back. A white target further away and a grey target closer can return the same amount, and a bright metal frame behind the target can return more than the target does, so the sensor switches on whatever is brightest rather than on what is nearest. The usual fix is to move the sensor closer or to shade the background, both of which change the machine rather than the setting.

A sensor with background suppression measures where the light comes from instead. The emitter and the receiver look at the target from slightly different angles, so the position of the returning spot on the receiver depends on the distance to the target. The sensor compares the two signals and switches at a distance it has been set to, which is independent of how reflective the target is. That is why the same setting detects a black matt part and a white glossy one, and why an object behind the switching point is ignored even if it reflects strongly.

The three sensing forms differ in what they put into the gap. A diffuse sensor works alone. A retro-reflective sensor sends a beam to a reflector and detects the object when the beam is broken, which needs a reflector opposite but only one cable. A through-beam pair puts the emitter and the receiver at opposite ends of a span and detects when the beam between them is broken, which gives the longest range and the most reliable detection, at the cost of wiring both ends.

How to select the right sensor

Start with the distance between the sensor and the part. That single figure separates the models more than any other: the diffuse models cover 100, 300, 500 and 900 mm, the line-beam model covers 125 mm, the retro-reflective reaches 6 m and the through-beam pair reaches 15 m.

Then decide the form from the geometry. A sensor looking down at a part on a conveyor uses a diffuse model and needs nothing on the far side. A sensor looking across a roller bed where a part is not tall enough for a reliable reflection uses a through-beam pair. A sensor that must span several metres with only one cable run uses the retro-reflective form with a reflector at the far end.

Then match the output to the controller. An NPN output sinks current and suits a sinking input; a PNP output sources current and suits a sourcing input. The maximum output current is 100 mA, which drives a controller input and not a lamp or a valve.

Finally consider the environment. IP67 covers splash and dust but not continuous immersion or high-pressure washing. Ambient light up to 3,000 lx from an incandescent source is tolerated, so direct sunlight or a high-power lamp on the sensing face is a case for shading or for changing the mounting angle. The sensitivity adjustment is a knob on the diffuse models and a push-button on the others, which matters where the setting has to be altered during changeover rather than at commissioning.

Limitations and points to confirm

The sensing distances for the diffuse models are quoted against a white matte target at 90% reflectance. A dark, rough or tilted target returns less light, so the usable distance is shorter than the published one and the practical setting should leave margin. The other direction of error is a glossy target, which can return enough light from an angle to be detected outside the intended switching point.

Two diffuse variants appear in the per-model drawings of the supplied material but not in the range table or the summary table, so their availability and their complete specification are open. They are named on this page with that noted rather than left out, because a buyer who has seen the drawings would otherwise have no way to place them.

The spot diameters are published with the working distance at which each was measured, and those distances differ between models. The figures cannot be compared directly: a small spot measured at 300 mm and a larger spot measured at 50 mm describe different things about how tightly the beam is focused.

Finally, the material does not state whether the output action can be changed in the field or is fixed when the unit is ordered. For a line where the same sensor is used in both an open and a closed arrangement, that is worth confirming before the order rather than at commissioning.