Proximity Sensors for Conveyor Systems: Position, Jams and Belt Monitoring

Conveyor sensing looks like one problem and is really two, with almost opposite requirements. Detecting the product on the belt is a precision job close to the material. Protecting the conveyor itself is a reliability job that has to work for years without attention. Specifying both the same way is the usual reason one of them keeps failing.

Job one: detecting the product

Counting, presence, gap detection, triggering a diverter or a reject. What matters here is the target.

If the product is metal, an inductive sensor is the cheapest and most robust answer. It ignores dust between itself and the target, which matters on an open conveyor.

If the product is not metal, which covers cartons, bottles, bags, plastics and timber, you need capacitive at short range or photoelectric at longer range. Photoelectric in through-beam mode is the reliable choice across a wide belt, because it does not depend on the product reflecting anything back. Diffuse mode on a dark or matt product is where most intermittent counting faults come from.

If the belt is wide, the distance decides the technology. Inductive stops being an option past about 30 mm. Our note on extended-range inductive sensing covers where that boundary sits and when to stop pushing it.

If the line is fast, check switching frequency against the actual product rate, and check that the controller input can keep up. Counts that are correct at low speed and drift low as the line speeds up are almost always the input scan rate rather than the sensor.

Job two: protecting the conveyor

This is the higher-value sensing, because a conveyor failure costs far more than a miscount.

Underspeed and belt slip. A drive turning while the belt is not is the classic conveyor failure. It wears the drive pulley, and in dusty plant the friction is a genuine fire risk. Detection is a magnetic or inductive sensor reading a toothed wheel or bolt heads on the tail pulley shaft, compared against the drive. Our guide to speed and underspeed monitoring covers target geometry and the controller requirements, which matter more than the sensor choice.

Belt drift and tracking. A belt running off to one side damages itself and the structure. Sensors either side of the belt edge detect it before it becomes expensive.

Blocked chute and transfer point detection. Where material backs up at a transfer, early detection prevents spillage and a cleanup. This is among the harshest positions on the whole conveyor: material flow, impact and dust all at once. Mount with standoff and protect mechanically.

Pull cord and guard status. Safety-relevant, and specified normally closed so a severed cable produces the stopped state.

What the environment does

Dust is the main enemy for optical sensing. Lenses foul, and in cement or grain handling they foul quickly. Where photoelectric is unavoidable, use through-beam, specify considerably more range than the gap requires so there is margin to lose as the lens dirties, and plan cleaning access. Inductive and magnetic sensing largely ignore dust, which is why they dominate on conveyor protection.

Vibration loosens mountings. A sensor that drifts out of position reads intermittently and gets blamed on electronics. Use locknuts both sides and check alignment as routine maintenance rather than after failure.

Impact happens. Material falls off belts. A guard bracket on any sensor in the drop zone costs very little.

Long cable runs. Conveyor galleries are long, and a sensor can be well over a hundred metres from the panel. Check the supply voltage actually reaching the sensor, use screened cable earthed at one end, and keep signal cable away from drive cabling. Where an analogue signal is involved over that distance, specify a current output rather than voltage.

Where this comes up

The protection requirements are broadly the same whether the conveyor is moving packages, ore, clinker, bagasse or grain. The product detection requirements change completely. Our industry guides cover the specifics for cement, sugar, steel and packaging.

Specifying

Tell us which of the two jobs you are solving, the product material and size, the belt width and speed, the mounting distance available, the environment, and what the output feeds. If this is underspeed protection rather than monitoring, say so, because the fail-safe requirement changes the specification.