Capacitive Sensors for Plastic, Powder and Non-Metal Detection

An inductive proximity switch sees metal and nothing else. That is not a limitation, it is the operating principle — it works by inducing eddy currents in a conductive target. Present it with a plastic bottle, a paper carton, a wooden pallet or a bin of powder and it will sit there indifferent.

Capacitive proximity switches solve that problem. They respond to a change in the dielectric in front of the sensing face, which means they detect essentially anything: plastic, liquid, powder, granulate, wood, glass, cardboard, and metal too.

How the sensing actually works

The sensing face forms one plate of a capacitor. The surroundings form the other. When a material enters the sensing field, the dielectric constant of the space in front of the face changes, the capacitance changes, and an internal oscillator circuit detects the shift and switches the output.

Two consequences follow, and they explain most of what is useful and most of what goes wrong.

Materials with a high dielectric constant are detected more easily. Water is detected extremely well. Metals behave as if they have an effectively infinite dielectric constant and are detected at the full rated distance. Dry plastics and powders have low dielectric constants and are detected at a reduced distance. This is why a sensor that detects a full bottle reliably may not detect an empty one.

The sensor cannot inherently distinguish target from contamination. A film of water, a dust build-up or a layer of product on the sensing face changes the dielectric just as a target does. This is the dominant failure mode in the field and it is a design problem rather than a device problem.

Sensing through a container wall

This is the application capacitive switches are genuinely unmatched at, and the one that brings most enquiries to us.

Mount the sensor on the outside of a non-metallic tank, hopper or pipe and set the sensitivity so that the wall alone does not trigger it but wall plus contents does. You then have a level switch with no penetration, no seal, no wetted part and nothing inside the vessel to clean. For food, pharmaceutical and chemical duty that is a substantial advantage.

What determines whether it will work:

• Wall thickness and material. The wall consumes part of the sensing range. A thin polypropylene wall costs little; a thick glass-reinforced one may consume most of it.
• The dielectric contrast between full and empty. Water against air is easy. Dry granulate against air is harder and needs care.
• Whether product clings to the inside of the wall. If it does, the sensor reads full permanently. This is the single most common reason a through-wall installation fails, and it must be assessed before ordering rather than after.

Our note on capacitive proximity sensors for level detection goes into the level application in more depth.

Where capacitive switches earn their place

• Packaging lines — detecting cartons, bottles, film and labels that inductive sensors cannot see.
• Level in non-metallic vessels — high and low level switching without breaching the tank.
• Powder and granulate handling — hoppers, silos, feed chutes and blocked-chute detection.
• Plastics processing — presence of mouldings, sprue detection, bin-full signalling.
• Timber and panel handling — board presence and edge detection.
• Glass handling — where optical sensing is defeated by transparency.

The Accent capacitive range

Our capacitive proximity switches are M30 bodies with 10 mm sensing range, available across the full output matrix. The ordering code is straightforward once you know the pattern:

• D32C-3010-NM-3S — NPN, normally open, 3-wire, 300 mA, short-circuit protected
• D32C-3010-NB-3S — NPN, normally closed, short-circuit protected
• D32C-3010-PM-3S — PNP, normally open, short-circuit protected
• D32C-3010-PB-3S — PNP, normally closed, short-circuit protected

The same four are available without short-circuit protection as the -3 suffix rather than -3S. In almost every case the protected version is the right choice — the price difference is small and a shorted output on an unprotected sensor destroys it.

Reading the code: N or P selects NPN or PNP, M or B selects normally open (make) or normally closed (break), and 3S indicates 3-wire with short-circuit protection. If you are unsure whether your input card wants NPN or PNP, our guide to PNP and NPN sensors covers it.

Two mistakes worth avoiding

Setting the sensitivity at commissioning and never revisiting it. A capacitive sensor adjusted on a clean, dry morning will behave differently in humid weather and differently again with six months of product film on the face. Set it with margin, and specify a mounting position where the face can be wiped.

Mounting it where splash or condensation lands on the face. Water has a very high dielectric constant. A sensor that gets splashed will trigger on the splash. Shield it, angle it downward, or move it.

Specifying one

Tell us the target material, whether you are sensing it directly or through a wall, the wall material and thickness if so, the required sensing distance, whether the target is stationary or moving and how fast, the output your controller needs, and the environment — washdown, dust, temperature. If the target is a powder or granulate, the bulk density is worth including.