TL;DR: An inductive proximity sensor detects metallic targets only — it ignores plastic, cardboard, and liquids completely. A capacitive proximity sensor detects any material with a dielectric constant greater than air: metal, plastic, glass, liquids, and granular solids. Both are 3-wire sensors using the same NPN/PNP wiring convention. Choose inductive for metal-only detection in harsh environments; choose capacitive when the target is non-metallic or when you need to sense through a container wall.
The most common selection mistake in discrete sensor applications is reaching for a proximity sensor without knowing which type to buy. The confusion is understandable — both sensors look similar (M12 or M18 or M30 cylindrical bodies), both use 3-wire cables, both output a switched discrete signal to the PLC. The difference is entirely in the physics of detection — and that difference dictates every application decision.
How Inductive Proximity Sensors Work
An inductive proximity sensor contains a coil and oscillator circuit behind the sensing face. The oscillator generates a high-frequency electromagnetic field that projects from the face. When a conductive (metallic) target enters this field, eddy currents are induced in the metal. These eddy currents absorb energy from the oscillator, damping its amplitude. The sensor's electronics detect the amplitude change and switch the output.
Because the operating principle requires eddy current induction, the target must be electrically conductive. Ferrous metals (mild steel, cast iron) give the best response. Non-ferrous metals (aluminium, stainless steel, copper) give 30–80% of the rated range. Non-metallic objects — plastic, glass, wood, cardboard, water — produce zero eddy currents and are completely invisible to an inductive sensor.
The sensing face is typically marked as shielded (flush) or unshielded (non-flush):
- Shielded: the electromagnetic field is contained within the sensor face. Can be flush-mounted in a metal bracket without false triggering from the bracket itself. Shorter range.
- Unshielded: the field projects beyond the sensor body. Longer range, but requires a metal-free zone around the sensing face (the "exclusion zone" in the datasheet).
How Capacitive Proximity Sensors Work
A capacitive proximity sensor contains two internal electrodes connected to a high-frequency oscillator. The electrodes form a capacitor with the material in front of the face — the capacitance depends on the dielectric constant of that material. Air has a dielectric constant (εr) of approximately 1.0. Any solid or liquid has a higher dielectric constant: water ≈ 80, most plastics ≈ 2–5, glass ≈ 6, grain/powder ≈ 2–4.
When a target enters the sensing field, the increased dielectric constant raises the capacitance. The oscillator detects the change and switches the output. Because the principle works on dielectric change rather than conductivity, any material above air dielectric will trigger the sensor — metal, plastic, liquid, or granular.
A key feature unique to capacitive sensors is the sensitivity adjustment potentiometer on the body. This lets you tune the detection threshold for your specific target material and container. You typically turn sensitivity down to just above the point where the sensor ignores the empty container wall, so only the material inside triggers it.
Side-by-Side Comparison
| Inductive | Capacitive | |
|---|---|---|
| Detection principle | Eddy current damping | Dielectric constant change |
| Target materials | Metallic conductors only | Any material (metal, plastic, liquid, powder) |
| Detects through walls? | No | Yes (thin non-metallic walls) |
| Typical sensing range | 1–20 mm (depends on material) | 2–25 mm (depends on sensitivity setting) |
| Sensitivity adjustment | None | Potentiometer on body |
| Best for | Metal part detection in oily/coolant environments | Non-metallic targets, level detection through tank wall |
| Environmental resistance | Excellent (IP67–IP69K, immune to coolant/oil) | Good, but surface contamination can cause false triggers |
| Output types | NPN / PNP, NO / NC | NPN / PNP, NO / NC |
| Relative cost | Lower | Slightly higher |
NPN vs PNP Wiring — Both Sensor Types
Both inductive and capacitive sensors use the same 3-wire connection:
- Brown wire: +24 V DC supply
- Blue wire: 0 V / common
- Black wire: signal output
The output type — NPN or PNP — determines which direction the signal wire switches:
NPN (current sinking, open collector to 0V): The signal wire pulls LOW (to 0V) when the sensor detects a target. The PLC input must be sourcing type (PNP input, positive common). NPN sensors are common in equipment made in Asia.
PNP (current sourcing, switches to +24V): The signal wire pulls HIGH (to +24 V) when the sensor detects a target. The PLC input must be sinking type (NPN input, 0V common). PNP sensors are standard in European and North American practice.
Most PLC input modules work with PNP sensors directly. If you have an NPN sensor and a PNP-input PLC module, you need a signal converter or to swap to a PNP sensor — mixing NPN and PNP is the most common wiring mistake on the factory floor.
The interactive capacitive proximity sensor and inductive proximity sensor animations show the NPN output switching in real time — watch the signal wire go low when the target enters range.
When to Use Inductive
- Detecting whether a metal part is seated in a fixture before a press, robot, or clamp operates
- Position feedback on cylinders, slides, or cam lobes (all metallic)
- Counting metal objects on a conveyor (bottle caps, nuts, metal stampings)
- Environments with cutting fluid, coolant, oil mist — inductive sensors are generally more resistant to surface contamination than capacitive (no sensitivity drift from liquid on the face)
- Applications where you must not detect plastic fixtures or cardboard packaging accidentally
When to Use Capacitive
- Detecting plastic bottles, bags, or cartons that an inductive sensor cannot see
- Level sensing through a tank wall — mount the sensor against the outside of a plastic or glass vessel; the dielectric change when liquid is present inside triggers the sensor without penetrating the vessel
- Granular material in hoppers, chutes, or silos (grain, pellets, powder)
- Liquid level in a pipe or vessel without a probe inserted into the process
- Applications where the target material varies and you need a sensor that works on everything
Common Application Examples
Beverage filling line: A capacitive sensor mounted outside the PET bottle confirms the bottle is filled. The water inside (εr ≈ 80) is detected through the 2 mm bottle wall. An inductive sensor on the same line detects whether the metal bottle cap is present after capping — the two sensor types complement each other in the same machine.
Injection moulding machine: Inductive sensors detect the position of the metallic mould carrier. Capacitive sensors detect whether plastic material is present in the hopper, giving a low-material alarm before the machine runs dry.
Conveyor sorting: An inductive sensor detects metallic components; a capacitive sensor detects all components (metallic and non-metallic) — both signals feed PLC inputs for a two-stage sorting gate.
Frequently Asked Questions
Q: Can a capacitive sensor detect metal?
A: Yes. Metal has a high dielectric constant and is a very good conductor, so a capacitive sensor will detect metallic targets reliably. However, an inductive sensor is better for metal-only detection in oily environments because it is immune to surface contamination and does not require sensitivity adjustment. Use capacitive only when you also need to detect non-metallic targets.
Q: Can an inductive sensor detect aluminium or stainless steel?
A: Yes, but with reduced range. The datasheet correction factor for aluminium is typically 0.35–0.45 (so a sensor with a 10 mm nominal range has about 3.5–4.5 mm effective range on aluminium). Stainless steel is 0.7–0.85. The nominal sensing range is always specified for mild steel — read the correction table for the actual target material.
Q: My capacitive sensor keeps triggering falsely. What is wrong?
A: The three most common causes are: (1) sensitivity set too high — back off the potentiometer until the sensor ignores background objects; (2) condensation or liquid on the sensing face — the film changes the dielectric and causes false triggers; (3) ground loops in the cable shield — if you are running cable in a conduit with other signals, ensure shields are grounded at one end only. A fourth cause is proximity to another capacitive sensor — adjacent sensors can crosstalk; increase the spacing to at least 3× the sensing range.
Q: What does the correction factor on an inductive sensor datasheet mean?
A: The nominal sensing range (Sn) is measured with a mild steel square target. The correction factor multiplies that range for other metals: stainless steel 0.7–0.85, brass 0.35–0.50, aluminium 0.35–0.45. If your actual target is not mild steel, multiply Sn by the correction factor to get the effective range for your application — and add a 20% margin so vibration and mounting tolerance do not push the target outside detection range.
Practice connecting discrete sensors in PLC ladder logic with the free motor start-stop scenario — it models sensor inputs exactly as you would wire them on the machine.
Explore the interactive animations for both sensor types:
