Inductive, capacitive, photoelectric, magnetic, and ultrasonic proximity sensors are the five families that handle the bulk of non-contact detection in factory automation, with each family tied to a specific physical principle: high-frequency magnetic field, capacitance change, light beam, magnetic field, or acoustic reflection [S1][S2].
Selection is driven by four binding constraints in this order: target material, required sensing distance, output interface (NPN/PNP/analog/IO-Link), and the surrounding environment (dust, oil, washdown, temperature) [S1][S3]. Checking the target material first is critical because inductive sensors detect only metallic targets and are effectively blind to non-metallic materials such as plastic [S3].
Inductive Sensors: The Metal-Only Default
Inductive proximity sensors detect ferrous and non-ferrous metals through eddy-current losses in an oscillator coil, with ferrous metals (steel, iron) hitting 100% of rated range and non-ferrous metals (aluminum, brass, copper) dropping to roughly 60-80% of the rated value [S3]. The standard target is mild steel; spec sheets derate for aluminum and brass, and any thin plate below 1 mm further reduces effective range [S3].
Typical sensing distance runs 1-50 mm shielded and 2-80 mm unshielded, with switching frequency up to 3 kHz and operating temperature commonly -25 to +70 deg C [S3]. Standard industrial supply is 10-30 V DC with three-wire PNP or NPN outputs, making them the default pick for machine-tool, conveyor, and end-of-stroke position sensing on metal targets [S1][S3]. For a deeper primer on the family, see the proximity sensor overview.
Capacitive Sensors: For Plastics, Liquids, and Bulk Solids
Capacitive proximity sensors detect any material with a dielectric constant above air, including plastics, glass, wood, paper, liquids, powders, and metals, by forming a capacitor with the target and sensing the resulting frequency shift [S1][S3]. They are the only mainstream technology that sees through a tank wall to a liquid level or counts molded plastic parts on a belt, but they are also the most prone to humidity and dust drift [S1][S3].
Typical sensing distance is 2-30 mm shielded, 4-60 mm unshielded, with switching frequency around 10-300 Hz and a wider drift window of about +/-15% across temperature and humidity swings [S3]. Most capacitive units ship with a sensitivity trimpot or teach button, which must be tuned after mounting in non-metallic surfaces or when the target dielectric changes [S3]. The full feature breakdown lives in the capacitive sensor reference.
Photoelectric, Magnetic, and Ultrasonic: The Long-Range and Specialty Trio
Photoelectric sensors use through-beam, retro-reflective, or diffuse-reflective light configurations and are the right call when sensing distance exceeds roughly 80 mm, up to 10 m on through-beam pairs, and when the target may be transparent, colored, or irregular [S1][S2]. They outperform inductive and capacitive units on packaging lines, automatic doors, and long conveyor spans, but suffer from dust buildup on the lens and from highly reflective or transparent targets unless a specific optical mode is chosen [S1].
Magnetic proximity sensors detect a magnetic field from a permanent magnet on the moving part, giving a long mechanical life and tolerance to dust, oil, and water; they are the standard cylinder-end-position sensor on pneumatic actuators and a common limit-switch replacement [S1][S4]. Ultrasonic sensors use 25-200 kHz acoustic waves and detect distance independent of color and transparency, including clear glass and liquid surfaces, with ranges typically 0.1-6 m, which is why they dominate bin-level and roll-diameter measurement where optical sensors fail [S5]. For inductive long-reach variants, the inductive sensor page covers shielded vs unshielded mounting tradeoffs.
Selection Criteria: Target, Distance, Output, Environment
Target material is the first filter: metal-only points to inductive, any non-metal to capacitive or ultrasonic, magnet-equipped parts to magnetic, and long-range or color-agnostic detection to photoelectric [S1][S3][S5]. The second filter is sensing distance: spec the sensor at 1.25 to 1.5 times the worst-case mechanical gap, which absorbs thermal expansion, target position tolerance, and supply-voltage sag, and prevents marginal triggering at end of life [S3].
Output type and environment close out the decision: three-wire PNP is the dominant 24 V DC industrial interface, NPN remains common in Asian-built machinery, analog 0-10 V or 4-20 mA outputs are used for distance measurement, and IO-Link is now the standard upgrade path for smart sensors with diagnostic data [S1][S2]. Environmentally, an IP67 rating covers most washdown, IP69K is required for high-pressure cleaning in food and beverage, and stainless-steel housings with PTFE faces are specified for chemical-exposure zones, all of which can push a sensor from a $20 unit to a $200 unit with the same 8 mm range [S2][S4].
Inductive vs Capacitive vs Photoelectric vs Magnetic vs Ultrasonic
On detection target, inductive is metal only, capacitive and ultrasonic cover metal and non-metal, photoelectric covers most opaque and many transparent objects, and magnetic requires an actuator magnet [S1][S3][S5]. On typical range, inductive sits at 1-80 mm, capacitive at 2-60 mm, magnetic at 2-80 mm (set by magnet strength), photoelectric at 0.1-10 m, and ultrasonic at 0.1-6 m [S1][S3][S5].
On switching speed, inductive leads at up to 3 kHz, photoelectric sits at 500 Hz to 1 kHz depending on mode, capacitive at 10-300 Hz, magnetic at up to 5 kHz, and ultrasonic at 1-50 Hz, which is the binding constraint for high-speed counting [S3][S5]. On environmental weakness, inductive is upset by strong AC fields, capacitive by moisture and dust, magnetic by loss of magnet magnetization above roughly 80-150 deg C, photoelectric by lens contamination, and ultrasonic by temperature gradients and soft acoustic absorbers like foam [S1][S3][S5]. For proximity-style measurement, the displacement sensor family overlaps with long-range photoelectric and ultrasonic units when a continuous analog output is needed.
Who Should Not Pick the Mainstream Inductive Default
If the target is plastic, glass, paper, wood, liquid, or any powder, an inductive sensor will simply not detect it, and forcing it to work by bolting a metal flag to the target adds cost, mass, and a wear point [S1][S3]. In washdown or outdoor environments with heavy condensation, a capacitive sensor set to its maximum sensitivity will false-trigger on water film, and a derated unit or a shielded-mount inductive unit is the safer call [S3][S4].
For high-cycle end-of-stroke duty on a moving mass, a limit switch is often the correct answer instead of a proximity sensor: mechanical limit switches carry higher contact ratings, give a positive mechanical stop reference, and let a maintenance tech see the failure with the power off, while a proximity sensor leaves no visible wear and requires a tester to diagnose [S4]. The decision tree is target material first, distance second, output third, environment fourth, and only then price; reversing that order is how plants end up with photoelectric sensors covered in dust or capacitive sensors triggered by humidity [S1][S2].
Sourcing, Standards, and Shortlist Logic
Most industrial proximity sensors are specified to IEC 60947-5-2 for low-voltage switching devices and carry an IP rating per IEC 60529, with IP67 the typical floor for factory use and IP69K reserved for food, beverage, and pharmaceutical washdown [S1][S4]. Hazardous-area duty requires ATEX or IECEx certification with explicit zone marking on the nameplate, and the sensor datasheet must be cross-checked against the zone classification, not assumed from the housing [S1]. For chemical and offshore sites, also verify NACE MR0175 material compliance if the sensor wetted parts see sour service, and confirm the cable jacket and connector rating for the solvent list [S2].
The shortlist logic that survives plant audits: pick two candidate technologies, size the rated range at 1.5x the worst-case gap, lock the output type to the PLC card, and verify IP, temperature, and certification before price. For a process-plant detector that has to live outdoors near a pump skid, the pump procurement strategy spec walks through the same vendor-qualification discipline that catches a $20 sensor with the wrong cable jacket. Two trackable signals to watch in the next procurement cycle: a rising share of IO-Link smart sensors replacing plain PNP units, and a steady migration from M12 4-pin to M12 4-pin A-coded for sensor and A-coded for actuator as the connector standard consolidates across vendors [S1][S4].