Choosing a detection sensor: match the technology to the job

A metal gear tooth, a clear bottle and granules inside a plastic hopper present different sensing problems. The best choice depends on what must be detected, what surrounds it and how the signal will be used. A longer sensing range or a smaller housing is useful only when the complete application works reliably.

Begin with the required information. A proximity sensor may provide a simple presence signal; a measurement device may report distance or another value. These functions can share a housing, but one should not be assumed from the other. The sensor fundamentals guide explains this distinction.

Start with the application, then narrow the choices

Metal parts close to the sensing face: An inductive sensor is a useful first candidate. Test the actual alloy and smallest target, including gear teeth at maximum speed.

Product inside a non-metallic vessel: Consider a capacitive device suited to the contents and wall. Test empty, full and coated-wall conditions.

Clear packaging: Evaluate a photoelectric sensor designed for transparent objects, or ultrasonic sensing where the target geometry permits it. A general-purpose photo-eye is not automatically suitable.

Changing product colours against a conveyor: Compare background suppression with through-beam or retroreflective arrangements, depending on the available mounting positions.

Restricted space: A miniature head or fibre-optic sensing arrangement may solve the access problem. Keep the sensing principle and the installation constraints in the same decision.

Compare the main sensing methods

Ranges and speeds belong to individual models. A technology name does not specify a universal distance, response time or switching frequency. Compare datasheets under conditions that resemble the actual application.

Inductive sensing: check the metal and the mounting

An inductive sensor detects changes caused by a metal target in its electromagnetic field. In a common implementation, eddy currents affect the oscillator. Detection is not limited to iron: aluminium, copper and brass can also be detected. Their usable sensing distances may differ, so check the model's material reduction factors. Factor 1 designs reduce this variation for the metals specified by the manufacturer.

The catalogue rating refers to a defined test target. It does not impose a universal minimum target thickness of 1 mm on every application. Small targets may reduce the available distance. Also distinguish flush from non-flush mounting; nearby metal and adjacent sensors can affect operation. Leave the clearances required by the selected device.

Capacitive sensing: include the unwanted material in the test

A capacitive sensor responds to a change in capacitance around its sensing element. Suitable devices can detect liquids, granules, powders and other non-metallic materials. Detection through a non-metallic vessel wall can be useful, provided the wall, contents and installation are compatible.

The same sensitivity that detects the product can also respond to moisture, deposits or nearby objects. Set sensitivity or teach-in using representative empty and full conditions. When comparing capacitive sensor models, assess the unwanted conditions as carefully as the wanted target.

Photoelectric sensing: choose the light path

Photoelectric sensors detect a change in received light. The mounting arrangement determines whether the light is interrupted, returned by a reflector or reflected by the target. A laser sensor uses a particular light source; laser illumination alone does not define the sensing arrangement.

Through-beam: Separate emitter and receiver units face each other. An object reduces the light reaching the receiver. With opaque targets, surface colour generally matters less than it does in diffuse sensing. The installation needs access to both sides, suitable wiring and alignment. Transparent or very small objects require appropriate model selection.

Retroreflective: The emitter and receiver share one housing, while a reflector returns the beam from the opposite side. Retroreflective sensors avoid electrical wiring at the reflector. Check polarisation for shiny targets and dedicated clear-object detection features for transparent packaging. Use the specified reflector rather than assuming any reflective surface will work. The retroreflective sensing guide covers the operating principle.

Diffuse: A diffuse photoelectric sensor receives light reflected by the object itself. No opposite-side component is needed, but black, white, glossy and angled surfaces can produce very different returns. Background suppression (BGS) helps separate a nearby product from a more distant surface. Common triangulation designs use distance-related geometry, rather than brightness alone. This improves discrimination without making every target colour and finish irrelevant.

Ultrasonic sensing: distinguish an echo from a blocked echo

In a common reflective measurement arrangement, an ultrasonic sensor evaluates the time taken for a sound pulse to return from the target. Optical colour and transparency are less significant than echo quality. Check the blind zone, operating range and target geometry. Angled surfaces and some sound-absorbing foams or textiles can make direct echo measurement difficult.

A reflective ultrasonic barrier works differently. It monitors an expected echo from a fixed background surface and detects a target that obscures that surface. This can be useful for sound-absorbing objects under suitable conditions. Detecting the loss of a background echo does not mean that the sensor can measure distance to every absorbing target. For multiple sensors, review mutual interference and any available synchronisation features.

Specify the electrical signal as well as the sensing method

The PLC or receiving device must accept the sensor output. PNP and NPN describe electrical output arrangements; NO and NC describe switching behaviour. On photoelectric devices, Light-ON activates the output with sufficient received light, while Dark-ON activates it when light falls. Relate this setting to target presence using the actual optical arrangement.

Review the interface requirements of two-wire sensors and NAMUR sensors separately. For continuous measurement, select a suitable analogue or digital measurement interface. The 4–20 mA and 0–10 V analogue sensor ranges provide examples of these outputs. Confirm the measurement span and engineering-unit scaling.

Check pulse duration before trusting a counting specification

A switching-frequency rating is only part of the timing requirement. In a simplified example, a target with an effective width of 10 mm moving at 1 m/s presents an approximately 10 ms detection opportunity: duration = width / speed. Sensor response, the input filter and the controller's signal capture method must suit that pulse. The gap between targets must also allow the output to reset.

Include the environment and maintenance in the specification

An IP rating does not establish resistance to every oil, chemical or washdown condition. Consider dirty optical surfaces, metal chips, capacitive deposits, temperature and vibration. Cable versus connector selection affects replacement and servicing. Compare sensor prices alongside brackets, reflectors, wiring and maintenance. Where personnel protection is required, select an appropriate safety sensing system.