What is a sensor? From physical conditions to usable signals

A sensor detects a physical condition and turns it into information that an electrical or electronic system can read. That condition might be temperature, pressure, light, distance or the presence of a part. Some sensors answer a simple “Is it there?” question. Others report “How much?” as a measured value.

On a production line, a photoelectric sensor can detect an arriving carton, a temperature probe can monitor a heated tool, and an encoder can track a shaft. A PLC or other controller uses these signals to decide what the machine should do. Sensing, decision-making and actuation are separate jobs, even when a compact device combines several of them.

Detection and measurement solve different problems

A switching sensor changes state when a condition is met. A measuring sensor reports a value over a specified range. A proximity sensor therefore does not necessarily tell you the distance to a target; many only indicate presence. Some devices provide a measured value and a separate threshold output. Start with the information your controller needs, then compare the sensing technologies that can provide it.

Read the output specification before connecting anything

Discrete outputs: These provide an on/off state. Normally open and normally closed describe the switching behaviour under normal powered operation: an NO output activates when the target is detected, whereas an NC output deactivates. This does not establish what will happen after loss of power or a fault.

PNP and NPN: On common three-wire DC sensors, PNP outputs source current from the positive supply and NPN outputs sink current toward 0 V. The PNP or NPN choice must match the receiving input circuit. PNP/NPN and NO/NC describe different properties: a PNP sensor can have an NO or NC output. Two-wire sensors and NAMUR devices need their own specified wiring and interface arrangements.

Analogue outputs: Signals such as 4–20 mA and 0–10 V represent a measured value. A linear 4–20 mA pressure transmitter configured for 0–10 bar gives 5 bar at 12 mA. The receiving channel must support the signal and use the correct scaling. A live zero of 4 mA helps distinguish a valid zero reading from some fault conditions, but fault interpretation still depends on the device specification.

Digital communication: An IO-Link device can exchange measurement values, parameters and diagnostic information with a compatible master. Digital does not simply mean one bit: a digital interface can carry a numerical reading as well as device status. Accessing IO-Link communication requires a suitable IO-Link master; a standard switching input cannot provide that functionality.

A sensing element is not always a complete industrial sensor

Many industrial sensors package a sensing element, signal-conditioning electronics and an output stage in one housing. A bare element may behave quite differently. A Pt100 changes resistance, while a thermocouple produces a small voltage related to a temperature difference. Neither should be treated as a generic 24 V switching sensor; it needs a suitable measurement input or signal converter.

A transmitter conditions a measured signal and sends it through a defined interface. For example, a pressure transmitter may provide a 4–20 mA output. The controller reads that signal, and a SCADA system can display and log the resulting process value. The terminology overlaps in product catalogues, so the datasheet and output specification matter more than the name on the box.

Common sensor technologies

Inductive: An inductive sensor detects metal targets through changes in its electromagnetic field. Typical jobs include part detection, position confirmation and gear-tooth sensing. It will not normally detect plastic or wood directly. Target material, size and mounting conditions affect the usable sensing distance; equal range on every metal is a product feature, not a universal rule.

Capacitive: A capacitive sensor responds to changes in capacitance and can detect suitable metals, plastics, powders, granules and liquids. Some applications allow detection through a non-metallic container wall. Wall thickness, material properties and sensitivity settings determine whether this works reliably. Moisture and deposits on the sensing face can also change the response.

Photoelectric: A photoelectric sensor detects changes in received light. Through-beam sensors use a separate emitter and receiver; retro-reflective sensors use a reflector; diffuse sensors receive light returned by the target itself. Transparent objects, dark surfaces and shiny backgrounds require particular care. Fibre-optic sensors can bring the sensing point into a confined space.

Ultrasonic and laser: An ultrasonic sensor uses sound pulses and echoes; suitable models detect objects or measure distance and level. Check the blind zone, target angle and sound-absorbing surfaces. A laser sensor uses light and may suit small targets or precise positioning. A laser light source does not automatically mean the device provides a distance measurement.

Temperature: Temperature sensing includes RTDs, thermocouples, thermistors and infrared devices. Pt100 is an RTD; an NTC thermistor decreases in resistance as temperature rises. Probe construction, response time, process compatibility and the receiving instrument all affect selection. The Pt100 versus thermocouple comparison explains why these technologies are not interchangeable.

Pressure, flow and level: A pressure sensor measures gas or liquid pressure, while a flow meter measures flow. Level instruments either report a continuous level or detect a selected point. These are different process variables. Hydrostatic pressure can be used to calculate liquid level, but the conversion also depends on liquid density.

Position, motion and magnetic field: Encoders provide position or movement information, from which speed may also be calculated. An accelerometer measures acceleration, a gyroscope measures angular rate, and a magnetometer measures magnetic field. Hall sensors detect magnetic fields and can form part of a current-sensing system. A magnetometer is not, by itself, a direct motion sensor.

Other applications: Humidity sensors, gas detectors, load cells, torque sensors, light sensors and microphones cover other measurement needs. Biosensors combine biological recognition with a readable response. Gas detection requires the right target gas, range and cross-sensitivity characteristics; a single general-purpose device cannot be assumed to measure every gas.

What makes a sensor suitable for the application?

Define the target or process variable, working range, required response time and available input interface. Then check supply voltage, wiring, target size and mounting clearance. Matching the body diameter is insufficient. The choice between a fixed cable and a connector also affects replacement and maintenance.

Accuracy, resolution and repeatability are different specifications. A display with extra decimal places does not prove a more accurate measurement. For fast-moving parts, check the counting frequency, input filtering and PLC processing as well as the sensor response. Measurement applications may also require a defined calibration schedule.

Evaluate temperature, vibration, washdown, dust, chemicals and cable movement. An IP rating addresses ingress protection; it does not establish compatibility with every chemical or approval for hazardous areas. A personnel-protection function requires an appropriate safety sensor and an assessed safety system, rather than an ordinary process sensor chosen only for its sensing range.

Troubleshooting: follow the signal from the target to the controller

The LED changes, but the PLC does not: A local indicator may show power or detection without proving that the input receives a valid signal. Check output type, input common, pin assignments, cable continuity and channel configuration. The manufacturer's connection diagram takes precedence over assumptions based on wire colour.

Detection is intermittent: Check alignment, target distance, mounting movement, dirty optics and moisture or material buildup. With unstable measurements, also examine the supply, connections, cable routing, shielding and scaling. Hysteresis separates the activation and deactivation thresholds to reduce chatter near a switching point; excessive hysteresis can reduce the precision required by the application.