Sensors & Instrumentation

The 4–20 mA current loop

The 4–20 mA current loop is the de-facto standard signal for process instrumentation, defined in IEC 60381-1. It transmits a measurement as a proportional current: 4 mA represents the bottom of the measurement range (0%), and 20 mA represents the top (100%). The 16 mA span corresponds to the full engineering range of the sensor.

Current is used instead of voltage for several practical reasons. First, current is constant around a loop — the same 4–20 mA flows through the transmitter, the wiring, and the PLC input module regardless of wire resistance (within loop compliance limits). This makes the signal immune to voltage drops caused by cable resistance, which is critical for long cable runs. Second, a broken wire produces 0 mA — a value impossible during normal operation — making open-circuit faults immediately detectable. Third, the 4 mA "live zero" allows the transmitter to be loop-powered (no separate supply wires needed) while still transmitting zero process value.

Two-wire, three-wire, and four-wire transmitters

A two-wire (loop-powered) transmitter draws its operating power from the loop itself. The supply voltage (typically 24 V DC) minus the transmitter voltage drop minus the burden resistor voltage drop must leave enough voltage to drive 20 mA through the loop. Minimum loop voltage is specified by the transmitter manufacturer (typically 10–12 V DC at 20 mA).

A three-wire transmitter has a separate power supply (usually 24 V DC) and a shared 0 V return, with the signal current flowing on a separate wire. It does not depend on the PLC input module to supply power, so it can be used with input modules that do not provide a loop supply.

A four-wire transmitter has fully separate power and signal circuits, providing maximum isolation. Typically used for high-accuracy instruments or when the transmitter output is a voltage signal converted to current by an external converter.

Temperature measurement: RTDs and thermocouples

Temperature is the most commonly measured process variable in industrial automation. The two dominant sensor types are resistance temperature detectors (RTDs) and thermocouples. Choosing between them requires understanding their operating principles, accuracy, range, and robustness.

RTDs (Resistance Temperature Detectors)

An RTD measures temperature by exploiting the predictable change in electrical resistance of a metal with temperature. The most common industrial RTD is the platinum PT100, with a nominal resistance of 100 Ω at 0 °C. The resistance-temperature relationship follows the IEC 60751 standard curve, with a temperature coefficient of approximately 0.385 Ω/°C (the "European curve", also called the DIN curve).

RTDs offer excellent accuracy (typically ±0.1 to ±0.5 °C) and long-term stability. Their main limitations are fragility (platinum elements are fine wires or films that can fail under vibration), cost (platinum is expensive), and limited temperature range (practical maximum around 600–850 °C for standard PT100 elements).

RTD input modules use a Wheatstone bridge or constant-current source to measure resistance. Two-wire, three-wire, and four-wire RTD connections are available; four-wire connections eliminate lead resistance error and are used for highest accuracy.

Thermocouples

A thermocouple consists of two dissimilar metal wires joined at one end. The junction voltage (the Seebeck effect) is proportional to the temperature difference between the measurement (hot) junction and a reference (cold) junction, which must be at a known temperature. Thermocouple input modules include cold-junction compensation circuitry to correct for the reference junction temperature.

Common thermocouple types include: Type K (chromel-alumel, –200 to +1260 °C, most common general-purpose type), Type J (iron-constantan, –40 to +750 °C, common in older US installations), and Type T (copper-constantan, –200 to +350 °C, good for cryogenic applications). Each type uses specific extension wire — mixing types introduces additional thermocouple junctions and measurement errors.

Proximity sensors

Proximity sensors detect the presence or absence of an object without physical contact. The three most common industrial types each suit different applications.

Inductive proximity sensors

Inductive sensors detect ferrous and non-ferrous metals by generating an electromagnetic field and detecting the eddy currents induced in a metal target. They are the workhorse of discrete position sensing in machine automation: detecting part presence, verifying cylinder positions, and counting metal parts on conveyors.

Key specifications: sensing range (the distance at which a standard target is reliably detected), output type (NPN or PNP, NO or NC), switching frequency (maximum pulses per second), and IP rating. Shielded sensors have reduced sensing range but can be flush-mounted in metal; unshielded sensors have greater range but require clearance around the sensing face.

Capacitive proximity sensors

Capacitive sensors detect any material — metallic and non-metallic — that changes the capacitance of an internal oscillator circuit. They are used for level detection (liquid, powder, granules) through non-metallic container walls, and for detecting non-metallic objects such as plastic parts, glass, and wood.

Photoelectric sensors

Photoelectric sensors use a light beam (typically infrared LED) and receiver to detect objects. Three common modes: through-beam (emitter and receiver in separate housings, most reliable and longest range), retroreflective (emitter and receiver in same housing, beam reflected by a prism reflector), and diffuse/proximity (emitter and receiver in same housing, beam reflected by the target). Photoelectrics can detect virtually any material at ranges from a few millimetres to tens of metres.

IO-Link

IO-Link (IEC 61131-9) is a point-to-point serial communication standard connecting sensors and actuators to an IO-Link master port. Unlike a fieldbus, IO-Link is not a network — each device connects to a dedicated port. Communication uses a standard 3-wire unshielded cable (maximum 20 m per segment), making installation simple.

IO-Link adds bidirectional digital communication to any sensor or actuator. As well as process data (the primary measurement), IO-Link transmits parameter data (set-points, filter settings, scaling), identification data (device type, serial number, firmware version), and event data (diagnostic codes, process alarms). Remote parameterisation — changing sensor settings from the PLC without touching the device — eliminates field trips for commissioning changes.