Sensor

What is a sensor?

A sensor is a device that detects a physical quantity, such as temperature, pressure, flow, level, position or vibration, and converts it into an electrical signal that a measurement, control or automation system can process. That signal goes to a PLC, DCS or controller, which uses it for control, supervision and monitoring of a process or machine. Sensors form the bottom layer of all industrial automation: without reliable measurements, no control system can work properly. Modern smart sensors deliver diagnostic and configuration data alongside the measured value, which makes them part of the network and of the security scope.

🧠 What exactly does a sensor do?

A sensor goes through three steps:

1. Sensing - a sensing element responds to a physical quantity, for example a platinum resistor that changes with temperature or a diaphragm that deflects under pressure
2. Converting - electronics amplify, linearise and digitise that signal, or convert it into a standard signal
3. Forwarding - the signal goes to a Controller, PLC, HMI or SCADA system, either directly or via O and a fieldbus

In a control loop the sensor is usually the first step. It supplies the process value that is compared with a setpoint; based on the deviation, the controller adjusts an actuator or control valve. A faulty or drifting measurement therefore feeds straight into the control of the process.

🔄 What is the difference between a sensor and a transmitter?

In everyday practice the terms are used interchangeably, but they do not mean the same thing. The sensing element detects; a transmitter combines that element with electronics that produce a standardised output signal.

Machine builders mostly talk about sensors (proximity switches, photoelectric sensors, encoders); the process industry talks about transmitters and “instrumentation”.

Sensors are found in almost every sector: in the food and beverage industry for temperature and hygiene monitoring, in pharmaceuticals under GMP requirements, in energy management for measuring consumption and voltage, in building automation for climate control and lighting, and in the automotive industry for position, pressure and speed.

🧱 What types of industrial sensors are there?

For temperature, the choice is often between a PT100 and a thermocouple. A class A PT100 to IEC 60751 has a tolerance of ±(0.15 + 0.002·|t|) °C, so ±0.35 °C at 100 °C. A class 1 type K thermocouple to IEC 60584 has a tolerance of ±1.5 °C between −40 and 375 °C (±0.4 % of the temperature above that), but it measures up to well over 1,000 °C.

Inductive and capacitive proximity switches are covered by IEC 60947-5-2. That standard defines the switching distance using a square plate of mild steel 1 mm thick; aluminium or copper targets require a correction factor, so the real switching distance is considerably shorter.

🔌 Which output signals do sensors use?

The old split between “analogue and digital” is too coarse. In practice there are three families:

• 4-20 mA - defined in IEC 60381-1. Because 0 % corresponds to 4 mA, a broken wire (0 mA) is immediately recognisable. Under NAMUR NE 43, a current of ≤ 3.6 mA or ≥ 21 mA indicates a fault.
• HART - superimposes a digital signal (FSK at 1,200 and 2,200 Hz, 1,200 bit/s) on the 4-20 mA loop without disturbing the analogue value. This lets a handheld communicator or asset management system configure and read the transmitter. A wireless variant exists as WirelessHART.
• IO-Link - a point-to-point connection to IEC 61131-9 between an IO-Link master and a single sensor or actuator, over a standard unshielded 3-wire cable of up to 20 metres, usually with an M12 connector. The data rate is 4.8, 38.4 or 230.4 kbit/s. Since IO-Link version 1.1, a replacement sensor automatically receives its parameters back from the master (data storage).

🧭 How do you choose the right sensor?

Good sensor selection starts with the process, not with the catalogue. Work through these steps:

1. Quantity and measuring range - what do you need to measure, and between which minimum and maximum? Choose the range so that the normal value sits somewhere in the middle.
2. Accuracy and response time - a control loop with fast dynamics needs a short response time; energy or custody transfer metering is mainly about accuracy.
3. Medium and environment - temperature, corrosion, fouling, hygiene requirements (food, pharma), vibration and ingress protection (IP67 or higher in wet areas).
4. Explosion hazard - in an Ex-zone the sensor must be suitable under ATEX and IEC 60079, for example intrinsically safe.
5. Signal and integration - does the output signal fit your I/O, your fieldbus and your engineering tools?
6. Diagnostics and lifecycle - does the sensor support status signals to NAMUR NE 107, and how long will the type remain available?
7. Safety and security - does the sensor have to perform a safety function, and can its configuration be changed remotely?

Calibration is part of the decision: every sensor drifts over time. Define how often you calibrate, against which reference and with what permitted deviation. In the Netherlands, laboratories accredited by the RvA to ISO/IEC 17025 provide traceable calibrations. Meters used for billing, such as flow meters for fuel transfer, fall under the European Measuring Instruments Directive (MID, 2014/32/EU).

🏭 How are sensors used in functional safety?

In a SIS, a sensor performs a safety function: it detects excessive pressure, for instance, after which the logic solver closes a valve. In the process industry IEC 61511 applies, and the sensor must be suitable for the required SIL. There are two routes: the manufacturer develops the sensor to IEC 61508 (a certified “SIL-capable” transmitter), or the user demonstrates suitability through documented operating experience in similar applications (“prior use”).

To cope with failures, sensors are often installed redundantly, for example three transmitters with 2-out-of-3 voting (2oo3): one faulty transmitter then causes neither a spurious trip nor a missed trip. In machinery, ISO 13849 and IEC 62061 apply. A safety light curtain to IEC 61496 is therefore something different from an ordinary photoelectric sensor: the principle looks similar, but the light curtain is designed and certified as a safety component.

📡 What makes a sensor “smart”, and what does that mean for IIoT?

A smart sensor has its own microprocessor and firmware. Besides the measured value, it delivers diagnostics, serial number, calibration data and sometimes a secondary value, such as the internal temperature of a pressure transmitter. This makes sensors an important source for condition monitoring and predictive maintenance.

A vibration sensor on a pump or motor illustrates this. Under ISO 20816-3 (2022, the successor to ISO 10816-3), RMS vibration velocity in mm/s is classified into zones A to D. For a machine of 15 to 300 kW on a rigid foundation, the boundary between zone B (acceptable for long-term operation) and zone C (unsuitable for long-term continuous operation) is 2.8 mm/s.

In IIoT architectures, this additional data often bypasses the PLC and travels via an edge device or a second channel such as NAMUR Open Architecture to analytics and maintenance systems.

🔐 What security risks do smart sensors have?

Classic 4-20 mA sensors without digital communication have virtually no digital attack surface. Smart sensors do: they can be configured remotely, run firmware and sometimes sit directly on an Ethernet or wireless network.

• Unsecured protocols - wired HART, Profibus and IO-Link have no authentication. Anyone with access to the network or a HART multiplexer can change range, damping or zero, so the control system sees a wrong value.
• Software vulnerabilities - in 2020 CISA published a critical vulnerability (CVSS 9.8) in the FieldComm Group HART-IP reference code; earlier advisories concerned vulnerable device drivers (DTMs).
• Measures - enable write protection (hardware jumper or password), record sensors in your asset inventory, place gateways and multiplexers in a separate zone and monitor configuration changes.

For new products, the European Cyber Resilience Act places requirements on manufacturers of products with digital elements, including network-connected sensors. The obligation to report actively exploited vulnerabilities has applied since 11 September 2026; the full requirements apply from 11 December 2027. Component requirements for industrial devices are set out in IEC 62443-4-2.

❓ Frequently asked questions

What is the difference between a sensor and an actuator?

A sensor measures a physical quantity and converts it into a signal for the control system. An actuator does the opposite: it converts a control signal into a physical action, such as opening a valve or starting a motor. Together with the PLC, sensor and actuator form the basis of every control loop.

Why does an analogue sensor signal start at 4 mA rather than 0 mA?

With 4-20 mA, 4 mA represents 0 % of the measuring range, so a broken wire (0 mA) can be distinguished from a genuine zero reading. In addition, a two-wire transmitter can draw its own power supply from that minimum current. Under NAMUR NE 43, values below 3.6 mA or above 21 mA are treated as a fault.

What is the difference between a PNP and an NPN sensor?

A PNP sensor switches the positive supply (24 V) to the PLC input and is therefore called “sourcing”; an NPN sensor switches the negative and is called “sinking”. PNP is the usual standard in Europe, while NPN is more common in Asia. The sensor must match the type of input card, otherwise the connection will not work.

How often should a sensor be calibrated?

There is no fixed period: the calibration interval of a sensor depends on the risk of a wrong measurement, the stability of the type and legal or quality requirements such as GMP. Many companies start with annual calibration and lengthen or shorten it based on the deviations they find. Sensors in a safety function have a proof test interval that follows from the SIL calculation.

When should you choose IO-Link instead of an analogue sensor?

IO-Link makes sense when you want to use diagnostics, parameters and identification of the sensor alongside the measured value, or when you want to replace sensors quickly and without errors. An IO-Link sensor uses the same 3-wire cable as a switching sensor, so cabling does not become more expensive. For long distances or hazardous process areas, 4-20 mA with HART and Ethernet-APL remain more common.

Can a sensor be hacked?

A simple analogue sensor cannot, but a smart sensor with firmware and a network connection can. An attacker could, for example, change the measuring range or zero point of a transmitter, so that the control system and even a safety system see a wrong process state. Write protection, network segmentation and monitoring of configuration changes reduce that risk.

📌 In summary

A sensor converts a physical quantity into a signal that control systems can process, and is therefore the foundation of every measurement, control loop and safety function. Select sensors based on process, accuracy, environment and signal, calibrate them periodically, and treat smart sensors as network devices that need to be secured as well.