REFERENCESENSORS / SIGNALS / MEASUREMENT

Industrial Sensors for Automation & Measurement

Industrial sensors respond to quantities such as pressure, temperature, position, distance, level, flow rate and acceleration. The sensing element and associated electronics can provide a switching, analogue, pulse or digital output to a PLC, controller or measuring system.

Reliable industrial measurement depends on the sensing principle, signal conditioning, output circuit, field wiring, input configuration and the characteristics of the complete measuring system.

COVERSPressurePositionTemperatureFlow & level4–20 mAPNP / NPNAccuracy & calibration

MEASURING SYSTEM

From measurand to measurement result.

The measurand is the quantity intended to be measured. The sensor is the element of the measuring system directly affected by the phenomenon, body or substance carrying that quantity.

Signal conditioning and output electronics may provide excitation, amplification, linearisation, filtering, compensation or isolation. The receiving system then interprets the resulting switching state, analogue signal, pulse train or digital data to determine a process state or measured quantity value.

Measurand

Pressure, temperature, displacement, level, flow rate, force, rotational speed or acceleration.

Sensor

The element directly affected by the phenomenon, body or substance carrying the quantity to be measured.

Conditioning

Excitation, amplification, filtering, linearisation, compensation or isolation where required.

Output / Interface

Switching output (PNP/NPN), 4–20 mA, 0–10 V, pulse or frequency output, or digital communication.

Interpretation

The receiving system interprets a switching state or derives a measured quantity value from the measurement signal.

CORE AREAS

Sensing, signal and measurement functions.

Separate the sensing principle, the electrical interface and the metrological performance of the measurement. These layers are related, but they describe different parts of the measuring system.

Sensors

Pressure, temperature, proximity, position, level, flow and vibration sensing methods and sensor technologies.

SENSINGPRINCIPLES→ 02

Signals

4–20 mA, 0–10 V, PNP/NPN, sourcing and sinking, pulse and frequency outputs, and sensor wiring.

OUTPUTSWIRING→ 03

Measurement

Accuracy, resolution, repeatability, linearity, hysteresis, response, calibration and drift.

ERRORSQUALITY→ 04

Reference

Standards, units, terminology, conversion relationships, tables and engineering checks.

STANDARDSUNITS→

INDUSTRIAL SENSOR TYPES

Select by measured quantity.

Industrial automation sensors can be compared first by measured quantity, then by sensing principle, output type, measurement range and operating conditions.

Pressure Sensors

Gauge, absolute and differential pressure measurement for process, hydraulic, pneumatic and machine systems.

GAUGE · ABSOLUTE · DIFFERENTIAL

Temperature Sensors

RTDs, thermocouples, thermistors and semiconductor sensors for contact and process temperature measurement.

RTD · THERMOCOUPLE · THERMISTOR

Position & Distance Sensors

Inductive, capacitive, optical, magnetic, ultrasonic and displacement methods for position or distance.

LINEAR · ANGULAR · NON-CONTACT

Proximity Sensors

Presence and object detection using inductive, capacitive, photoelectric, magnetic or ultrasonic principles.

INDUCTIVE · CAPACITIVE · OPTICAL

Flow & Level Sensors

Flow-rate measurement and continuous or point level detection for liquids, gases and bulk materials using electrical or mechanical sensing principles.

CONTINUOUS · POINT · FLOW RATE

Vibration & Acceleration Sensors

Piezoelectric, MEMS and velocity-based sensing for machine condition, motion and dynamic measurement.

PIEZOELECTRIC · MEMS · VELOCITY

SIGNALS

Match the sensor output to the receiving input.

A sensor output and the receiving input form an electrical interface. Output type, supply arrangement, reference potential, cable characteristics and input configuration determine whether the signal is interpreted correctly.

4–20 mA

Current-loop transmission for process variables and long industrial field wiring.

ANALOGUE→ A2

0–10 V

Voltage transmission where reference potential, input impedance, electrical noise and cable length are controlled.

VOLTAGE→ D1

PNP / NPN

Sourcing and sinking transistor switching outputs used by industrial sensors and PLC input circuits.

DISCRETE→ P1

Pulse / Frequency

Pulse count or frequency representing position, speed, flow or event rate.

DYNAMIC→

MEASUREMENT QUALITY

Range alone does not describe measurement performance.

Accuracy, resolution, repeatability, linearity, hysteresis and response time describe different aspects of measurement performance. The uncertainty of a measurement result also depends on the measuring system, calibration, installation, environmental conditions, signal integrity and the measurement procedure.

Accuracy

Closeness of agreement between a measured quantity value and a true quantity value of the measurand.

accuracy describes closeness, not resolution

Resolution

The smallest change in the measured quantity that produces a distinguishable change in output or reported value.

resolution ≠ accuracy

Repeatability

How closely repeated readings agree when the same input is measured under the same conditions.

same input → agreement of readings

Linearity

Closeness of the calibration curve to a specified straight line over the stated measurement range.

maximum deviation from reference straight line

Hysteresis

Difference at the same input depending on the direction from which the value is approached.

upscale ≠ downscale

Response time

For a step change in input, the time taken for the sensor or measuring system output to settle within specified limits around its final steady value.

step input → settled output

TECHNICAL REFERENCES

Core industrial sensor topics.

01PNP vs NPN sensor outputsCurrent direction, PLC input compatibility and three-wire connections.↗024–20 mA current loopLive zero, loop supply, burden resistance and fault interpretation.↗032-wire vs 3-wire vs 4-wire sensorsPower and signal paths for switching and analogue devices.↗044–20 mA vs 0–10 VTransmission behaviour, loading, wiring and typical industrial use.↗05Sensor accuracy vs resolutionWhy a finer displayed increment does not prove a more accurate measurement.↗06Sensor signal conditioningExcitation, amplification, filtering, isolation and conversion.↗

COMMON QUESTIONS

Industrial sensor fundamentals.

What is an industrial sensor?

In metrology, a sensor is the element of a measuring system directly affected by the phenomenon, body or substance carrying the quantity to be measured. In industrial automation, the term commonly also refers to the complete device containing the sensing element, signal conditioning and an electrical output interface.

Why is 4–20 mA widely used for industrial measurement?

A 4–20 mA loop carries an analogue measurement as current and uses 4 mA as a live zero, allowing the lower-range measurement value to be distinguished from loss of loop current caused by an open circuit or loss of power.

What is the difference between PNP and NPN sensor outputs?

In common three-wire DC sensor circuits, a PNP output sources current to the load when active, while an NPN output sinks current from the load to 0 V. The PLC input circuit must be compatible with the sensor output type.

What affects industrial sensor measurement performance?

Measurement performance depends on sensor characteristics, the measuring interval, signal conditioning, installation, environmental conditions, calibration, the receiving instrument and the measurement procedure.