Sensor | Types of Sensor

A measurement system can include a sensing element, signal conditioning, conversion, processing and a display or data interface. The required blocks depend on the measurand, sensor output and intended result. Some sensors produce a directly usable signal, while others need excitation, amplification, compensation or analogue-to-digital conversion.

This page focuses on the sensor and the main ways sensors are classified.

Sensor

A sensor’s response carries information about the quantity being measured. For example, a thermocouple responds to the temperature difference between its measuring and reference junctions by producing a thermoelectric voltage.
Calibration establishes the relation between indications and reference quantity values, with stated conditions and measurement uncertainty. Some sensors are factory calibrated, while others need system-level or periodic calibration. Calibration does not necessarily include adjustment, and it does not by itself remove drift, noise or installation error.
A measuring transducer produces an output quantity related to an input quantity. A thermocouple is both a temperature-sensing element and a measuring transducer. Cold-junction compensation, signal conditioning and a display may be needed to form a complete temperature sensor measurement system, but they do not turn the thermocouple into a sensor.

Sensor and transducer therefore overlap, but the terms describe different roles. A sensor is directly affected by the measurand. A measuring transducer provides an output quantity with a specified relation to its input. Depending on context, one component or assembly can satisfy both definitions. Sensors do not have to include every circuit used to display or process the measurement.

Characteristics of Sensors

Useful sensor performance depends on the application. Common characteristics include:

1. Sensitivity: Sensitivity is the change in indication divided by the corresponding change in the measured quantity. If a temperature sensor changes by 1 mV for a 1^oC input change, its sensitivity over that interval is 1 mV/^oC. Higher sensitivity is useful only when the range, noise and interface can handle it without saturation.
2. Linearity: A linear transfer function can simplify conversion, but many accurate sensors are non-linear. A specified calibration curve, lookup table or compensation equation can map a repeatable non-linear response to the measurand.
3. Resolution: Resolution describes the smallest change in the measured quantity that produces a detectable change in indication. Resolution, accuracy, precision and sensitivity are distinct specifications. The resolution must suit the required measurement uncertainty.
4. Measurement quality: Noise, hysteresis, drift, cross-sensitivity, repeatability and response time affect the result. Their acceptable values depend on bandwidth, environment and the required uncertainty.
5. System fit: Power consumption, loading, range, overload tolerance, stability, size, reliability and environmental limits must match the application. Improving one characteristic can reduce another.

Types of Sensors

Sensors can be classified by measurand, sensing principle, required excitation, output format, contact method or application. The following types of sensors show several common groupings.

Sensor classification

Based on the quantity being measured:

• Temperature: Resistance Temperature Detector (RTD), Thermistor, Thermocouple
• Pressure: Bourdon tube or diaphragm sensing elements, manometers and pressure gauges
• Force/ torque: Strain gauge elements and load cells
• Speed/ position: Tachometers, encoders and LVDTs
• Light: Photo-diode and Light dependent resistor

Other measurands include humidity, flow, acceleration, magnetic field, chemical concentration and radiation.
(2) Active and passive sensors: These labels depend on the field. In one instrumentation convention, a self-generating sensor element produces its output signal from energy in the measurand, while a modulating sensor element needs external excitation. A piezoelectric element, for example, produces charge when strained or accelerated. It does not create operating power for a complete instrument, and its charge or voltage commonly needs a powered high-impedance interface. In remote sensing, active instead means that the instrument emits probe energy, while passive means that it receives naturally emitted or reflected energy. The convention must always be stated.

Under the instrumentation convention used above, externally excited sensors change resistance, inductance, capacitance or another property in response to the measurand. The readout supplies excitation and measures that change. Many resistive, inductive and capacitive sensing elements work this way, including structures related to resistors, inductors and capacitors. The required signal-conditioning electronics may need power even when the sensing element is self-generating.

(3) Analog and digital sensor: An analogue-output sensor provides a continuously variable electrical quantity, such as voltage, current, resistance or capacitance. A thermocouple, RTD and Strain gauge element have analogue responses. A digital sensor may provide an on-off state, pulse frequency, duty cycle or encoded data word through an interface such as I²C or SPI. Encoders can provide pulses or digital position codes. A digital interface often follows an internal analogue sensing element and analogue-to-digital converter.

(4) Reversible sensing and actuation: Some transduction materials support energy conversion in both directions. Through the direct piezoelectric effect, mechanical stress produces electric charge. Through the converse effect, an applied electric field produces mechanical strain. A device must still be designed and connected for its sensing or actuation role. Piezoelectric elements can therefore serve in microphones, vibration or ultrasonic sensors, buzzers, speakers and actuators. This reversibility does not mean that every sensor can recreate its original measurand from the output signal.