Definitions of Sensor and Transducer
What is a Sensor and Transducer ?
In the majority of cases these tasks were accomplished by transforming the physical input energy into an electrical output signal that could be measured and/or displayed on a suitable device. Sensors came, therefore, to be known as ‘energy transformers’, devices capable of converting a physical input quantity into an electrical output quantity. This process has been identified in various disciplines as a process of ‘energy transduction’ which has also led to sensors’ other common name, transducers.
In fact, the differences in nomenclature go even further-as well as the aforementioned historical reasons there are also linguistic-geographical ones (there are differences in the terminology used in some European countries as well as some differences between USA and British terminology) and many within the various fields of physics too. Transducers are sometimes referred to as ‘transmitters’ in the process industry–e.g. pressure transmitter, a ‘detectors’ in the optical field–e.g. light detector, or as ‘probes’ in fluid-mechanical measurements–e.g. pressure probe. It is beyond the scope of this book to propose a rationalization of the existing nomenclature but definitions of what is intended here by sensor and transducer are required both to help the reader and to permit comparison with other books in this subject.
Definitions of Transducer
A transducer is here defined as an elementary device capable, within a given field of measurement, of converting a physical non-electrical input quantity (the measurand) into an electrical output quantity. The transducer itself does not contain any further processing beyond this energy conversion.
Definitions of Sensor
A sensor is here defined as a non-elementary device, usually based on a transducer, capable of converting a physical non-electrical input quantity into an electrical output quantity and of processing it, in accordance with a given algorithm, to provide an output suitable for interfacing to a process control system such as a computer.
The main difference lies therefore in the non-elementary nature of the sensor, that is in its capability to embody functions other than the basic energy conversion. This leads to a further classification of sensors, that of:
- Intelligent sensors, those that can interact with the control computer to provide data manipulation (such as ‘feature extraction’ as in the case of vision sensors)
- Non-intelligent sensors, those that can provide the computer with the output data only (and which therefore require longer communication times).
Generalities Of Sensor And Transducers
Both sensors and transducers can be classified according to their input/output characteristics. With respect to their input physical quantity these devices can be termed:
- Absolute-when, given a fixed origin, the electrical output signal can represent all the possible values of the input physical signal with no ambiguity
- Incremental-when an origin cannot be fixed for all points within the field of measurement and each point is taken as the origin for the next one
The nature of the output function, on the other hand, determines whether the device is:
- Analogue-when the output signal is continuous and proportional to the input physical quantity
- Digital-when, given a continuous physical input quantity, the output signal can only take a number of discrete values
The nature of the electrical output quantity also determines the device performance characteristics, which can be divided into static and dynamic ones.