7 What is Sensing
Measurements are at the heart of the scientific revolution. Generally speaking, a measurement is the estimation of the magnitude of some attribute of an entity. It usually involves a measuring instrument (e.g., a ruler, a thermometer, etc.). Due to the fundamental (and practical) physical limitations, all measurements are imperfect.
The basic physical component of an instrument that is used to perform a measurement is a transducer. Generally speaking, transducers are devices that convert one type of energy into another. For our purposes, they are devices that convert between electrical signals and physical stimulus (in either direction). Depending on the direction of this conversion transducers can be either sensors (from a physical stimulus to an electrical signal) or actuators (from an electrical signal to a physical stimulus). A single sensor (or actuator), in fact, can be composed of multiple transducers inside.
7.1 Instruments
The instruments used for measurement allow us to asses the relationship between an attribute of the item under study (e.g., the length of a beam), and a referenced unit of measurement (e.g., a standard meter). Traditional instruments are physical / hardware based, with fixed/limited versatility and respond to a specific stimulus. An oscilloscope, for instance, is an example of a traditional instrument. With advances in computing hardware and software, instruments can now be completely virtual (digital) and/or significantly miniaturized. Below are diagrams depicting common components of different types of instruments (typical, analog and digital):
7.2 Sensors
Sensors can be classified in many different ways depending on the objective of the classification. Some of those, as expressed by Fraden (2010) are as follows:
- Passive or Active
- Absolute or Relative
- By their characteristics:
- Specifications
- Materials
- Detection means
- Conversion phenomena
- Application area
- Stimulus
7.2.1 Sensor Characteristics
Because there may be multiple transduction processes in place to get from a physical stimulus to an electrical signal inside a sensor, it is convenient to characterize this input/output relationship as a black box and define common terms that can be used to describe it.