Sensors and actuators are tools which enable computers to gather information about and affect change in the physical world. The following information is based on Chapter 3 of the book “New digital musical instruments: control and interaction beyond the keyboard” by E.R. Miranda and M.M. Wanderley.
Different definitions of what a sensor is are available in the literature; the two most frequent definitions are reported here. Note that the second one includes the first as a special case.
A sensor can be defined as “a device that converts any type of energy into electrical energy” [Fraden 2004]; more generally, a sensor can be seen as “an input device that provides a usable output in response to a specific physical quantity input” [Nyce 2004].
A sensor can also be seen as a modifier, i.e. a device that modifies an electrical signal in proportion to a physical input.
Sensors allow humans to transmit information to a computer, and thus are one of the most important considerations in the design of musical controllers, musical installations, and multimedia applications. SensorWiki.org contains an extensive and detailed catalogue of currently available sensors and sensor technology, with information about applications in these fields.
Actuators (or effectors) can be considered as the opposite of sensors; they convert electrical energy into other kinds of energy.
There are many types of actuators which differ in technology and in output.
The most common actuators are electro-magnetic, and which convert electrical energy into movement : motors, solenoids, coils etc. The type of movement these actuators create are usually linear or circular. These actuators can also be used to create arrays of pins that can move vertically or horizontally to stimulate the skin.
Other kind of actuators not based on motion can convert, for example, electric energy into heat in order to produce temperature modifications.
Actuators are mostly used to add “feedback” capabilities to controllers or interfaces. The feedback can be in form of vibration (vibrotactile feedback) or of actual displacement of mobile parts of the controllers, which can dynamically react (exerting forces) to the input of a user (haptic feedback).
Actuators are also used in the creation of robotic musical instruments. In these instruments computer-controlled actuators act upon traditional musical instruments or other physical objects to create sound.
The word “transducer” is sometimes used as a synonym of sensor but in general this term identifies devices capable of translating a form of energy into another of different nature.
Given the previous definition, any sensor or actuator is also a transducer.
For a more formal definition of sensors and transducers please refer to The International Vocabulary of Metrology.