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Level 1 – sensors, transducers, and actuators
According to the IEC, the sensor is defined as the following:
"The primary element of a measurement chain, which converts the input variable in a signal suitable to the measurement."
The transducer is the following:
"The device that accepts information in the form of a physical variable (its input variable) and converts it to an output variable of the same or of different magnitude, according to a defined physical law."
Since the sensor and the transducer are often physically within the same component, the two terms are often used as synonyms. The actuator is a transducer that transforms a command signal into a physical action on the process. Basically, its function is complementary to that of the sensor. It receives a signal as an input in the physical domain of the control device and sends energy as an output in the physical domain of the command variable. They are often made by means of multiple transducers.