It can convert the output analog signal of the non-standard sensor into a standard signal. Then it is a transmitter. The specific working principles and methods are resistive, inductive, magnetoelectric, photoelectric, piezoelectric, capacitive, and vibration frequency. Type, magnetoelasticity and eddy current etc.
The traditional output DC signal of the transmitter has 0-5V, 0-10V, 1-5V, 0-20mA, 4-20mA, etc. So far, the most widely used is the 4-20mA current to transmit the analog quantity. The so-called "change" refers to the conversion of various physical quantities from the sensor into an electrical signal. For example: use thermocouple to convert temperature into electric potential; use current transformer to convert large current to small current. Because electrical signals are the easiest to process, modern transmitters convert various physical signals into electrical signals. Therefore, what we mean by the transmitter usually becomes "electricity".
The so-called "send" refers to the various electrical signals that have been turned into, in order to facilitate the reception and transmission of other instruments or control devices, the electrical signals from the sensors are unified through electronic circuits (such as 4-20MA). The method is implemented by multiple operational amplifiers. This "change" + "send" constitutes the most commonly used transmitter in modern times.