The automation instrument is composed of several automation components, which has a relatively perfect function of automation technology tools. It generally has several functions at the same time, such as measurement, display, recording or measurement, control, alarm, etc. The automation instrument itself is a system, and it is also a subsystem of the whole automation system. The automatic instrument is a kind of “information machine”, its main function is the conversion of information form, transforming the input signal into the output signal. The signal can be expressed in time domain or frequency domain, and the transmission of the signal can be modulated into continuous analog quantity or intermittent digital quantity.