Intelligent sensors in process instrumentation

Publisher Summary This chapter discusses the general features of intelligent field instruments and explains several examples of them such as flowmeters, differential pressure transmitters, and a control valve positioner. In addition, the current status of the field bus is described because of its close relationship with intelligent field instruments. The development of intelligent field instruments has made progress with sophisticated signal processing functions and communication functions by using microcomputers. Both measuring characteristics and communication capabilities have been greatly improved—for example, field instruments have become able to make calculations using the multiple sensors signal, to pass accurate judgment on the operational conditions of the process and to acquire knowledge of the control process. The use of microcomputers has been an essential condition of the realization of intelligent field instruments. The following are considered to be general reasons for the provision of intelligent field instruments; (1) Improvements in measurement accuracy (a) linearization of the relationship between input and output signals, (b) automatic zero-point calibration, (c) automatic compensation of errors caused by environmental disturbances such as changes in ambient temperature, and (d) automatic compensation of errors caused by changes in the process condition, such as fluid temperature and fluid pressure in flow measurement, and (2) Improvements in operational capability and maintenance ability (a) remote maintenance operation utilizing digital communication functions, (b) integration of different range sensors by widening sensor's rangeability, (c) storage and readout of sensor data and process control data, and (d) self-check and self-learning functions.