Applications of the Process Industries

The earliest applications of computer control were in the process industries. The process industries are those which change the composition of materials to produce metals of higher value. Here automation is in some ways easier, and is fully developed. In process industries instruments are available to monitor the continuous flow of a product and to send the data to the computer, which can then direct changes in the process by adjusting valves and switches.

The elements of a control system are measurement, control, actuation and communication. Measurement refers to the sensing of variables such as flow rate, temperature, pressure, level, and chemical composition, and the transmission of the measurement to the controller. Control is the decision-making operation. It compares the measured statе of the process with the desired conditions and decides how the variables should be manipulated. Actuation is the means by which the operating variables are manipulated. Typical actuators are valves, rheostats, switches and relays. Communication includes the display of information to the plant operators as well as the transmission of important variables to the plant management.

The organization of a plant control system is comprised at different levels. The lowest level is occupied by the control computer that regulates a single process unit holding it to desired operating conditions and moving the unit to a safe condition in emergencies. The next step is a supervisory computer responsible for coordinating several units, for scheduling operations, and for optimizing the plant's performance. At the top level is the manager control computer, which supplies the manager with current information about manufacturing operations.

Improved communications are making it possible to use systems in which elements of control system are located throughout the plant and communicate with each other through networks. Communication over these networks is by a digital signal.