Processes in Engineering have to be controlled in order to do what is required. The simplest way to control is by manual. An operator has to be present to operate valves, start equipment, watch out for errors, and so on. As processes become more and more complicated, the use of human labor to control becomes more and more difficult. Human judgment is prone to error due to fatigue. Control systems are designed to replace humans in routine and repetitive tasks.
Semi-automatic and automatic systems are put into place to make it easier to control processes.
Devices are installed to measure values such as pressure, liquid level, temperature, flow, speed, and many other parameters. These devices are called sensors.
Usually the measured value has to be converted to another form in order to be useful. The control systems may use electrical, electronic, pneumatic, hydraulic, and mechanical means to transmit power, compare user set values in order to be useful. Devices called transducers do the conversion of measured value to another medium. For example, a temperature transducer to produce an electrical signal senses temperature.
A transmitter to produce a standard signal recognized by the Industrial Instrumentation Standard processes the electrical signal. For example, the signal is processed to give 4 mA signal output when the temperature is 0 degree Centigrade and 20 mA at 100 degree Centigrade.
In this way, the controller can recognize the 4 to 20 mA electrical signal as equivalent to the temperature of 0 to 100 degree centigrade.
Once the signal has been converted to the industrial standard, it is compared with the set value. For example, when the measured value is 30 degree Centigrade, and the set value is 40 degree Centigrade, there is difference in value. This difference is converted as an output signal to actuate a heater or other output devices to bring back the temperature to the required value of 40 degree Centigrade. The controller is the device for comparing the measured value to the required set value.
Each type of control medium has it own pros and cons. Pneumatic controllers are used in explosive environment, is rugged, and not easily corroded. Electronic controllers can be made very compact and small, and can be very accurate, but they are susceptible to electrical disturbances. Hydraulic controllers are heavily built for withstanding high pressures, are rugged, but are quite messy when their fluid leaks. The control medium is chosen to suit.
Once the system is found to have drifted from the set value, it has to be brought back. The final controlling element is the equipment for doing this. The actuating device for the final control element can be powered by the same medium or a different medium from the controller. Most of the actuating devices are valves. They can be operated by compressed air, electrical motor, hydraulic fluid, etc.
The above scenario describes a simple automatic system. However, it can get more complicated as the controls get more precise. If the process has to be maintained at a certain value that will not drift very much from the set value, then various actions have to be controlled. Proportional, Integral and Derivative actions built into the controllers will offer precise control actions.