Let’s consider the industrial process control system shown below:
The requirement is to control the temperature ϴ in the tank. The voltage r, obtained from a potentiometer, is calibrated in terms of the desired temperature ϴc. This voltage represents the input quantity to the feedback control system. The actual temperature ϴ, the output quantity, is measured by means of a thermocouple immersed in the tank. The voltage eth produced in the thermocouple is proportional to ϴ. The voltage eth is amplified to produce the voltage b, which is the feedback quantity.
The voltage e = (r-b) is the actuating signal and amplified by the amplifier having the gain KA to produce the solenoid voltage e1. The current is in the solenoid, which results from applying e1, produce a proportional force fs that acts on the solenoid armature and valve to control the valve position x. The valve position in turn controls the flow of hot steam q from the boiler into the heating coil in the tank. The resulting temperature ϴ of the tank is directly proportional to the steam flow with a time delay that depends on the specific heat of the fluid and the mixing rate.
The block diagram representation below for this system shows the function of each unit and the signal flow through the system.
To illustrate the operation of the system, consider an increase in the tank temperature is needed. The voltage r is increased to a value that represents the value of the desired temperature. This change in r causes an increase in the actuating signal e. This increase in e, through its effect on the solenoid and valve, causes an increase in the amount of hot steam flowing through the heating coil in the tank. The temperature ϴ therefore increases in proportion to the steam flow. When the temperature rises to a value essentially equal to the desired temperature, the feedback voltage b is equal to the reference input r (b ≈ r). The flow of steam through the heating coil is stabilized at a steady-state value but maintains ϴ at the desired value.
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