CONTROL LOOP DIAGRAMS
The actuating signal passes through the two control elements: the temperature controller and the
temperature control valve. The temperature control valve responds by adjusting the manipulated
variable (the cooling water flow rate). The lube oil temperature changes in response to the
different water flow rate, and the control loop is complete.
Process Time Lags
In the last example, the control of the lube oil temperature may initially seem easy. Apparently,
the operator need only measure the lube oil temperature, compare the actual temperature to the
desired (setpoint), compute the amount of error (if any), and adjust the temperature control valve
to correct the error accordingly. However, processes have the characteristic of delaying and
retarding changes in the values of the process variables. This characteristic greatly increases the
difficulty of control.
Process time lags is the general term that describes these process delays and retardations.
Process time lags are caused by three properties of the process. They are:
resistance, and transportation time.
Capacitance is the ability of a process to store energy. In Figure 9, for example, the walls of
the tubes in the lube oil cooler, the cooling water, and the lube oil can store heat energy. This
energy-storing property gives the ability to retard change. If the cooling water flow rate is
increased, it will take a period of time for more energy to be removed from the lube oil to reduce
Resistance is that part of the process that opposes the transfer of energy between capacities. In
Figure 9, the walls of the lube oil cooler oppose the transfer of heat from the lube oil inside the
tubes to the cooling water outside the tubes.
Transportation time is time required to carry a change in a process variable from one point to
another in the process. If the temperature of the lube oil (Figure 9) is lowered by increasing the
cooling water flow rate, some time will elapse before the lube oil travels from the lube oil cooler
to the temperature transmitter. If the transmitter is moved farther from the lube oil cooler, the
transportation time will increase. This time lag is not just a slowing down or retardation of a
change; it is an actual time delay during which no change occurs.
Stability of Automatic Control Systems
All control modes previously described can return a process variable to a steady value following
a disturbance. This characteristic is called "stability."
Stability is the ability of a control loop to return a controlled variable to a steady, non-cyclic
value, following a disturbance.