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: capacitance, 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 its temperature.

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.

Control loops can be either stable or unstable. Instability is caused by a combination of process time lags discussed earlier (i.e., capacitance, resistance, and transport time) and inherent time lags within a control system. This results in slow response to changes in the controlled variable. Consequently, the controlled variable will continuously cycle around the setpoint value.

Oscillations describes this cyclic characteristic. There are three types of oscillations that can occur in a control loop. They are decreasing amplitude, constant amplitude, and increasing amplitude. Each is shown in Figure 10.

Decreasing amplitude (Figure l0A). These oscillations decrease in amplitude and eventually stop with a control system that opposes the change in the controlled variable. This is the condition desired in an automatic control system.

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Figure 10 Types of Oscillations

Constant amplitude (Figure 10B). Action of the controller sustains oscillations of the controlled variable. The controlled variable will never reach a stable condition; therefore, this condition is not desired.

Increasing amplitude (Figure 10Q. The control system not only sustains oscillations but also increases them. The control element has reached its full travel limits and causes the process to go out of control.

Summary

The important information in this chapter is summarized below.

Control Loop Diagrams Summary

A controlled system is the system or process through which a particular quantity or condition is controlled.

Control elements are components needed to generate the appropriate control signal applied to the plant. These elements are also called the "controller."

Feedback elements are components needed to identify the functional relationship between the feedback signal and the controlled output.

Reference point is an external signal applied to the summing point of the control system to cause the plant to produce a specified action.

Controlled output is the quantity or condition of the plant which is controlled. This signal represents the controlled variable.

Feedback signal is a function of the output signal. It is sent to the summing point and algebraically added to the reference input signal to obtain the actuating signal.

The actuating signal represents the control action of the control loop and is equal to the algebraic sum of the reference input signal and feedback signal. This is also called the "error signal."

The manipulated variable is the variable of the process acted upon to maintain the plant output (controlled variable) at the desired value.

A disturbance is an undesirable input signal that upsets the value of the controlled output of the plant.

Process time lags are affected by capacitance, which is the ability of a process to store energy; resistance, the part of the process that opposes the transfer of energy between capacities; and transportation time, the time required to carry a change in a process variable from one point to another in the process. This time lag is not just a slowing down of a change, but rather the actual time delay during which no change occurs.