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 10A). 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.
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 10C). 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.