Process Time Lags and Stability of Automatic Control Systems

Processes have the characteristics of delaying and retarding changes in the values of the process variables. Process time lags generally describe these process delays and retardations. Process time lags are caused by three properties of the process. They include:

• Capacitance
• Resistance
• Transportation time

Capacitance

is the ability of a process to store energy. This energy-storing ability gives the ability to retard change.

Resistance

This that part of the process that opposes the transfer of energy between capacities, for example in a lubrication oil cooling system, the walls of the lubrication oil cooler opposes the transfer of heat from the lubrication oil inside the tubes to the cooling water outside the tubes.

Transportation time

This is the time needed to carry a change in a process variable from one point to another in the process. The time lag not only slows down or retards a change, it is during this actual time delay, that there is no change occurring.

Related: Feedback control systems

Stability of Automatic Control System

The characteristic of a control mode that returns a process variable to a steady value following a disturbance is called ‘’stability’’ in other words, stability is the ability of control loop to retain a controlled variable to a steady, non-cyclic value, following a disturbance.

Control loops can either be stable or unstable. The instability is caused by a combination of process time lags (capacitance, resistance, transportation time) and inherent time lags within a control system. This results in slow response to changes in the controlled variable. Subsequently, the controlled variable will continuously cycle around the setpoint value. Oscillations describe this cyclic characteristic. There are 3 types of oscillations that can occur in a control loop. They are decreasing amplitude, constant amplitude, and increasing amplitude.

Decreasing amplitude

Here the 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

The action of the controller sustains oscillations of the controlled variable. The controlled variable will never reach a stable condition; hence this condition is not desired.

Increasing amplitude

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.

Related articles:

• Key Causes of Uncertainty in Control Systems
• The Goals of Robust Control Techniques
• Centralised vs. Decentralised Control Systems