Reduction in Sensitivity to Parameter Changes and Disturbances

Amajor reason for using closed-loop control systems rather than open-loop control is the reduction in system sensitivity both to internal and external changes affecting the system. Internal changes would be characterized by changes in the parameter values of certain components in the system. These parameters could include resistor or capacitor values in electrical circuits, masses or spring constants in mechanical systems, efficiency of heating coils in ovens or furnaces, etc. Parameter changes reflect something inside the system that is changing. External changes that affect performance will be referred to as disturbances. These disturbances reflect how the changes in the system’s environment can influence performance. Examples of disturbances include waves on a ship steering system, electrical noise in a communication system, wind gusts on an antenna or aircraft, changes in road slope in a vehicle speed control system, etc. Parameter changes within the system and external disturbances acting on the system are both considered bad in that they change the system’s behavior from what is desired. These changes can cause ships to head in the wrong direction, aircraft to fly at the wrong altitude, room temperatures to be too hot or too cold, or a person’s blood pressure to be too high or too low. Whatever the source of the change, it is desired that the effects of the change on the system’s performance be kept small. The examples presented here will illustrate the advantages of a closed-loop configuration over an open-loop configuration in reducing the sensitivity both to parameter changes (internal changes) and disturbances (external, environmental changes). The model for the system to be controlled in these examples is the transfer function representing ship steering for an ocean-going cargo ship. The transfer function for the plant is