Closed Loop Servo

A servo system is a control circuit, which maintains the speed of a motor within a set programmed range. The system is responsible for adjusting the speed accordingly to keep within the set parameters. A closed loop servo utilises a feedback signal, which conveys the actual speed of the motor shaft. This value subtracts from the command value to provide the difference. The difference, which is the error value, is the amount the system has to compensate to meet the desired value.

System Basics

System Basics

The purpose of the loop is to make the output value F equal to the input value C. In mechanical systems there are usually factors such as friction and load, which prevent this from happening, hence the output value is usually a value close to the input value and contains an error. The purpose of the system is to determine the error and compensate accordingly so that the input C equals F.


A command value C at the input is the desired value that we would like at the output. However, what we actually get at the output is value F, which feeds back to a summing circuit. The summer, represented by the circle with a cross, subtracts the feedback value F from the input value C to provide an error value E, as shown by the following formula.

E = C - F