Q. Explain Integral-error or reset control?

A control system is said to possess integral-error control when the generation of the output in some way depends upon the integral of the actuating signal. By designing the servoamplifier such that it makes available an output voltage that contains an integral term and a proportional term, integral-error control can be obtained. By modifying the transfer function of the servoamplifier in the block diagram, the complete block diagramof the servomechanismwith integral-error control is shown in Figure, where Ki is the proportionality factor of the integral-error component. The direct transmission function is then given by

and the corresponding closed-loop transfer function becomes

where Q_i = K_iK_pK_m and K = K_pK_aK_m, as defined earlier. Since the integral term stands alone without combiningwith the viscous-friction F term(as was the case with error-rate and output-rate controls), its influence on system performance differs basically from the previous compensation schemes. Also note that the order of the system is changed from second to third, because of the integral-error compensation. The inclusion of the integral term implies that a third independent energy-storing element is present. The position lag error, which exists with error-rate and output- rate control, disappears with integral-error control. This is a characteristic of integral control which greatly improves steady-state performance and system accuracy. However, it may make the dynamic behavior more difficult to cope with successfully.