Q. What is error-rate control?

A system is said to possess error-rate damping when the generation of the output in some way depends upon the rate of change of the actuating signal. For the system of Figure, if the

amplifier is so designed that it provides an output signal containing a term proportional to the derivative of the input, as well as one proportional to the input itself, error-rate damping will be introduced. For the system that includes error rate, the only modification needed is in the transfer function of the servoamplifier. Instead of the gain Ka, the new transfer function will be K_a+ sK_e, where Ke denotes the error-rate gain factor of the amplifier. The complete block diagram is shown in Figure. In this case the direct transmission function becomes

where K = K_pK_aK_m and Q_e = K_pK_eK_m are known as the loop proportional gain factor and the loop error-rate gain factor, respectively.

Note that the steady-state solution for a step input r0 is the same whether or not error-rate damping is present. The advantage of error-rate damping lies in the fact that it allows higher gains to be used without adversely affecting the damping ratio, and thereby makes it possible to satisfy the specifications for the damping ratio as well as for the steady-state performance. Also, the system's natural frequency is increased, which in turn implies smaller settling times.