Putting it all together. I f we compare the composite feedforward torque signal of Fig. 9 c) with the torque output of only the P.I .V. controller with a bandwidth of 20Hz and a damping ratio of 1, we notice how strikingly similar they are. Fig. 10 shows this comparison. This suggests that we could have near zero following error if our Feed forward control is accurate. Figure 10. Comparison of Feedforward Torque with the Output of only P.I .V. Control. Feedforward control goes a long way towards reducing settling times and minimizing overshoot, however there are several of assumptions that ultimately limit its effectiveness. For example, servo amplifiers all have current limits and finite response times. For motion bandwidths in the sub 50 Hz range, the current loops can be safely ignored, however as the need to push the motion bandwidths higher, the current
loops need to be accounted for as well. In addition, the single most limiting factor in servo motion control is the resolution and accuracy of the feedback device. Low-resolution encoders contribute to poor velocity estimations that lead to either limit cycling or velocity ripple problems. Finally, compliant couplers that connect the load to the servomotor must also be accounted for as they too limit the useable motion bandwidths. In summary, disturbance rejection control can be obtained by one of a number of ways, the two most common are P.I .D. and P.I .V. control. The direct use of P.I .D. control can often meet low performance motion control loops and are generally set by either the Ziegler Nichols or by trial and error methods. Overshoot and rise times are tightly coupled making gain adjustments difficult. P.I .V. control on the other hand, provides a method to significantly decouple the overshoot and rise time, allowing for easy set up and very high disturbance rejection characteristics. Finally, feedforward control is needed in addition to disturbance rejection control to minimize the tracking error.