Post date: Mar 4, 2014 12:00:07 AM
While intuitive actions of proportional, derivative and integral actions are widely discussion in PID control, one additional aspect is that the derivative term increases the damping of the closed-loop system. This leads to the less oscillatory response, and also allows for the use of a higher proportional gain.
Example, consider a second order system with only an inertial load (no damping). This could be something like 1/(J*s^2). If only a proportional term K is used, then the closed-loop transfer function would be of the form 1/(J*s^2 + K). The step response will exhibit oscillations with no decay. If instead we use a PD control term K(1+s), then the closed-loop TF will be of the form 1/(J*s^2 + K*s + K). Hence, there is a damping term, and the step response will be much better.
Note, in practice, the derivative signal may inject noise, so the term oscillatory here refers to the low frequency component.