The gap metric is introduced to design the robust controller for the quantitative analysis of uncertainty as much
as possible. The robust margin of the robust controller is defined in a tracking system based on the gap metric, and the robust
controller is designed according to the gap metric in the tracking system. Uncertainty the plant and the controller should be
considered simultaneously to make the designed controller has some ability of disturbance rejection, when a control system
is being synthesized. Simulation results show that the robust controller having bigger robust margin has abilities of both
stronger disturbance rejection ability and better tracking performance compared with common PID controller when additive
uncertainty is existing.