7.1.2 Adjusting PID Gains

The P gain of a PID controller will set the overall system response. When first tuning a controller, the I and D gains should be set to zero. The P gain can then be increased until the system responds well to set point changes without excessive overshoot or oscillations. Using lower values of P gain will loosely control the system, while higher values will give tighter control. At this point, the system will probably not converge to the set point.

After a reasonable P gain is selected, the I gain can be slowly increased to force the system error to zero. Only a small amount of I gain is required in most systems. Note that the effect of the I gain, if large enough, can overcome the action of the P gain, slow the overall control response and cause the system to oscillate around the set point. If this occurs, reducing the I gain and increasing the P gain will usually solve the problem.

After the P and I gains are set, the D gain can be set. The D term will speed up the response of control changes, but it should be used sparingly because it can cause very rapid changes in the controller output. This behavior is called set point kick. The set point kick occurs because the difference in system error becomes instantaneously very large when the control set point is changed. In some cases, damage to system hardware can occur. If the system response is acceptable with the D gain set to zero, then omit the D gain.

Figure 7-1. PID Control System