Hardware Reference
In-Depth Information
to zero, and increase the Kp value until the system becomes unstable; then back off a bit until it becomes stable again.
This particular system becomes stable around a Kp value of 2.27. For a slower system or one that needs a slower
reaction to error, a lower gain will be required. After the proportional component is set, move on to the integral.
Figure 7-5. Proportional control: Kp = 1 (left) and Kp = 7 (right)
Figure 7-6 demonstrates the addition of the integral component, making a PI controller. The left side of the figure
shows that a lower Ki gain produces a slower controller that approaches the setpoint without overshoot. The right
side of the figure, with a gain of 2, shows a graph with a faster rise, followed by overshoot and a ringing before settling
at the setpoint. Setting a proper gain for this part is dependent on the needs of the system and the ability to react to
overshoot. A temperature system may need a lower gain than a system that controls positing; it is about finding a
good balance.
Figure 7-6. Proportional integral control: Kp = .5; Ki = .1 (left) and Ki = 2 (right)
The derivative value is a bit more difficult to tune because of the interaction of the other two components. The
derivative is similar to a damper attempting to limit the overshoot. It is perfectly fine omit the derivate portion and simply
use a PI controller. To tune the derivative, the balance of the PI portions should be as close as possible to the reaction
required for the setup. Once you've achieved this, then you can slowly change the gain in the derivative to provide some
extra dampening. Figure 7-7 demonstrates a fully functional PID with the PID Tuner program. In this graph, there is a
small amount of overshoot, but the derivate function corrects and allows the setpoint to be reached quickly.
 
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