by Aaron Dahlen
Why is the sky blue? Why do birds sing? Why do I
have 10 fingers? We all asked these questions when we
were children. When I started to learn about control
systems, I felt just like a kid again. I asked all sorts of
questions. Why does a system overshoot? Why
can’t I turn the gain up higher? Why is this
system oscillating? It’s wonderful to be like a
child again. There is so much to be learned.
In Part 1, we layed the foundation for the Proportional
Integral Derivative PID controller. Using a simple, intuitive
approach, we explored what each term represents. We examined how simple op-amp circuits could be used to construct
the individual elements. In Part 2, we will see a complete
analog PID system. We will explore the dynamic operation of
the PID control system. We will explore how to operate and
tune a PID controller. Using our servo motor example, we will
explore system stability. We will learn new terms, such as overshoot,
dampening, and oscillation. We will see how the individual P, I, and
D terms come together to form a complete control system.
Last month, we came to several conclusions about each of
the P, I, and D terms. These concepts are very important to our
present discussion, so we will review them again. You can follow
along with the block diagram shown in Figure 1.
Proportional Concepts (PRO)
A system will try to correct the error between the set point and the measured
output. It does this by commanding the system in a direction that opposes the
error. The intensity of the correction is determined by proportional gain. The
proportional component provides a correction only if there is an error!
Integral Concepts (INT)
NUTS & VOLTS
The integral section operates when an error is present. It accumulates this error over time.
Therefore, a small error can become a large correction, given enough time. As the error is accumulated,
the system will be forced to correct the error. Finally, the integrator will overshoot the set point. An error
opposite of the original is required to discharge the capacitor.
Derivative Concepts (DIF)
The output of the differentiator is proportional to the speed of the system. If the system is moving fast, the output
62 FEBRUARY 2005