Welcome to the world of control systems.This is
truly one of the most exciting and fascinating
aspects of electronics.
by Aaron Dahlen
In this series of articles, we will explore how to implement both analog and
digital control systems. We will be using a PID (Proportional Integral
Derivative) controller. With a PID controller, we can control thermal, electrical,
chemical, and mechanical processes. The PID controller is found at the heart
of many industrial control systems.
In this first of three installments, we will answer the “why”
questions. We will also lay a foundation to better understand what
a PID controller is. In subsequent installments, we will explore
how to tune the PID controller and how to implement a digital
PID using the ZILOG Encore! microprocessor.
The goal of this series is to introduce you to the world of
control electronics. Concepts will be explained in a simple,
intuitive fashion and useful, practical examples will be presented.
The math will be kept to an absolute minimum. This is not to say
that the math is not important. Quite the opposite — control
systems may be modeled and analyzed mathematically. The
mathematics is nothing short of amazing and I would
encourage you to peruse it. There are hundreds of books
that explain the theory and mathematics of control systems.
These books will introduce you to powerful tools, such as
Laplace transforms, root locus, and Bode plots. Again, this
series of articles hardly scratches the surface. There is much more
to be learned.
What Is PID Control?
The term PID is an acronym that stands for Proportional
Integral Derivative. A PID controller is part of a feedback system.
A PID system uses Proportional, Integral, and Derivative drive
elements to control a process. Some of you already know what P, I,
and D stand for. Don’t worry if you don’t; we will soon cover these terms with
easy-to-understand examples.
Why Do I Need PID Control?
You need the PID because there are some things that are difficult to control
using standard methods. Let me illustrate with an example. My first experience
with control systems was a failure. My goal was to regulate the output of a power supply using a
PIC microcontroller. The PIC read the output voltage with an AD converter and adjusted a PWM to
regulate the output. The control strategy was very simple: If the voltage was below a set-point, turn
on the PWM. If the measured voltage was above the set-point, then turn off the PWM. The PIC power
supply almost worked. It did produce the DC output voltage that I wanted. Unfortunately, it also has a
significant AC ripple riding on the DC signal.
The control strategy I just described is called on-off or bang-bang control. Many types of systems use this control
strategy. Take the furnace in my house as an example. When the temperature is below the set-point, the furnace is on.
When the temp is above the set-point, the furnace is off. Just like my power supply, the plot of temperature over time
results in a sine wave.
JANUARY 2005
65
