130-28176 Parallax Inc, 130-28176 Datasheet - Page 255

• Manufacturer Part Number: 130-28176
• Description: KIT PARTS PROCESS CONTROL
• Manufacturer: Parallax Inc

Specifications of 130-28176

• Accessory Type: Education Kit
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: Board of Education Full Kit
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant

Chapter 8: Proportional-Integral-Derivative
Control
PID is an acronym for Proportional-Integral-Derivative Control. In this chapter, we will
explore each of these control methods and how they work together to efficiently control a
system.
One objective of a process control method is to hold a system constant. In the previous
chapter we used various means of cycling the heater of our incubator to maintain a
desired temperature. Using differential gap, we created an allowable band, or range of
temperatures, in which the heater would cycle, causing the temperature to cycle on and
off, above and below the desired setpoint. In Chapter 6, we saw how we could use pulse-
width modulation (PWM) to add variable amounts of energy to our system in duty cycles
between 0% (fully off) and 100% (fully on). While these types of control have their
advantages and disadvantages, PID control allows greater control of a system but can be
more difficult to implement and tune (or adjust) for optimum performance. Holding a
process constant involves continually adding energy that exactly equals the system
energy losses. If the system losses were constant, then process control would be as simple
as applying one steady state level of drive. However, the factors that affect a process do
change. They change in unpredictable magnitudes and at unpredictable rates.
Compounding this problem is that a system has reaction delays that must be understood.
An instant change in energy losses due to a disturbance is not felt immediately.
Furthermore, an instant change in drive does not create an instant output response.
Process control can be as much an art form as it is a science. The first step to
understanding PID control is to realize that every system has both gains and losses of
energy.
A system is said to be in equilibrium when the energy gained equals the energy lost.
In equilibrium, our incubator would maintain a constant temperature. But this is seldom,
if ever, the case. Depending on the heater drive and conditions surrounding the incubator,
temperature will either be increasing or decreasing. Conditions of the system will change
the energy loss, such as room temperature changing, air movement changes around the
E
Equilibrium: E
System
= E
In
− E
In
Out
= E
Out