Chapter hvac engineering Fundamentals: Part 1 Introduction

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Chapter Eight
Figure 8.5
Proportional control, unstable.
Figure 8.6
Proportional plus integral control.
The integral term has the effect of continuing to increase the output
as long as the error persists, thereby driving the system to eliminate
the error, as shown in Fig. 8.6. The integral gain
K
i
is a function of
time; the shorter the interval between samples, the greater the gain.
Again, too high a gain can result in instability.
The derivative mode is described mathematically by
K
d
de
/
dt
, where
de
/
dt
is the derivative of the error with respect to time. A control mode
which includes all three terms is called
PID
(proportional-integral-
Design Procedures: Part 6
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