Installation guide

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Adjusting set point with PID loop
When your unit is equipped with a humidity or dew point
transmitter, you can adjust and control the set point through the
keypad/display, using a proportional, integral, and derivative (PID)
control loop.
With a PID loop, you can “tune” your system for maximum
performance using the proportional (Kp), integral (Ki), and
derivative (Kd) gain terms. These gain factors work in the following
way: the overall demand in a PID system is made up of three
distinct parts added together—the proportional, the integral, and
the derivative. Each of these parts is calculated and then multiplied
by its corresponding gain factor. These gain factors are the setup
variables you have access to from the Set Up menu. By making
a gain factor larger, you increase its overall influence on system
demand. Once each PID component is multiplied by its gain factor,
all three terms are added together to determine the overall demand
percentage.
The proportional term
The proportional term is the difference between the RH set point
and the actual humidity multiplied by the proportional gain. For
example, with a Kp of 80 and the actual humidity 5% below the
RH set point, the proportional contribution to the demand is:
¨¨UIFJTBOJOUFSOBMTDBMBSVTFEUPJODSFBTF
the usable span of Kp).
There is a problem with using only proportional gain to control the
RH. In almost all applications there is some constant load on the
humidifier just as there is a constant load on heating equipment. If
the proportional term is all that is used, the actual humidity must be
less than the set point for the humidifier to be on.
What happens is the humidifier finds a “happy medium” where the
actual humidity is something less than the set point, which allows
the humidifier to continue to run. This difference between the set
point and the actual running humidity level is called the “droop.”
This droop can be corrected using the next term, the integral gain.
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VL3_IOM.pdf 70 11/19/2009 9:11:15 AM