DriSteem Vapor-logic3, Installation And Operation Manual

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Tune your system with PID loop
When your unit is equipped with a humidity or dew
point transmitter (when you are able to adjust the set
point through the keypad/display) control is
accomplished through a PID control loop. PID stands
for Proportional, Integral and Derivative.
With a PID loop, you are able to “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 that are added together. There is the
proportional piece, the integral piece and the
derivative piece. Each one of these parts is calculated
and then multiplied by a gain factor. These gain
factors are the setup variables that you have access to.
By making a gain factor larger, you increase its
component’s overall influence on the system demand
equation. Once the multiplication has taken place, all
three terms are added together to determine the
overall demand percentage. One other term that is
user-controlled in the PID equation is the
proportional band.
The proportional term
The proportional band is the band (in % RH, or °C
for dew point control) in which the humidifier will
modulate.
For example, if starting with a set point of 35% and a
proportional band of 10%, the humidifier operation
will be as follows: On initial start-up, assume an
actual humidity level of 15%. The proportional band
is 10% so the humidifier will modulate when the
actual humidity is in the range 25-45%. If the actual
humidity is lower than 25%, the humidifier will be
full on. If the actual humidity is above 45%, the
humidifier will be full off. In the example, starting at
an actual humidity level of 15%, the humidifier will
be full on. As the actual humidity level climbs into the
range of the proportional band (25-45%), the
humidifier will begin to modulate down. Once the set
point of 35% is reached, the humidifier will be full
off.
Looking at this scheme a little closer, a problem can
be found. In almost all applications there will be some
constant load on the humidifier just as there is a
constant load on the heating equipment. With this
control scheme, the actual humidity must be less than
the set point for the humidifier to be on. What ends up
happening 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.
Operation:
PID tuning
Control functions: PID tuning.
Operation
VL3-IOM-0301-METRIC.pdf 46 11/19/2009 9:22:39 AM