DriSteem Vapor-logic3, Installation And Operation Manual

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The integral term
The integral gain affects how fast the humidifier will
react to a droop condition. The higher the number, the
faster the reaction. (An integral term of zero disables
this variable and allows the unit to run on the
proportional band only.) The way this term works is
as follows: When the actual humidity is in the
proportional band, the humidifier demand is
somewhere between 0 and 100%. Every half second
the demand is updated. With an integral gain term
greater than zero and an actual humidity below set
point, each time the demand is updated it is increased
slightly. If the actual humidity is above set point, the
demand will be decreased slightly. The amount it is
increased or decreased is dependent on the difference
between the actual humidity and set point. (The closer
you are to the set point, the smaller the addition or
subtraction.)
When looking at this control scheme, something
interesting is found. The total demand signal for the
humidifier is the sum of the proportional part, the
integral part and the derivative part. As the actual
humidity approaches the set point, the integral portion
makes up the majority of the demand and the
proportional part makes up very little. Once the set
point is achieved and the unit stabilizes, the entire
demand is made up of the integral part because the
proportional part is zero. If the actual humidity goes
over the set point, the integral term will start to
decrease. In addition, the proportional term will go
negative and actually start to subtract from the total
system demand. These two items work in conjunction
with each other to bring the humidifier back to set
point.
The derivative term
A derivative term’s basic function in a PID loop is to
anticipate where the demand is going and to help it
get there. Its basic operation is as follows: As the
humidifier is running, the PID loop is generating an
error term. (The error term is the mathematical
difference between the RH set point and the actual
measured RH.) If the actual measured RH is below
set point and is rising, the derivative term will
subtract from the demand in anticipation of the
approaching set point. If the actual measured RH is
below set point and is falling, the derivative term will
add to the demand in anticipation of the need to get
the demand up faster and start climbing toward set
point. The same is true for situations above the set
point. If the actual measured RH is above set point
and is rising, the derivative term will subtract from
the total demand in anticipation of needing to reduce
demand and get closer to set point. If the actual
measured RH is above set point and falling, the
derivative term will add to the overall demand in
anticipation of the approaching set point. In the
majority of control situations, the derivative term is
not needed and is simply set to zero. The proportional
term and integral term will provide very tight,
accurate control without the addition of the derivative
term.
PID setup tips
A large proportional band (10 to 20%) will yield
tighter and more stable control with longer response
times. A small proportional band will produce quicker
response times but control may become unstable. As a
rule of thumb, start with a band of 10%. If the unit
does not “hunt” and a quicker response is needed, the
band can be lowered. If the unit hunts or the fast
response is not needed, increase the band. This will
result in more system stability. (To increase or
decrease the effect of the proportional band on system
performance, the proportional gain factor may be
adjusted. However, for the majority of systems, the
factory default setting of 80 is sufficient.) A moderate
integral gain term (30 to 50) will also yield tighter
and more stable control with a longer response time.
A large integral gain will quicken the response but
may cause the system to become unstable. As a rule
of thumb, start with an integral term of 40. If the unit
does not hunt and quicker response is needed, raise
the integral gain. If the unit hunts or does not need the
fast response, decrease the integral gain.
Control functions: PID tuning (cont.).
Operation
Operation:
PID tuning
VL3-IOM-0301-METRIC.pdf 47 11/19/2009 9:22:39 AM