%3*45&&. 7BQPSMPHJD *OTUBMMBUJPO BOE 0QFSBUJPO .BOVBM t 1BHF
Operation
The derivative term
The derivative term is the measured change in error over time
multiplied by the derivative gain (differentiating error with respect
to time).
If the actual measured RH is below set point and is rising, the
derivative term subtracts 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 adds to the demand in anticipation of the need to
get the demand up faster and start climbing toward set point.
If the actual measured RH is above set point and falling, the
derivative term adds to the overall demand in anticipation of the
approaching set point. It generally is used to increase damping and,
in some cases, improves the stability of the system.
However, in the majority of control situations, the derivative term
is not needed and is simply set to zero. The proportional term and
integral term provide tight, accurate control without the addition
of the derivative term.
PID band
The last user-controlled term in the PID equation is the PID band.
The PID band defines the range of measured RH values (°F/°C
for dew point control) where the PID loop is in operation. The
PID loop is in operation when the measured RH is in the range
of (RH set point – PID band) to (RH set point + PID band). If
the measured RH is below the PID band, the PID calculations
are suspended and the demand is set to 100%. Conversely, if the
measured RH is above the PID band, the demand is set to 0%.
For example, if starting with an RH set point of 35% and a PID
band of 10%, the PID loop operates when the actual humidity is
in the range of 25% to 45%. If the actual humidity is lower than
25%, the humidifier is full on. If the actual humidity is above 45%,
the humidifier demand is 0%. The PID band aids in speeding the
response time of the system. It allows the RH to get somewhat close
to the set point and then lets the PID loop precisely control the RH
when it is within the PID band. The default value for the PID band
is 10%.
Large spaces where the humidification system influences the RH
very slowly typically benefit from a smaller PID band. Small spaces
where the humidification system can quickly influence the RH
typically benefit from a larger PID band. Rarely should it be set to
less than 10%.
1 * % U V O J O H
