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22 Direct-Fired Indoor and Outdoor Units • MUA-SVP01A-EN
Functions
REVERSE ACTING
The controller output AO-1 is modulated by a PID loop from 0% to 100% based on
the User Control setpoint and the input signal to IN-9. As the input signal
increases the output voltage decreases. Additionally, a secondary control acts to
override the PID loop to scale the output as follows.
If the input variable equals the User Control setpoint, the output will be
determined by the PID loop. In this situation the override has no effect on the
output value.
If the input variable is above the User Control setpoint, the output could decrease
from 100% to 0% as the input variable increases from the User Control setpoint
to the Max User setpoint. In this situation the output value is the lesser of the PID
output or the override output.
If the input variable is below the User Control setpoint, the output could increase
from 0% to 100% as the input variable decreases from the User Control setpoint
to the Min User setpoint. In this situation the output value is the greater of the
PID output or the override output. See the following example.
Assume the user wishes to control the fan speed using a variable frequency drive
(VFD). The VFD input is 0-10Vdc. The control variable will be building pressure.
As the building pressure increases, the fan speed should decrease. The building
pressure transducer has a range of -0.1" to +0.1". The user would like the building
Figure 2. Direct acting
80ºF 100%
override
scaler
75ºF 0%
Max User SP
75ºF PID 0-100%
Min User SP
User Ctrl SP
>
=
<
0-100%
Output
75ºF 100%
override
scaler
70ºF 0%