AERCO BMS 168 - Manual (Page 40)

specific installation. The factory defaults
preset by AERCO work well for most
applications. In instances when there is a
large error between the setpoint and the
actual supply water temperature, the BMS
may appear to require PID tuning. However,
It is best to observe BMS operation over a
period of time prior to making any PID
changes. Contact AERCO or an AERCO
representative prior to making any PID
setting changes.
The functions provided by the PID function
keys are described in the following
paragraphs.
4.4.7.1 TEMP BANDWIDTH
Header Temperature Bandwidth (HDR
TEMP B.W.) concerns the system’s
response to the setpoint error. Setpoint
error is the difference between the supply
water temperature setpoint and the actual
supply water temperature. A constant
setpoint error will yield a constant and
proportionate correction factor for the
duration of the error. If there is a deviation
from the constant error, the correction factor
will be changed in proportion to the
deviation. For instance, a temperature
bandwidth of 50°F is chosen. The header
temperature setpoint is 180°F and the
actual incoming supply water temperature is
130°F
. This is a 50° error and the following
is true:
Temp. Error_____
X 100 = Firing Rate in %
Prop Bandwidth
Therefore:
50 X 100% = Firing Rate
50
1 X 100 = 100 % Firing Rate
With an error of 30° and a bandwidth of 50,
the following would be true:
30/50 X 100 = .6 X 100 = 60% Firing Rate.
4.4.7.2 INTGL RATE GAIN
Integral gain responds to the setpoint error
over a period of time. Integral references
the proportional bandwidth error signal and
sums itself with respect to the period of time
that an error exists. Based on the example
in the previous paragraph (4.4.7.1), if the
integral is 0.15 repeats per minute and the
firing rate is 60%, and a temperature error
exists for 1 minute, then the following is
true:
(0.15 reps/min.) x (60% firing rate) = 9%
actual firing rate
60% firing rate +9% firing rate = 69% firing
rate
If the error continues and is present for
another minute, another 9% correction
factor will be added:
69% firing rate +9% firing rate = 78% firing
rate
If, after a load change, the supply water
temperature stabilizes at a temperature
above or below the setpoint, the integral
gain should be increased. If, after a load
change, the supply water temperature
overshoots and oscillates excessively,
integral gain should be reduced.
4.4.7.3 DERIV GAIN
Derivative gain is a function of time. It
senses and responds to the rate of change
of the setpoint error. A slow rate of change
will yield a small amount of derivative gain.
Conversely, a fast rate of change will yield a
large derivative gain. Too high a derivative
gain setting will produce a large output for a
short time. This can result in overshoot of
the setpoint. Too low a derivative gain
setting will have the opposite effect,
producing a small output for a longer period,
and may result in slow system response or
the system undershooting the setpoint.
4-14