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9.5.
Analog Output Control
The analog outputs on the GP1 boards can be set to closed loop PID or staged control. Most applications such as variable air
delivery and variable speed exhaust fans will use the closed loop PID option. The closed loop PID method used by the M2
has four constants, manual reset (ManRS), proportional (P), integral (I) and derivative (D) as shown in figure 19. The PID
control constants as well as the output minimum and maximum values, may be adjusted if necessary.
Figure 19
PID Operation Diagram for VAV/CAVB Air Delivery and Exhaust Fan Control
.
9.5.1.
Manual Reset (ManRS)
The output value when the P, I, and D values are all 0. This is the approximate output expected which allows quicker settling
at setpoint. On CAV units with bypass damper, this is also the damper position when blower is off. ManRS can be adjusted
between a minimum value and 100%.
9.5.2.
Proportional Constant (P)
To handle the present, this is the value of the gain' that is multiplied times the error. The error is the difference between the
output and the setpoint. A large value of P" will cause the output to reach the setpoint faster, however, this faster rate can
cause the output to overshoot the setpoint. On the other hand, a low value of P" will reduce overshoot, but will cause the
output reaction to be too slow. The P"constant ECTO value can be adjusted between 0 –127 with 0 being off and 127 being
the highest value.
9.5.3.
Integral Constant (I)
To handle the past, this gain is proportional to the amount of time that the error is present. This gain tries to integrate out any
offset. A high value of I" can provide fast correction but can cause overshoot and ringing. The I' gain should be set to the
lowest value possible that corrects the offset. I" can be adjusted between 0−127 with 127 being the lowest value. The I"
constant ECTO value is inverted. A value of 0 turns the integral factor off. A value of 127 is the minimum and 1 is the maxi-
mum.
9.5.4.
Derivative Constant (D)
To handle the future, this gain is proportional to the rate of change of the error and provides a damping factor. The D"
constant ECTO value can be adjusted between 0− 127 with 0 being off and 127 being the highest value. Most M2 applica-
tions do not require using any D" gain.
9.5.5.
Tuning
In the event that the PID loop requires tuning, the following two methods are recommended depending on the severity of the
problem:
1. In most cases the parameters will only need a small adjustment. In that case use the following table as a basic guide.
Change Desired
Reduce Response time
Reduce Overshoot
Reduce Settling Time
Reduce Offset
(steady state error)
2. If method 1 adjustment does not stabilize the system, you may use the following tuning method:
NOTE − To use this method the system must be allowed to operate at the manual reset value (% output) without damag-
ing the system.
A
Set the P, I and D constants to 0 (off).
506457−01 05/10
M2
Static
∑
Pres. SP
Feedback
Static Pres. Sensor
ECTO P" Parameter
Increase
Decrease
−
Increase
Page 40
P Gain
∑
I Gain
VFD or Bypass Damper
D Gain
ECTO I" Parameter
Decrease
Increase
Decrease
Decrease
9.
Supply Air Delivery
ECTO D" Parameter
Decrease
Increase
Decrease
−
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