User Guide

System 350 P352P Electronic Proportional Plus Integral Static Pressure Controls 3
Minimum Output Adjustment
This adjustment is used to set the minimum voltage or
milliampere (mA) output that the P352P will provide. It
can be set from 0 to 60% (up to 6 VDC or up to
12 mA).
Example: A controlled device that responds to a
4 to 20 mA output would require that the minimum output
be adjusted to 20% or 4 mA (see Figure 3).
The “MIN OUTPUT” adjustment is located on the lower
circuit board to the right of the throttling range
potentiometer. One bar will light on the bar graph LED
(front of control) for each 10% increase in range.
Minimum
Output
mA
Throttling Range
(in. W.C.)
20
10
0
Setpoint
4
1 2 3
(P352PQ-1C)
0.05 0.1 0.15 (P352PQ-2C)
Figure 3: Minimum Output Set to 20% (4 mA) and
Throttling Range Set to 3 in. W.C. (P352PQ-1C) or
0.15 in. W.C. (P352PQ-2C)
Throttling Range (Proportional Band)
The throttling range can be adjusted from
0.5 to 5 in. W.C. (P352PQ-1C) or
0.025 to 0.25 in. W.C. (P352PQ-2C) (see Figure 3).
Adjustment is made using the potentiometer marked
“THROT RANGE” located on the lower printed circuit
board to the left of the “MIN OUTPUT” potentiometer.
Integration Constant
Proportional controls cannot hold a process at the exact
setpoint. A proportional offset is always present and is due
to the fact that the control output reaches 0% at setpoint.
Any load on the system will cause the point of control to be
offset from the setpoint. The greater the load on the
system, the further the control point will be offset from the
setpoint (this is commonly referred to as “proportional
offset”).
Some proportional only controls are designed with their
setpoint located midway through the proportional band to
help compensate for this offset, but a fixed 50% controlled
device action at setpoint does not necessarily match the
system load. Depending on the load, the control point can
still be held some distance from setpoint. This results in a
plus/minus error from the setpoint rather than a single-
ended error.
The P352P has an integration feature, which measures the
control point’s distance from setpoint and automatically
adjusts the amount of control output. Over a period of
time, the P352P’s output is brought into balance with the
system load at setpoint
(see Figure 4). The rate of correction automatically
increases as the process deviates further from setpoint.
On traditional proportional plus integral controls, the
amount of correction is sometimes in danger of becoming
too large. When the output of the actuated device (valve or
damper) is fully open or closed and the setpoint still cannot
be reached, the integration error continues to grow. The
result is called
“integral windup.” The P352P avoids “integral windup”
with a patented circuit that puts a dynamic ceiling on the
integrator. This resets the integration error when the
sensor goes just above the setpoint plus the throttling range
(in DA [Direct Action] mode) or just below the setpoint
minus the throttling range (in RA
[Reverse Action] mode). This will allow the process to
recover from an out-of-range condition without a long
period of overshoot.
The P352P has three field-selectable integration constants
as well as an off position. The DIP switch block that
selects the integration constant is located on the lower
circuit board next to the RA/DA mode jumper blocks (see
Figure 2). The field-selectable integration constants
include:
• OFF (Switch 1 to On position, all others Off)
provides proportional only operation.
Note: In open-loop (without feedback) applications,
select “OFF” proportional only operation.
• C3 (Switch 2 to On position, all others Off) is the
slowest integration constant (equal to 9 min.) and is
suitable for most proportional plus integral
applications. It is recommended that C3 be the initial
setting.