Johnson Controls System 350T A350P Manual page 4

Temperature
Proportional
Offset
Setpoint
Temperature
Proportional
Offset = 0
Setpoint
Figure 4: Comparison Between Proportional Only and Proportional Plus Integral Control
The A350P control has an integration feature that
forces the control point to match the setpoint. Over
time, the A350P will control the heating/cooling
equipment to balance the system load at the control
setpoint. (See Figure 4.)
On traditional proportional plus integral controls, the
amount of correction will become too large if the
system load exceeds the capacity of the equipment.
When 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 A350P control 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 mode) or just below the setpoint minus
the throttling range (in RA mode). This allows the
process to recover from an out-of-range condition
without a large overshoot.
4 System 350 A350P Electronic Proportional Plus Integral Temperature Control Product/Technical Bulletin
Proportional Only Control
Load Change
Time
Proportional Plus Integral Control
Load Change
Time
System Load
Control Point
Follows the Load
System Load
Integration adjusts the
proportional output to
bring the process to
setpoint regardless of load.
The A350P control has three field-selectable
integration constants and an off position. The
integration DIP switch selects the integration constant.
(See Figure 2 for location.)
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.
•
Slow (C3): Switch 2 to On position, all others
Off is the slowest integration constant (26 minute),
and is suitable for most proportional plus integral
applications. Slow is the recommended initial
setting.
•
Medium (C2): Switch 3 to On position, all
others Off selects a 13-minute integration
constant. If the rate of system recovery to setpoint
is sluggish with the control set to slow, and if the
system has enough capacity to drive the process
to setpoint at a faster rate, the medium setting
may be used.
•
Fast (C1): Switch 4 in On position, all
others Off is the fastest integration constant
(6.5 minutes). Use fast only in instances where the
rate of change at the sensor is extremely rapid
and system capacity can compensate for that
rapid change.
Throttling
Range