Proportional Control; General - Honeywell AUTOMATIC CONTROL Engineering Manual

SETPOINT
CONTROLLER
LOAD
DAMPER
POSITION
T1

PROPORTIONAL CONTROL

GENERAL

Proportional control proportions the output capacity of the
equipment (e.g., the percent a valve is open or closed) to match
the heating or cooling load on the building, unlike two-position
control in which the mechanical equipment is either full on or
full off. In this way, proportional control achieves the desired
heat replacement or displacement rate.
In a chilled water cooling system, for example (Fig. 31), the
sensor is placed in the discharge air. The sensor measures the
air temperature and sends a signal to the controller. If a
correction is required, the controller calculates the change and
sends a new signal to the valve actuator. The actuator
repositions the valve to change the water flow in the coil, and
thus the discharge temperature.
VALVE
CHILLED
WATER
RETURN
AIR
Fig. 31. Proportional Control Loop.
T2 T3
Fig. 30. Floating Control.
CONTROLLER
SENSOR
DISCHARGE
AIR
COIL
C2718
CONTROL POINT
T4 T5 T6
TIME
In proportional control, the final control element moves to a
position proportional to the deviation of the value of the
controlled variable from the setpoint. The position of the final
control element is a linear function of the value of the controlled
variable (Fig. 32).
100%
OPEN
ACTUATOR 50%
POSITION OPEN
CLOSED
Fig. 32. Final Control Element Position as a Function of
the Control Point (Cooling System).
The final control element is seldom in the middle of its range
because of the linear relationship between the position of the
final control element and the value of the controlled variable.
In proportional control systems, the setpoint is typically the
middle of the throttling range, so there is usually an offset
between control point and setpoint.
21 ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONTROL FUNDAMENTALS
ON
"CLOSE"
SWITCH
DIFFERENTIAL
OFF
DEADBAND
OFF
"OPEN"
SWITCH
DIFFERENTIAL
ON
FULL LOAD
NO LOAD
OPEN
CLOSED
T7
C2094
POSITION OF FINAL
CONTROL ELEMENT
73 74 75 76
CONTROL POINT (°F)
THROTTLING RANGE
77
C2095