Table of Contents
Advertisement
PNEUMATIC CONTROL FUNDAMENTALS
SENSOR-CONTROLLER SYSTEMS
A sensor-controller system is made up of a pneumatic
controller, remote pneumatic sensors, and a final control
element. The controller provides proportional or proportional-
integral control of temperature, humidity, dew point, or pressure
in HVAC systems. Sensors do not have a setpoint adjustment
and provide a linear 20 to 100 kPa signal to the controller over
a fixed sensor range. The controller compares the sensor input
signal with the setpoint signal. The difference is the pilot input
to a signal amplifier, which provides a branchline pressure to
the controlled device. Thus the controller acts as a general-
purpose pneumatic amplifier.
PNEUMATIC CONTROLLERS
Controllers generally use diaphragm logic, which allows
flexible system application, provides more accurate control,
and simplifies setup and adjustment for the needs of each
system. Controllers may be proportional only or proportional-
integral (PI). The integral function is also called "automatic
reset". Proportional and PI controllers are available with single-
sensor input or dual-sensor input for resetting the primary sensor
setpoint from a second sensor. They are also available with
integral or remote setpoint adjustment.
The single-input controller consists of a signal amplifier
feeding a capacity amplifier. The capacity amplifier is discussed
under PILOT BLEED SYSTEM. A dual-input controller has
inputs from a primary temperature sensor and a reset
temperature sensor. The reset sensor resets controller setpoint.
Reset can be negative or positive.
Figure 21 depicts a single-input controller as it would appear
in a simple application. Figure 22 depicts a dual-input controller
with manual remote setpoint control. In Figures 21 and 22 the
sensors are fed restricted main air from the controllers. Where
sensors are located extremely remote from the controller, a
remote restrictor may be required.
MAIN AIR
(125 kPa)
HOT WATER
SINGLE INPUT
VALVE
CONTROLLER
Fig. 21. Single-Input Controller.
M
TEMPERATURE
SENSOR
M15140
MAIN AIR
M
(125 kPa)
HOT WATER
VALVE
Fig. 22. Dual-Input Controller
with Manual Remote Setpoint.
PROPORTIONAL-INTEGRAL (PI) CONTROLLERS
Variations of single-input and dual-input controllers can
provide proportional-integral (PI) control. PI controllers are
used in critical applications that require closer control than a
proportional controller. A PI controller provides close control
by eliminating the deviation from setpoint (offset) that occurs
in a proportional controller system. PI controllers are similar
to the controllers in Figures 21 and 22 and have an additional
knob for adjusting the integral reset time.
CONTROLLER ADJUSTMENTS
Controller operation is adjusted in the following ways:
— Adjusting the setpoint
— Changing between direct and reverse control action
— Adjusting the proportional band (throttling range)
— Adjusting the reset authority
— Adjusting the integral control reset time
The setpoint can be manually adjusted with a dial on the
controller. Remote setpoint adjustment is available for all con-
trollers. Control action may be direct or reverse, and is field
adjustable. The proportional band setting is typically adjust-
able from 2.5 to 50 percent of the primary sensor span and is
usually set for the minimum value that results in stable control.
In a sensor with a span of 110 kelvins, for example, the mini-
mum setting of 2.5 percent results in a throttling range of
2.75 kelvins (0.025 x 110 = 2.75 kelvins). A change of 2.75
kelvins is then required at the sensor to proportionally vary the
controller branchline pressure from 20 to 90 kPa. A maximum
setting of 50 percent provides a throttling range of 55 kelvins
(0.50 x 110 = 55 kelvins).
ENGINEERING MANUAL OF AUTOMATIC CONTROL
72
PRIMARY
SENSOR
RESET SENSOR
M
MANUAL REMOTE
SETPOINT CONTROL
M15141
Advertisement
Chapters
Table of Contents
