Honeywell AUTOMATIC CONTROL SI Edition, Инструкция по эксплуатации

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ELECTRONIC CONTROL FUNDAMENTALS
ENGINEERING MANUAL OF AUTOMATIC CONTROL
129
Fig. 18. Modulating Control.
TRANSDUCER
In some applications, a transducer converts a controller output
to a signal that is usable by the actuator. For example, Figure 19
shows an Electronic-to-Pneumatic (E/P) transducer: electronic-
to-pneumatic that converts a modulating 2 to 10V dc signal from
the electronic controller to a pneumatic proportional modulating
20 to 90 kPa signal for a pneumatic actuator.
1 2 3 . 4
DIGITAL
DISPLAY
LED PANEL
ANALOG METER
0
20
40
60
80
100
C3089
INDICATING DEVICE
An electronic control system can be enhanced with visual
displays that show system status and operation. Many electronic
controllers have built-in indicators that show power, input signal,
deviation signal, and output signal. Figure 20 shows some types
of visual displays. An indicator light can show on/off status or,
if driven by controller circuits, the brightness of a light can
show the relative strength of a signal. If a system requires an
analog or digital indicating device and the electronic controller
does not include this type of display, separate indicating devices
can be provided.
INPUT
POWER
SENSORS
REMOTE
SETPOINT
INPUT
CIRCUIT
REGULATED
VOLTAGES
INPUT
DEVIATION
SIGNAL
CONTROL
CIRCUIT
HIGH/LOW
LIMIT
SENSORS
RESET-
COMPENSATION
SENSOR
OVERRIDE
CONTROL
POWER
SUPPLY
CIRCUIT
OUTPUT
CIRCUIT
OUTPUT TO
ACTUATOR OR
ACCESSORY
DEVICE
C3087
CONTROL
SIGNAL
(INTERNAL)
MODULATING
VALVE ACTUATOR
CHILLED WATER
SUPPLY TO COIL
C3081
POWER
ELECTRONIC
CONTROLLER
+
-
OUTPUT
Fig. 19. Electric-to-Pneumatic Transducer.
Fig. 20. Indicating Devices.
INTERFACE WITH OTHER SYSTEMS
It is often necessary to interface an electronic control device
to a system such as a microprocessor-based building management
system. An example is an interface that allows a building
management system to adjust the setpoint or amount of reset
(compensation) for a specific controller. Compatibility of the two
systems must be verified before they are interconnected.
ELECTRONIC CONTROLLER FUNDAMENTALS
GENERAL
The electronic controller is the basis for an electronic control
system. Figure 21 shows the basic circuits of an electronic
controller including power supply, input, control, and output.
For greater stability and control, internal feedback correction
circuits also can be included, but are not discussed. The circuits
described provide an overview of the types and methods of
electronic controllers.
Fig. 21. Electronic Controller Circuits.
POWER SUPPLY CIRCUIT
The power supply circuit of an electronic controller provides
the required voltages to the input, control, and output circuits.
Most voltages are regulated dc voltages. The controller design
dictates the voltages and current levels required.
All power supply circuits are designed to optimize both line
and load regulation requirements within the needs and con-
straints of the system. Load regulation refers to the ability of
the power supply to maintain the voltage output at a constant
value even as the current demand (load) changes. Similarly,
line regulation refers to the ability of the power supply to main-
tain the output load voltage at a constant value when the input
(ac) power varies. The line regulation abilities or limitations of
a controller are usually part of the controller specifications
such as 120V ac +10%, –15%. The degree of load regulation
involves the end-to-end accuracy and repeatability and is usu-
ally not explicitly stated as a specification for controllers.
+
–
OUTPUT
2-10V dc
E/P
TRANSDUCER
PNEUMATIC
VALVE
ACTUATOR
20-90 kPa
ELECTRONIC
CONTROLLER
VARIABLE RESTRICTION CONTROLLED
BY E/P TRANSDUCER ELECTRONICS
M15133
140 kPa
AIR SUPPLY
M