Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
ELECTRONIC CONTROL FUNDAMENTALS
122
TYPICAL SYSTEM
Figure 1 shows a simple electronic control system with a
controller that regulates supply water temperature by mixing
return water with water from the boiler. The main temperature
sensor is located in the hot water supply from the valve. To
increase efficiency and energy savings, the controller resets
the supply water temperature setpoint as a function of the
outdoor air temperature. The controller analyzes the sensor data
and sends a signal to the valve actuator to regulate the mixture
of hot water to the unit heaters. These components are described
in COMPONENTS.
MAIN SENSOR
(HOT WATER SUPPLY)
OUTDOOR AIR
SENSOR
REMOTE
SETPOINT
ADJUSTMENT
CONTROLLER
HOT WATER
SUPPLY TO
HEATING
SYSTEM
FROM
BOILER
RETURN
FROM
SYSTEM
AC
POWER
INPUT
INPUTS
ELECTRONIC
CONTROLLER
FINAL CONTROL
DEVICE
C3096
Fig. 1. Basic Electronic Control System.
COMPONENTS
An electronic control system includes sensors, controllers,
output devices such as actuators and relays, final control
elements such as valves and dampers, and indicating,
interfacing, and accessory devices. Figure 2 provides a system
overview for many electronic system components.
SENSORS
A sensing element provides a controller with information
concerning changing conditions. Analog sensors are used to
monitor continuously changing conditions such as temperature
or pressure. The analog sensor provides the controller with a
varying signal such as 0 to 10V. A digital (two-position) sensor
is used if the conditions represent a fixed state such as a pump
that is on or off. The digital sensor provides the controller with
a discrete signal such as open or closed contacts.
Some electronic sensors use an inherent attribute of their
material (e.g., wire resistance) to provide a signal and can be
directly connected to the electronic controller. Other sensors
require conversion of the sensor signal to a type or level that
can be used by the electronic controller. For example, a sensor
that detects pressure requires a transducer or transmitter to
convert the pressure signal to a voltage or current signal usable
by the electronic controller. Typical sensors used in electronic
control systems are included in Figure 2. A sensor-transducer
assembly is called a transmitter.
TEMPERATURE SENSORS
For electronic control, temperature sensors are classified as
follows:
— Resistance Temperature Devices (RTDs) change resistance
with varying temperature. RTDs have a positive tempera-
ture coefficient (resistance increases with temperature).
— Thermistors are solid-state resistance-temperature sensors
with a negative temperature coefficient.
— Thermocouples directly generate a voltage as a function of
temperature.
Resistance Temperature Devices
In general, all RTDs have some common attributes and
limitations:
— The resistance of RTD elements varies as a function of temp-
erature. Some elements exhibit large resistance changes,
linear changes, or both over wide temperature ranges.
— The controller must provide some power to the sensor and
measure the varying voltage across the element to determine
the resistance of the sensor. This action can cause the element
to heat slightly (called self-heating) and can create an
inaccuracy in the temperature measurement. By reducing
the supply current or by using elements with higher nominal
resistances the self-heating effect can be minimized.