Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATION CONTROL
ELECTRONIC CONTROL FUNDAMENTALS
122
TYPICAL SYSTEM
Figur e 1 shows a simple electr onic contr ol system with a
contr oller tha t r egula tes suppl y water temper atur e by mixing
retur n water with w ater fr om the boiler . The main temper atur e
sensor is loca ted in the hot w ater suppl y fr om the v alve. To
incr ease efficiency and ener gy savings, the contr oller r esets the
suppl y water temper atur e setpoint as a function of the outdoor
air temper atur e. The contr oller anal yzes the sensor da ta and
sends a signal to the v alve actua tor to r egula te the mixtur e of
hot water to the unit hea ter s. These components ar e descr ibed
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 de vices such as actua tor s and r elays, final contr ol
elements suc h as v alves and damper s, and indica ting ,
interf acing , and accessor y devices. Figur e 2 pr ovides a system
overview for man y electr onic system components.
SENSORS
A sensing element pr ovides a contr oller with inf or ma tion
concer ning c hang ing conditions. Analo g sensor s ar e used to
monitor continuously changing conditions such as temperature
or pr essur e. The analo g sensor pr ovides the contr oller with a
var ying signal suc h as 0 to 10V. A digital (tw o-position) sensor
is used if the conditions r epr esent a f ixed sta te suc h as a pump
tha t is on or of f. The digital sensor pr ovides the contr oller with
a discrete signal such as open or closed contacts.
Some electr onic sensor s use an inher ent a ttr ibute of their
ma ter ial (e.g., wire resistance) to pr ovide a signal and can be
dir ectly connected to the electr onic contr oller. Other sensor s
requir e conver sion of the sensor signal to a type or le vel tha t
can be used b y the electr onic contr oller. For e xample , a sensor
that detects pressure requires a transducer or transmitter to
convert the pr essur e signal to a v oltage or cur rent signal usa ble
by the electr onic contr oller. Typical sensor s used in electr onic
contr ol systems ar e included in F igur e 2. A sensor -tr ansducer
assemb ly is called a tr ansmitter .
TEMPERATURE SENSORS
For electronic control, temperature sensors are classified
as follows:
— Resistance Temperature Devices (RTDs) change resistance
with varying temperature. RTDs have a positive temperature
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 g ener al, all RTDs ha ve some common a ttr ibutes and
limitations:
— The resistance of RTD elements varies as a function of
temperature. 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.