Controller Configuration; Application Flexibility; Coordinated Multifunction Capability; Precise And Accurate Control - Honeywell AUTOMATIC CONTROL Engineering Manual

APPLICATION FLEXIBILITY

Since microprocessor-based controllers are software based,
application flexibility is an inherent feature. A wide variety of
HVAC functions can be programmed and, in addition, the
controller can perform energy management, indoor air quality
(IAQ), and/or building management functions. Changes in
control sequences can easily be accommodated through
software whether dictated by system performance or by
changes in the owner's use of the facility.
COORDINATED MULTIFUNCTION
CAPABILITY
Although basic environmental control and energy
management operate as independent programs, it is best to
have them incorporated as an integrated program in order to
provide more efficient control sequences. For example, sensing
the temperatures of several zones to determine the average
demand, or the zone with the greatest demand for cooling,
will provide improved efficiency and control over merely
sampling a representative zone for a chiller reset program. An
added feature is that the sensors providing zone comfort control
can serve a dual function at no added cost. These benefits
require controller-to-controller communications which is
discussed in the Building Management System Fundamentals
section.

CONTROLLER CONFIGURATION

The basic elements of a microprocessor-based (or micro-
processor) controller (Fig. 3) include:
— The microprocessor
— A program memory
— A working memory
— A clock or timing devices
— A means of getting data in and out of the system
In addition, a communications port is not only a desirable
feature but a requirement for program tuning or interfacing
with a central computer or building management system.
Timing for microprocessor operation is provided by a
battery-backed clock. The clock operates in the microsecond
range controlling execution of program instructions.
Program memory holds the basic instruction set for controller
operation as well as for the application programs. Memory
size and type vary depending on the application and whether
the controller is considered a dedicated purpose or general
purpose device. Dedicated purpose configurable controllers
normally have standard programs and are furnished with read
only memory (ROM) or programmable read only memory
MICROPROCESSOR-BASED/DDC FUNDAMENTALS

PRECISE AND ACCURATE CONTROL

Proportional control has the inherent problem of offset. The
wider the throttling range is set for control stability, the greater
the offset. With the microprocessor-based controller, the offset
can easily be corrected by the simple addition of integral action.
For even more accurate control over a wide range of external
conditions, adaptive control algorithms, available in some
microprocessor-based controllers, can be employed. With
adaptive control, system performance automatically adjusts as
conditions vary. The need for manual fine tuning for seasonal
changes is eliminated. These items are discussed in the Control
Fundamentals section.

RELIABILITY

Digital controllers should be conservatively designed and
should incorporate self-checking features so they notify the
operator immediately if anything goes wrong. Input and output
circuits should be filtered and protected from extraneous signals
to assure reliable information to the processor.
(PROM.) General purpose controllers often accommodate a
variety of individual custom programs and are supplied with
field-alterable memories such as electrically erasable,
programmable, read only memory (EEPROM) or flash
memory. Memories used to hold the program for a controller
must be nonvolatile, that is, they retain the program data during
power outages.
CLOCK
PROGRAM
MICROPROCESSOR
MEMORY
COMMUNICATIONS
PORT
SIGNAL
CONDITIONING AND
A/D CONVERTER
D/A CONVERTER
Fig. 3. Microprocessor Controller Configuration for
Automatic Control Applications.
135
ENGINEERING MANUAL OF AUTOMATIC CONTROL
WORKING
MEMORY
BINARY
INPUTS &
OUTPUTS
SENSORS
INPUT
AND
MULTIPLEXER
TRANSDUCERS
TRANSDUCERS
OUTPUT
AND
MULTIPLEXER
ACTUATORS
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