Introduction; Definitions - Honeywell AUTOMATIC CONTROL Engineering Manual

INTRODUCTION

This section discusses the types of microprocessor-based
controllers used in commercial buildings. These controllers
measure signals from sensors, perform control routines in
software programs, and take corrective action in the form of
output signals to actuators. Since the programs are in digital
form, the controllers perform what is known as direct digital
control (DDC). Microprocessor-based controllers can be used
as stand-alone controllers or they can be used as controllers
incorporated into a building management system utilizing a
personal computer (PC) as a host to provide additional

DEFINITIONS

Analog-to-digital (A/D) converter: The part of a micropro-
cessor-based controller that changes an analog input
signal to a digital value for use by the microproces-
sor in executing software programs. Analog input
values typically come from temperature, pressure,
humidity, or other types of sensors or transducers.
Application software: Programs that provide functions such
as direct digital control, energy management, lighting
control, event initiated operations, and other alarm
and monitoring routines.
Configurable controller: A controller with a set of selectable
programs with adjustable parameters but without the
ability to modify the programs.
Digital-to-analog (D/A) converter: The part of a micropro-
cessor-based controller that changes digital values
from a software program to analog output signals
for use in the control system. The analog signals are
typically used to position actuators or actuate trans-
ducers and relays.
Direct digital control: A control loop in which a digital
controller periodically updates a process as a function
of a set of measured control variables and a given set
of control algorithms.
MICROPROCESSOR-BASED/DDC FUNDAMENTALS
functions. A stand-alone controller can take several forms. The
simplest generally controls only one control loop while larger
versions can control from eight to 40 control loops. As the
systems get larger, they generally incorporate more
programming features and functions. This section covers the
controller as a stand-alone unit. Refer to the Building
Management System Fundamentals section for additional
information on use of the controller in networked and building
management systems.
Microprocessor-based controller: A device consisting of a
microprocessor unit, digital input and output connec-
tions, A/D and D/A converters, a power supply, and
software to perform direct digital control and energy
management routines in a HVAC system.
Operating software: The main operating system and programs
that schedule and control the execution of all other
programs in a microprocessor-based controller. This
includes routines for input/output (I/O) scanning,
A/D and D/A conversion, scheduling of application
programs, and access and display of control program
variables.
System-level controller: A microprocessor-based controller
that controls centrally located HVAC equipment such
as variable air volume (VAV) supply units, built-up
air handlers, and central chiller and boiler plants.
These controllers typically have a library of control
programs, may control more than one mechanical
system from a single controller, and may contain an
integral operating terminal.
Zone-level controller: A microprocessor-based controller that
controls distributed or unitary HVAC equipment such
as VAV terminal units, fan coil units, and heat pumps.
These controllers typically have relatively few con-
nected I/O devices, standard control sequences, and
are dedicated to specific applications.
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ENGINEERING MANUAL OF AUTOMATIC CONTROL