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Design, Write, and Compile Program
The actual process of designing and writing the control loop
programs can be a very complex or a relatively straightforward
procedure, depending on the language processing software
provided for the controller. The microprocessor-based controller
understands instructions only at the most elementary language
level, i.e., strings of 1s and 0s or machine code. Because of
this, language processing software is often required. This
software translates the instructions of a control program written
in an easier-to-use high-level language into actual machine
code. The terms compiler, assembler, object oriented, or
interpreter are used to describe types of language processing
software packages. The assembler is normally associated with
a lower level assembly language while the compiler, object
oriented, or interpreter is normally associated with a higher
level language. Most system level controllers today are
programmed using an object oriented (graphical) language.
Object-oriented languages are custom software packages
tailored to the requirements of a specific vendor's controller.
Control sequences are built by selecting preprogrammed control
blocks, for example the PID algorithm, and linking them with
TYPICAL APPLICATIONS
ZONE-LEVEL CONTROLLER
Zone-level controllers can be applied to a variety of types of
HVAC unitary equipment. Several control sequences can be
resident in a single zone-level controller to meet various
application requirements. The appropriate control sequence is
selected and set up through either a PC for the system or through
a portable operator's terminal. The following two examples
discuss typical control sequences for one type of zone-level
controller used specifically for VAV air terminal units. For
further information on control of terminal units, refer to the
Individual Room Control Applications section. As stated in
the introduction, the following applications are for stand-
alone controllers. See the Building Management System
Fundamentals section for network applications.
EXAMPLE 1. VAV COOLING ONLY
In a pressure independent VAV cooling only air terminal unit
application the zone-level controller controls the primary airflow
independent of varying supply air pressures. The airflow
setpoint of the controller is reset by the thermostat to vary
airflow between field programmable minimum and maximum
ENGINEERING MANUAL OF AUTOMATIC CONTROL
MICROPROCESSOR-BASED/DDC FUNDAMENTALS
other control blocks. Although this process requires little or no
knowledge of programming, it does require in-depth knowledge
of the control blocks and the specific HVAC process.
Debug, Install, Enter Data Files, and Test
Regardless of the custom control program used, each program
must be debugged to assure proper operation. When programs
are written on a host machine, special debug and simulation
programs are frequently employed prior to installing the
program in the controller. Debug programs test for syntax
(language) and procedural errors. Simulation programs allow
inputs and outputs to be simulated and a static test of the
program to be run. After debug and error correction, the program
and associated data files are loaded into the controller and a
full system check is made under normal operating conditions
to assure proper operation.
Some systems allow graphically constructed programs to be
monitored live in their actual executing environment with inputs,
outputs, and intermediate signal values updating continuously.
settings to satisfy space temperatures. On a call for less cooling,
the damper modulates toward minimum. On a call for more
cooling, the damper modulates toward maximum. The airflow
control maintains the airflow at whatever level the thermostat
demands and holds the volume constant at that level until a
new level is called for. The minimum airflow setting assures
continuous ventilation during light loads. The maximum setting
limits fan loading, excessive use of cool air, and/or noise during
heavy loads.
EXAMPLE 2. VAV COOLING WITH
SEQUENCED ELECTRIC REHEAT
In a VAV cooling air terminal unit application with sequenced
electric reheat, an adjustable deadband is provided between the
cooling and the reheat cycle. During cooling the control mode
is constant discharge temperature, variable volume. On a call
for less cooling, the damper modulates toward minimum flow.
The damper remains at minimum cooling through a deadband.
On a call for reheat, the damper goes from minimum flow to
reheat flow to ensure proper air distribution and prevent
excessively high discharge temperatures and to protect the
reheat elements. In this sequence, duct heaters are cycled and
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