Controller Programming; Distributed Power Demand; Building Management Software; General - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

MICROPROCESSOR-BASED/DDC FUNDAMENTALS

Distributed Power Demand

The distributed power demand program (Fig. 12) is only
applicable to microprocessor controllers with intercommuni-
cations capability. The demand program is resident in a single
controller which monitors the electrical demand and transmits
the required load shed or restore messages to other controllers on
the communications bus or within the network. Each individual
controller has prioritized load shed tables so that when a message
to shed a specific number of kilowatts is received it can respond
by shedding its share of the load. The basic demand program
normally utilizes a sliding window demand algorithm and has
provision for sequencing so that the same loads are not always
shed first when a peak occurs.
It should be noted that there is interaction between the power
demand program, duty cycle program, time schedule programs,
and optimum start and stop pro;grams. Therefore, a priority
structure program is necessary to prevent control contentions.
AVERAGE
DEMAND
DEMAND DEMAND
INTERVAL INTERVAL
1 2
TIME
Fig. 12. Typical Power Curve Over Four
Successive Demand Intervals.

CONTROLLER PROGRAMMING

GENERAL

The term programming as it pertains to microprocessor-
based controllers relates primarily to setting up the controller
for the given application. Zone-level controllers require
initialization, selection of control algorithms and parameters,
definition of control sequences, and establishing reference data
bases. For zone-level controllers, the programming effort can
be as simple as selecting the applicable control sequence from
a library of programs resident in a configurable controller. For
highly customized applications, usually encountered at the
POWER
AVERAGE
CURVE
DEMAND
DEMAND
LIMIT
DEMAND DEMAND
INTERVAL
INTERVAL
3 4
C2429

BUILDING MANAGEMENT SOFTWARE

Microprocessor-based controllers are used extensively as
data gathering panels (DGP) for building management
systems. Since a microprocessor-based controller is already in
place to provide DDC, IAQ, and EMS functions, many sensors
and data files can be shared with building management system
(BMS) functions. The distribution of many BMS functions into
controllers throughout the premises increases the overall system
reliability. The following BMS software is normally included
in the controller.
Alarm lockout: Permits designated alarm points to be locked
out from reporting process depending on the status
of another point, e.g., discharge temperature alarm
can be locked out when fan is off and during initial
startup periods.
Alarm monitoring: Scans all analog and digital points and
tests for alarm status. Sets of high and low limits for
analog inputs are stored in the controller.
Communications module: Controls transmissions between
controllers and between controllers and a central
computer based on an established bus protocol.
Global points: Allows designated points to share their data
with other bus connected devices.
Run time: Accumulates equipment on or off time and trans-
mits totals periodically to the central system. On-off
cycle counting can also be accumulated as a mainte-
nance indicator. Alarm annunciation occurs if run time
or cycle count limits are exceeded.
Time and event programs: Initiates a predetermined series of
control actions based on an alarm condition, a point
status change, time of day, or elapsed time. Points acted
upon can be resident in any controller.
system controller level, a problem oriented language or a subset
of a high-level language can be used to define control loops
and sequences.
The means of entering a program can vary from a keypad
and readouts on the controller to an operator terminal in a large
centrally based computer configuration. Sophistication of the
entry device is directly related to how well defined and fixed
the control application is compared to the degree of customization
or end-user modifications required. If considerable customization
or modification is required, data entry could require a centrally
based computer or a portable PC.
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