System-Level Controller - Honeywell AUTOMATIC CONTROL Engineering Manual

MICROPROCESSOR-BASED/DDC FUNDAMENTALS
staged by a PI algorithm with software heat anticipation. See
Figure 15. During reheat, the control mode changes to constant
volume, variable discharge temperature.
MAX
CONSTANT VOLUME,
FLOW
VARIABLE DISCHARGE TEMPERATURE
REHEAT
FLOW
MIN
FLOW
COLD
HEATING
SETPOINT
Fig. 15. Control Sequence for VAV Cooling with
Sequenced Electric Reheat.

SYSTEM-LEVEL CONTROLLER

System-level controllers are variable-function devices
applied to a wide variety of mechanical systems. These
controllers can accommodate multiloop custom control
sequences and have control integrated with energy management
and building management functions. The examples that follow
cover direct digital control functions for a system-level
controller. Integrated building management functions are
covered in the Building Management System Fundamentals
section.
Where the examples indicate that user entered values are
furnished (e.g., setpoint), or that key parameters or DDC
operator outputs will have display capability, this represents
sound software design practice and applies whether or not the
controller is tied into a central building management system.
Data is entered or displayed in non-BMS applications by a
portable operator's terminal or by a keypad when display is
integral with the controller.
A five-step approach can be used to define DDC programs.
1. Develop a system flow schematic as a visual representa-
tion of the process to be controlled. The schematic may
be provided as a part of the plans and specifications for
the job. If not, a schematic must be created for the sys-
tem.
2. Add actuators, valves, sensors, setpoints, and operational
data required for control and operation.
3. Write a detailed sequence of operation describing the
relationship between inputs, outputs, and operational
data points.
4. Develop a detailed flowchart of the control sequence
using either DDC operators or a programming logic flow
diagram. Programs written totally in a high-level
language use the logic flow diagram.
5. Write the program using either DDC operators (Table
1) or high-level language statements.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONSTANT
DISCHARGE
TEMPERATURE,
VARIABLE
DEAD BAND VOLUME
HOT
COOLING
SETPOINT
SPACE LOAD C2686
An example of this approach follows for control of a hot
water converter:
Step 1—Develop flow schematic of the process to be controlled
(Fig. 16).
HOT WATER SUPPLY
HOT WATER RETURN
Fig. 16. Schematic of Steam to Hot Water Converter.
Step 2—Identify required sensors, actuators, and operational
data (Fig. 17). Refer to the Chiller, Boiler, and
Distribution System Control Applications section for a
symbol legend.
OUTSIDE
OUTSIDE
AIR
AIR
60
22
0
HOT WATER RESET
SCHEDULE
PUMP-ON
SETPOINT
AUTO
52
HOT
WATER
RETURN
ON
HOT WATER
PUMP
Fig. 17. Schematic Illustrating Sensors, Actuators, and
Operational Data for Steam to Hot Water Converter.
If the DDC system is provided with a BMS having a color
monitor, a graphic may be required to be displayed with live,
displayable and commandable points (12 total). If a BMS is
not provided, the points may be required to be displayed on a
text terminal (fixed or portable) at the system level controller.
146
STEAM
STEAM TO
HOT WATER
CONVERTER
M15035
HOT WATER
SETPOINT
SETPOINT
120
152
170
SP
IN OUT
PID
PERCENT
58
OPEN
152
STEAM TO
HOT WATER
CONVERTER
140
M15036A
STEAM
VALVE