Analyze Control Application; Partition Into Control Loops; Determine Inputs And Outputs - Honeywell AUTOMATIC CONTROL Engineering Manual

MICROPROCESSOR-BASED/DDC FUNDAMENTALS
STEP 1
REQUIREMENTS
DRAWINGS AND
SEQUENCES OF
STEP 2
INTO CONTROL
STEP 3
FOR EACH LOOP
WRITE, AND COMPILE
STEP 4
STEP 5
STEP 6
Fig. 13. Custom Control Program Development.

Analyze Control Application

The systems analysis step in writing a custom control pro-
grams requires that the control engineer thoroughly understand
the process controlled. The output of the systems analysis is
normally a system drawing and a concise and clearly stated
sequence of operation.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
START
ANALYZE
CONTROL
APPLICATION
SYSTEM
OPERATION
PARTITION
LOOPS
DETERMINE
INPUTS AND
OUTPUTS
DESIGN,
PROGRAM
RUN DEBUG
AND
SIMULATION
PROGRAMS
INSTALL
PROGRAM
AND
ASSOCIATED
DATA FILES
END
C2440

Partition Into Control Loops

The next step is to partition the entire process into individual
control loops. The Control Fundamentals section defines a
control loop as a process in which a controller compares the
measured value of a controlled variable to a desired value or
setpoint. The resulting output of the controller goes to an
actuator that causes a control agent to lessen the deviation
between actual and desired values (Fig. 14). Control loops can
be complex when limit control is needed or when several
actuators are controlled in sequence to maintain the controlled
variable. At this step a flow chart should be drawn showing all
relationships influencing the controlled variable between the
controller and actuators.
CONTROLLER
ERROR
CONTROL
ALGORITHM
D/A
SETPOINT
FEEDBACK
Fig. 14. Simple Control Loop.
A typical central fan system may require several control
loops including various combinations of:
— Discharge-air temperature control
— Mixed-air temperature control
— Hot-deck temperature control
— Cold-deck temperature control
— Humidity or dewpoint control
— IAQ control
— Ventilation control
— Supply fan static pressure control
— Return fan airflow control

Determine Inputs and Outputs

The next step in custom control programming is to determine
the inputs and outputs associated with each control loop. This
establishes the data file associated with the program.
144
DISTURBANCE
(LOAD CHANGE)
CORRECTIVE
FINAL
SIGNAL
CONTROL
ELEMENT
(ACTUATOR)
PROCESS
MANIPULATED
VARIABLE
SENSING
CONTROLLED
ELEMENT
VARIABLE
C2441