MICROPROCESSOR-BASED/DDC FUNDAMENTALS
144
ENGINEERING MANUAL OF AUTOMATIC CONTROL
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.
START
ANALYZE
CONTROL
APPLICATION
REQUIREMENTS
PARTITION
INTO CONTROL
LOOPS
SYSTEM
DRAWINGS AND
SEQUENCES OF
OPERATION
DETERMINE
INPUTS AND
OUTPUTS
FOR EACH LOOP
DESIGN,
WRITE, AND COMPILE
PROGRAM
RUN DEBUG
AND
SIMULATION
PROGRAMS
INSTALL
PROGRAM
AND
ASSOCIATED
DATA FILES
STEP 1
STEP 2
STEP 3
STEP 4
STEP 5
STEP 6
C2440
END
CONTROL
ALGORITHM
SETPOINT
D/A
ERROR
CORRECTIVE
SIGNAL
FINAL
CONTROL
ELEMENT
(ACTUATOR)
MANIPULATED
VARIABLE
PROCESS
DISTURBANCE
(LOAD CHANGE)
FEEDBACK
CONTROLLED
VARIABLE
SENSING
ELEMENT
CONTROLLER
C2441
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.
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.
Содержание AUTOMATIC CONTROL SI Edition
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