Honeywell VRX180 - Manual (Page 75)

Programming and Operating Concepts
Video Recorder – User Manual 61
1. Diagram the function blocks
To configure this application using the instrument, your task is to build up a simple current control loop.
Note that this control loop must monitor and control the temperature of the furnace zone to a local set
point of 500 ºF. Using a 4 to 20 mA signal applied to a gas valve actuator, the furnace zone’s
temperature will be controlled by regulating the flow of gas to the zone’s burner.
The instrument will measure temperature, in a range between 0 and 1000 ºF, by means of a Type J
thermocouple.
To support this application, a 4 to 20 mA control loop with a thermocouple process variable must be
configured. Three function blocks—one for specifying a thermocouple analog input, a second for a
standard PID control loop, and a third defining a 4 to 20 mA analog output—are needed to produce this
control strategy’s function block diagram.
Each function block should first be arranged as in Figure 3-12. Analog input and output function blocks
are represented by right-pointed triangles. Control loop function blocks are represented by right-pointed
parallelograms.
AI
AO LP
AI = ANALOG INPUT
LP = CONTROL LOOP
AO = ANALOG OUTPUT
Figure 3-12 Basic Function Blocks Required For Control Configuration Of Figure 3-11
2. Label input parameters
Properly label each function block. First, assign to each function block a name that identifies it within the
hardware and feature capacities of the instrument being worked with. You may assign any of the analog
inputs, control loops, and analog outputs that your instrument has to the blocks comprising the function
block diagram drawn. For simplicity, AI1, LP1, and AO1 will be used in this example.
Refer to Figure 3-13. Note that AI5, LP2, and AO2 could just as easily have been used.