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Programming Function Blocks and Features
Video Recorder – User Manual 100
AI3
FUEL
∆
P
SIGNAL
AI3 OV
AI2 OV
AIR
∆
P
SIGNAL
AI2
×
+
RATIO
SETPOINT
RATIO
BIAS
WILD
VARIABLE
SP2
LP2
PV
FEEDBACK
4 TO 20 mA
TO AIR FLOW
CONTROL
VALVE
LP2 OV
OUTPUT TYPE =
CAT
IN LOW LIMIT = 0
IN HIGH LIMIT =
100
OUT LOW LIMIT = 4
OUT HIGH LIMIT =
20
AO2IN
TYPE =
RATIO
SETPOINT #1 = 500
WILD VARIABLE = AI3 OV
RATIO SETPOINT = 0.50
RATIO BIAS = 0.75
GAIN = 10
RESET = 1
RATE = 0
AO2 BC
UNITS = FPS
INPUT TYPE =
LINEAR
DIRECT/INDIRECT/SQRT = SQRT
RANGE LOW = 0
RANGE HIGH = 40
CIRCUIT LOW = 1
CIRCUIT HIGH = 5
ELECTRICAL UNITS = VOLTS
UNITS = FPS
INPUT TYPE =
LINEAR
DIRECT/INDIRECT/SQRT = SQRT
RANGE LOW = 0
RANGE HIGH = 40
CIRCUIT LOW = 1
CIRCUIT HIGH = 5
ELECTRICAL UNITS = VOLTS
Working set point of loop when use of WILD
VARIABLE is desired to establish proper control
of PV. If ratio control using WILD VARIABLE
must be temporarily suspended, loop’s working
set point may be switched ON LINE to SETPOINT
#1 via “SP (Set Point Select)” key oninstrument’s
front door.
Figure 4-1 Function Block Configuration of a Typical Ratio Control Loop
When the ratio control loop in Figure 4-1 is brought ON LINE, its ON LINE display must be accessed to
allow the instrument’s SP (Set Point Select) key to be operational. To make the working set point of the
ratio control loop equal the equation (WILD VARIABLE X RATIO SETPOINT) + RATIO BIAS, then
select the TUNE LOOP menu item TOGGLE SETPOINT SOURCE so that an “SP2” is seen in the loop’s
ON LINE display. Note that within the ratio control loop’s programming menu (accessed under
PROGRAM CONTROL LOOPS), you will not find a SETPOINT #2 menu choice. The ratio control loop
function block operates with the formula (WILD VARIABLE X RATIO SETPOINT ) + RATIO BIAS
assuming all the functionality of the ratio loop’s second set point. Pressing the SP key to switch the
working set point of a live ratio control loop from SP2 to SP1 will essentially suspend any ratio control
and cause the loop to behave like a standard PID controller. This standard PID loop will have a local set
point equal to the value programmed for SETPOINT #1. To control the process variable using a fixed set
point that is independent of the loop’s WILD VARIABLE input, toggle the ratio control loop’s working set
point to SP1.