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Programming Function Blocks and Features
Video Recorder – User Manual 98
Table 4-10 Loop Prompts (continued)
Prompt Range/Selections Definition
RATIO SETPOINT Enter a number will equal the ratio that the ratio control loop
will maintain between its process variable and a “wild
variable.” The wild variable may be the Output Value of any
analog input or calculated value function block over which the
ration loop will have no control.
The RATIO SETPOINT menu choice is essentially a multiplier
applied to the wild variable’s value. This multiplication’s result
will be a working set point for the loop that will ensure the
desired “process variable-to-wild variable” ratio. For example,
suppose you intend to use the ratio control loop to maintain a
50% fuel-air ratio in the operation of a furnace. If the loop will
be directly controlling air flow (i.e. - air flow will be the loop’s
process variable) and fuel flow will be used as a wild variable
to generate the ratio loop’s working set point, the RATIO
SETPOINT menu choice will have to be set to 0.50. Hence, if
fuel flow is measured to be 60, the working set point that will
be used in the ratio loop’s control of air flow will be (60) x
(0.50) = 30.
To make a ratio control loop’s working set point equal to the
product of RATIO SETPOINT and a specific wild variable
value, the loop’s working set point must be toggled to SP2
using the “SP (Set Point Select)” key while viewing the loop’s
ON LINE display. Refer to Figure 4-1.
SETPOINT SELECT OFF, discrete parameter, 0,
1
When the DISCR VS KEY SEL discrete is ON (1) this
parameter takes the place of the On-Line Tune Loop menu’s
Toggle Set point Source prompt.
SETPOINT SELECT = ON (1) = Set point2
SETPOINT SELECT = OFF (0) = Set point1
When DISCR VS KEY SEL is OFF(0), SETPOINT SELECT
has no effect.
SETPOINT#1 &
SETPOINT#2
Set point #1 can be a
number only. Set point #2
can be a number, an analog
parameter, or OFF.
Set point #1 and #2 are independent set points. Either may be
the active set point for the loop. When viewing a live ON
LINE mode control loop display, a loop may be switched
between SETPOINT #1 and SETPOINT #2 by accessing the
Tune Loop menu and selecting TOGGLE SETPOINT
SOURCE. Operation of the loop using SETPOINT #1 will be
indicated by the presence of an “SP1” symbol on the loop’s
display. “SP2” will be the symbol observed when the loop is
operating using SETPOINT #2.
While SETPOINT #2 may be programmed as a numerical
value, it may also be used as a remote set point input to the
loop. You may link SETPOINT #2 to any of several analog
parameters that include the Output Values of analog input or
calculated value function blocks. If used as a remote set point
input, SETPOINT #2’s value may not be manually changed
from within any live ON LINE mode control loop displays.
SOFT PID YES, NO Soft PID action (YES) causes the control algorithm to not
calculate proportional output corresponding to errors resulting
from changes to set point. The algorithm will adjust its Reset
(Integral) term to a value required to maintain the present
output when the set point is changed. Normal proportional
action should occur for all changes and variations to the
controlled variable.