Micromod MOD: Modcell 2050R Users Guide - Manual (Page 190)

IB-23C650
RATIO BIAS CONTROL
The independent (wild) flow changes from time to time, and in order to
maintain the correct proportions of the blend the other (dependent) flow must
be controlled to change in a like manner. The dependent flow must change in
the same relationship, or ratio, to changes in the independent flow
.
The wild
flow is not necessarily uncontrolled; it is only wild insofar as the ratio system is
concerned. For instance, in air-to-fuel ratio control in boilers, air flow is
controlled to maintain the required air-to-fuel ratio. The wild fuel flow is in turn
controlled by another system, based on the boiler steam demand. Thus it is
actually the controlled flow in another ratio system.
Instrument Set-up
To implement ratio bias control, simply follow Figure 20 for process input
connections. In the BASE CFG **MENU**, Table 1, enable input two. Do not
enable "A/M Ratio Bias Station". Ratio and bias values are entered in the
TUNE ** MENU, Table 12, as Remote Set-Point Ratio or Remote Set-Point
Bias.
Control Method
The method used to accomplish ratio control is to let the changeable variable
(also called the independent variable) adjust the remote set-point of the
controller that manipulates the other variable (also called the dependent
variable or controlled variable). This instrument includes an adjustable
multiplier and adjustable bias on the remote set-point so that the required
relationship between dependent and independent variables can be established
and maintained.
Ratio Function
Ratio, as it applies to process control systems, is defined as follows:
Dependent Variable
Ratio = or
Controlled Variable
Wild Variable
(1)
Independent Variable
In the ratio control system shown in Figure 20 the set-point of the dependent
variable controller is:
Dependent Variable Set-Point = Independent Variable × Ratio (2)
It is important to consider proper scaling of the two variables when using
equations 1 and 2. If the flow streams in the system shown have the same
flow range, computation of the flow ratio is very simple. The two variables
may be expressed in either percent of span or engineering units to obtain the
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