Table of Contents
Advertisement
If the function is given in tabular form it is tempting to simply set up
the DFG to the values of x and f(x) from the table, especially if the num-
ber of data points in the table happens to coincide with the number of seg-
ments available in the DFG.
The difficulty with this approach is that the
distribution of the data points that define the function
~ll
probably not
be the best distribution of breakpoints for straight-line approximation.
For example, the original data may have been obtained for equally-spaced
values of the input variable x, whereas it is generally
~
a good idea
to use equally-spaced breakpoints for segment approximation.
It is better
to plot the data points, pass a smooth curve through them, and determine
a good breakpoint location from this smooth graph.
In any case, it is de-
sirable to know "what the function looks like" before trying to set it up,
which means it should be plotted before setup.
Hence, setup procedure in this chapter assumes from the start that the func-
tion is defined graphically.
The procedure consists of determining good
breakpoint locations, tabulating the values of x and f(x) at these points,
and setting up the function from this table of values.
The 580 DFG's have
a setup panel that permits the direct setup for values of x and f(x), both
the input x and output f(x) may be read directly on the DVM during setup.
An
alternate procedure, illustrated in Paragraph 8.4.4, skips the table and
consists of setting up the function of an
X~y
plotter.
The procedure is
exactly the same, but the operator looks at the plotter, instead of the DVM.
Even if the function is set up from a table of discrete values read out in
the DVM, it is a good idea to obtain a continuous plot of f(x) versus x to
make sure that the function has been set up correctly.
Such a plot should
be a part of the problem documentation,
along with the circuit diagram,
listing of pot-settings, assignment sheets, etc.
The ,580 setup panel in-
cludes an' integrator capable of generating a ramp input to the DFG for con-
venient plotting without patching changes.
In fact, the DFG can be com-
pletely set up and the resulting curve plotted with the-patch panel off.
8.4.1
Breakpoint Location
Although there exist analytical methods for dealing with breakpoint loca-
tion, they generally require too much computation to be of practical use.
With a bit of experience, a good programmer can come very close to the op-
timum breakpoint location Simply by inspection of the curve.
The follow-
ing general rules may serve as a rough guide to the techniques
o
10
Keep in mind the total number of breakpoints available on
the DFG.
Most functions of practical interest may be ade-
quately represented with 10 segments; a few require 20 seg-
ments
o
Most computers (including the 580) have ten-segment
DFG's capable of being "paired" to handle the occasional
twenty-segment function
0
8-3
Advertisement
Table of Contents
