VXI VT1422A - Manual (Page 491)

Generating User Defined Functions 489 Appendix F
A typical use of this function would be to output an analog voltage or current
at each Scan Trigger of the VT1422A and over the range of the haversine.
For example, suppose a new position is desired for an analog output and it
is moved from 1 mA to 3 mA over a period of 100 ms. If the TRIG:TIMER
setting or the EXTernal trigger was set to 2 ms, then forcing fifty intervals
over the range of the haversine is desired. This can be easily done by using
a scalar variable to count the number of times the algorithm has executed
and to scale the variable value to the -
π
/2 to
π
/2 range. 3 mA is multiplied
by the custom function result over each interval which will yield the shape
of the haversine (0.003*sin(x)+0.001).
This is illustrated in the example below. The program (sine_fn.cpp) on the
CD illustrates the actual program used to generate this haversine function.
Simply supply the algebraic expression in my_function(), the desired range
over which to evaluate the function (which determines the table range), and
the name of the function. The Build_table() routine creates the table for the
function and the ALG:FUNC:DEF writes that table into VT1422A memory.
The table MUST be built and downloaded BEFORE trying to use the
function.
The following is a summary of what commands and parameters are used in
the program example. Table F-1 shows some examples of the accuracy of
the custom function with various input values compared to an evaluation of
the actual transcendental function found in 'C'. Please note that the Mx+B
segments are located on boundaries specified by 2/64 on each side of X=0.
This means that if the exact input value that was used for the beginning of
each segment is selected, the exact calculated value of that function at that
point will be determined. Any point between segments will be an
approximation dependent upon the linearity of that segment. Also note that
values of X = 2 and X = -2 will result in Y=infinity.
−π/2 π/2
+1
−1
−2 +2
Figure F-1. A Haversine Function