HP VXI 75000 C Series, User'S And Scpi Programming Manual

Page: 129 / 438

Creating and Running Custom Algorithms 129 Chapter 4
Writing the
Algorithm
The most convenient method of creating your custom algorithm is to use
your text editor or word processor to input the source code. The following
algorithm source code is on the examples disc in a file called "mxplusb".
/* Example algorithm that calculates 4 Mx+B values upon
* signal that sync == 1. M and B terms set by application
* program.
*/
static float M, B, x, sync;
if ( First_loop ) sync = 0;
if ( sync == 1 ) {
writecvt( M*x+B, 10 );
writecvt(-(M*x+B), 11 );
writecvt( (M*x+B)/2,12 );
writecvt( 2*(M*x+B),13 );
sync = 2;
}
Running the
Algorithm
A C-SCPI example program "file_alg.cs" shows how to retrieve the
algorithm source file "mxplusb" and use it to define and execute an
algorithm. When you have compiled "file_alg.cs", type file_alg mxplusb to
run the example and load the algorithm.
Modifying a Standard PID Algorithm
While the standard PID algorithms can provide excellent general closed
loop process control, there will be times when your process has specialized
requirements that are not addressed by the default form of the HP E1415’s
PID algorithms. In this section we show you how to copy and modify a
standard PID algorithm. Both of the HP E1415’s standard PID algorithms
PIDA and PIDB are also available as source files supplied with your
HP E1415. Also included is a source file for a PIDC algorithm. PIDC has
more features than PIDB but is not pre-defined in the HP E1415’s driver like
PIDA and PIDB. It is only available as a source file.
PIDA with digital
On-Off Control
The HP E1415’s PID algorithms are written to supply control outputs
through analog output SCPs. While it would not be an error to specify a
digital channel as the PID control output, the PID algorithm as written would
not operate the digital channel as you would desire.
The value you write to a digital output bit is evaluated as if it were a boolean
value. That is, if the value represents a boolean true, the digital output is set
to a binary 1. If the value represents a boolean false, the digital output is set
to a binary 0. The HP E1415’s Algorithm Language (like ’C’) specifies that
a value of 0 is a boolean false (0), any other value is considered true (1).
With that in mind we’ll analyze the operation of a standard PIDA with a
digital output as its control output.
How the Standard PIDA
Operates
A PID is to control a bath temperature at 140 degrees Fahrenheit. With the
Setpoint at 140 and the process variable (PV) reading 130, the value sent to
the output is a positive value which drives the digital output to 1 (heater on).