| Measuring a Single Temperature (Hydra) |
An Introduction to
Instrument Programming in LabVIEW Using GPIB/IEEE-488
We assume that you have some background -
however sparse - in programming. In this lesson, we will examine how to
communicate with an instrument using LabVIEW and a GPIB (IEEE-488) connection.
We will focus on the Fluke Hydra Data Acquisition Unit (DAU), but what is
covered here is applicable to many other instruments. Note that LabVIEW is a
copyrighted name owned by the National Instruments Corporation, and that is the
corporation that sells LabVIEW. It is the most widely used programming
environment for instrument control although there is considerable instrument
programming done in C/C++ and Visual Basic. LabVIEW is a visual programming
environment (which means that you do not write sequential lines of code) and
seems to be uniquely matched to the ways of thinking and solving problems that
engineers and scientists commonly employ.
The first step in
instrument programming is to reset the instrument under program control in
LabVIEW. What you need for that process is the following.
First, you need to start
LabVIEW and generate a new "vi". LabVIEW files often have a "vi" extension,
and that stands for "Virtual Instrument". You are about to embark on
developing a virtual instrument that uses the Hydra DAU to take the actual
measurements. What you write in LabVIEW will be a program that controls
that process. You need to know the GPIB
address for the instrument. In many cases - especially with more recent
instruments - the GPIB address will flash for a moment as the instrument
cycles through its startup procedure. In the case of the Hydra,
you need to set that address. Within our department, the standard
address is 3, and you will find most programs use that address. Once you have that taken care
of, the Hydra should be ready to be accessed from within LabVIEW.
The Hydra
In the lab, you will find
that the numbered terminals for the Hydra have been brought out to a connector
block attached to the top of the Hydra. Those terminals are shown below in the
photo of the Hydra and we are referring to the orange terminal blocks there.
In the example in this note, we will be
taking a temperature measurement from Channel 1 of the Hydra. That means that
you should connect the thermocouple leads to the Hydra.
WARNING: You cannot take a
temperature measurement from the front panel terminals (Channel 0). You might
try, and you might even force the thermocouple wires into those terminals, but
it will not work!
Taking a Temperature
Measurement
Taking a temperature measurement is more
complex than just resetting the instrument. The steps you need to take are as
follows (assuming that the instrument has been reset). Here's the diagram for
the vi.
Here's what you do in this vi.
Send the following
commands using the GPIB Write
block in LabView. In the diagram below, there are three GPIB Write blocks.
Note that each of them uses the same address (3) and the same mode (3) as in
the reset vi above. Here are the commands that are sent to the instrument
in each of the GPIB Write blocks.
FUNC 1,TEMP,K
- which sets the function on Channel 1 to be TEMP (TEMPerature
measurement) using a type K thermocouple. *TRG
- which triggers a measurement when the instrument receives this
command. LAST? 1
- which requests the instrument to prepare the last measurement on
Channel 1 for transmission to the computer. In LabView you sequence
these commands by taking the error signal from each block and feeding it
to the next block. See the diagram reproduced below.
After setting up the
instrument, taking a measurement and preparing the data, you must get the
instrument to send the string to the computer. In LabView you use a
GPIB Read block. It's
found in the same general place as the GPIB Write block.
The vi above will take a single
temperature measurement once the instrument has been reset. That allows you to
put the temperature mesurement inside a loop and put the reset outside the
loop. If you use this in an application where speed is critical, you might also
want to consider putting the GPIB Write block that sets the channel and the
measurement type outside the loop, separating it from the rest of the operations
above.
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