PyLVDT Files

Installation:
This project uses Linux ALSA utilities for reading an LVDT sensor. It uses
Python3 with Tkinter as the GUI and the python3-scipy and python3-numpy
libraries. idle-python3.x is recommended as an IDE.
Copy the "LVDT Measurement.zip" file into a convenient folder and extract. This
will create a folder named "LVDT Measurement". Change the permission of file
"pyLVDT.py" to executable and run this program.
To uninstall, just delete the "LVDT Measurement" folder.
The file "configure.py" contains a constant "EXTENSION" which is used to set
the LVDT plunger extension used for setting the span value.
The sound card setup is relatively complex and a GUI is provided to help in the
setup. Click on "ALSA..." on the toolbar to bring up the GUI. The current ALSA
state will be save on disk and will be restored when the program is exit.
First click on "Initialize" to set all the ALSA controls to a known state. This
will turn all switches to on and set all volume controls to 0 dB.
Since only two input channels and one output channel is required for the LVDT,
turn off all "Playback" switches, except for "Master", "Front" and "Headphone".
The "Input Source" selector must be set to "Line". Click on "Okay" to save the
current settings and close the dialog. The settings will be saved in the file
"Control Presets.pickle" in the "alsa" folder.
A quick check to see if the ALSA settings are correct is to first plug a speaker
or headphone into the headphone jack and then click on one of the
"Measurement..." menu and then the "Measure extension" option on the submenu.
A loud tone should be heard on the left channel.
Test the inputs by connecting the left output of the headphone directly into the
left and right inputs of the "Line" input jack and again clicking on the
"Measure extension" option. The "Readings" line of the GUI should show left and
right values well above 0.1.
The "LVDT Connections.svg" file shows how to connect a five wire LVDT.
The polarity of the connections is not important. It's only important that the
primary is connected to the headphone and each secondary winding goes to one
audio channel. For a six wire LVDT, connect one wire of each secondary winding
to the input common.
Before measurements can be made, the LVDT has to be calibrated. First set the
plunger to the zero position and click on "Measurement..." then "Set Zero" to
set the zero value. If everything is working correctly, the "Readings" line
should show left and right channel readings above 0.1.
Next set the plunger to the extension as set in the "EXTENSION" value in the
"configure.py" file. The LVDT is a very high precision device and if high
accuracy measurements are required, the actual extension must be set using a
guage block to set the exact travel between the zero and full extension value.
Click on "Set Span" to measure the extension and complete the LVDT calibration.
After calibration is done, the "Tare..." function can be used to set an
arbitray zero to use the LVDT as a high precision indicator.
For better quality measurements, a higher input gain may be used. Open the ALSA
GUI and adjust the "Capture Volume" left and right controls. Don't adjust the
volume below 0 dB since this doesn't increase the input range and just degrades
the measurement quality. If the gain is set too high, an "Over range" message
will be displayed in the "Readings" line and the measurement will be invalid.
The signal levels used are relatively low and it's very easy to introduce noise
through ground paths. Use shielded audio cables for all connections and only
connect the LVDT cable shield to the headphone ground. The input cable shields
should only connect to the secondary windings common.