Parallel port dynamometer with Python

Cremona, around 200000 BC.
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fig-002-potentiometer-python
I just found in a corner of my garage this dynamometer prototype. It works by reading the state of a potentiometer via a parallel port, using python. The goal was measuring the thrust generated by some small candy rockets. I was probably a freshman in engineering school back then, and you can guess I had not much aquaintance with data acquisition systems! If you need to measure rockets thrust I'd suggest you of course to use load cells. Check out this document HERE for a homemade load cell and HERE for a tri-axial load cells calibration.

The idea is that the force will be proportional to the displacement (there are two springs at the bottom), and we probe this displacement by connecting a potentiometer. The potentiometer is put as a branch of a NE555 monostable timer:
  1. The current time is saved in a variable;
  2. The monostable 555 is started with a pulse from the parallel port. The 555 output will be on for a duration that depends on the potentiometer;
  3. When the 555 output goes down, the time is probed again;
  4. The resistance of the potentiometer is found by checking how much time has passed (in the order of ms).

In the following pictures you can see the system (sorry, I have no schematic at the moment), the python script and an image output (produced by the script).

CLICK TO OPEN THE IMAGES

fig-001-potentiometer-python

fig-002-potentiometer-python

fig-003-potentiometer-python

fig-004-potentiometer-python

fig-005-potentiometer-python

fig-006-potentiometer-python


Cheers!
Stefano

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