Digital Tension Scale, Part II

Posted 17 November 2018

In a previous post on this subject, I described a digital tension scale arrangement using a load cell incorporated into a 2-hook tension measurement setup, interfaced to a common HX711 ADC board, and a Sparkfun Pro Micro ATMega32U4 microcontroller.

After (finally) getting the setup to work and getting some initial real-time tension measurements on our rowing machine, I decided to see if I could improve the usability of the overall system, with the goal of constructing a fully automatic battery powered tension scale, with the ability to communicate wirelessly to my PC for data acquisition and programming.

Desired Features:

  • Easy calibration:  The system should offer a calibration option when connected to a PC/Smartphone, but otherwise should use the last calibration data (stored in EEPROM) for measurements.
  • Battery operated: This implies a low-power mode to extend battery life if using primary batteries, and/or a charging arrangement if using secondary (rechargeable) cells.
  • Local display:  A low power display (LCD?) for local tension measurement display
  • Wireless capability:  A wireless connection to a PC or smartphone for real-time data acquisition.
  • Small size:  I would like to mount the entire system on the 2-hook tension measurement assembly itself.

Easy Calibration:

The calibration procedure associated with my previous post was a PITA, to say the least, so I decided to attack this problem first.  I modified the software to allow the user to skip calibration entirely or to calibrate the tensionometer automatically using any known weight.  The new software is shown below:

When run on my PC, this produced the following output

Here are a couple of photos showing the calibration process with my current setup

‘Tare Weight’ configuration, used to zero out the contribution from the bucket

Calibration configuration. Bucket now contains 1.8L water (1.8 Kg)

After calibrating, I tested the the system by measuring tension vs time with an elastic strap I am using as part of my rotator cuff surgery rehab, as shown in the following short video clip

The data from this experiment was captured on my PC and plotted in Excel, as shown below

Tension vs time for the orange elastic strap shown in the video

Then, at the request of my physical therapist, I measured the real-time tension for single & double orange straps, and single/double green straps, as shown below

Single and double green strap tension vs time

Single and double orange strap tension vs time

 

Stay tuned!

Frank

 

 

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