Monthly Archives: November 2018

Digital Tension Scale, Part III

Posted 25 November, 2018

Over the Thanksgiving weekend I had a chance to do some more work on my digital tension scale project.  My wife and I drove to St. Louis to visit our kids and grand-kids, and its a 6-hour drive each way.  I make sure we have an audio book going for my wife, and as a consequence I get 6 hours of (mostly) uninterrupted geek time to work on things like this project.

As I mentioned in Part II, my goal is to construct a battery-operated tensionometer that can be mounted directly on the dual-hook S-shaped tension block, as defined by the features delineated in Part II.  After doing some more web research, I came up with the following possible components for the system:

Display:

Apparently, the Nokia 5110 84×48 pixel monochrome LCD display used in prehistoric times as the display in Nokia cellphones has found a second career as a simple, low power display for battery-operated devices like the one I envision.

 

Nokia 5110 Monochrome LCD display

Battery & Battery Charger:

The LCD display will operate quite nicely from 3.3V, so as long as I can come up with a 3.3V micro-controller (like the Teensy 3.2) and a small, capable LiPo charger, I should be in business.  For this component, I plan to use the Adafruit PowerBoost 1000C and something like the 3.7V 2500 mAh battery as shown below

Adafruit PowerBoost 1000C single-cell LiPo charger

3.7V 2500 mAh LiPo battery

 

I don’t think I’ll need the 5V boosted output from the PB1000C and the entire thing may be a bit of overkill for this project, but I had them hanging around from a previous project, so…

MicroController:

For this I plan to use one of Paul Stoffregen’s magical Teensy 3.2’s.  Again this is probably way overkill for the project, but…

Wireless Connection:

This component is the one for which I have the least understanding and confidence.  I currently use a Pololu Wixel for wireless serial comms and programming with my autonomous robot, but I haven’t figured out how to use it with a Teensy, and I thought maybe there were better solutions out there by now anyway.  So, after some more web searching I found that many ‘makers’ are using the HC-05/6 Bluetooth modules for this purpose.  Hopefully with this module I’ll be able to use a Bluetooth connection from the tensionometer to my laptop or even my cellphone to perform calibrations and collect real-time tension data.

Combining all these with the HX-711 load cell amplifier, I came up with the following system schematic.

 

Stay tuned!

Frank

 

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