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When fitting and adjusting a prosthetic leg, finding the weight bearing line is important. Instead of aproximation, we want to mark the line with a laser. To do this we make a device that can measure the total weight placed on the artificial leg, and find the distribution of it. Then we calculate where the center of the weight is placed, and show the laser line according to our calculation.
If you get 10kg total weight, with 6kg on the left sensor and 4kg on the right, you have a 60/40% ratio. In theory the laser should then be placed 10% (of the length of the foot, measuring from the side) left of center. An actual measurement could give a total weight several times 10kg, but the numbers are chosen to simplify the example.
To develop the hardware and firmware for this, a prototype mockup is in the making. I live about 250km from the FabLab in Lyngen, Norway, so I made this with what I had at home. Do not despair, it will be prettier once it's FabLab-able. Pictures and description of the device is discribed in the .PDF. Source code included, but not schematics. Eagle files will be provided eventually, please contact me if you want/need a sketch of the circuitry.
Here is a quick diagram of the contraption:
Technical details on stepper part:
I would really like to know more about the microstepping controller. In the current setup I have, it steps 8 microsteps at a time, and I think finer 'grain' steps would be an improvement. The specs for the stepper in the setup now is:
Rated Voltage DC 12V
Working Voltage DC 10.8~13.2V
Rated Current/Phase 259mA
No. of Phase: 4 Phase
Coil DC Resistance 50Ω/phase±7%
Step Angle 7.5deg./step
Excitation Method 2-2 Phase excitation (Unipolar driving)
Max. Pull-out Pulse Rate 600pps 650pps
Max. Pull-in Pulse Rate 420pps 430pps
As for the sensing, a capacitive grid sounds encouraging. I have failed using capacitive sensing, but I lack the knowledge and my setup was crude. Has there been done any work on such a grid that I can look into, and maybe try to replicate?
