Measuring Load on Hemispherical Surface TfCD

For the TfCD technology exploration assignment there was chosen to make an Instructable for a simplified prototype that will measure the correlation between the angle and the applied load in a cup-head formation.

The cup and head were modelled using the hemispherical design of existing Biomet components. The models were eventually milled out of foam. To determine the angle of femoral stem towards the cup, a triaxle SPARKFUN accelerometer was used. To determine the load of the head on the cup, a (brand?) pressure sensor was placed at the bottom of the cup. Eventually these sensors were connected to each other using Arduino in the following Arduino circuit scheme. To measure the variables a software program was written. The LED in the circuit is used as output and give an indication of the load of the force as it increases in intensity as the load increases acting as a warning. In the serial monitor of the Arduino program the exact forces and corresponding angles can be viewed.

Material:

180*90*50 polystyrene foam (EPS)

Wooden stick 10 cm

Small piece of wire

Arduino board

Arduino cable

Breadboard

Approximately 2 meters of electric wire preferably in different colors.

220Resistor

10KResistor

1 LED

1 Triaxial accelerometer

1 Force sensitive resistor (max 100N)

Soldering tin

Soldering Iron

Step 1: CAD

Make a model using CAD software, we used Solidworks 2014. U can download our models.

Step 2: Milling

To make the hemispherical surface as accurate as possible, it is recommended to use rapid prototyping. We used a CNC milling machine, milling polystyrene foam (EPS). When using this method remember that your piece of foam needs to be 180*90*50 mm.

*Hint: if you want to prevent unnecessary time sandpapering the models. Dimension the radius of the head 0,5 mm smaller. We milled it using the exact dimensions. Which resulted in a long time to sandpaper our models.

Step 3: Finishing Foam Model

When the models are milled, sandpaper your model using a soft sandpaper. For instance Sia P280.

Step 4: Drilling

Turn the cube were the cup is integrated upside down. Check the dimensions and drill a 4 mm hole in the exact center.

Step 5: Flat File

Now draw a stroke of 20 mm wide at the bottom of the cube, and delicately remove the foam within this stroke using a flat file. Just far enough to reach the surface of the cup.

Step 6: Handle

Take the back of the foam head, and define its center, and place the wooden stick in the center.

Step 7: Preparing Arduino

Prepare the breadboard with arduino according to the circuit scheme. It should look like this. We've added the Fritzing file.

breadboard_sketch.fzz
Download

Step 8: Coding and Testing

Download the code via this link (). Start up your Arduino CC software, copy paste the code and check on your computer if the sensors are working by reading out the serial monitor.

code_accelorometer_with_force.txt
Download

Step 9: Soldering Parts

When everything is working you can make changes to the code or add more sensors and features. Or you can solder the parts. Make sure the wires are long enough to fit the prototype. It is easier to take parts of the wiring then add.