The first step in controlling our prosthetic foot is balance. The question we face is, how can we create a simple control system to govern balance? The group chose to use a fixed adaptive PID control which increments the values on the PID based on the position of the roll and pitch in regard to its variance from the set point value.
Like any good engineering process the design must be rigorously validated to prove it meets the objectives. Unfortunately, some of the issues with the design included jitteriness and speed. This is most likely due to the fact that the PID needs additional tuning through a mathematical calculation.
Upon the completion of our prototype code we needed a method to model our programming. This model could be made in one of two ways, either through computer graphical simulation or a mechanical model. We decided to construct a servo based mechanical model to give us the best ability to debug and tune the PID coefficients the easiest. This model was created using FDM like previous material samples. Below is a picture of our testing platform:
The most important aspect of this design is the ball joints. The ball joint cups were designed in such a way that they could snap onto the balls while still promoting smooth motion. This type of technique is really only applicable when using fairly soft plastics like ABS (which we used) because it allows the plastic to stretch a little bit and then contract back to its original size therefore encasing the ball inside the cup. Any inconsistencies with 3D printing can lead to the part tolerances not being exact, but the group was able to mitigate the inconsistencies.