Carbon Fiber Feet

Between 2017-2018, I was involved with the Tik-Tok robot, a project to build a walking robot that was as robust as the ATLAS robot but at a tenth the cost and energy use. Specifically, I worked on the feet of the robot to potentially transition it from metal to carbon fiber to save weight and allow for a more robust foot profile. There were several constraints on the foot design, specifically the fixed ankle geometry and hollow feet requirement. The connection points from the feet to the rest of the robot was already manufactured, so the foot had to fit with the existing geometry. The foot needed to be hollow so that pressure sensors could go on the inside of foot. In this project, I gained experience working with Solidworks, designing for assembly, manufacturing with composites, and laying up PCB boards.

To manufacture the foot, it was done in two pieces so that it could be hollow. To make the two pieces, molds were designed in Solidworks, then 3D printed. Because of the unique geometry of the foot and without access to an autoclave or oven, wet carbon fiber (Fibreglass 1069-A 3K, 2x2 Twill Weave Carbon Fiber Fabric) was used. Two plies of the material were laid up on the carbon molds and room temperature cured for a day under vacuum pressure. The two halves of the foot were cured independently. Holes are then drilled out of the larger foot so that the pressure sensors can poke through the bottom. After a flexible PCB board is placed inside, the two halves of the foot are bonded together with epoxy. To stiffen the foot, a foaming adhesive is used to fill the inside of the foot. To protect the sensors, a 15mm layer of Smooth-on Flex Foam-it! 17 is used. There is also a sole to protect the foam from being punctured- this was made using smooth-on PMC-780 Dry because it adhered well enough to the Flex Foam-it! 17 material.

The layup of the flexible PCB board was done using Autodesk Eagle. Pressure sensors were placed throughout the bottom of the foot to ensure coverage, and communication to the rest of the robot is done through a MCB23517 microchip located inside each foot. A flexible PCB board was used because of the curvature on the bottom of the foot. A difficult constraint of this layup was the immobile electronic components- there was only one possible location for the microchip, and there needed to be as many pressure sensors as possible to ensure detection of impact forces. To deal with this, the board was laid out in a snaking way to allow for small misalignments of the PCB board with the holes drilled out in the foot. The entire board was laid out using just two layers.

If you would like to read more in detail, feel free to look at this paper for challenges and improvements with regards to the manufacturing the foot and this paper for challenges and improvements with regards to the flexible board design and overall assembly from start to finish.