Feet

Robot Evolution

The feet enable the robot to turn reciprocating motion into forward motion.  

Successful feet have two qualities:

  • They must allow the robot to move forward.
  • They cannot inhibit the soft nature of the robot.

Foot Prototypes

All of the foot prototypes were designed to work the same way. They have two contact surfaces, a high friction rubber coated surface and a low friction rubberless surface. The feet achieve directional friction by pivoting between these two surfaces. When the foot moves forward, the low friction surface slides across the ground uninhibited. As the robot moves the foot backward, the high friction surface makes contact with the ground and inhibits movement, the rest of the robot is then moved forward.

(left) Cardboard sled leaning on low friction side. (right) Upside down image of cardboard sled showcasing rubber pads.

The proof of concept prototype was a cardboard sled. In this prototype, each end of the robot was outfitted with a cardboard sled. The high friction surface was the rubber pads  (seen in the image on the right) and the low friction surface was the sloped cardboard.  When the sled was dragged forward, the entire sled pivoted forward so that only the bare cardboard touched the ground.  When pushed back the sled pivoted and the rubber pads made contact with the floor. This prototype allowed the robot to move forward reliably and confirmed viability of this concept. However, the sleds were bulky and too cumbersome to be used.

The second foot prototype was designed to be a smaller less obstructive version of the cardboard sled. Like the cardboard sled, it was designed to switch between a rubber coated high friction surface and a low friction surface. These sleds were 3D printed and designed to go underneath the robot. The sleds were connected to the body of the robot with a piece of fabric that was glued to the body of the worm. This prototype failed because the feet did not pivot reliably. Some feet remained bent in the low friction position despite not being in contact with the ground (see 2nd foot from the left). It was also noticed that the rigid feet could not conform to the body of the inflated robot. Furthermore, it was thought that the stiff extrusions on the bottom of the robot would inhibit motion if the robot was to crawl over debris.

The next foot prototype was built out of the same type of fabric used to make the sleeve. It was a skirt that had a coating of rubber on the underside. Bristles were cut into the skirt to allow individual sections to pivot. The foot was ineffective because the rubber bled through the fabric and resulted in little differentiation between the high and low friction surfaces. However, this design confirmed that it was possible to create a foot that conformed to the shape of the robot.

The next iteration was made with two pieces of fabric glued together using hemming tape. The tape formed a barrier that the rubber could not permeate.  While this kept the frictional differentials intact, it also made the fabric too stiff to pivot between the high and low friction surfaces. To solve this issue, the fabric was bent away from the robot and the bristles were shortened to allow for easier pivoting. This decreased the angular displacement needed to pivot between states and lifted the robot up so that all of the weight was on the feet. The robot moved forward reliably and the foot pads were soft enough to conform to the shape of the inflated robot. This prototype was successful.

Foot Placement

The feet were initially evenly spaced along the robot. The intention was that a larger number of feet would equate to more traction. It was thought that all of the feet would contribute to the forward movement. However, during testing, all of the movement was focused around two individual feet, one in the front and one in the back.    

The next iteration used less feet and placed them on the front and back thirds of the robot. Again, the resulting movement was centered around two feet.

To save time, a sled was developed to test a new foot placement on the existing sleeve. The feet were sewn to a piece of fabric that was then wrapped around a block of rubber and butterfly clipped to the ends of the sleeve. This prototype was successful, a large amount of expansion was converted to forward motion. Unfortunately, the sled was too heavy to be viable.

Next, the same foot placement was used; however, the feet were sewn directly onto the sleeve. This was very successful.

Throughout testing, it appeared that one primary foot on each side of the robot did most of the work. It was decided that 2-3 rows of bristles per foot allowed for reliable forward movement.

Up until this point the feet were sewn directly onto the sleeve. However, this meant that the feet had to be sewn onto the fabric before the sleeve was made. To simplify the manufacturing process going forward, the feet were pre-assembled on fabric patches that could be attached via hemming tape or hot glue after the sleeve was completed.

The ability to attach patches post production increases the potential applications for fabric-reinforced actuators exponentially. In this project the patches were only used to attach the feet; however, in future development, patches can be used to connect small instruments such as sensors, thermometers, and cameras to the exterior of the robot. Therefore, on-site modifications can be made once needs are identified, allowing for individually tailored solutions to unique problems.

The design is not perfect, dirt and dust stick to the rubber side and decrease its surface friction. Dirty feet were still able to move the robot forward; however, the movement was noticeablely less efficient compared to a clean foot. Additionally the feet were designed to move over smooth ground. This currently limits the environments that the robot would be effective in.  Building versatile feet was reserved for a future enhancement.