IDEA 170 - Introduction to Mechanical Design & Engineering
Hopper
Using mechanical systems to design a biologically inspired "hopping" mechanism
Materials and Tools
After being briefed on the design parameters we were given a set list of materials and tools that could be used to design our hopper. Materials ranged from MDF (micro density fibreboard) and dowels to balloons and golf balls. Tools included anything we had access to in the lab, provided we had been properly trained and certified to use them. These included machines such as the laser cutter, 3D printer, CNC mill, and any hand power tools.
IDEATION/Prototyping
Biological Inspiration
Before creating a CAD model, I first brainstormed what biological organism I wanted my hopper to resemble. I decided I wanted to model it off of a spider, specifically the skeletal and muscular structure of the legs that enables it to jump. It was this design constraint, along with my design goal to have my hopper land on its feet, that led to the first sketches of my hopper.
Experimentation
I had various prototypes I designed throughout this project, some of which worked better than others, and all of which helped inform my ultimate design. In these iterations, I used a balloon, dowels, MDF, rubber tubing, green elastic bands, magnets, and wood glue.
Delay Mechanism
When thinking about how I would go about designing my “Spider”, I was most concerned with finding a successful and reliable timer more so than figuring out how I would actually get it to jump. I theorized a few ways which I could delay the release of my hopper, and accidentally stumbled upon a unique but consistent mechanism.
In this mechanism, two magnets would squeeze together the end of a long balloon, slowly releasing the air held inside. This would result in the balloon gradually deflating, shrinking from the top down. I theorized that I could integrate this mechanism into a design, using the deflation of the balloon as the timer which would allow for the release/deployment of the hopping mechanism. My first design idea involved an elastic band that went around a set of legs and the balloon, and could release once the balloon deflated.
Testing
Spider 1.0
Once I was sure my balloon was able to withstand elastic pressure, I created my first design on solidworks. I printed out a set of 5” long square legs and attached them to a circle that would hold the balloon in place. The plan was simple: an elastic exercise band tied in a circle would wrap around the vertical part of the legs - pulling them together. However, the balloon would hold them apart until eventually the air where the elastic band sat was released and the band could constrict, thereby pulling the legs with it and causing the horizontal part of the legs to exert force against the floor and push the hopper off the ground.
After a few futile attempts to get this design to work, I quickly realized just how unstable and delicate it was; the legs could not support the circular frame, it had to be balanced against something in order to stand upright. Just the process of putting the elastic band over the legs and balloon was quite a hassle, and resulted in too many popped balloons. Like the balloon, I was a bit deflated, but determined nonetheless to continue with my delay mechanism as I knew there was still plenty of time to test out new designs before the final hopper presentation.
Spider 2.0
My next hopper barely made it past the laser-cut phase before I realized it would not work. The concept was to use a scissor like motion to propel the body off the ground, however the concept was poorly thought out and the model was too unsteady and light to withstand any real force. The timing mechanism was going to be the same, with the balloon resting between the upper prongs and an elastic band pulling the legs together.
Spider 3.0
After the previous two failed attempts, I decided to shift gears and change the conceptual design of my hopper, going with a flat base which allowed it to be more sturdy. This design pulled from an original cardboard hopper I created in the beginning of the year; a piece of green elastic pulled the legs together, and would wrap under the legs and base while another green band would run around the top of the legs and balloon holding them together until the air released.
Timing System
I still believed that I could implement the balloon timer into my hopper, which is why I made a large base so the balloon could fit easily.
Design Mechanics
An integral part of this design was the hinge, and making sure there was enough surface area for it to properly grasp onto the base
Spider 3.1
In my first design meeting, I was given really good feedback on how I could change my design, as it worked mechanically but was incredibly flimsy and did not exert much force. I moved from a single-legged spider to a dual-legged hopper, reinforcing the overall design and making it more stable. However, I still struggled with implementing my delay mechanism.
Design Mechanics
I also changed the depth of my hinges, increasing the indent from .1 to .5 inches
Spider 3.1
This model (3.1) ended up being my final design, with a few minor modifications
Spider 3.2
I still struggled with getting enough force still in my second iteration of the final spider. I figured the best solution to this would be to increase the number of legs which I believed would increase the force (more legs = more elastic bands = more tension). This however turned out to be a nightmare of a hopper and had strayed completely from my original design goal to be minimalistic. I did however try a new strategy for the tension in this version, using a tied balloon and feeding that through the legs. I thought, well why not try implementing this into my earlier, stable but not incredibly forceful hopper?
Details
One change that I made though from the previous design was making the hole that the elastic went through larger. This wasn’t an essential design change but it made it so much easier to put the elastic in and change it out when I was testing different band lengths, since the smaller the band the more force it will release when stretched over a constant distance.
Final Hopper - Spider 3.1.1
The results were beyond better than what I expected. The spider hopped perfectly on my first test with the balloon band.
The Spider was close to being done, but it still needed a timer. My original method of a deflating balloon was no longer a viable option as the hopper was too strong.
Luckily, in the second design meeting I was given a great suggestion to use a friction timer, and was able to accomplish this with a small wooden dowel inserted into a piece of thick rubber tubing, slowly being pulled out by the force from the balloon and consistently releasing the hopper 8 - 25 seconds after I removed my hands.
Energy Calculations
These are the data points I got using a linear analyzation method to calculate my hopper’s potential energy. Because of the design, I had a minimum stretch of 7cm and a maximum of 23.7cm
The theoretical height and actual height my hopper jumped are very different because a lot of the energy created is used in the rotational movement of the legs and is dissipated when they collide into one another.