07.2 - Design Process
1. Physical Prototype & Iterations
1.1 Physical Prototype Overview
Our physical prototype involves 2 systems: (1) an automatic transmission set up with a series of 2 planetary gears and (2) a Jansen Leg Mechanism. The purpose of our prototype is to demonstrate how we are going to mechanically change the speed of the Jansen Leg Mechanism.
1.1.1 Planetary Gears Model
i. Planetary Gear CAD and Model
The planetary gears consist of a series of 2 planetary gears that share the same sun gear. We have 6 planetary gears rotating around the sun gear with 3 planetary gears meshing with each of the 2 rings gears. We initially were planning to use only 1 stage of planetary gears. However, we decided to connect 2 together to make a series since we wanted to change the speed of the Jansen mechanism. We can either use the singular planetary gear or the compounded effect of a planetary gear series to decrease and increase the speed respectively. Below is the labeled input, output, and components of the automatic transmission system.
Note: An automatic transmission systems contains stages of planetary gears. We replicated this design to add to our Jansen Mechanism.
We decided that for prototyping purposes we will 3D print our gears to model what it would look like when we use them as our driving mechanism. However, for our final prototype, we plan to order gears to prevent from degradation due to use. We noticed that our prototype is pretty stiff, so by ordering gears that mesh more effectively, we will not have that problem. Below are the top and side views of the CADs used to print our automatic transmission system.
ii. Gear Speeds
Our automatic transmission will be capable of changing between 2 speeds. Our input is always constant, but depending on which gear is being held/restrained, the driving gears, D1 and D2 will move at different speeds. The first speed can be outputted by holding D2 stationary as D1 rotates. This engages only R1’s planetary gears, and barely moves the planetary gears connected to the purple carrier. (on our actual model) Since the speed is only being outputted by a singular planetary gear track, the output speed is low. However, the second speed is fast because none of the gears are being restrained. Both R1 and R2 are engaged and all the 6 planetary gears rotate. Due to this effect, the speed of the output increases. Below is an animation of the gears moving around the sun gear in the 2 different speed modes. Also, below is our complete transmission system with the 2 driven gears.
1.1.2 Jansen Leg Model
To demonstrate the motion of the Jansen Mechanism and explain its role relative to the automatic transmission gearbox, we designed and laser cut an example Jansen leg. The mechanism was entirely made out of plywood and wooden dowels were used to pin joints together.
Our Jansen mechanism is made up of 2 ternary links and 4 binary links. The Jansen mechanism has multiple 4 bar linkages including the ground link which is connected from O1 to 2 in the figure below. The link from joint 2 is a crank mechanism that traces a path of 360 degrees.
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Combining the 2 systems that have been presented, we get a Jansen leg that can mechanically change its speed through an automatic transmission. Presently, we are spinning the gears manually to test the concept, but on our final prototype, we will be powering the rotation with a stepper motor. We will also be restraining and releasing the D2 gear through a servo mechanism. By a button press we will be able to turn the servo to lodge itself in the gear. This will change the speed of the mechanism. Joint 1 of the Jansen Mechanism will be fixed to the rotating gear while Joint O1 will be fixed. Joint 1 will trace a circle and move the toe (Joint 7) in closed loop path, which will allow the leg to move forward.
1.2 Prototype Iterations
This section outlines the design modifications made through prototyping and testing. Each of the iterations provides insights that informed improvements to both the mechanical performance and manufacturing process of the Jansen Mechanism.
1.2.1 Iteration 1: Cardboard Prototype to Solidify Concept
Before laser cutting the wood, we wanted to check whether the design of the Jansen mechanism was mechanically viable. And although the Jansen mechanism could walk in theory, a physical demonstration was necessary to aid in the visualization process. The main objectives for this stage was to get the links to move smoothly without getting jammed and produce a smooth walking motion. Additionally, we wanted to analyze how the linkages interacted with each other mechanically, as well as get an overall feel of the mechanism’s motion. This process was also helpful in making note of potential design and assembly issues before jumping into manufacturing.
Construction of Cardboard Jansen Mechanism:
Six Jansen links were cut from cardboard based on Theo Jansen’s specified proportions. Wooden skewers served as revolving joints at each pivot, this providing full rotational motion between the connected links.
Main Takeaways from Cardboard Jansen Mechanism:
The cardboard prototype successfully demonstrated the walking motion of the Jansen mechanism. During testing, we observed the following:
Smooth and sequential movement of all linkages
Proper translation of the foot along the expected trajectory
No significant binding or jamming in the four-bar loops
This low resolution prototype essentially verified the key facets of our design including validating that the lengths of our links were accurate.
We have attached an animation of the prototype to demonstrate the walking motion of it to document proof of concept before starting with laser cutting.
1.2.2 Iteration 2: Material Selection → Shift from PLA to PETG Material
PETG was chosen over PLA when manufacturing the planetary gears because it provides more of a resistance to wear and allows more flexibility when subjected to repeated loading.
Because the planetary gears within the Jansen mechanism stayed in contact with each other as they turn, this generated friction and heat. This side effect caused the PLA to wear out quickly. PETG offers more toughness and a stronger layer of bonding within the material, helping it counteract any shear during the torque phases induced by walking. Additionally, PETG material is better at handling the shock when the Jansen legs hit the ground.
Additionally, the switch from PLA to PETG material for the planetary gears helped prevent shrinkage problems due to tolerancing . When we first printed the carrier gear in PLA, the gears wouldn’t rotate around the spokes because they were too tight of a fit. PETG material shrunk less than PLA material, so we used PETG for our gears and carrier. This allowed the gears to rotate around the spokes of the carrier.
2. Proposed Bill of Materials
Parts | Quantity | Purpose |
|---|---|---|
TIW Plywood | 1 | Laser Cut Jansen Mechanism Legs and Mounts for Motors |
Cylindrical Steel Dowel Pins | 20 | Used as joint pins for the Jansen Leg |
M3 Screws + Washers | 20 | Fastening Electronic, Hardware Components |
Washers | 20 | Spacers in between the links of the Jansen Leg |
Arduino UNO | 1 | Turn Stepper Motor and Servo |
Motor Driver | 1 | Turn Stepper Motor |
Stepper Motors | 2 | Motor is needed to move input of automatic transmission to move legs |
Servo | 2 | Stops D2 gear from moving and changes speeds |
Battery | 1 | Power the Electronics |
Jumper Wires | 50 | Power and Connect Electronics |
Front Wheel | 1 | Used to guide Jansen Legs at the back or front |
Planetary Gear | 6 | Automatic Transmission Gears |
Sun Gears | 1 | Automatic Transmission Gears |
Driver Gears | 2 | Automatic Transmission Gears |
Ring Gear | 2 | Automatic Transmission Gears |