07.2 - Design Process
1. Early Prototyping and Iterations
This section outlines the design modifications made during the initial prototyping and testing phase. Each of the iterations provides insights that informed improvements to both the mechanical performance and manufacturing process of the Jansen mechanism and speed-changing system.
1.1 Iteration 1: Cardboard Prototype to Solidify Concept
Before laser cutting our initial Jansen mechanism design for prototyping, 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 Iteration 2: Material Selection → Shift from PLA to PETG Material
PETG was chosen over PLA when manufacturing the components of our build because it provides more of a resistance to wear and allows more flexibility when subjected to repeated loading.
When manufacturing our planetary gears, we ran into issues regarding strength of material. We initially used PLA for all of our prints. However, because the planetary gears within the Jansen mechanism stayed in contact with each other as they turn, this generated friction and heat. This 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.
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. From this point onward, we decided to continue using PETG for all of the prints in our project.
2. Jansen Legs with Automatic Transmission
Our first physical prototype involved 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 was to demonstrate how we are going to mechanically change the speed of the Jansen Leg Mechanism.
2.1.1 Planetary Gears Model
i. Planetary Gear CAD and Model
The planetary gears stages consisted 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 could have either used 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. For our final prototype, however, we decided that if we continue using the automatic transmission as our speed changing system, we should order commercially manufactured 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 was capable of changing between 2 speeds. The input was always constant, but depending on which gear was being held/restrained, the driving gears, D1 and D2 moved at different speeds. The first speed could be outputted by holding D2 stationary as D1 rotates. This engaged only R1’s planetary gears, and barely moved the planetary gears connected to the purple carrier. (on our actual model) Since the speed was only being outputted by a singular planetary gear track, the output speed was low. However, the second speed was fast because none of the gears were being restrained. Both R1 and R2 were engaged and all the 6 planetary gears rotated. Due to this effect, the speed of the output increased. 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.
2.1.2 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, we got a Jansen leg that could mechanically change its speed through an automatic transmission. For prototyping purposes, we did spin the gears manually to test the concept, but on our final prototype, we planned to power the rotation with a stepper motor. We also wanted to use a servo to restrain and release the D2 gear. By a button press we would be able to turn the servo to lodge itself in the gear. This would change the speed of the mechanism. Joint 1 of the Jansen Mechanism would be fixed to the rotating gear while Joint O1 would be fixed. Joint 1 would trace a circle and move the toe (Joint 7) in a closed loop path, which would allow the leg to move forward.
3. Preliminary Bill of Materials for Automatic Transmission–Powered Jansen Leg Mechanism
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 |
Planetary Gear | 6 | Automatic Transmission Gears |
Sun Gears | 1 | Automatic Transmission Gears |
Driver Gears | 2 | Automatic Transmission Gears |
Ring Gear | 2 | Automatic Transmission Gears |
4. Considerations for Final Prototype
4.1 Shifting from Automatic to Manual Transmission
We initially planned to build an automatic transmission using a multi-stage planetary gear system. However, the high level of precision required to manufacture reliable automatic transmission components made gear fabrication a significant challenge. When using 3D printed planetary gears, maintaining consistent tooth alignment with the ring gear was a struggle. These issues were especially amplified due to the use of multiple gear stages in the automatic transmission system. Small inaccuracies in the printed tolerances compounded across the multiple stages, leading us to anticipate significant resistance when rotating the input driving the Jansen legs.
To better accommodate manufacturing constraints and follow the project timeline, we decided to transition to a manual transmission design. A manual transmission reduced the complexity of the system and entirely removed the need for multiple planetary gear stages. This provided more control over gear engagement.
After transitioning from an automatic to a manual transmission, the final design incorporated 4 gears mounted on bearings rotating about a common axis. We included a dog clutch to engage a select gear. This configuration allowed the gears to rotate freely when disengaged, minimizing unnecessary friction and wear, while enabling direct power transfer when the clutch was engaged.
The axle connecting the gear system to the Jansen Legs was press-fit to the output shaft of the transmission, creating a rigid mechanical connection. We also noticed some slippage when the shaft was circular, so we added a key to the shaft allowing for proper torque transmission.
4.2 Four Leg Mechanism vs Two Leg Mechanism
The project initially considered a 4-Leg Mechanism capable of moving forward. However, the design was simplified to better focus on the primary goal: demonstrating a mechanical power transmission system and speed control using a manual gear transmission. By reducing the number of legs, the system complexity, part count, and alignment concerns were significantly decreased. This promoted reliable integration between the transmission system and the Jansen Mechanism.
The 2-leg configuration provided a stationary base for observing the leg kinematics and evaluating the effect of different gear ratios. We were able to do this without introducing issues of gait stability and coordination and ground friction. Also, since we were able to reduce the number of legs, we reduced the number of gears in our design. This allowed us to easily identify meshing problems within the system. Overall, this simplification allowed us to reduce the troubleshooting time needed and ensure that the core functionality of the transmission-leg system could be successfully demonstrated.
4.3 Addressing Friction in Joints
After initial prototyping efforts, the design was intended to incorporate bearings in the Jansen leg joints to achieve smooth and low-friction rotation. However, due to manufacturing and time constraints, the joints were instead produced using 3D printing. While this approach did reduce cost and simplify the integration of the joints, the 3D printed joints had higher friction due to high surface roughness and material properties compared to bearings. To address this, we used lubrication oil on the joints to reduce friction and improve smoothness of motion.
4.4 FUTURE PROTOTYPING: Torque on Motor Used for Driving Transmission
The system was initially designed to be purely mechanical and manually operated to show change in speeds. With additional resources available, a stepper motor was integrated to explore automated operation of the transmission. While the motor was able to rotate the gears when the transmission was not connected to the Jansen Leg gears shaft, once connected to the Jansen legs, the torque produced by the motor was not able to turn the Jansen shaft. Future work would involve using a motor that can provide sufficient torque at low speeds. An option we were considering was Brushless DC Motors. We further explain our proposed electronics and code in the 07-4 Implementation section.