16.3 Preliminary Prototype
Based on preliminary analysis of both the Klann and the 8-bar mechanisms, we selected the Klann mechanism for our preliminary prototype. The prototype was designed to demonstrate the variation in gait patten we can make from a simple change in the ground link.
After some analysis on the Klann mechanism, we realized that changing the angle of ground link A did not produce much of a distinct difference as compared to changing the angle of ground link C. To show these different gait patterns, we used a slot to be able to rotate the ground link C about the ground link O2 and were able to visualize the gait patterns.
Modeling and Design
The physical prototype consisted of the main linkage system, a back plate assembly that housed the slot for configuration selection, and a marker mount attached to the end-effector link to be use to trace the motion profile of the system onto a taped piece of paper. The links for the Klann mechanism were fabricated with 1/8” plywood, while the backplate was laser cut with 1/4” plywood to provide some structural rigidity. The links were assembled using M4 (4mm) hardware, specifically using two nuts on each bolt at every joint to ensure minimal axial translation of the mechanism.
To demonstrate the key feature of our walking mechanism, the ability to alter the length of the gait profile with a single change of the ground link, we cut a radial slot in the base plate spanning from 25° to 70° relative to our global axis. This slot was used to select between two binary configurations, enabling the ground link C to rotate in the slot in between the two poses and thus create the distinct motion profiles required for the “dot” and “dash” gait patterns.
To visualize and validate our prototype, we designed a marker mount for the output link of our Klann mechanism so that the marker tip is perpendicular to the base plate. This will allow us to draw out the motion profiles for both configurations of our Klann linkage to visually analyze if these profiles can be used for our final product. The marker mount was 3D printed as a two-piece assembly, with a bottom piece that is fixed to the linkage with fasteners, and a top piece that clamps around the body of the marker, providing a firm grip to ensure consistent marking on the paper.
The CAD assembly below shows the entire prototype and the implementation of the Klann mechanism with the slotting system for the gait variation.
Below is an image showing the motion profiles generated by the prototype.
Iterations Documentation
During our prototyping phase, we encountered several unexpected issues that we took note of to effectively implement into our final project
Joint Tolerances
A critical design oversight that compromised the efficiency and easy of motion for our assembly was the lack of tight tolerances at our joints. The laser cut holes for our M4 bolts were too loose around the bolts and caused significant play for our mechanism, resulting in difficulty turning the mechanism and results that were not very repeatable. To improve upon this for our final project, we will laser cut test piece of different diameters to find the optimal size.
Linkage Spacing
Our initial prototype had incorrect spacing at the joints connecting the adjacent links, causing interference between bolt heads and nuts. To temporarily address this, washers and cut spacers were used to provide additional clearance and allow for links to complete their full motion path.
For the final product, we will spend significantly more time in the design phase, including detailed such as fasteners, spacers, and other hardware in our CAD to ensure proper spacing and that all parts are working smoothy. We also plan to 3D print custom spacer lengths based on our CAD to eliminate unintended interference between components.
Unnecessary Ground Links B and C
During analysis and initial CAD design, additional ground links A and C, which connect the origin of the crank link to the ground positions A and C, were constructed. This was used for our kinematic analysis and visual demonstration of the linkage system however, during assembly it was found that these links were unnecessary as the back plate incorporated locating features for the ground joints A and C, making the actual links themselves unnecessary.
Unnecessary Links 3 and 6
In the kinematic analysis, there are two ternary links, links 3 and 6, that were separated into 2 links each to allow for ease of kinematic calculations and flexibility of the motion profile. However, we unintentionally cut these pieces separately for our prototype, adding additional play and degrees of freedom to the mechanism, causing in a deviated motion profile. We quickly fixed this by constructing the ternary links 3 and 6 together to allow for the Klann mechanism to move as intended.
Marker on End-Effector Link
One final oversight was the lack of space on the end-effector to accommodate the marker mount. Since link 6 was designed before the 3D-printed mount, it didn’t account for its integration requirements. After fabricating the marker mount, we realized a modification was needed. A semicircular extrude-cut was added to Link 6 to allow the marker to slide smoothly through the mount and link without obstruction. This was a simple fix and the inclusion of a marker will be more integrated and complete for our final project.
Hardware Issues
The final prototype also exhibited slightly loose tolerances, mainly due to imperfect joint design. To address this in the next iteration, we plan to replace standard nuts with lock nuts and precision spacers. This will help prevent the unintentional tightening or loosening of joints during movement, reducing unwanted degrees of freedom and improving the mechanism's performance.
Complete Prototype
Overall, the prototype provided some validity and verification for the use of the Klann mechanism to change gait profiles with the alteration of a ground link. The design and fabrication of the prototype allowed us to find many areas of improvement and allowed for us to execute a more refined final product in the process. Although the prototype included the Klann mechanism, after some more in-depth kinematic analysis that we were working on concurrently with the prototype, we ultimately decided to use an 8-bar walking mechanism due to several key parameters analyzed in the Kinematic Analysis section of this report. Ultimately however, this prototype was a critical checkpoint for our design and analysis process.
Based on the challenges we faced with this prototype, we plan to conduct a more refined design and fabrication phase for our final assembly. We will conduct test cuts to find correct tolerances and dimensions for critical pieces, mainly, the holes for the joints. Additionally, as noted in the Iterations section, a properly designed and laser-cut base plate will be included to ensure structural stability and to place the system upright. Finally, a paper feeding mechanism will be designed, incorporating a belt and pulley system to advance the paper, enabling the Morse code encoder to accurately encode messages onto the paper.
A video of the prototype in motion is shown below: