The proposed work for this project is to design a Strandbeest using the Jansen linkage mechanism to navigate complex terrains such as stairs in addition to flat surfaces. To build such a robot mechanism, the Jansen linkage is the most critical aspect of the design process, as it enables the specific motion profiles necessary to achieve the objectives of this project. A motion profile of the Jansen mechanism was initially analyzed using MotionGen to determine which linkage needs to vary in length to switch to walking up a stair from a flat surface, as shown in Figure 1. To suffice the aim of stair climbing, we experimented with changing the different linkage lengths to create a motion profile that can execute a higher step height (Figure 2). Upon changing individual linkage lengths and analyzing the motion profiles generated in MotionGen, it was determined that changing certain linkages caused an issue with the input crank (Link A) not being able to make a full revolution. Also, changing multiple linkages at the same time was not realizable in terms of implementation due to time constraints. However, we found that changing the length of Link H, gave us a higher step height in the motion profile analysis (Figure 3), and proceeded to utilize this.

Figure 1: Jansen Linkage mechanism with MotionGen 

Figure 2: Jansen linkage mechanism with enlarged dimension of link A to generate a different motion profile

Figure 3: Jansen Linkage mechanism with a modified Link H to achieve higher step height   

To initiate the first prototype, we set our main objective to build a regular Jansen mechanism leg. A prototype of the Jansen linkage mechanism was designed and assembled in SolidWorks, as shown in Figure 4. The purpose of the prototype is to gauge the size of our final design as well as to determine any unforeseen issues that will be encountered later. Building the prototype and testing it by hand enables us to get a feel of how the mechanism will work and efficient ways to assemble the linkages. The physical prototype is shown in Figure 5.

Figure 4: Jansen linkage mechanism  design and motion study simulation video generated with SolidWorks  

 Figure 5: Physical prototype of the Jansen mechanism                

The links and spacers were cut out from a 6-mm acrylic using a laser cutter, and ball bearings were press-fitted into the linkages to connect them using M6 bolts and nuts. We used a servo The physical prototype worked as expected from the Solidworks simulation video shown (click on the link to view the video) in Figure 4. We used a stepper motor to actuate the crankshaft. However, the stepper motor used draws 2 amps, and the motor driver (L298N) has a current rating of 1.7 amps. This caused an overheating issue, and a different motor driver was used for the final prototype. A 7.2V battery was used to supply power to the motor, and a Raspberry Pi Pico was used as the microcontroller to communicate with the motor driver.

The next task of this project is to take the knowledge gained from the physical prototype design and make a modification by including the new proposed change to the linkage to vary the motion profile of Link H using a servo motor. This will enable the mechanism to travel in different terrains by actively controlling the motion profile of Link H. Since there will be four Jansen linkage mechanisms to build the Strandbeest, this prototype helps us get closer to our final objective of building the Strandbeest. Since it becomes necessary to transmit power from one motor to all the linkages, a crankshaft will be used to distribute power from a motor to all the Jansen linkages. The walking steps of the mechanism also have to be designed in such a way that the front right and the rear left legs move at the same time, while the front left and rear right move at the same time to generate a four-legged walking motion. To achieve such a walking step from a single motor, a crankshaft with a 90-degree angle between the crank throws will be used for the final design. The crankshaft was designed concurrently with the iterations of the Jansen linkage mechanism and the length of the crank is determined to be 45 mm, which is the same length as the input link connected to the motor in Figure 5 Building this prototype enabled us to determine the dimensions of the crankshaft necessary to transmit power to all four Jansen linkages, thus getting us closer to our final design and submission.

Bill of Materials

For the first prototype, the materials shown in Figure 6 were purchased. The biggest expense was purchasing ball bearings that are necessary to connect the different links. A 12 X 24 in acrylic was used to cut the linkages. 

Figure 6: Bill of materials for the first prototype

Takeaway

As a result of our first prototype, many lessons and takeaways were drawn for final assembly and design. The most important thing that we were able to prove with the prototype was the ability to transmit power from the motor to the linkage at scale, ensuring that we have a high enough motor output to drive the legs of the mechanism. However, we did notice that there was not enough contact surface between the leg and the ground and will need additional "shoes" to ensure that the mechanism is transmitting the motion to the ground in order to move. We were also able to test the play in the linkages as well as out-of-plane stresses to ensure that they were within tolerance. The play in the joints was within specification, but we did notice some flexure in the acrylic links. In order to mitigate this effect, it was necessary to change the link geometry or use a thicker/different material for the final prototype. Based on the first prototype, we translated these learnings into building more rigid bodies through tighter tolerances and smarter materials selection. We planned to integrate all the legs into one mechanism close to what is shown in Figure 7-9. This will require a full chassis integration and motor housing. We also planned to further implement the modified leg to enable stair-climbing. 

Figure 7: Isometric view of assembled Jansen linkage mechanisms    

Figure 8: Top view

Figure 9: Front view