05 - Spider Walking Mechanism

Introduction

There already exists a multitude of walking linkage mechanisms, from the Jensen linkage to the Klann linkage, there is no shortage of mechanisms that achieve this profile. Instead of replicating what many other linkage systems have done, we wanted to tackle a problem more complex while still keeping the theme of walking linkage systems. Therefore, we turned to nature and looked at how other organisms moved. We were particularly fascinated with the spider, with movements that seemed to pivot and shift rather than strut like a typical walking linkage system, and so for our project we decided to mimic the movements as best we can. The biggest issue, is the fact that a spider leg moves with multiple degrees of freedom and in different directions, as such a direct copy would involve knowledge beyond the scope of this class. Instead be decided to approach this goal by combining two different 1 DOF linkage systems in different phases to achieve a similar 3D motion, limiting the scope to contents of our class and also creating a challenge that not only involves the design of linkages, but also the fine tuning of the relationship between multiple linkages using just one input. In summary, our goal is to create the closest representation of a spider as we can, capturing the complex motion profile of the legs using two 2-D linkage systems in parallel to create a more realistic walking motion.

Complexities

Many complexities arise from replicating the walking profile of a spider. Because a spider's leg has multiple degrees of freedom, they are able to move in very complex position profile.  Furthermore there are very few resources that are applicable to this situation, and therefore our solution will have to be relatively novel. In order to replicate a spider as closely as possible, there will also be 8 individual legs. Each leg would add another layer of friction and possibility of error, potentially creating issues for actuation, troubleshooting, among many others. 

Description of Proposed Mechanism

The proposed mechanism is a combination of two different linkages, a 4-bar oscillator and a Klann linkage. Each leg will combine both linkages in sync in order to facilitate the desired position profile. Both linkages will also be modified to better fit our desired position profile. The Klann linkage will be heavily modified to create a more vertical path while the 4-bar oscillator will be modified to match the phases of the Klann linkage and to create the desired distance of travel. These mechanisms will be built using gears serving as the crank arm to save space and reduce complexity. The gear driving the 4-bar oscillator will drive a bevel gear driving the Klann linkage. Both gears will be the same size to better sync the motion of each respective linkage. There will be 8 legs in total all with the same size gears. These gears will the be connected to the outside perimeter of a big actuator gear that will actuate all of the legs at once. Each leg can then synchronized by placing each gear in different position or different times in their phases. 

Scope of Work

• While our primary goal would be to completely replicate the walking motion of a spider, realistically it would likely be outside of our scope. As such, instead of creating a mechanism that can actually walk on its own, our main goal will be to simply create a working mechanism combining two linkages to create the more complex 3D motion profile. So instead of focusing on the functional purpose of the robot, we want to lean in to the artistic purpose of the robot and demonstrate the possibility of the combination of the two linkages. 
• Prior to fabrication we will need to analyze and modify each individual linkage. While we do not need to consider mechanical advantage and thus do not need to worry about velocity or acceleration analysis, it is important to perform a position analysis to ensure that the linkages work well together and would fit the motion need for lateral motion. 
• It will be exciting to work on the novel solutions we will need to come up with to combine each linkage and the way will need to connect them together. It will likely be most challenging to actually manufacture the robot, since there would be so many components and small parts.

Preliminary Design Ideas

Our first task was to decide on a leg mechanism that both visually represented a spider leg and had a position profile that fit our application. After a bit of research we were between the Jansen Linkage, Klann Linkage, and a custom 4 bar linkage. We ruled out the custom 4 bar linkage as we could not get it to have a motion profile that could go down to the ground and remain their with relatively little up and down motion for approximately 50% of its cycle. When it came to choosing between the Jansen and Klann linkages, we saw more potential in the manipulability of the Klann linkage to achieve our target motion profile. Additionally, the Klann linkage appeared visually skinnier and more "spider-like" so we figured it would be the best for both functionality and aesthetic.

As for the oscillating linkage, we always knew we wanted a linkage system that could oscillate back and forth but we were not entirely sure how to achieve that. Our first idea was a double linear slider linkage system but we decided against that because the production and analysis of the system would be overly complicated and we felt there was likely a simpler solution. Eventually we came across a diagram of a windshield wiper mechanism that we realized was very close to what we were looking for. So inspired by this mechanism we figured our a simple 4 bar linkage that we could manipulate to achieve our desired oscillating motion that had a link that the Klann linkage could be mounted to. 

Combining the two systems through 1 actuator was a pretty big obstacle to overcome. Though we quickly decided on a bevel gear, we struggled to find a bevel gear set that was both the right size and had the proper tolerances to allow for the pivoting that Klann linkage would do. We were also weary of attempting to produce our own bevel gears as they would likely have a lot of inconsistencies and resistance. 

Project Video