Mimicking the motion of a Coconut Crab using 1-degree of freedom planar mechanisms

Team Members: Cecilia Corral, Haoran Xie, Pradeep Radhakrishnan and Sadhan Sathyaseelan

Introduction

Our goal is to mimic the motion of a coconut crab using one-degree of freedom planar mechanisms. An example of a coconut crab is shown in the video below. 

As you may notice in the video, there are two pairs of legs on either side of the crab body in addition to a pair of claws on the front and a pair of small legs on the rear side. The two pairs on either side are the main source of motion. The front claws and the rear legs serve as support apart from the obvious predatory function of the front claws. Though the crab can be categorized into a multi-degree of freedom system, we felt that it is possible to divide the motion into two categories - planar and lateral movements - by repeated analysis of the video (see above). Therefore, we are investigating through this project, the ability of 1-dof planar mechanism to mimic the planar motion of each leg in the coconut crab. If this attempt is successful, our goal of proving that simple mechanisms can predict complex motions will be achieved.

Tasks

In order to achieve our goal, we had decided to divide our work into several tasks and those are listed below. 

Mapping the Trajectory

The trajectory of each joint was mapped for every step made by the crab from the video. An example image with the joints on each leg marked is shown below. 

The joints are connected by links. At the time of mapping the trajectory, we did not assign any type for these joints. To make it easier for us to map the trajectory, we developed a MS -EXCEL VBA program to enable us to easily mark these joints and extract (x,y) coordinates of these points. A total of 5 points were extracted. The MS EXCEL VBA program and the values can be found in the attachment below.  [Book1.xlsm]

Synthesis

The trajectory mapped 5 points, which leads to 5 different link positions. In order to apply the Graphical three-position synthesis, we are considering the first , third and the final positions as shown in the figure below for the rear leg. 
It should be pointed out that the graphical three position synthesis is usually applicable only for one link in a mechanism consisting of revolute joints. But since our requirement is to have one mechanism replicating the motion of three links, we decided to independently carry out three-position synthesis for these links and merge them on to the same mechanism. The links are assigned measurements 2.5in, 4in and 3in for the top, middle and bottom links respectively. The three position synthesis for the rear leg is shown in the figure below. The corresponding AutoCad file is also attached.