Phase 2 - Grasping Mechanism

Overview

During this phase of the project, a grasping mechanism to pick up and hold the payload that was designed during phase 1 of the project.  This phase will incorporate concepts of degree of freedom analysis, mechanism classification, and graphical linkage synthesis. Each team member will create a hand sketch, and after deciding which one to continue forward with, the environment for the claw will be fully dimensioned. From there, a GLS file can be created, which would pave the way for a 3D model and, ultimately, the final, physical model. 

Constraints

• The design must have at least one four bar linkage. 

• The mechanism must be actuated by one 180 degree MG-996R servo. 

• The servo motor cannot be directly attached to a link that contacts/grips the payload. 

• This design cannot utilize any gears. 

• The mechanism must be mounted on the test rig via the two #8 screw holes, placed one inch apart.

• The payload must be able to get picked up anywhere along the midline of the test rig. 

• The links must be made of 1/4" plywood that was cut via the laser cutter. 

• The servo must interface with the provided Arduino shield. 

• The Arduino must be programmed such that rotating one of the potentiometers opens and closes the claw. 

• Grasping mechanism must be operational by Friday, March 10th, 2023. 

Sketches

• Andrew's design features a ground link that moves out with the arm and touches the payload.  Attached is the servo with the crank link.  This is connected to the coupler, which is also connected to the rocker link, which doubles as the second payload contact.  A benefit of this design is that all links are placed together to save space.  A drawback of the design is that that it is very tall and takes away from how high it can be lifted. In addition, the unnecessary added material would create more lift for our arm to have to lift later on. 

• Jorge's design  was simple but had a few broken constraints. It appeared to be relatively lightweight, which was beneficial for his design. However, the servo appears to be connected somehow to the 3D printed piece that connects our claw to the rig, which would not be possible. It had a few theoretical benefits, but the broken constraints present was something the team had to consider when assessing his design.

• James's design, like Andrew's, had the second point of payload contact on the rocker. It was slightly more compact, which is desirable, as we wish to be able to pick up the payload from a low point in order to lift it a fair distance. The small size would also decrease weight. However, it suffers from the same flaw that many of the others did, where the ground link was in the back, which poses the problem of it potentially hitting the payload as the arm is lowered. Also, the claw's gripping ability does not seem to be as strong as a design like Paul's. 

• Paul's design, unlike any of the others, had the claw sticking out perpendicularly from the coupler. This design is very easy and streamlined, which is a benefit. One unique feature that adds to this design is the rigid pattern on the face of the links that contact the payload, which could serve to increase friction and decrease the chances of the payload slipping out. 

Decision Matrix

Upon doing the decision matrix, we decided to go with Paul's design, due to his simple and compact design, as well as the ridges he modeled on his claw which promised to increase the grasping mechanism's ability to pick up the payload. However, we also desired to incorporate some of the other design's strengths when continuing forward to build the best grasping mechanism possible. For example, we plan on mirroring Paul's design so that the rocker will be closer to the rig as opposed to the fixed link, so that any chances of hitting a payload when lowering the grasping mechanism is minimized.   

Degrees of Freedom Analysis

Graphical Linkage Synthesis (GLS)

GLS Design Considerations: When designing the GLS for the claw, the main considerations were that we want the claw to be lightweight and strong. The claw also needed to open wide enough to fit the payload and close tight enough to grab it. During the design, the decision was made to fix the outside jaw, this is to eliminate interference when raising the claw away from the payload. In the process of designing, alterations had to be made so the links would not interfere with each other. One of the largest considerations in the design is the lengths of the links, we wanted them short because it would make our mechanism stronger and lighter, but it would reduce range of motion. So, the final design is a balance between mass, strength, stability, and range of motion.

Comparison Photos

Video

Reflection

Overall, our grasping mechanism worked really well.  It was able to successfully pick up our payload, as well as all the other team's payloads.  Because of this, not much that is needed to improve the design.  In the future the team may alter to the design for mass reduction, but nothing is planned to be changed in way the claw operates.