Problem Statement
This section willfocus will focus on the design and construction of the manipulator, which should lower, open, and grab an object, then rotate and place the object on a drop-off surface 90 degrees relative to the pick-up surface. Ideally, the end effector should be perfectly synchronized with the rotating linkage system, opening as the linkage system expands and closing as the linkage system contracts. For reference purposes, a diagram of the linkage system, end effector system, and its various components are provided below.
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An ideal mechanism path would achieve perfect linear motion of the output point as the linkage system reaches 90 degrees and 0 degrees relative to the ground, allowing for the isolation of the grabber mechanism movement relative to the rotational component of the overall system. This ideal linkage system input path is demonstrated in the curved slot and full linkage system below.
Ideal Linkage System Input Link Path Animations
The force profile/mechanical advantage of the end effector should ideally peak at the maximum close position to maintain adequate contact with the object being picked up. The plot below demonstrates a possible ideal plot of mechanical advantage, with a peak mechanical advantage at small linear displacements (indicating a closed configuration for iteration one). For future iterations, a better alternative would be to design a system that achieves a mechanical advantage that approaches infinity as the linear displacement decreases.
Ideal Mechanical Advantage Plot for Iteration One Type End Effectors
Mobility Analysis
Positonal
Mobility Calculations for Linkage-End Effector System
Positional Analysis
We chose to focus on the kinematic/position analysis for the overall linkage system because its motion determines the opening/closing/position of the end effector and the rotation/displacement of the package volume.
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First Iteration Mechanical Advantage Plot Second Iteration Mechanical Advantage
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Physical Prototype Design Process
Workflow:
- Identify constraints Identify constraints
- Set boundaries on what type of object is being moved (shape, size) and where it's being moved
- Clarify assumptions- we are moving the object onto a conveyor belt, which moves the object outside of the grasp of the mechanism without the mechanism needing to retract its position as it's placing
- Construct ideal input pathways
- Establish general trends (kinematic, force)
- Created parameterized code that generated force and motion profiles
- Design prototype using general trends trends and ideal input pathways
- Construct prototype
- Conduct in-depth analysis for areas of interest
- Iterate on prototype design using analysis results
Future Timeline moving forward:
Week Weekend of 11/10-11/13: Design iteration, final design changes, final analyses conducted
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Physical Prototype of First Iteration End Effector
In-Progress Physical Prototype of Second Iteration End Effector
Prototyping Reflection
Bill of Materials:
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The most important thing we learned from prototyping is that not everything that works in Solidworks will work in real life. The sharp corner in the input link path slot that was projected to work in Solidworks caused the system to bind in the physical prototype. Furthermore, material choice is important, especially in planar systems that require a low coefficient of friction for links to slide against each other or against slots. When we originally used wood, the friction between the slot and the dowel massively increased the input force required to move the mechanism, greatly decreasing the mechanical advantage of the system. When talking to the TAs, we learned that we can create PETG inserts (PETG has one of the lowest coefficients of friction out of all the materials in TIW) that interface between the slots and the dowels to prevent the system from binding due to friction. We have since learned from the first iteration as the second iteration of the end effector was made out of acrylic, which made a noticeable difference in terms of smooth mechanism movement. Furthermore, for the first iteration of the end effector specifically, prototyping made our group realize that the slots in the end effector assembly were redundant and over-constraining the system/adding unnecessary friction.
In terms of workflow, prototyping emphasized the importance of good analysis before jumping into design. Since all three of us are more versed in modeling than in mathematical simulation, we were tempted to jump straight into physical design without doing the proper mathematical analysis. This caused us to waste a lot of time and energy adjusting parameters to create effects we were not familiar with and also to end up with a product that performed badly in terms of kinematic and mechanical advantage. This is the reason why both iterations of the end effector generated low mechanical advantage values at the critical closed position. We have since asked for help with the mathematical side of the analysis and we now have a good understanding of where to go from here.
Future Tasks
Having received some feedback from both the TAs and our own analyses we conducted, we recognize that we have a long way to go in terms of design iteration. Since we now have a better understanding of how to approach the end effector especially, those force graphs need to be optimized using the methods mentioned in the mechanical advantage analysis section (i.e. stacking levers, shifting the slot path position). The linkage system also needs a redesign of the slotted paths, incorporating a variable radii curve instead of sharp transitions from curved paths to linear paths. Design is projected to be done by the end of this weekend (before 11/13), at which point we can begin iterating on the final product using the 2-3 weeks we have left before the final demo day.
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