Pick and Place Mechanism

Overview: For my open-ended class project, I explored the concept of a linkage-based robotic arm as a cost-effective and versatile alternative to traditional multi-actuator arms. Recognizing that many industrial robotic arms are often over-specked for simpler tasks due to their complexity and cost, I aimed to design a mechanism capable of executing complex motions with a single input. 

Design and Application: The approach focuses on developing a planar robotic arm that mimics human-like movements such as rotation at the shoulder, extension through a modified elbow, and object manipulation akin to a hand. This design is particularly advantageous for applications like moving objects between conveyor belts in warehouses, offering a more economical and robust solution compared to multi-actuator arms. The project's objective is to provide a practical and efficient tool for tasks requiring precise motion control. The associated video showcases the diverse applications of robotic arms in real-world scenarios.

The subsequent paragraphs detail the initial project concept. Through additional rounds of brainstorming, iteration, and testing, modifications were made to both the scope and the mechanisms employed in the final iteration.

1. Scope: The robotic arm project is designed to efficiently pick up objects from an assembly line, traverse horizontally along the same axis, and deposit items at a new location along that axis, all while optimizing spatial usage. Enabling the deployment of multiple arms for sorting and product assembly, my goal is to incorporate a motor in the base, a primary arm utilizing four-bar linkages, and a claw gripper. The claw could feature an escapement mechanism for cycling between open and closed positions, along with gears that facilitate the rotation of two four-bar linkages, allowing it to handle objects of varying sizes and weights. My plan extends to automating the picking and placing process to simulate real assembly line functionality.

2. Analysis: Prior to fabrication, a comprehensive analysis will be essential. This involves understanding the motion profiles of the linkages, rotary motion from the base to the claw, force profiles of the grabber, arm geometry/dimensions, and an overall design simulation using SolidWorks. Efficiency is prioritized by utilizing minimal materials and analyzing the rotation of the base concerning other components such as wires.

3. Significance: The excitement in this project lies in the journey from conceptualization to drawings, CAD models, and finally, fabrication. I'm enthusiastic about applying lecture knowledge on four-bar linkages to a practical application that streamlines manufacturing processes. The primary challenge lies in designing a linkage system that accommodates the arm's complex motion, an escapement mechanism for the gripper, and coordinating these seemingly independent mechanisms with a single rotary input. Automation and the application of mechanical engineering knowledge for the build present additional challenges within my constrained timeframe.

The brainstorming process was divided into two separate parts. The first step was brainstorming the arm mechanism. I started with the desired position profile, which is based on the problem I decided to address. An image from my brainstorming stage about the desired position profile can be seen below on the left, and a sketch on the linkages on the right.

Software Used: Since the desired position profile was known, I used the software MotionGen to generate different arm designs. MotionGen was helpful because it can show the position profile of a link based on an input. After creating a successful concept design in MotionGen, I planned on using  SOLIDWORKS to validate and itterate on the design.

Claw: Brainstorming for the claw did not start until the arm design was finalized and prototyped. Once I knew that I had a working arm, I then started designing the claw. The claw brainstorming had the most variance as I researched different methods of coupling the grab and release of the claw to the constant movement of the arm. A functioning claw would require the successful implementation of several constraints; it needed to open and close at very particular instances along the arm traversal without independent actuators. Although designing a cam system would have been the easiest way to achieve this motion, I wanted to be creative about my implementation. Ideas included a linear gear system and some sort of escapement mechanism. Initial sketches of a proposed linear gear system are shown below.

Although the linear gear system could have worked, it was theoretically unreliable since it used the force of gravity to transfer the pinion gear between separate linear tracks that would enable extension and retraction. In addition, with the scope of this class centered on achieving complex motion through linkage systems, I decided to attempt an implementation with the escapement mechanism. Once I finally began to implement the claw, after many unsuccessful attempts with the escapement mechanism, I decided to transition to a ratchet and spring-tensioned claw in which mechanical levers attached to the claw would engage to enable opening or closing at the respective ends, shown below.

The manufacturing and assembly of my prototypes utilized the resources available in Texas Inventionworks, encompassing 3D printing and laser cutting. I express gratitude to Texas Inventionworks, along with my TIW teacher assistants Victor Guzman and Ashwin Hingwe for their invaluable assistance.

Virtual Design: The prototyping journey began with the creation of the arm mechanism, followed by the integration of the claw mechanism. As highlighted in the preceding brainstorming section, I utilized MotionGen, a linkage design software, to delineate the overall motion of the arm. The primary objective was to achieve both extension for grasping and retraction for lifting an object. Upon reaching a satisfactory conceptualization, I translated it into a 2D layout using SOLIDWORKS, adjusting dimensions to achieve an optimal profile. Subsequently, I transitioned to the virtual development of 3D parts and conducted a comprehensive motion analysis on the assembly, ensuring proper tolerances and eliminating interference. Once content with the virtual representation, I proceeded to fabricate the first physical prototype.

Prototype 1: This prototype served as a proof of concept for the arm mechanism, constructed from 3mm- and 6mm-thick wood components using the laser cutter. Pin joints were simulated using ¼ inch dowels, and while the motor and claw were not yet attached, this prototype validated the geometric validity of our design. Manual manipulation of the input link allowed us to observe the position profile of the claw connection point and ensure the viability of our MotionGen-generated design.

Prototype 2: The second prototype, a more robust iteration, featured laser-cut wood links along with upgraded components such as metal shafts, collars, ball bearings, thrust bearings, 3D printed spacers, and a motor. This prototype included a non-functional claw for visualization purposes. It aimed to confirm the functionality envisioned for the final machine, identify areas of unwanted friction, verify tolerances for both the frame and the claw, and visualize the claw's operation. While confirming the viability of our design, we noted a hitch in the arm's movement, attributed to factors such as the out-of-plane wiggle room between the slotted link and its pin. Planned solutions included 3D-printed spacers with lock collars, the use of new bearings and linkages, and addressing weight distribution issues.

Claw Development: The development of the claw involved adding mounting points for two pre-tensioned springs on the bottom, ensuring the default closed position. A ratchet mechanism allowed controlled opening, engaging as the claw moved to the object-releasing side. A lever attached to the ratchet and input link facilitated opening, while another lever engaged a spring-tensioned ratchet foot to rapidly close the claw around the object on the object-grabbing side. The final prototype, also the first iteration of the claw, had time constraints limiting further iterations.

For detailed descriptions of the final prototype and the claw prototype, refer to section 6, "Final Design and Demonstration."

Goals: As outlined in the initial project proposal, the pre-fabrication analysis encompasses several crucial aspects. It involves understanding the motion profiles of the linkages and the rotary motion from the base to the claw. Additionally, the force profile of the grabber, the arm's geometry/dimensions, and the overall design are analyzed through SolidWorks simulation. The goal is to utilize the minimum amount of materials, ensuring the arm occupies as little space as possible, and to analyze the rotation of the base concerning other components, such as wires.

Methods: The analysis is conducted using SolidWorks, where the entire mechanism assembly, including the motor, is modeled. A common input, represented by the input theta, is applied to the modeled motor. This allows the determination of the position of a point, velocity of a point, and force of a point.

Position Profile: Analytically determining the position profile of the claw serves as a sanity check, aligning with the driving force behind our brainstorming process. The prototyping stage aimed to match the position profile of the prototype to the expected one. The analysis sets the origin at the first joint on the base frame from the ground up, represented by (0,0) on the graph for the position of the grabber. The graph shows the x- and y-positions of the claw in inches, indicating both vertical and horizontal movement. The linear claw displacement as a function of the input angle (ranging from 0° - 360°) is also graphed, showcasing the overall positive displacement of the grabber.

Velocity Profile: The velocity of the grabber versus the input angle is graphed to visualize the x-direction and y-direction velocities, along with the overall magnitude. The plot indicates that the claw maintains control of the object throughout its transversal, with maximum y-velocity around -5.8 in/s, maximum x-velocity about -6 in/s, and a maximum magnitude of approximately 9 in/s.

Mechanical Advantage: A plot of mechanical advantage versus the grabber x-position provides insights into the strongest and weakest points of the design. Asymptotes converging to infinity correspond to the most extended left and right side positions, and the weakest point is identified when the mechanical advantage is 0.5 during traversal through the origin.

Force Profile: A graph of the upward force of the grabber versus its x-position aligns with the position analysis. At extreme positions, the mechanism demonstrates the ability to lift a substantial amount of weight. The minimum force is around 10 lbs, approximately 3 in away from the center, highlighting a potential weak spot in the design where higher forces may impede the mechanism's function.

Scope: The objective of the project was to develop a complex linkage-based mechanism and gain practical experience in the design, manufacturing, and analysis of linkages. I chose to create a robotic arm using a linkage system and a ratchet-based claw for applications in a manufacturing setting, specifically for transferring objects between conveyor belts. The documentation is organized into sections: Introduction and Background, Initial Project Proposal, Brainstorming, Manufacturing and Prototyping, Kinematic Analysis, Final Design and Demonstration, and Appendix. Throughout the process, I utilized tools such as SOLIDWORKS, 3D printers, and laser cutters.

Takeaways: The successful prototype achieved the goal of creating an arm with side-to-side movement and a claw capable of lifting objects theoretically weighing up to 10 pounds. I gained valuable experience in the engineering design process, including designing, prototyping, testing, and analyzing links and linkage systems. Time management played a crucial role due to my involvement in various coursework and extracurricular activities. I learned to set realistic goals within a semester, prioritizing the creation of a successful prototype before expanding the project further.