2) Design Process

Objectives:

        The primary objective of our mechanism is to scoop liquid from a pool and pour it into a tank. This can be achieved by successfully implementing the following design considerations. First, the mechanism should follow a semi-circular path that moves the end effector into a pool and up to a tank where the liquid will be disposed of. Second, the mechanism’s velocity profile should be non-constant, with the highest speed occurring when the end effector is returning to the pool and the slowest speed occurring when the mechanism is scooping and transferring the liquid. Third, the mechanism should be powered by a single DC motor rotating at a constant velocity. Fourth, the mechanism should be designed to mimic the appearance of the artwork it was inspired by. Finally, the overall footprint of the mechanism should be no larger than desktop sized.

Mechanism Design:

        We used our desired end effector path, shown in figure 1.2, to influence our mechanism layout. From the textbook Dr. Deshpande recommended, Mechanisms and Mechanical Devices, in lecture we were able to search through and find an initial design that we could adjust, figure 2.1. The initial mechanism created a relatively constant velocity D curve at the end effector. This mechanism had the type of path we were desiring but we needed a non-constant velocity profile at the end effector. From this point we needed a way to rapidly test out different adjustments to the initial mechanism. We were able to use PMKS to achieve this testing.

        Our first step in PMKS was to recreate the found mechanism. We then began to change the ground links, joint placements, types of joints, and linkage lengths. PMKS allowed us to iterate through different changes and visualize the effects on the path of each joint and see their velocity profiles. The figures below show the initial mechanism and our iterations until our final selected layout.

Figure 2.1. D-Drive Four Bar Linkage

Figure 2.2. Iteration One

Figure 2.3. Iteration Three

Figure 2.4. Final Layout

        Once we finalized the mechanism layout we were able to then begin the kinematic analysis. We used MATLAB to solve for all unknowns and create the plots seen in the Kinematic Analysis section of this report. The appendix shows all of the hand calculations and MATLAB code used along with all of the created plots. 

CAD Model:

        With the pin locations and linkage lengths identified in our mechanical design phase, the next step was to create a 3D structure that could effectively utilize the end effector path we had designed to scoop and pour liquid into a tank. When designing the components, it was previously planned that laser cut acrylic would be used for the mechanism linkages and 3D printing would be used to create the pins and all mechanical interfaces, such as the motor support and the bucket. The primary consideration for linkage design was to have the linkages be sturdy enough to not flex significantly or risk breaking while operating. This was achieved by increasing the linkage width to an acceptable dimension, as the length and thickness were pre-determined by the material and mechanism design. An overall view of the CAD model can be seen in the following figure.

Figure 2.5. Overall View of CAD Model

        The pin design aimed to satisfy three primary requirements. First, the pins should be sturdy enough to not break or bend in order to restrict the mechanism from rotating out of alignment and binding up. Second, the pins should constrain the linkages with narrow tolerances such that the linkages can rotate properly but not be restricted. Finally, the pins, particularly those attached to the end effector and the motor, should properly translate mechanical motion throughout the system. This can be exemplified by the motor interface, which must rigidly link the motor to structural components yet translate rotational motion into the input link, as well as the end effector, which must be rigidly attached to the last link yet allow for the bucket to swing freely.

Figure 2.6. Close-Up View of the Motor Interface

        The last critical design step completed during the CAD phase determined how the bucket would actually pour liquid into the tank. While the motion of the end effector would allow our mechanism to properly scoop liquid and move it out of the pool, it did not accommodate a pouring motion very well. To remediate this, a swinging bucket and catch tunnel was designed such that the bucket would naturally remain parallel to the ground but be tipped to pour out its contents as it moved past the catch tunnel. This was achieved by adding an extended lip on the bottom edge of the bucket and designing the water catch such that the mechanism could operate very closely to it but only make contact with the extended lip to tip the bucket.

Figure 2.7. Close-Up View of Bucket and Catch

        One of the biggest challenges during the CAD phase was planning how to place the linkages within the acrylic structure such that the link with the wide end effector attached to it could move without running into adjacent links. Similarly, the motion of one of our links passed through the platform/tank in which the liquid was to be poured, as seen earlier in Figure 2.5. After a fair amount of trial and error, the placement as displayed by the top-down view in Figure 2.8 was selected, which places the end-effector link in the center of the structure and located the other links as closely to one of the side walls as possible so as to not interfere with the end effector.

Figure 2.8. Top-Down View of Linkage Placement

        The structural element of the mechanism consisted of a clear acrylic box that was held together at the edges by dove-tail joints. These joints were secured and waterproofed using superglue and a sealing glue, as well as a back-up layer of tape.

Figure 2.9. Animation of Completed CAD Model (Front Wall Hidden)