Robotic Arm Module by Mark Kershisnik
Figure 1: Outline of graphical synthesis goal
Figure 2: Final construction
Robot Arm Module:
The arm mechanism is a six bar linkage with four binary links and two ternary links. The goal of the mechanism is to move the claw from the extended position to the contracted position so that the bottle is delivered in the correct orientation for pick up and for launch. The mechanism can cross over into an unwanted geometry, but this is prevented by using hard stops attached to the claw attachment link and at the ground link. The mechanism is to be driven by an electric motor in two directions and as such requires some slightly more sophisticated motion control. Ideally, the mechanism would be improved by reconfiguration to employ the Grashoff condition such that a full rotation of the input link provides the desired motion and also returns the linkage to the initial position, however the geometry of the entire machine would not allow for this.
Synthesis of the Arm Module motion:
The arm mechanism was originally conceptualized as a four-bar linkage. When the preliminary designs of the launcher module were delivered, it was clear that it would be advantageous to add an element of controlling the orientation of the item to be "recycled," and a simple four bar does not deliver a complex enough motion to achieve this. Hence I began work on modifying a four-bar linkage, knowing that the topology must include four binary links and two ternary links for a one degree of freedom motion (among several other configurations as outlined in Norton's Design of Machinery, see Norton Table 2-2). The mechanism was contrived mainly by graphical synthesis using a home-made compass, which allowed simultaneous dimensional synthesis. Several heuristics were discovered in the process regarding the geometry of motion and they will be discussed below.
It is clear that the motion can be divided into two general branches: the movement of the main four-bar sub-chain to bring the claw link up to the final position, and the rotation of the claw link about the end of the long arm link for proper orientation. The first heuristic that was inferred was that the claw link simply needs to be rotated about one point and this was accomplished using the idea of a third class lever. The second heuristic relationship discovered when the driving link (lower link attached to ground) was converted to a ternary link. It was quickly realized that if the driver link is simply coupled to the claw link (to create the lever action), the only synthesis that needs to be done is to analyze the distances in the beginning and end positions and ensuring that the linkage can go through the desired range of motion. At this point, a preliminary SolidWorks model was drawn up and dimensions of the claw and coupler link were tuned to create the desired output.
SolidWorks Modeling
To fine tune the motion, the mechanism was drawn up in a SolidWorks assembly. The motion was analyzed mostly qualitatively for smoothness and avoidance of unwanted toggles amongst other things. Two improvements were made to the design this way, the first including sizing the long coupler link to produce a desirable amount of claw link rotation between design positions. The second improvement was not realized until after preparing to construct a real physical model of the linkage. The driver link was originally designed to be a simple 'L' shape (See Figure 3 below), however this would lead to problems in assembly as the link would run into the structure holding the other links. A solution to this problem was not difficult to come by and proved to be an elegant fix (See Figure 4).
Figure 3: Image of SolidWorks rendering of original Driver Link (green)
Figure 4: Image of SolidWorks rendering of improved Driver Link