Design Overview

v1: Mouth Kinematic Design: Double 4-bar Mechanism

In order to create the biting motion that we desired, we found a 4-bar mechanism which produced the correct motion path for both jaws of the dinosaur. We use this 4-bar twice (mirrored across the horizontal) in order to achieve movement for the top jaw and for the bottom jaw. The two 4 bars are shown below in their respective mounting positions (with the bright red, green, yellow and black lines denoting the links and motion the path for the lower jaw—mounted in the “top” position; and the faded orange 4-bar/path denoting the upper jaw mechanism).

For our initial version (3D model) we used this arrangement in order to generate the movement of the dinosaur’s mouth. However, it became apparent that the hardware we were using to assemble the mechanism would result in a very bulky dinosaur, due to the usage of nuts and bolts to create pin joints and the spacing that resulted from this method of assembly. Ideally, we would have used press fit bearings and e-clips to create a smaller pin joint assembly which would also allow a smoother rotation about each joint. The image below shows one half of the original dinosaur design, note the distance needed in order to avoid linkage intersections and appropriate spacing for friction alleviation (half of the jaw mechanism spans nearly 6 inches).

v2: Mouth Kinematic Design: Single 4-bar Mechanism

In this first version of our bank, both the top and bottom jaws move such that the jaws “meet” during the biting action (fully hiding the inserted coin from the operators view and allowing for the coin to be “swallowed” via the 5-bar tongue mechanism). Because of the path of the 4-bar mechanisms the input link of the bottom jaw and the input link of the top jaw must rotate in opposite directions. In order to account for this and still allow for our design to be driven by a single motor, we would need to design in a set of gears to change the direction of rotation between the two sets of 4-bars which are coupled so that they can be driven together. This addition would create even more bulk in the already complicated dinosaur head assembly.

In an effort to simplify and reduce the size of the dinosaur head (and overall mechanism) we decided to move only the bottom jaw, since this movement would be necessary to accommodate for the 5-bar tongue motion, while the top jaw motion was simply a nice addition to the movement of the dinosaur as a whole. This change greatly reduced the number of components needed and significantly reduced the amount of friction that would be in the system, because we essentially were able to cut down the number of moving parts in the head by two. This also helped to make the size of the dinosaur head smaller (total width ~7inches) because there the space that was occupied before by the top jaw assembly was removed from the system.

Removing the top jaw motion from our design alleviated several issues/concerns that our team had regarding our project: it removed the need to have a gear system (to change the direction of rotation between the top and bottom jaw), reduced the number of components (lightening the overall jaw mechanism, which is mounted in the head of the dinosaur), reduced friction, and reduced the overall size of the dinosaur bank.

The jaw mechanism was assembled as one unit (as seen on the right) which could easily be mounted and moved around so that it was located in the correct position relative to the rest of the dinosaur’s body. This design choice was made deliberately, because we were still in the process of designing the outer aesthetics (the body) of the dinosaur bank.

v2: Overall Design

Assembly Issues: Jaw Mechanism

In assembling our mechanism we ran in to a couple interesting roadblocks which we had not anticipated in the design phase (or subsequent iterations). Aside from friction, which we had attempted to counteract through the usage of bushings, nylon washers, and spacers, we also saw movement synchronization issues and cantilevering in the jaw mechanism which we successfully adapted for in our final product.

Upon assembling one half of the jaw (consisting of two identical 4-bar mechanisms mounted in-line horizontally and constrained to a mounting plate) we noticed that the two bars experienced a toggle position in which it was very easy for the two mechanisms to go out of phase of one another, this resulted in the overall jaw mechanism seizing up. To fix this, we designed in two gears connected with an idler between the input links of each 4-bar. We used GearDXF to design the custom gears and idler for this and then laser cut a them out of acrylic. This coupled the motion of the 2^nd 4-bar mechanism to that of the 4-bar which was being driven by the gear box, thereby constraining it move in phase with its “twin.”

The 2^nd issue that we faced upon assembly occurred after the entire jaw assembly had been created. As designed the jaw is driven from one side of the full jaw mechanism and the two sets of 4-bars (each attached to one side of the jaw box) are coupled together by each side of the moving jaw itself. Because this is the only connection point between the two halves and because of its location (the point farthest away from the input) the mechanism was subject to heavy cantilevering. In our initial discussions we had considered having the jaw be connected centrally, however had moved away from this idea due intersecting links—this was when we still had the top and bottom jaws moving. Now that only the bottom jaw moved, we were able to connect the two halves of the mechanism across the center of the jaw box. We accomplished this with hollow aluminum tubes which we cut to length and crimped onto the screw ends that formed the pin joints of the input links in the front-most 4-bars on each side of the jaw box assembly. This helped to support the mechanism across its entirety and greatly reduced the cantilevering issues that we were experiencing.

Changing from Dinosaur to Nessi

The design aesthetic change from dinosaur to Nessi came in the final assembly stage. The initial assembly of the jaw mechanism (in its original orientation) was flipped because the motion of the jaw was jamming and it was thought that potentially this could be remedied by changing how the jaw needed to be driven (clockwise versus counter-clockwise)—eventually this was ruled out and the issue was identified to be associated with cantilevering. However, due to time constraints and the difficulty of assembling the jaw box, the flipped jaw box mechanism was left as is. Because of this new positioning the gearbox would not be fully hidden by the body of the dinosaur, so Michelle made the decision to change the look from dinosaur to lochness monster (which had more horizontal width). This design change was fairly simple to implement since none of the body components, aside from the jaws, had been laser cut. We modified the body of the creature to be longer and more squat, removed the legs, and added waves to create the habitat for the creature to be mounted in. This design change allowed us to better hide the gearbox, without drastically changing the design or function of our product.

Robot Mechanism Design

I. Jaw Mechanism