10/23/2018 Amanda Maher
Written by Amanda Maher
I found this fun simulator that allows you to make your own four bar linkage and trace the motion of the coupling link.
Here is a screenshot of one linkage configuration I was playing around with.
The software obeys Grashof’s condition.
where S is the shortest link, L is the longest link, and P and Q are the other two links.
All this means is that the length of the shortest link and the longest link summed together must be less than or equal to the sum of the length of the other two links in order for the shortest link to rotate fully with respect to its neighboring link. If you are playing with this and your four bar linkage disappears at certain points in the rotation, you know how to manipulate the link dimensions to make Franz Grashof proud.
Now you can make your own four bar linkage and generate motion. With that skill, you could work with a software Disney created five years ago and make your own rapid prototyped, mechanically animated character without a deep background in mechanical design.
So let’s say you’ve got an idea for a little toy you want to make. To begin the creation of your character, you will input the design for your movable character into Disney’s software. Then, you will specify pin joints and select their preferred movement based on your ideal movement. From this point, Disney’s system will have to choose a driving mechanism from a database of motion curves and try to match up the motion curve you dreamed up with something that already exists in their system.
Initially, the database was created by exploring different possibilities of the motion curve’s behavior in parametric space:
By dipping into the database, the software can generate assemblies of links and gears that will create motion to match your motion curve as closely as possible.
To create these assemblies, different component interactions are categorized into four types of connections: pin/hinge connections that are attached to each other but can rotate in plane, point on line connections, phase connections that connect gears to each other, and fixed state connections that allow you to temporarily freeze parts of characters.
The path optimization function of the software is able to modify the assembly to a point where it efficiently matches your desired input. After this, the system is able to detect errors in the assemblies, as they may be over or under constrained, and the software will give feedback and allow you to edit the assembly if need be. In this way the software is able to have a conversation of sorts with you, giving you suggestions and allowing you to change what you need to make a functioning physical animation.
In addition to creating linkages, the system must also create gears that are compatible with the assembly. The software lets the user select pairs of gears that should be connected to each other. Parallel and sequential gear connections will be generated based on your initial best guess of where the gears should be placed. The rate of motion can be changed for particular links through the usage of asymmetric gears. The gears are then transformed to a set of constraints to be added to the assembly.
This gif shows a user changing gear parameters, and the software responding with two linking gears to animate a character called “Clocky.”
After calculation for the optimization of the gears, there are further calculations to ensure that none of the components intersect with each other. The software allows for you to edit the assembly if there are intersections detected, but the researchers at Disney have also created a constraint solving algorithm, which will generate random solutions to resolve the issue of the interfacing parts.
The fabrication of these guys relies heavily on 3D printing, with some characters being printed in parts and assembled later, and other characters with more complex internal structure being printed in one assembly. Ultimately, each one of them will be driven with either a servomotor, a wheel operated by a handle, or a wheel that is in contact with the ground.
Some fun and compelling characters are generated as a result, my favorite from the lot presented in the video is named Bernie. With the aid of a string he can mostly walk on his own.
Thanks to Professor Ferreira for introducing this application to us in our ME 370 Mechanical Design I course. You can check out the video summary of Disney’s research here and the accompanying research paper here.