Rigging Your 3D Models In The Real-World

Computer animation is a task both delicate and tedious, requiring the manipulation of a computer model into a series of poses over time saved as keyframes, further refined by adjusting how the computer interpolates between each frame. You need a rig (a kind of digital skeleton) to accurately control that model, and researcher [Alec Jacobson] and his team have developed a hands-on alternative to pushing pixels around.

3D Rig with Control Curves
Control curves (the blue circles) allow for easier character manipulation.

The skeletal systems of computer animated characters consists of kinematic chains—joints that sprout from a root node out to the smallest extremity. Manipulating those joints usually requires the addition of easy-to-select control curves, which simplify the way joints rotate down the chain. Control curves do some behind-the-curtain math that allows the animator to move a character by grabbing a natural end-node, such as a hand or a foot. Lifting a character’s foot to place it on chair requires manipulating one control curve: grab foot control, move foot. Without these curves, an animator’s work is usually tripled: she has to first rotate the joint where the leg meets the hip, sticking the leg straight out, then rotate the knee back down, then rotate the ankle. A nightmare.

[Alec] and his team’s unique alternative is a system of interchangeable, 3D-printed mechanical pieces used to drive an on-screen character. The effect is that of digital puppetry, but with an eye toward precision. Their device consists of a central controller, joints, splitters, extensions, and endcaps. Joints connected to the controller appear in the 3D environment in real-time as they are assembled, and differences between the real-world rig and the model’s proportions can be adjusted in the software or through plastic extension pieces.

The plastic joints spin in all 3 directions (X,Y,Z), and record measurements via embedded Hall sensors and permanent magnets. Check out the accompanying article here (PDF) for specifics on the articulation device, then hang around after the break for a demonstration video.

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Stewart Platform Ball Bearing Balancer

PID balancing a ball on a plate

For their Mechanical Engineering senior design project at San Jose State University, [Tyler Kroymann] and [Robert Dee] designed and built a racing motion simulator. Which is slightly out of the budget of most hackers, so before they went full-scale, a more affordable Arduino powered Stewart platform proof of concept was built. Stewart platforms typically use six electric or hydraulic linear actuators to provide motion in six degrees of freedom (6 DOF), surge (X), sway (Y), heave (Z), pitch, roll, and yaw. With a simple software translation matrix, to account for the angular displacement of the servo arm, you can transform the needed linear motions into PWM signals for standard hobby servos.

The 6 DOF platform, with the addition of a resistive touch screen, also doubled as a side project for their mechatronic control systems class. However, in this configuration the platform was constrained to just pitch and roll. The Arduino reads the resistive touch screen and registers the ball bearing’s location. Then a PID compares this to the target location generating an error vector. The error vector is used to find an inverse kinematic solution which causes the actuators to move the ball towards the target location. This whole process is repeated 50 times a second. The target location can be a pre-programmed or controlled using the analog stick on a Wii nunchuck.

Watch the ball bearing seek the target location after the break.

Thanks to [Toby] for sending in this tip.

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DMG Lib: Digital Mechanism and Gear Library

Reader, [klemens], suggested DMG Lib to us when we posted about a similar site. DMG-Lib is an amazing source of information. It’s primary downside is that a great portion of the text is in a language other than English, though in some ways this is a plus. Latin, Italian, German, and many other languages held the position of being the chief scientific language of the world long before English, and this repository holds entire books about mechanisms in those languages. Some of the books range all the way back to the 1500s. The mechanism animations are very good on this site and play smoothly. While it’s a little harder to search than KMODDL due to the language oddities, it’s still an extremely useful and interesting site to add to the hacker’s information toolbox.

KMODDL: A mechanism maker’s dream site.

Computers are relatively new still, but we’ve had mechanics for a very long time. KMODDL  keeps us from reinventing the wheel. It contains collections of mechanisms with descriptions, pictures, and even videos. We were working on a arbalest design not too long ago, and we were having trouble coming up with a clever ratchet design for one of the parts. We spent a few moments in KMODDL looking through the ratchet section of the Reuleaux collection, and  soon after we had the basic building blocks of our design. Sure there are books you could buy that do a similar thing, but KMODDL is completely free, very in depth, and easier to search. Plus, with a useful tool like this you might not even have to take apart all your appliances anymore to see how they work. My first sewing machine might have lived a longer life had I seen this first. Anyone know of more resources like this?