Monday, September 22, 2014

Lambda Top Spins for almost 10 minutes...




This ridiculously smooth top utilizes an instrument grade ruby (now also a stainless steel BB) to create a nearly perfect point on which the top can spin. From the Kickstarter:

ANGULAR MOMENTUM (L) = RADIUS (MASS x VELOCITY)

This equation is all you need to know about tops. For a given top, the size (radius) and weight (mass) are fixed, so your only variable is velocity. If you want more spin, you need more speed. How fast can you spin it? The (major) factors that reduce spin time are friction and the geometry between the top's "contact point" and the target surface. You get to pick the surface, but I get to pick the contact point :)


The other nifty aspect of this little toy is that is that it is optimized to have a lower polar moment, while still having a large mass to maintain momentum. Back to the Kickstarter

POLAR MOMENT OF INERTIA (Ip) = p(r)r^2dV

6061 aluminum is used for the spindle to decrease the polar moment of inertia...in other words, reducing the amount of force it takes to get the top up to a given velocity. If you remember, the more velocity the more angular momentum...and a longer spin time.

Solid brass is heavy, really heavy. It also machines beautifully. If you recall, mass is another important component to angular momentum, making brass an ideal material for the outer ring.

This all adds up to a top that spins for a ridiculous amount of time. up to 12 minutes in fact, however the video above is about 10 minutes of spinning.

Monday, September 8, 2014

Cubli

The video shows a research project at ETH Zurich called Cubli. It takes advantage of reaction wheels, in several unique ways. Typically a reaction wheel is used on an orbiting space craft in order to maintain attitude. Basically they are a type of flywheel that is continuously spinning. A change in angular speed/momentum of the reaction wheel results in an adjustment of the craft, due to the conservation of angular momentum.

Cubli takes this to the next level. Spacecraft are typically outside the effects of a large gravitational pull, and angular corrections are calculated only in the inertial frame of the craft. Cubli is working within another inertial reference frame, so all of its attitude adjustments must take into account that frame, that is gravity. Watching the video you can see the reaction wheels spin up and then quickly brake in order to bring the cube first up on edge, then on its vertex. The positioning of the cube is maintained using the reaction wheels, and the reaction wheels can also be used to control the direction of the fall, creating a "Walking" action.

There are more videos after the break, as well as a couple other pretty awesome projects at the Institute for Dynamic Systems and Control at ETH Zurich.

Monday, September 1, 2014

Sound Mirror

This one is a little bit of acoustics and a little bit of calculus. The British military experimented with acoustic detection of approaching aircraft. These parabolic concrete plinths (Called Sound Mirrors) were built as the predecessor to radar during World War I. Many mirrors can be found on the coasts of England, and many more would have been built but for the invention of radar

The concept of operation is that a listener or microphone would be placed at the focus of the mirror. Due to the definition of a parabola, all sound coming from long distance (infinity) would reflect off of the mirror, and be directed toward the listener. In effect, this is a very simple means of amplifying and detecting plane engine noise from long distances. Similar equipment (albeit more compact and portable) is still used today as a means of spying on people or listening to the sounds of the NFL

This exhibit at Brooklyn's SIGNAL Gallery is a recreation of these mirrors with microphones embedded in their center. Two such mirrors are facing each other to create an interactive experience in the large gallery space. The sound picked up by the microphones is then played throughout the space via speaker.