The other day I was able to cast the silicone molding media to make the mold for the QD embedded plastic pieces. The molding media I bought doesn't require a vacuum as I previously thought, so I was able to cast it at home in my kitchen. It is very simple to do as it is simply a 1:1 volumetric ratio. I borrowed some multipurpose mold release from out school's "Tinker" lab, and it seemed to work just fine.
The specs for this design have been developed. Recently I've been working on making the molds to cast the QD plastic in. I purchased a tin-cure silicone molding media which meets the temperature requirements of the vacuum oven the plastic will be cured in. I selected tin cure over platinum cure because we have access to a vacuum chamber to de-gas the molding media as it cures, and it's a bit cheaper than the platinum cure. It also works with a wider variety of materials. The library life is not as long as the platinum cure, but that is not really a concern for this application.
I milled the positive for the mold out of aluminum. I then lasercut some acrylic to form a cavity around the positive in which to cast the mold media which will become the negative. The result will be a simple open faced mold. If the results from this mold are no good, we may try a 2 part mold, but I think a 2 part mold would be overkill for this part.
Up Next:
I have a design for the box, but I need to make sure it can be made with an efficient use of materials, and easy to assemble, as we will be making many of these devices.
I need to further embody the electrical design by selecting a UV source. I am having trouble designing a UV LED array that meets the light intensity requirements to activate the QDs while being at a reasonable cost. The UV LEDs are quite expensive, and I'd need a lot of them. I'm considering reverting to a fluorescent source if it turns out to be cheaper.
My boss put me on a team to design and fabricate the hardware required for a new activity being developed. The activity is centered on Quantum Dot (QD) technology. The basic concept is that these nano-scale QDs emit single wavelengths of light when excited by UV light. The emitted wavelength is related to the radius of the QD, so larger QDs emit longer wavelengths. Typically, QDs are stored in a liquid solution which is toxic and dangerous for children to handle. However, it is possible to embed QDs in a plastic polymer which is safe for children to handle. The plastic is made by creating a solution of QDs in the polymer, pouring the solution into a mold, and vacuum baking the solution to harden the plastic in the mold.
The plastic itself is clear until you illuminate it with UV light. Once illuminated with UV light, the QDs inside the plastic will emit light, causing the plastic to glow! We plan on making lots of plastic pieces with different sized QDs in them (the QD sizes will correspond to different colors) and having students arrange the plastic pieces on a grid to make a picture of something. These plastic pieces will essentially be pixels in that sense. They will insert their arrangement of pixels into a UV light-box consisting of of a UV source, a UV transparent platform to lay the arrangement on, and a UV resistant shield for viewing the glowing arrangement.
There are three issues I have identified as being critical to the success of the project:
Safety. We need to ensure that the children cannot activate the UV source while the UV shield is not installed, to be sure that the children will not be exposed to the UV rays. We also need to assess the effectiveness of the UV shield.
Light Source Requirements. We need to ensure that our light source emits UV light at the correct wavelength to excite the QDs, and that the intensity of the UV rays is high enough that the pixel arrangement will glow visibly and impressively to the students. We are currently leaning towards using an array of UV LEDs underneath the arrangement to ensure all the pixels receive an equal light intensity.
Casting the Pixels. Currently the pixels are cast in an off the shelf silicone mold used for baking. The plastic parts from the mold are roughly rectangular, but have many imperfections and low dimensional repeatability. We need to develop a mold/molding process that results in repeatable, rectangular plastic pieces that will fit together on a grid. Fitting together multiple pieces in a grid will result in a tolerance stackup. So the smaller we decide to make the pixels, the more we can fit in the grid, and therefore the tighter the tolerance on each pixel will have to be to guarantee the same degree of alignment on the grid.
One of my tasks as an engineering education developer at BU is to beef up the content of some activities with some higher level concepts. Recently I've been working on an activity where students get to design, build, and test their own wind turbines. Right now the activity works great with middle school students, but we want to improve the activity and make it more challenging so that we can bring it to high schools as well. The activity uses store bought wind turbine kits with a variety of blade types. Students can choose the material of the blades, the size of the blades, how many blades, and how to angle the blades. Then, once they've built their turbine, it's placed in front of a box fan and connected to a volt meter to measure the power. After their first design, students are allowed to make changes and test a second time before the teacher gathers everyone for a recap.
Our goal is to enable students to use data obtained from their first test to quantitatively evaluate and improve the performance of their design. This requires some type of quantitative analysis of the wind turbine. So I began doing some math. I just finished a course in aerodynamics, where we learned all about airfoils and wings, so I felt I had a good handle on the concepts at hand. After all, a wind turbine is just a bunch of rotating wings, right?
Well regardless, that's how I modeled it. But unlike regular airplane wings, rotating wings (aka blades) don't all feel the same wind. The ends of the blades travel much faster than the inward portions, meaning the airspeed for that section of blade is faster. So the normal approach doesn't work in this case, because the wind is quite different everywhere across the wing.
This image shows how much the actual speed of the blade varies with distance from the hub. Image Credit: http://www.cfinotebook.net/notebook/operation-of-aircraft-systems/propeller.php
So what can be done to solve the problem? Well when engineers come across a problem like this, they usually try to reduce it to something they already know how to solve. To give an example of how this is done, imagine taking a thin slice of the blade. Because it is so thin, the wind speed on one end is almost the same as the wind speed on the other end. Because the wind speed is *essentially* the same all across this tiny wing, we can use our original method on this tiny wing and get a good approximation. And it turns out if you add up an infinitely large number of infinitely thin blades, you can actually end up with a complete, finite blade; and no error! This technique of adding radial sections of the blade is called the Blade Element Method (BEM).
Here is a picture showing the 'slice' of the blade. Image Credit: http://fessrg.ucsd.edu/Research/Wind/
BEM seems to be pretty widely used for getting preliminary performance estimates when designing a new propeller, helicopter rotor, wind turbine, or really anything else that has rotating wings. However there are limitations. The most significant limitation is that BEM cannot account for any radial flow - that is, if there is air moving along the blade from one tiny wing to the next. When engineers are satisfied with results from preliminary estimates such as BEM, they may move on to more accurate and sophisticated methods like a numerical simulation. But for my purpose, I think BEM is just fine.
One of my responsibilities as a teaching assistant at BU Academy was to cover any classes the teacher (Mr. Gary Garber) could not make. This week he is traveling with the BU Academy FIRST Robotics team to the FIRST competitions and I'm covering some of his classes.
Right now, the class is just starting standing waves. It can be tricky to visualize how the waves actually reflect and interfere with each other. If you're using a spring or a rope to demonstrate this effect, damping is often an issue and other parameters can be difficult to control. If you're using a simulation, the waves usually travel too quickly and it can be difficult to see what's going on. I decided to write a quick MATLAB script to help my students visualize how standing waves form.
The script basically plots several cycles of a sine wave as it travels down a 'string'. Both the input and the string's response (what we actually see the string doing) are plotted on top of each other so that they can be compared. I took some screencasts and some screenshots to better illustrate the process. It's important to note that this simulation doesn't model resonance or harmonics; it simply reflects the incoming waveform off the wall and adds it to the rest of the wave form.
This video shows the typical standing wave we are used to seeing. The nodal points can clearly be seen. I took a few other videos showing different inputs, but they don't all fit here. Those videos have been left as an exercise for the reader.
Download the MATLAB file here.
Part of my job as an engineering education developer includes designing and manufacturing the parts needed for the activities. One of the activities I helped create was on the structural design principle of tensegrity. Tensegrity is a way to build structures that are lightweight and strong. They rely on flexible tensile elements such as strings, ropes, or cables; as well as on stiff compressional elements such as rods. Tensegrity structures are pre-stressed and self stabilizing, and none of the compressional members touch. They are suspended in a web of tensional elements, a phenomenon referred to as "floating compression" by artist Kenneth Snelson.
The activity itself is like a board game to teach students the entrepreneurial aspects of engineering. The activity centers around building a mini tensegrity tower. These towers use wooden dowels with notches cut in either end; rubber bands fit in these notches and hold the rods together in a simple tensegrity structure. The problem is that the balance and symmetry of the tower depends on the quality of the rods. All the rods must be made to a high standard, or else the tensegrity towers become very asymmetrical and unstable. This is because with different length rods, the rubber bands are stretched to different lengths and therefore exert different restoring forces. The imbalance of restorative forces within the structure causes the structure to shift in order to find a new equilibrium. This throws everything off.
To make the towers as balanced as possible, a manufacturing method was needed to produce the rods very quickly and precisely. I decided to make a simple vise to hold the rods. The vise is clamped into the vise of a mill, which can be used to machine the notches on one end. Then, the inner vise can be flipped upside down to machine the other side of the rods.
There were a few caveats to this approach; the length of the rods compared to the size of the vise made it difficult to clamp into the mill's vise jaws. Additionally, the length of the dowels sticking out of the vise made it difficult to machine cleanly without using extra clamps to restrict the motion. All in all, however, it worked pretty well and I think these towers will perform much better.
