Moongel, Drum Dials, and Aurora Strings
by Bryan Bergeron
Experimenters who have spent significant time designing and building robots know that often the greatest challenge is locating affordable parts. For example, oftentimes I know exactly the sort of sensor I need, but can’t seem to find it in the online robotic shops. When pressed for time, I sometimes cannibalize a perfectly good working robot for a $50 sensor. As a result, my collection of bots resembles a hospital ward filled with hobbled patients.
Time and budget allowing, I have occasionally succumbed to the lure of the specialty electronic supply companies. At the top of my list is Omega Measurement (http://www.omega.com) which sells high quality sensors for anything imaginable. Just be prepared to pay for it. My collection of Omega catalogs and ‘how to measure’ books is so useful that it commands an entire shelf in my bookcase. In addition to providing an industry-recognized name for a particular type of sensor, the books give me an idea of what constitutes professional grade tolerance and how these compare to what’s available from the general robotics supply houses.
If you’re a regular reader of this column, you know that I’m a fan of repurposing, especially when it comes to devices that were designed for uses unrelated to robotics. Take a few of my recent finds, discovered while I visited my local Guitar Center. The top find of the day was Moongel (RTOM Corp): gum-stick sized sheets of silicone gel designed to dampen vibrations of drum heads. One problem with drums is that they tend to resonate when nearby instruments — such as amplified bass guitars — hit their resonant frequency. The silicon gel adds mass to the paper-thin drum heads, thereby dampening the sympathetic vibration. I’ve also found that the blue gel sheets make excellent vibration-isolation platforms for miniature cameras and accelerometers.
Moongel is available at most musical instrument stores and online sites that sell percussion instruments. You can purchase a container of Moongel sheets on Amazon for about $7. Moongel is also sold in 7” and 14” diameter discs about 1/4” thick as practice pads for drummers. I’ve used the larger disc ($35) as a test platform for calibrating accelerometers. The pad is useful for isolating the sensors from vibrations conducted through my benchtop. It doesn’t provide the isolation of a slab of marble floating on a bed of mercury, but for the price it suffices.
Another find at the music store was a set of drum tuners from Tama (http://www.tama.com) and Drum Dial (http://www.drumdial.com). These mechanical instruments — designed to directly measure drumhead tension — are an inexpensive ($60) means of checking the tension on fabric or Mylar covered wings, and control surfaces on R/C planes, boats, and hovercraft. I’ve used a drum tuner to calibrate the tension on Kevlar sheets laced with conductive thread connected to a Lilypad Arduino from SparkFun (http://www.sparkfun.com). Take a look at the drum dial in use on YouTube to see if it will work with your application.
My last find during my outing to the music store was colored electric guitar strings by Aurora (http://www.stringsbyaurora.com). These coated steel strings are available in colors from black and white to gold, red, purple, and nitro lime ($15 for six cables). Why would you want these? Mainly for aesthetics. Why use a plain silver wire for your new claw gripper when you could have nitro tangerine? More practically, if you’re demonstrating, say, a hand with 20 or more control cables articulating the joints, using a uniquely colored control cable for each servo or function can clarify operation and aid in troubleshooting. The main limitation of colored guitar strings is that they’re not designed for heavy loads. If you need thick control cables, consider bass guitar strings from the same manufacturer.
The takeaway is that it pays to keep your eyes open for tools and technology that can be applied to your robotics projects. Otherwise, you’re constrained by the economic decisions of the parts buyers for robotics supply houses. Have fun hunting. SV