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Nuts & Volts Magazine (July 2012)

When Less is More: Designing for Minimum Mass

By Bryan Bergeron    View In Digital Edition  

If you’re a typical N&V reader, then sizing a component means defining its value, precision, power handling capacity, operating voltage, physical size, and precision. Take a typical carbon film resistor listed in the Mouser catalog (www.mouser.com) — a 2K ohm 1/4W 5% tolerance with axial leads. Although you may not have noticed in the past, if you download the full datasheet, you’ll see that the mass is about 226 mg per resistor. Now, go to the equivalent SMT resistor. You’ll see mass listings from 2-16 mg, depending on SMT size.

To be fair, the mass of the axial resistor includes the full lead, but we’re still talking at least an order of magnitude difference in mass. If you look carefully at the datasheet for most components, you’ll find the mass values. Unfortunately, most parts search engines don’t allow you to compare component mass.

Why does this matter? Because there are situations where mass really makes a difference. Take micro R/C work, such as my in-progress micro-quadcopter. Every gram saved translates directly to greater maneuverability, longer flight time, and the ability to provide more functionality in the same space. As such, SMT components rule.

So, if you’re planning a move to the micro world, what are some additional means of shaving weight? Second to component selection on my list is mechanical connectors. Substitute nylon nuts, bolts, and washers for your sturdy (but massive) stainless steel hardware. Just make certain you’re within the safety limits of the holding capacity of the nylon hardware. (See my previous editorial on where to find specs on hardware.)

My third-tier approach is to minimize or discard all electrical connectors. Use micro-connectors instead of full-sized ones. For example, I’m a fan of the JST connector for all of my power connections. SparkFun (www.sparkfun.com) sells these little gems which are rated at 100V at 2A AC/DC for $1. These connectors save space, as well as mass. Unfortunately, the datasheet doesn’t list the mass, but it’s essentially zero compared with typical Molex power connectors.

Related to minimizing connector mass, I also work to minimize the mass contribution of wiring. This means using a PCB tracing instead of wire when possible. Short of that, I use my lookup table of wire gauge vs. maximum handling current. For most of my short runs, I’ve found that 36 gauge wire-wrap wire works for all of my signal level needs. I sometimes double-up on power runs, using two 36 gauge wires in parallel to handle modest 5V demands — especially when powering a few ICs. Motors and other heavy draw devices get the usual treatment.

My last tactic for minimizing the mass footprint of a device is to modify or replace the supporting structure. For example, I use an electric drill to pepper holes into aluminum sheet metal. When that isn’t feasible, I sometimes replace metal with plastic or even carbon fiber, depending on the application. I recently replaced the aluminum spars on my UAV prototype with carbon fiber tubing for a mass reduction of about 10% in the overall structure. Carbon fiber tubing isn’t terribly expensive, but be sure to use a composition that provides adequate strength and stiffness. Aluminum tubing often bends when overstressed; graphite tends to shatter. My favorite source for graphite and other exotic lightweight building materials is Kite Builder (www.kitebuilder.com). Make sure to check out their chart of weight vs. relative stiffness for their products. Another source for lightweight components, connectors, and 4-40 nylon hardware in bulk is Tower Hobbies (www.towerhobbies.com). They’re also a good source for R/C airplanes and helicopters — great subjects for teardowns to learn about designing for minimum weight.

Good luck with your own weight loss program.  NV