Software has traditionally been the biggest time suck in the product development process. But it’s the mechanical parts that now are flummoxing a new wave of Internet entrepreneurs and tinkerers alike who are working on designs using open source hardware and software.
“At least the electronics and software are known challenges,” says Pierre Michael, an EE who is currently beta testing a cocktail-dispensing robot he designed and successfully funded on Kickstarter. “The pump was probably a good 60 percent chunk of the design effort.”
The robot, called Bartendro, automatically dispenses mixed drinks from a rack of bottles filled with ingredients ranging from Cinnamon Schnapps to Kahlua. The hardware (Raspberry Pi and Arduino) and software are all open-source. The developers wrote the firmware code in C and used Python to program the Raspberry Pi.
Figuring out an affordable and accurate liquid dispensing technology was a challenge for the designers of the Bartendro robotic drink making machine.
That was the easy part. Figuring out how to dispense the fluids accurately was a bit more… troublesome.
"Originally we wanted to use inverted bottles in the design. It seemed so clean and easy to just open a valve and let gravity do the work,” Pierre told EE Times. But he and his co-developer, Rob Kay, a software architect with a computer engineering degree, quickly realized what any mechanical engineer learns in a first-year fluid dynamics course: Flow rate is proportional to the volume of liquid in the bottle.
They could have compensated by using flow meters for control, but sanitation was a concern (there is no such thing as non-invasive flow meters). Moreover, the cost would have blown the $100 budget Pierre and Kay set for the entire pump module.
The duo finally hit upon a peristaltic pump as a potential solution -- while invasive, these positive displacement pumps do not damage blood cells. But any off-the-shelf products they were able to find were astronomically expensive.
“At that point we started thinking, ‘How hard could it be to make one ourselves?’ The concept looked simple enough -- it’s just a hub with some rollers on it squeezing a tube,” says Pierre. So while continuing to search for a cheaper off-the-shelf alternative, Pierre and Kay went through five design iterations in their shop. In the end, they still weren’t satisfied with the design because it wasn’t something that was made to high tolerances.
Eventually they located a supplier that made the kind of pump they wanted for the right price. It required only a few modifications, like pressing magnets into the hub and machining some of the components. Hall sensors measure the position of the rotor.
"It’s pretty elegant,” says Pierre proudly. “One revolution of the rotor dispenses about 3 millimeters, and we can detect 1/8 of a revolution. We experimented with adding a lot more Hall sensors, but we found that the increased resolution wasn’t giving us any better results. In fact, we don’t actually have the proper equipment to measure anything that small accurately -- and, frankly, it’s been good enough for even the most discerning palate.”
Compensating for the mismatch between motor shaft diameter and the propeller hole flummoxed the designers of the Mesh Network Propeller Beanie used for a popular Design West 2013 Speed Training Workshop.
David Ewing, chief technology officer at Synapse-Wireless was the mastermind behind the design of the mesh network propeller beanies used in a hands-on speed training session at Design West in April. He faced a similar challenge fitting together the motor and propeller blades.
To compensate for the mismatch in the tiny DC motor’s 1 mm shaft and propeller’s 2 mm hole, Ewing was overly optimistic in thinking he could find a “shaft adapter” to match his specs.
After trolling hobby sites for things like “radio control,” he discovered that props sized for a beanie are not typically matched with motors as small as his. After all, the propeller is not designed to fly both beanie and beanie-wearer across the room.
“Budget was a concern, too. With the motor itself not costing more than $1, it would be silly to pay more than a few cents for the adapter,” says Ewing.
It was time to bring in the professionals: Ewing, an EE, tapped senior mechanical engineer Edward Jory. Not able to find a suitable off-the-shelf solution, Jory quickly sketched the design for a “prop adapter,” which would also serve to rubber-band the prop. At $6 a pop, cost was an issue.
The idea of press-fitting a piece of plastic tubing on the motor shaft and into the propeller was intriguing -- a simple and cheap solution, if a tubing of the right inner and outer diameters and material could be found. Nothing quite filled the bill, but the engineers ultimately found a 10 cent nylon PCB spacer with an outer diameter just a tad bigger than the specs. A bit of drill work on the prop bore, and a nice, tight fit.
Even when you’re only building just one or a few of something, the mechanical design can still be a diabolical challenge. That’s what Jason Kridner -- a software architecture manager at Texas Instruments and a well known figure in the open-source community -- discovered when he needed to come up with an enclosure for a device he calls the "Beaglestache camera." The DIY device combines a Beaglebone, PS3camera, and facial recognition software to automatically apply one of many mustache styles to a face, take a photograph, and tweet it out to the Internet.
Jason Kridner hacked up a cardboard box to make an enclosure for his Beaglestache camera.
“The mechanical stuff is really the hardest part, which is basically why I hacked something out of a cardboard box. But, even so, making that box accounted for about 50 percent of the total development time!” Kridner told us. “And, you know, it’s going to be the hardest thing for me to improve.”
Kridner, in fact, had a third-party company assemble a second 'stache camera for EE Times to use at Design West. The professional-looking plastic enclosure was a definite improvement over a cardboard box, but with one tiny problem: The on-off switch was sealed up inside of the box.
Mechanical design is indeed a beast.