This section was edited by Associate Editor Alan S. Brown.

POWER TRANSMISSION AND MOTION CONTROL

Improving the Wheelchair

The idea behind MagicWheels seems obvious: Build a mechanically powered wheelchair with gears so it takes less muscle power to roll up a ramp, come down a hill, or navigate uneven terrain. Yet it took Seattle inventor and ASME member Steve Meginniss 10 years to bring the concept to market.

Meginniss, who invented the induction-charged, high-speed Sonicare toothbrush, was looking for a new project to work on in his basement. He learned about the wheelchair from the University of Washington technology transfer office. It took a decade and a surprising number of innovations before the company he founded, Magic Wheels Inc., could sell it.

Gearing a wheelchair is not as easy as it sounds. At first, Meginniss thought he might be able to take advantage of the two-speed bicycle hub gears that are popular in Europe. These simple hub gears are planetary mechanisms that start with a sun gear in the center. Two planetary gears circle around it, while their outside edges connect to a ring gear that imparts motion to the wheel.

The world's first geared wheelchair uses a hypocycloidal
drive that switches gears with a light nudge of the hand
and holds its position on a hill or ramp without rolling
backward. 

It might sound like a great way to build a two-gear wheelchair, but it's not. Shifting hub gears involves pedaling backward and then forward, which creates a noticeable backlash. On wheelchairs, that's not acceptable. "We have to be able to drive forward and backward with minimal backlash between them," said Meginniss.

Even more important, though, a wheelchair must hold its position when moving up a hill. Even gentle hills—and ramps to buildings and cars—present challenges to people wheeling a chair. If they get tired or need to reposition their hands, they don't want to slip backward.

Meginniss solved the hill holding problem by designing a hy-pocycloidal drive, which looks like a planetary gear without the sun. In low gear, it creates a huge eccentric load that locks the wheel into place so it will only move forward. His team also devised a friction system that, because of its size, needs to provide 10 to 15 times the 1,000 pounds per square inch of friction found on a car's brakes, but that releases easily enough so the wheelchair can move forward. "The friction system has to be more robust than one used on an earth-moving machine and weigh practically nothing," noted Meginniss.

Two other technologies make MagicWheels work. One is the shift. Not only does it have to move with a touch for people with limited arm mobility, but it must be compact. Meginniss says it is the first transmission that works in the plane of the wheel rather than back and forth along the hub. The second technology is the unit's aircraft-grade carbon fiber-reinforced wheels. They are exceptionally strong, yet thin enough to contain the entire gear mechanism in a space no deeper than that taken up by spoked wheels.

The result is a wheelchair that weighs about 10 pounds more than conventional mechanical systems, yet is much easier for active people to use day in and day out. Many of them prefer it to electrical systems because it keeps their working muscles strong and healthy, and they can retrofit it onto existing wheelchairs. Meginniss said the units retail for $4,995, but often cost less through insurance or government medical programs. 

Servo, Controller Together at Last

If anything is true about integrated servo drives, it is that users want them small, powerful, smart, and easy to wire. Thanks to advances in electronics and a slick design, Bosch Rexroth Corp.'s IndraDrive Mi servo motion control system goes a long way toward granting each of those wishes.

The most visible change in the new IndraDrive Mi is that it mounts directly on the motor lengthwise. This is a departure from existing rear-mounted designs. They blow air through the driver's power transistors to dissipate heat generated by high-speed switching to control servo motion.

Yet air cooling alone does not keep the largest transistors cool, said Bosch Rexroth's electric drives and controls product manager, Rami Al-Ashqar. As a result, designers must either reduce the voltage output of the integrated servo/controllers or use a central control cabinet, which involves more complicated wiring.

According to Al-Ashqar, Bosch Rexroth has switched to power electronics that have fewer thermal issues than conventional transistors. By laying the driver lengthwise along the servo motor, the power transistors use the motor casing as a heat sink to bleed off excess heat. The unit does not need a fan or an external cooling system.

Not only is this solution simple, but the IndraDrive Mi can deliver up to 700 volts. This is as much as Rexroth's cabinet-mounted drives. Higher voltages provide greater torque and control. The design also shaves about 30 percent off the drive's footprint when compared with typical rear-mounted servo/controllers.

The drivers install using a standard interface, called SERCOS (for "standard serial real-time communications"), originally developed for precision-motion machine tools and widely used to transmit information between industrial motion controls and digital servo drives. The IndraDrive Mi uses a version called SERCOS II, which communicates up to 16 megabits a second, four times faster than the original SERCOS. However, plans are in the works to upgrade to the new Ethernet-enabled SERCOS III, which is capable of 100 megabits a second. Because SERCOS systems transmit information over an optical fiber, Rexroth can combine fiber with a power wire in a single cable without risk of interference. The single cable simplifies installation.

The company says that users can connect up to 20 IndraDrive Mi systems together without additional distribution boxes or modifications. Moreover, they can connect more than one string of drives in parallel to a single supply unit.

IndraDrive Mi is designed to fit onto Rexroth servos, although designers can also mount the driver onto a machine and use it with other motors as well. An optional embedded PLC adds up to 100 functions to the drive, including an integrated motion logic system conforming to IEC 61131-3.

Bosch Rexroth is making a big deal out of the new system. Its initial prototypes have already won several engineering awards because the IndraDrive Mi gives users just what they have been looking for: smaller, smarter, easier-to-install drives with no compromise on power.

Air Powers Engine

Air has been powering torpedoes since 1866 and mine locomotives even earlier. Now, inventor Angelo Di Pietro of Engineair Pty. Ltd. of Melbourne, Australia, says he is developing an air motor efficient enough for wider practical use.

Vehicles driven by compressed air have an obvious advantage: They generate no pollution. They can run indoors or in crowded locations where it's a good idea to avoid noxious fumes (like Enginair's first testbed, Melbourne's wholesale produce market). Air-powered vehicles fuel up much faster than battery units, which take hours to recharge.

In addition, Di Pietro said, his motor generates high torque at low revolutions per minute and produces little noise. It also weighs a fraction of traditional piston motors and costs less to produce. Di Pietro claims that the Engineair motor is one-seventh the size of most piston-based air motors and is more efficient.

The secret is in the design. More than 35 years ago, Di Pietro worked on rotary Wankel engine development at Mercedes-Benz. After a few years, he moved to Australia and founded a construction company. Yet he kept coming back to the idea of a rotary engine based on air rather than combustion gases.

The Di Pietro motor is a rotary piston. It consists of a cylindrical piston (which drives the shaft) that rotates eccentrically inside a cylindrical stator.

The space between the rotor and stator is divided into six expansion chambers by pivoting dividers, which open and close as they follow the motion of the rotor as it rolls against the stator wall. A thin film of air cushions the stator as it rolls (so it takes only 1 psi to overcome friction).

A slotted timer mounted on the output shaft controls the flow of air in and out of the expansion chambers. The greater the inlet flow, the higher the engine torque. Shorter inlet cycles let in less air, but the expansion of the air in the chamber is more efficient.

Di Pietro points to a study at Monash University in 2002 which found that his air engine used 770 liters per minute per horsepower. He has built several new prototypes since then, and believes that he will eventually be able to quadruple efficiency over those levels.

Potential applications range from commercial vehicles used in the mining, petrochemical, and pharmaceutical industries to scooters, buses, boats, trains, and cars. Di Pietro plans to put the engine into a carbon fiber two-seat car and use it to commute to work.

Drives in New Areas

The ability of alternating current motor drives to match power with load promises a fast way to reduce energy use and emissions. Once reserved for large industrial pumps, fans, and conveyors, compact drives are now showing up in smaller applications and even high-end consumer appliances.

According to Ilpo Ruohonen, Mika Paakkonen, and Mikko Koskinen, product developers in ABB Ltd.'s large motor drives unit, engineers now expect drives to be simple to install, start up, commission, and operate. They say that original equipment manufacturers rank simplified drive control and setup at the top of their features list, followed by convenient operator interfaces.

Drives are smaller than they used to be, making them easier to add to new designs. By switching to better power electronics and by consolidating parts, manufacturers have shrunk drive sizes tenfold over the past decade. This lets designers reduce drive panel sizes or fit drives directly onto cranes, industrial machinery, or even domestic washing machines. Fewer parts and integrated electronics also push down cost.

Better power electronics deliver a second benefit. In the past, power transistors generated lots of heat as they switched on and off to control motor motion. The greater the drive voltage, the more elaborate the air blowers and heat sinks needed to keep them cool.

Today's power semiconductors run cooler and handle higher temperatures. This lets ABB and others cram higher drive voltage into smaller packages. ABB is also working with alternate techniques, such as liquid cooling, to whisk away even more heat from high-voltage drives.

Although drives are smaller, they are more powerful. Software monitors, diagnoses, and configures drives, while archiving and analyzing operating information. Engineers can set up drives entirely within software, and then download parameters to the appropriate drives.

ABB and other developers are also seeking ways to replace paper-based manuals with more intelligent control panels. ABB's new keypad, for example, combines eight icon-based soft keys with a series of "wizards" that guide users through maintenance, diagnostics, and system start-up by asking questions in plain text language.

ABB estimates that plain language slices 15 minutes off drive commissioning. For an OEM that buys 4,000 ac drives annually, that comes to a half-year of engineering time. They can save even more time using handheld devices that install preconfigured drive parameters in seconds.

Like other drive makers, ABB is now including more functionality to support such specific applications as fans, pumps, mixers, and cranes. Such application-specific drives lower integration costs, improve machine productivity, and reduce start-up time while minimizing the need for expert programmers.

Like several other drive leaders, ABB has developed technology that enables conventional ac motors to work like dc servo drives. ABB's new machinery drive works control open- and closed-loop synchronous and asynchronous motors. Engineers can install the modular system on site and plug in a memory card with drive parameters later, eliminating connections to PCs and on-site programming.

Next-Generation Bearings

When NASA and the U.S. military launched the Versatile Affordable Advanced Turbine Engine program in 2003, they realized they needed better turbine shaft bearings. Since then, the program, known as VAATE, has funded one of the largest bearing research programs ever ($2.4 million in 2008 alone) at The Timken Co. in Canton, Ohio.

Timken continues to make progress in building bearings for extreme conditions. The company's director of advanced engineering technology, Stephen P. Johnson, said he hopes to apply VAATE technology to other industries where temperature, corrosion, and wear are critical.

An X-ray diffractometer tests material characterization of bearings at Timken.

As those industries run equipment hotter and faster to increase efficiency and power, they face many of the same problems as builders of jet engines. Bearing makers index bearing surface speeds by multiplying bearing diameter times rotation rate for a number they call DN. Ten or 15 years ago, a 100-millimeter-diameter bearing in a turbine might run 20,000 to 25,000 revolutions per minute, Johnson said. This gave it a DN of 2.0 million to 2.5 million. Today's engines are looking at DN of 3.0 million, 3.5 million, and even 4.0 million.

Shaft operating temperatures have also risen, from 350°F to 450°F. Once the engine shuts down, the bearings must also withstand up to 200°F of "soak back," the heat radiated by hot engine parts after the cooling system shuts down.

Until now, turbine makers used bearings made of M-50 high-speed machine tool steel, which retains its outstanding hardness and wear resistance even at high temperatures. Unfortunately, hardness is M-50's undoing, since it comes at the expense of ductility. This makes it prone to fracture if debris gets into the lubrication system.

"This happens more than you would think," Johnson said. Lubrication systems can carry small, hard particles into the bearings from the transmission and turbine blades. Sand and grit can work through seals, especially in aircraft flying in the desert. On a heavily loaded, high-speed bearing raceway, even small particles can apply enough force to initiate a crack. Because M-50 rollers are through-hardened, a single crack can propagate right through them.

M-50 is also vulnerable to corrosion. That isn't an issue on commercial jets, whose continuous operation burns off any moisture in the system. Military jets, on the other hand, may sit for days in humid conditions before they are used. The problem is even worse for naval aircraft. Even M-50 spare parts that have been greased and vacuum packed corrode over time.

Timken tried to solve the intertwined problems of hardness, ductility, and corrosion by looking at M-50 NiL, an alloy with extra nickel (for corrosion) and less carbon (for improved ductility). Although it hardened the bearing surface, the alloy proved too ductile to resist wear at very high speeds.

Timken switched to a ductile stainless steel, whose chromium forms corrosion-resistant oxides on the surface. It then applies multiple treatments to harden its surface. Ordinarily, treating the surface with carbon ties up the alloy's chromium so it cannot form oxides. "One of the things we've been able to do is preserve the corrosion resistance," said Johnson. Timken has also developed ways to form hard layers that penetrate deep into the bearing interior, hardening the stainless steel enough so that it doesn't deform during high-speed operation.

Johnson hopes to apply those microstructural manipulations to other industries that want to run hotter and faster to reduce energy use and pollution. He points to turbodiesels, the predominant vehicle motor in Europe, which get twice the mileage of gasoline engines.

"Turbodiesels now use hydrodynamic bearings, which spin on a film of oil," Johnson explained. "First, they have to overcome the drag of a fluid-filled bearing, which causes a lag in turbo performance. Then, as they spin faster, they begin to burn oil. A rolling element bearing could handle the speed and the heat."

Could automakers afford it? "The industry will pay a premium for products that deliver value that they can recoup," Johnson said. "The roller bearing may be the difference between high-speed turbodiesels working and not working. The challenge is applying what we learn in aerospace to build a bearing small enough to fit inside a turbocharger."