TECHFOCUS This section v\/as edited byAssociate Editor Alan S- Brown.

power transmission and motion control

WHAT ROBOTS LEARN

A^ the c.i iks that we ask ofrobots—especially servicerobots designed to helphumans—-grow more

comphcated, teaching robots how toperform those tasks has grown evermore complex. That's why robotdevelopers may want to look at thework of a group of Stanford Univer-sity researchers building autonomoushelicopters. They have taught theirrobotic helicopters to fiy the sameway humans learn—by copyingsomeone who knows how to do it.

These robots don't just fly. Theyperform a wliole range of aerobaticstunts, such as traveling flips, rolls,loops with pirouettes, stall-turns

with pirouettes, inverted tail slides,and maneuvers with such exoticnames as the knife-edge, slapper,hurricane, and the tic-toe, whichinvolves swinging from side to side.Not only do the robotic helicoptersperform these tricks, but they dothem better than their hunian pilots.Just staying aloft is impressive.

Unlike airplanes that glide on wings,helicopters are inherently unstablesystems. All they really want to dois fall out of the sky. "If you don'tprovide feedback, it will crash." saidPieter Abbeel. He and fellow gradu-ate students Adam Coates, Timo-thy Hunter, and Morgan Quigleydeveloped the helicopter under thedirection of Andrew Ng, an assistant

professor of computer science.At first, Abbeel and Coates tried to

teach helicopters aerobatics by writ-ing computer code for each specificmaneuver. This proved workable(but not elegant) for novice-levelflips and rolls. It failed entirelyfor the tic-toe and other complexmaneuvers. Nor could the robotsimply copy the moves of the team'sexpert radio control pilot, GarrettOku. Performance changed withwind speed, sudden gusts, tempera-ture, and humidity.

Finally, the programmers devel-oped artificial intelligencealgorithms to analyze Oku'sroutines. Even though

+Tracking in Two DinnensionsOPTICAL ENCODERS ARE DEVICES THAT SIT ON MOVING

PARTS AND CONTINUOUSLY MONITOR THEIR POSITION BY

READING MARKS ON A RULER-LIKE INCREMENTALTRACK.

The information goes to a controller, which uses it to adjustthe part's speed and location.

But suppose you want to control a gantry or stage in twodimensions? The usual solution is to mount two encod-ers on the stage. Now, encoder speciaUst Dr. JohannesHeidenhain GmbH of Traunreu, Germany, offers a sec-ond option. It lias developed a new encoder, the lDplus,which simultaneously tracks a part's position along a gridthat defmes a part's x and y axes.

The lDplus does this by using two or three opticalscanning units. This may sound like an obvious solution,but the problem in the past has always been accuracy.The lDplus is accurate to ^ 1 micrometer.

"The sensor is the easy part," said Kevin Kaufenberg, aproduct manager at Heidenhain's U.S. subsidiary. "Thedifllcult part is creating long, accurate )'-axis markinglines that run parallel to one another and perpendicularto the incremental track." The hnes themselves are onlyabout 200 nanometers thick, spaced about S micrometersfrom one another, and run the entire length of the track-

It took nearly two years to figure out how to etch suchfine lines onto a scale. While most of the technology isproprietary, Kaufenberg said that the process begins witha precision-flat glass substrate. The company uses a sys-tem similar to those used to define the circuitry of semi-conductor chips to etch a grid of lines into the glass.

An encoder with two scanning units can read the grid

22 mechanical engineering | November 2008

and simultaneously measure both the x and y positions.Adding a third sensor enables the controller to calcu-late the angle of rotation of the bracket that houses theencoder and use the information to compensate for lin-ear guiding errors and thermal drift.

"Let's say you have an H-stage gantry," Kaufenbergsaid. "With three sensors, you could measure any deflec-tions in the middle bar of the H. If you're using air bear-ings, you can make sure the center bar is not skewed. Ifyou're doing a repetitive precision ta.sk all day and thegantry starts to heat up, you can also measure and correctfor heat-induced deflections."

Kaufenberg sees potential applications in everythingfrom wafer inspection and wire bonders to medical testingand the production and testing of large flat panel displays.While the lDplus costs more than a conventional one-di-mensional encoder, it costs significantly less than the twoencoders typically used for the same job.

Oku's piloting varied with eachflight, the AI system was able toabstract the ideal trajectory Okusought. Eventually, the autonomoushelicopter learned to fly the routinebetter and more consistently thanOku himself

The helicopter carries acceleronie-ters, gyroscopes, and magnetometersthat measure its position, direction,orientation, velocity, acceleration,and spin 20 times per second. TheAl system analyzes the data in orderto adjust the vehicle's flight path.

According to Ng, the goal is tocreate autonomous helicopters thatcan search for land mines in battlezones or map the spread of wildfiresin real time. "In order for us to trusthelicopters in these sort of mission-critical applications, it's importantthat w'c have very robust, very reh-able helicopter controllers that canfly maybe as well as the best humanpilots in the world can." Ng said.

Stanford's autonomous helicoptersare a step in that direction. Theyalso showcase the type of algorithmsthat may simplify teaching robotsto provide complex services to theirhuni.m masters.

Editor's Note: This month's coverarticle, "Moving on Their Own,"examines technical issues beingresearched to make possible futuregenerations of autonomous roboticdevices, capable of taking on increas-iiifily complex tasks.

POSITIONER LIGHTENS UP

baldor Electric Co. of FortSmith, Ark., has developeda new x-y positioning basethat integrates two linear

stepper motors into a single plane ina Hghtweight honeycombed platen,or base. Ordinary positioning basesuse two separate sets of motors andtracks, one on top of the other. Thismakes them bulky and heavy. Thenew Honeycomb series is thinner,lighter, and flatter, and you can hangit upside down without any addi-tional bracing.

According to product managerJohn Mazurkiewicz. Baldor's previ-ous line of dual-axis linear motors

+ Encoders for Drag RacingMAGNETIC ROTARY ENCODERS MADE BY GREAT BRITAIN'S RENISHAW PLC AREUSED AS POSITION SENSORS IN A WIDE RANGE OF APPLICATIONS. FROM FOR-MULA ONE RACE CARS TO OIL FIELDS Among the more colorful uses has beenin the clutch housing of one of the world's fastest motorcycles.

|aska Salakari of the Salakazi Racing team in Finland competes in the SuperTwin Top Fuel class of motorcycle drag racing on a bike made by KTM Pow-er Sports AG in Mattighofen, Germany. Leading vehicles in this class finish aquarter-mile in under 7 seconds.

This takes reaction times measuredin milliseconds, and a driver movingthat flist over such a short distancedoesn't have time to react and adjustthe clutch during a race. Salakazi Rac-ing has equipped its dragster with anautomatic Prowork three-disc, four-stage clutch fitted with a Proworkdigital controller. When the ridersnaps open the throttle, the controllerengages the clutch according to howit has been preprogranmied.

The program has to be right because every revolution that a wheel spins with-out traction is time lost on the track. The team has placed a Renishaw magneticrotary encoder to monitor the clutch so that after each race the team can evalu-ate its settings and fine-tune them, if necessary.

The team uses a compact RM22 system designed and manufactured by Ren-ishaw's Slovenia-based partner company, RLS d.o.o. The solid-state encoderhas a small actuator magnet that tits to the end of a shaft and a Hall sensorchip embedded in epoxy inside a metal housing/mount. The encapsulateddesign provides resistance to shock, vibration, acceleration, and deceleration.

Heat is also an issue. The KM22 has a maximum operating temperature ofonly 125°C, and the engine gets much hotter than that. This is where thedevice's small size comes into play. According to the team's technical chief.Petri Mäkinen, the 22-millimeter-dianieter unit is small enough to fit insidethe machined aluminum clutch housing. The housing is thick enough toprovide some thermal protection, and the team removes the encoder quicklyafter each race to download its information.

A small encoder enables a motorcycleracing team to analyze clutch shifts duringraces that last only 7 seconds.

was up to U) inches thick andweighed as much as 140 pounds. Fiveof the seven Honeycomb platens areonly 1.1 inches thick, and the othertwo are 1.8 inches thick. They alsoweigh 70 to 80 percent less thansimilarly sized x~y positioners.

Like other linear motors, the Hon-

eycomb motors have superb accu-racy. According to Mazurkiewicz,they move at speeds up to 1.5 meters(5 feet) per second. The positionerhas a resolution of 2.5 micrometersand repeatability of better than 2micrometers (unidirectional) and3 micrometers (bidirectional) in a

November 2008 | mechanical engineering 23

TECHFOCUS

four-phase power configuration.The platen consists of an alumi-

num honeycomb core sandwichedbetween two sheets of aluminum.This is the same type of designused to stiffen aircraft wings andtails. Compared with platens madefrom steel or cast iron, it is muchstiifer and flatter. It is also much lessmassive, so it is easier to mount intight spots.

The design is straightforward. Themoving stage, or forcer, integratestwo linear stepper coils mounted atright angles and floats on an air bear-ing. It acts as the motor's rotor. Theplaten contains permanent magnetsand acts as the stator. The air bearingsupports quite a load, as much as 400pounds if it is mounted upside down.

According to Mazurkiewicz,Honeycomh positioners cost 50 to 60percent less than conventional two-axis/two-motor systems. Equallyimportant, they have the speed andaccuracy needed for 3-D prototyp-ing and rapid manufacturing, testand inspection measurement, andpick-and-place applications.

MOTOR UNTRACKS MAGNETS

Anew linear motor turnsthe usual recipe for suchdevices on its head. Linearmotors are essentially

rotary motors with their statorsrolled flat. The shuttle, or primarysection, contains an electromagnet.It rides along the secondary section,or stator, containing permanentmagnets, like a train riding on atrack. The attraction and repulsioncreated as the shuttle's electromagnetswitches polarity propels the primarysection forward.

The new motor, made by SiemensEnergy & Automation Inc., achievesmotion in a very different way. Thenew IFN6 synchronous linear motorseries packs both permanent magnetsand electromagnets into the pri-mary section. The secondary track issimple steel and houses no magnetsof any kind.

"We found out that the position

2A mechanical engineering | November 2008

A new-style linear motor houses all itsmagnets in its shultle. simplifying installa-tion of its plain steel secondary section.

of the permanent magnets did notmatter as long as they were withinthe flux loop," said Jeff Gerlach. aSiemens consulting business develop-er. So if the permanent magnet is noton the track, what makes the shuttlemove? "'The teeth along the second-ary track," Gerlach said. "The elec-tromagnets in the primary inducea magnetic field in the teeth of thesecondary, and the shuttle is attractedtoward that. There are no magnetsin the track, no copper wire, nomagic—-just magnetic steel."

This seems like a complicated wayoí doing things, so why go throughall the bother? It conies down toeconomics, said Siemens' applica-tions engineering manager, StephenCzajkowski.

Eliminating magnets from thesecondary track not only eliminatesexpensive permanent magnets fromlong secondary sections, but alsosimplifies installation. "Workers haveto take off their watches, glasses,and anything else that might getmagnetized during installation," hesaid. Nor do the new tracks attractferrous chips and debris thrown offby machining. The new technologyalso removes the need for water lines,tanks, and pumps to cool magnetsalong the length of the track.

"The system is really targetedfor applications with long trav-els," Czajkowski said. "After 5 or 6meters, it is a lot more cost effective."

Of course, the new design operates

somewhat differently from conven-tional linear motors. Because theprimary section contains both per-manent magnets and electromagnets,it is more massive than conventionalshuttles. While it accelerates smooth-ly, it is not as fast as other linearprimaries. Nor does the air-cooledsystem generate as much force asother linear motors, although modelsin the series range from 900 to 8,080newtons.

According to Czajkowski, potentialapplications include water jet cut-ting, especially of large composites,gantries, laser cutting, and aluminummachining. The system supportsmultiple primaries on a single sec-ondary track moving in the same oropposite directions.

POWER ELECTRONICS GROWINGRAPIOLY.SAYSSTUOY

he market for powerI electronics products and

systems, which approachedSIO billion in 2007, will

grow 11.6 percent annually to reach$17.7 billion in 2013, according to anew study by market researcher BCCResearch of Wellesley, Mass.

Classic power electronics devicesconvert electrical power from oneform to another, such as alternatingto direct current, low to high fre-quency, and transmission to line volt-ages. This is generally done with suchsemiconductor switching devices asdiodes, thyristors. and transistors.

But power electronics devicesincreasingly fulfill other roles.Because they control and conditionelectrical power, they have a key rolein determining the performance andefficiency not only of electronics,but also of motors, pumps, and otherelectrical equipment. According toBCC, power electronics representsone of the few

mexpensivetools that canreduce fuel con-sumption acrossthe entire indus-trial spectrum.

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