Inertial Sensors - Xsens

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COVER STORYXsens Technologies B.V., a Netherlands-based supplierof 3-D motion-tracking products, began researching theconcept of a sensor-based suit for biomechanical measurementsin 2005. A 25-person research team, including abouta dozen Ph.D.s, was assigned to the project of applying accelerometers,gyroscopes, digital processors and Bluetoothtransmitters to the Lycra suit, and then tying the hardwaretogether with a laptop-based software model.By August 2007, the company was demonstratingthe technology at the Siggraph computer graphicsconference.“I went to Siggraph and they were unveilingthis killer app,” Edwards says. “It was a way to domo-cap (motion capture) unlike anything I’dever seen.”Indeed, until that time, Edwardsand others who did “previs”thought the state-of-the-artwas an optical method thatinvolved suits with luminousmarkers. The optical technique,however, contained a level of complexitythat was especially vexingfor smaller companies.“If you have an optical system,you have to rent more space, use expensive cameras and hiretrained operators,” says Erik Wilbrink, vice president ofsales and marketing for Xsens, “whereas with our system,people are motion-capturing and getting real-time datawithin 15 minutes.”The Xsens MVN suit contains 51single-axis gyroscopes, 34 dualaxisaccelerometers and 17 Blackfindigital signal processors from AnalogDevices. Xbus Master controllers synchronizeand power the sensors, anddo wireless data transmission.Source: Xsens TechnologiesSource: The Third FloorThe Third Floor, which specializes in film previsualization, takesdata from the MVN suit and turns it into a scene full of characters,environments and props that can be re-arranged with alaptop computer.Collecting the DataBut real-time feedback is just one of many advantages TheThird Floor got from the MVN Motion Capture Suit.The suit, which incorporates 17 matchbox-sized “trackers,”enables directors to take a set of data points and turnthem into a scene full of characters, environments andprops that can be re-arranged with a laptop computer.Containing three single-axis iMEMS gyroscopes, twodual-axis iMEMS accelerometers, and one Blackfin digitalsignal processor (DSP) from Analog Devices, each trackeris sewn into fabric tunnels within the suit. Wires in thetunnels connect the trackers in long daisy chains to anXbus Master — a controller that synchronizesand powers them while simultaneously sendingdata to a laptop computer through a Bluetoothconnection.Essentially, the suit comprises a six-degreeof-freedommotion-sensing platform. The twodual-axis accelerometers collect accelerationdata in the x, y and z planes, with one extra axisremaining for redundancy. The three gyrosmeasure rate of rotation about each of the axes.During a movie shoot, pre-vis experts gather data fromthe inertial measurement units (IMUs), which are locatedin the middle of actors’ body segments, such as mid-calf,mid-forearm, mid-thigh and other spots around the body.46 DESIGN NEWS SEPTEMBER 2010 [www.designnews.com]
COVER STORYData from those points is sent to a biomechanical softwaremodel in the laptop, which tallies motion data at each pointin real time. Computer graphics imagery (CGI) softwarethen superimposes characters, objects and environmentsover the motion data.“The sensors give us acceleration data and orientationdata,” says Hein Beute, product manager for the entertainmentmarket at Xsens. “When you feedthat data into a biomechanical model, thenyou know the orientation of the upper armand lower arm and legs. Within a millisecond,you can reconstruct an actor’s pose.”During motion capture, the sensor-basedsuits offer two important advantages overoptical techniques: They can gatherdata over huge areas and they eliminatethe problem of “occlusion,”which occurs when one body partblocks another, preventing the transmission of data.Because the MVN suit transmits data by RF, rather than optically,occlusion is not an issue.“When we tell people they don’t have to deal with occlusionanymore, there’s a big sigh of relief,” Wilbrink says.“With occlusion, there’s missing data. And the only solutionto that is to add more cameras, which makes it morecomplex.”Living Story BoardEngineers at The Third Floor employed the MVN suit forIron Man 2’s action sequences and fight scenes. The sensorbasedtechnology was especially important for a complex20-minute scene known as the Japanese garden sequencenear the end of the movie, which involved multiple characters,props and environments.“We had bi-pedal humans and robotic characters, andthey were supposed to be running around and punchingand flipping each other,” Edwards says. “Within verylittle time, the actors got into their motion capture suitsand did the fight choreography, punching and flipping totheir hearts’ content.” The director’s team thencompiled single elements of the battle into alarger scene that involved integration ofmultiple clips.Seventeen of the matchbox-sizedsensor packs are placed within thesuit, as well as in the actors’ glovesand atop their shoes.Source: Adam NelsonThe Third Floorused a similar approachin another recentmovie, Alice in Wonderland.There,engineers usedthe MVN MotionCaptureSuits on actorsrunning onSource: Xsens TechnologiesTrackers containing accelerometers, gyroscopes and digitalsignal processors are placed at the mid-points of leg and armsegments, as well as elsewhere around the body. Accelerationand orientation data is sent wirelessly to a software model thattallies that data.treadmills, then took the data and later “composited” theactors’ movements into a virtual world.“If you can put a lot of detail into your pre-vis, which iswhat we always try to do, then directors treat it more seriously,”Edwards says. “All of a sudden, what you’ve builtis more than a story board. It’s a living, breathing designdocument for each and every shot.”Engineers from Xsens say they foresee the technologybeing employed in biomechanical research, movement science,ergonomics, human factors design, sports analysisand virtual-reality applications. Computer games are also agrowing market for the technology. Gearbox Software employedit for character animation in the “Borderlands” videogame, and Sony Computer Entertainment /Guerilla Gamesused it for “Killzone 2.” Moreover, the MVN suit is capturingthe interest of a growing number of Hollywood studios.In addition to Iron Man 2 and Alice in Wonderland, it’s beenused in movies made by Industrial Light & Magic, SonyPictures Imageworks and Double Negative.“It’s amazing to see the directors’ and cinematographers’eyes light up when they realize, ‘You mean we can get in thesuits and act this out right now?’”Equally important, though, is the effect the suit couldhave on smaller film companies with tighter budgets.“That’s the most exciting part,” Edwards says. “With this,you don’t need to be a super-elite filmmaker to take advantageof the best tools.”48 DESIGN NEWS SEPTEMBER 2010 [www.designnews.com]
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Inertial Sensors - Xsens

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