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The Mechanics of Motion Sensing 119

Posted by kdawson
from the ride-that-dragon dept.
Dr. Eggman writes, "The AP has a short technology piece on the mechanics that go into the motion-sensing capabilities of the Wii and PS3 controllers. It also details some of the past uses of the technology and gives a nice overview of just how far the technology has come from the earliest missile-guidance sensor equipment."
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The Mechanics of Motion Sensing

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  • Interesting. (Score:4, Interesting)

    by Mendak Jemuna (998832) on Monday November 20, 2006 @05:24PM (#16920986)
    Interesting, but I have one question. Do the silicon springs recalibrate every so often, or will they wear out and break? My old N64 controller did this.
    • Re:Interesting. (Score:5, Informative)

      by drinkypoo (153816) <martin.espinoza@gmail.com> on Monday November 20, 2006 @05:43PM (#16921278) Homepage Journal
      They're so minute and move so little you will never have to worry about them breaking. Because they are so tiny you can never build up appreciable inertia in them. They are functionally considered solid state devices and they should last pretty much forever.
      • Re:Interesting. (Score:5, Interesting)

        by fatphil (181876) on Monday November 20, 2006 @06:43PM (#16922206) Homepage
        Should - and do. One of Freescale Semiconductor's clients' more popular uses for accelerometers is in industrial applications -- BIG kit -- on things that vibrate constantly. Things that vibrate more and more as bearings dry or wear (hence the need for the accelerometer - so you can perform detect this and perform maintenance before your printing press, or whatever, self-destructs). The accelerometers outlive any of the parts they are supposed to look after.

        Freescale has some pretty cool electonmicrographs of their 2-axis and 3-axis accelerometers - www.freescale.com, sensors, accelerometers, and browse around - I can't actually find the images now. I have a PPT on my work laptop, but that's no use. Grab a ZSTAR for <$50, and simply have a play with one yourself - (that includes everything - hardware and development kit).
      • Re:Interesting. (Score:5, Interesting)

        by naoursla (99850) on Monday November 20, 2006 @07:16PM (#16922604) Homepage Journal
        I asked a MEMS researched about this once. He said it comes down to fatigue. If you bend a paperclip back and forth a bunch of times the metal fatigues and eventually breaks. Fatigue occurs because the metal actually migrates around the bend. MEMS components are so small and move so quickly that they do not rest in one position long enough for fatigue to occur. The same principle hold for the springs in accellerometers and the mirrors in DMD projectors.
        • by kilraid (645166)
          I don't buy this explanation. Say you accelerate a missile, you will have a considerable duration of acceleration, imposing a force upon the MEMS (micro-electro-mechanical systems) components. Yet they survive it.
          • by naoursla (99850)
            Good point. The original question I asked was in reference to the mirrors on a DMD device. Maybe the same principle does not hold for accellerometers.
        • There's another way to avoid damage due to stress. As an alternative, some accelerometers monitor the movement of heated air in a cavity (source [omimo.be]). Surprisingly, they can be made MEMS and are cheaper than the mechanical ones (though usually with a factor-of-two loss of accuracy).
      • Re:Interesting. (Score:4, Informative)

        by EmbeddedJanitor (597831) on Monday November 20, 2006 @07:54PM (#16923090)
        Most accelerometers can survive 3000G or so.
    • Re: (Score:2, Informative)

      by indigest (974861)
      "Silicon spring" is misleading because it implies there are moving parts within the accelerometer that can break. In actuality, the proof mass is held perfectly still using a feedback loop to cancel the externally applied force. The magnitude of this applied force is read out as the acceleration. No calibration is ever needed thanks to the feedback loop.
      • Re: (Score:2, Informative)

        by Plutonite (999141)
        It is not held perfectly still in both piezoresistive and capacitive accelerometers. You need to have displacements to measure the acceleration, and stiction is the problem solved with feedback/force units.

        Or maybe I just misunderstood you..
      • "Silicon spring" is misleading because it implies there are moving parts within the accelerometer that can break. In actuality, the proof mass is held perfectly still using a feedback loop to cancel the externally applied force. The magnitude of this applied force is read out as the acceleration. No calibration is ever needed thanks to the feedback loop.

        Actually - you are only partly right. You still need to calibrate the device in order to determine what magnitude of applied (feedback) force correlates t

    • No fatigue (Score:3, Interesting)

      I was worried that something like metal fatigue would set in. But metal fatigue is a result of microscopic fractures in the crystalline structure of the metal. These sensors have moving parts that are fabricated from a single flawless crystal. So nothing like metal fatigue can take place.
  • It also details some of the past uses of the technology and gives a nice overview of just how far the technology has come from the earliest missile-guidance sensor equipment."

    See, I think we should re-incorporate early guidance systems into modern-day game controllers.

    I mean, think about it: game controllers with trained pigeons in 'em! That'd take force-feedback to a whole new level!

    • "I mean, think about it: game controllers with trained pigeons in 'em! That'd take force-feedback to a whole new level!"

      You'll need a speaker-hole so that the birds in the controllers could communicate with the miniature pterodactyl inside the console box that actually pulls the strings and sticks to make the game work.
    • Sure, a guidance system will have accelerometers, but by far the more important part is the gyro. The Wii does not have a gyro. While silicon gyros do exist, they're still relatively large and expensive.
    • The missile knows where it is at all times. It knows this because it knows where it isn't. By subtracting where it is from where it isn't, or where it isn't from where it is (whichever is greater), it obtains a difference, or deviation. The guidance subsystem uses deviations to generate corrective commands to drive the missile from a position where it is to a position where it isn't, and arriving at a position where it wasn't, it now is. Consequently, the position where it is, is now the position that it wa
  • Globalvr's new Ultrapin features Globalvr's Patent Pending U-Shock Board Which allows players to interact with the cabinet for a totally realistic pinball experience. You can BUMP and NUDGE the cabinet to affect the ball in play - just like real pinball
    http://www.globalvr.com/products_ultrapin_intro.ht ml [globalvr.com]
    • I know where you can BUMP and NUDGE to affect the gameplay... an actual PINBALL game!!!

      It's always irked me to see some hand-held P.O.S. that touts "realistic pinball action". (...action ...action ...ction ...tion)

      If I want to play pinball, I'll play pinball.

      However, if I want to ride a dragon... well I can't really do that now, can I?

      THAT is why the new controllers are so incredible; they give a new, visceral edge to creative gaming... rather than try to "come full circle" with a gaming platform t

      • I've played the silver ball for 36 years and with one exception I would agree with you. That exception is 3D Ultra Pinball Thrillride -- a video game that plays better than a pinball machine. It is as good as any pinball game in terms of game play, variety and realism. It is better because you never have to clean it or replace worn out components. I picked up a surplused copy for about $10. Much as I love ST:TNG (the show and the pin), a $10 product that never wears out, is fully mute-able and is legal
  • Other applications (Score:4, Interesting)

    by inKubus (199753) on Monday November 20, 2006 @05:36PM (#16921166) Homepage Journal
    It would be cool to incorporate this type of controller into a portable device. You could have portable laser-tag like games with real-time mapping, or incorporate some type of ball and it automatically keeps score and records the game. You could have "operations" players indoors that have some different role (view the maps, for instance, and direct the players)

    With networking and GIS in a portable device, it's almost necessary to have new interfaces to use all the new space it knows about. In gaming it's the most fun but these types of thing could be used in more professional ways also.

    • Laser Tag with a doom-style map on the gun/on a wristband?
      Brilliant!
      Then again, it also makes a great set for a horror movie (big arena, psychopath with a real gun, etc, etc *groan*)
    • by MankyD (567984) on Monday November 20, 2006 @05:45PM (#16921322) Homepage
      It would be cool to incorporate this type of controller into a portable device. You could have portable laser-tag like games with real-time mapping
      While it would definitely be cool, you can't really use accelerometers as location devices. Accelerometers and, to a lesser degree, odometers both do a pretty terrible job of keeping track of where an object is located. That's why the Wii also includes a sensor bar.

      They'll generally give you pretty good readings for a single movement, like 'the object just moved 1 meter forwards', however as soon as you turn or travel a long distance, they suffer from drifting, skidding, and general measurement errors.

      A different sort of tech would be needed for mapping. You can do some research into Robotics, such Markov Localization, for some more information. GPS and related techs are better for real time location reporting.
      • It would be cool to incorporate this type of controller into a portable device. You could have portable laser-tag like games with real-time mapping

        While it would definitely be cool, you can't really use accelerometers as location devices.

        I guess then all those inertial guidance and navigation systems the military has deployed don't really work then? (Or, IOW, you are incorrect. Considerably.) Accelerometers by themselves can't be used as location devices - but if you use a clock and a computer and int

        • by dcam (615646) <<moc.tpecnocrebu> <ta> <divad>> on Monday November 20, 2006 @09:33PM (#16924026) Homepage
          They'll generally give you pretty good readings for a single movement, like 'the object just moved 1 meter forwards', however as soon as you turn or travel a long distance, they suffer from drifting, skidding, and general measurement errors.


          That's true of odometers, but again with accelerometers it's just a matter of engineering your system properly. (The USAF equips its fighters with inertial navigation systems - and they remain accurate through a bloody dogfight!.)


          No the OP was right. It is also true of accelerometers. And the fact that they are measuring data during a dog fight it irrelevant, it is the amount of time/distance they are measuring data. They include a random walk error that is small for a short time/distance, but compounds over time.

          You will find that typically this is corrected with something that can give an absolute position (eg GPS). Your absolute positioning device typically also has a know error. The values from both of these are generally married using a Kalman Filter or Extented Kalmna Filter.

          I've written code to do this in the past.

          I suggest that you do some research on inertial navigation and inertial guidance - because what you say is true at the level of the casual gamer, its not true (as you imply) of accelerometers in general.


          I suggest that you also do some. 4 years of Mechatronic Engineering would be good start.
          • by big tex (15917)
            4 years of Mechatronic Engineering would be good start.

            OK, you made that up.
            It sounds like you design robotic enemies for Godzilla.

            • by dcam (615646)
              No. Click the about Us link at the bottom of the page.

              But yes it does mean robots.
            • by dcam (615646)
              Trying that again, should have previewed.

              No [usyd.edu.au]. Click the about Us link at the bottom of the page.

              But yes it does mean robots.
            • by KDR_11k (778916)
              Naah, it's Computer Science, Mechanic and Electronic Engineering. Not as in-depth as either of these but the versatility is needed in some areas, e.g. automobile design. Or designing robotic enemies for Godzilla. Except you don't need stop motion or Cg to make them move.
          • by Animats (122034)

            Low-end solid state accelerometers aren't that good. Accuracy is only 1% or so. You can't really get position by integrating them twice; you'll get huge amounts of drift as false velocity builds up. If you have some external reality check, even an odometer, you do much better.

            Low-end rate gyros aren't that good either. We did badly in the DARPA Grand Challenge because our heading measurements were about +-3 degrees off, which was enough to mess up the maps being built up from the laser rangefinder.

            • by dcam (615646)
              Cool. It looks like the ACFR, where I did my BE, has been involved [usyd.edu.au] in DARPA. I haven't been in contact with any of them since I left uni, so I don't know what the project is. It isn't surprising that UTS is involved, the guy who heads UTS's Mechatronics dept is a former ACFR man.

              The ACFR has for some time had a bit to do with vehicles. The've used a UTE as a testing platform for a while. Most of their work has been to do with industry, mining trucks and straddle carriers. They've done some solid work though
          • No the OP was right. It is also true of accelerometers. And the fact that they are measuring data during a dog fight it irrelevant, it is the amount of time/distance they are measuring data. They include a random walk error that is small for a short time/distance, but compounds over time. You will find that typically this is corrected with something that can give an absolute position (eg GPS). Your absolute positioning device typically also has a know error. The values from both of these are generally marr

            • by dcam (615646)
              Haha thanks for the lesson. Love the ad hominem attack.

              I've got more than sufficient education on inertial systems...

              Clearly not. ...when you get out of high school, look me up

              Gee, somehow I forgot to look up up 10 years ago, how remiss of me.

              Ok I'll make it simple for you, so that with some high school physics and some elementary logic you can understand.

              Lets being by assumimg you aren't an idiot and use SI units.

              All sensors have some error. So lets take an example. I have a accelerometer, with a 0.1 m/s/s
          • by glindsey (73730)
            Dude, Mechatron was my favorite Transformer!

            Seriously, though, I had no idea "mechatronic engineering" was a discipline. Fascinating...
            • by dcam (615646)
              When I started my undergraduate degree (1997) it was kind of new. It had been started only 4 years before. At that point you began doing Mechanical Engineering and after 1 year had the choice of transferring into to Mechatronics, ie there was no first year for mechatronics. For the second year, it was identical to Mech Eng, except you dropped Materials for a dedicated Mechatronics subject. After that it tended to diverge quite a bit.

              Now it is a completely separate degree from the first year. It also needs b
    • by justkarl (775856) *
      That makes sense. Dosen't Wii come with wireless networking built-in? Or am I confusing it with the PS3?
    • Most importantly, you could have real time weapon switching!!!
  • Very odd (Score:2, Interesting)

    by also-rr (980579)
    Given the 10+ year development and 20+ year use lifecycles of military equipment it's surprising to see consumer gear _behind_ military tech.

    The main reasons for this, of course, are obvious. It's not often that your games console needs the same provable failsafes as a weapon of mass destruction and equally it doesn't cost £10m a unit (launch day eBayed PS3 excepted) so replacing it within a short timeframe isn't going to give you a massive budget deficit.
    • Re: (Score:3, Insightful)

      by the.Ceph (863988)
      Consumer gear is often behind military tech. Perhaps you have heard of velcro, or GPS, or the internet.
    • Re: (Score:3, Interesting)

      by DerekLyons (302214)

      Given the 10+ year development and 20+ year use lifecycles of military equipment it's surprising to see consumer gear _behind_ military tech.

      It's hard to actually say who is ahead or behind, partly because tech in the real world isn't a linear scale (like it is in various games), partly because the military deploys such a wide variety of accelerometers. The ones used in, say the Trident-II's MK6 guidance, are certainly much larger than these (about the size of a film can) and are 'old style' (asymmetrical

      • The ones used in, say the Trident-II's MK6 guidance, are certainly much larger than these (about the size of a film can) and are 'old style' (asymmetrical floats in fluid) - they are also much more robust and less sensitive to vibration.

        Now that's interesting. Is it the fluid that makes them less sensitive to vibration? (It seems like it would be.) In effect, the fluid would be working as a low-pass filter, so only large movements would be detected.

        Does the Wii-mote get around the vibration issue by doing t

        • The ones used in, say the Trident-II's MK6 guidance, are certainly much larger than these (about the size of a film can) and are 'old style' (asymmetrical floats in fluid) - they are also much more robust and less sensitive to vibration.

          Now that's interesting. Is it the fluid that makes them less sensitive to vibration? (It seems like it would be.) In effect, the fluid would be working as a low-pass filter, so only large movements would be detected.

          It's partly the fluid, partly the suspension system (the

  • by xxxJonBoyxxx (565205) on Monday November 20, 2006 @05:42PM (#16921274)
    I thought the "snare" was the first motion-activated weapon. Or was it the "covered pit"?
  • MEMS (Score:5, Informative)

    by Mike1024 (184871) on Monday November 20, 2006 @05:52PM (#16921418)
    A good picture of a two-axis accelerometer can be seen here: http://users.wpi.edu/~cfurlong/me-593Mech.html [wpi.edu] (second picture down). Sensing is usually performed by capacitive combs, structures which act as capacitors, with their capacitance varying with displacement.

    MEMS accelerometers have dropped in price in recent years because there's a big market: the automotive sector. A typical new car needs two accelerometers, one for the traction control system measuring roughly plus-or-minus 2 to 4g, and one for airbag deployment measuring more like 50g.

    Two big manufacturers are Analog Devices [analog.com] and ST Microelectronics [st.com], though others exist.

    The high demand of the automotive sector has driven prices right down; sensors which would have cost hundreds of dollars in the past can now be purchased in bulk for less than $4. In fact, you could order one right now; component retailers [digikey.com] will sell you one for less than $15.
    • I thought nintendo was supposed to be using the Gyration miniature gyroscopes. Link here [gamecubicle.com]
      • by Mike1024 (184871)
        I thought nintendo was supposed to be using the Gyration miniature gyroscopes.

        Analog devices make MEMS gyroscopes [analog.com] too. Nintendo could have gone to any vendor, of course.

        There's not as much market for gyroscopes as for accelerometers, hence they're more expensive. Sometimes they can be found in car satellite navigation systems as a way of increasing resolution above what GPS can offer - ever heard of a roundabout [google.co.uk]? They're useful there - and there are other applications as well. Games consoles, for instance!

        I
        • Not that it's a particularly big market or anything, but MEMS gyros are also used in R/C helicopters as a stabilization/pilot-assist device. They keep the tail of the helicopter pointed in basically the same direction, by adjusting the pitch of the tail rotor dynamically. Without a gyro, the pilot has to constantly make this adjustment in real-time, or risk putting the heli into a spin.

          For somewhere around $90-120, you can run out today and get yourself a single-axis "Silicon Micro Machine" gyro that's set
          • by Mike1024 (184871)
            The gain on most of them is adjustable, but I assume that you could set it up so that the magnitude of the signal was basically proportional to the deviation from a particular position.

            It's likely they work through PID Control, which stands for proportional, integral, derivative. If this is the case, increasing the gain makes the system respond faster, but can make the system less stable. Wikipedia [wikipedia.org] has a page on the subject.
            • by Kadin2048 (468275)
              Fascinating; thanks for the link. As much as I've used servo and feedback networks in hobby applications, I've never really delved into their design very much. It would make a lot of sense if the heli gyros used a control system like that, since they're essentially no different from any other type of industrial-control feedback loop.
  • Typically it's in my pants while watching Pamela Anderson's sweater meat bounce around like a raver on extacy. Come on! Laugh! It's funny! You KNOW you WANT to.
  • Oh CNN (Score:4, Insightful)

    by bunions (970377) on Monday November 20, 2006 @05:56PM (#16921484)

    Sony Corp.'s "Sixaxis" controller for the PS3 also has an accelerometer. The six axises the name refers to are the three dimensions of space, plus three axises of spin. The company hasn't revealed who makes the chip.


    Axises?? How can you mispluralize one of the most entertaining plurals around? Axes, dammit.
  • Who was first? (Score:5, Interesting)

    by Glacial Wanderer (962045) on Monday November 20, 2006 @06:04PM (#16921618) Homepage
    The Nintendo Wii Remote one-ups the Sony controller by including an infrared camera.

    I think they meant to say: The Sony controller dumbed down the Wii Remote by excluding the infrared camera.
  • datasheet (Score:3, Informative)

    by tonigonenstein (912347) on Monday November 20, 2006 @06:13PM (#16921770)
    Right from the source: http://www.st.com/stonline/products/literature/ds/ 11115.pdf [st.com] These things are not exactly new. They are used in the automotive sector, or for "stabilizers" in camcorders.
  • So what happens if in a game if you point the camera away from the sensor bar? For example, in tennis when you swing the remote I would imagine the camera would lose track of the bar at some point during the swing. How does the system know if you're swinging the racket in the right plane? (Or does it even know?)
    • Re:camera problems (Score:5, Informative)

      by anotherone (132088) on Monday November 20, 2006 @06:30PM (#16922016)
      It has accelerometers AND the sensor thing. Tennis actually just uses the accelerometers, it works quite well no matter where you're facing.
      • Again though, that tells the game if you have the timing of the swing down, not to mention the acceleration and rotation, but what about the accuracy? How does it know whether the racket (Wiimote) was swung in the right plane so that the ball doesn't go above or below it?
  • by abscr (645403) on Monday November 20, 2006 @06:27PM (#16921970)
    so I can finally keep Mario from falling down the bottomless pit by jerking the controller above my head?
  • A few years ago I went to a talk on making a position sensor for fire fighters. Remember GPS works poorly inside buildings. The idea was to use microaccelorometers to track the position of the firefighter inside a building. The author mentioned they placed the device in the heel of the boot. This way they could sense when the motion was stopped so they could rezero the accelerometers. Eliminating zero drift has always been a problem with guidance systems.
  • Who decided that things that measure tilt should be called "accelerometers"? They can be used to measure acceleration, but thats not what the sensor directly measures!
    • by JCondon (1029908) on Monday November 20, 2006 @07:13PM (#16922548)
      Tilt sensors indirectly measure orientation (tilt). They look at the change in the gravitational acceleration vector with respect to the sensor coordinate frame.
    • by CityZen (464761)
      They're called accelerometers because that's what they measure.

      If you hold them still, then they measure acceleration due to gravity. That is, they "point down," thus measuring tilt.

      If you wave them about, then it measures the applied force plus acceleration due to gravity. If you know which way is down, then you can subtract out gravity and just measure the applied force. If you don't, well, then, good luck.
      • by xenocide2 (231786)
        I guess my big hangup is that an object experiences 1G at rest, and 0G in a freefall.
        • by Spunkee (183938)
          1G is equal to earth's gravitional acceleration. I don't think anything can experience 0G. Everywhere in known space something is causing a gravitational force.
          • by LKM (227954)
            Hm... but if you're falling, you're accelerating at 1G, thus cancelling out gravitation, aren't you?
            • Hm... but if you're falling, you're accelerating at 1G, thus cancelling out gravitation, aren't you?

              Sort of... When you are in a free fall you are being affected by the Earth's gravity, which in your case is 1G. When you stand on the ground, you are still being affected by 1G, but this time you are fighting that pull with your legs. So you are not cancelling out gravity, a freefall just means you have nothing to counter your 1G acceleration/freefall. Even if you leave the planet, gravitational forces still affect you. The really cool thing is that gravity is instantaneous - meaning that it is not a wave t

    • by Spunkee (183938)
      To tilt requires acceleration. They measure acceleration first. Then tilt indirectly.
  • "Put me down Steve"
  • I think this is another ploy in the american plan to monitor the politically divergant opinions of europeans and other non-americans. :P
  • by bVork (772426) <rpantella+slashd ... com minus author> on Tuesday November 21, 2006 @07:01AM (#16928320)
    I found this absolutely fascinating video that shows exactly how the sensor bar detects the Wiimote's position - and how you can possibly hack up your own "sensor bar": http://www.youtube.com/watch?v=JTGSkYRDpWY [youtube.com]
  • The Nintendo Wii Remote one-ups the Sony controller by including an infrared camera. It picks up signals from a sensor bar the owner attaches to the television set. This enables the remote to "know" where it is in relation to the screen, so the player can use the controller to point to things on the screen -- a useful feature in shooting games (and a lot of games are shooting games).

    Here's another interesting piece of information. According to this source [wordpress.com], the sensor bar is not even communicating with th

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