How Motion Control Works

Moses unleashes some motion-controlled calamity.
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It's one of the craziest Old Testament moments of all time: Moses — a bona fide prophet in three (yes, three!) major world religions — raises his rod high in the air and commands the winds to part the Red Sea.

This is where it gets awesome, because the winds do exactly that; otherwise, it's just a story about an old man yelling at some water.


Now presented with an escape route, Moses leads the children of Israel across the seafloor, flanked on both sides by impossible walls of storm-pitched water. When they emerge on the far shore, the bearded one raises his rod once more and commands the waters to crash back in on the pharaoh's pursuing army.

Explanations for this magnificent feat vary, of course. Faithful literalists chalk the whole thing up to divine intervention, while others look to atmospheric anomalies or the power of myth.

But let us consider another option: motion-control technology.

Oh sure, this explanation also requires the existence of ancient technology — machinery capable of mastering the wind and diverting large bodies of water. But Moses' trusty rod stirs it all to life, so let's crack that fancy walking stick open and see what sort of technology it might contain, shall we?


As the Gyroscopes Turn

Maybe the snake ate one of these?
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Yes, gyroscopes.

You may remember this gadget from your childhood toy chest, where it lived in the dregs beside a wad of knotted string. The more industrious among you may have spun it on the floor like a top or danced it across a string before banishing it to the shadows.


But gyroscopes — gyros for short — are more than mere forgotten toys. Unlike the faceless Mr. Potato Head of your childhood, this little device plays a vital role in some of the most complex machinery on Earth — or in orbit around it. An airplane's autopilot system depends on gyros to keep the craft level, and control moment gyroscopes (CMGs) help to make sure that the International Space Station and the Hubble Space Telescope are facing the right direction as they spin around the planet.

It all comes down to a physical property known as precession, the cone-shaped motion of a spinning body's axis. Once you start a gyroscope spinning, its axle continues to point in the same direction.

The article "How Gyroscopes Work" goes into greater depth on precession — and includes some helpful illustrations. The important thing to remember is that gyros allow us to create machines capable of measuring or maintaining orientation in a physical space, a handy ability to have when your plane is, say, engulfed by clouds.

How does this factor into motion control and Moses' fabulous rod? Well, just look to the iPhone 4, Wii Motion Plus and the PlayStation Move. These smartphones and game controllers depend in part on tiny gyros that enable a device to detect movements relative to a fixed point — such as your gaming console or the smartphone in your hand.

So just imagine the bearded Hebrew prophet raising a Wii controller instead of a stick and you get the general idea.


Accelerometer? I Hardly Knew Her!

Check batteries before use.
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Moses points his rod skyward, stirring the engines of Yahweh to life. Waters part. Former slaves get their exodus on. Gyros may well factor into the design of Moses' fabulous staff, but much like your smartphone, it might also boast an accelerometer.

An accelerometer is a device that can measure the rate of acceleration, caused by gravity or by movement. Moses waves his rod around in the air; Moses throws it like a baton and catches it on the flip side. If there were an accelerometer in there, it could clock the rod's movement and speed.


Several types of accelerometers live inside our gadgets. For instance, piezoelectric accelerometers (such as the one in an iPhone) depend on materials such as crystals, which generate electric charges when stress is applied — a measurable occurrence known as the piezoelectric effect.

Other accelerometers work by calculating the electrical resistance of materials under mechanical stress or changes in resistance due to a magnetic field. More advanced accelerometers depend on nanoscale micro-electro-mechanical system (MEMS) technology.

Accelerometers in your smartphone or strapped to your gym shorts tell you how far you've run on the treadmill. Accelerometers inside your Wii remote sense the tilt, movement and speed of your frantic arm movements — and when these inputs are relayed to the gaming console itself, your motions influence the action in the game.

So was Moses' rod equipped with accelerometer nanotechnology? One movement might cause the waters to part, while another causes them to collapse in a torrent of biblical payback.


Lights, Cameras, Action

No keyboard or controller required.
Enamul Hoque/Rod Steele/Getty Images

Here's another possibility: What if Moses' magnificent rod really was nothing more than a walking stick? Perhaps all that hidden, sea-parting machinery merely activated when the bearded prophet made the appropriate motions.

Again, the sort of technology capable of parting the sea is a little bit beyond even our modern age — but machines that respond to our movements? Why those are all around us.


Visit the local grocery store, where you'll notice a little motion detector that uses a simple form of radar to know when to activate the door. That motion detector emits microwave radio energy (or ultrasonic sound waves), which bounces back off the floor and returns to the sensor. Walk in front of its motion detector and those energy waves will rebound a little earlier — right off your body. The detector registers a change in the time it takes for the waves to return, so it opens the doors to let that changer through.

Your motion controls an automatic door. The same radar technology turns on security floodlights when someone walks by or trips a home security alarm when a sufficiently large object moves through a room. Light-based systems have a beam of light or a laser beam that spans the space between a light source and a sensor. Break that beam with your movements and the alarm blares.

The Xbox 360's Kinect sensor represents an even more remarkable motion-detection system. The gadget relies on an infrared projector and a sensor to visualize even a dimly lit man cave of a gaming room in 3-D. Once the Kinect system maps out the room, it moves on to the humans.

The Kinect detects and tracks 48 points on each player's body and maps them to a digital reproduction of that player's body. Even a casual shrug of the shoulders — or the dramatic raising of a mystical Old Testament rod — becomes a control input.

So there you have it: three key ways that motion-control systems allow us to boss around gadgets and programs with our body movements. Any or all of them could conceivably give Moses' rod the power to activate a sea-parting super machine.

Of course, Moses also turns the rod into a snake at one point. You're on your own figuring that trick out.


Lots More Information

Robert Lamb, Senior Staff Writer
HowStuffWorks 2009

Author's Note: How Motion Control Works

I drafted Moses to appear in this article early on in the research process. See, the science is fascinating, but the technology is also ubiquitous. We move our bodies and watch machines obey us every day. Big deal, right?

So, I turned to Moses, that famed figure of Hebrew, Christian and Islamic tradition. You also might remember him as the wizardly looking dude played by Charlton Heston in "The Ten Commandments" or voiced by Val Kilmer in the animated film "The Prince of Egypt." In working him into the article, I found it rather nifty to think that while modern technology could facilitate a rod-based control system, a Kinect-like system could just as easily depend on nothing more than a wooden stick.

Related Articles

  • Banks, David. "Playstation's Move Makes Motion Controlled Gaming Fun, Not Frustrating." WIRED: Geek Dad. Nov. 2, 2010. (May 4, 2012)
  • "Control Moment Gyroscope Platform." (May 4, 2012)
  • SENSR. "Practical guide to Accelerometers." (May 4, 2012)