Gyroscopes, Not Just Toys Anymore by Paul Doherty
At one time or another, you've probably played with a toy gyroscope, a wheel on an axle mounted inside a frame. To make the wheel spin, you wind a string around the axle, and then pull. Once it's spinning, the gyroscope behaves somewhat like a living thing. If you balance it precariously on a fingertip, it will perform a twirling dance, yet still remain improbably balanced. If you try to twist the spinning gyroscope around one way, it will resist your twisting and perversely move in a different direction.
Playing with gyroscopes and watching their apparently strange behavior has entertained children for decades. However, gyroscopes are more than just toys. They are also important parts of the navigation systems of aircraft and spacecraft. In this article, I will discuss how aircraft and spacecraft use gyroscopes, and then illustrate how the navigation gyroscopes work by showing you a few simple experiments you can perform with toy gyros.
In the weightless environment of the space shuttle, a spinning toy gyroscope was recorded on videotape. The gyro spun around an axis that kept pointing toward the same distant star. Even when an astronaut pushed on the gyroscope, it stubbornly maintained the orientation of its axis as it flew across the cabin. In the absence of twisting forces, a gyroscope 's axis will always point in whatever direction it was pointing when you started it spinning.
If you try this experiment on earth, you will discover something quite different: if you place a spinning gyroscope with its axle horizontal and with one end of the axle on a small stand, the axle of the gyro will not continue to point in the same direction, but will move around in a horizontal circle.
The difference between your experiment on earth and the experiment aboard the space shuttle is that your gyroscope is subjected to a twist by a pair of forces: gravity pulling down on the gyroscope and the small stand pushing up on it. A pencil placed in this position would respond to this pair of twisting forces by crashing to the floor. But the axis of a spinning gyroscope responds to the twisting pair of upward-downward forces by moving sideways around in a circle. The circling motion of the gyroscope axle is called precession.
Engineers who want a gyroscope axle to keep pointing in the same direction must eliminate all twisting forces on the gyroscope. They do this by supporting the gyroscope axle at both ends in a mount called a gimbal. A gimbal is a set of frames mounted in bearings. It not only keeps the twisting forces on the gyroscope small; it also allows the gyroscope axis to swivel and point in any direction. A gimbal-mounted gyro acts like the gyro on the space shuttle&emdash; it keeps pointing in a constant direction. It is this ability that makes gyroscopes important elements in navigation systems of aircraft.
An aircraft pilot relies on a gimbal-mounted gyro when consulting an artificial horizon, a device that shows the pilot which way is up. Now you may think that it's silly to ask which way is up. Since you can feel gravity through the seat of your pants, the answer seems obvious. My friend Jeff thought so too &endash; until one night when he was on a jet training flight over Atlanta. " I was flying up through a cloud," he says," and I was tremendously surprised when I broke out of the top of the cloud to find the lights of the city above me." Jeff had trusted his human sensors to tell him which way was up. Deprived of their usual visual clues, they had been fooled by the acceleration of the airplane. He was upside down and accelerating toward the ground when he broke through the cloud. After that, he never relied on his own perceptions to determine which way was up, but trusted his artificial horizon instead.
An artificial horizon contains a gimbal-mounted gyroscope which can point in any direction with respect to the aircraft. The gyroscope's wheel is spun by either an electric motor or by air blown across the rotor. When the gyroscope starts spinning its axle is perpendicular to the ground. When the plane banks to the left, the gyroscope swivels in its frame so that the axle continues to point up. Another way of looking at the situation is that the gimbal allows the plane to rotate about the gyroscope, while the gyroscope's axle remains stubbornly stationary.
The instrument readout on an artificial horizon is a ball with a black hemisphere representing the ground and a white hemisphere for the sky. The line between the black and the white is the artificial horizon. When the airplane banks to the left, the artificial horizon stays horizontal. From the point of view of the pilot, who rotates with the airplane, the artificial horizon seems to rotate to the right&emdash;just like the real horizon. When the plane dives, the pilot sees the black hemisphere rise.
Unfortunately, the gyroscope doesn't really orient itself with respect to the earth. A gyroscope &emdash; whether it is spinning on your table top or in an artificial horizon &emdash; is orienting itself relative to the distant stars(See "Spinning the Universe"). As the earth spins, the stars rise and then set. After a twelve-hour flight, a star that was originally overhead is underfoot, and a pilot following a perfect gyroscope would be flying upside down. The solution to this problem is to twist the gyroscope around slowly so that it follows the earth. The mount for the gyroscope in an artificial horizon is a combination of a gimbal, a pendulum, and a damping mechanism. The twisting of the pendulum weight by the gravity of the earth combined with friction inside the artificial horizon makes the gyroscope slowly precess so that it follows the earth rather than the stars.
You can get an idea of how an artificial horizon combines a gyroscope, a pendulum, and damping by experimenting with a toy gyroscope. Tie a six inch string to one end of a toy gyroscope. Start the toy gyroscope spinning as fast as you can. Hold the gyroscope axle horizontal to the ground and hold on to one end of the string, then let go of the gyro. The gyroscope will remain horizontal and slowly spiral down as frictional twisting forces damp its motion. The gyroscope axle ends up perpendicular to the earth just as the gyroscope does in an artificial horizon.
Unfortunately, accelerations of the aircraft can fool the pendulum in an artificial horizon. If the airplane stays in the same rapid turn for over a minute, the pendulum will be deflected to a new, non-vertical position. Luckily, high acceleration maneuvers are rarely kept up for a long enough time to substantially affect the artificial horizon. To decrease problems associated with gyroscope precession, and with the affects of accelerations on artificial horizons, aircraft use a complete inertial navigation system.
An inertial navigation system uses at least three gyroscopes and three accelerometers. It keeps track of every acceleration and rotation of the aircraft and figures out not only which way is up, but also where the airplane is. Modern inertial navigation systems are so accurate that after a transcontinental flight they can correctly determine on which end of the runway a jet is parked.
So far, we have talked about using gyroscopes only to detect orientation. When you hold a spinning gyroscope by its frame and turn it over you can feel the gyroscope resisting your efforts. They can also be used to control orientation. You can experience the push of a rotating gyroscope at the Exploratorium exhibit Bicycle Wheel Gyro. (Or you can build a version of this exhibit from a bicycle wheel(see side box)). In this exhibit, you sit on a rotating chair and hold a spinning bicycle wheel with its axis vertical. When you try to twist the wheel upside down, the bicycle wheel gyroscope pushes on you and starts you spinning. Flip it back and you stop. You can rotate in either direction just by twisting the gyroscope. When you try to twist this gyroscope, it precesses and pushes back on you, making you spin.
To build your own Bicycle Wheel Gyro exhibit, you need the front wheel of a bicycle, a rotating chair or stool, and two file handles from the hardware store. Screw the file handles on to the axle of the bicycle wheel. You now have a heavy gyroscope that you can spin up to a high speed by brushing your hand along the tire tread. The file handles will give you the good grip you will need to keep hold of the spinning wheel. Sit on the chair, set the wheel spinning and hold the axle vertically. Flip the axle over, and notice that you start spinning.
Some spacecraft twist gyroscopes to control their orientation. For example, the Soviet space station Mir uses six, 130 kilogram, rapidly-spinning gyroscopes to control its orientation. The American space station Skylab also used gyroscopes for orientation control, a situation that had unexpected consequences for the astronaut's exercise program. Initially, Skylab's gyros presented a maintenance problem &emdash; they kept wearing out long before their predicted lifetimes. Something was overworking the gyros. After investigating, engineers realized that some of the overwork was due to astronauts who had discovered, much to their delight, that they could jog around inside a ring of storage lockers. Each time a foot pushed off the wall to accelerate an astronaut on his way, a twist was given to the space station. The gyroscopes had to correct this. No one had accounted for astronaut jogging when the gyroscopes were designed.
Smaller spacecraft do not use large, heavy gyroscopes for orientation control. Instead, they fire rocket motors. However, rocket motors use up irreplaceable fuel, while gyroscopes can be powered by solar cells. In addition, rocket motors pollute the clean vacuum of space, which is required for many experiments performed in space. Gyroscopes on the other hand, keep space clean.
One reason that gyroscopes are not usually used to control spacecraft orientation is due to the fine print in the description of the behavior of gyroscopes: "A gyroscope keeps its axle pointed in the same direction, in the absence of torques." Torques are the twisting forces mentioned earlier. When you twist the lid off a peanut butter jar, you are applying a torque to the lid. If there are any torques on a gyro, its direction of spin will precess and the gyros axis will point in a new direction. All mechanical gyroscopes in use today have some frictional torques on them: either from the bearings in their gimballed mounts, or from air drag against the moving gyroscope. Over hours or days these frictional torques cause the gyroscopes to precess away from their original headings. A spacecraft which depended on the original heading would then be in trouble.
You can dramatically demonstrate of the effects of frictional torques on a gyroscope with an American football. Rest the football on the ground, and spin it very fast. As the spinning ball rubs against the ground, friction exerts a twisting torque that will cause the ball to stand up on one end as it spins.
Some spacecraft, such as the Pioneer series launched by the United States, solve the problem of frictional torques by spinning the entire spacecraft. A Frisbee® is a flying toy which holds its orientation as it flies by spinning. It illustrates nicely how a spinning spacecraft holds its orientation. Just try to fly a Frisbee® without spinning it, and watch what happens.
Frictional torques can almost be eliminated when an object spins in the vacuum of space. Once it is spinning, the whole spacecraft becomes a gyroscope and holds its orientation constant. When a spacecraft spins to hold its orientation, it does not need an inertial navigation system. Without an inertial navigation system, the spacecraft design becomes much simpler, and there are fewer things to fail.
Unfortunately, spinning a spacecraft does not entirely eliminate complexity. Instead, the complexity is shifted to other parts of the spacecraft design. After all, on a spinning spacecraft, how can you keep a large parabolic radio antenna pointed at the earth to send data and receive instructions? You have to "de-spin" the antenna. The non-rotating antenna is connected to the rotating part of the spacecraft through bearings, but that introduces a new difficulty: getting electrical signals through the rotating connector.
Another problem with rotating spacecraft involves the craft's solar cells. A spinning craft cannot have large solar panels facing the sun, unless the panels are also de-spun. But if you de-spin too many parts of the spacecraft, you might as well build a non-rotating spacecraft. Pioneer spacecraft that go toward the sun are covered with solar panels. As the spacecraft rotates, some of these panels are always facing the sun. Pioneer spacecraft that go away from the sun carry radioisotope thermal generators and do not depend on solar power.
One other important point: spinning spacecraft cannot be used when there are astronauts on board, or the astronauts will get very dizzy. Unless, of course, the spacecraft is large enough that the spin can be used to produce what is known as artificial gravity. Such spacecraft are larger than we can currently produce.
The spinning Pioneer spacecraft are among the most reliable spacecraft ever launched: Pioneers 6 through 9 were designed to live for a year, but most of them have astounded even their designers by continuing to work and return data, twenty years later. Pioneers 10 and 11 gave us our first close-up looks at Jupiter and Saturn and today continue to send back data from the outer fringes of the solar system, spinning all the time to keep their orientation.
Gyroscopes are fun to play with. Their behavior is just puzzling enough to keep kids like me interested for hours. But gyroscopes also have a serious side. A commercial airliner contains several gyroscopes to tell it which way is up and where it is located. The spacecraft that cross the darkness of the night sky in majestic silent arcs either contain gyroscopes or are themselves large gyroscopes, rotating to maintain their orientation with respect to the distant stars.
Scientific Explorations with Paul Doherty
20s September 2006