Gyroscopes can be very perplexing objects because they move in peculiar ways and even seem to defy gravity. These special properties make gyroscopes extremely important in everything from your bicycle to the advanced navigation system on the space shuttle. A typical airplane uses about a dozen gyroscopes in everything from its compass to its autopilot. The Russian Mir space station used 11 gyroscopes to keep its orientation to the sun, and the Hubble Space Telescope has a batch of navigational gyros as well. Gyroscopic effects are also central to things like yo-yos and Frisbees!
In this edition, we will look at gyroscopes to understand why they are so useful in so many different places. You will also come to see the reason behind their very odd behavior!
If you have ever played with toy gyroscopes, you know that they can perform all sorts of interesting tricks. They can balance on string or a finger; they can resist motion about the spin axis in very odd ways; but the most interesting effect is called precession. This is the gravity-defying part of a gyroscope. The most amazing, and also the thing that is unbelievable about gyroscopes, is the gyroscopic bicycle wheel is able to hang in the air like this:
How can it do that? This mysterious effect is precession. In the general case, precession works like this: If you have a spinning gyroscope and you try to rotate its spin axis, the gyroscope will instead try to rotate about an axis at right angles to your force axis, like this:
The Cause of Precession
Why should a gyroscope display this behavior? It seems totally nonsensical that the bicycle wheel's axle can hang in the air like that. If you think about what is actually happening to the different sections of the gyroscope as it rotates, however, you can see that this behavior is completely normal!
Let's look at two small sections of the gyroscope as it is rotating -- the top and the bottom, like this:
When the force is applied to the axle, the section at the top of the gyroscope will try to move to the left, and the section at the bottom of the gyroscope will try to move to the right, as shown. If the gyroscope is not spinning, then the wheel flops over, as shown in the video on the previous page. If the gyroscope is spinning, think about what happens to these two sections of the gyroscope: Newton's first law of motion states that a body in motion continues to move at a constant speed along a straight line unless acted upon by an unbalanced force. So the top point on the gyroscope is acted on by the force applied to the axle and begins to move toward the left. It continues trying to move leftward because of Newton's first law of motion, but the gyro's spinning rotates it, like this:
This effect is the cause of precession. The different sections of the gyroscope receive forces at one point but then rotate to new positions! When the section at the top of the gyro rotates 90 degrees to the side, it continues in its desire to move to the left. The same holds true for the section at the bottom -- it rotates 90 degrees to the side and it continues in its desire to move to the right. These forces rotate the wheel in the precession direction. As the identified points continue to rotate 90 more degrees, their original motions are cancelled. So the gyroscope's axle hangs in the air and precesses. When you look at it this way you can see that precession isn't mysterious at all -- it is totally in keeping with the laws of physics!
Uses of Gyroscopes
The effect of all this is that, once you spin a gyroscope, its axle wants to keep pointing in the same direction. If you mount the gyroscope in a set of gimbals so that it can continue pointing in the same direction, it will. This is the basis of the gyro-compass. If you mount two gyroscopes with their axles at right angles to one another on a platform, and place the platform inside a set of gimbals, the platform will remain completely rigid as the gimbals rotate in any way they please. This is this basis of inertial navigation systems (INS). In an INS, sensors on the gimbals' axles detect when the platform rotates. The INS uses those signals to understand the vehicle's rotations relative to the platform. If you add to the platform a set of three sensitive accelerometers, you can tell exactly where the vehicle is heading and how its motion is changing in all three directions. With this information, an airplane's autopilot can keep the plane on course, and a rocket's guidance system can insert the rocket into a desired orbit!