B.E. Mechanical Engineering 3rd Year
Have you ever wondered why you don’t tip off a running bicycle but fall off very easily on a stationary one? There is a force that keeps your bicycle from falling when it is in motion. The same principle governs the mechanics of a gyroscope. A gyroscope (gr. guros = wheel; lat. scopium = scope) is a device consisting of a disc (rotor) that can spin about an axis which is itself free to alter in direction. The orientation of the axis is not affected by tilting of the mounting thus making gyroscopes ideal for use in stable balancing operations.
Fig: Working of a gyroscope
Gyroscopes have found their use in many fields ranging from aeronautics to motion sensors in mobile phones. The gyroscopes used in mobile phones, called MEMS (Micro Electro Mechanical System) gyroscopes, sense the angular rotation of the device. It consists of a tiny mass, which is constantly moving, surrounded by plates, attached to springs. These are manufactured by Nano-manufacturing techniques such as nanolithography. When the device experiences angular motion, a force (called the Coriolis force) displaces the mass in a direction perpendicular to both angular velocity and linear velocity. This motion of the mass changes the distance between the plates and the mass which in turn changes the capacitance and thus angular rate is sensed. In aircrafts, gyroscopes and accelerometers are the main components of the autopilot system. In the past, mechanical gyroscopes were used in airplanes, but they are today completely replaced by laser gyroscopes or solid state gyroscopes.We study the motion of a gyroscope by a simple example. Consider a wheel and axle being supported from its end by a cord hung from the ceiling as shown in the figure. At rest, if we let go of it, it will rotate downward and stay in a vertical orientation. This is due to the fact that gravity exerts a torque on it and causes it to rotate about the point of contact with the cord. On the other hand, if the wheel and axle is rotating, it will not rotate downward when we let go. In contrast, it will rotate in a horizontal plane about the point of contact. This is because the rotating wheel has angular momentum whose direction is along the axis of the wheel. The moment of gravity on the wheel has a direction perpendicular to that of the angular momentum vector. This torque tries to changes the angular momentum of the wheel and axle. (This is analogical to force tending to change linear momentum.) But, being perpendicular to each other, the torque can only change then direction of the angular momentum. Thus the axis of the wheel rotates due to the torque, and this motion is called precession.
Fig: Gyroscope used as flight turn coordinator
Fig: Schematic of a MEMS gyroscope