How Good Is Your Gyro ?

Q: In my design course in aerospace engineering, my group is building an unmanned aerial vehicle (UAV), which means the aircraft must fly on its own. My task is to choose a gyro for the aircraft to measure angular rates about the aircraft axes. The problem is that when I look at the spec sheet for gyros there are many numbers that I do not know how to interpret. Can someone at IEEE Control Systems Magazine please explain gyro specifications? Mohinder and Angus: We’re happy to answer your question, which is really a great question. Rate gyroscopes, which I will call “gyros,” are sensors that measure angular (rotational) rates with respect to an inertial frame of reference. An inertial reference frame is a coordinate frame in which Newton’s laws of motion are valid. Gyros are used in many applications, including inertial navigation, robotics, and automobiles. In the inertial navigation system (INS) of a vehicle, the equations of motion are numerically integrated using the outputs of gyros and accelerometers to estimate the vehicle’s attitude, position, and velocity. Because inertial navigation has co-evolved with the supporting gyro technology, the methodologies for relating gyro performance to mission requirements are fairly well established. These methodologies may not be directly transferable to other applications, such as robotics, automotive systems, or pointing stabilization of a telescope. Instead of navigation, these applications use gyros for stabilization. However, the same gyro error models and specification methods used for inertial navigation can be used in stabilization applications. Before defining the gyro error model and its specifications, we need to define gyro classifications and types. Gyros can be classified according to the physical phenomena involved in their operation. The earliest types of gyros used spinning wheels and the law of conservation of angular momentum. These devices have been redesigned in several ways to reduce angular drift errors due to bearing torques, such as “floated” designs, electrostatic suspension to reduce bearing torques, and dynamically tuned flexible structures to decouple bearing forces from gyro torques. More recent designs use sensing mechanisms based on alternative physical phenomena, such as How Good Is Your Gyro?