Gyros are one of the sensor types used in most inertial navigation systems (INS). One of the other sensor types used in most inertial navigation systems are accelerometers, which are great at measuring straight-line motion, but they’re no good at rotation—that’s where gyros come in. Gyros don’t care about linear motion at all, only rotation. When describing different inertial navigation systems the term ‘gyro’ can mean different things, depending on what type of system is being described.
In a traditional sense, a gyroscope employs one or more spinning rotors held in a gimbal or suspended in some other system that is designed to isolate it from external torque. That type of gyroscope works because once the rotor is spinning, it wants to maintain its axis or rotation. In other words, if you projected a line through the spin axis of the gyro, no matter how you try to twist and turn the gyro the projected line would always try to stay pointing towards the same spot. Obviously the gyro could be forced to move if you could apply a torque to it—but that’s what the gimbal is designed to prevent.
A gimbal uses a number of concentric rings mounted inside one another that are connected via orthogonally arranged pivots. This design allows the gyro to freely rotate in three-axes, assuming the rings aren’t in gimbal lock. Gimbal lock occurs when two axes become aligned. In this state, the gimbal has two degrees of freedom instead of three, so it is possible external torque applied in a certain direction might affect the axis of rotation.
Because the gyro’s rotor wants to maintain its initial axis of rotation, sensors can be mounted to the gimbal to measure the relative change in orientation of the external frame to which it is attached. In this way, it’s possible to maintain a picture of how the external frame is orientated relative to the gyro’s axis. The image on the left illustrates this.
The gyros used in strap-down navigators, don’t suffer from gimbal lock. That’s because they’re not gyros in the traditional sense of things. Instead, they are MEMS devices that measure angular velocity—typically in units of °/s (said: degrees per second). So, regardless of the direction, a MEMS gyro is pointing, as long as it’s not rotating about its measurement axis, it will output a value of 0 °/s. If however the gyro was rotating about its measurement axis and taking about one second to perform each revolution, it would output a value of 360 °/s.
This would be positive or negative depending on the direction of the rotation.
MEMS angular rate sensor
MEMS (microelectromechanical system) gyros come in many shapes and sizes. The measurement axis of this angular rate sensor is shown by the red arrow. Rotation in the direction of the arrow gives a positive value While the opposite direction gives a negative measurement. Linear movement isn’t registered.
Angular rate sensors (gyros) measure angular rate in o/s (degrees per second). They do not “measure” direction. although you can use their measurements to work out what direction the sensor is facing if you know which way it was facing to begin With. This sensor and the one to the left would both read 0 o/s.
This sensor is rotating to the right and would produce a positive value. The measurement output depends on how fast it is rotating—the higher the number, the faster the rotation. If the sensor saw an average value of 90 o/s for 0.5 seconds, we could work out that the sensor had rotated 45o clockwise.
By mounting three gyros on three mutually perpendicular axes it’s possible to keep track of an object’s orientation in 3D space. When combined with accelerometers it’s possible to track the relative position, orientation and velocity of an object—and as long as we know the start position, we can work out the current position.
From this you can see that just as with accelerometers, a gyro by itself doesn’t tell the INS which way it is orientated. When it’s first powered up, all the gyro knows is how fast it’s rotating. It is the job of the INS to keep track of all those measurements. So if the INS sees an average velocity of 360 °/s for exactly 0.25 seconds about the z-axis, it knows that regardless of what direction it was pointing to start with, it has now turned through 90° (360 °/s ÷ 0.25 s = 90°). Of course, if the INS knew it was facing north before the movement and it also knows positive gyro values indicate clockwise rotation, then it can easily work out that it is now facing east.
Just as it’s normal to use three accelerometers, it’s normal for the INS to contain three gyros orientated to measure rotation about three mutually perpendicular axes. In this way the INS can measure its orientation in 3D space. It does this using a process called dead reckoning.
