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Inertial Technology

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Inertial Technology

An inertial navigation system (INS) includes a computer to process navigation computations, an inertial sensor sub assembly or inertial measurement unit (IMU) containing accelerometers and gyroscopes orthogonally mounted on three axis. The INS is initially provided with its position or in some cases velocity from another source (an alignment), and thereafter computes its own position and velocity by integrating information received from the accelerometers and gyros. The primary advantage of an INS is that it requires no external references in order to determine its position, orientation, or velocity once it has been aligned. All inertial navigation system will exhibit drift as even the best accelerometers and gyros have some noise in their measurements. Usually the drift in position or velocity of an inertial navigation system is constrained, or controlled through the use of a measurement that can control the drift. In most cases this external aiding computation is made within a Kalman filter.

An INS can detect a change in its geographic position (a move east or north, for example), a change in its velocity (speed and direction of movement), and a change in its orientation (rotation about any axis). It does this by measuring the linear and angular accelerations applied to the system.

Since it requires no external reference (after initialization), it is immune to jamming and deception and is most useful where other systems are impacted by their environment. One example of this would be GPS under thick forest canopies. Signals from the GPS satellites cannot easily penetrate the leaves and branches, so precise positioning in the woods is very difficult to achieve with just GPS.

Gyroscopes measure the angular rotation in the inertial reference frame. By using the original orientation of the system and integrating the angular rotation at a specific rate, the system’s orientation is known at all times.

Accelerometers measure the linear acceleration in the inertial reference frame. By sensing the change in acceleration over a known time the displacement, or movement of the system is known at all times.

Because these measurements and calculations are with respect to the “inertial reference frame” and the end user is usually interested in where the car, ROV or airplane is, we translate the inertial reference frame through an “earth” reference frame to a “body” or “vehicle” reference frame to allow us to navigate or position these vehicles in known geographical coordinates. All of these calculations within Zupt’s systems are contained within what we call our “navigation solution”.

The type of sensors we use to sense angular rotation will vary on the application. We use fiber optic (FOG), ring laser (RLG) and many other types of gyroscopes. The same is true for accelerometers as we use a variety of force rebalance, pendulum rebalance or MEMs accelerometers depending on the application and quality required. The quality of an IMU does not depend on the type of sensor, but the quality of the sensor. It is possible to buy very good FOGs, it is also possible to buy less capable FOGs. The same is true for RLGs, Dynamically tuned gyros (DTG), and other rate sensors.

The industry has a lose definition of the grades of inertial navigation systems:

Strategic, Navigation, Tactical or Consumer

The table below provides some guidelines as to the capability and costs of these systems:

Inertial Sensor Grade Price Range Position Error(km/h) Gyro Bias Error (°/h) Accelerometer Bias Error (mg)
Strategic >$500K <0.03 0.00001 0.001
Navigation $90K to >$130K <1.7 0.005 to 0.015 0.03 to 0.050
Tactical $6K to $35K 18 to 40 0.5 to 10 0.5 to 1.0
Consumer >$350 >40 1000 20

IMU’s are sometimes defined by discussing the gyro specification:

  • Bias usually stated in – °/hr
  • Short term bias stability – °/hr
  • Noise – angle random walk – °/sqrt hr
  • Scale factor accuracy – ppm
  • Bandwidth (50Hz or 100Hz) – Hz

When a manufacturer describes an IMU as a 1°/hr IMU they are not referring to the capability of the IMU to derive “heading” to +/-1° or +/-1°/hr, they are referring to the bias or drift of the rate sensors within the IMU.

For more help understanding inertial navigation systems, or specifying the correct capability and price range of an IMU or INS for yourapplication contact Zupt