Inertial measurement unit
An inertial measurement unit ( English inertial measurement unit , IMU ) is a spatial combination of several inertial sensors such as accelerometers and angular rate sensors . It provides the sensory measurement unit of an inertial navigation system ( English inertial navigation system , INS ). In applications of IMU are any of these aircraft and missiles for air navigation . In ships, in robotics and in image stabilization , they are used for motion detection. In the case of guided missiles and unmanned aerial vehicles (UAVs), they also serve to stabilize the aircraft in terms of control technology. In the case of motorcycles, the IMU provides further principles for the interpretation of the driving dynamics by the on-board electronics. Electronic chassis, engine control, rear wheel lift protection or ABS can therefore work more precisely, also depending on the lean position as well as acceleration and pitching movement in / against the direction of travel, in each case relative to the straight line of gravity.
Correspondingly precise and long-term stable inertial measuring units are essential components of the inertial navigation systems of long-range missiles or cruise missiles and are often subject to trade restrictions and import and export restrictions.
To record the six possible kinematic degrees of freedom , an IMU has three orthogonally positioned acceleration sensors (translation sensors) for recording the translational movement in the x, y and z axes and three rotation rate sensors (gyroscopic sensors) that are positioned orthogonally to one another for the Detection of rotating (circular) movements in the x, y or z axis. An inertial measuring unit supplies three linear acceleration values for the translational movement and three angular speeds for the rotation rates as measured values . In an inertial navigation system (INS), the measured values of the IMU for the linear accelerations, after compensation of the acceleration due to gravity , the linear speed is determined by integration and the position in space related to a reference point. The integration of the three angular velocities provides the orientation in space in relation to a reference point.
To determine the integration constants , to improve the accuracy and to correct the drift of the sensors mentioned above, additional magnetometers (magnetic field sensors) and GNSS sensors are integrated in some IMUs .
Inertial measurement units can be divided into two large basic groups in terms of the type of construction:
IMU with stable platform
IMUs with stable platform ( English stable platform ) are characterized in that all the inertial sensors inside are housed on a stabilized platform in space. This structure is based on the gyroscopic instruments and also represents the older design of IMUs. The stabilized platform can be moved freely in space via a cardanic suspension and is moved in the suspension via three servomotors so that external rotary movements are compensated. The servomotors are moved in such a way that the yaw rate sensors deliver a minimal signal, while the manipulated variable of the motors is an expression for the orientation. The advantage is that the acceleration sensors, which are also accommodated on the stabilized platform, after correction by the value of the acceleration due to gravity , supply the linear acceleration information of the translational movement directly as a sensor signal. The disadvantage of this measurement setup is the mechanically complex and sensitive setup and the problem of requiring highly dynamic drive systems for the cardanic suspension.
Strapdown IMUs are characterized by the fact that the inertial sensors are permanently connected to the outer frame. There are three sensors each for the rotation rates and accelerations related to the three IMU axes. In the simple case of a linear movement in exactly one of these axes, the current position could be calculated comparatively easily by double integration of the acceleration in the direction of movement, which is to be continuously measured at a suitable sampling rate. In the general case, on the other hand, the acceleration sensors not only supply a signal in the case of linear accelerations, but also have a signal component due to the rotational movement which must be compensated for in order to determine the translational acceleration values. For this it is necessary to integrate the sensor signal values of the three rotation sensors and to use the information obtained in this way about the orientation in space to calculate the linear acceleration values from the measured values of the acceleration sensors. Only after this calculation step can the acceleration values be compensated for by the value of the acceleration due to gravity. This algorithm is called the strapdown algorithm.
The advantage of strapdown IMUs is the elimination of the mechanically complex cardanic suspension and its control. This means that significantly more compact IMUs can be implemented. In the case of low demands on accuracy and long-term stability, inexpensive IMUs of this type are also available for applications such as the flight stabilization of flight models .
Depending on the requirements for accuracy and long-term stability, different types of inertial sensors are used; the accuracy of the inertial sensors used has a clear effect on the accuracy of an inertial measuring system.
Rotation rate sensors
The rotation rate sensors used in IMUs are optical systems with high demands on accuracy and stability. They are either implemented as a fiber-optic gyroscope ( English fiber optic gyroscope FOG ) or as a laser gyroscope (ring laser, English ring laser gyroscope , RLG ).
With low stability requirements, micro-electro-mechanical systems ( MEMS ) are used, which offer the advantage of being implemented directly in integrated circuits and allowing the construction of very compact IMUs. However, the long-term stability is several orders of magnitude worse than that of optical systems: For example, the drift of a laser gyro system used in weapon systems is 0.0035 ° per hour. In the case of a MEMS-based rotation rate sensor based on semiconductors, the drift is approx. 70 ° per hour.
The acceleration sensors used in IMUs also depend on the required accuracy and long-term stability. Among other things, piezoelectric acceleration sensors based on quartz rods , which are slightly bent by the acceleration and slightly detuned an electrical oscillating circuit , are common. There are also micro-electro-mechanical systems (MEMS) for measuring acceleration, which, like the rotation rate sensors, can be implemented directly in integrated circuits.
- David Titterton, John Weston: Strapdown Inertial Navigation Technology . IEE Radar, Sonar, Navigation and Avionics Series. 2nd Edition. Institution of Engineering and Technology, 2005, ISBN 978-0-86341-358-2 .
- Oliver J. Woodman: An introduction to inertial navigation . UCAM-CL-TR-696. University of Cambridge, Computer Laboratory, 2007 ( online ).
- Jörg Böttcher: Compendium of measurement technology and sensor technology: Inertial measurement units (IMU). Retrieved September 27, 2019 .
- Introduction into Inertial Measurement Technology