In the world of navigation having precise measurements of orientation is crucial, for ensuring efficient movement. One popular tool for measuring an objects acceleration and angular rate is the measurement unit (IMU). Navigation grade IMUs are widely used in aerospace, land, sea and military applications. This article aims to give you an understanding of Navigation Grade IMUs by discussing their definition distinguishing features from IMU types, practical applications, calibration methods and integration, with navigation systems.
What is a Navigation Grade IMU?
An IMU is an electronic device consisting of accelerometers and gyroscopes that measure the linear acceleration and rotational velocity of an object. The combination of these measurements provides information on the object’s position and orientation relative to a reference frame. Navigation Grade IMUs are designed to provide highly accurate and stable measurements of an object’s orientation.
The main difference between Navigation Grade IMUs and other types of IMUs is in their level of accuracy, stability, and reliability. Navigation Grade IMUs offer higher accuracy and stability than commercial-grade IMUs, making them suitable for applications where highly precise positioning is required.
Types of IMUs
There are several types of IMUs available in the market today, with variations in design, technology, and accuracy. These include MEMS-based IMUs, Fiber Optic Gyroscopes (FOGs), and Ring Laser Gyros (RLGs).
MEMS-based IMUs utilize micro-electromechanical systems (MEMS) technology to detect changes in motion. They are typically small, lightweight, and relatively inexpensive. However, their accuracy is limited, making them unsuitable for high precision applications.
Fiber optic gyroscopes (FOGs) are based on the Sagnac effect, which states that the rotation of a closed loop interferometer causes a phase shift in light traveling through it. This effect is used to measure the rate of rotation. FOGs offer higher accuracy than MEMS-based IMUs, but they are expensive and relatively large.
Ring Laser Gyros (RLGs) use the same principle as FOGs but replace the fiber with a laser beam circulating around a ring cavity. RLGs offer the highest accuracy of all the IMU types but are also the most expensive and largest.
Differences Between Navigation Grade IMUs and Other Types of IMUs
Navigation Grade IMUs differ from other IMUs in several ways, including accuracy, stability, reliability, and cost.
Accuracy: Navigation Grade IMUs provide highly accurate measurements of an object’s orientation, with error rates as low as 0.01 degrees per hour. In contrast, commercial-grade IMUs typically have error rates ranging from 1-5 degrees per hour.
Stability: Navigation Grade IMUs also offer high stability, meaning that they maintain their accuracy over long periods. This stability is achieved by using advanced technologies, such as closed-loop feedback systems, environmental compensation, and internal calibration procedures.
Reliability: Navigation Grade IMUs are designed to operate in harsh environments and withstand extreme conditions, making them highly reliable compared to commercial-grade IMUs.
Cost: Navigation Grade IMUs are more expensive than commercial-grade IMUs due to their high precision, advanced technology, and reliability.
Uses of Navigation Grade IMUs
Navigation Grade IMUs are used in various applications, including:
Aerospace Industry: Navigation Grade IMUs play a crucial role in the aerospace industry, where they are used in spacecraft navigation, attitude control, and guidance systems. They are also used in aircraft autopilots, drones, and unmanned aerial vehicles (UAVs).
Autonomous Vehicles: Navigation Grade IMUs are used in autonomous vehicles, including self-driving cars and unmanned ground vehicles (UGVs). They provide precise information on the vehicle’s orientation, which is essential for accurate positioning and control.
Marine Navigation: Navigation Grade IMUs are used in marine navigation systems to provide accurate information on a ship’s orientation, heading, and motion. They are also used in offshore drilling platforms and underwater vehicles.
Military Applications: Navigation Grade IMUs are used extensively in military applications, including missile guidance, target tracking, and reconnaissance.
Calibration Techniques for Navigation Grade IMUs Navigation Grade IMUs require calibration to maintain their accuracy and stability. Calibration involves correcting errors in the IMU measurements caused by factors like temperature, bias, scale factor, and misalignment. The most common calibration techniques include:
Bias Calibration: This involves measuring the output of the IMU when it is stationary to determine any offset or bias that affects the measurements.
Scale Factor Calibration: This involves measuring the changes in output when the IMU is subjected to different acceleration levels to correct for scaling errors.
Misalignment Calibration: This involves measuring the difference between the IMU measurement axis and the reference axis to correct for misalignment errors.
Integration with other Navigation Systems
Navigation Grade IMUs are often integrated with other navigation systems to improve their accuracy and reliability. The most common systems include:
GPS: Navigation Grade IMUs can be integrated with GPS to provide accurate positioning information. The IMU measurements can be used to compensate for GPS errors caused by factors like ionospheric delay, signal blockage, and multipath.
GNSS: Global Navigation Satellite Systems (GNSS) like Galileo, GLONASS, and BeiDou can also be integrated with Navigation Grade IMUs to provide accurate positioning information. The IMU measurements can be used to compensate for GNSS errors caused by factors like atmospheric interference, satellite clock error, and receiver noise.
Inertial Navigation Systems (INS): Navigation Grade IMUs can be integrated with INS to provide a high-precision navigation solution. An INS uses the IMU measurements to estimate the vehicle’s position, velocity, and attitude, which are then fused with other sensor data to provide a complete navigation solution.
Conclusion
Navigation Grade IMUs play an essential role in various applications, including aerospace, land, sea, and military navigation. They differ from other types of IMUs in their level of accuracy, stability, reliability, and cost. Navigation Grade IMUs offer higher accuracy and stability than commercial-grade IMUs, making them suitable for high precision applications. Calibration techniques like bias calibration, scale factor calibration, and misalignment calibration are used to maintain their accuracy, while integration with other navigation systems like GPS, GNSS, and INS improves their performance.
As technology advances, Navigation Grade IMUs will continue to play a critical role in navigation systems, providing accurate and reliable orientation measurements for safe and efficient movement.
