Inertial Measurement Units are essential for UAV control and navigation. This article explores the technologies available and the suitability for each type of UAV application.Published: 30 Nov 2018 | Author: Mike Rees
An Inertial Measurement Unit (IMU) is an electronic device that uses accelerometers and gyroscopes to measure acceleration and rotation, which can be used to provide position data.
IMUs are essential components in unmanned aerial systems (UAVs, UAS and drones) – common applications include control and stabilization, guidance and correction, measurement and testing, and mobile mapping.
The raw measurements output by an IMU (angular rates, linear accelerations and magnetic field strengths) or AHRS (roll, pitch and yaw) can be fed into devices such as Inertial Navigation Systems (INS), which calculate relative position, orientation and velocity to aid navigation and control of UAVs.
IMUs are manufactured with a wide range of features, parameters, and specifications, so the most suitable choice will depend on the requirements for a particular UAV application. This article outlines some of the key options and considerations, such as the underlying technology, performance, and ruggedness, in selecting an IMU for drone-based applications. It also highlights some of the leading IMU manufacturers for UAS (jumps to this section).
There are many types of IMU, some of which incorporate magnetometers to measure magnetic field strength, but the four main technological categories for UAV applications are: Silicon MEMS (Micro-Electro-Mechanical Systems), Quartz MEMS, FOG (Fiber Optic Gyro), and RLG (Ring Laser Gyro).
Silicon MEMS IMUs are based around miniaturized sensors that measure either the deflection of a mass due to movement, or the force required to hold a mass in place. They typically perform with higher noise, vibration sensitivity and instability parameters than FOG IMUs, but MEMS-based IMUs are becoming more precise as the technology continues to be developed. MEMS IMUs are ideal for smaller UAV platforms and high-volume production units, as they can generally be manufactured with much smaller size and weight, and at lower cost.
FOG IMUs use a solid-state technology based on beams of light propagating through a coiled optical fiber. They are less sensitive to shock and vibration, and offer excellent thermal stability, but are susceptible to magnetic interference. They also provide high performance in important parameters such as angle random walk, bias offset error, and bias instability, making them ideal for mission-critical UAV applications such as extremely precise navigation. Higher bandwidth also makes FOG IMUs suitable for high-speed platform stabilization. Typically larger and more costly than MEMS-based IMUs, they are often used in larger UAV platforms.
RLG IMUs utilise a similar technological principle to FOG IMUs but with a sealed ring cavity in place of an optical fiber. They are generally considered to be the most accurate option, but are also the most expensive of the IMU technologies and typically much larger than the alternative technologies.
Quartz MEMS IMUs use a one-piece inertial sensing element, micro-machined from quartz, that is driven by an oscillator to vibrate at a precise amplitude. The vibrating quartz can then be used to sense angular rate, producing a signal that can be amplified and converted into a DC signal proportional to the rate. Quartz MEMS technology features high reliability and stability over temperature, and tactical-grade quartz MEMS IMUs rival FOG and RLG technologies for SWaP-C (size, weight, power and cost) metrics. These factors make it ideal for inertial systems designed for the space- and power-constrained environments of UAVs.
The performance and accuracy of an IMU are influenced by a combination of factors, including the sensor technology, the thermal properties of the packaging, and the software used. The following parameters can be used when comparing the performance and accuracy of specific IMUs, to help determine suitability for a given UAV application:
These factors are dependent on the technologies used in the IMU and the physical properties of the accelerometers, gyroscopes and magnetometers. If an IMU is manufactured with temperature compensation, this will improve the stability of the measurements.
For high-accuracy applications such as UAV surveying and mapping, a high data output rate is also important as this will reduce errors due to interpolation between readings.
In general, inertial sensors can be grouped into performance classes according to bias stability specifications which help to categorise the performance of a module. The higher the grade of sensor, the more accurate it tends to be.
Typically, tactical- and industrial-grade IMUs are preferred for demanding applications such as UAV navigation, as they offer levels of high performance and accuracy.
Tactical-grade IMUs have gyroscopes with an extremely low in-run bias stability, meaning that the offset error is more stable over time. This is a necessary quality for mission-critical and high-precision applications such as UAV navigation and antenna and weapon platform stabilization.
Industrial-grade IMUs provide lower accuracy than tactical-grade-IMUs, but feature better performance and reliability than systems intended for consumer applications. They offer good tolerance to shock and vibration, and exhibit good repeatability over time and temperature. Many industrial-grade IMUs are factory-calibrated to improve temperature performance.
MEMS-based IMUs are commonly used for industrial-grade applications, such as UAV platforms requiring small form factor components over other performance parameters for stabilization, control, and navigation.
In addition to consumer, industrial and tactical-grade IMUs, higher grades are also available: navigation-grade, and strategic or military-grade.
IMUs for UAVs will often need to be highly rugged in order to withstand demanding environments, such as high levels of shock and vibration, as well as wide temperature ranges. Enclosures may be made of materials such as precision-machined anodized aluminium, and be rated to various standards of ingress protection (environmental sealing) such as IP67, to provide protection from board stresses and hazards due to harsh environmental conditions. IMUs with low vibration sensitivity provide higher accuracy navigation solutions in harsh UAV applications where vibration levels are high.
ITAR (International Traffic in Arms Regulations)-free IMUs are not restricted by export controls that require a license to transfer any system around the world that contains ITAR-controlled technology. Using an ITAR-free IMU in an unmanned system design will simplify international sales of the final product.
IMUs that are designated as Significant Military Equipment (SME) and thus controlled by ITAR regulations include those specifically designed for unmanned aircraft that are also ITAR-controlled, as well as those designed for weapons or weapons systems.
Size, Weight and Power requirements (SWaP) are key considerations in the design and development of UAV platforms, and many manufacturers aim to reduce the overall size and weight of their systems in order to fly greater distances, extend flight times, or increase payload capacity. This has led many IMU suppliers to develop miniature IMUs with extremely low weight and small dimensions to meet the SWaP requirements of UAVs.
Engineers designing systems for UAV applications need to define and prioritize the performance, SWaP and cost factors that are most important to the success of their design in order to select the appropriate IMU and technology. Comparing IMUs based on their specific technologies and performance strengths and weaknesses will be a key factor of the design process.
The inertial sensor market spans an enormous range in terms of product price and performance; here is an overview of some of the leading IMU manufacturers:
Systron Donner Inertial
Systron Donner Inertial (SDI) provides high performance, solid-state MEMS IMUs that deliver industry-leading reliability under the most demanding conditions. SDI’s innovative Quartz MEMS technology boasts superior accuracy and industry-leading signal/noise performance. The SDI500 MEMS IMU demonstrates true tactical grade performance, with 1°/hr bias stability and very low 0.02°/√hr angle random walk. The SDI500 is rated for rugged military unmanned aircraft environments, and is an ITAR-controlled product. Learn more >
SBG Systems is a supplier of MEMS-based ITAR-free inertial sensors that balance cost, size and performance for unmanned systems applications. The Ellipse 2 Micro Series is SBG’s smallest high-performance IMU, weighing in at 10 grams with a small form factor designed for SWaP-conscious unmanned systems. The IMUs can be interfaced with a GNSS receiver for increased accuracy position, heave and heading output. Learn more >
Inertial Labs develops high-performance MEMS-based inertial solutions with small size and low power requirements. The Inertial Labs IMU-P is an ITAR-free, fully temperature calibrated and compensated IMU that is available in industrial and tactical grades. It is designed for a variety of applications including navigation and control for UAVs.
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Advanced Navigation supplies MEMS and FOG-based navigation solutions for both commercial and military UAV applications. The Motus is a miniature ultra-high accuracy MEMS IMU that provides inertial performance exceeding that of even some FOG IMUs. It weighs in at just 26 grams, and features a bias stability of 0.4 °/hr. Learn more >
Inertial Sense is a provider of micro-sized high-performance MEMS IMUs for a wide range of UAV applications such as navigation and stabilization. The μIMU is a precision calibrated IMU that incorporates a L1 GPS (GNSS) receiver, providing UTC time synchronized outputs at up to 1kHz. It is available as a board-level unit or as a rugged self-contained module in a machined aluminium enclosure. Learn more >
VectorNav Technologies is a developer of state-of-the-art MEMS tactical and industrial-grade sensors for demanding environments. The VN-110 is a compact tactical-grade combination IMU/AHRS (Attitude & Heading Reference System) encased in a ruggedized aluminium housing. With less than 1˚/hr in-run gyro bias stability, its industry leading algorithms provide high-accuracy position, velocity, and attitude estimates along with compensated inertial measurements.
EMCORE is a supplier of high-accuracy FOG IMUs that are designed with the low SWaP requirements of UAVS and unmanned systems in mind. The EN-150 is currently the smallest closed-loop FOG-based IMU available on the market, offering high accuracy and high performance.
The EN-300 is a low-noise, high-stability FOG IMU that provides the high performance required for GPS denied navigation, precise targeting and line-of-sight stabilization. It is available in three performance variants to suit a wide range of applications. Learn more >
LORD Microstrain is a manufacturer of miniature inertial sensors designed for efficient integration into existing unmanned platforms. The 3DM-CV5-10 is a fully calibrated and temperature-compensated miniature industrial-grade IMU. Measutring 38 mm x 24 mm x 9.7 mm and weighing just 8 grams, it is LORD’s smallest, lightest and highest performance IMU in its class. Learn more >
NovAtel offers a range of IMUs from leading manufacturers, which can be coupled with a GNSS receiver to provide highly accurate and continuous position, velocity and attitude data. Available options include commercial MEMS IMUs, high-end tactical grade MEMS IMUs, tactical-grade FOG IMUs, and tactical-grade RLG IMUs. Learn more >
Gladiator Technologies specializes in low-noise, high-performance MEMS-based IMUs for demanding environments. Their range of rugged IMUs feature environmental sealing, precision alignment and internal vibrational isolation, as well as bias, scale factor and misalignment modelling over the entire operating temperature range. Learn more >
Memsense provides both off-the-shelf and custom high-performance MEMS IMUs for demanding SWaP-constrained UAV applications. The MS-IMU3020 is a miniature low-drift IMU that weighs just 20g. The MS-IMU3030 is an ultra-compact IMU with a gyro bias stability of 0.55 degrees per hour and acceleration bias stability of 3μg.
Unmanned Systems Source
Unmanned Systems Source is a US distributor of UAV and unmanned systems component manufacturers. They supply a wide range of IMUs, across different performance grades, and their team can help determine which IMU will be suitable for particular application requirements. Learn more >
Xsens manufactures high-reliability MEMS-based IMUs for professional UAS applications, including navigation, control and stabilization. The MTi-100 is a high-performance IMU tuned for precise operation under vibration and magnetic distortion conditions, with a sophisticated motion processing core that provides results surpassing traditional Kalman filtering. It is available as an OEM board or as an IP67 encased module. Learn more >
MEMSIC supplies MEMS IMUs for precise, reliable dynamic measurement in demanding UAV applications. The IMU380ZA is a miniature, lightweight IMU designed for embedded applications. Measuring 24 x 37 x 9.5 mm, the module is factory-calibrated to provide consistent performance in a wide variety of extreme environments. It is available with several range options. Learn more >