Feature: Sensor Technology
Figure 3: Robots at an Ocado grocery warehouse in the UK. (Image credit: Techwords under Creative Commons 4.0 licence)
other forms of autonomous vehicles. And this means that many more
autonomous vehicles of all types are going to make use of the latest generation of IMUs.
Why motion sensing underpins the growing demand for autonomous vehicles Many autonomous mobile devices use technologies such as radar, LiDAR and vision cameras to ‘see’ the world around them. But if an autonomous vehicle is to locate
itself relative to the objects that it detects, it needs to know its position, its attitude and its heading, and this is the data that an IMU produces (see Figure 3). Most IMUs are supplied as complete
modules, in either an open-frame form factor, or sealed in an enclosure. A module will typically be composed of:
• T ree accelerometers measuring acceleration relative to gravity in the x, y and z dimensions
• T ree gyroscopes, measuring rate of turn in the x, y and z dimensions
• A magnetometer, measuring heading relative to the Earth’s magnetic fi eld An IMU module might additionally include a GNSS (Global Navigation Satellite System) receiver, or an interface to an external GNSS receiver, for geolocation data. Alongside these components, an IMU can feature a barometer for measurement of elevation. T e outputs from the IMU enable an
autonomous mobile vehicle to track both its orientation and its motion. T e orientation data shows where it is headed relative to magnetic North or South, and the degree to which its nose is pointing up or down relative to the horizon. T is is why it can be described as the
balancing organ of an autonomous vehicle. In addition, motion data can be calculated
on the principle of dead reckoning from the combination of orientation, heading and acceleration data over time. T is allows the device to track its approximate position relative to a known starting point in the absence of accurate geolocation data from a GNSS receiver.
Semiconductor technology provides aff ordable IMU performance Various technologies have been developed for measuring acceleration, rate of turn and
heading. T e choice that a manufacturer makes depends on the accuracy and stability required in the application, and the bill-of- materials budget. In mainstream autonomous vehicles
for use in logistics, mining, agriculture, subsea operations, aerial surveying and similar applications, IMU modules based on MEMS accelerometers, gyroscopes and magnetometers generally provide the best balance of price and performance. A MEMS sensor is made in a fabrication plant using processes and equipment derived from the semiconductor industry. T is makes these sensors highly aff ordable, and readily available in high volumes. In an advanced IMU module, the merged
output of the accelerometer, gyroscope, magnetometer and GNSS input (if available) is more valuable than the multiple discrete measurement outputs in isolation. T is is because sophisticated sensor fusion algorithms implemented inside the module enhance the accuracy and stability of the individual sensors’ measurements. For instance, all MEMS gyroscopes
suff er from a certain amount of driſt over time caused by the eff ect of gravity, but accelerometer outputs, which measure acceleration relative to gravity, can be used to compensate for this gravity eff ect, dramatically reducing the amount of driſt in the gyroscope’s output. In the best MEMS sensor-based IMU
modules, sensor fusion enables measurement accuracy to be tightly specifi ed, and IMU performance to be stable over time and temperature. Firmware in the module can also provide a high level of immunity from magnetic and electrical noise. IMU modules also insulate autonomous
mobile device manufacturers from changes in the individual sensor ICs. An accelerometer, gyroscope or magnetometer might be upgraded or discontinued and replaced with a newer model, but the output from the module will remain the same, freeing the device manufacturer from the need to re-qualify the components of an inertial measurement system.
IMU module market supports adoption of autonomous vehicle technology A thriving market for MEMS-based IMU
Figure 4: The Xsens Sirius from Movella, a robust inertial navigation system for use in harsh environments
modules supplied by manufacturers such as Movella Technologies means that there is a wide choice of modules off ering a range of features and functions and at various price points. Movella is typical in off ering products from basic IMUs to vertical reference units (VRUs) and Attitude and Heading Reference Systems (AHRS), up to complete Inertial Navigation Systems (INS) which include GNSS satellite positioning capability (see Figure 4). T e products are available in open-frame formats or in a waterproof enclosure off ering IP68 protection. T e best module manufacturers support
their hardware products with a full-featured soſt ware development kit (SDK) to accelerate integration of the IMU into an autonomous device design. A module should also be backed by comprehensive user documentation, application notes, a knowledge base and access to technical support. T e accurate orientation and motion
data available from the latest generation of high-quality IMU modules, backed by GNSS receiver inputs, provides a vital enabler of the growth in demand for autonomous vehicles, from the largest devices such as self- driving mining trucks to the smallest, such as pipe crawlers which autonomously map underground pipe networks.
www.electronicsworld.co.uk February 2025 15
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