Inertial measurement unit sensors lower risk for autonomous vehicles

Position sensor insights

• While many vehicles are already equipped with autonomous technologies such as assisted parking and automatic braking, fully autonomous driving requiring no driver interaction is still some time off.
• Inertial measurement sensors, or IMUs, using an accelerometers and gyroscopes, will expedite the development of fully autonomous driving.
• IMUs help lower risk for vehicle navigation, even if other systems, such as LiDAR, radar, and cameras, fail.

The future of vehicles is autonomy, and advanced position sensor technologies are helping vehicles designed to drive themselves—safely and accurately. There are many predictions regarding when truly autonomous cars will become a reality.

However, while we wait, many of today’s vehicles are equipped with a variety of advanced driver assistance systems (ADAS) like lane departure warnings, assisted parking, and automatic braking. Although these technologies are very useful, they are considered an SAE autonomy level of just 1 or 2, which means that they still require driver engagement 100% of the time.

The big question is when will we see the technological leap to level 5, or fully autonomous vehicles that require no driver interaction. Unfortunately, we are still several years away from this technology becoming widely accepted and adopted into any type of vehicle. Why? The critical challenge is ensuring safety.

To achieve full autonomy requires absolute confidence that a vehicle will continue to safely and accurately operate in any type of weather or road conditions and will avoid significant injury to passengers, pedestrians, or property.

This would require a built-in guidance and navigation technology that can ensure the vehicle will operate safely if and when vehicle perception sensors like LiDAR, radar, or cameras fail, or if there is an intermittent disruption in GNSS satellite signals due to weather, terrain, or environment.

Safe-stop sensing system for autonomous vehicles

This vehicle sensing technology is the inertial measurement unit sensor, or IMU sensor. Because an IMU sensor is based on gravity and the laws of physics rather than external conditions, it can continue to send data so the vehicle can safely maintain course until it can come to a secure stop or the other navigation systems start functioning again, even if the perception sensors fail because of weather. By eliminating data interruption and increasing operational safety, the IMU will expedite the reality of Level 5 fully autonomous driving.

IMU sensors rely on gravity and physics of motion. Unlike vision, radar, LiDAR and other perception sensors, as well as steering systems based on GNSS signals from satellites, IMU based guidance systems are not affected by harsh environmental conditions and physical obstructions like tunnels and foliage and will continue to operate. Courtesy: Aceinna—Inertial Measurement Systems

The bottom line is this: Without IMU sensors to provide the safety cushion, autonomous vehicles will never be able to effectively work in city streets and highways.

What is an IMU sensor, and how does it work?

Most IMU sensors are composed of two different sets of sensors—accelerometer sensors and gyroscope sensors. The accelerometer sensors measure linear acceleration in three orthogonal axes. Integrating acceleration over time will provide velocity, and integrating velocity over time will change result in a change in position.

The gyroscope sensors measure the angular rate of three orthogonal axes. Integrating the angular rate along the three axes over time will generate change in roll, pitch, and yaw, which is the change in attitude of an object.

An IMU module with gyroscopic and accelerometer sensors can provide measurement over 6 degrees of freedom (often called 6-DOF, see below).

An IMU uses its accelerometer and gyroscope sensors to capture measurements over six degrees of freedom, three axes of movement (forward and backward, left and right, and up and down), as well as three axes of rotation (roll, pitch and yaw). Courtesy: Aceinna—Inertial Measurement Systems

Why do some IMUs also include a magnetometer?

An accelerometer can be used to successfully calculate roll and pitch values with respect to earth’s gravitational force, and correct gyroscope drift.

However, it cannot be used to detect absolute heading (yaw) because the change of yaw is orthogonal to the gravity vector. A magnetometer measures the magnetic field strength in three dimensions. By using the Earth’s magnetic field, it can help to determine heading (i.e., yaw) as well as roll and pitch of the object.

Integrating a magnetometer in the IMU can help with detection of the initial heading of an object and correct integration errors of the yaw gyroscope in the sensor fusion algorithm.

IMU performance measurement; IMU triple redundancy

Bias instability is one of the most critical performance parameters of the gyroscope.

It is a direct measure of how much the gyroscope drifts over time. Because the rate output of the gyroscope is being integrated to calculate change in angles (roll, pitch, and yaw), any error associated with drift results in accumulated error in relative angles. Furthermore, these angular errors translate into position errors over time. For automotive applications a high performance IMU is a necessary component for the autonomous vehicle to achieve high accuracy positioning.

In a triple redundant IMU, three IMUs are used to construct a triple-redundant sensor architecture that provides additional levels of reliability and accuracy.

Aceinna’s Triple Redundant IMU sensors provide the extra level of safety, accuracy and reliability that is required to ensure safe operation of autonomous vehicles on highways and in dense urban environments. Courtesy: Aceinna—Inertial Measurement Systems

If for some reason one or more sensors is not accurately functioning, the system can be programmed to recognize the defective sensor data and avoid using it. The defective sensor output or errant dataset will be ignored or de-rated in importance. This architecture ensures the reliability of the system and simultaneously improves the performance.

IMU sensors may not attract the same amount of attention and media coverage as other sensors— i.e. LiDAR Radar, and cameras.  However, in many ways IMUs are the critical safety sensor component required for the successful operation of the Level 4 and 5 autonomous vehicles that will be appearing on streets within the next decade.

James Fennelly is product manager, AceinnaCEINNA—Inertial Measurement Systems. Edited by David Miller, content manager, Control Engineering, CFE Media and Technology, dmiller@cfemedia.com.

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