MEMs Six Degrees of Freedom Sensors are Built to Cover More ADAS Safety BasesJune 03, 2020 by Robin Mitchell
Partial or conditional autonomous vehicles leave critical blind spots. MEMs 6DoF inertial sensors plus GNSS, camera, RADAR, and LiDAR data may be a better bet.
This week, Murata announced its latest MEMS six degrees of freedom (6DoF) device, the SCHA600, designed to advance the safety of automotive designs. The inertial sensor infuses its data with GNSS and a number of perception sensors—from cameras to RADAR and LiDAR—to deliver high performance when it comes to bias stability and noise.
In order for an autonomous system to operate independently in any environment, it must be aware of its surroundings. Autonomous systems must respond quickly to a rapidly-changing environment. If, for example, a car is to suddenly stop, the autonomous car behind it must recognize the rapid deceleration and take evasive action, whether it be emergency braking or lane changing.
The specifications of this new MEMs inertial sensor can shed light on one interesting dimension of ADAS technology: 6DoF.
The Faults in Past ADAS Technology
Murata posits that there have been four stages of vehicle technology progressing toward fully autonomous driving—driver assistance, partial automation, conditional automation, and high automation. These stages involve a range of devices to varying degrees: cameras, RADAR, ultrasound, LiDAR, and GPS positioning.
Stages of autonomous driving. Screenshot used courtesy of Murata
But not all of these devices account for the six degrees of freedom, or a device's ability to measure motion along three perpendicular linear axes while also rotating around each of these axes.
For instance, cameras are limited in measuring depth perception. Other technologies like LiDAR are good for object detection but are not ideal for precise positioning. Additionally, the resolution of GPS is too small to know precisely which side of a road a vehicle is on. The usefulness of GPS significantly drops when considering driving areas that can’t receive a GPS signal.
Accelerometers and Six Degrees of Freedom
This is where accelerometers come in. These devices can determine the current forces in multiple axes. From there, accelerometers can measure the acceleration in each axis. This measurement is then turned into an estimate of velocity that, in turn, can be used to determine displacement. Such a system can be combined with a GPS to create an accurate positioning system that uses the accelerometer to produce in-between readings. Meanwhile, the GPS can provide a rough confirmation of speed and position.
Even then, three degrees of freedom—X, Y, and Z—are not enough; cars turning around corners or driving up a hill may not properly detect such motion with a simple 3-axis accelerometer.
A 6 degree of freedom (6DoF) accelerometer does not just measure the acceleration in the X, Y, and Z planes. It also measures acceleration in the yaw, pitch, and roll axes.
The translational and rotational movements that combine to form the six degrees of freedom. Image used courtesy of Honeywell
Movement in these axes may not affect the X, Y, and Z planes. Thus, integrating a MEMS device in an autonomous vehicle can cover all six movements and directions, producing a better position result.
Introducing the SCHA600
What does a 6DoF MEMs device look like in practice? One example comes from Murata with a MEMS device that has been designed for safety-critical automotive applications. When integrated with GNSS and perception sensors, the SCHA600 allows for a high degree of vehicle autonomy, enabling centimeter-level accuracy of the vehicle's position and dynamics.
SCHA600. Image used courtesy of Murata
Data communication with the SCHA600 is achieved with SPI while extensive self-diagnostic features allow safe operation. Murata says the ISO26262-compliant sensor is also AEC-Q100 qualified to ensure that it can withstand all vibration and shock expected on the road. The device also has a self-test function that monitors whether the sensor is operating correctly for each measurement cycle.
The SCHA600 has a gyro RMS noise level below 0.007°/s, an Allan variance down to 0.9°/h at room temperature, and ±125°/s or ±300°/s angular rate measurement depending on the chosen application.
The Allan Deviation in °/h of Z-Gyro SCHA634-D01, a member of the SCHA600 family. Image used courtesy of Murata
Housed in a SOIC IC package at 18.7 mm x 8.5 mm x 4.5 mm, the accelerometer has a ±6g measurement range and operating temperature range of -40°C to +110°C. Typical applications for the SCHA600 include advanced driver assistance systems, dead reckoning navigation, and platform stabilization.
The Place of 6DoF in ADAS
The ability to measure 6 degrees of freedom is incredibly important for fully autonomous systems.
MEMs 6DoF inertial sensors like Murata's SCHA600 are not just applicable for automotive applications; even drones would find such devices useful since they are designed to move in all 6 axes.
Do you work with autonomous vehicle technology? What design measures do you take to ensure accuracy and safety? Share your experiences in the comments below.