Attitude and heading reference systems, traditionally based on large mechanical components and instruments, are experiencing further miniaturization enabled by MEMS technologies. Applications for attitude and heading reference systems include control and stabilization, measurement and correction, and navigation. We recently spoke with Per Slycke, Chief Technology Officer at Xsens Technologies based in the Netherlands. In this detailed interview, Per reviews the current status of MEMS based attitude and heading reference systems and discusses ongoing and future trends.
MEMS Investor Journal: What are attitude and heading reference systems (AHRS) and what do they do?
Per Slycke: Attitude and Heading Reference System, or AHRS, essentially provides a sense of direction with respect to Earth – for example, gravity (such as up or down) or North (forward or backward) to whatever object they are mounted on. Basically, an AHRS provides real-time 3D orientation data – roll, pitch and yaw. One could say that, in a sense, an AHRS is like an electronic balance organ like the vestibular system in human ears, with the exception that humans cannot sense North or South.
Often an AHRS also provides the “raw” data from the sensors that it uses such as 3D acceleration, angular velocity (rate of turn) and magnetic field.
MEMS Investor Journal: What are the main components of a MEMS based AHRS?
Per Slycke: The sensor assembly of an AHRS consists of a gyroscope triad, an accelerometer triad and a magnetometer triad. Signal processing is done on the board as well and current high-quality AHRS’s have a powerful digital signal processor (DSP) and microcontroller unit (MCU) that apply individual device sensor calibration parameters such as temperature compensation, cross coupling between acceleration and angular velocity, and run a sensor fusion filter, often based on a recursive Kalman filter, to output orientation (roll, pitch, yaw) and improved motion data directly from the AHRS.
The sensor fusion algorithm usually also estimates sensor component calibration parameters while in use to further improve the accuracy of the orientation estimates. Advanced AHRS systems, such as some of products from Xsens, also can include additional sensors such as GPS receivers and barometric pressure sensors to further improve the motion tracking.
MEMS Investor Journal: What is the primary function of each AHRS component?
Per Slycke: The gyroscope triad is the most important part of the AHRS and strongly influences the accuracy of the AHRS. The gyroscope triad measures the rate of turn (angular velocity) and can thus provide the change in orientation. Drift in the gyroscope integration over time however must be referenced by other sensor systems. In the case of an AHRS, these complementary compensating sensors are the accelerometers for attitude (roll/pitch) and magnetometers for heading (yaw). The DSP and MCU on the AHRS fuse all these signals together in a Kalman filter and the output is a stable and robust 3D orientation.
MEMS Investor Journal: What about the magnetometers? Which technologies are typically used for the magnetometer components?
Per Slycke: Traditionally, AHRS systems have often used fluxgate based magnetometers (compasses), but, with the miniaturization over the past decade, other magnetic sensing technologies that became available have gained popularity.
Today, the most commonly used magnetic sensing technology in miniature MEMS industrial-grade AHRS system is of the anisotropic magnetoresistive (AMR) type. The advantage of AMR sensors with respect to, for example, sensors based on the Hall effect is primarily higher sensitivity. Since the Earth’s (horizontal) magnetic field is so weak, a very sensitive magnetic sensor is needed especially for locations that closer to the North or South poles. For this reason, Hall effect sensors need “magnetic field concentrators” built-in to the device which can complicate accurate 3D vector measurements.
Other novel magnetic sensing technologies such as sensors based on the giant magneto impedance (MI) effect are also becoming increasingly popular, in particular because of their high bandwidth and resistivity to permanent saturation in very strong fields that AMR sometimes suffer from. Another advantage the MI sensor has got going for it is the low requirements on peripheral circuitry to support the sensor.
MEMS Investor Journal: What are the emerging trends with AHRS technologies? How are the technologies changing and improving?
Per Slycke: Developments for MEMS accelerometers have pretty much stabilized at this time and it is now a race to the bottom in terms of size and cost. High performance 3D accelerometers, originally developed for automotive applications, still have room to further improve.
The more exciting developments today are in the MEMS gyroscope market. It is changing very rapidly, as you know, with recent introductions of 3D gyroscopes in consumer electronic devices such as the iPhone. However, the performance of “consumer grade” gyros is still not comparable to automotive and industrial grade gyros – there is a performance difference of approximately factor of 5, at least as measured for navigation and AHRS applications where the goal is to track motion in 3D. In addition, the size and price difference is even larger in favor for the consumer grade devices.
For MEMS gyros, the main performance parameter that still needs improvement is bias stability as well as the initial bias. Both of these parameters are important for navigation and AHRS functionality. Automotive gyroscopes were originally designed for ESP systems, where the long term bias stability is less important than the short term bias stability.
On the hand, for control, stabilization and navigation applications, gyro bias stability is one of the leading performance requirements. Because of the recent advances by consumer grade gyro manufacturers, we are now very happy to see our repeated requests in the past decade or so to automotive grade MEMS gyro makers to improve long term bias characteristics starting to pay off. Automotive inertial MEMS manufacturers are now creating a new class of “industrial grade” sensors. New products in this segment can to a certain degree rival the performance of very expensive fiber optic laser gyros, or FOGs.
A major future trend we see is that of a further physical integration of all required components in an AHRS – gyros, accels, mags and processors in the same device. Although Xsens designs and manufactures integrated hardware products, our software can actually be used in a much wider range of devices. We see some exciting opportunities there, while we will certainly extend the functionality and ease of use of our existing product lines. An example is our recent launch of an advanced wireless body-area networked IMU and AHRS product.
MEMS Investor Journal: In which applications are your attitude and heading reference systems primarily used?
Per Slycke: Applications of Xsens AHRS are in three major market segments: entertainment (full body motion capture for movie special effects and game developers), movement science (motion capture for sports, rehabilitation and clinical applications) and industrial applications. The latter market can be divided in three fields of use within the context of industrial applications: control and stabilization, measurement and correction, and navigation.
MEMS Investor Journal: For each application, what is the grade of accelerometer, gyro and magnetometer components needed (e.g. consumer grade, automotive grade, aerospace grade) along with key performance requirement for each component in each application?
Per Slycke: The figure below shows some of the dynamics in the gyroscope market where the recent advent of the “consumer grade” MEMS gyroscopes has pushed automotive grade gyroscopes to a higher performance class. In the “industrial grade” range of performance, the requirements for almost all types of applications can be met, especially when combined with advanced sensor fusion algorithms and complementary sensors such as magnetometers, GPS, barometers, optical, LIDAR, laser scanners, sonar and other. This includes human motion tracking applications for professional purposes such as healthcare, medical, film and game applications. Only the most stringent, traditionally military, applications require gyroscopes with higher performance and, for those applications, customers are willing to pay for this higher performance.
In the near future, performance of “consumer grade” devices will continue to improve, enabling a wide range of very interesting new applications. The Nintendo Wii, Sony Move and iPhone 4 are just the beginning.
MEMS Investor Journal: For some of the major applications, please explain briefly why an AHRS is needed and what is the context of its application?
Per Slycke: In control and stabilization, primary uses are stabilization of aerial cameras and antenna systems on ships. An AHRS is needed to stabilize the platform where the camera or antenna is mounted for case where, for example, the platform is on a rocking ship or on a survey aircraft. It is important to have an AHRS that has a low and known latency in order to successfully control the platform.
In measurement and correction, applications are mainly found in imaging systems, such as underwater acoustics (sonars, echo sounders) and portable laser scanners. An AHRS is needed to correct the sensor data captured by the sonar or laser scanner or to determine where the imaging device is pointing at. Together with a synchronized GPS module there is information available to automatically determine the position of the image.
In navigation, an IMU and AHRS can be used to aid a GPS module for those times when the GPS experiences outages and to make navigation more robust under high dynamics and challenging GPS conditions. Likewise, altitude estimation typically can be improved by an order of magnitude.
Other growth markets are in healthcare applications and professional 3D tracking systems for the movie and game industry as well as a whole range of exciting consumer devices on the lower end.
MEMS Investor Journal: For each application, what is the approximate sales price of an AHRS unit?
Per Slycke: Professional grade miniature MEMS based AHRS units suitable for industrial applications sell for around $1,300 at volumes of 100 units. For AHRS units with integrated GPS and associated advanced embedded sensor fusion firmware, prices are in the range of $3,200.
MEMS Investor Journal: What are the emerging trends with AHRS applications? What are some of the latest applications that you’ve seen?
Per Slycke: Professional grade miniature MEMS AHRS systems are poised to take over the traditional market of fiber optical laser based IMUs, especially now that gyroscopes are becoming more accurate and manufacturers can combine many sensor systems, other than just inertial sensors and magnetometers.
This means that AHRS systems in industrial applications enter the domain of portable applications, such as portable laser scanners and miniature unmanned vehicles. Lower costs and yet higher accuracy will drive the use of AHRS systems in more applications, where an optical based IMU and AHRS would have been too heavy, too large and too costly until now.
Mr. Per Slycke is the CTO at Xsens and is responsible for R&D, strategic product management and new business development. As a co-founder of Xsens in 2000, Mr. Slycke has helped build the company into a leader in the field of advanced 3D motion trackers based on MEMS sensors for industrial applications such as marine systems, robots, unmanned vehicles as well as emerging professional markets for human motion analysis in movement science, entertainment motion capture solutions and training and simulation systems. Mr. Slycke holds an M.S. degree from the University of Twente in Physics.
Copyright 2010 MEMS Investor Journal, Inc.