Inertial sensors are used to transduce inertial force into measurable electrical signals to measure acceleration, inclination, and vibration of an object. Inertial sensors are sensors based on inertia and relevant measuring principles. These range from Micro Electro Mechanical Systems (MEMS) inertial sensors, measuring only few mm, up to ring laser gyroscopes that are high-precision devices with a size of up to 50cm.
Micromachining technology has made it possible to produce MEMS (Micro Electromechanical System) inertial sensors using single-crystal silicon sensor elements. Micro-Electro-Mechanical-Systems (MEMS) is the integration of mechanical elements such as gears and motors, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology already used for conventional electronic integrated circuits. As a general rule of thumb, MEMS typically have dimensions ranging from nanometers to centimeters; however, very little has been done with MEMS below one micrometer. Some well-known examples of MEMS-enabled functionality in everyday life are airbag deployment in automobiles; motion and orientation detection in smartphones; and blood pressure measurement in IV lines and catheters.
Generally speaking, micromachined inertial sensors include accelerometers used for measurements of linear acceleration, velocity and position or tilt angle, shock, jerk transduction, and gyroscopes used to measure the angular rate of moving objects. The MEMS based inertial sensors are small, light, low-cost sensors, very low power, on the order of milliwatts, often operating from low-voltage supplies ( 3 V), and suitable for monolithic multi-axis devices, such as an accelerometer for X, Y, and Z axes. They are also very rugged, and can withstand shock of tens of thousands of g’s , very desirable in military applications like weapons and munitions.
Silicon has been the material of the choice in micromachining including inertial sensors due to its many desirable mechanical and electrical properties and its compatibility with IC fabrication technologies. In the field of inertial sensors, another material has established as reasonable alternative to silicon for fabrication of MEMS devices. This material is single crystal Quartz which has mechanically strong and thermally stable insulator material with piezoelectric property.
These micron-sized sensors meet all major system design drivers like low-cost, high performance, high precision, and small form-factor. Based on the same principles as of macroscopic inertial sensors, MEMS inertial sensors can detect the slightest change in position, orientation, and acceleration of an object several metres long using a sensor unit as small as few micro-meters in dimensions. Recent MEMS accelerometers have an excellent price/performance ratio, and their performances can be in the same range than accelerometers based on previous technologies, which is why many high-precision IMUs contain MEMS accelerometers.
The inertial sensors are generally part of a larger control system in any application or device. Mere information of acceleration or angular movement of an object is of no use. The information collected from inertial sensor is always used to control movement of the device itself or is used for activating an actuator, like to open an airbag of a car.
The precision of MEMS gyroscopes has been improved in the last years with the help of batch production techniques and new gyroscope designs, they remain the biggest problem of MEMS inertial sensing technology and the main reason why previous technologies such as fiber-optic gyroscopes (FOG) or ring laser gyroscopes (RLG) are still vastly used in domains where high-precision sensors are necessary.
Applications of inertial sensors
Microelectromechanical system (MEMS)-based inertial sensor commercialisation goes back around 50 years and has involved many interesting developments, new applications and acquisitions. Development of capacitive and piezoresistive silicon linear accelerometers began in the 1970s. Airbags were the first high-volume application for these inertial sensors in the late 1980s and early 1990s. As with many MEMS devices, the aerospace industry led the development of angular rate sensors, also known as MEMS gyroscopes. Like the accelerometer, the automotive market provided the first high-volume applications for MEMS gyroscopes, starting in the mid-1990s with navigation assist, vehicle dynamic control (VDC) and roll-over protection (ROP) systems. Now, both of these sensors can be found in every smartphone, tablet, vehicle and wearable device.
Inertial sensors based on MEMS technology are fast becoming ubiquitous with their adoption into many types of consumer electronics products, including smart phones, tablets, gaming systems, TV remotes, toys, and even (more recently) power tools and wearable sensors. MEMS-based motion tracking enhances the user interface by allowing response to user motions, complements the GPS receiver by providing dead-reckoning indoor navigation and supporting location-based services, and holds the promise of enabling handset optical image stabilization in next-generation handsets by virtue of its lower cost and small form factor.
In the automotive industry, accelerometer is used for airbag release control, traction control, seat belt control, active suspension, antilock braking system (ABS) and monitoring vehicle vibrations. While the gyroscope is used for rollover protection, automatic indicators, power steering and to control vehicle dynamics.
In consumer segment, inertial sensors are used in a variety of applications like platform stabilization in video cameras, virtual reality headsets, pointing devices for computers, intelligent toys and gaming keypads. All smartphones and tablets nowadays have inertial sensors for detecting screen rotation, gaming and augmented reality applications.
The inertial sensors are also used to monitor position and orientation of robotic manipulators and unmanned robotic vehicles. In medical applications, these sensors are used for monitoring patients with specific conditions like for monitoring patients having Parkinson’s disease. High-end inertial sensors are used in military and aerospace applications like smart ammunition, aircraft dynamics control, crash detection, seat ejection system in aircrafts, and microgravity measurement.
Inertial sensors are also used in a wide range of military applications, from airplanes to guided missiles, robots, radar stations, and missile, torpedo and ground vehicle applications and GPS denied environments. MEMS are also enabling development of Smart munitions, for example a bullet equipped with a MEMS inertial IMU could calculate its position, compare it to the one of its target, and redirect itself using e.g. small fins on the back of the bullet. Another example is microrobotics, small Robots enabled by MEMS IMU to enable precise navigation.
MEMS inertial sensors classification
There are mainly two types of MEMS inertial sensors – accelerometers that measure linear acceleration in one or more axis, and gyroscopes that measure angular motion. These sensors are manufactured for use in specific applications as each application requires inertial sensors having different bandwidth, resolution, and dynamic range. For example, the inertial sensor used in the automotive airbag release system must have a bandwidth up to 0.5 KHz, resolution of around 500 mG and dynamic range of about +/-100G. While inertial sensor used in a space microgravity measurement instrument can have a bandwidth of 0-10 Hz, but must have a resolution as precise as < 1 µG and dynamic range less than +/- 1G.
Accelerometers consist of a mechanical sensing element that can measure acceleration in one or more axis. The sensing element consists of a proof mass attached to a reference frame by a mechanical suspension system. In MEMS sensors, proof mass is an extremely small seismic mass and the suspension system is built from silicon springs.
In all types of micro-machined accelerometers, the displacement of the proof mass is measured by position measuring interfaces, like in a capacitive measurement, there are movable plates attached to proof mass which move along the proof mass between fixed capacitive electrodes. There are many types of sensing mechanisms that are used in the design of accelerometers. Some of the common sensing methods include piezoresistive, capacitive, piezoelectric, optical, and tunnelling current.
Piezoresistive Accelerometers – In these type of accelerometers, the proof mass is attached to a piezoresistor. The resistor is connected to read-out electronic circuit. When there is displacement in proof mass, there is a change in the resistance of the piezoresistor proportional to the applied force. These types of accelerometers are the first one to see bulk production. The biggest drawback of these type of accelerometers is their thermal stability. The peizoresistance can significantly change due to thermal noise and can lead to false outputs.
Capacitive Accelerometers – In capacitive accelerometers, capacitive sense fingers are attached to the proof mass which move along a given axis with the displacement of the proof mass. Each movable plate is placed between two electrodes. When there is an acceleration, the proof mass displaces in the direction opposite to the direction of motion and the variable plate moves along the proof mass. The change in the position of variable plate along an axis cause change in its distance with fixed electrode plates and cause symmetrical change in capacitance. This is then measured as electrical output by a read-out electronics. The capacitive accelerometers are thermally stable, but are prone to electromagnetic interference, where they can give false outputs due to parasitic capacitance.
Piezoelectric Accelerometers – Most of the macroscopic accelerometers use piezoelectric materials for detecting motion of proof mass. Many micro-machined accelerometers also use the same principle. These accelerometers have great bandwidth, but have extremely poor resonant frequency due to leakage currents. The piezoelectric material produces electrical signals proportional to the displacement of the proof mass in a given axis.
Tunnelling Accelerometers – These types of accelerometers use tunnelling current for measuring the displacement of the proof mass. The tunnelling current between a sharp tip and an electrode changes exponentially by the tip-electrode distance. The following equation gives the tunnelling current:
Resonant Accelerometers – In a resonant accelerometer, the proof mass is attached to a resonator. The displacement of the proof mass changes the strain of the resonator and so its resonant frequency. The change is frequency is converted to digital electrical signals using a frequency counter circuit. These accelerometers are quite immune to noise and are highly reliable as frequency changes can be directly converted to digital format.
Optical Accelerometers – These accelerometers use optical fibres and wave guides attached to the proof mass. However, optical fibre type accelerometers are not suitable for batch fabrication as the fibre needs to be manually installed near the proof mass in the sensor assembly. Another type of optical accelerometers use LED and PIN photo detectors to measure displacement of the proof mass. The optical accelerometers have the advantage that they are free from electrostatic and electromagnetic interference. But, because they usually involve a complex assembly and read-out circuitry, they are not much popular.
Gyroscopes- A gyroscope measures the rotation of an object. The MEMS gyroscopes use the principle of Coriolis force. When a mass moves in a rotating system, it experiences a force perpendicular to axis of rotation and the direction of motion. This is called Coriolis force. A MEMS gyroscope consists of a mechanical structure that is driven into resonance due to Coriolis force and excites secondary oscillation in the same or a secondary structure. The secondary oscillation is proportional to the rotation of the structure in a given axis. The Coriolis force has relatively small amplitude compared to its driving force. That is why, all MEMS gyros use a vibrating structure that use the phenomenon of Coriolis force.
The vibrating structure consists of proof mass that is connected to a inner frame by a pair of springs. The inner frame is connected to an outer frame by another set of orthogonal springs. There are capacitive sense fingers between the inner frame and the outer frame attached along the orthogonal springs. The Coriolis force is proportional to both the angular velocity of the rotating object as well as velocity of the object towards or away the axis of the rotation. The proof mass is continuously driven sinusoidally along the inner springs. When the system experiences rotation, the resonating proof mass experiences Coriolis force along the orthogonal springs attached between inner and outer frame. This changes the distance between the capacitive sense fingers and so an electrical signal proportional to Coriolis force is output. As the Coriolis force is proportional to angular velocity, the electrical signal due to it is also proportional to the angular velocity of the system.
Inertial Measurement unit (IMU)
An IMU, inertial measurement unit, is a sensor package containing 3 discrete sensors that can be used to track movement and orientation of objects. It is the Fusion of three sensor types: Gyroscopes -> Angular Velocity (rad/s or deg/s); Accelerometer -> Linear Acceleration (m/s^2 or g) and Magnetometer -> Magnetic field strength (micro-Tesla or Gauss) . Using a combination of multiple outputs allows us to build robust, complex systems that can achieve higher levels of accuracy . This is needed where GPS is not available (underwater, indoors, or due to interference or spoofing), for implementing image stabilization, or if the system needs to cross-check the GPS results.
Due to the performances of the MEMS inertial sensors, and particularly to the integration of the gyroscope errors, MEMS IMUs are globally still less performant than previous IMUs. However, some recent inertial measurement units based on MEMS technology can be used for navigation. ARDEC, along with the U.S. Army Aviation and Missile Research Development and Engineering Center, managed the development of common IMU devices for a wide range of projectile and missile applications. MEMS IMUs have replaced larger devices such as traditional gyroscopes, air-bearing gyros and ring-laser gyros for the inertial guidance reference in precision munitions.
The Analysis and Evaluation Division developed detailed models that predict and display the forces acting upon these components in the gun-launch environment. These models were able to identify structural concerns, which were addressed to assure survivability under forces in excess of 15,000 Gs.
A promising breakthrough in MEMS inertial sensing technology is the use of multi-directional MEMS inertial sensors, which have the particularity to measure an acceleration or rotation in several directions at once. These enable even more miniaturization, but also more precise sensors since it would make the combination of different signals from different sensors to know the direction of acceleration or rotation obsolete. A six-axis IMU measures linear acceleration along X, Y,and Z axes, and rotation around the same three orthogonal axes.
Advances in MEMS-based inertial sensors
MEMS-based inertial sensors span forty years of research and commercialisation that started with single-axis linear accelerometers, followed by gyroscopes and then magnetic and barometric pressure sensors. Through advances in MEMS design, processing and packaging, more sensing features have been incorporated into a single chip or via a few MEMS chips in a small surface mount package. Not only have multiple sensors and axes been compressed into a tiny IMU package, but the performance of the inertial sensors over temperature and in aggressive applications has also improved. MEMS gyroscope zero drift bias has improved from degree per second errors in the 1990s to fractions of a degree per hour errors in 2020. After successful development and commercialisation, billions of inertial MEMS sensors are manufactured every year and found in drones, smartphones, tablets, toys, vehicles and wearables.
High accuracy gyroscopes, called hemispherical resonant gyroscopes (HRGs), were being produced for aviation and spacecraft by Delco, Litton (acquired by Northrop Grumman), Safran and other companies. These led to the smaller micromachined ring gyroscope chips developed in the 1990s at the University of Michigan with Delco Electronics (spun off as Delphi Technologies) as well as British Aerospace and now manufactured by Silicon Sensing Systems. The micromachined ring formed part of the MEMS chip. It required vacuum sealing to achieve a Q (measure of resonant signal peak sharpness) of 2,000 and thus deliver improved performance. The higher the Q, the better the performance.
1890 wine glass HRG1990s single-axis MEMS silicon nitride ring gyroscope2020 quartz MEMS HRG
The single-axis MEMS gyroscopes of the 1990s had zero rate drift and noise levels of 3 to 5 degrees per second. MEMS HRGs, such as the one in the right image of figure , are still under development at the University of Michigan. The single-axis glass micro HRG shown has a Q of over 5 million and a bias stability of 0.0014 degree/hour. The new MEMS HRG work at the University of Michigan has been spun off for commercialisation as Enertia Microsystems. While these high-performance HRGs were developed for aerospace navigation, the advent of autonomous vehicles and tighter specification for advanced driver-assistance systems (ADAS) could see wider market adaptation for this ever-improving inertial technology. A single-axis ring gyroscope was coupled with a linear accelerometer on the same chip in 1998. Only a single-axis gyroscope could be made on a MEMS chip with resonant ring technology. To make multi-axis gyroscopes on MEMS chips required the use of planar, gimballed capacitive micromachines.
Gyroscopic sensing in all three axes, namely yaw, roll and pitch, was available on a single MEMS chip in the mid-2000s. These were quickly combined with 3-axis linear accelerometers for 6 degrees of freedom (DOF) inertial sensors. Silicon comb fingers could electrostatically drive and sense motion in the X-Y plane while underlying conductive electrodes below the micromachine could sense motion in the Z-axis. By 2010, low-cost, consumer grade 3 and 6 DOF devices were offered by Bosch, Fairchild Semiconductor (acquired by ON Semiconductor), InvenSense (acquired by TDK), Maxim Integrated (acquired by Hanking Electronics) and STMicroelectronics. As shown in figure , the key to putting both vacuum packaged resonant gyroscopes and damped linear accelerometers on one MEMS chip was dual cavity wafer-level packaging (WLP).
A single chip 6 degrees of freedom (DOF) MEMS IMU.
For long-term high reliability, improved methods of wafer-to-wafer bonding are required to ensure that small diameter gases such as helium do not leak through the sealing interface10. Even the 5-parts per million (ppm) level of helium in the Earth’s atmosphere has been found to penetrate some WLP vacuum seals at the high end of the operating temperature range (85 to 125˚C), and nobel gases such as helium are not absorbed by chemically reactive getters. To maintain high Q for decades of use, resonators in conventional and MEMS HRGs must take care in vacuum sealing the devices. As shown in figure, WLP has enabled a dramatic shrinkage in IMUs over the last couple of decades. The small surface mount large grid array (LGA) packages on the right are incorporated into smart phones and watches for personal navigation.
1999 50 x 50 mm printed circuit board (PCB) 6 DOF IMU, incorporating MEMS ring gyroscope, through-hole single in-line packages (SIPs) and dual in-line packages (DIPs)2016 3 x 3 mm 6 DOF large grid array (LGA) IMU produced by Hanking Electronics
Further improvement in MEMS inertial sensors have employed mounting both the MEMS and CMOS chip in thin, mechanically and thermally isolated platforms inside a cavity ceramic package. The sensing chips on the isolation platform are solder vacuum sealed, with a thin film getter patterned on the Kovar lid, in hermetic ceramic packages. These micromachined insulating platforms dampen vibration and shock and can even employ heating coils to keep the sensing chips in a constant, slightly elevated temperature range, resulting in a 10-fold improvement in performance over temperature versus conventional ceramic and plastic packages.
The printed circuit board (PCB) of the large sensor module could accommodate barometric pressure sensors, high- or low-g accelerometers, gyroscopes and other sensors needed for automotive applications. The small surface mount IMUs that have been developed for mobile applications in the last few years are also integrating magnetic and barometric pressure sensors. The magnetic sensors can use the Hall-effect, magneto-resistance, Lorentz force and other magnetic sensing methods. Magnetic sensors can measure the Earth’s magnetic field and are often coupled with the linear accelerometers in the IMU via sensor fusion algorithms. The linear accelerometers can detect the Earth’s gravity or tilt of the IMU. Coupling the magnetic sensors and accelerometers can provide personal orientation with respect to magnetic north, also called an e-compass.
Similar to the accelerometers and gyroscopes, the three small magnetic sensors are placed orthogonally with respect to each other in the x, y and z axes as a 3 DOF magnetic sensor. By stacking the magnetic sensor chips on top of the MEMS motion sensors, 9 DOF inertial sensors have been made in small surface mount packages for smart phone applications. MEMS barometric pressure sensors, such as the one shown in figure 4, have also been incorporated in IMUs for altitude measurement and are available in 5 x 5 mm surface mount packages. They enable the user to navigate as well as be located and tracked without the need for GPS.
In the future, MEMS gyroscopes may employ an optical mechanism instead of the resonant Coriolis effect. Optical gyroscopes were first developed in the 1970s using the Sagnac effect and coils of fibre optic cable. The Sagnac effect splits a beam of light, and the two separate beams follow the same path but in opposite directions through the coils of fibre optic cable. On their return to their starting or splitting point, the two beams are allowed to exit the ring and undergo interference. The interference fringes of the relative phases of the two exiting beams are shifted according to the angular velocity of the apparatus.
These optical ring laser gyroscopes (RLGs) do not have moving parts. Improvements in RLG performance intensified in the 1990s and continue today. Higher accuracies, capable of even detecting the Earth’s rotation, have been achieved using ring diameters of 1 to 4 m. Smaller, MEMS-based RLG prototypes have been developed by several groups using oxide and nitride waveguides. Conventional RLGs can wind three-dimensional coils of fibre optic cable with relatively large diameters, while MEMS-based RLGs are limited to two-dimensional waveguides with diameters less than 10 mm. A single-chip RLG has not yet been commercialised and one would expect the accuracy to be lower than the larger diameter RLGs that are sold commercially.
Smartphone Accelerometers Can Be Fooled by Sound Waves
Inertial sensors were thought to be immune to jamming and spoofing since they do not rely on radio frequency signals. However, A research team led by Kevin Fu of the University of Michigan have found that they could fool accelerometers using sound waves—in particular, a single tone played at an accelerometer’s resonant frequency. With it, they can cause two signal processing components within the phone to generate a false report of the accelerometer’s behavior.
The University of Michigan team tested 20 models of capacitive micro-electromechanical (MEMS) accelerometers from five manufacturers: Bosch, STMicroelectronics, InvenSense, Analog Devices, and Murata Manufacturing. An accelerometer contains a physical mass placed on springs. When a device moves, the mass does too. The movement causes the capacitance—the ability to store charge—to change in the springs, which can be interpreted as movement. The researchers launched a series of attacks on the accelerometers by using sound waves to impart a physical force to move the mass of accelerometer in a particular way.
The team subjected the sensors to two types of attacks using sound waves at the resonant frequency. The first, called output biasing, exploits a feature of the low pass filter, a signal processing component that filters out high frequency interference. This technique can be used to slightly alter readings produced by the accelerometer for several seconds, as reported by Amy Nordrum in Spectrum. The second, called output control, takes advantage of the phone’s amplifier, which typically handles the raw signal even before it reaches the low pass filter. This method can be used to take control of the accelerometer indefinitely and produce false signals. They found that 75 percent of the accelerometers could be fooled by an attack that allowed them to slightly alter the sensors’ signals for a brief moment, and 65 percent were vulnerable to a more severe attack that allowed the team to control their signals indefinitely.
The researchers then tried their hacking on several products, They loaded a a music video on a Samsung Galaxy S5, with the accelerometer’s resonant frequency embedded in it, and remotely prompted the phone to play the video. At the same time, they ran a game on the phone called Spy Toys that relies on the accelerometer to control a toy car. While the video played, the toy car accelerated or decelerated in accordance with the pulses of the signal they had embedded in the video.
In their final demo, they used an off-the-shelf speaker to play a tone that caused a FitBit to log 2,100 steps in just 40 minutes, earning them 21 reward points on a health tracking site (they declined to cash in their points, citing ethical concerns). Fu suggests accelerometer designers choose a resonance in the ultrasound range to prevent such attacks as it is more difficult to generate with off-the-shelf speakers. And encasing devices in foam is a good way to stop sound waves from reaching a device’s accelerometer, though not always practical. “If we can’t be sure [sensors are] trustable, we need to limit the kind of security decisions we’re making off of them,” said Patrick McDaniel, University of Pennsylvania.
In future MEMS-based sensors for pressure, vibration, and location shall also be key enabling devices for the Internet of Things (IoT) as they make practical the long-term monitoring of key parameters in industry, commercial, and consumer/home environments. According to a December 2014 report “Internet of Things begins to impact High-Value MEMS” by specialist Jérémie Bouchaud from IHS Technology, “the worldwide market revenue for MEMS directly used in industrial IoT equipment will rise to $120 million in 2018, up from $16 million in 2013. In addition, MEMS will also be used to support the deployment of the IoT, such as devices employed in data centers. This indirect market for industrial IoT MEMS will increase to $214 million in 2018, up from $43 million in 2013,”
“The impressive opportunities in mass applications such as the Internet of Things and autonomous/ semiautonomous vehicles for land, water, and air are also due to the multiple sensors which have allowed remote guidance and navigation (with or without GPS), such as electronic compasses, and MEMS-based motion, acceleration, and inertial measurement units. In broad context, the availability of these high-performance devices has completely changed the design tradeoff balance, transforming sensors from large, costly, heavy, power-hungry transducers with incompatible electrical interfaces to their complete opposite,” says Bill Schweber
The global MEMS Inertial sensor market by application, the market is segmented into consumer electronics, healthcare, industrial, aerospace & defense, and automotive among others. The MEMS accelerometer sensors are widely used in electronic devices such as laptops and smartphones. The accelerometer is integrated in laptops as it detects the sudden free fall and immediately turn off the hard drive to prevent its damage. The accelerometers are integrated in smartphones to provide various features such as image stability, shock detection, menu navigation, text scroll, gaming control, silent mode activation, and motion dialing, and others. Moreover, the capability to accurately track the motion using sensors based on MEMS technology has led to the adoption of MEMS sensors in the health and fitness devices.
The geographical analysis of the global MEMS & sensors market is studied for North America, Europe, Asia-Pacific, and the rest of the world. The product terrain of the Mems Inertial Sensors market is categorized into Accelerometers,Gyroscopes andMagnetometers. The application scope of the concerned products is classified into Automotive,Consumer Electronics,Medical,Industrial andOthers
North America is one of the early adopters of technology and holds a significant market share among other regions due to the presence of leading market players such as Texas instruments, InvenSense, Qualcomm, Honeywell, Allego microsystems, and Bosch. The other factors which are impacting the growth of MEMS & sensors market in this region are increasing adoption of smart devices such as smartphones, smart wearables, PDAs, and computing devices.
Whereas, Asia-Pacific holds the largest market among other regions due to the proliferating Chinese market owing to the growing number of smart consumer electronics devices and the presence of major key players such as Mega Chips Corporation, Panasonic, Denso Corporation, and Hitachi Ltd. This region is expected to witness a consistent growth rate throughout the forecast period (2018–2023)
Some of the key players of global MEMS inertial market are Asahi Kasei Microdevices Corp., Robert Bosch GmbH, InvenSense Inc., STMicroelectronics N. V., Alps Electric Co. Ltd., Analog Devices Inc., Freescale Semiconductor Ltd., Kionix Inc., Memsic Inc., Texas Instruments Inc., Epson Electronics America Inc., Fairchild Semiconductor International Inc., Honeywell Aerospace, Colibrys Ltd. based on the data collected and analyzed.
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