What is MEMS Accelerometer?

MEMS accelerometer are micro electro mechanical systems that measure the static or dynamic force of acceleration.  Static force refers to the earth’s gravitational pull.  On the surface of the Earth, all objects fall with an acceleration defined as one “g”, which is approximately equal to 9.81 m/s² (32.17 ft/s²).  Quite often, “g” is used as a unit of acceleration rather than expressing acceleration in m/s² or ft/s².  Dynamic forces result when movement or vibration is applied to the accelerometer itself.  Accelerometers enable such functionality as screen orientation, hard-disk-drive protection, gesture recognition for interactive games, activity monitoring for power management, and much more.

How they Work
Scientists have now refined accelerometer design and sensitivity to the point where small accelerometers can actually be attached to notebooks. These notebooks are then used at earthquake sites to provide both quake measurements as well as a place to immediately write down pertinent information that the accelerometer may reveal while researchers are visiting the quake site.

Surprisingly enough, delicate accelerometers can also be used to track animals. By measuring wavelength frequencies and muscle acceleration, among other things, researchers are able to keep track of an animal’s behavioral patterns even when that animal may be out of sight.

And by using an accelerometer to determine the frequency of the animal’s limb strokes or using accelerometer data to measure their Overall Dynamic Body Acceleration, animal scientists are equally able to discover how much energy an animal uses in the wild and how quickly they expend that energy. These qualities have made accelerators very popular with marine biologists and others who study water animals, since their subjects will usually be hidden from view for the majority of their research.

There are medical applications for accelerometers as well. Many walkers and runners now enjoy a specialized type of pedometer that employs accelerometer technology in order to calculate speed and distance. They have also been used in emergency situations to determine the number and depth of chest compressions during CPR.

Other fields that regularly employ accelerometers include navigation, transportation, consumer electronics, and structural integrity which feed into construction, architecture, and other building-related trades. In short, wherever a measurement of vibrations, pressure, energy expenditure, or of course acceleration is useful, an accelerometer is key.

Analog Devices accelerometers and iSensor® MEMS accelerometer subsystems provide accurate detection while measuring acceleration, tilt, shock, and vibration in performance driven application. Our portfolio leads the industry in power, noise, bandwidth, and temperature specifications, and offers a range of MEMS sensor and signal conditioning integration on chip. Our MEMS-based Circuits from the Lab® reference designs have been built and tested by ADI experts to help you jumpstart your next system design.

MEMS accelerometers use nanotechnology in order to enhance the natural abilities common between all accelerators; hence, these devices are extremely fine-tuned and accurate. MEMS stands for Micro Electro Mechanical Systems, and when discussing the technicalities of accelerometers it refers specifically to a mass-dis placer that can translate external forces such as gravity into kinetic motion energy. This part of the accelerometer usually contains some type of spring force in order to balance the external pressure and displace its mass, thus leading to the motion that produces acceleration.

MEMS accelerometers are actually the simplest type, since they consist of little more than a seismic mass, also known as a proof mass, as well as a cantilever beam. Oftentimes residual gas becomes sealed inside the device, which can cause damping over time but is usually not too severe unless the Q-factor is too low. In this case the damping process can cause a loss of sensitivity.

The mechanism works like this: external force is applied and shifts the position of the proof mass from a neutral position to an active position; typically the amount of this deflection is measured by analog or digital readouts. The variations can be charted by using a set of beams that are fixed in place contrasting with a set of beams that have been attached to the surface of the proof mass somehow. Such a simple system makes the accelerometer not only reliable but also relatively inexpensive to manufacture.

In some cases there have been questions about the accuracy of the spring inside the mechanism, since it is known that spring devices can warp or deteriorate over time; in this case it is possible to apply a series of piezo resistors to the springs to detect any deformities and ensure their accuracy. If these additional parts are inserted, the manufacturing process becomes somewhat longer and more complicated and the device becomes slightly pricier. Top-notch MEMS accelerators are built with quantum tunneling in order to achieve the highest sensitivity possible. These accelerators are accurate enough that they can be measured optically and despite their high cost are the ideal laboratory equipment.

Less commonly, a MEMS accelerator may have a small heater inside the base of a dome-shaped structure which heats the air inside and causes it to rise. The dome is equipped with a thermocouple to mark where the heated air hits the dome’s upper shell, and by measuring how far off-center the air is, the level of deflection can be ascertained. This deflection measurement is the amount of acceleration that has been applied to the sensor. However, this type of accelerator is generally considered far inferior to the previous model.

Most accelerometers function on one axis, but two-axis and three-axis models have been invented. The three-axis model is naturally more expensive but also far more accurate; if this model isn’t used then three one-axis accelerometers will typically be combined after construction, with far less accurate results. There is also a direct relationship between the number of g’s that can be measured and the accuracy and sensitivity of the device. Usually the higher the device can measure, the more the accuracy suffers.

Nanotechnology has already enhanced many industrial areas, and now its effects can be seen in devices as specialized as the accelerometer. One of the crucial uses for MEMS accelerometers in particular has been airbag deployment systems; they literally save lives because they are able to judge when two cars have struck each other and even ascertain the severity of the collision, adjusting airbag size and rate of deployment accordingly.


Accelerometers: What They Are & How They Work in Your Mobile device

When we use a compass app on our smartphone, it recognizes which direction the device is pointing. Through astronomy apps, it recognizes where in the sky we’re watching to accurately show patterns. Smartphones and other smart technology recognize their positioning through the use of a mems accelerator, a tiny device made up of axis-based motion detecting.

The motion sensors in accelerometers also used to detect earthquakes, and medical devices such as bionic limbs and other artificial body parts. Some devices, part of the quantified self-movement, use accelerometers.

A mems accelerometer is an electromechanical device used to measure acceleration forces. Such forces may be static, like the continuous force of gravity or, as is the case with many mobile devices, dynamic to sense movement or vibrations.

Acceleration is the measurement of the change in velocity, or speed divided by time. For example, a car accelerating from a standstill to 60 mph in six seconds is determined to have an acceleration of 10 mph per second (60 divided by 6).

The purpose of the accelerometer

The application of accelerometers extends to multiple disciplines, both academic and consumer-driven. For example, accelerometers in laptops protect hard drives from damage. If the laptop were to suddenly drop while in use, the accelerometer would detect the sudden free fall and immediately turn off the hard drive to avoid hitting the reading heads into the hard drive platter. Without this, the two would strike and cause scratches to the platter for extensive file and reading damage. Accelerometers are likewise used in cars as the industry method way of detecting car crashes and deploying airbags almost instantaneously.

In another example, a dynamic accelerometer measures gravitational pull to determine the angle at which a device is tilted with respect to the Earth. By sensing the amount of acceleration, users analyze how the device is moving.

Accelerometers allow the user to understand the surroundings of an item better. With this small device, you can determine if an object is moving uphill, whether it will fall over if it tilts any more, or whether it’s flying horizontally or angling downward. For example, smartphones rotate their display between portrait and landscape mode depending on how you tilt the phone.

How they work

An accelerator looks like a simple circuit for some larger electronic device. Despite its humble appearance, the accelerometer consists of many different parts and works in many ways, two of which are the piezoelectric effect and the capacitance sensor. The piezoelectric effect is the most common form of accelerometer and uses microscopic crystal structures that become stressed due to accelerative forces. These crystals create a voltage from the stress, and the accelerometer interprets the voltage to determine velocity and orientation.

The capacitance accelerometer senses changes in capacitance between microstructures located next to the device. If an accelerative force moves one of these structures, the capacitance will change and the accelerometer will translate that capacitance to voltage for interpretation.

Accelerometers are made up of many different components, and can be purchased as a separate device. Analog and digital displays are available, though for most technology devices, these components are integrated into the main technology and accessed using the governing software or operating system.

Typical accelerometers are made up of multiple axes, two to determine most two-dimensional movement with the option of a third for 3D positioning. Most smartphones typically make use of three-axis models, whereas cars simply use only a two-axis to determine the moment of impact. The sensitivity of these devices is quite high as they’re intended to measure even very minute shifts in acceleration. The more sensitive the accelerometer, the more easily it can measure acceleration.

Accelerometers, while actively used in many electronics in today’s world, are also available for use in custom projects. Whether you’re an engineer or tech geek, the accelerometer plays a very active role in a wide range of functionalities. In many cases you may not notice the presence of this simple sensor, but odds are you may already be using a device with it.


MEMS (Micro Electro-Mechanical Systems) Technology

In less than 20 years, MEMS (micro electro-mechanical systems) technology has gone from an interesting academic exercise to an integral part of many common products. But as with most new technologies, the practical implementation of MEMS technology has taken a while to happen. The design challenges involved in designing a successful MEMS product are described in this article by Harvey Weinberg from Analog Devices.

In early MEMS systems a multi-chip approach with the sensing element (MEMS structure) on one chip, and the signal conditioning electronics on another chip was used. While this approach is simpler from a process standpoint, it has many disadvantages:
* The overall silicon area is generally larger.

* Multi chip modules require additional assembly steps.

* Yield is generally lower for multi chip modules.

* Larger signals from the sensor are required to overcome the stray capacitance of the chip to chip interconnections, and stray fields necessitating a larger sensor structure.

* Larger packages are generally required to house the two-chip structure.

Of course, history teaches us that integration is the most cost effective and high performance solution. So Analog Devices pursued an integrated approach to MEMS where the sensor and signal conditioning electronics are on one chip.

Figure 1

The latest generation ADXL2O2E is the result of almost a decades worth of experience building integrated MEMS accelerometers. It is the world’s smallest mass-produced, low g, low cost, integrated MEMS dual axis accelerometer.
The mechanical structure of the ADXL2O2E is shown in Figure 1 along with some key dimensions in Figure 2.



Figure 2

Polysilicon springs suspend the MEMS structure above the substrate such that the body of the sensor (also known as the proof mass) can move in the X and Y axes. Acceleration causes deflection of the proof mass from its centre position. Around the four sides of the square proof mass are 32 sets of radial fingers.

These fingers are positioned between plates that are fixed to the substrate. Each finger and pair of fixed plates make up a differential capacitor, and the deflection of the proof mass is determined by measuring the differential capacitance.
This sensing method has the ability of sensing both dynamic acceleration (i.e. shock or vibration) and static acceleration (i.e. inclination or gravity).

The differential capacitance is measured using synchronous modulation/demodulation techniques. After amplification, the X and Y axis acceleration signals each go through a 32KOhm resistor to an output pin (Cx and Cy) and a duty cycle modulator (the overall architecture can be seen in the block diagram in Figure 3). The user may limit the bandwidth, and thereby lower the noise floor, by adding a capacitor at the Cx and Cy pin.

The output signals are voltage proportional to acceleration and pulse-width-modulation (PWM) proportional to acceleration.
Using the PWM outputs, the user can interface the ADXL2O2 directly to the digital inputs of a microcontroller using a counter to decode the PWM.




Figure 3

Challenges in MEMS Design

The mechanical design of microscopic mechanical systems, even simple systems, first requires an understanding of the mechanical behavior of the various elements used. While the basic rules of mechanical dynamics are still followed in the miniaturized world, many of the materials used in these structures are not well mechanically characterized. For example, most MEMS systems use polysilicon to build mechanical structures. Polysilicon is a familiar material in the IC world, and is compatible with IC manufacturing processes.

Until recently, little work has been done to fully understand polysilicon’s mechanical properties. In addition, many materials mechanical properties change in the microscopic world. Again, polysilicon is a good example. In the macro world it is rarely used as a mechanical element. It is too brittle and fragile to withstand all but small mechanical deflections. But in the
extremely small movements of MEMS structures (less than a few pm), it turns out to be an almost ideal material.

The electronic design of MEMS sensors is very challenging. Most MEMS sensors (the ADXL2O2E included) mechanical systems are designed to realize a variable capacitor. Electronics are used to convert the variable capacitance to a variable voltage or current, amplify, line arise, and in some cases, temperature compensate the signal. This is a challenging task as the signals involved are very minute.

In the case of the ADXL2O2E for example, the smallest resolvable signal is approximately 2OzF and this is on top of a common mode signal several orders of magnitude greater than that! Of course, for cost reasons the
electronics must be made as compact as possible at the same time.

The integrated approach presented further challenges.
Many standard production steps that improve the mechanical structure degrade the electronics and vice versa. For example, the usual method for flattening out the Polysilicon mechanical structure is annealing (where the structure is exposed to controlled high temperatures). While the annealing process is beneficial to the mechanical structure, it can degrade or destroy the BiMOS transistors used in the signal conditioning electronics. So compatible mechanical and electronic process methods had to be devised.

Another roadblock for the MEMS designer has been the unavailability of standard design software. Modern integrated circuits are rarely designed by hand. Complex CAD and simulation software is used to help design and optimize the designer’s concepts.

MEMS design software is still in its infancy, and most MEMS manufacturers develop part or all of their CAD and simulation software to suit their particular needs.

The fabrication process design challenge is perhaps the greatest one. Techniques for building three-dimensional MEMS structures had to be devised. Chemical and trench etching can be used to “cut out” structures from solid polysilicon, but additional process steps must be used to remove the material underneath the patterned polysilicon to allow it to move freely.

Standard plastic injection molded IC packaging cannot be used because of the moving parts of the MEMS structure. A cavity of some type must be maintained around the mobile MEMS structure. So alternative low-cost cavity packaging was developed.

In addition, this package must also be mechanically stable as external mechanical stress could result in output changes.

Even mundane tasks, such as cutting the wafer up into single die, becomes complicated. In a standard IC the particle residue created by the sawing process does not effect the IC. In a moving MEMS structure these particles can ruin a device.

The Users Challenge
MEMS sensors, like almost all electronic devices, do not exhibit ideal behavior. While most designers have learned how to handle the non-ideal behavior of op-amps and transistors, few have learned the design techniques used to compensate for non-ideal MEMS behavior. In most cases, this type of information is not available in textbooks or courses, as the technology is quite new. So generally designers must get this type of information from the MEMS manufacturer.

Analog Devices, for example, maintains a web site with design tools, reference designs, and dozens of application notes specific to its MEMS accelerometers to ease the users work.

as with all new technologies both designers and users of MEMS devices have a learning curve to overcome. The effort is worthwhile, as the latest generation MEMS devices high performance and low cost have enabled innovative new products in dozens of markets.