ACCELEROMETERS

ACCELEROMETERS

1.      INTRODUCTION

Acceleration is the rate of change of either the magnitude or the direction of the velocity of an object, and it is measured in units of length per time squared (i.e., m/s2) or units of gravity (g) (1 g¼9.81 m/s2). Devices that measure acceleration, or accelerometers, are used in high performance devices such as missile guidance systems, airbag deployment systems in automobiles, in handheld electronics, as well as in biomedical applications such as motion analysis or assessment of physical activity.

In the most general terms, an accelerometer consists of a mass, spring, and damper. The acceleration of the mass is measured by the deformation of the spring, and oscillation of the spring is controlled by the damper. The acceleration of the mass should not affect the actual acceleration of the object being monitored, so size is an important design criterion.

The earliest devices that were designed in the 1930s were based on strain gages in a Wheatstone bridge configuration on a small frame supporting a mass weighing over 3 pounds (1). Current accelerometers are typically electronic devices based on piezoelectric or semiconductor technology. Depending on the device and its design, an accelerometer can measure very small accelerations caused by vibration (micro g), or very large accelerations during motion or impact (thousands of g’s).

2. THEORY

Most accelerometers contain a known mass (sometimes referred to as a seismic mass or a proof mass) and some means for measuring either the resultant force or the displacement when the mass is accelerated. These types of accelerometers work by the principle of Newton’s second law of mechanics, which states that the force (F) acting on an object that is accelerating will be proportional to its acceleration (a), and the proportionality constant is the mass (m) of the object: F¼ma.

An example of a simple accelerometer is a mass attached to a spring and ruler, where the deflection of the spring is proportional to the force acting on it. As the force is directly proportional to the acceleration of the mass, acceleration in the direction of the ruler can be calculated by measuring the spring deflection. Most electronic accelerometers use a similar principle, measuring the displacement of the mass in the desired direction and producing an output signal proportional to the acceleration.

Piezoelectric materials, which develop an electric charge when deformed, were used to make some of the first modern accelerometers in the 1940s and 1950s and are the most common and cost-effective types of accelerometers being used today. Such a device is typically comprised of a piezoelectric crystal supporting a small mass. Acceleration produces a force proportional to the mass, which causes a change in the electrical output of the piezoelectric crystal.

Piezoresistive accelerometers measure the force produced by the acceleration of an internal mass by the change in resistance of the material (often silicon based) when a force is applied to it. Recently developed thermal accelerometers do not use amass/spring configuration, but instead sense acceleration by changes in heat transfer between micromachined elements within a small, insulated space.

3. PERFORMANCE

The optimum performance of an accelerometer is based on the linearity of its response, its frequency bandwidth, its sensitivity to acceleration, and minimal cross-sensitivity. All electronic devices have some useable range in which their response is linear (i.e., the electrical output is directly proportional to the parameter being measured).

Mass-spring-damper accelerometers have a natural frequency at which they oscillate that is particular to their design and construction, also called the resonant frequency. The usable frequency range is the flat area of the frequency response curve, which is a plot of the deviation in output versus frequency. Frequencies below about one-third to one-half of the resonant frequency are generally within the range in which the device response is linear,

and this range is referred to as bandwidth (see Fig. 1).

Sensors that can sense constant accelerations are said to have a ‘‘DC response’’ (0 Hz). Those that are functional below several hundred Hz are referred to as ‘‘low frequency’’ sensors. The upper limit of the bandwidth is determined by the physical properties of the device such as the dimensions of the mass and beam, the location of the mass on the beam, and the elastic modulus of the material that it is made from. The lower limit is governed by the amplifier and electronic response of the device. These parameters are all optimized to provide the widest possible bandwidth or range of operation for a specific device.

The sensitivity of the accelerometer is defined as the ratio of the change in output to the relative change in acceleration. As sensitivity is determined by the same physical properties as bandwidth, there is a tradeoff between sensitivity and bandwidth for most devices (except thermal accelerometers). A small mass usually means a lower sensitivity, which is typical for most high-frequency accelerometers. Accelerometers generally have an output that is some voltage that is proportional to acceleration, reported as a ratio of the output to input. Output units are typically mV/V/G. As the output is dependent on the input, or excitation voltage, care must be taken to provide a properly regulated input voltage.

Accelerometers are designed to be sensitive to accelerations only in a specified direction. However, the accelerometer will also sense accelerations in other directions to a certain degree, which is called cross-sensitivity or transverse sensitivity, which can contribute to errors in measurement and is usually quantified and reported by device manufacturers. All devices have some sensitivity to fluctuations in temperature that may affect their performance, which is also reported by the manufacturer, and the thermal environment should be taken into consideration when choosing a device for a particular application.

Electronic devices can be damaged by overloading or by accelerating it beyond its normal range of operation. Some devices have overload protection built in to their design, which will protect against accelerations during typical handling. However, a trade-off exists between protective mechanisms and damping of the device response, so these mechanisms have limitations. Dropping a device can expose it to accelerations of up to 400 g, which is difficult to accommodate for in a low-range device. The overrange limit is usually specified by the manufacturer.

As gravity is acceleration toward the center of the Earth, accelerometers that have a DC response are sensitive to gravity. An accelerometer at rest or in steady-state motion (i.e., not subject to other accelerations caused by motion) with its sensitive axis pointing toward the center of the Earth will have an output equal to 1 g. This property is commonly used to calibrate the gain and zero offset of an accelerometer by positioning the sensitive axis at a known

angle relative to vertical and measuring the output.

High-frequency accelerometers, such as piezoelectric accelerometers, have a small physical size to ensure a high natural frequency (above 10 kHz). They are used in applications such as vibration and impact monitoring.

Devices that provide the best DC response include piezoresistive, capacitive, force balance, and thermal accelerometers. These devices would be used in navigation and robotic applications as tilt sensors, constant acceleration measurements, or sensors taking measurements over prolonged periods.

Table 1 summarizes some typical performance characteristics of devices used for biomedical

applications.

4. PHYSICAL FEATURES

Semiconductor accelerometers are often MEMS, which are a combination of mechanical and electrical components made from silicon using micromachining processes. Piezoelectric devices are usually machined and packaged with necessary electronic components for filtering and

amplifying their output, and are therefore referred to as Integrated Electronics Piezo Electric (IEPE) devices.

Semiconductor accelerometers measure acceleration in a variety of ways. Piezoresistive devices have resistive elements embedded in the spring components, which sense deflection as a change in resistance. Piezoelectric devices have a layer of material on the springs that responds to deflection with a change in electrical output. Others work like plate capacitors, with the silicon being machined into tiny comb-like structures that move with the attached mass when they are accelerated. The change in the distance between the combs causes a change in the electrical output. Capacitive devices have advantages of low power consumption, a wide range of operation, and low thermal sensitivity.

Some designs combine these technologies to form a force-balanced capacitive accelerometer, typically used in automotive technology. These devices measure the force required to maintain a proof mass in a central, preset position. One drawback of this design is that because they are closed-loop accelerometers, the feedback loop slows down the response time, resulting in a smaller bandwidth than other devices. However, they have other features that are desirable for high-precision applications, such as accuracy and high signal-to-noise ratios.

Thermal accelerometers have recently been developed that sense acceleration by changes in heat transfer between micromachined elements within a small, insulated space on a silicon wafer. Some rely on thermal sensing for a seismic mass; as the mass is accelerated toward a heat source within the device, heat flows from the source to the mass and is sensed as acceleration. Other thermal accelerometers have no mass or spring elements, and therefore they are not subject to the typical trade-off between size/ weight and sensitivity. They are made using MEMS technology, and are more cost effective than other mass and spring MEMS systems because of their ease of manufacturing. They are highly shock-resistant (reportedly up to 50,000 g) and are less prone to noise and drift than other MEMS devices. The main disadvantages of thermal devices are their limited frequency response (usually less than 100 Hz) and sensitivity of some designs to the temperature of their environment.

All electronic devices require power and a means to measure their output. The power requirements for accelerometers vary depending on the type of device. In general, piezoelectric devices require a current in the milliamp range, and have a preamplifier and signal conditioners packaged within the device. Especially for small devices, this input can have a significant thermal effect on the device, and therefore they require a warm-up period

before use.

Accelerometers are usually packaged in a case that allows them to be firmly adhered to the object being measured in order to minimize the effects of damping or vibration. The sensitive axis is indicated on the outside of the package. Multiple axis accelerometers are available, either as multiple single-axis devices within one package, or as an inherently multiple-axis device.

 Usually, the axes are aligned orthogonally (at right angles to each other), and the multiple axes are indicated on the package. The outer case is usually metal, such as stainless steel, titanium, or a composite, which serves to protect and seal internal components. Accelerometers come in a variety of forms and sizes, such as through-hole and surface mount packages for use on a circuit board (Small Outline Integrated Circuits, or SOICs). Others are in a self-contained form that can be attached directly to a data acquisition system or in a package that can be attached to the object whose acceleration is being measured. The forms typically include a tapped hole or stud (threaded or unthreaded), so they can be bolted securely to the object being monitored. Adhesives such as hot glue or double-sided tape can also be used to hold the device in position.

Depending on the inherent stiffness of the device, the amount of torque applied when bolting it in place can change the output. Care must be taken to ensure consistent mounting torque when taking measurements over time or when comparing measurements between locations

or devices.

Actual accelerometer sizes can be as small as a few hundred microns for MEMS devices, but the final size of the product is larger, typically several millimeters, because it is packaged together with the necessary electronics to amplify and process the output. The mass of the device should be small relative to the mass of the object being monitored. Otherwise, the behavior of the object may be affected, a condition referred to as mass loading. For some applications where size constraint is critical, or for high-frequency measurements, a miniature device may be required, generally considered to weigh less than 100 grams. Although these devices use advanced manufacturing methods and can be highly sensitive and accurate, there are trade-offs between size and stability, ruggedness and cost.

5. APPLICATIONS

The measurement of the acceleration of limb segments has been used for human motion analysis, motion detection, vibration, and impact studies. To measure the linear acceleration as well as the angular acceleration and angular velocity of the center of mass of a body segment in two or three dimensions, groupings of six to twelve linear accelerometers can be placed on a body segment such as the upper body (6) or the lower limb. However, it has been found that because of the magnitude of experimental errors, accelerometers alone may not be a reliable method of calculating limb kinematics, such as position and orientation.

 As semiconductor and MEMS technology develop, it is likely that accelerometers will become smaller, more sensitive, and more accurate. In fact, the performance capabilities of these devices has greatly improved over the past decade, and it is possible that this will lead to more

widespread use for human motion analysis.

Vibration studies have been conducted to examine the shock-absorbing properties of joints and soft tissue structures such as the spine and lower leg. Accelerometers can quantify the accelerations being sensed by these parts of the body and the shock attenuation that takes place within the soft tissue structures. This information is useful for establishing the long-term effects of exposure to low-level vibrations such as lower back pain and other musculoskeletal disorders. Although industrial vibration sensors require a high bandwidth, those used for human body vibration monitoring are typically below 60 Hz. Studies of accelerations caused by impact during normal walking and running have found accelerations of up to 10 g during running, with a frequency range of up to 100 Hz, which has significance in the area of athletic shoe design, as well as the study of tissue damage caused by high impact.

Detection of motion and the initiation of movement have been investigated by researchers for position feedback in systems being investigated to restore gait for paraplegics and stroke victims. With functional neuromuscular stimulation (FNS), the lower limbs are stimulated to move, and accelerometers provide feedback on body segment and joint positions. Another application of accelerometers in motion detection is for monitoring balance during walking. Accelerometers and gyroscopes, which measure angular velocity, can be combined to form an inertial sensing system that works similarly to the human vestibular system, providing complete information on linear and angular accelerations. Several researchers and manufacturers are investigating ways to combine three-axis accelerometers and gyroscopes into a six-degree of freedom inertial sensing device.

One of the most common biomedical applications of accelerometers is in the area of physical activity monitoring. The use of accelerometer-based monitors is gaining widespread acceptance as a means to monitor physical activity of patients undergoing rehabilitation or treatment for mobility-related disorders, as well as activity of the general public. Accelerometer-based pedometers log the number of acceleration spikes identified as ‘‘steps,’’ And are readily available to consumers through several manufacturers.

Other accelerometer systems are used by physical activity researchers to discern not only the number of movements, but the type of activity, such as sitting, standing, lying down, and dynamic activity. These are sometimes referred to as activities of daily living (ADL) monitors. The accelerometer unit is strapped somewhere on the trunk and is wired to a separate data logger, which stores and displays the cumulative data. These typically use a time-frequency analysis to establish the duration of the different activities. The latest research efforts in physical activity monitoring are focused on discriminating between different dynamic activities, such as walking and stair climbing, which cannot be achieved with currently available devices.

Human motion, biofeedback, and wireless computer interfaces all make use of accelerometers as tilt sensors, which measure the direction of an object relative to the ground. Future applications include tilt sensors for easier manipulation of handheld devices such as cell phones and personal digital assistants (PDAs). Researchers are investigating data entry to a device by a combination of tilting it in different directions and allowing multiple devices to communicate by tapping them together like champagne glasses, which holds promise for use in for adaptive  devices for the disabled and feedback devices for rehabilitation.