What You Need to Know to Choose an Accelerometer

Simply put, an accelerator is an electromechanical device that measures change in velocity or the force of acceleration caused by gravity or movement over time. Whether sensing orientation and vibration, or detecting the speed of movement of an object it is attached to, selecting accelerometers involves a variety of considerations.

Accelerometers have a myriad of applications. When used in aviation and aerospace, highly sensitive accelerometers are used as inertial navigation system components. In industrial settings, they often detect and monitor vibration in machinery that rotates. The ones we are most familiar with in day-to-day living are resident in our tablet computers, digital cameras, games, and smartphones to ensure screen images are viewed in an upright position.

So, how do you know which accelerometer to choose? There are many questions to answer, starting with your design requirements:

• Do you need analog or digital?
• Do you need single or multiple axes?
• What level of swing?
• Do you need highly sensitive or mid-range?
• What bandwidth?
• Did you check impedance and buffering?

The choice between analog and digital is dictated by the hardware used. Analog accelerometers have an output that is a continuous voltage proportional to the acceleration. Digital accelerometers typically use pulse width modulation so that there is a square wave at a certain frequency. In this case, the time period of high voltage is proportional to the amount of acceleration.

The question becomes, what are you using in your design? A BASIC Stamp module from Parallax or a microcontroller with digital inputs will require a digital output accelerometer; when using a completely analog-based circuit, analog is usually the best choice.

For most designs, two axes are sufficient. Naturally, a 3D project will require a 3-axis accelerometer, or possibly using two 2-axis accelerometers mounted at right angles. Most accelerometers are designed to sense movement in one direction, so 3D position sensing uses three crystals mounted in different orientations each with their own floating mass.

When measuring tilt using Earth's gravity, a ±1.5 g accelerometer will be more than sufficient regarding swing. If measuring the motion of a car or a robot, ±2 g is recommended. In addition, if your project involves sudden starts or stops, consider an accelerometer that handles ±5 g or more. Greater sensitivity in the accelerometer results in more accurate readings, so the more sensitive, the better. Sensitivity is the ratio of change in acceleration (input) to change in the output signal. This defines the ideal, straight-line relationship between acceleration and output. Sensitivity is specified at a particular supply voltage and is typically expressed in units of mV/g for analog-output accelerometers, LSB/g, or mg/LSB for digital-output accelerometers. It is usually specified in a range (min, typ, max) or as a typical figure and percent deviation. For analog-output sensors, sensitivity is ratiometric to supply voltage; doubling the supply doubles the sensitivity.

Bandwidth ensures reliable readings and the question becomes how many times per second do you need the accelerometer to take a reading? Slow-moving, tilt-sensing apps need a bandwidth of 50 Hz, while vibration measurements may require several hundred hertz.

Impedance and buffering require close attention to documentation. Check to see specifications regarding A/D conversion and maximum output impedance to make sure the particular accelerator is right for your design.

Some examples

Initially, accelerometers were used in scientific markets. Enter cost reductions, advances in signal filtering, and power savings advances, and they found their way into games such as Wii, where the controller contained accelerometers that sensed tilt, movement, speed, and direction. Accelerometers in iPhones that note movement and tilt of the device are similar to those in the Wii, although on a smaller scale. Measuring the movement and tilt of the device allows the smartphone to tell which way the screen is held and to adjust the visual output accordingly.

More recently, laptop makers have used accelerometers to protect hard drives from damage. In this application, an accelerometer detects the sudden free-fall, and switches the hard drive off and into locked position so the heads do not crash on the platters. Similarly, in automotive applications, high-g accelerometers detect car crashes so the vehicle can immediately deploy airbags.

Looking at specific accelerometers, the ADXL362 from Analog Devices is an ultra-low-power, 3-axis MEMS (micro-electromechanical system) accelerometer that consumes less than 2 µA at a 100 Hz output data rate and 270 nA when in motion-triggered wake-up mode (a MEMS accelerometer measures the static or dynamic force of acceleration). The ADXL362 does not alias input signals by undersampling; it samples the full bandwidth of the sensor at all data rates.

The ADXL362 always provides 12-bit output resolution; 8-bit formatted data is also provided for more efficient single-byte transfers when a lower resolution is sufficient. Measurement ranges of ±2 g, ±4 g, and ±8 g are available, with a resolution of 1 mg/LSB on the ±2 g range. For applications where a noise level lower than the normal 550 µg/vHz of the ADXL362 is desired, either of two lower noise modes (down to 175 µg/vHz typ) can be selected at minimal increase in supply current.

In addition to its ultra-low power consumption, the ADXL362 has many features to enable true system-level power reduction. It includes a deep multimode output FIFO, a built-in micro-power temperature sensor, and several activity detection modes including adjustable threshold sleep and wake-up operation that can run as low as 270 nA at a 6 Hz (approximate) measurement rate. A pin output is provided to directly control an external switch when activity is detected, if desired. In addition, the ADXL362 has provisions for external control of sampling time and/or an external clock.

The ADXL362 operates on a wide (1.6 to 3.5 V) supply range, and can interface to a host operating on a separate, lower supply voltage. The ADXL362 is available in a 3 x 3.25 x 1.06 mm package. In the ADXL362, acceleration is reported digitally and the device communicates via the SPI protocol. Built-in digital logic enables autonomous operation and implements functionality that enhances system-level power savings.

Applications include hearing aids, home healthcare devices, motion-enabled power-save switches, wireless sensors, and motion-enabled metering devices.

Another low-power example is the SCA3060-D01 digital low-power accelerometer by Murata (Figure 1), used in non-safety-critical automotive applications. Features include a 3.0 to 3.6 V supply voltage, ±2 g measurement range, 16-bit SPI digital interface, very-low current consumption (3.3 V, 150 μA typ), 64 samples/axis buffer memory for output acceleration data, and advanced features enable significant power and resource savings at system level. The device features an interrupt signal triggered by motion, has a small 7.6 (W) x 3.3 (H) x 8.6 (L) mm footprint, and uses proven capacitive 3D-MEMS technology. The capacitive detection principal is based on the variation of the distance between two surfaces. The capacitance, or charge storage capacity of a pair of surfaces, depends on their distance and on the overlapping surface area.

The SCA3060-D01 is targeted at non-safety-critical automotive applications such as inertial navigation, vehicle alarms, inclination sensing, motion activation, and black-box systems.

For such applications as notebooks and cell phones, Freescale Semiconductor’s Xtrinsic MMA8452QT-ND 3-axis, 12-bit/8-bit digital accelerometer (Figure 2) is a smart, low-power, three-axis, capacitive, micro-machined accelerometer with 12 bits of resolution. The accelerometer has numerous embedded functions and flexible user-programmable options.

The MMA8452Q has user-selectable full scales of ±2 g/±4 g/±8 g with high-pass filtered data, as well as non-filtered data, available in real-time. The device can be configured to generate inertial wake-up interrupt signals from any combination of configurable embedded functions, allowing the MMA8452Q to monitor events and remain in a low-power mode during periods of inactivity. The MMA8452Q is available in a 3 x 3 x 1 mm QFN package.

Typical applications include e-compass applications, static orientation detection (portrait/landscape, up/down, left/right, back/front position identification), notebook, e-reader, laptop tumble and free-fall detection, real-time orientation detection (virtual reality and gaming 3D user position feedback), real-time activity analysis, motion detection for portable product power saving (auto-sleep and auto-wake for cell phone, PDA, GPS, gaming), and shock and vibration monitoring. Freescale also provides sensor drivers for Android, Linux, and other operating systems (Figure 3).

For gaming applications, Analog Devices’ ADXL337 is a small, thin, low-power, complete 3-axis accelerometer with signal-conditioned voltage outputs. This 3-axis ±3 g accelerometer features a small low-profile package (3 x 3 x 1.45 mm LFCSP), low-power (300 μA typ), single-supply operation (1.8 to 3.6 V), 10,000 g shock survival, and excellent temperature stability. The user selects the bandwidth of the accelerometer using the CX, CY, and CZ capacitors at the XOUT, YOUT, and ZOUT pins. Bandwidths can be selected to suit the application, with a range of 0.5 to 1,600 Hz for x and y axes, and a range of 0.5 to 550 Hz for the z axis.

The cost-sensitive, low-power, motion- and tilt-sensing applications that Analog Devices targets with this part include mobile devices, gaming systems, disk drive protection, image stabilization, and sports and health devices. The product measures acceleration with a minimum full-scale range of ±3 g. It can measure the static acceleration of gravity in tilt-sensing applications, as well as dynamic acceleration resulting from motion, shock, or vibration.

With accelerometers now an essential design feature in numerous consumer products, these devices are offered in a wide variety of technologies, sizes, shapes, ranges, and more. By answering the questions posed in this article (such as analog or digital, number of axes, size, sensitivity, bandwidth, among others) you can narrow down the choices to those best suited for your application. For more information on the accelerometers discussed here, use the links provided to access product information pages on the Digi-Key website.