Todd Biesiadecki
Advisor: Erick Maxwell
Group: TB-Eye
Integrated Circuit Accelerometers
Introduction
As electronics become more and more a part of everyday life, the demand for circuits to
receive and interpret mechanical signals has grown. Because of this, integrated-circuit
accelerometers have found widespread use in applications including medical diagnostics,
automotive airbag systems, and entertainment systems. This paper describes some common
commercial applications of accelerometers, summarizes the technological concepts behind them,
and gives instructions for implementing them in a real-world application.
Commercial Applications
Accelerometers are capable of detecting a variety of physical phenomena, and have
therefore found use in many different applications. Low-range multiple-axis accelerometers have
been used in cellular phones, handheld electronic devices, and video game systems to detect tilt.
High-g accelerometers are often employed to detect shock a variety of applications including
automotive collision detection, brain injury prediction, and computer hard drive protection.
Accelerometers are also able to measure periodic vibrations, making them suitable for monitoring
the status of machinery such as fans, motors, and compressors [1].
The ADXL78, designed by Analog Devices, is an example of a high-g accelerometer IC
currently on the market. It is capable of measuring acceleration values of up to 70g in either
direction along one axis. The output of the circuit is an analog voltage signal proportional to the
measured acceleration. The device has a bandwidth of 400 Hz and a sensitivity of 27 mV/g. It
requires a power supply of 5 V and draws a current of 1.3 mA [2]. Because of its high range and
low bandwidth, it is ideal for applications in which high-g impacts are in view. The ADXL78
costs $5.58 per unit when purchased in quantities of 1000 units or more [3]
The MMA2301, made by Freescale Semiconductor, is another high-g accelerometer that
is now on the market. It has a 200g range along one axis, and outputs a radiometric analog
voltage signal. Its bandwidth is 400 Hz and its sensitivity is 10 mV/g. It runs on a 5V power
supply and draws between 3 mA and 6 mA or current [4]. Since it has a high acceleration
measurement range, it is best suited for applications involving impact detection. This device costs
$3.99 per unit when purchased in quantities of 1000 units or more [5].
Underlying Technology
The structure of an accelerometer varies depending on the device’s targeted application.
One of the common types of accelerometers is the capacitive accelerometer. In these devices, a
change in velocity causes one of the plates of a capacitor to move, which alters the distance
between the plates of the capacitor. In some designs, the movable plate moves translationally and
is anchored by a spring [2]. In others, the plate is attached to a torsion rod and moves rotationally
[6]. The rest of the circuit interprets the resulting change in capacitance and converts it into an
analog or digital output signal. Capacitive sensors are often preferable over other types of sensors
because they offer higher resolution and better sensitivity. Accelerometers of this type are used in
applications such as airbag systems, instrumentation, and navigation [7].
Another common type of accelerometer is the piezoelectric accelerometer. These types of
devices make use of the piezoelectric effect of some materials to convert periodic accelerations
into electrical signals. When pressure is applied to a piezoelectric material, charged particles
within the crystal structure are displaced. This displacement of charges can be read as an
electrical signal. Piezoelectric accelerometers are often advantageous because of their wide
bandwidth, good linearity, and temperature insensitivity. Since they cannot measure static
accelerations, they are undesirable in applications in which DC accelerations are in view [8].
Implementation
The implementation of an integrated-circuit accelerometer onto a circuit is relatively
simple. Many accelerometer ICs are surface-mounted packages, and therefore need to be soldered
onto a PCB. The data sheets for the accelerometers will provide the designer with pinouts and
instructions on how the IC should be oriented. Many accelerometer applications include a
microcontroller that can read the acceleration data and send the appropriate commands to other
parts of the system in response. The designer should ensure that a microcontroller with the
appropriate input interfaces and suitable power consumption specifications is chosen. Also, to
minimize measurement error, the accelerometer should be very closely coupled to the object
whose acceleration is being measured. Finally, to avoid excessive power consumption, the
designer may use the microcontroller to sample the accelerometer slowly or turn it off when
measurements are not being used [9].
References
[1]
Analog Devices. “The Five Motion Senses: Using MEMS Inertial Sensing To Transform
Applications”, analog.com, 2009. [Online]. Available: http://www.analog.com/en/memssensors/high-gaccelerometers/products/whitepapers/over_Five_Motion_Senses/resources/fca.html
[2]
Analog Devices, “Single-Axis, High-g, iMEMS Accelerometers: ADXL78” ADXL78
datasheet, 2010.
[3]
Analog Devices, “Analog Devices, “ADXL78: Single-Axis, High-g, iMEMS®
Accelerometers,” analog.com, September 2011. [Online]. Available:
http://www.analog.com/en/mems-sensors/high-gaccelerometers/adxl78/products/product.html
Freescale Semiconductor, “Surface Mount Micromachined Accelerometer:
MMA2301KEG” MMA2301KEG datasheet, November, 2009.
[4]
[5]
Digi-key, “Digi-key MMA2301EGNR2-ND,” digikey.com, September, 2011. [Online].
Availible:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MMA2301EGR2ND
[6]
Silicon Designs, Inc., “Technology,” silicondesigns.com. September, 2011. [Online].
Available: http://www.silicondesigns.com/tech.html
[7]
Beliveau, A.; Spencer, G.T.; Thomas, K.A.; Roberson, S.L. (1999, Oct.). Evaluation of
MEMS capacitive accelerometers. Design & Test of Computers, IEEE [Online].
16(4),.48-56. Available:
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=808209&isnumber=17507
[8]
Piezocryst, “Basics: Piezoelectric Sensors,” piezocryst.com, September, 2011. [Online].
Available: http://www.piezocryst.com/piezoelectric_sensors.php
[9]
Analog Devices, Appl. Note AN-601, October, 2002.