Keywords: MEMS, Accelerometer, Gyroscope Sensors
`
` APPLICATION NOTE 5830
`ACCELEROMETER AND GYROSCOPES
`SENSORS: OPERATION, SENSING, AND
`APPLICATIONS
` By: Majid Dadafshar, Senior Member of Technical Staff (Field Application Engineering)
`
`Abstract: Microelectromechanical systems (MEMS) in consumer electronics are growing faster each year,
`with increasing demands from the mobile market, which is dominating the growth for this emerging
`technology. MEMS sensors are, in fact, becoming the key elements in designing differentiating products for
`consumer and mobile markets like game consoles, smartphones, and tablets. MEMS give the user a new
`way to interface with their smart device. This paper is an overview of MEMS: the principle of their operation,
`the sensing mechanism, and a variety of potential applications.
`
`A similar version of this article appeared March 2014 in EDN.
`
`Introduction
`Microelectromechanical systems (MEMS) combine mechanical and electrical components into small
`structures in the micrometer scale. They are formed by a combination of semiconductor and
`microfabrication technologies using micro machine processing to integrate all the electronics, sensors, and
`mechanical elements onto a common silicon substrate. Major components in any MEMS system are the
`mechanical elements, sensing mechanism, and the ASIC or a microcontroller. This article presents an
`overview of MEMS accelerometer sensors and gyroscopes. We discuss the principles of their operation,
`their sensing mechanism, the growing variety of applications for them, and the profound impact they are
`already having on our daily lives.
`
`MEMS as Inertial Sensors
`MEMS sensors have many applications in measuring either linear acceleration along one or several axis, or
`angular motion about one or several axis as an input to control a system (Figure 1).
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`Figure 1. Angular versus linear motion.
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`All MEMS accelerometer sensors commonly measure the displacement of a mass with a position-
`measuring interface circuit. That measurement is then converted into a digital electrical signal through an
`nalog-to-digital converter (ADC) for digital processing. Gyroscopes, however, measure both the
`displacement of the resonating mass and its frame because of the Coriolis acceleration.
`
`Basic Accelerometer Operation
`2
`Newton’s Second law of motion says that the acceleration (m/s ) of a body is directly proportional to, and in
`he same direction as, the net force (Newton) acting on the body, and inversely proportional to its mass
`(gram).
`
` Acceleration = Force (Newton)
`2
` (m/s ) Mass (gram)
`
` (Eq. 1)
`
`It is important to note that acceleration creates a force that is captured by the force-detection mechanism of
`the accelerometer. So the accelerometer really measures force, not acceleration; it basically measures
`acceleration indirectly through a force applied to one of the accelerometer's axes.
`
`An accelerometer is also an electromechanical device, including holes, cavities, springs, and channels, that
`is machined using microfabrication technology. Accelerometers are fabricated in a multilayer wafer process,
`measuring acceleration forces by detecting the displacement of the mass relative to fixed electrodes.
`
`The Accelerometer's Sensing Mechanism
`A common sensing approach used in accelerometers is capacitance sensing in which acceleration is related
`to change in the capacitance of a moving mass (Figure 2). This sensing technique is known for its high
`accuracy, stability, low power dissipation, and simple structure to build. It is not prone to noise and variation
`with temperature. Bandwidth for a capacitive accelerometer is only a few hundred Hertz because of their
`physical geometry (spring) and the air trapped inside the IC that acts as a damper.
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` C = (ε × ε × A)/D (Farad)
`0
`r
`
`ε = Permitted free space
`0
`εr = Relative material permitted between plates
`A = Area of overlap between electrodes
`D = Separation between the electrodes
`
` (Eq. 2)
`
`Figure 2. Moving mass and capacitance.
`
`The capacitance can either be arranged as single-sided or a differential pair. Let’s look at accelerometers
`arranged as a differential pair (Figure 3). It is composed of a single movable mass (one planar surface),
`that is placed along with a mechanical spring between two, fixed, reference silicon substrates or electrodes
`(another planar surface). It is obvious that the movement of the mass (Motion x) is relative to the fixed
`electrodes (d1 and d2), and causes a change in capacitances (C1 and C2). By calculating the difference
`between C2 and C1 we can derive the displacement of our mass and its direction.
`
`Figure 3. Acceleration associated with a single moving mass.
`
`The displacement of the movable mass (micrometer) is caused by acceleration, and it creates an extremely
`small change in capacitance for proper detection (Equation 1). This mandates using multiple movable and
`fixed electrodes, all connected in a parallel configuration. The configuration enables a greater change in
`capacitance, which can both be detected more accurately, and ultimately makes capacitance sensing a
`more feasible technique.
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`Let’s summarize quickly. Force causes a displacement of the mass which, in return, causes a capacitance
`change. Now, placing multiple electrodes in parallel allows a larger capacitance, which will be more easily
`detected (Figure 4). V1 and V2 are electrical connections to each side of the capacitors and form a voltage-
`divider with the center point as the voltage of our mass.
`
`Figure 4. Acceleration associated with multiple moving masses.
`
`The analog mass voltage will go through charge amplification, signal conditioning, demodulation, and
`lowpass filtering before it gets converted into a digital domain using a sigma-delta ADC. The serial digital bit
`stream from the ADC is then passed to a FIFO buffer that converts the serial signal into a parallel data
`2
`stream. That parallel data stream can then be transformed using a serial protocol like I C or SPI before it is
`sent to the host for further processing (Figure 5).
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`Figure 5. Electrical circuit of an accelerometer.
`
`A sigma-delta ADC is well suited for accelerometer applications because of its low signal bandwidth and
`high resolution. With an output value defined by its number of bits, a sigma-delta ADC can be translated
`into “g” units for an accelerometer application very easily. The “g” is a unit of acceleration equal to the
`earth’s gravity at sea level:
`
`10
`- 1 =
`For example, if the X-axis reading of our 10-bit ADC is equal to 600 out of the available 1023 (2
`1023), and with 3.3V as the reference, we can derive the voltage for the X-axis specified in “g“ with the
`following equation:
`
` X - voltage = (600 × 3.3)/1023 = 1.94V
`
` (Eq. 3)
`
`Each accelerometer has a zero-g voltage level that is the voltage that corresponds to 0g. We first calculate
`the voltage shifts from zero-g voltage (specified in the data sheet and assumed to be 1.65V) as:
`
` 1.94V - 1.65V = 0.29V
`
` (Eq. 4)
`
`Now, to do the final conversion we divide 0.29V by the accelerometer’s sensitivity (specified in the data
` sheet and assumed to be 0.475V/g):
`
` 0.29V/0.475V/g = 0.6g
`
` (Eq. 5)
`
`A Multiaxis Accelerometer
`Let’s take another look at our Figure 3 and add an actual manufactured accelerometer (Figure 6). Now we
`can clearly relate each component of an accelerometer to its mechanical model.
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`Figure 6. A mechanical model of an actual accelerometer.
`
`By simply mounting an accelerometer differently (90 degrees, shown in Figure 7) we can create a 2-axis
`accelerometer needed for more sophisticated applications.
`
`Figure 7. A 2-axis accelerometer.
`
`There are two ways to construct a two-axis accelerometer: lay out the two different single-axis
`
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`accelerometer sensors perpendicular to each other, or use a single mass with capacitive sensors arranged
`to measure movement along both axes.
`
`Selecting an Accelerometer
`When selecting an accelerometer for a given application, it is important to consider some of its key
`characteristics:
`
`1. Bandwidth (Hz): the bandwidth of a sensor indicates the range of vibration frequencies to which the
`accelerometer responds or how often a reliable reading can be taken. Humans cannot create body
`motion much beyond the range of 10Hz to12Hz. For this reason, a bandwidth of 40Hz to 60Hz is
`adequate for sensing a tilt or human motion.
`2. Sensitivity (mV/g or LSB/g): sensitivity is a measure of the minimum detectable signal or the change
`in output electrical signal per change in input mechanical change. This is valid in one frequency only.
`3. Voltage noise density (µg/SQRT Hz): voltage noise changes with the inverse square root of the
`bandwidth. The faster that we read accelerometer changes, the worse accuracy we get. Noise has a
`higher influence on the performance of the accelerometers when operating at lower g conditions with
`a smaller output signal.
`4. Zero-g voltage: this term specifies the range of voltages that can be expected at the output under 0g
`of acceleration.
`5. Frequency response (Hz): this is the frequency range specified with a tolerance band (±5%, etc) for
`which the sensor will detect motion and report a true output. The specified band tolerance lets the
`user calculate how much the device's sensitivity deviates from the reference sensitivity at any
`frequency within its specified frequency range.
`6. Dynamic range (g): this is the range between the smallest detectable amplitude that the
`accelerometer can measure to the largest amplitude before distorting or clipping the output signal.
`Accelerometer Versus Gyroscope
`Before describing some MEMS applications, we must understand the differences between an accelerometer
`and a gyroscope. Accelerometers measure linear acceleration (specified in mV/g) along one or several axis.
`A gyroscope measures angular velocity (specified in mV/deg/s). If we take our accelerometer and impose a
`rotation to it (i.e., a roll) (Figure 8), the distances d1 and d2 will not change. Consequently the
`accelerometer’s output will not respond to change in angular velocity.
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`Figure 8. Accelerometer immunity to rotation.
`
`We can construct the sensor differently, so the inner frame containing the resonating mass is connected to
`the substrate by springs at 90 degrees relative to the resonating motion (Figure 9). Then we can measure
`Coriolis acceleration though capacitance sensing on the electrodes mounted between the inner frame and
`substrate.
`
`Figure 9. Inner and substrate representation relative to a moving mass.
`
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`Accelerometer and Gyroscope Applications
`Accelerometers have been used for a long time in automobiles for detecting car crashes and for triggering
`airbags at just the right moment. They have many applications in mobile devices like switching between
`portrait and landscape modes, tap gestures to change to the next song, tapping through clothing when the
`device is in a pocket, or anti-blur capturing and optical image stabilization.
`
`Indoor Navigation
`Acceleration is the rate of change of velocity
`
`2
`2
`α = δv/δ t = δ x/δt
`
` (Eq. 6)
`
`We can derive velocity and distance information from the acceleration output by single or double integration,
`respectively. By adding the measurements provided by gyroscopes, we can then use a special technique to
`track the position and orientation of an object relative to a known starting point. This information is used for
`indoor navigation without external reference or GPS signal (Figure 10).
`
`Figure 10. Accelerometer for indoor navigation.
`
`Optical Image Stabilization
`Human hands shake at a very low frequency (10Hz to 20Hz). When taking a picture with our latest small,
`lightweight smartphones and cameras, we cause jitter that blurs the image. Features like optical zoom
`aggravate this problem and produce even more blurring.
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`Consider a sensor with 0.08 degree horizontal drift when using an SVGA camera with 800x600 pixel
`resolution for a 45 deg viewing angle. The 45/800 = 0.056 deg, which corresponds to 1.42 pixel blurring. As
`the camera resolution improves, the blurring covers more pixels and causes more image distortion.
`
`Figure 11. Image blurring is removed using optical image stabilization.
`
`Gyroscope-based optical image stabilization (Figure 11) with correctional software can compensate for
`image blurring by sending a mechanical gyroscope’s measured data to a microcontroller and linear motor
`to move the image sensor.
`
`Gesture-Based Control
`We can use MEMS accelerometer sensors for gesture-based control of a wireless mouse, or wheel chair
`®
`directional control, or a gyroscope in a Wii console. Other examples include a smart device using gestures
`to control a cursor on TV, or “virtual” knobs, or even gesture commands to control external devices with a
`handheld wireless sensor unit.
`
`Conclusion
`MEMS accelerometer sensors and gyroscopes have long been used for a wide range of applications in
`shipping, space, industrial robotics, and automobiles. But their application versatility has now spread to
`smartphones where they give us a new way to interface for motion and gestures with our smart device.
`Understanding MEMS behavior and the characteristics of an accelerometer or gyroscope allows designers
`to design more efficient and low-cost products for high-volume applications. These MEMS devices also
`allow us to create new applications that are profoundly changing how our movements, body motion, and
`gestures impact the way we live.
`
`Wii is a registered trademark and registered service mark of Nintendo of America Inc.
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` APPLICATION NOTE 5830, AN5830, AN 5830, APP5830, Appnote5830, Appnote 5830
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