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Accelerometers go mainstream

ACCELEROMETERS TRADITIONALLY APPEAR IN RELATIVELY FEW MAINSTREAM ELECTRONICS APPLICATIONS, SUCH AS AIRBAG DEPLOYMENT AND HARD-DISK PROTECTION. DESIGNERS ARE NOW REVIEWING HOW BEST TO EMPLOY A TECHNOLOGY THAT IS BECOMING EVER LOWER COST AND MORE CAPABLE.

Now in their second decade of commercialisation, MEMS (microelectro- mechanical systems) technologies make possible devices as diverse as cardiac monitors, inkjet-printer dispensers, and optical multiplexers. But for many electronics engineers, the most rapidly emerging application for MEMS fabrication lies with building inertial sensors, or accelerometers. Traditional uses include the shock sensors that control airbag deployment in vehicles and protect the heads and platters within hard-disk drives. Such devices began to win design acceptance about 15 years ago. Meantime, numerous, mainly dual- and triple-axis devices have become available to tackle an ever-growing range of automotive, consumer, and industrial applications. So, how useful and easy is it to exploit inertial-sensor measurements, and what’s available to minimise a busy designer’s burden?

Before considering these issues, it is worthwhile taking a look at the techniques that build a MEMS accelerometer (see sidebar “Anatomy of an accelerometer”). It is also helpful to relate acceleration values to physical events. The familiar value of 1g is the force that gravity exerts on an object at the Earth’s surface—equivalent to an acceleration of 9.8057 m/sec². Because it is natural to think of acceleration as a dynamic event, it may not be obvious that most types of accelerometer—such as the inclinometers that measure tilt angles—can measure this “static acceleration”. You can check a device’s response by orientating the accelerometer’s axis of interest towards the centre of the Earth and then rotating the axis 180° skywards to apply ±1g. Similarly, you can test a sensor’s zero-g responses by laying it on a flat surface. Ideally, a three-axis device should then output zero-g in its X and Y planes, and 1g in the Z plane. These tests can provide a simple basis for assessing device sensitivity and offsets between axes.

At the most sensitive end of the measurement spectrum, applications for sub- 1g measurements include the geophones that measure sonar echoes in seismic surveys. Such sensors may have full-scale spans of just ±0.1g—by comparison, inclinometers typically measure ±2g. A racing-car driver can experience 5g in high-speed cornering, with most humans losing consciousness at around 6g. A car crash of just 10g will break human bones, while 30g are sufficient for a seatbelt to break your ribs. Sensors for under-vehicle testing may receive as much as 100g, while a laptop dropping onto a concrete floor from a height of 1m may endure as much as 2000g—explaining why so many accelerometers are finding their way into portable electronics that contain hard disk drives. Applications beyond 10,000g typically lie within ballistics and explosive-force measurements.

MARKETS DETERMINE QUALITIES

The application heavily determines the most desirable attributes for an accelerometer beyond the normal considerations of low power, size, and cost. For instance, there is a point when the sensing elements within your accelerometer will resonate. This is a primary term in determining device bandwidth, with the maximum useful response typically being a fraction of the structure’s resonant frequency. The device’s structure also greatly influences the sensor’s dynamic range as well as the device’s ability to survive shock levels way beyond its measurement span. Notice that many MEMs accelerometers specify shock survivability only when under power, as electrostatic damping requires energy. Other issues that you should specifically consider include the sensor’s linearity, noise floor, zero-shift and gain drift with temperature and time, and—in the case of multiple-axis devices—the degree of physical misalignment and electrical imbalance between axes.

Bruno Cristofari, regional sales manager for Europe at Measurement Specialties, says that the vast majority of his company’s sales go into relatively small-volume industrial applications—typically for monitoring vibration within rotating machines. Cristofari says that for such use, the sensor should be small, lightweight, and highly robust: “Because mounting a sensor on a vibrating object adds mass and changes the vibration phenomena that you initially wanted to measure, the [key objective] here is to avoid mass loading.” He says that a useful rule-of-thumb is that your sensor must not exceed 10% of the total mass of the object that is vibrating. Such sensors also need a rugged housing and wiring that will withstand its environment. Cristofari says that the wiring often weighs more than the sensor. A wireless link would help, but making such a module sufficiently small and rugged currently presents significant barriers to this approach.

Because shock and vibration are dynamic events, the sensor’s frequency response should be as wide and flat as possible, preferably extending to dc to capture the low-frequency energy content that’s invariably present in rotating machines. Cristofari observes that piezo-electric devices have much higher frequency-response characteristics than their MEMS equivalents, but no dc response. Some piezo-electric devices with integral amplifiers can offer frequency responses as low as 0.3 Hz, while MEMS designs reach dc and often offer a useful compromise choice. Buffered and scaled voltage- or current-output signals dispense with the need for users to add charge or microvolt-level amplifiers, at the expense of component cost and temperature range—industry-standard accelerometers with onboard electronics have a maximum 175°C continuous rating.

While the process-control industry has almost universally adopted bus-based communications, Cristofari says that to date, the vibration market only applies such technology for niche applications within the aerospace industry. Often costing around $2000 per channel, aerospace-qualified accelerometers represent the polar extreme, in marketing terms, from mass-volume airbag sensors that lie around the $2 area. Cristofari notes: “Outside of vibration monitoring for rotating machinery, the market in between these extremes is highly fragmented— applications drive the exact specification with little commonality”. Accordingly, Measurement Specialties offers accelerometers that employ piezoresistive MEMS, piezo-film, and piezoelectric ceramic construction, with or without signal-conditioning electronics, in packages that span IC-like formats to rugged aluminium enclosures.

For its MEMs devices, the company’s second-generation bulk-silicon micromachining process builds a seismic mass that four flexible beams suspend. Each beam contains two ion-implanted resistors that connect to form a Wheatstone bridge. Under acceleration, the seismic mass moves, causing four of the resistors to increase in value while the opposite four decrease, which produces a voltage change that’s proportional to acceleration. The resistor-interconnection method cancels out the effects of any off-axis acceleration, and the on-chip design minimises zero shift. Other refinements include gas damping to tune the sensor’s frequency response and mechanical end-stops to prevent damage under severe shocks. Cristofari summarises the advantages of piezo-resistive MEMS over capacitive devices: “In our favour—less electrical noise, outstanding overrange protection, and higher useful bandwidth.”

GYROS TAKE OFF

Bob Scannell, business-development manager for Analog Devices’ iSensor products, explains that his company divides its MEMS products into two lines: “In relative terms, automotive and consumer customers are few but work with very high-volume applications, while industrial customers are numerous but require much lower product volumes.” Typically, automotive- and consumer-level designers seek low-cost parts that they can apply efficiently and are willing to design interface circuitry for their deeply embedded applications. Devices that target this segment most often have analogue outputs and few features. Because MEMS devices require substantial calibration to optimise their performance, the great majority of industrial users demand fully calibrated parts. With features such as digital outputs and programmability, these system-ready devices are more expensive in per-unit terms, but realise overall savings for system integrators.

Scannell says that gyroscope products are experiencing rapid growth in applications that range from preserving satellite-antenna orientation onboard ships to guiding agricultural equipment: “High-end tractors must maintain around 1-in. accuracy to maximise crop yield,” he asserts. Although no currently available MEMS gyro is sufficiently accurate for use as a primary navigational reference, the embedded calibration and compensation within the company’s ADIS16250 allows the device to serve short-term angularestimation duties. Scannell says that combining the rapid responses of a MEMS gyroscope with reference data from a GPS can build a low-cost navigational system that’s good enough for applications such as guiding vehicles.

The ADIS16250 is a three-axis angular rate sensor that—like all MEMS gyros—exploits the Coriolis effect, referring to the force that a mass moving at a velocity experiences when moving within a rotating reference frame. It employs a resonator gyro design, where each of a pair of polysilicon sensor structures contains a mass—or dither frame—that the control system electrostatically drives to resonance at 14 kHz. This resonance provides the velocity to produce a Coriolis force when the sensor rotates. Moveable fingers that lie in between fixed fingers at two of the outer extremes of each frame and at right angles to the dither motion form a capacitive structure that senses the Coriolis motion. The detector’s signal then passes through gain and demodulation stages to derive the angular-rate output signal. Scannell says that integrating the analogue-signal generation is crucial to achieving maximum sensitivity.

The ADIS16250 allows you to program its 14-bit-resolution yaw-rate outputs to dynamic-range settings of ±80, ±160, and ±320 °/sec; at maximum sensitivity, the device resolves 0.01832 °/sec. The gyro’s analogue bandwidth is 50 Hz, which you can lower by applying a capacitor between its filter and rate pins. Programmable finite-impulse-response filters that operate on each of the outputdata registers provide for additional filtering. Key specifications include an in-run bias stability of 0.016 °/sec at 25°C—this metric refers to the deviation from zero output value when the device is perfectly still.

Interfacing via an SPI port, the ADIS16250 includes auto-zero for bias drift calibration, dual alarm settings, a low-power mode with interrupt-driven wake-up, programmable sample rate, and self-test. There’s also an auxiliary 12-bit ADC input and DAC output together with two GPIO pins. Available in a 20- terminal 11115.5-mm LGA package that operates from a 5V supply, the ADIS16250 has a temperature coefficient of 225 ppm/°C across its 40-to-85°C operating range. The otherwise similar ADIS16255 reduces this term to just 25 ppm/°C. Guide prices are $41.98 (1000) and $55.90 (1000), respectively.

Combining a three-axis angular rateof- change sensor with a three-axis linear acceleration sensor constructs a 6-DOF (6°-of-freedom) inertial-measurementunit, with applications ranging from the traditional guidance and control to image stabilisation in high-resolution cameras. One example is the new ADIS16350, a calibrated 23x23x23-mm module that includes automatic bias calibration, programmable condition monitoring, digital filtering, sample-rate selection, and selftest facilities via its SPI port (Figure 1). Featuring a programmable dynamic range of ±75 to ±300 °/sec, the module houses a three-axis acceleration sensor that relies on the differential capacitances between fixed and central plates together with a three-axis angular-rate sensor. The guide price is $275 (1000), and a dedicated evaluation board is available (see sidebar “Evaluation kits accelerate learning”).

Recent iSensor product announcements include the ADIS16209, a dualaxis inclinometer that directly outputs angular data to within 0.1° of accuracy via its SPI port. According to Scannell, this specification makes the device around three times more accurate than any of its MEMS-based competitors. Much of the device’s performance comes from fully embedding compensation for axial misalignment, bias, cross-axial sensitivities, integral non-linearities, temperature drift, and variations on the device’s 3.3V supply. The device is sampling now and should be in production by press time.

ANATOMY OF AN ACCELEROMETER

According to Newton’s second law, acceleration of a constant mass implies a force, where the level of this force is equal to mass times acceleration (F = ma). Most accelerometers detect the restoring force that an elastic restraint such as a mechanical spring or electrostatic field exerts on a fi xed mass. Under acceleration, the mass moves a distance that its degree of acceleration and the spring’s stiffness determines, so knowing the mass and the spring’s elasticity makes it possible to determine acceleration simply by measuring the distance of the mass’s displacement. This is the principle behind the ADXL50 from Analog Devices, which in 1993 became the first commercially available device to combine a surfacemicromachined MEMS accelerometer with integral signal-processing circuitry. Previously, devices employed bulk micromachining, a chemical etching process that typically constructs a membrane of around 10 µm thickness in a silicon wafer. This etching process leaves a test mass of silicon that moves under acceleration in the centre of this membrane. Thin-fi lm piezo resistors around the edge of the membrane are sensitive to strain and change their resistance under acceleration, allowing a Wheatstone-bridge measurement. With various proprietary enhancements, this technique appears in top-specifi cation sensors from companies such as Measurement Specialties (see feature article). From a semiconductor manufacturer’s viewpoint, the key drawback of bulk micromachining is that the chemical etching process is incompatible with semiconductor processing. Fully compatible with IC production techniques, surface micromachining works by depositing multiple thin films and silicon/siliconoxide layers that a chemical etch process selectively removes. This leaves complex fi ne-line mechanical structures—in the first-generation ADXL50, the feature sizes are around 1 to 2 µm, which is similar to the semiconductor processes of the time. Four anchor points secure this chip’s sensing element, which comprises a fl exible beam-and-tether assembly that carries 42 “fingers”, or moveable plates. These moveable plates lie between fi xed plates to create a differential capacitor whose 0.1-pF total value changes by ±0.01 pF with the beam’s displacement by accelerations within the range ±50g (Figure A). The ability to integrate the signal-conditioning circuitry alongside the sensor element delivers obvious cost and size benefi ts, together with performance improvements such as far greater temperature stability and fully scaled and compensated output signals. Importantly, silicon is very flexible at the sensor’s small feature sizes, allowing the ADXL50 to survive multiple shocks of as much as 2000g. This, and a full self-test capability, are essential features in applications such as airbag deployment. The chip’s basic operating principle relies upon applying equal amplitude 1-MHz square waves at 180° to opposite plates of the fi xed outer plates while biasing one plate to 0.2V and its partner to 3.4V. At equilibrium—or zero g—the capacitances balance, coupling equal levels of the anti-phase excitation signals into the central plate to cancel one another. The sensor array’s output is a biased high-impedance square wave that requires buffering, synchronous demodulation and filtering to produce a voltage that’s mid-way between the bias levels at zero g (nominally 1.8V dc). Refi ning the basic system, the synchronous demodulator drives a pre-amplifi er with a 1.8V reference level and feeds back its output to the fi xed outer plates via a resistor, creating a bias potential that opposes the sensor’s motion. This control loop acts to hold the sensor’s plates at or very close to its zero-g position, with the magnitude of the feedback voltage being proportional to acceleration. This approach results in a system with a stiffer spring constant, better linearity, and less temperature dependence than was previously possible. The sensitivity that the ADXL50’s sensor achieves is pretty remarkable—it can detect capacitance changes a small as 10E-18F, which equates to a physical beam displacement of 0.2 Ångström, or 0.2 nm. Electrically, the chip achieves a span of about ±1V around its mid-way value for ±50g, which equates to a sensitivity metric of 19 mV/g. The later ADXL150 refines the sensor design by folding the tethers and halving the number of anchors, doubling sensitivity by reducing sensor size and the anchors’ parasitic capacitances. This chip also dispenses with the ADXL50’s feedback bias circuitry in favour of an open-loop design that allows its 100-kHz antiphase excitation signals to swing the full power-supply level, which together with better demodulation reduces the output signal’s noise density to around 1 mg/√Hz, or one-sixth of the original chip’s level (Reference A). Despite its popularity, MEMS construction is by no means the only way to build accelerometers, with alternative technologies offering advantages in some applications. For instance, ceramic and quartz materials have piezo-electric qualities that traditionally offer better high-frequency performance than MEMS at the expense of low frequency response. One relatively new material is “piezo film”—PVDF (polyvinylidene fluoride) film—which generates a voltage that’s proportional to mechanical stress with an output level of about 10 times that of ceramic or quartz (Reference B). It’s possible to construct highreliability PVDF sensors that measure forces from nanostrains to explosive levels at relatively low cost. One example that illustrates the operating principle is the MiniSense 100 from Measurement Specialties, a cantileverbeam device capable of measuring ±100g that costs around $1.50 in low volumes. REFERENCES Samuels, Howard, “Single- and dual-axis micromachined accelerometers,” Analog Dialogue, vol. 30 no. 4 (1996), pg 3, www.analog.com. “Piezo film sensors— technical manual,” Measurement Specialties Inc., www.meas-spec. com.

 

ESC DRIVES AUTOS

Tommi Vilenius, product manager for VTI’s automotive digital platform, recalls that the infamous elk-rollover test at the 1997 Swedish press launch of the original version of Mercedes’ A-Class generated huge interest in applying accelerometers within European sports-utility vehicles. Today, Vilenius estimates that the European automotive industry currently installs ESC (electronic-stability-control) systems in about 40% of its output, while the US legislature mandates such systems in all cars and light trucks from the 2011 model year. With US safety researchers estimating an annual figure of 5000 to 10,000 lives saved from this move and the European Union promoting the technology, it is only a matter of time before the ESC becomes as commonplace as seatbelts.

ESC systems require at least one accelerometer to measure lateral acceleration and work with the braking system to constrain excessive acceleration within this vector. You might think that tilt would be important, but Vilenius points out that by the time that the vehicle is leaning undesirably, it is already unstable. Areas where tilt sensors are becoming important include electronic parking brakes and hill-start-assistance systems. Replacing parking-brake handles or foot pedals with a button on the dashboard frees up cabin space that is then available for more creative uses. In this application, an accelerometer measures the vehicle’s inclination to avoid pulling in the brake too hard, which could otherwise bind and create harshness on release. Vilenius forecasts that this feature will be commonplace by 2012 to 2015. Hill-start-assistance systems that prevent the vehicle from rolling backwards as the driver moves off from a standstill employ an accelerometer to measure longitudinal acceleration. By measuring if the vehicle is moving or at rest, this sensor can also provide an additional input into the anti-lock braking system. This feature is especially useful in four-wheel-drive vehicles with full-time central-locking differentials that can simultaneously lock the front and rear wheels.

Noting that there’s a trend to integrate the accelerometer sensor cluster to serve all of the vehicle’s safety-related electronics, Vilenius says that vertical acceleration sensors are popular in active-suspension- control systems. Non-safety-related uses include reducing noise, vibration, and harshness, as Jaguar’s active-enginemounting system demonstrates. Characterising the sensor’s bandwidth is crucial in such applications to ensure that the phase-locked-loop locks correctly under all conditions; otherwise, the system that feeds back anti-phase motion to cancel the engine’s vibration may oscillate and create far worse harshness.

VTI’s capacitive MEMS devices take advantage of a bulk three-dimensional, high-aspect-ratio etching process that exploits the sensor’s mass in a gas-filled cavity to filter unwanted frequencies. Recognising the trend towards smart sensors, the company recently introduced its digital-accelerometer family for the automotive market. Available in 1-, 2- and 3-axis versions, these products specifically target ESC and inclinometer applications from 2 to 5g and come in a 7.6x3.3x8.6-mm package (Figure 2). One example is the SCA2120-DO5, a ±2 g YZ-axis device that runs from a 3.3V supply and interfaces via an SPI port. The maximum active-mode current consumption is 5 mA, reducing to typically 120 µA in sleep mode. Importantly for minimising power consumption as well as system latency, the wake-up time is 100 msec. The digital subsystem makes it possible to include comprehensive diagnostic capabilities; in normal operation, the device has 12-bit resolution and outputs 900 counts-per-g at a 2-kHz rate. Key specifications include a lifetime total offset error of ±100 mg over the -40-to- +125°C operating range, a bandwidth of 30 to 50 Hz, a maximum linearity error of ±20 mg within a ±1g span, a worst-case cross-axis sensitivity mismatch of ±3.5%, and maximum noise equivalent to 5 mg rms. The part withstands shock levels as high as 20,000g while under power and has AEC-Q100 automotive-standard qualification.

Interestingly, VTI has just announced a “chip-on-MEMS” fabrication technology that bonds an ASIC on top of each MEMS die at the wafer scale. Apart from demonstrating packages of just 4 mm² footprint and 1 mm thickness, the final test and calibration steps become waferscale processes. The production process tests each MEMS device, hermetically seals the wafer, and places ASICs on known-good dice. The process deposits 300 µm solder-balls onto each ASIC before testing and slicing the wafer to create individual components. VTI is working on refining the technique to permit high-volume production and hopes to use it to stack multiple 20-µm thick ASICs on top of its sensors to create highly complex system-level devices.

EVALUATION KITS ACCELERATE LEARNING

As with other application areas, the semiconductor vendors are keen to provide hardware and software support that helps secure design wins. Analog Devices offers dedicated evaluation boards for its ADXL-family accelerometers, most of which are simple PCBs that carry the chip of interest, capacitors to adjust its bandwidth, and connection points. The SPI-compatible iSensor products—apart from the ADIS16350, which has its own PCB— use the ADIS/EVAL base board to accommodate a range of device-specifi c plug-in assemblies. At $175, the 19277-mm base board’s primary function is to interface a PC parallel port to the device-of-interest’s SPI interface. Its major circuitry comprises a hex Schmitt-trigger inverter and power supplies. There’s also a reset switch, a potentiometer, a DIL switch that connects with six test points, and a small prototyping area. Two twelve-pin headers connect to a range of devicespecifi c modules, such as the ADIS16250/PCB ($75). Other deliverables include a parallel-port cable and a CD that furnishes datasheets, application notes, and system software. Sadly, attempts to get our sample of the ADIS16250 evaluation environment operational failed dismally. Firstly, the software installed seemingly satisfactorily on a Windows XP Pro machine but failed to return any data, despite extensive attempts to troubleshoot the connection. Suspecting that these diffi culties may be due to the giveio parallel-port driver, I tried a 2000 Pro machine with a standard confi guration. The software refused to install, returning a series of “fi le update” and “reboot required” messages that had no useful effect. Trying yet another 2000 Pro machine produced precisely the same results as the fi rst—contrary to what the “readme fi rst” fi le indicates, it appears that it is not possible to circumvent the installer updating system files that it considers out-of-date. Supposedly, the software will also run on NT4 and Windows 98. Trying this with an old NT4 machine that had minimal resident software resulted in the blue screen-of-death at every reboot attempt. A Windows 98 machine with no prior instances of giveio generated a threatening list of errors on reboot, but allowed the software to install—then failed to communicate in exactly the same way as the fi rst attempt on XP Pro. Finally, publication pressures made pursuing this problem impossible. Freescale similarly offers a range of kits for its devices, mostly mounting the part on a small PCB that provides the user with easy electrical connections. By way of contrast, the company’s recent ZStar platform that showcases the tri-axial MMA7260QT comprises two boards (Figure A). The accelerometer PCB also carries an SO8QG8 microcontroller, while the USB stick uses an HC908JW32 for system management. The boards communicate via a pair of MC13191 chips that operate in the 2.4 GHz band. The protocol is IEEE 802.15.4, which provides the physical-layer basis for ZigBee systems. Freescale offers a range of RF products that allow you to build proprietary and standard links using these technologies. Software running on the sensor board acquires and packetises data for transmission, with the USB stick’s microcontroller providing the RF-to-USB bridge. The sample that we requested did not arrive in time to permit testing, but checks confi rm that the RD3152MMA7260Q kit is available from distribution for $99. STMicroelectronics sent us three examples of its MEMS-device evaluation boards to review. Not yet showing on the Web site, the EK331AL is a 4135-mm assembly that carries the LIS331Al device and support circuitry to provide a simple demonstration of this analogue sensor’s capabilities. The EK302DL and the similar EK3LV02DL that respectively showcase the LIS302DL and LIS3LV02DL digital-output devices pack an ST72F651 microcontroller, its crystal, three pushbuttons, and four LED packages onto a tiny 4638-mm PCB. The microcontroller interfaces the device with a PC via full-speed USB. On my test XP Pro machine, the software installed and ran faultlessly, comprehensively demonstrating the chip’s abilities from register read/writes through general-purpose acquisition to plotting and mapping data. The kit includes a good documentation package, with a helpful user manual, an application note, a schematic, and Gerber layout fi les. It also usefully includes example source code that demonstrates interfacing the sensor with a user application. The kit is available now from distributors including Digi-Key (www.digi-key. com) at around €65. Emphasising ease-ofuse, the PSoC FirstTouch starter kit bundles the PSoC Express development environment along with a USB stick that houses a CY8C24894. This USB stick provides the interface between the host PC and a header that carries the target CY8C21434, a buzzer, three LEDs, a light sensor and a thermistor. Together with a PCBtrack capacitive slider, this hardware allows you to exercise four programs that help familiarise with the environment—you can be up and running within a few minutes of opening the box, which is a massive accessibility improvement in comparison with the full-blown (and also free) PSoC Designer suite. Notes that accompany the demo programs describe how you can modify routines to suit your own needs. Furthermore, among the long list of input devices that PSoC Express already accommodates, you will fi nd drivers for several common accelerometers from Analog Devices, Freescale, and STMicroelectronics. PSoC FirstTouch is available now for $29.95.

 

CONSUMERS RULE

Arguably today’s highest-profile application for inertial sensors is the motion sensor interface that appears as part of Nintendo’s Wii games console. Tripleaxis motion sensing is a major feature of the system’s wireless remote controller, bringing a new level of user interaction. The retail price of around €39 per remote controller emphasises that component cost is minimal, with Nintendo releasing versions that use the ADXL330 from Analog Devices and the LIS3L02AL from STMicroelectronics. With many competitors still using 6-in. wafer technology for MEMS production, Fabio Pasolini, business-development manager at STMicroelectronics, says that his company’s early investment in 8-in. technology is essential to address consumer- market volumes at minimum cost. He observes that the LIS3L02AL is just one example of ST’s focus on today’s trend towards user-interface accelerometer applications in consumer electronics. Pasolini predicts that features such as the ability to interpret the user tapping, for instance, a mobile phone to accept or reject calls, will soon be commonplace: “Accelerometers will help system designers to build user interfaces that adapt to the user, rather than requiring the user to learn the interface.”

In common with ST’s other accelerometers, the LIS3L02AL is a singlepackage device that combines a surfacemicromachined MEMS chip built using standard IC epitaxial-growth technology with a separate ASIC that performs signal conditioning. Pasolini states that in comparison with single-chip fabrication, ST’s two-chip approach provides greater flexibility, allowing the company to choose its optimum processes for any application. In a steady state, the nominal value of the MEMS capacitive halfbridge is of the order of a few picoFarads, with a maximum variation under acceleration of around 100E-15F. The ASIC part employs a conventional CMOS process that helps constrain the device’s room-temperature current consumption to 1.5 mA maximum. Measuring ±2g in three axes, the LIS3L02AL has a useful bandwidth that extends to its sensor’s minimum 1.5-kHz resonant frequency. The sensor provides 0.5 mg resolution within a 100-Hz bandwidth and includes an embedded self-test feature that typically provides a 50 mV output-voltage change. You can band-limit the output voltages simply by adding capacitance to each output channel. The device operates from 2.4 to 3.6V and -40 to +85°C and comes in an 8-terminal LGA that measures 5x5x1.6 mm.

The broadly similar LIS3LV02DL allows you to program the device for ±2 or ±6g full-scale range via its dual-mode I2C/SPI port. The device effectively adds three delta-sigma ADCs via a multiplexer that connects to the output of the sensor array’s charge amplifier (Figure 3). The charge amplifier operates at 61.5 kHz, and the delta-sigma converters at onethird this rate. Reconstruction filters attenuate high-frequency quantisation noise and assemble the programmable 12- or 16-bit-resolution output-data stream. The output-data rate depends on the user-programmable decimation factor and spans 40 to 2,560 Hz. The I/O pins are 1.8V-compatible, and the device operates from 2.16 to 3.6V within a 16-terminal, 4.4x7.5x1-mm LGA.

ST’s new nano-accelerometer family tackles space-constrained consumer devices and comprises the three-axis, analogue- output LIS331AL and the similar LIS331DL that substitutes a dual-mode I2C/SPI port. These devices employ an LGA that measures just 3x3x1 mm. The analogue-output version provides simultaneous voltages for each ±2g axis, while the digital part adds programmable ±2-to-±8g ranges and the ability to detect user-taps and free-fall conditions— essential features for simplifying the user interface as well as protecting hard-disk drives in portable electronics. Both products store factory calibration for sensitivity and zero-g offset in nonvolatile registers and include an embedded self-test function. Sampling now, the devices will enter volume production during Q4/2007 with pricing for each below $1.50 in high volume.

Drawing on around 26 years experience with MEMS fabrication and a long history of supplying accelerometers for airbags, Freescale increasingly targets consumer applications with low-g products that span the ±1.5-to-10g range. Luc Darmon, the company’s European marketing manager for analogue and RF MEMS devices, differentiates the respective customer needs: “Automotive customers demand high reliability, long life, and guaranteed responses. The consumer market is all about price.” Darmon identifies six types of accelerometer measurement—the traditional ones being movement, shock, and vibration, and free-fall detection, positioning, and tilt the newer applications. Although digital-output accelerometers are rapidly becoming popular, he points out that analogue-output sensors allow users to connect simple voltagecomparator circuits to construct alarms that are independent of processor crashes or software glitches—important considerations in safety-critical applications. Also, analogue-output accelerometers and external ADCs can often improve on digital-output devices to meet industrial- accuracy expectations of as much as 14 bits.

The MMA7260QT is one example of Freescale’s first series of low-g sensors to specifically target gaming, movement, and other user-interface applications. Drawing a maximum of 800 A from a 2.2-to-3.6V supply, this triple-axis analogue-output device has four pinselectable sensitivities from 1.5 to 6g; its maximum sensitivity equates to 800 mV/g at 3.3V. Like many analogue-output accelerometers, the device’s sensitivity and offset voltages scale with the supplyvoltage level. This attribute cancels errors in supply variations when interfacing to ADCs that use the supply as a reference. Useful bandwidth within the X and Y planes is 350 Hz, and down to 150 Hz in the Z plane—reflecting the respective 6- and 3.4-kHz resonant frequencies of the surface-micromachined elements. Single-pole, switched-capacitor filters attenuate the internal 11-kHz sampling frequency, making additional filtering unnecessary. The device’s sleep-mode pin allows external logic to reduce its power consumption to a typical level of just 3 µA, which together with a maximum wake-up time of 2 msec especially suits the device to battery operation. The device is available in a 16-terminal, 6x6x1.45-mm QFN package with a guide price of $2.76 (1000).

The new MMA73xx family of analogueand digital-output parts adds various features but most noticeably shrinks package dimensions—specifically, Darmon says that at just 0.8 mm thick, the MMA7450L’s 14-pin 3x3-mm LGA is the thinnest accelerometer on today’s market. The package contains separate sensor and ASIC circuits to create a tripleaxis device. A pin selects between I²C and SPI communications, and the device normally outputs its ADC’s native 8-bit data for a sensitivity of 64 bits-per-g in ±2 g mode. Because sensitivity sequentially halves for the ±4 and ±8g ranges to just 16 bits/g, an optional 10-bit mode is available for the least sensitive range. With all specifications relating to 2.8V operation, the device draws around 400 µA from a 2.4-to-3.6V supply, reducing to around 5 µA in standby. Special features include single- and double-click recognition for user-interface applications, together with level or pulse motion-detection modes that recognise free-fall, shock, and vibration. The device includes useraccessible registers for offset calibration and a self-test facility. The guide price is $3.23 (1000).

PROCESS YOUR SIGNALS

If you choose the analogue route and require external signal processing, one approach that offers great flexibility and the potential to reconfigure hardware on-the-fly to perform multiple duties is to use a PSoC (programmable Systemon- Chip), such as the PSoC family from Cypress. David van Ess, senior applications engineer at the company, cites the ability to dynamically reconfigure the hardware as a major cost-saver in many of his customers’ applications: “Many times, you don’t need a subsystem to be running continuously, and can interleave multiple processes using the same measurement and control block.” Because PSoC chips include a multiplexer on most I/O pins, it is possible to use a single pin to access analogue and digital resources such as ADCs, DACs, and counter/timers.

As we went to press, Maxim announced its MAXQ7666, a mixed-signal microcontroller for automotive sensor applications. Comprising an 8-channel differential input that connects to a programmablegain amplifier and a 12-bit, 500-ksps ADC, its analogue subsystem resolves signals down to 8 V. Features such as on-chip and remote temperature sensing allow users to compensate for the sensor’s temperature coefficients, while the digital block’s output to the CAN bus makes it easy to integrate within ESC and other applications.

YOU NEED MORE?

Some accelerometer applications are truly specialist. One example is tyre-pressure monitoring, where a conventional pressure sensor measures the tyre’s air pressure (Reference 1). Optionally, an accelerometer measures the force in the Z axis to derive the dynamic loading on the revolving tyre. For instance, the SP30 from Infineon Technologies is a third-generation tyre-pressure sensor that integrates the sensing elements alongside an LF radio receiver. But what if your application demands a device that no off-the-shelf product can fulfil? Vincent Gaff, public-relations manager at custom MEMS maker Tronics Microsystems, says that his company works closely with its customers to fulfil such needs. Typically, his customers come from the aerospace and industrial sectors and require mediumvolume, very high-performance products with production runs of between 10,000 and a few million units. These volumes can’t justify a monolithic approach to signal conditioning, which may not in any case extract the maximum performance from a sensor.

Built using micromachined SOI (silicon-on-insulator) technology, Tronics offers three process geometries for its capacitive devices: its original 20 µm process for low-g applications such as medical monitoring, a more sensitive 60 µm process that’s the most frequent choice, and a very thick 100 µm process that suits certain aerospace and defence applications. The generic process is a plasma technology that the company terms “deep reactive ion etching”, which—together with very high-vacuum assembly techniques—is capable of constructing extremely sensitive devices. Gaff asserts that his company’s ability to encapsulate elements within vacuum pressures to less than 1 mTorr is critical to achieving optimum sensitivity. One example is the geophone elements that Tronics builds for seismic exploration. Banner specifications include a sensitivity of ±100 mg, resolution of 1 µg, dynamic range of 120 dB, and thermal noise of just 0.01 µg √Hz. The company is also working with a partner to develop MEMS-based inertial-measurementunits for civil navigation. The technology uses a form of ring gyro with feature widths of around 20 µm (Figure 4).

Expect to see a raft of product introductions from semiconductor companies that are keen to develop within the MEMS marketplace. Examples include OKI, which recently released its ML8953, a three-axis ±3 g device built using piezoresistive MEMS technology that achieves 10 mg/bit sensitivity and 200 Hz bandwidth. An on-board microcontroller with a dual-mode I²C/SPI interface performs temperature compensation and pitchand roll-angle calculation, and supports multiple data-acquisition modes with programmable threshold alarms. Epson uses proprietary quartz MEMS fabrication to construct its latest gyro, the XV-8000LK, which targets the automotive market for dead-reckoning navigational devices. The 5V device provides an analogue output with an angular rate-of-change sensitivity of 25 mV/°/sec within a maximum range of ±60 °/sec.

CONTACT DETAILS

You can reach Contributing Technical Editor David Marsh at forncett@btinternet.com.