In a conference room at Analog Devices (ADI), Howard Wisniowski holds a demo board a little bigger than a commemorative stamp about a meter above the table top. An ADI motion sensor and associated circuitry are on the board. Wisniowski drops the board into his other hand.
As soon as the board starts to fall free, the motion sensor detects a change in acceleration. Before the board reaches Wisniowski’s lower hand, an LED flashes red and a tiny transducer on the board squeaks out an SOS in Morse Code. The speed of that response, reoriented to the horizontal plane, is just what you would want in a notebook disk-drive head assembly or an automotive airbag sensor.
Those are just two of the new applications areas opened up by the economies of scale, high sensitivities, and small footprints made possible by microelectromechanical-systems (MEMS) manufacturing technologies adapted to CMOS process flows.
THE INSIDE STORY
The sophistication in MEMS accelerometers lies partly in the electronics and partly in the geometry of the mechanical configuration. Accelerometers can be fabricated and packaged to measure acceleration in a single plane or in two or three orthogonal planes. Conceptually, the acceleration-sensing portion generally comprises a “proof” mass at one end of a cantilever beam.
The deflection of the system of multiple proof masses and beams under acceleration is often measured by sensing a change in capacitance between a set of fixed beams and a set of deflecting beams—somewhat analogous to a macro-scale variable capacitor. Since many capacitive transducers have a nonlinear capacitance versus displacement characteristic, the electronics in the sensor are called upon to convert the signal to a linear output. Alternative sensing elements may be piezoelectric, rather than capacitive.
Important accelerometer datasheet characteristics include bandwidth and resonant frequency, noise floor, cross-axis sensitivity, drift, linearity, dynamic range, shock survivability, and power consumption. Generally, resonant frequency is several times higher than the upper bandwidth limit. Bandwidth and sensitivity tend to be inversely related.
In addition to the usual noise sources in electronic devices, MEMS sensors are so small that Johnson noise, caused by Brownian effects on the proof mass, can have a significant effect. A nice derivation of Brownian effects can be found in “Sensors— An Overview of MEMS Inertial Sensing Technology” at www.sensorsmag.com/sensors/content/printContentPopup.jsp?id=334974.
So far, we’ve been considering linear accelerometers, which have a huge market in transportation applications, particularly airbag-related deceleration sensing. But a large market also exists for MEMS angular accelerometers in disk drives, where they compensate for angular shock and vibration. Unlike their linear cousins, these devices locate their centers of gravity at the centroid of the support springs, making them sensitive to angular acceleration.
Acceleration, vibration, shock, and tilt relate to linear rate motion. Rotation is a measure of angular rate motion. This mode differs from the others because rotation may occur without a change in acceleration. To understand how that works, picture a three-axis inertial sensor.
Say that the sensor’s X and Y axes are parallel to the Earth’s surface. The Z axis is pointing toward the center of the Earth. In this position, the Z axis measures 1 g. The X and Y axes register 0 g. Now rotate the sensor to move only about the Z axis. The X and Y planes simply rotate, continuing to measure 0 g while the Z axis still measures 1 g.
That’s why MEMS gyroscopes are used to sense this rotational motion. Because certain end products must measure rotation in addition to other forms of motion, gyroscopes may be integrated in an inertial measurement unit (IMU) that embeds a multi-axis gyroscope and multi-axis accelerometer.
For a good heuristic video on the complementarities of accelerometers and gyros, check out www.invensense.com/support/videolibrary.html on InvenSense’s Web site. (The InvenSense site also has some excellent white papers on topics like image stabilization and MEMS gyroscopes.)
Accelerometers are all about displacement and vibration in a plane. With MEMS gyros, it’s about displacement caused by Coriolis force. While it may have nothing to do with water going down the bathtub drain, the Coriolis force does work on smaller scales than hurricanes, and it’s demonstrated on a more moderate, though still far from microscopic, scale in those Foucault pendula that every science museum seems to have.
Assuming that all you remember about Foucault’s pendulum is that it has something to do with the rotation of the earth, here’s the short explanation. Suppose you have a vibrating mass particle (the pendulum ball) moving at resonance with velocity v0 cos(Ot) that’s fixed to a body (planet Earth) rotating at rate Oin. The Coriolis effect induces a time-varying acceleration at the same frequency as the driving acceleration, but at right angles to the velocity vector of the mass particle. That is, the Coriolis acceleration is a cross product: a(t) = [ 2Oin × v0] cos(Ot).
Now, mentally change the huge Foucault pendulum to a vibrating tuning fork and you have a similar effect (Fig. 1). The tuning fork’s normal vibration mode is in one plane and the Coriolisinduced displacements are in another plane that’s orthogonal. Shrink that to MEMS size, drive the tuning forks with an external signal, use separate tuning forks for three axes, and you have the basic concept of a three-axis MEMS accelerometer.
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