Some recent breakthroughs in accelerometer technology have enabled the production of very small accelerometers with very wide bandwidth. Tiny MEMS accelerometers are so small and low in mass that they have no mechanical or mass loading effect on the instrument, making them attractive for these applications but to date their use has been limited due to the narrow bandwidth of commercially available acceleration sensors. 5 Piezo- and electromagnetic transducers are the basis for many of today’s acoustic pickup applications. The concept of using vibration sensing transducers as acoustic pickups in musical instruments is not new. Accelerometers as Vibration Measurement Sensors Cross-section of a typical two-chip accelerometer. For a single-chip solution, the capacitance of the sense element can be as low as 1 to 2 femtofarads per g, which equates to measurement resolution in the attofarad range! In a two-chip structure, the capacitance of the MEMS element must be high enough to overcome the parasitic capacitance effects of the bond wires between the MEMS and the conditioning ASIC (application specific integrated circuit). The MEMS sensing element can be connected to the conditioning electronics on the same chip (Figure 3) or on a separate chip (Figure 4). Sensors can be produced to measure fractions of one g or hundreds of g’s with bandwidths as high as 20 kHz. The mass of the sense element and the damping of the structure can also be modified by design.
One mechanical parameter than can be controlled and varied is spring stiffness. Structures with very different mechanical characteristics can be created using this flexible technology. MEMS structures are typically formed from single-crystal silicon, or from polysilicon that is deposited at very high temperatures on the surface of a single-crystal silicon wafer. The dimensions of these MEMS structures, on the order of microns (Figure 2), require very high precision silicon photolithography and etching process technologies. This applied acceleration (Figure 1) changes the capacitance between the fixed and moving beam fingers.
The core element of a typical MEMS accelerometer is a moving beam structure composed of two sets of fingers: one set is fixed to a solid ground plane on a substrate the other set is attached to a known mass mounted on springs that can move in response to an applied acceleration.
This article describes some of the key technologies deployed in MEMS accelerometer products and discusses how this technology can bring a new dimension to acoustic transducers.
In recent years, MEMS microphones 3 have begun to proliferate the broad consumer market, including cell phones, Bluetooth headsets, personal computers, and digital cameras. MEMS micromirror optical actuators are used in overhead projectors, HDTVs, and digital theater presentations. MEMS accelerometers 2 are also used for motion sensing in consumer applications, such as video games and cell phones. Pressure sensors, one of the first high volume MEMS applications, now monitor pressure in hundreds of millions of engine manifolds and tires and MEMS accelerometers have been used for over 15 years for airbag deployment, rollover detection, and automotive alarm systems. Micromechanical structures are created by etching defined patterns on a silicon substrate to form sensor elements or mechanical actuators that can move fractions of a micron. MEMS 1 (microelectromechanical systems) technology builds on the core fabrication infrastructure developed for silicon integrated circuits. Sonic Nirvana: Using MEMS Accelerometers as Acoustic Pickups in Musical Instruments