Proponen construir sombrilla espacial contra calentamiento

La posibilidad de que el calentamiento global accione un cambio abrupto del clima es algo que la gente no desea pensar, pero el astrónomo Roger Angel, de la universidad de Arizona, sí piensa en ello.

Angel, uno de los mas grandes expertos en óptica del mundo, dirige el laboratorio auxiliar de espejos del observatorio del Centro de Óptica Adaptativa Astronómica.Ha ganado grandes honores por sus muchas ideas conceptuales extraordinarias que se han convertido en soluciones prácticas de ingeniería para la astronomía.Desde el año pasado, Angel ha estado pensando en la forma sacar a la Tierra de una emergencia.

Ha estado estudiando que tan práctico sería desplegar una sombrilla en el espacio, en una crisis de calentamiento global, en la cual llegaría a estar claro que la Tierra irremediablemente estaría en camino hacia un cambio desastroso del clima dentro de una década o dos.

Roger Angel presentó la idea a la Academia Nacional de las Ciencias de Estados Unidos en abril, y ganó una beca del instituto de conceptos avanzados de la NASA, para investigación adicional en julio.

Angel ahora ha publicado un primer articulo detallado del estudio titulado "viabilidad de refrescar a la tierra con un enjambre de naves espaciales pequeñas cerca de L1", en la revista Proceedings.

El plan sería lanzar una constelación de trillones de naves espaciales pequeñas a millones de kilómetros de la tierra en una órbita alineada con el sol, llamada órbita L-1 (Lagrange –1).La nave espacial formaría un gran enjambre cilíndrico con un diámetro de la mitad de la Tierra, y cerca de 10 veces más largo.

Cerca del 10 por ciento de la luz del Sol que pasaría a lo largo de los 120 mil kilómetros del enjambre, entre la Tierra y el Sol, serían desviados lejos de nuestro planeta.

El efecto sería reducir uniformemente la luz del sol cerca del 2 por ciento sobre el planeta entero, bastante para balancear el calor ocasionado por el aumento del bióxido de carbono atmosférico en la atmósfera de la Tierra.

Sombrilla solar

Los investigadores han propuesto varias alternativas para refrescar el planeta, incluyendo la diseminacion41 de aerosoles en la atmósfera de la Tierra.

La idea para una sombrilla en el espacio en L1 para desviar la luz del sol de la tierra, fue propuesta primero por James Early del laboratorio nacional Lawrence Livermore en 1989.

"Las ideas anteriores se crearon para estructuras más grandes, más pesadas y que habrían necesitado su fabricación y lanzamiento desde la luna, que es bastante futurista", señaló Angel.

"Quise hacer la sombrilla de naves espaciales pequeñas, ligeras y extremadamente finas, como aviadores pequeños, que se pudieran montar y lanzar totalmente desde la Tierra, apilados de millones a la vez.

Cuando alcanzaran L1, se repartirían en el enjambre. No se tendrían que montar en el espacio.

Las pequeñas naves diseñadas por Angel estarían hechas de una película transparente perforada con agujeros pequeños. Cada aviador tendría 60 centímetros de diámetro, muy delgados, y pesarían alrededor de un gramo, serian como una mariposa grande.

Utilizaría espejos con tecnología de "MEMS" (sistemas micro-eléctrico-mecánicos) como velas minúsculas que se inclinarían para estabilizar la orbita y posición de las naves en formación.

La transparencia y de dirección de las naves evitaría que fuera desviada por la presión de radiación. La presión de la radiación es la presión de la luz del Sol.

Emergencia planetaria

La masa total de todas las naves que componen la estructura de la sombrilla espacial sería de 20 millones de toneladas.

A 10 mil dólares por kilogramo, el lanzamiento químico convencional de los cohetes sería prohibitivamente costoso.

Angel propone usar una manera más barata desarrollada por el laboratorio nacional Sandia; lanzadores electromagnéticos espaciales, que podrían bajar el costo a sólo 20 dólares por kilo. La sombrilla se podría desplegar con 20 lanzadores electromagnéticos que lanzaran una nave cada 5 minutos por 10 años.

Los lanzadores electromagnéticos funcionarían idealmente con energía hidroeléctrica, pero incluso en el peor de los casos con electricidad generada con carbón c ada tonelada de carbón usada para producir electricidad atenuarían el efecto de mil toneladas de carbono lanzado a la atmósfera.

Propulsado una vez más allá de la atmósfera y de la gravedad de la Tierra con el lanzador electromagnético, las naves serían dirigidas a la órbita L-1 mediante propulsión solar accionada por iones, un nuevo método probado en el orbitador espacial en la luna SMART-1 de la Agencia Espacial Europea y la prueba de Deep Impact 1 de la NASA. "El concepto se construye con tecnologías existentes," afirmó Angel.

"Parece factible que podría ser desarrollada y desplegada en 25 años a un coste de algunos millones dólares. Con cuidado, la sombrilla solar debe durar cerca de 50 años. El coste medio es tan cerca de 100 mil millones de dólares al año, o los que es lo mismo sobre dos décimas de un por ciento del producto doméstico global", y enfatizo, "la sombrilla no es ningún substituto para desarrollar energía renovable, la única solución permanente.

http://www.eluniversal.com.mx/articulos/35870.html

ADAN F CHAPARRO C
CI: 17501640
EES

APPLICATIONS MEMS

The major application of MEMS technology to date is in sensors. These include sensors for medical (blood pressure), automotive (pressure, accelerometer), and industrial (pressure, mass air flow) applications. Commercial sensor applications in Japan are in the same areas that both Europe and North America are concentrating on. In most cases the markets for these products are international.

There are extensive efforts in Japan to apply MEMS to actuators. Dr. Higuchi and his associates at Kanagawa Academy of Science and Technology (KAST) have developed an instrument that is in commercial use to fertilize eggs (1990). The instrument uses a piezoelectric vibrating element to avoid the problem of egg deformation that occurs with conventional methods.

While the commercial applications of actuators have been limited, there is a vast array of actuator needs that MEMS researchers are addressing. These include muscle-like electrostatic actuators, microrobots, noncontacting wafer transport systems, and ultraprecise positioning.

Most researchers estimate that it takes approximately five years to commercialize a product based on a new technology. There are some estimates that it takes two years to do the research prototype, four years to do the engineering prototype, and four years to get the final design to market. There is a large variation in time requirements based on how much process development and trial-and-error development is required, as well as the complexity of the device and how much invention is required.

Many Japanese researchers look on high-aspect-ratio technology (LIGA, polyimide ultraviolet) as new technology for MEMS applications. A substantial number of those visited by JTEC look on the refinement of conventional machining as a new technology for MEMS. This includes conventional milling and EDM. Some researchers consider nanotechnology as a technology potentially competitive with MEMS.

Most Japanese researchers agree that the driving forces for MEMS are size, cost, and intelligence of the sensor. One of the challenges of dealing with MEMS is learning how to effectively package devices that require more than an electrical contact to the outside. Pressure sensors are the most commercially successful MEMS-type sensors to use nonintegrated circuit-type packaging. Hall sensors, magnetoresistive sensors, and silicon accelerometers have used IC-based packaging. The IC packaging is viable since the measurand can be introduced without violating package integrity. Some optical systems use IC-type packages with windows. MEMS will require the development of an extensive capability in packaging to allow the interfacing of sensors to the environment. The very advantage of small size becomes a liability when a device is open to the environment. At the time of the JTEC visits, most Japanese predicted that MEMS sensors would be on the market in three to five years, and that micromedical sensors would probably be the most likely application. Some researchers were predicting that these micromedical sensors would be chemical sensors.

U.S. researchers forecast that in the near future (ten years), MEMS systems will have applications in a variety of areas, including:

* Remote environmental monitoring and control. This can range from sampling, analyzing, and reporting to doing on-site control. The applications could range from building environmental control to dispensing nutrients to plants.

* Dispensing known amounts of materials in difficult-to-reach places on an as- needed basis. This could be applicable in robotic systems.

* Automotive applications will include intelligent vehicle highway systems and navigation applications.

The Japanese forecast for MEMS actuators was not at all clear at the time of the JTEC visit. There was much interest expressed in exploring arrays of actuators as a method of obtaining useful work. Some researchers expressed interest in pursuing low mass applications such as directing light beams, based on the success of the Texas Instruments optical array (Sampsell 1993).

One of the major concerns with some true MEMS systems (those on the micron level) is that they must at some point be coupled to a macroworld. Some researchers see an application for a "milli" system, where the problems of coupling to the macroworld are made easier. If one can have a useful product that is all on the microlevel with only an electrical output, then the concern is eliminated.

A broad overview of the potential applications of MEMS is seen in MITI'S "Techno-Tree of Micromachine" (Figure 7.1.

In its Micromachine Technology Project, MITI has targeted two major application areas for MEMS -- maintenance of power plants and medical applications. The advance maintenance system for power plants (see Figure 6.5) consists of:

* Mother ship (Figure 7.2)
* Microcapsule (Figure 7.3)
* Inspection module (Figure 7.4)
* Operation module (Figure 7.5)

Figure 7.1. MITI "Techno-Tree of Micromachine."

Figure 7.2. Mother ship.

Figure 7.3. Microcapsule.

Figure 7.4. Inspection module.

Figure 7.5. Work module.

The purpose of this elaborate system is to do repairs in heat exchanger tubes with no or minimum down time. It should be noted that even if only a portion of this task is completed, a large number of the resulting MEMS components could be utilized in other industrial applications.

http://www.wtec.org/loyola/mems/c7_s3.htm

ADAN F CHAPARRO C
CI:17501640
EES

BIOMEMS

Recent progress in microelectromechanical systems - the microelectronics, microfabrication and micromachining technologies known collectively as MEMS - is being applied to biomedical applications and has become a new field of research unto itself, known as BioMEMS. The technology is originally based upon the same technology that has been used to make computer chips ever more powerful and less expensive. MEMS technology has enabled low-cost, high-functionality devices in some commonly used areas, such as inexpensive printer cartridges for ink jet printing and chip-based accelerometers responsible for deployment of automotive airbags. BioMEMS applies these technologies and concepts to diverse areas in biomedical research and clinical medicine. BioMEMS is an enabling technology for ever-greater functionality and cost reduction in smaller devices for improved medical diagnostics and therapies.

With strong traditions of innovation and research, The Cleveland Clinic Foundation is strategically positioned to investigate and develop BioMEMS. Furthermore, the research environment at the Lerner Research Institute is unique as it provides the necessary multidisciplinary collaboration that will be required for the implementation of new and novel ideas into successful BioMEMS.

The inherent characteristics of BioMEMS promise the production of miniature, smart, and low-cost biomedical devices that could revolutionize biomedical investigation and clinical practice. Consequently, a primary thrust is the research into development of BioMEMS and associated nanotechnology for clinical applications such as surgical instruments, tissue repair, artificial organs, diagnostic tools, and drug delivery systems.

http://www.lerner.ccf.org/bme/biomems/

ADAN F CHAPARRO C
CI:17501640
EES

Sonic Nirvana: Using MEMS Accelerometers as Acoustic Pickups in Musical Instruments

MEMS (microelectromechanical systems) technology builds on the core fabrication infrastructure developed for silicon integrated circuits. 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. 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.

MEMS accelerometers2 are also used for motion sensing in consumer applications, such as video games and cell phones. MEMS micromirror optical actuators are used in overhead projectors, HDTVs, and digital theater presentations. In recent years, MEMS microphones3 have begun to proliferate the broad consumer market, including cell phones, Bluetooth headsets, personal computers, and digital cameras.

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.

MEMS Accelerometer Technology
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 applied acceleration (Figure 1) changes the capacitance between the fixed and moving beam fingers.

Figure 1. MEMS accelerometer structure.

Figure 2. ADXL50 MEMS accelerometer structure.

The dimensions of these MEMS structures, on the order of microns (Figure 2), require very high precision silicon photolithography and etching process technologies. 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. Structures with very different mechanical characteristics can be created using this flexible technology. One mechanical parameter than can be controlled and varied is spring stiffness. The mass of the sense element and the damping of the structure can also be modified by design. Sensors can be produced to measure fractions of one g or hundreds of g’s with bandwidths as high as 20 kHz.

Figure 3. ADXL202 ±2 g accelerometer.

The MEMS sensing element can be connected to the conditioning electronics on the same chip (Figure 3) or on a separate chip (Figure 4). 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).

Figure 4. Cross-section of a typical two-chip accelerometer

Accelerometers as Vibration Measurement Sensors
The concept of using vibration sensing transducers as acoustic pickups in musical instruments is not new. Piezo- and electromagnetic transducers are the basis for many of today’s acoustic pickup applications. 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.

Some recent breakthroughs in accelerometer technology have enabled the production of very small accelerometers with very wide bandwidth. The ADXL0017 (Figure 5) high-g (±70 g to ±500 g), single-axis accelerometer has 22-kHz bandwidth and comes in a 5-mm × 5-mm × 2-mm package. This is ideal for monitoring vibration to determine the state-of-health of motors and other industrial equipment by detecting changes in their acoustic characteristics. In the early stages of bearing wear, a clear vibration signature that develops in the audio band can be detected with a high-g vibration sensor attached to the system housing. This particular sensor, which measures acceleration on the order of 10s of g’s, is not sensitive enough for use as an acoustical vibration sensor for musical instruments. Also, it only senses along one axis of motion, while an ideal acoustic sensor will measure the response along all three axes. It does demonstrate, however, that full audio bandwidth acceleration transducers can be produced using MEMS technology.

Figure 5. ADXL001 Frequency response curve.

Low-g accelerometers can measure acceleration down to milli g’s, but are typically bandwidth-limited around 5 kHz. This limitation may be associated with the fact that few commercial applications require significant bandwidth (the primary applications involve the detection of human motion or gravity-driven acceleration), so there has been little motivation to develop sensors suited specifically for audio band measurement.

A 3-axis accelerometer has three separate outputs that measure acceleration along the Cartesian X, Y, and Z axes. The ADXL3308 3-axis, low-g accelerometer has wider effective bandwidth than other traditional low-g accelerometers. Its bandwidth is up to 6 kHz on X- and Y axes, and around 1 kHz on the Z axis. While not ideal, this expanded bandwidth allows the part to gather useful information in the audio band. The output is analog, so it can be easily instrumented and used with standard audio recording equipment. Housed in a standard surface-mount package, it takes advantage of the mature semiconductor manufacturing infrastructure. Measuring less than 4 mm × 4 mm × 1.45 mm (Figure 6), the product can fit into places unimaginable with traditional accelerometer technology. Its very small size does not cause mass loading or other changes in the response of the system being measured. Later we will explore how this low-g accelerometer can be applied as an acoustic pickup for a guitar.

Low-g accelerometers can measure acceleration down to milli g’s, but are typically bandwidth-limited around 5 kHz. This limitation may be associated with the fact that few commercial applications require significant bandwidth (the primary applications involve the detection of human motion or gravity-driven acceleration), so there has been little motivation to develop sensors suited specifically for audio band measurement.

Figure 6. MEMS accelerometer, 4 mm × 4 mm × 1.45 mm.

Acoustic Feedback
Beginning with the introduction of omnidirectional condenser and dynamic microphones in the mid 1920s9 by Søren Larsen, the Danish scientist who first discovered the principles of audio feedback (known as the Larsen effect), acoustic feedback has been a demon few audio engineers are able to totally control, making it unavoidable in live sound. The Beatles experimented with this audio artifact, then decided to add it to their memorable introduction to “I Feel Fine” in 1964.10 Rock ’n’ Roll then set out to tame the beast by embracing it, making acoustic feedback a striking characteristic of rock music. Electric guitar players such as Pete Townshend and Jimi Hendrix deliberately induced feedback by holding their guitars close to the amplifier. As the fad waned, audio engineers continued their struggle with acoustic feedback’s undesirable ear-shattering effects, particularly in live sound applications. In the perfect world of a well-appointed and acoustically treated recording studio, a high-end omnidirectional microphone will record instruments with an astonishing degree of realism and fidelity. Artists who know and cherish this sound have long sought the ability to reproduce it on stage. Although recording a live show with studio sound quality is every musician’s dream, it has been virtually impossible. Even if sound reinforcement rigs sounded good, arenas had excellent acoustics, and sound engineers knew everything there was to know about mixing sound and had the best gear available, there would still remain one obstacle on the road to sonic nirvana: feedback.

Acoustic Pickups
Acoustic feedback is typically minimized by using directional microphones. This works to a certain extent, but requires constant management by sound engineers to adapt to the changing characteristics of a stage venue.

Musical instruments can be amplified using pickups. The technologies vary, but the basic idea is to sense the vibrations of the instrument’s body directly, rather than the sound it produces in the air. The advantage is obvious: these pickups generate almost no acoustic feedback as they are not sensitive to airborne sound. The shortcomings are many: finding a good-sounding location on an instrument body is notoriously difficult, the sonic characteristics of piezo pickups are far from perfect, and their high output impedance requires special instrument inputs or direct boxes. In addition, they can be large and can interfere with the natural acoustic behavior of the instrument.

This leads to the idea of a low-mass contact microphone. Suppose that we used a surface transducer that measured the acceleration of the instrument’s body, preferably on more than one axis.11 This transducer would have good linearity and be so lightweight that it would not acoustically affect the instrument being measured. Suppose further that the transducer had similar output level, output impedance, and power requirements as a traditional microphone. In short, suppose that a musician could just plug this transducer into a microphone preamp or mixer input, just like any other microphone.

Contact Microphones
An attentive reader will notice the mention of acceleration in the preceding paragraph. Our ears respond to sound pressure, so microphones are designed to sense sound pressure. To simplify matters greatly, the sound pressure in the immediate vicinity of a vibrating body is proportional to acceleration.12 What if an accelerometer had enough bandwidth to be used as a contact microphone?

To explore this concept, a 3-axis accelerometer was mounted on an acoustic guitar to act as a pickup. The vibration of the instrument was measured and compared to the built-in piezo pickup and to a MEMS microphone mounted near the guitar. The guitar used was a Fender Stratacoustic acoustic with a built-in Fender pickup. An analog output MEMS accelerometer was mounted on a lightweight flex circuit (Kapton® with etched traces) and attached to the guitar body using beeswax at the bridge location, as shown in Figure 7. The X-axis of the accelerometer was oriented along the axis of the strings, the Y-axis was perpendicular to the strings, and the Z-axis was perpendicular to the surface of the guitar. A MEMS microphone with a flat frequency response out to 15 kHz was mounted 3" from the strings for use as a reference.

Figure 7. Accelerometer mounted on Fender Stratacoustic acoustic guitar.

A short sound segment was recorded using the accelerometer, the built-in piezo pickup, and the MEMS microphone. The time domain waveforms for each transducer are shown in Figure 8. No postprocessing was done on any of the audio clips.

Figure 8. Time domain waveforms using different transducers.

Figure 9 shows an FFT-based spectrum of the piezo pickup measured at one of the peaks in the time domain waveform. This spectrum shows a response with a strong bass component. Indeed, the actual audio file sounded excessively full, with a lot of bass response. This sounds pleasing (depending on your taste), as the cavity resonance creates a fuller bass sound than that heard when listening to the instrument directly.

Figure 9. Spectrum of piezo pickup.

The MEMS microphone output is very flat and reproduces the sound of the instrument very well. It sounds very natural, well balanced, and true to life. The FFT-based spectrum measured at the same point in time as the piezo pickup is shown in Figure 10(a). The frequency response of the MEMS microphone is shown in Figure 10(b) for reference.

Figure 10(a). Spectrum of MEMS microphone.

Figure 10(b). Frequency response of MEMS microphone.

The output from the MEMS accelerometer is very interesting. The immediate weak points are that the noise floor was too high and audible at the beginning and end of the track, and that the bandwidth of the Z-axis was clearly limited to lower frequencies. The sound reproduction from each axis was noticeably different.

The X- and Y-axes sounded bright and articulate and had clearly discernible differences in tonality. As expected, the Z-axis obviously sounded bass dominated. Figure 11 shows the X-axis spectrum (a), the Y-axis spectrum (b), and the Z-axis spectrum (c).

Figure 11(a). Spectrum of X-axis.

Figure 11(b). Spectrum of Y-axis.

Figure 11(c). Spectrum of Z-axis.

The X-, Y-, and Z axes mixed together produced a fair representation of the instrument with some brightness. By adjusting the mix, a variation in tonal balance can be achieved with natural sound reproduction. The extended upper harmonics are still missing due to the bandwidth limitation of the current accelerometers, but the sound reproduction was still surprisingly true.

Conclusion
Low-g MEMS accelerometers do not suffer from traditional feedback problems and demonstrate clear potential as high-quality acoustic pickups for musical instruments. A 3-axis accelerometer mounted on a Fender Stratacoustic acoustic guitar achieved promising sound reproduction. The three axes have different tonal characteristics related to the vibration modes of the instrument in the different directions of the body. The three output channels can be mixed to generate realistic sound reproduction. In addition, these channels can be mixed in different ways resulting in creative tonal effects.

While the performance of the accelerometer in this experiment is very promising, there are a few drawbacks. The noise floor of the sensor is audible; a problem that can be minimized using noise gating or other techniques, but the ideal sensor will have a noise floor comparable to conventional microphones. The high frequency response of the sensor needs to be extended, ideally up to 20 kHz to capture the full tonal range of the instrument.

MEMS accelerometer technology has clear potential for acoustic pickup applications in musical instruments, especially in live performances where acoustic feedback could be a problem. A very small, low-power MEMS device can be mounted unobtrusively anywhere on the instrument without affecting its natural vibration characteristics. In fact, multiple sensors can be mounted at different points around the instrument to provide the sound engineer additional flexibility to reproduce the natural character of the instrument without fear of acoustic feedback in live sound application—one step closer to “Sonic Nirvana.”


ADAN F CHAPARRO C
CI: 17501640
EES

http://www.analog.com/library/analogdialogue/archives/43-02/mems_microphones.html