机械设计及其自动化毕业设计（论文）外文资料翻译

毕业设计（论文）外文资料翻译学院： 机电工程学院 专业： 机械设计及其自动化班级： 机械四班 姓名： 学号： Microscopic Machines1 The surgeon picks up a syringe and approaches the man on the operating table. The patients coronary arteries are dangerously clogged with fatty deposits, which must be removed to prevent him from suffering a heart attack. The doctor injects a cloudy solution into the vein in the mans arm. The solution contains thousands of microscopic “robot surgeons”, each equipped with a tiny motor to propel it through the bloodstream, chemical detectors for locating the life-threatening blockages, and miniature scalpels for cutting them away. Within half an hour, the swarms of tiny robots have navigated through the patients blood vessels to his heart, located the trouble spots, and sliced the lumpy, yellowish deposits off the artery walls. Normal blood flow has been restored.2 for the time being, such medical scenarios will have to remain on the technological dream list - and they may never become reality. No one has built anything remotely like these fictional micro robots. But scientists and engineers in the United States and elsewhere have already made a variety of gears, levers, rotors, and other mechanical parts the size of specks of dust. Such components - made of the element silicon or of metals or other materials - may someday be assembled into tiny robots and various other kinds of microscopic machines designed to perform specific functions. These micro machines would be so small that dozens could easily fit inside a sesame seed. 3 The recent advances in the miniaturization of machine parts represent the beginnings of a new branch of engineering, whose practitioners think small - extremely small. Micro machine technology is still so new that it doesnt yet have a widely accepted name. Some researchers call it micro engineering, while others refer to it as micro dynamics or micromechanics. Whatever they call their new discipline, these engineers work in a realm where objects are measured in fractions of a millimeter. (One millimeter is about 0.04 inch.) At that scale, a grain of sand looks like a boulder and mechanical principles such as friction, wear, and lubrication take on new, poorly understood meanings. 4 Such factors may present problems that cannot be overcome. If they can be surmounted, however, micro engineering may usher in a revolutionary new machine age. We may see the creation of all kinds of teensy devices combining electronic detectors called sensors with mechanical parts called actuators that do work. In addition to performing microscopic surgery, such micro machines might pump minute amounts of chemicals, focus laser beams in optical computers, and power tiny tools whose uses can only be guessed at for now.5 A handful of relatively simple micro devices have already made it to the marketplace. Some computer printers, for example, form letters by spraying tiny amounts of ink onto the paper through microscopic nozzles developed by engineers at the International Business Machines (IBM) Research Laboratory in San Jose, Calif. But most currently available micro devices are sensors which react to changes in their environment, for example, by bending under pressure. Engineers at the Honeywell Corporations Physical Sciences Center in Bloomington, Minn., have developed micro sensors that measure airflow in the ventilation systems of buildings or in the instruments that hospitals use to monitor patients breathing. Other companies have developed tiny sensors for measuring pressure in automobile engines or in the human heart. 6 Meanwhile, researchers are working on various kinds of microscopic actuators that may be perfected in the 1990s. Some of these will perhaps work like minuscule hands or tweezers for manipulating tiny objects, such as individual cells under a microscope. Miniature pumps and valves are also a possibility and would have a variety of applications. Medical researchers envision an artificial pancreas for treating diabetes that would pump tiny amounts of insulin as needed into the blood stream.7 Micro engineering came to national attention in June 1988 when electrical engineer Richard Smaller and his colleagues at the University of Californias Berkeley Sensor & Actuator Center announced that they had made a tiny silicon motor, the first electrically powered micro device containing a rotating part. The devices rotor, the part that spins, was smaller than the width of a human hair. (A human hair is about 0.05 millimeter in diameter.) The cogs of the rotor were the size of red blood cells. When the researchers used static electricity to activate electrodes surrounding the rotor, the rotor began to spin haltingly. Although the movement was crude, and the rotor later jammed, the experiment showed that engineers visions of microscopic machines could become reality.8 The achievement at Berkeley came almost 30 years after researchers first began to think small. In1959. Nobel Prize-winning physicist Richard P. Feynman predicted that scientists would someday build machines and tools as tiny as dust specks and then use them to manufacture even smaller things. Feynman had no idea how that feat would be accomplished, however, and to many ears his speculations were the widest kind of blue-sky fantasy. But with the coming of the microelectronics revolution in the computer industry in the 1970s, what had been fantasy suddenly seemed like a distinct possibility. 9 The history of the computer industry is a story of constant miniaturization, as engineers learned to cram more and more electronic components into a smaller amount of space. In the 1960s, electronics manufacturers began building complex circuits on fingernail-sized pieces of silicon. By the 1970s, these tiny circuits, which had become known as microchips, contained thousands of elements. Today, a single microchip can hold millions of components.10 The production of silicon microchips begins with a procedure called microlithography, which involves several steps. First, a large, detailed drawing of the chip is made, and the drawing is photographed. The photographic image is then greatly reduced and imprinted - usually as a stencil like pattern of metallic lines - on a glass plate. The finished plate is known as a mask. Next, a palm-sized silicon wafer gets a coat of a photo resist, a plastic material that, when exposed to ultraviolet light, is chemically weakened. When the mask is placed over the coated wafer and exposed to ultraviolet light, the ultraviolet rays that are not blocked by the mask transmit the image of the chip to the wafer. The regions of the photo resist that are weakened by the process are then etched, or eaten away, by solvents or gases. The etching exposes the underlying layer of silicon in a pattern that corresponds to the mask pattern. 11 Once this process has been completed, the engineers usually deposit additional thin films of silicon, metal, or insulating materials onto the exposed silicon pattern and repeat the etching process several more times. In this way, they can build up extremely complicated patterns and structures on the wafer, all no more than a few thousandths of a millimeter thick. Each patterned layer is connected to the next, becoming part of the final device or part of the formation of the next layer.12 Silicon is an excellent semiconductor, a material that conducts electricity better than insulations such as glass but not as well as conductors such as copper. Silicon is an ideal material for electronic microchips because it can be processed together with an insulating material such as silicon dioxide. But silicon also possesses unusual mechanical properties. 13 Although it is brittle and fragile when it is in the form of wafers, at microscopic dimensions silicons crystal structure makes it highly resistant to stress. At that scale, it is in fact stronger than steel. Thus, by 1980, some engineers were suggesting the possibility of crafting mechanical devices as well as electronic components from silicon. That idea received a major boost in 1982 from Kurt Peterson, an IBM researcher and now a chief scientist at Lucas/nova Sensor, a micro engineering company in Ferment, Calif. Peterson argued that by adopting and modifying the miniaturization techniques pioneered by the electronics industry, it would be possible to create a variety of micro devices from silicon and mass-produce them. Moreover, he said, silicons mechanical and electronic properties made it an ideal material for the production of integrated devices consisting of actuators combined with sensors and other electronic devices.14 Today, a growing number of researchers in the United States, Europe, and Japan are forging a new area of science and technology. These micro engineering pioneers are showing ever-increasing skill at making miniature parts out of silicon as well as other materials. To make micro parts out of silicon, engineers use standard microlithography along with a variety of chemicals that etch into silicon wafers at different rates and in different directions. By controlling the etching times of these chemicals, or by using a chip on which has been deposited an underlying layer of a chemical-resistant material, engineers can dig out extremely precise microscopic pits or holes and form tiny walls and other structures. 15 Using these etching techniques, researchers have learned to chemically “chisel” around and underneath pre_ patterned gears and rotors to separate them from their base and allow them to move freely. This procedure involves depositing a “sacrificial layer” of a material such as silicon dioxide on the original blank chip and then overlaying it with another silicon layer from which the moving part is to be fashioned. After this “sandwich” is exposed to ultraviolet light projected through the microlithography mask and the silicon layers are etched, chemicals are used to dissolve the sacrificial layer. As that layer dissolves, the rotor, lever, or other part in the upper layer of silicon is liberated. This technique has enabled engineers to create a variety of gears, rotors, sliding mechanisms, and other microscopic devices with moving parts. And because 1,000 or more copies of a device can be etched onto a single silicon wafer, researchers say that it may one day be possible to manufacture micro machines by the tens of thousands at a cost of only a few cents apiece.16 Although silicon seems likely to remain the primary micro engineering material for some years, some metals also show promise. Several researchers, including electrical engineer Henry Buckle of the University of Wisconsin in Madison, have been scoring successes in using metals such as tungsten and nickel to create micro machine parts. Metal seems to have one big advantage over silicon for many micro machine parts - structural rigidity. Although silicon is extremely strong at microscopic dimensions, the silicon etching techniques developed so far work well only for thin structures. Most silicon micro machine parts are so thin that they tend to warp from internal stresses. Researchers hope to find ways of making silicon parts thicker, but for now only metal can be made into thick parts. 17 Buckle and his colleagues have made all-metal micro gears that are slightly larger than the silicon gears made in some laboratories. But at a diameter of 0.1 to 0.2 millimeter - 2 to 4 times the width of a human hair - they are still smaller than grains of salt. The researcher made the gears using a technique called electroplating. In this procedure, an electric current is used to draw dissolved ions (electrically charged atoms) of nickel or chromium into tiny molds. The molds have been etched into a layer of Plexiglas (a type of hard plastic) on a metal plate. After the metal ions have filled the molds and solidified, a sacrificial layer underneath the Plexiglas is dissolved to free the part.18 Instead of ultraviolet light, the Buckle team used X-rays generated by an atomic-particle accelerator. These exceptionally powerful and short-wavelength X-rays, directed through a mask, enabled the researchers to etch deep molds with perfectly vertical sides essential for the production of thick parts. Buckle thinks gears and other micro parts made of metal, because they are thicker and stronger, may be better suited for powering drills and other tiny tools than similar parts made of silicon. So far, X-rays have not been widely used in the making of silicon parts because the rays tend to damage the silicon. And there is another reason as well: cost. The use of X- rays, especially those generated by a particle accelerator, to make micro parts is an expensive proposition that may not be commercially practical. Due to these drawbacks, most micro machine engineers are sticking with methods employing ultraviolet light and chemical etching agents. 19 Although micro engineering may indeed herald the beginning of a new machine age, researchers must first solve several fundamental problems. Besides the warping that can make silicon parts curl up like potato chips, there are other difficulties involved with operating in the micro world. In that hidden realm, a fine grain of flour could bring a rotor grinding to a halt - perhaps with a screech that a nearby flea could hear - in just a few seconds. In addition, familiar phenomena such as friction, air resistance, electrical charge, wear, and the behavior of fluids must be redefined because their effects are different at microscopic dimensions than in the everyday world. At tiny scales, where the spaces between parts are vanishingly small, standard lubricants can work instead like adhesives. Blood, which to a human surgeon flows freely, might seem like molasses to a robot surgeon that is no bigger than a red blood cell.20 Overcoming such obstacles will undoubtedly keep engineers busy for years to come, but there is already cause for optimism. Even now, engineers are solving some of the warping and sticking problems that doomed their earliest micro motors to short lives. For example, Muller and his colleagues have learned to deposit silicon layers in a way that lessens the stresses that lead to warping. Through refinements in the microlithography process, they have also crafted sets of gears so precise that they mesh with absolutely no slippage.21 The sheer tinniest of micro motors, however, may present another, altogether different problem: They may be too small to do much of anything requiring the application of force. Think of trying to move a stone with a whisker and you will see the difficulty facing researchers. “Little machines dont produce big forces,” says engineer Stephen Senatorial of the Massachusetts Institute of Technology (MIT) in Cambridge. “So scaling machines down may make them so weak they cant do any useful mechanical work.” Part of the answer to that concern may lie in making parts thicker.22 Another possibility is to build somewhat bigger micro machines, ones that are perhaps 20 times the width of a human hair. At the University of Utah in Salt Lake City, engineer Stephen Jacobson has been doing just that. Jacobson and his co-workers have been using conventional machine tools to painstakingly assemble small metal and plastic components into what they call “wobble motors,” since the devices rotors wobble as they spin. This form of motion can reduce the effects of friction and thus produce more torque, or rotational force. Jacobson thinks wobble motors would be better suited than other kinds of micro motors for controlling the movements of the tiny robotics machines foreseen by many engineers. Wobble motors and other larger-sized micro machines being developed have one big disadvantage of their own, however. As yet, they cannot be mass-produced. Unless a way can be found to manufacture them by the thousands, such devices will almost certainly be too expensive to be widely used. 23 for all varieties of micro devices, the list of challenges goes on. In developing micro machines that would have to withstand harsh environments, such as those of outer space or the interior of jet engines, engineers must find ways of protecting delicate parts against damage. Researchers must also learn how to connect micro machines requiring human guidance to control systems large enough for people to operate.24 Because of the many obstacles that must be overcome, the micro machine age - assuming there will be one - is unlikely to arrive until sometime after the year 2000. In the mean time, however, many kinds of micro sensors have been perfected and coming into widespread use. So far, most of these are pressure sensors for automotive and medical uses. Most such sensors consist of an ultra thin disk called a diaphragm built on a tiny silicon chip. The diaphragm, which is also usually made of silicon, is created by etching away an intermediate layer of material. The diaphragm is engineered so that the slightest bending caused by a pressure charge alters the electrical resistance of a pattern of resistors (components that control voltage) in a precise way. Circuitry on the chip, or attached to it, detects the change and translates it into an electrical signal, which in turn is translated into a pressure measurement. 25 Most of the pressure sensors used in automobiles goes under the hood to monitor engine pressure for the on-board computers that help control the cars combustion and exhaust emissions. Other kinds of sensors are also being used in many new cars, measuring everything from engine coolant temperature to the moment-by-moment position of the crankshaft. Cars build in coming years will most likely include micro sensors that keep track of all of a vehicles temperatures, pressures, airflow, mechanical motions, and other operating factors.26 Medical micro sensors are often used to measure blood pressure inside patients hearts. The sensor is attached to the end of a plastic filament，in which the physician snakes through the blood vessels to the heart. But other kinds of medical sensors may soon be available to doctors. For example, engineer Kensal Wise and his associates at the Center for Integrated Sensors and Circuits at the University of Michigan in Ann Arbor are working on a new kind of brain probe. This silicon probe is thinner than a sewing needle and has 32 micro sensors that can eavesdrop on the electrical activity in small groups of neurons (nerve cells). Wise thinks the technology used in the probe might eventually be used to make tiny medical devices that can be inserted into the brain to detect and control epileptic seizures and other neurological disorders. 27 Wise foresees a dazzling future for micro sensors. He envisions sensors that “see”, “hear”, “feel”, “taste”, and “smell” the world with a precision and sensitivity exceeding that of the human senses. Used in the manufacturing industries the not-so-distant future, hundreds of widely distributed sensors could continuously gather data about the temperatures of vats, the flow of fluids through pipes, the pressure in tanks, and the progress of chemical reactions. Computers and technicians would instantly receive and analyze these data to make adjustments in the factorys processing equipment. 28 Micro sensors should also be of great value in space exploration, especially aboard satellites and unmanned spacecraft, in which compactness and light weight have always been high priorities. Besides taking up less space than conventional sensors, micro sensors weigh less and use less power. Also, they usually respond faster to changes in their environment.29 While many of the applications of micro sensors are already with us, or easily foreseen, the uses of true micro machines - sensors plus actuators - are far less certain. “Were in the process of discoverin