MEMS Micro-Electro-Mechanical Systems, or MEMS, is a technology that in its most general form can be defined as miniaturized mechanical and electro-mechanical elements that are made using the techniques of microfabrication. The critical physical dimensions of MEMS devices can vary from several millimeters to below one micron. Similarily, the types of MEMS devices can vary from relatively simple structures having no moving elements, to extremely complex electromechanical systems with multiple moving elements under the control of integrated microelectronics. The one main criterion of MEMS is that there are at least some elements having some sort of mechanical functionality whether or not these elements can move. The miniaturized structures such as sensors, actuators, and microelectronics are the functional elements of MEMS. Microsensors and microactuators are the main elements which comes under MEMS. Microsensors and microactuators are appropriately categorized as “transducers”, which are defined as devices that convert energy from one form to another. Microsensors- Microsensers are the devices which convert a measured mechanical signal into an electrical signal. Over the past several decades MEMS researchers and developers have demonstrated an extremely large number of microsensors for almost every possible sensing modality including temperature, pressure, inertial forces, chemical species, magnetic fields, radiation, etc. Many of these micromachined sensors have demonstrated performances exceeding those of their macroscale counterparts. For example, a pressure transducer usually outperforms a pressure sensor made using the most precise macroscale level machining techniques. Microsensors not only achieve stellar device performance, but reduce the production cost. Silicon based discrete microsensors were quickly commercially exploited and the markets for these devices continue to grow at a rapid rate. Microatuators- A microactuator is a microscopic servomechanism that supplies and transmits a measured amount of energy for the operation of another mechanism or system. It includes microvalves for control of gas and liquid flows, optical switches and mirrors to redirect or modulate light beams, independently controlled micromirror arrays for displays, microresonators for a number of different applications, micropumps to develop positive fluid pressures, microflaps to modulate airstreams on airfoils, as well as many others. Even though these microactuators are extremely small, they frequently can cause effects at the macroscale level; that is, these tiny actuators can perform mechanical feats far larger than their size would imply. For example, researchers have placed small microactuators on the leading edge of airfoils of an aircraft and have been able to steer the aircraft using only these microminiaturized devices. Microelectronics:- Microelectronics relates to the study and manufacture of very small electronic designs and components. These devices are typically made from semiconductor materials. Microelectronic integrated circuits can be thought of as the "brains" of a system and MEMS augments this decision-making capability with "eyes" and "arms", to allow microsystems to sense and control the environment. Sensors gather information from the environment through measuring mechanical, thermal, biological, chemical, optical, and magnetic phenomena. The electronics then process the information derived from the sensors and through some decision making capability direct the actuators to respond by moving, positioning, regulating, pumping, and filtering, thereby controlling the environment for some desired outcome or purpose. Furthermore, because MEMS devices are manufactured using batch fabrication techniques, and can be placed on a small silicon chip at a relatively low cost. MEMS technology is extremely diverse and fertile, both in its expected application areas, as well as in how the devices are designed and manufactured. Already, MEMS is revolutionizing many product categories by enabling complete systems-on-a-chip to be realized. Nanotechnology- Nanotechnology is the ability to manipulate matter at the atomic or molecular level to make something useful at the nano-dimensional scale. Basically, there are two approaches in implementation: the top-down and the bottom-up. In the top-down approach, devices and structures are
made using many of the same techniques as used in MEMS except they are made smaller in size, usually by employing more advanced photolithography and etching methods. The bottom-up approach typically involves deposition, growing, or self-assembly technologies. The advantages of nano-dimensional devices over MEMS involve benefits mostly derived from the scaling laws, which can also present some challenges as well. Nanotechnology allow us a). to put essentially every atom or molecule in the place and position desired – that is, exact positional control for assembly. b). to make almost any structure or material consistent with the laws of physics that can be specified at the atomic or molecular level. c). to have manufacturing costs not greatly exceeding the cost of the required raw materials and energy used in fabrication (i.e., massive parallelism). Although MEMS and Nanotechnology are sometimes cited as separate and distinct technologies, in reality the distinction between the two is not so clear-cut. In fact, these two technologies are highly dependent on one another. The well-known scanning tunneling-tip microscope (STM) which is used to detect individual atoms and molecules on the nanometer scale is a MEMS device. Similarly the atomic force microscope (AFM) which is used to manipulate the placement and position of individual atoms and molecules on the surface of a substrate is a MEMS device as well. In fact, a variety of MEMS technologies are required in order to interface with the nano-scale domain. Microprocessor based controllers- Microprocessors are now rapidly replacing the mechanical camoperated controllers and being used in general to carry out control functions. They have the great advantage that a greater variety of programs become feasible. In many simple systems there might be just an embedded microcontroller, this being a microprocessor with memory all integrated on one chip, which has been specifically programmed for the task concerned. A more adaptable form is the programmable logic controller. This is a microprocessor-based controller which uses programmable memory to store instructions and to implement functions such as logic, sequence, timing counting and arithmetic to control events and can be readily reprogrammed for different tasks. Figure 1 shows the control action of a programmable logic controller, the inputs being signals from, say, switches being closed and the program used to determine how the controller should respond to the inputs and the output it should then give. Fig. 1 Programmable logic controller The following examples of control systems illustrate how microprocessor-based systems have not only been able to carry out tasks that previously were done 'mechanically' but also able to do tasks that were not easily automated before. 1. The automatic camera- The modem camera is likely to have automatic focusing and exposure. Figure 2 illustrate the basic aspects of a microprocessor based system that can be used to control the focusing and exposure.
Fig.2 Basic elements of the control system for an automatic camera When the switch is operated to activate the system and the camera pointed at the object being photographed, the microprocessor takes in the input from the range sensor and sends an output to the lens position drive to move the lens to achieve focusing. The lens position is fed back to the microprocessor so that the feedback signal can be used to modify the lens position according to the input from the range sensor. The light sensor gives an input to the microprocessor which then gives an output to determine, if the photographer has selected the shutter controlled rather than aperture controlled mode, the time for which the shutter will be opened. When the photograph has been taken, the microprocessor gives an output to the motor drive to advance the film ready for the next photograph. The program for the microprocessor is a number of steps where the microprocessor is making simple decisions of the form: is there an input signal on a particular input line or not and if there is output a signal on a particular output line. The decisions are logic decisions with the input and output signals either being low or high to give on-off states. A few steps of the program for the automatic camera might be of the form: begin if battery check input OK then continue otherwise stop loop read input from range sensor calculate lens movement output signal to lens position drive input data from lens position encoder compare calculated output with actual output stop output when lens in correct position send in-focus signal to viewfinder display etc. 2. The engine management system- The engine management system of a car is responsible for managing the ignition and fuelling requirements of the engine. With a four-stroke internal combustion engine there are several cylinders, each of which has a piston connected to a common crankshaft and each of which carries out a four-stroke sequence of operations.
When the piston moves down a valve opens and the air-fuel mixture is drawn into the cylinder. When the piston moves up again the valve closes and the air-fuel mixture is compressed. When the piston is near the top of the cylinder the spark plug ignites the mixture with a resulting expansion of tile hot gases. This expansion causes the piston to move back down again and so the cycle is repeated. The pistons of each cylinder are connected to a common crankshaft and their power strokes occur at different times so that there is continuous power for rotating the crankshaft. Four-stroke sequence The power and speed of the engine are controlled by varying the ignition timing and the air-fuel mixture. With modern car engines this is done by a microprocessor. Figure shows the basic elements of a microprocessor control system. For ignition timing, the crankshaft drives a distributor which makes electrical contacts for each spark plug in turn and a timing wheel. This timing wheel generates pulses to indicate the crankshaft position. The microprocessor then adjusts the timing at which high voltage pulses are sent to the distributor so they occur at the 'right' moments of time. To control the amount of air-fuel mixture entering a cylinder during the intake strokes, the microprocessor varies the time for which a solenoid is activated to open the intake valve on the basis of inputs received of the engine temperature and the throttle position. The amount of fuel to be injected into the air stream can be determined by an input from a sensor of the mass rate of air flow, or computed from other measurements, and the microprocessor then gives an output to control a fuel injection valve. Elements of an engine management system