======================COMPONENTS_SENSORS=========================== 10X scope probe scope Zin = 1Meg BNC __________/\ _____________ 1X \/ R 10k | _____ ___||_________ | \ | || 2-20pF | | \______|____/\ ______|______|______ Probe 10X \/ R 9Meg | _|_ \ / gnd V ---------------------------------------------------------------------------- hall effect usually silicon or other semiconductor has a current I flowing through it. magnetic field is applied perpendicular to current charge carriers in the slab feel a force F=Bqv, B is the strength of the magnetic field, q charge (for electrons, this is 1.6x10^-19 Coulombs) v is the velocity of charge carriers ^ | | Y /---|----/| / | / | ---> I / / ------> I (current) |--------| / | X | / |--------|/ | | | B (magnetic field line) ^ positive charges accumulate on one side of the slab, V = BI --- nqt t is the thickness of the slice, n is the charge carrier density per cubic metre. ---------------------------------------------------------------------------- Radiation sensors variety of radiation sensors for different types of radiation, including nuclear radiation as well as visible light, infrared and ultraviolet. photodiode and phototransistor, charge coupled devices (CCDs), and pyroelectric sensors. ---------------------------------------------------------------------------- photodiode is a reverse biased p-n (diode) junction. When no light falls on device only a small amount of current flows (the dark current). When light falls on device, carriers are generated, current flows. Photodiodes typically work in visible light - near infrared region of spectrum. are high impedance devices, and operate at relatively low currents (typically 10uA dark current, rising to 100uA when illuminated). They have fairly linear responses to increasing illumination, fast response times. ---------------------------------------------------------------------------- Phototransistors phototransistor has a much higher current output than a photodiode for comparable illumination levels. , not as fast as photodiodes ( 100kHz being top limit), and also has higher dark current. The phototransistor is essentially a transistor with base current supplied by current produced by illumination of base-collector junction; it can be considered to be similar to a photodiode supplying base current to a transistor (figure 3). ---------------------------------------------------------------------------- CCDs Charge coupled devices (CCDs) Charge coupled devices can be built as large linear and two dimensional arrays; latter are often used for small video cameras. consist of a large number of electrodes (gates) on semiconductor substrate. A thin insulating layer between metal gates and semiconducting substrate. ---------------------------------------------------------------------------- Pyroelectric sensors operate on pyroelectric effect in polarised crystals (e.g. zinc oxide).crystals have built in electrical polarisation level which changes with amount of incident thermal energy. generally high impedance devices, One potential problem is that crystals that exhibit pyroelectric effect may exhibit piezoelectric effects One common application of these devices is in human motion detectors for intruder alarms. A lens cuts sensor's field of view into discrete sections. As someone moves across field of view, thermal radiation from their body falls on sensor, resulting in discrete pulses as person moves from one part of field of view to next. It is thus possible to build relatively cheap motion detectors, which is tuned to respond to a particular rate of motion. ---------------------------------------------------------------------------- Magnetic sensors There are many ways of sensing magnetic fields. Optical sensors can be based on crystals that exhibit a magneto-optic effect, or specially doped optical fibres. Coils can be used, although microfabricated coils are generally two dimensional, which often isn't useful for many applications. continuing development of high-temperature superconductors is also broadening possibilities for sensors based on superconducting quantum interference devices (SQUIDs), which are capable of detecting magnetic fields of heart or brain. There are also a variety of other devices. Many measurements can be made, however, using Hall effect sensors. These are very common, and are outlined below. A Hall effect sensor is shown diagrammatically in figure 5. The sensor consists of a conducting material, usually a semiconductor, and a current is passed between two contacts on opposite sides of device. Two sensing contacts are placed on two other sides of device, opposite each other,perpendicular to current .A magnetic field perpendicular to plane of contacts is causes a deviation in current flow across device. This in turn is detected as a potential difference between two sensing contacts. Figure 5. Hall effect sensors operate typically in range 0.1 milli-Tesla to 1T (the earth's magnetic field is about 0.05mT). Hall effect IC packages are available which typically give an output of about 10mV per mT. ---------------------------------------------------------------------------- Thermal sensors thermocouples and thermoresistors (thermistors). Thermocouples two dissimilar metals (e.g. copper and iron) junctions are held at different temperatures, The open circuit voltage (i.e. as measured by an ideal voltmeter with infinite input impedance) is related to temperature difference (Ta - Tb ), and difference in Seebeck coefficients of two materials (Pa - Pb ); equation 1: V = ( Pa - Pb )( Ta - Tb ) Equation 1. V will typically be of order of millivolts, or tens of millivolts, for metal thermocouples with temperature differences in order of 200degC. ---------------------------------------------------------------------------- Thermoresistors The electrical resistivity of metals varies with temperature. Above -200degC, resistivity varies almost linearly with temperature. In this approx linear region, variation of resistivity (r) with temperature (T) can be adequately described by a quadratic equation: r=R(1+aT+bT2) Equation 2. Where R is resistivity of material at a reference temperature (0degC), and a and b are constants specific to metal being used. Platinum is often used, as its resistance variation is particularly linear with temperature (i.e. b is particularly small). metal thermoresistors generally have relatively small resistances, and their rate temperature coefficient of resistance, TCR) is not particularly large, Due to negative TCR, it is possible for resistor to go into a self-heating loop: current flowing through resistor heats it up, resistivity drops, more current flows, it gets hotter, etc. ---------------------------------------------------------------------------- chemical sensors are based around metal oxide semiconductor field effect transistor (MOSFET) devices. , ISFET (ion sensitive field effect transistor) have been around (since about 1970), ---------------------------------------------------------------------------- biosensor refers to any sensor that uses an active biological (or sometimes biologically derived) component in transduction process. This may be a sensory cell taken from a living organism, and mounted on an electrode. Alternatively, antibodies may be used, which will lock on to material of interest, and hold it in an appropriate position for sensing. A further option is to use an enzyme that catalyses a reaction that can be detected by suitable means. Since there is considerable interest in monitoring blood glucose levels (to provide closed loop control of blood glucose, by means of an artificial pancreas, for diabetics), blood glucose sensors have received much attention. One of these is based on glucose oxidase enzyme; so this sensor will be outlined. One thing to note is that a lot of research has gone into these sensors; biosensors and blood glucose sensors in particular. Whilst progress has been made, there are still a lot of problems to be solved. One big problem in this area is that sensor performance drifts or degrades over time, often in unpredictable ways. So device has to be calibrated regularly, or just before use. Clearly a blood glucose sensor that only gives reliable readings over a period of a hundred days cannot be used in an implanted artificial pancreas. Thus, while there are many potential uses for chemical sensors, their use is often complicated by calibration requirements. ---------------------------------------------------------------------------- ISFET sensors ISFETs sense concentration (activity level) of a particular ion in a solution. These devices are generally based on enhancement mode metal-oxide-semiconductor field effect transistor (MOSFET) structure, In ISFET, The PMOS gate metal is replaced with an ion selective membrane and device is immersed in a solution. Ions in solution interact with ion selective membrane. When there is a high concentration of positive ions in solution, a lot of them will accumulate on gate, widening channel between source and drain. With a low concentration of positive charged ions, channel will be narrow. In order to ensure that FET channel is biased to an optimum size, about which sensing can take place, solution is maintained at a reference potential by an electrode placed in it. Generally reference potential is adjusted to maintain a constant current flowing from drain to source,so ionic concentration will be directly related to solution reference potential with respect to substrate potential (in circuit shown in figure 7). One significant problem in design and fabrication of ISFETs is ensuring that ion selective membrane adheres to device. If integrity of membrane is compromised, then device is useless; this problem has considerable effct on yield (0f functioning devices on a wafer) of fabrication process. Enzyme-based biosensors Enzymes are highly specific in reactions that they catalyse. If an enzyme can be immobilised on a sensing substrate, and reaction products detected, then one has basis of a highly selective biosensor. The enzyme-based biosensor described below is for monitoring glucose levels; this application has received considerable investigation since glucose is important in diabetes, and also in many industrial fermentation processes. The operation of a glucose oxidase based sensor is shown schematically in figure 8. The enzyme is immobilised on a platinum electrode, and covered with a thin polyurethane membrane to protect enzyme layer, and reduce dependence of sensor on blood oxygen levels. Glucose oxidase, in its oxidised form, oxidises glucose entering sensor to gluconic acid; resulting in conversion of enzyme to its reduced form. The enzyme does not remain in this form for long. It interacts with oxygen entering through membrane. The products of this interaction are oxidised form of enzyme, and two hydrogen ions and two oxygen ions. When platinum electrode is biased to correct potential, it will reduce one of oxygen ions so that end products are oxygen and water; resulting electrode current can be measured, and will be proportional to concentration of glucose in external medium. (Nb/ This is a very much simplified explanation, and there are also many other ways to monitor reaction). Figure 8. One thing to note is that because various molecules have to physically move through materials of sensor, such biosensors can be quite slow to respond to changes in external medium. ---------------------------------------------------------------------------- Microelectrodes for neurophysiology Microelectrodes of fine wire or electrolyte filled micropipettes have been used for some time to study nervous system on a cellular (individual neuron) basis. These, in particular metal wire microelectrodes, are prime targets for application of microengineering techniques. The small signal amplitudes involved (in region of 100uV) and high interface impedances (1-10 MOhm at 1 kHz) between metal and tissue mean it is advantageous to place amplifier as close as possible to recording site. In addition, characteristics of microfabricated devices can be more reproducible than for hand-made metal wire microelectrodes, and their small size enables accurate insertion of many recording sites into small volumes of tissue to study networks of neurons, or for neural prosthesis applications. The microelectrodes operate by detecting electrical potential generated in tissue near an active nerve fibre, due to action potential currents flowing through fibre membrane. There are three common types of micromachined microelectrode (figure 9). Array-type microelectrodes (figure 9a) are used to form floor of cell culture dishes: signals are recorded from neurons which are placed or grown over these. Probe-type microelectrodes (figure 9b) have recording sites on a long thin shank, which is inserted into tissue under investigation. Regeneration electrodes (figure 9c) are placed between ends of a severed peripheral nerve trunk; nerve fibres then re-grow (regenerate) through device. These microelectrodes can be quite difficult to use. For array microelectrodes, appropriate cell culture methods have to be developed and practised. Probe types have to be mounted on amplifier boards, and different situations require many different size / shaped probes. Regeneration electrodes have to be fixed to stumps of nerve trunk, and require connecting to outside world. All devices can potentially generate huge amounts of data, which has to be collected and analysed. ---------------------------------------------------------------------------- Mechanical sensors Two different types of mechanical sensors will be discussed here. The first uses physical mechanisms to directly sense parameter of interest (e.g. distance, strain). The second type uses microstructures to enable mechanical sensors to detect parameters of interest that cannot be measured directly with first type of sensor (e.g. acceleration). Piezoresistors The change in resistance of a material with applied strain is termed piezoresistive effect. Piezoresistors are relatively easy to fabricate in silicon; being just a small volume of silicon doped with impurities to make it n-type or p-type. ---------------------------------------------------------------------------- Piezoelectric sensors force applied to piezoelectric material, charge induced on surface proportional to applied force Common piezoelectric crystals used include zinc oxide and PZT (PbZrTiO3 - lead zirconate titanate), which can be deposited on microstructures, and patterned. ---------------------------------------------------------------------------- Capacitive sensors two parallel conducting plates, C = eA / d ---------------------------------------------------------------------------- Optical sensors Silicon is a reflective material, as aluminium Thus optical means may be used to sense displacement or deformation of microengineered beams, membranes, etc. A laser is directed at surface to be monitored in such a way that interference fringes are set-up. By analysing these fringes, displacement or deformation may be detected and quantified. One area where this is often employed in in atomic force microscopy, to monitor deflection of beam upon which sensing 'tip' is mounted. ---------------------------------------------------------------------------- Resonant sensors These are based on micromachined beams or bridges which are driven to oscillate at their resonant frequency. Changes in resonant frequency of device would typically be monitored using implanted piezoresistors, or optical techniques. resonant frequency of bridge related to force applied to it (between anchor points), its length, thickness, width, its mass, and modulus of elasticity of material from which it has been fabricated. force applied to bridge changes resonant frequency resonant device may be used as a biosensor, coating with material that binds to substance of interest As more of substance binds to device, its mass will be increased, again altering resonant frequency. ---------------------------------------------------------------------------- Accelerometers Microengineered acceleration sensors, accelerometers, consist of a mass suspended from thin beams (figure 11). As device is accelerated, a force (force = mass x acceleration) is developed which bends suspending beams. Piezoresistors situated in device where beams meet support (where strain is greatest) can be used to detect acceleration. Another alternative is to capacitively sense displacement of mass. ---------------------------------------------------------------------------- Pressure sensors Microengineered pressure sensors are usually based around thin membranes. On one side is an evacuated cavity (for absolute pressure measurement), and other side is exposed to pressure to be measured. The deformation of membrane is usually monitored using piezoresistors, or capacitive techniques. ---------------------------------------------------------------------------- Microactuators Microactuators are required to drive resonant sensors, above, to oscillate at their resonant frequency. They are also required to produce mechanical output required of particular microsystems: this may be moving micromirrors to scan laser beams, or switch them from one fibre to another; to drive cutting tools for microsurgical applications; to drive micropumps and valves for microanalysis or microfluidic systems; or these may even be microelectrode devices to stimulate nervous tissue in neural prosthesis applications. Within following section a variety of methods for achieving microactuation are briefly outlined: electrostatic, magnetic, piezoelectric, hydraulic, and thermal. Of these, piezoelectric and hydraulic methods currently look most promising, although others have their place. Electrostatic actuation runs a close third, and is possibly most common and well developed method, but it does suffer a little from wear and sticking problems. Magnetic actuators usually require relatively high currents (and high power), and on microscopic scale, electrostatic actuation methods usually offer better output per unit volume (the limit is somewhere in region of going from 1cm cubed devices to a few mm cubed - depending on application). Thermal actuators also require relatively large amounts of electrical energy, and heat generated also has to be dissipated. When dealing with very smooth surfaces, typical of micromachined devices, sticking or cold welding of one part to another can be a problem. These effects can increase friction to such a degree that all output power of device is required just to overcome it, and they can prevent some devices from operating at all. Careful design and selection of materials can be used to overcome these problems; but they still cause trouble with many micromotor designs. Another point to be aware of is that when removing micromachined devices from wet etch baths, surface tension in liquid can be strong enough to stick parts together. ---------------------------------------------------------------------------- Electrostatic actuators For a parallel plate capacitor, energy stored, U, is given in equation 4 (where C is capacitance, and V is voltage across capacitor). Equation 4. When plates of capacitor move towards each other, work done by attractive force between them can can be computed as change in U with distance (x). The force can be computed by equation 5. Equation 5. Note that only attractive forces can be generated in this instance. Also, to generate large forces (which will do useful work of device), a large change of capacitance with distance is required. This has lead to development of electrostatic comb drives (figure 12a). Figure 12. Comb Drives. These are particularly popular with surface micromachined devices. They consist of many interdigitated fingers (figure 12a). When a voltage is applied an attractive force is developed between fingers, which move together (figure 12b). The increase in capacitance is proportional to number of fingers; so to generate large forces, large numbers of fingers are required. One potential problem with this device is that if lateral gaps between fingers are not same on both sides (or if device is jogged), then it is possible for fingers to move at right angles to intended direction of motion and stick together until voltage is switched off (and in worst scenario, they will remain stuck even then). Wobble motors are so called because of rolling action by which they operate. Figure 13a,b shows a surface micromachined wobble motor design. The rotor is a circular disk. In operation electrodes beneath it are switched on and off one after another. The disk is attracted to each electrode in turn; edge of disk contacting insulator over electrode. In this manner it rolls slowly around in a circle; making one revolution to many revolutions of stator voltage. Problems can arise if insulating materials on stator electrodes wear rapidly, or stick to rotor. Also, if rotor and bearing aren't circular (this is possible since many CAD packages draw circles as many sided polygons), then rotor can get stuck on its first revolution. Figure 13. A problem with surface micromachined motors is that they have very small vertical dimensions, so it is difficult to achieve large changes of capacitance with motion of rotor. LIGA techniques can be used to overcome this problem - for instance wobble motor shown in figure 13c,d, where cylindrical rotor rolls around stator. ---------------------------------------------------------------------------- Electrostatic actuators For a parallel plate capacitor, energy stored, U, is given in equation 4 (where C is capacitance, and V is voltage across capacitor). Equation 4. When plates of capacitor move towards each other, work done by attractive force between them can can be computed as change in U with distance (x). The force can be computed by equation 5. Equation 5. Note that only attractive forces can be generated in this instance. Also, to generate large forces (which will do useful work of device), a large change of capacitance with distance is required. This has lead to development of electrostatic comb drives (figure 12a). Figure 12. Comb Drives. These are particularly popular with surface micromachined devices. They consist of many interdigitated fingers (figure 12a). When a voltage is applied an attractive force is developed between fingers, which move together (figure 12b). The increase in capacitance is proportional to number of fingers; so to generate large forces, large numbers of fingers are required. One potential problem with this device is that if lateral gaps between fingers are not same on both sides (or if device is jogged), then it is possible for fingers to move at right angles to intended direction of motion and stick together until voltage is switched off (and in worst scenario, they will remain stuck even then). Wobble motors are so called because of rolling action by which they operate. Figure 13a,b shows a surface micromachined wobble motor design. The rotor is a circular disk. In operation electrodes beneath it are switched on and off one after another. The disk is attracted to each electrode in turn; edge of disk contacting insulator over electrode. In this manner it rolls slowly around in a circle; making one revolution to many revolutions of stator voltage. Problems can arise if insulating materials on stator electrodes wear rapidly, or stick to rotor. Also, if rotor and bearing aren't circular (this is possible since many CAD packages draw circles as many sided polygons), then rotor can get stuck on its first revolution. Figure 13. A problem with surface micromachined motors is that they have very small vertical dimensions, so it is difficult to achieve large changes of capacitance with motion of rotor. LIGA techniques can be used to overcome this problem - for instance wobble motor shown in figure 13c,d, where cylindrical rotor rolls around stator. [Electrostatic Comb Driven Resonator ---------------------------------------------------------------------------- Magnetic actuators Microstructures are often fabricated by electroplating techniques, using nickel. This is particularly common with LIGA. Nickel is a (weakly) ferromagnetic material, so lends itself to use in magnetic microactuators. An example of a magnetic microactuator is linear motor shown in figure 14. The magnet resting in channel is levitated and driven back and forth by switching current into various coils either side of channel at appropriate time. Figure 14. From figure 14, one common problem with magnetic actuators is clear: coils are two dimensional (three dimensional coils are very difficult to microfabricate). Also, choice of magnetic materials is limited to those that can be easily micromachined, so material of magnet is not always optimum. This tends to lead to rather high power consumption and heat dissipation for magnetic actuators. In addition, with microscopic components (up to about mm dimensions), electrostatic devices are typically stronger than magnetic devices for equivalent volumes; whereas magnetic devices excel for larger dimensions. ---------------------------------------------------------------------------- PIEZOELECTRIC EFFECT certain dielectric materials converts an Input voltage to a mechanical motion or vice versa. important uses include filters, beepers, sonar, ultrasonics, micropositioners, gyros, microphones, miniature fans, strain gauges, accelerometers, and furnace igniters. + _______|_______ __|_______________|____ ____---- ----___ | _______________________ | |____---- |_______________| ----___| | - 0 _______|_______ ________|_______________|_____________ | | |______________________________________| |_______________| | 0 - ____ _______|_______ ___ | ----__|_______________|____---- | |____ ___| ----_______________________---- |_______________| | + fatigue life of piezoelectric Why piezoelectricity some atomic lattice structures have as an a cubic or rhomboid cage made of atoms, this cage holds a single semi-mobile ion which has several stable quantum position states inside cell. The ion's post ion state can be caused to shift by either deforming cage (applied strain) or by applying and electric field. The coupling between central ion and cage provides basis for transformation of mechanical strain to internal electric field shifts and vice versa. electric field small known charge (Q) placed near a charge will experience an accelerating force (F) electric field (E) is ratio F/Q (a vector). strain? rod of length (L) stretched to length (L + delta L), strain defined as ratio (delta L)/(L). elastic modulus (or Young's modulus) "stiffness" of materials. Young's modulus can be computed as follows: Y = (L/A)*(F/deltaL) tensile strength is stress (measured in Newtons/m^2 or psi) at which a solid material will break from tension. poling/depoling piezoceramic materials? A. The piezoelectric property of ceramics does not arise simply from its chemical composition. In addition to having proper formulation piezoceramics must be subjected to a high electric field for a short period of time to force randomly oriented micro-dipoles into alignment. This alignment by application of high voltage is called "poling". At a later time, if an electric field is applied in opposite direction it exerts a "dislodging stress" on micro-dipoles. Low level applied fields result in no permanent change in polarization (it bounces back upon removal). Medium fields result in partial degradation of polarization (with partial loss of properties). High applied fields result in repolarization in opposite direction. Damping is term used for general tendency of vibrating materials or structures to lose some elastic energy to internal heating or external friction. piezoelectricity? some atomic lattice structures have as a "cell") a cubic or rhomboid cage made of atoms, this cage holds a single semi-mobile ion has several stable quantum position states inside cell ion's post ion state can be caused to shift by either deforming cage (applied strain) or by applying and electric field. coupling between central ion and cage provides the basis for transformation of mechanical strain to internal electric field shifts and vice versa. strain rod of length (L) stretched to a new length (L +delta L) strain defined as the ratio (delta L)/(L). "millimeters per meter", or "microns per meter (microstrain)" for convenience of visualization. elastic modulus (or Young's modulus) ? material property of all elastic solids, Young's modulus (Y) describe "stiffness" of materials. rod or plate of cross section (A) and length (L) pulled with force (F) resulting in elongation (delta L) Young's modulus can be computed as follows: Y = (L/A)*(F/deltaL) In piezo applications Y is frequently used to estimate the equivalent spring constant of a rod or a plate of material (i.e. that quantity (F/deltaF) that is in contact with a piezo actuator). tensile strength stress (measured in Newtons/m^2 or psi) at which a sample will break from tension. poling/depoling piezoelectric property of ceramics and piezoceramics must also be subjected to a high electric field for a short period of time to force randomly oriented micro-dipoles into alignment This alignment at high voltage is called "poling". At a later time, if an electric field is applied in the opposite direction it exerts "dislodging stress" on the micro-dipoles. Low level fields result in no permanent change Medium fields result in partial degradation o Highapplied fields result in repolarization in the opposite direction. piezoceramic All piezo actuators function down to zero degrees Kelvin Quantitatively, the piezo coupling of most common piezoceramics does decrease as temperature drops. At liquid helium temperatures, the motion of most materials drops to about half that measured at room temperature. static applications? Piezo transducers not suitable for stati measurements. effective for measurements lasting less than 0.1second. cut up sheet of piezoceramic into size I want? A. Ceramic is best cut using a special diamond saw. Small prototype parts can be cut from piezoceramic sheet stock by using a razor blade and a straight edge to score the piezo surface and then making a controlled break. Even with practice this method does not yield straight-sided parts or repeatable cuts. Use at your own risk. bond/attach piezoceramic sheet to a structure like an aluminum beam? A. Good quality long lasting bonds can be achieved with a number of adhesives and are invariably determined by the application. We suggest that you contact several epoxy manufacturer and discuss the application, being sure to include: 1. The metal surfaces to be joined 2. Temperature of operation 3. Any unusual shear stress requirements 'superglue'? Good quality temporary bonds may be made with cyanoacrylate (e.g. "super glue"). An added benefit of cyanoacrylate bonds is that the bond easily achieves electrical contact. The length of time the bond will last will be application dependent, from seconds to years. For a short time the performance of the part is very close to that achieved using the best bonds, which makes it useful for exploratory work. make electrical contact to the side of the piezoceramic that is bonded down? A. The most common method is to make a conductive bond between a metal substrate and the piezo part. Then one electrical lead is attached to the substrate, and one to the outward face of the piezoceramic sheet. In cases where a conductive bond is not possible (i.e. when the substrate is glass or plastic), a wire must be soldered to the "down" side of the ceramic at some location and a corresponding 'dish', 'cutout', or 'overhang' must be used to allow room for the wire when bonding the piezo sheet to the substrate. attach wire leads to the piezoceramic? A. All of the PSI piezoceramic parts come with a thin metallic electrode already on the ceramic. Wire leads can be soldered (use ordinary 60/40 resin core solder) anywhere on the electrode to suit the application/experiment. Most PSI ceramics have nickel electrodes and require the use of an additional liquid flux for uniform results. access center shim? A. Support the bender underneath; using a milling machine, take .001 - .002" passes to remove ceramic and expose center shim. etch electrode? You can chemically etch the electrode with sandblasting, sandpaper, or laser. stretch a sheet before it breaks? A. A sheet can be stretched to a strain of approximately 500 microstrain ( micrometers per meter) in regular use. Higher surface strains can be achieved, but the statistics of survival get worse. Proceed with caution. bend a bimorph before it breaks? A. If a bender is cantilevered to a distance of one inch, the tip can be pushed a distance of 0.055 inch before the bender is heard to "snap". If sheet loses some properties, can it be repoled? A. Yes. For 5H material, an electric field of 40 - 50 volts/mil will restore nearly all lost polarization. For 5A, use 50 - 100 volts/mil. frequency limit of piezoceramic sheet? A. There is no inherent frequency limit for a piezoceramic sheet. In practice the frequency limits of signal applications are usually determined by resonances associated with the shape and/or size of the transducer design. A typical sheet of PSI-5A material has a thickness mode vibration in the neighborhood of 13 MHz and a planar dilatation mode at around 14 KHz. At ultrasonic frequencies large surface area parts draw considerable current and resistive heating of the electrodes becomes the limiting factor. hook up bender element so it works? A. This depends on how the two piezoceramic plates are polarized. If the plates are poled for series operation (i.e. poling arrows anti-parallel) then a wire is attached to each of the outer electrodes of the bender. ±180 volts is then applied between the wires. If the plates are poled for parallel operation (i.e. poling arrows parallel, pointing in the same direction) then the two outer electrodes are shorted together forming one lead, and a wire is attached to the center metal shim forming the second lead. ±90 volts may be applied between these leads. (See Tutorial) highest voltage that I can drive a piezoceramic sheet to? A. For low frequency operation (0 to 5 KHz) a conservative recommendation for applied bi-polar voltage for a single sheet of PSI-5A ceramic is ±90 volts. Voltage applied in the poling direction only can be raised up to 250 volts. Use caution! How much mechanical power can I get out of one sheet? A. In theory, one standard PSI-5A sheet (1.5" x 2.5" x .0075") used as an "extender" can do .00035 joules of work on the outside world in a quasistatic cycle (i.e. a slowly executed sinusoidal cycle). When operated just under its first longitudinal resonance of 15 KHz, the theoretically available output power from the sheet would be around 5 watts. In practice it is difficult to collect more than 100f this work. Resonant designs can be considerably more efficient. How much electrical power can I get out of one piezo sheet in principle? A. Assuming that we stretch a PSI-5A 1.5" x 2.5" x .0075" sheet to ±500 microstrains quasistatically at a frequency just below its fundamental longitudinal resonance of 15 KHz, and that we collect 1000f the stored electrical energy at its height twice per cycle we would get approximately 9 watts of electrical power from the sheet. The mechanical energy input under these assumptions would be in excess of 100 watts. Resonant designs can be considerably more efficient. However, the mechanical apparatus for achieving the above mentioned 15 KHz high strain excitation is not available, and there is no known electronic method for extracting 1000f the available energy. How much electrical power can be extracted from a typical piezo bender element in practice? A. A "Double Quick Mount" bending element bolted to a rigid surface provides a convenient demonstration of a cantilever mount generator. Applying 80 gram force to its tip at a frequency of 60 Hz produces an open circuit voltage of 15V peak between its two electrical leads. When the leads are connected to a 8 Kohm resistive load, the output to the load is 5.3 Vrms, representing a power output of 3.6 mW. Can piezoceramic sheet be used to pick up vibrations in machinery? A. Yes. Almost any size or shape of piezoceramic element will give off a measurable signal when fastened somewhere on machinery. See 'strain gages'. as strain gage? A. Yes. Piezoceramic is one of the most sensitive strain gage technologies existent, and it is the only one which is self-powered. repeat voltage outputs from a piezo strain gage? A. Outputs from piezoceramics which are 'following' surface vibrations are generally very repeatable and stable. If the sensor is initially calibrated it can be trusted for years of accurate service. repeat motion of a piezo actuator? A. A piezoceramic actuator which is cyclically driven at a constant cycle time between the same two points will perfectly repeat its path every time. However, if the cycle time or either endpoint is changed, hysteresis and creep effects cause non-repeatable motions. effects temp piezoceramic transducers? A. Temperature changes cause a voltage to appear across the electrodes of any piezo transducer. This is due to the pyroelectric properties of piezoceramic. Temperature also affects every property of piezoceramics (elastic, dielectric and piezoelectric coupling). There is no general trend. Each dependence must be looked up or better yet measured in the context of your experiment. (See Tutorial). resonant freq of a piezoceramic sheet? A. There is no one 'resonance'. There are many resonances. The number of them and their location in the frequency spectrum depend on the shape and thickness of the part. For a flat sheet as shipped, three obvious resonances are the ones associated with the length, width, and thickness of the sheet. drive with 'square wave' ? A. The answer is application dependent. If the square wave voltage is low (i.e., less than 30 V), then the answer is usually yes. If the square wave voltage is higher, there is a good chance for shockwave, damage, cracking, reduced life, or other failures. Careful control of the square wave rise time/fall time is the cryogenic temperatures? A. Yes. All piezo actuators continue to function right on down to zero degrees Kelvin. This may seem counter-intuitive at first; however, you must remember that basis for piezoelectric effect is inter-atomic electric fields, and electric fields are not affected by temperature at all. Quantitatively, piezo coupling of most common piezoceramics does decrease as temperature drops. At liquid helium temperatures, motion of most materials drops to about half that measured at room temperature. pyroelectric effect a voltage will arise between electrodes in response to temperature shifts. static applications can be used effectively for transient force measurements lasting less than 0.1 second. fatigue life of piezoelectric material? A.The "fatigue life" is pretty difficult to estimate; although we've had a piezo fan running constantly here since 1982, no conclusive tests have been done. It would depend on mounting, voltages, etc. cut up a sheet of piezoceramic into size I want? best cut using a special diamond saw. Small prototype parts can be cut from piezoceramic sheet stock by using a razor blade and a straight edge to score piezo surface and then making a controlled break. Even with practice this method does not yield straight-sided parts or repeatable cuts. Use at your own risk. bond/attach piezoceramic sheet to a structure like an aluminum beam? We suggest that you contact several epoxy manufacturer discuss application, being sure to include: 1. The metal surfaces to be joined 2. Temperature of operation 3. Any unusual shear stress requirements Note: University of Missouri at Rolla has a very helpful section on this. superglue' Good quality temporary bonds may be made with cyanoacrylate (e.g. "super glue"). An added benefit of cyanoacrylate bonds is that bond easily achieves electrical contact. The length of time bond will last will be application dependent, from seconds to years. electrical contact to side of piezoceramic that is bonded down? most common method is to make a conductive bond between a metal substrate and piezo part. attach wire leads to piezoceramic? All of PSI piezoceramic parts come with a thin metallic electrode already on ceramic. Wire leads can be soldered (use ordinary 60/40 resin core solder) anywhere on electrode to suit application/experiment. Most PSI ceramics have nickel electrodes and require use of an additional liquid flux for uniform results. Q. How can I access center shim? stretch a sheet before it breaks? approximately 500 microstrain ( micrometers per meter) in regular use. If sheet loses some properties, can it be repoled? A. Yes. For 5H material, an electric field of 40 - 50 volts/mil will restore nearly all lost polarization. For 5A, use 50 - 100 volts/mil. frequency limit of piezoceramic sheet? material has a thickness mode vibration in neighborhood of 13 MHz and a planar dilatation mode at around 14 KHz. highest voltage that I can drive a piezoceramic sheet to? low frequency operation (0 to 5 KHz) a conservative recommendation for applied bi-polar voltage for a single sheet of PSI-5A ceramic is ±90 volts. Voltage applied in poling direction only can be raised up to 250 volts. Use caution! mechanical power can I get out of one sheet? A. In theory, one standard PSI-5A sheet (1.5" x 2.5" x .0075") used as an "extender" can do .00035 joules of work on outside world in a quasistatic cycle (i.e. a slowly executed sinusoidal cycle). When operated just under its first longitudinal resonance of 15 KHz, theoretically available output power from sheet would be around 5 watts. In practice it is difficult to collect more than 100f this work. Resonant designs can be considerably more efficient. electrical power can I get out of one piezo sheet in principle? A. Assuming that we stretch a PSI-5A 1.5" x 2.5" x .0075" sheet to ±500 microstrains quasistatically at a frequency just below its fundamental longitudinal resonance of 15 KHz, and that we collect 1000f stored electrical energy at its height twice per cycle we would get approximately 9 watts of electrical power from sheet. The mechanical energy input under these assumptions would be in excess of 100 watts. Resonant designs can be considerably more efficient. However, mechanical apparatus for achieving above mentioned 15 KHz high strain excitation is not available, and there is no known electronic method for extracting 1000f available energy. electrical power can be extracted from a typical piezo bender element in practice? A. A "Double Quick Mount" bending element bolted to a rigid surface provides a convenient demonstration of a cantilever mount generator. Applying 80 gram force to its tip at a frequency of 60 Hz produces an open circuit voltage of 15V peak between its two electrical leads. When leads are connected to a 8 Kohm resistive load, output to load is 5.3 Vrms, representing a power output of 3.6 mW. effects temperature on piezoceramic transducers? A. Temperature changes cause a voltage to appear across electrodes of any piezo transducer. This is due to pyroelectric properties of piezoceramic. Temperature also affects every property of piezoceramics (elastic, dielectric and piezoelectric coupling). There is no general trend. Each dependence must be looked up or better yet measured in context of your experiment. ( find piezo devices? 'watch beepers' are piezoceramic audio transducers, most battery operated smoke detector alarms, fish finders, some cigarette lighters, many gas grill igniters. first demonstration of piezoelectric phenomena and Piezo crystallographic structure was published in 1880 by Education Pierre and Jacques Curie. In scientific circles of day, this effect was considered quite a "discovery," and was quickly dubbed as "piezoelectricity" in order to distinguish it from other areas of scientific phenomenological experience such as "contact electricity" (friction generated static electricity) and "pyroelectricity" (electricity generated from crystals by heating). The Curie brothers asserted, however, that there was a one-to-one correspondence between electrical effects of temperature change and mechanical stress in a given crystal, and that they had used this correspondence not only to pick crystals for experiment, but also to determine cuts of those crystals. To them, their demonstration was a confirmation of predictions which followed naturally from their understanding of microscopic crystallographic origins of pyroelectricity (i.e., from certain crystal asymmetries). The Curie brothers did not, however, predict that crystals exhibiting direct piezoelectric effect (electricity from applied stress) would also exhibit converse piezoelectric effect (stress in response to applied electric field). This property was mathematically deduced from fundamental thermodynamic principles by Lippmann in 1881. The Curies immediately confirmed existence of "converse effect," and continued on to obtain quantitative proof of complete reversibility of electro-elasto-mechanical deformations in piezoelectric crystals. LABORATORY CURIOSITY MATHEMATICAL CHALLENGE 1882 - 1917 The first serious applications work on piezoelectric devices took place during World War I. In 1917, P. Langevin and French co-workers began to perfect an ultrasonic submarine detector. Their transducer was a mosaic of thin quartz crystals glued between two steel plates (the composite having a resonant frequency of about 50 KHz), mounted in a housing suitable for submersion. Working on past end of war, they did achieve their goal of emitting a high frequency "chirp" underwater and measuring depth by timing return echo. The strategic importance of their achievement was not overlooked by any industrial nation, however, and since that time development of sonar transducers, circuits, systems, and materials has never ceased. ---------------------------------------------------------------------------- FIRST GENERATION APPLICATIONS WITH NATURAL CRYSTALS 1920 - 1940 The success of sonar stimulated intense development activity on all kinds of piezoelectric devices, both resonating and non-resonating. Some examples of this activity include: * Megacycle quartz resonators were developed as frequency stabilizers for vacuum-tube oscillators, resulting in a ten-fold increase in stability. * A new class of materials testing methods was developed based on propagation of ultrasonic waves. For first time, elastic and viscous properties of liquids and gases could be determined with comparative ease, and previously invisible flaws in solid metal structural members could be detected. Even acoustic holographic techniques were successfully demonstrated. * Also, new ranges of transient pressure measurement were opened up permitting study of explosives and internal combustion engines, along with a host of other previously unmeasurable vibrations, accelerations, and impacts. In fact, during this revival following World War I, most of classic piezoelectric applications with which we are now familiar (microphones, accelerometers, ultrasonic transducers, bender element actuators, phonograph pick-ups, signal filters, etc.) were conceived and reduced to practice. It is important to remember, however, that materials available at time often limited device performance and certainly limited commercial exploitation. SECOND GENERATION APPLICATIONS WITH PIEZOELECTRIC CERAMICS 1940 - 1965 During World War II, in U.S., Japan and Soviet Union, isolated research groups working on improved capacitor materials discovered that certain ceramic materials (prepared by sintering metallic oxide powders) exhibited dielectric constants up to 100 times higher than common cut crystals. Furthermore, same class of materials (called ferroelectrics) were made to exhibit similar improvements in piezoelectric properties. The discovery of easily manufactured piezoelectric ceramics with astonishing performance characteristics naturally touched off a revival of intense research and development into piezoelectric devices. The advances in materials science that were made during this phase fall into three categories: 1. Development of barium titanate family of piezoceramics and later lead zirconate titanate family 2. The development of an understanding of correspondence of perovskite crystal structure to electro-mechanical activity 3. The development of a rationale for doping both of these families with metallic impurities in order to achieve desired properties such as dielectric constant, stiffness, piezoelectric coupling coefficients, ease of poling, etc. All of these advances contributed to establishing an entirely new method of piezoelectric device development - namely, tailoring a material to a specific application. Historically speaking, it had always been other way around. This "lock-step" material and device development proceeded world over, but was dominated by industrial groups in U.S. who secured an early lead with strong patents. The number of applications worked on was staggering, including following highlights and curiosities: * Powerful sonar - based on new transducer geometries (such as spheres and cylinders) and sizes achieved with ceramic casting. * Ceramic phono cartridge - cheap, high signal elements simplified circuit design * Piezo ignition systems - single cylinder engine ignition systems which generated spark voltages by compressing a ceramic "pill" * Sonobouy - sensitive hydrophone listening/radio transmitting bouys for monitoring ocean vessel movement * Small, sensitive microphones - became rule rather than exception * Ceramic audio tone transducer - small, low power, low voltage, audio tone transducer consisting of a disc of ceramic laminated to a disc of sheet metal * Relays - snap action relays were constructed and studied, at least one piezo relay was manufactured It is worth noting that during this revival, especially in U.S., device development was conducted along with piezo material development within individual companies. As a matter of policy, these companies did not communicate. The reasons for this were threefold: first, improved materials were developed under wartime research conditions, so experienced workers were accustomed to working in a "classified" atmosphere; second, post war entrepreneurs saw promise of high profits secured by both strong patents and secret processes; and third, fact that by nature piezoceramic materials are extraordinarily difficult to develop, yet easy to replicate once process is known. From a business perspective, market development for piezoelectric devices lagged behind technical development by a considerable margin. Even though all materials in common use today were developed by 1970, at that same point in time only a few high volume commercial applications had evolved (phono cartridges and filter elements, for instance). Considering this fact with hindsight, it is obvious that while new material and device developments thrived in an atmosphere of secrecy, new market development did not - and growth of this industry was severely hampered. JAPANESE DEVELOPMENTS 1965 - 1980 In contrast to "secrecy policy" practiced among U.S. piezoceramic manufacturers at outset of industry, several Japanese companies and universities formed a "competitively cooperative" association, established as Barium Titanate Application Research Committee, in 1951. This association set an organizational precedent for successfully surmounting not only technical challenges and manufacturing hurdles, but also for defining new market areas. Beginning in 1965 Japanese commercial enterprises began to reap benefits of steady applications and materials development work which began with a successful fish-finder test in 1951. From an international business perspective they were "carrying ball," i.e., developing new knowledge, new applications, new processes, and new commercial market areas in a coherent and profitable way. Persistent efforts in materials research had created new piezoceramic families which were competitive with Vernitron's PZT, but free of patent restrictions. With these materials available, Japanese manufacturers quickly developed several types of piezoceramic signal filters, which addressed needs arising in television, radio, and communications equipment markets; and piezoceramic igniters for natural gas/butane appliances. As time progressed, markets for these products continued to grow, and other similarly valuable ones were found. Most notable were audio buzzers (smoke alarms, TTL compatible tone generators), air ultrasonic transducers (television remote controls and intrusion alarms) and SAW filter devices (devices employing Surface Acoustic Wave effects to achieve high frequency signal filtering). By comparison to commercial activity in Japan, rest of world was slow, even declining. Globally, however, there was still much pioneering research work taking place as well as device invention and patenting. HIGH VOLUME MARKETS 1980 - Present The commercial success of Japanese efforts has attracted attention of industry in many other nations and spurred a new effort to develop successful piezoceramic products. If you have any doubts about this, just track number of piezo patents granted by U.S. Patent Office every year - there has been a phenomenal rise. Another measure of activity is rate and origin of article publication in piezo materials/applications area - there has been a large increase in publication rate in Russia, China and India. Solid state motion is presently single most important frontier. The technical goals of frontier are to obtain useful and reasonably priced actuators which are low in power and consumption and high in reliability and environmental ruggedness; or, more simply stated, "solenoid replacements," or "electrostatic muscles." The search for perfect piezo product opportunities is now in progress. Judging from increase in worldwide activity, and from successes encountered in last quarter of 20th century, important economic and technical developments seem certain. Typical electric squirrel cage motors generate AC voltage of purest sinewave.use no brushes and do not produce any RFI.(Radio Frequency Interference) A They can not be overloaded; if too much of a load is applied to generator, it simply quits generating. Removing load will usually cause generator to start again Speeding up motor will help if doesn't start right away. By adding capacitors in parallel with motor power leads, and driving it above nameplate RPM, 1725 RPM ones need to turn at approximately 1875 RPM,3450 RPM ones at 3700 RPM) motor will generate AC voltage!capacitance helps to induce currents into rotor conductors and causes it to produce AC current. 200 uf starting capacitor across permanent 160 uf running capacitor (Using Normally Closed contacts) to get it generating. When 120 volts was produced, relay contacts opened up and removed 200 uf from circuit. That worked, but it was not dependable. I just gave up on that one. capacitors used must be type designated as "running" capacitors ,NOT "starting" capacitors Starting capacitors are used for a very short time,less than a second or two, ---------------------------------------------------------------------------- Chemical sensors & biosensors large proportion of chemical sensors are based around metal oxide semiconductor field effect transistor (MOSFET) devices. For this reason, ISFET (ion sensitive field effect transistor) will be discussed in this section. ISFET devices have been around for some time now (since about 1970), term biosensor refers to sensor that uses active biological (or sometimes biologically derived) component in transduction process. This may be a sensory cell taken from a living organism, and mounted on an electrode. Alternatively, antibodies may be used, which will lock on to material of interest, and hold it in an appropriate position for sensing. A further option is to use an enzyme that catalyses a reaction that can be detected by suitable means. Since there is considerable interest in monitoring blood glucose levels (to provide closed loop control of blood glucose, by means of an artificial pancreas, for diabetics), blood glucose sensors have received much attention. One of these is based on glucose oxidase enzyme; so this sensor will be outlined. One thing to note is that a lot of research has gone into these sensors; biosensors and blood glucose sensors in particular. Whilst progress has been made, there are still a lot of problems to be solved. One big problem in this area is that sensor performance drifts or degrades over time, often in unpredictable ways. So device has to be calibrated regularly, or just before use. Clearly a blood glucose sensor that only gives reliable readings over a period of a hundred days cannot be used in an implanted artificial pancreas. Thus, while there are many potential uses for chemical sensors, their use is often complicated by calibration requirements. ISFET sensors ISFETs sense concentration (activity level) of a particular ion in a solution. These devices are generally based on enhancement mode metal-oxide-semiconductor field effect transistor (MOSFET) structure, shown in figure 6. Figure 6. This has a metal gate electrode, insulated from semiconductor (silicon) wafer by a thin layer of silicon dioxide (oxide). The bulk of semiconductor (i.e. substrate) is doped with impurities to make it p-type silicon; in this material current is carried by positive charge carriers called holes (since they are, in fact, absence of negatively charged electrons; not easy to explain briefly). Either side of gate are small areas of silicon doped with impurities so that negatively charged electrons are main carriers in these n-type silicon regions: source and drain. n-type and p-type silicon are used to form diodes; current will flow from p-type to n-type, but not other way round. So to keep bulk of silicon substrate from interfering with transistor (gate, drain, source), this is connected to most negative part of circuit (often connected inside transistor package to source, so although device may look symmetrical, you should use it according to pin-out on data sheet). When in use, a positive voltage is applied to gate. This repels holes from region near gate, and attracts electrons, forming a small channel between drain and source where majority charge carriers are electrons. Current can flow through this channel, amount of current that can flow depends on how large channel is, and thus voltage applied to gate. In ISFET, gate metal is replaced with an ion selective membrane (figure 7), and device is immersed in a solution. Ions in solution interact with ion selective membrane. When there is a high concentration of positive ions in solution, a lot of them will accumulate on gate, widening channel between source and drain. With a low concentration of positive charged ions, channel will be narrow. Figure 7. In order to ensure that FET channel is biased to an optimum size, about which sensing can take place, solution is maintained at a reference potential by an electrode placed in it. Generally reference potential is adjusted to maintain a constant current flowing from drain to source, so ionic concentration will be directly related to solution reference potential with respect to substrate potential (in circuit shown in figure 7). One significant problem in design and fabrication of ISFETs is ensuring that ion selective membrane adheres to device. If integrity of membrane is compromised, then device is useless; this problem has considerable effct on yield (0f functioning devices on a wafer) of fabrication process. Enzyme-based biosensors Enzymes are highly specific in reactions that they catalyse. If an enzyme can be immobilised on a sensing substrate, and reaction products detected, then one has basis of a highly selective biosensor. The enzyme-based biosensor described below is for monitoring glucose levels; this application has received considerable investigation since glucose is important in diabetes, and also in many industrial fermentation processes. The operation of a glucose oxidase based sensor is shown schematically in figure 8. The enzyme is immobilised on a platinum electrode, and covered with a thin polyurethane membrane to protect enzyme layer, and reduce dependence of sensor on blood oxygen levels. Glucose oxidase, in its oxidised form, oxidises glucose entering sensor to gluconic acid; resulting in conversion of enzyme to its reduced form. The enzyme does not remain in this form for long. It interacts with oxygen entering through membrane. The products of this interaction are oxidised form of enzyme, and two hydrogen ions and two oxygen ions. When platinum electrode is biased to correct potential, it will reduce one of oxygen ions so that end products are oxygen and water; resulting electrode current can be measured, and will be proportional to concentration of glucose in external medium. (Nb/ This is a very much simplified explanation, and there are also many other ways to monitor reaction). Figure 8. One thing to note is that because various molecules have to physically move through materials of sensor, such biosensors can be quite slow to respond to changes in external medium. Microelectrodes for neurophysiology Microelectrodes of fine wire or electrolyte filled micropipettes have been used for some time to study nervous system on a cellular (individual neuron) basis. These, in particular metal wire microelectrodes, are prime targets for application of microengineering techniques. The small signal amplitudes involved (in region of 100uV) and high interface impedances (1-10 MOhm at 1 kHz) between metal and tissue mean it is advantageous to place amplifier as close as possible to recording site. In addition, characteristics of microfabricated devices can be more reproducible than for hand-made metal wire microelectrodes, and their small size enables accurate insertion of many recording sites into small volumes of tissue to study networks of neurons, or for neural prosthesis applications. The microelectrodes operate by detecting electrical potential generated in tissue near an active nerve fibre, due to action potential currents flowing through fibre membrane. There are three common types of micromachined microelectrode (figure 9). Array-type microelectrodes (figure 9a) are used to form floor of cell culture dishes: signals are recorded from neurons which are placed or grown over these. Probe-type microelectrodes (figure 9b) have recording sites on a long thin shank, which is inserted into tissue under investigation. Regeneration electrodes (figure 9c) are placed between ends of a severed peripheral nerve trunk; nerve fibres then re-grow (regenerate) through device. Figure 9. These microelectrodes can be quite difficult to use. For array microelectrodes, appropriate cell culture methods have to be developed and practised. Probe types have to be mounted on amplifier boards, and different situations require many different size / shaped probes. Regeneration electrodes have to be fixed to stumps of nerve trunk, and require connecting to outside world. All devices can potentially generate huge amounts of data, which has to be collected and analysed. [Stanford Transducers Lab Projects.] [Michigan's Center for Neural Communication Technology.] [Living Neural Networks, from CalTech.] [previous page] [contents page] [next page] [author's home page] Copyright D Banks 1999. All rights reserved. ueng@dbanks.demon.co.uk