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Biomedical engineering lecture notes ppt
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Biomedical Instrumentation and Measurements
Answered Apr 9, Teaching Powerpoint files - by topic sheet. Subjects include computer science, web design, psychology, Online it In the Holiday Lectures on Science, leading medical researchers explain how advances in genomics are revolutionizing their work, leading to a better understanding of disease and to improved treatments. Navpreet Kaur. In late , they developed a definition that is widely accepted:. A clinical engineer is a professional who brings to health care facilities a of education, experience, and accomplishment which.
They must have at least a B. Another new term, also coined in recent years, the 'biomedical equipclinical engineers. AAMI definition. Typically, the BMET has two community college. This person is not to be confused with the medical technologist. The latter is usually used in an operative sense, for example in blood chemistry and in the taking of electrocardiograms.
These definitions are all noteworthy, but whatever the name, this age of the marriage of engineering to medicine and biology is destined to benefit all concerned. Improved communication among engineers, technicians, and doctors, better and more accurate instrumentation to measure vital physiological parameters, and the development of interdisciplinary tools to help fight the effects of body malfunctions and diseases are all a part of this new field.
With this point in mind, the authors of this book use the term biomedical engineering for the field in general and the term biomedical instrumentation for the methods of measurement within the field. Another major problem of biomedical engineering involves communication between the engineer and the medical profession.
The language and jargon of the physician are quite different from those of the engineer. In some cases, the same word is used by both disciplines, but with entirely different meanings. Although it is important for the physician to understand enough engineering terminology to allow him to discuss problems with the engineer, the burden of bridging the communication gap usually falls on the latter.
The result is that the engineer, or technician, must learn the doctor's language, as well as some anatomy and physiology, in order that the two disciplines can work effectively together. To help acquaint the reader with this special aspect of biomedical engineering, a basic introduction to medical terminology is presented in Appendix A.
This appendix is in two parts: Appendix A. In addition to the language problem, other differences. Since the physician. Also, engineers,. Since the development and use of biomedical instrumentation must be a joint effort of the engineer or technician and the physician or nurse ,. By being aware of their possible existence, the engineer or technician can take steps to avert these pitfalls by adequate preparation and care in establishing his relationship with the medical profession.
The field of medical instrumentation is by no means new. Many instruments were developed as early as the nineteenth century for example, the electrocardiograph, first used by Einthoven at the end of that century.
This process occurred primarily during the s and the results were often disappointing, for the experimenters soon learned that physiological parameters are not. They also encountered a severe communication problem with the medical profession. Many developments with excellent potential seemed to have become lost causes.
It was during this period that some progressive companies decided that rather than modify existing hardware, they would design instrumentation specifically for medical use. Although it is true that many of the same components were used, the philosophy was changed; equipment analysis and design were applied directly to medical problems.
A large measure of help was provided by the U. Mercury, Gemini, and Apollo programs needed accurate physiological monitoring for the astronauts; consequently, much research and development money went into this area.
The aerospace medicine programs were expanded considerably, both within NASA facilities, and through grants to universities and hospital research units.
Some of the concepts and features of patient-monitoring systems presently used in hospitals throughout the. The use of adjunct fields, such as biotelemetry, also finds some basis in the NASA programs. Along with the medical research programs at the universities, a need developed for courses and curricula in biomedical engineering, and today almost every major university or college has some type of biomedical engineering program.
However, much of this effort biomedical instrumentation per se. Biomedical instrumentation provides the tools by which these measurements can be achieved.
In later chapters each of the major forms of biomedical instrumentacovered in detail, along with the physiological basis for the measureis tion The physiological measurements themselves are summarized involved. Some forms of biomedical instrumentation are unique to the field of medicine but many are adaptations of widely used physical measurements. A thermistor, for example, changes its electrical resistance with is that of an engine or Only the shape and size of the device might be different.
Another example is the strain gage, which is commonly used to measure the stress in structural components. It operates on the principle that electrical resistance is changed by the stretching of a wire or a piece of semiconductor material.
When suitably excited by a source of constant voltage, an electrical output can be obtained that is proportional to the amount of the strain. Since pressure can be translated into strain by various means, blood pressure can be measured by an adaptation.
In the design or specification of medical instrumentation systems, each of the following factors should be considered. The objective should be to provide an instrument that will give a usable reading from the smallest expected value of the variable or parameter. Linearity should be obtained over the most important segments, even if it is impossible to achieve it over the entire range. Mechanical friction in a meter, for example, can cause of the indicating needle to lag behind corresponding changes in the ing direction.
It is important to display a waveshape that is a faithful reproduction of the original physiological signal. An instrument system should be able to respond rapidly enough to. This condition is referred to as a ''flat response'' over a given range of fre-. Accuracy is a measure of systemic error. Errors can occur in a multitude of ways. Although not always present simultaneously, the following errors should be considered: Errors due to tolerances of electronic components.
Mechanical errors in meter movements. Errors due to poor frequency response. Reading errors due to parallax, inadequate illumination, or excessively wide ink traces on a pen recording. Two additional sources of error should not be overlooked.
The first concerns correct instrument zeroing. In most measurements, a zero, or a baseline,. Another source of on the parameter to be measured, and measurements in living organisms and is. It is important that the signal-to-noise ratio be as high as possible. In the hospital environment, power-line frequency noise or interference is common and is usually picked up in long leads.
Also, interference due to elec-. Although thermal noise. Often measurements must be made on patients or experimental animals in such a way that the instrument does not produce a direct electrical connection between the subject and ground. This requirement is often necessary for reasons of electrical safety see Chapter 16 or to avoid interference between different instruments used simultaneously. Electrical isolation can be achieved by using magnetic or optical coupling techniques, or radio telemetry.
Telemetry is also used where movement of the person or animal to be measured is essential, and thus the encumbrance of connecting leads should be avoided see Chapter Most instrumentation systems require calibration before they are actually used.
Each component of a measurement system is usually calibrated individually at the factory against a standard. An example would be that of a complicated, remote blood-pressure monitoring system, which is calibrated against a simple mercury manometer.
The object is to learn the nature and characteristics of the. The end product of such an exercise is usually a set of input-output equations intended to define the internal functions of the box.
One of the most complex black boxes conceivable is a living organism, especially the living human being. Within this box can be found electrical, mechanical, acoustical, thermal, chemical, optical, hydraulic, pneumatic,.
To further complicate the situation, upon attempting to measure the inputs and outputs, an engineer would soon learn that none of the input-output relationships is deterministic. That is, repeated application of a given set of input values will not always produce the same output values. In fact, many of the outputs seem to show a wide range of responses to a given set of inputs, depending on some seemingly relevant conditions, whereas others appear to be completely random and totally unrelated to any of the inputs.
The living black box presents other problems, too. Many of the important variables to be measured are not readily accessible to measuring devices. The result is that some key relationships cannot be determined or that less accurate substitute measures must be used.
Furthermore, there is a high degree of interaction among the variables in this box. Thus, it is often impossible to hold one variable constant while measuring the relationship between two others.
In fact, it is sometimes difficult to determine which are the inputs and which are the outputs, for they are never labeled and almost inevitably include one or more feedback paths.
The situation is made even worse by the application of the measuring device itself, which often affects the measurements to the extent that they may not represent normal conditions reliably. The function of medical instrumentation is to aid the medical chnician and researcher in devising ways of obtaining reliable and meaningful.
Still other problems are associated with such measurements: This means that many of the measurement techniques normally employed in the instrumentation of nonliving systems cannot be applied in the instrumentation of humans. Additional factors that add to the difficulty of obtaining valid measurements are 1 safety considerations, 2 the environment of the hospital in which these measurements are performed, 3 the medical personnel usually involved in the measurements, and 4 occasionally even ethical and legal considerations.
Because special problems are encountered in obtaining data from living organisms, especially human beings, and because of the large amount of interaction between the instrumentation system and the subject being measured, it is essential that the person on whom measurements are made be considered an integral part of the instrumentation system. In other words, in order to make sense out of the data to be obtained from the black box the human organism , the internal characteristics of the black box must be considered in the design and application of any measuring instruments.
Consequently, the overall system, which includes both the human organism and the intrumentation required for measurement of the human is called the man-instrument system. An instrumentation system is defined as the set of instruments and equipment utilized in the measurement of one or more characteristics or phenomena, plus the presentation of information obtained from those measurements in a form that can be read and interpreted by man.
In some cases, the instrumentation system includes components that provide a stimulus or drive to one or more of the inputs to the device being measured. There may also be some mechanism for automatic control of certain processes within the system, or of the entire system.
In some applications, this type of instrumentation may be classed as 'troubleshooting equipment. Measurements are used to determine the ability of a. Instrumentation is used to monitor some process or. Instrumentation to aid the physician in the diagnosis of disease and other disorders also has widespread use. Similar instrumen-. Biomedical instrumentation can generally be classified into two major. Clinical instrumentation is basically devoted to and treatment of patients, whereas research instrumentation is used primarily in the search for new knowledge pertaining to the various systems that compose the human organism.
Although some instruments can be used in both areas, clinical instruments are generally designed to be more rugged and easier to use. Emphasis is placed on obtaining a limited set of reliable measurements from a large group of patients and on providing the physician with enough information to permit him to. On the other hand, research instrumentation is normally more complex, more speciaUzed, and often designed to provide a much higher degree of accuracy, resolution, and so on.
Clinical instruments are used by the physician or nurse, whereas research instruments are clinical decisions. An in vivo measurement is one that is made on or within the living organism itself. An example would be a device inserted into the bloodstream to measure the pH of the blood directly.
An in vitro measurement is one performed outside the body, even though it relates to the functions of the body. An example of an in vitro measurement would be the measurement of the pH of a sample of blood that has been drawn from a patient. Although the man-instrument system described here applies mainly to in vivo measurements, problems are often encountered in obtaining appropriate samples for in vitro measurements and in relating these measurements to the living human being.
The transducer may measure temperature, pressure, flow, or any of the other variables that can be. In essence, then, the purpose of the signal-conditioning equipment is to process the signals from the transducers in order to satisfy the functions of the system and to prepare signals suitable for operating the display or recording. The input to the display device is the modified electric signal from the signal-conditioning equipment.
Its output is some form of In the man-instrumentaequipment may include a graphic pen recorder that. Equipment for these functions is often a vital part of the man-instrument system. Also, where automatic storage or processing of data is required, or where computer control is employed, an on-line analog or digital computer may be part of the instrumentation system. It should be noted that the term.
This system usually consists of a feedback loop in which part of the output from the signal-conditioning or display equipment. Within the human body can be found electrical, mechanical, thermal, hydraulic, pneumatic, chemical, and various other types of systems, each of which communicates with an external environment, and internally with the other systems of the body. By means of a multilevel control system and communications network, these individual systems are organized to perform many complex functions.
Through the integrated operation of all these systems, and their various subsystems, man is able to sustain Ufe, learn to perform useful tasks, acquire personality and behavioral traits, and even reproduce himself. In addition, these various inputs and outputs can be measured and analyzed in a variety of ways.
Most are readily accessible for measurement, but some, such as speech, behavior, and appearance, are difficult to analyze and interpret.
Biomedical engineering lecture notes ppt
Next to the whole being in the hierarchy of organization are the major functional systems of the body, including the nervous system, the cardiovascular system, the pulmonary system, and so on. Each major system is discussed later in this chapter, and most are covered in greater detail in later chapters. Just as the these. These functional systems can be broken down into subsystems and organs, which can be further subdivided into smaller and smaller units.
The process can continue down to the cellular level and perhaps even to the molecular level. The major goal of biomedical instrumentation is to make possible the measurement of information communicated by these various elements. The problem is, of course, that many of the inputs at the various organizational levels are not accessible for measurement.
The interrelationships among elements are sometimes so complex. Thus, the models in use today contain so many assumptions and constraints that their application is often severely limited. All operations of this highly diversified and very efficient chemical factory are self-contained in that from a single point of intake for fuel food , water, and air, all the.
Moreover, the chemical factory contains all the monitoring equipment needed to provide the degree of control necessary for each chemical operation, and. The Cardiovascular System engineer, the cardiovascular system can be viewed as a complex, closed.
Reservoirs in the system veins acteristics to satisfy certain control. The four-chamber pump acts as two synchronized but functionally isolated two-stage pumps. The first stage of each pump the atrium collects fluid blood from the system and pumps it into the second stage the ventricle.
The action of the second stage is so timed that the fluid is pumped into the system immediately after it has been received from the first stage. One of the two-stage pumps right side of the heart collects fluid from the main hydraulic system systemic circulation and pumps it through an oxygenation system the lungs.
The other pump left side of the heart receives fluid blood from the oxygenation system and pumps it into the main hydraulic system. The speed of the pump heart rate and its efficiency stroke volume are constantly changed to meet the overall requirements of the system. The fluid blood , which flows in a laminar fashion, acts as a communication and supply network for parts of the system.
Carriers red blood cells of fuel suppHes and waste. The fluid also contains mechanisms for repairing small system punctures and for rejecting foreign elements from the system platelets and white blood cells, respectively. Because part of the system is required to work against gravity at times, special one-way valves are provided to prevent gravity from pulling fluid against the direction of flow between pump cycles.
The variables of prime. The passageway divides to carry air into each of the bags, wherein it again subdivides many times to carry air into and out of each of many tiny air spaces pulmonary alveoli within the bags. The dual air input to the system nasal cavities has an alternate vent the mouth for use in the event of nasal blockage and for other special purposes.
In the tiny air spaces of the bags is a membrane interface with the body's hydraulic system through which certain gases can diffuse.
Oxygen is taken into the fluid blood from the incoming air, and carbon dioxide is transferred from the fluid to the air, which is exhausted by the force of the pneumatic pump.
The pump operates with a two-way override. An automatic control center respiratory center of the brain maintains pump operation at a speed that is adequate to supply oxygen and carry off carbon dioxide as required by the system.
Manual control can take over at any time either to accelerate or to inhibit the operation of the pump. Automatic control will return, however, if a condition is created that might endanger the system.
System variables of primary importance are respiratory rate, respiratory airflow, respiratory volume, and concentration of CO2 in the expired air. This system also has a number of relatively fixed volumes and capacities, such as tidal volume the volume inspired or expired during each normal breath , inspiratory reserve volume the additional volume that can be inspired after a normal inspiration , expiratory special valving.
Its center a self-adapting central information processor or computer the brain. The computer is self adapting in that if a certain section. Almost as fascinating as the central computer are the millions of communication lines afferent and efferent nerves that bring sensory information into, and transmit control information out of the create art, poetry,.
By means of the interconnection patterns, signals from a large number of sensory devices, which detect light, sound, pressure, heat, cold, and certain chemicals, are channeled to the appropriate parts of the computer, where they can be acted upon. Similarly, output control signals are channeled to specific motor devices motor units of the muscles , which respond to the signals with some type of motion or force.
Feedback regarding every action controlled by the system is provided to the computer through appropriate sensors. Information is usually coded in the system by means of electrochemical pulses nerve action potentials that travel along. The pulses can be transferred from one element of a network to another in one direction only, and frequently the transfer takes place only when there is the proper combination of elements acting on the next element in the chain.
Action by some elements tends to inhibit transfer. Both serial and parallel coding are used, sometimes. In addition to the central computer, a large number of simple decision-making devices spinal reflexes are present to control directly certain motor devices from certain sensory inputs.
A number of feedback loops are accomplished by this method. In many. In some cases, however, animal subjects are substituted for humans in order to permit measurements or manipulations that cannot be performed without some risk.
Although ethical restrictions sometimes are not as severe with animal subjects, the same basic problems can be expected in attempting measurements from any living system. Most of these problems were introduced in earher sections of the chapter.
However, they can be summarized as follows. In other situations the medical operation required to place a transducer in a position from which the variable can be measured makes the measurement impractical on human subjects, and sometimes even on animals. Where a variable is inaccessible for measurement, an attempt is often made to perform an indirect measurement. This process involves the measurement of some other related variable that makes possible a usable estimate of the inaccessible variable under certain conditions.
In using indirect measurements, however, one must be constantly aware of the limitations of the substitute variable and must be able to determine when the relationship. In fact, such variables should be considered as stochastic processes. In other words, measurements taken under a fixed set of conditions at one time will not necessarily be the.
Here, methods must be employed in order to estimate relation-. The foregoing variability in measured values could be better explained if more were known and understood about the interrelationships within the body. Physiological measurements with large tolerances are often accepted by the physician because of a lack of this knowledge and the resultant inability to control variations.
Better understanding of physiological relationships. Because of the large number of feedback loops involved in the major physiological systems, a severe degree of interaction exists both within a given system and. Even when attempts are collateral loops appear and some aspects of feedback loop are still present. Also, when one organ or ele-. In many situations the physical presence of the transducer changes the reading significantly. For example, a large flow. Similarly, an attempt to. This penetration can easily kill the cell or damage it so that it can no longer function normally.
Another problem arises from the interaction discussed earlier. Often the presence of. For example, local cooling of the skin, to estimate the circulation in the area, causes feedback that changes the circulation pattern as a reaction to the. The psychological effect of the measurement can also affect the Long-term recording techniques for measuring blood pressure have shown that some individuals who would otherwise have normal pressures show an elevated pressure reading whenever they are in the physician's of-.
In designing a measurement system, the biomedical instrumentation engineer or technician must exert extreme care to ensure that the effect of the presence of the measuring device is minimal. Because of the limited amount of energy available in the body for many physiological variables, care must also be taken to prevent the measuring system from loading'' the source of the measured variable. Since many transducers are sensitive to movement, the movement of the subject often produces measuring of a.
For example, resistance measurements require the flow of electric current. Some transducers generate a small amount of heat due to the current flow. In most cases, this energy level is so low that its effect is insignificant.
However, in dealing with living cells,. Similarly, the measurement should not cause undue pain, trauma, or discomfort, unless it becomes necessary to endure these condi-. Fortunately, however, new developments resulting in. In addition, greater knowledge of the physiology of the various systems of the gresses in his. When measurements are made on human beings, one further aspect must be considered. During its earlier days of development biomedical apparatus was designed, tested, and marketed with little specific governmental control.
True, there were the controls governing hospitals and a host of codes and regulations such as those described in Chapter 16, but today a number of new controls exist, some of which are quite controversial. On the other hand, there is little control on the effectiveness of devices or their side effects. Food and drugs have long been subject to governmental control by a U. In a new addition, the Medical Devices Amendments Public Law , placed all medical devices from the simple to the complex under the jurisdiction of the FDA.
Since then, panels and committees have been formed and symposia have been held by both physicians and engineers. They should always be fully conversant with what is going on and aware of issues and regulations that are brought about by technological, be economic and political realities.
Each of the major body systems is discussed by presenting physiobackground information. Then the variables to be measured are considered, followed by the principles of the instrumentation that could be used. Finally, appUcations to typical medical, behavioral, and biological logical. The physiological systems from which these variables originate were introduced in Chapter 1.
The principal physiological variables and their methods of measurement are summarized in Appendix B and discussed in detail in. As stated in Chapter a transducer is required to convert each variable into an electrical signal 1 which can be amplified or otherwise processed and then converted into Physiological variables occur in. To conduct its function properly, one or more parameters of the electrical output signal say, its voltage, current, frequency, or pulse width must be a nonambiguous function of the nonelectrical variable at the input.
Ideally, the relationship between output and input should be linear with, for example, the voltage at the output of a pressure transducer being proportional to the applied pressure. A linear relationship is not always possible. For example, the relationship between input and output may follow a logarithmic funcversion.
The two transducer types will nevertheless be described separately in the following sections. In theory active transducers can utilize every for converting nonelectrical energy. It is a characteristic of active transducers that frequently, but not always, the same transduction principle used to convert from a nonelectrical form of energy can also be used in the reverse direction. For example, a magnetic loudspeaker can also be used in the opposite direction as a microphone.
Sometimes different names are used to refer to essentially the same to convert. These principles with the exception of the Volta effect and electrical Chapter 4 are described in later. If an electrical conductor is moved in a magnetic field in such a way that the magnetic flux through the conductor is changed, a voltage is induced which is proportional to the rate of change of the magnetic flux.
Conversely, if a current is sent through the same conductor, a mechanical force is exerted. The result, which depends on the polarities of voltage and current on the electrical side or the directions of force and motion on the mechanical side, is a conversion from electrical to mechanical energy, or vice versa. All electrical motors and generators and a host of other devices, such as solenoids and loudit.
The output voUage in each case is propor-. The most important biomedical apsound microphones, pulse transducers, and electromagnetic blood-flow meters, all described in Chapter 6. Magnetic induction also plays an important role at the output of many biomedical instrumentation systems. Analog meters using d'Arsonval movements, light-beam galvanometers in photographic recorders, and pen motors in ink or thermal recorders are all based on the principle of magnetic induction and closely resemble the basic transducer configuration shown in.
These microphones use an electret to create an electrostatic field between two capacitor plates. Electrets which are the electrostatic equivalent of magnets are normally in the form of foils of a special plastic material that have been heat-treated while being exposed to a strong electric field. It is conceivable that the principle of the electret microphone could also be applied advantageously to biomedical transducers.
The natural materials in which this piezoelectric effect can be. Piezoelectric properties can be introduced into wafers of barium titanate a ceramic material that is frequently used as a dielectric in disk-type capacitors by heat-treating them in the presence of a strong electric field. The piezoelectric process is reversible. The top trace shows the force applied to the removed again. While the electrical field. To meet this condition, even for large values of T, it may be necessary to make the amplifier input impedance very large.
In some applications, elec2. As an alternative, an external capacitor can be. This effectively increases the capacity of the transducer but also reduces its sensitivity. Because the output voltages of piezoelectric transducers can be very high they have occasionally even been used as high-voltage generators for ignition purposes ,.
Changes of the capacity, and thus the sensitivity, can also be caused by the mechanical movement of attached shielded or coaxial cables which can introduce motion artifacts. Special types of shielded cable that reduce this efthis. If the product oiR and C is of the same order of magnitude as T, the resulting voltage is a compromise between the extremes in the two previous traces, as shown in the appHed force changes as.
Principles of ultrasound and biomedical applications are covered in Chapter 9. The polarity depends on which of the two junctions is warmer.
The device formed in this fashion is called a thermocouple, shown in Figure 2. In the case of the thermocouple it. Actually, the delivery of electrical energy causes the transfer of heat from the hotter to the colder junction; the hotter junction gets cooler while the colder junc-. Because the thermocouple measures a temperature difference rather than an absolute temperature, one of the junctions must be kept at a known reference temperature, usually at the freezing point of fect.
Frequently, instead of an icebath for the reference Thermo-voltage.
Because of their low sensitivity, thermocouples are seldom used for measurement of physiological temperatures, where the temperature range is so limited. Instead, one of the passive transducers described later is usually preferred.
Thermocouples have an advantage at very high temperatures where passive transducers might not be usable or sometimes where transducers of minute size are required. The use of the thermoelectric effect to convert from thermal to electrical energy is called the Seebeck effect.
In the reverse direction it is called the Peltier effect, where the flow of current causes one junction to heat and the. When operated into a small load.
Passive transducers utilize the principle of controlling a dc excitation voltage or an ac carrier signal. The voltage at the output of the circuit reflects the physical variable. There are only three passive circuit elements that can be utilized as passive transducers: An ordinary potentiometer, for example, can be used to convert rotary motion or resistive.
Similarly, the special linear potentiometers shown in Figure 2. In resistors this characteristic is a disadvantage; however, in resistive temperature transducers it serves a useful purpose.
Temperature transducers are resistivity. In certain semiconductor materials the conductivity light striking the material. This effect. This type of transducer is very sensitive, but has a somewhat limited frequency response. A different type of photoelectric transducer carriers generated.
Although less sensitive than the photoresistive cell, the photodiode has improved frequency response. A photo diode can also be used as a photoelectric transducer without a bias voltage. In this case it operates as an active transducer.
The principle of a strain gage can easily be understood with the help of Figure 2. Figure 2. If it is made of a material having a resistivity of r ohm-cm, its resistance is ment. Either an increase in L or a in an increase in resistance.
The ratio of the resulting. From D. Bartholomew, Electrical Measurement and Instruments. The basic principle of the strain gage can be utilized for transducers in number of different ways. In the mercury strain gage the resistive material consists of a column of mercury enclosed in a piece of silicone rubber tubing. The use of this type of strain gage for the measurement of physiological variables the diameter of blood vessels was first described by Whitney.
A disadvantage is that for practical dimensions the resistance of the mercury columns is inconven-.
Metal strain gages can be of two different types: In the unbonded strain gage, thin wire is stretched between insulating posts as shown in Figure 2.
In order to obtain a convenient resistance n is a common value , several turns of wire must be used. If the moving member is. At the same time, resistance changes of the strain gage due to changing temperatures tend to compensate each other. In the form shown, the is.
This strain gage is then cemented. Related to the bonded wire strain gage is ihQ foil gage. In this gage the conductor consists of a foil pattern on a substrate of plastic which is manufactured by the same photoetching techniques as those used in printed circuit boards.
This process permits the. If even made can be gages strain silicon measured is also made of silicon be strain is to the structure whose surface e. Such patterns can be obtained using the photolithographic and diffusion techniques developed for the manufacture of integrated circuits. The gages are isolated from the silicon substrate by reverse-biased diode junctions.
Therefore, at least two strain-gage elements are usually used, with the second element either employed strictly for temperature compensation, or that. In principle, the inductance of a coil can be changed either physical dimensions or core. However, in the inductive transducer the core. The inductance can then be measured using an ac signal.
Another passive transducer involving inductance is the variable reluctance transducer, in which the core remains stationary but the air gap in the magnetic path of the core is varied to change the effective permeability. This principle is also used in active transducers in which the magnetic path includes a permanent magnet.
The magnitude of the output voltage changes with amount of displacement of the core from its central or neutral position. Because nonlinearities in the magnitudes of Its. Both effects have occasionally been. As with the transducers using an inductive element, it is sometimes not apparent whether a capacitive transducer is of the passive type or is actually an active transducer utiHzing the principle of electric induction.
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