传感器英文翻译

1、Accelerometer Principles67 ratings | 4.01 out of 5| Print DocumentOverviewThis tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series will teach you a specific topic of common measurement applications by explaining theoretical concepts and providing practical examples. There are several physical processes that can be used to develop a sensor to measure acceleration. In applications that involve flight, such as aircraft and satellites, accelerometers are based on properties of rotating masses. In the industrial world, however, the most common design is based on a combination of Newton's law of mass acceleration and Hooke's law of spring action.Table of Contents1.Spring-Mass System2.Natural Frequency and Damping3.Vibration Effects4.Relevant NI Products5.Buy the BookSpring-Mass SystemNewton's law simply states that if a mass, m, is undergoing an acceleration, a, then there must be a force F acting on the mass and given by F = ma. Hooke's law states that if a spring of spring constant k is stretched (extended) from its equilibrium position for a distance D x, then there must be a force acting on the spring given by F = kDx.FIGURE 5.23 The basic spring-mass system accelerometer.In Figure 5.23a we have a mass that is free to slide on a base. The mass is connected to the base by a spring that is in its unextended state and exerts no force on the mass. In Figure 5.23b, the whole assembly is accelerated to the left, as shown. Now the spring extends in order to provide the force necessary to accelerate the mass. This condition is described by equating Newton's and Hooke's laws:ma = kDx(5.25)where k = spring constant in N/mDx = spring extension in mm = mass in kga= acceleration in m/s2Equation (5.25) allows the measurement of acceleration to be reduced to a measurement of spring extension (linear displacement) becauseIf the acceleration is reversed, the same physical argument would apply, except that the spring is compressed instead of extended. Equation (5.26) still describes the relationship between spring displacement and acceleration.The spring-mass principle applies to many common accelerometer designs. The mass that converts the acceleration to spring displacement is referred to as the test mass or seismic mass. We see, then, that acceleration measurement reduces to linear displacement measurement; most designs differ in how this displacement measurement is made.Natural Frequency and DampingOn closer examination of the simple principle just described, we findanother characteristic of spring-mass systems that complicates the analysis. In particular, a system consisting of a spring and attached mass always exhibits oscillations at some characteristic natural frequency. Experience tells us that if we pull a mass back and then release it (in the absence of acceleration), it will be pulled back by the spring, overshoot the equilibrium, and oscillate back and forth. Only friction associated with the mass and base eventually brings the mass to rest. Any displacement measuring system will respond to this oscillation as if an actual acceleration occurs. This natural frequency is given bywhere f N= natural frequency in Hzk = spring constant in N/mm = seismic mass in kgThe friction that eventually brings the mass to rest is defined by a damping coefficient , which has the units of s-1. In general, the effect of oscillation is called transient response, described by a periodic damped signal, as shown in Figure 5.24, whose equation isX T (t) = Xoe-µt sin(2p f N t) (5.28)where Xr(t) = transient mass positionXo= peak position, initiallyµ = damping coefficientfN= natural frequencyThe parameters, natural frequency, and damping coefficient in Equation (5.28) have a profound effect on the application of accelerometers.Vibration EffectsThe effect of natural frequency and damping on the behavior of spring-mass accelerometers is best described in terms of an applied vibration. If the spring-mass system is exposed to a vibration, then the resultant acceleration of the base is given by Equation (5.23)a(t) = -w2xosin wtIf this is used in Equation (5.25), we can show that the mass motion is given bywhere all terms were previously denned and w= 2p f, with/the applied frequency.FIGURE 5.24 A spring-mass system exhibits a natural oscillation with damping as response to an impulse input.FIGURE 5.25 A spring-mass accelerometer has been attached to a table which is exhibiting vibration. The table peak motion is xand the mass motionois D x.To make the predictions of Equation (5.29) clear, consider the situation presented in Figure 5.25. Our model spring-mass accelerometer has been fixed to a table that is vibrating. The x o in Equation (5.29) is the peak amplitude of the table vibration, and Dx is the vibration of the seismic mass within the accelerometer. Thus, Equation (5.29) predicts that the seismic-mass vibration peak amplitude varies as the vibration frequency squared, but linearly with the table-vibration amplitude. However, this result was obtained without consideration of the spring-mass system natural vibration. When this is taken into account, something quite different occurs.Figure 5.26a shows the actual seismic-mass vibration peak amplitude versus table-vibration frequency compared with the simple frequency squared prediction.You can see that there is a resonance effect when the table frequency equals the natural frequency of the accelerometer, that is, the value of Dx goes through a peak. The amplitude of the resonant peak is determined by the amount of damping. The seismic-mass vibration is described by Equation (5.29) only up to about f N/2.5.Figure 5.26b shows two effects. The first is that the actual seismic-mass motion is limited by the physical size of the accelerometer. It will hit"stops" built into the assembly that limit its motion during resonance. The figure also shows that for frequencies well above the natural frequency, the motion of the mass is proportional to the table peak motion, , but not to the frequency. Thus, it has become a displacement sensor. xoTo summarize:1. f < f N- For an applied frequency less than the natural frequency, the natural frequency has little effect on the basic spring-mass response given by Equations (5.25) and (5.29). A rule of thumb states that a safe maximum applied frequency is f < 1/2.5f N.-For an applied frequency much larger than the natural frequency, 2. f > fNthe accelerometer output is independent of the applied frequency. As shown in Figure 5.26b, the accelerometer becomes a measure of vibration displacement xof Equation (5.20) under these circumstances. It isointeresting to note that the seismic mass is stationary in space in this case, and the housing, which is driven by the vibration, moves about the mass. A general rule sets f > 2.5 f N for this case.Generally, accelerometers are not used near the resonance at their natural frequency because of high nonlinearities in output.FIGURE 5.26 In (a) the actual response of a spring-mass system to vibration is compared to the simple w2prediction In (b) the effect of various table peak motion is shownEXAMPLE 5.14An accelerometer has a seismic mass of 0.05 kg and a spring constant of 3.0 X 103N/m Maximum mass displacement is ±0 02 m (before the mass hits the stops). Calculate (a) the maximum measurable acceleration in g, and (b) the natural frequency.SolutionWe find the maximum acceleration when the maximum displacement occurs, from Equation (5.26).a.or becauseb. The natural frequency is given by Equation (5.27).2、Measuring Pressure with Pressure Sensors79 ratings | 4.00 out of 5| Print DocumentOverviewThis tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series will teach you a specific topic of common measurement applications by explaining theoretical concepts and providing practical examples. This tutorial introduces and explains the concepts and techniques of measuring pressure with pressure sensors.For more information, return to the NI Measurement Fundamentals Main Page. Table of Contents1.What is Pressure?2.The Pressure Sensor3.Pressure Measurement4.Signal Conditioning for Pressure Sensors5.DAQ Systems for Pressure Measurements6.ReferencesWhat is Pressure?Pressure is defined as force per unit area that a fluid exerts on its surroundings.[1] For example, pressure, P, is a function of force, F, and area, A.P = F/AA container full of gas contains innumerable atoms and molecules that are constantly bouncing of its walls. The pressure would be the average force of these atoms and molecules on its walls per unit of area of the container. Moreover, pressure does not have to be measured along the wall of a container but rather can be measured as the force per unit area along any plane. Air pressure, for example, is a function of the weight of the air pushing down on Earth. Thus, as the altitude increases, pressure decreases. Similarly, as a scuba diver or submarine dives deeper into the ocean, the pressure increases.The SI unit for pressure is the Pascal (N/m2), but other common units of pressure include pounds per square inch (PSI), atmospheres (atm), bars, inches of mercury (in Hg), and millimeters of mercury (mm Hg).A pressure measurement can be described as either static or dynamic. The pressure in cases where no motion is occurring is referred to as static pressure. Examples of static pressure include the pressure of the air inside a balloon or water inside a basin. Often times, the motion of a fluid changes the force applied to its surroundings. Such a pressure measurement is known as dynamic pressure measurement. For example, the pressure inside a balloon or at the bottom of a water basin would change as air is let out of the balloon or as water is poured out of the basin.Head pressure(or pressure head) measures the static pressure of a liquid in a tank or a pipe. Head pressure, P, is a function solely on the height, h, of the liquid and weight density, w, of the liquid being measured as shown in Figure 1 below.Figure 1. Head Pressure MeasurementThe pressure on a scuba diver swimming in the ocean would be the diver's depth multiplied by weight of the ocean (64 pounds per cubic foot). A scuba diver diving 33 feet into the ocean would have 2112 pounds of water on every square foot of his body. The translates to 14.7 PSI. Interestingly enough, the atmospheric pressure of the air at sea level is also 14.7 PSI or 1 atm. Thus, 33 feet of water create as much pressure as 5 miles of air! The total pressure on a scuba diver 33 feet deep ocean would be the combined pressure caused by the weight of the air and the water and would be 29.4 PSI or 2 atm.A pressure measurement can further be described by the type of measurement being performed. There are three types of pressure measurements: absolute, gauge, and differential. Absolute pressure measurement is measured relative to a vacuum as showing in Figure 2 below. Often times, the abbreviations PAA (Pascals Absolute) or PSIA (Pounds per Square Inch Absolute) are use to describe absolute pressure.Figure 2. Absolute Pressure Sensor[3]Gauge pressure is measured relative to ambient atmospheric pressure asshown in Figure 3. Similar to absolute pressure, the abbreviations PAG (Pascals Gauge) or PSIA (Pounds per Square Inch Gauge) are use to describe gauge pressure.Figure 3.Gauge Pressure Sensor[3]Differential pressure is similar to gauge pressure, but instead of measuring relative to ambient atmospheric pressure, differential measurements are taken with respect to a specific reference pressure as shown in Figure 4. Also, the abbreviations PAD (Pascals Differential) or PSID (Pounds per Square Inch Differential) are use to describe differential pressure.Figure 4. Differential Pressure Sensor[3]The Pressure SensorBecause of the great variety of conditions, ranges, and materials for which pressure must be measured, there are many different types of pressure sensor designs. Often pressure can be converted to some intermediate form, such as displacement. The sensor then converts thisdisplacement into an electrical output such as voltage or current. The three most universal types of pressure transducers of this form are the strain gage, variable capacitance, and piezoelectric.Of all the pressure sensors, Wheatstone bridge (strain based) sensors are the most common, offering solutions that meet varying accuracy, size, ruggedness, and cost constraints. Bridge sensors are used for high and low pressure applications, and can measure absolute, gauge, or differential pressure. All bridge sensors make use of a strain gage and a diaphragm as seen in Figure 4.Figure 4. Cross Section of a Typical Strain Gage Pressure Sensor [3]When a change in pressure causes the diaphragm to deflect, a corresponding change in resistance is induced on the strain gauge, which can be measured by a Data Acquisition (DAQ) System. These strain gauge pressure transducers come in several different varieties: the bonded strain gauge, the sputtered strain gauge, and the semiconductor strain gauge.In the bonded strain gauge pressure sensor, a metal foil strain gauge is actually glued or bonded to the surface where strain is being measured. These bonded foil strain gauges (BFSG) have been the industry standard for years and are continually used because of their quick 1000 Hz responsetimes to changes in pressure as well as their large -452°F to -525°F operating temperature.Sputtered strain gauge manufacturers sputter deposit a layer of glass onto the diaphragm and then deposit a thing metal film strain gauge on to the transd ucer’s diaphragm. Sputtered strain gauge sensors actually from a molecular bond between the strain gauge element, the insulating later, and the sensing diaphragm. These gauges are most suitable for long-term use and harsh measurement conditions.Integrated circuit manufacturers have developed composite pressure sensors that are particularly easy to use. These devices commonly employ a semiconductor diaphragm onto which a semiconductor strain gauge and temperature-compensation sensor have been grown. Appropriate signal conditioning is included in integrated circuit form, providing a dc voltage or current linearly proportional to pressure over a specified range.The capacitance between two metals plates changes if the distance between these two plates changes. A variable capacitance pressure transducer, seen in Figure 5 below, measures the change in capacitance between a metal diaphragm and a fixed metal plate. These pressure transducers are generally very stable and linear, but are sensitive to high temperatures and are more complicated to setup than most pressure sensors.Figure 5. Capacitance Pressure Transducer [4]Piezoelectric pressure transducer, as shown in Figure 6, take advantage of the electrical properties of naturally occurring crystals such as quartz. These crystals generate an electrical charge when they are strained. Piezoelectric pressure sensors do not require an externalexcitation source and are very rugged. The sensors however, do require charge amplification circuitry and very susceptible to shock and vibration.Figure 6. Piezoelectric Pressure Transducer [4]A common cause of sensor failure in pressure measurement applications is dynamic impact, which results in sensor overload. A classic example of overloading a pressure sensor is known as the water hammer phenomenon. This occurs when a fast moving fluid is suddenly stopped by the closing of a valve. The fluid has momentum that is suddenly arrested, which causes a minute stretching of the vessel in which the fluid is constrained. This stretching generates a pressure spike that can damage a pressure sensor. To reduce the effects of “water hammer”, sensors are often mounted with a snubber between the sensor and the pressure line. A snubber is usually a mesh filter or sintered material that allows pressurized fluid through but does not allow large volumes of fluid through and therefore prevents pressure spikes in the event of water hammer. A snubber is a good choice to protect your sensor in certain applications, but in many tests the peak impact pressure is the region of interest. In such a case you would want to select a pressure sensor that does not include overprotection. [3]Pressure MeasurementAs described above, the natural output of a pressure transducer is a voltage. Most strain based pressure transducers will output a small mV voltage. This small signal requires several signal conditioning considerations that are discussed in the next section. Additionally, many pressure transducers will output a conditioned 0-5V signal or 4-20 mA current. Both of these outputs are linear across the working range of thetransducer. For example both 0 V and 4 mA correspond to a 0 pressure measurement. Similarly, 5 volts and 20 mA correspond to the Full Scale Capacity or the maximum pressure the transducer can measure. The 0-5V and 4-20 mA signals can easily be measured by National InstrumentsMulti-function Data Acquisition (DAQ) hardware.See Also:Data Acquistion (DAQ) HardwareSignal Conditioning for Pressure SensorsAs with any other bridge based sensor, there are several signal conditioning considerations. To ensure accurate bridge measurements, it is important to consider the following:∙Bridge completion∙Excitation∙Remote sensing∙Amplification∙Filtering∙Offset∙Shunt CalibrationEach of these considerations are addressed thoroughly in the Measuring Strain with Strain Gauges tutorial linked below.Once you have obtained a measurable voltage signal, that signal must be converted to actual units of pressure. Pressure sensors generally produce a linear response across their range of operation, so linearization is often unnecessary, but you will need some hardware or software to convert the voltage output of the sensor into a pressure measurement. The conversion formula you will use depends on the type of sensor you are using, and will be provided by the sensor manufacturer. A typical conversion formula will be a function of the excitation voltage, full scale capacity of the sensor, and a calibration factor.[+] Enlarge ImageFor example, a pressure trandsducer with a full scale capacity of 10,000 PSI and a calibration factor of 3mv/V and given an excitation voltage of 10V DC produces a measured voltage of 15 mV, the measured pressure would be 5000 PSI.After you have properly scaled your signal, it is necessary to obtain a proper rest position. Pressure sensors (whether absolute or gauge) have a certain level that is identified as the rest position, or reference position. The strain gauge should produce 0 volts at this position. Offset nulling circuitry adds or removes resistance from one of the legs of the strain gauge to achieve this "balanced" position. Offset nulling is critical to ensure the accuracy of your measurement and for best results should be performed in hardware rather than software.See Also:Measuring Strain with Strain GaugesDAQ Systems for Pressure MeasurementsUsing SCXI with Pressure MeasurementsNational Instruments SCXI is a signal conditioning system for PC-based data acquisition systems as shown in Figure 7. An SCXI system consists of a shielded chassis that houses a combination of signal conditioning input and output modules, which perform a variety of signal conditioning functions. You can connect many different types of sensors, including absolute and gauge pressure sensors, directly to SCXI modules. The SCXI system can operate as a front-end signal conditioning system for PC plug-in data acquisition (DAQ) devices (PCI and PCMCIA) or PXI DAQ modules.[+] Enlarge ImageFigure 7. A Typical National Instruments SCXI SystemSCXI offers an excellent solution for measuring pressure. The SCXI-1520 universal strain-gauge module is ideal for taking strain based pressure measurements. It provides 8 simultaneous sampled analog input channels each with bridge completion, programmable excitation (0-10 V), remote excitation sensing, programmable gain amplification (1-1000), a programmable 4-pole Butterworth filter (10 Hz, 100 Hz, 1 kHz, 10kHz), offset nulling, and shunt calibration. The SCXI-1314 terminal block provides screw terminals for easy connections to your sensors. Additionally, the SCXI-1314T includes a built-in TEDS reader for Class II bridge-based smart TEDS sensors.Recommended starter kit for Pressure SCXI DAQ System:1.SCXI-1600 DAQ module2.SCXI chassis3.SCXI-1520 modules and SCXI-1314/SCXI-1314T terminal blocks4.Refer to /sensors for recommended sensor vendorsFor a customized solution, see the SCXI Advisor linked below.Using SC Series DAQ with Strain Based Pressure SensorsFor high performance integrated DAQ and signal conditioning, the National Instruments PXI-4220 shown in Figure 8, part of the SC Series, provides an excellent measurement solution. SC Series DAQ offers up to 333 kS/s measurements with 16-bit resolution, and combines data acquisition and signal conditioning into one plug in board. The PXI-4220 is a 200 kS/s, 16 bit DAQ board with programmable excitation, gain, and 4-pole Butterworth filter. Each input channel of the PXI-4220 also includes a 9-pin D-Sub connector for easy connection to bridge sensors, and programmable shunt and null calibration circuitry. The PXI-4220 provides the perfect solution for dynamic pressure measurements with low channel counts.Figure 8. National Instruments PXI-4220Recommended starter kit for Pressure SC Series DAQ System:1.PXI chassis2.PXI embedded controller3.PXI-4220 modules4.Refer to /sensors for recommended sensor vendorsFor a customized solution, see the PXI advisor linked below.Using SCC with Strain Based Pressure SensorsNational Instruments SCC provides portable, modular signal conditioning for DAQ system as seen in Figure 9 below. The SCC series provides a great low channel count and low cost solution that directly interfaces to National Instruments M Series DAQ boards. SCC modules can condition a variety of analog I/O and digital I/O signals, including bridge sensors. SCC DAQ systems include an SC Series shielded carrier such as the SC-2345 or the SC-2350, SCC modules, a cable, and a DAQ device. The SC-2350 shielded carrier provides additional support for TEDS sensors.[+] Enlarge ImageFigure 9. National Instruments SCC Carrier and ModulesThe SCC-SG24 Load Cell Input module accepts up to two full-bridge inputs from load cells or pressure sensors. Each channel of the module includes an instrumentation amplifier, a 1.6 kHz lowpass filter, and a potentiometer for bridge offset nulling. Each SCC-SG24 module also includes a single 10 V excitation source.Recommended Starter Kit for Pressure SCC DAQ System:1.M Series DAQ board2.SC-2345/SC-2350 module carrier3.SCC-SG24 modules (1 per 2 pressure sensors)4.Refer to /sensors for recommended sensor vendorsSee Also:Sensors - Affiliated Product AdvisorsSCXI Product AdvisorPXI Product AdvisorReferences[1] Johnson, Curtis D, “Pressure Principles” Process Control Instrumentation Technology, Prentice Hall PTB.[2] , “Strain Gauge Pressure Transducers”,/products/trans/t-presstrans.htm (current November 2003).[3] , “Honeywell Sensotec Frequently Asked Questions”, /pdf/FAQ_092003.pdf (current November 2003). [4] , "Pressure Measurement: Principles and Practice", /articles/0103/19/main.shtm l (current January 2003).3、Measuring Strain with Strain Gauges896 ratings | 4.27 out of 5| Print DocumentOverviewThis tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series will teach you a specific topic of common measurement applications by explaining theoretical concepts and providing practical examples.This tutorial introduces and explains the concepts and techniques of measuring strain with strain gauges.You can also view an on demand webcast on strain gauge measurements. For more information, return to the NI Measurement Fundamentals Main Page. Table of Contents1.What Is Strain?2.The Strain Gauge3.Strain Gauge Measurement4.Signal Conditioning for Strain Gauges5.DAQ Systems for Strain Gauge Measurements6.Relevant NI ProductsWhat Is Strain?Strain is the amount of deformation of a body due to an applied force. More specifically, strain (e) is defined as the fractional change in length, as shown in Figure 1 below.Figure 1. Definition of StrainStrain can be positive (tensile) or negative (compressive). Although dimensionless, strain is sometimes expressed in units such as in./in. or mm/mm. In practice, the magnitude of measured strain is very small. Therefore, strain is often expressed as microstrain (me), which is e x 10-6.When a bar is strained with a uniaxial force, as in Figure 1, a phenomenon known as Poisson Strain causes the girth of the bar, D, to contract in the transverse, or perpendicular, direction. The magnitude of this transverse contraction is a material property indicated by its Poisson's Ratio. The Poisson's Ratio n of a material is defined as the negative ratio of the strain in the transverse direction (perpendicular to the force)/e. to the strain in the axial direction (parallel to the force), or n = eT Poisson's Ratio for steel, for example, ranges from 0.25 to 0.3.The Strain GaugeWhile there are several methods of measuring strain, the most common is with a strain gauge, a device whose electrical resistance varies in proportion to the amount of strain in the device. The most widely used gauge is the bonded metallic strain gauge.The metallic strain gauge consists of a very fine wire or, more commonly, metallic foil arranged in a grid pattern. The grid pattern maximizes the amount of metallic wire or foil subject to strain in the parallel direction (Figure 2). The cross sectional area of the grid is minimized to reduce the effect of shear strain and Poisson Strain. The grid is bonded to a thin backing, called the carrier, which is attached directly to the test specimen. Therefore, the strain experienced by the test specimen is transferred directly to the strain gauge, which responds with a linear change in electrical resistance. Strain gauges are available commercially with nominal resistance values from 30 to 3000 Ω, with 120, 350, and 1000 Ω being the most common values.Figure 2. Bonded Metallic Strain GaugeIt is very important that the strain gauge be properly mounted onto the test specimen so that the strain is accurately transferred from the test specimen, through the adhesive and strain gauge backing, to the foil itself.A fundamental parameter of the strain gauge is its sensitivity to strain, expressed quantitatively as the gauge factor (GF). Gauge factor is defined as the ratio of fractional change in electrical resistance to the fractional change in length (strain):The Gauge Factor for metallic strain gauges is typically around 2. Strain Gauge MeasurementIn practice, the strain measurements rarely involve quantities larger than a few millistrain(e x 10-3). Therefore, to measure the strain requires accurate measurement of very small changes in resistance. For example, suppose a test specimen undergoes a strain of 500 me. A strain gauge witha gauge factor of 2 will exhibit a change in electrical resistance of only2 (500 x 10-6) = 0.1%. For a 120 W gauge, this is a change of only 0.12 W.To measure such small changes in resistance, strain gauges are almost always used in a bridge configuration with a voltage excitation source. The general Wheatstone bridge, illustrated below, consists of four resistive arms with an excitation voltage, VEX, that is applied across the bridge.Figure 3. Wheatstone BridgeThe output voltage of the bridge, VO, will be equal to:From this equation, it is apparent that when R1/R2= R4/R3, the voltageoutput VOwill be zero. Under these conditions, the bridge is said to be balanced. Any change in resistance in any arm of the bridge will result in a nonzero output voltage.Therefore, if we replace R4in Figure 3 with an active strain gauge, any changes in the strain gauge resistance will unbalance the bridge and produce a nonzero output voltage. If the nominal resistance of the strain。