Newnes Sensor Technology Handbook 2005 Yyepg Lotb Part 13 pdf

Chapter 17 470 most shock events are completed in such a short time. However, these outputs must be considered if the amplifier passes these low frequencies (usually less than 1 Hz) or if the outputs are sufficiently large that they would overload the amplifier. The amplitude and frequency content of thermal transient response is a function of the magnitude and rate of change of temperature. The pyroelectric characteristics of piezoelectric crystals are known and the output for any particular accelerometer-amplifier combination can be experimentally determined under specified temperature transient conditions. There are three mechanisms that cause thermal transient outputs; they are called primary, secondary, and tertiary pyroelectric effects. The thermal transient response of an accelerometer is the resultant of all three. Primary pyroelectric effect is the output caused by uniform temperature change in a constrained crystal. It occurs on surfaces perpendicular to the axis of polarization. Some natural piezoelectric crystals (e.g., quartz) do not produce primary pyroelectric output. Compression designs using ferroelectric ceramics have a large primary output. On the other hand, ferroelectric shear accelerometers do not produce a primary pyroelectric response because their electrode surfaces are parallel to the axis of polarization. Their pyroelectric response is comparable to natural crystals. Secondary pyroelectric response is caused by thermal deformation of the crystal from uniform heating. Some natural crystals (e.g., tourmaline) have a large secondary output, but still small compared to ferroelectrics. Tertiary pyroelectric response is produced by all accelerometers. It is caused by a temperature gradient across a crystal. The tertiary component is dependent on mechanical design, polarization axis, and electrode orientation, not on the specific crystal material. In shear designs, the crystal elements are usually well isolated from the case structure so that thermally induced case strains do not produce an output. This, plus the absence of primary pyroelectric response, makes shear type accelerometers less affected by transient temperature changes than compression types. The Endevco Isobase design isolates the crystal from the accelerometer base, and decreases the thermal conductivity from the environment to the crystal. The Isobase design reduces the pyroelectric output by approximately one order of magnitude. The use of low-frequency roll-off in amplifiers will reduce the amplitude of slowly varying pyroelectric outputs to an insignificant level and block any steady state outputs. In most applications, amplifiers with a low-frequency cut-off of 3 Hz or above will have no significant output errors caused by pyroelectric effects. However, amplifiers with extended low-frequency response can pass some transient pyroelectric signals. Sensors for Mechanical Shock 471 ANSI recommends a pyroelectric test procedure for the complete transducer-amplifier combination. In this procedure, the transducer is mounted on a test block whose mass is much greater than the transducer mass. The test block and the transducer are then subjected to a sudden 50°F ambient temperature change while the amplifier output (Figure 17.3.7) is monitored. Figure 17.3.7: Thermal transient response of accelerometer. The results of such a test on various transducers are presented in Table 17.3.1. Two different methods are compared in Table 17.3.1. In the older method, only the mounting block was immersed; in the newer method the block and the accelerometer are both immersed. Table 17.3.1: Thermal transient response of several accelerometers Model Type Sensitivity Total Immerse PC/g Immersion Block Only 2215E Compression 1650 51 0.05 2220D Shear 2.80 18 negligible 2222C Shear 1.20 20 negligible 2275 Isobase 11.0 12 negligible 2221D Shear 17.2 0.6 negligible 7701-50 Isoshear 50 0.2 negligible In obtaining these data, the transducer and mounting block were stabilized at 82°F and then plunged into an ice bath at 32°F. The transient output following immersion was a series of two to five low-frequency polarity reversals of approximately sinu- soidal shape within a time span of 1 to 10 seconds. The results are indicative of the Chapter 17 472 maximum pyroelectric sensitivity of various transducers only under the described test conditions. Actual pyroelectric outputs for any particular accelerometer-amplifier combination should be experimentally determined under the temperature conditions present in the measurement. A word of caution: Compression accelerometers (built with P-8 or P-10 crystals) which have been subjected to changing ambient temperatures while disconnected should not be connected to the amplifier without first shorting the transducer termi- nals. Damaging (not lethal, but shocking) potentials of several hundred volts have been observed in open-circuited accelerometers due to pyroelectricity. Transverse Motion For any single-axis accelerometer, one axis provides maximum response to an acceleration input. In an ideal accelerometer there would be no output signal for acceleration inputs along any other axes. In practice it is very difficult to produce this ideal transducer due to small manufacturing tolerances. However, a quality accelerometer will have minimal transverse sensitivity. Typical practical values are 3% or less in the axis of maximum transverse sensitivity. Almost all Endevco accelerometers are tested for transverse sensitivity as a normal inspection step in their manufacture. The input motion must be unidirectional. Most electrodynamic vibration exciters do not have adequate low transverse motion to test high quality accelerometers. Because of this, Endevco developed a special mechanical, large displacement shaker which operates at 12 Hz and 7 g. Direction of motion is well-controlled, and the moving assembly permits rotating the accelerometer while it is being vibrated. Thus, the transverse sensitivity can be measured to produce a plot similar to Figure 17.3.8 and the maximum transverse sensitivity noted on the accelerometer calibration card. The value supplied is only a measure of the misalignment of the accelerometer. Further errors can be caused by improper surface preparation and mounting (flatness, smooth- ness, perpendicularity). Figure 17.3.8: Typical transverse sensitivity plot. Sensors for Mechanical Shock 473 Electromagnetic Interference Radio frequency and magnetic fields have no effect on piezoelectric elements. How- ever, if an accelerometer includes magnetic materials, a spurious output may be observed when it is vibrated in a high magnetic field or subjected to high intensity changing magnetic flux. Adequate isolation must be provided against RF ground loops and stray signal pick-up. Insulated mounting studs can be used to electrically isolate accelerometers from ground. High intensity RF or magnetic fields may require special shielding of the accelerometer, cable, and amplifier. Other types of accelerometers will have varying sensitivities to EMI/RFI environments depending on their construction, internal shielding, and connecting cables. Some shock environments also include a high intensity electromagnetic pulse. Since this causes a rapidly changing magnetic field around the accelerometer, it can gen- erate a high voltage pulse in the accelerometer and connecting cable. The induced voltage may exceed the shock signal, and may be high enough to damage signal conditioning electronics. Although it cannot prevent damage, a placebo sensor can sometimes be used to minimize the distortion of the acceleration data. A placebo accelerometer is identical to the measurement accelerometer in all respects except it has zero sensitivity to acceleration. Therefore, any signal from the placebo must be noise induced by the environment. By placing the placebo near the measurement unit, it is exposed to the same environment. Thus the signal from the placebo can be subtracted from the signal from the measurement unit, and the result is the acceleration signal. This is not a foolproof technique, though, as some environmental effects may change the performance of the measurement unit without producing noise (change sensitivity, cause zero offset, etc.). 17.4 Applicable Standards IEST Institute of Environmental Standards and Technology, 5005 Newport Drive, Suite 506, Rolling Meadows, IL 60008-3841, USA, 1-847-255-1561; www.iest.org; IEST RP-DTE011.1, 2003 ISA ISA, The Instrumentation, Systems and Automation Society, 67 Alexander Drive, Research Triangle Park, NC 27709, USA, 1-919-990-9314; www.isa.org; ISA-RP37.2- 1982, ISA-RP37.5-1975, ISA-Dtr37.14.01 (draft in process). Chapter 17 474 17.5 Interfacing Information The fast rise time, high frequency content and high amplitude of most shock pulses requires special attention to the mounting of the accelerometer. Electrical connections and signal conditioning must be capable of accurately processing the transient signal without introducing extraneous noise. Mechanical Interface, Mounting For specimens having small cross-sectional dimensions, the attachment method, as well as the size and mass of the accelerometer, can alter the stiffness of the specimen. Ideally, the dimensions of the accelerometer should be small compared to the dimensions of the structure in the local area where the accelerometer is attached. If the accelerometer dimensions are too large, the local stiffness of the structure increases and the resonance frequency and amplitude of vibration are correspondingly changed. Similarly, the use of a fixture or accelerometer mounting stud may produce these stiffening effects. Choose microminiature accelerometers and use cement mounting for these small structures. In addition to the accelerometer’s effect on the dynamics of the mechanical system, an error source can also be introduced if the accelerometer is not securely attached to the structure. As frequency content increases, one must take special steps to attach the accelerometer. Accelerometers that are designed for stud mounting will ideally be mounted using the provided mounting stud and properly torquing them into the specified tapped hole. Accelerometers may be cemented directly to the test surface with several types of epoxies and quick-set cements. The strength of the bond should be evaluated, particularly if severe shock amplitudes are expected. If strains may be present on the surface, cements should be chosen with appropriate elastic properties. To avoid degradation, bonds must be thin, for example, 0.1 mm or thinner. Cyanoacrylate adhesives provide extremely thin bonds and minimal response change with miniature designs. Threaded studs are used to mount most accelerometers. Several important consider- ations are: 1. The surface condition of both the accelerometer and test specimen must be flat, smooth and clean. For most applications, the surface should approximate a 0.0003 inch TIR flatness and less than 32 microinch rms roughness. Sensors for Mechanical Shock 475 Figure 17.5.1: An assortment of special mounting blocks and adapters. 2. Coating all mating surfaces with a thin film of oil or acoustic couplant im- proves coupling at high frequencies and is recommended when frequency components exceed 2000 Hz. 3. The manufacturer’s recommendations for mounting torque should always be followed. 4. Insulated mounting studs may be needed for electrical isolation in some applications. When used, the resonant frequency is reduced and the effect on frequency response can be significant. For example, the resonant frequency of a 30 kHz, 1-ounce accelerometer typically reduces to 25–26 kHz when an insulated stud is used. Below 5 kHz the difference is less than 1%. 5. If the accelerometer cannot be mounted to a rigid block or boss, an accelerometer with low base strain sensitivity should be used. The use of mounting blocks and fixtures almost always degrades frequency response above 1000–2000 Hz for all but the microminiature accelerometers. When tests are performed at higher frequencies, a frequency response calibration of the accelerometer with fixturing is recommended. If biaxial or triaxial measurements are required, biaxial or triaxial accelerometers provide better frequency response than uniaxial accelerometers on a fixture or block. Figure 17.5.2 shows typical deviations in frequency response for a 30-gram accelerometer having a 30-kHz resonance frequency when mounted by different techniques. Chapter 17 476 Endevco Technical Paper Number 312, by James Mathews, explores the effect of various mounting methods on lighter weight accelerometers at different temperatures. Generally, the best adhesive is a cyanoacrylate such as Aron Alpha, or Super Glue. They cure very quickly at room temperature, and provide a broad frequency range and good temperature range. Disadvantages are the need for a solvent to break down the glue bond for removal, time-consuming removal, and difficulty getting a good bond on a rough surface. Dental cement is also useful in many applications. It provides good transmissibility and high strength. However, like the cyanoacrylates, it is difficult to remove from the test structure and from the accelerometer. Not Recommended for shock measurement: Waxes, such as petro-wax and beeswax, double-sided tape, hot glue and magnetic mounts are not recommended for accelerometer mounting for shock measurements. Petro-wax is often a convenient alternative, but has rather strict limitations. It is very easy to use, but requires a thin (less than 0.1 in.) wax layer. It is also very quick to use and easy to remove. Because of limited bonding strength, Petro-wax should not be used at peak vibration levels above about 20 g, frequencies above 2 kHz, or temperatures above 200°F. Double-sided tape is also quick and convenient, and has a temperature range similar to cyanoacrylates. However, it has low bonding strength limiting the amplitude range. Cable motion of some top connector or high profile accelerometers can cause transverse forces that may break the bond. Figure 17.5.2: Effect of attachment means on frequency response of a 30-gram accelerometer. 10 1.0 1.1 A B C D FREQUENCY–Hz AMPLITUDE FREQUENCY RESPONSE, MOUNTED BY: A – magnet B – back-to-back tape C – insulated mounting stu d D – noninsulated study 100 1000 10000 Sensors for Mechanical Shock 477 Hot glue from a glue gun may be useful, is better than waxes or tape, but is problem- atic for shock measurements. Hot glue mounting is appropriate for temperatures from –18°C to +93°C. Above 93°C, hot glue loses its stiffness and frequency response decreases rapidly. It is very convenient and provides quick cure time. The quick cure time is also one of its greatest disadvantages, requiring the user to mount the accelerometer as soon as the glue is applied. Maintaining a thin bond line for maximum transmissibility is difficult with hot glue applications. Magnetic mounts are not recommended for shock measurement. For vibration measurements, if the mounting surface is a magnetic material, accelerometers are often mounted using a magnetic mounting attachment. They are often used on industrial machinery that has cast iron or steel structures. They offer ease of use, quick mounting, broad temperature range, good holding strength, and are available in a variety of sizes. Disadvantages are size and weight (may increase mass loading effects), reduced bandwidth and the need for careful application. If a user “slaps” the accelerometer/ magnet onto a hard surface, the high frequency, high amplitude shock can cause cata- strophic damage to an accelerometer. TIP: If possible, it is advisable to have the accelerometer calibrated for frequency response, at the expected usage level and temperature, mounted as it will be mounted in service. Ideally, shock measurement sensors should be calibrated using a shock input. Electrical Interface, Signal Conditioning Signal conditioning electronics must be properly matched to the accelerometer used. Shock measurement may be accomplished by almost any type of sensor, so the user must be sure to match the signal conditioner to the type of sensor. Charge mode (“High Impedance”) accelerometers require a charge amplifier; IEPE (“Low Imped- ance”) accelerometers require a signal conditioner that provides constant current excitation and a coupling capacitor; strain gage (piezoresistive or metal strain gage) and variable capacitance accelerometers require a bridge amplifier (usually an “instrumentation amplifier”) with excitation voltage source; velocity sensors require an instrumentation amplifier but do not need any excitation voltage source. Regardless of the type of sensor, the signal conditioner must have adequate low- and high-frequency response and any filtering must be designed for transients. Chapter 17 478 17.6 Design Techniques and Tips, with Examples High Mechanical Resonance Frequency Mechanical resonance of a shock sensor should be at least three times (five times is recommended) the highest frequency in the transient being measured. Frequency response at one-fifth of the resonance frequency, for most shock sensors, is up by about four percent; at one-third the resonance frequency, response will be up about 12%. Rugged High frequency or short rise time of the shock pulse may excite the resonance of the sensor, so the sensor should be able to withstand occasional resonance excitation without permanent damage. Even relatively low level excitation at frequencies near the resonance frequency can cause excessive response because of the high Q resonance of most shock sensors. Piezoresistive (silicon strain gage) accelerometers are often broken by this kind of excitation. Although piezoelectric accelerometers may survive resonance excitation, they may be damaged such that their sensitivity decreases. Repetitive resonance excitation of piezoelectric accelerometers may cause low cycle fatigue failure of the crystal or other internal components. Damped Resonant Response Some piezoresistive and variable capacitance accelerometer designs incorporate mechanical damping to flatten their frequency response curves and reduce the Q of their resonance responses. The ideal damping ratio is 0.707 times critical to provide minimal phase distortion and ideal frequency response. Mechanical Filtering There are two shock accelerometer designs (Endevco 7255A and 7270AM6) that incorporate mechanical filtering between the mounting surface and the sensing mechanism. This provides mechanical low-pass filtering to protect the sensing mechanism and to flatten frequency response. It is combined with an integral matched electronic filter to provide flat response up to 10 kHz and then produce rapid rolloff at higher frequencies. External mechanical filter adapters have also been utilized in some special applications. Figure 17.6.1 shows a mechanical schematic of an internal mechanical filter. [...]... silicon sensing elements that are used in some shock sensors References 1 Ed Jon S Wilson, Shock and Vibration Measurement Technology, Publ Endevco (Part No 29005), San Juan Capistrano, CA, 2002 2 IEST RP DTE-011.1, Sensor Selection for Shock and Vibration Measurement, Publ Institute of Environmental Sciences and Technology, Chicago, 2003 3 IEST RP DTE-012.1, Handbook of Dynamic Data Acquisition and Analysis,...Sensors for Mechanical Shock Figure 17.6.1: Internal mechanical filter model Electronic Filtering Electronic low-pass filtering is often used in shock measurement systems It can be incorporated into the sensor design, included in the signal conditioning, provided for anti-aliasing prior to an analog to digital converter, or performed with digital signal processing as part of the data... Strain Gages Dr Thomas Kenny, Department of Mechanical Engineering, Stanford University Strain gages are used in many types of sensors They provide a convenient way to convert a displacement (strain) into an electrical signal Their “output” is actually a change of resistance It can be converted to a voltage signal by connecting the strain gage(s) in a bridge configuration A few sensors use only a single strain... driving the diaphragm is the result of one or more sound waves impinging on its surface from various directions An acoustic wave generates both pressure variations and local particle motions in the fluid medium In certain circumstances, the particle velocity is in the same direction as the propagation of the wave and also in-phase with the pressure variation One of these situations is when a wave is propagating... International Electrotechnical Commission (IEC) American National Standards Institute (ANSI) These standards are the following: ■ IEC 61672-1 (2002-05) Sound Level Meters -Part 1: Specifications ■ IEC 61672-2 (2002-05) Sound Level Meters -Part 2: Pattern Evaluation Tests ■ ANSI S1.4-1983 (R2001) with Amd S1.4A 1995 Specifications for Sound Level Meters ■ ANSI S1.43-1997 (R2002) Specifications for Integrating-Averaging... bias voltage and the internal electronics of the preamplifier are ICP®-powered, a technology commonly used for accelerometers with integrated circuits where an external current source of 2–4 mA provides the power Figure 18.9.2: Array microphone 495 Chapter 18 Modern versions offer TEDS (Transducer Electronic Data Sheet) technology, in which technical specifications including sensitivity, manufacturer,... Pistonphone Calibrator A pistonphone is a mechanical device in which a vibrating piston generates an acoustic pressure field within a cavity at a particular frequency, typically 250 Hz, having a fixed sound pressure level, typically 124 dB When a microphone is partially inserted into an opening in this cavity, its diaphragm will be exposed to this same sound pressure level The acoustic output level generated... importance are in the human hearing range, 20 Hz–20 kHz These microphones are often highly directional This is a desirable feature when the intention is to reproduce the sound produced by a particular singer or instrument, particularly in the presence of background noise Also, their ability to represent the measured sound level with precision is less important than their ability to reproduce the sound electrically... can be modified to provide a frequency response that is optimized for random incidence measurements Free field microphones, however, do not generally have an acceptable random incidence response Thus, a particular measurement microphone will be designed for use under one of the following acoustic field conditions: ■ pressure ■ free field ■ random incidence In many cases pressure microphones having diameters... amplitudes The upper measurement limit is established as the amplitude, in dB, for which the total harmonic distortion of the output signal exceeds 3%, a level that can be calculated analytically for a particular microphone design The actual limit is not only dependent upon it’s physical limitations, but it is effected by both the sensitivity of the microphone (mV/Pa) as well as the output voltage of . some shock sensors. References 1. Ed. Jon S. Wilson, Shock and Vibration Measurement Technology, Publ. Endevco (Part No. 29005), San Juan Capistrano, CA, 2002. 2. IEST RP DTE-011.1, Sensor Selection. Environmental Sciences and Technology, Chicago, 2003. 3. IEST RP DTE-012.1, Handbook of Dynamic Data Acquisition and Analysis, Publ. Institute of Environmental Sciences and Technology, Chicago, 1993. 4 12%. Rugged High frequency or short rise time of the shock pulse may excite the resonance of the sensor, so the sensor should be able to withstand occasional resonance excitation without permanent damage.

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