1、Institute of Environmental Sciences and Technology IEST-RP-DTE011.2 Design, Test, and Evaluation Division Recommended Practice 011.2 Mechanical Shock and Vibration Accelerometer Selection 1827 Walden Office Square, Suite 400 | Schaumburg, IL 60173 USA Phone: (847) 981-0100 Fax: (847) 981-4130 E-mail
2、: informationiest.org Web: www.iest.org 2 Copyrighted material Institute of Environmental Sciences and Technology IEST-RP-DTE011.2This Recommended Practice was prepared by and is under the jurisdiction of Working Group 011 of the IEST Design, Test, and Evaluation Division (WG-DTE011). The following
3、WG voting members contributed to the development of this edition of this Recommended Practice. Jon Wilson, DTE-011 Chair, Jon S. Wilson Consulting, LLC David Banaszak, Air Force Research Laboratory Vesta Bateman, Mechanical Shock Consulting Anthony Chu, TE Connectivity Randy Patrick, Army Yuma Test
4、Center Stanley Poynor, Lockheed Martin Missiles SHEAR TYPE (RIGHT). 15 5 SCHEMATICS OF AN IEPE ACCELEROMETER . 15 6 FREQUENCY RESPONSE DROOP 15 7 WHEATSTONE BRIDGE . 19 8 MONOLITHIC SILICON SENSING ELEMENT OF A PR ACCELEROMETER . 20 9 BLOCK DIAGRAM OF A VC ACCELEROMETER 21 10 EXPLODED VIEW OF A MICR
5、OMACHINED VC ACCELEROMETER . 22 11 PHASE RESPONSE OF A SINGLE-DOF SYSTEM VS. NORMALIZED FREQUENCY 26 12 TRANSVERSE SENSITIVITY 27 13 ADHESIVE MOUNTING 33 14 MAGNETIC MOUNTING . 33 15 ANGULAR ACCELEROMETER USING A LINEAR PAIR . 41 TABLE 1 COMPARISON OF PE, IEPE, PR, AND VC ACCELEROMETERS . 23 APPENDI
6、X A REFERENCES AND BIBLIOGRAPHY 42 IEST-RP-DTE011.2 Institute of Environmental Sciences and Technology Copyrighted material 5 Institute of Environmental Sciences and Technology Design, Test, and Evaluation Division Recommended Practice 011.2 Mechanical Shock and Vibration Accelerometer Selection IES
7、T-RP-DTE011.2 1 SCOPE AND LIMITATION The purpose of this Recommended Practice (RP) is to provide guidelines for selecting accelerometers to measure shock and vibration in laboratory and field testing environments. Many special applications are not covered (e.g., pyroshock 1,2, consumer products) bec
8、ause of their unique nature and the rapid advancements taking place in their disciplines. Even in applications not specifically addressed, however, these recommendations may be helpful. There are basically two classes of motion sensors: fixed-reference and mass-spring (relative motion). Non-contact
9、transducers, such as laser interferometric displacement and laser Doppler velocity transducers, are fixed-reference designs. Although they offer some unique properties, these instruments are used to measure shock and vibration only in applications where a fixed reference is available, and where thei
10、r cost, size, and physical space and geometry requirements are acceptable. Similarly, video and high-speed photographic displacement measurement techniques are becoming more sophisticated, thereby increasingly allowing their application to the motion analysis of structures. These fixed-reference tec
11、hniques, which have different constraints, are discussed elsewhere 3,4,5. This RP concentrates on the more common mass-spring type accelerometers, with the sensing element(s) represented by the spring. The following recommendations apply to dynamic measurements with frequencies ranging from DC (0 Hz
12、) to more than 20 kHz. Only measurements of linear (translational) motion are considered; measurement of angular or rotational motion is addressed as an application at the end of the document. 2 REFERENCES It is recommended that the user reference manufacturers specification and application data in
13、the selection and use of equipment. Please see Appendix A: Bibliography for informative resources. 3 TERMS AND DEFINITIONS accelerometer A transducer whose instantaneous output is proportional to the instantaneous acceleration input. acoustic sensitivity The rated output produced by a non-acoustic t
14、ransducer in the presence of a specified acoustic field. back-to-back comparison method A method of performing a sensitivity/frequency response calibration of an accelerometer by mounting the unit under test to the sensitive surface of a reference standard and comparing the outputs of the two device
15、s. base strain sensitivity The rated output produced by an accelerometer in the presence of a specified amount of strain input induced by the bending motion at the mounting interface. 6 Copyrighted material Institute of Environmental Sciences and Technology IEST-RP-DTE011.2 bias voltage The DC volta
16、ge that appears at the output of the accelerometer in quiescent state (its level is determined and preset by the manufacturer); generally interferes with receiving recording devices and normally decoupled by a high value capacitor in series. charge amplifier (converter) A preamplifier designed for u
17、se with high-impedance piezoelectric transducers, commonly referred to as a “charge amp;” consists of an operational amplifier with a capacitor in its feedback loop; output voltage of the amplifier is proportional to the input charge generated by the transducer and independent of the cable capacitan
18、ce. charge sensitivity The rated output produced by a high-impedance piezoelectric transducer per unit of acceleration input (e.g., pC/g). closed-loop accelerometer An accelerometer in which the output generated by deflection in the mass-spring system is used as feedback in a circuit that closes the
19、 loop by physically driving the deflected mass back to its equilibrium position. NOTE: Generally, closed-loop accelerometer designs offer better amplitude linearity than open-loop designs. However, with the use of microprocessors and the ability to correct data at the source, closed-loop acceleromet
20、ers are finding much less application. compensation resistor A resistor placed in parallel or in series with any of the legs of a piezoresistive accelerometer to correct for the imbalance in the bridge circuit or sensitivity errors at temperature, or both. compliance voltage The DC supply voltage av
21、ailable to the integral electronics piezoelectric (IEPE) accelerometer circuitry; usually lower than the power supply voltage due to the voltage drop in the constant current device(s). discharge time constant The RC time constant determined by the design of a transducers circuit topology; time requi
22、red for the step response to reduce to 37% of its original value. dynamic range The ratio of the highest level to the lowest level of signal to be measured, expressed in dB. electrical isolation A condition in which the output and ground leads of the accelerometer are electrically isolated from the
23、mounting surface. electromagnetic rebalancing A type of closed-loop accelerometer that uses an electromagnetic mechanism to rebalance the deflected mass. electromagnetic sensitivity The rated output produced by an accelerometer in the presence of a specified electromagnetic field. electrostatic reba
24、lancing A type of closed-loop accelerometer that uses an electrostatic mechanism to rebalance the deflected mass. ferroelectric A subset in the piezoelectric-type materials that typically has a high dielectric constant. Example: lead zirconate titanate (PZT). fixed-reference transducer A two-termina
25、l transducer in which one terminal is fixed at a point in space as reference and the other terminal is attached to the object in motion. gage factor An indicator of efficiency for strain gages, which is the ratio of the relative resistance change to the relative length change. hermetically sealed A
26、sealing method used in transducer design; usually accompanied by a welded construction and a glass-to-metal seal in the connector. IEST-RP-DTE011.2 Institute of Environmental Sciences and Technology Copyrighted material 7 integral electronics piezoelectric (IEPE) A piezoelectric transducer with a bu
27、ilt-in preamplifier to convert the high-impedance input into a low-impedance voltage output. integral mechanically isolated accelerometer An accelerometer with a mechanical isolator in a single package. In general, the contained isolation material in these instruments can be nonlinear, resulting in
28、errors attributable to extraneous frequencies in their response. Performance documentation and assurance should be requested from their manufacturer. NOTE: External isolators manufactured by the testing organization are also of concern. laser Doppler velocity transducer An instrument that uses the D
29、oppler shift of laser light that has been backscattered from a vibrating object to produce a real-time analog signal proportional to instantaneous velocity. laser interferometric displacement transducer An instrument that uses an interferometric fringe counting technique for vibration-displacement m
30、easurement in low-frequency ranges. linear variable differential transformer (LVDT) A variable inductance transducer that provides an AC voltage output proportional to the displacement of a core passing through its internal windings. linearity (nonlinearity) The constant (or non-constant) behavior o
31、f the ratio of the electrical output of a transducer to the mechanical input; typically stated as a percentage of full scale based on a best-fit straight line, or as a percentage of reading. mass-loading error An error in the structural response induced by the added mass of the accelerometer. This i
32、s a function not only of the accelerometer mass, but also of the dynamic stiffness of the structure at the mounting location and the frequency range over which measurements are being acquired. mass-spring (relative motion) accelerometer An accelerometer that has only one terminal attached to its bas
33、e and is attached to the object in motion; the motion of the device is inferred from the motion of the mass (of the mass-spring system) relative to the base. mean time between failure (MTBF) An established method of calculating and predicting the service life of an instrument. mechanically isolated
34、accelerometer An accelerometer with built-in or external mechanical isolation. Substantial documentation should be provided by the manufacturer or other sources to verify that the mechanically isolated accelerometer is appropriate for the intended application (i.e., shock or vibration). This informa
35、tion should include amplitude and frequency performance that is appropriate for the application 6. mechanically isolated and electrically filtered accelerometer An accelerometer with mechanical isolation and an electrical filter. Substantial documentation should be provided by the manufacturer or ot
36、her sources to verify that the mechanically isolated accelerometer is appropriate for the intended application (i.e., shock or vibration). This information should include amplitude and frequency performance that is appropriate for the application 6. Usually, the electrical filter is included to atte
37、nuate the mechanical isolator performance, and the user is advised that these accelerometer additions may mask the shock environment that is to be measured. micro electro-mechanical systems (MEMS) A silicon based sensor that uses etching and micromachine fabrication techniques in forming its compone
38、nts; typically less than 1 mm square. NOTE: MEMS technology can be applied to piezoresistive, variable capacitance, or other types of design. micromachining A manufacturing process that produces the entire sensing mechanism from a single piece of silicon by using anisotropic etching or other micro-m
39、anufacturing techniques. 8 Copyrighted material Institute of Environmental Sciences and Technology IEST-RP-DTE011.2 noise monitor A dummy, or placebo, accelerometer that possesses all of the electrical and physical characteristics of a real accelerometer except for dynamic response, usually produced
40、 by replacing or desensitizing the sensing element(s); useful in assessing the quality of the output signal in the presence of environmental noise. noise-treated cable A type of cable designed to minimize triboelectric noise in a high-impedance piezoelectric accelerometer system; also known as low-n
41、oise cable. open-loop accelerometer An accelerometer in which the output generated by the deflection in the mass-spring system is used directly to indicate acceleration (cf. closed-loop accelerometer). overrange capability The ability of an accelerometer to withstand excessive mechanical input beyon
42、d its stated full-scale range without sustaining permanent damage. picoCoulomb (pC) A common unit of charge from a PE accelerometer; 10-12 Coulomb. piezoelectric (PE) accelerometer An accelerometer that utilizes piezoelectric elements as the spring in its mass-spring system and produces an electric
43、charge proportional to the applied input. piezoelectric coefficients The measure of the amount of charge produced per unit of input force; unit is in picoCoulomb per Newton. piezoresistive (PR) accelerometer An accelerometer that utilizes piezoresistive elements as a portion of the spring in its mas
44、s-spring system, resulting in a change in resistance proportional to the applied input. piezoresistive elements Strain gage elements, usually doped silicon ingots, used in a bridge configuration. pyroelectric effect The phenomenon in which an electric charge is produced when an accelerometer is subj
45、ected to a thermal transient input; relates primarily to piezoelectric accelerometers. pyroshock The response of a structure to a high-frequency, high-magnitude shock pulse as a result of an explosive event; also known as pyrotechnic shock. RAD rating An indicator used by the nuclear industry to rep
46、resent the level of integrated gamma radiation. reciprocity method A little-used primary vibration calibration method that uses a shaker designed with two coils, one to provide motion, one to measure the motion; the shaker is a reciprocal transducer, able to sense or drive, with a sensitivity factor
47、 relating the ratio of the drive current to the ratio of pickup voltage to velocity. resolution The lowest level of signal that can be measured by a transducer, usually stated in terms of equivalent engineering unit; other terms, such as residual noise and threshold, are also used to represent the s
48、ame parameter. resonance frequency For undamped accelerometers, the frequency for which the response of a system undergoing forced, steady-state vibration is a maximum. For damped accelerometers, the frequency with 90 phase shift. Damped systems do not have their peak at the resonance frequency. Dam
49、ped accelerometers may not have a peak in the frequency response curve. NOTE: The mounted resonance frequency specified by the manufacturer is usually lower than the unmounted resonance (the natural frequency of the device). When an accelerometer is mounted to a structure, the resonance may be lower than the “mounted resonance” as a result of imperfect mounting compliance. IEST-RP-DTE011.2 Institute of Environmental Sciences and Technology Copyrighted material 9 self-generating A term referring to accelerometers that use only piezoelectric element(s) as the sensing
