eC MeN Crs ee UUs) Accessory Equipment Vibration Transducers types 4321, 4366, 4367, 4368, 4369, 4370, 4371, 4374, 4375, 4381, 4384, 8305, 8306, 8308, 8309 and 8310 | FEATURES: = Artificially aged for good long term stability |= Acceleration ranges cover 20 ums" to 1000 kms —»_Uni-gain® types = Frequeney ranges cover from near DC to 60kHZ ces. | (+ 10% limit) Temperature range cover -200°C to + 400°C ‘= Shock and vibration measurement and analysis = Transducer calibration = Low sensitivity to extraneous environmental influences = Vibration monitoring = Robust, sealed construction ‘= Production control «= Individual calibration data supplied = Vibration test control a‘The Brilel & Kjer accelerometer range incorporates transducers suit able for most application require- ments both in the laboratory and un- der field conditions. In addition to a group of wide range, multi-purpose transducers @ number of transducers are available for special purposes; hhigh temperature, high shock, very low acceleration levels, fluctuating ‘temperatures, calibration of other ac- celerometers and measurements on delicate structures. The application areas of the individual transducers naturally overlap, but by reference to the summary table on the back cover the user can isolate the transducers of interest. Pull specifications can be found inside the back cover of this brochure. General ‘An accelerometer is an electrome- chanical transducer which produces fan electrical output proportional to the vibratory acceleration to which it, is subjected. ‘The active element of B & K accel- ‘erometers consists of one or more pi- ezoelectrie discs or slices. These are top or side loaded by one or more seis- ‘mic masses and are held in position by a stiff spring. When the accelerometer is subjected to vibration the combined seismic mass exerts a variable force on the piezoelectric element which due to the piezoelectric effect produces a cor- responding electrical charge. For frequencies from very near DC up to approximately one third of the resonance frequency of the accelerom- eter assembly, the acceleration of the seismic mass is equal to the accelera- tion of the whole transducer. Conse- quently the charge produced by the piezoelectric element is proportional to the acceleration to which the trans- ducer is subjected. This charge can be measured electronically at the output terminals of the accelerometer and used for accurate determination of the vibration amplitude, frequency and waveform, Design and Construction For operation of accelerometers over wide dynamic and frequency ranges with low sensitivity to extrane- ‘ous environmental influences, careful 146 Fig. 1. Schematic of B& K accelerometer configurations M = Seismic Mass, P = Piezo ‘lecrle Element, B = Base, R= Clamping Ring and 8 = Spring design is necessary. In the design of B&K piezoelectric accelerometers three basic constructions are used. ‘These are illustrated in Fig.1 and each offers particular advantages. Centre Mounted Compression Design is a simple, sturdy construc- tion that gives a moderately high sen- sitivity to mass ratio and can with- stand high levels of continuous vibra. tion and shock. The piezoelectric element-mass-spring system is mount ed on a cylindrical centre post extend- ing from the base of the accelerome- ter. However, despite the use of a rela- tively thick base, its sensitivity to base strain and temperature transients is somewhat higher than that of other B&K designs. Compression design is employed with B&K Accelerometers Types £8805, 8306, 8308, 8309 and 8310. Delta Shear Design’ is a Briel & Kjer innovation that combines the very best characteristics of traditional ‘compression and shear design acceler- cometers. It gives a high sensitivity to ‘mass ratio with moderately high reso- nance frequency and particularly low sensitivity to base strain and tempera- ture transients. It employs three pi ezoelectric elements each with their ‘own seismic mass which for reduced sensitivity to extraneous environmen tal forces are arranged in the shear mode around a triangular centre post. All mating surfaces are worked to a very fine degree of precision enabling the piezoelectric elements and seismic ‘masses to be solidly clamped to the centre post using a high tensile strength, preloading ring. In this way the use of adhesives for bonding the piezoelectric elements and seismic ‘masses are avoided, enabling an ex- ceedingly good amplitude linearity and long term stability to be ‘maintained. Delta Shear design is used with BE&K Accelerometers Types 4321, 4366, 4367, 4368, 4369, 4370, 4371, 4375, 4981 and 4384. The excellent overall characteristics of these acceler- ometers makes them eminently suit- able for most vibration work. Planar Shear Design is an exten. sion of the well proven Delta Shear design. It employs two piezoelectric el- ements each with their own seismic ‘mass which are rigidly clamped to a rectangular centre post using a high tensile preloading ring. This construc tion lends itself to miniaturization en. abling small, lightweight accelerome- ters with a high sensitivity to mass ratio and resonance frequency to be constructed. At the same time sensi- tivity to extraneous environmental forces is very much reduced, com- pared with traditional shear ‘design counterparts. Planar Shear design is used with the B& K Miniature Accelerometer Type 4314. Piezoelectric Element ‘The suitability of piezoelectric ac- celerometers for different vibration ‘measurement applications also de- pends: on the piezoelectric material used. With B & K accelerometers four different types are employed. PZ 23 belongs to the lead zirconate titanate family of ferroelectric ceram- ice and may be used at temperatures up to 250°C, Owing to its good all round characteristics, it is used in most B & K accelerometers PZ27 is similar to PZ23 but with improved sensitivity and temperature response. PZ 48 is a specially formulated ferro- electric ceramic with particularly flat temperature response and may bewed at high temperatures. Is used with the B & K 400°C and high shock accelerometers. PZ 100 is a carefully selected and prepared quartz crystal. It exhibits ex- cellent long term stability plus partic. arly flat temperature response and therefore is used with the B & K refer- ence standard accelerometer. Characteristics Charge and Voltage Sensitivity ‘A piezoelectric accelerometer may be treated as a charge or voltage source, Its sensitivity, which is the ra- tio of its electrical output and the ac- caleration causing the output, may therefore be expressed in terms of charge per unit of acceleration: pC/ms? or in terms of voltage per unit of acceleration: Sig = mV/s? ‘The sensitivity depends not only on the type and size of piezoelectric ele- ment, but also on the weight of the seismic mass loading it. Consequently increased sensitivity is usually accom- panied by increased physical size and weight, What balance of sensitivity and size is appropriate is largely de- termined by the application and the frequency range required for ‘measurements. Uni-Gain® Sensitivity Certain B&K accelerometers have had their measured sensitivity espe- cially adjusted during manufacture to be within 2% of a convenient unified value, for example 1, 10, 100 or 1000 pC/ms*, The use of these accel- crometers with fixed gain preampli ers makes sensitivity adjustment of the measuring system an easy matter. Setting-up time is reduced to a mit mum and calculations during mea- surements are avoided. ‘Transverse Sensitivity ‘Accelerometers are also slightly sen- sitive to acceleration in a plane normal to their main sensitivity axis. This is due to minute irregularities in th structure and the alignment and in polarization of the piezoelectric ele- ment. At B & K particular attention is paid to selection of homogenous piezo- lectric ceramics and in careful ma- chining, polishing and lining-up of ac- Fig. 2 Typical frequency reponse curves of B& K accelerometers celerometer parts, thereby ensuring that the majority of B & K accelerom- eters have a maximum transverse sen- sitivity of less than 3 or 4% of their ‘main axis sensitivity at 30 He. Because the transverse sensitivity varies for different directions in the base plane, most B& K accelerome- ters are individually measured and ‘marked with a red spot to indicate the direction of minimum transverse sen- sitivity. See Main Specification Table for details, Frequency Response ‘Typical frequency range character- istics for each accelerometer are shown in Fig.2. The upper frequency limit is usually taken to be one third ‘of the mounted resonance frequency for less than 1 dB (12°) error, or one fifth of the resonance frequency for less than 0,5 dB (6%) error. This as- ssumes that the accelerometer is prop- erly fixed to the test specimen, as the use of poor mounting techniques can have a marked effect on the mounted resonance frequency of accelerome- ters. See section headed “Mounting”. ‘The low frequency response of an accelerometer depends primarily on the type of preamplifier used in the vibration measurement set-up. With voltage types, the preamplifier input resistance effectively decreases the electrical time constant of the acceler- ometer. Consequently voltage pream- plifiers having an exceedingly high in- put resistance have to be used to per- ‘mit measurements at low frequencies. With charge preamplifiers there is no auch problem as capacitive feedback ‘on the preamplifier input effectively increases the accelerometer time con- stant, enabling @ reduced lower fre- ‘quency measurement limit to be easily obtained. Phase Respon: ‘Owing to their low damping factors, B&K accelerometers may be used at all frequencies up to 0,3 times their ‘mounted resonance frequency without noticeable phase distortion being in- troduced. This is especially important with regard to investigation of shock ‘and transient vibrations where poor phase linearity can seriously distort the reproduced waveform, Dynamic Range ‘The dynamic range of an accelerom- ter defines the range over which its electrical output is directly propor- tional to the acceleration applied to its base. ‘Upper Limit. This is determined by the mechanical strength and preload- ing of the piezoelectric element. In general, the smaller the accelerometer, the higher the vibration level at which it may be used. ‘The maximum shock and continu fous vibration limits specified for B&K Accelerometers (see Fig) ap- ply for vibration in the direction of the accelerometer main sensitivity axis ‘and at frequencies up to one third of the accelerometer mounted resonance frequeney. With vibrations containing high level components of a higher fre- quency, a mechanical filter should be used for mounting the accelerometer. This will reduce unwanted high fre- quency stimulation of the accelerome- resonance frequency, thus nabling it to be used within specified 147