Accelerometers Uni-Gain? Shock and vibration measurement and analysis? Vibration monitoring? Modal and structural analysis? Vibration test control?
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Accelerometers Uni-Gain? Shock and vibration measurement and analysis? Vibration monitoring? Modal and structural analysis? Vibration test control? Uni-Gain types for easy interchangeability? DeltaTron types with integral preamplifier? Acceleration ranges cover 20 m ms—2 to kms—2? Low sensitivity to extraneous environmental influences including temperature fluctuations?
Low sensitivity to base bending effects? Individual calibration supplied? When the accelerometer is subjected to vibration, the combined seismic mass exerts a variable force on the piezoelectric element. Due to the piezoelectric effect, this force produces a corresponding electrical charge. Consequently, the charge produced by the piezoelectric element is proportional to the acceleration to which the transducer is subjected. All the piezoelectric accelerometer types described in this Product Data sheet are supplied with an individual calibration chart and, in most cases, an individually measured frequency-response curve.
Data from these charts are summarised in the Specifications. In contrast the sensitivity and other parameters for the Uni-Gain accelerometers are guaranteed within tight tolerances for easy interchangeability without recalibration see specifications on pages 20 and Except for the sensitivity, everything in this Product Data applies to both types. These types measure acceleration in three mutually perpendicular directions. Type uses the planar shear design, Type uses the inverted centremounted compression design and Type uses the centre-mounted compression design.
The piezoelectric elements of most of the accelerometers are PZ 23 lead zirconate titanate elements. Miniature Accelerometer Type has a lead zirconate titanate element PZ The housing material of all the accelerometers is the same as the base material given in the Specifications except Type , which has a nickel-chromium alloy housing. Characteristics Charge and Voltage Sensitivity A piezoelectric accelerometer may be treated as a charge or voltage source.
Its sensitivity is defined as the ratio of its output to the acceleration it is subjected to, and may be expressed in terms of charge per unit acceleration e.
The sensitivities given in the individual calibration charts have been measured at Hz with an acceleration of ms—2. For a With the exception of Triaxial Accelerometers Types , V, A and A—, the direction of main axis sensitivity for these accelerometers is perpendicular to the base plane of the accelerometers. Types , V, A and A— have three mutually perpendicular axes of sensitivity. DeltaShear Accelerometers The Delta design involves three piezoelectric elements and three masses arranged in a triangular configuration around a triangular centre post, as illustrated in Fig.
The Delta Shear design gives a high sensitivity-to-mass ratio compared to other designs, a relatively high resonance frequency and high isolation from base strains and temperature transients.
The excellent overall characteristics of this design make it ideal for both general purpose accelerometers and more specialised types. DeltaTron Accelerometers DeltaTron accelerometers operate on a constant-current power supply and give output signals in the form of voltage modulation on the power supply line. Types has an insulated base. All DeltaTron accelerometers are individually calibrated Uni-Gain types.
This transverse sensitivity is measured during the factory calibration process using a 30 Hz and ms—2 excitation, and is given as a percentage of the corresponding main axis sensitivity. These calculations assume that the accelerometer is properly fixed to the test specimen, as poor mounting can have a marked effect on the mounted resonance frequency.
The low-frequency response of an accelerometer depends primarily on the type of preamplifier used in the measurement setup. All the standard piezoelectric accelerometer types are supplied with an individual calibration chart.
With the exception of Types , A, A— and all V- types, all types have individually measured frequency response curves. DeltaTron types are supplied with individual frequency curves from 5 to Hz as well as typical curves below this range. Transverse Resonance Frequency Typical values for the transverse resonance frequency are obtained by vibrating the accelerometers mounted on the side of a steel or beryllium cube using Calibration Exciter Type Dynamic Range The dynamic range defines the range over which its electrical output is directly proportional to the acceleration applied to its base.
Upper Limit In general, the smaller the accelerometer, the higher the vibration level at which it can be used. The upper limit depends on the type of vibration, and is determined by the pre-stressing of the piezoelectric element as well as by the mechanical strength of the element. For accelerometers with built-in preamplifiers, the maximum shock and continuous vibration limits given in the Specifications are measuring limits.
For transportation and handling, the maximum non-destructive shock is specified. The maximum shock and continuous vibration limits are specified for vibration in any direction and for frequencies of up to one third of the mounted resonance frequency.
When measuring short duration transient signals, care must be taken to avoid ringing effects due to the high-frequency resonance of the accelerometer. Lower Limit Theoretically, the output of a piezoelectric accelerometer is linear down to the acceleration of the seismic mass due to the thermal noise, but a practical lower limit is imposed by the noise level of the measurement system and by the environment in which measurements are made.
Electrical Impedance Fig. Since the leakage resistance is very high, the accelerometers can be regarded as purely capacitive and the capacitances given in the Specifications are measured at Hz. DeltaTron accelerometers can be regarded as voltage sources, the ideal output impedance for an output source being zero.
The output impedance of these accelerometers is specified as a maximum resistance in Ohms W. At lower temperatures, the accelerometer piezoelectric element will exhibit temperature-dependent variations in charge and voltage sensitivity, as well as impedance. Details of these variations are given on the individual calibration chart supplied with each accelerometer see Fig.
To make measurements on surfaces with very high temperatures, some form of cooling is needed. Temperature Transients Piezoelectric accelerometers exhibit a small sensitivity to temperature fluctuations.
This effect is significant when low-frequency, low-level accelerations are being measured. This output will be approximately inversely proportional to the LLF. Humidity Fig. Use of moisture-impervious Teflon cables and sealing, as shown in Fig.
The acoustic sensitivity is specified as the equivalent acceleration given by a dB sound pressure level and measured in the frequency range 2 to Hz. Normal types of accelerometer cable can be used, but special cables are recommended for accumulated doses exceeding 1 kGy.
For greater exposure levels or for use under heavy neutron radiation, Industrial Accelerometer Type is recommended and special cables are available see separate Product Data sheet. Base Strains These may be introduced into the accelerometer by distortion of the structure being measured.
To minimise base strain outputs the DeltaShear design is used. Recommended Mounting Technique Fig. The accelerometers are screwed using a threaded steel stud onto a clean metal surface meeting the requirements specified in Fig. Under normal circumstances the absolute minimum depth of 4 mm will not be sufficient to accommodate the mounting stud, but is the minimum depth required to hold a stud securely. The optimum torque for tightening 10—32 UNF steel studs is 1. Type , due to its small size, cannot be mounted using a stud.
QS The tolerances on the clean metal mounting surface shown in Fig. Type has an M5 metric screw stud as an integral part of its base. The tolerances shown in Fig. When using the recommended technique, it should also be noted that if the mounting surface is not perfectly smooth, the application of a thin layer of grease to the base of the accelerometer, before screwing it down on the mounting surface, will improve the mounting stiffness.
Refer to pages 23 and 24 for an overview of the various cables and connector types. Types and V require TNC connectors. Type is supplied in four variants:? A: 10 m cable with BNC connector B: variable cable length with BNC connector H: 10 m cable without connector V: variable cable length without connector 9 Miniature Accelerometers Types , and V have integral cables, with a minimum length of 0.
Furthermore, extension connectors and cable AO are supplied. Types and V require sub-miniature connectors. Type is supplied with a subminiature to miniature plug coaxial cable AO All cables include a special noise-reduction treatment and are individually tested with regard to mechanical and electrical performance.
The max. DeltaTron accelerometers are supplied with a double-screened cable to reduce the electromagnetic interference to the absolute minimum. This also helps to reduce dynamically induced noise generated by the cables.
Details of these can be found in their respective Product Data sheets. Using charge preamplifiers, very long connection cables can be used without altering the specified sensitivity of the accelerometer and preamplifier combination. Sensitivity Conditioning Networks — Allow direct dial-in of transducer sensitivity on the preamplifier, giving unified system sensitivities.
High- and Low-pass Filters — Permit selection of different lower and upper frequency limits on the preamplifier to exclude unwanted signals and the influence of the accelerometer resonance from measurements. Each accelerometer undergoes an extensive calibration procedure and artificial ageing process so as to ensure completely predictable performance and stable operation. Accurate numerical details of the calibration are reported on the calibration chart supplied with each transducer see Fig.
Subsequent Calibration Fig. In addition to the general features so far described, some of these accelerometers have been designed for more specialised applications, and the special features of these accelerometers are discussed below. The accelerometers are tested at V and typically show that the resistance to ground loop effects is 50 M W. A strong, spiral-wound, mini-noise cable AO is available for use with these accelerometers. They require an external constant-current power supply and operate as voltage sources.
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