Vibration calibrators , sometimes also called reference shakers, are electromechanical instruments which enable
calibration
In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of kno ...
of vibration sensors and measuring instruments to
traceable standards. They produce sinusoidal mechanical vibration signals with known
amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
s and
frequencies. The vibrating part of the instrument is usually a cylindrical steel stud with an internal thread for attachment of the test object. An
electrodynamic
In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...
or
piezoelectric
Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word '' ...
actuator system is used to produce the vibrations. With older instruments it was necessary to adjust the vibration amplitude according to the weight of the test object. However, modern instruments contain a built-in reference
accelerometer
An accelerometer is a tool that measures proper acceleration. Proper acceleration is the acceleration (the rate of change of velocity) of a body in its own instantaneous rest frame; this is different from coordinate acceleration, which is acc ...
and
closed-loop control
Control theory is a field of mathematics that deals with the control of dynamical systems in engineered processes and machines. The objective is to develop a model or algorithm governing the application of system inputs to drive the system to a ...
, with which the amplitude is kept constant up to a maximum specified weight of test object. Older models can be used to calibrate objects weighing up to a maximum of approximately 100 g, whereas the latest instruments can work stably with test objects weighing over 500 g.
Vibration calibrators are most often used for testing and checking vibration sensors and measuring instruments at the site of their operation and are, therefore, usually transportable and battery operated.
The most commonly occurring vibration frequency of calibrators is 159.2 Hz, which is equivalent to a
radian frequency
In physics, angular frequency "''ω''" (also referred to by the terms angular speed, circular frequency, orbital frequency, radian frequency, and pulsatance) is a scalar measure of rotation rate. It refers to the angular displacement per unit tim ...
of 1000 rad/s. The vibration displacement, velocity and acceleration of sinusoidal signals are connected with each other through the factor of the radian frequency. Advantageously, at 1000 rad/s the numerical values of the amplitudes for all three vibration quantities are the same. For example, a vibration acceleration of 10 m/s² at 159.2 Hz is equivalent to a vibration velocity of 10 mm/s and a vibration displacement of 10 μm.
With some instruments it is possible to choose between several frequencies or to finely tune a specific frequency range. Frequencies between 16 Hz and 10 kHz are common on the market.
Occasionally, vibration calibrators also contain a signal conditioner for connection to various types of vibration sensors, and additionally a display for reading the sensitivity.
A procedure for the calibration of vibration calibrators is described in the international standard ISO 10816-44.
References
* T. Usuda, A.O. Oota, H. Nozato, Y. Hino. (2009) "Transportable Vibration Calibration System Employing E-trace Scheme", Proceedings of the IMAC-XXVII
External links
Vibration Calibrators, Brüel & Kjær{Dead link, date=June 2022 , bot=InternetArchiveBot , fix-attempted=yes
Portable Vibration Calibrator, The Modal ShopPortable Vibration Calibrators, Agate Technology Measurement