A New Solution for Shock and Vibration Calibration of Accelerometers

Shock and vibration phenomena are present around us in everything that moves. The accelerometer is a class of instruments commonly used to measure that motion, producing an electrical output signal related to the applied motion. Accurate accelerometer calibration is a way to provide physical meaning to this electrical output and it is a prerequisite for quality motion measurements. Systems and standards on comparison methods for accelerometer calibration are discussed, providing an overview on current technology available for calibrating and testing accelerometer performance characteristics. INTRODUCTION The manufacturer of an accelerometer subjects the design to a wide variety of tests to determine output due to a large number of inputs. Output characteristics commonly measured include sensitivity, frequency and phase response, resonant frequency, amplitude linearity, transverse sensitivity, temperature response, time constant, capacitance, and environmental effects such as temperature response, base strain sensitivity, magnetic sensitivity, etc. Often, when we talk about accelerometer calibration we are referring essentially to the measurement of sensitivity. The most common way to calibrate accelerometer sensitivity is by comparison to a reference transducer, generally another accelerometer designed to have stable low noise sensitivity in the conditions of calibration. Comparison methods are performed by back-to-back measurements. The sensor under test (SUT) is mounted in a back-to-back arrangement against a reference accelerometer and both sensors are subject to a common mechanical excitation. Since the motion input is the same for both devices, the ratio of their outputs is also the ratio of their sensitivities and the sensitivity and can be expressed by the following equation: Ssut = Sref • (Vsut/Vref) • (Gref / Gsut) where: Ssut is the SUT sensitivity (in mV/G, mV/(m/s); pC/G, or pC/(m/s)) Sref is the reference transducer sensitivity (in mV/G, mV/(m/s); pC/G, or pC/(m/s)) Vsut is the SUT channel output (in mV) Vref is the reference channel output (in mV) Gsut is the SUT conditioner gain (in mV/mV or mV/pC) Gref is the reference conditioner gain (in mV/mV or mV/pC). Two calibration methods are available depending on the nature of the excitation, periodic (vibration calibration) or transient (shock calibration), and are described by the ISO-16063 Part 21 [1] and Part 22 [2]. A welldesigned implementation of an accelerometer calibration system should not only follow the general guidelines provided by these standards, but also look carefully into the design and performance of each component of the system measurement chain, including the shock and vibration exciters, reference transducers, signal conditioner, digital data acquisition and controlling software. ACCELEROMETER VIBRATION SENSITIVITY CALIBRATION Vibration calibration uses oscillatory (sinusoidal) excitation normally provided by an electromechanical exciter or shaker (Figure 1) and the procedure for measurement of accelerometer sensitivity is described by the ISO 16063-21, “Methods for the calibration of vibration and shock transducers Part 21: Vibration calibration by comparison to a reference transducer” [1]. The shaker is driven by a sinusoidal vibration signal and the sensitivity of the SUT is measured at that particular frequency. Sweeping through the desired range of frequencies then generates a frequency response curve of the SUT (Figure 2), while changing the level of excitation at a certain frequency allows a linearity curve of the SUT to be measured. Figure 1: Back-to-back technique