XVIII IMEKO WORLD CONGRESS Metrology for a Sustainable Development September, 17 – 22, 2006, Rio de Janeiro, Brazil PRIMARY ACCELEROMETER CALIBRATION PROBLEMS DUE TO VIBRATION EXCITERS Gustavo P. Ripper 1 , Guilherme A. Garcia 1 , Ronaldo S. Dias 1 1 INMETRO / DIAVI / LAVIB, Rio de Janeiro, Brazil, lavib@inmetro.gov.br Abstract: The main purpose of this paper is to discuss the problems that are imposed to primary accelerometer calibration due to the use of imperfect vibration exciters. A review of the main problems is presented. Solutions are discussed and some calibration results are presented. Keywords: vibration, accelerometer, calibration, metrology shaker. 1. INTRODUCTION Most of the primary accelerometer calibrations that are currently performed use interferometric measuring techniques [1]. The reciprocity technique has been substituted along the last years because it is very time consuming and has a limited frequency range of application. Nowadays, most National Metrology Institutes (NMI) offer calibration services from a few hertz to some kilohertz. Different optical-processing techniques can cover this broad frequency range and some of these allow automation of the complete calibration process. The vibration exciter (shaker) is one of the most important items of the calibration system. Assuming a stepped sine calibration, the ideal exciter should furnish uniaxial, stable and distortion-free vibratory movement at any desired frequency and amplitude. No additional source of uncertainty should be generated by the exciter. Unfortunately this ideal condition is not achieved in real life. Most commercial vibration exciters have limitations to their use in primary calibrations over a broad frequency range. At low frequencies, they suffer the influence of the maximum displacement limit. At mid-frequencies, some projects present resonances at or close to calibration frequencies. At high frequencies, problems due to cross- motion and heating usually show-up and can strongly affect the calibration result. Some NMIs developed their own calibration shakers to overcome many of these problems [2-10]. NIST/USA and PTB/Germany have designed many different exciters during the recent years. Some of these designs use the electrodynamic moving-coil principle, while others use piezoelectricity to generate motion. Air bearing guides were also implemented in many projects to keep low levels of cross motion and to avoid the resonances that typically appear in flat-spring suspensions. The international standard ISO 16063-11:1999 [1] imposed tighter transverse motion limits for shakers to be used in primary interferometric calibrations of vibration transducers. These limits contributed to the development of some new projects by different shaker manufacturers. Models using air-bearing guides are already commercially available today. APS, Bouche Labs, Endevco, TMS and TIRA are some of the companies that currently produce exciters with this kind of bearings [11-16]. 2. INTERFEROMETRIC CALIBRATION The interferometric calibration of an acceleration measuring set (accelerometer + conditioning amplifier) comprehends the measurement of the displacement amplitude on the reference surface of the accelerometer by interferometry and the measurement of the corresponding voltage output of the conditioning amplifier. Fig.1 shows a typical calibration system based on the fringe counting method. Fig. 1. Interferometric calibration system - Fringe counting method Conditioning Amplifier Power amplifier Frequency Counter Function generator Voltmeter Oscilloscope Data logger DSA Interferometer IEEE-488 Thermocouple Accelerometer Vibration exciter