Implementation of an Accelerometer Transverse Sensitivity Measurement System. By: Ian Veldman 3 to 5 September 2012

Transcription

1 Implementation of an Accelerometer Transverse Sensitivity Measurement System By: Ian Veldman 3 to 5 September 2012 NMISA 2012

2 Overview Introduction Transverse Sensitivity System description Measurement procedure Validation methodology Measurement results Conclusions NMISA 2012

3 Introduction Accelerometers are widely used in everyday devices and applications Cell phones Transport (cars, trucks, planes, boats) Satellites Photography (image stabilisation) Plant & equipment maintenance Heath and Safety (human vibration)

4 Introduction (2) Accelerometers are not ideal devices The have limited operation ranges (frequency and level) Imperfections in the construction The crystal element is not 100% perpendicular with geometric axis

5 Transverse Sensitivity The transverse sensitivity of an accelerometer is defined as the sensitivity to acceleration applied at right angles to its geometric axis. Meaning that the sensitive axis of the transducer is not necessarily aligned with the geometric axis. As a result, any motion not in line with the geometric axis will produce an output. If the transducer is placed in a rectangular co-ordinate system the vector S max representing the maximum transducer sensitivity can be resolved into the components the geometric axis sensitivity S N the maximum transverse sensitivity S T, max

6 Transverse Sensitivity The transverse sensitivity is expressed as a percentage of the geometric axis sensitivity. For high quality accelerometers, manufacturers supply devices with low transverse sensitivities, typically 1 % with the direction of lowest transverse sensitivity (β TMin ) indicated with a red dot The manufacturer supplies these low transverse sensitivity devices through selection.

7 System description The transverse sensitivity calibration system was developed in compliance with ISO It was developed as an extension of the existing primary low frequency accelerometer calibration system. It utilises the existing long stroke (152 mm peak to peak) electro-dynamic exciter, connected to an air bearing linear translation stage (ABT). A stepper motor controlled turntable is mounted on top of the ABT.

8 System description (2) System Parameters Vibration frequency range: 5 Hz to 20 Hz Transverse acceleration range: 5 m/s 2 to 50 m/s 2 Analogue inputs: Two simultaneously sampled 12 bit channels Sampling Frequency: 500 khz Turntable rotation angle: 0 to 360 Reference: Heterodyne laser interferometer

9 Measurement procedure ISO describes a number of apparatus for testing the transverse vibration sensitivity of an accelerometer; A vibration exciter with an octahedron to obtain eight different excitation angles. A vibration generator with turn table Using a test system with X- and Y-vibration generators Using a test system with a tri-axial vibration generator

10 Measurement procedure For the NMISA system, once the Unit Under test (UUT) is mounted on the turn table and all the hardware connections are completed, the in-house developed software program is executed. The desired Reference, UUT, vibration frequency and angle step size is selected. The Program performs a set of procedural steps as part of each angular position; Move the turntable to the angular position of interest. Ramp the exciter to the selected vibration, in frequency and amplitude. Sample all the analogue inputs, streaming the data directly to the PC hard disk drive. Calculate the measurement parameters

11 Measurement procedure Measurement parameters The acceleration amplitude in m/s 2 The UUT output voltage in volts (V) The transverse sensitivity in V/(m/s 2 ) The relative transverse sensitivity (RTS) Record the RTS in the result sheet. Plot the RTS on a polar diagram. These steps are executed for each angular position from 0 to 360 using the selected steps size.

12 Measurement procedure Transverse sensitivity calculation S = T uˆ aˆ out T where S T is the transverse sensitivity û out is the amplitude of the output signal of the transducer vibrating perpendicularly to its sensitivity axis â T is the amplitude of the acceleration in the test direction

13 Measurement procedure Relative transverse sensitivity calculation * S = T S S T x100 Where * is the relative transverse sensitivity S T S T is the transverse sensitivity S is the sensitivity of the transducer on the geometric axis of sensitivity

14 Validation methodology An accelerometer with an accompanying transverse sensitivity certificate was purchased. The PCB, model 3701G2FA3G/ATS-7 (PCB UUT) accelerometer is designated as a laboratory standard. The system performance will validated using this accelerometer. Once the system is fully characterised, periodic verification will be performed using this accelerometer as a reference.

15 Measurement results Initial measurement using the PCB UUT did not produce the expected measurement results. The certificate supplied with the unit specified a maximum RTS of 0,129 %. Using he new system, a maximum RTS of 0,04 % was measured by the NMISA.

16 System Verification 10 Intervals To confirm the principle operation of the system, a tri-axial accelerometer was used 4 as the UUT. 70 The X-axis of 31the accelerometer was 6 selected as the transverse sensitivity axis. 30 It was expected that the 20 RTS measurement result would be 100 % As demonstrated in the graph, a maximum RTS of 100% was recorded at 120 and Transverse 16 Hz

17 System Verification - Bandpass filtering Transverse Sensitivity The transverse sensitivity of a PCB model Without Banpass Filtering M15 laboratory standard accelerometer was Hz Hz Hz 65 4 measured The RTS 60 was measured 2 at three 14vibration frequencies; Hz, 116 Hz and 20 Hz respectively A difference of more than 2 % between the RTS at 10 Hz and the RST at 20 Hz was 24 measured Such a large difference in RTS 27 was indicative of a measurement problem The investigation was focused on the signal to noise ratio

18 System Verification - Bandpass filtering Transverse Sensitivity The ISO standard With recommend Bandpass Filtering the use of a 24 1 db/octave narrow analogue 0.9 band-pass filter Hz 0.7 The 10 Hz 64 to 16 Hz measurements Hz for the PCB M15 accelerometer were repeated, 13 with an analogue band-pass 0.3 filter The results obtain using this setup 18 indicated a huge improvement in the results The maximum RTS measured, with the analogue band-pass filter, was 0,8 26 % at all three 47 the frequencies Hz

19 Conclusions A transverse sensitivity calibration system was successfully implemented by the NMISA. The system and software operation was verified using a tri-axial accelerometer. The requirement for a good signal to noise ratio was confirmed through the application of an analogue narrow band, band pass filter.

20 Thank you for your attention!