Vibration measurements to support failure analysis and design improvements

A customer approached Philips Innovation Services (PInS) for a problem related to loss in signal output from an optical sensor, ‘probably related to mechanical vibrations’. A measurement plan was developed with the customer, to identify the sensitivity of the sensor response to external vibrational loads. Time and frequency domain analyses provided valuable insights in the problem. Suggestions were given for design improvements that the customer will evaluate for future implementation.

Introduction

A customer was developing an optical sensor for a medical application. The sensor had been designed to be used by doctors during surgical interventions.

The optical signal was being affected by the various sources of mechanical vibration, involving electronic equipment, fans and operating room floors. The main disturbances to the sensor, however, were caused by a mechanical switching element which is housed in the sensor box.

The customer needed a measurement to quantify at which vibration levels and frequencies the sensors optical signal output deteriorates and to gain information of possible directions for design improvement.

Approach

The customer approached PInS to measure the vibration amplitudes and frequencies at which optical signal quality drops. In addition, an indication for future design improvements was desirable.

After a quick introduction to the problem at the customer site, a measurement plan was developed for vibrational analysis of the optical sensor box. A schematic of the measurement setup is visible in Figure 1 and part of the measurement setup is shown in Figure 2. The measurements were done at the customer site with PInS equipment.

Vibration measurements - Schematic of measurement setup

Figure 1. Schematic of measurement setup

Vibration measurements - Picture of shaker attached to customer setup, for evaluation of response to mechanical vibrations

Figure 2. Picture of shaker attached to customer setup, for evaluation of response to mechanical vibrations

Time domain vibration measurements were done to detect amplitudes and frequencies at which the  optical signal quality drops. This helped the customer identify sources that generate these conditions and to develop a risk analysis plan.

To indicate directions for design improvements, frequency domain measurements were done. A transfer function is given in Figure 3. This function shows the transfer from the acceleration of external excitation to acceleration of the sensor housing. Thus, amplitudes higher than one (100) indicate frequencies at which damping in the setup is not effective. On certain frequencies, amplifications of more than a factor 10 were measured.

Vibration measurements - Frequency response function from acceleration of external excitation (y1) to acceleration of the sensor housing in different directions (x2, y2, z2)

Figure 3. Frequency response function from acceleration of external excitation (y1) to acceleration of the sensor housing in different directions (y2, x2, z2)

 

Conclusions

The measurement project, from introduction to analysis and presentation of results, was carried out within a few days. The following was achieved:

  • Vibrational measurements indicate amplitudes and frequencies at which the optical signal quality decreases below acceptable limits.
  • The measurements revealed frequency ranges where attenuation was expected, but vibration amplification occurred instead. This insight gives valuable input for future design improvements.
  • New measurements are planned to identify the components that are due for redesign for future product releases.

Dynamic vibration measurements provide information of possible sources of malfunction. This can help on solving stringent customer issues. Besides, they provide an opportunity to understand how the customer and PInS can collaborate for achieving excellence in product design.

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