Laser-Based System for Measuring and Analyzing Vibrations

Unlike accelerometers, noncontact sensors do not alter dynamics of lightweight objects.

A system for noncontact measurement and analysis of vibrations of an electronic circuit board or a similar lightweight, nominally flat, rectangular object has been developed. This system, which utilizes laser displacement sensors, offers an important advantage over a traditional vibration- measurement system that utilizes accelerometers mounted on the vibrating object under test: Accelerometers are not suitable for vibration testing of a lightweight object because they contribute mass to the object and thereby alter the dynamic behavior of the object and thereby, further, give rise to errors in vibration-test data. In contrast, the laser displacement sensors in the present noncontact system do not contribute mass and do not significantly alter the dynamics.

Laser Displacement Sensors based partly on triangulation are used to measure bending vibrations at various points on an electronic circuit board or a similar lightweight object.
The laser displacement sensors in this system are commercially available units, each of which includes a laser diode, a lens, and a charge-coupled-device (CCD) image detector. The beam from the laser diode is aimed at the object under test. Laser light reflected from the beam spot on the object is focused by the lens onto the CCD, forming an image of the beam spot on the object, as shown in the upper part of the figure. The distance from the sensor to the object is then determined by triangulation, using the position of the beam spot on the CCD.

The system includes four laser displacement sensors (for the sake of simplicity, only two are shown in the figure). The sensors are positioned at a distance of ≈1 cm from the circuit board or other object under test, which is mounted on a shaker table to induce bending vibrations, which involve displacements perpendicular to the nominal plane of the object. The sensors are aimed at the object along axes perpendicular to the nominal plane of the object so as to measure the vibrational displacements perpendicular to the nominal plane of the object. The four sensors acquire displacement data simultaneously from four different positions along the horizontal dimension of the object. They can measure displacements of as much as ±1 mm.

The four sensors are mounted on a bracket that can be raised or lowered to enable acquisition of data at different vertical positions (eight different vertical positions in the original application). Thus, from measurements made at the four horizontal positions with the sensors at various increments of vertical position, it is possible to acquire data to characterize vibrations at an array of points that define a grid spanning the object [in the original application a grid of four horizontal increments of 0.7 in. (≈1.8 cm) and eight vertical increments of 0.4 in. (≈1 cm) spanning the circuit board].

In operation, the sensor readings are sampled at a rate of as much as 7.8 kHz as the shaker table is excited to produce sinusoidal vibrations that are swept through the vibrational-frequency range of interest (20 Hz to 2 kHz in the original application). The data thus acquired are processed by special- purpose software that yields information on the peak displacement and the resonance frequency of the first vibrational mode and generates a motion picture showing the displacement in this mode. In the original application, testing showed that the system captured the resonance frequency with an error of no more than 1.5 percent and the peak displacement with an error of no more than 0.7 percent.

This work was done by Krissa Elizabeth Arn of the Massachusetts Institute of Technology.


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Laser-Based System for Measuring and Analyzing Vibrations

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