Laser-Based System for Measuring and Analyzing Vibrations

Massachusetts Institute of Technology

Friday, August 01 2008

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.

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.