Sponsored by a NASA Small Business Innovation Research (SBIR) Phase II contract, MetroLaser, Inc., has developed a novel, compact and low-cost laser Doppler vibrometer (LDV) under the brand name VibroMet. This LDV is a turnkey system for noncontact measurement of vibration at a single point on any vibrating structure. The system consists of two parts: a laser head and an electronic control unit (see figure) that are connected by a flexible electrical cable. Some of the notable features of the system include the following:
- Simple "point and shoot" operation for object distances from a few centimeters to 3 meters;
- Detection of signals from untreated, low-reflectivity surfaces, including black paper;
- A compact laser head that measures 2 by 3.5 by 6 in. (5.1 by 8.9 by 15.2 cm) and weighs less than 2 lb (<0.9 kg);
- Measurement over a wide dynamic range.
The frequency and velocity ranges of the standard VibroMet are 5 Hz to 80 kHz (at 3 dB) and 10 µm/s to 0.1 m/s, respectively. The detection ranges are expected to increase to the intervals from 2 Hz to 2 MHz and 80 µm/s to 1 m/s, respectively, in the next generation of this system.
The laser head is based on a novel configuration (patent pending) in which a diode laser and an acousto-optic modulator are the only optical elements (there are no mirrors or beam splitters). This simple optical configuration results in a robust instrument that is easy to align and straightforward to manufacture. The laser head contains all the electronic circuitry for amplification and demodulation of the Doppler vibration signal. The electronic control unit includes dc power supplies, a radio-frequency driver, a signal-strength display, and selectable signal-conditioning circuits for use as antialiasing filters.
This system can be used in many applications; examples include testing of prototypes, monitoring the health of machinery, and quality control on assembly lines. Recently, MetroLaser collaborated with the Department of Civil and Environmental Engineering of the University of California at Irvine in performing a series of experiments to explore the utility of an LDV in detection of earthquake-induced damage. The VibroMet was used to measure the natural vibrational frequencies of concrete structures that are representative of highway bridge supports. The results of the experiments conclusively demonstrated that this system can accurately measure damage induced by simulated earthquakes, which may not be detectable by visual inspection. The results of the experiments also demonstrated that natural frequencies in concrete structures can be measured by use of random excitations, such as those from vehicular-traffic vibrations or wind. Therefore, monitoring of highway bridges for damage induced by earthquakes can be accomplished quickly and without interruption of service by use of this system.
A survey by means of this system is being planned for 70 new bridges in the soon-to-be-opened Eastern Corridor Toll Road in Orange County, California. Tracking the degradation of the bridge supports over time and especially after earthquakes will both validate this system for widespread use and provide safety engineers with valuable information.
In addition to having expanded measurement ranges as mentioned above, the next generation of this system will be more compact. It is envisioned that the system will eventually be powered by batteries and the size of the laser head will be reduced to that of a pen.
This work was done by De Yu Zang, James Millerd, David Rosenthal, Eric Johnson, and Cecil Hess of MetroLaser, Inc., and Matt Tonge, an independent consultant, under an SBIR contract monitored by Dryden Flight Research Center.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
18010 Skypark Circle, #100
Irvine, CA 92614-6428
Refer to DRC 098-067