Laser Sensors Measure Force

Michael D. Wheeler

When it comes to measuring force, the more precise the measurement, the better. Just ask anyone who makes a living designing high-precision acceleration sensors for aircraft navigation or fast tactile sensing systems in robots. For years, however, researchers have been frustrated by the lack of progress in precisely measuring forces such as torque, pressure, mass and inertial acceleration. Now a research team from the University of Kassel has demonstrated how the photoelastic effect of Nd:YAG lasers can be used to measure forces with a high degree of accuracy and a broad measurement range.

The researchers employed small mirrored laser crystals pumped by a low-power, 808-nm laser diode, along with a polarizer and a photodiode for recording results. This setup enabled them to closely study the two phenomena that result when mechanical force is applied to the laser crystal: birefringence and changes in the beat frequency within the beam. By monitoring these events, they gathered accurate measurements for both static and time-variable forces.

Simple design

The key to applying the laser's photoelastic effect to measurements of force relates to the simplicity of the design and the attributes of the Nd:YAG laser, said lead researcher Wolfgang Holzapfel.

Researchers have developed a simple force sensor based on the photoelastic effect of Nd:YAG lasers. The device quantifies forces applied to the laser crystal by measuring their effects on the beam, including birefringence and changes in the beat frequency.

"Overall, the setup is of great simplicity, and diode-pumped monolithic Nd:YAG technology offers excellent handling properties, which means easy adjustment of the test setup and diminishing influence of seismic and acoustic disturbances during force measurement." The Nd:YAG also has the advantage of high gain and low threshold, with minimal crystal heating to affect measurement, he said.

The researchers are investigating applications of the force sensors for commercial and scientific use.