Contributing Editor, MEMS Investor Journal
Conventional FTIR spectrometers are characterized as bulky and sensitive to vibration and mechanical shock. This sensitivity causes spectrometer mirror plates to come out of adjustment thereby rendering the instrument to be unusable until it is readjusted. These weaknesses confine spectrometer application to laboratories with operation and readjustment performed by highly trained personnel.
Now, Germany’s Fraunhofer Institute for Photonic Microsystems (IPMS) has devised an improved translatory design where the application of MEMS is advantageous not so much due to the smaller device size, but mainly because of its relative insensitivity to shock and vibration. “With MEMS, we have the advantage of low mass and high frequency, making it much more difficult to couple outside energy into the system and thus disturb mirror motion,” says the Institute’s Deputy Director Dr. Harald Schenk.
Another advantage of the new design is that it overcomes previous confinements to small strokes in order to avoid deformation of the thin mirror plates. Causes of deformation include the mirror suspension mechanism itself and inertial forces. The new MEMS actuator delivers strokes of 1 millimeter for a 5 millimeter diameter mirror. According to Dr. Schenk , this is an order of magnitude more than previously available.
He adds, “While this is a significant improvement, it is small in comparison to what can be achieved by precision engineering. In our case, however, it is enough for low- to medium-resolution systems, making the FTIR spectrometer practical for a large variety of portable applications such as food quality control and environmental studies. Instead of sending the samples in question to a lab, evaluations are performed on site.”
The FTIR spectrometer team solved the small stroke issue in three ways. “To address deformation of mirrors by springs, we designed a pantographic spring suspension configuration that translates bending to torsion,” Dr. Schenk explains. “It uses a solid body mechanism, with the result being that deformation and mechanical stress can be kept very small even for large deflections.” Secondly, inertial forces were addressed by adding silicon structures to the mirror to serve as added mass. These structures have the effect of minimizing the effect of inertial forces on the mirror plate. And, thirdly, to counteract limits in stroke by air friction a special vacuum packaging was developed.
Dr. Schenk reports that at present the complete system is being set up for evaluation and qualification. “The potentially low fabrication cost for medium and high volume production should see these robust, fast and highly miniaturized optical modulators being used in a variety of applications,” he says.
Copyright 2010 MEMS Investor Journal