Application of fringe counting interferometry to MEMS micropump characterisation

fibre optic intelferometric technique for non-contact dynamic displacement measurements of a micropump membrane smface is reported. The interferometer operates at 1500 nm, has a bandwidth of 200KHz, a focussed spot size of 20 J.tm and a noise equivalent displacement of 0.35 nm. The advantage of this intelferometric technique is that it is fibre optic based and hence can be used in inaccessible areas which would be generally considered inapprop1iate for conventional intelferomettic measurement techniques. Membrane displacement profiles have been obtained while pumping different gases and liquids using custom designed automated f1inge counting and interpolation software to interpret the digitised ftinge pattems from the inte1ferometer. Measurements show significant differences in membrane velocity, displacement and settling time between the different pumping media. In addition, transient underdamped vibration of the membrane smface was detected dming the rapid excursion and recursion phases of the pump movement while pumping air. The damping ratio and resonant frequency of the structure were determined from analysis of these transients in the displacement waveform. The amplitude of the membrane displacement was observed to be dependent on the pumping frequency when pumping air. This behaviour was caused partly by frequency-dependent effects in the pump dtive circuitry and partly by insufficient settling time between pump cycles, pariicularly at high pumping frequencies.