Dynamic measurements of a micropump using a fibre optic based interferometer

Spatially resolved, non-contact, displacement measurements are reported from the membrane surface of a piezoelectric-driven micropump. The measurement system uses a fibre optic interferometric technique, which incorporates an air path to the pump in the signal arm, allowing measurements to be made remotely. The interferometer operates at 1523nm, has a bandwidth of 200Khz, a focussed spot size of 22 mum and a noise equivalent displacement of 0.36nm. Membrane displacement profiles while pumping air and water have been obtained using custom designed automated fringe counting and interpolation software to interpret the digitised fringe patterns from the interferometer. Measurements show significant differences in membrane velocity, displacement and settling time between the two different pumping media. Transient underdamped vibration of the membrane surface was also detected in the rapid excursion and recursion phases of the pump cycle while pumping air. Analysis of the vibration transients allowed the resonant frequency and damping ratio of the system to be calculated. In addition, the amplitude of the membrane displacement was demonstrated to be dependent on the pumping frequency when pumping air. Analysis of the driving voltage and displacement profiles indicated that this frequency dependant relationship was primarily due to two effects: insufficient settling time between pump cycles and capacitive loading of the driving voltage at high pumping frequencies.