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We present a novel electronic circuit for a capacitive non-contact volume sensor to be used for the determination of
dispensed liquid droplets in the nanoliter range. Beside the ability of online droplet volume measurement the presented
sensor features also sensitivity to certain droplet parameters like droplet velocity, liquid type and even small droplet
deformations during the droplet's flight. The sensor principle is based on the capacitive change caused by a droplet while
it passes an open plate capacitor. In contrast to the already published results [1, 2], the presented circuit amplifies the
change of capacitance by a measurement bridge that feeds a differential amplifier. The required bridge regulation is
realised by an adjustable analogue voltage divider combined with an all-pass filter to modulate the reference signal in
phase and amplitude. The implementation of the analogue adjustment is crucial to enable the required high sensitivity.
The circuit is able to amplify changes in capacitance in the range from ▵C ≈ {0.6 to 2.7 fF} which are caused by single
droplets of volumes of V ≈ {20 to 70 nl}). The output voltage signals can reach up to Umax= 2.3V. The sensor sensitivity
to the droplet volume reaches Si = 82 mV/nl with an accuracy of ▵V = ± 4 nl. With this sensitivity even droplet
deformation occurring within the measurement capacitor can be observed as multiple signal peaks. These are caused
from lateral (towards the electrodes) and longitudinal extension of the droplet's shape that influence the effective
capacitance by ▵Cmax≈ 0.2 fF.
Andreas Ernst,Bernhard Vondenbusch,Roland Zengerle, andPeter Koltay
"Non-contact volume determination of free flying nanoliter droplets
using an adjustable capacitive measurement bridge", Proc. SPIE 8066, Smart Sensors, Actuators, and MEMS V, 806620 (5 May 2011); https://doi.org/10.1117/12.886705
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Andreas Ernst, Bernhard Vondenbusch, Roland Zengerle, Peter Koltay, "Non-contact volume determination of free flying nanoliter droplets using an adjustable capacitive measurement bridge," Proc. SPIE 8066, Smart Sensors, Actuators, and MEMS V, 806620 (5 May 2011); https://doi.org/10.1117/12.886705