Toward a low-cost disdrometer: Measuring drop size with a cantilever piezo film

Raindrop size distribution (DSD) is the key factor to derive reliable rainfall estimates. It is highly related to a number of integral rainfall parameters, including rain intensity (R), rain water content (W) and radar echo (Z). Disdrometers are the sensors commonly used to measure DSD based upon microwave or laser technologies; for example, JWD (Joss-Waldvogel Disdrometer), Parsivel and 2DVD (Two-Dimensional Video Disdrometer). These sensors may have their own strengths and weakness, but they are all relatively expensive. This hinders the possibility to have a high-density network for observing DSD at large scales. In this work, the ultimate goal is to develop a lightweight and low-cost disdrometer with descent accuracy.

We started with establishing a model that can well simulate the signal response of a single drop falling on a cantilever piezo film. A series of experiments were conducted to test the reaction of drops at different sizes (i.e. diameters ranging from 2 - 4 mm) and as drops fall onto various locations of the film. We then modelled the collision by assuming the piezo film to be a damped cantilever beam and drop force to be a step force. The drop force can be derived based upon the measurement of the deflection of beam end, which can be further used to calibrate the damp ratio. Preliminary results suggest that the signal response of a single drop hits can be well simulated based upon the proposed model under current experimental setting. We then developed an algorithm to optimize the simulation of signal responses with four four variables; these include drop’s weight, film thickness, film damping ratio and drop force. The result shows that the simulated drop force constitutes a strong linear relationship with the real drop’s weight.

We are now experimenting on the capacity of the developed model to work with a more complex yet realistic setting. For this purpose, we have created a more realistic rainfall condition by employing a micro pump. This pump can help control the size and timing of drops, so we can generate continuous single drops of consistent quality. In addition, we utilise a simple 1-D laser device to simultaneously measure the size of drops by analyzing the fluctuation in the laser signal. This would enable better understanding the actual size distribution of drops.  We expect that the outcome of the experiments  will provide useful insights on developing low-cost disdrometers with a cantilever piezo film.