Development of an Inductive Rain Gauge

Abstract

Measuring weather data in an urban environment is an important task on the journey towards smart cities. Heavy rain can cause flooding in cities and prevent emergency services from reaching their destination because roads or underpasses are blocked. In order to provide a high-resolution site-specific overview in urban areas during heavy rainfall, a dense measurement network is necessary. To achieve this, a smart low-cost rain gauge is needed. In this paper, the current status of the development of an inductive rain gauge is presented. The sensor is based on the eddy current principle and evaluates the frequency of an electrical resonant circuit. For this purpose, a coil is placed under a metal plate. When raindrops hit the plate, it starts to oscillate, which changes the distance to the coil accordingly and causes changes in the frequency of the resonant circuit. Since the sensor is cost-effective, operates self-sufficiently in terms of energy and transmits data wirelessly via LoRaWAN, it can be used flexibly. This enables dense, area-wide coverage over the urban area of interest. The first experimental investigations show a correlation between the size of the rain droplets and the frequency change. Small droplets cause a shift of about 8 kHz and larger droplets of up to 40 kHz. The results prove that raindrops can be detected and categorized using this measurement principle. These data will be used as a basis for future work on calculating precipitation.

Keywords: LoRaWAN; eddy current; internet of things; rain gauge; smart sensor.

Figure 1

Sensor concept.

Figure 2

Circular current lines on the plate.

Figure 3

Magnetic Vector Potential for different distances.

Figure 4

(a) Mutual inductance and resonant frequency for distances h between 2–100 mm; (b) Mutual inductance and resonant frequency for distances h between 2–3 mm.

Figure 5

Deflection for light rain. (a) Top view of the plate with deflection as a colormap; (b) Deflection for different application points along the cross section of the plate.

Figure 6

Parallel LC-Circuit with a comparator.

Figure 7

Measured frequency of the resonant circuit for distances between 2–3 mm.

Figure 8

(a) Top view of the sensor with solar module; (b) Side view of the sensor with 25° inclined mounting base.

Figure 9

Frequency over time of measurements 1–3.

Figure 9

Frequency over time of measurements 1–3.