Ground moisture sensor

(42) Date of commencement of the patent: 15. 4.1997 (45) Date of issue: 29.12.1997 (51) Int.Cl.⁶ : GO1N 27/22 (56) Documentation:

US 5424649A FR 2598810A1 US 4259632A (73)

KAUFMANN RÜDIGER DR.

A-6020 INNSBRUCK, TIROL (AT).

(54) SOIL HUMID SENSOR (57) A soil moisture sensor has at least two electrodes (2a, 2b) which can be plugged into the ground and a device (1) for applying an alternating voltage to the electrodes. Furthermore, a device is provided for forming an output signal dependent on the electrical capacitance (C) between the electrodes (2a, 2b). According to the invention, it is provided that the electronic components of the soil moisture sensor are arranged on a circuit board (7) and the electrodes (2a, 2b) are formed as conductor tracks on preferably the same board (7).

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The invention relates to a soil moisture sensor (hereinafter referred to as sensor), with:

at least two electrodes (2a, 2b) which can be plugged into the floor

- A device (1) for applying an AC voltage to the electrodes and

- Means for forming a dependent of the electrical capacitance (C) between the electrodes (2a, 2b) output signal.

Such sensors are known, for example, US Pat. No. 5,424,649A, FR 2598810 A1 and US Pat. No. 4,259,632 A.

In order to provide a more cost-effective and structurally simpler sensor, a sensor is provided according to the invention, which is characterized in that the electronic components of the soil moisture sensor are arranged on a circuit board and the electrodes are formed as conductor tracks on preferably the same board.

The sensor is suitable for measuring soil moisture in soil as well as in sandy or stony soils. Depending on the design, the measuring volume is about 15-100 ml. The main advantages of this sensor are the simple and therefore low-cost design, the low power consumption and a standard voltage output, which provides a simple connection to the sensor Measuring and control systems possible (for irrigation, lawn sprinklers, etc.)

The sensor is designed for applications that do not rely on high accuracy and precision. It is intended as an alternative to cheap sensors such as conductive blocks (eg plaster), but has a much better long-term stability.

Due to the high dielectric constant of the water, the electrical capacitance between sensor electrodes buried or buried in the ground can be used to measure soil moisture. For other sensors measuring frequencies between 30 MHz and 3 GHz are used, as in this range the dielectric constant is almost exclusively determined by the volumetric water content and is independent of soil type, soil composition (mineral soils, humus) and salinity.

In contrast, the present sensor measures the capacitance at operating frequencies between 100 kHz and 5 MHz. At these low frequencies, the dielectric constants of soils are further strongly affected by electrical dipoles in the soil matrix and by surface polarizations. However, it has been shown that these indirect, very complex effects are essentially related to soil moisture. As a result, although the calibration curves of the sensor are somewhat dependent on the particular type of soil, the low operating frequency allows a much simpler design of the measuring electronics and drastically reduces power consumption.

Further advantages and details of the invention will be explained in more detail with reference to the following description of the figures.

Fig. 1 shows an electrical block diagram. Figures 2 to 4 show different designs.

A quartz oscillator generates as device 1 for applying an alternating voltage, a 2 MHz square wave signal (Fig. 1). This square wave signal is applied through a resistor R to the active electrode 2a, the opposite electrode 2b is connected to ground. The electrode capacitance is thus connected in the form of an RC element whose time constant causes a phase distortion of the square wave signal. This phase distortion is converted into a pulse width by comparing the distorted signal through an AND gate 3 (Schmitt trigger inputs) with the reverse polarity oscillator signal (180 out of phase). The pulse width signal of this phase detector is smoothed by a sieve member Ri, Ci, a scaling and buffer amplifier 4 finally provides the output signal (standard 0-1V).

The oscillator 1 and the phase detector (AND gate 3) are realized with high-speed CMOS gates. The operating voltage is stabilized by a Micropower 5V regulator (types with low longitudinal voltage allow the operation of 6V batteries). The output amplifier is a type with a simple power supply, which is adjustable in zero point and in the gain via actuators 4a, 4b.

This design allows operating frequencies between 100 kHz and 5 MHz. At about 2 MHz, a low current consumption (eg 4 mA) can be combined with a short setting time (eg 4 ms) after switching on.

In the designs according to FIG. 2 and FIG. 3 the electrodes 2a, 2b are designed as rods. They are sheathed with an insulating layer 5, so that only capacitive currents can flow (at 2 MHz operating frequency otherwise the Elektrolytleitfähigkeit the soil would have to be considered, which would require a more complex electronics). The sensor rods are embedded in an abg edichtetes housing 6, in which the electronics is located. The sensors can be made in different sizes. An improved shielding against external electrical influences can be achieved by isolating only the active measuring electrode 2a and placing it between two bare ground electrodes 2b (FIG. 3). This construction proves itself especially when several sensors operate directly next to one another

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As an option, these sensors can also be equipped with a temperature sensor for simultaneous measurement of the

Soil temperature included.

In the further development according to FIG. 4 the manufacturing cost is substantially reduced by the fact that the entire sensor including the electrodes 2a, 2b is housed on a single printed circuit board (board 7). The electronics at the upper end of the board and the cable connections are shed (eg in epoxide 8). The electrodes 2a, 2b at the lower end are on the board one or two-sided executed, the isolation is carried out by a mechanically robust coating (eg Metal primer and epoxy). The active measuring electrode 2a is surrounded by the ground electrode 2b, which can be left blank even in this design for an enhanced shielding effect. In this case, the surface is passivated by tinning or gilding. This design may optionally include a temperature sensor.