WO2003089916A1

WO2003089916A1 - A sensor

Info

Publication number: WO2003089916A1
Application number: PCT/AU2003/000468
Authority: WO
Inventors: Neil Mclay Wilson; Richard John Cave
Original assignee: Agrilink Holdings Pty Ltd
Priority date: 2002-04-19
Filing date: 2003-04-17
Publication date: 2003-10-30
Also published as: AUPS180502A0

Abstract

A sensor for sensing at least one parameter of a medium including sensing electrodes (6, 8) arranged to form a capacitor, each electrode having an insulating film (9) such that when the sensor is located in the medium each electrode is electrically insulated from the medium. The sensor also includes means for applying a measurement signal to the capacitor to obtain at an output a sensed signal, and means for processing the sensed signal to derive the at least one parameter of the medium. A capacitance value which is attributable to the insulating film is used to process the sensed signal. In a typical application the sensor is used for measuring moisture content and/or salinity of a soil medium.

Description

A SENSOR

Field of Invention

The present invention broadly relates to a sensor for sensing at least one parameter of a medium. In a typical application, the sensor is used for measuring moisture content and/or salinity of a soil medium.

Background of the Invention

Measurement of soil parameters, such as moisture content and soil salinity, enables an agriculturalist to visualise a crop's response to irrigation and other practices, and to better understand crop and soil water relationships. Information obtained from such measurements may be used by an agriculturist to assist with day to day soil management decisions to thereby improve productivity and sustainability.

Clearly, an essential step in the management of soil is the monitoring of soil moisture content and/or soil salinity. Provided that the information obtained from such monitoring is accurate, such information may be used to determine the likely risk of salinity (and whether treatments are required) and/or when to irrigate and how much irrigation to apply.

In recent years, capacitance based soil parameter sensors have been developed which employ radio frequency signals to determine a soil mediums dielectric constant to thereby infer soil moisture content and/or salinity. Sensors of this type typically rely on measuring a frequency change in a radio frequency signal of an oscillator circuit having a capacitive sensing element (for example, an electrode) which projects an electric field into a 'sphere of influence' of the soil medium being measured. The capacitive sensing element may be provided in the form of plates that are located within an access tube which is able to be inserted into the soil medium. In this scenario, the plates are separated from the soil medium by the housing of the access tube.

In capacitive sensors of this type, although the housing may provide the sensing elements with effective protection from effects of corrosion and moisture, separation of the sensing elements from the soil medium using the housing has a detrimental effect on accuracy of these sensors. This detrimental effect is particularly evident at high moisture and/or salinity levels.

Furthermore, sensors having sensing elements located within the access tube are adversely influenced by irregularities in geometry of the access tube. For example, very small changes in diameter and/or wall thickness of the access tube may have a detrimental effect on accuracy of sensor measurements. Moreover, eccentricity of the access tube or lateral movement of the sensing elements within the access tube may have a similar detrimental effect.

Furthermore, it appears that the sphere of influence of sensors having internal sensing elements is reduced as compared to sensors having external sensing elements. Accordingly, sensors having internal sensing elements are more susceptible to measurement errors that may be introduced by air gaps in the soil medium (for example, air gaps formed by cracks). This is because such air gaps may form a larger relative proportion of the sphere of influence and (as a result of being devoid of, or full of water) may not be indicative of soil moisture content. Indeed, such air gaps may lead to spurious results, the effect of which may be more pronounced in capacitance based soil sensors having internal sensing elements.

Although sensors having sensing elements mounted externally, are not as susceptible to measurement errors that may be attributed to a smaller 'sphere of influence' , such sensing elements are generally manufactured using materials (for example, stainless steel) that have been selected on the basis of corrosion resistant properties. Materials having such properties are typically quite expensive and may be difficult to machine.

Where a material has been selected for it's corrosion resistant properties, such a material may have electrical properties (for example, electrical conductivity) which are not ideally suited for use as a sensing element. Moreover, such materials may be more expensive than materials having superior electrical properties.

It is an object of the present invention to provide a capacitance based sensor which ameliorates at least one of the aforementioned deficiencies of existing capacitance based sensors.

The discussion of the background to the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge in Australia as at the priority date of any of the claims.

Summary of the Invention

According to a first aspect of the present invention there is provided a sensor for sensing at least one parameter of a medium, said sensor including: - sensing electrodes arranged to form a capacitor, each electrode having an insulating film such that when said sensor is located in said medium each electrode is electrically insulated from said medium; means for applying a measurement signal to said capacitor to obtain at an output a sensed signal; and - means for processing said sensed signal to derive said at least one parameter of said medium; wherein a capacitance value attributable to said insulating film is used to process said sensed signal.

According to a second aspect of the present invention there is provided a capacitance based sensor for sensing at least one parameter of a soil medium, said sensor including: at least one pair of sensing electrodes having an insulating film such that when said pair of sensing electrodes is located in said medium said electrodes are electrically insulated from said medium by said insulating film; an input circuit for applying a measurement signal to said at least one pair of sensing electrodes to obtain at an output a sensed signal; and means for processing said sensed signal to derive said at least one parameter including using values of capacitance and/or conductance attributable to said medium; wherein said processing means uses a further capacitance value attributable to said insulating film.

According to a third aspect of the present invention there is provided a method of sensing at least one parameter of a medium, said method including: applying a measurement signal to at least one pair of sensing electrodes located in said medium, each pair of electrodes being separated from said medium by an insulating film; obtaining a sensed signal from an output of said pair of electrodes; and - processing said sensed signal to derive said at least one parameter, said processing using a capacitance value attributable to said insulating film.

According to a fourth aspect of the present invention there is provided a computer readable memory, encoded with data representing a computer program, that can be used to direct a computer, said program including means for processing data received from a sensor including sensing electrodes arranged to form a capacitor, each electrode having an insulating film such that when said sensor is located in a soil medium each electrode is electrically insulated from said medium, said data containing values of capacitance and/or conductance attributable to said medium, said processing to derive values of salinity and/or moisture content for said medium, wherein a capacitance value attributable to said insulating film is used to process said data.

According to a fifth aspect of the present invention there is provided a computer program including computer program code means to make a computer execute processing of data received from a sensor including sensing electrodes arranged to form a capacitor, each electrode having an insulating film such that when said sensor is located in a soil medium each electrode is electrically insulated from said medium, said data containing values of capacitance and/or conductance attributable to said medium, said processing to derive values of salinity and/or moisture content for said medium, wherein a capacitance value attributable to said insulating film is used to process said data.

A particular advantage of the present invention is that it provides a capacitance based sensor having improved accuracy for measuring high moisture content and/or salinity as compared to capacitance based sensors having sensing elements located with a housing (such as an access tube).

General Description of the Invention

A sensor in accordance with an embodiment of the present invention may measure values for conductance and/or capacitance attributable to a medium by using a further value attributable to the capacitance of the film and use the values attributable to the medium to derive values of salinity and/or moisture content for the medium. As will be described in more detail following, the two electrodes (in the form of rings) are separated from the medium by the film to thereby provide corrosion protection and prevent problems associated with sealing the sensor from moisture.

In operation, a radio frequency measurement signal (that is, an AC voltage having a high frequency) is applied across the two electrodes. In a preferred form of the invention, the radio frequency signal may have a fixed frequency.

The sensor measures the resulting AC current passing through a sample of the medium located within a sphere of influence created by an electric field between the two electrodes by the application of the AC voltage.

The sensor then preferably measures the amplitude and phase of the resulting AC current and uses these measurements to derive values of capacitance (using the proportion of the measured AC current at 90° phase) and conductance (using the proportion of the signal at 0° phase) between the two electrodes.

Since the electrodes are separated from the medium by the film, the equivalent current of the electrodes in the medium may be represented by way of the equivalent current shown in Figure 5.

By using a value for C_c (that is, a capacitance value which is attributable to the film), which has been determined possibly by a factory calibration measurement which is stored in the sensor, the sensor is able to use this value to calculate the value of C_s and/or G_s (that is, the capacitance and conductance which is attributable to the medium) using a calculation function.

As will be appreciated, the capacitance which is attributable to the medium is mostly affected by moisture content and the conductance by salinity. However, there is some interaction between moisture content and conductance, and salinity and capacitance, and it is envisaged that such interaction may need to be separated by data processing. Moreover, there are also temperature dependant effects which, in the present invention, may be removed by measuring soil temperature and performing suitable processing.

Thus, having obtained values for C_s and/or G_s the sensor is then able to use these values, preferably together with a soil temperature value (T) and a soil type indicator to obtain values for the moisture content and/or salinity of the soil medium. In a preferred form of the present invention the process of obtaining values for moisture content and/or salinity entails indexing T, the soil type indicator, C_s and/or G_s and into a look up table.

Although the description that follows will describe the sensor in terms of a combined moisture content and salinity sensor having a fixed measurement frequency of 12 MHz, it is to be appreciated that other embodiments (for example, a moisture content sensor only, or a salinity sensor only) having other measurement frequency may also be used. Indeed, it is envisaged that measurements may be performed using a measurement frequency which is above 27 MHz.

It is envisaged that the present invention will find particular application in the area of soil moisture content and/or soil salinity measurement.

Preferred Embodiment of the Invention

The present invention will now be described in relation to various embodiments illustrated in the accompanying drawings. However, it is to be appreciated that the following description is not to limit the generality of the above description.

In the drawings: Figure 1 is a schematic diagram of a sensor according to a preferred embodiment of the invention;

Figure 2 is a perspective view of a sensor according to a preferred embodiment of the invention;

Figure 3 is an enlarged cross sectional diagram of a preferred arrangement of a portion of the sensor of the preferred embodiment of Figure 2.

Figure 4 is a flow diagram of a measurement cycle suitable for use with the preferred embodiment of Figure 1 ;

Figure 5 is a schematic diagram of an equivalent circuit formed by the sensor electrodes being located in a soil medium; and

Figure 6 is a perspective diagram of an embodiment of a sensor having two pairs of sensing electrodes.

Detailed Description of the Invention

Figure 1 illustrates a capacitance based sensor 2 for sensing at least one parameter of a soil medium. The capacitance based sensor 2 includes at least one pair of sensing electrodes 4, including a first electrode 6 and a second electrode 8 (ref fig 2).

The first electrode 6 and the second electrode 8 are preferably ring shaped electrodes made from any suitable conductive material. In this respect, although the preferred embodiment will be described in terms of a sensor 2 having a single pair of sensing electrodes 4, additional pairs of sensing electrodes may also be used. Indeed, it is envisaged that a sensor having two pairs of sensing electrodes may also be implemented (ref fig. 6 for an illustration of an example).

Each sensing electrode 6, 8 includes an overlapping or protective film 9 (refer figs. 2 and 3) such that when the pair of sensing electrodes 4 is located within a medium to be measured, the film 9 electrically insulates the electrodes 4 from the medium. The film 9 will be described in more detail herein.

As is shown in figure 1 , the sensor 2 also includes sensor electronics which includes a processing means 10 (shown here as a controller unit), input circuit 12 and sensing circuit 14. The controller unit 10 may be any suitable device. In one form of the invention, the controller unit 10 includes a microcontroller (such as a Cypress PSOC microcontroller) having on-board field programmable hardware blocks. The controller unit 10 may also include, or have access to, a programmed memory unit (not shown) which contains program code for performing calculation/processing functions which will be described herein. In one form of the invention, the memory may be provided in the form of an EPROM which is addressed by the control unit 10.

As is illustrated in figure 1 , the input circuit 12 includes a fixed frequency oscillator 16, a frequency divider 18, a band-pass filter 20, a buffer 22 and a delay buffer 23. The sensing circuit 14 includes a current sensing resistor 24, a mixer 26, a DC amplifier 28 and an analog to digital (A/D) converter 30.

In a sensor 2 having a controller unit 10 including on-board field programmable blocks, the fixed frequency oscillator 16, DC amplifier 28, and A D converter 30 may be implemented using these blocks. Moreover, it is envisaged that the controller unit 10 may provide other functions (for example, temperature sensing), the purpose of which will be explained in more detail herein. The fixed frequency oscillator 16 may generate a 48MHz clock signal which is provided to frequency divider 18. The frequency divider 18 uses the clock signal to generate an intermediate signal having a frequency (herein also referred to as a measurement frequency) which is a quarter of the frequency of the clock signal (that is, 12MHz).

The frequency divider 18 and delay buffer 23 may be implemented with a digital circuit formed using, for example, D-type flip flops.

In the embodiment illustrated, the intermediate signal provided by frequency divider 18 is supplied to the band-pass filter 20 and the delay buffer 23. The band-pass filter 20 has a centre frequency of 12MHz and a pass-band which is suitable for attenuating harmonics of the measurement frequency. The bandpass filter 20 may be provided in any suitable form (for example, an LC circuit having an appropriate filter characteristic).

The output of the band-pass filter 20 is provided to low impedance buffer amplifier 22. Buffer amplifier 22 provides a measurement signal which is coupled to first electrode 6 and calibration impedances 32, 34. , In a preferred form of the invention, the buffer amplifier 22 exhibits a low and constant impedance.

A controllable switch 36 couples the second electrode 8 and a terminal 38. The switch 36 is able to be controlled by the controller unit 10 to connect the second electrode 8 to the terminal 38. Terminal 38 is connected to the current sensing resistor 24 to provide a current path to ground from the output of the buffer amplifier 22 via the electrode pair 4 and the terminal 38 when switch 36 is in a closed position.

Similarly, each calibration impedance 32, 34 is connected to a respective controllable switch 40, 42. Each switch 40, 42 is able to be controlled by the controller unit 10 to connect a respective calibration impedance 32, 34 to the terminal 38 to thereby provide a respective further current path to ground from the output of the buffer amplifier 22 via a respective calibration impedance 32, 34 and the terminal 38 when the respective switch 40, 42 is in the closed position. The calibration impedances 32, 34 may be resistive and/or capacitive components each having a known impedance.

The terminal 38 is connected to a first input of mixer 26. A second input of the mixer 26 is connected to an output of delay buffer 23, which output, in the embodiment illustrated, provides a reference signal by applying a selectable phase shift to the intermediate signal. Delay buffer 23 provides a reference signal to the mixer which has been phase shifted by 0°, 90°, 180° or 270°. Controller unit 10 provides control signals to the delay buffer 23 which are operable to select a desired phase shift.

The output of the mixer 26 is connected to the DC amplifier 32. The DC amplifier 32 has a programmable gain which is controllable by the controller unit 10. The DC amplifier 32 output is connected to the A/D converter 30 such that controller unit 10 is able to periodically sample the A/D converter 30 output. Ideally, the DC amplifier 32 and the A/D converter 30 are arranged to provide 14 bits of dynamic range.

As described earlier the controller unit 10, is also connected to (or contains) a temperature sensor 44 which is able to monitor the temperature of the soil medium. The controller unit 10 accepts a signal from the temperature sensor 44 and processes this signal to obtain a temperature value.

Finally, the controller unit 10 also includes a communications port 46, which in the illustrated embodiment is a serial interface (for example, an SDI-12 or RS 485 interface) that is suitable for enabling the controller unit 10 to interface with external devices (not shown) to transfer information between the controller unit 10 and an external device.

As will be appreciated, the sensor 2 is able to obtain electrical power from a power source (not shown), such as a battery, which is suitable for providing electrical power to the components of the sensor which require such power. Indeed, the sensor 2 may also include power supply filtering, such as bypass capacitors and rectification devices (for example, diodes), suitably arranged to condition the electrical power.

Referring to figure 2, the sensor 2 includes a cylindrical housing 48. In the embodiment illustrated, the sensing electrode pair 4 comprising first electrode 6 and the second electrode 8 are arranged coaxiaily and adjacently on an outer surface of the cylindrical housing 48 to provide an arrangement which forms a capacitor. The housing 48 may be made of any suitable insulating material (for example, PVC). In the preferred embodiment the purpose of the cylindrical housing 48 is to provide mechanical support for the sensing electrode pair 4 as well as provide a protective housing for the sensing electronics. In this respect, the housing 48 has been illustrated in figure 2 so as to render visible a circuit card assembly 50 having the sensing electronics. Ordinarily, the circuit card assembly 50 would be sealed within the housing 48, and thus would not be visible.

With reference to both figure 2 and figure 3, the first electrode 6 and the second electrode 8 are each separated from the medium by means of a film 9. In the preferred embodiment of the invention the film 9 has a substantially constant wall thickness that is typically less than 0.01cm.

In the embodiment of the sensor 2 which has been illustrated, the film 9 is a transparent continuous thin coating of an electrical insulating material (for example, an epoxy resin). The film 9 is applied over a portion of the outer surface of the sensor housing 48, the first electrode 6 and the second electrode 8 so as to provide a protective barrier. In this respect, reference to the term "protective barrier" throughout this specification is to be understood to be reference to the ability of the film 9 to provide corrosion protection and a moisture seal. Advantageously, the film 9 may also have other suitable characteristics (such as surface finish, toughness and abrasion resistance) such that the film 9 is able to withstand abrasion which may be encountered during the process of inserting the sensor into the medium.

Although the film 9 has been described in terms of a continuous thin coating, it is to be understood that the film 9 may be implemented using other mechanisms. Indeed, the film 9 may include an adhesive sheet (for example, a transfer) which is wrapped around the a portion of the sensor housing 48 so as to cover the first electrode 6 and the second electrode 8 and thereby provide a suitable protective barrier.

Furthermore, although in the preferred embodiment the film 9 has been illustrated as a continuous film which spans across both the first electrode 6 and the second electrode 8, it is envisaged that in an alternative embodiment of the invention, a separate film may be applied to each electrode 6,8.

Having described the sensor 2 in terms of the components illustrated in the figures 1 to 3, the description will now turn to the operation of the sensor 2.

In use, the sensor 2 is inserted into a medium to be measured so as to locate the sensing electrodes 4 in the medium. The process of inserting the sensor 2 into the medium may entail drilling a hole in the medium which is suitable for accommodating the sensor 2, or alternatively it may entail forming a 'slurry' in the medium and positioning the sensor 2 into the slurry. In operation, the sensor 2 is able to measure values for conductance and capacitance which are attributable to the medium and use these to derive values for the medium's moisture content and salinity. In the embodiment illustrated in figure 4, a measurement cycle includes four processes, namely:

1. a circuit calibration process 52;

2. a self check process 54;

3. a measurement process 56; and

4. a calculation process 58.

Each of these processes will now be described in more detail.

1. Circuit Calibration Process

In order to perform conductivity and capacitance measurements without relying on high stability and factory adjusted circuitry, the sensor 2 utilises a circuit calibration process 52 which employs calibration impedances 32, 34 either singularly, or in combination.

Advantageously, because the preferred embodiment of the invention employs a scheme which derives the measurement signal using a clock signal having a frequency which is four times greater than the measurement signal, and a delay buffer 23 (which is preferably a digital device), the preferred embodiment of the present invention is able to perform a calibration process using one, or both, of the calibration impedances 32, 34, as opposed to only one.

Turning to figure 4, and referring to figure 1 , prior to performing the calibration process, the sensor 2 must be suitably configured to perform a circuit calibration process 52. Here then, the controller unit 10 actuates switches 36, 40, 42 so as to disconnect the circuit branch including electrode pair 4 from the terminal 38, and instead connects either individually, or in combination, calibration impedance 32, 34 to the terminal 38 and therefore to the current sensing resistor 24.

Thus, in the configuration described, a current path is provided between the output of buffer amplifier 22 and current sensing resistor 24, such that in response to the application of the clock signal from the frequency oscillator 16, a current is able to flow through the current sensing resistor 24, which current is independent of any effects due to the positioning of the sensor 2 in the medium.

Upon application of a clock signal from the fixed frequency oscillator 16, an AC current develops in the current sensing resistor 24. As will be appreciated, the AC current through the sensing resistor 24 results in an AC voltage drop appearing across the sensing resistor 24 (that is across terminal 38 and ground) which is received by the mixer 26 as a sensed signal. That is, terminal 38 is effectively an output which obtains a sensed signal which is communicated to the mixer 26.

As described earlier, the controller unit 10 is able to control the delay buffer 23 such that a selected phase shift is able to be applied to the intermediate signal so to generate a reference signal. In this way, the delay buffer 23 may be controlled to provide a reference signal having a phase shift (that is, 0°, 20°, 180° or 270°) which is sequentially selectable.

Thus, having configured the sensor 2 for circuit calibration process 52, and selecting an initial reference signal, both the sensed signal and the reference signal are detected by the mixer 26. ln this respect, since both the sensed signal and the reference signal are at the same frequency, the mixer 26 effectively acts as a synchronous detector and thus generates a DC output signal which is proportional to the amplitude component of the sensed signal that is in-phase with the reference signal.

The DC output signal is fed to DC amplifier 28 and amplified to provide an amplified signal. The amplified signal is sampled by the A/D converter 30 which converts the amplified signal into a sensed value, which in the preferred embodiment is a digital value.

The digital value is periodically sampled by the controller unit 10 and stored at an addressable location. Having obtained a digital value using a reference signal having a selected phase shift, the above described process is repeated using reference signals having the remaining phase shifts. At this stage the microcontroller 36 stores the following measured values:

V = |v

Where v₀, v₉₀ ,v₁₈₀ and v₂₇₀ are the digital values obtained using a reference signal having a phase shift of 0°, 90°, 180° and 270° respectively.

Once digital values have been obtained using a reference signal having each of the selected phase shifts, the controller unit 10 proceeds to calculate correction values for each phase shift.

Here then, at step 52-3 controller unit 10 initially calculates a 'raw' in-phase component (I) using the difference operation:

¹ = ^v ~ ^vi8o 0 ) At step 52-4 the controller unit 10 calculates a 'raw' quadrature component (Q) using the difference operation:

Q = v₉₀ - v 270 (2)

Advantageously, use of the difference operations (1) and (2) removes static DC errors which may exist in the sensor 2.

At this stage, the controller unit 10 has effectively calculated rectangular / and Q values. The microcontroller converts the rectangular / and Q values into polar form, using step 52-5, to provide an equivalent magnitude (\A_m\) and phase angle (φ _m) value. At step 52-6, a ratio of \A_m\ to a first predetermined value (\A_E\) is used to calculate a magnitude error term (k). A difference between φ _m and a second predetermined value (φ_E ) is used to determine a phase error term Aφ . Here, \A_E\ and φs are predetermined values which have been prestored at a memory location which is able to be addressed by controller unit 10. In this respect, the predetermined values correspond to an expected magnitude and phase angle value for the selected calibration impedance(s) 32, 34.

The two error terms (that is, k and Aφ ) are stored by the controller unit 10 during step 52-7.

2. Self Check Process

The self-check process 54 is a process which is preferably used to confirm that the sensor is operating correctly, and optionally allow a first-order adjustment of any non-linearity errors in the sensor 2.

In the preferred embodiment of the invention, in the event that only one of the calibration impedances 32 or 34 has been used to perform a circuit calibration process 52, the sensor is able to use the other calibration impedance 34 or 32 impedance (that is, the calibration impedance which was not used for the calibration process 52) to perform the self check process 54.

Here, the switches for the respective calibration impedance 32, 34 are actuated so as to connect the other calibration impedance 34 or 32 to the terminal 38 and disconnect the calibration impedance 32, 34 which was used for the circuit calibration process 52. In this way, the sensor 2 is able to measure the impedance of the other calibration impedance and use the results of such a measurement to confirm that the measured calibration impedance has an impedance which falls within a fixed range around its correct value.

Although the self-check process 54 has been described in terms of a separate process, it is envisages that where both calibration impedances 32, 34 are used during the circuit calibration process 52, the self check process 54 may be performed during the circuit calibration process 52.

In an alternative embodiment of the invention the self check process 54 may also allow a first-order adjustment of any non-linearity errors in the sensor 2.

3. Measurement Process

After having completed a circuit calibration 52 and self check 54 process, the sensor 2 is then able to perform a measurement process 56.

Turning to figure 4, and referring to figure 1 , prior to performing the measurement process 56, the sensor 2 illustrated and described must be configured to perform a measurement process 56 using step 56-1.

In this case, the configuration of the sensor 2 to perform a measurement process 52 entails actuating switches 36, 40, 42 so as to isolate the circuit branch(s) including calibration impedances 32, 34 from current sensing resistor 24, and instead connects the electrode pair 4 to the current sensing resistor 24.

Thus, in the configuration described, a current path is provided between the output of buffer amplifier 22 and current sensing resistor 24, via the electrode pair 4, such that in response to the application of the clock signal a current flow through the current sensing resistor 24, via the electrode pair 4, which current is dependent upon effects due to positioning of the sensor 2 in the medium.

Thus, upon application of a clock signal from the fixed frequency oscillator 16, an AC current develops in the current sensing resistor 24. As described previously, the AC current through the sensing resistor 24 results in an AC voltage drop appearing across the sensing resistor 24, which is sensed by the mixer 26 as a sensed signal.

As described earlier in relation to the calibration process 52, the controller 10 is able to control the delay buffer 30 so as to apply a selected phase shift (that is, 0°, 90°, 180° or 270°) to the intermediate signal to thereby generate a reference signal.

Thus, having configured the sensor 2 for the measurement process 56 at step 56-1 , and selecting an initial reference signal (that is a reference signal having a phase shift of 0°), both the sensed signal and the reference signal are detected by the mixer 26 at step 56-2.

Since both the sensed signal and the reference signal are at the same frequency, the mixer 26 effectively acts as a synchronous detector and thus generates a DC output signal which is proportional to the amplitude component of the sensed signal that is in-phase with the reference signal. The DC output signal is fed to DC amplifier 28 and amplified to provide an amplified signal to the A/D converter 30. The output of the A/D converter is sampled by the controller unit 10 and is stored as a digital value.

The step 56-2 is repeated for reference signals having a phase shift of 90°, 180° and 270°.

At the completion of step 56-2, the controller unit 10 stores the following values in the form of an array:

v_c = v_r v_r v_r v_r (3)

Where v_ro , v_r9o , ' v_r C₁₈o and v_r27o are the dig ^ital values obtained using *^ a reference signal having a phase shift of 0°, 90°, 180° and 270° using the electrode pair 4.

At steps 56-3 to 56-5 the digital values are processed to derive a polar form result (that is magnitude and phase) using a similar process to that described in relation to the calibration process (refer to steps 52-3 to 52-5) to obtain (\A_C\ and c).

Once measurements have been obtained for each separate reference signal, the above described process is repeated at step 56-6 for a sensor configuration in which the electrode pair 4 is isolated from the sensing resistor 24. That is, switch 36 is actuated by the controller unit 10 to disconnect the electrode pair 4 from the current sensing resistor 24, and the DC output of the mixer 26 is again measured using separate reference signals for each phase shift. In this way, the effects of any stray resistance and capacitance is able to be compensated for.

At the completion of this process then, the controller unit 10 stores the following values in an array: stray ^Vstray₀ rtrøy₉₀ stray_m stray₂₁o (4)

Where v_strayo , v _stmy∞ , v _straym and v,_w%w are the digital values obtained using reference signals having a phase shift of 0°, 90°, 180° and 270° respectively with the electrode pair 4 and the calibration impedances 32, 34 disconnected from current sensing resistor 24.

Here also, at steps 56-6 the digital values v_stray are processed to derive a polar form result (that is magnitude and phase) using a similar process to that described in relation to the calibration process (refer to steps 52-3 to 52-5) to obtain \A_stmy\ andø^_fl),

Measurements made with electrode pair 4 not connected (that is, \A_stmy\ and φ _stray ) to the sensing resistor 24 are then subtracted from the corresponding measurements made with the electrode pair 4 not connected to the sensing resistor (that is, \A_C\ and _c ) at step 52-7 to obtain \A_meas\ and _meas.

Here then, at step 56-8 the resultant polar values \A_meαs\

are corrected using the correction terms which were determined using the calibration process (ref. Step 52-6). Thus,

and φ r cor = φ T ineas - Aφ T

At step 56-9, the result for A_cor and Q_cor is converted back to rectangular form thus: _or = ^A _corCθS _cor and

Qcor = ^Acor^Si≠ccr

l_cor and Q_cor are stored by the controller unit 10.

Advantageously, processes 52, 54 and 56 may be performed multiple times so as to obtain an average value for I_cor and Q_cor. In this way, the effects of noise on the sensor measurements may be reduced.

4. Calculation Process

During the calculation process 58, the values for conductance and/or capacitance which are attributable to the soil are derived via a process which uses a value of a capacitance which is attributable to the film. In the preferred embodiment of the invention, the value of the capacitance which is attributable to the film is the capacitance C_c (refer to fig. 5).

In the preferred embodiment of the present invention, the capacitance which is attributable to the film 9 (refer fig.5, C₀) is stored in memory (or other suitable means) which is addressable by the controller unit 10. In one form of the invention, the capacitance C_c has been determined during a factory calibration process which has been performed prior to the insertion of the sensor 2 into the soil medium.

The process of deriving values of conductance G_s (ref fig.5) and/or capacitance C_s (ref fig.5,) which are attributable to the soil using the capacitance C_c (refer to fig. 5) entails solving a circuit equation of the equivalent circuit of figure 5 which is a function of C_c, I_cor and Q_cor. This is because I_cor is proportional to the conductance across the electrode pair 4, and Q_cor is proportional to the capacitance across the electrode pair 4.

Hence, since C_c, I_cor and Q_cor are known, values for Cs and/or Gs are able to be derived at step 58-1.

At step 58-2, the values for Cs and/or Gs, together with a temperature (T) value obtained from temperature sensor 44 are logged in a data log (not shown). The data log may be a data file (for example, a text file), which is managed by the controller unit 10. In an alternative embodiment of the present invention, data which contains values for T, Cs and/or Gs is communicated, possibly in the form of a data file, via communications port 46 to an external processing device (not shown) equipped with suitable software for storage (for example on a computer readable media or in a computer readable memory) and/or processing by that device.

Having obtained values for T, Cs and/or Gs the sensor, or possibly the external device is able to obtain soil moisture content and/or salinity values at step 58-3.

In the preferred embodiment of the invention step 58-3 entails indexing T, Cs, and/or Gs (and optionally a soil type indicator) into a look-up table so as to retrieve a soil moisture content and/or salinity value.

It is to be understood that various additions, alterations and/or modifications made be made to the present invention as described without departing from the ambit of the invention.

Claims

Claims:

1. A sensor for sensing at least one parameter of a medium, said sensor including: - sensing electrodes arranged to form a capacitor, each electrode having an insulating film such that when said sensor is located in said medium each electrode is electrically insulated from said medium; - means for applying a measurement signal to said capacitor to obtain at an output a sensed signal; and - means for processing said sensed signal to derive said at least one parameter of said medium; wherein a capacitance value attributable to said insulating film is used to process said sensed signal.

2. A sensor according to claim 1 or 2 wherein said medium includes a soil.

3. A sensor according to claim 1 , 2 or 3 wherein said at least one parameter of said soil medium includes values of salinity and/or moisture content.

4. A sensor according to claim 3 wherein said processing of said sensed signal includes using values of capacitance and/or conductance attributable to said medium.

5. A sensor according to claim 4 wherein said deriving of said at least one parameter includes indexing said values of capacitance and/or conductance attributable to said medium into a database.

6. A capacitance based sensor for sensing at least one parameter of a soil medium, said sensor including: at least one pair of sensing electrodes having an insulating film such that when said pair of sensing electrodes is located in said medium said electrodes are electrically insulated from said medium by said insulating film; an input circuit for applying a measurement signal to said at least one pair of sensing electrodes to obtain at an output a sensed signal; and means for processing said sensed signal to derive said at least one parameter including using values of capacitance and/or conductance attributable to said medium; wherein said processing means uses a further capacitance value attributable to said insulating film.

7. A sensor according to claim 6 wherein said at least one parameter of said medium includes values of salinity and/or moisture content.

8. A sensor according to claim 7 wherein said measurement signal includes a radio frequency signal having a fixed frequency.

9. A sensor according to claims 6, 7 or 8 wherein said processing means includes a mixer for mixing said sensed signal with a reference signal having the same frequency as said measurement signal to produce a voltage value.

10. A sensor according to claim 9 including means for selectably controlling phase of said reference signal relative to said measurement signal.

11. A sensor according to claims 9 or 10 wherein said voltage value is proportional to an amplitude component of said sensed signal that is in- phase with said reference signal.

12. A sensor according to any one of claims 6 to 11 wherein said processing means uses a temperature value obtained for said medium.

13. A sensor according to any one of claims 6 to 12 wherein said processing means uses a soil type indicator.

14. A method of sensing at least one parameter of a medium, said method including: applying a measurement signal to at least one pair of sensing electrodes located in said medium, each pair of electrodes being separated from said medium by an insulating film; - obtaining a sensed signal from an output of said pair of electrodes; and processing said sensed signal to derive said at least one parameter said processing including using a capacitance value attributable to said insulating film.

15. A method according to claim 14 wherein said medium includes a soil.

16. A method according to claim 15 wherein said at least one parameter of said medium includes values of salinity and/or moisture content.

17. A method according to claim 16 wherein said processing of said sensed signal includes using values of capacitance and/or conductance attributable to said medium.

18. A method according to claim 17 wherein deriving of said at least one parameter includes indexing said values of capacitance and/or conductance attributable to said medium into a database.

19. A computer readable memory, encoded with data representing a computer program, that can be used to direct a computer, said program including means for processing data received from a sensor including sensing electrodes arranged to form a capacitor, each electrode having an insulating film such that when said sensor is located in a soil medium each electrode is electrically insulated from said medium, said data containing values of capacitance and/or conductance attributable to said medium, said processing to derive values of salinity and/or moisture content for said medium, wherein a capacitance value attributable to said insulating film is used to process said data.

20. A computer program including computer program code means to make a computer execute processing of data received from a sensor including sensing electrodes arranged to form a capacitor, each electrode having an insulating film such that when said sensor is located in a soil medium each electrode is electrically insulated from said medium, said data containing values of capacitance and/or conductance attributable to said medium, said processing to derive values of salinity and/or moisture content for said medium, wherein a capacitance value attributable to said insulating film is used to process said data.

21. A data file containing values of salinity and/or moisture content for a soil medium, said data file being assembled using a computer program of claim 20.

22. A computer readable media containing a data file according to 21.