Capacitive sensor

Abstract

The present invention relates to a capacitive sensor comprising a capacitor with two conductive electrodes for placement partly in a liquid during operation, wherein the capacitance of the capacitor varies with a liquid level between the conductive electrodes. A measurement bridge is connected to the capacitor and provides a measured output signal Vout, where a signal generator provides a frequency input signal Vin is transmitted to the capacitor. A signal mixer receives the output signal Vout and the frequency input signal Vin and extracts an amplitude signal I and a phase signal Q, wherein a processor estimates the capacitance and resistance values Cliquid, Rliquid based on the amplitude and phase signals I, Q. The processor further estimates a capacitance value Cliquid and a resistance value Rliquid of the impedance and calculates a value of the liquid level l based on the capacitance Cliquid value. The present invention further relates to a method of determining a liquid level of a liquid inside a container with the capacitive sensor.

Description

[0001] Capacitive sensor

[0002] Technical field

[0003] The present invention relates to a capacitive sensor used to measure level of liquid in a container. In particular, the present invention relates to a capacitive sensor which is still able to function even if a change in permittivity of the fluid takes place.

[0004] Background of the invention

[0005] Capacitive sensors are generally known. A capacitive sensor is a device that may be used to detect the presence of a liquid and the amount of liquid as it enters between the electrodes of the capacitor and thereby causing the electrostatic capacitance to change.

[0006] To establish the level of liquid in a container, the change in capacitance is converted into a measurement for level because C = f(l), where I is the liquid level.

[0007] The function f(l) depends on the type of capacitor used, the size of the capacitor and the permittivity of the fluid media in the container. If the capacitor is a cylindrical capacitor, then the equation will look as this: where e0is the vacuum permittivity and eris the relative permittivity, and the relative permittivity eris the ratio of the absolute permittivity e and the vacuum permittivity e0, L is the length of two electrodes facing each other, and a and b are the radius of respectively the inner and the outer electrodes.

[0008] Permittivity e is a measure of the electric polarizability of a dielectric material, and a material with high permittivity polarizes more in response to an applied electric field than a material with low permittivity, thereby storing more energy in the material. The relative permittivity eris a characteristic of the fluid media inside the container, and the relative permittivity erof the media in the container plays an important role in determining the capacitance of the applied capacitor.

[0009] When a fluid media is entered into the container, the fluid level I increases, air is displaced from the space between the surfaces of the two electrodes of the capacitor and therefore the capacitance of the capacitor is changed. As the capacitance changes as a function of the fluid level I it is possible to calculate the fluid level I as long as all other parameters stay constant.

[0010] However, the conductivity and therefore the relative permittivity erof the fluid media in the container may change due to contamination of the fluid media e.g. the fluid media may be contaminated by water, metal shavings, salt or other particles that affects the conductivity of the fluid media. Such a contamination will using a traditional capacitive sensor result in an incorrect estimation of the fluid level I as the change in relative permittivity erwill result in a change in capacitance and therefore be interpreted as a change in fluid level I inside the container. Mechanical errors in the capacitor may also affect the capacitance of the capacitor, thereby causing an error in the liquid level measurement.

[0011] US 2011 / 0113878 Al discloses a capacitive liquid level sensor comprising a detecting unit comprising a first detecting electrode which is always in the liquid, a second detecting electrode which measures a level of the liquid, and a third detecting electrode which is always out of the liquid. The sensor also comprises a detecting circuit which repeats an operation of charging for a time being in proportion to a ratio of length of a part of the second detecting electrode dipped into the liquid to a total length of the second detecting electrode, and an operation of discharging the charged electric charge for a time which is in proportion to a ratio of a length of a part of the second detecting electrode being out of the liquid to the total length of the second detecting electrode. With this configuration, a voltage being in proportion to the liquid level may be obtained as output. However, this device is rather complex.

[0012] JP H02276917 A discloses a capacitive sensor for measuring the fuel level in a fuel tank using a vibrating mechanical probe excited at an excitation frequency, where the output signal is measured using a sample-hold circuit and a timing circuit. A processor in the measuring circuit determines the capacitance and resistance components of the sampled signal, which are indicative of the remaining fuel and moisture content inside the fuel tank. However, this solution does not use standard measuring equipment but requires a specially constructed probe unit, which increases the costs and complexity of the sensor unit.

[0013] The article "Liquid Level Sensor Based on Phase-Shifting of Radio-Frequency Wave" by Esmaili, Parisa et al. discloses a liquid level sensor for capacitive measurement of the liquid level using electrodes made of insulated twisted pairs of wires. However, as the wires are insulated it is not practically to measure the conductivity of the fluid in the container.

[0014] US 2013 / 0298667 Al discloses an apparatus for capacitive measurement of the liquid level in a container using cylindrical probe electrodes, where the electronics unit transmits an input signal to the probe and performs a frequency sweep within a frequency range of the transmitted input signal. The measured signal is converted into a suitable digital signal for the microprocessor. An evaluation algorithm implemented in the electronics is used to determine the optimal measuring frequency based on one or more predetermined criterions, such as a linear dependence between the measured signal and the fuel level. The electronics unit then measures the liquid level at that optimal measuring frequency. This increases the amount of data processing in the electronics unit, particularly if the frequency sweep is performed before each measurement.

[0015] Hence, it would be advantageous if a capacitive sensor was able to differ between a change in conductivity of the fluid media inside a container where the level is measured by the capacitive sensor, and a change in capacitance of the capacitor cause by change in level of fluid media inside the container. Summary of the invention

[0016] Thus, an object of the present invention relates to provide a capacitive sensor which is less sensitive to changes in conductivity of a liquid in a container where liquid level is measured. An object of the present invention is also to provide at least an alternative solution that overcomes the abovementioned problems of the cited prior art.

[0017] Thus, one aspect of the invention relates to a capacitive sensor comprising

[0018] - a capacitor comprising at least two conductive electrodes configured to be placed partly into a liquid during operation, the capacitance of the capacitor varies with a level I of liquid between the conductive electrodes, and

[0019] - a measurement circuit comprising

[0020] - a measurement bridge connected to the capacitor, the measurement bridge providing a measured output signal Vout descriptive of the impedance of the capacitor when placed in the liquid,

[0021] - a signal generator providing a frequency input signal Vin, the frequency input signal Vinbeing transmitted to the capacitor,

[0022] - a processor configured to estimate a capacitance value Chquid and a resistance value R.iiqUid of the impedance based on the measured output signal Vout, and to calculate a value of the liquid level I based on the capacitance Ciiquid value, wherein the measurement circuit further comprises a signal mixer configured to receive the output signal Vout from the measurement bridge and the frequency input signal Vinfrom the signal generator and to extract an amplitude signal I and a phase signal Q, and wherein the amplitude and phase signals I, Q are transmitted to the processor and used by the processor to estimate the capacitance and resistance values CiiqUid, R.iiqUid. This provides a capacitive sensor using standard components and a minimum amount of data processing to determine the liquid level, thus reducing the costs and complexity of the capacitive sensor.

[0023] According to any embodiment of the first aspect, the signal mixer may be an I-Q mixer generating two output signals (I, Q), a phase part and a quadrature phase part, descriptive of the amplitude and phase signals I, Q. The I-Q mixer may multiply the output signal Vo with the in-phase part (I) of the input signal Vi and separately with the quadrature part (Q) of the input signal Vi: I(t) = Vo (t)* cos (cot); Q(t) = Vo(t)*sin(cot), where Vo(t) is the output signal of the measurement bridge, and co is the angular frequency of the frequency input signal Vi. This allows the output signal to be modulated independent of its carrier frequency, which can be processed by the processor.

[0024] According to any embodiment of the first aspect, the signal generator may be an oscillator providing a range of frequency input signals. According to any embodiment of the first aspect, the oscillator may provide frequency input signals within the range of 10kHz to 20MHz. This allows the amplitude and phase signals I, Q to be sampled as a series of I / Q data points over a predetermined range of frequencies. According to any embodiment of the first aspect, the processor may be configured to estimate the capacitance value Chquid and the resistance value R-hquid of the capacitor by curve fitting, such as by applying a least squares algorithm. This allows the processor to construct the curve or function that provides the best fit to the series of I / Q data points. The applied curve fitting method outputs an estimated value of the variables R.iiqUid and Chquid when there is a minimum of residuals between the measurements I, Q and the formula for the measurement bridge.

[0025] According to any embodiment of the first aspect, the capacitor may comprise two conductive plates or wires between which there is a space and liquid progress into the space during use, wherein the progress of liquid between the two conductive plates or wires is used to determine a value for the liquid level I. According to any embodiment of the first aspect, the capacitor may be formed either as a cylindrical capacitor, a plate capacitor, or a two-wire capacitor. This allows the configuration of the capacitor to be adapted to the desired application and placement of the capacitive sensor.

[0026] Another aspect of the present invention relates to a method to determine a liquid level of a liquid inside a container with a capacitive sensor, comprising the following steps:

[0027] - providing a capacitor inserted into the container, wherein at least two conductive electrodes of the capacitor extend partly through the liquid inside the container,

[0028] - providing a frequency input signal Vinin a measurement unit and transmitting the frequency input signal Vinto the capacitor,

[0029] - receiving an output signal Vout in the measurement unit, the output signal Vout being descriptive of an impedance of the capacitor when placed in the liquid,

[0030] - transmitting the output signal Vout and the frequency input signal Vinto a signal mixer in the measurement unit,

[0031] - the signal mixer unit extracts an amplitude signal I and a phase signal Q, which are transmitted to a processor in the measurement unit,

[0032] - the processor estimates a resistance value R-hquid and a capacitance value CiiqUid of the impedance and calculates a value of the liquid level I based on the capacitance value CiiqUid. This provides a method of determining the liquid level using a capacitive sensor that is almost independent of the conductivity of the liquid.

[0033] According to any embodiment of the second aspect, the processor may use a curve fitting method, e.g., a least squares algorithm, or solving two equations with two unknowns and averaging for all measured samples to estimate the resistance and capacitance values iiquid, Chquid- The processor may use any known method of curve fitting to estimate the resistance and capacitance values hquid, CiiqUi , e.g. by applying a "Least squares algorithm". The resistance and capacitance values Riiquid, Chquid may also be estimated by simply solving the two equations with two unknowns and averaging all the samples. The skilled person would know how to solve such two equations with two unknowns using mathematics.

[0034] So, the selected method of Least squares is not a requirement for finding the resistance R-hquid and the capacitance Chquid.

[0035] Brief description of the figures

[0036] Figure 1 illustrates an embodiment of a suitable measurement bridge.

[0037] Figure 2 illustrates a measurement circuit according to the invention.

[0038] Figure 3 illustrates in more details how an I-Q-mixer works.

[0039] Figure 4 shows the I and Q samples and the least squares fitted I and Q values.

[0040] Figure 5 shows the found resistance vs. the number of least squares iterations.

[0041] Figure 6 shows the found capacitance vs. the number of least squares iterations.

[0042] Figure 7A-7C show three embodiments of a capacitor sensor according to the invention.

[0043] The present invention will now be described in more detail in the following.

[0044] Detailed description of the invention

[0045] Definitions

[0046] Prior to discussing the present invention in further details, the following terms and conventions will first be defined:

[0047] In general - when this expression is used when mentioning a feature relating to the present invention, it must be understood that the feature may be used with all embodiments of the invention, even if the mentioning is made in the detailed part of the document.

[0048] To obtain a precise value for a liquid level in a container when using a capacitor measuring capacitance as an estimation of the liquid level, it is for many systems necessary to compensate for changes of the conductivity of the liquid, the changes may be caused by change in chemical composition of the liquid, addition or removal of particles from the liquid, change in temperature, etc.

[0049] According to the present invention, the uncertainty for the estimated value for the liquid level is reduces as the capacitor and the liquid are modelled as a system comprising two electronic elements: a conducive part comprising a resistor with resistance R-hquid and a capacitor having capacitance CiiqUid.

[0050] The model capacitor does not involve a resistance but causes a phase-shift for a response, whereas the model resistor causes a change in size of a response and this change will include a change in conductivity of the liquid. A change in conductivity of the liquid causing a change in relative permittivity will therefor cause a change in the size of a response without contributing to a phase-change.

[0051] To estimate the capacitance CiiqUid and the resistance R.iiqUid, a capacitive sensor according to the invention comprises measurement circuit comprising a measurement bridge, a signal generator and a processor and a signal mixer receiving frequency input signals Vinand output signals Vout and splitting the resulting mixed signals into a real and a complex part.

[0052] Figure 1 illustrates an embodiment of a suitable measurement bridge. However, it is generally known how to construct a measurement bridge for measuring the capacitance of a capacitor, and alternative constructions of measurement bridges may be used. The measurement bridge must be able to receive a frequency input signal Vinand produce a corresponding output signal Vout.

[0053] Fig. 1 illustrates how the unknown impedance is modelled by a resistance R-hquid placed in parallel with the unknown capacitance CiiqUid, and the equation for estimation of the unknown impedance is:

[0054] (Equation 1)

[0055] Normally, the signal generator of the measurement circuit is able to provide frequency input signals Vinhaving a frequency between 10kHz and 20.000kHz. However, the range may vary depending on the liquid and the size and basic construction of the capacitor.

[0056] Figure 2 illustrates a measurement circuit according to the invention and Figure 3 illustrates in more details how the I-Q-mixer works.

[0057] According to the shown embodiment, the signal mixer treating the frequency input signals Vin and the measured output signals Vout to obtain suitable incoming signals for the processor is a signal mixer in form of an I-Q-mixer. An I-Q-mixer combines the incoming signals Vinand Vout, and splits the resulting output signals into a phase signal I and a quadrature signal Q. If e.g. 1 = 1 and Q=0, then the measured output signal has an amplitude = 1 and a phase = 0. If 1 = 0 and Q=l, then the incoming signal has and amplitude =1 and a phase = 90.

[0058] The two unknown variables R.iiqUid and Chquid are determined by equations:

[0059] R-hquid = fR(x,I,Q) (Equation 2)

[0060] CiiqUid = fc(x,I,Q) (Equation 3)

[0061] Where equation 2 and 3 are derived from equation 1 and the equations will depend on which capacitor is used for the sensor. Normally either a cylindrical capacitor, a plate capacitor, or a two-wire capacitor is used, but the type of capacitor is not essential for the invention. To determine the two unknown variables R-hquid and Chquid, a number of curve points is established for the incoming signals Vinand Vout at different frequencies. The processor may use any known curve fitting method to establish an estimate for the two unknown variables Rnqui and Cnqui .

[0062] The mathematical problem of having two equations and two unknown variables may be solved in many ways. According to the below example, the solution is provided by the method "Least squares Algorithm" which algorithm seeks to minimize the residual between the treated measurements I and Q and the formula for the measurement bridge as the algorithm will adjust R and C in a loop minimizing the sum of residuals: h = yt- ftXi.R. C)

[0063] Where y, are a complex value containing the I,Q measurement and Xi are the frequency parameters.

[0064] The sum of residuals is defined as:

[0065] Where p is a vector with the parameters to estimate (R,C) defined as:

[0066] The solution is found by running the loop until p converges:

[0067] Pi+i= pi - STEP * G(p)

[0068] Figures 4, 5 and 6 illustrate an example where the unknown variables iiquid and CiiqUid are estimated from measured and treated data in the form of values of I and Q, and resulting curves are shown as a function of the input frequencies Vin.

[0069] Figs. 5 and 6 show the number of least squares iterations x to the estimated resistance and capacitance, and fig. 4 shows the outputs in form of approximating values for respectively the real (Green line - I fittet) and for the complex (Red line - Q fittet) values. Using equation 2 and 3 with the found complex values, we can calculate Rhquid and Chquid-

[0070] The measurements are made by a cylindrical capacitive sensor according to the invention which sensor is partially emersed in water in a container. During the measurements the liquid level I in the container as well as the conductivity and therefore the relative permittivity erof the liquid are kept constant.

[0071] According to the algorithm the values for Rhquid and CiiqUid are estimated to be respectively Riiquid = 1000 ohm and C|iqUid = 100 pF.

[0072] Figures 7A-7C illustrate three different types of capacitive sensors with different capacitors which may be used with a capacitive sensor according to the invention. The capacitive sensor illustrated in fig. 7A comprises a cylindrical capacitor where a first conductive plate 2 having a cylindric shape with radius rl is positioned inside a second conductive plate 3 having a cylindric shape with radius r2 where r2 is larger than rl. Between the two conductive plates 2, 3 is a space 4 and during use liquid will enter the space 4.

[0073] The capacitive sensor illustrated in fig. 7B comprises a plate capacitor comprising two plates, a first conductive plate 2 and a second conductive plate 3 facing each other. The two conductive plates 2, 3 may be parallel and have the same shape and size, but they do not need to be parallel and have same shape and size. Between the two conductive plates 2, 3 is a space 4 and during use liquid will enter the space 4. The size, shape and distance between the conductive plates 2, 3 may be dependent on the application in which the capacitive sensor is used.

[0074] Depending on which capacitor is used with the capacitive sensor according to the invention the equations used to determine values for R-hquid and Chquid will change, but a skilled person will be able to determine relevant equations or estimations.

[0075] The capacitive sensor illustrated in fig. 7C comprises a two-wire capacitor comprising a first conductive plate in form of a first wire 2 and a second conductive plate 3 in form of a second wire 3. Between the two conductive plates 2, 3 is a space 4 and during use liquid will enter the space 4.

[0076] A capacitive sensor according to the invention may be used for measuring any type of liquid, both conducting and non-conducting liquids.

[0077] The capacitive sensor of the invention is particularly suitable e.g. for measuring a liquid level in a refrigeration system or other similarly closed system containing e.g. nonconducting and e.g. non-corrosive liquids.

Claims

Claims1. A capacitive sensor comprising- a capacitor comprising at least two conductive electrodes configured to be placed partly into a liquid during operation, the capacitance of the capacitor varies with a liquid level (I) between the conductive electrodes, and- a measurement circuit (1) comprising- a measurement bridge connected to the capacitor, the measurement bridge providing a measured output signal Vout descriptive of the impedance of the capacitor when placed in the liquid,- a signal generator providing a frequency input signal Vin, the frequency input signal Vinbeing transmitted to the capacitor,- a processor configured to estimate a capacitance value Chquid and a resistance value R.iiqUid of the impedance based on the measured output signal Vout, and to calculate a value of the liquid level I based on the capacitance Ciiquid value, characterised in that- the measurement circuit (1) further comprises a signal mixer configured to receive the output signal Vout from the measurement bridge and the frequency input signal Vinfrom the signal generator and to extract an amplitude signal I and a phase signal Q, and wherein the amplitude and phase signals I, Q are transmitted to the processor and used by the processor to estimate the capacitance and resistance values CiiqUid, R-hquid-2. A capacitive sensor according to claim 1, characterised in that the signal mixer is an I-Q mixer generating two output signals (I, Q), a phase part and a quadrature phase part, descriptive of the amplitude and phase signals I, Q.

3. A capacitive sensor according to claim 2, characterised in that the I-Q mixer multiplies the output signal Vo with the in-phase part (I) of the input signal Vi and separately with the quadrature part (Q) of the input signal Vi:I(t) = Vo(t)*cos(cot); Q(t) = Vo(t)*sin(cot), where Vo(t) is the measurement bridge output signal, and co is the angular frequency of the input signal Vi.

4. A capacitive sensor according to any preceding claim, characterised in that the signal generator is an oscillator providing a range of frequency input signals.

5. A capacitive sensor according to any preceding claim, characterised in that the oscillator provides frequency input signals within the range of 10kHz to 20MHz.

6. A capacitive sensor according to any preceding claim, characterised in that the processor is configured to estimate the capacitance value Chquid and the resistance value R-hquid of the capacitor by curve fitting, such as by applying a least squares algorithm.

7. A capacitive sensor according to any preceding claim, characterised in that the capacitor comprises two conductive plates (2, 3) or wires between which there is a space (4), during use liquid progress into the space (4) and the progress of liquid between the two conductive plates (2. 3) or wires is used to determine a value for the liquid level I.

8. A capacitive sensor according to any preceding claim, characterised in that the capacitor is formed either as a cylindrical capacitor, a plate capacitor, or a two-wire capacitor.

9. A method to determine liquid level of a liquid inside a container with a capacitive sensor, comprising the following steps:- providing a capacitor inserted into the container, wherein at least two conductive electrodes of the capacitor extend partly through the liquid inside the container,- providing a frequency input signal Vinin a measurement unit (1) and transmitting the frequency input signal Vinto the capacitor,- receiving an output signal Vout in the measurement unit (1), the output signal Vout being descriptive of an impedance of the capacitor when placed in the liquid,- transmitting the output signal Vout and the frequency input signal Vinto a signal mixer in the measurement unit (1),- the signal mixer unit extracts an amplitude signal I and a phase signal Q, which are transmitted to a processor in the measurement unit (1),- the processor estimates a resistance value R-hquid and a capacitance value CiiqUid of the impedance and calculates a value of the liquid level I based on the capacitance value CiiqUid.

10. A method according to claim 9, characterised in that the processor uses a curve fitting method, e.g., a least squares algorithm, or solving two equations with two unknowns and averaging for all measured samples to estimate the resistance and capacitance values R-liquid, Cliquid-

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