Toroidal conductivity sensor

Abstract

The present invention provides a toroidal conductivity sensor.SOLUTION: A toroidal conductivity sensor with increased range and linearity. The size of the drive coil may be smaller than the receive coil. An internal conductive loop is provided through both the drive coil and the receive coil.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to conductivity sensors, and more particularly to toroidal conductivity sensors.

[0002] Background technology This section provides background information related to the present disclosure that is not necessarily prior art.

[0003] Conductivity sensors are often used in industries such as pharmaceuticals, chemicals, food, metals, mining, and various other industries to measure the conductivity of various liquids. These conductivity sensors typically measure the resistance of an ionic liquid solution and use that measurement to determine conductivity.

[0004] Toroidal sensors are often used for this task. The sensors typically contain two spaced-apart toroidal transformer coils. One coil is usually called the drive or transmitting coil, and the other coil is called the receiving or sensing coil. When these toroidal sensors are immersed in a conductive fluid, the drive coil is electrically excited by an AC power source, which generates a changing magnetic field.

[0005] This changing magnetic field induces a current loop in the fluid under test. The magnitude of the induced current is a function of the fluid's conductivity. Current flow in the fluid induces a current in the receiving coil, which is analyzed to measure the conductivity.

[0006] Among the drawbacks of prior art toroidal sensors are problems with linearizing the sensing signal, limited sensing range, and relatively bulky geometric size.

[0007] Summary of the Invention This section provides a general overview of the disclosure, but is not an all-inclusive disclosure of its entire scope or features.

[0008] In accordance with the teachings of the present invention, a technique is provided for measuring a wide range of electrical conductivities.

[0009] The present invention has the following advantages: 1. Linearize the sensing signal. 2. Expand the detection range. 3. Provide a method to auto-calibrate electronic equipment before installation or for periodic auto-calibration. 4. Reducing the geometric size of the proposed sensor.

[0010] The following combinations may be employed: 1. Toroids with two different dimensional sizes: a. There is no need to match permeance between the driving core and the receiving core. b.This means that you are free to use a smaller core size on the driver side than on the receiver side, eliminating volume space limitations within the probe. c. This allows the toroidal probe to be geometrically smaller than previous models. 2. A resistive element having a variable resistance is connected in parallel with the receiving toroid. a. The variable resistor element value is selected based on the conductivity range and conductivity of the media. b. By varying the value of the resistive element, the strength of the signal can be varied, which depends on the current conductivity range. 3. An inner conductive loop wound through both the drive core and the receiver core, this conductive loop containing a variable resistor element. a. The resistance of the resistor is selected according to the measured value of the medium.

[0011] advantage 1. An electric field is induced in the medium through the center of the toroidal probe. This field drives an ionic current in the medium through the center of the hole. This field loops around the toroidal probe to close the ionic current. By changing the permeability and dimensions of the driving core, it is possible to reduce unwanted coupling between the two cores. This improves signal resolution and reduces noise in the measurement signal. This allows the use of an ideal inductance for each conductance range. This means that measurements are taken with the ideal inductance value for the receiving coil over a wide dynamic conductivity range. 2. Conventionally, two sensors with the same permeance or permeability are used. The drawback of this is that the range is narrower. For example, a high permeance will cover a low conductivity range, but a low permeance will cover a low conductivity range, and vice versa. In other words, with a high permeance, the maximum detectable signal will be reached very quickly, and the upper sensing range will be missed. 3. See the governing equations below.

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[0012] In-situ calibration. In-situ calibration allows for calibration of the sensor without the use of external devices such as decade boxes or pre-calibrated solutions. To accomplish this, wires are looped through the drive and receive toroids and connected internally to a potentiometer such as those described above, which is packaged internally with the toroids.

[0013] The advantage is in-situ calibration without having to take the sensor out to combine with a decade box or solution.

[0014] Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

[0015] The drawings described herein are for purposes of illustrating selected embodiments only, not all possible implementations, and are not intended to limit the scope of the present disclosure. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a perspective view of a conductivity sensor made in accordance with the present invention; [Figure 2] FIG. 2 is a perspective view of a drive coil and a receiver coil. [Figure 3] FIG. 2 is a circuit diagram of a drive circuit. [Figure 4] FIG. 2 is a circuit diagram of a receiving circuit. [Figure 5] FIG. 1 is a circuit diagram of an induction loop. [Figure 6] FIG. 6 is a circuit diagram showing an arrangement of the circuits of FIGS. 3, 4, and 5 used to measure a fluid property such as conductivity using a sensor.

[0017] Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

[0018] MODE FOR CARRYING OUT THE INVENTION Exemplary embodiments will now be described more fully with reference to the accompanying drawings.

[0019] 1, there is shown a conductivity sensor 10 constructed in accordance with the teachings of the present invention. Sensor 10 includes a non-conductive housing 12 made of plastic or the like. Housing 12 has a central bore 14 through which a fluid F to be measured flows when sensor 10 is immersed in the fluid. Surrounding bore 14 are a drive coil 16 and a receive coil 18.

[0020] As better shown in Figure 2, the drive coil 16 has an annular magnetic core surrounded by multiple loops of wire wound around the core with numerous loops passing through opening 17. The ends of the wire for the drive coil are labeled DL and DH. Similarly, the receive coil 18 is an annular magnetic core surrounded by multiple loops of wire passing through opening 19 terminating at ends RL and RH. As can be seen, the wire passes through the openings in the core and loops around the periphery of the core.

[0021] Additionally, there is an inner conductive loop 20, which is a single wire wrapped around both the receiving core and the driving core. The wire ends of the conductive loop 20 are labeled ICL and ICH. As best shown in FIG. 1, the inner conductive loop 20 is a wire within the housing 12 that loops through the opening 17 in the driving coil 16 and the opening 19 in the receiving coil 18, but is not exposed to the fluid F. In other words, the inner conductive loop is inside the housing 12, as best shown in FIG. 1, and does not come into contact with the fluid F.

[0022] The ends of wires DL, DH, RL, RH, ICL and ICH are connected to a sensor circuit 22, which may be internal or external to housing 12.

[0023] 3 shows the drive circuit for drive coil 16. An AC current source 24 generates a drive signal through resistor 26, which enters drive coil 16 within housing 12. This induces a signal in inner conductive loop 20 as well as in fluid medium F.

[0024] 4 shows the receive circuitry for receive coil 18. The receive circuitry includes a variable resistor RP connected across receive coil 18. This measured signal is connected to an operational amplifier 27, which in turn is connected to suitable signal processing circuitry 28, which may include filters and analog-to-digital converters, as well as a processor.

[0025] 5 shows the circuit of the inner conductive loop 20. Resistance RW represents the resistance of a fluid F, such as water. Connected in parallel with RW is a variable resistor RI, the purpose of which will be explained in more detail later in this specification.

[0026] FIG. 6 shows the completed sensor circuit 22, including the driver circuit, receiver circuit, and inner conductive loop.

[0027] Sensor 10, sometimes referred to as an inductive conductivity sensor (ICS), is a device that measures the conductivity of a liquid medium without direct contact with the medium. This offers advantages over contact conductivity sensors, hereafter referred to as CCS, in harsher environments where the electrodes of a CCS may experience failure due to interaction with harder chemicals in the medium, such as acids. ICS, such as toroidal sensors, offer advantages because they do not directly contact the medium. CCSs tend to have advantages in media with low conductivity, while ICSs have advantages in media with high conductivity. For these reasons, ICSs are the sensors of choice for media with high conductivity, and CCSs are the sensors of choice for media with low conductivity. Resistance to a variety of media is an advantage in media with low conductivity. This can be achieved by extending the measurement range of an ICS. This is done in several unique ways, as described below.

[0028] While there are toroidal ICS on the market that can measure a wider range, they rely on larger units with greater power consumption and space requirements. These larger units can provide signal filtering and measurement stabilization that extend the range of the toroidal ICS. This comes at the cost of bulk and greater power consumption. The goal of the following technology is to create a toroidal ICS that can provide the same dynamic as a larger one, but in a smaller package and with minimal power consumption. The goal is to create a sensor that uses primarily passive components for measurement, and with a small number of those components. Signal conditioning is minimal. This results in an efficient sensor that can provide a wider measurement range due to the physics of the measurement principle.

[0029] ICS often have accuracy errors over a wide range. The method of toroidal ICS measurement is susceptible to change. This source of measurement variability comes from the variability of relative magnetic permeability to various conditions, such as the current flowing around it. This value is dynamic with respect to the conductivity of the medium. This means that ICS are prone to loss of accuracy over a wide range. Methods for stabilizing measurements over a wider range are discussed herein.

[0030] In many operating environments, space is often at a premium. In this regard, a small sensor is an advantage. Techniques for reducing the size of the drive coil to maximize space and measurement range are also disclosed. Many people want to know if their electronic equipment is working properly and still within calibration. This document describes how an ICS can perform calibration methods within itself.

[0031] Increased measurement range The measurement range depends on the strength of the impedance of the receiving coil 18 relative to the impedance of the medium F being measured. This means that the relative strength of the signal is related to the strength of the measurement range. One way to extend the measurement range is to have circuitry and filters to process signals with a high dynamic range. Another method is to change the strength of the receiving coil's inductance, thereby changing its impedance. This changes the ratio between the medium's impedance and the receiving coil's impedance. Under normal circumstances, it is not feasible for a miniature sensor to have multiple cores. Pairing the receiving coil 18 in parallel with a variable resistor RP (see Figure 4) makes it possible to change the effective inductance of the receiving coil 18. Changing the effective inductance of the receiving coil 18 has the same effect on the measurement signal as using a receiving coil with a different inductance value. This relationship can be used to generate a stable signal over a wider range. This is done by "switching" to a different receiving coil by adjusting RP when the measurement signal becomes too weak.

[0032] Processing this wider dynamic range is currently done with amplifiers and filters. However, this also has the effect of amplifying noise. While there are filtering methods that can be used, the space and components are too large for a handheld sensor to actually implement the filtering method. The present method for varying the strength of the receive coil circumvents this need by having a good measurement range and ensuring that the measurement signal always falls within this "golden zone" by varying the strength of the receive coil. This has the effect of only amplifying the measurement signal, not amplifying the noise. This minimizes the required electronics and power consumption while allowing the ICS to maintain the wider dynamic range of a heavier desktop unit.

[0033] As mentioned above, this can be achieved by connecting a variable resistance element RP in parallel with the receiving coil 18. The magnitude of the effective inductance of the receiving coil 18 depends on the resistance of the parallel resistance element RP. This means that by changing the magnitude of the variable resistance element RP, the effective inductance of the receiving coil can be changed. This is like having multiple cores within one core.

[0034] As a specific example, when the fluid is in the 10 ohm range, the RP is set to approximately 100 ohms. This allows the typical inductance of the receiving core to be around 0.1 uH, which is ideal for measuring the RW resistance range. As the RW increases to approximately 100 ohms, the RP becomes approximately 1000 ohms. This results in a receiving inductance of approximately 2 uH, which is ideal for measuring RWs from approximately 100 to 1000 ohms. In this way, receiving inductors of various inductances can be obtained. This functions as if there were multiple cores on the measurement toroidal sensor, allowing the user to switch to the ideal core for the current measurement range.

[0035] Inner Conductive Loop Measurements made using ICS depend on the relative permeability of the receiving coil 18. The relative permeability of the receiving coil is dynamic and variable with respect to various parameters. One parameter of interest is the current flowing around the receiving coil. Because this value is dynamic, the permeability of the receiving coil is also somewhat dynamic. The total power flowing through the receiving coil 18 is fixed and cannot be easily increased. This means that power is shared between the receiving coil 18 and the impedance of the medium F. This results in nonlinearities in the measurement, which can be understood as introducing inaccuracies into the measurement. According to the teachings of the present invention, reducing the dynamic variability of the measurement artificially reduces the apparent measurement range, but does not actually reduce the measurement range.

[0036] As seen in FIG. 1, ICL 20 is an internal loop inside housing 12. The ICL does not directly contact medium F; however, it passes through openings 17, 19 in both drive coil 16 and receive coil 18. This ICL 20 mimics what occurs in medium F. In this configuration, sensor circuit 22 of sensor 10 senses a parallel resistive element RI that depends not only on the conductivity of medium F but also on the resistance of ICL 20.

[0037] This has the effect that the new resistance is the effective resistance created by the two current flows: the Media F ion flow and the ICL20 current flow. The parallel effective resistance is always lower than either input. This means that the present invention introduces an artificial saturation point to reduce the dynamic variability of the measured parameter. This creates a system that is accurate over a wider range.

[0038] Assume the fluid F resistance (RW) is in the 50K ohm range, but the sensor can only measure up to 6K ohms. To eliminate this problem, if we use a 50K ICL, we find that the measured resistance will only be 4.545K ohms. This is fine, we have chosen a water resistance outside the 50K ohm measurement range and brought that resistance into the measurement range. By using the correct value of IRL, we can ensure that the water resistance is always within the measurement range, even if it is well outside the measurement range.

[0039] Sensor size reduction Space is not a luxury in portable units. This is especially true for toroidal sensors, which must fit through small pipe holes of a given size. This means that keeping the geometry small is paramount to ensuring that toroidal sensors can be used in as many systems as possible. Measurement sensitivity depends on the strength or size of the receiving core, not the driving core. The driving core is only needed to generate voltage. It is necessary to ensure that some voltage can be driven across the driving core. To maintain a wide dynamic range, it is necessary to have a sufficiently large receiving coil, but the driving coil is less critical. If the inductor can generate voltage with a relatively low number of turns, the driving coil can be made smaller. This allows for a smaller ICS sensor. As can be seen particularly in Figure 2, the coils come in various sizes, with the driving coil 16 being much smaller—at least 75% smaller in volume—than the receiving coil 18. This creates an ideal scenario while reducing the size of the ICS. This means the receiving core can be made larger and still fit into the same volume. This increases the measurement sensitivity of the probe by increasing the size of the receiving core while maintaining or even reducing the overall size of the toroidal sensor.

[0040] Because the only thing that matters when driving the toroid is the voltage across the drive inductor coil, it is possible to use a small drive coil for measurements. This means that the geometric size of the toroid is not as important. A larger permeability reduces the current required to drive the coil, but does not reduce the sensitivity of the sensor. A smaller geometric size can be used as long as the drive voltage can be excited and the power consumption is within an acceptable range.

[0041] calibration It is often desirable to know if the electronics are within calibration and operating correctly. The presence of a variable resistance element (discussed above) in the ICL allows for a method of calibrating the ICS electronics. This can be done because the value of the variable resistance element on the ICL is known. This allows the ICS to have a reference, which is the calibration the ICS can have. With this reference, the electronics can be recalibrated after extended use. This allows the ICS to maintain its accuracy for a longer period of time. To calibrate the electronics, calibration is performed by eliminating the secondary ion flow through the medium. In this way, there is only current flow through the ICL. This allows the user to calibrate the electronics in the sensor circuit 22 to an internal value.

[0042] As a specific example, the ICL can be used to calibrate the sensor circuit 22 by removing the sensor 10 from the fluid or by knowing the conductivity of the fluid F. The output of the sensor circuit 22 should be, for example, 100 when the sensor 10 is not in the fluid. However, if the output instead reads 110, appropriate adjustments can be made to the sensor circuit 22, for example, by adding a calibration factor so that the sensor output reads 100.

[0043] analysis In the previous section, we saw how the total impedance depends on the cell inductance of the receiving core in relation to the conductance of the medium. We can define dimensionless constants to parameterize the relationship, and the parameterization variables we introduce are

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[0044] The condition defined in 3.29 can be expressed in terms of this dimensionless constant as follows:

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[0045] Revisiting the nonlinear factors,

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[0046] Now, we can see that the conductance of the medium decreases at some point, so the above condition is met. When the above condition is met, we are in the linear region, where the induced current is mainly affected by the impedance of the water.

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[0047] If we are not in the high resistance regime, we must use the full expression for the induced current.

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[0048] Equation 3.34 is the intermediate conductance

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[0049] The above formula allows the drive frequency to be calculated as the required receiving cell inductance,

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[0050] conclusion Using the methods described above, it is possible to create an ICS that can measure over a wide dynamic range. These methods allow the ICS to operate at low power, using primarily passive components to make measurements. The ICS can be made smaller by using a variety of core sizes. The use of ICLs minimizes dynamic fluctuations in the measurement parameters. An on-board reference resistor allows the ICS to maintain its accuracy for longer. Combining these methods allows for increased accuracy, lower power consumption, and a smaller geometry, while maintaining the wide measurement range expected of larger desktop units.

[0051] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the present disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment and, where applicable, may be interchangeable and used in selected embodiments even if not specifically shown or described. The same may be modified in many ways. Such variations should not be considered a departure from the present disclosure, and all such modifications are intended to be included within the scope of the present disclosure.

Claims

1. 1. A toroidal conductivity sensor comprising: a housing having an internal bore; a drive coil having an apertured core surrounded by a loop of a first wire; a receive coil having an apertured core surrounded by a loop of a second wire; the drive coil and the receive coil disposed within the housing and surrounding the bore; a sensor circuit connected to the second and third wires, the sensor circuit configured to detect the conductivity of a fluid flowing through the bore in the housing.

2. a variable resistor coupled in parallel between the receive coil and the sensor circuit and configured to vary the inductance of the receive coil to increase the range and linearity of the sensor circuit. The conductivity sensor according to claim 1 .

3. an inner conductive loop having a wire looped around both the drive coil and the receive coil, the inner conductive loop being disposed within the housing; The conductivity sensor according to claim 1 .

4. 4. The conductivity sensor of claim 3, wherein the drive coil has an annular core with an opening and the receive coil has an annular core with an opening, and the wire is looped through the openings in the drive coil and receive coil and is contained within the housing and is not exposed to the bore in the housing.

5. The conductivity sensor of claim 1 , wherein the size of the drive coil is smaller than the size of the receive coil.

6. The conductivity sensor of claim 5 , wherein the drive coil is at least 75% smaller by volume than the size of the receive coil.

7. The conductivity sensor of claim 3 , wherein the inner conductive loop includes a variable resistor configured to facilitate calibration of the sensor circuit.

8. 1. A method for measuring the conductivity of a fluid using a toroidal conductivity sensor, the sensor having an internal bore, a drive coil having an apertured core surrounded by a first loop of wire, and a receive coil having an apertured core surrounded by a second loop of wire, the drive coil and the receive coil being disposed within the housing and surrounding the bore, the sensor further including sensor circuitry for measuring the conductivity of the fluid, coupling a variable resistor between the receiving coil and a sensor circuit; and adjusting the variable resistor to change the inductance of the receive coil to increase the range and linearity of the sensor circuit.

9. When the fluid is within the range of 10 ohms, the variable resistor is set to approximately 100 ohms, resulting in an effective inductance of the receive coil of approximately 0.1 uH; When the fluid is within the range of 100 ohms, the variable resistor is set to approximately 1000 ohms, resulting in an effective inductance of the receive coil of approximately 2 uH. The method of claim 8.

10. providing a drive coil that is smaller than the receive coil; 10. The method of claim 9.

11. 1. A method for measuring the conductivity of a fluid using a toroidal conductivity sensor, the sensor having an internal bore, a drive coil having an apertured core surrounded by a first loop of wire, and a receive coil having an apertured core surrounded by a second loop of wire, the drive coil and receive coils disposed within the housing and surrounding the bore, the sensor further including sensor circuitry for measuring the conductivity of the fluid, The method includes providing an inner conductive loop having a wire looped around both the drive coil and the receive coil, the inner conductive loop being disposed within the housing.

12. 12. The method of claim 11, wherein the drive coil has an annular core with an opening and the receive coil has an annular core with an opening, and the wire is looped through the openings in the drive coil and receive coil and contained within the housing and is not exposed to the bore in the housing.

13. The method of claim 12 , wherein the inner conductive loop is used to calibrate the sensor circuit.

14. The method of claim 13 , wherein the inner conductive loop further includes a variable resistor that is adjusted to facilitate calibration.

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