WO1993014394A1

WO1993014394A1 - Conductivity sensor using energy conservation principle

Info

Publication number: WO1993014394A1
Application number: PCT/US1993/000680
Authority: WO
Inventors: Ivan P. Da Silva; Wayne D. Bennett, Iii
Original assignee: PALM INSTRUMENTS Inc
Current assignee: PALM INSTRUMENTS Inc
Priority date: 1992-01-21
Filing date: 1993-01-19
Publication date: 1993-07-22
Anticipated expiration: 1994-07-21

Abstract

A conductivity sensor (10) for measuring conductivity of a material (15) includes an inductive probe (13) having an inductor (21, 22) with an inductance L, a supply of current (35) for driving a current I through the inductor to establish a magnetic field having flux lines extending into the material, an energy monitor (40) for monitoring the energy stored in the magnetic field which is dissipated due to the conductivity of the material (15) and a processor (60) for providing a direct measurement of conductivity of the material (15) based upon an output signal from the energy monitor (40).

Description

TITLE: CONDUCTIVITY SENSOR USING

ENERGY CONSERVATION PRINCIPLE

Technical Field of the Invention The present invention relates, generally, to an inductive sensor for use in determining the conductivity of a material. More particularly, the present invention relates, as is indicated, to a conductivity sensor which uses energy conservation principles to determine directly the conductivity value of different materials such as liquids, fluids, gases, or solids.

Background of the Invention Conductivity sensors are known in the art. Typically, conductivity sensors are used in a manufacturing or other processing environment to measure the conductivity of a material. Based on the measured conductivity, one or more steps in the process can be carried out. Accordingly, it is important that the conductivity sensor provide accurate and repeatable results. Moreover, in many applications, it is important that the conductivity sensor provide accurate measurements even when used in aggressive, corrosive environments. For example, in processes such as alkaline cleaner processes and other industrial chemical processes, it is often necessary to measure directly the conductivity of a corrosive liquid, e.g., an acid, a corrosive solution, a strong alkaline solution, or some other chemical product. One specific application of a conductivity sensor in an aggressive environment is in the electrolytic processing of materials such as in electroplating and electropolishing. The conductivity sensor probe is placed into an electroprocessing bath filled with a corrosive liquid and is used to measure the conductivity of such liquid. The electroprocessing bath typically will include highly corrosive liquids such as caustics and strong acids. Therefore, it is important that the conductivity sensor probe and its ability to measure the conductivity of the liquid not be affected by the corrosiveness of the liquid.

One particular shortcoming of conductivity sensors in the past is that, over time, the conductivity sensor probe became eroded and/or permanently damaged by the corrosive liquid. As a result, the conductivity sensor would not provide an accurate measurement of the conductivity of the liquid. Thus, in the context of electrolytic processing of materials, for example, the erroneous conductivity measurements oftentimes would result in the system process producing a defective product.

Other drawbacks or shortcomings associated with conductivity sensors in the past relate to their high cost and excessive complexity. In addition, conductivity sensors oftentimes were not able to withstand ordinary wear associated with use such as in an electrolytic processing bath. An example of a known type of conductivity sensor used in an electrolytic processing environment involves a pair of toroidal type sensors which are used as probes. During operation, a voltage is applied across one probe and a portion of the liquid so as to produce a current through the liquid. The other probe is then used to detect the current flowing through the liquid. As a result, the equivalent resistance of the liquid could be determined.

A particular problem with the conductivity sensors which utilized the toroidal type sensors is that the sensor probes are expensive to manufacture, are difficult to install in proper relation, and are difficult to maintain in correct operation. The toroidal type sensor probes generally include a hollow center and a large projection portion which served as the sensor tip. The liquid or other material in which the sensor probes are placed would include solid particles or the like which would ultimately leave deposits in the center of the toroidal type sensors, thus affecting the measurement accuracy of the sensor. Furthermore, oftentimes the projecting sensor tip included on each probe would become damaged and sometimes break when dropped or when other parts in the processing bath, for example, would accidentally strike the tips.

An example of a known conductivity sensor is described in U.S. Patent No. 3, 152,303.

The '303 patent describes an electrodeless radio frequency conductivity probe for use with fluids. However, the conductivity probe does not provide a direct reading of conductivity.

Instead, the conductivity probe provides only a relative reading which must be compared to measurements taken in fluids of known conductivity.

In view of existing conductivity sensors, it will be appreciated that there is a strong need in the art for a conductivity sensor which can provide a direct measurement of conductivity and which can provide measurement reliability and repeatability even in aggressive environments. There is a strong need in the art for a conductivity sensor which can withstand the effects of corrosive liquids such as caustics, strong acids and other aggressive chemicals. Furthermore, there is a strong need for a conductivity sensor which is economical to manufacture and is simple to install and maintain, regardless of the specific application. There also is a strong need for a conductivity sensor which is durable and resists damage under typical operating conditions.

The present invention overcomes the aforementioned shortcomings of known conductivity sensors. The present invention is summarized and described in detail below. Summary of the Invention

The present invention relates to a conductivity sensor which utilizes the principle of energy conservation to determine directly the conductivity of an material such as a liquid, fluid, or solid. The sensor includes an inductive probe which is positioned in or adjacent the material being measured. A known amount of energy is pumped into the inductive probe such that the energy is stored in the form of a magnetic field established by the inductive probe. The flux lines of the magnetic field extend into the material itself and, as a result, a certain amount of the stored energy is dissipated within the material based on the conductance of the material. Such energy dissipation or loss is monitored using the inductive probe, and, based on such energy loss, the conductivity sensor quantitatively determines the conductivity of the material. The conductivity sensor also includes temperature compensation circuitry which permits the conductivity measurements to be normalized with respect to temperature.

According to one aspect of the present invention, a conductivity sensor for measuring conductivity of a material comprises an inductive probe having an excitation inductor with an inductance L; current means for driving a current I through the excitation inductor, the inductor being positioned in physical proximity to said material such that energy is stored in a magnetic field produced by the excitation inductor as a result of such current, and wherein the flux lines extend into said material; energy monitoring means for monitoring the portion of the energy which is dissipated in the material due to the conductivity of the material, and means for providing a direct measurement of conductivity of said material based upon a signal produced by the energy monitoring means.

Brief Description of the Drawings In the annexed drawings:

Fig. 1 is a block diagram of a conductivity sensor used in accordance with the present invention; Fig. 2 is a circuit diagram showing a simplified equivalent circuit for the inductive probe portion of the conductivity sensor of Fig. 1 as the probe is coupled magnetically to the material under test in accordance with the present invention; Fig. 3 is an exemplary timing diagram for the conductivity sensor of Fig. 1 in accordance with one embodiment of the present invention;

Fig. 4 is a detailed schematic diagram of the conductivity sensor of Fig. 1 in accordance with one embodiment of the present invention; Fig. 5A is a block diagram representing an embodiment of the energy monitoring circuit in the conductivity sensor which utilizes a multiplier in accordance with the present invention;

Fig. 5B is a block diagram representing an embodiment of the energy monitoring circuit without temperature compensation in accordance with the present invention; Fig. 5C is a block diagram of an embodiment of the energy monitoring circuit without a multiplier and without temperature compensation in accordance with the present invention;

Fig. 6 is a plan view in partial cross-section of the inductive probe of Fig. 1 in accordance with one embodiment of the present invention;

Fig. 7 is a detailed schematic diagram of the temperature compensation portion of the support electronics of Fig. 1 in accordance with one embodiment of the present invention; and Fig. 8 is a schematic diagram of a different embodiment of the inductive probe in accordance with the present invention.

Detailed Description of the Preferred Embodiment

Referring now in detail to the drawings, wherein like reference numerals designate like parts in the several figures, and initially to Fig. 1, a conductivity sensor in accordance with the present invention is generally designated 10. The conductivity sensor 10 includes an inductive probe 13 and a sensor circuit 14. The inductive probe 13 is positioned preferably in, or at least adjacent to, the material 15 whose conductivity is to be measured. More particularly, the probe 13 is positioned in or adjacent to the material 15 such that when the inductive probe 13 is pumped with a current I by the sensor circuit 14, the flux lines of the magnetic field which stores the energy delivered to the inductive probe 13 cuts a path through or along the material 15.

The material 15 can be a liquid, fluid, emulsion, solid, etc., or any combination thereof. The conductivity sensor 10 measures the conductivity of the material 15 based on the amount of energy dissipated in the material 15, regardless of whether the material is a liquid, fluid, etc. Therefore, although the conductivity sensor 10 of the present invention will be described herein as it is used to measure the conductivity of a liquid, it will be appreciated that the conductivity sensor 10 can be used to measure the conductivity of other materials without departing from the scope of the invention.

In the exemplary embodiment of Fig. 1, the conductivity sensor 10 is shown as it is used to measure the conductivity of a liquid 17 in a bath 19. Such bath 19 can be, for example, an electrolytic processing bath filled with a corrosive liquid 17. The corrosive liquid 17 can consist of one or more caustics or strong acids, for example. The conductivity for such a bath typically will be on the order of 0.5 to 2 mhos. Alternatively, the bath 19 may consist of one or more other types of liquid used in different manufacturing processes.

In the preferred embodiment, the inductive probe 13 is completely immersed in the liquid 17. The inductive probe 13 includes an excitation inductor 21, having a known inductance L, and a sensor inductor 22, preferably with a known inductance L also. As will be further described herein, the excitation inductor 21 is used in the conductivity sensor 10 to store energy in a magnetic field such that the energy is inductively coupled to the liquid 17 surrounding or adjacent the inductive probe 13. At the same time, the sensor inductor 22 is used by the sensor circuit 14 to monitor the energy as it is dissipated in the liquid 17.

Preferably, the inductive probe 13 is positioned in the liquid 17 (or other material under test) such that the magnetic field which forms about the inductive probe 13 will exist substantially entirely in the liquid 17. It is preferable that the probe 13 be positioned away from any foreign objects or the like which may present a discontinuity or otherwise detrimentally distort or alter the magnetic flux in the field. Various spacers or the like can be used to position the probe in the bath 19. For example, a support arm 22' can be used to secure the probe 13 away from the walls of the bath 19 such that the walls will not affect the magnetic field created about the inductive probe 13 in the event the walls are made of a material which can interfere with the magnetic field, for example, stainless steel. Describing briefly the inductive probe 13, included are two pairs of terminals 23a,

23b, and 24a, 24b which are used to couple electrically the inductors 21 and 22, respectively, to the sensor circuit 14. Preferably, the inductors 21 and 22 are enclosed within a chemical resistant housing 25 which protects the inductors from corrosive materials. Also, the housing 25 is made of a material such as most plastics which do not prevent the magnetic flux lines from extending into the material under test. The housing 25 is described in detail below.

The inductors 21 and 22 preferably each include a known equal number of turns of wire wound about a rod shaped core as is described below with respect to Fig. 6. However, it will be appreciated by those having ordinary skill in the art that one or both of the inductors 21 and 22 may have a different core, for example, an air core, a toroid-shaped core, etc., and may include a different number of turns. In addition, some other type conductive material, such as conductive ribbon, can be used in place of the wire to form the inductors without departing from the scope of the invention. Furthermore, it is not necessary that the excitation inductor 21 or the sensor inductor 22 actually be inductive coils. Virtually any type of inductive device may be utilized within the inductive probe 13, provided that the inductive probe 13 is capable of producing and detecting a magnetic field which is magnetically coupled to the liquid 17 or other material under test as is described in detail below. While the inductive probe 13 is described herein primarily as including a separate excitation inductor 21 and sensor inductor 22, each with its own separate pair of terminals, it will be appreciated that other configurations for the inductors are possible. For example, the excitation inductor 21 and the sensor inductor 22 may be the same inductor with only a single pair of terminals. In such case, the single inductor is multiplexed between serving as the excitation inductor and the sensor inductor. In addition, while the sensor 10 is represented in Fig. 1 with the sensor circuit 14 located primarily external to the probe housing 25 and located outside of the bath 19, it will be appreciated that all or part of the sensor circuit 14 can be included within the probe housing 25.

Li the preferred embodiment, the terminals 23a and 23b of the excitation inductor 21 are connected to output lines 30 and 31, respectively, of the current driver 35 which is included in the sensor circuit 14. The current driver 35 serves to periodically pump a known current I through the excitation inductor 21. Preferably the current I is a direct current, although an alternating current can be utilized without departing from the scope of the invention. The energy provided by such current I will be stored by the excitation inductor 21 in the form of a magnetic field as is known. The amount of such stored energy, B _2l, can be represented by the equation:

E^_! = (1/2) L21*? where:

Em (j°^ul^es)

L21 is the known inductance value of inductor 21 (henry) I is the current pumped through the inductor (amps)

As the excitation inductor 21 is pumped periodically with the current I, of the magnetic field storing the energy E^- will be magnetically coupled to the surrounding liquid 17 as a result of the magnetic flux lines which form around the probe 13. It will be appreciated that there will be a quantifiable amount of energy dissipated within the liquid 17 as a result of the magnetic coupling. As is explained in detail below, the actual amount of energy dissipated is a function of the conductivity of the liquid 17 (or other material whose conductivity is being measured). The sensor inductor 22 is used to monitor the energy established and dissipated in the liquid 17 such that this energy can be monitored across the terminals 24a and 24b of the sensor inductor 22 using basic transformer principles. Based on such energy loss detection, the conductivity sensor 10 is able to determine quantitatively the conductivity of the liquid 17 as described below. In the preferred embodiment, the current driver 35 periodically pumps at a relatively high frequency the excitation inductor 21 with the known current I. Such high frequency, i.e., in the range of twenty kilohertz to one-hundred kilohertz, reduces the physical size requirements for the inductors 21 and 22 as will be appreciated. As a result, the physical dimensions of the inductive probe 13 can remain small as is generally preferred. However, it will be appreciated that other pumping frequencies can be employed without departing from the scope of the invention.

As is illustrated in Fig. 1, the sensor circuit 14 includes an energy monitoring circuit 40 having an input 41 which is connected by lines 42 and 43 to terminals 24a and 24b of the sensor inductor 22 in the inductive probe 13. In the preferred embodiment, the input 41 is a high impedance input which allows the monitoring circuit 40 to detect the voltage induced across the terminals 24a and 24b without any detrimental loading effects.

As the excitation inductor 21 in the inductive probe 13 is periodically or otherwise controllably pumped with the current I, the magnetic field which is coupled from the excitation inductor 21 induces a voltage in the liquid 17 which, in turn, is inductively coupled across the inductor 22 and is monitored by the energy monitoring circuit 40 across lines 42 and 43. The lines 42 and 43, specifically, are connected to the input 45 of an envelope detector 46 in the energy monitoring circuit 40. The envelope detector 46 detects the voltage envelope from the high frequency signal present across the inductor 22 as is discussed below with respect to Fig. 3. The output of the envelope detector 46 is connected to the input of a temperature compensation circuit 47 by way of line 48. The temperature compensation circuit 47 includes a comparator circuit 49 (described in detail with respect to Fig. 4) which compares the voltage of the signal received on line 48 to a preselected reference voltage V_ref (Fig. 4). As a result of such comparison, the comparator circuit 49 provides a square-wave output signal on line 51 to a low pass filter 53. The low pass filter 53 is used to integrate the output signal on line 51. The output on line 54 from the low pass filter 53 represents the output voltage V_ptdix which is provided to the support electronics 60 where the signal V_probe is processed in the manner presented below in order to provide a direct reading of the conductivity of the liquid 17.

The support electronics 60 typically will include a microprocessor or other computing device which will calculate the conductivity of the liquid 17 based on the simplified equations presented below. As a result, the conductivity of the liquid can be shown on a display and/or used to control a process as represented by block 61. For example, the support electronics 60 can function as an interface to enable connection of the sensor 10 to existing process equipment that is compatible with other types of conductivity sensors. The support electronics 60 can include, for example, circuitry that is able to process data from the conductivity sensor 10 and, based on such data, provide control information to different external devices such as pumps, computers, displays, etc., which control or facilitate operation of the overall process. The sensor circuit 14 also includes a temperature controlled power supply 62 which, along with a temperature sensor 64 included in the inductive probe 13, allows for the conductivity measurements to be normalized with respect to the temperature of the liquid. As is known, the conductivity of the material under test will change with a change in temperature. The temperature sensor 64 is used to provide a signal on line 65 to the temperature controlled power supply 62 which represents the temperature of the liquid 17. The signal on line 65 is coupled to the temperature controlled power supply 62 which is shown as being separate from the support electronics 60, although in a different embodiment the temperature controlled power supply can be included in the support electronics 60.

Based upon the magnitude of the signal on line 65, the temperature controlled power supply 62 provides feedback compensation to the comparator circuit 49 via line 70 which enables the conductivity measurement to be norrήalized as is described in more detail below with respect to Fig. 4. Such approach for providing temperature compensation guarantees high reliability of the conductivity sensor 10 and the sensor 10 can be easily adapted for use in place of or in combination with previously known conductivity probes. Different circuitry may be used for temperature compensation without departing from the scope of the invention. The temperature sensor 64 can comprise a semiconductor-type temperature sensor or any other type of temperature sensor as will be appreciated. The temperature sensor 64 preferably is enclosed within the inductive probe housing 25, although different embodiments of the invention can include the temperature sensor 64 located at another location in the bath 19.

In addition to providing temperature feedback compensation on line 70' to the comparator 49, the temperature controlled power supply 62 is also used to provide a supply voltage along line 69 to the input of DC power supply 70. The DC power supply 70 includes a voltage regulator which regulates the supply voltage from line 69 and provides a stable voltage supply V_dd at its output on line 71. The output voltage V_dd on line 71 in turn, serves as the power supply to the trigger circuit 72. The trigger circuit 72 is a pulse generator which provides a control signal on line 73 which causes the current driver 35 to periodically pump a current I through the excitation inductor 21 via lines 30 and 31.

To facilitate understanding of the operation of the conductivity sensor 10, reference is made to Figs. 2 and 3. In Fig. 2, a circuit 100 is shown which represents the simplified electrical circuit equivalent of the inductive probe 13 immersed within the liquid 17, as is shown in Fig. 1. The inductive probe 13 itself has an equivalent circuit 101 which includes the excitation inductor 21 having a known number of turns N_p (preferably fifteen or more turns) and a finite resistance represented by a resistor 102. Such resistance can represent losses in the probe 13 due to, for example, line resistance, hysteresis loss, skin effect, LC losses, etc. A shunt capacitor 104 which is provided at the output of the current driver 35 is coupled in parallel across the inductor 21 and the resistor 102 to form a tank circuit 105 into which the current I from the current driver 35 is pumped. Using the capacitor 104 in parallel with the excitation inductor 21 to form a tank circuit 105 allows the use of a physically smaller excitation inductor 21 in the probe 13 and, therefore, the use of a physically smaller inductive probe 13. At a given pumping frequency, the resultant oscillation in the tank circuit 105 will increase the frequency of the current applied through the inductor. However, the conductivity sensor 10 will function in substantially the same manner described herein, without the shunt capacitor 105 and with only the excitation inductor 21 and, both embodiments are within the contemplated scope of the invention.

The circuit 101 also includes an equivalent diode 106 which represents a high input impedance which is seen looking into the current driver 35 from the tank circuit 105. Furthermore, the equivalent circuit 101 includes the sensor inductor 22, as is shown, which has a known number of turns N which, preferably, is equal to N_p.

The liquid 17 has an equivalent circuit 107 which comprises an equivalent inductor 108 joined in parallel with an equivalent resistor 109. Resistor 109 represents the resistance of the liquid 17 or, alternatively, the inverse of the conductivity of the liquid 17 as is known. The inductor 21 serves as a primary winding which is inductively coupled according to conventional transformer theory to the secondary windings presented by the inductors 22 and 108 as is illustrated. Those having ordinary skill in the art will recognize that the equivalent circuit 100 is similar in design and operation to that of a flyback transformer such as those transformers that are used in switching power supplies. During operation, the excitation inductor 21 is periodically pumped with a current I such as that shown by waveform 150 in Fig. 3. The voltage which is generated across the excitation inductor 21 as a result of the current I correspondingly induces a secondary voltage across the inductor 108 according to basic transformer principles. In turn, the voltage across the inductor 108 will be reflected back to the sensor inductor 22 according to the same known transformer principles. The magnetic coupling between the inductors 21, 22 and the equivalent inductor 108 is provided as a result of the inductive probe 13 and the liquid 17 or other material under test being in close physical proximity such that the magnetic fields formed by the equivalent inductors are magnetically or inductively coupled. In the preferred embodiment, the liquid 17 completely surrounds the inductors 21 and

22 within the probe 13, and it will be appreciated that the equivalent inductor 108 will have an equivalent number of turns N_m, equal to one. In another embodiment, the number of turns in the equivalent inductor 108 can be different and can be determined empirically or by other means, as will be appreciated. As the current driver 35 periodically pumps the tank circuit 105 with a known amount of energy E^, a current 1^ will be induced in the equivalent inductor 108 as represented in Fig. 2. As a result, energy will be dissipated by the equivalent resistor 109, representing the resistance of the liquid, in the form of .PR losses. The resultant voltage which is reflected back across the sensor inductor 22 will be representative of the energy dissipated in the liquid 17 due to the equivalent resistance of resistor 109.

The conductivity sensor in the preferred embodiment monitors the voltage V_U2 across the sensor inductor 22 during the time T^ (Fig. 3) when the current I is not applied to the tank circuit 105. During such time, the voltage V^ across lines 42 and 43 will decay exponentially from its peak value V,^ (Fig. 3) as a result of the energy dissipated due to the conductivity of the liquid. The conductivity sensor 10 processes the signal received across lines 42 and 43 based on such peak voltage V_peΛ and/or the voltage decay and ultimately provides a signal V_probe on line 54 which can be used to represent directly the conductivity of the liquid 17 as described below. Fundamentally, it will be appreciated that as the conductivity of the liquid 17 increases, for example, the value of the equivalent resistor 109 decreases and the voltage V-^_p^ will decrease as the conductivity increases. Thus, the energy stored in L^ has no other way to dissipate during T_OFF except through the circuit tank loss R102 and the liquid conductance coupled to !--_>,• by L₁₀₈, where the energy is:

E _2I = E_Rιo₂ + Ejurø and E-f = _Vi,

R where R, in this case, represents the equivalent parallel resistance between R102 and R109, and since energy E^ is applied equally during all pulses (f_p) and equivalent R as defined above, the only way to keep the equation true is by changing voltage V.

Referring specifically to Fig. 3, an exemplary timing diagram is shown illustrating the relationship between the current I waveform 150, which would appear on line 31 of the conductivity sensor 10, the voltage waveform 151, appearing across lines 30 and 31 representing the voltage V-^- across the inductor 21, and the corresponding waveform 152, appearing across lines 42 and 43 representing the voltage W_l_₇₂ which is induced across the sensor inductor 22. The waveform 152 is illustrative of the voltages which are induced across each of the inductors 22 and 108. The voltage waveform 151, on the other hand, is the same as the waveform 152 with the exception that it is inverted in phase as determined by the polarity of the respective coils in the equivalent circuit of Fig. 2. The amplitude of the respective waveforms may differ depending on the actual amount of energy dissipation due to the liquid resistance and the respective known transformer ratios, although in the preferred embodiment the number of turns is the same in both inductors 21 and 22. As a result, the turn ratios N_p/N,,, and N^N,-. preferably will be equal.

As shown by the waveform 150, the inductor 21 and capacitor 104 are pumped with current I at a frequency f_p having a preselected duty cycle as determined by the on time T_∞ and the off time T_off. The energy E^ stored in the magnetic field of the inductor 21 will build up during time T^,, where time T_∞ preferably is a long enough time to allow the current I through the tank circuit 105 to begin flowing entirely through the inductor 21. At such time, the voltage across the inductor 21 will go to zero as represented in segment 153 in the waveform 151. Segment 154 in the waveform 151 generally illustrates the voltage generated across the inductor 21 during such time T_^. as the voltage increases and then decreases as the tank circuit 105 is energized during each cycle.

As the current I is interrupted in each cycle, at time tl for example, the tank circuit 105 will be "released" in the sense that the tank circuit 105 is permitted to oscillate freely at a frequency f^. Initially, it is noted that the sudden interruption in current I will cause a voltage spike 155 and 155' to appear in the respective voltages V_Uι and V_I2i, for example. Thereafter, during time T_off the tank circuit 105 will be permitted to oscillate freely as is illustrated in segment 156 of the waveform 152 for example. The actual oscillation frequency f_tui_t will be a function of the preselected values of the inductor 21 and the capacitor 104, as will be appreciated.

The signal represented by waveform 152 is provided to the input of the envelope detector 46 as described above. The envelope detector 46 includes a rectifying input which eliminates the lower "negative" portion of the waveform 152. Therefore, the envelope detector will analyze and produce the envelope of only the upper "positive" portion of the waveform 152 producing an output signal V_mv on line 48 illustrated by waveform 160. As a result, the envelope detector 46 will ignore segment 154' of the waveform 152 which reflects the voltage across the inductor 22 as the excitation inductor 21 is energized each cycle during T_∞. While some energy will be dissipated in the liquid 17 during T_OT, it is not necessary to monitor the energy dissipation during such time in order to determine the conductivity of the liquid 17.

Each cycle of the waveform 160 has the same initial peak voltage V_p^ as the voltage V_L22 shown in waveform 152. While the theoretical peak voltage in each cycle of the waveform 152 may occur within the voltage spike 155' , it has been found that the first upward peak of the waveform at time tl⁺, for example, closely approximates the peak voltage across the inductor 22 so long as f____. is several times greater than f_p.

From the initial value V_vc___, each cycle in the waveform 160 will reflect the exponential decay of the voltage measured across the inductor 22 as a result of the energy dissipated in the resistance of the liquid. Those having ordinary skill in the art will appreciate that the time constant for such exponential decay is given by R109//R102/L21, where R109 is the resistance of the liquid 17, R102 is the equivalent loss in the tank circuit, and L21 is the inductance of the inductor 21. Preferably, the pumping frequency f_p is selected to be low enough so that the waveform 160 in each cycle will decay to zero prior to the next pumping cycle as is shown in Fig. 3. The time constant is based on the inductance value L21, the core material, the number of turns in the inductor 21, etc., being preselected according to basic transformer principles such that the inductance of the equivalent inductor 108 and resistor 102 in the equivalent circuit 100 will have a minimal effect on total inductance and resistance seen by the tank circuit 105. Therefore, the known value of the inductor 21 and the value of the resistor 109 are the dominant factors in determining the time constant for the exponential decay as will be appreciated.

Thus, it will be apparent that the peak voltage V_peik and rate the of decay of the output signal from the envelope detector 46 will be related to the resistance and conductivity of the liquid 17. The present invention uses different processing techniques in different embodiments to achieve a direct reading of conductivity based on the output of the envelope detector 46 as described below.

In the embodiment of Fig. 1, wherein the conductivity sensor 10 includes a temperature compensation circuit 49, the output signal V_w is compared in the comparator 49 to a reference voltage V_ref as is shown relative to the waveform 160 in Fig. 3. When the waveform signal 160 exceeds the reference voltage, the output of the comparator 49 on line 51 goes to logic "high" forming a pulse 163 as is illustrated in the waveform 162 in Fig. 3.

In the event the waveform signal 160 goes below the reference voltage V_ref, the output of the comparator 49 goes to a logic "low" level. The reference voltage V_ref is preselected based on the expected maximum conductance to be measured by the sensor 10 as will be appreciated.

The width of each pulse 163 is a function of the voltage V,^ and of the time constant for the exponential decay, as is the resistance of the liquid 17 as is described above.

Therefore, as the resistance 109 of the liquid increases, the width of each pulse 163 increases, and vice versa. In order to provide temperature compensation, the logic "high" level V_coot of each pulse 163 is controlled as a function of temperature. As a result, the output of the

^• comparator 49 can be adjusted to normalize the conductivity measurements as a function of temperature as described in more detail below. The pulsed output signal 162 from the comparator 49 is integrated by the low pass filter 53 to provide the DC signal V_p-^ on line 54 (waveform 164 in Fig.3) which can be shown to be related to the resistivity, or inversely, the conductivity of the liquid 17. i view of the analysis presented above and using conventional circuit analysis techniques, it has been determined and verified through experimentation that the output voltage V_profae from the sensor 10 shown in Fig. 1, will be directly linearly related to the conductivity of the liquid 17 at least over a limited range on the order of one decade in resistance. However, with additional processing of the signal on line 54 by the support electronics, a direct linear measurement of the conductivity of the liquid over a broad range, i.e., on the order of ten decades, can be accomplished. Specifically, with respect to the embodiment shown in Fig. 1, it has been determined using conventional circuit analysis techniques that a direct value of the conductivity of the liquid 17 can be found from the following equations:

R109 V_PROBE2 V₀ PR_OBE

conductivity = 1/R109 (Siemens), where for a given I

V_pπ-_be is the voltage measured on line 54,

No _robe. Ks ^and Ko are parameters preferably provided to the user of the sensor 10 from the manufacturer and relate as follows:

Vo_probe is the voltage measured on line 54 when the sensor 10 is used to measure a material having infinite resistance, which may be approximated by the value measured on line 54 when the inductive probe 13 is located in air;

K_s = (J8_p/NJ.²*^lA*I_ xH²*f_p*K_v which is a constant over a given temperature range, for example 0 degrees C to 100 degrees C, using a ferrite core in the inductive probe: where K., is an inductive probe 13 constant that provides the resistance value R109 of the material under test in units of ohms given in a cross section area of 1 square centimeter and a length of 1 centimeter; KQ is a constant to compensate for attenuation presented by the temperature compensation circuit 47. The support electronics 60 can be programmed using conventional techniques to perform the above calculation for R109 and to invert the value R109 to determine directly the conductivity value of the liquid 17. The result can be utilized in the display and/or controller 61. Referring now to Fig. 4, shown is a detailed schematic of the conductivity sensor 10 of Fig. 1. The temperature controlled power supply 62 (described in detail with respect to Fig. 7) provides a DC output voltage V_coot on lines 69 and 70'. The magnitude of V_cont is a function of the temperature of the liquid 17 as determined from the signal on line 65 from the temperature sensor 64. The output voltage V_coot on line 70' serves as the supply voltage to the comparator 49 in which it is used to compensate for changes in temperature of the liquid 17 so that the conductivity measurements are normalized with respect to temperature.

The voltage V_c00t on output line 69 is supplied to the input of the DC power supply 70 as is shown. The DC power supply 70 includes a series resistor 200 having one of its leads connected to line 69. The other lead is connected to the leads of a shunt capacitor 202 and zener diode 203 connected in parallel between the resistor 200 and a first common ground COM1. The zener diode 203 is reversed biased as is shown and, together with the capacitor 202 and the resistor 200, provides a regulated DC supply voltage V_dd at node 204. The supply voltage V^ is used to provide a regulated DC supply voltage to the other components throughout the sensor circuit 10 as is illustrated. In a different embodiment, some other means for providing a regulated DC supply voltage can be used to power the sensor 10 without departing from the scope of the invention.

The regulated supply voltage V_dd at node 204 is coupled to the trigger circuit 72 input by way of line 71. The trigger circuit 72 provides a series of pulses to the current driver 35 as mentioned above in order to periodically pump the current I into the inductive probe 13. In the preferred embodiment, the trigger circuit 72 includes a stable multivibrator 206, such as a LM555 timer IC manufactured by National Semiconductor, to generate the series of pulses as is conventional. The supply voltage V_dd on line 71 is used to provide the supply voltage to the on-shot 206. Capacitors 207, 208 and resistors 209,210,211 are preselected to provide the desired output frequency f_p and duty cycle as is conventional. In the preferred embodiment, the V_dd output signal on line 73 from the trigger circuit

72 toggles between zero and V_dd=5 volts. The frequency f_p of the signal preferably is in the range of twenty kilohertz to one-hundred kilohertz so as to reduce the physical size requirements of the inductive probe 13 as described above. The duty cycle of the signal should be large enough to allow the full current I to pass through the excitation inductor 21 during each cycle, yet preferably small enough to allow the voltage across the sensor inductor 22 to decay to zero during each cycle after the current I is interrupted. Nonetheless, it will be apparent that other frequencies, and/or duty cycles can be used without departing from the scope of the invention.

The current driver 35 in the conductivity sensor 10 includes apnp-typedrive transistor 220 which has its emitter connected to the V_M voltage supply and has its collector connected through the series resistor 221 and output capacitor 104 to the common ground COM1. The base of the transistor 220 is connected to line 73 by way of a filtering capacitor 223 in parallel with a resistor 224. Therefore, when the signal on line 73 is at a logic "low" level, i.e. zero volts, the base of the transistor 220 will be forward biased and the transistor 220 will turn on allowing current I to flow from the V^ voltage supply through the drive output on line 31. The drive output on line 31 is then coupled to the inductive probe 13 by way of connector 230 which is used to connect the several lines from the sensor circuit 14 to the inductive probe 13. The value of the resistor 221 and V^, will determine the maximum magnitude of the current I which is pumped into the tank circuit 105 (Fig. 2) as will be appreciated.

When the signal on line 73 is at a logic "high" level, i.e. +5 volts, the base of the transistor 220 will be reversed biased, therefore turning off the transistor 220. As a result, the current I delivered on line 31 will be interrupted. In such manner, the present invention is capable of controllably pumping the inductive probe 13 with a known current I as represented by waveform 150 in Fig. 3. However, many other techniques for pumping the inductive probe 13 may be used as will be appreciated and are well within the intended scope of the present invention. As described above, the periodic pumping of the inductive probe 13 with a current I results in an induced voltage across the sensor inductor 22 such as that illustrated by waveform 152 in Fig. 3. Such signal as detected across lines 42 and 43 is coupled to the input 45 of the envelope detector 46. The envelope detector 46 in the exemplary embodiment includes a diode 239 which is used to rectify the signal on line 43 so as to eliminate the lower negative portion of the waveform 152 as described above with respect to Fig. 3. Preferably, the diode 239 offers relatively fast time-recover performance, i.e. , 5-10 nanoseconds, in order to handle the high frequency signal from the sensor inductor 22. A suitable diode would be a 1N4148 manufactured by National Semiconductor.

The anode of the diode 239 is connected directly to line 43 and the cathode is connected to a shunt capacitor 240 which filters out the high frequency components of the signal on line 43 and forms the envelope represented in waveform 160 in Fig. 3. The envelope detector 46 also includes voltage limiting resistors 241 and 242 which are coupled in series across the capacitor 240 and which establish the maximum voltage of the output signal from the envelope detector 46 along line 48. Resistor 241 is preferably a potentiometer presenting its maximum resistance in series between the resistor 242 and COM2. The center top of the potentiometer is coupled to the output line 48 such that by adjusting the potentiometer 241, the attenuation presented by the envelope detector 46 can be controlled. In accordance with the present invention, the conductivity sensor 10 can be calibrated by adjusting the potentiometer 241 to account for any losses within the sensor 10 itself. For example, losses due to hysteresis, line resistance, capacitive losses, higher order harmonics, component tolerances, etc. , can be compensated for by an initial calibration performed by the manufacturer or the user. After the sensor 10 has been calibrated by the manufacturer, for example, the sensor can be used to measure the conductivity of any material without the need to recalibrate.

The comparator 49 includes a voltage comparator 250 such as an LM311 available from National Semiconductor. The voltage comparator 250 is used to compare the output from the envelope detector 46 to the reference voltage V_ref as mentioned above. Specifically, the output signal from the envelope detector 46 is connected to the inverting input of the comparator 250 and, the reference voltage V_ref is coupled to the non-inverting input. A voltage divider consisting of resistors 253 and 254 connected in series between the supply voltage V,-,, and COM2 provides the reference voltage at node 252. The comparator 49 also includes a resistor 260 coupled between the comparator output and node 252 to provide a small hysteresis in the voltage comparator 250. A pull-up resistor 261 is coupled between the comparator output and the comparator supply voltage V_cont.

Thus, when the signal on line 48 exceeds the reference voltage V_ref, the output of the voltage comparator 250 at line 51 will be pulled "high" to the level of V_coot as determined by the temperature controlled power supply 62. When the signal on line 48 drops below the reference voltage V_ref, the output of the voltage comparator 250 will go "low", i.e., to zero volts, as is illustrated in the waveform 162 in Fig. 3.

The output signal on line 51 from the comparator 48 is coupled to the input of the low pass filter 53 as is shown in Fig.4. The low pass filter 53 in the exemplary embodiment consists of an RC circuit having a resistor 270 with one lead connected to line 51 and the other lead connected to the parallel shunt combination of resistor 271 and capacitor 272 at node 273. The output of the low pass filter 53 is then provided on line 54 to the support electronics 60 by way of connector 280 for further processing if desired.

It will be appreciated that by varying the supply voltage V_c00l to the voltage comparator 250 as a function of the temperature of the liquid, a DC offset will be produced at the output of the low pass filter 53. Therefore, the value of V_probe on line 54 can be normalized with respect to temperature. Furthermore, it is noted that the conductivity sensor 10 as shown in Fig. 4 includes two common grounds COM1 and COM2 for the purpose of improving the signal-to-noise performance of the sensor by reducing the effects of ground loops. However, in another embodiment, a single common ground can be utilized without departing from the scope of the invention.

Figs. 5A, 5B, and 5C each relate to a different embodiment of the energy monitoring circuit 40 for the sensor circuit 10 shown in Fig. 1, designated 40', 40" and 40'", respectively. i Fig. 5A, the signal across lines 42 and 43 from the sensor inductors 22 is input to the envelope detector 46 in the same manner described above. The output of die envelope detector is coupled to a conventional peak detector 285 which detects the peak of the envelope V-^ illustrated in waveform 160 in Fig. 3. The peak detector 285 produces at its output a DC voltage equal to -,^ which is coupled to a conventional multiplier circuit 287 which squares the value of V^ to produce an unnormalized output signal V_peak ² on line 290. The output signal on line 290 is coupled from the multiplier 287 to the temperature compensation circuit 289 which, based on the signal V_coot received on line 70', provides an appropriate scaling factor to the signal on line 290. Such scaling factor can be implemented witii a simple adjustable gain amplifier as will be appreciated. As a result, a normalized output V^,_^ is provided to V,-^. Li order to calculate a direct value of conductivity based on the signal V_probe on line

54', the support electronics 60 is programmed to implement the following simplified equation which has been determined to provide a direct, accurate reading of conductivity of the material 15 even over a broad range of conductivity values such as that mentioned above: _ _ = .Ks lKo - JCs LKo.

R109 V_pROBE V₀ p_Ro_BE

conductivity = 1/R109 where V^,^ = the value of the signal V_probe on line 54'

Vo_probe is the voltage measured on line 54' when the sensor 10 is used to measure a material having infinite resistance, which may be approximated by the value measured on line 54' when the inductive probe 13 is located in air;

K_s =

as described above. Ko is the same as described above. The energy monitoring circuit 40" shown in Fig. 5B is identical to that shown in Fig. 5A with the exception that no temperature compensation is provided. Otherwise, the signal Vpi-ot_* is processed in the same manner by the support electronics as described with respect to the circuit in Fig. 4 and where K„ = 1. The result is a non-temperature normalized direct conductivity reading which is accurate over a broad range of values. Such conductivity reading can be displayed by display 61 as desired.

Briefly referring to Fig. 5C, another embodiment of the energy monitoring circuit 40'" is shown. The monitoring circuit 40'" operates in the same manner as these embodiments shown in Figs. 5A and 5B, except that the output V,^ from the peak detector 285 serves as the signal V_probe on line 54'". As in the other embodiments, V_probe is coupled to the support electronics 60 for further processing if desired to give a direct reading of conductivity. It has been determined that using the energy monitoring circuit 40'", the conductivity of the material can be defined, again over a broad range, by the following simplified equation:

— L_ ⁼ Ks — ^— Ks

R109 V_PROBE V_{0 PR}O_BE 2

conductivity = 1/R109 where V^.,*,. = the value of the signal V_probe on line 54'" Vo _rob i the voltage measured on line 54"' when the sensor 10 is used to measure a material having infinite resistance, which may be approximated by the value measured on line 54"' when the inductive probe 13 is located in air; _s = ( _p/N ²*^*!^^!²*^*^ as described above.

Therefore, it is shown that there are several ways for using the present invention to provide a direct measure or value of conductivity over a broad range. Other methods will become apparent based on the disclosure herein and are within die intended scope of die present invention. Turning now to Fig. 6, shown is an exemplary embodiment of the physical construction of the inductive probe 13. As was mentioned above, d e inductive probe 13 includes inductors 21 and 22, each having a known number of turns of wire 300 wound around a rod shaped core 302. Bifilar wire can be used to form the inductors 21 and 22 around d e core if desired. The inductors 21 and 22 along with the temperature sensor 64 are enclosed within the sealed housing 25 such as chemically resistant plastic pipe or the like. A sealed, chemically resistant sheatii 304 prevents the corrosive liquid 17 from damaging the leads between the inductive probe 13 and tfiose components of the sensor circuit 14 which are positioned outside die housing 25 and preferably are maintained outside of the bath 19. The lines at the end of the sheath 304 are coupled to a connector 305 which enables the probe 13 to be connected to the support electronics 60 and any external components of the sensor circuit 14.

The probe 13 also includes a seal 306 to prevent the liquid 17 from entering die housing 25 where die respective lines in the sheatii 304 exit the housing. Such seal 306 can be made using any of a number of conventional sealing techniques which are known to withstand caustics, strong acids, and the like.

The inductive probe 13 in the preferred embodiment is rod shaped as shown in Fig. 6, although other shaped cores for the inductors and other shaped housings can be used. The rod shaped inductive probe 13 such as that shown in Fig. 5 will be substantially less expensive to manufacture compared to existing toroidal type sensors, and is much easier to use and to preserve in operating condition. Installation is accomplished by dropping the probe 13 into die liquid 17 or other material to be tested. The chemical resistant plastic housing 25 and cable sheath 304 enables the probe 13 to be used in different aggressive mediums such as those discussed above. Furthermore, the mechanical strength of the housing 25, the unobtrusive shape of the housing, and die avoidance of large projections such as those found on many toroid shaped sensors makes die probe 13 much more damage resistant.

Referring to Fig. 7, shown is an exemplary embodiment of the temperature controlled power supply 62 used for generating the voltage V_coot in the sensor circuit 10. The supply 62 includes differential amplifiers 550, 551, 552. The non-inverting input of amplifier 551 is connected to line 65 from die temperature sensor 64. A pull-up resistor 554 provides an appropriate bias. The non-inverting input of amplifier 550 is connected to a V_Ωώttaop reference voltage provided by the support electronics 60. V^^_p is specified by the manufacturer of the sensor 10 and represents the equivalent voltage from the temperature sensor 64 at the minimum temperature the sensor 10 is expected to operate.

To provide for different degrees of temperature compensation as may be desired when measuring the conductivity of different materials, a potentiometer 557 is included. The potentiometer 557 is coupled from die output of amplifier 550 to the inverting input of the amplifier 551 through resistors 558, 559 as shown. By adjusting the potentiometer, it is possible to control the extent of temperature compensation.

The outputs from the amplifiers 550 and 551 are inputs, as shown, to amplifier 552, which provides an output on line 560. The signal on line 560 is added to another voltage signal provided at the input to the amplifier 570 through a resistive network 571 from the supply voltage V^^ to the sensor circuit 10, which is provided from the support electronics 60. The output from amplifier 570 drives transistor 573 and, together, with current limit protector 574 (i.e., a LM317), maintain the temperature dependent voltage V_cont at the emitter of transistor 573.

Although the invention has been shown and described with respect to a certain illustrative embodiment, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. For example, in anotiier embodiment of the present invention, the conductivity sensor 10 pumps the excitation inductor 21 with a continuous sinusoidal current rather than the periodic pulsed DC current described above. In such case, for example, the energy loss and conductivity of the material 15 can be determined simply by detecting the magnitude of the signal induced across me sensor inductor 22 as will be appreciated. As another example, the inductive probe 13 may include only a single inductor which serves as both the excitation inductor 21 and the sensor inductor 22 as is mentioned above. Referring briefly to Fig. 7, shown is such an alternate embodiment of the inductive probe 13. Included in the probe 13' is a single inductor 700 which has a single pair of terminals 702a and 702b. Lines 30,31 and 41,42 from die current driver 35 and the envelope detector 46, respectively, are connected in parallel across the inductor 700 as shown. Otherwise, the remainder of die conductivity sensor is identical to tiiat described above with respect to Fig.1. Thus, when the current driver 35 pumps the inductor via lines 30 and 31 with current I, the inductor 700 serves as the excitation inductor. When the current I is interrupted as shown in waveform 150 CFig. 3), the inductor 700 serves as the sensor inductor across lines 41 and 42. Therefore, it will be appreciated that a single inductor 700 can be utilized in accordance with the present invention.

Most materials which the conductivity sensor 10 described herein can be used to measure the conductivity of will not be affected by the operating frequency of the inductive probe 13. However, it will be appreciated that die conductivity sensor 10 is to be utilized to measure the conductivity of a material such as copper, gold, silver, etc., skin effects may occur in the material. In such case, it is preferable to take into consideration how such conductivity measurement is affected by the occurrence of skin effects in the material as a result of the maximum operating frequency of the inductive probe 13. i order to compensate for such skin effects, an appropriate correction factor can be applied to die measured conductivity value. Such correction factors may be determined experimentally or empirically by one having ordinary skill in the art.

The present invention includes all such equivalents and modifications, and is limited only by the scope of the following claims.

Claims

CLAIMSWe claim:

1. A conductivity sensor for measuring conductivity of a material, comprising: an inductive probe having an inductor witii an inductance L; current means for driving a current I through said inductor to establish a magnetic field having flux lines extending into said material; energy monitoring means for monitoring the energy stored in said magnetic field which is dissipated due to the conductivity of said material; and processing means for providing a direct measurement of conductivity of said material based upon an output signal from said energy monitoring means.