CA2065811A1 - Method and apparatus for measuring dielectric properties of materials - Google Patents
Method and apparatus for measuring dielectric properties of materialsInfo
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- CA2065811A1 CA2065811A1 CA 2065811 CA2065811A CA2065811A1 CA 2065811 A1 CA2065811 A1 CA 2065811A1 CA 2065811 CA2065811 CA 2065811 CA 2065811 A CA2065811 A CA 2065811A CA 2065811 A1 CA2065811 A1 CA 2065811A1
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Abstract
ABSTRACT OF THE DISCLOSURE
A sensor for use in on-line, real-time sensing of the condition of a fluid, such as the level of contamination of an engine lubricating oil or the concentration of methanol in gasoline, comprises a body having a fluid flow passage extending therethrough for passing a sample of the fluid through the body, a slow wave sensing element disposed in the body for reducing the phase velocity of RF energy applied to the fluid, an input signal coupler for exciting the sensing element at the resonant frequency of the element, and an output signal coupler for producing an RF signal representative of the RF energy in the element.
A sensor for use in on-line, real-time sensing of the condition of a fluid, such as the level of contamination of an engine lubricating oil or the concentration of methanol in gasoline, comprises a body having a fluid flow passage extending therethrough for passing a sample of the fluid through the body, a slow wave sensing element disposed in the body for reducing the phase velocity of RF energy applied to the fluid, an input signal coupler for exciting the sensing element at the resonant frequency of the element, and an output signal coupler for producing an RF signal representative of the RF energy in the element.
Description
~3~ ~
~he present invention generally relates to an apparatus for sensing the condition of fluids and, more specifically, a sensor for detecting the level of contamination of lubricating fluids, the concentration of methanol in gasoline as well as other applications.
S
BACKGROUND OF THE INVENTION
Lubricating oils are routinely used in a wide range of applications. fn many ciTcumstances, particularly for oils used in combllstion engines, the oil degrades with use and must be routinely replaced. Oil degradation is associated with three phenomena: the accumulation of undesirable contaminants in the oil, the consumption of additives in the oil and the chemical and physical changes that occur in the oil which are frequently related to consumptlon of protective additives. Unnecessary or premature oil replacement is costly while late oil replacement can lead to machine damage. There has been a lollg felt need for a simple on-line measurement device to measure oil qualit~ and provide guidance for lubricating oil management.
Many attempts to provide such a device have been made. The critical cornponent in all of these devices is the detector and, more specifically, the plinciple on which the detector operates.
United States Patent No. 4,785,287 granted to Homnna et al on No~ember 15, 1988 describes an oil condition detecting apparatus in which the sensor detects ultrasonic properties of the oil. The patent describes an arrangement in which an ultrasonic wave of predetermined waveform is propagated through the oil and assumes a change in waveform in accordance with the condition of the oil. The device detects the change in waveform to provide an indication of the condition of the oil. This type of device tends to be bulkyand requires relatively complex electronics.
Sensors which detect the capacitive properties of fluid have received by far the greatest amount of attention because they are relatively simple and inexpensive. United States Patent No. 4,733,556 granted to Meitzler et al on March 29, 1988 describes a typical on-line o;l condition sensing device having afirst capacitor which is exposed to the flow of lubricating oil through an internal ~ . .
. . . . , - ~ . ;
.: . . .
~$3 combustion engine so that its electrical capacitance value is dependent on the dielectric constant of the oil flowing through it. A second capacitor is isolated from the fluid flow but has its capacitance value detelmined by the dielectric constant of a reference material. An electrical circuit is connected to both S capacitors and compares the respective capac;tance values to provide an outputrepresentative of the difference therebetween. The magnitude of the difference in the capacitance values is an indication of the degree of deterioration of the oil.
Socie~ of Automotive Engineers (SAE~ Paper 910497, dated February 25, 1991 and entitled "A Capacitive Oil Deterioration Sensor", describes the sarne or very 10 similar device. Other devices which operate on this principle are United States Patent No. 4,806,847 granted to Atherton et al on February 21, 1989, United States Patent No. 4,443,754 granted to William H. King on April 17, 1984, PublishedEuropean PatentApplication No. 0 080 632published June 8,1983 and Published European Patent Application No. 0 121 739 published October 17, 1984.
Notwithstanding the popularity of capacitive sensors, they suffer from the need for temperature compensation and greater sensitivity. To reduce temperature related problems, the complete electronics package must be bui}t into the sensor body so that the sensor and the electronics fo~n an integrated 20 package. Even with these precautions, high lubrication oil temperatures still cause problems for the electronics when used for oil contamination measurements.
Resonant cavity sensors have been used to detect the dielectric properties of materials, although few attempts have been made to employ them in oil 25 condition sensing devices. As is well known, resonant cavity sensors tend to be relatively sensitive and tend not to suffer from fouling, temperature variations and vibrations and other such problems associated with optical, ultrasonic and capacitive sensors. As evidenced by the references discussed below, the major drawback of resonant cavity sensors are the requirement for high operating 30 ~requencies in the order of several hundred MHz ancl hi8her and the size of sensors. Resonant cavity sensors are mucll larger than capacitive sensors and, therefore, would not generally be considered practical for use in oil condition detection systems. Accordingly, this ~ype of sensor has tended to be rejected infavour of other types of sensors, normally capacitive sensors.
SAE Paper 910497, dated February 2S, 1991 and entitled "Two Alternative, Dielectric-Effect, Flex;ble-Fuel Sensors", describes one attempt at incorporating S a resonant cavity sensor in a fuel condition detecting apparatus. The sensor cell is in the form o~ a shorted coaxial, radio freqlency, transmission-line and comprises a pair of coaxial inner and outer cylinders between which fuel is caused to flow. The two cylinders define a resonant cavity which is excited with RF
energy. The author acknowledges that the sensor is structurally very similar to 10 a simple coaxial capacitive sensor structure. The prima~y diference resides in the manner in which the sensor is interrogated. The device would normally operate in the frequency range of 500 MHz but the high dielectric constants of methanol - and methanoVgasoline ~lels allows cells "of practical size" that operate in the 20 to 100 MHz frequency range. The paper refers to two sensor cells but does not 15 describe either in much detail. One sensor was fonned with coaxial brass cylinders with a length of 88 cm ~approximately 35 inches). The other sensor wasformed of stainless steel and while it is described as being smaller, no dimensions are given. These dimensions are clearly not practical for oil condition measurement. Given that oils have much lower dielectric constants, one would 20 expect much higher operating frequencies and/or much larger sensor dimensions.
Thus, the practicality of using resonant cavity sensors for fluids having lower dielectric constants is questionable.
The drawbacks of cavity resonators for detecting the dielectric properties of materials are acknowledged by W. Meyer in a paper entitled "Helical 25 Resonators for Measuring Dielectric Properties of Materials" published in theMarch 1981 issue of IEEE Transactions on Microwave Theory and Techniques, Vol. MIT-29, No. 3. Meyer was concerned with need the for experimental data respecting microwave low-temperature dielectric properties of materials. He points out that dielectric measurements are a power~ul tool for gaining some 30 insight into molecular structure and mechanisms because of the high measurement sensitivity in the microwave region, especially when using superconducting resonators. He indicates that these measurements are :, . . .
. ~ ~, . .
, accomplished with cavity resonators in the microwave range but, below 6 GHz, these cavities become too large for easy handling and therefore few measurements could be taken down to the 1ûO MHz range. To overcome this problem, Meyer investigated the properties of helical resonators in the form of S a quarter-wave shorted transmission line in which the centre conductor is wound into a helix. A lNbe is positioned coaxially within the resonator and provided with an intake pipe by which a sample material, whose dielectric properties are to be investigated, can be introduced into the tube. A coupling mechanism is provided to excite the nnaterial w;th RF energy at a frequency of about 200 MHz.Meyer is not at all concerned with continuous detection of the condition of fluids and the disclosed device, being a batch based sensor, obviously could not be used for this purpose. Accordingly, Meyer does not provide any directions as to how the principles disclosed could be used by those skilled in the art of fluid condition detection. ~he Meyer device requires a very high quality factor (Q) 15 and the measurernent method is valid only for small sample sizes such that there is only a small shift in the resonallt frequency and loss tangent. These restrictions are necessary in order to satisfy the perturbation formalism. Although Meyer contemplates the use of a multiple quarter-wave helical resonator to measure thedielectric properties of materials, he clearly did not anticipate filling the entire 20 structure with a fluid, let alone causillg a fluid to flow through the structure on a contimlous basis because this would cause a large shift in resonant frequency,Q and resonant loss tangent and violate the per~urbation formalisrn.
SUMMARY OF THE INVENTION
The present invention provides an on-line device for sens;ng the condition of a fluid on a continuous basis and overcomes many s:)f the disadvantages of the prior art in that it is of very simplc construction, is not susceptible to temperature S variations, is compact and reliable and is capable of operation in relatively low RF frequency ranges. The present invention is capable of sensing the condition and/or composition of two or more miscible or immiscible fl-uids, the condition and/or composition of a fluid or fluids and suspended or dissolYed solids and/orgases and combinations thereof.
Vehicle manufacturers have been evaluating alternate fuels both as a means of controlling the production of exhaust pollutants and as a means of reducing dependence on crude-oil based fuels. Mixtures of gasoline and methanol are being marketed as an alternative fuel for spark ignition engines.
The composition of these gasol;ne/methanol mixtures is quite variable. In order 15 to maintain efficient combustion of these mixtures in a spark ignition engine, it is necessary to adjust engine timing according to the gasoline/methanol cornposition of the fuel. The present invention provides a mechanism by which on-line fuel composition measurements may be obtained.
The present invention is generally defined as a sensor for sensing the 20 condition of a fluid, comprising a body having a fluid flow passage extendingtherethrough for passing a sample of the fluid, a slow wave sensing element disposed in the body for reducing the phase velocity of RF energy applied to thefluid, input signal coupling means for exciting the sensing element, and output signal coupling means for producing an RF signal representative of the RF energy25 in the element.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other ~eatures of the invention will become more apparent from the following description in which reference ;s made to the appended drawings, 30 wherein:
FIGURES 1~6 are cross-sectional views of different embodiments of the fluid condition sensor according to the present invention;
FIGURES 7a, b and c illustrate alternative slow wave strllcture sensing elements;
FIGURE 8 is a graph of resonant frequency versus percent soot in oil; and FIGURE 9 is a graph of the return loss versus frequency of clean lubricating oil, 4% anti-freeze/water in oil and 1% diesel soot in oil;
S FIGURE 10 is a graph of the sensor resonant frequency as a function of liquid dielectric constant; and FIGURE 11 is a graph showing Return Loss vs. Frequency for various concentrations of methanol in gasoline.
By way of background, changes in the permittivity and/or permeability of a fluid can be correlated with changes in its composition. For the sake of clarity the following definition of permittivity will be used in the following discussions.
Permittivity (~) is a complex parameter consisting of a real component ~' called15 the dielectric constant and an imaginary component ~" called the dielectric loss factor related by the equation ~ ". The ratio ~ ' is called the loss tangent.
The permittivity of a fluid is a characteristic property dependent on the fluid composition. Differences in permittivity can be used to identify changes in 20 the fluid or the presence of contaminants in the fluid. In general, changes in either or both ~' and ~" result in a change in the radio frequency (RF) properties of a resonant structure containing the fluid. The changes in RF properties of a resonant structure that can be measured by transmitting RF energy through the fluid include: the complex propagation constant, the reflection coefficient, the25 transmission coefficient, the input impedance and the angular frequency. The ability to measure small changes in the dielectric properties of a fluid contained within a resonant RF structure is greatly enhanced by increasing the strength ofthe electromagnetic field that exists in the resonator. The amount of R~ energy absorbed by materials is proportional to ~"E2, where E is the s~rength of the 30 electric field. It is clear that small differences ;n ~" can be measured more easily by increasing the electric field in the resonant structure. This can be accomplished by designing a resonant structure which has a high quality factor , ,, (Q~. This factor is a measure of the abili~ of the structure to store RF energy and increased stored energy leads to a higher electric field.
The present invention provides a resonant structure based sensor for use in detecting the permittivity and/or permeability of a ~uid on a continu~us basis and which is particularly useful in the measurement of gasoline/methanol concentrations and the measurement of contaminant levels in lubricating oil or hydraulic fluid resulting from thermal degradation, oxidation, or the ingress of external contarninants. It has been found that this caxl be achieved by providing a sensing chamber which houses a sensing element in the ~orm of a "slow wave" RF resonant structure, i.e. a structure wherein the phase veloci~ ofapplied RF energy is effecthrely reduced. In accordance wi-th the preferred embodiment of the present invention, the slow wave structure is in the forrn of a multi~le quarter-wave helical resonator. The structure operates to cause the RF
energy to propagate along the wire helix rather than between the walls of the shield or cavi~ in which it is disposed. This arraDgement results in an apparentcompression of the wavelength. The present invention takes advantage of this latter phenomenon in order to provide a sensor with dimensions that are greatly reduced over those conventional design.
A helical resonator can be corufigured as e;ther a "shorted" or "transmission" type of structure and a variety of coupling methods can also be employed to interrogate the helical resonator (e.g. magnetic field loop or inductive coupling, electric-field antenna or capacitive coupling, etc.) and a variety of RF-based measurement methods can be used to assess flu~d perrnittivi~.
Examples include: (a) Changes in e' cause the resonant frequency to shift. ~his shift in frequency can be measured and correlated with a corresponding change in fhlid composition; (b) changes in both ~' and ~" will cause a change in both resonant frequency and Q. Both the shift in resonant frequency a~nd/or Q can be correlated to changes in fluid composition; (c) if the interrogating frequency remains constant and the fluid c' changes, then there is a change in the compleximpedance of the resonator. This will in turn cause an increase in the arnount of transmitted power reflected by the resonator (this is true whether the resonatoris operated in either the shorted or transrnission mode). The amolmt of reflected .
power can be correlated with changes in fluid composition; (d) following on ~rom(c), if the resonator is operated in a transmission mode, an increase in the amount of reflected power will cause a corresponding decrease in the amount of transmitted power. The amount of transmitted power Call, therefore, be correlated to changes in fluid composition; and (e) an increase in fluid ~" willcause an increase in the amount of power adsorbed by ~he fluid. This will resultin a corresponding decrease in the amount of power transmitted through the sensor and a reduction in Q. Thus, transrnission loss can be correlated with 1uid composition. These examples are illustrative, but not exhaustive, of the varietyof E~F-based measurement methods that can be used, in association with a shielded multiple quarter-wave helical resonator, to interrogate the properties of a fluid.
A helical resonator can resonate at approximately odd multiples of its fundamental requency. There is, therefore, the potential to interrogate theproperties of the fluid at multiple -frequencies using the sarne resonator. The dielectric properties of materials can vary with frequency and thus it is possible to obtain several different measurements of the contents of the body containing the resonator. This additional information can be used in alternative techni~uesto identi~ the composition of a I1uid within a helical resonator.
With reference to FIGURE 1, the sensor 10 of the present invention will be seen to comprise of an electrically conducting body or shield 12 which defines a through passage 14 having an inlet 16 and an outlet 18. Ihis passage is arranged to be connected to a source of Iluid to be monitored in any suitablemarner. The shield may be formed of brass or other suitable low resistance material. In the embodiment shown, the shield may have an outside diameter of 20 mm, an inside diameter of 18 mm and a length of 152 mm.
The preferred slowwave structure is a multiple quarter-wave helical resonator 20 coaxially disposed within the shield. The axial length of the shield should be sufficient to minimize RF energy losses. In the embodiment shown, the resonator m~y be formed of Inconel, Copper or other suitable material, the wire being 1.6 mm in diameter, the helix being 13 mrn in diameter and 50 mm in length. A BNC RF connector 22 and RF coupler 24 having a 7 mm capacitive disk antenna 26 incased in teElon may be provided to excite the resonator.
The resonator can be supported in any variety o manners within the shield structure. As shown in FIGURE 1, the opposite ends of the helix can be attached directly to the shield structure. Alternatively, only one end of thehelix can be attached to the shield side wall. FIGURE 2 illustrates the helix supported by a tube 30, made of a low dielectric loss material such as quartz, teflon or alumina. In this embodiment, the tube itself de~mes the fluid Mow passage so that the resonator is not in contact with the fluid. In the embodiment of FIG~E 3, ~here is provided a high dielec~ric constant, low dielectric loss orferromagnetic material insert 32 in which the helix 20 is supported or embedded.'~e insert defines the fluid flow passage and, again, the helix can be isolated from the fluid being monitored. FIGURES 4a and 4b illustrate ~he resonator supported on a material hav~ng a high magnetic permeability and low magnetic loss (ferrite) insert 34. FIGURE 6 illustrates still another embodiment in which the helical resonator 20 is supported on high electrical conductivity metal rod 46 within a metal or dielectric body 48.
The design of quarter-wave helical resonators is well known to those skilled in the art of RF electronics and, therefore, need not be repeated herein.
It should be pointed out that the equations commonly used for the design of air filled helical resonators and shield structures do not account for the presence of dielectric inserts and/or dielectric fluids passing through the helical resonator.
'rheoretical treatment of the basic theoretical equations could of course be re-derived to account for each of the geometries described herein. HoweYer, experimental measurements are usually the most expedient remedy. It should also be noted that although cylindrical helical structures are assumed, other cross-sectional geometries are possible. Such as, for example, square, elliptical and the like.
The ratio of the shield diameter to that of the helix and its material of construction will effect the quality factor, Q, of the resonator. The shield should be constructed of low resistivity material to minimize ohmic losses.
Although the shield can be constructed from an extruded cylindrical tube, such -' ~
3 ~ :3. ~1 as copper or brass, it can be square in cross section provided the corner seams are of high quality. The shield can be open ended as shown in FIGURES 1-4 or have endcaps (not shown).
Although the preferred slow wave structure is a monofilar ~i.e.
single heliY) helical resonator as discussed above, it will be understood that the present invention contemplates other geometrical shapes. FIGURES 7a, b and c illustrate three alternative slow wave structures. FIGURE 7a and c illustra~e t vo multifilar~ helical resonators. In FIGURE 7a, ~he structure is in the form of concentric, contra-wound helices 50 and 52 while in Fl~lllRE 7c, the structure is in the form of co-wound or bifilar helices 58 and 60. FIGURE 7b illustra~es a modified helical structure constructed by milling away parts of a solid tubular structure.
I~ere are four common methods o~ coupling an RF input signal to a helical resonator. FIGIJ~E 1 illustrates capacitive coupling. In this case, a probe is inserted through the shield so that ;t projects radially into the shield.
This orientation places the probe parallel to the orientation of the resonator electrical field lines. An inductive coupler uses a magnetic loop or coil 25 oriented such tbat the magnetic field lines pass through the axis of the helLx. This orientation is illustrated in FIGURE 4. It should be noted that the ferrite coreis not a requirement for the inductive loop shown. A third method of coupling is by direct connection to the helical resonator. As is well known to those skilled in the art, the point of attachment is critical for optimum performance. The preseffl invention is not limited to any particular coupling mechanism. Various other forms of coupling may be utilized. FIGIJRE 5 illustrates the sensor connected in a transmission mode and including a transmitting coupler 4~ and a receiving coupler 44.
FIGURES 8-10 illustrate various responses of the present invention in the presence of different fluids. FIGUIU3 ~ is a graph of resonant frequency versus percent diesel soot in oil. FIGURE 9 is a graph of the return loss versusfrequency or clean oil, 4% anti-freeze/water in o;l and 1% diesel soot in oil.
FIGUlRE 10 is a graph illustrating the relationship between resonant frequenc~
and dielectric constant of the liquid (see 'Dielectric Materials and Applications", 1954, Arthur R. Von Hippel, published by Technology Press of MIT and John Wiley & Sons, New York and Chapman and Hall I imited, London). It can be seen that the resonant frequency of a helical resonator varies inversely as a square root of the dielectric constant of the material (fluid) filling the sensor cell, S as would be predicted by theory. What in fact has occurred is that the electromagnetic dimensions of the helical rssonator and shield have effectively been increased by a factor proportional to the square root of the dielectric constant of the material filling the sensor cell. If low loss, high permittivi~ or permeability material is used as an insert in the sensor cell, as illustrated in10 FIGURES 3 and 4, these inserts effectively increase the electromagnetic dimensions of the helical resonator. Conversely, the helical resonator cou}d be designed to be geometrically smaller by a factor equal to the square root of theeffective dielectric constant and/or permeability of the inserts. Clearly, the sensor can be made physically smaller or the resonant frequency altered by using low 15 loss, high permittivity or permeabili~y inserts. In addition, the inserts can be used to locally enhance or concentrate the electromagnetic fields to increase the measurement sensitivity. This feature could be used to advantage for some measurement applications where small sample volumes are desirable.
FIGURE 1 further illustrates the provision of an optional tuning stub 40.
20 By inserting or withdrawing the stllb, the fundamental resident frequency of the multiple quarter-wave helical resonator can be al~ered over a limited frequency range. If an inductive coupler is used to excite the helical resonator, then a variable capacity circuit connected (externally) in parallel to the resonator circuit can be used to optimize the impedance match of the resonator to the RF source 25 and measurement circuits. The impedance and resonant frequency of the helicalresonator are determined by its geometry. The external capacitive circuit allows~fine tuning of the resonator impedance to take into account irnperfections in resonator construct;on. A corresponding external variable inductance circuit canbe used in conjunction with a capacitive coupler for fine tuning the resonator 30 impedance.
Although a helix as short as a qwarter-wave length can be used as a resonator, helices that are integer multiples of a qwarter-wave may be more ~ ~ ~ 3 convenient for certain applications. The helix dirnensions given in FIGURE 1 andmentioned earlier are for a half wave resonator. It has been ~ound experimentally that the mechanical stabilit~ of the helix and its proximity to the coupler could be enhanced by the structure shown. In this case, the metal S coupler is held at a constant distance from the helix by the teflon that encases the coupler. A slight radial pressure from the coupler body on the helix prevents vibration under conditions of very high fluid flow. The lack of vibration has been verified for the geometry shown in FIGURE 1 for fluid linear velocities up to 3 mls.
FIGURE: 11 ;s a graph showing Return Loss vs. Frequency for various concentrations of methanol in gasoline and demonstrates how, as the percentage of methanol is increased, both the resonant frequency of the sensor of the present invention progressively decreases and the resonator Q changes, thus enabling thesensor to be advantageously used as a basis for adJusting engine timing according 15 to the gasoline/methanol composition of the fuel.
It will be understood that various modifications and alterations may be rnade to the present invention without departing frorn the spirit of the presentinvention.
. ~
- , . . . , '', ~
~he present invention generally relates to an apparatus for sensing the condition of fluids and, more specifically, a sensor for detecting the level of contamination of lubricating fluids, the concentration of methanol in gasoline as well as other applications.
S
BACKGROUND OF THE INVENTION
Lubricating oils are routinely used in a wide range of applications. fn many ciTcumstances, particularly for oils used in combllstion engines, the oil degrades with use and must be routinely replaced. Oil degradation is associated with three phenomena: the accumulation of undesirable contaminants in the oil, the consumption of additives in the oil and the chemical and physical changes that occur in the oil which are frequently related to consumptlon of protective additives. Unnecessary or premature oil replacement is costly while late oil replacement can lead to machine damage. There has been a lollg felt need for a simple on-line measurement device to measure oil qualit~ and provide guidance for lubricating oil management.
Many attempts to provide such a device have been made. The critical cornponent in all of these devices is the detector and, more specifically, the plinciple on which the detector operates.
United States Patent No. 4,785,287 granted to Homnna et al on No~ember 15, 1988 describes an oil condition detecting apparatus in which the sensor detects ultrasonic properties of the oil. The patent describes an arrangement in which an ultrasonic wave of predetermined waveform is propagated through the oil and assumes a change in waveform in accordance with the condition of the oil. The device detects the change in waveform to provide an indication of the condition of the oil. This type of device tends to be bulkyand requires relatively complex electronics.
Sensors which detect the capacitive properties of fluid have received by far the greatest amount of attention because they are relatively simple and inexpensive. United States Patent No. 4,733,556 granted to Meitzler et al on March 29, 1988 describes a typical on-line o;l condition sensing device having afirst capacitor which is exposed to the flow of lubricating oil through an internal ~ . .
. . . . , - ~ . ;
.: . . .
~$3 combustion engine so that its electrical capacitance value is dependent on the dielectric constant of the oil flowing through it. A second capacitor is isolated from the fluid flow but has its capacitance value detelmined by the dielectric constant of a reference material. An electrical circuit is connected to both S capacitors and compares the respective capac;tance values to provide an outputrepresentative of the difference therebetween. The magnitude of the difference in the capacitance values is an indication of the degree of deterioration of the oil.
Socie~ of Automotive Engineers (SAE~ Paper 910497, dated February 25, 1991 and entitled "A Capacitive Oil Deterioration Sensor", describes the sarne or very 10 similar device. Other devices which operate on this principle are United States Patent No. 4,806,847 granted to Atherton et al on February 21, 1989, United States Patent No. 4,443,754 granted to William H. King on April 17, 1984, PublishedEuropean PatentApplication No. 0 080 632published June 8,1983 and Published European Patent Application No. 0 121 739 published October 17, 1984.
Notwithstanding the popularity of capacitive sensors, they suffer from the need for temperature compensation and greater sensitivity. To reduce temperature related problems, the complete electronics package must be bui}t into the sensor body so that the sensor and the electronics fo~n an integrated 20 package. Even with these precautions, high lubrication oil temperatures still cause problems for the electronics when used for oil contamination measurements.
Resonant cavity sensors have been used to detect the dielectric properties of materials, although few attempts have been made to employ them in oil 25 condition sensing devices. As is well known, resonant cavity sensors tend to be relatively sensitive and tend not to suffer from fouling, temperature variations and vibrations and other such problems associated with optical, ultrasonic and capacitive sensors. As evidenced by the references discussed below, the major drawback of resonant cavity sensors are the requirement for high operating 30 ~requencies in the order of several hundred MHz ancl hi8her and the size of sensors. Resonant cavity sensors are mucll larger than capacitive sensors and, therefore, would not generally be considered practical for use in oil condition detection systems. Accordingly, this ~ype of sensor has tended to be rejected infavour of other types of sensors, normally capacitive sensors.
SAE Paper 910497, dated February 2S, 1991 and entitled "Two Alternative, Dielectric-Effect, Flex;ble-Fuel Sensors", describes one attempt at incorporating S a resonant cavity sensor in a fuel condition detecting apparatus. The sensor cell is in the form o~ a shorted coaxial, radio freqlency, transmission-line and comprises a pair of coaxial inner and outer cylinders between which fuel is caused to flow. The two cylinders define a resonant cavity which is excited with RF
energy. The author acknowledges that the sensor is structurally very similar to 10 a simple coaxial capacitive sensor structure. The prima~y diference resides in the manner in which the sensor is interrogated. The device would normally operate in the frequency range of 500 MHz but the high dielectric constants of methanol - and methanoVgasoline ~lels allows cells "of practical size" that operate in the 20 to 100 MHz frequency range. The paper refers to two sensor cells but does not 15 describe either in much detail. One sensor was fonned with coaxial brass cylinders with a length of 88 cm ~approximately 35 inches). The other sensor wasformed of stainless steel and while it is described as being smaller, no dimensions are given. These dimensions are clearly not practical for oil condition measurement. Given that oils have much lower dielectric constants, one would 20 expect much higher operating frequencies and/or much larger sensor dimensions.
Thus, the practicality of using resonant cavity sensors for fluids having lower dielectric constants is questionable.
The drawbacks of cavity resonators for detecting the dielectric properties of materials are acknowledged by W. Meyer in a paper entitled "Helical 25 Resonators for Measuring Dielectric Properties of Materials" published in theMarch 1981 issue of IEEE Transactions on Microwave Theory and Techniques, Vol. MIT-29, No. 3. Meyer was concerned with need the for experimental data respecting microwave low-temperature dielectric properties of materials. He points out that dielectric measurements are a power~ul tool for gaining some 30 insight into molecular structure and mechanisms because of the high measurement sensitivity in the microwave region, especially when using superconducting resonators. He indicates that these measurements are :, . . .
. ~ ~, . .
, accomplished with cavity resonators in the microwave range but, below 6 GHz, these cavities become too large for easy handling and therefore few measurements could be taken down to the 1ûO MHz range. To overcome this problem, Meyer investigated the properties of helical resonators in the form of S a quarter-wave shorted transmission line in which the centre conductor is wound into a helix. A lNbe is positioned coaxially within the resonator and provided with an intake pipe by which a sample material, whose dielectric properties are to be investigated, can be introduced into the tube. A coupling mechanism is provided to excite the nnaterial w;th RF energy at a frequency of about 200 MHz.Meyer is not at all concerned with continuous detection of the condition of fluids and the disclosed device, being a batch based sensor, obviously could not be used for this purpose. Accordingly, Meyer does not provide any directions as to how the principles disclosed could be used by those skilled in the art of fluid condition detection. ~he Meyer device requires a very high quality factor (Q) 15 and the measurernent method is valid only for small sample sizes such that there is only a small shift in the resonallt frequency and loss tangent. These restrictions are necessary in order to satisfy the perturbation formalism. Although Meyer contemplates the use of a multiple quarter-wave helical resonator to measure thedielectric properties of materials, he clearly did not anticipate filling the entire 20 structure with a fluid, let alone causillg a fluid to flow through the structure on a contimlous basis because this would cause a large shift in resonant frequency,Q and resonant loss tangent and violate the per~urbation formalisrn.
SUMMARY OF THE INVENTION
The present invention provides an on-line device for sens;ng the condition of a fluid on a continuous basis and overcomes many s:)f the disadvantages of the prior art in that it is of very simplc construction, is not susceptible to temperature S variations, is compact and reliable and is capable of operation in relatively low RF frequency ranges. The present invention is capable of sensing the condition and/or composition of two or more miscible or immiscible fl-uids, the condition and/or composition of a fluid or fluids and suspended or dissolYed solids and/orgases and combinations thereof.
Vehicle manufacturers have been evaluating alternate fuels both as a means of controlling the production of exhaust pollutants and as a means of reducing dependence on crude-oil based fuels. Mixtures of gasoline and methanol are being marketed as an alternative fuel for spark ignition engines.
The composition of these gasol;ne/methanol mixtures is quite variable. In order 15 to maintain efficient combustion of these mixtures in a spark ignition engine, it is necessary to adjust engine timing according to the gasoline/methanol cornposition of the fuel. The present invention provides a mechanism by which on-line fuel composition measurements may be obtained.
The present invention is generally defined as a sensor for sensing the 20 condition of a fluid, comprising a body having a fluid flow passage extendingtherethrough for passing a sample of the fluid, a slow wave sensing element disposed in the body for reducing the phase velocity of RF energy applied to thefluid, input signal coupling means for exciting the sensing element, and output signal coupling means for producing an RF signal representative of the RF energy25 in the element.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other ~eatures of the invention will become more apparent from the following description in which reference ;s made to the appended drawings, 30 wherein:
FIGURES 1~6 are cross-sectional views of different embodiments of the fluid condition sensor according to the present invention;
FIGURES 7a, b and c illustrate alternative slow wave strllcture sensing elements;
FIGURE 8 is a graph of resonant frequency versus percent soot in oil; and FIGURE 9 is a graph of the return loss versus frequency of clean lubricating oil, 4% anti-freeze/water in oil and 1% diesel soot in oil;
S FIGURE 10 is a graph of the sensor resonant frequency as a function of liquid dielectric constant; and FIGURE 11 is a graph showing Return Loss vs. Frequency for various concentrations of methanol in gasoline.
By way of background, changes in the permittivity and/or permeability of a fluid can be correlated with changes in its composition. For the sake of clarity the following definition of permittivity will be used in the following discussions.
Permittivity (~) is a complex parameter consisting of a real component ~' called15 the dielectric constant and an imaginary component ~" called the dielectric loss factor related by the equation ~ ". The ratio ~ ' is called the loss tangent.
The permittivity of a fluid is a characteristic property dependent on the fluid composition. Differences in permittivity can be used to identify changes in 20 the fluid or the presence of contaminants in the fluid. In general, changes in either or both ~' and ~" result in a change in the radio frequency (RF) properties of a resonant structure containing the fluid. The changes in RF properties of a resonant structure that can be measured by transmitting RF energy through the fluid include: the complex propagation constant, the reflection coefficient, the25 transmission coefficient, the input impedance and the angular frequency. The ability to measure small changes in the dielectric properties of a fluid contained within a resonant RF structure is greatly enhanced by increasing the strength ofthe electromagnetic field that exists in the resonator. The amount of R~ energy absorbed by materials is proportional to ~"E2, where E is the s~rength of the 30 electric field. It is clear that small differences ;n ~" can be measured more easily by increasing the electric field in the resonant structure. This can be accomplished by designing a resonant structure which has a high quality factor , ,, (Q~. This factor is a measure of the abili~ of the structure to store RF energy and increased stored energy leads to a higher electric field.
The present invention provides a resonant structure based sensor for use in detecting the permittivity and/or permeability of a ~uid on a continu~us basis and which is particularly useful in the measurement of gasoline/methanol concentrations and the measurement of contaminant levels in lubricating oil or hydraulic fluid resulting from thermal degradation, oxidation, or the ingress of external contarninants. It has been found that this caxl be achieved by providing a sensing chamber which houses a sensing element in the ~orm of a "slow wave" RF resonant structure, i.e. a structure wherein the phase veloci~ ofapplied RF energy is effecthrely reduced. In accordance wi-th the preferred embodiment of the present invention, the slow wave structure is in the forrn of a multi~le quarter-wave helical resonator. The structure operates to cause the RF
energy to propagate along the wire helix rather than between the walls of the shield or cavi~ in which it is disposed. This arraDgement results in an apparentcompression of the wavelength. The present invention takes advantage of this latter phenomenon in order to provide a sensor with dimensions that are greatly reduced over those conventional design.
A helical resonator can be corufigured as e;ther a "shorted" or "transmission" type of structure and a variety of coupling methods can also be employed to interrogate the helical resonator (e.g. magnetic field loop or inductive coupling, electric-field antenna or capacitive coupling, etc.) and a variety of RF-based measurement methods can be used to assess flu~d perrnittivi~.
Examples include: (a) Changes in e' cause the resonant frequency to shift. ~his shift in frequency can be measured and correlated with a corresponding change in fhlid composition; (b) changes in both ~' and ~" will cause a change in both resonant frequency and Q. Both the shift in resonant frequency a~nd/or Q can be correlated to changes in fluid composition; (c) if the interrogating frequency remains constant and the fluid c' changes, then there is a change in the compleximpedance of the resonator. This will in turn cause an increase in the arnount of transmitted power reflected by the resonator (this is true whether the resonatoris operated in either the shorted or transrnission mode). The amolmt of reflected .
power can be correlated with changes in fluid composition; (d) following on ~rom(c), if the resonator is operated in a transmission mode, an increase in the amount of reflected power will cause a corresponding decrease in the amount of transmitted power. The amount of transmitted power Call, therefore, be correlated to changes in fluid composition; and (e) an increase in fluid ~" willcause an increase in the amount of power adsorbed by ~he fluid. This will resultin a corresponding decrease in the amount of power transmitted through the sensor and a reduction in Q. Thus, transrnission loss can be correlated with 1uid composition. These examples are illustrative, but not exhaustive, of the varietyof E~F-based measurement methods that can be used, in association with a shielded multiple quarter-wave helical resonator, to interrogate the properties of a fluid.
A helical resonator can resonate at approximately odd multiples of its fundamental requency. There is, therefore, the potential to interrogate theproperties of the fluid at multiple -frequencies using the sarne resonator. The dielectric properties of materials can vary with frequency and thus it is possible to obtain several different measurements of the contents of the body containing the resonator. This additional information can be used in alternative techni~uesto identi~ the composition of a I1uid within a helical resonator.
With reference to FIGURE 1, the sensor 10 of the present invention will be seen to comprise of an electrically conducting body or shield 12 which defines a through passage 14 having an inlet 16 and an outlet 18. Ihis passage is arranged to be connected to a source of Iluid to be monitored in any suitablemarner. The shield may be formed of brass or other suitable low resistance material. In the embodiment shown, the shield may have an outside diameter of 20 mm, an inside diameter of 18 mm and a length of 152 mm.
The preferred slowwave structure is a multiple quarter-wave helical resonator 20 coaxially disposed within the shield. The axial length of the shield should be sufficient to minimize RF energy losses. In the embodiment shown, the resonator m~y be formed of Inconel, Copper or other suitable material, the wire being 1.6 mm in diameter, the helix being 13 mrn in diameter and 50 mm in length. A BNC RF connector 22 and RF coupler 24 having a 7 mm capacitive disk antenna 26 incased in teElon may be provided to excite the resonator.
The resonator can be supported in any variety o manners within the shield structure. As shown in FIGURE 1, the opposite ends of the helix can be attached directly to the shield structure. Alternatively, only one end of thehelix can be attached to the shield side wall. FIGURE 2 illustrates the helix supported by a tube 30, made of a low dielectric loss material such as quartz, teflon or alumina. In this embodiment, the tube itself de~mes the fluid Mow passage so that the resonator is not in contact with the fluid. In the embodiment of FIG~E 3, ~here is provided a high dielec~ric constant, low dielectric loss orferromagnetic material insert 32 in which the helix 20 is supported or embedded.'~e insert defines the fluid flow passage and, again, the helix can be isolated from the fluid being monitored. FIGURES 4a and 4b illustrate ~he resonator supported on a material hav~ng a high magnetic permeability and low magnetic loss (ferrite) insert 34. FIGURE 6 illustrates still another embodiment in which the helical resonator 20 is supported on high electrical conductivity metal rod 46 within a metal or dielectric body 48.
The design of quarter-wave helical resonators is well known to those skilled in the art of RF electronics and, therefore, need not be repeated herein.
It should be pointed out that the equations commonly used for the design of air filled helical resonators and shield structures do not account for the presence of dielectric inserts and/or dielectric fluids passing through the helical resonator.
'rheoretical treatment of the basic theoretical equations could of course be re-derived to account for each of the geometries described herein. HoweYer, experimental measurements are usually the most expedient remedy. It should also be noted that although cylindrical helical structures are assumed, other cross-sectional geometries are possible. Such as, for example, square, elliptical and the like.
The ratio of the shield diameter to that of the helix and its material of construction will effect the quality factor, Q, of the resonator. The shield should be constructed of low resistivity material to minimize ohmic losses.
Although the shield can be constructed from an extruded cylindrical tube, such -' ~
3 ~ :3. ~1 as copper or brass, it can be square in cross section provided the corner seams are of high quality. The shield can be open ended as shown in FIGURES 1-4 or have endcaps (not shown).
Although the preferred slow wave structure is a monofilar ~i.e.
single heliY) helical resonator as discussed above, it will be understood that the present invention contemplates other geometrical shapes. FIGURES 7a, b and c illustrate three alternative slow wave structures. FIGURE 7a and c illustra~e t vo multifilar~ helical resonators. In FIGURE 7a, ~he structure is in the form of concentric, contra-wound helices 50 and 52 while in Fl~lllRE 7c, the structure is in the form of co-wound or bifilar helices 58 and 60. FIGURE 7b illustra~es a modified helical structure constructed by milling away parts of a solid tubular structure.
I~ere are four common methods o~ coupling an RF input signal to a helical resonator. FIGIJ~E 1 illustrates capacitive coupling. In this case, a probe is inserted through the shield so that ;t projects radially into the shield.
This orientation places the probe parallel to the orientation of the resonator electrical field lines. An inductive coupler uses a magnetic loop or coil 25 oriented such tbat the magnetic field lines pass through the axis of the helLx. This orientation is illustrated in FIGURE 4. It should be noted that the ferrite coreis not a requirement for the inductive loop shown. A third method of coupling is by direct connection to the helical resonator. As is well known to those skilled in the art, the point of attachment is critical for optimum performance. The preseffl invention is not limited to any particular coupling mechanism. Various other forms of coupling may be utilized. FIGIJRE 5 illustrates the sensor connected in a transmission mode and including a transmitting coupler 4~ and a receiving coupler 44.
FIGURES 8-10 illustrate various responses of the present invention in the presence of different fluids. FIGUIU3 ~ is a graph of resonant frequency versus percent diesel soot in oil. FIGURE 9 is a graph of the return loss versusfrequency or clean oil, 4% anti-freeze/water in o;l and 1% diesel soot in oil.
FIGUlRE 10 is a graph illustrating the relationship between resonant frequenc~
and dielectric constant of the liquid (see 'Dielectric Materials and Applications", 1954, Arthur R. Von Hippel, published by Technology Press of MIT and John Wiley & Sons, New York and Chapman and Hall I imited, London). It can be seen that the resonant frequency of a helical resonator varies inversely as a square root of the dielectric constant of the material (fluid) filling the sensor cell, S as would be predicted by theory. What in fact has occurred is that the electromagnetic dimensions of the helical rssonator and shield have effectively been increased by a factor proportional to the square root of the dielectric constant of the material filling the sensor cell. If low loss, high permittivi~ or permeability material is used as an insert in the sensor cell, as illustrated in10 FIGURES 3 and 4, these inserts effectively increase the electromagnetic dimensions of the helical resonator. Conversely, the helical resonator cou}d be designed to be geometrically smaller by a factor equal to the square root of theeffective dielectric constant and/or permeability of the inserts. Clearly, the sensor can be made physically smaller or the resonant frequency altered by using low 15 loss, high permittivity or permeabili~y inserts. In addition, the inserts can be used to locally enhance or concentrate the electromagnetic fields to increase the measurement sensitivity. This feature could be used to advantage for some measurement applications where small sample volumes are desirable.
FIGURE 1 further illustrates the provision of an optional tuning stub 40.
20 By inserting or withdrawing the stllb, the fundamental resident frequency of the multiple quarter-wave helical resonator can be al~ered over a limited frequency range. If an inductive coupler is used to excite the helical resonator, then a variable capacity circuit connected (externally) in parallel to the resonator circuit can be used to optimize the impedance match of the resonator to the RF source 25 and measurement circuits. The impedance and resonant frequency of the helicalresonator are determined by its geometry. The external capacitive circuit allows~fine tuning of the resonator impedance to take into account irnperfections in resonator construct;on. A corresponding external variable inductance circuit canbe used in conjunction with a capacitive coupler for fine tuning the resonator 30 impedance.
Although a helix as short as a qwarter-wave length can be used as a resonator, helices that are integer multiples of a qwarter-wave may be more ~ ~ ~ 3 convenient for certain applications. The helix dirnensions given in FIGURE 1 andmentioned earlier are for a half wave resonator. It has been ~ound experimentally that the mechanical stabilit~ of the helix and its proximity to the coupler could be enhanced by the structure shown. In this case, the metal S coupler is held at a constant distance from the helix by the teflon that encases the coupler. A slight radial pressure from the coupler body on the helix prevents vibration under conditions of very high fluid flow. The lack of vibration has been verified for the geometry shown in FIGURE 1 for fluid linear velocities up to 3 mls.
FIGURE: 11 ;s a graph showing Return Loss vs. Frequency for various concentrations of methanol in gasoline and demonstrates how, as the percentage of methanol is increased, both the resonant frequency of the sensor of the present invention progressively decreases and the resonator Q changes, thus enabling thesensor to be advantageously used as a basis for adJusting engine timing according 15 to the gasoline/methanol composition of the fuel.
It will be understood that various modifications and alterations may be rnade to the present invention without departing frorn the spirit of the presentinvention.
. ~
- , . . . , '', ~
Claims (17)
1. A sensor for sensing the condition of a fluid, comprising:
a body having a fluid flow passage extending therethrough for passing a sample of said fluid;
a slow wave sensing element disposed in said body for reducing the phase velocity of RF energy applied to said fluid;
input signal coupling means for exciting said sensing element; and output signal coupling means for producing an RF signal representative of the RF energy in said element.
a body having a fluid flow passage extending therethrough for passing a sample of said fluid;
a slow wave sensing element disposed in said body for reducing the phase velocity of RF energy applied to said fluid;
input signal coupling means for exciting said sensing element; and output signal coupling means for producing an RF signal representative of the RF energy in said element.
2. A sensor as defined in claim 1, said slow wave sensing element being a helical multiple quarter-wave length resonator.
3. A sensor as defined in claim 1, said slow wave sensing element including a multifilar helical multiple quarter-wave length resonator structure.
4. A sensor as defined in claim 1, said slow wave sensing element including concentric, contra-wound multiple quarter-wave length resonator helices.
5. A sensor as defined in claim 1, said slow wave sensing element including a modified helical structure constructed by milling away parts of a solid tubular structure.
6. A sensor as defined in claim 1, said body defining an electromagnetic shield and being formed of low resistivity material to minimize RF energy losses.
7. A sensor as defined in claim 1, said resonator being secured to said body.
8. A sensor as defined in claim 7, wherein one end of said resonator is secured to said body.
9. A sensor as defined in claim 7, wherein each of the opposite ends of said resonator are secured to said body.
10. A sensor as defined in claim 7, further including a tube formed of RF
energy transparent energy and coaxially disposed within said body and defining said fluid flow passage, said resonator being wound on and supported by said tube between said tube and said body.
energy transparent energy and coaxially disposed within said body and defining said fluid flow passage, said resonator being wound on and supported by said tube between said tube and said body.
11. A sensor as defined in claim 10, said tube being formed of a material selected from the group consisting of quartz, teflon and alumina.
12. A sensor as defined in claim 7, further including a tubular dielectric insert disposed in said body and defining said fluid passage, said sensing element being imbedded or supported in said insert.
13. A sensor as defined in claim 1, said input signal coupling means including a capacitive probe extending through said body into said passage, said probe being oriented so as to be parallel to electrical field lines in said body.
14. A sensor as defined in claim 1, said input signal coupling means includes a magnetic loop oriented such that magnetic field lines within said body pass through the axis of said loop.
15. A sensor as defined in claim 1, said input signal coupling signal iucludes an electrical conductor connected directly to said resonator.
16. A sensor as defined in claim 1, further including a rod formed of high conductivity metal and coaxially disposed within said body, said resonator beingcoaxially disposed around said rod and being attached at its opposed ends to said rod or otherwise supported around said rod by suitable low dielectric loss material.
17. A sensor as defined in claim 16, wherein the outer body of said sensor being formed of suitable low dielectric loss or low permeability loss material.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2065811 CA2065811A1 (en) | 1992-04-10 | 1992-04-10 | Method and apparatus for measuring dielectric properties of materials |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2065811 CA2065811A1 (en) | 1992-04-10 | 1992-04-10 | Method and apparatus for measuring dielectric properties of materials |
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| CA2065811A1 true CA2065811A1 (en) | 1993-10-11 |
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| CA 2065811 Abandoned CA2065811A1 (en) | 1992-04-10 | 1992-04-10 | Method and apparatus for measuring dielectric properties of materials |
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Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US9588063B2 (en) | 2014-06-02 | 2017-03-07 | Senfit Oy | Sensor, measuring device, and measuring method |
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1992
- 1992-04-10 CA CA 2065811 patent/CA2065811A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US9588063B2 (en) | 2014-06-02 | 2017-03-07 | Senfit Oy | Sensor, measuring device, and measuring method |
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