Patent Publication Number: US-2005120779-A1

Title: Directional coupler sensor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of U.S. Provisional Application No. 60/387,474, filed Jun. 10, 2002. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to measurement sensors and, more particularly, to a sensor for measuring a property of a substrate, such as the internal and external moisture content of biological systems such as hair.  
     BACKGROUND OF THE INVENTION  
      Human hair is made up of a complex protein called alpha-keratin. Protein molecules of the hair are arranged in organized patterns and are held together by weak bonds, such as hydrogen, saline and hydrophobic bonds, and stronger ionic bonds and sulphur bridges. These bonds lend stiffness or rigidity to the hair and enable styling of the hair with waves and curls as these bonds are broken and then re-established in different orientations through the styling process. As is known, styling of hair may be accomplished by breaking the bonds by adding energy to the hair, such as by heat with a curling iron or blow dryer, or by getting the hair wet or damp. When the hair is wet or damp and the hydrogen bonds are broken, the hair becomes elastic and can be stretched and given a particular form since the position of the keratin chains has been altered. As the hair dries, the bonds reform in different places, maintaining the hair in its new shape. Blow-drying or setting assists in controlling the styling process so that, once dry, the hair will retain the form it has been given.  
      Hair is hydroscopic and permeable so it will absorb water from the environment. Under normal conditions, water accounts for about 12% to 15% of the composition of hair. Normal hair can absorb more than 30% of its own weight in water. If the hair is damaged, this percentage can approach 45%, however damaged hair has less ability to retain water within the hair fibers which gives hair its healthy appearance. As more water is absorbed within the hair fiber due to humidity or prior damage, the hydrogen bonds may loosen so that the hair has a decreased ability to maintain its set.  
      During styling, if the hair is too wet, it will not hold its shape and water must be removed before styling will be effective. Conversely, when the hair is too dry, the hydrogen bonds will have already been formed and poor styling will result since the keratin chains cannot be repositioned and set. It has previously been determined that optimum styling results may be achieved when the moisture of hair is in the range of approximately 30-40% by weight. It is thus desirable to be able to tell when to begin styling (i.e., when moisture in the hair is in a range of 30-40%) to obtain the optimum styling results.  
      Likewise, it is also important to know when to stop styling hair which has been wetted to break the hydrogen bonds. If the hair is too dry, it will not be flexible and potential damage of the hair may result when styling is continued. It has been determined that the process of drying hair exhibits two stages which are relevant to styling. In a primary drying stage, water is evaporated from the outside of the hair fibers and no styling benefit is achieved. In a secondary drying stage, water from inside the hair fibers is diffused to the environment. It is during this transition to the secondary drying stage when optimum styling of hair may be achieved. A moisture level of about 30% is a balance between providing enough water to disrupt the hydrogen bonds to allow the hair to shape and not enough water that must be removed for the hydrogen bonds to be reformed.  
      Moisture sensing devices have been developed in the past to determine the moisture level in hair, and have relied on various techniques including resistance and capacitance measurements to obtain the desired indication. However, these methods only work well for a known cross sectional quantity and density of the hair being measured. As the hair density or compactness is varied, these measurement techniques fail. Additionally, these techniques rely primarily on the moisture content outside of the hair fiber for the measurement, and do not have the ability to accurately measure moisture content within hair fibers as well.  
      Thus, there is a need for a sensing device which can accurately and reliably determine the moisture content of a substrate, such as hair, including moisture both inside and outside of the hair fiber.  
     SUMMARY OF THE INVENTION  
      The present invention overcomes the foregoing and other shortcomings and drawbacks of moisture sensors and methods of determining moisture content heretofore known. While the invention will be described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. On the contrary, the invention includes all alternatives, modifications and equivalents as may be included within the spirit and scope of the present invention.  
      In accordance with the principles of the present invention a directional coupler sensor is provided for measuring the moisture content of a substrate, such as hair. As the moisture content of a substrate increases, so does its effective relative electrical impedance. The sensor of the present invention is designed to measure the relative impedance of a substrate, and from that measurement, the moisture content of the substrate can be determined.  
      The sensor of the present invention incorporates a high frequency directional coupler having a pair of generally parallel strips that define a coupling gap therebetween. A high frequency signal generator is electrically coupled to one of the strips and generates an electromagnetic field across the coupling gap to couple power to the other strip with the substrate placed across, i.e., generally normal to the longitudinal axis of, the coupling gap. The signal generator is preferably operable to generate signals in the VHF to UHF frequency ranges, although other frequency ranges are possible as well. The VHF frequency range is between about 30 MHz and 300 MHz, and the UHF frequency range is between about 300 MHz and about 3 GHz. The signal generator generates a coupled power signal in the coupled strip that has an amplitude related to the impedance, and therefore the moisture content of the substrate placed across the coupling gap.  
      In accordance with another aspect of the present invention, a pressure sensor is provided to ensure a proper packing pressure of the substrate placed across the coupling gap. The pressure sensor incorporates a film transducer in one embodiment that generates an output voltage signal that varies with the packing pressure applied to the substrate placed across the coupling gap. The measurement of the moisture content of the substrate is triggered upon the crossing of a pre-set pressure threshold. This ensures that the desired compactness of the substrate placed across the coupling gap is achieved to obtain accurate, reliable and consistent results. Alternatively, the packing consistency can be achieved by a mechanical system as well. The features and objectives of the present invention will become more readily apparent from the following Detailed Description taken in conjunction with the accompanying drawings.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the invention.  
       FIG. 1  is a functional block diagram of a directional coupler sensor in accordance with the principles of the present invention;  
       FIG. 2A  is a circuit representation of a high frequency signal generator for use in the sensor of  FIG. 1  in accordance with one embodiment of the present invention;  
       FIG. 2B  is a circuit representation of a directional coupler for use in the sensor of  FIG. 1  in accordance with one embodiment of the present invention;  
       FIG. 2C  is a circuit representation of a moisture content detector for use in the sensor of  FIG. 1  in accordance with one embodiment of the present invention;  
       FIG. 2D  is a circuit representation of a pressure sensor for use in the sensor of  FIG. 1  in accordance with one embodiment of the present invention;  
       FIG. 2E  is a circuit representation of a voltage regulator for use in the sensor of  FIG. 1  in accordance with one embodiment of the present invention;  
       FIG. 3  is a top plan view of the sensor of  FIG. 1  shown integrated onto a printed circuit board;  
       FIG. 3A  is a cross-sectional view taken along line  3 A- 3 A of  FIG. 3 ;  
       FIG. 4  is a perspective view of a directional coupler sensor system in accordance with one embodiment of the present invention;  
       FIG. 4A  is an enlarged front elevational view of a hair clamping device for use in the sensor system of  FIG. 4 , illustrating the clamping device in an open position to receive hair in the device;  
       FIG. 4B  is a view similar to  FIG. 4A , illustrating the clamping device in a closed position to clamp hair in the device;  
       FIGS. 5A and 5B  are side elevational views of a hair brush incorporating the directional coupler sensor of the present invention;  
       FIG. 6  is a graph illustrating the relationship between output voltage of the directional coupler sensor and relative humidity for various switches of hair;  
       FIG. 7  is a graph illustrating the relationship between moisture content of hair by weight and relative humidity of hair; and  
       FIG. 8  is a graph illustrating the relationship between output voltage of the directional coupler sensor and pressure applied to pack the hair.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Referring now to the Figures, and to  FIGS. 1 and 2 A- 2 E in particular, a directional coupler sensor  10  is shown in accordance with the principles of the present invention. For the sake of simplicity, the sensor  10  will be described herein in connection with measuring the moisture content of hair. However, it will be appreciated by those of ordinary skill in the art that the present invention has use in a wide variety of applications and is therefore not limited to the analysis of hair or the measurement of moisture content in a substrate. Rather, the sensor  10  of the present invention is readily adaptable to analyze a wide variety of substrates and to measure different moisture related properties of those substrates as will be readily appreciated by those of ordinary skill in the art.  
      For example, in the measurement of the moisture content of a substrate, the sensor  10  of the present invention operates under the principle that as the moisture content of a substrate increases, so does its effective relative electrical impedance. As will be described in greater detail below, the sensor  10  is designed to measure the relative impedance of a substrate, and from that measurement, the moisture content of the substrate can be determined. The moisture content value may be presented on a visual display, indicated through a user-perceptible audible tone and/or used as a control signal to control a function of a device.  
      As shown in FIGS.  1 ,  2 A- 2 E,  3  and  3 A, the sensor  10  incorporates a high frequency directional coupler  12  having a pair of generally parallel strips  14   a  and  14   b  that define a coupling gap  16  therebetween. In one embodiment of the present invention, the parallel strips  14   a ,  14   b  are supported on an FR4 printed circuit board  18  ( FIGS. 3 and 3 A) having a ground plane  20  formed on a lower surface of the board  18 . In one embodiment of the present invention, the height “h” of the PCB  18  is 0.062 in., each strip  14   a ,  14   b  has a width “w” of 0.15 in. and a length “l” of 0.350 in., and the coupling gap  16  has a gap distance “s” of 0.020 in. Of course, it will be appreciated by those of ordinary skill in the art that other dimensions of the PCB  18 , strips  14   a ,  14   b  and gap  16  are possible as well depending on a particular application as will be described in detail below.  
      A high frequency signal generator  22  is electrically coupled to strip  14   a  and is operable to generate an electromagnetic field across the coupling gap  16  that couples power to strip  14   b  with the substrate placed across, i.e., generally normal to the longitudinal axis of, the coupling gap  16  in a packed manner as will be described in detail below. The signal generator  22  generates a coupled power signal in the coupled strip  14   b  that has an amplitude related to the impedance, and therefore the moisture content, of the substrate placed across the coupling gap  16 . The signal generator  22  is phase locked to a crystal reference  24  ( FIG. 2A ) to maintain frequency and therefore measurement accuracy, stability and repeatability and has an adjustable power  26 . The signal generator  22  is preferably operable to generate signals in the VHF to UHF frequency ranges, i.e., between about 30 MHz and about 3 GHz, although other frequency ranges are possible as well. In accordance with one embodiment of the present invention, the signal generator  22  may operate at about 1 GHz, such as at about 915 MHz, since it is contemplated that the water content of a substrate may be most accurately determined by its measured impedance in the near GHz range.  
      In accordance with one aspect of the present invention, the sensor  10  utilizes the reverse power coupling variation of the high frequency directional coupler  12  to measure the change in the impedance of the material placed across the coupling gap  16 . As the substrate is packed across the coupling gap  16 , the directional coupler  12  becomes mismatched, and this mismatch causes a monotonic increase in the reverse power coupling of the directional coupler  12  as the impedance across the gap  16  is increased as the result of increased moisture content of the material. The amplitude of the reversed power in the reflected power leg  28  ( FIGS. 1 and 2 B) from strip  14   b  is generally a direct measure of the impedance, and hence the moisture content, of the substrate placed across the coupling gap  16 . As will be described in detail below, the moisture content of the substrate, i.e., its water content by weight, can be determined from the measured impedance of the sample.  
      Further referring to  FIGS. 1 and 2 A- 2 E, the forward power signal from strip  14   a  is electrically coupled to one port of a mixer  30  through a forward power leg  32  ( FIGS. 1 and 2 B) and an attenuator  34 . For example, the forward power signal may be attenuated to about −10 dBm by the attenuator  34 . The coupled power signal from strip  14   b  is phase shifted by phase shifter  36  and is electrically coupled to another port of the mixer  30  through the reflected power leg  28 . The mixer  30  may act as a coherent receiver in that it is most responsive to coupled signals that are in phase with the forward power signal. The phase shifter  36  assures the proper phase coherence of the reflected power signal relative to the forward power signal for the mixer  30  to produce the maximum discernable mixer output. With the mixer forward power set to the appropriate level through the adjustable power  26 , the output of the mixer  30  monotonically increases with an increase in the reflected coupled power. The mixer  30  demodulates or reduces to DC base band the value of the coupled power though the directional coupler  12 . The DC output of the mixer  30  is filtered and amplified by amplifier  38  to produce a measurable output voltage that is related to the moisture content of the substrate placed across the gap  16 . The amplifier  38  includes an adjustable gain  40  and an adjustable DC offset  42 .  
      Referring now to  FIGS. 4, 4A  and  4 B, use of the sensor  10  to determine the moisture content of hair will now be described in connection with a hair moisture sensor system  44 . Hair moisture sensor system  44  may be used by a professional salon, for example, to quickly, accurately and reliably indicate to a stylist when the moisture content of a customer&#39;s hair is in the range of approximately 30-40% by weight so that the optimum styling results may then be achieved.  
      As shown in  FIGS. 4A and 4B , a hair clamping device  46  is provided having pivoted jaws  48  and  50  that each terminate in a handle  52  that may be easily grasped and manipulated by the stylist. The jaws  48  and  50  may be biased to an open position as shown in  FIG. 4A  so that a bundle of hair  54  is readily received between the jaws  48 ,  50  and is oriented with the hair fibers  54  extending across, i.e., generally normal to the longitudinal axis of, the coupling gap  16  of the directional coupler  12  which is supported by jaw  50 . As shown in  FIG. 8 , it has been determined that the packing pressure of C  5  the hair  54  across the coupling gap  16  is important to ensure reliability in the moisture content measurement. With low packing density below about three (3) lbs., i.e., a packing density in the pressure region  56 , the output voltage signal of the mixer  30  may be unstable due to insufficient packing density of the hair fibers  54  across the coupling gap  16 . At higher packing pressures above about seven (7) lbs., i.e., a packing density in the pressure region  58 , the output voltage signal of the mixer  30  begins to fluctuate as the hair fibers  54  will exhibit the result of difference in packing density across the coupling gap  16 . At these higher pressures, excess moisture is also quickly expelled resulting in unreliable lower readings. It has been determined that packing pressures in the range of about three (3) lbs. to about six (6) lbs., i.e., a packing pressure in the pressure region  60 , provides an output voltage signal from the mixer  30  that is stable to produce reliable and repeatable measurements of the moisture content.  
      In accordance with another aspect of the present invention, as shown in  FIGS. 1, 2D ,  4 A and  4 B, a pressure sensor  62  incorporating a film pressure transducer  64 , is supported by the jaw  48  in juxtaposition to the directional coupler sensor  12 . The pressure transducer  64  is operable to generate an output voltage signal that varies with the packing pressure applied to the hair  54  placed across the coupling gap  16 . As shown in  FIGS. 1 and 2 D, the output voltage signal from the pressure transducer  64  is amplified by amplifier  66  having an adjustable gain  68  and DC offset  70 , and that amplified output voltage signal is either provided directly at the output of the pressure sensor  62  through jumper  72 , or it is applied as an input to a comparator  74  through jumper  76 . A trigger voltage corresponding to a desired trigger pressure is set as a reference voltage  77  to the comparator  74 . The measurement of the moisture content is triggered upon the crossing of the pre-set pressure threshold  77 . This ensures that the desired compactness of the hair fibers  54  placed across the coupling gap  16  is achieved to obtain accurate, reliable and repeatable results. It will be understood by those of ordinary skill in the art that packing consistency can be achieved by a mechanical system (not shown) as well without departing from the spirit and scope of the present invention.  
      With reference to  FIGS. 1 and 4 , the measured signal from the sensor  10 , and the trigger signal or pressure signal from the pressure sensor  62 , are electrically coupled through a cable  78  to a processing system  80 , such as a conventional PC or laptop computer. The processing system  80  is operable to convert the measurement signal generated by the sensor  10  into a moisture content value that may be presented on the display  82  of the system  80 . As described in detail above, the measurement signal is triggered in response to the trigger signal generated by the pressure sensor  62 . One or multiple measurements signals may be taken in response to the trigger signal.  
      Referring now to  FIGS. 6 and 7 , the amplified output voltage of the sensor  10  is calibrated by first subjecting multiple switches of hair to a known moisture content via the use of relative humidity. The sensor  10  is then used to generate a measurement signal for each switch of hair at the various relative humidities, as shown in  FIG. 6 . Since hair exhibits a generally linear relationship between moisture content by weight and relative humidity as shown in  FIG. 7 , the processing system  80  is operable to convert the amplified output voltage of the sensor  10  into a value representing the moisture content by weight of the hair using a look-up table or algorithm. Since the water absorption capability of damaged hair and healthy hair will differ, the sensor  10  of the present invention may be used to provide a signal that is generally related to the health of the hair. Generally, the health of hair is characterized by such factors as smoothness, shine, absence of frigility, absence of fissuring and absence of cuticular breakdown. As each of these factors is directly or indirectly related to the moisture content of the hair, the sensor  10  of the present invention is able to provide an accurate and reliable indication of the health of measured in vivo or in vitro hair.  
      The sensor  10  of the present invention provides a consumer friendly self-assessment tool that permits a consumer to periodically measure the general health of the consumer&#39;s hair. Based on these measurements, the consumer is able to take corrective actions as necessary which tend to improve the health of the consumer&#39;s hair. These actions may include changing hair care products, changing hair styling techniques, or both, so that the general health of the consumer&#39;s hair can be consistently monitored and improved. The sensor  10  also provides a useful monitoring tool to hair stylists and hair technicians as well.  
      In accordance with another aspect of the present invention, as shown in  FIGS. 5A and 5B , the sensor  10  is incorporated into a hair care product, such as a brush  84 , used for grooming hair. The brush  84  includes an elongated body portion  86  terminating in a handle  88 . Bristles  90  extend in a conventional manner from the body portion  86  of the brush  84  to enable grooming of the hair. In accordance with the principles of the present invention, as shown in  FIG. 3 , the signal generator  22 , mixer  30 , voltage regulator  92  ( FIG. 2E ) and electronics of the pressure sensor  62  are all integrated onto the PCB board  18  which is supported on a fixed base  94  of a hair clamping device  96  ( FIGS. 5A and 5B ). The fixed base  94  positions the directional coupler  12  near the bristles  90  so that measurements are easily taken while the hair is being brushed. The hair clamping device  96  includes a spring biased clamp member  98  that positions the pressure transducer  64  in juxtaposition to the directional coupler  12 . A lever  100  is operatively connected to the movable clamp member  98  to enable a user to clamp hair across the coupling gap  16  when a sensor measurement is desired by moving the clamp member  98  toward the fixed base  94  as shown in  FIG. 5B . The hair brush  84  may include LED&#39;s, and/or produce an audible signal, to provide an indication to the user about the moisture condition, health or other condition of the hair based on the sensor measurement. While not shown, in will be appreciated that the sensor  10  of the present invention may be incorporated into other hair care products as well, such as a comb, curling iron, or similar hair care product that preferably engages the user&#39;s hair during grooming to provide a measurement of the moisture content, health or other status of the hair based on the sensor measurement.  
      The directional coupler sensor  10  of the present invention is well suited to measure the moisture content, health or other condition of hair since it possesses sensitivity to variations in impedance in close proximity, such as about 0.1 in., to the surfaces of the strips  14   a  and  14   b . The height of this effective measurement probing depth from the surfaces of the strips  14   a ,  14   b  is a function of the electromagnetic field that couples the strips  14   a  and  14   b . The height of the measurement probing depth may be changed for a particular application by changing the height of the PCB  18 , the dielectric constant of the PCB  18 , the dimensions of the strips  14   a ,  14   b , the coupling gap distance “s”, and/or the power supplied by the signal generator  22 . By varying any or all of these parameters, the height of the coupling field can be altered to thereby change the effective measurement probing depth.  
      It is contemplated that sensor  10  may comprise multiple directional couplers  12  electrically coupled to at least one signal generator  22  to measure the respective moisture content of multiple substrates in accordance with the principles described in detail above. It is further contemplated that at least two of the multiple directional couplers  12  may have different effective measurement probing depths by varying one or more of the parameters described in detail above.  
      All documents cited in the Detailed Description of the Invention are, in relevant part, incorporated herein by reference; the citation of any document is not to be construed as an admission that it is prior art with respect to the present invention.  
      While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.