Abstract:
A distance measurement system, comprising at least one resonant circuit, at least one magnetic element with predetermined magnetic properties, a transmitter operable to transmit an electromagnetic pulse, a receiver operable to detect oscillations emitted by said resonant circuit in response to said electromagnetic pulse, and an analyzer operable to analyze an amplitude envelope property of said oscillations, to thereby determine a distance between the resonant circuit and the magnetic element.

Description:
RELATIONSHIP TO EXISTING APPLICATIONS 
     The present application is a continuation in part of PCT/IL00/00757 filed Nov. 15, 2000, and is a continuation in part of U.S. patent application Ser. No. 09/585,318 filed Jun. 2, 2000, now U.S. Pat. No. 6,245,109, which is a continuation of U.S. patent application Ser. No. 09/443,113 filed Nov. 18, 1999, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a distance measurement system and more particularly but not exclusively to determining a distance between the core of a resonant circuit and a ferromagnetic or paramagnetic element. 
     BACKGROUND OF THE INVENTION 
     Mechanical fixtures such as joints and bearings often suffer from performance degradation due to material wear. When the wear becomes pronounced this may result in loss of efficiency or damage to the system in which it is installed. This wear may be difficult to measure, as it generally involves small displacement changes and develops slowly over time. Additionally, the fixtures may be in inaccessible locations. Thus, in order to detect such wear, a sensor is needed which can detect small changes in the distance between two interior locations. 
     Koeplin, et al. in U.S. Pat. No. 4,914,951 describes an ultrasonic sensor for measuring the distance from a transducer to an object. These sensors have particular applicability for motor vehicles. The invention uses at least one electroacoustic transducer to transmit an ultrasonic signal and to receive the ultrasonic signal reflected by an object. An electric generator is used to activate the transducer. A receiving stage is provided for the echo signals picked up by the transducer. A control unit uses the generator to activate the transducer for a predetermined transmission time. The transducer then decays for a time following its activation to provide a reception window for receiving the reflected echo signals. Ultrasonic signals are not always suitable for detecting material wear. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is thus provided a distance measurement system, comprising at least one resonant circuit, at least one magnetic element with predetermined magnetic properties, a transmitter operable to transmit an electromagnetic pulse, a receiver operable to detect oscillations emitted by the resonant circuit in response to the electromagnetic pulse, and an analyzer operable to analyze an amplitude envelope property of the oscillations, to thereby determine a distance between the resonant circuit and the magnetic element. In an embodiment, the magnetic element is ferromagnetic. In another embodiment the magnetic element is paramagnetic. In an additional embodiment, the analyzer is operable to determine the amplitude envelope property from an absolute value of amplitudes of the oscillations. In another embodiment the analyzer is operable to determine the amplitude envelope property from relative amplitude values of more than one cycle of the oscillations. In an embodiment the amplitude envelope property is an amplitude rate of decay. In an additional embodiment the analyzer is operable to determine the distance between the resonant circuit and the magnetic element additionally from frequency properties of the oscillations. 
     In a preferred embodiment the distance measurement system comprises more than one resonant circuit, to thereby determine more than one distance between the resonant circuits and the at least one magnetic element. Another embodiment is operable to determine the distance between the resonant circuits and the at least one magnetic element in more than one dimension. In another embodiment the more than one resonant circuits are operable to resonate at different frequencies. 
     Another embodiment comprises more than one magnetic element, to thereby determine more than one distance between the at least one resonant circuit and the magnetic elements. Another embodiment preferably is operable to determine the distance between the at least one resonant circuit and the magnetic elements in more than one dimension. 
     Another embodiment comprises more than one resonant circuit and more than one magnetic element, to thereby determine more than one distance between the resonant circuits and the magnetic elements. Another embodiment preferably is operable to determine the distance between the resonant circuits and the at least one magnetic element in more than one dimension. 
     In a preferred embodiment the analyzer comprises a look-up table, comprising relationships between measured oscillations and distances. In another embodiment the relationships are per-system relationships. In an additional embodiment, the relationships comprise in-situ calibrations. 
     According to a second aspect of the present invention there is thus provided a method for assembling a distance measurement system, comprising the steps of: placing a resonant circuit at a first location, placing a magnetic element with predetermined magnetic properties at a second location, providing a transmitter for transmitting an electromagnetic pulse to the resonant circuit, providing a detector for detecting oscillations emitted by the resonant circuit in response to the electromagnetic pulse, and providing an analyzer for analyzing an amplitude envelope property of the detected oscillations, to thereby determine a distance between the first location and the second location. In an embodiment, the analyzer comprises a look-up table of relationships between measured oscillations and distances, and wherein the look-up table values are established for each one of a predetermined set of distances by performing for each predetermined distance the steps of: transmitting an electromagnetic pulse to the resonant circuit, detecting oscillations emitted by the resonant circuit in response to the electromagnetic pulse, and measuring an amplitude envelope property of the detected oscillations, to thereby establish a look-up table value for the distance. 
     According to a third aspect of the present invention there is thus provided a method for measuring the distance between a first location comprising a resonant circuit and a second location comprising a magnetic element, comprising the steps of: transmitting an electromagnetic pulse to the resonant circuit, detecting oscillations emitted by the resonant circuit in response to the electromagnetic pulse, and analyzing an amplitude envelope property of the detected oscillations, to thereby determine a distance between the first location and the second location. In a preferred embodiment, the step of analyzing an amplitude envelope property further comprises comparing information detected from the emitted oscillations to information in a look-up table, of relationships between measured oscillations and distances. Another embodiment comprises obtaining the amplitude envelope property from an absolute value of amplitudes of the oscillations. Another preferred embodiment comprises obtaining the amplitude envelope property from relative amplitude values of more than one cycle of the oscillations. Another embodiment, comprises making use of an amplitude rate of decay as the amplitude envelope property. Another embodiment comprises additionally detecting the distance from frequency properties of the detected oscillations. 
     According to a fourth aspect of the present invention there is thus provided a distance measurement system utilizing eddy currents for energy dissipation, the system comprising: at least one resonant circuit, at least one magnetic element with predetermined magnetic properties, a transmitter operable to transmit an electromagnetic pulse, a receiver operable to detect oscillations emitted by the resonant circuit in response to the electromagnetic pulse, and an analyzer operable to analyze an amplitude envelope property of the oscillations as an indicator of eddy current induced energy dissipation, to thereby determine a distance between the resonant circuit and the magnetic element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings, in which: 
     FIG. 1 is a simplified block diagram of a preferred embodiment of a distance measurement system. 
     FIG. 2 is a simplified diagram of a resonant circuit. 
     FIG. 3 is a simplified diagram of a sensor subsystem. 
     FIG. 4 shows an example of a damped oscillation. 
     FIG. 5 shows a second example of a damped oscillation. 
     FIG. 6 is a simplified sensor subsystem with more than one resonant circuit. 
     FIG. 7 is a simplified flow chart of the distance measurement process. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     When a resonant circuit is energized by a short pulse, the circuit begins to oscillate at the free resonant frequency. The free oscillation frequency of a resonant circuit is: 
     
       
           f= 0.5*Π*( L*C ) 0.5    
       
     
     If no additional external energy is provided, the amplitude of the oscillation will decay. The amplitude rate of decay, absolute amplitude, and exact resonance frequency are influenced by two main factors. The first factor is the internal equivalent resistance of the circuit due to circuit elements. The second factor is the external equivalent resistance due to eddy currents at elements that interfere with the magnetic field of the resonant circuit. Magnetic elements in proximity to the resonant circuit create such interference, and influence the resonant circuit response to the energizing pulse. The characteristics of the oscillation induced in the resonant circuit by the energizing pulse may be used to measure the distance between a resonant circuit and a magnetic element. This physical principle is the basis for the distance measurement system described below. 
     Reference is now made to FIG. 1, which is a simplified block diagram of a preferred embodiment of a distance measurement system. A distance measurement system  10  comprises two subsystems, a sensor subsystem  12  and an analysis subsystem  18 . The sensor subsystem  12  comprises at least one resonant circuit  14  and at least one magnetic element  16 , separated by a distance d that is to be detected by the measurement system. The analysis subsystem  18  comprises a transmitter  20 , receiver  22 , and analyzer  24 , all of which are controlled by controller  26 . 
     Transmitter  20  transmits a short electromagnetic pulse that energizes the resonant circuit  14 . In response to the pulse, sensor subsystem  12  emits a damped oscillation, which is received by receiver  22 . The characteristics of the emitted signal depend upon the properties of resonant circuit  14  and magnetic element  16 , and upon the distance d between them. The received signal is analyzed by analyzer  24 , which analyzes its characteristics to determine the distance d, as will be described in detail below. Controller  26  controls and coordinates all analysis subsystem  18  functions. 
     Reference is now made to FIG. 2, which is a simplified diagram of a preferred embodiment of a resonant circuit  30 . The resonant circuit shown is a simple RLC circuit comprising a capacitor C  32 , an inductor L  34 , and a resistor R  36 . The resonant frequency of resonant circuit  30  depends primarily on the values of capacitor C  32  and inductor L  34 , as defined in the above equation. The value of resistor R  36 , along with other dissipative effects such as eddy currents in elements which interfere with the circuit&#39;s magnetic field, determines the circuit&#39;s amplitude rate of decay, as discussed above. 
     Reference is now made to FIG. 3, which is a simplified diagram of a preferred embodiment of a sensor subsystem  40 . Magnetic element  42  comprises a material with known magnetic properties. In one preferred embodiment the material is ferromagnetic. In another preferred embodiment the material is paramagnetic. Resonant circuit  44  comprises a capacitor  46  and a coil  48  wound on a core  50 . When the resonant circuit  44  is energized by a pulse of electromagnetic radiation, the circuit emits a damped oscillatory electromagnetic signal. The characteristics of the emitted signal may be affected by the distance between magnetic element  42  and resonant circuit  44 . 
     Reference is now made to FIG. 4, which shows an example of a damped oscillation. The oscillation shown has two main characteristics. The first characteristic is the rate at which the amplitude of the oscillation decreases over time. The second characteristic is the frequency of the oscillation. 
     As shown in FIG. 4, the amplitude of the generated damped oscillations may be represented by a decreasing series of values, A 0  to A n.  The amplitude of the initial oscillation is A 0  and the amplitude of the n th  oscillation is A n , where n equals four in FIG.  4 . The amplitude of the damped oscillations is a function of the energy absorbed at each cycle by the internal equivalent resistance of the resonant circuit  44 , and by the external equivalent resistance due to eddy currents at elements that interfere with the magnetic field of resonant circuit  44 , primarily the magnetic element  42 . The amount of interference by magnetic element  42  is a function of the distance d between the magnetic element  42  and the resonant circuit  44 . Distance d may thus be determined from measurements of the amplitudes A i , of the oscillations produced by sensor subsystem  40 . Any reference to amplitude values in this or later embodiments may be to peak-to-peak amplitude, negative peak amplitude, positive peak amplitude, or any other amplitude indicator. 
     In one preferred embodiment distance d is determined from the absolute value of the amplitudes of one or more cycles of oscillation produced by sensor subsystem  40 . Reference is now made to FIG. 5, which shows a second example of a damped oscillation. The absolute amplitudes of the oscillations A i ′ differ from the absolute amplitudes A i  in FIG.  4 . Changing the distance d between the resonant circuit  30  and the magnetic element  42  affects the absolute signal amplitudes. Distance d may be detected by analyzing the absolute amplitudes of the emissions received from the sensor subsystem  40 . 
     In a second preferred embodiment, the distance d is measured by analyzing the relative values of the amplitudes of successive cycles of emissions received from the sensor subsystem  40 . In the preferred embodiment, the relative amplitudes are compared by examining the ratios: 
     
       
           A   r ( ik )= A   i   /A   i+k    
       
     
     where A i  is the amplitude of the i-th oscillation, and A i+k  is the amplitude after k subsequent oscillations. Higher resolution may be obtained by increasing k, however k is preferably not increased to the point where noise effects compromise amplitude measurements. 
     In another preferred embodiment, frequency information is additionally used to determine the distance between the sensor subsystem elements. The distance d may change the oscillation frequency due to the proximity of the magnetic element  42  to the resonant circuit  44 . The oscillation frequency of resonant circuit  44  is determined primarily by the values of the capacitor  46  and the coil  48 . However the distance of the magnetic element  42  from the resonant circuit  44  also influences the values of the reactive elements of the circuit. The values of the reactive elements, and hence the emitted frequency, depend upon both the magnetic characteristics of the magnetic element  42 , and its distance from the resonant circuit  44 . If the magnetic properties of magnetic element  42  are known, the emitted frequency may be used as an additional indicator of the distance d. 
     Reference is now made to FIG. 6, which is a simplified preferred embodiment of a sensor subsystem  60  with more than one resonant circuit. The subsystem comprises one magnetic element  62  and several resonant circuits  64 . 1  . . .  64 . 6 . In a preferred embodiment each resonant circuit is tuned to a different frequency. The number of resonant circuits may vary over different configurations. Each resonant circuit  64 .i is located at a distance d i  from the magnetic element  62 . Each resonant circuit emits a damped oscillation in response to a transmitted electromagnetic pulse. The total emission from the sensor subsystem  60  depends upon the distances d i . In the current embodiment the distances d i  may be measured simultaneously by analyzing the sensor subsystem emissions. In the preferred embodiment the resonant circuits  64  are designed to emit at different free oscillation frequencies in order to distinguish the signal emitted by each resonant circuit  64 . If the free oscillation frequencies are adequately separated and the resonant circuits tuned accordingly, the emission parameters of each resonant circuit may be easily distinguished. 
     The preferred embodiment can be used for determining a distance, or for monitoring a change in a distance, between any objects which are placed several millimeters to several meters apart, while providing highly accurate distance measurements with a resolution of several microns or less. The effectiveness of the preferred embodiment at various distance ranges depends mainly on the size and configuration of the sensor subsystem elements employed. 
     In another preferred embodiment the sensor subsystem comprises more than one magnetic element. Each magnetic element i is located at a distance d i  from the resonant circuit. The resonant circuit emits a damped oscillation in response to the transmitted electromagnetic pulse. The characteristics of the sensor subsystem emission depend upon the distances d i . The distances d i  may be measured simultaneously by analyzing the total emission output from the sensor subsystem. 
     In another embodiment the sensor subsystem comprises more than one magnetic element and more than one resonant circuit. Each magnetic element i is located at a distance d ik  from resonant circuit k. The resonant circuits emit damped oscillations in response to the transmitted electromagnetic pulse. The characteristics of the total emission from the sensor subsystem depend upon the distances d ik . The distances d ik  may be measured simultaneously by analyzing the total emission output from the sensor subsystem. 
     In a preferred embodiment, a dedicated look-up table is used to facilitate the analysis of the emitted signal, and to eliminate variations due to production tolerances. The look-up table stores information about emitted signal characteristics at several distances. Intermediate distances may be calculated by interpolation. The look-up table may be used to compensate for non-linearities in the resonant circuit response as a function of distance from the magnetic element. In the preferred embodiment the signal characteristic stored in the look-up table is the value of a preselected amplitude ratio, for example A 0 /A 20 . In one preferred embodiment the values stored in the look-up are expected signal characteristics. In another preferred embodiment, the values stored in the look-up table are specific to each device. These values are determined by testing each device after manufacture. In another embodiment, the distance d is determined by linear interpolation of the amplitude. 
     Reference is now made to FIG. 7, which is a simplified flow chart of an embodiment of the distance measurement process using the system described above. First an electromagnetic pulse is emitted in the vicinity of the sensor subsystem in step  72 . The sensor subsystem emits a damped oscillation in response to the pulse, which is detected in step  74 . The amplitudes of the required cycles of the detected oscillation are measured in step  76 . Dividing the measured amplitudes forms an amplitude ratio in step  78 . The look-up table is used to convert this ratio into a distance measurement in step  80 . 
     The embodiments of distance measurement systems described above all utilize the effect of eddy currents on resonant circuit emissions to measure the distance between one or more resonant circuits and one or more magnetic elements. Various signal parameters, such as absolute amplitude, relative amplitude, and frequency may be utilized to determine the distance between each resonant circuit and each magnetic element. 
     It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined by the appended claims and includes both combinations and subcombinations of the various features described hereinabove as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description.