Abstract:
At least one exemplary embodiment of the present invention includes a capacitive sensing system, comprising a sensing conductor coupleable to a grounded target by a gap capacitance C d , said grounded target separated from said sensing conductor by a gap having a width. The capacitive sensing system also comprises a circuit connected to said sensing conductor, an input signal having an input frequency f osc  provided to said circuit through an input resistance R d , an output signal of said circuit having an output voltage varying linearly with the width of the gap when the impedance of the gap capacitance 1/(2πf osc C d ) approaches or exceeds the input resistance R d . It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. This abstract is submitted with the understanding that it will not be used to interpret or limit the scope.

Description:
BRIEF DESCRIPTION OF THE DRAWINGS  
         [0001]    The invention and its wide variety of potential embodiments will be readily understood via the following detailed description of certain exemplary embodiments, with reference to the accompanying drawings in which:  
           [0002]    [0002]FIG. 1 is an electrical diagram of an exemplary embodiment of a system  1000  of the present invention;  
           [0003]    [0003]FIG. 2 is an electrical diagram of an exemplary embodiment of a system  2000  of the present invention;  
           [0004]    [0004]FIG. 3 is an electrical diagram of an exemplary embodiment of a system  3000  of the present invention;  
           [0005]    [0005]FIG. 4 is an electrical diagram of an exemplary embodiment of a system  4000  of the present invention;  
           [0006]    [0006]FIG. 5 is an electrical diagram of an exemplary embodiment of a system  5000  of the present invention;  
           [0007]    [0007]FIG. 6 is a block diagram of an exemplary embodiment of an information device  6000  of the present invention; and  
           [0008]    [0008]FIG. 7 is a flow diagram of an exemplary embodiment of a method  7000  of the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0009]    At least one exemplary embodiment of the present invention includes a capacitive position sensing system that comprises a sensing conductor coupleable to a grounded target by a gap capacitance C d , said grounded target separated from said sensing conductor by a gap having a width. The capacitive sensing system also comprises a circuit connected to said sensing conductor, an input signal having an input frequency f osc  provided to said circuit through an input resistance R d , an output signal of said circuit having an output voltage varying linearly with the width of the gap when the impedance of the gap capacitance 1/(2πf osc C d ) approaches or exceeds the input resistance R d .  
         [0010]    At least one exemplary embodiment of the present invention includes a system that comprises a delay element connected to an op-amp, said delay element comprising a stray capacitance C s  between a guard conductor and a sensing conductor. The delay element also comprises a stray capacitance C c  between the guard conductor and a grounded shield, an input resistance R d  connected serially between an oscillating voltage input and said op-amp, and a variable resistance R c  connected in series between an output of said op-amp and the guard conductor. A resonant frequency of said system is approximately equal to a frequency f osc  of the oscillating voltage input.  
         [0011]    At least one exemplary embodiment of the present invention includes a method that comprises providing an oscillating signal to a system comprising a delay element serially connected to an input of an op-amp, the delay element comprising a stray capacitance between a guard conductor and a sensing conductor, and a variable resistor. The method also comprises adjusting a resistance of the variable resistor such that a resonant frequency resulting from the delay element approximately equals a frequency of the provided oscillating signal.  
         [0012]    [0012]FIG. 1 is an electrical diagram of an exemplary embodiment of a capacitive sensing system  1000  of the present invention. A grounded target  1100  can be sensed by as few as one sensor  1200 , which is connected via a cable  1300  to sensor electronics  1400 . When sensor  1100 , which can comprise as few as one sensing electrode  1210  surrounded by a guard  1250 , is placed in proximity of grounded target  1100 , the sensing electrode  1210  can be capacitively coupled to the target  1100  (and hence ground) by an effective gap capacitance  1240  given by:  
         C d =ε o S/g   (1)  
         [0013]    where ε o  is the permittivity of free space, S is the effective surface area  1220  of the electrode, and g is the gap  1230  between the sensor electrode  1210  and the target  1100 .  
         [0014]    By designing a suitable circuit  1400  whose output voltage  1480  varies inversely with gap capacitance  1240  (C d ), the gap  1230  can be measured. The circuitry  1400  can be connected to the sensor  1200  through a cable  1300 , which can be up to several meters or more in length. At the sensor end of the cable  1300 , the center conductor  1310  of the cable  1300  can be connected to the sensing electrode  1210 . The guard electrode  1250  can surround the center conductor  1310  and the sensing electrode  1210 . A grounded shield  1270  can surround the guard  1250 .  
         [0015]    The center conductor  1310  can be coupled to the guard conductor  1250  through an effective stray capacitance  1260  (C s ), and the guard conductor  1250  in turn can be coupled to the grounded shield  1270  through an effective stray capacitance  1280  (C c ).  
         [0016]    A signal source  1410 , such as an oscillator, providing a sinusoidal signal of amplitude V osc  and frequency f osc , can be fed to the cable  1300  through a resistor  1420  (R d ) of high impedance. The resistor  1420  (R d ) and gap capacitance  1240  (C d ) form a voltage divider network, and this voltage can be fed to the non-inverting input of an op-amp  1430  (A 1 ), which can serve as a buffer by connecting its output and inverting input terminals.  
         [0017]    The actual performance of the op-amp  1430  (A 1 ) is represented as an ideal op-amp (with infinite open-loop bandwidth and gain) in series with a first-order lag network  1440  with bandwidth (f u ), where f u  is the unity gain bandwidth of the op-amp. Variable resistor  1450  (R c ) is in series with the actual op-amp  1430 .  
         [0018]    As the gap  1230  (g) between the sensor  1210  and target  1100  is varied, the amplitude (V o ) of the sinusoidal output signal  1460  also can vary. This sinusoidal voltage signal then can be demodulated via demodulator  1470  and subsequently filtered (not shown) using standard techniques so that the output level  1480  of the circuit is proportional to the amplitude V o , and is a DC voltage if the gap is not varying with time.  
         [0019]    If the first-order lag network  1440  and variable resistor (R c ) are absent, the relationship between the output and oscillator amplitudes can be described by the following equation:  
                 V   o     /     V   osc       =       1     1   +       (     2                 π                   R   d          C   d          f   osc       )     2                   (   2   )                               
 
         [0020]    In some situations, it can be desirable for the output voltage V o  to vary linearly with gap g. Using Equations (1) and (2), however, this occurs only if the product 2πR d C d f osc &gt;&gt;1. Yet when the first-order lag network  1440  and variable resistor (R c ) are absent, this in general is not the case. For instance, as the gap  1230  (g) increases, C d  becomes small, and a gap will be reached for which this relationship is no longer satisfied. As a result, when the first-order lag network  1440  and variable resistor (R c ) are absent, the useful range of the capacitive sensing system can be limited.  
         [0021]    One function of the guard conductor is to minimize the effect of the stray capacitance between the center conductor and ground. If the guard conductor were absent, any such stray capacitance would appear electrically in parallel to the gap capacitance C d , and would therefore cause a reduction in sensitivity and linearity of the output V o  relative to the gap. To minimize the effect of this stray capacitance, the guard conductor can be driven by the output of the buffer so that its electrical potential is very nearly the same as the center conductor and sensing electrode. As such, essentially no stray currents between them will flow, and the stray capacitance is effectively nulled.  
         [0022]    It has been discovered that when the first-order lag network  1440  and variable resistor (R c ) are absent, capacitive sensing system  1000  can suffer from the following limitations:  
         [0023]    1. To achieve a linear relationship between the voltage V o  and the gap g, the product 2πf osc R d C d  must be much greater than unity. For larger gaps in which gap capacitance C d  becomes small, linearity may be achieved by:  
         [0024]    increasing the area S of the sensing electrode, which can increase the overall size and cost of the sensor;  
         [0025]    increasing the operating frequency f osc , which can increase the complexity, power requirements, and cost of the electrical circuitry;  
         [0026]    increasing the series resister R d , which can degrade the performance by increasing the electrical noise and reducing the gain;  
         [0027]    replacing the sinusoidal voltage source V osc  with a precisely controlled sinusoidal current source, which can increase the complexity and cost of the electrical circuitry;  
         [0028]    2. The finite bandwidth of op-amp A 1  can reduce the effective nulling of the stray capacitance, thereby reducing the sensitivity and linearity;  
         [0029]    3. The finite bandwidth of the op-amp A 1  can produce a small lag in output that when introduced to the non-inverting terminal through the guard stray capacitance C s , can result in a second-order attenuation of the output that limits the operating frequency well below the unity gain bandwidth of the op-amp;  
         [0030]    4. Stray capacitance between the non-inverting input terminal of op-amp A 1  and its power input terminals can reduce the sensitivity and linearity;  
         [0031]    5. There can be additional stray capacitance such as the fringing of the electric field at the sensing electrode that can reduce the sensitivity and linearity.  
         [0032]    [0032]FIG. 2 is an electrical diagram of an exemplary embodiment of a system  2000  of the present invention. In this figure, gap capacitance (C d ) and the stray capacitances  1280  (C c ) and  1260  (C s ) depicted in FIG. 1 are shown as discrete components. Also, variable resistor  1450  (R c ), in combination with the guard-to-shield stray capacitance  1260  (C c ), are shown to form a first-order lag network which can function as an adjustable delay element, in cascade with the delay of the op-amp  1440 .  
         [0033]    When the output of the adjustable delay element is fed-back to the non-inverting input of the op-amp  1430  through the guard stray capacitance  1260  (C s ), system  2000  forms a resonant circuit. As such, when the frequency f osc  of the oscillator  1410  is varied over a range, the circuit output amplitude V o  is amplified until the natural frequency f n  is reached, and further increases in frequency result in attenuation of the output. This amplification due to resonance tends to compensate for the non-linearity that is exhibited without the variable resistor  1450  when the impedance of the gap capacitance C d  approaches the impedance of the resistor R d .  
         [0034]    An analysis of the circuit, results presented below, shows that when the oscillator frequency f osc  is chosen to be equal to the natural frequency f n  of the circuit, the non-linearity is exactly cancelled, and the output amplitude V o  varies linearly with gap  1230  (g). This is true even when the value of the impedance of the gap capacitance  1240  (C d ) approaches the value of the impedance of the input resistance  1420  (R d ).  
         [0035]    The following observations and assumptions simplify the analysis of this circuit:  
         [0036]    The unity gain bandwidth of the op-amp f u  and the bandwidth of the low pass network (½πR c C c ) are much greater than the operating frequency f osc  so that the delays of the op-amp and the low pass network add, yielding an effective bandwidth f s  of the cascaded networks given by:  
           f   s [1/ f   u +2 πR   c   C   c ] −1    (3)  
         [0037]    The stray capacitances C s  and C c  are much greater than the sensor capacitance C d .  
         [0038]    When the guard conductor is driven by op-amp A 1 , the phase-shifted signal appears at the non-inverting input of the op-amp through the stray capacitance C s . It can be shown that this causes the circuit to behave as a damped, resonant circuit whose natural or resonant frequency f n  is given by:  
               f   n     =       [       f   s       2                 π                   R   d          C   s         ]       1   /   2               (   4   )                               
 
         [0039]    It can also be shown the damping ratio of this resonant system is given by  
             ζ   =           C   d     2          [       2                 π                   R   d          f   s         C   s       ]         1   /   2               (   5   )                               
 
         [0040]    For optimum performance, the variable resistor R c  can be adjusted such that the natural frequency f n  is approximately equal to the oscillator frequency f osc . In one exemplary embodiment, f n =f osc =125 kHz. When the two frequencies essentially match, the amplitude of the output signal V o  is related to that of the oscillator voltage V osc  by the well-known equation:  
         V o /V osc =½ζ  (6)  
         [0041]    Combining equations (5) and (6) yields an equation for the output voltage as a function of sensor capacitance:  
                 V   o     /     V   osc       =         1     C   d            [       C   s       2                 π                   R   d          f   s         ]         1   /   2               (   7   )                               
 
         [0042]    Equation (7) predicts that the output V o  is inversely proportional to the sensor capacitance C d  for the improved circuit with resonant amplification, and therefore the output voltage is linear with gap. Equation (7) can be contrasted to Equation (2), which predicts that when the first-order lag network  1440  and variable resistor (R c ) are absent, the output voltage amplitude V o  is linear with gap only if 2πR d C d f osc &gt;&gt;1.  
         [0043]    Thus, when the first-order lag network  1440  and variable resistor (R c ) are present, and particularly selected such that it causes f n  to equal and/or approximately equal f osc , the linearity and range of the circuit can be greatly improved. Moreover, the size of the sensor, power requirement, cost, and/or complexity of the sensing system can remain the same. Limitations (1) through (3) that were presented above for the system with the first-order lag network  1440  and variable resistor (R c ) absent can be eliminated.  
         [0044]    Referring to FIG. 2, there is a parasitic capacitance between the non-inverting input of the op-amp and the positive and negative power inputs. The parasitic capacitances are shown as the components  1434  (C p ) and  1438  (C n ) in the schematic for the positive and negative power inputs, respectively. This parasitic capacitance is electrically in parallel with the sensor capacitance C d  and therefore causes a reduction in sensitivity and linearity of the voltage output V o  relative to the gap g. To minimize the effect of this parasitic capacitance, the circuit can be modified such that the power input terminals of the op-amp are at approximately the same AC potential as its non-inverting input.  
         [0045]    [0045]FIG. 3 is an electrical diagram of an exemplary embodiment of a system  3000  of the present invention that includes this improved circuit. The parasitic capacitances are shown as the components  1434  (C p ) and  1438  (C n ) in the schematic for the positive and negative power inputs, respectively. The high frequency output of op-amp  1430  (A 1 ) is added to the positive and negative supply voltages at the input terminals of buffer amplifiers  1432  (A 2 ) and  1436  (A 3 ), respectively. The outputs of these buffers drive the power inputs to the op-amp  1430  (A 1 ). As such, the amount of parasitic current that flows through C p  and C n  is essentially reduced to zero. This improvement can eliminate limitation (4) presented above for the system  1000  which suffers from a reduction in linearity due to the parasitic capacitance between the non-inverting input of the op-amp and the positive and negative power inputs.  
         [0046]    Other stray capacitances can reduce the linearity of the sensing system. For instance, stray electrical fields at the sensing electrode would appear as a shunt capacitor in parallel with the sensor capacitance C d . FIG. 4 is an electrical diagram of an exemplary embodiment of a system  4000  of the present invention, and includes an improved circuit that includes an adjustment for any residual stray capacitances, shown collectively as  1242  (C z ) in the schematic. Op-amp  1430  (A 1 ), previously operated as a unity-gain buffer, is re-configured to produce non-inverting gain, where the stage gain is given by (1+R o /R b ). By making R b  a variable resistor  1444 , the gain of this stage can be made to vary over a range.  
         [0047]    When the gain of the op-amp stage is greater than unity, stray current flows via the stray capacitance  1260  (C s ) between the sensing conductor and the guard conductor because the potential of the guard is no longer equal to that of the sensing conductor. If the stray current through C s  exactly balances the stray currents flowing through the residual stray capacitance C z , then the effect of the C z  is nullified. This condition occurs when the gain G is chosen such that  
           G− 1= R   a   /R   b   =C   z   /C   s    (8)  
         [0048]    For a properly designed system, C z &lt;&lt;C s , and therefore R a &lt;&lt;R b . As such, the gain G of the op-amp stage is typically slightly greater than unity. Because the residual capacitance C z  can be difficult to measure or calculate, best performance of the circuit can be attained by experimentally adjusting R b  to achieve the optimum linearity. This improvement can eliminate limitation (5) presented above for the system which suffers from a reduction in linearity and gain due to the parasitic capacitance such as fringing of the electric field at the sensing electrode.  
         [0049]    In certain exemplary embodiments, typical values/part numbers for certain components of system  4000  can be as follows:  
         [0050]    R d : 1.5 M Ohm  
         [0051]    R c : 10 Ohm  
         [0052]    R a : 3.32 Ohm  
         [0053]    R b : 10 K Ohm  
         [0054]    A 1 : Burr Brown OPA671AP  
         [0055]    A 2 : Burr Brown BUF634T  
         [0056]    A 3 : Burr Brown BUF634T  
         [0057]    C s : 320 pF typical stray capacitance  
         [0058]    C c : 1950 pF typical stray capacitance  
         [0059]    C d : 0.1 pF typical gap capacitance  
         [0060]    [0060]FIG. 5 is an electrical diagram of an exemplary embodiment of a system  5000  of the present invention. System  5000  can include a target subsystem  5100  that is capacitively coupled to a sensing subsystem  5200 , which can be electrically coupled to a processing subsystem  5300  and/or an information device  5400 . Processing subsystem  5300  also can be connected to information device  5400 , which can be connected via a network  5500  to another information device  5600 , which can log information to a storage  5700 , such as an archive or memory.  
         [0061]    An embodiment of sensing subsystem  5200  can be any of systems  1000 ,  2000 ,  3000 , and/or  4000 . Sensing subsystem  5200  can sense gap, displacement, position, proximity, vibration, velocity, acceleration, jerk (the first derivative of acceleration with respect to time), pulse (the second derivative of acceleration with respect to time), and/or time (e.g., time of coupling, duration of coupling, time at which particular velocity occurs, time over which a acceleration occurs, etc.), etc.  
         [0062]    Target  5100  can be any of a wide range of devices, including a machine having a rotating shaft or reciprocating component, a servo-positioner, and/or a magnetic bearing. Target  5100  also can be a vibrating structure such as, for example, structural steel in a building, a pipe in a power plant, a vehicle engine, etc.  
         [0063]    Processing subsystem  5300  can process an output signal of sensing subsystem  5200  to determine an amplitude of a voltage of that signal. Upon receiving the output signal, processing subsystem  5300  can correlate the amplitude of that signal to, for example, to a measurement and/or determination (e.g., target is or is not capacitively coupled to sensor, target will likely be coupled in 100 microseconds, target is moving away from sensor, etc.) of gap, displacement, position, proximity, vibration, velocity, acceleration, jerk, and/or time, etc. Processing subsystem  5300  can store, communicate, and/or further process the amplitude, measurement, determination, and/or recognition. For example, processing subsystem  5300  can communicate an alert (e.g., sound an annunciator, send a paging message, and/or flash an alert box on a monitor, etc.) when a measurement exceeds a predetermined (e.g. minimum, maximum, threshold, etc.) value.  
         [0064]    In one embodiment, processing subsystem  5300  can comprise a commercially available general-purpose microprocessor. In another embodiment, processing subsystem  5300  can comprise an Application Specific Integrated Circuit (ASIC) that has been designed to implement in its hardware and/or firmware at least a part of a method in accordance with an embodiment of the present invention. In yet another embodiment, processing subsystem  5300  can comprise a Field Programmable Gate Array (FPGA).  
         [0065]    Processing subsystem  5300  also can comprise a memory comprising instructions that can be embodied in software, which can take any of numerous forms that are well known in the art. Processing subsystem  5300  also can include a communications interface, such as a bus, a connector, a telephone line interface, a wireless network interface, a cellular network interface, a local area network interface, a broadband cable interface, etc. Processing subsystem  5300  can be implemented in any of a wide range of configurations, such as, for example, integrated with sensing subsystem  5200 , as a stand-alone device (such as a personal computer or the like), as a subsystem (e.g. plug-in card) of a personal computer or the like, etc.  
         [0066]    Network  5500  can be a public switched telephone network (PSTN), a private network, a wireless network, a cellular network, a local area network, the Internet, etc.  
         [0067]    Information devices  5400 ,  5600  also can comprise a microprocessor, a memory, instructions, and/or a communications interface. Information devices  5400 ,  5600  can be embodied in any of wide range of devices, such as a traditional telephone, telephonic device, cellular telephone, mobile terminal, Bluetooth device, communicator, pager, facsimile, computer terminal, personal computer, etc. Information devices  5400 ,  5600  can be used to program, interact with, and/or monitor sensing subsystem  5200  and/or processing subsystem  5300 .  
         [0068]    [0068]FIG. 6 is a block diagram of an exemplary embodiment of an information device  6000  of the present invention. Information device  6000  can represent any of information devices  5400 ,  5600 , or even processing subsystem  5300 . Information device  6000  can include well-known components such as one or more communication interfaces  6100 , one or more processors  6200 , one or more memories  6300  containing instructions  6400 , and/or one or more input/output (I/O) devices  6500 , etc.  
         [0069]    In one embodiment, communication interface  6100  can be a bus, a connector, a telephone line interface, a wireless network interface, a cellular network interface, a local area network interface, a broadband cable interface, a telephone, a cellular phone, a cellular modem, a telephone data modem, a fax modem, a wireless transceiver, an Ethernet card, a cable modem, a digital subscriber line interface, a bridge, a hub, a router, or other similar device.  
         [0070]    Each processor  6200  can be a commercially available general-purpose microprocessor. In another embodiment, the processor can be an Application Specific Integrated Circuit (ASIC) or a Field Programmable Gate Array (FPGA) that has been designed to implement in its hardware and/or firmware at least a part of a method in accordance with an embodiment of the present invention.  
         [0071]    Memory  6300  can be coupled to processor  6200  and can comprise any device capable of storing analog or digital information, such as a hard disk, Random Access Memory (RAM), Read Only Memory (ROM), flash memory, a compact disk, a digital versatile disk (DVD), a magnetic tape, a floppy disk, and any combination thereof. Memory  6300  can also comprise a database, an archive, and/or any stored data and/or instructions. For example, memory  6300  can store instructions  6400  adapted to be executed by processor  6200  according to one or more activities of a method of the present invention.  
         [0072]    Instructions  6400  can be embodied in software, which can take any of numerous forms that are well known in the art. Instructions  6400  can control operation of information device  6000  and/or one or more other devices, systems, or subsystems.  
         [0073]    Input/output (I/O) device  6500  can be an audio and/or visual device, including, for example, a monitor, display, keyboard, keypad, touchpad, pointing device, microphone, speaker, video camera, camera, scanner, and/or printer, including a port to which an I/O device can be attached, connected, and/or coupled.  
         [0074]    [0074]FIG. 7 is a flow diagram of an exemplary embodiment of a method  7000  of the present invention. At activity  7100 , a sinusoidal signal can be applied to the capacitive sensing circuit. At activity  7200 , a target can be capacitively coupled to a sensor conductor across a gap. At activity  7300 , a variable resistor of the circuit can be adjusted such that the resonant frequency of the system f n  matches that of the oscillator frequency f osc . At activity  7400 , one or more stray capacitances can be nulled, such as by adjusting a second variable resistor. At activity  7500 , the circuit can output a voltage that varies linearly with the gap. At activity  7600 , the output voltage can be processed. At activity  7700 , an amplitude of the voltage can be correlated, such as, for example, to a measurement and/or determination of gap, displacement, position, proximity, vibration, velocity, acceleration, jerk, and/or time, etc.  
         [0075]    The following reference is incorporated herein by reference in its entirety: Baxter, Larry K., Capacitive Sensors, Design and Applications, 1997, IEEE, New York.  
         [0076]    Although the invention has been described with reference to specific embodiments thereof, it will be understood that numerous variations, modifications and additional embodiments are possible, and accordingly, all such variations, modifications, and embodiments are to be regarded as being within the spirit and scope of the invention. Also, references specifically identified and discussed herein are incorporated by reference as if fully set forth herein. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive.