CROSS-REFERENCE TO RELATED APPLICATIONS 
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     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     BACKGROUND OF THE INVENTION 
     The present invention relates generally to active sensors for electronically sensing the presence of an object and in particular to such a sensor having improved noise immunity. 
     The presence or absence of an object may be detected by measuring the interaction of the object with an electromagnetic field generated in a sensing volume. The object, when in the sensing volume, introduces a new or changed impedance into the circuit generating the electromagnetic field through capacitive or inductive coupling. Sensors that provide the source of the electromagnetic field used for sensing will be termed “active” sensors. 
     In a capacitive presence sensor, for example, an object may increase a capacitive coupling between an electrode of the generating circuit and environmental ground return paths. In an inductive presence sensor, the object may inductively couple to an antenna of the generating circuit to change the effective inductance of that antenna. 
     This change in impedance, caused by the introduction of an object within the sensing area, is manifest as an energy transfer from the generating circuit to the object, such energy transfer being detected by a sensing circuit, for example, as increased current flow. The amount of energy transfer may be compared against a threshold to produce a binary, switched output indicating the presence or absence of an object within the sensed area. 
     Such electromagnetic field presence sensors do not require direct physical or electrical (ohmic) contact with the object and thus can be easily sealed against water and dirt for use in hostile industrial environments. 
     A tradeoff exists between the degree of sensitivity of such presence sensors and thus their ability to be triggered by small or remote objects, (e.g. a hand separated from the sensor by a thick glove), and their susceptibility to noise. As the sensitivity of the sensor is increased (increasing the sensing volume or decreasing the size of the object sensed) by setting the threshold to detect smaller energy transfers, there is an increased chance that electrical noise from the environment or conducted through the power line provided to the sensing circuitry will cause false triggerings of the sensor. 
     Averaging circuitry may be added to the sensing circuitry so as to diminish the effect of noise relative to the longer term signal generated and measured by the presence sensor. Such averaging circuitry, however, also slows the response of the presence sensor to changes in the presence or absence of an object it is detecting, thus limiting the application of such switches in cases where fast response is required. 
     SUMMARY OF THE INVENTION 
     The present inventors have recognized that electrical noise not only tends to be limited in the time domain, that is, to occur in bursts of limited duration, but that it is also limited in the frequency domain to occur, during any given burst, in a relatively narrow set of frequencies. Accordingly, an improved presence sensor can be constructed by applying to the sensing volume, a broadband electromagnetic signal and separately analyzing frequency bands of that signal to independently ascertain whether an object is present. Conflicts in these determinations at different frequencies, such as may be caused by electrical noise, is resolved by means of a voting circuit which adopts the output indicated by a majority of the determinations. 
     Specifically, the invention provides a method of sensing the presence of an object in a sensing volume including the steps of generating an electromagnetic signal composed of a plurality of different frequencies and electromagnetically communicating the electromagnetic signal to a sensing volume. Energy transfers to the sensing volume at the plurality of frequencies are separately detected and the energy transfers at the plurality of frequencies are compared to detect the presence of an object in the sensing volume and to provide an output signal. 
     Thus it is one object of the invention to provide a broadband presence sensor that may better resist frequency limited electrical noise. 
     The energy transfer at each frequency may be compared against a threshold indicating an energy transfer associated with the presence of the object to produce a frequency linked presence signal at each of the frequencies. The number of frequency linked presence signals indicating the presence of an object may be compared to the number of frequency linked presence signals indicating the absence of the object to determine the output signal. The comparison of the output signals observe a simple majority. 
     Thus it is another object of the invention to provide a simple voting method for eliminating artifacts caused by electromagnetic interference such as may provide a high degree of noise immunity even when multiple frequencies of the electromagnetic signal are obscured by electromagnetic noise. 
     The electromagnetic signal may be communicated to the sensing volume by an electrode capacitively coupled to an object in the sensing volume or by an inductor inductively coupled to the object in the sensing volume. 
     Thus it is another object of the invention to provide a technique that may be used for different types of electromagnetic presence sensors. 
     Each of the frequency linked sensor signals may be separately weighted in the comparison process. 
     Thus it is another object of the invention to provide a sensing of an object that is tailored to the particular frequency dependent characteristics of the object. 
     The amount of energy transfer may be detected by measuring changes in current or voltage at the different frequencies of the electromagnetic signal through or across a known impedance. 
     Thus it is another object of the invention to provide for a simple mechanism of measurement of energy transfer. 
     The foregoing objects and advantages may not apply to all embodiments of the inventions and are not intended to define the scope of the invention, for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a presence sensor such as may incorporate the present invention, providing a housing holding a sensing circuit and having a upper surface supporting a sensing electrode or inductor and an output cable conducting an output signal indicating the presence of an object in a sensing volume above the upper surface; 
     FIG. 2 is a schematic representation of the sensing circuit and electrode of FIG. 1 showing the effect of an object in the sensing volume and showing the introduction of noise into the sensing circuit; and 
     FIG. 3 is a detailed diagram of the sensing circuit of the present invention showing the generation of multiple frequencies to form the electromagnetic signal and their separation to provide separate frequency linked sensing signals that are combined by a voting circuit to produce the output signal. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, a presence sensor  10  per the present invention includes a housing  12  supporting on one face, one or more electrode pads  14 . Although the electrodes are shown for clarity, generally they are electrically insulated from an adjacent sensing volume  16 . Cabling  18  may exit the presence sensor  10  providing power conductors  22  for conducting power to internal sensing circuitry (not shown) and at least output  25  providing a presence signal indicating the presence or absence of an object within the sensing volume  16 . 
     Referring now to FIG. 2, the housing  12  holds sensing circuit  20  connecting to the electrode pad  14 , the power conductors  22 , and the output  25  providing the presence signal. During operation, an object  24  (such as a human hand) may move into the sensing volume  16  thereby establishing a capacitive coupling  26  with the electrode pad  14  indicated by capacitance C po  (capacitance between the pad and the object). Capacitance C po  provides a path of energy transfer from the electrode pad  14  into the object  24  and through a capacitive coupling  28  between the object and its environment indicated by capacitance C oe  (capacitance between the object and earth). A completed circuit between the sensing circuit  20  and the object  24  is provided by capacitive coupling  30  indicated by capacitance C se  (capacitance between the sensing circuit and earth). Alternatively, but not shown, the sensing circuit  20  may be directly coupled to earth. Capacitance C oe  and C se  result from the normal proximity and connection of the object  24  and sensing circuit  20  to their environments. 
     A noise source  32  may introduce a noise current into a junction between the sensing circuit  20  and capacitance C se  causing a perturbation in the voltage level of the sensing circuit  20  with respect to earth. This perturbation can, for example, cause additional current to flow from the sensing circuit electrode pad  14  to the object  24  insofar as the energy transfer through the object  24  to earth will be in some part proportional to the voltage difference between electrode pad  14  and earth. Noise source  32  is intended to show one mechanism for the introduction of noise into the signals sensed by the sensing circuit  20  but generally the present invention will also address other avenues of noise introduction well known in the art including capacitive coupling or induction into other leads or points in the circuit. 
     The present inventors have recognized that in many situations, the noise source  32  is band limited, meaning that the noise is represented by a limited number of different frequencies over an arbitrary time interval. Accordingly, a broad-spectrum sensing signal may be used to decrease the influence of such noise signals. 
     Accordingly, referring now to FIG. 3, the sensing circuit  20  may include a plurality of frequency generators  34 , each producing a relatively narrow band signal having spaced center frequencies f 0  through f n . These signals may be produced by separate oscillator circuits of a type well known and combined by a summing circuit  36  to produce a composite waveform  38 . Alternatively, the composite waveform  38  may be produced by digital synthesis of a single wave being the combination of the desired signals using a digital signal processor (DSP) of a type well known in the art. The frequencies are preferably in the range of 150 kHz to one MHz. 
     In yet a further alternative embodiment, different ones of the frequency generators  34  may be activated in sequence (with the outputs of the other frequency generators  34  effectively suppressed) so that an instantaneously narrow band signal is output from the summing circuit  36  but so that the composite waveform  38  is nevertheless composed of many frequencies when viewed over a period of time. This approach can simplify the synthesis of the composite waveform  38  and can simplify the decoding of frequency linked presence signals described below. 
     The composite waveform  38  is communicated to the electrode pad  14  where it creates a changing voltage such as may capacitively couple with the object  24 . Alternatively in an inductive version of the invention, the composite waveform  38  may be conducted to an inductive coil antenna  40  providing a fluctuating magnetic field such as may inductively couple to the object  24 . 
     The energy transferred from the frequency generators  34  and summing circuit  36  (or from an output of the DSP) to the object  24  may be detected by a sensor  42 . In one embodiment, the sensor  42  is a resistor whose terminal voltage values indicate current flowing through the electrode pad  14  to the object  24 . The output of the sensor  42  may thus provide a modified composite waveform  38 ′, the modification typically being a change (amplitude increase or decrease or phase shift) in the voltage of the modified composite waveform  38 ′ compared to the composite waveform  38 , the change indicating the energy transfer to the object  24 . Other sensing systems can be easily substituted for this including other current sensing devices or voltage sensors across more complex impedances than a resistor as shown. 
     The modified composite waveform  38 ′ passes to a sequence of band-pass filters  44  having center frequencies corresponding to the frequencies f 0  through f n  of the frequency generators  34 . Each band pass filter  44  includes a peak detectors so as to produce an envelope signal  46  indicating the amplitude of the modified composite waveform  38 ′ at a particular frequency f 0  through f n  and a nominal bandwidth about those center frequencies. Again the band-pass filters  44  may be implemented as analog circuits or by means of a digital circuit including but not limited to a DSP executing a Fourier transform or the like. 
     The envelope signals  46  pass to comparators  48  which compare the envelope signals  46  to corresponding threshold value  50 , a predetermined voltage below which an envelope signal  46  from the band-pass filters  44  would tend to indicate no object  24  is present in the sensing volume  16 , and above which the envelope signal  46  from the band-pass filters  44  would tend to indicate that an object  24  is present in the sensing volume  16 . The comparators  48  may be readily implemented either in analog circuitry according to well-known techniques or in digital circuitry, preferably according to a processing of a signal by the DSP. 
     Binary signals  52  from the outputs of the comparators  48  thus provide frequency linked presence signals each independently indicating the presence or absence of the object  24  in the sensing volume  16 , as measured in a narrow frequency range. The binary signals  52  are combined in a voter circuit  56  which may operate under a simple majority principle to provide a single presence sensing output  25  corresponding to the state of the majority of the outputs of the comparators  48 . Thus if most of the comparators  48  provide a signal indicating the presence of an object  24 , the output  25  will indicate the presence of that object as well. Again the voter circuit  56  may be implemented as analog circuitry (for example by summing the binary voltages and comparing them against a threshold equal to 50% of the maximum sum) or by digital circuitry such as a simple program executed on the DSP. 
     The output  25  may be a simple digital signal or may be a more complex network compatible message for communication on a standard industrial networks such as DeviceNet or the like. 
     The threshold values  50 , against which the envelope signals  46  at the different frequencies are compared, will generally be different, reflecting the relative contribution of each frequency f 0  through f n  to the modified composite waveform  38 ′. The threshold values  50  need not adhere to this proportion, however, and may alternatively be set empirically to better discriminate the particular objects  24  intended to be sensed, or may automatically be calibrated through a process of adding and removing the object  24  from the sensing volume  16  to determine a division line between voltages indicating a presence of an object  24  and the lack of a presence of an object  24  and thus to establish the threshold. Adjustment of the threshold values  50  allows an arbitrary weighting to be imposed on the frequency linked presence signals. 
     When a simple majority voting rule is used by the voter circuit  56 , an odd number of frequencies f 0  through f n  is desired of no less than three frequencies. Other voting rules than simple majority may be used to provide more or less noise immunity including two-thirds majority rules that may provide for either more or less noise immunity depending on whether two-thirds of the signals must indicate a presence of the object or two-thirds of the signals may fail to indicate a presence of the object. 
     It will be understood from the above description that the techniques of the present invention can be applied not only to active sensors that produce a binary presence signal but also to active sensors that provide an analog output indicating, for example, a distance to a remote object as deduced by the amount of energy transfer. In this case the voting circuit compares the analog output reading at each frequency and ignores any minority, conflicting output readings that may have been corrupted by noise. It will be thus understood that the term presence sensor, as used herein, is intended to embrace active sensors that produce both binary and analog type presence outputs and that the invention is not limited to one type or the other. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but that modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments also be included as come within the scope of the following claims.
An electromagnetic field presence sensor independently evaluates the presence or absence of an object in a variety of frequency ranges. Conflicting indications of the presence of the object in these different ranges, such as may be caused by electromagnetic interference, is resolved through a voting system. In this way, band limited noise may be resisted while improving the sensitivity of the sensor and without reducing its response speed.