Abstract:
A device for discharging fastening elements, and a method of preventing a device from discharging fastening devices into human flesh, are disclosed. The device includes a coil proximate a location of discharge, a capacitive element coupled in parallel with the conductive coil to form a resonant tank circuit, an oscillator that drives the tank circuit, a frequency detector, an amplitude control circuit and a processor. The detector detects a frequency of oscillation of the tank circuit as affected by a material proximate the coil. In response to an electrical signal from the oscillator, the control circuit generates a control signal that is provided back to the oscillator. Based upon the frequency and an additional signal functionally related to the control signal, the processor provides an output signal that prevents the device from discharging when the material proximate the coil is human flesh.

Description:
FIELD OF THE INVENTION 
     The present invention relates to nail guns and similar construction, manufacturing or assembly devices, and more particularly relates to an apparatus and method for restricting operation of such devices under certain operational circumstances. 
     BACKGROUND OF THE INVENTION 
     A variety of construction, manufacturing, or assembly tools operate by discharging fastening devices towards a target material. Such tools include, for example, nail guns and staplers. Typically, the fastening devices that are discharged from these tools are projected at high velocities, so that the fastening devices effectively penetrate, and become secured with respect to, the target material. 
     Often these tools must be used at a rapid pace by construction workers and other operators. To facilitate such rapid use, the tools often include mechanisms that reduce the amount of effort that the operator must put forth in order to cause the tools to discharge the fastening devices. For example, nail guns often include pressure sensing devices near the tips of their barrels so that the nail guns discharge fasteners immediately once the nail guns are pressed onto the target material, without any additional triggering action on the part of the operator. 
     Due to the rapid pace at which the tools are used, combined with possible fatigue of the operators, or even due to carelessness on the part of the operators, the tools can be misdirected toward the operators themselves or toward other human beings. 
     To avoid the discharge of fastening devices when the tools are so misdirected, it would be advantageous for the tools to have a feature that allowed the tools to automatically determine whether the tools were being misdirected and, while determining this to be the case, rendered the tools disabled from discharging fastening devices. It would further be advantageous if such a feature in the tools did not significantly restrict the pace at which the tools could be used in construction, manufacturing, or assembly. 
     SUMMARY OF THE INVENTION 
     The present inventor has realized that a coil can be placed on the tip of a nail gun or similar device and be employed as part of a sensor to determine whether the tip of the nail gun is abutting human flesh as opposed to a standard target material such as wood or metal. The coil forms part of a resonant tank circuit of the sensor, and produces a magnetic field that causes eddy currents to occur within the abutting material in accordance with Lenz&#39;s law. The eddy currents in turn can produce a change in the quality factor of the tank circuit, and the inductive or capacitive nature of the material will cause a change in the resonant frequency of the tank circuit. The sensor is able to determine a resistance of the abutting material based upon the change in the quality factor and a reactance of the abutting material based upon the change in the resonant frequency. By comparing the measured resistance and reactance values with known values associated with different materials, the sensor is able to generate a signal indicating when the abutting material is human flesh or some other non-construction material, such that the nail gun should be disabled and allowed to fire only upon an operation override. 
     In particular, the present invention relates to a device for discharging fastening elements. The device includes a body having a location at which the fastening elements are discharged, and a sensor circuit supported by the body. The sensor circuit includes a conductive coil proximate the location and further includes a capacitive element, a frequency detector, an oscillator, an amplitude control circuit and a processor. The capacitive element is connected in parallel with the conductive coil so that the capacitive element and the conductive coil form a resonant tank circuit. The frequency detector is connected to the resonant tank circuit, detects a frequency of oscillation of the resonant tank circuit as affected by a material proximate the conductive coil and outputs a frequency signal indicative thereof. The oscillator has an output terminal and a control terminal, where the output terminal is connected to the resonant tank circuit, and where the oscillator drives the resonant tank circuit at the resonant frequency of the resonant tank circuit as affected by the material proximate the conductive coil. The amplitude control circuit is coupled to the oscillator, receives an electrical signal from the output terminal, and in response generates a control signal that is provided to the control terminal of the oscillator. The processor receives the frequency signal and an additional signal that is functionally related to the control signal. The processor provides an output signal that prevents the device from discharging at least one of the fastening elements when the processor determines that the frequency signal and the additional signal indicate that the material proximate the conductive coil is a particular material into which the fasteners should not be discharged. 
     The present invention additionally relates to a tool for discharging fastening devices. The tool includes means for discharging the fastening devices, and means for determining when the fastening devices are to be discharged, where the determining means is coupled to the discharging means. The tool additionally includes means for generating an oscillatory signal, where a resonant frequency of the oscillatory signal depends both upon characteristics of the generating means and also upon a material proximate at least one portion of the generating means, and where the generating means is supported by the discharging means. The tool further includes means for detecting a frequency of the oscillatory signal and producing a first signal indicative thereof, where the detecting means is electrically coupled to the generating means. The tool additionally includes means for producing a second signal indicative of a quality factor of the oscillatory signal, where the quality factor depends at least in part upon the material proximate the at least one portion of the generating means, and where the producing means is coupled to the generating means. The tool further includes means for providing a third signal to prevent the determining means from causing the discharging means to discharge at least one of the fastening devices, where the third signal is provided in response to the first and second signals. 
     The present invention additionally relates to a method of preventing a tool from discharging a fastening device into human flesh. The method includes exciting a resonant tank circuit having a coil with an electrical signal to produce an oscillatory signal within the resonant tank circuit and an electromagnetic field that envelops a material that is proximate the coil, where the electrical signal is continually adjusted to be at a resonant frequency of the resonant tank circuit as affected by the material. The method additionally includes generating a frequency signal indicative of a frequency of oscillation of the oscillatory signal, which is the resonant frequency of the resonant tank circuit as affected by the material. The method further includes generating a control signal for controlling an amplitude of the electrical signal so that the oscillatory signal tends toward a constant amplitude. The method additionally includes processing the frequency signal and an additional signal that is functionally related to the control signal to determine whether the material has a resistance and a reactance characteristic of human flesh. The method further includes, when the processing of the frequency signal and the additional signal indicates that the material has the resistance and the reactance characteristic of human flesh, producing an output signal that causes the tool to become disabled from discharging the fastening device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a nail gun having a coil on a tip of the nail gun in accordance with one embodiment of the present invention; 
     FIG. 2 is a perspective view of the tip of the nail gun of FIG. 1 including the coil, shown in cut-away, alongside an exemplary portion of the human body and an exemplary, standard target material; 
     FIG. 3 is a schematic diagram of a sensor circuit including the coil in the tip of the nail gun of FIGS. 1 and 2, which is capable of detecting a resistance and a reactance of a material abutting the tip of the nail gun and generating a flesh detection signal in response thereto; and 
     FIG. 4 is a plot of magnetic field strength versus distance through the target material of FIG. 2 along line  3 — 3  when the tip of the nail gun including the coil of FIGS. 1 and 2 abuts the target material; 
     FIG. 5 is a graph of resistance versus reactance showing exemplary characteristic resistances and reactances associated with different materials including standard target materials and human flesh, which information can be employed by the sensor circuit of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a nail gun  10  is shown to include a barrel  12 , a handle  14  and a trigger  16 . The nail gun  10  is representative of a variety of different types of tools employed in construction, manufacturing or other assembly processes to affix fasteners to target materials including, for example, staplers. The nail gun  10 , which can be held by an operator at handle  14 , further includes (or is coupled to) a nail supply  18  and a power supply  20 . The nail supply  18  is shown to be a cartridge full of nails, although in alternate embodiments other sources of nails can be employed. The power supply  20  is shown to be an electric power cord, although in alternate embodiments the power supply can be a battery, air pressure supply, or other source of power. 
     Referring to FIGS. 1 and 2, the barrel  12  includes a tip  22  out of which the nail gun  10  discharges nails. At the tip  22  is a pressure sensor  24 . Although an operator can manually fire the nail gun  10  by pressing the trigger  16 , the nail gun is designed to allow automatic triggering by way of the pressure sensor  24 . That is, when the tip  22  of the nail gun  10  is pressed against a standard target material such as a wooden beam  25 , the pressure sensor  24  detects the pressure on the tip  22  and produces a signal that automatically triggers the nail gun to discharge a nail. The standard target material can be, instead of the wooden beam  25 , any of a number of different materials such as metal, plaster, or concrete. 
     In accordance with one embodiment of the present invention, also at the tip  22  is a wire coil  26  that can be made from standard copper wire or another conductor. As shown in FIG. 2, the coil  26  is typically in front of the pressure sensor  24  on the barrel  12  so that, when the nail gun  10  abuts a target material, the coil  26  in particular also abuts or is in close proximity to the target material. 
     The coil  26  forms part of a sensor circuit  30  shown in FIGS. 1 and 3. The sensor circuit  30  disables the nail gun  10  from automatically discharging nails at times when the nail gun is misdirected toward human flesh such as a human hand  29  (see FIGS. 2 and 3) instead of toward a standard target material such as the wooden beam  25 . Although the sensor circuit  30  disables the nail gun  10  from automatically discharging nails in such circumstances, in the embodiment of FIG. 1 the operator is able to override the disabling of the nail gun by manually pressing the trigger  16 . Thus, if it is determined by the operator that the sensor circuit  30  has incorrectly determined a material proximate the tip  22  to be human flesh when it is not, the operator can override this determination. 
     In alternate embodiments, no manual override is possible, or another device other than the trigger  16  governs the overriding of the determination of the sensor to circuit  30 . In further alternate embodiments, the nail gun  10  is not designed to allow automatic discharging of nails, but rather is designed to allow only manual triggering of the discharging of nails (e.g., there is no pressure sensor  24  and manual triggering occurs by way of the trigger  16 ). In such embodiments, the sensor circuit  30  would preclude any manual triggering of the discharging of nails whenever the sensor circuit determined that the nail gun  10  was misdirected toward human flesh. 
     Referring to FIG. 3, the sensor circuit  30  operates to distinguish human flesh such as the hand  29  from other materials such as the wooden beam  25  by sensing two characteristics using the coil  26 , namely, resistance (or conductance) and reactance. The sensor circuit  30  shown in FIG. 3 is an exemplary embodiment of a sensor circuit that is capable of measuring both resistance and reactance; however, alternative embodiments are also possible. 
     As shown in FIG. 3, the effective circuit of the coil  26  in proximity to a material that is at least partly conductive, such as the wooden beam  25  or the human hand  29 , can be modeled as the coil  26  having inductance L 1 , inductively coupled (as if in a transformer) to a second inductor  13  having inductance L 2 , which is connected in parallel with an imaginary element  15  and a resistor  17  having reactance J 1  and resistance R 1 , respectively. The inductor  13 , imaginary element  15 , and resistor  17  are not discrete elements, but are merely respectively representative of the equivalent lumped values incorporating the distributed inductance, reactance and resistance of many looping current paths of eddy currents that can pass through either of the materials  25 , 29 . The reactance of the imaginary element  15  can include both inductance and capacitance (+JX or −JX, respectively). Generally, however, the resistance R 1  of the resistor  17  will reflect a total resistance (or conductance, 1/R 1 ) in the region proximate the coil  26 . 
     When an oscillating current is provided to the coil  26 , a changing magnetic field or flux  27  is produced by the coil. FIG. 4 shows an exemplary amplitude of the magnetic flux  27  along a transverse plane through a target material such as the wooden beam  25  caused by oscillatory current flow through the coil  26 . As shown, the amplitude of the magnetic flux  27  is concentrated within the target material and drops off rapidly beyond the outer edges  28  of the target material. 
     Whenever a conductive or partially-conductive material such as materials  25 , 29  is proximate the coil  26 , the oscillating magnetic flux  27  will induce eddy currents within the material. The magnitude of the eddy currents is proportional to the conductivity of the material. For example, if the material proximate the coil  26  was metal and was perfectly conductive, then theoretically the eddy currents would be sufficiently strong as to generate a magnetic flux (back EMF) opposing the magnetic flux  27  to completely cancel the magnetic flux  27  within the coil  26 . To the extent that the material is not perfectly conductive, the eddy currents will be lower, and so the magnetic flux  27  will be reduced but not canceled. Thus, a measurement of the back EMF that is created in the coil  26  by the eddy currents within the material abutting the coil provides an indication of the conductivity and thus the resistance of that material. 
     The back EMF created in this coil  26  and thus the resistance R 1  of the effective resistor  17  is detectable as a decrease in the quality factor of an resonant tank circuit  31  employing the coil  26 . The resonant tank circuit  31  is formed from the parallel combination of the inductance L 1  of the coil  26 , and the capacitance C 2  of a capacitor  33  within the sensor circuit  30 . In a preferred embodiment, the capacitance value C 2  is selected to tune the combination of L 1  and C 2  into parallel resonance at approximately 4.5 MHz. 
     As is known in the art, the quality factor of the resonant tank circuit  31  provides a measure generally indicating how long the resonant tank circuit would continue to oscillate without the input of additional energy (free oscillation). Without eddy currents, the resonant tank circuit  31  formed from the coil  26  and the capacitor  33  would be expected to oscillate for a time limited only by the intrinsic resistance associated with the coil and the capacitor. With eddy currents, the resulting back EMF adds an effective power dissipating resistance to the resonant tank circuit, shortening the time of free oscillation. Thus, a measure of the quality factor of the resonant tank circuit  31  provides an indication of the resistance (or conductance or conductivity) of whatever material is proximate the coil (such as materials  25  or  29 ). 
     Although the resistance of a material proximate the coil  26 , such as materials  25  or  29 , can be determined by measuring the quality factor of the resonant tank circuit  31 , quality factor measurements do not provide an indication of the reactance of the material proximate the coil. However, because the resonant frequency of the resonant tank circuit  31  varies based upon the values of the effective reactance J 1  (which can include inductance and/or capacitance) as well as the inductance L 2  of either material  25  or  29 , measurement of changes in the resonant frequency of the resonant tank circuit  31  can be used as an indication of the reactance of the material. Typically, if the reactance is positive (e.g., primarily due to the inductance), the resonant frequency will be increased above its normal level, while if the reactance is negative (e.g., primarily due to capacitance), the resonant frequency will be decreased below its normal level. 
     The sensor circuit  30  includes circuit elements that are capable of detecting (or detecting changes in) both the quality factor and the resonant frequency, which respectively are then used to determine the effective resistance R 1  and the effective reactance (due to the effective inductance and/or capacitance) of a material proximate the coil  26  such as the materials  25  or  29 . With respect to determining the resonant frequency of the resonant tank circuit  31  as affected by a material such as materials  25  or  29 , the sensor circuit  30  includes a frequency detector  34  that is coupled to the resonant tank circuit and produces a frequency signal (f OUT ) indicative of the resonant frequency of the resonant tank circuit. The frequency detector  34  can be any one of a number of different types of frequency counters or detection circuits known to those skilled in the art. 
     As for determining the quality factor, measurement of the quality factor of a resonant circuit is well known in the art. To improve the accuracy of the quality factor measurement, the measurement should be made at the resonant frequency of the resonant tank circuit  31  as affected by any material proximate the coil  26  such as the materials  25  or  29 . Therefore, in a preferred embodiment, an operational transconductance amplifier (OTA)  32  is employed as an oscillator to provide the desired feature of tracking the resonant frequency of the resonant tank circuit  31  as affected by the proximate material, and to drive the resonant tank circuit at that resonant frequency. 
     As shown in FIG. 3, the OTA  32  is connected at its output  38  to a first junction  37  between the capacitor  33  and the coil  26  of the resonant tank circuit  31 , which is also the junction at which the frequency detector  34  is coupled. A remaining junction  39  between the capacitor  33  and the coil  26  is connected to ground. The output  38  of the OTA  32  is also connected to a non-inverting input  35  of the OTA  32 . In this positive feedback configuration, the output current at the output  38  of the OTA  32  will naturally oscillate at the resonant frequency of the resonant tank circuit  31  as affected by a material proximate the coil  26  such as materials  25 , 29 . Consequently, the output current at the output  38  is an oscillator signal  41  that drives the resonant tank circuit  31  at its resonant frequency (as affected by any proximate material such as materials  25 ,  29 ) so that the resonant tank circuit will continue to oscillate. It will be further understood that, by driving the resonant tank circuit  31  at its resonant frequency, undesired capacitive and inductive influences on the measurement are often reduced because some of the inductive components of the detected signal will cancel the capacitive components of that signal. 
     In addition to driving the oscillation of the resonant tank circuit  31  at its resonant frequency (as affected by any proximate material such as materials  25 , 29 ), the OTA  32  also precisely controls the amplitude of the oscillator signal  41  driving the resonant tank circuit to be at a constant value. In this way, the effect of amplitude on the quality factor measurement is eliminated and apparent changes in quality factors such as might be caused by a slight detuning of the oscillator signal  41  with respect to the resonant frequency of the resonant tank circuit  31  are reduced. 
     In order for the OTA  32  to control the amplitude of the oscillator signal  41 , the OTA operates in conjunction with additional circuit elements that provide the OTA with an amplifier bias current I abc  based upon the oscillator signal  41  at the output  38  of the OTA. As is understood in the art, the output current (e.g., the oscillator signal  41 ) of an operational transconductance amplifier such as the OTA  32  can be modeled as a gain factor G m  times the voltage across an inverting input  36  and the non-inverting input  35  (indicated by a minus and plus sign, respectively) of the operational transconductance amplifier. The value G m  is determined by the amplifier bias current I abc . 
     In the present embodiment, the amplifier bias current I abc  is determined as follows. The oscillator signal  41  on the output  38  of OTA  32  is received by an amplitude detector  40 , which includes a precision synchronous rectifier  45  coupled in series with a low-pass filter  46 . The amplitude detector  40  provides at its output  47  a DC voltage proportional to the amplitude of the oscillator signal  41  at the output  38 . The synchronous rectifier  45  is realized in the preferred embodiment by a multiplier that accepts at both of its two factor inputs the output  38 . Any noise signal on the output  38  that is a synchronous with the oscillator signal  41  will average to zero in the low pass filter  46 . The DC voltage provided at the output  47  of the amplitude detector  40  is received by an inverting input of a standard high-gain operational amplifier  42 , the non-inverting input of which is provided with a precision reference voltage  44  designated as V r . 
     The amplifier  42  operates open-loop, and hence it will be understood that if the voltage on the inverting input of the amplifier  42  is greater than V r , the output of the amplifier  42  will be a negative value. On the other hand, if the voltage on the inverting input of the amplifier  42  is negative with respect to V r , the output of amplifier  42  will be positive. The output of the amplifier  42 , termed V OUT , is applied to a limiting resistor  43  to become the amplifier bias current I abc . 
     The connection of the output of the amplifier  42  V OUT  to the OTA  32  provides feedback control of the amplitude of the oscillator signal  41  to the value of V r . As connected in this manner, the value of V OUT  further is an amplitude error signal indicative of the quality factor of the resonant circuit  31  as affected by any material proximate the coil  26  such as materials  25  or  29 . This is because V OUT  generally indicates how much additional energy must be input into the resonant tank circuit  31  to maintain oscillation at the desired amplitude of V r , which is a measure of the quality factor of the resonant tank circuit. 
     Using V OUT  and f OUT  respectively as indications of the quality factor and resonant frequency of the resonant tank circuit  31  as affected by any material proximate the coil  26  such as materials  25  or  29 , the sensor circuit  30  is able to determine the effective resistance and reactance of the proximate material and additionally determine whether the material is likely to be human flesh as opposed to some other material. Specifically, the signals V OUT  and f OUT  are provided to a processor  50 . The processor  50  converts the values of V OUT  and f OUT  respectively into corresponding resistance and reactance values using known relationships. The resistance and reactance values are then compared with resistance and reactance values that are known to be approximately those corresponding to human flesh. 
     If the values are indeed approximately those corresponding to human flesh, the processor  50  produces a flesh detection signal  52 . The flesh detection signal  52  can, as discussed above, be used to prevent automatic (or, depending upon the embodiment, manual) discharging of nails by the nail gun  10 . Also, in certain embodiments, the flesh detection signal  52  governs the switching on of a lamp  55  (or other indicator) on the nail gun  10  indicating that the material proximate the tip  22  of the nail gun is human flesh (see FIG.  1 ). In alternate embodiments, the flesh detection signal  52  is continuously provided from the processor  50 , but the value of the flesh detection signal varies depending upon the resistance and reactance values that are determined. 
     A variety of specific embodiments of the processor  50  are possible. For example, in one embodiment, the processor  50  includes one or more comparators that compare the values of resistance and reactance based on V OUT  and f OUT  with known threshold values that are indicative of human flesh. In another embodiment, the processor  50  includes, in a memory, an array or other representation of a graph  60  of resistance (R) versus reactance (+/−JX) such as that shown in FIG.  5 . Certain regions of the graph  60  are understood to correspond to target materials such as metal or wood (e.g., regions  62  and  64 , respectively), while other regions of the graph such as region  66  are understood to correspond to human flesh. The values of resistance and reactance shown in FIG. 5 as being indicative of metal, wood, and flesh are merely intended to be exemplary, and actual values may vary from the values shown. 
     Depending upon the embodiment, the processor  50  is capable of converting values of V OUT  and f OUT  into corresponding values of resistance and reactance in a variety of ways. In one embodiment, the processor  50  includes look-up tables representing levels of resistance corresponding to particular values of V OUT , and levels of reactance corresponding to particular values of f OUT  The processor  50  is capable of interpolating in between discrete values of the look-up tables. In alternate embodiments, the processor  50  converts values of V OUT  and f OUT  into resistance and reactance values by way of formulas. In additional alternate embodiments, no conversion is made; rather, the received values of V OUT  and f OUT  are directly compared with values of V OUT  and f OUT  that are known to correspond to human flesh. Generally, the processor  50  can be any device that is able to detect human flesh based upon the input values of V OUT  and f OUT    
     The exact correspondences between V OUT  and resistance, and f OUT  and reactance, as well as the particular levels of resistance and reactance that are indicative of human flesh, will depend upon the particular embodiment of the nail gun  10 , sensor circuit  30  and coil  26 . However, each of these relationships and values can be either calculated or experimentally determined by one skilled in the art. 
     Many other modifications and variations of the preferred embodiment which will still be within the spirit and scope of the invention will be apparent to those with ordinary skill in the art. In order to apprise the public of the various embodiments that may fall within the scope of the invention, the following claims are made.