Abstract:
A composite tool and instrument is configured as a hammer having a metal striking head electrically coupled to capacitive sensing electronics disposed in a handle of the hammer. The head can thus serve as a sensing plate portion of a capacitive sensor responsive to a change in the effective dielectric constant of a building surface caused by the proximity of a structural member hidden behind that surface. The head can also serve as a sensing electrode portion of a sensor used to sense the proximity of AC power lines hidden behind the surface. In a preferred composite tool, the hammer has a handle made of an electrically insulating polymeric material and has an electrical conductor axially threaded through the handle so as to electrically couple the striking head to sensing electronics located in a cavity at the end of the handle distal from the head.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention provides means for finding a support member hidden behind a building surface and for inserting a nail or similar fastener into the support member. In the preferred embodiment a composite tool for construction and remodeling of houses and other buildings is provided by the combination of a capacitive stud sensor with a carpenter &#39;s hammer. 
     2. Background Information 
     Many buildings have walls, ceilings or floors constructed so that supporting structural members (e.g., wall studs) are concealed behind a surface (eg., a plasterboard wall panel). It is often desirable to determine the location of the hidden structural members—e.g., when one wishes to drive a nail into wall stud in order to hang a picture or other object on the wall. Many approaches to this problem have been proposed and used. These include tapping on the wall and estimating the stud &#39;s location from the quality of the sound, as well as moving a pivotally-mounted permanent magnet along the wall to find the ferromagnetic nails or screws that a previous worker had used to hold up the wallboard. 
     Of particular interest to the present invention is the prior art of locating a hidden structural member by using a capacitive sensor responsive to a change in the effective dielectric constant of the wall created by the presence of that member. Notable among the prior patent art in this area are: 
     U.S. Pat. No. 4,099,118, wherein Franklin et al. teach a portable capacitive sensor to be moved along the wall and to visually indicate the proximity of a stud to an operator. 
     U.S. Pat. No. 4,464,622, wherein Franklin teaches a capacitive stud sensor comprising improved adjustment and calibration means. The disclosure of Franklin is herein incorporated by reference. 
     U.S. Pat. Nos. 4,853,617, and 4,992,741 wherein Douglas et al. teach an instrument comprising a metal detector and a capacitive sensor, the instrument also having a bar-graph display usable by the operator to more accurately determine the location of a hidden structural member. Douglas et al. also provide circuitry responsive to the AC power line frequency (e.g., 50 or 60 Hz) in order to warn the operator of the proximity of energized and unshielded electrical lines behind the wall. The teachings of Douglas et al. are herein incorporated by reference. 
     U.S. Pat. No. 5,352,974, wherein Heger teaches an improved capacitive sensor that informs its operator when a wall is too thick or too thin for stud detection and when the operator has (incorrectly) calibrated the instrument by placing it over a stud, rather than over a section of the wall between two adjacent studs. 
     U.S. Pat. No. 5,562,240, wherein Campbell teaches a tool comprising a nail gun, or the like, having a proximity sensor attached thereto, the composite tool further comprising a visual indicator to inform the operator when a firing end of the nail gun is aligned with a wall stud or other nail-receiving structural element. 
     Also of interest to the present invention is improved apparatus and method for making capacitive proximity measurements, as taught by the inventor in his U.S. Pat. No. 5,730,165, the disclosure of which is herein incorporated by reference. 
     BRIEF SUMMARY OF THE INVENTION 
     A composite tool of the invention comprises a hammer having a metal striking head electrically coupled to sensing electronics disposed in a handle of the hammer. The head can thus serve as a sensing plate portion of a capacitive sensor responsive to a change in the effective dielectric constant of a building surface caused by the proximity of a structural member hidden behind that surface. The head can also serve as a sensing electrode portion of a sensor used to sense the proximity of AC power lines hidden behind the surface. In a preferred embodiment, the hammer has a handle made of electrically insulating polymeric material, the handle having an electrical conductor axially threaded therethrough, the conductor electrically coupling the striking head to sensing electronics disposed within the handle. 
     The composite tool comprises indicator means displaying the proximity of a structural member, or of energized AC wiring to an operator. In a preferred embodiment, the indicator means comprises a visual bar-graph display capable of displaying an effective wall thickness when the head of the hammer is touched to the wall and moved thereacross. In another embodiment, the indicator means comprises a light emitting diode, or other such light source having a controllable flash rate and operated responsive to the effective wall thickness to signal the proximity of a structural member to an operator. Moreover, in some embodiments the composite tool of the invention may comprise other visual, audible, or tactile signaling means for indicating to an operator that the head of the hammer is adjacent an AC power line hidden behind a wall. 
     In addition to providing a capacitive proximity instrument for sensing and displaying the effective thickness of a building wall, a preferred embodiment of the invention also comprises a second capacitive sensing means for sensing the proximity of an operator and for activating the first capacitive proximity measurement means responsive thereto. In a preferred embodiment, the proximity instrument is built into a hammer having a metal striking head and an electrically insulating handle. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a partly cut-away perspective view of a stud-sensing hammer of the invention proximate a hidden stud. 
     FIG. 2 is a sectional view of a second embodiment of the invention, the section taken as depicted by the double-headed arrow  2 — 2  in FIG.  1 . 
     FIG. 3 is schematic circuit diagram showing the sensing electronics of the invention 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A hammer  10 , modified by the inclusion of a capacitive sensing circuit  12  configured to measure an electrical capacitance between a plate  14  and an electric ground  16 , can be used to determine the location of objects, such as a wall stud  18 , hidden behind a wall  20  or other opaque surface. As hereinbefore noted, a variety of capacitive sensors have been applied to this use, but these have generally required a person  22  seeking the stud  18  to use a sensor separate from the hammer. The present invention simplifies the user &#39;s task by using the metal striking head  24  of the hammer  10  as the sensing plate  14 . 
     In a preferred embodiment the hammer  10  comprises a metal striking head  24  and a dielectric handle portion  26  depending therefrom. A wire  28  or other metallic member, such as a conductor trace on a circuit board, is disposed generally along the axis  30  of the handle  26  and is either metallically connected to or capacitively coupled to the head  24  so as to couple it to the rest of the capacitive sensing circuitry  12 , which may be disposed, along with a portable energy supply  32 , in a cavity  34  formed in that end  36  of the handle  26  distal from the head  24 . As depicted in FIG. 2, a threaded plug  38 , or other suitable closure, may be used to seal these elements within the handle  26 . Although the preferred source of energy is a lithium primary battery that is expected to power the sensing circuitry  12  for several years, other known portable energy sources could be used. Applicable portable energy sources include a high value capacitor of the type commonly called a “capattery”; a secondary battery rechargeable from the AC power lines; or a secondary battery rechargeable by means of a solar cell. 
     Although capacitive sensing circuitry of the sort earlier taught by Franklin in U.S. Pat. No. 4,464,622, or by Douglas et al. in U.S. Pat. Nos. 4,853,617, and 4,995,741, could be used in practicing the invention, a preferred embodiment employs capacitive sensing circuitry  12  of the type referred to as charge transfer sensing circuitry. The inventor has provided teaching of this sort of circuitry for other uses in his U.S. Pat. No. 5,730,165. Turning now to FIG. 3, one finds a schematic depiction of a charge transfer measurement circuit  12  comprising a first sensing plate  14   a  (i.e., the hammer head  24 ) used to measure a first electrical capacitance to an electrical ground  16 . As is known from the inventor&#39;s earlier teachings, this is advantageously done by repeatedly charging the plate  14   a  by means of a charging transistor  40  and subsequently discharging the plate  14   a  by means of a discharging transistor  42  into a charge detector  44 . This process is preferably carried out under the control of a microcontroller  46 , which has the capability of selectively measuring either the capacitance of the plate  14   a  to ground  16  or of measuring the capacitance of a second sensing plate  14   b,  which may is used to form a “wake-up” area disposed on a portion of the handle  26  distal from the head. When the user places a finger or portion of his or her hand over the wake-up area a preferred sensor makes a transition from a power-conserving sleep mode, in which the wake-up plate is periodically monitored, to a frilly operational mode. It will be realized by those skilled in the art that the wake up plate  14   b  may comprise a metal plate disposed on the surface of the handle so that the user is coupled thereto upon directly touching the plate, or it may comprise a metal plate disposed beneath a thin insulating layer so that the user is capacitively coupled thereto when placing a portion of his or her body adjacent the plate  14   b.    
     Because the sensor of the invention is a portable device that must operate from an exhaustible power supply  32 , in a preferred mode of operation the microcontroller  46  spends much of its time in a well known power-conserving “sleep” mode. At each of a predetermined set of times (e.g., after the expiration of a regular interval) the microcontroller  46  wakes up, measures the capacitance to ground of the wake-up plate  14   b  and compares the result of that measurement with a predetermined threshold value stored in memory. If a user  22  is holding the hammer  10  so that a portion of his hand rests on the second sensing plate  14   b,  the capacitance measured at that plate  14   b  will be above the threshold, and the microcontroller will begin repetitively measuring and displaying the capacitance to ground of the hammer head plate  14   a.  On the other hand, if the capacitance measured at the TOUCH plate  14   b  is below the threshold value, the microcontroller  46  returns to sleep mode. In a preferred embodiment the TOUCH plate  14   b  is disposed on a portion of the surface of the handle  26  that the user does not normally touch while hammering, but that is sufficiently close to his or her normal hand position (e.g., is disposed a few centimeters above where the user&#39;s thumb would normally grip the handle) so that a single, comfortable motion is all that is needed for the user to touch the second sensing plate  14   b  and thereby activate the display. In a less desirable embodiment, the second sensing plate  14   b  would be disposed on the handle so that the user touched the plate  14   b  whenever the hammer was being held. In this case, the display would operate during hammering, which could be annoying, but would still provide an energy conserving means by allowing the apparatus to be in sleep mode when not in active use. 
     When the preferred hammer  10  is being held by a user  22  who is touching the TOUCH sensing plate and the capacitance to ground of the head  24  is being repeatedly measured, the microcontroller  46  provides an output representative of that capacitance to an indicator means  48  adapted to visibly or audibly indicate to the user  22  a current relative value of capacitance. An extremum in this capacitance is associated with the proximate presence of a stud  18  behind the wall  20 . 
     In a preferred embodiment the indicator means  48  may comprise a columnar display  50 , which may comprise a plurality of light emitting diodes or a liquid crystal display, is disposed on the back of the hammer handle  26  so that a user  22  can see the measured capacitance represented in a bar-graph fashion on the handle by looking at the columnar display to see the relative height of the active portion. In another embodiment, as depicted in FIG. 2, the hammer handle  26  comprises a transparent or translucent portion  52  proximal the head  24  and an opaque portion  54  distal from the head. In this embodiment the indicator means  48  may comprise a single light emitting diode  56  disposed adjacent the interface between the two portions  52 ,  54  of the handle, can be turned on and off by the microcontroller  46  at a rate indicative of the measured capacitance. This flashing light will be reflected and refracted at the surface of the handle, so that the operator viewing the handle will see displayed thereon a representation of the measured capacitance. In this embodiment the user  22  moves the hammer head  24  across the wall  20  while observing the rate at which the LED flashes, wherein a maximum in the flash rate may indicate that the hammer head  24  is disposed adjacent a stud  18 . 
     In some known hammers a steel I-beam extends along substantially the entire axis of the hammer handle  26 . The I-beam is inserted into the conventional handle-attaching hole in the hammer head  24  and is glued in place in a plastic molding operation that also enrobes the rest of the I-beam with a layer of varying thickness so as to form a handle having a desired shape. It will be understood that the capacitance measurement circuitry, in order to be compatible with this structure, may involve the use of circuit boards that can be fitted about or adjacent the beam and cast into the handle at a selected location along the handle. In one embodiment, the capacitance measurement circuitry is cast into a partially transparent portion of the handle  26  lying between the head and an opaque rubber cushioning grip. 
     As hereinbefore noted, it is known in the art of electronic stud finding to provide an additional safety feature to the operator by including circuitry responsive to the AC mains frequency and by then warning the operator when the stud finder is disposed adjacent an energized conductor so that he or she does not drive a nail into the wiring. Such means for detecting the presence of energized and unshielded wires may be readily incorporated into the stud sensing hammer of the invention, and suitable alerting means may be provided to warn the operator of a proximate energized conductor. One could, of course, provide a visual warning e.g., by showing a rapidly oscillating bar graph on a columnar display  50  or by using a rapid and repetitive characteristic flashing sequence of an LED  56 . Alternately, one could provide a piezoelectric element  58  driven by the sensing circuitry and disposed on a surface of the handle to provide an audible or tactile warning of the hazard. 
     Although the present invention has been described with respect to several preferred embodiments, many modifications and alterations can be made without departing from the invention. Accordingly, it is intended that all such modifications and alterations be considered as within the spirit and scope of the invention as defined in the attached claims.