Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of application Ser. No. 09/538,087, filed Mar. 29, 2000; now U.S. Pat. No. 6,593,754 B1, was patented on Jul. 15, 2003, which claims benefit to provisional application Ser. No. 60/127,322, filed Apr. 1, 1999. 

   FIELD OF THE INVENTION 
   This invention relates to electronic instruments for detecting a stud or other object behind an opaque surface, such as wall board. 
   BACKGROUND OF THE INVENTION 
   Carpenters, electricians, do-it-yourselfers and others are often faced with the problem of locating the position of the wall studs behind the wall board material forming the wall surface. They are interested in hanging pictures, drilling holes and so on. However after the walls are finished and painted the location of the hidden substructure (i.e. the studs) is not visually detectable. The same is true of finding the location of hidden wooden frames in furniture and boats from the outside surface of the structure. 
   U.S. Pat. No. 4,099,118 issued Jul. 4, 1978 discloses an electronic wall stud sensor which is suitable for detecting a wall stud behind a wall surface. It utilizes one or more capacitor plates, a fixed frequency oscillator, a dual one-shot multivibrator, a field effect transistor, and a complicated calibration procedure. Each individual circuit must be calibrated at the time of manufacture, which is a costly procedure for mass production. 
   U.S. Pat. No. 4,464,622 describes a wall stud sensor similar to U.S. Pat. No. 4,099,118 but with a plurality of capacitor elements and means for detecting the presence of alternating current in the wall. Finding the presence of alternating current in walls is often not practical or possible with modern wiring methods. U.S. Pat. No. 5,352,974 describes a stud sensor similar to U.S. Pat. No. 4,099,118 but with means for storing calibration data for thick or thin walls. However, in most cases, the user will not know if the wall is thick or thin. The circuit used is complex and uses special purpose hardware. The sensor also uses a plurality of capacitor plates. Both of these devices require factory calibration. 
   U.S. Pat. No. 5,485,092 describes a device for investigating surface and subsurface structures. It uses four-sided conductive elongated plates and rectangular sensor plates connected together in a special arrangement. The different surfaces are charged at different rates and a differential amplifier and peak detector are used to determine information about the subsurface. It requires a complicated charging scheme and an expensive voltmeter for readout, which requires an interpretation of the results which would be difficult for an inexperienced person. 
   Prior art sensors were required to be a relatively large size so as to make them sufficiently sensitive for their intended purpose. Prior circuits required a relatively large sensor, and to isolate the sensor from the user&#39;s hand, which contributed to the relatively large size of the sensors. 
   Thus, there is a need for a low cost subsurface object locator that is easy to use, works well in the environment for which it is designed, simply and reliably identifies the location of substructure components in an efficient manner, is easy to manufacture, requires no calibration or adjustments by the factory or operator, and can be made of a relatively small size. 
   SUMMARY OF THE INVENTION 
   The invention provides a compact device capable of efficiently finding the location of hidden objects or substrata such as studs, joists and other similar objects below the surface of walls, floors and similar type structures. The device may also be used to find the location of braces, wood frames or other substructures in wooden furniture such as tables and cabinets, wooden boats and similar type structures. 
   In particular, the invention provides a pocket-sized object finder having a housing containing a battery, electronic circuitry and a capacitor plate. The housing has front, rear, side and end walls that define a cavity of a width no more than two inches and one third the length of the housing. The battery powered circuitry is responsive to variations in capacitance effected in the capacitor plate arising from the presence of the object. The capacitor plate is disposed in the housing along the flat rear wall, which is adapted for sliding along the wall so as to capacitively couple the capacitor plate to the object. A signal indicator visible from the front wall of the housing is activated in response to the change in capacitance and illuminates when the object is detected. A pocket clip is located at the front wall. 
   In one preferred form, the object finder has multiple signal indicators located at a tapered end of the housing. Successive signal indicators taper in the direction the housing is tapered and activate serially, without overlapping, at essentially a leading edge of the object and deactivate serially, again without overlapping, at essentially a trailing edge of the object. 
   In another preferred form, the housing has a removable access door over an access opening to a battery compartment of the cavity in which the battery is disposed. Preferably, the pocket clip is a part of the access door. 
   Thus, the present invention provides a compact object finder that using a small area capacitor plate allowing the over form factor of the finder to be smaller than prior finders. The small capacitor remains sufficient sensitive to detect objects at the same or even a greater depth than prior devices. The present object finder can be easily retained to a person&#39;s body either by grasping by hand, placing into a shirt or pants pocket, or by clipping it to one&#39;s clothing. The device is also easy to use and operate without manual calibration or adjustments on the part of the operator. 
   Other features and advantages of the invention will be apparent from the detailed description and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a first embodiment of a circuit for practicing the invention; 
       FIG. 2  shows various waveforms and details of operation of the circuit of  FIG. 1 ; 
       FIG. 3  is a schematic diagram of a second embodiment of a circuit for practicing the invention; and 
       FIGS. 4A-4B  are schematic diagrams illustrating a comparison of the operation of the first embodiment and the second embodiment; 
       FIG. 5  is a perspective view of an electrical instrument design incorporating the invention; 
       FIG. 6  is top plan view of the instrument of  FIG. 5 ; 
       FIG. 7  is a right side plan view of the instrument; 
       FIG. 8  is a left side plan view of the instrument; 
       FIG. 9  is a bottom plan view of the instrument; 
       FIG. 10  is a front plan view of the instrument; 
       FIG. 11  is a rear plan view of the instrument; 
       FIG. 12  is an exploded perspective view illustrating how the sensor plate, circuit board, switch and batteries are assembled in the instrument housing; and 
       FIG. 13  is a schematic showing use of the instrument against a wall to detect a stud. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring first to  FIG. 1  there is illustrated a circuit diagram of the invention. Shown on this figure is a portion of a wall structure  10 , studs  11 ,  12  and wall board  13  to be illustrative of one way of operating the invention. In this case it is desired to locate the positions of the hidden studs  11  and  12 . As shown in  FIG. 1  there is a metallic sensor plate  21  connected to a CMOS oscillator  20  which produces a square (or rectangular) wave output. The circuit consists of a timer IC  22 , the sensor plate and resistors. The frequency of the square (or rectangular) wave produced by the oscillator  20  is determined by the values of resistors R 1  and R 2  and the capacitance presented by the plate  21 . When the sensor plate is above a section of the wall  13  with no studs it will cause the oscillator  20  to run at frequency f 1 . When the sensor is above a section of the wall  13  that has a stud below it the oscillator will have a different frequency f 2 . 
   The square (or rectangular) wave output of the oscillator  20  goes to a microprocessor circuit  30  via line  26 . The microprocessor circuit  30  is programmed to measure the frequency difference f 1  minus f 2 . The frequency difference has been found to be a reliable and consistent means of identifying subsurface objects such as studs and has been found to be relatively independent of the wall material. This makes the device self calibrating, obviating the need for any special factory calibration. If the frequency difference exceeds an amount deemed sufficient to indicate the presence of a stud, an LED is turned on. 
   The circuit  30  actually has four LEDs D 2 , D 3 , D 4 , D 5  that can be activated at different amounts of frequency change. This is illustrated in more detail in  FIG. 2 , discussed later on. More or fewer LEDs could be used as indicators depending upon resolution and cost considerations. The circuit is powered by batteries  40  (four 1.5V pancake cells) through protective diode D 1  (e.g., a 1N270 diode) and line  42 . Resistor R 3  is used to limit the current in the LEDs. Resistor R 4  is used for a power on reset for circuit  30 . Normally open switch  45  is pressed to enable power to circuit from the batteries  40  to circuit  30 . 
   Although visual LED indicators D 2 -D 5  are described here, it should be clear that audible indicators could be used as well. For example, different audible tones could be produced corresponding to various frequency differences encountered in scanning the wall, as the leading edge of a stud was approached, the frequency could go up, and as the trailing edge of the stud was passed the frequency could go down. In fact, there are occasions where audible indications may be better, such as in cases where the visible indicators may be hard to see. 
   Referring now to  FIG. 2  there is illustrated the relation of the signal indicator means (LEDs) D 2 -D 5  to the position of the sensor plate along the wall. As the sensor moves along the wall the frequency changes in accordance with curve  52 . As the frequency decreases, the circuit  30  ( FIG. 1 ) senses this change and turns on one or more of the LEDs D 2 -D 5 . The LEDs could be turned on so as to overlap in on-times or not. In the preferred embodiment, the on-times do not overlap to preserve battery power. 
   To use the device described, the plate  21  is placed on or in close proximity to wall  13  where there are no studs and the switch  45  is pressed. This causes circuit  30  to be activated and it will measure the first frequency f 1  from the oscillator  20  and save it in memory. After this step is performed, which takes less than a second, the lowest LED D 3  (green) comes on and stays on as a power indicator, while the switch  45  is pressed. This signals to the operator that the device can now be moved across the wall being probed. As the sensor is moved across the wall the circuit  30  is continuously measuring the second or subsequent frequency f 2  from oscillator  20  and comparing it to the first frequency f 1  by taking the frequency difference. When the difference exceeds a first threshold, the next LED up, LED D 4  (amber) will be lit and LED D 3  will go out. When the difference exceeds a second threshold, greater than the first threshold, the next LED D 5  (amber) will be turned on and LED D 4  will go out. When the difference exceeds a third threshold, greater than the second threshold and which indicates the presence of the leading edge of the stud, the highest LED D 2  (red) goes on and the LED D 5  goes out. LED D 2  stays on as the thickness of the stud is traversed by the device. When the trailing edge of the device is reached, the LEDs go off and on in the reverse sequence. Thus, a user trying to find a stud, will mark the leading edge of the stud when LED D 2  comes on, and will mark the trailing edge of the stud when the LED D 2  goes off. 
   When a user first puts the device against a wall or other surface to be probed, there is no way of telling if it is initially placed over a stud or other subsurface object or not. The device assumes that it is not. However, if by chance it is, then the subsequently found frequency difference will be negative and unless special provision is made in the programming of the microprocessor, an error will result. It is an easy matter, however, to program the microprocessor so that if the f 1 -f 2  frequency difference is found to be negative, it means that the device was initially placed over a stud or other subsurface object. The device could be programmed to flash the LEDs or beep a buzzer in that event to alert the user to start over, placing the device in a different initial position. 
   Referring to  FIG. 3 , there is illustrated a circuit diagram of a second embodiment of the invention. Shown on this figure is a portion of a wall structure  60 , studs  61 ,  62  and wall board  63  to be illustrative of one way of operating the invention. In this case, it is desired to locate the positions of the hidden studs  61  and  62 . As shown in  FIG. 3 , there is a metallic sensor plate  71  connected to a CMOS oscillator  70 . The frequency of the oscillator  70  is determined by IC  72 , the values of resistors R 1  and R 2  and the capacitance presented by the plate  71 . 
   The capacitance of the plate  71  is determined by the surrounding medium including the wall material, the studs, the circuit and the person holding the device. It is desirable to reduce the stray capacitance as much as possible since this will improve the sensitivity of the plate  71 . The capacitance of plate  71  is influenced considerably by the operator and the housing of the device. 
   Capacitance is related to its potential with respect to other objects. If an additional plate  75  is introduced in the vicinity of plate  71  with the same potential as plate  71 , it will reduce the “stray” effects. This improves the sensitivity of the plate  71  and allows it to sense further into the wall. 
   The potential of plate  71  changes as the oscillator  70  operates. In a typical situation it may vary from 0 to 5 volts in amplitude. Hence the guard plate  75  must have its potential vary in the same way. This is accomplished by using a buffer amplifier  78 , with a gain of one, which has the voltage of the sensor plate  71  at its input and produces a near exact replica of it at its output, which is connected to plate  75  via line  77 . Hence plate  75  is driven at the same potential as plate  71 . 
   Referring to  FIG. 4A , a side view of a sensor plate  100  is shown to illustrate how a sensor with a single plate operates. The sensor plate  100  is connected to the oscillator  103 , which causes its potential to vary. The electrical E-field lines  102  are free to go in any and all directions. 
     FIG. 4B  illustrates how the second embodiment described above operates. In  FIG. 4B , a sensor plate  110  is connected to an oscillator  116  and a guard plate  114  is driven from amplifier  118  so it has the same potential as the sensor plate. The E-field  112  is now prevented from going in the direction of the guard plate  114 . This is because both plates are at the same potential and by electrical laws there can be no E-field between conductors of the same potential. With fewer E-field lines, there is less capacitance of plate  110 . Hence it will be more responsive to dielectric changes in the direction opposite to the guard plate  114 . The guard plate  114  may be somewhat larger than the sensor plate  110  so as to extend beyond the edges of the sensor plate  110 , which redirects the E-field lines emanating from the edges of the plate  110  in the direction toward the surface being probed. 
   Referring back to  FIG. 3 , the remainder of the circuit of  FIG. 3  acts in the same way as the first embodiment of FIG.  1 . When the sensor plate is above a section of the wall  63  with no studs it will cause the oscillator  70  to run at frequency f 1 . When the sensor is above a section of the wall  63  that has a stud below it the oscillator will have a different frequency f 2 . The output of the oscillator  70  goes to a microprocessor circuit  80  via line  76 . 
   The microprocessor circuit  80  is programmed to measure the frequency difference f 1  minus f 2 . As in the first embodiment, this can be done by any suitable means. For example, the microprocessor circuit  80  will typically include a counter. The counter can be programmed to count the number of times the oscillator output signal to the microprocessor goes high in a certain period, which yields a measure of the frequency of the oscillator output. If the frequency difference between the first measured frequency and the subsequently measured frequencies exceeds an amount deemed sufficient to indicate the presence of a stud, an LED is turned on. 
   The circuit  80  actually has four LEDs D 2 , D 3 , D 4  and D 5  that can be activated at different amounts of frequency change. This is illustrated in more detail in FIG.  2 . More or fewer LEDs could be used as indicators depending upon resolution and cost considerations. 
   The circuit is powered by batteries  90  through protective diode D 1  and line  92 . Resistor R 3  is used to limit the current in the LEDs. Resistor R 4  is used for a power on reset for circuit  80 . Switch  95  is pressed to enable power to circuit from the battery  90  to circuit  80 . 
   The microprocessor is capable of detecting very small changes in the frequency of the oscillator, which improves the sensitivity of the device and permits making the device relatively small.  FIGS. 5-13  illustrate the design of an electrical instrument, which may include either of the two previously described circuits. As illustrated, this instrument is generally pen-light sized, able to easily fit into a breast pocket. In the preferred embodiment, the device is approximately 1 inch wide, {fraction (19/32)} inches thick (not including the pocket clip) and 5{fraction (9/16)} inches long. The housing  105  is provided with a pocket clip  109 , integrally molded as part of the battery cover  107 , to help hold the device in a user&#39;s breast pocket, since the device is small enough to fit, being at least three times longer than it is wide, and in the case of the embodiment disclosed, being over five times longer than it is wide. To be of material benefit over prior art devices, it is preferred that the locator be less than two inches wide, which is more than accommodated by the preferred embodiment, since it is only 1 inch wide. 
   The instrument in  FIGS. 5-13  has been labeled with reference numbers as if it includes the first circuit, of FIG.  1 . If so, the metal sensor plate  21  can be provided on the bottom side of a printed circuit board  101 , and fixed in the housing, for example by an adhesive, so the exposed surface of the plate is against bottom wall  103  of housing  105  so as to minimize any air gap between the plate  21  and the surface being probed. The plate  21  may be provided as the copper layer commonly provided as part of an ordinary printed circuit board. If the device is made to include the second circuit of  FIG. 3 , the circuit board  100  can be provided with metal (typically copper) layers on both sides, with the layer on side  106  being the guard plate  75 , and the layer  71  being on the lower side, as is layer  21 . The lower side sensor plate may be etched so as to make it smaller than the upper side guard plate. The other circuits, i.e., the oscillator, microprocessor, LED and buffer amp (if applicable) circuits, are provided on circuit board  115 , which is secured in the housing  105  as far away as possible from the sensor plate  21 . 
   The top end of the housing  105  tapers in width to a blunt point  111 , to give an operator a better approximation of the center of the device. Transparent or translucent windows  113  centered laterally on the front surface of the housing  105  are aligned with the respective LEDs D 2 -D 5  and also taper in width toward the top to a sharper point, also to help the operator locate the center of the housing, and therefore the edge of a stud or other subsurface object. 
   Variations and modifications to the preferred embodiments described will be obvious to persons skilled in the art without deviating from the spirit of the invention. Therefore, the invention should not be limited to the preferred embodiments described, but should be defined by the claims which follow.

Technology Category: g