Electromagnetic hidden object detector

STATEMENT OF GOVERNMENT RIGHTS

The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory.

BACKGROUND OF THE INVENTION

The present invention relates generally to detectors for locating hidden objects. More particularly, the present invention relates to detectors for locating objects hidden behind wails, ceilings and floors, for locating metallic and non-metallic buried objects, and further for locating cavities within solid objects.

I. Detection of objects hidden behind wood walls, ceiling and floors

A common problem faced by anyone attempting to hang a picture or cabinet is how to precisely locate between-wall studs, so a sturdy hook may be attached or clearance may be provided for the cabinet. General methods for locating studs and joists include tapping with a hammer, searching for nails with a magnetic compass, and random piercing with a nail. Hammer tapping and magnetic compass searching are unreliable and time-consuming, and random piercing is destructive.

These conventional methods were vastly used when electronic wall stud sensors became commercially available. The user places the sensor flat against the wall and scans it laterally across the extent of the wall. When it passes over a stud, a vertical series of LED's indicate the presence of the stud behind the wall. The sensor is based on dielectric density sensing. U.S. Patent No. 4,099,118 describes a portable electronic wall stud sensor having capacitor plates and circuitry for detecting changes in the capacitive charge due to changes in the dielectric constant in the wall adjacent the sensor. U.S. Patent No. 4,464,622 describes a similar capacitive sensor with calibration means and means for detecting an AC line in the wall.

Dielectric density sensing has limitations. If a small air gap forms between the sensor and the wall, the device becomes inoperative due to the substantial change in density adjacent the two sensing plates that are internal to the unit. It is therefore difficult or impossible to locate studs on rough or highly textured surfaces. Another limitation is that stud detection is directly affected by the dielectric constant of the intervening wall material. Sheetrock, plywood, particle board, and dense hardwoods vary in dielectric constant to such an extent that a dielectric sensor generally only works on sheetrock and not on plywood walls, wood floors, stair steps, furniture or cabinetry. Furthermore, these conventional sensors cannot detect cavities behind walls or within objects.

II. Detection of objects behind masonry and cement structures

Locating hidden metallic and non-metallic objects and cavities behind masonry and cement structures presents further complications. Conventional methods of locating embedded objects rely on trial and error methods, which include drilling several holes in the structures, in the general area where the objects are believed to be hidden. Oftentimes, this method causes damage to the objects and to the drill equipment. Conventional magnetic methods have limited applications, such as detecting copper wiring or aluminum conduit.

III. Detection of Underground Objects

Previously, metallic underground pipes were used almost exclusively in the transportation of natural gas. The location of the buried metallic pipes was relatively simple since metal reflects high frequency electromagnetic waves which can be easily detected. However, underground metal pipes have inherent problems. They are subject to corrosion to differing degrees, they are difficult to install, and they are becoming more difficult and expensive to purchase. As a result of these limitations, other types of pipes have become popular. Polymeric pipes, being virtually non-corrosive, light, easily installed and relatively inexpensive are rapidly replacing metallic pipes.

An ever increasing problem facing the natural gas distribution companies, municipal government agencies, other public utilities and contractors is the rapid and accurate location of buried polymeric pipe lines. Since underground plastic pipes cannot be located with conventional metal detectors, sub-surface detectors of non-metallic and metallic objects have evolved.

Several methods of sensing are available. U.S. Patent No. 4,072,942 describes an apparatus for the detection of buried objects using a broadband, high resolution short pulse transmitter and a bistatic or monostatic noncontacting antenna that radiates a transmitted signal through the ground for reflection from a buried object, a sampling type receiver, and a locking circuit to lock the range sweep. U.S. Patent No. 4,381,544 describes a process for geotechnic exploration comprising the steps of emitting electromagnetic pulses from an airborne platform, receiving the reflections, and amplifying, processing and displaying the signals. U.S. Patent No. 4,132,943 describes a radar transmitter for directing a beam of microwave energy through the atmosphere. A gas seep in the atmosphere irradiated by the beam is excited to emit microwave energy at a frequency characteristic of the particular species of gas. A radar receiver tuned to the frequency produces video signals representative of the emitted microwave energy. U.S. Patent No. 3,392,384 describes an apparatus for detecting hidden objects including an antenna system that radiates over a large area and twin receiving systems. U.S. Patent No. 4,052,666 describes a method of determining vegetation height and water content from the intensity and state of elliptical polarizaton of a reflected train of microwaves. U.S. Patent No. 4,983,914 describes a sensing arrangement with a coil of varying inductance.

There is still a great and still unsatisfied need for a new locator for detecting objects buried underground, which is portable, easy to use, relatively inexpensive, and which has a low power emission that helps comply with the requirements of Part 15 of the Federal Communications Commission's Rules. Furthermore, this new detector should be readily usable in security applications such as for locating guns and similar objects in suitcases.

SUMMARY OF THE INVENTION

A new electromagnetic detector is designed to locate an object hidden behind a separator or a cavity within a solid object. The detector includes a PRF generator for generating 2 MHz pulses, a homodyne oscillator for generating a 2 kHz square wave, and for modulating the pulses from the PRF generator. A transmit antenna transmits the modulated pulses through the separator, and a receive antenna receives the signals reflected off the object.

The receiver path of the detector includes a sample and hold (S/H) circuit, an AC coupled amplifier which filters out DC bias level shifts in the S/H circuit, and a rectifier circuit connected to the homodyne oscillator and to the AC coupled amplifier, for synchronously rectifying the modulated pulses transmitted over the transmit antenna. The homodyne oscillator modulates the signal from the PRF generator with a continuous wave (CW) signal, and the AC coupled amplifier operates with a passband centered on that CW signal.

In one sense the invention can broadly be described as an electromagnetic detector for locating an object or a cavity behind a separator, comprising in combination means for transmitting electromagnetic signals in the direction of the separator; means for receiving electromagnetic signals reflected from the object or the cavity and the separator; means for detecting change in the electromagnetic reflectance of the separator, the detector characterized in that the means for detecting a change in the electromagnetic reflectance of the separator detects changes in amplitude of the reflected electromagnetic signals from a predetermined/fixed detection range behind the separator.

In another sense the invention may broadly be described as a method for detecting an object or cavity behind a separator comprising the steps of transmitting a sequence of pulses in the direction of the separator; detecting the pulses reflected from said separator averaging said detected pulses over repeated pulses; and detecting changes in the electromagnetic reflectance of the separator, the method characterized by the sequence of pulses being unipolar.

BRIEF DESCRIPTION OF THE DRAWINGS

<ul id="ul0001" list-style="none" compact="compact"><li>Figure 1 is a block diagram of a new electromagnetic hidden object detector according to the present invention;</li><li>Figure 2 is a timing diagram of the detector of Figure 1;</li><li>Figure 3 is a schematic representation of a range gate location and reflection mechanism forming part of the operation of the detector of Figure 2;</li><li>Figure 4A-D illustrate various pulse shapes for off-surface invariance, where Figure 4A shows a preferred pulse shape with 2 unipolar peak and exponential tail, Figure 4B shows a post shoot or ringing pulse, Figures 4C and 4D show the resulting indicator signals;</li><li>Figure 5 is a schematic representation of a wire antenna forming part of the detector of Figure 1;</li><li>Figure 6 is a schematic circuit diagram of the detector of Figure 1;</li><li>Figure 7 is a block diagram of another embodiment of a new homodyne electromagnetic hidden object detector according to the present invention;</li><li>Figure 8 illustrates various timing charts at different sections of the detector of Figure 7;</li><li>Figures 9A and 9B represent a circuit diagram of the detector of Figure 7;</li><li>Figure 10 is a schematic view of an automatic tool arrangement which includes two generally identical detectors of Figures 1 or 7; and</li><li>Figure 11 is a schematic view of another automatic tool arrangement which includes a common transmitter unit and two receiver units.</li></ul>

DETAILED DESCRIPTION OF THE INVENTION

The general operation of the inventive electromagnetic hidden object detector is based on emitting a pulse from a transmit antenna, waiting for a brief period of time corresponding to about 2 inches of round trip time of flight at the speed of light, and then opening a gate connected to a receive antenna to allow the reflected pulse to be sampled. This process is repeated at a 1MHz rate, allowing approximately 10,000 receive pulses to be averaged prior to driving a signal amplitude display.

The high level of averaging reduces the random noise accompanying the sampled signal to such an extent that extremely low amplitude signals can be detected. Repetitive operation also leads to extreme simplification of the entire circuit. The invention utilizes an ultra-wideband receiver described in U.S. Patent Application Ser. No. 08/044,745 filed April 12, 1993 by Thomas E. McEwan entitled "Ultra-Wideband Receiver.

Referring now to the drawings and more particularly to Figure 1 thereof, there is illustrated a block diagram of the electromagnetic hidden object detector 1 according to the present invention. Pulses from a 1MHz pulse repetition frequency (PRF) generator 10 are input into two parallel paths, a transmit path 12 and a gating path 14. In the transmit path 12, the PRF generator 10 drives a step generator 16, which provides a transmit pulse with a +5V to 0V, 200ps transition that is applied to a transmit antenna (T) 18. The electrical length of the antenna 18 is set to be short relative to the spectral content of the voltage step, so differentiation occurs in the antenna 18, and a 200ps wide pulse is radiated. The radiated pulse can be considered to be about one half the cycle of an RF sinewave.

A receive antenna (R) 20 picks up the pulse reflected from a hidden object, i.e., a stud 22 behind a wall board 24, and applies it to a sample/hold (S/H) circuit 26 that is gated by a gating pulse from the gating path 14. The gating pulse is delayed by approximately 0.5ns from the time that the transmit antenna 18 radiates the pulse. Pulses from the PRF/PRI generator 10 which are input into the transmit path 12 are simultaneously input into the gating path 14 where they pass through a range delay generator 30 followed by a step generator 32, which produces a 200 ps gating pulse for controlling a gating switch 34. The delay generator 30 is set at about 0.5ns so that the range of the electromagnetic hidden object detector 1 is about 1-2". The gating pulse closes the switch 34 so that reflected pulses from the 1-2" range are input into the sample/hold circuit (S/H) 26. The step generator 32 is connected to an RC circuit 33 to provide a gate pulse to the S/H circuit 26.

In the preferred embodiment, the S/H circuit 26 is formed of a capacitor 28 connected to ground. Reflections, or lack thereof, occurring 1-2" from the antennas 18, 20 are thereby sampled. The size of the capacitor 28 in the sample/hold 26 circuit is sufficiently large that each sample only partially charges it, and approximately 10,000 samples are required for the circuit to reach an equilibrium with the receive antenna signal. The product of the impedance of the receive antenna 20 and the capacitance of capacitor 28 yield a time constant which is much greater than the width of the gate pulse, so it takes many pulses to charge capacitor 28.

The timing relationship is shown in Figure 2. The five waveforms are shown over a one pulse repetition interval (PRI). The transmit step generator 16 produces a +5V to 0V, 200 ps step, which produces a 200 ps wide radiated pulse from the transmit antenna 18. The reflected pulse from the receive antenna 20 coincides with the gating pulse. Each received pulse produces an incremental voltage change V on the capacitor 28 of the S/H circuit 26. The capacitor voltage is the output of the averaging S/H circuit 26. The increment <maths id="math0001" num=""><math display="inline"><mrow><mtext>V = 1/N</mtext></mrow></math><img file="EP0700528B1_D0001.tif" /></maths> of the total received pulse, where N is the number of samples averaged, typically about 10,000.

The noise voltage at the sample/hold circuit 26 is reduced by a factor related to the square root of the number of samples averaged, 100 times in this case, and by a factor related to the effective time constant of the averaging circuit relative to the PRF of the system and the instantaneous bandwidth of the sampler - a factor stemming from the sampled data nature of the sample/hold circuit. In all, greater than 60dB noise reduction is obtained compared to a circuit with the full bandwidth of the radiated pulse.

The sample/hold output is applied to a voltage summation element or summer 36, which subtracts background reflections as described herein. The output of the summer 36 is amplified by an amplifier (A) 38, typically 60dB gain and having a passband of DC-16Hz, and applied to a display 40. The display 40 includes an arrangement of light emitting diodes (LED's) 42 that sequentially light in proportion to the applied voltage, which is linearly related to the reflection magnitude of the electromagnetic pulse.

The display 40 begins to respond at a level corresponding to approximately 100 microvolts appearing at the receive antenna 20. Since systematic errors in the sample/hold circuit 26, the summer 36, and the amplifier 38 may amount to several tens of millivolts, this error must be subtracted out in order to detect small changes, such as a 100 microvolt change caused by a distant hidden object 22. In addition, front surface reflections from the wall 24 contribute to the error voltage.

Therefore, when power is applied to the detector 1, a power-on-reset circuit 44 closes a "calibrate" switch 46 for one second so an integrator 48 in the feedback path 50 of the amplifier 38 servo's the output of the amplifier 38 until an equilibrium is reached, such that the output of the amplifier 38 is forced to equal the reference voltage applied to the integrator 48. Since integrators have extremely high DC gain, the voltage difference between the output of the amplifier 38 and the reference is reduced to a negligible value. The display 40 is also referenced to the same reference voltage as the integrator 48, so the indicator scales its response relative to the voltage deviation from the reference voltage. This power-on calibrate sequence leaves the detector 1 ready to respond to changes in voltages caused only by a change in the pulses reflected off the wall 24 or the object 22.

Figure 3 depicts the geometry of the antennas 18, 20 and the effective physical location of the range gate 54 is projected into space. Its position is effected by the square root of the relative dielectric constant of the intervening building material (i.e., ε_r about 2 - 3). As shown schematically in a top view, the transmit antenna (T) 18 and the receive antenna (R) 20 are contained in a housing 52, and are positioned adjacent to the wall 24, and operate with a range gate 54 determined by the delay generator 30 (Figure 1). The range gate 54 typically extends about 1 inch behind the wall. The range gate 54 is actually curved because it corresponds to a fixed radius set by the echo acceptance time of the S/H circuit 26.

The propagation impedance in free space <maths id="math0002" num=""><math display="inline"><mrow><mtext>Zo(space) = √</mtext><mover accent="true"><mrow><msub><mrow><mtext>µ</mtext></mrow><mrow><mtext mathvariant="italic">0</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">/ε</mtext></mrow><mrow><mtext mathvariant="italic">0</mtext></mrow></msub></mrow><mo>¯</mo></mover></mrow></math><img file="EP0700528B1_D0002.tif" /></maths> where µ₀ is the permeability of vacuum and ε₀ is the permittivity of vacuum. The propagation impedance in a material (such as wood) having ε_r = 2 is <maths id="math0003" num=""><math display="inline"><mrow><msub><mrow><mtext>Z</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><mtext> (wood)=√</mtext><mover accent="true"><mrow><msub><mrow><mtext>µ</mtext></mrow><mrow><mtext mathvariant="italic">0</mtext></mrow></msub><mtext>/</mtext><msub><mrow><mtext mathvariant="italic">ε</mtext></mrow><mrow><mtext mathvariant="italic">r</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">ε</mtext></mrow><mrow><mtext mathvariant="italic">0</mtext></mrow></msub></mrow><mo>¯</mo></mover><msub><mrow><mtext> = Z</mtext></mrow><mrow><mtext>o</mtext></mrow></msub><mtext>(space)/√</mtext><mover accent="true"><mrow><msub><mrow><mtext mathvariant="italic">ε</mtext></mrow><mrow><mtext mathvariant="italic">r</mtext></mrow></msub></mrow><mo>¯</mo></mover><msub><mrow><mtext> = Z</mtext></mrow><mrow><mtext>o</mtext></mrow></msub><mtext>(space)/ √2</mtext></mrow></math><img file="EP0700528B1_D0003.tif" /></maths>. The free space propagation impedance is 377 ohms and the propagation impedance of wood (ε_r =2) is 266 ohms. This difference in impedance causes a difference in the reflection magnitude when an object such as a stud is present. A profile of the propagation impedance Z₀ in the range gate location 54 is shown in Figure 3.

In a one dimensional analogy to propagation along a transmission line, which can be equated to time domain reflectometry (TDR), reflections off a stud become equivalent to reflections from a transmission line discontinuity. The reflection coefficient, Γ, defined as <maths id="math0004" num=""><math display="inline"><mrow><mtext>(Y - 1)/(Y + 1)</mtext></mrow></math><img file="EP0700528B1_D0004.tif" /></maths> where <maths id="math0005" num=""><math display="inline"><mrow><mtext>Y = Z(wall)/Z(space)</mtext></mrow></math><img file="EP0700528B1_D0005.tif" /></maths>, can be applied to determine what fraction of the radiated pulse is returned. For example, if the wall material is wood with an ε_r=2, the reflection magnitude is 0.17. Thus the difference in reflection magnitude between the presence and absence of a stud is 0.17. If the object 22 were metal, the reflection would be total, or 1.0. Thus, metal is easily discerned from wood by a 5.9 times greater reflection magnitude. Even if the metal has a much smaller cross-section, as may be the case with a wire, it is still easily discerned in practice as long as the polarization of the wire and the detector antenna match - which is generally the case for wires behind walls and for a vertical orientation of the detector 1.

The present invention overcomes a serious limitation stemming from a variable reflection magnitude from the first surface 25 of the wallboard 24. In conventional studfinders, the circuit loses its power-on calibration if the studfinder is moved from the wall by even the slightest distance "x" (Figure 4A). Once out of calibration, the reliable detection of the stud is compromised. It is therefore most desirable that the indicated reflection amplitude from the wallboard 24 remain constant over a housing-to-wall distance of several inches.

The problem of varying first surface reflection magnitude is caused by radiating a pulse, shown in Figure 4B, that contains either post-shoot or ringing - a common effect when radiating pulses through an antenna. What radiates later in time falls into the sampler's gate when reflected off objects closer than the intended range gate 54, i.e. there is a displaced range gate 54x. Accordingly, ringing components reflect off the front surface of the wall 24 and fold into the reflections from the object 22 in simultaneity. Indeed, these front surface reflections can exceed the rear surface reflections. During power-on calibration, the front surface reflections are subtracted out so the detector can properly detect an object - until the detector is lifted slightly off the surface and the calibration is altered by changes in front surface reflections. Thus, the detector has little tolerance for rough surfaces or non-contact operation. Because of the changing polarity of the pulse, the indicator signal can change polarity, as shown in Figure 4C.

The solution for this problem is illustrated in Figures 4A, 4B, 4D by the radiated waveform, which has an exponential tail of the same voltage polarity as its peak voltage. If the peak of the pulse is spatially located behind the wall for stud detection, the center of the tail may then be spatially located at the front surface of the wall. When the calibrated studfinder is lifted from the wall (increase "x"), the diminishing surface return, caused by increasing the distance, is compensated by the increasing tail amplitude that becomes positioned at the wall front surface. Figure 4D shows the indicator signal (amplified reflection signal) for the tail-pulse case, showing a constant signal over a 2 inch variation in "x". Thus, in the preferred embodiment, the detector 1 emits a pulse having the waveform shown in Figure 4A. This can be accomplished by the proper design of the transmit antenna 18.

Figure 5 is a schematic view of the preferred configuration of the antennas 18, 20. The antennas 18, 20 are formed of wires 56, 58 situated above a ground plane 60 and may be considered to be either leaky transmission lines or bent monopoles. The value of the distal termination resistors R_T affects the shape of the tail pulse, so R_T can be fine-tuned to obtain a flat response characteristic versus distance from the wall surface.

In an illustrative embodiment, the ground plane is a copper ground plane circuit board. The antennas are #24 AWG enameled copper wire. Each antenna has a length L of about 1.5" and a height H of about 0.8".

The transmit antenna 18 is driven by a voltage step so its radiated waveform tends to be a pulse with a slight tail caused by the distal reflection from R_T, which is set to be higher than the propagation impedance of the 1.5 inch wire. A similar effect occurs at the receive antenna 20, where there is no termination at the sample/hold input. The combination of high impedance and parasitic capacitance at the sample/hold input tend to integrate the receive pulse, further stretching the tail of the pulse. The propagation impedance of the 1.5 inch wire is about 200 ohms, and the value of R_T is about 330 ohms.

Figure 6 shows a prototype embodiment of the detector 1. The PRF generator 62 is formed of three inverters (I1) and is followed by a pulse width limiter 64. The pulses pass to step generator 66 formed of a low cost TV tuner transistor <maths id="math0006" num=""><math display="inline"><mrow><mtext>Q1=BFW92</mtext></mrow></math><img file="EP0700528B1_D0006.tif" /></maths>, whose output is connected to the transmit antenna 68 which is a wire loop. The pulses from the PRF generator 62 also follow a second path through the range delay generator 70 which is formed of a variable resistance plus stray capacitance and input capacitance of a buffer gate. The delayed pulse is input into step generator 72, formed of another transistor <maths id="math0007" num=""><math display="inline"><mrow><mtext>Q2=BFW92</mtext></mrow></math><img file="EP0700528B1_D0007.tif" /></maths>, which produces the gating pulse.

The reflected signals are picked up by the receive antenna 74 and input into S/H circuit (capacitor) 76 which is gated by the gating pulse through a Schottky diode <maths id="math0008" num=""><math display="inline"><mrow><mtext>D1=MBD701</mtext></mrow></math><img file="EP0700528B1_D0008.tif" /></maths>. The output from the S/H circuit 76 is input into amplifier (I2) 78. A second amplifier (I2) 80 is connected though calibrate switch (MOSFET) Q3 (part of I3) from the output of amplifier 78 back to its input to form the baseline subtractor /integrator circuit.

The "power on" reset circuit 82 (I3) turns on transistor Q3 so that the output of amplifier 78 is fed back through operational amplifier 80 to subtract the background from the input of amplifier 78. The input of amplifier 78 serves as the summer for the S/H circuit 76 output and the calibrate signal from amplifier 80. The output of amplifier 78 drives the indicator circuit 84 which is formed of a plurality of comparators (I4) referenced to different levels which drive associated LED's. The highest level LED "metal" is turned on by comparator (I5); the high reflectivity of metal produces a high indicator signal. A low battery test circuit 86 (I5) and voltage regulator circuit 88 (I6) are also included.

In a preferred embodiment, <maths id="math0009" num=""><math display="inline"><mrow><mtext>I1 =74HC04</mtext></mrow></math><img file="EP0700528B1_D0009.tif" /></maths>, <maths id="math0010" num=""><math display="inline"><mrow><mtext>I2=TLC272</mtext></mrow></math><img file="EP0700528B1_D0010.tif" /></maths>, <maths id="math0011" num=""><math display="inline"><mrow><mtext>I3=CD4007</mtext></mrow></math><img file="EP0700528B1_D0011.tif" /></maths>, <maths id="math0012" num=""><math display="inline"><mrow><mtext>I4=LM324</mtext></mrow></math><img file="EP0700528B1_D0012.tif" /></maths>, <maths id="math0013" num=""><math display="inline"><mrow><mtext>I5=LM358</mtext></mrow></math><img file="EP0700528B1_D0013.tif" /></maths>, and <maths id="math0014" num=""><math display="inline"><mrow><mtext>I6=78L05</mtext></mrow></math><img file="EP0700528B1_D0014.tif" /></maths>. The detector of this invention propagates an electromagnetic pulse, and since electromagnetic propagation scales by the square root of dielectric constant, the detector is substantially independent of building materials. Further, the propagating pulse will easily radiate across an air gap of several inches. The sensitivity of this invention is such that a stud can be detected behind several inches of concrete with the unit held an inch off the surface of the concrete (about 40dB signal-to-noise ratio). As mentioned previously, the detector 1 can similarly be applied to locate a variety of hidden objects.

Referring now to Figures 7, 8 and 9 of the drawings, there is illustrated another electromagnetic hidden object detector 100 according to the present invention. The detector 100 operates in a generally similar way to the detector 1 of Figure 1, and further includes additional features the implementation of which will be described hereafter in greater detail. One such feature of the detector 100 is to significantly reduce, or to completely eliminate the near wall proximity effect with the receive antenna that causes a DC level shift in the receiver, resulting from the impedance shift of the receiver antenna. This objective is achieved by using an AC coupled amplifier in the receiver path that prevents DC signals from passing from the averaging sample and hold circuit to the display. This AC coupled amplifier filters out the DC bias level shifts in the S/H circuit 26 caused by near wall proximity effect to the receive antenna 20.

Another new feature of the detector 100 is the materials thickness control, which allows the detector 100 to locate hidden objects behind either a relatively thin or thick separator, including without limitation, a wall, ceiling, floor, ground, etc. This feature will particularly enable t including without limitation he detector to be used with concrete and stone wall separators. This thickness control feature is also achieved by impressing an AC modulation upon the transmitter pulses, and then synchronously rectifying this AC modulation in the receiver (homodyne technique), thereby permitting the use of an AC coupled amplifier in the receiver, to eliminate near wall proximity errors.

The detector 1 has been modified for homodyne operation. The homodyne technique involves modulating a signal from a PRF generator prior to radiation and detection with a continuous wave (CW) signal. The receive amplifier then operates with a passband centered on the CW signal and is thus AC coupled. After amplification, the signal is synchronously detected using the same CW signal.

Turning now more specifically to Figure 7, the detector 100 includes a homodyne oscillator 102 which typically operates at several kHz (in the present example 2 kHz), and a PRF generator 104 (similar to the PRF generator 10 shown in Figure 1), which generally operates in the range of 1 MHz to several MHz (i.e., 2 MHz in the particular example described hereafter). It should however be understood that the homodyne oscillator 102 generates a signal whose frequency is faster than the response time of the display 106. It should also become clear that the homodyne signal can alternatively be an arbitrary sequence of pulses having a mean frequency on the order of several kHz and a zero average.

The signals from the homodyne oscillator 102 and the PRF generator 104 are fed into a step generator 106 where the homodyne oscillator 102 amplitude modulates the step signal generated by the step generator 106, in effect turning the step generator ON and OFF at the desired homodyne frequency, which in this example is 2 kHz. Therefore, the signal output by the step generator 106 and transmitted by the transmit antenna 118, includes periodic packets of pulses having a frequency of 2 kHz, such that each pulse typically comprises bursts (such as 1,000 pulses) at a frequency of 2 MHz, with a 0.5 millisecond burst interval.

As the pulses are transmitted over the transmit antenna 118, they are reflected off a hidden object 122 for reception by the receive antenna 120. The signals reflected off the object 122 are formed of a sequence of periodic pulses, corresponding to the transmitted packets, and having a period of 2 kHz. The 2MHz pulses or bursts within the 2 kHz envelopes are reflected off the hidden object as described above in connection with the detector 1. At the receive antenna 120, the amplitude of the 2 kHz envelope is related to the reflection from the hidden object 122.

For a proper display of the waves reflected off the object 122, it is desired to reference the 2 kHz envelope to a predetermined reference level from the integrator 48, thus allowing the detector 100 to basically operate similarly to the detector 1. For this purpose, a receiver sample and hold circuit 126, similar to the sample and hold circuit of detector 1, averages the 2 MHz bursts (pulses) over a period of about 1 millisecond, so that only the 2 kHz homodyne frequency remains on the sample and hold capacitor 126. The homodyne frequency is amplified by an AC coupled amplifier 129, and is thereafter synchronously rectified into a DC level by means of a synchronous rectifier 130. The advantage of the AC amplifier 129 is that the detector DC bias levels, i.e., the DC bias levels at the sample and hold circuit 126 are not allowed to pass through (i.e., are filtered out).

These DC bias levels vary with the power supply fluctuations and, more importantly, with materials brought into near proximity to the receive antenna 120. The near proximity effect of the materials changes the characteristic impedance of the receive antenna 120, and thus the average rectified gate pulse signal. The rectified DC level at the output of the synchronous rectifier 130 represents the reflected pulses from the hidden objects, and the subsequent operation of the detector 100 is similar to that of the detector 1.

In operation, the rectifier switch 130S closes during one half of the homodyne oscillator cycle, and charges the capacitor 130C during this half cycle of the homodyne oscillator cycle. During the complementary (i.e., remaining) half cycle of the homodyne oscillator cycle the switch 130S is open, and the rectifier 130 does not detect the signals from the homodyne oscillator 102. As a result, the average signal applied to the capacitor 130C represents the peak amplitude of the signal (square wave) at the output of the AC coupled amplifier 129, thereby generating a DC voltage which corresponds to the reflection signal from the hidden object 122 and not from the DC voltage from the sample and hold circuit 126.

The DC voltage developed on the capacitor 130C represents the summation of the desired signals reflected from the hidden object 122, as well as undesirable reflections from various sources including the detector housing and direct antenna-to-antenna coupling. Therefore, at turn on, the detector is held at a distance from the separator and the power-on-reset circuit 44 closes the switch 46, causing a feed back action through the integrator 48, and its associated DC reference voltage. Consequently, the output of the DC coupled amplifier 38 is caused to be equal to the DC reference voltage.

Shortly after turn-on, the switch 46 is opened and the voltage at the output of the integrator is maintained at a steady level which represents the correction for the undesirable reflections (or signals) mentioned above. The summer 36 continues to subtract this correction voltage from the voltage at the output of the capacitor 130C, thereby providing a signal that very closely corresponds to the reflections from the hidden object 122. It should be noted that a similar mode and logic of operation will apply to the detection of cavities.

Referring to Figure 8, it illustrates various timing charts at different sections of the detector 100. Starting with Chart A, it corresponds to, and represents the voltage at point A, which is the output of the homodyne oscillator 102, and shows a pulse signals with a period of 500 µs, corresponding to a 2 kHz frequency. Chart B corresponds to the voltage at point B, which is the output of the PRF generator 104, and shows a pulse signals with a period (not to scale) of 500 nanoseconds, corresponding to a 2 MHz frequency.

Chart C illustrates the voltage at point C, which is the output of the step generator 106. Chart D shows the voltage at point D, which is the output of the sample and hold circuit 126, as a pulse or square wave, with an associated DC component. Chart E represents the voltage at point E, which is the output of the AC coupled amplifier 129. Chart F is the reference voltage at point F, which is the output of the DC coupled amplifier 38 during power on reset, and corresponds to the reference voltage V_ref.

Charts F' and F'' represent the voltages at point F and illustrate the operation of the detector 100, corresponding to the detection of a cavity (Chart F') and a solid object (Chart F''), with the axis x-x representing the center of the object and the cavity. The phantom lines represent the various levels at which the LED's of the display indicators 162A and 162B light up. For example, with reference to Chart F'', the fourth LED (4) will light up as soon as the detector 100 is about 1.5 inches away from the center of the object 122. As the detector 100 approaches the center of the object, the third LED (3) will light up, and then the second LED (2), and finally the first LED (1) will light up indicating the position of the center of the object 122. Thereafter, as the detector 100 is progressively distanced away from the center of the object 122, the LED's will light up in the reverse order.

Turning now to Figures 9A and 9B, they represent a circuit diagram of a prototype embodiment of the detector 100. The transmission path of this circuit is generally similar to the transmission path of the detector 1 shown in Figure 6, and further includes the homodyne oscillator 102, which typically includes two 74HCO4 inverters 150, 151 by National Instrument. The output of the homodyne oscillator is simultaneously connected to the step generator 106 and the synchronous rectifier 130 along the receive path.

The receive path of the circuit of the detector 100 is generally similar to that of the detector 1, and further includes the AC coupled amplifier 129 and the synchronous rectifier 130. The AC coupled amplifier 129 is connected between the averaging sample and hold circuit 126 and the synchronous rectifier 130, which, in turn, is connected to the summer 36. The AC coupled amplifier 129 includes two MC14069UB inverters 152, 153 by Motorola, used in a linear mode as amplifiers. The synchronous amplifier 130 includes a bipolar transistor 155, such as transistor 2N2222 by National Semiconductor, that is turned ON or OFF by the homodyne oscillator 102.

The DC coupled amplifier 38 includes two MC14069UB inverters 156, 157 by Motorola used in the linear mode as amplifiers. Similarly, the integrator 48 and the power-on-reset circuit 44 include MC14069UB inverters, such as inverters 159, 160 respectively. The power-on reset circuits 44 further includes an RC circuit 161 which acts as a self referencing buffer. The output of the DC coupled amplifier 38 drives the display 106. It should become clear to those skilled in the art after reviewing the present invention, that while the circuits of the disclosed embodiments have been described in term of discrete components for simplicity and clarity purpose, these circuits can alternatively be miniaturized by integrating these components on an integrated circuit.

The display 106 includes a plurality of LED's 162A and 162B that sequentially light in proportion to the applied voltage, which is linearly related to the reflection magnitude of the electromagnetic pulse. The display 106 further includes a network of resistors 163 and inverters 164 (such as 74HCO4 inverters), whose internal discrimination level defines the threshold at which the LED's 162A and 162B are turned ON. In this particular illustration, the LED's 162A indicate the presence of a hidden object, while the LED's 162B indicate the presence of cavities within a solid object.

For cavity detection, the voltage applied to the display 106 is reversed (i.e., has an opposite polarity) relative to the voltage indicating solid objects. In operation, the detector 100 may be calibrated to a solid portion of the separator 24, and as the detector 100 is moved along the separator 24, the cavities will produce less reflection than the calibration position, thereby driving the display indicators 162B.

Therefore, the detectors 1 and 100 are capable of locating hidden metallic objects such as conduits, electrical wiring and nails, and non-metallic objects, such as pipes, studs and joists behind wood walls, ceiling, floors, masonry and cement structures, regardless of the conditions of these structures or their distance to the detector. This feature is accomplished by transmitting short electromagnetic pulses and sampling reflections from the hidden object 122 by range gating (i.e., setting a predetermined detection range or range gate 54 as shown in Figure 3) the receive sample and hold circuit 126 at a specific location from the separator 24. Additionally, the detector 100 can be moved relative to the separator 24, and it is not necessary to keep it in close proximity to the separator 24.

The new detectors 1 and 100 will not be directly affected by the dielectric constant of the intervening wall, ceiling and floor materials, and will work on almost all structures, including sheetrocks, plywood, particle board, dense hardwoods, such as wood floors, stair steps, furniture or cabinetry having different dielectric constants, and tiles. This feature is achieved by using an electromagnetic sensor which is affected only by the square root of the dielectric constant of the matter be scanned. As a result, the detector is much less dependent on the dielectric constant variations, than a capacitive sensing studfinder, thus providing more accurate measurements.

The present detectors 1 and 100 present a first surface cancellation effect, with a fixed and controllable detection or depth adjustment. The first surface cancellation effect is implemented by controlling the transmitted waveform, and by the proper selection of the characteristics of the pulses from the pulse generators and the dimensions and geometry of the antennas.

Additionally, the new detectors 1 and 100 are portable, light weight, simple to use, reliable, relatively inexpensive, and have a low power emission, which helps comply with the requirements of Part 15 of the Federal Communications Commission's (FCC) Rules. Part 15 requires less than 500 microvolts per meter_field strength at a 3 meters distance from the transmit antenna. The power emission of the detectors 1, 100 has been measured to be 100 microvolts per meter.

The present detectors 1 and 100 do not interfere with the operation of telecommunications and wireless equipment and other detectors in the proximity. These detectors accomplish the required low power emission through the use of a receiver that averages a very large number of pulses, and do not rely on the high power avalanche mode pulse generators to form the transmitter.

The present detectors 1 and 100 are adaptable for automating the construction process, such as for use construction tools or equipment in order to control their operation, thereby improving the efficiency of the construction process.

The present detectors 1, 100 can locate objects buried underground. It is portable, easy to use, relatively inexpensive, and has a low power emission which helps comply with the requirements of Part 15 of the Federal Communications Commission's Rules. While the capacitive type detectors do not operate properly on moist soil, the present detectors 1, 100 provide precise readings because the emitted signal propagates with a relatively low loss through both dry and moist soil.

The detectors 1 and 100 according to the present invention are readily usable in security applications such as for locating guns, weapons in suitcases or briefcases. These detectors will not only detect metallic objects but will also detect other objects having a high density, such as plastic explosives. In general, clothing emits less reflection than solid objects, and thus they easily distinguishable from these solid objects. When the detectors are used in security applications, it is possible to adjust or program their sensitivity for a more accurate performance.

Referring now to Figure 10 there is illustrated a schematic view of an automatic tool arrangement 150, which includes two generally identical detectors 151 and 152 similar to either of the previous embodiments, i.e., detector 1 or 100. The detectors 151 and 152 are mounted on a desired tool or tool housing 155, such as a nail gun. In operation, as one of the detectors, i.e., 151 approaches the object (i.e., a stud) 122, the display indicator 162A (Figure 7) will light up to indicate the presence of the object 122, while the detector 152 does not detect the presence of the object 122.

As the detector 152 approaches the object 122, its display indicator(s) will light up. However, as long as the detectors 151 and 152 are not substantially symmetrically positioned relative to the object 122 (such as the center of the object 122 if the latter is not too wide), then the display indicators of the detectors 151 and 152 will not provide the same indication level. For instance, the fourth LED of the detector 151 might light up, while only the second LED of the detector 152 is simultaneously lit. As the detectors 151 and 152 approach the desired position, for instance they become symmetrically positioned relative to the center of the object 122, then the display indicators 162A of both detectors 151 and 152 will provide the same indication levels. In the foregoing examples, the third LED of both detector displays will light up simultaneously. As soon as this desired position is reached, one or both detectors 151 and/or 152 generate a control signal to the tool (i.e., nail gun) 155, for activating a nail dispenser 156 that forms part of the tool 155, in order to dispense one or more nails.

It should become apparent to those skilled in the art after reviewing the present description that the foregoing arrangement can be modified and used in conjunction with various applications without departing from the scope of the intended invention. The following are a few non comprehensive examples of such applications. In some cases, it would be desired to dispense two parallel columns of nails, not necessarily at the center of the objects 122. Therefore, the detectors 151 and 152 can be regulated, such as by programming, to issue a control instruction to the nail gun 155, to dispense one or more nails, individually or in a delayed sequence, as soon as a desired or predetermined spatial relationship between the detectors 151 and 152 and the object 122 is reached.

Merely for illustration purpose, if the object has a known dimension, such as a width of four inches, and it is desired to dispense a series of nails at a distance of one inch from each edge 160, 161 of the object 122, then the nail gun (or tool) will be instructed (or programmed) to dispense nails as soon as the third LED of the detector 151 is lit simultaneously with the first LED of the other detector 152, until the first column of nails is completed. Similarly, the nail gun 155 will be instructed to dispense nails as soon as the third LED of the detector 152 is lit simultaneously with the first LED of the other detector 151, until the second column of nails is completed.

The nail dispensing sequence can be interrupted either manually, or, if desired, automatically, as soon as the positions of the detectors 151 and 152 change relative to the object 122, either intentionally or by accident. In some applications, when the exact positioning of the nails is not entirely required, it would be desirable to include a predetermined margin of error in the program instructions to the nail gun 155, such that it will continue to dispense nails as long as the LED indicators of both detectors 151 and 152 indicate the relative position of the nail gun 155 is within an acceptable tolerance level.

Figure 11 shows an alternative automatic tool arrangement 163, which operates similarly to the tool arrangement 150, with the two detectors 151 and 152 replaced with a single transmission unit 164 and two separate receiver units 165 and 166. In this embodiment, the transmitter unit 164 includes a homodyne oscillator 102, a PRF generator 104, two step generators 106 and 32, and RC circuit 33, and a transmit antenna 118, that are connected as described above (Figure 7). The RC circuit 33 forms a short gate pulse from the much wider pulse developed by the step generator 32, and forms a pulse differentiator.

Each of the receiver units 165 and 166 includes a receive antenna 120, a sample and hold circuit 126, an AC coupled amplifier 129, a synchronous rectifier 130, a summer 36, a DC coupled amplifier 38, and a display 106, connected as described above in relation to Figure 7. In one design, each of the receiver unit 165 and 166 includes a power-on-reset circuit 44, an integrator 48 and a calibrate switch 46, as shown in Figure 7, or alternatively, these components could be shared between the two receiver units 165 and 166.

The transmit unit 164 is simultaneously connected to each receiver unit 165, 166, via a gating connection 167 and a rectifier connection 168 as illustrated in Figure 7. The transmit unit 164 is preferably mounted half way between the receiver units 165, 166. It should however become clear that the transmitter unit 164 could be adjustably mounted on the tool or tool housing 155 between, or relative to the receiver units 165, 166. In the preferred embodiment, the transmit unit 164 sends out a common gate pulse to both receiver units 165, 166. The foregoing dual arrangements of Figures 10 and 11 can also be used for a more accurate location of hidden objects.

The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms described, and obviously many other modifications are possible in light of the above teaching. The embodiments were chosen in order to explain most clearly the principles of the invention and its practical applications, thereby to enable others in the art to utilize most effectively the invention in various other embodiments and with various other modifications as may be suited to the particular use contemplated.