WO1992006392A1

WO1992006392A1 - Apparatus having magnetic field inducing target signal transmitter and position reference transmitter for detecting concealed pipes

Info

Publication number: WO1992006392A1
Application number: PCT/US1991/007101
Authority: WO
Inventors: Behnam Motazed; Bryon Smith; David White
Original assignee: Southern California Gas Company
Priority date: 1990-09-27
Filing date: 1991-09-27
Publication date: 1992-04-16
Also published as: EP0503054A1

Abstract

The device comprises a target signal transmitter (32) to induce an electromagnetic signal in the hidden object (36), a position reference transmitter (40) to transmit a signal for positioning purposes, a sensor unit (4) to detect changes in the magnetic flux of the hidden object (36) due to the induced electromagnetic signal and the positioning signal from the position reference transmitter (40), and a central unit (2) to process and display a map of the hidden object. The initial processing of the data results in a gray-scale representation of the object, but additional data processing may be performed.

Description

APPARATUS HAVING MAGNETIC FIELD INDUCING TARGET SIGNAL TRANSMITTER AND POSITION REFERENCE TRANSMITTER FOR DETECTING CONCEALED PIPES

The present invention relates to a device for the non-destructive mapping of concealed, cylindrical objects such as underground gas lines by active electromagnetic sensing. The mapper induces an electric current of known frequency into the underground object and detects the resulting magnetic field using a spherically shaped antenna.

The determination of the location and orientation of underground pipes or cables is essential to the maintenance and installation of utility networks. It is important that a damaged pipe or cable which is in need of repair be accurately located. It is also essential to locate undamaged pipes and cables which are in close proximity to the damaged one to avoid interfering with the undamaged ones during repair. Locating underground pipes and cables is also essential whenever excavation or performing other ground penetrating activities in order to avoid disturbing the underground ob'jects.

Previous methods to locate these hidden objects have included destructive coring, exploratory excavation, lengthy and unreliable thermal induction, electrical resistance, Subsurface Interface Radar (SIR), localized characterization of the earth's magnetic field, and active electro¬ magnetic induction. Magnetic detectors both active and passive are in common use.

Active electromagnetic sensing entails inducing an electric current in an object and then detecting or sensing the magnetic field that results from the induced electric current. Passive electromagnetic sensing entails detecting a magnetic field that emanates from an object due to signals induced in the object from ambient electromagnetic sources, such as radio transmissions or 50/60 Hz electric power signals. It is not practical to use passive electromagnetic sensing if such ambient sources are weak. Another form of passive sensing entails detecting a ferro- agnetic object by sensing the local perturbations caused by the object in the Earth's magnetic field.

Previous methods using active electromagnetic induction have used simple cylindrical antennas or sensors and have required the operator of the device to start from a known point, generally where the electric current is introduced into the cable, and trace the buried object. Typically, these active electromagnetic induction devices simply employ a single electric current with a single frequency. Additionally, the user of such known devices must perform a number of additional operations while collecting data, such as maintaining a constant sensor orientation, observing changes in the sensor's output signal, identifying and remembering where any fluctuation occurs, and inferring the position of the target line in light of the changes. Additionally, the operator of these systems must use a good deal of intuition to interpret the results which are scaled readings of the values of intensities of the magnetic field and are usually indicated as meter deflections or loudspeaker responses. The intensities of the magnetic field will vary depending on the location of the pipe and the orientation of the sensor.

The sensors that have traditionally been used for detecting induced signals are known as pipe and cable locators and consist of ferrite cylinders wrapped with coils of copper wire about the circumference of the cylinder. The magnetic flux in the vicinity of the target object induces a voltage into the sensor coils. The ferrite core increases the magnetic flux through the coils and therefore boosts the signal. The voltage induced in the coil is filtered to remove noise by filtering circuitry in the sensor. The induced voltage is also amplified by amplification circuitry in the sensor. The filtering and amplification circuitry of the sensor is conventional and well known to those of skill in the art. The signal strength is then shown on an analog meter or made available as a loudspeaker response.

The signal strength detected by the sensor is proportional to the magnetic flux flowing through the ferrite core of the sensor. Therefore, the sensor measures the magntiude of the magnetic field vector coincident with the sensor's axis. Because of this directional sensitivity of the sensor and the shape of the magnetic field around the target object, the sensor output varies according to the orientation and position of the sensor relative to the target object. Thus, the location of the target object may be determined by the fluctuations in the magnetic field intensity as indicated by the sensor as the sensor moves over the target object. These traditional sensors are limited in that they measure only one component of the magnetic flux at a time. The traditional sensor must remain in a particular orientation with respect to the target object in order for the data to be interpreted.

Devices presently in use do not simultaneously characterize all components of the magnetic field surrounding the pipe, nor do they accumulate, display or retain data from a region of observation. These and other problems of the currently-used mappers have been solved by the present invention.

The subsurface mapper of the present invention is a position-tracked sensing system that is able to detect the location of objects which are hidden from sight of an unaided human eye.

The mapper detects the hidden object by sensing, measuring, recording, and displaying the magnetic flux of an electromagnetic field which has been induced into the object. The mapper displays the signals received by a sensor as a gray-scale representation in real-time. This allows for immediate interpretation of the data representation, even while collecting additional data. Unlike previous devices, there is no requirement for the operator to maintain constant sensor orientation with respect to the target object nor is there any need for the operator to remember any fluctuations in sensor response while measuring a particular region. The mapper also provides the user with constant visual feedback as to the position of the mapper relative to the data map being collected.

The mapper of the present invention can determine the location and orientation of underground pipes or utility cables. In general, the device of the present invention may be used in any situation where the hidden object is capable of carrying an induced electromagnetic signal, i.e. an electrical current, and is in an environment wherein an electromagnetic signal may not be transmitted or is transmitted to a lesser degree, e.g. the finding of a ferrous pipe in a non-ferrous surrounding. The electrical conductivity as well as the magnetic permeability of the object to be located must be considered in terms of whether this method is reliable and useful in that application. In describing the device of the present invention, the reference of use will be set forth as the finding of ferrous cylinders such as pipes embedded within soil, but it will be understood that many other applications of the invention may be present, including the location of non-ferrous objects so long as the non-ferrous objects are capable of generating an induced magnetic field.

As used in the instant specification and claims, "target object" means the buried object which is being located and mapped. "Target area" means the area wherein the target object is believed to be located. The term "operator" as used herein is not limited to a human operator, nor is the mapper limited to being moved by a human operator. The mapper may be operated by, for instance, a mechanical mobile unit which is able to traverse the target area wherein the target object may be located.

The portable subsurface mapper of the present invention comprises: a target signal transmitter

SUBSTITUTESHEET for inducing a first and a second magnetic field, wherein said first magnetic field has a first frequency and said second magnetic field has a second frequency, said second frequency being greater than said first frequency, and wherein said second magnetic field consists of an impulse of one period in length of said second frequency transmitted once each period of said first frequency; a sensor unit for detecting both said first magnetic field and said second magnetic field, and generating data with respect to the detected first magnetic field and second magnetic field; and a central unit for processing the data generated by said sensor unit and coveying to a user the location of the hidden object.

Preferably, the device of the present invention comprises a position reference transmitter for transmitting a position reference signal to said sensing unit, and said sensor unit further comprises a means for detecting the position reference signal.

In another embodiment of the present invention, the device comprises: a target signal transmitter for inducing at least two magnetic fields into the object at different frequencies; a position reference transmitter for transmitting a position reference signal; a sensor unit for detecting both said magnetic fields from said object and said position reference signal, and generating data with respect to the detected magnetic field and the detected reference signal; and a central unit for processing the data generated by the sensor unit and conveying to a user the location of the hidden object. A spherical electromagnetic antenna is used for detecting the first magnetic field. The spherical electromagnetic antenna allows for sensing of the magnetic field in three axes. This spherical electromagnetic antenna completely characterizes the magnetic field of the signal of the target object by measuring all three of the orthogonal vector components simultaneously and at the same position. The spherical electro¬ magnetic antenna of the present invention comprises a solid ferrite ball with copper windings about all three orthogonal axes.

The central unit contains a computer and a display unit.

SUBSTITUTESHEET The sensor unit contains a spherical electro¬ magnetic antenna for detecting said first magnetic field, a pair of cylindrical antennas for detecting said second magnetic field and preferably a means for detecting the position reference signal. The means for detecting the position reference signal will depend on the signal transmitted by the position reference transmitter. The cylindrical electromagnetic antennas for detecting said second magnetic field are arranged orthogonally with respect to one another, parallel to the plane of the target area, and with their center points on a line that is perpendicular to the plane of the target area. The sensor unit also contains a microprocessor for processing the signals received and communicating the processed signals to the central unit's computer.

The target signal transmitter induces at least two electromagnetic signals of different frequency into the target object. The magnetic fields generated by the induced electromagnetic signals in the vicinity of the target object are measured by the sensor unit to determine the location of the target object. In order to locate a target object, the target signal transmitter is positioned to induce two electromagnetic signals of different frequency into the target object which is located below the surface, either by direct or by remote induction. These different types of induction will be discussed in more detail below. The sensor unit scans over the target area, receiving magnetic flux data from the electromagnetic signals induced in the target object by the target signal transmitter. The magnetic data are communicated to the computer in the central unit where they are converted to be shown on the display as a gray-scale representation. From the gray-scale representation, information about the location and orientation of the pipe is immediately available. Additional processing of the data may be performed in order to resolve the data further.

A position reference transmitter can be set up near or in the target area. The position reference transmitter sends a signal to a sensor means for detecting the signal from the position reference transmitter which is located in the sensor unit. This allows the position of the sensor unit to be determined by the computer in the central unit.

The central unit comprises a computer, a disk drive, a display, a keyboard, and a power - source. The computer controls the data collection and any desired initial processing of the data. The computer is conventional and performs conventional operations on the specific data. The preferable brand of computer for use in the present invention

•^ is the Megatel Quark PCII. The computer contains and controls the ports in communication with the sensor unit, the function keys of the keyboard, the disk drive, and the loudspeaker. The computer also contains conventional graphics electronics

*L5 which control the display and allow the position and electromagnetic data to be displayed as the gray-scale representation. The graphics electronics are part of the Megatel Quark PCII. The computer can also support an optional full keyboard and

20 optional headphones which can provide an audio feedback alternative to the speaker during selected sensing modes. Alternative computers such as the Agiliε TM 11P0 or 22P3 or the Gespac MPL-4079 are suitable for use in the present invention, but would require external graphics electronics similar to those which are part of the Megatel Quark PCII.

The display may be any type of display which can display gray-scale representation and is visible in bright sunlight to allow the invention to be used outside. Applicant has found that a liquid crystal display with graphics capabilities has good resolution and low power consumption, can display the gray-scale representation in a satisfactory manner, and is visible in bright sunlight. The preferable display is the KL6440GSR-FW from Kyocera America, Inc. Other displays, such as liquid crystal flat panel displays available from Sharp Electronics Corp. or Panasonic Industrial Company, are suitable for use in the present invention.

The keyboard that is incorporated into the central unit is a limited type of keyboard with enough keys to control the operation of the system. The keyboard consists of membrane switches arranged linearly and positioned across the bottom of the display. This allows key labels and functions to be shown on the display, since the function controlled by each key changes, depending on the state of the system. Applicant has found that five keys are generally sufficient to provide enough versatility for use during operations. Membrane switch keypads such as those sold by

Bergquist Switch Inc. or Lucas Duralith Corporation are suitable for use as part of the present device. The optional full-size keyboard will provide access to extended, more technical operations of the system such as software development or troubleshooting. Any keyboard that meets the specific requirements of the computer is suitable for use with the present invention. If the brand of computer used is the Megatel Quark PCII, any IBM PC-compatable keyboard, such as the Microtype available from Mechanical Enterprises Inc. or the ZEOS/RS available from Zeos International, Ltd., is suitable for use with the present invention.

The disk drive boots the computer and allows for storage of the collected data for any desired delayed post-processing or for transfer to another computer. The drive and drive media may be any that are sufficient to perform these functions in the environment where the present invention is to be used. Applicant has found that static RAM cartridge drives perform well for the required functions. Suitable RAM drives include those sold under the names ThmCard TM, available from Databook Inc., and BC Reader/Writer, available from Mitsubishi Plastics Industries Limited.

Suitable RAM cartridges depend on the drive chosen and include those sold under the names Melcard, available from Mitsubishi Electric Corporation, Beecard, available from Mitsubishi Plastics Corporation, or ThmCard TM, available from

Databook Inc.

Any power source which is sufficient to run the central unit may be used. Applicant has four, that batteries are convenient due to the mobility of the unit. A battery pack can be carried by the deployment apparatus or worn by a human operator.

The sensor unit is connected to the central unit with a cable to allow for transfer of communication information and power. Conventional radio transmitters and receivers may also be used to connect the sensor unit to the central unit. The sensor unit contains both position and electromagnetic sensors. The position sensor receives the signal from the position reference transmitter, which is then interpreted as position information and communicated to the computer as position data. The electromagnetic sensors observe the magnetic flux due to the electromagnetic signals induced into the target object. The signals are then communicated to the computer as electromagnetic data. The sensor data and the position data are processed in the computer in a conventional manner to produce a spatially correlated data array showing both the position of the sensor and the intensity of the magnetic field sensed by the sensor. The data array is shown on the display of the central unit as a gray-scale representation with the current sensor position highlighted.

The electromagnetic sensor for the first magnetic field is a spherical eletromagnetic antenna which comprises a solid ferrite ball with copper windings in the three orthogonal axes. The number of windings about the ball and the size of the wire used for the windings are selected to tune the sensor to the frequency of the electromagnetic signal transmitted by the target signal transmitter. This tuning of the sensor to the frequency of the transmitted signal is calculated and implemented in accordance with standard electromagnetic principles. It has been found that if the frequency of the transmitted electromagnetic signal is 10 kHz, the number of windings about each axis is suitably 1000 to 2000 and, preferably, 1500 windings. The size of the wire used for this frequency is suitably 40 to 48 gauge and, preferably, 42 gauge. The ideal frequency for the transmitted signal varies depending on the material of the object being located and its external surroundings. Suitably, the frequency of the actively applied signal ranges from 1 kHz to 150 kHz. For most situations, a frequency from 8 kHz to 12 kHz is preferable, and a 10 kHz signal is most preferable The target signal transmitter and sensor units of the present invention are preferably tuned to 10 kHz.

Each coil in the spherical antenna measures the magnitude of one component of the first induced magnetic field. The spherical antenna, through conventional circuitry, transfers the value of the magnitude of the voltage of each coil in analogue form to a signal processing board. The spherical antenna and the signal processing board are in the sensor unit as well as conventional filtering and amplification circuitry.

A second antenna is used to detect the second magnetic field. Preferably, this second antenna is a pair of conventional cylindrical electromagnetic antennas.

Measuring the sign of the magnetic field components is done by comparing the phase of the first magnetic field against the second magnetic field. This allows the device to determine which side of the underground object it is on. If the two signals are in phase, a positive sign is assigned. If they are not in phase, a negative sign is assigned. By comparing the emulated signal with the magnetic signal detected by the sensor, determining whether or not the signals are in phase, and assigning a positive or negative sign respectively, the sensor circuitry applies a magnitude and a sign to each measured field component. On the signal processing board, the three signals generated for each sensor position (one from each coordinate axis) are filtered, amplified and compared for phase. The three normalized analogue voltages are then digitized and made available as signed integers at the board's serial communications port.

Alternatively, the three normalized analogue voltages are digitized and made available as unsigned integers at the board's serial communications port and the three phase angles are digitized and made available as unsigned integers at the same port.

It is important to note that the field vector measurements are made with respect to the coordinate system that is coincident with the spherical antenna. This coordinate frame depends on the orientation of the conductors wound around the ferrite ball. As the orientation of the sensor changes, so does the orientation of this coordinate system. To avoid the requirement for maintaining the sensor in a constant orientation, the field measurements are taken at arbitrary orientations, and then the data is rotated into a common, local coordinate frame. The operations systems for rotating data in this manner are conventional and well known to those of skill in the art. Cylindrical antennas suitable for the present invention are conventional and typically comprise a ferrite rod with a fixed number of copper wire windings around the rod. The number of windings and the gauge of the wire are calculated to be in tune with the frequency of the electric signal in the hidden object. Preferably, a ferrite rod of 1 cm diameter and 4 cm length is wound with copper wire of 36 to 40 gauge to obtain 1000 to 2000 windings for a frequency of between about 50 and 200 kHz.

The position reference transmitter is located in the target area and remains stationary during the mapping operation. The location and orientation of the position reference transmitter define the target area over which sensor data is taken and the coordinate frame into which the data will be rotated. During the mapping operation, the position and orientation of the sensor unit are measured in relation to the position reference transmitter. The position reference transmitter is selected from the group consisting of magnetic position reference transmitter, acoustical position reference transmitter, microwave position reference transmitter, and laser position reference transmitter. Preferably, the position reference sensor is acoustical.

The means for detecting the position reference signal or the position registration unit is conventional and usually sold in conjunction with a position reference transmitter as a positioning system. The positioning system is conventional and performs conventional operations as part of the subsurface mapper. A suitable brand of positioning system for use in the present invention is GP8-3D Sonic Digitizer by Science Accessories Corporation. The target signal transmitter induces two electromagnetic signals into the target object. The signals are measured by the electromagnetic sensors in the sensor unit. The target signal transmitter can induce the electromagnetic signal either by direct connection to the target object or by remote induction of a signal in the target object. Which type of induction method to use is determined by the accessibility of the target object. If a portion of the target object is accessible, direct connection can be used. If the target object is inaccessible, remote induction is used. Direct connection is when an electrically conductive connection is made between the transmitter and an exposed portion of the target object in any conventional manner. Remote induction is when the transmitter is positioned above the target object such that an antenna in the transmitter is parallel with the longer axis of the target object. When the transmitter transmits a signal, the antenna generates an electromagnetic field which penetrates the ground and travels along the target object. Such pieces of equipment are conventional.

Because of the strength of the electromagnetic fields produced both by direct and remote induction, it is preferred that the transmitter be located about three to five meters away from the target mapping area.

The frequency of both electromagnetic signals emitted by the target signal transmitter should be sufficient to induce a large enough magnetic field for detection by the sensor. The frequency will vary depending on the material of the target object and the external environmental conditions. It has been found that an actively applied signal of from about 5 kHz to about 10 kHz works satisfactorily for the first frequency and about 50 kHz to about 200 kHz for the second. It is preferred to use a signal of about 10 kHz for the first frequency and about 100 kHz for the second.

The mapping operation and transition of the data to the display form are described in more detail below.

The position and magnetic flux data are received by the central unit while mapping the target area. Initial processing of that data is performed by the computer to extract the details required to determine the location and orientation of the target object. The computer receives both position and magnetic flux data from the sensor unit, correlates the data, and displays in real-time a position icon and a gray-scale representation showing the degree of magnetic flux recorded by the sensor. The position icon is generated in a conventional manner using the position signal from the position reference transmitter.

To show the data in the form of a gray-scale 5 representation, conventional graphics electronics and software are used to convert the data received from the sensor. The display is divided into a number of cells. As the sensor completes its traversal over the portion of the target area _jL represented by a particular cell in the display, the position icon moves out of the cell and the empty cell is replaced with a gray block which represents the sensor response in that area. Once the sensor operation is complete over the

_]_5 entire target area, the data map shown by the cells of the display is completely filled. Each pixel on the gray-scale representation can display one of sixteen gray levels, covering the range from darker to brighter. Cells in the display

2o corresponding to areas of large values of magnetic flux are of bright-colored regions and the cells corresponding to lower values are darker. When mapping data is represented in this way, bright- colored areas in the representation roughly correspond

25 to the location of the target object. Alternatively, large values of magnetic flux could be represented as dark-colored regions and small values of magnetic flux could be represented as bright-colored regions, in which case the dark-colored areas in the representation would roughly correspond to the location of the target area.

At this point in the processing of the data, the gray-scale representation can show the preliminary location and orientation of any target object present and this preliminary information may be sufficient to enable location of the target object.

However, it is generally considered desirable to perform additional processing of the data in order to highlight subtle features in the data set not illustrated by the gray-scale representation. This additional processing also allows for an estimation of the depth of the target object.

Conventional image enhancement, thresholding and skeletonizing operations are applied to the pixel-digitized gray-scale representation, generating binarized chains of line-like features. The image enhancement of the data performs conventional operations on the data such as quantization of the magnetic images and transformations of the image. Also conventional is the segmentation of the image data by thresholding. It has been found that simple brightness thresholding produces satisfactory results in resolving the data.

A depth estimation algorithm performs operations on segment chains to infer the depth of the hidden object. Segment chains are selected that are roughly perpendicular to the axis of the target object. For each pixel in the chain, a normal vector is constructed to the magnetic vector represented by the pixel. As shown in Figure 1A, the intersection of these normal vectors corresponds to the center of the hidden object, from which the depth can be calculated.

One depth estimate is provided for each pair of pixels (vectors) selected. The depth estimate is refined by performing the operation on several pixel pairs and averaging the results. This depth estimation algorithm is suitable for use only on signed magnetic field measurements, such as those provided by the sensor of the present invention. If the algorithm is applied to unsigned data, as available from conventional pipe and cable locators, the normal lines will not always intersect, as shown in Figure IB.

The present invention may be more fully understood with reference to the following drawing figures:

Fig. 1 shows the central unit and the sensor unit of the mapper of the present invention;

Fig. 1A illustrates depth estimation operation applied to signed data from the present invention;

Fig. IB illustrates depth estimation operation applied to unsigned data from a conventional locator;

Fig. 2 shows the mapper of the present invention in position for location of a target object;

Fig. 3 is a more detailed drawing of the electromagnetic receiver of the sensor unit of the mapper of the present invention;

Fig. 4 illustrates a central unit and a sensor unit combined into a hand-held device; and

Fig. 5 illustrates a spherical antenna in accordance with the present invention. Fig. 1 shows the components of the central unit 2 and the sensor unit 4 and the power source 5. The computer 6, disk drive 8, display 10, keypad 12, loudspeaker 14, optional headphones 16, and optional full keyboard 18 are shown connected to power and signal distribution board 20 of the central unit 2.

Sensor unit 4 is connected to central unit 2 by means of cable 22 and power source 5 is connected to central unit 2 by cable 23. The electromagnetic receiver 24 is shown along with position receiver 26. The electromagnetic receiver comprises the two electromagnetic sensors and the circuitry for processing the electromagnetic sensor data.

Fig. 2 shows all of the components of the subsurface mapper in position for mapping. Target signal transmitter 32 is connected by means of electrically conductive connection 34 to target object 36. Target object 36 is located in target area 38. Position reference transmitter 40 is located just outside target area 38 and is set to emit the positioning signal. The central unit 2 and sensor unit 4 are in position to begin covering the target area 38 in order to locate target object 36.

Fig. 3 is a more detailed drawing showing the electromagnetic receiver 24. As shown in Fig. 3, phase signal antenna 42 is connected to circuitry 44 which is circuitry to amplify and filter for noise the signal from the spherical antenna 46, circuitry to convert the signal from the cylindrical antenna 42 to an emulated or recreated first signal as well as filter and amplify the signal from the phase antenna, all of which are fed to a microprocessor. All this circuitry is conventional. The spherical electromagnetic antenna 46 has copper windings about its three orthogonal axes; it detects the magnetic flux from the target object and passes the resulting signal to the electromagnetic signal processing circuitry 24. The phase signal antenna 42 detects the phase synchronization signal from the target signal transmitter and passes it to the signal processing circuitry 44 over the phase signal connector 48. The signal processing circuitry filters and amplifies the signal from the spherical electromagnetic antenna 46 and emulates the target signal, synchronizing it according to the phase synchronization signal. The electromagnetic signal is then compared to the emulated signal for phase determination. The electromagnetic signal is digitized and signed according to the phase determined and made available to the central unit over the connector to the central unit 22.

Fig. 4 illustrates central unit 2 and sensor unit 4 in a portable configuration. The portable pipe mapper as shown in Fig. 4 has central unit 2 which is connected to sensor unit 4. Central unit 2 has display 10 and 5 Button Keypad 12. Sensor unit 4 has electromagnetic receiver 24 with phase antenna 42 and spherical electromagnetic antenna 46. Fig. 5 illustrates preferred spherical antenna 60 made in accordance with the present invention having ball 62 with windings 64 in three orthogonal axes therearound. Channels 66 have been made in the ball to accommodate the windings. In a preferred embodiment, Fig. 3, the two antennas are located adjacent to each other and about 2.5 cm apart. Alternating current is used to create the magnetic fields.

In the sensor unit the microprocessor uses the first and second magnetic fields to generate a first and second signal, respectively. The second signal generated by the microprocessor is a recreation of the first signal. By comparing the recreated first signal, i.e. the second signal, to the received first signal, the sensor is able to determine which side of the pipe it. is on by comparing the phase of the recreated first signal, i.e. second signal, to the received first signal. The microprocessor is able to recreate the first signal from the second magnetic field because the second magnetic field is at a frequency one period apart from the frequency of the first magnetic field. In other words, by comparing the phase of both signals, the sensor knows which side of the hidden object is located. This recreated first signal is also referred to as emulated signal.

In other words, the complex magnetic field which is made up of both signals which have been induced in the target object, induces a complex signal in both the spherical and phase antennas. This complex signal is co posied of a train of first signal, followed by a single pulse of a second signal. The electronics associated with the spherical antenna in the sensor unit only filter through or is sensitive to the first signal, as induced in each of the three coils, while the electronics associated with the phase antenna in the sensor unit only filter through the second signal. The phase antenna electronics use the single pulse of the second signal as an event in order to create a second signal which is a regenerated form of the first signal. In a sense "emulating" the first signal at the same frequency, as produced by the electromagnetic transmitter. The microprocessor in the sensor unit uses the first signal sensed at each coil on the spherical antenna, and compares it to the second signal (so-called emulated signal) from the phase antenna circuitry. If the microprocessor determines that the first signal and the second signal are exactly rising and falling at the same time, then it considers them to be in phase, otherwise they are out of phase. This change in phase, a physical phenomena, occurs as the sensor unit is moved from one side of the pipe to the other, thereby giving the capability to the sensor unit to distinguish whether it is on one side of the pipe or the other.

Good results have been obtained where instead of inducing a second magnetic field and using the second magnetic field to create the second signal (recreated first signal), radio waves or some other conventional electromagnetic waves are used to send a second signal from the target signal transmitter to the sensing unit. In the case of radio waves, the two cylindrical antennas are then replaced by a conventional radio wave receiver and antenna combination. From the radio waves the device then generates the second signal which is compared to the received first magnetic field to determine where the sensor unit is with respect to the object.

In this case, both the target object transmitter and the sensor unit have a transmitter and receiver, respectively, which are compatable with each other, e.g. radio transmitter and receiver. The sensor unit still uses the second signal to recreate the first signal and compares the recreated first signal to the first magnetic signal. It will also be appreciated by those in the medical profession that the device of the present invention can be used to determine the location and orientation of cylindrical objects located in the human body and invisible to the naked eye. Such is possible if the hidden object is capable of having a magnetic field emanating therefrom.

Claims

What is claimed is:

1. A device for detecting a hidden, cylindrical object which is capable of carrying an induced magnetic field comprising:

(a) a target signal transmitter for inducing a first and a second magnetic field into the object, wherein said first magnetic field has a first frequency and said second magnetic field has a second frequency, said second frequency being greater than said first frequency;

(b) a sensor unit for detecting said first magnetic field and said second magnetic field, comparing the frequency of said first magnetic field with the frequency of said second magnetic field to determine which side of the cylidrical object the device is on, and generating data with respect to the detected first magnetic field and second magnetic field; and

(c) a central unit for processing the data generated by said sensor unit and conveying to a user the location of the hidden object.

2. The device of claim 1 wherein the first frequency is about 10 kHz and said second frequency is about 100 kHz. -3 J -

3. The device of claim 1 wherein the ratio of the first frequency to the second frequency is about 1:2 to about 1:20.

4. The device of claim 1 wherein said sensor unit comprises a spherical electromagnetic antenna for detecting said first magnetic field and a pair of cylindrical electromagnetic antennas for detecting said second magnetic field, said spherical antenna located about 2.5 cm from said cylindrical antenna.

5. The device of claim 1 further comprising a position reference transmitter for transmitting a position reference signal to said sensing unit, and said sensor unit further comprises a means for detecting the position reference signal.

6. The device of claim 5 wherein the position reference transmitter is selected from the group consisting of magnetic position reference transmitter, acoustic position reference transmitter, microwave position reference transmitter, and laser position reference transmitter.

7. The device of claim 5 wherein the position reference transmitter is an acoustic position reference transmitter.

8. The device of claim 1 wherein- the target signal transmitter has a radio transmitter and said second magnetic field is radio waves which are transmitted to said sensor unit and said sensor unit has a radio antenna and receiver for receiving said radio waves from said transmitter.