Abstract:
A procedure and a system for the detection of a plurality of passive electronic markers are disclosed. A marker location device capable of scanning for multiple types of electronic marker thereby increasing operator efficiency and reducing erroneous marker indications is described. In some embodiments, scanning may be performed at the operating technician&#39;s direction or may be performed in the background while the operating technician is performing other tasks.

Description:
CROSS-REFERENCE TO CD-ROM APPENDIX 
   CD-ROM Appendix A, which is a part of the present disclosure, is a CD-ROM appendix consisting of two (2) text files. CD-ROM Appendix A contains, two computer program listings for embodiments of controllers of a marker locator as described below. The total number of compact disks including duplicates is two. The attached CD-ROM Appendix A is a CD-ROM formatted for an IBM-PC operating a Windows operating system. Appendix B, which is part of the present specification, contains a list of the files included on the compact disk. 
   A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. 
   These and other embodiments are further discussed below. 
   BACKGROUND 
   1. Field of the Invention 
   The present invention relates to the detection of concealed electronic markers, and specifically, to a procedure and device for discriminating among a plurality of buried electronic markers. 
   2. Discussion of Related Art 
   Utility conduits are often buried underground or concealed in walls and not readily accessible. It is often necessary to locate these concealed utility conduits in order to repair and replace them. It is also important to know the location of utility conduits in order to avoid them while excavating an area. Examples of hidden utility conduits include pipelines for gas, sewage or water and cables for telephone, television or power. 
   There are various ways to locate concealed objects, for example, using line locators or marker locators. Line locators are appropriate when seeking electrically conductive objects, such as metallic pipelines and cables. Line locators may also be used for finding non-electrically conducting conduits when the conduit is marked with a conducting trace wire or trace tape buried along the conduit. The process of applying an AC signal to the conductor at an accessible point and detecting the resulting electromagnetic radiation is well known in the art. When an AC signal is applied, the conductor acts as an antenna radiating an electromagnetic field along its entire length. 
   A line locator used above ground detects electromagnetic emissions from conductors underground. A disadvantage with relying solely on the line locator device is that it may fail to identify and distinguish among various utility conduits and conductors. Additionally, line locator devices can not be used to locate non-conductive lines, such as, for example, gas lines, fiber optic lines and plastic water lines when not marked with trace wires. 
   Conduits may also be marked with electronic markers, either at surface level or underground. Buried electronic markers may be used to locate and identify a number of concealed objects such as cables, pipes, access points, underground stock piles, survey points and septic tanks. 
   Generally, electronic markers consist of two types, namely, active markers and passive markers. Active markers radiate a signal detectable at the surface; however, they require a power source. Passive markers, on the other hand, require no power source and become active when induced by an external electromagnetic field, which can be generated with a portable source. 
   A marker locator is a device for detecting and determining the location of concealed or buried markers. Passive markers typically include a multi-turn wire loop (coil) tuned with a capacitor to a pre-determined resonant frequency. 
     FIG. 1  illustrates a marker locator as operated by a location technician. Location technician  6  holds marker locator  1  directed towards ground level  7  to find the location of hidden passive markers  10  and  12 . The hidden passive markers  10  and  12  can each be coded with a resonant frequency in order to identify the type of utility lines  11  and  13  that each frequency respectively marks. 
   Commonly, a passive marker is the combination of a wire coil and a capacitor enclosed within a non-metallic protective enclosure. The combination creates an inductance-capacitance (LC) circuit defined by an inductance developed by the wire coil and a capacitance held by the capacitor. The LC circuit operates in a resonance mode at its resonant frequency f given by the equation: 
             f   =     1     2   ⁢   π   ⁢     LC                 (     Equation   ⁢           ⁢   1     )             
 
where L is the inductance of the wire coil and C is the capacitance of the capacitor.
 
     FIG. 2A  shows an example of a ball-type passive marker. Passive marker  10  is a spherical passive marker housing three LC circuits  10 A,  10 B and  10 C. The coils of each LC circuit  10 A,  10 B and  10 C are positioned in orthogonal Cartesian planes such that the three tuned circuits produce a uniform radio frequency (RF) field. 
     FIG. 2B  shows a disk-type passive marker. Passive marker  12  is a flat passive marker housing a single LC circuit  12 A with the coil positioned in the horizontal X-Y plane. 
     FIG. 3  shows the electrical schematic diagram of a single LC circuit. The coil acts as an inductor  16 , and is connected in parallel with a capacitor  18  to form a resonant tank circuit  14 . The frequency f of the passive marker is set by the resonant frequency of the passive LC circuit, which can be tuned to a preset value. 
   Different types of utility lines are each associated with unique resonate frequency values. Markers with different resonant frequencies may also be colored for quick identification when installed. Six distinct frequency/color combinations are commonly used: 77.0 kHz (Orange/Black) for Canadian telephone and Cable TV; 83.0 kHz (Yellow) for Gas; 101.4 kHz (Orange) for Telephone; 121.6 kHz (Green) for Sanitary/Waste water; 145.7 kHz (Blue) for Water; and 169.8 kHz (Red) for Power. Of course, these frequencies (and colors) have been designated by conventional use and are not meant to be restrictive. 
   Though passive electronic markers have several advantages over tracing wires, they are still subject to certain limitations. One such problem is the time consumed by separate searches for each type of marker. Another such problem is the “neighbor detection” problem where emissions of marker-types not being searched for overwhelm the receiver producing false-positive indications. A similar problem is the “near-far” problem where emissions from nearby markers can override signals from the farther placed marker possibly producing an erroneous marker indication. 
   In light of the foregoing description, it would be desirable to devise an improved method for locating markers. It would also be desirable to reduce the occurrence of erroneous marker indications. It would be further advantageous if a method existed that could facilitate detection of all markers in a given area more quickly than is conventionally known. 
   SUMMARY 
   In accordance with the present invention, a marker locator system that can scan for multiple marker types and a method to scan for multiple marker types are presented. 
   In some embodiments, the marker locator scans at the operator&#39;s request. In some embodiments, the marker locator scans in the background during the “idle” times of the marker locator. A marker locator according to the present invention includes (1) a transmitter stage capable of transmitting electromagnetic radiation at one or more of a plurality of fixed frequencies; (2) a receiver stage capable of receiving electromagnetic radiation from one or more of a plurality of fixed frequencies; and (3) a processor coupled to the transmitter and receiver stages, wherein the transmitter stage scans through a plurality of fixed frequencies in response to the processor. 
   In some embodiments, a marker locator includes: (1) a base subsection including a transmit antenna and a receive antenna; (2) a shaft coupled to the base subsection; (3) a top assembly coupled to the shaft, wherein the top assembly includes a control panel, the control panel includes a display screen and input buttons; and (4) electronic circuitry mounted in the base subsection, the shaft and the top assembly, wherein the electronic circuitry includes: (4a) a transmitter stage including a transmitting antenna; (4b) a receiver stage including a receiving antenna; and (4c) a processor electrically coupled to the transmitter stage and to the receiver stage. 
   In some embodiments, a marker locator includes a means for scanning a set of a plurality of marker frequencies, a means for generating a transmit pulse at one of the set of the plurality of marker frequencies, a means for transmitting the transmit pulse, and a means for receiving responses from one or more markers. 
   In some embodiments, a method of locating markers includes: transmitting a first pulse, the first pulse including electromagnetic radiation of a first frequency appropriate for a first marker type; receiving responses from one or more markers; and detecting a first marker response appropriate for the first marker type. 
   In some embodiments, a method of locating markers includes: performing a primary scan with a first set of sinusoidal waves of a first set of frequencies appropriate for a first set of marker types; and performing a background scan with a second set of sinusoidal waves of a second set of frequencies appropriate for a second set of marker types. 
   These and other embodiments are further discussed below with respect to the following figures. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  illustrates a marker locator in operation by a location technician. 
       FIG. 2A  shows an example of a ball-type passive marker. 
       FIG. 2B  shows an example of a disk-type passive markers. 
       FIG. 3  shows the electrical schematic diagram for a single LC circuit of a passive marker. 
       FIG. 4  shows a marker locator in a typical application scenario according to the present invention. 
       FIG. 5  shows a diagram of a control panel of a marker locator according to the present invention. 
       FIGS. 6A and 6B  show the temporal relationship between a transmitted signal from the transmitter stage of  FIG. 10  and a passive marker&#39;s reflected signal. 
       FIGS. 6C ,  6 D and  6 E show transmitted signals containing primary, background and neighbor pulses. 
       FIG. 7  illustrates the “neighbor detection” problem in locating markers. 
       FIG. 8  show an example of the “near-far” problem in locating markers. 
       FIG. 9  further expands on the “near-far” problem shown in  FIG. 8 . 
       FIG. 10  shows a hardware overview block diagram of a marker locator according to the present invention. 
       FIG. 11A  shows a hardware block diagram of one embodiment of a processing subsystem according to the present invention. 
       FIG. 11B  shows a marker locator connected to a remote PC according to the present invention. 
       FIG. 12  shows a hardware block diagram of an alternate embodiment of a processing subsystem according to the present invention. 
       FIG. 13  shows a hardware block diagram of another embodiment of the Processing Subsystem including both shared and dedicated hardware. 
       FIG. 14A  shows a hardware block diagram of a transmitter stage of a marker locator according to the present invention. 
       FIG. 14B  shows another hardware block diagram of a transmitter stage of a marker locator according to the present invention. 
       FIG. 14C  shows yet another hardware block diagram of a transmitter stage of a marker locator according to the present invention. 
       FIG. 15  shows a hardware block diagram of a receiver stage according to the present invention. 
       FIG. 16  shows a hardware block diagram of a detection circuit of the receiver stage shown in  FIG. 15 . 
       FIG. 17  shows a software state transition diagram according to the present invention. 
       FIG. 18  shows a software flow chart of a scan mode algorithm operating on a marker locator system shown in  FIG. 10  according to the present invention. 
       FIG. 19  shows a software block diagram of the background-scan feature operating on a marker locator system shown in  FIG. 10 . 
   

   In the figures, elements having the same designation have the same or similar functions. 
   DETAILED DESCRIPTION 
   A passive marker as shown in  FIGS. 2A and 2B  is self-contained, with no accessible physical connections. Radiating a signal from marker locator  1  towards the area where the marker is located activates the passive marker. As discussed above, a passive marker (marker  10  in  FIG. 2A  or  12  in  FIG. 2B ) absorbs and re-radiates electromagnetic energy radiated from marker locator  1  that falls within its resonant band as defined by the passive marker&#39;s LC circuit shown in  FIG. 3 . 
     FIG. 4  shows a marker locator in a typical application scenario according to the present invention. Marker locator  1  includes a base subsection  30  with transmit and receive antennas. In some embodiments, both transmit and receive antennas can be combined into a single antenna housed in base subsection  30 . Base subsection  30  is attached to shaft  32 . Shaft  32  holds top assembly  40  and handles  34  for location technician  6  to carry marker locator  1 . The front panel display can be integrated into top assembly  40 , into base subassembly  30  or remotely situated. The electronic circuitry of marker locator  1  can be distributed among the base subsection  30 , shaft  32  and top assembly  40 . Alternatively, the hardware of marker locator  1  can be vehicle mounted rather than portable. 
     FIG. 4  also shows several markers of various types in the vicinity of marker locator  1 . Markers with absorption bands centered at frequencies f 1 , f 2  and f 3  are shown although any number of markers, each of which can be activated at a particular center frequency, can be present. For example, F1-type marker  51  may mark a gas utility line, F2-type marker  52  may mark an underground power utility line, F3-type marker  53  may mark a water main and F4-type markers  54 ,  55  and  56  may mark three spots adjacent to a sanitary water main. 
     FIG. 5  shows a diagram of a front control panel  41  for an embodiment of marker locator  1 . As discussed above, control panel  41  may be dispersed throughout marker locator  1 , may be incorporated into top assembly  40  of marker locator  1 , or may be separated and electrically coupled to marker locator  1 . Control panel  41  can allow location technician  6  to input data and control the operation of marker locator  1 . Input  42  can include, for example, input buttons  42 , keypad, and/or keyboard devices. Control panel  41  can provide location technician  6  with outputs to receive information from marker locator  1 . Outputs can include, for example, speakers  45 , screen display  46 , and LEDs  47 . A touch screen can be used as display  46  to combine input and output functions in a single device. Additionally, port  48  may be used to interface to external devices to control and receive data from marker locator  1 , for example keyboards or computers. 
   During the period while marker locator  1  transmits a signal near a passive marker, that passive marker accepts, accumulates and re-radiates energy within its resonant frequency bandwidth. When the transmitter of marker locator  1  turns off, the marker continues to emanate energy that was still stored; however, the intensity of the emitted radiation is exponentially decaying. When the receiver of marker locator  1  detects the re-radiated energy from the passive marker, marker locator  1  alerts location technician  6  of the presence of a detected marker. 
     FIGS. 6A and 6B  show the temporal relationship between the transmitted signal  222  from marker locator  1  and the emitted signal  332  from a passive marker, for example, marker  52 .  FIG. 6A  shows the envelope of the sinusoidal transmit signal of the transmitter.  FIG. 6B  shows the envelope of the relatively weaker sinusoidal signal emanating from the passive marker. As the transmitter pulse  223  excites the passive marker, the passive marker accepts, accumulates and re-radiates energy  333 . During transmitter pause  224 , the passive marker stops accumulating energy but continues to emanate energy in the form of a proportionally decaying electromagnetic field  334 . Marker locator  1  can tune to receive radiation field  334  emanated by, potentially, each of a plurality of passive markers (e.g., markers  51  through  55  in  FIG. 4 ). The width of pulse  223  and the width of pause  224  are each typically longer than the LC time constant of a marker LC circuit on one of markers  51  through  55 . 
   Conventional marker locators are designed to search for only a single type of marker during the marker search process. To search for two different types of markers, location technician  6  performs multiple steps: (1) manually select the first type of marker; (2) perform a first physical search of the area for the first type of marker; (3) manually select a second type of marker to search; and then (4) repeat the physical sweep of the area for the second type of marker. 
   With embodiments of the present invention, marker locator  1  may search for a plurality of marker types without intermediate operator intervention. Marker locator  1  allows location technician  6  to search for two or more marker types during a single physical sweep. Marker locator  1  can alternatively sweep through a range of frequencies rather than a set of distinct frequencies. The list or range of marker types can be defined within marker locator  1 , can be defined by or selected by location technician  6 , or can be remotely set though an interface to an external device. Marker locator  1  according to the present invention can include multiple modes and features, including foreground-scan, background-scan and neighbor-detection. Marker locator  1  can perform foreground-scan. A foreground-scan feature can allow location technician  6  to define a primary list of marker types. Marker locator  1  uses the primary list when determining the frequency to transmit in a pulse of energy. Marker locator  1  can send a first pulse with a frequency representing the first marker type on the primary list. Marker locator  1  can then send a second pulse with a second frequency representing the second marker type on the primary list. Marker locator  1  continues the process until all frequencies representing each marker type in the primary list has been transmitted. Marker locator  1  then repeats the process until location technician  6  terminates the foreground-scan. 
     FIG. 6C  shows the envelope of a transmitted signal during a foreground-scan operation. Marker Locator  1  transmits a series of primary pulses (p). For example, the primary list contains two marker types, F1-type and F2-type. Marker locator  1  will transmit a primary pulse (p) with a frequency representing the first marker type (F1). Marker locator  1  will then transmit a primary pulse (p) with a frequency representing the second marker type (F2). Alternatively, marker locator  1  can send a fixed number of primary pulses (p) of each maker type before advancing to the next marker type on the primary list. Marker locator  1  continues the process until location technician  6  terminates the foreground-scan operation. 
   In some embodiments, marker locator  1  can perform background-scan. A background-scan feature can allow location technician  6  to scan for marker types when not scanning for marker types defined by location technician  6 . Background scanning may be performed based on a time schedule, based on signal received, or based on depth measurements as described below. 
   The background-scan feature allows location technician  6  to define a background list of marker types. Marker locator  1  uses both the foreground list and the background list when determining the frequency to transmit in a pulse of energy. As with the foreground mode, marker locator  1  sends a series of primary pulses (p); however, the series is periodically interrupted by a background pulse (b). Primary pulses (p) represent marker types from the primary list. Background pulses (b) represent marker types from the background list. Marker locator  1  allocates a majority of pulse slots to the primary pulse sequence and a minority of slots to the background sequence. Marker locator  1  continues transmitting primary pulses and background pulses in their respective pulse slots until location technician  6  terminates the background-scan operation. 
     FIG. 6D  shows the envelope of a transmitted signal during a background-scan operation base on a time schedule. As described above, marker locator  1  transmits a sequence of primary pulses (p) and background pulses (b).  FIG. 6D  shows a primary list containing a single marker type, F1, and a background list containing two marker types, F2 and F3. Marker locator  1 , in the example shown in  FIG. 6D , has allocated 3 of 4 pulse slots to primary pulses (p) and 1 of 4 pulse slots to background pulses (b). During each primary pulse (p) time slot, marker locator  1  transmits a frequency pulse representing the next marker type on the primary list. Here, the primary list contains a single marker type, therefore each primary pulse (p) contains a frequency representing the F1 marker type. Similarly, during each background pulse (b) time slot, marker locator  1  transmits a frequency pulse representing the next marker type on the background list. Here, the background list contains two marker types, therefore every second background pulse (b) contains a frequency representing the F2 marker type and every other background pulse (b) contains a frequency representing the F3 marker type. 
   In some embodiments, marker locator  1  can perform neighbor detection. A neighbor detection feature can allow location technician  6  to configure marker locator  1  to scan for neighboring marker types. Initially, marker locator  1  receives an initial response from a marker indicating a first marker type. By checking for neighboring marker types, marker locator  1  can increase the certainty of the initial response or can report that a marker of a neighboring marker type exists. By determining if a marker of a neighboring marker type caused the initial response, marker locator  1  can reduce erroneous indications provided to location technician  6 . 
     FIG. 6E  shows the envelope of a transmitted signal during a neighbor detection operation. Marker locator  1  can amend the primary list entered by location technician  6  with a list of neighboring marker types. The neighboring marker types are those marker types that lie adjacent to the marker types in the primary list entered by location technician  6 . For example, if the primary list contains a marker type of F2, then neighbors to F2, i.e., F1 and F3, would be included in a neighbor list. If the neighbor is already contained in a primary list, then the neighbor would not necessarily need to be in the neighbor list. Marker locator  1  can append the primary list with the list of neighbors. Alternatively, marker locator  1  can append or replace the background list with the neighbor list.  FIG. 6E  shows a series of three primary pulse (p) representing the F2 marker type from the primary list. The figure also shows that after each series of primary pulse (p), marker locator  1  transmits a neighbor pulse (n) representing the next marker type on the background list. With a primary list containing F2, a neighbor list containing F1 and F3, neighbors of F2, and a ratio between primary and neighbor pulses of 3 to 1, marker locator  1  sends the sequence F2 (p), F2 (p), F2 (p), F1 (b), F2 (p), F2 (p), F2 (p), F3 (b), then repeats the sequence until location technician  6  terminates the neighbor detection operation. 
   Embodiments of the present invention can include any combination of pulses corresponding to the frequencies of the various markers. In general, a primary list of markers is scanned. In the background, between pulses at frequencies corresponding to markers on the primary list, pulses with frequencies corresponding to markers on a background list and possibly near neighbors occur with less frequencies than pulses corresponding to markers on the primary list. 
   Utilizing some embodiments of the present invention, location technician  6  performs a single sweep for all selected markers rather than multiple sweeps for each marker type. As the technician performs the sweep of the area under search, marker locator  1  automatically cycles through the various marker resonant frequencies without additional operator intervention. In some embodiments, scanning of five discrete frequencies can take approximately 600 milliseconds each spatially located in the scan. 
   In some embodiments, marker locator  1  indicates to location technician  6  that marker locator  1  has completed one search cycle therefore directing the technician to move to the next physical location. The indication can be an audio indication, such as a beep from a speaker, or a visual indication, such as a flash from an LED or text and graphics on an LCD display. 
   When scanning for multiple marker types, embodiments of marker locator  1  can search for multiple marker types sequentially, in parallel or in a hybrid fashion. 
   To search sequentially, marker locator  1  searches each marker type in a sequential fashion. After each marker type has been searched, marker locator  1  advances to the first marker type, repeating the cycle. For example, if searching for F1, F2 and F3-type markers, marker locator  1  first transmits and “listens for” F1-type markers. Marker locator  1  emits an electromagnetic radiation pulse  223  containing a single predetermined resonant frequency for the F1-type marker followed by a pause  224  between pulses  223 . Marker locator  1  then advances to transmit and “listen for” F2-type markers, then in-turn F3-type markers. After completing the search for the last marker type, marker locator  1  begins the process again with the F1-type marker search. Before marker locator  1  advances to the next marker type search, marker locator  1  performs an individual marker-type search that can consist of the transmission of a single pulse or can consist of the transmission of a series of pulses. When a scan is complete at a particular location, marker locator  1  may wait for location technician  6  to relocate before the next scan is started. 
   To search in parallel, marker locator  1  utilizes a comb transmitter. A comb transmitter combines multiple sinusoidal signals at discrete frequencies into a single signal. To allow enough energy to pass to a marker, more power is required than with the single frequency signals of the sequential search described above. The combined multiple frequency signal may be constructed with out-of-phase resonant frequency signals to accommodate the dynamic range of marker locator  1  transmitter. Marker locator  1 , thereby, searches for multiple types of markers with each transmitted pulse  223 . Pulse  223  contains a set of predetermined resonant frequencies followed by a pause  224  between pulses  223 . Marker locator  1  combines the individual marker type pulse signals of each of the marker types to be searched into a single pulse  223 , thus multiple marker types, if present, will activate and response to pulse  223 . 
   To search in a hybrid fashion, marker locator  1  incorporates a combination of serial and parallel methods described above. That is, marker locator  1  subdivides the search list into two or more subgroups of marker types to search. A first pulse  223  or first series of pulses  223  contain a first subgroup of predetermined resonant frequencies representing the first subgroup of marker types being searched. The next pulse  223  or series of pulses  223  contain the next subgroup of frequency components representing the next group of marker types being to search. The process of searching for subsets of marker types is repeated once all marker types have been searched. 
   As previously discussed, marker locator  1  can perform a primary-scan among a predetermined or technician defined plurality of marker types. Marker locator  1  can supplement the primary-scan with a secondary-scan of marker types not included in the primary-scan. The secondary-scan can include all or just some of the other marker types not included in the primary-scan. The secondary-scan can include, for example, just neighboring marker types as described below. 
   Marker locator  1  can scan a primary subset of marker types selected by location technician  6 . For example, if a single utility provider maintains both cable TV lines as well as telephone lines, location technician  6  may set up a limited type list to scan. If searching for cable TV and telephone lines, location technician  6  can configure marker locator  1  to scan for both cable TV and telephone line type markers. 
   On the other hand, if preparing to excavate an area for new cable TV and telephone lines, location technician  6  would want to know what other hazards exists. Location technician  6  can configure marker locator  1  to scan for all but cable TV and telephone line type markers by enumerating all other marker types individually. Alternatively, location technician  6  can create a do-not-search list. Location technician can enable marker locator  1  to search for all marker types except for those marker types enumerated on the do-not-search list. 
   In some embodiments, marker locator  1  also scans for neighboring marker types. Neighboring marker types are those marker types that are higher and lower in resonant frequency compared to the marker types selected by location technician  6 . Immediate neighboring marker types are those marker types that are next higher and next lower in resonant frequency. With an immediate neighbor mode enabled, marker locator  1  conducts a search that includes both selected and immediate neighboring marker types. For example, if scanning at a first frequency, marker locator  1  also scans at adjacent frequencies belonging to marker types just higher and just lower in frequency. Thus, marker locator  1  can perform a search for marker types not specifically selected for location. 
   For the following immediate neighbor marker search example, assume that only five marker types exist. These five marker types have associated sequential resonant frequencies f 1 , f 2 , f 3 , f 4  and f 5  having F1, F2, F3, F4 and F5-type markers, respectively. If the primary search involves the F2-type marker, marker locator  1  will perform a secondary search for neighbors for both F1 and F3-type markers. If the primary search involves both F2 and F3-type markers, marker locator  1  will perform a secondary search for neighbors for both F1 and F4-type markers. If the primary search involves F1, F3 and F5-type markers, marker locator  1  will perform a secondary search for neighbors for both F2 and F4-type markers. Marker locator  1  can perform the primary search and secondary search in series, in parallel or in a hybrid fashion as described above. 
   In some embodiments, in addition to scanning for immediate neighbors, marker locator  1  can search for all marker types multiple marker types away. Some embodiments of marker locator  1  can conduct a search that includes both selected marker types and multiple neighboring marker types to each side of the selected marker types. For example, if scanning at a first frequency, marker locator  1  also scans at multiple adjacent frequencies belonging to marker types just higher and multiple adjacent frequencies belonging to marker types just lower in frequency. In some embodiments, marker locator  1  can have a variable called neighbor — width representing how wide the neighbor search should be, that is, the number of marker types away from the selected marker types to search. If searching for f 5  with a neighbor — width of two, then marker locator  1  will search for neighbors f 3 , f 4 , f 6  and f 7  in addition to the selected marker type f 5 . 
   In some embodiments, marker locator  1  scans for all defined marker types. With a scan-all mode enabled, marker locator  1  can search through each discrete marker type resonant frequencies f 1 , f 2 , f 3  through f n . 
   In some embodiments, marker locator  1  can use the ability to scan for multiple marker types and neighboring markers to reduce erroneous marker indications. Erroneous marker indications can occur when two or more types of markers lie in one region. 
   When location technician  6  searches for a particular type of utility line marked with a set of passive markers of one marker type, marker locator  1  will typically transmit and “listen for” the one resonate signal associated with the sought after type of passive marker. As a result, location technician  6  does not intend to activate and detect other types of markers marking other utility lines. For example, if looking for power cable markers at 169.8 kHz, a nearby water main marker, which is tuned to resonate at 145.7 kHz, should not activate, and thus, should not emanate a signal to marker locator  1  within the band of the power cable marker. In practice, however, the marker positioned near the surface identifying the water main can radiate electromagnetic energy appropriate for the power cable marker. Such saturation may cause a false indication of the presence of the first type of markers. If not compensated, a detector searching for one type of marker may erroneously indicate the presence of that marker due to detection of energy radiated from another type of marker. 
     FIG. 7  illustrates the “neighbor detection” problem described above.  FIG. 7  illustrates the effect of strong signal  801  emanating from an F2-type marker preset to resonant frequency f 2  buried near the surface. The absorption band of the F2-type marker can be broad enough to absorb and radiate electromagnetic radiation from marker locator  1  that is set to radiate at frequency f 1  appropriate for detection of F1-type markers. The signal received from a marker closer to marker locator  1  can be a stronger signal than the signal received from a more distant marker. Marker locator  1  transmits pulse  223  then pauses  224  before sending the next pulse. During pause  224 , the marker detector searches for F1-type markers with energy at resonant frequency f 1 . Marker locator  1  takes the measurement  804  that may appear to come from an F1-type marker emanating signal  803 , but is actually part of the energy spectrum radiated by the F2-type marker. 
   Marker locator  1  may receive a signal from a marker of a marker type not specifically being searched. If marker locator  1  erroneously detects a marker not being searched, marker locator  1  might provide an erroneous indication. 
   In order to prevent an erroneous indication, some embodiments of marker locator  1  search for marker signals from markers at neighboring frequencies. In the example of  FIG. 7 , marker locator  1  searches for neighbors to F1-type markers. Therefore, marker locator  1  searches for F2-type markers. Marker locator  1  takes measurement  802  at frequency f 2  indicating the presence of an F2-type marker emanating signal  801 . Marker locator  1  can then extrapolate measurement  802  at frequency f 2  to obtain a predicted measurement at frequency f 1 . The measurement at frequency f 1  is compared to the predicted measurement based on the strength of the signal radiated at frequency f 2 . If the signals were substantially equal, marker locator  1  would not indicate an F1-type marker but rather the presence of an F2-type marker. The search of neighbor marker types can continue reiteratively such that if a neighbor is detected, the frequency of the neighbor&#39;s neighbor is searched. 
   Marker locator  1  may receive signals from two or more markers. Again, if searching for one marker type and two markers of different types respond (one being sought after and another marker not being sought after), marker locator  1  might provide an erroneous indication. 
     FIG. 8  shows, for illustrative purposes, an example of the “near-far” problem. Marker locator  1  has activated an F4-type marker near the surface, thus creating a strong signal  811 . Marker locator  1  has also activated a weaker F3-type marker father away than the F4-type marker. Because the F3-type marker is much weaker than the F4-type marker, measurement  812  at frequency f 4  contains little contribution from the F3-type marker. However, because the F4-type marker is significantly stronger than the F3-type marker, a measured signal  813  from the F3-type marker emanating signal  814  may be hidden from proper detection unless the F4-type marker is properly considered. In sum, when one marker is buried at a shallow depth in comparison to a second marker, the closer marker may override the dynamic range of marker locator  1  receiver and thus cause erroneous marker indications. 
     FIG. 9  shows, for illustrative purposes, a continued example of the “near-far” problem as shown in  FIG. 8 . Consider the situation where location technician  6  seeks to find just F3-type markers. The combined contribution of both weak F3-type marker and strong F4-type marker at frequency f 3  results in measurement  821  representing a non-existent signal  820 . By searching for markers neighboring F3-type markers, marker locator  1  can determine that measurement  821  is a false reading. By comparing the measurement  821  at f 3  with the predicted measurement for signal  811  at f 3 , marker locator  1  determines that the neighboring F4-type marker emanating signal  811  overshadows the F3-type marker emanating signal  814 . Marker locator  1  then can indicate the presence of a possible erroneous marker detection and a weak F3-type marker hidden by the F4-type marker. 
     FIG. 10  shows an embodiment of hardware block diagram of marker locator  1  according to the present invention. Marker locator  1  includes multiple subsystems: the processing subsystem  100 , transmitter stage  200 , receiver stage  300  and operator input/output  400 . The processing subsystem  100  controls interaction among transmitter stage  200 , receiver stage  300  and operator input/output  400 . Each of the subsystems is further described below. 
   Operator input/output  400  contains devices necessary for accepting operator commands and control, as well as displaying information to the operator. Operator input/output  400  can include input and output devices such as, for example, combinations of keypad, keyboard, touch-screen, display, indicator and speaker units, as well as other input/output devices. The display, or equivalent output device, can show the received field strength of the marker, as well as the selected modes, the detected marker type and depth information, and additional appropriate information. 
     FIG. 11A  shows an embodiment of hardware block diagram of an embodiment of processing block  100  of  FIG. 10 . In the embodiment of  FIG. 11A , processing has been divided between two processors: main processor  101  and auxiliary processor  102  that share the computational requirements of marker locator  1 . For example, main processor  101  can control coordination among transmitter stage  200  of  FIG. 10 , receiver stage  300  of  FIG. 10  and auxiliary processor  102 . External oscillator  103  supplies timing reference signals to main processor  101 . Auxiliary processor  102  then interacts with operator input/output  400 . 
   In some embodiments of the present invention, main processor  101  can be a RISC microcontroller with serial interface capability, multiple Analog-to-Digital converter channels, and a hardware multiplier, such as the PIC17C44 manufactured by Microchip Technology Inc. (2355 W. Chandler Blvd., Chandler, Ariz. 85224). 
   In some embodiments of the present invention, auxiliary processor  102  can be a standard microcontroller with an integrated LCD driver module, serial interface capabilities and Analog-to-Digital converters, such as the PIC16C924 also manufactured by Microchip Technology. Generally, any number and type of processors capable of performing calculations for controlling marker locator  1  can be used. 
   In some embodiments, auxiliary processor  102  manages operator input, addresses a display, and drives a loudspeaker. In some embodiments, data from main processor  101  to auxiliary processor  102  is supplied via a serial link. Main processor  101  can generate a clock frequency, for example, from oscillator  103 . While main processor  101  controls the stringent timing of the marker location transmit and receive sequence, auxiliary processor  102  manages the less stringent peripheral tasks in support of main processor  101 . 
     FIG. 11B  shows a marker locator connected to a remote PC. In some embodiments, processor  100  provides one or more interfaces to external devices. An interface, for example, may be a serial or parallel, wired or wireless interface. Many interfaces, such as a direct serial interface, can be used to connect processor  100  to a remote computer  900 . Remote computer  900  may be, for example, a laptop PC, PDA, tablet PC, handheld PC or desktop PC. Remote computer  900  can have additional resources for storage, processing and display of operator set parameters, current mode of operation, and frequency verses field strength measurements. Remote computer  900  can also be used for remote control of marker locator  1 . Additionally, remote computer  900  can coordinate measurements and information from marker locator  1  with an external positioning sensors such as a GPS device. 
     FIG. 12  shows a hardware block diagram of an alternate embodiment of a processing subsystem. In some embodiments, main processor  101  and auxiliary processor  102  can share memory  110  and input/output devices  120 . Transmitter stage  200 , receiver stage  300  and operator input/output  400  can have direct or indirect access to processing  100  via shared bus lines. 
     FIG. 13  shows a hardware block diagram of another embodiment of the Processing Subsystem including both shared and dedicated hardware. Processing subsystem  100  may include shared volatile and non-volatile storage such as ROM  113 , Flash memory  114 , RAM,  115 , hard drive  116  and removable media  117 . Processing subsystem  100  may also include shared wireless input/output interface  125  (e.g., RF, infrared or optical interface), serial input/output  124 , and parallel input/output  123 . External interfaces allow connection to external devices, for example, to external storage, to printers, to GPS positioning devices, to command and control devices, to client computers, and to master and slave computers. Interfaces  125 ,  124  and  123  can be used to by-pass operator input/output  400 . Main processor  101  can still have dedicated memory  111  and input/output devices  121 . Auxiliary processor  102  can also have dedicated memory  112  and input/output devices  122 . 
   In some embodiments, location technician  6  selects multiple modes of operation with a keyboard, keypad, touch screen or a similar input device. Modes include, for example: (1) modes to seek an individual service marker type (e.g., gas markers alone); (2) modes to seek a set of service markers (e.g., just sewage and water markers); (3) modes to scan for any and all detectable markers; (4) modes to determine depth measurements; and (5) modes to operate with high gain. Input devices can also be utilized to initiate calculations or perform other queries of auxiliary processor  102 . 
     FIG. 14A  shows a hardware block diagram of transmitter stage  200  of  FIG. 10 . Main processor  101  of  FIG. 11A  incorporates the functions of a digital-to-analog converter (DAC) and of a voltage controlled oscillator (VCO) thereby providing an analog excitation signal  204  directly to transmitter stage  200 . Driver  212  initially amplifies analog excitation signal  204 . Driver  212  provides the amplified output to final amplifier  214  for a second amplification. Final amplifier  214  can be controlled by processing  100  to control gain during transmit pulse period and to shutdown transmission during the pause between pulses. Final amplifier  214  output signal is transmitted via transmitting antenna  216 . Transmitting antenna  216  transmits transmitted electromagnetic radiation. 
     FIG. 14B  shows another hardware block diagram of transmitter stage  200  of  FIG. 10 . Alternatively, main processor  101  of  FIG. 11A  provides a digital value  201  representing the excitation frequency to a digital-to-analog converter (DAC)  202 . DAC  202  creates an analog excitation voltage  203 . Voltage controlled oscillator (VCO)  210  converts analog excitation voltage  203  from DAC  202  into analog excitation signal  204 . Alternatively, processor  100  can incorporate the function of DAC  202 , thus providing an analog excitation voltage  203  directly to transmitter stage  200 . 
     FIG. 14C  shows yet another hardware block diagram of transmitter stage  200  of  FIG. 10 . Alternatively, main processor  101  of  FIG. 1A  provides digital value  201  representing the excitation frequency to a direct digital synthesizer (DDS)  205 . DDS  205  is a programmable device that integrates the functionality of DAC  202  and VCO  210  into a single component. DDS  205  uses direct digital synthesis, which generates a frequency and phase tunable output signal referenced to a fixed frequency from a precision reference clock source. DDS  205  divides down the reference clock signal to provide analog excitation signal  204 . 
   The excitation frequency is based on the sought after marker resonant frequency. If, for example, marker locator  1  seeks to find all power cable-type markers, converter  210  provides a 169.8 kHz excitation frequency signal to driver  212 . Transmit signal  222  emanates from marker locator  1  to activate markers. 
   In some embodiments of the present invention, transmitting antenna  216  is a loop antenna mounted on base  30  of marker locator  1 . Transmitting antenna  216  creates an electromagnetic field that excites the resonant circuit of a marker. In some embodiments, transmitting antenna  216  is a coil-type antenna. Those skilled in the art know that a variety of antenna designs are available to perform as transmitting antenna  216 . 
     FIG. 15  shows a hardware block diagram of an embodiment of receiver stage  300  of  FIG. 10 . Receiving antenna  301  receives emitted signal  332  from a nearby passive marker. The passive marker&#39;s emitted signal  332  includes received signal  334 , which consists of the decaying electromagnetic fields that emanate from the markers during the pause between marker locator  1  transmissions. In some embodiments, receiving antenna  301  includes a ferrite rod antenna coil that is mounted centrally within transmitting antenna  216  of  FIG. 14A . Alternatively to high permeability ferrite, a powdered iron magnetic material may be used. Receiving antenna  301  is coupled, by way of protection circuit  302 , to front-end amplifier  303 . Protection circuit  302  aids in curtailing overload and damage to receiver stage  300 . In some embodiments, main processor  101  of  FIG. 11A  provides gain control  320  to front-end amplifier  303  to adjust for differences in signal level caused by different laying depths of markers. 
   The output of front-end amplifier  303  is filtered by low pass filter  304  to reduce the noise bandwidth. Low pass filter  304 , through AC coupling and the inherent high pass characteristics of the coil antenna, functionally operates as a band pass filter. 
   The output of filter  304  is directed through Tx/Rx switch  305 , a self-biasing circuit used to limit transmitter signal  222  of  FIG. 14A  that can be coupled to the received signal  334 . Since transmitted signal  222  is typically much stronger than the received signal  334  generated by the subterranean markers, Tx/Rx switch  305  is used to mute input to the down stream components of receiver stage  300 . Muting occurs when transmit stage  200  is active. Muting also helps to prevent saturation in receiver stage  300  and shortens recovery time. Main processor  101  of  FIG. 11A  provides Tx/Rx control  321  to Tx/Rx switch  305  to engage and disengage muting. 
   Phase shift  306  adjusts the zero phase of the signal from switch  305 . Phase shift  306  permits adjustment of the phase to compensate for accumulated phase error. By compensating for accumulated phase error, receiver stage  300  provides higher receiver sensitivity in amplifier  303  and switch  305  of the receiver  300 . Phase error accumulates along the entire loop, from transmitter signal generation in the transmitter chain, through the transmitter antenna, back through the receiver antenna, to the receive chain up to mixer  308 . Phase error is also introduced from variations among individual components. 
   The output signal from phase shift  306 , with the corrected phase, is then coupled into premixer amplifier  307 , whose output signal in turn is coupled into mixer  308 . Mixer  308  demodulates the output signal of premixer amplifier  307  by mixing in a reference signal  323  provided by processor  100  of  FIG. 11A . The resulting mixed signal is demodulated to base band or near base band. The resulting output signal of mixer  308  is fed to detection circuit  309 . 
     FIG. 16  shows an embodiment of a detection circuit  309  comprising detection filter  310 , for example, a low pass filter, followed by an integrator  311 . Processor  100  of  FIG. 11A  can provide detection control  324  to integrator  311  to initiate and halt detection between pulses  223 . 
   The output signal of detection circuit  309  can be routed to Analog-to-Digital (A/D) converter  312  with offset addition for bipolar digitization. The output signal of converter  312  can be provided to processor  100 . In some embodiments, the detection can be performed digitally by using converter  312  to convert the analog signal from either premixer amplifier  307  or the output of mixer  308 . With a digital signal, software or firmware within the processing subsystem  100  can perform the functions of mixer  308  and detection circuit  309 . 
   By implementing many of the signal processing functions in software, marker locator  1 , according to some embodiments of the present invention, is flexible in its transmitting and tuning capabilities. As a result of the electronic software generation of marker frequencies, main processor  101  of  FIG. 11A  can rapidly cycle among discrete marker resonant frequencies or sets of discrete marker frequencies. The scanning time is limited by the response time of the markers and the desired noise reduction. 
   Flexible digital architecture allows marker locator  1  the versatility to incorporate various scanning features. In scanning modes marker locator  1  reprograms transmitter stage  200  and receiver stage  300  of  FIG. 10  to operate at discrete frequency or combinations of frequencies automatically. In areas with multiple types of buried markers, location technician  6  can more efficiently, accurately and thoroughly perform a search of the area of concern. 
     FIG. 17  shows a software mode state diagram. In some embodiments, marker location technician  6  can enable or disable scan software features. The operation of marker locator  1  starts with input from location technician  6 . Operator control  702  waits for and accepts location technician  6  input. A search for a single marker type invokes marker location state  710 . A search for multiple marker types invokes scan state  714  that invokes marker location  710  multiple times. When a wrong marker is detected, marker locator  1  can enter the wrong marker state  712  and alert location technician  6 . In some embodiments, once the marker scan has been completed in one location, marker locator  1  waits in idle state  720  until location technician  6  triggers the beginning of the next scan cycle. 
     FIG. 18  shows a software block diagram of the scan mode. Main processor  101  of  FIG. 11A  determines the status of scan mode  501 . If set for automatically scanning, the processor advances the marker type  503  to be searched to the next on the list, transmits the appropriate marker search signal  504 , then searches  505  for an indication of the presence of a marker. In step  506 , if a marker has been detected, marker locator  1  updates operator data  507  displayed to location technician  6 . 
   By incorporating a mode to automatically scan for more than one type of marker, location technician  6  speeds detection of all markers and is not forced to manually cycle through each marker type. Instead of setting a marker type, performing a manual sweep of the area, then repeating the process, location technician  6  simply sets marker locator  1  to scan, and performs a single sweep of the area. Thus, the scan mode of the present invention eliminates the need to repeat the manual sweep for each marker type. 
     FIG. 19  shows a software block diagram of the background-scan feature. The background-scan feature is similar to the operator selected scan mode described above. With the scan mode, location technician  6  sets the scan frequencies to scan. With the background-scan feature, marker locator  1  selects frequencies the technician is not seeking. 
   In some embodiments, background scanning can be initiated based on one or a combination of: (1) the idle times of marker locator  1 ; (2) a time schedule; (3) signals detected by marker locator  1 ; and (4) depth measurements. Background scanning can be initiated based on idle times of mark locator  1 . During idle times  601 , when the background-scan feature is enabled  602 , marker locator  1  will utilize the time in between other operations to perform a scan for “other” markers. Again, the processor advances the marker type to be searched to the next on the list  603 , transmit the appropriate marker search signal  604 , and then searches for an indication of the presence of a marker  605 . In step  606 , if a marker has been detected, marker locator  1  updates its operator data  607  displayed to location technician  6 . 
   In some embodiments, background scanning can be initiated based on a time schedule. For example, location technician  6  defines a list of enumerated marker types to search. Marker locator  1  translates the enumerated marker types into resonant frequencies. Marker locator  1  then begins searching for markers. Marker locator  1  scans for enumerated marker types. Every N seconds, marker locator  1  interrupts the current search for enumerated marker types to search all non-enumerated marker types. Non-enumerated marker types are all of the marker types not included in the enumerated marker type list. Upon completion of the non-enumerated marker type search, marker locator  1  continues with the enumerated marker type search until another N seconds pass. 
   In some embodiments, background scanning can be initiated based on signals detected by marker locator  1 . For example, location technician  6  defines a list of enumerated marker types to search. Marker locator  1  translates the enumerated marker types into resonant frequencies. Marker locator  1  then begins searching for markers. Marker locator  1  scans for enumerated marker types. If a marker from the enumerated marker type is potentially detected, then marker locator  1  interrupts the current search for enumerated marker types to search all non-enumerated marker types. A marker type is potentially detected when marker locator  1  detects a signal at the resonant frequency of a marker type above a set detection threshold. Marker locator  1  can internally set or location technician  6  can configure the detection threshold. Upon completion of the non-enumerated marker type search, marker locator  1 , continues with the enumerated marker type search until marker locator  1  makes another measurement above the detection threshold. 
   In some embodiments, background scanning can be initiated based on depth measurements. If known, location technician  6  can set the estimated marker depth, thereby, calibrating the received signal strength from markers at the estimated marker depth. When location technician  6  begins a depth measurement, marker locator  1  can first perform a background scan for non-enumerated marker types. If marker locator  1  detects a non-enumerated marker type, marker locator  1  can signal an alarm to location technician  6 , thereby helping to assure that location technician  6  does not take inaccurate measurement data. 
   In some embodiments, the concept of the background-scan feature is also applicable to the “wrong marker alert” feature. Depending on the dynamic range of the receiver, the band stop suppression of the receiver is limited. As describe earlier, a marker at a short distance away from marker locator  1  that is not being searched for may show up as a detected marker of a different type. When searching for a first type of marker, a marker of a second type near the receiver may saturate the receiver searching for markers of the first type. 
   In some embodiments, the purpose of the “wrong marker alert” feature is to aid in the prevention of an erroneous indication of a buried marker of a particular type when a marker of another type is found. Marker locator  1  uses its software based frequency generator to quickly scan all defined marker frequencies in the background. When location technician  6  selects a dedicated frequency (e.g., cable TV markers at 77.0 KHz), marker locator  1  scans the other frequencies in the background without notice to the technician. If marker locator  1  detects a neighboring type marker during the background scan, marker locator  1  determines if a potential erroneous indication has been given before providing a “wrong marker alert” warning to the technician. 
   In some embodiments, when a “wrong marker” has been detected, marker locator  1  device notifies the technician. The technician may act on the “wrong marker” indication by performing a search for other individual types of markers, or may engage the scan mode to search for multiple marker types simultaneously. 
   The attached CD-ROM Appendix A, herein incorporated by reference, contains two files: M-1200˜1.TXT and M-1200˜2.TXT. The M-1200˜1.TXT file includes assembly language programs for a Microchip Technology PIC16C924 auxiliary processor to performing display, keypad, keyboard and related functions. The M-1200˜2.TXT file includes assembly language programs for a main processor on a Microchip Technology PIC17c44 microcontroller. Appendix B, herein incorporated by reference, contains a list of the files included on the CD-ROM. 
   The above-described embodiments of the invention are exemplary only. One skilled in the art may deduce various modifications to the embodiments described here which are intended to be within the scope of this invention. As such, the invention is limited only by the following claims. 
   APPENDIX B 
   Volume in drive D is 011212 — 0949 
   Volume Serial Number is 2CBA-618D 
   Directory of D:\ 
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   Dec. 12, 2001 09:49a &lt;DIR&gt; 
   Dec. 10, 2001 12:23p 118,445 M-1200˜1.TXT 
   Dec. 10, 2001 11:34a 101,504 M-1200˜2.TXT 
   
       
       
         
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