Abstract:
A transmitter for a line locator system that controls the electrical current, voltage or power applied to the target line is disclosed. Control of the electrical output of the transmitter can be achieved passively or by means of a feedback control system. A transmitter connected directly to a line can include an operator control and monitoring of current being supplied to a line to be located. Some transmitters include a current regulating circuit that controls current supplied to a line. In some transmitters, feedback controllers and feedback loops are used to regulate output current, voltage or power. Some control loops are based on monitoring currents in circuits; some control loops monitor power output from an inductive mode transmitter antenna. An inductively coupled transmitter with power output control is also disclosed.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates to the field of underground line location systems and, in particular, to devices for electrically coupling power to concealed objects so that they can be located by an underground line locator. 
     2. Related Art 
     Underground line locators are used to locate the position of lines buried in the ground (i.e., underground lines) such as gas pipes, water pipes, telephone cables, and power cables. A line locator system typically includes a transmitter and a receiver. The transmitter can be electrically coupled to the underground line to be located to cause an electric current to flow in that underground line, which results in the emission of a magnetic field from that line. The receiver locates the underground line by detecting the radiated magnetic field from the underground line. 
     In a direct connection mode, the transmitter is conductively coupled to the line to be located, typically at a point in the line that is above the ground. The transmitter generates a voltage at one end of the line, causing an electrical current to flow along the conductive line, which produces an electromagnetic field around the line. The electromagnetic field penetrates the ground surface and exists above ground where the receiver can detect it. 
     In instances where direct connection to the line is not possible (e.g., the line is completely underground), the transmitters of line locators can operate in an inductive mode. The transmitter of an inductive mode line locator produces current in the line by mutual electromagnetic induction. A time-varying electromagnetic field is radiated by an antenna and induces a current in the line to be located. The current produces an electromagnetic field around the line that can be detected by the receiver. 
     To locate the line, an operator typically moves the receiver over the surface of the ground until the receiver indicates the location of the source of the magnetic field and, therefore, the location of the underground line. The ability of the receiver to locate a line depends on the strength of the electromagnetic field, which is proportional to the electrical current in the buried line. According to Ohm&#39;s Law, the current is inversely proportional to the impedance of the line. Because of the wide variability of the physical condition of buried lines and the wide variability of the environment in which those lines are buried, the impedance of buried lines can vary over a wide range. The current produced by the transmitter of a typical locator system varies with the different impedance encountered in each line. The strength of the magnetic field generated by the current within the line, then, varies with the impedance and determines whether or not a receiver above the ground can locate the line with any accuracy. 
     However, it may be necessary to control the output voltage of the transmitter in order to prevent damaging the underground line or to comply with regulations such as those of the Federal Communications Commission. At present, conventional transmitters in line locator systems control the voltage applied to the underground line, and therefore allow the output current to depend on the impedance of the line, while attempting to maximize the electrical power transmitted into the line. However, the resulting magnetic signal generated from the underground line varies with the impedance of the underground line such that the ability of the receiver to locate the line also depends on the impedance of the line, which can vary widely from location to location. 
     Therefore, there is a need for transmitters in line locator systems which result in magnetic signals which are independent of the impedance of the underground line. 
     SUMMARY 
     In accordance with the present invention, a transmitter for an underground line locator system which provides a constant output to the underground line is presented. The effects of the impedance of the underground line, therefore, is minimized. In some embodiments, the electrical power, current or voltage output of the transmitter can also be controlled. A transmitter according to the present invention includes a source that produces a substantially constant current when coupled to an underground line for a range of impedances of the underground line. In some embodiments, once an upper limit of voltage or power is reached, the voltage or power is held constant by the source instead of the current. 
     In some embodiments of the present invention, the transmitter is coupled directly, or conductively, to the line to be located. In some embodiments, the transmitter is designed to be a constant-current source, whereby the electrical current output from the transmitter does not depend on the impedance of the line over some finite range in impedance, even if the impedance changes during operation. In some embodiments, the transmitter incorporates a feedback control system, wherein the electrical voltage generated by the transmitter is changed to maintain a constant current in the target line. 
     In some embodiments, the transmitter monitors the current and voltage of the output and adjusts the voltage to provide a constant current essentially independent of the resistance of the line. In some embodiments, the transmitter holds the voltage to within preset limits of voltage or so as to fall within a range of power. Preset limits of power and voltage may be set by a user of the line locator system, or may be preset in the line-locator system. If the resistance of the line is such that the voltage required to achieve the desired current exceeds the preset limit, then the transmitter changes from constant current output to constant voltage output, and the current is allowed to decrease as required to maintain the maximum voltage. If the resistance of the line is such that the power output of the transmitter exceeds a preset limit, then the voltage is decreased such that the power does not exceed its limit. These embodiments allow operation of the transmitter as a constant-current source and additionally limit the maximum voltage that will be applied to the underground line, so that the line will not be damaged, or limit the maximum power applied to the line, to insure compliance with the regulations of the Federal Communications Commission or other regulatory body. 
     These and other embodiments are further discussed below with respect to the following claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of a line locator system with an embodiment of a transmitter according to the present invention directly connected to an underground line. 
         FIG. 2  shows a schematic diagram of a line locator system with an embodiment of a transmitter according to the present invention inductively coupled to an underground line. 
         FIG. 3  shows a schematic of a transmitter according to the present invention with a transmission line circuit to passively maintain a constant current in an underground line connected to the transmitter over a range in the impedance of the underground line. 
         FIG. 4  shows a graph of the electrical current in the underground line in an embodiment of a transmitter such as that shown in  FIG. 3  as a function of the impedance of the line for three frequencies of transmitter excitations. 
         FIG. 5  shows a graph of the power applied to the underground line by a transmitter as shown in  FIG. 3  as a function of the resistance of the line for three operating frequencies of the transmitter. 
         FIG. 6  shows a graph of the electrical voltage applied across an underground line by a transmitter such as that shown in  FIG. 3  as a function of the impedance of the line for three operating frequencies of the transmitter. 
         FIG. 7  shows a block diagram of an embodiment of the present invention wherein the output of the transmitter is monitored in order to control the transmitter to produce a constant output. 
     
    
    
     In the figures, elements having the same identifying designation have the same or similar function. 
     DETAILED DESCRIPTION 
     There is a significant improvement in line location utilizing a transmitter according to the present invention, which results in a constant and known generated magnetic signal from the underground line for a broad range of impedance of that line. Since the current, and not the power or voltage, in the underground line determines the strength of the generated magnetic field, and therefore the success and accuracy with which the line can be located, generating a constant current is an efficient and effective way of energizing an underground line in order to facilitate its location by a receiver of a line locator system. Using more current than is necessary results in reducing the life of the battery that powers the transmitter. Using too little current reduces the performance of the receiver and its ability to accurately locate the underground line. 
     In some embodiments, the voltage applied to the target underground line is also controlled so as not to damage the line or equipment attached to the line. In those embodiments, the current is controlled to remain constant as the impedance of the line changes, but the voltage is also monitored to insure that the voltage applied to the line does not exceed a maximum value. Finally, in various countries throughout the world, there are regulations concerning the maximum power that can be applied to an underground line. Therefore, in addition to monitoring the output current in order to keep that current constant, some embodiments also monitor the power applied to the underground line so that a maximum power limit is not exceeded. Controlling the transmitter as described above and in a manner that gives constant and repeatable performance as the impedance of the target underground line changes provides for efficient use of battery power and insures optimum performance of the locator system over a wide range in impedance of the target underground line. 
     The transmitters of line locators can be connected to the target line directly (conductively) or inductively. When the transmitter is connected inductively, an antenna within the transmitter is driven by the electronics within the transmitter in order to generate a magnetic field that induces the current in the target line. If the current in the inductive antenna is not controlled, there are two difficulties that can arise. First, the current induced in the target line can vary depending on the presence of metallic or conducting objects in the ground near the target line. This variation will result in a variation of the signal in the receiver that may interfere with determining the location of the target line. Second, the current in the antenna may increase, possibly to the point of destroying the transmitter electronics due to excessive heating, especially if the transmitter is placed on or near a metallic object such as a metal plate. In some embodiments of the invention, the current into the antenna of the transmitter is controlled to be constant regardless of the presence of conducting objects located close to the antenna. The transmitter electronics, then, eliminates changes in the current within the target line due to the presence of metallic objects, and prevents the transmitter from being damaged or destroyed if placed on or near a metallic surface. 
       FIG. 1  is a schematic diagram of an embodiment of a line locator system  100 . Line  101 , located under surface  102 , is to be located. Underground line  101  can be any underground current carrying structure, including pipes and wires. Line locator system  100  includes transmitter  110  and receiver  120 .  FIG. 1  is illustrative of an embodiment of a direct connection mode locator; transmitter  110  supplies current  111  directly to line  101  by conductive lead  112 . 
     Current  111  flows through line  101  generating electromagnetic field  121 . An impedance  113  is shown symbolically as an impedance in line  101 , and an electrical ground potential is symbolized by electrical ground  115 . In some embodiments, current  111  is direct current (DC) and electromagnetic field  121  is therefore constant (static). In embodiments where current  111  is time-varying, electromagnetic field  121  will also be time varying (dynamic). Some embodiments of the present invention use a time-varying signal (e.g., radio carrier wave or modulated electronic signal) in order to apply detection techniques available for time varying fields (e.g. band filtering, synchronous detection). 
     Magnetic field  121  is detected by detecting element  122 , which can, for example, be a coil, antenna, or magnetometer, of receiver  120 . Receiver  120  communicates the strength of magnetic field  121  to an operator. Some embodiments of receiver  120  can include multiple detector elements  122 , for example, multiple coils that may have selective orientations and thus determine a position of receiver  120  relative to the electromagnetic field produced by current  111 . Relative position and/or electromagnetic field strength can be communicated by, for example, visual display  124 . Visual display  124  can include, for example, a text screen (e.g., an LCD display), a meter, or an audio signal. Examples of embodiments of receiver are included in U.S. Pat. No. 6,130,539, “Automatic Gain Control for a Line Locator”, issued Oct. 10, 2000, to Steven Polak, assigned to the same assignee as is the present disclosure, herein incorporated by reference in its entirety; and U.S. application Ser. No. 09/136,767, filed Aug. 19, 1998, “Line Locator with Accurate Horizontal Displacement Detection”, by Gopalakrishnan Parakulam and Steven Polak, assigned to the same assignee as is the present application, herein incorporated by reference in its entirety. 
     Transmitter  110  includes a current controlled current source  116 . Current controlled current source  116  includes generator  114  and accompanying circuitry. Current source  116  controls current  111  to be roughly constant over a range of impedance values for line  101 . In some embodiments, source  116  can monitor the value of current  111  and communicate that value to meter  118 , which could be an electronic display, or through interface  119 , an interface to display  124 . Meter  118  may, in some embodiments, monitor the current output of source  116 . 
     In some embodiments the level of current  111  produced by source  116  can be set by control  117 , which may be a factory preset, a knob, or another control interface. In some embodiments, transmitter  110  can be controlled through communications link  119  from receiver  120 . Communications link  119  can be wireless or direct connection (e.g., by electrical wire). 
     In some embodiments of the invention, feedback  130  is provided to current source  116  so that the current can be directly controlled. In some embodiments, a source  116  is a passive source which holds current  111  constant without feedback  130 . In some embodiments, voltage across line  101  or power input to line  101  can be monitored through feedback  130  and current  111  may be clipped to insure that the voltage or power remain between predetermined limits of operation. 
       FIG. 2  shows a schematic of an embodiment of transmission system  100  using inductive coupling to generate current  111 . Generator  114  and other transmitter electronics create signal  210 . Signal  210  is radiated by antenna  218  to line  101 . In some embodiments, current controller  116  controls the output power of signal  210 . In some embodiments, current controller  116  controls current passing through components of transmitter  110 . In some embodiments, signal  210  is a square wave, which can save power and reduce complexity in signal generator  250 . Other embodiments make use of other waveforms. 
     Field  219  generates current  111  in underground line  101  by electromagnetic induction. Current  111  generates electromagnetic field  121  from line  101 . Electromagnetic field  121  is detected by means of detecting element  122  (e.g., an antenna or coil) and receiver  120 , which communicates the information to the operator. 
     The power output of transmitter  110  is equal to the product of the resistance of the line times the square of the current in the line  111 . In some embodiments, of both direct connection and inductive mode transmitters, current is controlled so as to keep total power output by transmitter  110  below a threshold. This threshold, for example, can be a regulatory limit (e.g., a radiated power limit set by the Federal Communications Commission (FCC)). 
       FIG. 3  shows a schematic diagram of an embodiment of a current controlled transmitter  110  coupled to underground line  101 . Transmitter  110  includes current source  116 , which includes generator  114  and transmission line  300 . Generator  114  includes a source  301  and internal impedance  310 . Transmission line  300  receives the output signals from generator  114  and maintains a relatively constant current over a range of impedance of underground line  111 . The impedance of transmission line  300  can be set equal to the geometric mean of the low and high limits to the impedance of underground line  111 . In some embodiments, transmission line  300  functions as a low-pass filter to select the fundamental frequency of a square-wave excitation received from generator  114 , whereby the output of the transmission line is a sinusoidal waveform. 
       FIG. 4  shows a graph of predicted performance of an embodiment of transmitter  110  shown in  FIG. 3 . As seen from  FIG. 4 , transmitter  110  holds a constant current for variation in resistance  113  of line  101  from about 1 to about 1,000 Ohms for transmitter  110  operating at 80, 83 and 86 kHz. In this example of the embodiment shown in  FIG. 3 , capacitor  330  can be a C=0.015 uF, Panasonic series ECQ-E(F), Digikey Part Number EF2153-ND 10, of dimension 3 mm wide×7.5 mm tall×4.4 mm thick. Inductors  320  and  321  can each be an L=220 uH, Toko Type 10RHB2, Digi-key part number TK5168-ND, of size 10.5 mm diameter×15.5 mm tall. Transmitter  110  is coupled to line  101  through terminals  302 . 
       FIG. 5  shows the power in line  101  as a function of impedance  113  for transmitter frequencies of 80, 83 and 86 kHz of the embodiment of transmitter  110  shown in  FIG. 3 .  FIG. 6  shows the line voltage across line  101  as a function of impedance  113  for transmitter frequencies of 80, 83 and 86 kHz of the embodiment of transmitter  110  shown in  FIG. 3 . As shown in  FIG. 6 , the voltage (and therefore the power as shown in  FIG. 5 ) is a linear function of line impedance  113  in the range of impedance between about 1 and about 1000 Ohms. 
     In some embodiments, as shown in  FIGS. 5 and 6 , either power or voltage can be monitored, for example through feedback  130 , in order to maintain the power or voltage within set limits. In some embodiments, source  116  includes circuitry which receives a signal indicating either power or voltage through feedback  130  and, if the set limit of power or voltage is exceeded, reduces current  111  accordingly. In some embodiments, this can be accomplished by reducing the current until the power or voltage is again within the set limits. Therefore, in such embodiments, current  111  is constant until the limit on power or voltage is reached, at which point the current is reduced such that the power or voltage remains below the limit. 
     In some embodiments, current  111  is controlled by controlling the voltage across line  101 . In some embodiments, once the voltage across line  101  reaches an upper limit, the voltage is held constant unless the current or power exceed their upper limits. 
     In some embodiments, transmitter  110  may include monitor  118  ( FIGS. 1 and 2 ) which monitors the output current, the voltage, or the power from source  116 . In some embodiments, monitor  118  may receive information regarding current  111 , voltage or power from other sources. In some embodiments, the signals measured by monitor  118  can be provided to feedback  130  in order to control source  116 . 
       FIG. 7  shows an embodiment of a transmitter  110  according to the present invention with active monitoring and feedback. Transmitter  110  of  FIG. 7  shows both a coupling line  112  for direct connection to line  101  (see, e.g.,  FIG. 1 ) and an antenna  218  for inductively coupling energy to line  101  (see, e.g.,  FIG. 2 ), either of which may be chosen as the output mode. Transmitter  110  further includes a microcontroller  701  which receives feedback signals and, in turn, generates control signals for holding an output signal constant even if the load of line  101  changes. 
     Microcontroller  701  generates a sinusoidal signal of a particular frequency which is input to amplifier  702 . The gain of amplifier  702  is controlled by microcontroller  701 . The output signal from amplifier  702  is input to power amplifier  703  and inverting power amplifier  704 . The output signals from amplifiers  703  and  704  are input to the primary of an impedance matching transformer  703 . The output signal taken from selected taps of the secondary of impedance matching transformer  713  are coupled either into direction connection  112  or into antenna  218  to couple energy into line  101 . The taps of the secondary of transformer  713  are selected through an impedance matching relay  705  which is controlled by microprocessor  701 . 
     The voltage across the selected taps is monitored by a voltmeter  706  and a voltage signal from voltmeter  706  is input to microcontroller  701 . Similarly, the current flowing through the secondary of transformer  713  is monitored by current meter  707  and a current signal from current meter  707  is input to microcontroller  701 . 
     Power can be supplied to transmitter  110  through voltage regulator  108 . Power switch  709  can be utilized to turn on and off transmitter  110 . Further, microcontroller  701  may indicate output level and battery level through output level LEDs  710  and battery level LEDs  711 , respectively. The output level of the output signal to direct coupling  112  or antenna  218  can be controlled with switch  712 , which inputs a control signal to microcontroller  701 . 
     Therefore, microcontroller  701  receives a current signal from current meter  707 , a voltage signal from volt meter  706 , and an output level signal from output level switch  712 . Based on the current signal, the voltage signal, and the output level signal, microcontroller  701  can set the gain of amplifier  702  and select the taps of the secondary of transformer  713 . Microcontroller  701  also provides a signal to amplifier  702  which has the selected frequency of the output signal coupled to line  101 . 
     In some embodiments, microcontroller  701  can start by setting a fixed gain for amplifier  702 , which can be a low gain, and adjusting the impedance matching relay to maximize the power output of the output signal from transformer  713 . Microcontroller  703  then adjusts the gain of amplifier  702  so that a property of the output signal from transformer  713  matches that level selected by the output level signal. In some embodiments, the current is held constant at a value selected by the output level signal. In some embodiments, the power or voltage can be held constant. The output signal can then be held constant regardless of changes in the impedance of line  101 . 
     In some embodiments, the current signal is held constant provided that the voltage or the power remains within a prescribed window. If the voltage or power becomes too high, then the gain of amplifier  702  can be reduced, thereby reducing the current signal and the voltage or of the output signal, such that the voltage or power stays within the prescribed window. 
     Other embodiments of the present invention may make use of many types of current circuits and methods to carry out the current control function. The embodiments bed above are exemplary only and are not intended to be limiting. One skilled in the art cognize various possible modifications that are intended to be within the spirit and scope disclosure. As such, the invention is limited only by the following claims.