The present invention relates generally to the field of receiver antennas used in line locating, specifically the tuning of receiver antennas.
The present invention is directed to an automatically tunable locator apparatus for locating an object. The apparatus comprises a means for generating a stable magnetic field, an antenna, a variable reactance means, and a processor. The antenna detects a parameter of the magnetic field at a frequency range. The variable reactance means changes the center frequency of the frequency range in response to a control signal. The processor receives the strength of the magnetic field and adjusts the control signal to maximize the strength of the magnetic field.
In another embodiment, the invention is directed to a method for tuning an antenna assembly. The method comprises providing a stable magnetic field at a desired frequency, detecting a first strength of the magnetic field at the antenna assembly, recording the first strength, adjusting a variable reactance means to manipulate a center frequency of the antenna assembly, detecting a second strength of the magnetic field of the antenna assembly, and determining which of the two magnetic field strengths is greatest.
In another embodiment, the invention is directed to a receiver tuning system. The system comprises an antenna assembly, a measurement system, an antenna tuning circuit, a variable reactance means, and a magnetic field source. The measurement system is operatively connected to the antenna assembly and comprises a processor. The antenna tuning circuit changes a center frequency of the antenna assembly. The variable reactance means is operably connected to the antenna and the processor such that the processor can vary the center frequency of the antenna tuning circuit. The magnetic field source is positioned such that the antenna assembly is located in the magnetic field.
A basic cable locating system consists of a transmitter and a receiver, or locator. The transmitter couples an AC current of a predetermined frequency onto the target cable. The locator responds to the magnetic field generated by the current on the cable and gives feedback to the operator allowing him to locate the cable.
The locator typically detects the magnetic field using a solenoid style magnetic loop antenna which is part of an antenna circuit. The antenna circuit includes at least an antenna and a tuning capacitor, and usually a resistor, arranged to create a parallel resonant circuit. This circuit has a resonant frequency that is a function of the antenna inductance and capacitance and a bandwidth which is a function of the inductance, capacitance, and resistance. The quality factor, Q, of the circuit is calculated by dividing the resonant frequency by the bandwidth and is a measure of how sharply tuned the circuit is. Note that in some designs the tuning capacitor is a parasitic capacitance created by the close proximity and large mutual area of the antenna windings.
Prior to this invention, receiver tuning has been done manually. The typical variable reactance elements used have been variable capacitors or inductors that are adjustable via mechanical means. This means that an operator must physically access the tuning element and turn a variable capacitor or inductor by hand to make an adjustment. Not only is this a cumbersome operation, it has the following complications:
The magnitude of the voltage across the receiver antenna can be reasonably represented by
V∝ω·Q·N·A·μ
e
·H
Where V is the voltage across the receiver antenna, ω is the angular frequency in radians, Q is the quality factor of the circuit, N is the number of turns of wire on the antenna, A is the cross sectional area of the antenna, μe is the effective permeability of the core, and H is the intensity of the magnetic field. Since ω is equal to 2*pi*frequency and the receiver is operating at a single frequency at any given time, we remove ω. We then remove H and assume a constant magnetic field intensity. This leaves
V∝Q·N·A·μ
e
To increase the performance of a receiver antenna circuit, one of these four variables must be manipulated. Increasing the turns count, N, will increase signal proportional to the number of turns, but will increase noise by the same amount. Similar performance gains are attainable by increasing antenna cross-sectional area and effective permeability. Increasing Q increases signal proportional to the Q increase, but unlike the other variables increases the signal to noise ratio by reducing receiver bandwidth and attenuating out of band frequencies.
In a conventional multi-frequency receiver it is standard practice to have a low Q or reasonably flat tune. This gives the receiver the wide bandwidth it needs to function over a large range of frequencies. It trades performance at each individual frequency for cost effectiveness and simplicity.
In a high performance cable locator it is desirable to use a high Q tuned antenna to increase signal output at the frequency of interest and better reject nearby frequencies. Such an antenna must be tuned such that the peak frequency response is reasonably close to the frequency being detected to gain these benefits.
With reference now to the figures in general, and
The antenna assembly 12 may comprise a single coil, a ferrite rod, a tri-axial antenna assembly or a combination of the above. The antenna assembly 12, once tuned with the tuning system 10 of the present invention, is used with a locator 11 to detect a location of an underground object, such as a cable or utility line (not shown).
With reference now to
The processor 24 is capable of adjusting the variable reactance means 20. The variable reactance means 20 comprises a potentiometer 32 and a varactor diode 34. The varactor diode 34 may include a capacitor 36. A control signal, such as a voltage across the potentiometer 32 is adjusted by the processor 24 to increase or decrease the capacitance of the varactor diode 34. As the capacitance is increased, the variable reactance means 20 adjusts the target frequency detected by the antenna 12. The processor 24 iteratively adjusts the potentiometer 32 to automatically identify the highest measured signal and record the corresponding potentiometer 32 setting in its memory.
Tuning the antenna 12 is accomplished by adjusting the inductive and/or capacitive portion of the antenna tuning circuit 14 so that, at the target frequency of interest, the capacitive reactance and the inductive reactances of the parallel resonance circuit 18 are of equal magnitude and opposite polarity. This maximizes signal magnitude at the center frequency, or frequency of interest, amplifying it by a factor of Q. Tuning also improves performance by reducing the bandwidth of the receiver which reduces the magnitudes of nearby interfering frequencies with respect to the magnitude of the target frequency, improving the signal to noise ratio.
Therefore, it may be desired that before the tuning process begins, the antenna 12 may be placed in a magnetic field of the proper frequency. Therefore, the antenna is placed within the means for generating a stable magnetic field 16 (
The magnetic field source 16 may comprise a Helmholz coil due to the uniformity of the magnetic field near the center of such a source. The coil will need to be sized appropriately for the particular antenna assembly 12. The magnetic field source 16 is used as a frequency reference in the tuning methods described herein. The magnetic field source 16 may also be used as an amplitude, phase, or directional reference.
With reference now to
With reference to
Referring now to
The left-right balance can be adjusted through use of the balancing variable reactance means 52. The balancing variable reactance means 52 comprises a balancing potentiometer 58 and a balancing varactor diode 60. The processor 24 may adjust the control signal, such as a voltage, on the potentiometer 58 to adjust the capacitance on the balancing diode 60 to achieve a target phase relationship between the left antenna 42 and right antenna 40. The processor may use an iterative process as disclosed above to determine the proper phase relationship.
While voltage is specifically mentioned as an exemplary control signal, one skilled in the art can envision that alternative control signals can adjust the tuning variable reactance means 50 and the balancing variable reactance means 52. These alternative control signals may comprise a current, a mechanical adjustment, or other signals.
One skilled in the art will appreciate that the antenna assembly 12 of the locator 11 is located within a housing (not shown) that may cause some interference with the signal. Use of the various variable reactance means 50, 52 allows the antenna tuning system 10 to compensate for misalignment of the antenna assembly 12 relative to the housing.
Automating the tuning process has several advantages over manual tuning. It is more accurate, more repeatable, faster, and reduces human error. Using the processor 24 to perform this task is far more repeatable and less error prone. Automatic tuning also does not require a person to be in close proximity to the sensitive antenna assemblies 12 during tuning which, particularly at higher frequencies, causes distortion of the magnetic field and may result in inaccuracy. The left-right balance is particularly sensitive to this distortion. Automatic tuning can also now be performed indoors; conditions can be tightly controlled which results in more predictable performance in the field.
Automatic tuning could be used with any variable reactance means 20, 50, 52 used as the tuning element. The methods may vary slightly to accommodate different variable reactance devices. The variable reactance tuning element(s) may be solid state for increased reliability and repeatability, resistance to vibration. Automatic tuning could also be used to tune an antenna assembly 12 to a point other than the maximum or minimum signal strength. Variable reactance could be adjusted to match a target signal level (matching two antennas) or phase.
With reference now to
Alternatively, an operator could place the locator 11 in a fixture and use an interface to remotely tune the antenna assembly 12. This would place the operator far enough from the antennas to minimize magnetic field distortion while making adjustments. Remote tuning retains many of the advantages over manually tuning the antenna assembly 12, particularly if paired with an appropriate magnetic field source 16.
Although the present invention has been described with respect to the preferred embodiment, various changes and modifications may be suggested to one skilled in the art, and it is intended that the present invention encompass such changes and modifications as fall within the scope of this disclosure.
This application claims the benefit of provisional patent application Ser. No. 61/559,205 filed on Nov. 14, 2011, the entire contents of which are incorporated herein by reference.
Number | Date | Country | |
---|---|---|---|
61559205 | Nov 2011 | US |