Patent Publication Number: US-10768330-B2

Title: System and method for locating a marker using a locator with multiple transmitters

Description:
RELATED APPLICATION 
     The present application claims the benefit of U.S. Provisional Patent Application No. 62/195,736 filed Jul. 22, 2015, the entire contents of each being incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to locating a marker and more particularly to locating a marker using a locator with multiple transmitters. 
     BACKGROUND 
     Markers are used to locate objects of interest that are buried underground (e.g., utility pipes and/or cables) or otherwise disposed in hard-to-reach locations. In general, markers are small, passive devices that are affixed in, on, and/or near the hard-to-reach objects during installation. After installation, markers allow the position of the object to be determined non-invasively (e.g., without digging into the ground). For instance, a resonant marker may be detectable by electromagnetic means at depths of up to 3 meters underground. 
     A locator is a portable, moveable instrument used to detect the location of the markers. A locator is equipped to transmit and receive signals (e.g., electromagnetic signals) to and from the markers. Based on these signals, the locator derives information about the position of the marker relative to the locator. The ability of a locator to accurately locate markers and/or the objects of interest that they are affixed to (e.g., utility lines) is highly desirable in many applications. For example, inadvertently digging into an electrical and/or gas line may result in injury, fires, toxic emissions, damage to the digging equipment, damage to the utility line, and/or the like. Furthermore, the ability to accurately locate markers provides greater knowledge about the layout of a site. For example, detailed knowledge about the location of underground lines at a given site may be helpful when planning construction and/or repair projects at the site. 
     Accordingly, it would be desirable to provide systems and methods for improved marker locators. 
     SUMMARY 
     A marker locator may include a first transmitter that generates a first activation signal, second transmitter that generates a second activation signal, a receiver that detects first and second response signals, and a processor that determines a depth of a marker based on the first and second response signals. The first transmitter is located at a first position, and the second transmitter is located at a second position apart from the first position. The first and second response signals respectively correspond to the first and second activation signals. the processor is coupled to the receiver. 
     A transceiver for locating a buried marker may include a plurality of transmitters that transmit a plurality of activation signals to the buried marker, and a receiver that detects a plurality of response signals corresponding to each of the activation signals from the buried marker. The plurality of transmitters are located in different positions. 
     A method for estimating a depth of a marker may include measuring an intensity of a first round-trip signal transmitted from a locator to the marker and retransmitted from the marker to the locator, measuring an intensity of a second round-trip signal transmitted from the locator to the marker and retransmitted from the marker to the locator, and estimating the depth of the marker by comparing the intensities of the first and second round-trip signals. The first round-trip is being transmitted by a first transmitter located at a first position, and the second round-trip signal being transmitted by a second transmitter located at a second position and having a different path length than the first round-trip signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified diagram of a marker locator system according to some embodiments. 
         FIG. 2  is a simplified diagram of a marker locator with a plurality transmitters according to some embodiments. 
         FIG. 3  is a simplified diagram of a receiver of a marker locator according to some embodiments. 
         FIG. 4  is a simplified diagram of a method for estimating the depth of a marker according to some embodiments. 
         FIG. 5  is a simplified diagram of a plurality of inductive loops according to some embodiments. 
         FIG. 6  is a simplified diagram of a computer system in which embodiments of the present disclosure may be implemented. 
     
    
    
     In the figures, elements having the same designations have the same or similar functions. 
     DETAILED DESCRIPTION 
     In the following description, specific details are set forth describing some embodiments consistent with the present disclosure. It will be apparent to one skilled in the art, however, that some embodiments may be practiced without some or all of these specific details. The specific embodiments disclosed herein are meant to be illustrative but not limiting. One skilled in the art may realize other elements that, although not specifically described here, are within the scope and the spirit of this disclosure. In addition, to avoid unnecessary repetition, one or more features shown and described in association with one embodiment may be incorporated into other embodiments unless specifically described otherwise or if the one or more features would make an embodiment non-functional. 
       FIG. 1  is a simplified diagram of a marker locator system  100  according to some embodiments. A marker  110  is affixed to an underground line  120  that is buried at a depth ‘d’. A locator  130  is used to estimate the depth ‘d’ using electromagnetic signaling. 
     A transceiver  140  of locator  130  includes a plurality of transmitters  141  and  142  that are spatially separated. The plurality of transmitters  142  and  144  transmit a plurality of activation signals  152  and  154  to marker  110 . Transceiver  140  further includes a receiver  143  that receives a plurality of response signals  162  and  164  from marker  110 . Activation signal  152  and response signal  162  form a round-trip signal  182 , and activation signal  154  and response signal  164  for a roundtrip signal  184 . 
     Because the plurality of transmitters  141  and  142  are spatially separated, the path lengths of round trip signals  182  and  184  are different. Locator  130  estimates the depth ‘d’ by measuring and comparing the intensity of round-trip signals  182  and  184 . Although it may be feasible to alter the path lengths of round trip signals  182  and  184  using a single transmitter by physically raising and lowering locator  130  or otherwise moving locator  130 , such an approach may be burdensome to the operator and/or may not consistently achieve high accuracy depth estimates. Accordingly, including a plurality of transmitters  141  and  142  in marker locator system  100  simplifies the process of accurately determining the depth of marker  110 . 
     Although  FIG. 1  depicts two transmitters  141  and  142  for simplicity, it is to be understood that transceiver  140  may include more than two transmitters in different locations. For example, transceiver  140  may include a transmitter array that includes a plurality of transmitters spaced at fixed intervals in one or more dimensions. Correspondingly, marker locator system  100  may estimate the depth of marker  110  using more than two activation signals, response signals, and/or round-trip signals. In some embodiments, using more than two transmitters may improve the accuracy and/or reliability of the depth estimates. 
     In general, marker  110  is a passive device. For example, marker  110  may be a resonant ball marker. In order to generate response signals  162  and  164 , marker  110  absorbs and retransmits energy from activation signals  152  and  154 . According to some embodiments, marker  110  may include an antenna and a resonant circuit (e.g., an LC tank circuit) that couples to electromagnetic signals at a resonant frequency. Consistent with such embodiments, activation signals  152  and/or  154  may each include bursts of electromagnetic radiation at the resonant frequency. Energy from each burst is received by marker  110  and stored in the resonant circuit. At the end of the burst, marker  110  releases the energy stored in the resonant circuit by transmitting an exponentially decaying signal corresponding to response signals  162  and  164 . In some examples, the radius of marker  110  and/or an antenna of marker  110  is 10 cm or less (e.g., 6 cm). 
     In some examples, activation signals  152  and  154  and response signals  162  and  164  may be low frequency electromagnetic signals that match the resonant frequency of marker  110 . For example, the resonant frequency of marker  110  may be between 30 and 250 kHz. Consistent with such examples, activation signals  152  and  154  and/or response signals  162  and  164  may be transmitted and/or received using magnetic induction loops, such as ferrite-core induction loops, air-core induction loops, and/or the like. 
     In some examples, activation signals  152  and  154  may include periodic burst sequences characterized by a duty cycle and a burst frequency. For example, the duty cycle may be 10-15% and the burst frequency may be a few kHz or less (substantially less than the resonant frequency of marker  110 ). Each burst in the periodic burst sequences may include a predetermined number of cycles (e.g., 25 cycles) at the resonant frequency of marker  110 . 
     Response signals  162  and  164  include a sequence of exponentially decaying waveforms trailing each burst in the burst sequence of activation signals  152  and  154 . Intensity measurements are obtained by integrating response signals  162  and  164  over one or more burst cycles. The exponentially decaying waveforms may be characterized by a frequency, phase, and lifetime. To improve the signal to noise ratio of each of the intensity measurements, locator  130  may correlate and/or filter response signal  160  based on the frequency, phase, and/or lifetime. 
     Based on the intensity measurements, locator  130  estimates the depth ‘d’ of marker  110 . According to some embodiments, locator  130  may provide feedback to an operator regarding marker  110  by any suitable mechanism, such as audio, visual, and/or haptic feedback. For example, locator  130  may indicate the estimated depth ‘d’ of marker  110  to the user through a display interface. The display interface may convey depth information in various formats including text, numbers, colors, bar graphs, arrows, meters, needles, maps and/or the like. In some examples, the depth information may be stored in memory and/or transmitted over a network for further processing and/or record-keeping. 
     To ensure that each of round-trip signals has a different path length, activation signals  152  and  154  may be transmitted from two or more positions separated along an elongate axis  170  of locator  130 . 
       FIG. 2  is a simplified diagram of a marker locator  200  with a plurality of transmitters according to some embodiments. According to some embodiments consistent with  FIG. 1 , marker locator  200  may be used to implement locator  130  and/or transceiver  140  of marker locator system  100 . Marker locator  200  includes a plurality (e.g., a pair) of transmitters  210  and  220  and a receiver  230 . Each of transmitters  210  and  220  and receiver  230  are coupled to a processor  240 . 
     Transmitters  210  and  220  are located at first and second positions, respectively, separated by a distance ‘a’ along an elongate axis of marker locator  200 . According to some embodiments, the distance ‘a’ may be fixed. For example, transmitter  210  and  220  may be coupled by a rigid member  250  and/or disposed within a same enclosure/housing. In some examples, the distance ‘a’ may be approximately 20 cm. However, in some embodiments, the distance ‘a’ may be variable and/or adjustable. Receiver  230  may be located between transmitters  210  and  220  and/or may be collinear with transmitters  210  and  220  (i.e., positioned along the elongate axis of marker locator  200 ). For example, receiver  230  may be located at a fixed or adjustable position near the lower transmitter  210  to be reduce the distance between receiver  230  and the ground. 
     According to some embodiments, transmitters  210  and  220  may include magnetic induction loops, such as ferrite-core induction loops, air-core induction loops, and/or the like. According to some embodiments, receiver  230  may include magnetic induction loops and/or other types of sensors, such as magnetometers, Hall effect sensors, magnetoresistive devices, and/or the like. Transmitters  210  and  220  and/or receiver  230  may further include signal processing circuitry to amplify, filter, convert (e.g., perform analog to digital or digital to analog conversion), and/or perform other signal processing operations. A particular embodiment of receiver  230  is discussed in greater detail below with reference to  FIG. 3 . To the extent that transmitters  210  and  220  and/or receiver  230  are not isotropic (i.e., they do not radiate or detect equally in all directions), they may generally be oriented along the elongate axis of marker locator  200 . 
     To determine the depth of a buried marker, such as marker  110 , transmitters  210  and  220  each generate activation signals, such as activation signals  152  and  154 . A first activation signal is emitted by transmitter  210 , and a second activation signal is emitted by transmitter  220 . At least a portion of the energy in the activation signals is captured and retransmitted by the buried marker to form response signals, such as response signals  162  and  164 . The response signals are detected by receiver  230 . Pairs of activation and response signals form round-trip signals, such as round-trip signals  182  and  184 . 
     Processor  240  controls which of transmitters  210  and  220  is active at a given point in time to distinguish between the round-trip signals. According to some embodiments, processor  240  may implement a time-division multiplexing scheme to separate the round-trip signals. For example, processor  240  may instruct transmitter  210  to transmit several dozen (e.g., 100) burst cycles while transmitter  220  is inactive. Subsequently, processor  240  may instruct transmitter  220  to transmit several dozen (e.g., 100) burst cycles while transmitter  210  is inactive, and so on. Processor  240  may control other aspects of the activation signals, including their amplitude, frequency, phase, duty cycle, burst start and end times, and/or the like. Processor  240  may send one or more synchronization signals to receiver  230 , such as a synchronization signal indicating the beginning or end of a burst, to facilitate detection of the response signals. 
     Processor  240  receives the first and second response signals from receiver  230 . Processor  240  may perform one or more signal processing operations on the first and second response signals, such as integration, averaging and/or analog to digital conversion. Processor  240  determines the intensity of each of the round-trip signals by measuring the magnitude of the first and second response signals. Advantageously, the intensity of each of the round-trip signals is measured without moving marker locator  200  and/or components within marker locator  200 . 
     Because transmitters  210  and  220  are separated by the distance ‘a’, the path length of the first round-trip signal is shorter than the path length of the second round-trip signal by the distance ‘a’. Assuming marker locator  200  is positioned relatively far from the buried marker such that the far-field approximation is valid, processor  240  may estimate the depth of the ball marker using the equation: 
                   Z   =       a   +         2   ·   C   ·     R   2       -     R   2     +     C   ·     a   2       -       C   2     ·     R   2               C   -   1               (     Eq   .           ⁢   1     )               
where Z represents the depth estimate, R represents the radius of transmitters  210  and  220  (assuming transmitters  210  and  220  are configured as inductive loops), and C is a value representing K 2/3 , where K is the ratio between the first and second intensity measurements. A derivation of Eq. 1 is provided below with reference to  FIG. 5 . Although Eq. 1 assumes that the transmitters  210  and  220  are inductive loops with the same radius R, it is to be understood that Eq. 1 may be modified in embodiments where these assumptions are relaxed. For example, Eq. 1 may be modified using one or more calibration and/or correction factors.
 
       FIG. 3  is a simplified diagram of a receiver  300  of a marker locator according to some embodiments. According to some embodiments consistent with  FIGS. 1-2 , receiver  300  may be used to implement receiver  230  and/or processor  240  of marker locator  200 . Receiver  300  is used to reduce or eliminate noise when measuring the intensity of one or more round-trip signals to determine the depth of a buried marker. 
     An antenna  310  detects electromagnetic signals, such as response signals  162  and  164 . Antenna  310  may include any suitable device for converting electromagnetic signals into electronic signals. For example, antenna  310  may include an inductive loop, a magnetometer, and/or the like. An amplifier  320  increases the signal level of the detected signal. In some examples, amplifier  320  may include a low noise amplifier (LNA) to maintain low noise levels during signal amplification. An analog to digital converter (ADC)  330  digitizes the analog electronic signal output by amplifier  320 . In general, ADC  330  provides sufficient amplitude resolution to accurately represent the signal level and sufficient time resolution (e.g., sampling frequency) to capture the time-dependent features of the detected electromagnetic signals. For example, ADC  330  may operate at 16-bit resolution at a frequency of 1 MHz. 
     A phase-sensitive detector stage  340  includes an in-phase mixer  341 , a quadrature mixer  342 , an in-phase matched filter  343 , a quadrature matched filter  344 , an in-phase integrator  345 , and a quadrature integrator  346 . In-phase mixer  341  and quadrature mixer  342  are matched to the resonant frequency of the buried marker and may also be matched to the exponential decay lifetime of the buried marker. Similarly, in-phase matched filter  343  and/or quadrature matched filter  344  may be matched to the exponential decay lifetime of the buried marker. In-phase integrator  345  and quadrature integrator  346  perform integration and/or averaging to generate intensity measurements corresponding to the in-phase and quadrature components of the detected signal, respectively. A converter  350  may convert the in-phase and quadrature representation of the intensity measurements to magnitude and phase representation, where the magnitude is given by √{square root over (I 2 +Q 2 )} and the phase is given by 
               tan     -   1       ⁡     (     Q   I     )           
where I and Q represent the in-phase and quadrature intensities, respectively.
 
       FIG. 4  is a simplified diagram of a method  400  for estimating the depth of a marker according to some embodiments. According to some embodiments consistent with  FIGS. 1-3 , method  400  may be performed by a processor, such as processor  240 , to estimate the depth of a marker, such as marker  110 . 
     At a process  410 , a first round-trip signal intensity is measured. The first round-trip signal intensity corresponds to the strength of a signal that is transmitted by a locator, such as locator  130 , to the marker and is retransmitted by the marker back to the locator. The first round-trip signal may be decomposed into an activation signal from the locator to the marker, such as activation signal  152 , and a response signal from the marker to the locator, such as response signal  162 . According to some embodiments, the first round-trip signal intensity may correspond to an averaged (and/or integrated) intensity of the response signal. 
     At a process  420 , a second round-trip signal intensity is measured, the second round-trip signal having a different path length than the first round-trip signal. Like the first round-trip signal intensity, the second round-trip signal intensity corresponds to the strength of a signal that is transmitted by the locator to the marker and is retransmitted by the marker back to the locator. The second round-trip signal may be decomposed into an activation signal from the locator to the marker, such as activation signal  154 , and a response signal from the marker to the locator, such as response signal  164 . According to some embodiments, the second round-trip signal intensity may correspond to an averaged (and/or integrated) intensity of the response signal. 
     The first and second round-trip signals of processes  410  and  420 , respectively, are transmitted by different transmitters located at different positions. Accordingly, the path length of the second round-trip signal is different than the path length of the first round-trip signal. Thus, the first and second round-trip signal intensities may be measured without moving the locator and/or components within the locator between measurements. In order to distinguish between the first and second round-trip signals, the first and second round-trip signals are multiplexed. For example, the first and second round-trip signals may be multiplexed using techniques that include time-division multiplexing, frequency-division multiplexing, code-division multiplexing, space-division multiplexing, and/or the like. 
     At a process  430 , the depth of the marker is estimated. The depth of the marker is estimated by comparing the first and second round-trip signal intensities. For example, the depth of the marker may be estimated based on a ratio of the first and second round-trip signal intensities. According to some embodiments, the depth of the marker may be estimated by solving Eq. 1. Upon completion of process  430 , method  400  may proceed back to process  410  to continuously and/or periodically, update the depth estimate. According to some embodiments, the a plurality of depth estimates may be obtained over time and filtered and/or averaged to refine the depth estimate. 
       FIG. 5  is a simplified diagram of a plurality of inductive loops  500 , including inductive loops  510  and  520 , according to some embodiments. According to some embodiments consistent with  FIGS. 1-4 , inductive loops  510  and  520  may be used to implement transmitter  210  and/or transmitter  220  of marker locator  200 . Inductive loops  510  and  520  are oriented along a z-axis  530  (e.g., a depth axis). Inductive loops  510  and  520  are separated by a distance ‘a’ along the z-axis. 
     The Biot-Savart Law provides that the magnetic field strength at a point of interest  540  generated by a current flowing through inductive loop  510  is given by the equation: 
                     B   1     =         μ   0       4   ⁢   π       *       2   ⁢   π   ⁢           ⁢     R   2     ⁢     I   ·   N           (       Z   2     +     R   2       )       3   /   2                   (     Eq   .           ⁢   2     )               
Where Z is the distance between the point of interest and inductive loop  510 , B 1  is the magnetic field strength at the position Z, μ 0  is the permeability constant, R is the radius of inductive loop  510 , N is the number of turns in inductive loop  510 , and I is the current flowing through inductive loop  510 .
 
     Similarly, the Biot-Savart Law provides that the magnetic field strength at depth ‘Z’ generated by a current flowing through inductive loop  520  is given by the equation: 
                     B   2     =         μ   0       4   ⁢   π       *       2   ⁢   π   ⁢           ⁢     R   2     ⁢     I   ·   N           (         (     Z   +   a     )     2     +     R   2       )       3   /   2                   (     Eq   .           ⁢   3     )               
Where B 2  is the magnetic field strength at a depth Z, μ 0  is the permeability constant, R is the radius of inductive loop  510 , N is the number of turns in inductive loop  510 , and I is the current flowing through inductive loop  510 . It is observed that when R is small compared to Z, the magnetic field strength scales approximately according to 1/Z 3  (the far-field approximation).
 
     The ratio of Eqs. 2 and 3 is given by the equation: 
     
       
         
           
             
               
                 
                   K 
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                               ( 
                               
                                 Z 
                                 + 
                                 a 
                               
                               ) 
                             
                             2 
                           
                           + 
                           
                             R 
                             2 
                           
                         
                         
                           
                             Z 
                             2 
                           
                           + 
                           
                             R 
                             2 
                           
                         
                       
                       ] 
                     
                     
                       3 
                       / 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ) 
                 
               
             
           
         
       
     
     Applying the substitution C=K 2 /3 to Eq. 4 and solving for Z results in Eq. 1, as described previously with respect to  FIG. 2 . 
       FIG. 6  is a simplified diagram of a computer system  600  in which embodiments of the present disclosure may be implemented. Computer system  600  may be adapted for estimating the depth of a buried marker. For example, the steps of the operations of method  400  of  FIG. 4  may be implemented using system  600 . System  600  can be a computer, phone, personal digital assistant (PDA), or any other type of electronic device. Such an electronic device includes various types of computer readable media and interfaces for various other types of computer readable media. As shown in  FIG. 6 , system  600  includes a permanent storage device  602 , a system memory  604 , an output device interface  606 , a system communications bus  608 , a read-only memory (ROM)  610 , processing unit(s)  612 , an input device interface  614 , and a network interface  616 . 
     Bus  608  collectively represents all system, peripheral, and chipset buses that communicatively connect the numerous internal devices of system  600 . For instance, bus  608  communicatively connects processing unit(s)  612  with ROM  610 , system memory  604 , and permanent storage device  602 . 
     From these various memory units, processing unit(s)  612  retrieves instructions to execute and data to process in order to execute the processes of the subject disclosure. The processing unit(s) can be a single processor or a multi-core processor in different implementations. 
     ROM  610  stores static data and instructions that are needed by processing unit(s)  612  and other modules of system  600 . Permanent storage device  602 , on the other hand, is a read-and-write memory device. This device is a non-volatile memory unit that stores instructions and data even when system  600  is off. Some implementations of the subject disclosure use a mass-storage device (such as a magnetic or optical disk and its corresponding disk drive) as permanent storage device  602 . 
     Other implementations use a removable storage device (such as a floppy disk, flash drive, and its corresponding disk drive) as permanent storage device  602 . Like permanent storage device  602 , system memory  604  is a read-and-write memory device. However, unlike storage device  602 , system memory  604  is a volatile read-and-write memory, such as random access memory. System memory  604  stores some of the instructions and data that the processor needs at runtime. In some implementations, the processes of the subject disclosure are stored in system memory  604 , permanent storage device  602 , and/or ROM  610 . For example, the various memory units include instructions for estimating the depth of a marker in accordance with some implementations. From these various memory units, processing unit(s)  612  retrieves instructions to execute and data to process in order to execute the processes of some implementations. 
     Bus  608  also connects to input and output device interfaces  614  and  606 . Input device interface  614  enables the user to communicate information and select commands to system  600 . Input devices used with input device interface  814  include, for example, alphanumeric, QWERTY, or T9 keyboards, microphones, and pointing devices (also called “cursor control devices”). Output device interfaces  606  enables, for example, the display of images generated by system  600 . Output devices used with output device interface  606  include, for example, printers and display devices, such as cathode ray tubes (CRT) or liquid crystal displays (LCD). Some implementations include devices such as a touchscreen that functions as both input and output devices. It should be appreciated that embodiments of the present disclosure may be implemented using a computer including any of various types of input and output devices for enabling interaction with a user. Such interaction may include feedback to or from the user in different forms of sensory feedback including, but not limited to, visual feedback, auditory feedback, or tactile feedback. Further, input from the user can be received in any form including, but not limited to, acoustic, speech, or tactile input. Additionally, interaction with the user may include transmitting and receiving different types of information, e.g., in the form of documents, to and from the user via the above-described interfaces. 
     Also, as shown in  FIG. 6 , bus  608  also couples system  600  to a public or private network (not shown) or combination of networks through a network interface  616 . Such a network may include, for example, a local area network (LAN), such as an Intranet, a wireless network, and/or a wide area network (WAN), such as the Internet. Any or all components of system  600  can be used in conjunction with the subject disclosure. 
     These functions described above can be implemented in digital electronic circuitry, in computer software, firmware or hardware. The techniques can be implemented using one or more computer program products. Programmable processors and computers can be included in or packaged as mobile devices. The processes and logic flows can be performed by one or more programmable processors and by one or more programmable logic circuitry. General and special purpose computing devices and storage devices can be interconnected through communication networks. 
     Some implementations include electronic components, such as microprocessors, storage and memory that store computer program instructions in a machine-readable or computer-readable medium (alternatively referred to as computer-readable storage media, machine-readable media, or machine-readable storage media). Some examples of such computer-readable media include RAM, ROM, read-only compact discs (CD-ROM), recordable compact discs (CD-R), rewritable compact discs (CD-RW), read-only digital versatile discs (e.g., DVD-ROM, dual-layer DVD-ROM), a variety of recordable/rewritable DVDs (e.g., DVD-RAM, DVD-RW, DVD+RW, etc.), flash memory (e.g., SD cards, mini-SD cards, micro-SD cards, etc.), magnetic and/or solid state hard drives, read-only and recordable Blu-Ray® discs, ultra density optical discs, any other optical or magnetic media, and floppy disks. The computer-readable media can store a computer program that is executable by at least one processing unit and includes sets of instructions for performing various operations. Examples of computer programs or computer code include machine code, such as is produced by a compiler, and files including higher-level code that are executed by a computer, an electronic component, or a microprocessor using an interpreter. 
     While the above discussion primarily refers to microprocessor or multi-core processors that execute software, some implementations are performed by one or more integrated circuits, such as application specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs). In some implementations, such integrated circuits execute instructions that are stored on the circuit itself. Accordingly, the steps of the operations of method  400  of  FIG. 4 , as described above, may be implemented using system  600  or any computer system having processing circuitry or a computer program product including instructions stored therein, which, when executed by at least one processor, causes the processor to perform functions relating to these methods. 
     As used in this specification and any claims of this application, the terms “computer”, “server”, “processor”, and “memory” all refer to electronic or other technological devices. These terms exclude people or groups of people. As used herein, the terms “computer readable medium” and “computer readable media” refer generally to tangible, physical, and non-transitory electronic storage mediums that store information in a form that is readable by a computer. 
     Embodiments of the subject matter described in this specification can be implemented in a computing system that includes a back end component, e.g., a data server, or that includes a middleware component, e.g., an application server, or that includes a front end component, e.g., a client computer having a graphical user interface or a Web browser through which a user can interact with an implementation of the subject matter described in this specification, or any combination of one or more such back end, middleware, or front end components. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (LAN), a wireless network, and a wide area network (WAN), an inter-network (e.g., the Internet), and peer-to-peer networks (e.g., ad hoc peer-to-peer networks). 
     The computing system can include clients and servers. A client and server are generally remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other. In some embodiments, a server transmits data (e.g., a web page) to a client device (e.g., for purposes of displaying data to and receiving user input from a user interacting with the client device). Data generated at the client device (e.g., a result of the user interaction) can be received from the client device at the server. 
     It is understood that any specific order or hierarchy of steps in the processes disclosed is an illustration of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the processes may be rearranged, or that all illustrated steps be performed. Some of the steps may be performed simultaneously. For example, in certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     Furthermore, the exemplary methodologies described herein may be implemented by a system including processing circuitry or a computer program product including instructions which, when executed by at least one processor, causes the processor to perform any of the methodology described herein.