Abstract:
There is provided a vehicle with the capability of estimating its position. An exemplary vehicle comprises a positioning unit configured to operate in a first mode for communicating with one or more first radio-responsive devices and operate in a second mode for communicating with one or more second radio responsive devices. The positioning unit is configured to estimate a coarse position of the vehicle based on data received from the first transceiver, and estimate a fine position of the vehicle based on data received from the second transceiver.

Description:
BACKGROUND 
     1. Field of Invention 
     Exemplary embodiments of the invention relate generally to a system and method for determining a position of a mobile unit. For example, embodiments relate to techniques for estimating a position of a railway vehicle in relation to a point of interest. 
     2. Discussion of Art 
     Conventional positioning of a moving vehicle may be based, for example, on speed and position measurements, Doppler effect, the Global Positioning System (GPS), or some combination of these. One common approach to estimating a vehicle position uses the Global Positioning System (GPS). However, GPS requires a line of sight between the receiver and the global navigation satellites. Furthermore, the accuracy provided by GPS for civilian use is limited. Another common system for determining vehicle position, referred to an inertial navigation system (INS), makes use of a gyroscope and accelerometer. One advantage of an INS is that there is no need for line of sight to an external device. However, inertial systems have unbounded position error that increases slowly with time, which means decreasing accuracy. 
     Other common systems for determining vehicle position include the use of speed and position measurements taken using a tachometer. Position estimates can also be achieved by transmitting measurement signals using transponders or track circuits. However, such techniques present various obstacles to obtaining accurate position estimates for a high-speed vehicle. For example, track circuit methods can only achieve accuracy equal to a length of a track block. In tachometer-based systems, position errors occur caused by slipping of the vehicle wheals. Position measurement errors may be corrected by using GPS signal. However, in many applications, including railway applications, line of sight may not always be available. 
     BRIEF DESCRIPTION 
     Briefly, in accordance with an example embodiment of the invention, there is provided a vehicle with a positioning unit configured to operate in a first mode for communicating with one or more first radio-responsive devices and operate in a second mode for communicating with one or more second radio responsive devices. The positioning unit is configured to estimate a coarse position of the vehicle based on data received while operating in the first mode, and estimate a fine position of the vehicle based on data received while operating in the second mode. 
     In accordance with another example embodiment of the invention, there is provided a system for estimating position that includes a first set of stationary radio-responsive devices disposed relative to a point of interest and a second set of stationary radio-responsive devices disposed relative to the point of interest. The system also includes a vehicle with a positioning unit configured to operate in a first mode for communicating with the first set of radio-responsive devices and operate in a second mode for communicating with the second set radio responsive devices. The positioning unit is configured to estimate a coarse position of the vehicle based on data received while operating in the first mode, and estimate a fine position of the vehicle based on data received while operating in the second mode. 
     In accordance with another example embodiment of the invention, there is provided a method of estimating position. The method includes, within a first area of interest, communicating with one or more first radio responsive devices to estimate a coarse position. The method also includes, within a second area of interest, communicating with one or more second radio-responsive devices to estimate a fine position. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of embodiments of the invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a block diagram of a positioning system, according to an exemplary embodiment of the invention; 
         FIG. 2  is a block diagram of a positioning system, according to an exemplary embodiment of the invention; 
         FIG. 3  is a block diagram of a vehicle that includes a positioning unit, in accordance with exemplary embodiments of the invention; and 
         FIG. 4  is a method of determining a position, in accordance with exemplary embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the invention relate to a two-stage positioning technique. In accordance with an exemplary positioning system, a coarse position estimate may be computed within a coarse area of interest, and a fine position estimate may be is computed within a fine area of interest. Within the coarse area of interest, a mobile positioning unit may use a narrow band communication technique to estimate the coarse position of the mobile positioning unit. Within the fine area of interest, the positioning unit may use a wide band communication technique to estimate the fine position of the positioning unit. In embodiments, the fine positioning within the fine area of interest may be computed using both narrow band and wide band communications to obtain a more accurate estimate of the positioning unit compared to using narrow band communications alone. In embodiments, the positioning unit may be disposed within a vehicle such as a railway vehicle. The positioning unit may be used to control a braking system of the vehicle to stop the movement of the vehicle at a point of interest with sub-meter accuracy. 
       FIG. 1  is a block diagram of a positioning system, according to an exemplary embodiment of the invention. As shown in  FIG. 1 , the positioning system  100  may include a vehicle  102  equipped with a positioning unit  104 . The vehicle  102  may include a rail-way vehicle such as a locomotive, an automobile, a marine vessel, or any other suitable type of vehicle. Further, it will be appreciated that embodiments are not limited to determining the position of a vehicle. For example, the positioning unit  104  may be disposed within other types of mobile devices, such as mobile phones, for example. 
     The positioning unit  104  may be configured to estimate the position of the vehicle  102  in relation to a point of interest  106 . The point of interest  106  may be a geographical location that is relevant in some way to the vehicle  102 . For example, the point of interest  106  may coincide with the location of a loading dock or a passenger platform. The area within the vicinity of the point of interest  106  may be divided into a coarse area of interest  108  and a fine area of interest  110 . The coarse area of interest  108  defines an area within which the positioning unit  104  will compute an estimated coarse position of the vehicle  102 . The fine area of interest  110  defines an area within which the positioning unit  104  will compute an estimated fine position vehicle  102 . 
     To determine the position of the vehicle  102 , the positioning unit  104  may communicate with radio responsive devices disposed in the vicinity of the point of interest  106 . As used herein, the term radio responsive device may be used to refer to active reflectors, passive reflectors, and wireless communication devices. An active reflector refers to a radio communication device that can receive, generate, and transmit radio signals using, for example, analog circuits. Examples of active reflectors include transponders, repeaters, and the like. A passive reflector refers to a radio reflective device such as a piece of metal. A wireless communication device refers to communication devices that can receive digital signals, process the data contained in the signals, and generate and transmit new digital signals that contain additional data. Examples of wireless communication devices include wireless routers, and the like. As used herein, the term “communicate” and variations thereof are used in relation to active reflectors, passive reflectors, and wireless communication devices. In other words, “communicating” with a radio responsive device includes sending an outbound signal to the radio responsive device and receiving a corresponding inbound signal from the radio responsive device, regardless of whether the inbound signal is generated by an active reflector or wireless communication device, or reflected from a passive reflector. 
       FIG. 1  shows two radio responsive devices referred herein as a coarse positioning device  112  and a fine positioning device  114 . Both the coarse positioning device  112  and the fine positioning device  114  may be disposed at fixed positions about the point of interest  106 . The positions of the devices  112  and  114  are known values. For example, coordinates describing the positions of the devices  112  and  114  in relation to the point of interest  106  may be stored to the positioning unit  104 , programmed into the logic of the positioning unit  104 , or otherwise accessible by positioning unit  104 . To compute an estimated position of the vehicle  102 , the positioning unit  104  determines a position of the vehicle  102  in relation to either the coarse positioning device  112  or the fine positioning device  114  or both. The position of the vehicle  102  in relation to the point of interest  106  can then be determined based on the known position of the coarse positioning device  112  or the fine positioning device  114  in relation to the point of interest  106 . 
     In embodiments, the positioning unit  104  determines the position of the vehicle  102  by determining a distance between the vehicle  102  and the radio responsive devices. For example, the positioning unit  104  may transmit an outgoing signal from the vehicle  102  to one of the radio responsive devices, receive a return signal from one of the radio responsive devices, and compute a time-of-flight of the signals. The time-of-flight refers to an amount of time elapsed between the transmission of the outbound signal and the receipt of the inbound signal. The time-of-flight can be used to compute a distance measurement. In embodiments wherein the vehicle  102  is a railway vehicle, the path of the railway may provide another set of data that can be used to determine the position of the vehicle  102  based on the distance measurement. In this way, a single distance measurement may be used to determine the position of the vehicle  102 . Data describing the path of the railway may be stored to the positioning unit  104 , programmed into the logic of the positioning unit  104 , or otherwise accessible by positioning unit  104 . As described below in relation to  FIG. 2 , the position of the vehicle  102  may also be computed using two or more distance measurements based on signals received from two or more radio responsive devices. 
     As noted above, the positioning unit  104  computes a coarse position within the coarse are of interest  108  and a fine position within a fine area of interest  110 . The coarse position is computed based on communications with the coarse positioning device  112 , which may be an active or passive reflector or a wireless communication device. To compute the coarse position, the positioning unit  104  communicates with the coarse positioning device  112  using a narrow-band signal. For example, the bandwidth of the narrow-band signal may be approximately 5 to 40 Megahertz. The extent of the coarse area of interest  108  corresponds to the distance at which signals can be communicated between the positioning unit  104  and the coarse positioning device  112 . 
     The fine position is computed based on communications with the fine positioning device  114 , which may be an active or passive reflectors or a wireless communication device. To compute the fine position, the positioning unit  104  communicates with the fine positioning device  114  using a wide-band signal that has a bandwidth greater than the narrow-band signal that is used to communicate with coarse positioning device  112 . For example, the bandwidth of the wide-band signal may be approximately 500 Megahertz to 2 Gigahertz. The extent of the fine area of interest  108  corresponds to the distance at which signals can be communicated between the positioning unit  104  and the fine positioning device  112 . 
     As indicated by the extent of the coarse area of interest  108 , the narrow-band signal used during coarse positioning enables a greater communication range compared to the wide-band signal. For example, the coarse area of interest  108  may extend approximately 1 kilometer around the coarse positioning device  112 , whereas the fine area of interest  110  may extend approximately 50 meters around the fine positioning device  114 . However, the wide-band signal used during fine positioning enables the computation of a more precise vehicle position compared to the narrow-band signal. For example, the use of the wide-band signal may enable the computation of position estimates with a precision of less than a meter, whereas the narrow-band signal may enable the computation of position estimates with a precision of a few meters. 
     In embodiments, the coarse positioning device  112  is a wireless communication device that communicates with the positioning unit  104  using an IEEE 802.11 standard protocol, such as WiFi. The positioning unit  104  may periodically transmit outbound signals to be received by the coarse positioning device  112  in an attempt to establish a communication link with the coarse positioning device  112 . When the vehicle  102  is within the coarse area of interest  108 , the outbound signals can be received by the coarse positioning device  112 . In response to the outbound signal, the coarse positioning device  112  may generate and transmit a corresponding inbound signal back to the positioning unit  104  at the same frequency as the outbound signal. The inbound signal may transmit one or more data packets to the positioning device  112 . Each inbound data packet may include an identifier that identifies the particular coarse positioning device  112  sending the data packet. The positioning unit  104  may then compute a coarse estimate of the vehicle position based on the round-trip time-of-flight of the outbound and inbound signals. The positioning unit  104  may continue to periodically send outbound signals to the coarse positioning device  112  in order to periodically re-compute the vehicle position as the vehicle  102  moves through the coarse area of interest  108 . Additionally, when the vehicle  102  enters the coarse area of interest  108 , the positioning unit  104  may begin periodically transmitting outbound signals to be received by the fine positioning device  114  in an attempt to establish communications with the fine positioning device  114 . 
     The positioning unit  104  may communicate with the fine positioning device  114  by transmitting Ultra-wideband (UWB) pulses to the fine positioning device  114 . In embodiments, the fine positioning device  114  is a passive reflector, which reflects the outbound pulses back to the positioning unit  104 . In embodiments, the fine positioning device  114  is an active reflector, which, in response to receiving the outbound pulses, generates and transmits a corresponding inbound signal back to the positioning unit  104  at the same frequency as the outbound signal. In embodiments, the signal generated by the active reflector may be amplitude modulated or phase modulated the create a unique signature that identifies the fine positioning device  114  generating the signal. In embodiments, the fine positioning device  114  is a wireless communication device that communicates with the positioning unit  104  using an standard UWB protocol, such as ISO/IEC 26907 and IEEE 802.15.4a, among others. The inbound UWB signals generated by the fine positioning device  114  may include one or more data packets, each of which includes an identifier that identifies the particular fine positioning device  114  sending the data packet. 
     The vehicle  102  is within the fine area of interest  110  when the inbound signals generated or reflected by the fine positioning device  114  can be detected by the positioning unit  102 . Upon the receipt of the inbound signals, e.g., UWB pulses, the positioning unit  104  can compute a fine estimate of the vehicle position based on the round-trip time-of-flight of the outbound and inbound signals. The positioning unit  104  may continue to periodically send outbound signals to the fine positioning device  114  in order to periodically re-compute the vehicle position as the vehicle  102  moves through the fine area of interest  110 . As described further below in reference to  FIG. 2 , an exemplary positioning system may include any suitable number of coarse positioning devices  112  and any suitable number of fine positioning devices  114 . 
       FIG. 2  is a block diagram of a positioning system, according to an exemplary embodiment of the invention. As in the positioning system  100  described in reference to  FIG. 1 , the positioning system  200  may include a vehicle  102  equipped with a positioning unit  104  that computes an estimate of the vehicle position based on communications with a number of radio responsive devices disposed at fixed positions about a point of interest  106 . The exemplary positioning system  200  shown in of  FIG. 2  includes a set of four coarse positioning devices  112  and a set of four fine positioning devices  114 . Other exemplary embodiments of a positioning system may include one, two, three, five, or more coarse positioning devices  112  and one, two, three, five, or more fine positioning devices  114 . As used herein, the term “set” as in the phrase “set of radio responsive devices” is used to refer to one or more. Furthermore, embodiments are not limited to positioning systems that include an equal number of coarse positioning devices  112  and fine positioning devices  114 . 
     As described above in relation to  FIG. 1 , the positioning unit  104  may determine the position of the vehicle  102  by transmitting radio signals to the radio responsive devices, receiving return signals from the radio responsive devices, measuring the time-of-flight, and computing a corresponding distance based on the time-of-flight measurements. In the embodiment shown in  FIG. 2 , the positioning unit  104  may compute a plurality of distance measurements for each level of positioning. For example, a distance measurement may be computed for each of the coarse positioning devices  112  and each of the fine positioning devices  114 . The plurality of distance measurements can be used to compute a more precise position of the vehicle, for example, using trilateration. To trilateration, each distance measurement corresponds with the radius of a circle centered at the corresponding radio responsive device from which the signal was received. The intersection of the circles provides the vehicle location. Three signals may be used to determine a specific point in two-dimensional space. Four signals may be used to determine a specific point in three dimensional space. 
     As an example, when the vehicle  102  is within the coarse area of interest  108 , the outbound signals sent by the positioning unit  104  can be received by each of the coarse positioning devices  112 . In an embodiment wherein the coarse positioning devices  112  are wireless communication devices, the coarse positioning devices  112  may generate and transmit a corresponding inbound signal back to the positioning unit  104  upon receiving the outbound signal from the positioning unit  104 . To enable the positioning unit  104  to associate each inbound signal with the proper coarse positioning device  112 , each coarse positioning device  112  may add a unique identifier to the return signal that it generates, as described above. The positioning unit  104  may then compute the distance between the vehicle  102  and each of the coarse positioning devices  112 , based on the round-trip time-of-flight of the outbound signal and the plurality of inbound, signals. The coarse estimate of the vehicle position may then be computed based, for example, on trilateration of the computed distances. 
     In a similar fashion, the fine estimate of the vehicle position may be computed using the signals received from the plurality of fine positioning devices  114 . In embodiments wherein each of the fine positioning devices is an active reflector, each fine positioning device  114  may use a different level of amplitude modulate or phase modulate for the signal that it generates, which enables the positioning unit  104  to associate each inbound signal with the proper fine positioning device  114 . 
       FIG. 3  is block diagram of a vehicle that includes a positioning unit, in accordance with exemplary embodiments of the invention. As shown in  FIG. 3 , the positioning unit  104  may include a processor  300  and a memory  302  comprising a non-transitory, computer-readable medium. The memory  302  may include volatile memory such as Random Access Memory (RAM) used during the execution of various operating programs, including operating programs used in embodiments of the present invention. The memory  302  can also include a storage system for the long-term storage of operating programs and data, including the operating programs and data used in embodiments of the present invention. For example, the memory  302  can include a hard disk drive, an optical drive, a universal serial bus (USB) drive, solid state memory, and the like. In embodiments, the processor  300  and the memory  302  may be implemented as an Application Specific, Integrated Circuit (ASIC). In embodiments, the positioning unit  104  may be implemented on a general-purpose computing device, for example, laptop computer, a smart phone, and the like. 
     The positioning unit  104  may include two physical layers or PHYs, referred to herein as PHY A  304  and PHY B  306 . Each PHY  304  and  306  is communicatively coupled to the processor  300  and enables the positioning unit  104  to communicate with the radio responsive positioning devices  112  and  114  ( FIGS. 1 and 2 ). As an example, PHY A  304  may be used to communicate with the coarse positioning devices  112  and PHY B  306  may be used to communicate with the fine positioning devices  114 . Each of PHY A  304  and PHY B  306  may include one or more transceivers, amplifiers, signal processors, and any other circuitry which may be used to enable the positioning unit  104  to transmit and receive radio signals. PHY A  304  and PHY B  306  may each be operatively coupled to a corresponding antenna  308 , which may be disposed in or on the vehicle  102 . In an embodiment, PHY A  304  and PHY B  306  may be coupled to the same antenna  308 . 
     PHY A  304  may be used to acquire information used for computing a coarse estimate of the vehicle position while the vehicle  102  is in the coarse area of interest  108 . PHY B  306  may be used to acquire information used for computing a fine estimate of the vehicle position while the vehicle  102  is in the fine area of interest  110 . In an embodiment, PHY A  304  communicates with the coarse positioning devices  112  using a first bandwidth, and PHY B  306  communicates with the fine positioning devices  114  using a second bandwidth larger than the first bandwidth. For example, the first bandwidth may be approximately 5 to 40 Megahertz and the second bandwidth may approximately 500 Megahertz to 2 Gigahertz. Further, PHY A  304  may communicate with the coarse positioning devices  112  using an IEEE 802.11 protocol such as WiFi. PRY B  306  may communicate with the fine positioning devices  114  by transmitting Ultra-Wideband (UWB) pulses and receiving corresponding echoes from the fine positioning devices  114 . 
     In embodiments, the positioning unit  104  may be communicatively coupled to a central control unit  310  of the vehicle  102 . The position estimates computed by the positioning unit  104  may be output to the central control unit  310 . The central control unit  310  may use the position estimate for a variety of purposes. In an embodiment, the position estimate may be communicated to a person such as a vehicle operator through a user interface. In an embodiment, the central control unit  310  may be operatively coupled to a braking system of the vehicle  102 . In such embodiments, the central control unit  310  may compute a braking signal based, at least in part, on the position estimate received from the positioning unit  104 . The braking signal may determine a degree of braking to be applied to the vehicle  102  may be computed based, for example, on the speed of the vehicle  102  and the distance of the vehicle  102  from the point of interest  106 . The braking signal may be sent from the central control unit  310  to the braking system  312  to engage the brakes of the vehicle  102  until the vehicle  102  is stopped at the point of interest  106 . Within the coarse area of interest  108 , the braking signal may be based on the coarse position estimate provided by the positioning unit  104 . Within the fine area of interest  110 , the braking signal may be based on the fine position estimate provided by the positioning unit  104 . 
     Further, the positioning unit  104  may be configured to automatically switch between outputting a coarse position estimate and outputting a fine position estimate based, at least in part, on whether the vehicle  102  is within the coarse area of interest  108  or the fine area of interest  110 . For example, when the positioning unit  104  is able to establish communications through PHY B  306 , the positioning unit  104  may automatically switch from outputting a coarse position estimate to outputting a fine position estimate. In some embodiments, when the vehicle  102  is within the fine area of interest  110 , the positioning unit  104  may compute both a fine position estimate and a coarse position estimate, in which case both PHY A  304  and PHY B  306  may be operating simultaneously to obtain information for computing the vehicle position. In other embodiments, when the vehicle  102  is within the fine area of interest  110 , the positioning unit  104  may compute only a fine position estimate, in which case only PHY A  304  may be operating. 
       FIG. 4  is a block diagram of a method of determining a position, in accordance with exemplary embodiments of the invention. The method  400  may be performed by the positioning unit  104  and is described herein with reference also to  FIGS. 1-3 . The method  400  may begin at block  402 , wherein the vehicle  102  is approaching the coarse area of interest  108 . During this time, the vehicle  102  continues to attempt to establish communications with the set of coarse positioning devices  112  even though the vehicle  102  may be outside the radio range of the coarse positioning devices  112 . Upon entering the coarse area of interest  108 , the process flow may advance to block  404 . 
     At block  404 , the positioning unit establishes communications with one or more of the coarse positioning devices  112 . This indicates that the vehicle  102  is within the coarse area of interest  108 . Based on the information received from the coarse positioning devices  112 , the coarse estimate of the vehicle position may be computed as described above. For example, the vehicle position may be determined by measuring the time-of-flight of the signals sent to and received from the coarse positioning devices  112 . The position of the vehicle  102  may be tracked as the vehicle  102  moves through the coarse area of interest  108  by periodically transmitting signals to and receiving signals from the coarse positioning devices  112 . As described in relation to  FIG. 3 , communications with the coarse positioning devices  112  may be accomplished using a dedicated physical layer, for example, PHY A  304 , which uses a narrow-band signal. 
     While tracking the position of the vehicle  102  through the coarse area of interest, the positioning unit  104  may output the coarse estimate of the vehicle position to the central control unit  310 . The central control unit  310  may begin engaging the braking system  312  of the vehicle  102  based on the coarse position of the vehicle as described above. 
     At block  406 , after entering the coarse area of interest, the positioning unit  104  may also begin attempting to communicate with the fine positioning devices  114 . Upon entering the fine area of interest  110 , the process flow may advance to block  408 . 
     At block  408 , the positioning unit  104  establishes communications with one or more of the fine positioning devices  114 , which indicates that the vehicle  102  is within the fine area of interest  110 . For example, the positioning unit  104  may detect echoes reflected from the fine positioning devices  114  or receive data packets generated by the fine positioning devices  114 . When the positioning unit  104  is able to detect the signals transmitted by or reflected from the fine positioning devices  114 , the fine estimate of the vehicle position may be computed as described above. For example, the vehicle position may be determined by measuring the time-of-flight of the signals sent to and received from the fine positioning devices  114 . The position of the vehicle  102  may be tracked as the vehicle  102  moves through the fine area of interest  110  by periodically transmitting signals to and receiving signals from the fine positioning devices  114 . As described in relation to  FIG. 3 , communications with the fine positioning devices  112  may be accomplished using a dedicated physical layer, for example, PHY B  306 , which uses Ultra-wideband pulses. 
     While tracking the position of the vehicle  102  through the fine area of interest  110 , the positioning unit  104  may output the fine estimate of the vehicle position to the central control unit  310 . In embodiments, the positioning unit  104  may automatically stop outputting a coarse position estimate and begin outputting a fine position estimate upon entering the fine area of interest  106 . In embodiments, the positioning unit may output both a coarse position estimate and a fine position estimate. Within the fine area of interest  110 , the engagement of the braking system  312  of the vehicle  102  may be based on the fine position estimate as described above. Eventually, the central control unit  310  may cause the vehicle  102  to stop within the vicinity of the point of interest  106 , for example, within  1  meter of the point of interest  106 . 
     When the vehicle  102  starts moving away from the point of interest  106 , the process flow described above may be performed in the reverse order. In other words, a fine position estimate may be computed while the vehicle  102  is in the fine area of interest  110  until the signals from the fine positioning devices  114  can no longer be detected. Once the vehicle  102  leaves the fine area of interest, the positioning unit may automatically switch to computing a coarse position estimate. The positioning unit  104  may track the vehicle position through the coarse area of interest  108  until the positioning unit  104  is out of radio range of the coarse positioning devices  112 . 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. The dimensions, values, and types of materials described herein are intended to illustrate embodiments of the invention, but are by no means and are exemplary in nature. Other embodiments may be apparent upon reviewing the above description. The scope of the invention, therefore, should be determined with reference to the appended claims, alone: with the fall scope of equivalents to which such claims are entitled. 
     In the appended claims, any usage of the terms “including” and “in which” are indicated the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” “third,” “upper,” “lower,” “bottom,” “top,” “up,” “down,” etc, are used merely as labels, and are not intended to impose numerical or positional requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. §112, sixth paragraph, unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure. 
     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the invention are not to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising,” “including,” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property. 
     Certain changes may be made in the above-described apparatus, without departing from the spirit and scope of the invention herein involved. Accordingly, it is intended that all of the subject matter of the above description or shown in the accompanying drawings shall be interpreted merely as examples illustrating the inventive concept herein and shall not be construed as limiting the invention.