Patent Publication Number: US-2021173036-A1

Title: Unmanned aerial vehicle as well as direction finding system

Description:
FIELD OF THE DISCLOSURE 
     Embodiments of the present disclosure generally relate to an unmanned aerial vehicle. Embodiments of the present disclosure further relate to a direction finding system. 
     BACKGROUND 
     In the state of the art, stationary direction finding systems are known that are used to observe a certain area. This is also known as spectrum monitoring since the electromagnetic spectrum, particularly the radio spectrum, assigned to the area to be observed is monitored in order to identify sources of radio signals, particularly interferers. 
     In fact, spectrum monitoring can effectively help enforcing compliance with (international) radio traffic regulations, as breaches can be recognized in time by means of radio surveillance or rather radio monitoring. Generally, radio traffic is regulated in order to ensure that when police, radio stations, air traffic control and ham radios are all on the air at the same time, they do not cause mutual interference. Compliance with the respective regulations is enforced by radio monitoring. 
     So far, stationary direction finding systems are used in order to locate (“direction finding”) a source of a certain signal, for instance a disturbing or interfering signal. However, certain sources cannot be identified effectively, as this would require to get closer to the source, which might not be possible for several reasons. 
     Moreover, it can also be necessary to adapt the relative orientation with respect to the source of the certain radio signal in order to improve the receiving quality of the direction finder. This is also not always applicable since the direction finder may relate to stationary systems that cannot be moved in order to adapt the relative orientation. 
     Accordingly, there is a need for an improved direction finding. 
     SUMMARY 
     Embodiments of the present disclosure provide an unmanned aerial vehicle with a main body and at least two rotor units configured to propel the unmanned aerial vehicle. In an embodiment, the unmanned aerial vehicle comprises at least two antenna units configured to receive a radio signal. The antenna units are located with respect to the main body such that the antenna units are assigned to different lateral sides of the main body. 
     The unmanned aerial vehicle according to the present disclosure is based on the finding that the unmanned aerial vehicle (UAV), also called drone, has at least two antenna units that can be used for direction finding purposes. This can be ensured as the respective antenna units are located at different sides with respect to the main body of the unmanned aerial vehicle. Thus, the antenna units receive the incoming signals in a different manner which can be used for direction finding purposes. In some embodiments, the angle of incidence can be determined due to the fact that two antenna units are provided that are located at different sides of the main body. In some embodiments, the radio signals received by the at least two radio antennas have different radio characteristics that can be evaluated in order to locate the source of a certain radio signal. As the antenna units are assigned to different lateral sides of the main body, it is ensured that they are not situated on an upper or a lower side of the main body. 
     Put differently, the main body typically corresponds to a payload of the unmanned aerial vehicle, which encompasses the central control circuit, such as a control unit, as well as any sensors assigned to the unmanned aerial vehicle. In contrast to the unmanned aerial vehicles known in the state of the art, the unmanned aerial vehicle according to the present disclosure has at least two antennas that are configured to receive a radio signal. The antenna units are distanced to the main body such that the radio signals received by each of the antenna units can be analyzed, resulting in a direction finding functionality provided by the unmanned aerial vehicle. This direction finding functionality could not be provided if the respective antenna units are located close to each other, for instance on the upper side or the lower side of the main body, as the antenna units would receive radio signals with substantially the same radio characteristics. 
     In general, direction finding may be based on combining the direction information from two or more suitably spaced antenna units, also called receivers, wherein the source of the radio signal may be located via triangulation techniques. Thus, it is ensured that the antenna units are suitably spaced antenna units with respect to each other since they are assigned to different lateral sides of the main body of the unmanned aerial vehicle. 
     The unmanned aerial vehicle is generally configured to operate in areas or regions in which a stationary direction finding system cannot operate, for instance mountain side, open sea or rather border regions. 
     According to an aspect, the at least two antenna units are assigned to the at least two rotor units. Thus, the at least two antenna units may be directly associated with the at least two rotor units. 
     For instance, at least two functional pairs are provided, each pair comprising one antenna unit and one rotor unit associated thereto. Put differently, each antenna unit is assigned to a dedicated rotor unit. 
     According to another aspect, the antenna units are located close to the rotor units. In some embodiments, each of the antenna units may be located in direct proximity of the dedicated rotor unit. Hence, a compact unmanned aerial vehicle is provided ensuring flying functionality as well as direction finding functionality. Typically, the rotor units are assigned to the outer portions of the unmanned aerial vehicle, ensuring flight stability. As the respective antenna units are located close to the respective rotor units, the antenna units are suitably spaced with respect to each other, yielding the direction finding functionality. 
     For instance, each of the antenna units corresponds to a support for its assigned rotor unit. Thus, the antenna unit may support the respective rotor unit, for example a bearing for the respective rotor unit. In other words, the antenna units each may correspond to stabilizers for the dedicated rotor units. 
     According to another aspect, each antenna unit and its assigned rotor unit together form an integrated module or assembly. Thus, the antenna unit and its assigned rotor unit may be a separately formed module that can be connected with the main body. For instance, the main body comprises several interfaces for such integrated modules so that the main body may be expandable. In some embodiments, the number of integrated modules, namely antenna units and/or rotor units, connected with the main body may be varied due according to the respective scenario intended. 
     For instance, more rotor units may be required due to a higher payload. In another scenario, more antenna units may be required for improving the direction finding functionality. In general, the respective capability of the unmanned aerial vehicle ca be adapted to the respective needs. 
     In some embodiments, the antenna units may be located such that the distance between the antenna units is maximized This ensures improved direction finding characteristics, as the time difference of the radio signals received by each antenna unit is maximized due to the maximized distance between the antenna units. 
     According to an embodiment, the unmanned aerial vehicle has at least two arms. Each arm holds one of the at least two antenna units and the dedicated rotor unit. Hence, the antenna units are spaced with respect to the main body by the corresponding arms. The respective arm may be part of the integrated module wherein the arm may provide the interface via which the integrated unit is coupled with the main body. 
     For instance, the unmanned aerial vehicle has more than two arms such that the respective antenna units and/or rotor units are located in a star-like manner with respect to the main body. 
     In some embodiments, the at least two arms extend from the main body in a radial and/or equidistant manner. This ensures that the unmanned aerial vehicle can be operated in a stable manner, as the different rotor units are spaced from each other in an equidistant manner, ensuring a homogeneous propulsion of the unmanned aerial vehicle. Accordingly, a flight stability is ensured. 
     The equidistant orientation of the at least two arms extending from the main body of the unmanned aerial vehicle in a radial manner results in an equidistant distance between the respective arms with respect to a circumference around the main body of the unmanned aerial vehicle. 
     In a first operation mode, the unmanned aerial vehicle may be configured to be operated as a flying unmanned aerial vehicle. In some embodiments, the unmanned aerial vehicle can be used to monitor areas that cannot be accessed by a stationary direction finding system, for instance open sea and/or mountain side. The rotor units are operated such that the unmanned aerial vehicle can fly. For flying operation, control signals may be received by the respective antenna units. Thus, the antenna units may be configured to receive control signals used for flying the unmanned aerial vehicle in the first operation mode. 
     In a second operation mode, the unmanned aerial vehicle may be configured to be operated as a direction finder, for example a stationary direction finder. In the second operation mode, which may be effective in addition to the first operation mode or (only) separate from the first operation mode, the unmanned aerial vehicle corresponds to a direction finder. Thus, the antenna units of the unmanned aerial vehicle are used to receive radio signals that are forwarded to a processing unit, such as a microprocessor, processor circuit, etc., of the unmanned aerial vehicle for being analyzed respectively. 
     The processing unit may be located within the main body. Hence, the main body may encompass a control unit and the processing unit. Generally, a control and/or analyzing circuit(s) or unit(s) may be located within the main body. 
     However, the unmanned aerial vehicle may also be operated in a stationary manner within the second operation mode. Thus, the several rotor units may not be controlled in the second operation mode. 
     The unmanned aerial vehicle may be configured to be operated as a mobile locator for direction finding. Thus, the unmanned aerial vehicle can also be used to get as close as possible to a certain source of a radio signal detected. This ensures that the location of the source can be identified in an improved manner In some embodiments, the unmanned aerial vehicle, for example the processing unit connected with the antenna units in a signal transmitting manner, may determine the respective coordinates of the source of the radio signal based on the radio signals received by the at least two antenna units. 
     Accordingly, direction finding can be improved by the unmanned aerial vehicle being operated as a flying unmanned aerial vehicle, namely a drone, since a different line of sight can be achieved by the unmanned aerial vehicle. In some embodiments, the unmanned aerial vehicle can be flown at different heights, for example higher heights than the ones that can be achieved by a stationary direction finding system. Put differently, the relative orientation can be adapted by the unmanned aerial vehicle, as the unmanned aerial vehicle is flown to a height that enables the different line of sight with respect to the source to be identified by direction finding techniques. 
     For instance, the unmanned aerial vehicle comprises a base interface configured to be connected to a mast. Thus, the unmanned aerial vehicle can also be attached to the mast, for example a mast top, while the unmanned aerial vehicle is not flying. 
     Thus, the unmanned aerial vehicle has at least a dual functionality, as it can be operated as a mobile direction finder, namely a flying one, and a stationary direction finder that is fixedly coupled to the mast. 
     The mast may relate to an extractable mast. Thus, the height can be adjusted by simply extracting the mast to the desired height provided that the height can be achieved by the extractable mast. 
     In some embodiments, the (extractable or non-extractable) mast corresponds to a rod of a certain base station, for instance a shelter mounted on a vehicle, a standalone base station, a ship or any other system used for providing a mast used for direction finding purposes. 
     Another aspect provides that the unmanned aerial vehicle comprises a communications interface or module that is configured to transmit direction finding data measured. Hence, the unmanned aerial vehicle collects direction finding data when being operated as a direction finder. The respective direction finding data measured can be transmitted by the communications module, for instance to a base unit having higher computational power for analyzing the respective data collected. 
     The base unit used for communication purposes may be assigned to the base station. 
     In some embodiments, the communications module is configured to use optical communication techniques. For instance, a laser communication is established which ensures a secure communication between the unmanned aerial vehicle and the base unit. Moreover, the communication may be done by a wire or a cable provided that the unmanned aerial vehicle is connected with the mast. 
     Generally, the communications module corresponds to a data link circuit or module that is capable of transmitting the direction finding data measured in a way that is not interceptable by a third party not authorized. 
     The communication module may establish a communication link, for instance by a simple cable or rather an optical link, in order to ensure secure communication. 
     Embodiments of the present disclosure further provide a direction finding system that comprises a mast and an unmanned aerial vehicle as described above. The unmanned aerial vehicle is fixedly connected with the mast. Thus, the unmanned aerial vehicle may be connected by its base interface with the mast, for example a mast top. 
     An aspect provides that the direction finding system comprises a base unit that is configured to communicate with the unmanned aerial vehicle. The base unit may comprise a corresponding communication interface that is configured to communicate with the unmanned aerial vehicle, for example its communications module. 
     For instance, the base unit comprises an optical receiver that receives the laser signals transmitted by the communication module of the unmanned aerial vehicle provided that the communication module is configured to communicate by laser signals. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of the claimed subject matter will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  schematically shows an unmanned aerial vehicle according to a first embodiment of the present disclosure; 
         FIG. 2  schematically shows an unmanned aerial vehicle according to a second embodiment of the present disclosure; 
         FIG. 3  schematically shows a representative embodiment of a direction finding system according to the present disclosure; and 
         FIG. 4  schematically shows another embodiment of a direction finding system according to the present disclosure, which comprises several unmanned aerial vehicles according to the present disclosure in an operative scenario. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings, where like numerals reference like elements, is intended as a description of various embodiments of the disclosed subject matter and is not intended to represent the only embodiments. Each embodiment described in this disclosure is provided merely as an example or illustration and should not be construed as preferred or advantageous over other embodiments. The illustrative examples provided herein are not intended to be exhaustive or to limit the claimed subject matter to the precise forms disclosed. 
     In  FIG. 1 , an unmanned aerial vehicle  10  is shown that comprises a main body  12  that encompasses a control and/or analyzing circuitry, for example, a control and/or analyzing unit  14 , of the unmanned aerial vehicle  10 . In an embodiment, a control and/or analyzing unit  14  includes a processing circuit, such as a processing unit, and a control circuit, such as a control unit, that are separately formed with respect to each other. 
     In some embodiments, the term “unit” used in the preceding paragraph with regards to the unit  14  and/or its components refers to a combination of hardware (e.g. a processor such as an integrated circuit or other circuitry) and/or software (e.g. machine- or processor-executable instructions, commands, or code such as firmware, programming, or object code). Furthermore, a combination of hardware and/or software may include hardware only (i.e. a hardware element with no software elements), software hosted at hardware (e.g. software that is stored at a memory and executed or interpreted at a processor), or hardware with the software hosted thereon. In some embodiments, the hardware may, inter alia, comprise a central processing unit (CPU), a graphics processing unit (GPU), a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), or other types of electronic circuitry. 
     The unmanned aerial vehicle  10  also includes several arms  16  that are connected with the main body  12 . The arms  16  each extend from the main body  12  in a radial manner, wherein the arms  16  are distanced from each other in an equidistant manner with respect to a circumference around the main body  12 . 
     In the shown embodiment, the unmanned aerial vehicle  10  comprises five arms  16 , wherein two neighbored arms  16  define an angle a between them that corresponds to 72°. However, the unmanned aerial vehicle  10  may generally comprise more or less than the shown five arms  16 . 
     In addition, the unmanned aerial vehicle  10  comprises several antennas, such as antenna units  18 , that are located at the free ends  20  of the arms  16 . The free ends  20  relate to the ends that are opposite to the main body  12  to which the arms  16  are connected. Accordingly, the antenna units  18  are distanced (spaced) from the main body  12  by a maximum distance. The maximum distance is defined by the length of the respective arm  16 . Furthermore, the antenna units  18  are also distanced from each other in a maximal manner 
     As shown in  FIG. 1 , the antenna units  18  extend in a (substantially) perpendicular direction with respect to the extension direction of the arms  16 . In some embodiments, prolongations of the arms  16  intersect each other in a center point that coincidences with the center of the main body  12 . 
     In contrast to the arms  16 , the antenna units  18  each define a longitudinal axis, wherein the longitudinal axes of the antenna units  18  extend parallel with respect to each other. In some embodiments, the longitudinal axes of the antenna units  18  are normal to a surface of the main body  12 . 
     The unmanned aerial vehicle  10  further comprises several rotors, such as rotor units  22 , that are also assigned to the free ends  20  of the arms  16 . The rotor units  22  are generally configured to propel the unmanned aerial vehicle  10 . In some embodiments, the rotor units  22  are supported by the antenna units  18 . In other words, each of the antenna units  18  corresponds to a dedicated support for its assigned rotor unit  22 . 
     As shown in  FIG. 1 , the entire unmanned aerial vehicle  10  has, in a top view on the unmanned aerial vehicle  10 , a star-like shape. 
     In some embodiments, the antenna units  18  as well as the associated rotor units  22  are assigned to lateral ends, namely the free ends  20 , of the arms  16  that extend in a radial manner from the centrally arranged main body  12  of the unmanned aerial vehicle  10 . 
     Accordingly, the antenna units  18  are located with respect to the main body  12  of the unmanned aerial vehicle  10  such that the antenna units  18  are assigned to different lateral sides of the main body  12 . 
     In a similar manner, the rotor units  22  are also assigned to different lateral sides of the main body  12 , as the rotor units  22  are directly assigned to the respective antenna units  18 . 
     Accordingly, the antenna units  18  and the rotor units  22  are located in close proximity of each other. 
     The antenna unit  18  and the assigned rotor unit  22  may together form an integrated assembly or module  24  that can be connected with the main body  12  of the unmanned aerial vehicle  10 . 
     In some embodiments, the integrated module  24  may also comprise the respective arm  16  to which the antenna unit  18  is connected. Thus, the arm  16  may provide an interface of the integrated module  24  via which the integrated module  24  can be connected with the main body  12 . 
     The main body  12  may comprise several corresponding interfaces such that several integrated modules  24  may be connected to the main body  12 . 
     Generally, the main body  12  may be configured in a modular manner such that the number of antenna units  18  and/or rotor units  22  can be extended if desired. In some embodiments, the number of integrated modules  24  can be adapted in a desired manner 
     The unmanned aerial vehicle  10  may be operated as a flying unmanned aerial vehicle. Hence, the unmanned aerial vehicle  10  may have a first operation mode, in which the unmanned aerial vehicle  10  is controlled to fly. Thus, the unmanned aerial vehicle  10  may receive respective control signals via its antenna units  18  that are forwarded to the control and/or analyzing unit  14  of the unmanned aerial vehicle  10  for controlling the respective rotor units  22  in an appropriate manner 
     In the first operation mode, namely the flying mode, the unmanned aerial vehicle  10  may also be configured to be used as a mobile locator  26  for direction finding. This means that the unmanned aerial vehicle  10  is flown to a certain area, for example an area that cannot be accessed by a stationary direction finding system. 
     When being operated as a mobile locator  26 , the unmanned aerial vehicle  10  may receive radio signals from the respective area monitored. Then, the unmanned aerial vehicle  10  is enabled to perform direction finding in order to locate a source of a certain radio signal, for instance an interferer. The respective direction finding data measured by the unmanned aerial vehicle  10  may be (temporarily) stored in a storage  28  that may also be encompassed by the main body  12 . 
     Further, the unmanned aerial vehicle  10  may comprise a communications interface or module  30  that can also be encompassed in the main body  12 . The communications module  30  may include, for example, one or more communications circuits for implement one-way (e.g., transmit or receive) and/or bi-directional (e.g., transmit and receive) communications as further described herein. 
     The unmanned aerial vehicle  10  is configured to communicate via the communication module  30  with a base unit (not shown in  FIG. 1 ) in order to forward the respective direction finding data measured for evaluating purposes as will be explained later. 
     The communication module  30  may use optical communication techniques in order to transmit the respective direction finding data measured. For instance, laser technologies are used to communicate the respective data. Thus, a secure communication is ensured that is not interceptable by a third party which is not authorized to intercept the respective communication. 
     In a second operation mode, which may be effective alternatively or additionally to the first operation mode, the unmanned aerial vehicle  10  is configured to be operated as a (stationary) direction finder. As already mentioned above, the unmanned aerial vehicle  10  may be operated as a mobile locator  26 . Thus, the unmanned aerial vehicle  10  can identify and locate a certain source of a radio signal. 
     In some embodiments, the unmanned aerial vehicle  10  may also be operated as a stationary direction finder. In this mode, the rotor units  22  of the unmanned aerial vehicle  10  are not controlled to propel the unmanned aerial vehicle  10 . 
     In some embodiments, the unmanned aerial vehicle  10  comprises a base interface  32  that may be assigned to the main body  12 . The unmanned aerial vehicle  10  may be connected to a mast  34  via the base interface  32 . The mast  34  may relate to a direction finding system  36  that comprises the unmanned aerial vehicle  10  as well as the mast  34 . In  FIG. 3 , the direction finding system  36  is shown in which the unmanned aerial vehicle  10  of  FIG. 1  is connected to the mast  34 . 
     In other words, the unmanned aerial vehicle  10  has a constructive design such that the main body  12  or rather the antenna units  18  may serve as support stands for the unmanned aerial vehicle  10  while not flying. In this stationary operation mode, the unmanned aerial vehicle  10  is, for example, fixedly connected with the mast  34 . 
     In some embodiments, the mast  34  may be extractable or non-extractable. Thus, the mast  34  corresponds to a rod of a certain base station, for instance a shelter mounted on a vehicle, a standalone base station, a ship or any other system used for direction finding. 
     Generally, the unmanned aerial vehicle  10  may be connected with a base unit  38  of the direction finding system  36  by a cable or a wire, ensuring secure communication. 
     In  FIG. 2 , another embodiment of the unmanned aerial vehicle  10  is shown. In the second embodiment, the antenna units  18  are also assigned to different lateral sides of the main body  12 . However, the rotor units  22  are not directly assigned to the respective antenna units  18 , but to a lower side of the main body  12 . 
     Generally, the unmanned aerial vehicle  10  may have different designs with regard to the respective arrangement of the rotor units  22  and the antenna units  18 . Moreover, the unmanned aerial vehicle  10  shown in  FIG. 2  comprises a cage  40  for protecting the components of the unmanned aerial vehicle  10 , for example the rotor units  22  and/or the antenna units  18 . In addition, the unmanned aerial vehicle  10  may comprise at least one further module  42  for collecting data, such as a camera. 
     In  FIG. 4 , another embodiment of a direction finding system  36  is shown that comprises several unmanned aerial vehicles  10  operated in a flying mode as well as one unmanned aerial vehicle  10  fixedly connected to the mast  34 . 
     As shown in  FIG. 4 , the mast  34  is assigned to a vehicle. The direction finding system  36  comprises the base unit  38  that is assigned to the vehicle. 
     The flying unmanned aerial vehicles  10  communicate with the base unit  38  by their communications modules  30  in order to forward the direction finding data measured via the antenna units  18 . Accordingly, these unmanned aerial vehicles  10  may correspond to mobile locators  26 . Furthermore, the flying unmanned aerial vehicles  10  may receive control signals via their antenna units  18 . The stationary unmanned aerial vehicle  10 , namely the one connected to the mast  34 , is only operated as a direction finder, as the respective rotor units  22  are not controlled. 
     Generally, the base unit  38  communicating with the communication module(s)  30  of the unmanned aerial vehicle(s)  10  may comprise high computational power. This ensures that the base unit  38  may process direction finding data measured by several unmanned aerial vehicles  10 . 
     Certain embodiments disclosed herein utilize circuitry (e.g., one or more circuits) in order to implement protocols, methodologies or technologies disclosed herein, operably couple two or more components, generate information, process information, analyze information, generate signals, encode/decode signals, convert signals, transmit and/or receive signals, control other devices, etc. Circuitry of any type can be used. It will be appreciated that the term “information” can be use synonymously with the term “signals” in this paragraph. It will be further appreciated that the terms “circuitry,” “circuit,” “one or more circuits,” etc., can be used synonymously herein. 
     In an embodiment, circuitry includes, among other things, one or more computing devices such as a processor (e.g., a microprocessor), a central processing unit (CPU), a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), a system on a chip (SoC), or the like, or any combinations thereof, and can include discrete digital or analog circuit elements or electronics, or combinations thereof. In an embodiment, circuitry includes hardware circuit implementations (e.g., implementations in analog circuitry, implementations in digital circuitry, and the like, and combinations thereof). 
     In an embodiment, circuitry includes combinations of circuits and computer program products having software or firmware instructions stored on one or more computer readable memories that work together to cause a device to perform one or more protocols, methodologies or technologies described herein. In an embodiment, circuitry includes circuits, such as, for example, microprocessors or portions of microprocessor, that require software, firmware, and the like for operation. In an embodiment, circuitry includes one or more processors or portions thereof and accompanying software, firmware, hardware, and the like. 
     In some examples, the functionality described herein can be implemented by special purpose hardware-based computer systems or circuits, etc., or combinations of special purpose hardware and computer instructions. 
     Of course, in some embodiments, two or more of these components, or parts thereof, can be integrated or share hardware and/or software, circuitry, etc. In some embodiments, these components, or parts thereof, may be grouped in a single location or distributed over a wide area. In circumstances were the components are distributed, the components are accessible to each other via communication links. 
     The present application may reference quantities and numbers. Unless specifically stated, such quantities and numbers are not to be considered restrictive, but exemplary of the possible quantities or numbers associated with the present application. Also in this regard, the present application may use the term “plurality” to reference a quantity or number. In this regard, the term “plurality” is meant to be any number that is more than one, for example, two, three, four, five, etc. The terms “about,” “approximately,” “near,” etc., mean plus or minus  5 % of the stated value. For the purposes of the present disclosure, the phrase “at least one of A and B” is equivalent to “A and/or B” or vice versa, namely “A” alone, “B” alone or “A and B.”. Similarly, the phrase “at least one of A, B, and C,” for example, means (A), (B), (C), (A and B), (A and C), (B and C), or (A, B, and C), including all further possible permutations when greater than three elements are listed. 
     The principles, representative embodiments, and modes of operation of the present disclosure have been described in the foregoing description. However, aspects of the present disclosure which are intended to be protected are not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. It will be appreciated that variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present disclosure. Accordingly, it is expressly intended that all such variations, changes, and equivalents fall within the spirit and scope of the present disclosure, as claimed.