Patent Publication Number: US-2019168534-A1

Title: CNC Marking Device

Description:
FIELD OF THE INVENTION 
     This invention relates to a CNC (computer numerically controlled) marking device adapted for use in a CNC machine. 
     BACKGROUND 
     CNC (computer numerically controlled) machines are used to form complex parts that are machined in a single operation using more than one tool. The machine selects a tool from a bank of tools that are at its disposal. For example, the machine may select a milling tool to form a profile on a workpiece and then subsequently select a drill tool to form holes in that workpiece and substantially complete the part. Commonly the single operation is performed in a closed environment, which is furnished with cooling/lubricating fluids used to aid the machining. 
     Parts that are machined using a CNC machine often need to be traced. This may be, for example, to ensure batch identification should one part of a batch develop a fault. To trace such parts they are often marked with an identification code. This process is often completed using a marking machine that each part is placed into. However, the simple movement of each part from one machine to the next may result in damage to the part if it is dropped or accidently bumped into other objects. It is for this reason that CNC machines are able to perform more than one section of the machining process. Returning to the above example, a margin for error would be introduced to the machining operation if the milling was performed in a different machine to the drilling. 
     It is therefore desirable to mark each part as it is formed. However, as mentioned, the CNC machine environment is closed and contains fluid and debris. 
     EP1361960 describes a handheld marking device that may be used to mark a surface for permanent identification purposes. Such a device may therefore be used to mark each part formed by the CNC machine. This could be a manual or semi-automated, i.e. robotic, operation that takes place as each part is unloaded from the CNC machine and therefore, in some circumstances, eliminates the need for a static marking machine in which each part must be loaded. However, such an operation still requires transportation of the part from the CNC closed environment to an area accessible by the marking machine. 
     US2010/0275794 describes a CNC machine having a gantry on which is mounted tooling, to effect working on a workpiece mounted on a tooling table, and a carriage, holding a marking device. A marker controller operates the marking device to mark a workpiece with data derived from a CNC controller and from an HMI device. Such an arrangement is complicated but can be simplified. 
     It is an object of the present invention to address the problems identified above, or at least to mitigate their effects. 
     SUMMARY 
     In accordance with the present invention there is provided CNC machine comprising: a holder for a workpiece to be machined; a tool array comprising one or more tools; a machining arm having a first connector and adapted to select and use one of said one or more tools to machine said workpiece; and a marking device having a second connector and positionable in said tool array and selectable by said machining arm by interengagement of said first and second connectors, wherein said marking device comprises: a housing; a marking head mounted in said housing, said marking head comprising a marking tip being moveable in a marking direction for marking said workpiece; a controller for controlling said marking pin; and a wireless communicator adapted to receive instructions from a transmitter unit, said instructions being passable to said controller to instruct said controller to control said movement of said marking tip. 
     Said wireless communicator may comprise a wireless communicator such as, for example, a Bluetooth® module. The wireless communicator may be further adapted to transmit data indicating the current or historic movement of said marking head and/or said marking tip. 
     The machine may further comprise a computer adapted to transmit said instructions to said wireless communicator. Said computer may be further adapted to receive said data indicating the current or historic movement of said marking head and/or said marking tip. 
     Said wireless communicator might be pneumatically or hydraulically driven through connection of the machining arm to the marking device and under the control of a controller of the CNC machine that controls said machining arm. 
     The marking device may further comprise a support frame on which said housing is mounted substantially enclosing said support frame, said frame providing a mount for said marking head and said controller. The housing may comprise a body, a base and a lid, wherein said base and said lid are fixable to said frame and said body is positionable between said base and said lid, so that fixation of said base and lid to said support frame clamps said body and forms said housing, which is preferably fluid-sealed. 
     The marking tip is preferably moved by electromagnetic means. 
     The marking device may include its own power source, which may be activated and deactivated by connection to said machining arm of the marking device. 
     The housing may also comprise a window through which said marking head and/or marking tip protrudes, a gaiter being connected between said marking head and said window to fluid-seal for said window. 
     Preferably, the marking head is also mounted for translational movement in a marking surface under the control of said controller, said marking surface being substantially orthogonal with respect to said marking direction. 
     The marking surface may be planar and said translational movement may be effected by a first motor and a second motor wherein said first motor is adapted to move said marking head in a first direction in said marking plane and said second motor is adapted to move said marking head in a second direction in said marking plane, wherein said first direction is substantially perpendicular to said second direction. 
     Said first and second motors may be fixed in the support frame and drive belts that slide a marking head mount in said first and second directions. 
     Alternatively, said marking surface may be cylindrical and said translational movement is effected by a first motor and a second motor. The first motor may be disposed on a tilting frame pivoted in the support frame, the first motor being arranged to tilt the tiling frame in a first direction with respect to the support frame, and the second motor may be mounted on a slide mount slidable by operation of the second motor in a second direction substantially orthogonal with respect to said first direction, the slide mount mounting said marking head. 
     The invention also provides a marking device suitable for use in a CNC machine as defined above, the marking device comprising: a marking head mounted in a housing of the device, said marking head comprising a marking tip being moveable in a marking direction for marking said workpiece; a controller for controlling said movement of said marking pin; and a wireless communicator adapted to receive instructions from a transmitter unit, said instructions being passable to said controller to instruct said controller to control said movement of said marking tip. 
     The marking head may be mounted for translational movement in a marking surface under the control of said controller, said marking surface being substantially orthogonal with respect to said marking direction. The device may further comprise an on-board power source to power the controller and drive said marking head. The marking device may further comprise a pneumatically operated switch adapted for operation by a pneumatic source of a CNC machine to connect said power source to and disconnect said power source from the controller and marking head. 
     The invention also provides a method of marking a workpiece in a CNC machine comprising: a holder for said workpiece; a machining arm adapted to select and use a tool to machine said workpiece; and a marking device selectable by said machining arm, said marking device comprising a marking head having a marking tip being movable in a marking direction under the control of a controller, and a wireless communicator connected to said controller; wherein said method comprises the steps of:
         a) the CNC machine selecting said marking device using said machining arm;   b) the CNC machine positioning said marking device with respect to said workpiece;   c) communicating with said wireless communicator to initiate marking;   d) the controller moving said marking tip in said marking direction to produce a mark on said workpiece; and   e) optionally, repeating steps b), c) and d).       

     Said marking head may be movable in the marking device in a marking plane being substantially orthogonal with respect to said marking direction, the method further comprising the step of:
         f) before step d), the controller moving said marking head in said marking plane while retaining the position of said marking device.       

     The method may further comprise the step of:
         g) repositioning said marking device so that a camera of said marking device is able to obtain an image of said mark.       

     Step c) may further comprise activating a power source in the marking device. In which case, said activation of the power source may be effected pneumatically under the control of the CNC machine. 
     Said controller may receive instructions from a computer remote from said marking device outside of said CNC machine. The method may further comprise the steps of:
         i. transmitting a signal from said wireless communicator to said computer to indicate that said marking device has been activated;   ii. transmitting said instructions from said computer to said marking device;   iii. performing step d) and, optionally, step f); and   iv. transmitting a signal from said wireless communicator to said computer to indicate that said mark has been produced.       

     Optionally, step g) further comprises the steps of:
         i. said repositioning of said marking device so that a camera of said marking device is able to obtain an image of said mark;   ii. transmitting a signal from said computer to said marking device containing further instructions for said controller to operate said camera to obtain said image;   iii. transmitting said image from said controller to said computer; and   iv. said computer checking said image.       

     After checking the image, the computer may notify the CNC machine that the power source is to be deactivated. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, where: 
         FIG. 1  is perspective view of a marking device for a CNC machine according to an embodiment of the present invention; 
         FIG. 2  is a perspective view of a marking device for a CNC machine according to an embodiment of the present invention; 
         FIG. 3  is a perspective view of internal features of a marking device for a CNC machine according to an embodiment of the present invention; 
         FIG. 4  is a perspective view of a marking device for a CNC machine according to another embodiment of the present invention; 
         FIG. 5  is a side view of the marking device of  FIG. 4 ; and 
         FIG. 6  is a schematic illustration of a CNC machine in accordance with embodiment of the invention. 
     
    
    
     DESCRIPTION 
     The following description provides detailed examples of some embodiments of the present invention. 
     In  FIG. 6 , a CNC machine includes an enclosed room  610  in which is disposed a robotic machining arm  600  fixed in place on a floor  612  of the room  610 . The arm  600  is movable in multiple directions and orientations (x, y, z directions) with respect to a workpiece  630 , which may be disposed and fixed by a clamp on a work table  640 . The arm  600  has on its end a universal first connector  650 , arranged to interconnect with a second connector on a specific tool for performing a specific function (machining operation) with respect to the workpiece  630 . A tool array  620  locates a plurality of different tools  622 , each having a said second connector  624 . Each tool  622  is potentially for performing a different role, for example, drilling, milling, grinding etc. It is feasible that the tool array  620  may be located outside the room  610  and is only brought into the machining room  610  when a new tool is required. This is because machining frequently involves washing the workpiece with coolant during machining, which frequently leads to the room  610  becoming splashed with coolant that may be deflected by each tool as they operate on the workpiece. 
     The machining arm  600  operates under the control of a CNC controller  700 , located outside of the room but most likely hardwired to the arm by a line  710 . Other supply lines (not shown) may connect to the machining arm to supply hydraulic power, pneumatic power or electrical power to the arm, and to a tool  622  connected to the arm. 
     A marking device  10  comprises one of the tools  622 , in accordance with an embodiment of the present disclosure, which is selectable by the machining arm as and when required under the control of the CNC controller  700 . It also has a corresponding second connector  624  for engagement with the first connector  650 . As described below, the marking device may include an on-board power source that is activated when the device  10  is engaged by the arm  600 . It also includes a wireless communicator (described further below) that is adapted to receive signals from, and optionally transmit signals to, an external controller  800 , which may be a personal computer or other device having an antenna  810 . 
       FIG. 1  shows a marking device  10  for use by a machining arm of a CNC (computer numerically controlled) machine (not shown in  FIG. 1 ). The CNC machine, as known, comprises a means for holding a workpiece (for example, a clamp), a tool array and the machining arm, all within a closed environment. The environment is usually populated with fluids and debris formed as a result of the machining process. The marking device  10  is used to mark a workpiece machined in the CNC machine with a unique code. This often takes the form of a matrix of dots but other codes are within the scope of this invention. Such dots may be formed as impressions in the workpiece by a tip formed of a hardened material that is located over the workpiece in a predetermined location and then punched into the surface of the workpiece. 
     According to some embodiments of the present invention, the marking device  10  comprises a housing  12 , which forms a, preferably, fluid-sealed environment. A fluid resistant housing is potentially satisfactory since tool arrays in CNC machines (where the marking device  10  might be stored when not in use) are generally isolated from the area of the machine where machining of the workpiece takes place and a hot wet environment pertains. Also, during use of the marking device, any fluid lubricant or cooling supply would likely be disengaged. Hence absolute fluid-sealing is not essential. 
     A first connector  14  is positioned at the top end of the marking device relative to a marking head  16  as used to mark the workpiece. The connector preferably comprises a collar  14   a  adapted to engage with a second connector (not shown) on the machining arm (not shown) of the CNC machine. The machining arm further comprises locking means positionable inside lugs  14   b.  The marking device is storable in the array of tools inside the CNC machine. A reveal  14   c  (optionally in the form of a cylindrical groove) in the connector is adapted to be engaged by a forked bracket on which the marking device is stored after disconnection from the shaft of the machining arm. 
     The marking head  16  of the marking device  10  is mounted inside the housing  12  and is moveable in a marking plane (x, y). Movement within the plane is effected by movement in a first direction x and a second direction y, the second direction y being substantially perpendicular to the first direction x. Movement in the first direction x is effected by a first motor  18  and movement in the second direction y is effected by a second motor  20 . 
     The marking head comprises a marking tip  22 , which is used to mark the workpiece. The head  16  projects from the housing  12  through a marking window  24 . Marking is performed by moving the marking tip  22  in a marking direction z onto the workpiece using, for example, electromagnetic means such as a solenoid. Alternative means of moving the marking tip are described below. A power regulator  26  is also mounted inside the housing  12  and is used to supply electrical power to move the first and second motors  18 ,  20  and optionally the marking tip  22 . The electrical power may be supplied by the machine arm or could, as described below, be supplied by an on-board power source such as a battery. 
     In some embodiments, the marking device also comprises a frame  28  positioned inside the housing to provide a support for the housing and a mount for the marking head, first and second motors and the power regulator  26 . The frame may be of metal construction and may be of, but is not restricted to, a planar construction. The housing  12  comprises a base  30 , a cylindrical body  32  and a lid  34 . The base  30  and the lid  34  provide a clamp for the body  32  when the base and the lid are fixably attached to the frame  28 . Suitable fixings may be, for example, bolts or screws  36 . The clamping of the body  32  between the base and the lid provides a fluid-sealed housing. O-rings  38  provided at the junction between the base and the body, and the lid and the body enhance the fluid seal. A flexible gaiter (not shown) is attached between the window  24  and the marking head  16  so as to provide a fluid seal over the marking window region. 
     As mentioned above, the body may be cylindrical in shape wherein the combination of the frame and the clamping arrangement provides a strong rigid configuration. The frame provides a transfer of force from the marking tip as it impacts the workpiece to the machining arm. 
     Turning to  FIG. 2 , the marking device  10  further comprises a power source  40  enclosed within the housing  12 . The power source is preferably a rechargeable battery capable of supplying power to the first and second motors (see  FIG. 1 ) and, optionally, the marking head  16 . The battery is removably placed into a holder  44  via a battery window  45  positioned in the base  30  adjacent to the window  24  for the marking means. The battery window  45  is covered by a battery cover  46  which is held in place by screws  48 , screwable into the base  30 . An O-ring may be supplied around the battery window  45  to enhance the seal made between the cover  46  and the base  30 . 
     In some embodiments, the marking device comprises a controller  50  adapted to control the first and second motors to effect movement of the marking head. As used herein, a “controller” is something that directs or regulates something, such as, for example, a motor. A controller can be used in connection with software that directs the controller&#39;s function. A controller can include memory such as a random access memory chip. A controller can be a microcontroller (or MCU for microcontroller unit) that contains one or more CPU cores, and may include memory and programmable input/output peripherals or a similar programmable controller on a single integrated circuit, including a system on a chip or SoC, or state machine. 
     The controller may be a pre-programmed computer that is able to translate computer code data into a series of movement commands for the first and second motors. Furthermore, the controller controls the movement of the marking tip. In the example of the marking tip being movable using electromagnetic means, the controller may send a pulse signal to the power regulator, or direct to the electromagnetic means, to actuate the marking tip and therein punch the tip into the surface of the workpiece. 
     The marking device may also comprise a communicator  42 , such as a wireless communicator (for example, a Bluetooth® module) for receiving and optionally transmitting data. The data primarily comprises instructions (e.g. the computer code) for the controller that are used to instruct the controller to move the first and second motors  18 ,  20  and the marking pin  22 . The communicator  42  is shown to be placed proximal to the power source  40 . However, the communicator  42  can be located anywhere inside the housing. The communicator  42  is connected to the controller using cabling (not shown) connected at terminals  52 . However, it is understood that the controller and the communicator may be positioned on a single printed circuit board  54 , or the like. In another embodiment, the controller  50  may comprise the communicator. 
     In a preferable embodiment of the marking device, the base  30  comprises a camera window  56 . A camera  58  positioned inside the housing  12  so that a fluid seal is formed with the base  30 . The camera is used to check the mark produced by the marking head. 
     The CNC machine arm may comprise a pneumatic supply which is switchable between an on and an off state by the machine. In some embodiments, the marking device comprises a pneumatically operated switch. In these embodiments, connection of the marking device to the machining arm completes a pneumatic circuit wherein the switching of the pneumatic supply operates the pneumatic switch between an on- and an off-state. The pneumatic switch is used to control the power source so the switch turns the marking device on or off. This feature serves a purpose of reducing battery consumption when the marking device is not marking the workpiece. 
     Alternatively, a Hall-effect reed switch ( 31 —see  FIG. 1 ) may be disposed in the housing, which reed switch  31  is operated when the housing is moved into the vicinity of a magnetic field. The magnetic field may be provided by a simple magnet, for example magnet  33  in  FIG. 6 , disposed at some convenient location in the CNC machine, and towards which the machining arm can move the marking device  10  when the marking device is to be actuated. The reed switch may be arranged as a toggle switch, so that it may be employed also to deactivate the power source at the end of a marking operation. 
     The advantage of the latter is that no pneumatic connection is required, which in some CNC machines might not be provided. Also, the switch is entirely located within the housing so it is protected from the environment within the CNC machine. A different switch, however, could be operated in other ways, for example by mechanical operation through the machine arm  600  being arranged to press the housing incorporating a mechanical switch against a stop on the table  640  to actuate and de-actuate the switch. 
     That is, the marking device may comprise a switch that is actuatable by movement of the machining arm after connection of the marking device to the machining arm, and de-actuatable by corresponding movement when marking of the workpiece has been completed and prior to return of the marking device to the tool array. 
     In  FIG. 3 , the marking head  16  is disposed on a mount  17 , forming part of a moving frame  60 . The moving frame is slidably attached to a first slide  62  and a second slide  64 . The first and second slides are mounted on a frame mount  66 . The first slide  62  comprises a first slide bar  68  journalled to slide on a first slide runner  70 . An upper portion  72  of the first slide bar  68  is connected to a first belt  74 . The first belt  74  runs around an arrangement of pulleys  76  and around a drive shaft (not shown) of the first motor  18 . Likewise, the second slide comprises a second slide bar  78  journalled to slide on a second slide runner  80 . An upper portion  82  of the second slide bar  78  is connected to a second belt  84 . The second belt  84  runs around an arrangement of second pulleys  86  and around a drive shaft (not shown) of the second motor  20 . The configuration of the first and second belts allows rotation of the first and second motors to move the first and second slide portions respectively. Thus the moving frame  60  is movable in the x and y direction and therefore within the marking plane. 
       FIG. 3  further shows bores  90  in the frame  28  configured to receive screws  36  for fixing the lid (see  FIG. 1 ) to the frame  28 . The pneumatic connection mentioned above is shown in  FIG. 3  to comprise a first connector  92  suitable for attachment to the machining arm and a second connector  94  suitable for attachment to the pneumatic switch  96 . Preferably, a flexible pipe (not shown) joins the first and second connectors but the join may comprise any component suitable for use in a pneumatic system. 
     The pneumatic system may also be used to drive the marking tip. In this configuration, a switched valve, controllable by the controller, would be placed between the pneumatic supply and the marking tip. The supply may be taken from the pneumatic switch or directly from the first connector. The marking head, in this configuration, comprises a piston having a piston head movable in said piston and resiliently biased away from the workpiece. The marking tip is in turn connected to the piston so that on opening of the switched valve, the marking tip is pneumatically forced toward the workpiece and on closing of the switched valve, the marking tip retracts from the workpiece. 
     To create a mark on a workpiece, the CNC machine of any of the above embodiments performs the method of:
         a) selecting the marking device using the machining arm;   b) positioning the marking device over the workpiece;   c) moving the marking head in the marking plane while retaining the position of the marking device; and   d) moving the marking tip in the marking direction to mark the workpiece; and   e) repeating steps c) and d).       

     Steps a) and b) are performed by the CNC machine under its control (using its own controller,  700  in  FIG. 6 ). Steps c) and d) are performed by the marking controller. The marking controller may be in wireless communication with an external controller  800  in the form of a computer (see  FIG. 6 ) separate from the CNC machine which comprises the requisite software to translate desired marking shapes, such as a dot matrix, into suitable code that enables the marking controller to appropriately perform steps c) and d). Such steps may be repeated for each dot in the dot matrix. The CNC machine (CNC controller  700 ) may be arranged to notify the computer  800  when the CNC machine has performed steps a) and b) and the marking device is ready to be operated by the computer. Likewise, the computer  800  may be arranged to notify the CNC machine (CNC controller  700 ) when a marking operation has been completed and the next stage of part processing can be commenced. Generally, this may comprise moving the machined part to another location for processing elsewhere. 
     Where a camera is included in the marking device  10 , a step f) (before notifying the CNC machine that the marking operation has been completed) comprises: positioning the camera of the marking device over the mark made by the marking device  10  so as to obtain an image of the mark. This image may be communicated to the computer which is arranged to detect the image and ensure that it is readable and that what is read corresponds with what was intended. 
     It is to be understood that part of the reason for employing a marking controller and a marking head that is movable in the marking device in said marking plane is because CNC machines are not readily adapted to produce unique and variable movements at the touch of a button, or especially to move in a fashion to describe a unique and variable dot matrix code. Thus, the present invention requires only that the machining arm is held stationary with said marking plane held at an appropriate distance from, and substantially parallel to, a surface of the workpiece to be marked. Thereafter, the arrangement of the marks on the workpiece made by the marking pin is under the control of the marking controller in the marking device and/or the computer  800  in communication with it. 
     However, it may well be that such functionality may ultimately become readily available in CNC machines, in which event the marking head may be fixed in said marking device with the machining arm making the necessary movements, under the control of the CNC&#39;s controller, to trace the required pattern of movements for the marking device over the workpiece. Nevertheless, in many applications a simplified controller  50  and communicator will still be required because a solenoid actuated marking pin provides the cleanest and most accurate mark in the surface of the workpiece. In many instances, the workpiece will be a highly valued part in which additional stresses are to be avoided where necessary. A precisely weighted single punch to a precise depth ensures a minimum additional stress imposed on the workpiece. 
     Indeed, in its simplest form, the communicator could be a pneumatic connection where each pulse of air is sensed by the controller as an instruction to effect a punch once the CNC machine has moved the marking head to a desired position. In this arrangement, no external or separate computing is required, since the CNC machine is itself fully enabled. Other forms of communicator are envisaged, such as infra-red or a wi-fi connection, including Bluetooth®. 
     Until such time as CNC machines are rendered adaptable to move their machining arms in unique and varying patterns, a marking device with the full functionality described above will remain necessary. However, while the arrangement of  FIGS. 1 to 3  above works very well in principle, with the marking head constrained to move in single x, y plane, it does result in a fairly large bodied marking device because of the requirement for large motors  18 ,  20  driving the belts  84 ,  74  and the carriages  68 ,  78 . 
       FIGS. 4 and 5  show an alternative arrangement which is more compact because it employs the arrangement of its marking head as disclosed, for example, in WO02/055319. Here, the marking device  10 ′ likewise has a housing  12 ′ comprising base  30 ′ and lid  34 ′ with a cylindrical body  32 ′. A frame  28 ′ is in the form of a shallow U-shaped section having side walls  28   a, b  and back wall  28   c.  Side walls  28   a, b  extend the complete distance between the base  30 ′ and lid  34 ′ and are secured thereto by screws  36 ′. Back wall  28   c  extends part of the way up from the base  30 ′, terminating at  28   d  near the lid  34 ′ so that circuit boards  54 ′ may be retained in grooves  28   e  in the side walls  28   a, b,  and on which boards the same electrical components as described above with the embodiment  FIGS. 1 to 3  are mounted. 
     However, between the side walls  28   a, b  is pivoted, about journal axis  100 , a tilting frame  102 . Between arms  102   a, b  of the tilting frame  102  is fixed a first motor  18 ′ which has an armature (not visible) mounted around a first lead screw  104 . One end of the first lead screw is fixed axially (with respect to the lead screw) in back wall  28   c,  so that when the first motor  18 ′ operates, it screws itself up and down lead screw  104  so as to tilt tilting frame  102  about its journal axis  100 . 
     Independently, a second motor  20 ′ is likewise disposed around a second lead screw  106  mounted between the arms  102   a, b  of the tilting frame  102 . Around the second motor  20 ′ is a slide mount  17 ′ which slides on rails  110  on a base  102   c  of the tilting frame  102 . The slide mount  17 ′ carries the marking head  16 ′. When the second motor  20 ′ operates, it screws itself along the second lead screw  106  in the y direction, taking the marking head  16 ′ with it. Because the tilting frame pivots, however, the marking head moves in an arc when the first motor operates, so that the surface described by the marking pin when the motors operate is not planar but cylindrical. For most purposes, this is satisfactory. 
     Otherwise the marking device  10 ′ is essentially the same. It has a different first connector  14 ′, a simple on-off switch  111  that is manually operated and LED lights  112  that inform the user that the marking device is live and the state of the battery  40 ′. However, by employing the tilting arrangement smaller motors can be employed and so the tool has a smaller diameter. It can therefore be employed by more CNC machines. 
     Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to,” and they are not intended to (and do not) exclude other components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
     Although the present invention has been described in considerable detail with reference to certain preferred embodiments, other embodiments are possible. The steps disclosed for the present methods, for example, are not intended to be limiting nor are they intended to indicate that each step is necessarily essential to the method, but instead are exemplary steps only. Therefore, the scope of the appended claims should not be limited to the description of preferred embodiments contained in this disclosure. All references cited herein are incorporated by reference in their entirety.