Abstract:
This disclosure describes methods and systems for remote control of stroke length and frequency of percussion apparatus, such as a rock hammer drill. At a high level, the hammer drill is allowed to stay at a default low stroke length and high frequency to avoid applying excessive cyclic stress to the housing of the hammer drill and can be controlled to operate at a long stroke length and low frequency when the hammer drill has engaged the target material. The long stroke length and low frequency during operation can be initiated when a sufficient forward feed pressure is provided. While the hammer drill is idling or retracting, the forward fee pressure is not sufficient for the long stroke length operation and thus the drill operates at the default state and at a safe stress level to avoid premature damage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 62/199,670 filed on Jul. 31, 2015, which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to a percussion apparatus, in particular, related to remote control of stroke and frequency of a reciprocating component of the percussion apparatus. 
       BACKGROUND 
       [0003]    A percussion apparatus, such as hammer rock drills, are designed to deliver a repetitive impact in the axial direction of a rotating component (e.g., a drill bit). The axial impact forces the rotating component to engage a target material. In many instances however, when the percussion apparatus disengages from the target material, the repetitive impact continues and the percussion energy is then absorbed by the housing or other structures of the apparatus. This typically occurs when the apparatus is retracted or idling. This continuous repetitive impact negatively affects the life of the percussion apparatus as the absorbed energy causes fatigue in the housing or other structures of the apparatus. 
       SUMMARY 
       [0004]    This disclosure describes methods and systems for remote control of stroke length and frequency of percussion apparatus, such as a rock hammer drill. At a high level, the hammer drill is allowed to stay at a default setting of short stroke length and high frequency to avoid producing excessive cyclic stress to the housing of the hammer drill and can be controlled to operate at a long stroke length and low frequency when the hammer drill has engaged the target material. The long stroke length and low frequency during operation can be initiated when a sufficient feed forward pressure is provided. While the hammer drill is idling or retracting, the feed forward pressure is not sufficient for the long stroke length operation and thus the drill operates at the default state and at a safe stress level to avoid premature damage. 
         [0005]    In a first aspect, there is provided a method for controlling a percussion apparatus for an extended life of operation, the method including operating the percussion apparatus at a first stroke length and at a first frequency, wherein the first stroke length and the first frequency generate a low stress level to reduce fatigue in the percussion apparatus. The method further includes receiving a user selection for a second stroke length and a second frequency, wherein the second stroke length is longer than the first stroke length and the second frequency is lower than the first frequency such that a high stress level increases fatigue in the percussion apparatus when the percussion apparatus has yet engaged with an operation target. In addition, the method includes providing a feed forward pressure to a sliding selector controlling the piston hammer stroke length and the frequency according to the user selection and in response to an actuation input and in response to the feed forward pressure lower than a threshold level, maintaining the first stroke length and the first frequency. The method further includes that in response to the feed forward pressure higher than the threshold level, increasing the first stroke length to the second stroke length and reducing the first frequency to the second frequency. 
         [0006]    In other embodiments, the actuation input comprises a command to increase the feed forward pressure above the threshold value at a remote control unit. 
         [0007]    In still other embodiments, increasing the first stroke length and reducing the first frequency further includes translating a stroke selection piston biased by a resilient member. 
         [0008]    In other embodiments, the stroke selection piston continuously receives a biasing force from the resilient member for remaining in a default mode corresponding to the first stroke length and the first frequency until the feed forward pressure overcomes the biasing force and actuates the stroke selection piston. 
         [0009]    In yet other embodiments, the method further includes retracting the percussion apparatus at the first stroke length and the first frequency. 
         [0010]    According to a second aspect, there is provided a remote control system for reducing cyclic percussion stress, the remote control system including a percussion apparatus having a sliding selector biased toward a default setting. The default setting corresponds to a first stroke length and a first frequency of a reciprocating component, wherein the sliding selector includes a stroke selection piston operable to change the first stroke length and the first frequency. The apparatus further includes a cylinder having a hammer piston controlled by the sliding selector and a source providing a feed forward pressure to the sliding selector, wherein the feed forward pressure increases in response to a user selection of a second stroke length and a second frequency and an actuation input supplying the feed forward pressure to the sliding selector. The apparatus actuates the stroke selection piston when the feed forward pressure is greater than a threshold value. 
         [0011]    According to some embodiments, the source includes a motor feed drive regulated with a filter and pressure control unit. 
         [0012]    In still other embodiments, the apparatus further includes a valve bank for generating the actuation input and adjusting the feed forward pressure. 
         [0013]    In yet other embodiments, the valve bank is operated by a remote control unit. 
         [0014]    In still other embodiments, the apparatus further includes a plurality of control ports controlled by the sliding selector for increasing the piston hammer stroke length and reducing the frequency to facilitate a drilling operation. 
         [0015]    According to some embodiments, the sliding selector is set at the default setting in response to the percussion apparatus retracting or idling. 
         [0016]    In still other embodiments, the first stroke length and the first frequency of the hammer piston produce a cyclic stress level in the cylinder lower than a fatigue stress level; and the second stroke length and the second frequency of the hammer piston produce a cyclic stress level greater than the fatigue stress level in the cylinder. 
         [0017]    According to a third aspect, there is provided a percussion apparatus having a reciprocating component producing an axial impact on a rotating component, the reciprocating component being housed in a cylinder. The apparatus further includes a sliding selector and a resilient member applying a continuous force biasing a selection piston toward a default setting, the default setting corresponding to a first stroke length and a first frequency of the reciprocating component. The sliding selector changes the first stroke length and the first frequency in response to a feed forward pressure when the feed forward pressure exceeds a threshold value, the threshold value corresponding to a value of the continuous force that the resilient member acts on the selection piston to allow for selecting an operation setting of a second stroke length and a second frequency. 
         [0018]    According to some embodiments, the percussion apparatus further includes a primary housing enclosing the selection piston and a secondary housing enclosing at least a portion of the resilient member, wherein the secondary housing is affixed to the primary housing. 
         [0019]    In other embodiments, the primary housing has a plurality of control ports hydraulically connected to the cylinder of the reciprocating component. 
         [0020]    In still other embodiments, the percussion apparatus further includes a pressure relief valve for limiting the feed forward pressure. 
         [0021]    In yet another embodiment, the percussion apparatus is a hammer drill and the reciprocating component is a hydraulically actuated hammer piston. 
         [0022]    In still another embodiment, the first stroke length and the first frequency produce a cyclic stress level lower than a fatigue stress level; and the second stroke length and the second frequency produce a cyclic stress level greater than the fatigue stress level. 
         [0023]    According to other embodiments, the first stroke length is shorter than the second stroke length and the first frequency is correspondingly higher than the second frequency. In yet another embodiment, the sliding selector is operable to further select a third stroke length and a third frequency, the third stroke length has a value between the first and the second stroke lengths, and the third frequency has a value between the first and the second frequencies. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0024]      FIG. 1  is an illustration of a hydraulic percussion tool, in which a hydraulic pressure fluid circuit for remote control of the hydraulic percussion tool is employed to advantage. 
           [0025]      FIG. 2  is a schematic of a hydraulic pressure fluid circuit for remote control of the hydraulic percussion tool of  FIG. 1 . 
           [0026]      FIG. 3A  is a cross sectional side view of a sliding selector. 
           [0027]      FIG. 3B  is a cross sectional side view of a hammer piston and a rotating tool bit. 
           [0028]      FIG. 4  is a flow chart illustrating the method of remote control of stroke length and frequency of a percussion apparatus. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    This disclosure presents an apparatus, method, and system of remote control for reducing fatigue failures in percussion tools, such as, for example, rock hammer drills. In many instances, a percussion tool has a reciprocating component that generates repetitive impact to a tool bit, such as a drill bit that engages a target material (e.g., often a hard surface). The repetitive impact is designed to be absorbed by the target material during operation, but when the tool bit is not engaged with the target material, the repetitive impact is dissipated internally, often to the cylinder that houses the reciprocating component or associated housing structures. Such impact can result in fatigue in the housing and eventually cause fracture or other forms of structural failure, thus shortening the life of operation of the percussion tool. This disclosure addresses this problem by reducing the stress level when the tool bit has yet engaged the target material thereby extending the overall life of the equipment. 
         [0030]    Hydraulically controlling the hammer stroke length and the frequency is known. For example, U.S. Pat. No. 4,062,411, which is incorporated herein by reference in its entirety, discloses using hydraulic means to move a valve that controls piston hammer blows. This disclosure, however, focuses on remote control of a percussion apparatus such that the apparatus operates in a default setting or mode to protect the apparatus from fatigue even if a selection has been made for a long stroke length (and thus high stress level) setting until an engagement command is given. 
         [0031]    In one embodiment, a hydraulic powered rock drill has two modes for its hammer stroke: a first or short stroke mode having a short stroke with high frequency and a second long stroke mode having a long stroke with low frequency. The long stroke mode has increased impact power and impact force, but can increase the likelihood of fatigue failure in the tool housing when the tool is not engaged with operation target. It should be understood, however, that a different number of modes may be utilized. For example, in some embodiments, the hydraulic powered rock drill has three, four or even more modes for its hammer stroke. In embodiments disclosed herein, the rock drill defaults to the short stroke mode of operation to avoid and/or otherwise minimize stress levels causing fatigue on the equipment. In operation, when a user selects the long stroke mode, but does not operate the rock drill (such as controlling or otherwise positioning the drill forward), the stroke length and the frequency setting will remain unchanged. However, when a feed forward pressure is applied and when such pressure exceeds a predetermined threshold level, the mode will automatically change from the first or short stroke mode to the second or long stroke mode. Likewise, when a feed forward pressure falls below the predetermined threshold level, the mode automatically changes from the second mode to the first mode. Therefore and as discussed more fully below, when the rock drill is idling or is retracting, for example, excessive stress on the equipment is lessened thereby reducing the likelihood of fatigue failure. Detailed examples are discussed below. 
         [0032]      FIG. 1  is an embodiment of a hydraulic percussion tool  100 . The percussion tool  100  includes a percussion apparatus  120  positioned to operate on a target  105 . The percussion apparatus  120  can be, for example, a drifter, a hammer drill, or other type of device. A positioner  115  supported by a support  110  holds and otherwise places the percussion apparatus  120  in a desired position. The support  110  may be a mobile vehicle or a stationary structure and provides power for operating the positioner  115  and the percussion apparatus  120 . A remote control unit or terminal  140  controls the percussion apparatus  120  via connection with the support  110 . In some examples, the connection between the remote control unit  140  and the support  110  can be wired (e.g., via wires or cables); in other embodiments, the connection may be wireless (e.g., via wireless network). In operation, a user may use the control unit  140  onsite, such as at or near the support  110 , or may be operating off-site using appropriate network technologies. 
         [0033]    In the embodiment illustrated in  FIG. 1 , the percussion apparatus  120  includes at least one or more control line  135  and a drill bit  125  for engaging the target  105 . In some embodiments, the control line  135  is connected to the hydraulic power of the overall system including the support  110  and the positioner  115 . In other embodiments, the control line  135  may derive independent hydraulic power at the percussion apparatus  120  and be remotely controlled by the remote control unit  140 . 
         [0034]      FIG. 2  is a schematic view of a hydraulic pressure fluid circuit  200  for remote control of the hydraulic percussion tool  100  of  FIG. 1 . In the embodiment illustrated in  FIG. 2 , the circuit  200  is in fluid communication with the percussion apparatus  120 , which includes a sliding selector  201  that is biased toward and otherwise positioned in a default mode to operate in the short stroke mode such that a hammer piston  210  operates with a short stroke length and a high frequency. As illustrated and as explained in greater detail below, the hammer piston  210  reciprocates in a drill cylinder  212  and repetitively impacts with the drill bit  125  to operate on the target  105 . 
         [0035]    With continued reference to  FIG. 2 , the hydraulic pressure fluid circuit  200  further includes a hydraulic power source, such as a motor feed drive  237 , which provides a circulating pressure for the system. The circuit  200  further includes a filter and pressure control unit  235  that regulates the pressure output from the motor feed drive  237 . For example, the filter and pressure control unit  235  may include one or more filters, valves, and adjustment mechanisms for regulating the hydraulic power output from the motor feed drive  237 . A valve bank  230  in the circuit  200  enables a user to provide the actuation input via the remote control unit  140 . According to some embodiments, the valve bank  230  includes a lever  225  or other mechanism having similar functions, which is remotely controlled by the remote control unit  140 . The lever  225  is used by a drill operator to move the percussion apparatus  120  into contact with the target  105 , to retract the percussion apparatus  120  from the target  105 , and stop the motion of the percussion apparatus  120 . 
         [0036]    In  FIG. 2 , pressure relief or adjustment valves  213  and  215  are placed at various locations in the circuit  200  to limit or otherwise control the allowable hydraulic pressure in the circuit  200 . For example, the adjustment valve  215  is used to set an upper pressure limit for feed forward pressure in the control line  135 . In some embodiments, the valve bank  230  controls the feed forward pressure according to the remote control unit  140 . As described more fully below, the circuit  200  further includes a hydraulic return line  137  for the sliding selector  201  to return hydraulic fluids in the circuit  200 . 
         [0037]    In operation, a user operates the system to apply a feed forward pressure to the percussion apparatus  120 . For example, the user may first select a mode, which includes a working stroke length and frequency. The working stroke length is longer than the default stroke length, and the working frequency is lower than the default frequency for the hammer piston  210  in order to produce high impact loads. Further, the user may provide an actuation input, such as an operation at the remote control unit  140  to command a feed forward operation. In other embodiments, the actuation input may be provided in response to operation of the percussion apparatus  120 , such as pressing the drill bit  125  against the target surface  105 . In response to the actuation input, the feed forward pressure increases and becomes, as discussed in greater detail below, greater than a threshold value to change the mode of operation (i.e., the stroke length and frequency). 
         [0038]    Referring now to  FIG. 3A , a cross-sectional view of the sliding selector  201  of  FIG. 2  is illustrated. In the embodiment illustrated in  FIG. 3A , the sliding selector  201  includes a stroke selection piston  310  and a resilient member  330  that applies a continuous force against the stroke selection piston  310 , both being operable to change the stroke length and the frequency of the hammer piston  210  such that the percussion apparatus  120  is operable between the different modes of operation. In particular, the selection piston  310  is movable in an axial direction, as indicated by arrows  325 , to control the flow of fluid through a plurality of ports  312 ,  320 ,  322 , and  324 , which selects and/or otherwise configures the percussion apparatus  120  in the desired mode of operation (i.e., short stroke mode, long stroke mode or otherwise). In the embodiment illustrated in  FIG. 3A , the control ports  312 ,  320 ,  322 , and  324  are formed in a first housing  340  and hydraulically connected to the selection piston  310 . 
         [0039]    In the embodiment illustrated in  FIG. 3A , three options of the stroke length and the frequency combinations are provided, including a long stroke length at low frequency, a medium stroke length at medium frequency, and a short stroke length at high frequency. The impact loads due to the percussion decreases as the stroke length decreases and the frequency increases. In other embodiments, more than three stroke lengths and frequency combinations may be provided. In other instances, the variation of the stroke length may be continuous and the change of the operation frequency corresponds to the change of stroke length. In  FIG. 3A , the control ports  320 ,  322 , and  324  respectively correspond to a short stroke-high frequency setting (i.e., the default setting), a medium stroke-medium frequency setting, and a long stroke-low frequency setting (i.e., the operation setting). In some embodiments, there may be additional settings in between the default setting and the operation setting. In other instances, the medium stroke-medium frequency setting may be omitted. In the embodiment illustrated in  FIG. 3A , the sliding selector  201  includes a resilient member  330  extending from within the second housing  345  so as to apply a continuous force biasing the selection piston  310  toward the default setting (e.g., a short stroke length and a high frequency) of the hammer piston  210 . 
         [0040]    At default settings, such as when the percussion apparatus  120  retracts or idles, the sliding selector  201  operates so that the hammer piston  210  operates at the default short stroke length and the high frequency. The stroke length and the frequency generate reduced stress levels in the drill cylinder  212  and minimize fatigue therein. For example, the default stroke length and the default frequency of the hammer piston  210  produce a cyclic stress level in the cylinder lower than a fatigue stress level. Actual stress levels, however, depends on the material and scale of the drill cylinder  212 . By contrast, the operation stroke length and frequency of the hammer piston  210  may produce a cyclic stress level greater than the fatigue stress level in the cylinder, if the percussion apparatus  120  is not engaged with the target  105 . Therefore, the sliding selector  201  can effectively avoid accumulating fatigue inducing stresses by reducing the situations of producing high repetitive impact loads while the percussion apparatus  120  has yet engaged with feed forward operations. 
         [0041]    With continued reference to  FIG. 3A , the selection piston  310  and the resilient member  330  are respectively housed in the first housing  340  and a second housing  345 . The second housing  345  is sealingly secured to the first housing  340 . An exit port  350  is attached to the second housing  345  for recirculating the hydraulic fluid via the return line  137 . The selection piston  310  further includes a conduit  326  that allows fluids to flow through to recirculate the hydraulic fluids in the circuit  200 . During operation, the valve bank  230  ( FIG. 2 ) supplies the feed forward pressure through a line  220  to a port  301  on the first housing  340 . The adjustment valve  215  is hydraulically connected to the port  301  to limit the allowable feed forward pressure to be applied into the system. 
         [0042]    In operation, the feed forward pressure produces a force on a shoulder  305  of the selection piston  310 . When the pressure exceeds a threshold value that is equivalent to the force exerted by the resilient member  330 , the feed forward pressure pushes the selection piston  310  toward the exit port  350  and the selection groove  328 , an area that is formed of a reduced diameter on the sliding selection piston  310 , moves toward the second housing  345  to limit and/or otherwise restrict hydraulic flow through the port  324 . This change of fluid flow selects the setting for the hammer piston  210  to be operating in a mode other than the short stroke mode, such as the long stroke mode (i.e., operating at a long stroke length and a low frequency). 
         [0043]    In the present embodiment, the default short stroke mode produces a cyclic stress level lower than a fatigue stress level (e.g., when the resilient member  330  pushes the selection piston  310  into the first housing  340  such that the selection groove  328  opens to all three control ports  320 ,  322 , and  324 ). On the other hand, the long stroke mode of operation occurs when only the control port  324  is selected (i.e., open?) and can produce a cyclic stress level greater than the fatigue stress level if the reciprocating impact energy is not transferred to the target surface. 
         [0044]    By comparison, a conventional percussion apparatus  120  can have a reciprocating component acting at a fatigue stress level whenever the apparatus disengages from the work surface, such as when retracting the apparatus or leaving the apparatus idle. The percussion apparatus  120  avoids such constant high stress level by automatically setting the stroke of the hammer piston  210  at the default setting whenever the feed forward pressure is less than the threshold level. Thus, the sliding selector  201  effectively reduces fatigue in the percussion apparatus  120  and extends its operational life compared to conventional models. 
         [0045]      FIG. 3B  is a cross sectional side view of the hammer piston  210  and the rotating tool bit  125 . In particular,  FIG. 3B  illustrates an example configuration of the assembly of the percussion portion of the percussion apparatus  120 . The housing  365  encloses the hammer piston  210  and the drill bit  125 , wherein the rotating shank of the drill bit  125  receives repetitive impact from the hammer piston  210 . The hammer piston  210  is actuated by the pressure differences in the spaces  361  and  363 . For example, when the space  361  has a higher hydraulic pressure than that of the space  363 , the hammer piston  210  is actuated toward the drill bit  125 ; otherwise when the hydraulic pressure in the space  361  is lower, the hammer piston  210  is actuated away from the shank of the drill bit  125 . 
         [0046]    The differences and timing of the pressure variations in the spaces  361  and  363  are controlled with the stroke control plate  321  connected to the sliding selector  201 , which has been discussed in detail in  FIG. 3A . In some embodiments, the stroke control plate  321  includes a plurality of ports communicating with the ports  312 ,  320 ,  322 , and  324  of the sliding selector  201 . The stroke control plate  321  allows the assembly to react to the pressure changes as the stoke selection piston  310  moves to connect and disconnect the ports  312 ,  320 ,  322 , and  324 , varying percussion frequency and stroke length. Although  FIG. 3B  provides an example of receiving the control signals from the sliding selector  201 , other configurations are possible. 
         [0047]      FIG. 4  is a flow chart  400  illustrating the method of remote control of stroke length and frequency of a percussion apparatus  100  at lower stress levels to extend total operation life thereof. At step  410 , the percussion apparatus  100  is operated under a default selection of a first stroke length and at a first frequency. The first stroke length is relatively short and the first frequency is relatively high such that they generate a low stress level for avoiding fatigue in the percussion apparatus. 
         [0048]    At step  420 , a user selection is received about a second stroke length and a second frequency. For example, the second stroke length and the second frequency correspond to an operational setting that generates high reciprocating impact forces. The second stroke length is longer than the default stroke length, and the second frequency is lower than the first frequency. Therefore, when the percussion apparatus  100  has yet engaged with the target surface, the second stroke length and the second frequency can cause a high stress level resulting in an increased likelihood of fatigue in the percussion apparatus  100 . The setting selection would further require an actuation input to change the actual output parameters of the percussion apparatus  100 . The actuation input depends on the user operation on a remote control unit (e.g., commanding an increase of the feed forward pressure), or depends on an automatic increase of feed forward pressure in response to the apparatus engaging the target surface. 
         [0049]    At step  430 , a feed forward pressure is provided to a sliding selector  201  controlling the piston hammer stroke length and the frequency according to the user selection and in response to an actuation input. For example, a user may operate on a remote control unit to create the actuation input to a valve bank for adjusting the feed forward pressure. When the feed forward pressure is lower than a threshold value (e.g., wherein the feed forward pressure cannot overcome a biasing load of a resilient member, such as the resilient member  330 ), the percussion apparatus  100  maintains the first stroke length and the first frequency. For example, the stroke selection piston  310  continuously receives a biasing force from the resilient member for remaining at a default state corresponding to the first stroke length and the first frequency until the feed forward pressure overcomes the biasing force and actuates the stroke selection piston, as in step  410 . In some embodiments, when the percussion apparatus  100  is retracted, the retraction prevents the feed forward pressure from exceeding the threshold value and thus maintaining the stroke length and the frequency at the default setting. 
         [0050]    At step  440 , when the feed forward pressure exceeds the threshold value, such as when the actuation input relates to a feed forward command from the user, the length of the hammer stroke increases to the second stroke length and the frequency reduces to a second frequency. For example, at step  450 , the feed forward pressure translates a sliding selection piston  310  biased by the resilient member  330  to select the operational setting. The selection piston  310  then allows hydraulic flow through a control port for the work setting. 
         [0051]    In some embodiments, a medium setting may be selected to configure medium stroke lengths and medium frequencies as needed in different situations. In one embodiment, the pressure required for moving the selection cylinder is about 200 psi (14 bar). This pressure may be regulated by the hammer stroke selector pressure reducing valve, such has the valve  230  in  FIGS. 2 and 3 . In many examples, the feed forward pressure can reach about 600-1200 psi (41-48 bar) range. Thus, the pressure required to select the working stroke length (i.e., the long stroke) of about 400 psi is much less than the feed forward pressure. Other values of the feed forward pressure may be specified depending on the configuration and output of the percussion apparatus. 
         [0052]    In the foregoing description of certain embodiments, specific terminology has been resorted to for the sake of clarity. However, the disclosure is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes other technical equivalents which operate in a similar manner to accomplish a similar technical purpose. Terms such as “left” and right”, “front” and “rear”, “above” and “below” and the like are used as words of convenience to provide reference points and are not to be construed as limiting terms. 
         [0053]    In this specification, the word “comprising” is to be understood in its “open” sense, that is, in the sense of “including”, and thus not limited to its “closed” sense, that is the sense of “consisting only of”. A corresponding meaning is to be attributed to the corresponding words “comprise”, “comprised” and “comprises” where they appear. 
         [0054]    In addition, the foregoing describes some embodiments of the disclosure, and alterations, modifications, additions and/or changes can be made thereto without departing from the scope and spirit of the disclosed embodiments, the embodiments being illustrative and not restrictive. 
         [0055]    Furthermore, the disclosure is not to be limited to the illustrated implementations, but to the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the disclosure. Also, the various embodiments described above may be implemented in conjunction with other embodiments, e.g., aspects of one embodiment may be combined with aspects of another embodiment to realize yet other embodiments. Further, each independent feature or component of any given assembly may constitute an additional embodiment.