Patent Publication Number: US-2006013274-A1

Title: Laser unit

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a laser unit using circulating cooling water and, in particular, relates to a laser unit in which a way of control of the cooling water is improved.  
      2. Description of the Related Art  
      As is well known, circulating cooling water is used to avoid overheating, of a laser medium, which may occur during operation of a laser unit or during laser oscillation.  FIG. 1  shows a schematic configuration of such a laser unit of the prior art. As shown, a laser oscillator  1  is a device for laser oscillation by receiving power from a laser power supply  3 . The power supply  3  is controlled by a control part  2 . The control part  2  sends a laser output command to the power supply  3  such that the laser oscillator  1  outputs the laser beam.  
      The output laser beam is used for laser machining, for example. When the control part  2  sends a laser stop command to the power supply  3 , the laser oscillator  1  stops the laser oscillation to stop outputting the laser beam. At this point, the laser output command may include two type of commands, i.e., for a continuous oscillation and a pulse-like oscillation. In this specification, even in the latter case, the situation in which the pulse-like oscillation is being repeated is referred as “during laser oscillation”. The control part  2  has a not shown manual operating part, such as an operating panel having a button, a display and a keyboard, arranged on the control part  2  in a known manner. Therefore, an operator may operate the control part  2  to output commands from the control part  2 .  
      As is known, while the laser oscillator  1  outputs the laser beam, the gaseous or solid laser medium generates heat and the temperature of the medium and a peripheral member is raised. Therefore, the laser unit has a channel for cooling water (hereinafter called circulating cooling water) flowing within the laser oscillator  1 . By circulating the cooling water in the channel, overheating of the laser medium and the peripheral member may be avoided and the temperature during laser oscillation may be made stable.  
      In order to pass the circulating cooling water, a water conveying device  10  is used. The water conveying device  10  is activated by a drive unit  4  controlled by the control part  2 . The water conveying device  10  and the drive unit  4  are constituted, for example, by a combination of a pump and a drive inverter for the pump. The circulating cooling water is taken from a tank  12  and is delivered by the water conveying device  10 . The delivered water is divided into two flows. The water of one of the flows is purified by passing through a filter  11  for the cooling water and flows into the laser oscillator  1 . The water absorbs heat of the laser medium and the peripheral member, then, flows into a heat exchanger  13 .  
      The circulating cooling water is returned to the tank  12  after the circulating cooling water is heat-exchanged with primary cooling water in the heat exchanger  13 .  
      The primary cooling water is introduced into the laser unit from an inlet  20  and flows in a channel other than that of the circulating cooling water. Then, the primary cooling water is heat-exchanged with the circulating cooling water and flows out from the laser unit through an outlet  21 . In other words, within the heat exchanger  13 , channels for the circulating water and the primary cooling water separate from each other. The primary cooling water and the circulating cooling water do not mix.  
      On the other hand, the water of another flow different from the above two flows is returned to the tank  12 , after metal ions in the water, etc. are removed by passing the flow through an ion-exchange resin  14 . In this way, the circulating water may be circulated in a channel or a circuit.  
      It should be noted that, while the water conveying device  10  is running and the cooling water is circulating, the circulating water for cooling the laser oscillator  1  is purified by the filter  11  or the ion-exchange resin  14 , however, when the device  10  is stopped, the circulating water is no longer purified and does not cool the laser oscillator  1 .  
      In operation of the laser unit employing such a cooling system, an operating procedure shown in  FIG. 2  is used in prior art. First, before starting the laser unit, the water conveying device  10  is in a stop condition and the cooling water does not circulate (or is in a stopped condition). Starting from this condition, at time A 1 , the water conveying device  10  is activated by a command from the operator so as to circulate the cooling water (or change the condition to a conveyance condition). Next, the laser oscillation is started at time B 1 , by a command from the operator and is stopped, at time C 1 , by another command of the operator. Further, at time D 1 , the water conveying device  10  is also stopped by a command of the operator so as to stop circulating the cooling water (or return the condition to the stopped condition).  
      In many cases, the laser unit thus operated is no longer operated and, therefore, the cooling water is in the stopped condition until just before the next laser oscillation is started. After a time period from stopping to restarting of the circulation of the cooling water is passed, or at time A 2 , the water conveying device  10  is activated again by the command from the operator, whereby the cooling water circulates again. After that, at time B 2 , C 2  and D 2 , the laser oscillation, the stop of the oscillation and the stop of the circulation of the water is carried out, respectively, as described above, resulting in the stopped condition.  
      The problem in this case is that, when the device  10  is stopped, the circulating water is no longer purified by the filter  11  or the ion-exchange resin  14  and does not cool the laser oscillator  1 , as described above. Therefore, when a time period of the stopped condition is relatively long, mildew may appear on the filter  11  or the ion-exchange resin  14  and the bacteria may propagate in the still water, which may deteriorate the quality of the cooling water. When the laser oscillator is activated under a condition in which the quality of the cooling water is deteriorated, the laser output may be decreased.  
      So far, a simple technique, for economically avoiding the growth of mildew on the filter or the ion-exchange resin so as not to deteriorate the quality of the cooling water, has not been found in published documents.  
     SUMMARY OF THE INVENTION  
      Accordingly, an object of the present invention is to provide a laser unit for economically avoiding the growth of mildew on the filter or the ion-exchange resin and the deterioration of the quality of the cooling water which may occur during an idle period of laser oscillation.  
      The present invention is intended to solve the above problems, by intermittently circulating the cooling water during the idle period of laser oscillation.  
      More specifically, the present invention provides a laser unit comprising: a circuit for cooling water for cooling a laser medium; a water conveying device for conveying the cooling water such that the cooling water circulates within the circuit; a control device for controlling the conveyance by the water conveying device; a purifying device, arranged in the circuit, for purifying the cooling water circulating in the circuit; wherein the control device controls the water conveying device, such that during a period of laser oscillation, the conveying device continuously conveys the cooling water; and during an idle period of laser oscillation, the conveying device intermittently conveys the cooling water so as to alternate between a conveyance condition in which the cooling water circulates in the circuit and a stopped condition in which the cooling water does not circulate in the circuit.  
      The laser unit may further comprise a heat exchanger arranged in the circuit and a channel for flowing primary cooling water heat-exchanged with the cooling water in the heat exchanger. From an economical point of view, it is preferable that the flow of the primary cooling water is stopped in the conveyance condition during the idle period of laser oscillation.  
      It is preferable that the water conveying device conveys the cooling water having one time to twenty times of the volume of a tank arranged in the circuit for the cooling water, within at most 48 hours, in the conveyance condition during the idle period of laser oscillation.  
      It is preferable that the purifying device is a particle filter having a mesh size equal to or less than 25 micrometer.  
      The water conveying device may be built into the body of the laser unit. Alternatively, the water conveying device may be located outside the body of the laser unit.  
      The stopped condition may be changed to the conveyance condition when a first predetermined period of time has passed from the beginning of the stopped condition. Also, the conveyance condition may be changed to the stopped condition when a second predetermined period of time has passed from the beginning of the conveyance condition.  
      The laser unit may further comprise a measuring device for measuring an electric conductivity of the cooling water and an ion-exchange resin arranged in the circuit for removing metal ions in the cooling water. In this case, it is preferable that the control device controls the water conveying device so as to control a flow rate passing through the ion-exchange resin such that the electric conductivity of the cooling water measured by the measuring device does not exceed a predetermined upper limit value. This upper limit value is advantageously equal to 3 micro-Siemens/cm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and advantages of the present invention will be made more apparent by the following description of the preferred embodiments thereof, with reference to the accompanying drawings, wherein:  
       FIG. 1 a  diagram showing an example of the configuration of a laser unit of prior art;  
       FIG. 2  is a diagram showing an operating procedure of the laser unit shown in  FIG. 1 ;  
       FIG. 3  is a graph indicating the growth of bacteria during a conveyance-stop period and the decrease of bacteria after restarting the conveyance of water;  
       FIG. 4  is a graph indicating a change of a number of bacteria during the intermittent conveyance per day of the water;  
       FIG. 5  is a graph indicating a dependency of an asymptotic number of bacteria on a filter mesh size when the intermittent conveyance per day of the water is carried out for a long time;  
       FIG. 6  is a diagram showing an operating procedure of a laser unit in which the cooling water circulates during an idle period of laser oscillation;  
       FIG. 7  is a diagram showing a schematic configuration of a laser unit according to one embodiment of the invention;  
       FIG. 8  is a diagram showing a schematic configuration of a laser unit according to another embodiment of the invention;  
       FIG. 9  is a diagram showing an operating procedure of the laser unit shown in  FIG. 7 ;  
       FIG. 10  is a flowchart for explaining a process of an intermittent conveyance mode of the embodiment of the invention; and  
       FIG. 11  is a diagram showing a schematic configuration of a laser unit according to still another embodiment of the invention, in which a water conveying device is located outside a body of the laser unit. 
    
    
     DETAILED DESCRIPTIONS  
       FIG. 3  is a graph indicating a relation between the number of bacteria and the circulation of the cooling water, concretely, an experimental result of the growth of bacteria when the cooling water does not circulate or the conveyance-stop period and a decrease of the bacteria after restarting the circulation of water. In this graph, an abscissa indicates a time after the conveyance of the cooling water is stopped and, an ordinate indicates the number of the bacteria per unit volume of the cooling water (a.u.). In other words, the graph indicates a change of the number of the bacteria detected by a microscopic examination. By the graph, the following points may be understood:  
      (1) The number of the bacteria increases from the stop of the conveyance of the water to the restart of the conveyance or after 72 hours.  
      (2) In particular, the number of the bacteria remarkably increases after 48 hours from the stop of the conveyance.  
      (3) The number of the bacteria decreases after the restart of the conveyance or the circulation of the cooling water.  
       FIG. 4  is a graph indicating an experimental result of a change of a number of bacteria in the laser unit. In the graph, an abscissa indicates a number of days from the start of the experiment and an ordinate indicates the number of the bacteria (counted as the number of colonies of the bacteria; hereinafter, the colony number is also used). In other words, the graph also indicates a change of the detected number of the bacteria. By the graph, the following points may be understood:  
      (4) When the conveyance is carried once time a day, the number of the bacteria reaches to a constant asymptotic number.  
      (5) The constant asymptotic number depends on a filter mesh size. As the mesh size is smaller (or the mesh is finer), the asymptotic number is lower.  
      As described above, when the conveyance is carried once time a day, the growth of the bacteria may be limited corresponding to the filter mesh size. In this regard,  FIG. 5  shows a graph indicating a relation between the filter mesh size and the asymptotic number of the bacteria colony (or the number of the bacteria in a stationary state), when the conveyance is carried once time a day or intermittently.  
      In addition, it could be understood by the experiment that the number of the bacteria cannot be controlled when the number is larger than 8,000 per unit. That is, in a condition that the conveyance of the cooling water is carried once time a day, the above asymptotic number of the bacteria is difficult to find if the number of the bacteria colony exceeds 8,000 per unit. As shown in  FIG. 5 , the filter mesh size corresponding to “the bacteria number of 8,000 per unit” is 25 μm. In other words, in the condition that the conveyance is intermittently carried once time a day, the growth of the bacteria may be limited by using a filter having the mesh size less than 25 μm.  
      A simple way for avoiding the deterioration of the quality of the cooling water during the conveyance-stop period is a continuous circulation, by operating the water conveying device, even when the laser is not used. That is, as in a process shown in  FIG. 6 , when the cooling water always circulates, the quality of the cooling water may be kept in a good condition, whereby the growth of mildew on the filter or the ion-exchange resin may be avoided. However, this way is not obviously economic. Therefore, one embodiment in which the cooling water may be economically and suitably purified is described below.  
      One embodiment according to the invention is described below, with reference to  FIGS. 7-10 . The component which is the same as described in  FIG. 1  is denoted by the same numeral as in  FIG. 1  and, the repeated description of the component is properly omitted.  
      In  FIG. 7 , a schematic configuration of a laser unit according one embodiment of the invention is shown. In comparison  FIG. 7  with  FIG. 1 , it is obvious that the configuration of the laser unit may be the same as a conventional laser unit. However, a control part  30  is used instead of the control part  2  in  FIG. 1 , as the content of controlling the water conveying device  10  is different, as described below.  
      That is, a laser oscillator  1  is a device for oscillating a laser beam by receiving a power by a laser power supply  3 . The power supply  3  is controlled by a control device or a control part  30 . The control part  30  sends a laser output command to the power supply  3  such that the laser oscillator  1  oscillates and outputs the laser beam. The output laser beam is used for laser machining, for example.  
      When the control part  30  sends a laser stop command to the power supply  3 , the laser oscillator  1  stops the laser oscillation to stop outputting the laser beam. At this point, the laser output command may include two type of commands, i.e., for a continuous oscillation and a pulse-like oscillation. As described above, even in the latter case, the situation in which the pulse-like oscillation is being repeated is referred as “during laser oscillation”. The control part  30  has a not shown manual operating part, such as an operating panel having a button, a display and a keyboard, arranged on the control part  30  in a known manner. Therefore, an operator may operate the control part  30  to output commands from the control part  30 .  
      The laser unit has a channel, for cooling water flowing within the laser oscillator  1 , in order to avoid overheating of the laser medium and a peripheral member. In order to pass the circulating cooling water, a water conveying device  10  is used, which is activated by a drive unit  4  controlled by the control part  30 . The water conveying device  10  and the drive unit  4  are constituted, for example, by a combination of a pump and a drive inverter for the pump. The circulating cooling water is taken from a tank  12  and is delivered by the water conveying device  10 . The delivered water is divided into two flows. As described with reference to  FIG. 1 , the water of one of the flows is purified by passing through a purifying device or a filter  11  for the cooling water and flows into the laser oscillator  1 . The water absorbs heat of the laser medium and the peripheral member, then, flows into a heat exchanger  13 .  
      The circulating cooling water is returned to the tank  12  after heat-exchanged with primary cooling water in the heat exchanger  13 . As described above, the primary cooling water is introduced into the laser unit from an inlet  20 , then, the water is heat-exchanged with the circulating cooling water and flows out from the laser unit through an outlet  21 .  
      The water of the other flow of the above two flows is returned to the tank  12  after removed metal ions in the water, etc., by passing through an ion-exchange resin  14 . The reason for removing metal ions is that all metal ions cannot dissolve in the cooling water when many metal ions are included in the water (or the electric conductivity of the water is high), whereby the metal ions may precipitate on the heat exchanger or the particle filter, resulting in the drop of the efficiency of heat exchange or the decrease of the flow rate of the cooling water by the constriction of the channel.  
      The drop of the efficiency of heat exchange may cause malfunction of the laser oscillator. Also, the decrease of the flow rate of the cooling water may change a suitable control time T 1  previously calculated by using the flow rate. It has been found, by experiment, that the electric conductivity of the cooling water must be less than 3 μS/cm to permanently operate the system. As a simple way to suitably keep the electric conductivity, a monitor unit  16  for measuring the electric conductivity of the water and an alarm device for indicating when the measured value exceeds a predetermined threshold value less than 3 μS/cm, may be used.  
      Further, in order to suitably keep the quality of the water, an electromagnetic valve  15  may be positioned upstream or downstream of the ion-exchange resin  14 . When the electric conductivity of the cooling water measured by the monitor unit  16  is larger than the predetermined threshold value, the electromagnetic valve  15  is opened to pass the water into the resin  14  so as to lower the electric conductivity. When the electric conductivity of the cooling water measured by the monitor unit  16  is less than the predetermined threshold value, the electromagnetic valve  15  is closed to not pass the water into the resin  14  and not to further lower the electric conductivity.  
      The cooling water thus flows and circulates in the channel of the circuit. Such a configuration is merely one example and may be variously changed. For example, the outlet  21  may be positioned on the circuit, or, the positions of the resin and the filter may be changed.  
      The ion-exchange resin and the particle filter are exemplary components included in a purifying means for the cooling water. Generally, other types of purifying means may be arranged. The heat exchanger also may have another constitution capable of heat exchanging with room air, instead of the constitution using the primary cooling water. Further, water conveying device may be positioned outside of the laser unit, as described below.  
      Next, an operation of the laser unit is described.  FIG. 9  shows a time chart of an example of a procedure of the operation. The operation from time A 1  to D 1  may be the same as the described operation regarding prior art. For example, the water conveying device  10  is activated at time A 1 , by a command of the operator and, then, the laser oscillation is started at time B 1 . Next, the laser oscillation is stopped at time C 1  and the water conveying device  10  is also stopped at time D 1 . After that, it is assumed that the laser oscillation is stopped for a considerably long time (for example, due to a summer vacation). In addition, it is economically advantageous to stop the flow of the primary cooling water in the channel when the laser oscillation is stopped.  
      The control part  30  activates a timer in the laser unit at this point or time D 1 . Then, after passing a predetermined period R 1  less than 48 hours, the control part  30  outputs an operation command to the water conveying device  10  without receiving any command from the operator. On receiving the operation command, the water conveying device  10  automatically restarts conveying of the cooling water. At this point, the conveyance of the primary cooling water may be restarted.  
      As a matter of convenience, a j-th start command of automatic water conveyance during the idle period of the laser oscillation is called as “conveyance start command j”. Therefore, at the first time (j=1), “conveyance start command  1 ” is outputted.  
      After a period of time T 1 , required for the water conveying device to circulate the cooling water having a predetermined volume larger than one time and smaller than twenty times of the volume of the tank  12 , the control part  30  outputs a stop command to the water conveying device  10  without receiving any command from the operator. If the primary cooling water is used, it is economically advantageous to stop the flow of the primary cooling water at the same time.  
      As a matter of convenience, a k-th stop command of automatic water conveyance during the idle period of the laser oscillation is called as “conveyance stop command k”. Therefore, at the first time (k=1), “conveyance stop command  1 ” is outputted.  
      One cycle including the start and stop of automatic water conveyance during the idle period of the laser oscillation is thus completed. After that, a sequence including the similar output command by the control part  30  is executed, without receiving any command from the operator. In other words, the sequence progresses as below: conveyance stop command  1 →passing of a period of time R 2 →conveyance start command  2 →passing of a period of time T 2 →conveyance stop command  2 →passing of a period of time R 3 →conveyance start command  3 →passing of a period of time T 3 →conveyance stop command  3 → . . .  
      In the above example, after the conveyance stop command  3  is outputted, the operator manually inputs the conveyance start command to activate the water conveying device  10 . In this case, the sequence is reset (or a timer within the control part  30  is reset). Then, at time B 2 , C 2  and D 2 , the laser oscillation, the stop of the laser oscillation and the stop of the water conveying device are sequentially carried out, respectively, similarly to the above. After that, the timer within the control part  30  restarts and the above sequence for the idle period of the laser oscillation is restarted.  
      In this embodiment, when the water conveying device  10  is stopped according to the stop of the laser oscillation, the sequence for the idle period of the laser oscillation is executed such that the conveyance condition and the stopped condition are alternately repeated. This alternation is continued until the operator stops the sequence by, for example, an interrupt operation. Therefore, by suitably predetermining the parameters of the sequence R 1 , T 1 , R 2 , T 2  . . . , various disadvantages induced by the long idle period of the laser oscillation, such as the deterioration of the quality of the cooling water, may be avoided with a relatively small consumption energy.  
      The time parameters R 1 , R 2 , R 3 , R 4  . . . and T 1 , T 2 , T 3 , T 4  . . . used in the embodiment may be predetermined in various ways and, optimum values may be determined as design matters. As an exemplary way to simplify the sequence, R 1 =R 2 =R 3 =R 4 = . . . =a constant value (a first constant period), T 1 =T 2 =T 3 =T 4 = . . . =a constant value (a second constant period), or the combination thereof may be possible.  
      Further, taking into account the characteristic of the above change of the number of the bacteria, it is preferable that the time parameters R 1 , R 2 , R 3 , R 4  . . . and T 1 , T 2 , T 3 , T 4  . . . are determined such that the water conveying device conveys the cooling water having an equal volume to the volume of the tank  12 , within at most 48 hours, even during the idle period of laser oscillation. At this point, the lower flow rate of the cooling water is preferable from an economical point of view. Further, the efficiency of limiting the growth of the bacteria is not raised when the flow rate is extremely high. In this sense, the flow rate of the cooling water substantially has an upper limit. As one benchmark for the upper limit, the conveyed volume per 48 hours, which is less than twenty times of the volume of the tank  12 , is possible. Obviously, it is possible to adjust the conveyed volume by adjusting the power of the pump.  
      The values of the parameters R 1 , R 2 , R 3 , R 4  . . . and T 1 , T 2 , T 3 , T 4  . . . are previously inputted in a memory of the control part  30  and a sequence executing means of the control part  30  executes the sequence by using the values. A known control circuit, capable of executing such a sequence regarding the above time schedule, may be used as the sequence executing means. Alternatively, when the control part  30  includes a CPU, software for executing the above sequence may be used. One example of the sequence is shown in  FIG. 10 , where R 1 =R 2 = . . . =R (the first constant period) and T 1 =T 2 = . . . =T (the second constant period). The key point of operation at each step of the sequence is explained below.  
      The program for executing the sequence is activated every time when the operator manually inputs the conveyance stop command.  
      Step S 1   
      A flag F representing the automatic intermittent conveyance mode is set to “1” (i.e., the intermittent conveyance mode is valid).  
      Step S 2   
      The timer is cleared and set to zero.  
      Step S 3   
      The cleared timer is started.  
      Step S 4   
      The flag F is checked. If F=1, the process progresses to step S 5 . If F=0, the process is terminated. The flag F is changed to “0” (the intermittent conveyance mode is invalid) by a manual operation of the operator when the laser oscillation should be restarted.  
      Step S 5   
      It is checked whether the timer&gt;R. If the timer&gt;R, the process progresses to step S 5 . Otherwise, the process returns to step S 4 .  
      Step S 6   
      The water conveying device is activated to cause “the conveyance condition”.  
      Step S 7   
      The timer is cleared.  
      Step S 8   
      The timer is started.  
      Step S 9   
      The flag F is checked. If F=1, the process progresses to step S 10 . If F=0, the process is terminated.  
      Step S 10   
      It is checked whether the timer&gt;T. If the timer&gt;T, the process is returned to step S 2  after the water conveying device is stopped (Step S 11 ). If the timer&lt;T, the process is returned to step S 9 .  
      The above sequence is repeated to intermittently convey the water, during, for example, night, week end or summer vacation, i.e., when the operation of the laser unit is stopped. Therefore, the deterioration of the quality of the water, the growth of the bacteria and/or the generation of mildew may be avoided or reduced. Further, the sequence requires less consumption of energy (normally, electric power) and thus is economically advantageous in comparison with a sequence in which the water conveying device  10  continuously runs.  
      When the operator changes the flag F from “1” to “0” after a vacation, for example, the change is recognized at succeeding step S 4  or S 9  and the laser unit is operated in a normal mode (i.e., the intermittent conveyance mode is invalid). Then, the normal operation of the laser unit including the laser oscillation and the conveyance of the cooling water is started by, for example, a manual input of the operator. After the normal operation ends and the operator stops the laser oscillation and the conveyance of the water, the program for executing the above sequence is started and the intermittent conveyance mode becomes valid (F=1). This cycle is repeated.  
      Finally, a layout of devices regarding the conveyance of the water, particularly the water conveying device  10 , will be briefly described. In the configuration shown in  FIG. 7 , the drive unit  4  for the water conveying device, the water conveying device  10 , the filter  11  for the cooling water, the tank  12 , the heat exchanger  13 , the ion-exchange resin  14  and pipes for connecting them are arranged in the laser unit and the inlet  20  and the outlet  21  for the primary cooling water serve as the boundary between the laser unit and the outside thereof. The laser unit thus configured may be bulky. In particular, it may be convenient that the water conveying device  10  using a pump or the like is located outside of the body of the laser unit.  
       FIG. 11  shows an example in which the water conveying device is located outside. In  FIG. 11 , the boundary between the body of the laser unit and the outside thereof is indicated using a broken line. As shown, the drive unit  4  and the water conveying device  10  are located outside of the body of the laser unit  100 (indicated by the broken line). An inlet  101  and an outlet  102  of the body of the laser unit are configured to allow the cooling water to flow into and from the channel in the body of the laser unit through the inlet  101  and the outlet  102 .  
      Although the filter  11 , the tank  12 , the heat exchanger  13  and the ion-exchange resin  14  are not shown in  FIG. 11 , these components are also located outside of the body of the laser unit. The description of the operation including the intermittent conveyance is omitted as it is similar to the above example.  
      According to the laser unit of the present invention, the deterioration of the quality of the water, which may occur during the idle period of the laser oscillation, is avoided by intermittent conveyance of the water. Therefore, the reliability of the laser unit may be economically improved.  
      While the invention has been described with reference to specific embodiments chosen for the purpose of illustration, it should be apparent that numerous modifications could be made thereto, by one skilled in the art, without departing from the basic concept and scope of the invention.