Abstract:
The invention pertains to an improvement for a tracking and focusing servo and defect management circuit that controls the tracking and focusing of an optical head on an optical media in an apparatus for reproducing a data, sound or image recorded on the optical media. The invention characterizes by the provision of a circuit for managing modulation of an RF signal from the optical pickup head caused when the optical pickup head encounters a non-continuous track in segment arrangement or a very long extended defect such as a extended flaw in the disc resulted discontinuous track segment thereon, and an adjusting circuit which manages the loop gain of the tracking servo circuit per the track arrangement of defect detection and a tracking profile, whereby the responsiveness of the optical pickup head is improved and track skipping prevented or recovered.

Description:
This application claims priority to pending U.S. provisional patent application entitled OPTICAL DISK TRACKING SERVO CIRCUIT ENABLED TO COMPENSATE FOR NON-CONTINUOUS TRACK SEGMENTS OR PROLONGED DEFECT filed Jun. 3, 2004 by Francis King and accorded Ser. No. 60/577,237, the benefit of its filing date being hereby claimed under Title 35 of the United States Code. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to systems and method for control an optical pickup head for reading data from and writing data to data storage medium. More particularly, this invention is related to an improved method for optical disk tracking servo and focusing servo circuits enabled to compensate for non-continuous track segments or prolonged defect data tracks. 
   2. Description of the Prior Art 
   Conventional techniques of servo control and the compensation for errors that occurs during the processes of reading data from a data track are limited by an intrinsic assumption of these compensation algorithm that the servo control is relying on a normal track data-reading feedback to restore a normal operational mode. Without such a normal feedback signal, the whole servo control scheme fails and the data reading process stops. Conventional technologies of servo control implemented for controlling the pickup heads are therefore unable to deal with a data-recording medium that has a large defective area covering a length of the data recording tracks exceeding certain threshold length or discontinued data tracks where a pickup head is caused to wait indefinitely for the normal feedback signals before a normal servo control mechanism can function properly. 
   Specifically, optical discs are recording media for storing data, e.g., video and/or audio information, on their surfaces in the form of spirally track with pits and land information. The reproduction of recorded information on such optical discs is through an optical pickup. The optical pick-up shines a light ray, e.g., a laser beam, to track along the spiral-recording track and converts the reflected light into an electrical signal. A servo control mechanism is implemented to control the motion and positioning of the pickup head. The control mechanism is provided to control the objective lens or a mirror of the optical pickup device when moved by a stepping motor with lead screw or a linear motor to position the pickup head on the right track with precisely controlled focus on the data track to retrieve the data from a designated point on a designated data track. 
     FIG. 1  shows a typical conventional optical data storage and access system. An optical pickup head  102  projects a laser beam on a rotating optical media  101 . An optical signal from the optical medium  102  is reflected and received by the optical pickup head (OPU)  102  that converts the reflected light from the optical media  101  into electrical signals. A radio frequency (RF) circuit  103  produces a tracking error (TE) signal and a focusing error (FE) signal to the respective tracking servo and focus servo processing circuits  104  and  105 . Each servo circuit receives error signals and applies the gain control and phase compensator to feed the control voltage to stepper or linear motor driver  118  and OPU  102  actuator driver. The servo management process (SMP)  109  sets the servo gain and receiving control profile for tracking and focusing from the OPU  102  drivers, e.g., the focusing driver  113 , the tracking driver  115 , the spindle motor driver  116 , the stepper/liner driver  118  and the spindle motor  117 , for further processing. Referring to  FIG. 1  again, the tracking servo circuit  104  receives the tracking error (TE) as a decoded signal for inputting to the SMP  109 . The SMP  109  then generates servo control correction signals to the tracking driver  115 . Similarly, the focus servo circuit  105  receives the focusing error (FE) as a decoded signal for inputting to the SMP  109 . The SMP  109  then generates focus control correction signals to the focusing driver  113 . The conventional servo control system as shown in  FIG. 1  is not able to handle large area defects or data read/write operations for data stored in the discontinued data track segments due to the limitation that a convention servo control must use servo feedback or reflected optical laser beam data unavailable at these areas. 
   Typical tracking servomechanism control methods are the so-called 3-spot method, push-pull method and Differential Phase Detection (DPD) method. A tracking or focusing error is determined from the intensity of reflected light or the like position on an optical disc by a spot light ray from an optical pickup. There are different techniques disclosed by several prior art patents to manage and correct the errors during the data reading process. Emoto disclosed in U.S. Pat. No. 4,687,916 “Dual beams Optical pick-up device for both focus and error tracking detection”. In U.S. Pat. No. 4,703,468, Baba, et al. “Optical disc tracking servo circuit having compensation for disc defects and external disturbances” and U.S. Pat. No. 4,722,079, Matsumoto “Optical disk player capable of distinguishing external disturbances and local defects and adjusting servo gain accordingly” disclosed schemes to separate defects and external disturbance and servo corrections. In United States patent application 20010055247 Tateishi, Kiyoshi; et al. “Servo control apparatus for optical disc driver”. In a United States Patent Application Publication 20030223335 Chen, Chih-Yuan “Method of defect detection for optical disc drives”, and United States Patent Application 20030103425 Shidara, Kiyoshi “Optical disk apparatus” disclosed enhanced methods to improve the tracking servo control. In all cases, when the external disturbance is removed, the servo control circuit still expects to have the normal tracking feed back signals. 
   If the track defect is very long, see  FIG. 3A , even with the defect mapping management method to map out the defect area to a different location, the available servo control methods can fail to keep or recover the optical pick-up to follow the prescribed track across the defect area and results a useless media. See  FIG. 3 , the track arrangement is non-continuously in track segments, the available servo control methods fails to keep the optical pick-up to follow the prescribed track across none available track area since no servo feedback signal is available. 
   Therefore, a need still exists to provide an improved data access device and CDROM, CDR, DVD and other data-card storage configurations that are compatible with the credit card size standard to process and store data therein such that more data can be available for card user authentication applications to overcome the above-mentioned difficulties and limitations. 
   SUMMARY OF THE PRESENT INVENTION 
   Therefore, an object of this invention is to provide new servo control circuits and control algorithms to overcome the above-discussed problems and limitations. The servo control circuits and control methods are provided to control the pickup head to continue a stable operation even when data tracks have a prolonged defect or the data tracks are non-continuous track segments. 
   Specifically, in order to achieve the objectives, this invention discloses an optical disc apparatus with a spindle motor for driving an optical pick-up (OPU) with servo control circuits. The servo control circuits receive control signals to control an optical beam and to drive an actuator for controlling a position of the optical beam on an optical disc. The servo control circuit comprising: a tracking error (TE), a Focusing error (FE) processing and defect detecting arrangement for detecting a defect or disturbance on the basis of reflected light of the optical beam from the optical disc. The servo control circuits further include an index or spindle FG input timing counter circuit, a servo tracking profiling circuit, memory devices for store timing and defect information, a storing device for tracking profile, error threshold adjusting circuits, tracking servo modulation circuits, focusing servo modulation circuits and servo management processor (SMP). 
   Defects occurring on account of dark spots, damages or others on the optical disc are detected from reflected light of an optical beam from the optical disc. When TE or FE detects an error signal that reaches the defect thresholds as that determined by the SMP, and additionally when the error signal detection can be repeated at the same location, a defect is recognized. The low gain at servo tracking is set to compensate servo tracking. The defect location timing and type is stored in memory. The servo control circuits are provided to deal with different kinds of defects. Defect type  1  considered a short and no extra servo modulation is needed. The servo control circuits manage a type- 2  defect when the defect time is long. Additional servo tracking modulation with tracking profile becomes necessary. As TE or FE reaches a different threshold set by the SMP and does not repeat at the same location, an external disturbance is recognized. A high gain servo tracking is set. 
   For servo control to access data recorded on non-continuous data track segments, a starting track index and an ending index are provided to define the active track region. When SMP recognizes the OPU light beam is entering active track region, SMP activates normal servo control functions and deactivate normal servo control function yet activate special servo control with predicated tracking profile at the exit of active track region. Under the circumstances when there is no physical index is available. SMP is provided to derive such index with the timing counter circuit and the FE feedback circuit. 
   Therefore, the optical beam is controlled to better track a target position on the optical media, which leads to servo control with stability. 
   Briefly, in a preferred embodiment, the present invention discloses a tracking and focusing servo circuit for controlling the tracking and focusing of an optical pickup head on an optical media, wherein the control is affected with a tracking and focusing error signal and a loop gain control signal obtained from an RF signal generated from said optical pickup head. The tracking servo circuit comprises comparators adapted to compare a signal level of the RF envelopes signal with an adjustable reference level and produce defect detection signal. In a preferred embodiment, the comparators adapted to compare a signal level of the RF envelop signal with an adjustable reference level and produce external disturbance detection signal. In a preferred embodiment, the tracking servo circuit further includes a focusing servo circuit comprises comparators adapted to compare a signal level of the focus error envelop signal with an adjustable reference level and produce focus error detection signal. In a preferred embodiment, the tracking servo circuit further includes a storage circuit, timing circuit, and A/D and D/A converters to store and reproduce tracking servo profile. In a preferred embodiment, the tracking servo circuit further includes a circuit adjusting means for setting the loop gain of the tracking servo circuit at an adjustable gain different from a normal circuit gain, upon detection of defect. In a preferred embodiment, the circuit adjusting means for activating extended recovery tracking and focusing error defect management at extended defect than the normal defect detection and recovery procedure. In a preferred embodiment, the circuit adjusting means for activating the tracking and focusing management circuit in the non-continuous segmented track arrangement 
   These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment, which is illustrated in the various drawing figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a servo control circuit that is typically implemented in a conventional data storage system. 
       FIG. 2  is a functional block diagram for showing an exemplary servo control system of this invention. 
       FIG. 3A  shows a small and a large defect in a spiral track. 
       FIG. 3B  shows non-continuous track segment arrangement. 
       FIG. 4  shows RF signal to track error signal with adjustable thresholds comparators and to defect type signal. 
       FIG. 5  shows RF signal to focus error signal with adjustable thresholds comparators and to focus error flag. 
       FIG. 6  shows the external index or spindle motor FG signal and timing counters. 
       FIG. 7  shows block diagram of servo management processor and memory arrangement. 
     FIGS.  8 A,B,C,D,E show a RF signal envelop profile, defect type set up and servo gain control respective to defect types 
       FIG. 9  shows an expanded RF signal profile and defect type threshold relationships. 
       FIG. 10  shows a functional block diagram of servo management processor and memory arrangement in a non-continuously track segment configuration. 
       FIG. 11  shows the generating pseudo track start and track end index for a non-continuous track segment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 2  for an example of a servomechanism control circuit of this invention. An optical pickup head  102  projects a laser beam on a rotating optical media  101 . An optical signal from the optical medium  102  is reflected and received by the optical pickup head (OPU)  102  that converts the reflected light from the optical media  101  into electrical signals. A radio frequency (RF) circuit  103  produces a tracking error (TE) signal and a focusing error (FE) signal to the respective tracking servo and focus servo processing circuits  104  and  105  respectively and more detail circuit configurations of the servo processing circuits  104  and  105  are described in  FIGS. 4 and 5  respectively below. Each servo circuit, e.g., circuits  104  and  105 ,receives error signals and transmits the corrective signals to the servo-management, processor SMP  109 . The SMP  109  then applies the gain control and phase compensator to feed the control voltage to stepper or linear motor driver and OPU  102  actuator driver. The servo management process (SMP)  109  sets the servo gain by either looking up a table of media property or using a dynamic calibration procedure as will be further described in  FIGS. 4 ,  5 ,  6 ,  7 , and  10  below. The control profile for tracking and focusing for OPU  102  power drivers are input to the SMP  109  for further processing and storage as further described in  FIG. 7 and 10 . The media management circuit  106  is implemented to indicate that the media is detected in the system using the signal as described in  FIG. 5 . The data signal process circuit  107  converts signal to data for host to use by SMP  109  through a host bus such as IDE/ATA, IDE/ATA serial, SCSI, or others as needed. The index or FG circuit  108  as described in  FIG. 6  generates once around index and timing counter for the servo management processor (SMP)  109  as described in  FIG. 7 and 10 . A defect detection circuit  110  with more detail descriptions provided in  FIG. 4  uses the threshold form SMP  109  to generate media detect signal and defect types back to SMP  109 . Focus error circuit  111  uses the threshold set by SMP  109  as described in  FIG. 5  to set focus error flag for SMP  109 . An adder  112  combines modulated servo from SMP  109  in  FIG. 7  or  10  and feedback from focus servo  105  to send a control signal to a driver  113  that controls the OPU  102  focus action. An adder  114  combines the modulated tracking servo from SMP  109  detailed in  FIG. 7  or  10  and feedback from tracking servo  104  to send control signals to a driver  115  that controls the motor  118  and the tracking action of OPU  102 . The SMP  109  controls the spindle motor  117  through the spindle motor driver  116 . Compared to a conventional servo control system, a new defect detection circuit  110  is added to further modulate the control signals. As shown in  FIG. 5 , additional FE comparator  111  is implemented to control the focusing driver  113 . This invention further uses a media control circuit  106  and SMP  6  to detect the media is in the device or not as shown in  FIG. 5 . This invention combines these added features with an index timing/counter circuits  108  as described in  FIG. 6  and SMP  109  memory management circuit detailed in  FIG. 7 and 10  to enable the servo control to manage media with large defects or discontinued track segments. 
     FIG. 3A  shows a relative small defect area  201  and a very large defect  202  at an optical disc. The conventional servo method usually can manage a small defect such as  201  by reducing the servo gain. However, when there is a very large and long defect such as  202 , it causes a conventional servo system to totally lose the servo stability since there is no tracking feedback across this large defect segment. A servo system of this invention as that shown in  FIG. 2  is implemented to handle both a large and small defects, e.g., defects  201  and  202 . 
     FIG. 3B  shows an optical media that has a non-continuously track segment arrangement  301 . There is no tracking feedback outside this  301  region. Conventional servo control method has limited usefulness in dealing with data access operations for data stored on these segmented non-continuous data tracks. A servo control mechanism as disclosed in this invention is implemented to read and write data form and to the data track on a discontinued or segmented data storage medium  301 . 
   Shown in  FIG. 4  is a detailed circuit of defect detect circuit  110  in  FIG. 2  that includes a tracking error RF envelope detector  406  and low pass filter  407  feed the error signal to the SMP  109 . This invention uses SMP  109  to set two D/A converters  401  and  402  at comparators  403  and  404  that define two types of defects, type  1  and type  2 . The type  1  defect threshold is higher than type  2  defect-threshold and has a higher servo gain feedback to correct the error. When the SMP  109  received a type  2  defect, the SMP  109  activates a very low gain response using DA converter  411  or shut off the servo gain  409  and temporarily disable an operation in carrying out the servo corrections. The SMP  109  can also deactivate the tracking servo  104  by switch  123  during the track jump action. Signal at servo gain  409  and phase compensator  410  feeds to SMP and memory  109  for further processing as shown in  FIGS. 7 and 10  that can provide further tracking servo modulation control in large defect or discontinued track segment environment. 
     FIG. 5  shows focus error detection circuits  111  in  FIG. 2  that includes an RF signal of focusing from the OPU  102  passes through an envelope detector  506  and a low pass filter  507  to comparators  503  and  504 . In this invention, two thresholds  501  and  502  set by SMP  109  are used to distinguish the RF signal is considered as an error by comparator  504  or considered as no media detected by comparator  503  that is used in media control circuit  106  in  FIG. 2 . The FE signal goes to the servo-gain control  509  with DA converter  508  and phase compensator  510  for further transmitting to the focus driver circuit  113 . The FE signal at phase compensator  501  is also inputted to the SMP  109  for further use in additional processes and for storage as shown in  FIGS. 7 and 10  for focusing servo modulation control in large defect or discontinued track segment environment. 
     FIG. 6  shows an external index is fed to a timing-and-counter circuit  108  in  FIG. 2 . The circuit  601  divides the index-to-index time to a predetermined number of counts N. A counter  602  counts starting from index and input real time count ( 0  to N) to SMP  109  detailed in  FIGS. 7 and 10 . If no external index is used, circuit uses the FG signal from spindle motor driver switched in by a switch. There are number of FG per revolution from spindle motor drive as M. Circuit  603  divides the FG signals by M to get the once around index. This kind of FG generated index varies its relative physical position when the spindle motor restarted. This invention uses the derived timing counter as marks for defect location or segmented track range for SMP  109  to manage the servo control in defect or segmented track environment as shown in  FIGS. 7 and 10 . 
     FIG. 7  shows the detail arrangement of the circuit SMP  109  and memory as that shown in  FIG. 2 . A MPU  701  interfaces with different signals and memory usages. Memories  702  and  703  are in round robin circular configuration with once around index as starting point. Each memory stores timing counter mark from  602  in  FIG. 6  and servo control voltage values from  410  in  FIG. 4  and from  510  in  FIG. 5 . Memory circuits  702  and  703  start with all zero values when a newly placed media is detected by  106  from  FIG. 2 . Memory  702  automatically stores the timing mark counter value from  602  in  FIG. 6  and the FE control voltage values from  510 . If a FE error flag is recognized, the memory  702  skips the update of control voltage value at the respected timing mark pointed location. Circuit fetcher  704  also fetches the FE servo control voltage value from  510  by matching the current timing mark time with the stored timing mark to a D/A converter  707 . MPU  701  can override the automatically fetch of data by fetcher  704  and supply a MPU supplied value. Such value can be a calculated estimated control value from previously stored history. MPU  701  also use switch  124  shown in  FIG. 2  to shut off the FE signal to focus servo circuits  105  in  FIG. 2  and let the servo circuit  105  to hold all its latest loop values. MPU  701  also uses switch  122  in  FIG. 2  to control the control values to adder  112  in  FIG. 2  is combination of  105  and  707  or just a selected  105  or  707 . 
   Similarly, memory  703  automatically stores the timing mark counter value from  602  in  FIG. 6  and the TE control voltage values from  410  in  FIG. 4 . If a defect is recognized, memory  703  skips the update of control voltage value from  410  at the respected timing mark pointed location. Circuit fetcher  708  also fetches the TE servo control voltage value from  410  by matching the current timing mark time with the stored timing mark to a D/A converter  708 . MPU  701  can override the automatically fetch of data by fetcher  705  and supply a MPU supplied value. Such value can be a calculated estimated control value from previously stored TE control history. MPU  701  also use switch  123  in  FIG. 2  to shut off the TE signal to track servo circuits  104  in  FIG. 2  and let the servo circuit  104  in  FIG. 2  to hold all its latest loop values. MPU  701  also uses switch  121  in  FIG. 2  to control the control values to adder  114  in  FIG. 2  is the combination of  104  in  FIG. 2 and 708  or just a selected  104  or  708 . MPU  701  also uses the defect type information from  403  and  404  in  FIG. 4  to set the high, low gain, or shut off gain at  411  to circuit  409  in  FIG. 4 . A normal servo gain applies when there is no defect detected. MPU  701  set different threshold values to D/A converters  401 ,  402 , and  411  in  FIG. 4 ,  501  and  502 , and  508  in  FIG. 5  through circuit matrix  706 . MPU  701  also use the information stored to separate the real defect that the defect type is repeatable at a logged timing location, or it is an external disturbance.  FIG. 7  shows this invention to modulated servo signal across a defect of discontinued track segment area. 
     FIG. 8A  shows an example RF signal and envelop profile measured after at low pass filter  407  shown in  FIG. 4  as part of circuit  110  in  FIG. 2  or after at low pass filter  507  shown in  FIG. 5  as part of circuit  111  in  FIG. 2 . Region  801  modulation is caused by an external disturbance. The signal degradation shown in Region  802  is caused by a minor defect. Region  803  signal drop is caused by a real serious defect that no reflection light beam back to OPU  102 . All this regions can be small or very large or ling in time. In the case at non-continuous track segment as shown in  FIG. 3 , the region  803  is very extensive long. In a servo control mode,  FIG. 8B  measured at  404  in  FIG. 4  shows the defect type  2  response and set active  813  respective to  803  in  FIG. 8A .  FIG. 8C  measured at  403  in  FIG. 4  shows the defect type  1  circuit response to set active as  821 ,  822 , and  823  respectively to  801 ,  802 , and  803  in  FIG. 8A . Since the type- 2  threshold is lower than the threshold for defect type  1 , type  2  detection does not respond to signals represented by regions  801  and  802 .  FIG. 9  shows an expanded track envelope profile of  FIG. 8A  and the relative defect type thresholds type  1  and type  2 ,  901  is for type  1  and  902  is for type  2 . The invention uses type  1  and type  2  defect information to control tracking and focusing servo for further processing with SMP  109  in  FIG. 2  that can provide further tracking servo modulation control in large defect or discontinued track segment environment as explained in  FIGS. 7 and 10 . 
   For a non-continuous track segment arrangement showed in  FIG. 3 ,  FIG. 10  shows an added circuit  1006  to circuits in  FIG. 7  for track start (TS)  1007  and track end (TD) 1008  signals. MPU  701  uses TS  1007  and TD  1008  for additional control to all circuits as OPU  102  in  FIG. 2  is in or out of the active region that has physical tracks. Servo circuits must react differently if it is following an active physical track or no existing physical tracks. Circuit  1006  provides such active and non-active track region information. Memory circuits  702  and  703  store or update data only from time mark trigged by TS  1007  and stop at TD  1008 . MPU  701  set the switch  123  and  124  in  FIG. 2  on to activate servo circuits after detect the signal  407  in  FIG. 4  and with the timing counter information from  108  in  FIG. 2  to turn off the switch  123  and  124  before signal TD  1008  becomes active or it is in a non track region. After the switch  123  and  124  in  FIG. 2  become off in non-track region, MPU can either hold servo drivers  113  and  115  in  FIG. 2  at their current values from  104  in  FIG. 2  or  410  in  FIG. 4 and 105  in  FIG. 2  or  510  in  FIG. 5 , feeds a calculate and predicated value through the  704 ,  707  and  705  and  708  in  FIG. 7  or  10 , or modulates servo control with the combination of holding previous value and the calculated values through switches  121 ,  122  and adders  112  and  114  in  FIG. 2 . This invention achieves a stable tracking and focusing servo control in a full circular way just as a full physical track is available. 
   In case no external track start TS and track end TD signal are available in a non-continuous track segment arrangement, with MPU  701  in  FIG. 7  or  10 , this invention can use available focusing FE signal from  507  in  FIG. 5  to define such signal as pseudo tack start and track end signals.  FIG. 11  shows an algorithm. MPU  701  spins up the media  101  in  FIG. 2  in process S 101  and turn on the Focusing part of OPU  102  in  FIG. 2  at process S 102 . Process S 103  checks the availability of focusing signal FE, A FE signal becomes active at  507  in  FIG. 5 . At the non-reflective region, there is no optical tracks available and focus signal is not active or signal amplitude is very small or none at  507  in  FIG. 5 . Process  104  determines the FE becomes form inactive to active. Once the media track area starts to pass under the OPU  102  and the FE becomes active, at process S 105 , MPU  701  can set the mark as track start TS signal using timing mark location generated at  108  in  FIG. 2  from process S 201 . Once the track region passes over the OPU  102 , the FE becomes as inactive. Process S 106  and S 107  determine that the FE is becoming from active to inactive. MPU  701  can use the timing mark counter  108  in  FIG. 1  again from process S 201  as reference and subtracting a predetermined value to mark it as a track end. As the media kept spinning, when ever the timing counter matches these two marked counter position, the track start and track end conditions are signaled. Process S 109  verifies the repeatability of the measured TS and TD are with in a predetermined tolerance. Process S 110  can take an average value by eliminating bad measurement. The pseudo TS and TD are determined and can be used in the servo control modulation in  FIG. 7  or  10 . 
   This invention discloses a control circuit for servo-controlling a data access pickup head that includes a defect type determination means for comparing an error signal with a defect threshold for determining at least two types of data track defects. In a preferred embodiment, the defect type determination means activating an extended recovery control process for managing a servo control of a defect type of an extended defect. In a preferred embodiment, the defect type determination means activating an external disturbance control process for managing a servo control of an external disturbance. In a preferred embodiment, the defect type determination means maintaining a normal servo control process when a error signal is below a threshold value. 
   This invention further discloses a control circuit for servo-controlling a data access pickup head that includes a track index determination means for controlling a servo control for accessing data on a segmented non-continuous data track. 
   Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. Various alternations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alternations and modifications as fall within the true spirit and scope of the invention.