Abstract:
A method, apparatus, and combination for reducing acoustical noise at start-up of a device is disclosed. The method includes the steps of determining an internal temperature of the device, and unlatching an actuator of the device based on the determined internal temperature of the device, by steps for unlatching the actuator of the device. The apparatus includes a thermistor providing a resistance value to a controller that selects an unlatch process based on the resistance value of the thermistor, and an unlatch procedure programmed into the controller executing the selected unlatch process. The device combination includes a base deck supporting an actuator confined by a magnetic latch for periods of inactivity, and unlatched from the magnetic latch for periods of activity by steps for unlatching the actuator.

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 60/494,822 filed Aug. 13, 2003, entitled Temperature Compensated VCM Driver Unlatch Algorithm to Improve Acoustic. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of data storage devices, and more particularly, but not by way of limitation, to incorporation of an acoustical noise reduction apparatus and method for an actuator of a data storage device. 
     BACKGROUND 
     One key component of many electronic systems is a device, such as a data storage device (DSD) to store and retrieve large amounts of user data in a fast and efficient manner. One method in which DSDs store digital data is in magnetic form on recording surfaces of one or more rigid data storage discs affixed to a spindle motor for rotation at a constant high speed. 
     Transducing heads aerodynamically supported over the recording surfaces by fluidic currents established by the rotation of the discs are controllably positioned by an actuator to read data from and write data to tracks defined on the recording surfaces. An outer framework that includes a base deck and top cover form an internal sealed compartment for housing mechanically operational components of the DSD. The base deck is essentially a platform to which DSD components, such as a disc stack assembly and the actuator are secured, and is of a size and shape to engage the electronic system. The top cover cooperates with the base deck to substantially seal the mechanically operational components of the DSD from external environments. 
     A major challenge for DSDs designs and designers is to limit and mitigate acoustical noise during start-up and active operating modes of the DSD. One source of acoustical noise occurs at start-up of the DSD as a result of unlatching the actuator in preparation for data exchange operations between each head and disc combination of the DSD. A reduction in the level of noise experienced by the DSD at start-up aids in compliance with acoustical noise limit specifications of the DSD. 
     As such, challenges remain and a need persists for improvements in methods and apparatus to mitigate acoustical noise of the actuator assembly of a DSD that disadvantageously erodes compliance with acoustical noise limit specifications of the DSD. 
     SUMMARY OF THE INVENTION 
     The present invention provides an economical method, apparatus, and combination for reducing acoustical noise during start-up activities of a data storage device through incorporation of a thermistor within an internal portion of the data storage device. In a preferred embodiment, the internal portion of the data storage device is an internal portion of a head-disc assembly of the data storage device. More specifically, the thermistor is supported by a flex circuit, which is attached to an actuator of the head-disc assembly of the preferred embodiment. 
     Preferably, a resistance value of the thermistor changes in response to the temperature surrounding the thermistor. The resistance value of the thermistor is compared by a controller to a predetermined temperature threshold value. The controller communicates with the thermistor via the flex circuit. Based on the comparison, the controller selects an unlatch process to unlatch an actuator of the head-disc assembly, while reducing the acoustical noise associated with unlatching the actuator. 
     In a preferred embodiment the method includes the steps of determining an internal temperature of the device based on the resistance value of the thermistor, and unlatching the actuator based on the determined internal temperature of the device. 
     Preferably, the apparatus includes the thermistor providing the resistance value to the controller, which selects an unlatch process based on the resistance value of the thermistor, and an unlatch procedure programmed into the controller executing the selected unlatch process. 
     Preferentially, the data storage device combination includes a base deck supporting the actuator confined by a magnetic latch for periods of inactivity of the data storage device, and unlatched from the magnetic latch for periods of activity of the data storage device by steps for unlatching the actuator. 
     These and various other features and advantages, which characterize the present invention, will be apparent from a reading of the following detailed description and a review of the associated drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top plan view of a disc drive incorporating a temperature dependent, unlatch apparatus and algorithm for unlatching a magnetic latch of the disc drive in accordance with a method of the present invention. 
         FIG. 2  is a functional block diagram of control circuitry of the disc drive of  FIG. 1 . 
         FIG. 3  is a graph showing a DC unlatch current profile used by a DC current unlatch process for unlatching the magnetic latch of the disc drive of  FIG. 1 . 
         FIG. 4  is a graph showing an AC unlatch current profile used by an AC current unlatch process for unlatching the magnetic latch of the disc drive of  FIG. 1 . 
         FIG. 5  is an acoustical noise graph showing the acoustical noise signal resulting from the use of the DC current unlatch process for unlatching the magnetic latch of the disc drive of  FIG. 1 . 
         FIG. 6  is an expanded view of the noise signal of  FIG. 5 . 
         FIG. 7  is an acoustical noise graph showing the acoustical noise signal resulting from the use of the AC current unlatch process for unlatching the magnetic latch of the disc drive of  FIG. 1 . 
         FIG. 8  is an expanded view of the noise signal of  FIG. 7 . 
         FIG. 9  is a flow chart of a DC current unlatch process used for unlatching the magnetic latch of the disc drive of  FIG. 1 . 
         FIG. 10  is a flow chart of an AC current unlatch process used for unlatching the magnetic latch of the disc drive of  FIG. 1 . 
         FIG. 11  is a flow chart of a temperature determined process used for unlatching the magnetic latch of the disc drive of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to one or more examples of the invention depicted in the figures. Each example is provided by way of explanation of the invention, and not meant as a limitation of the invention. For example, features illustrated or described as part of one embodiment may be used with another embodiment to yield still a different embodiment. Other modifications and variations to the described embodiments are also contemplated within the scope and spirit of the invention. 
     Referring to the drawings,  FIG. 1  provides a top plan view of a data storage device (DSD)  100 . The DSD  100  includes a base deck  102  cooperating with a top cover  104  (shown in partial cutaway) to form a sealed housing for a mechanical portion of the DSD  100 , referred to as a head-disc assembly (HDA)  106 . 
     A spindle motor assembly  108  (also referred to as a motor  108 ) rotates a number of data storage discs (disc or discs)  110  with a magnetic recording surface (surfaces)  112  at a substantially constant operational speed. Each disc  110  includes at least one magnetic recording surface  112 . The discs  110  are secured to the motor  108  by a disc clamp  114  to form a disc stack assembly  116 . 
     A rotary actuator (actuator)  118  supports and rotates a number of read/write heads (head or heads)  120  adjacent the surfaces  112  when current is applied to a voice coil (coil)  122  of a voice coil motor (VCM)  124 . The heads  120  are secured to a suspension  126 , which is attached to an actuator arm  128  of the actuator  118 . The suspension  126  assures that a consistent, predetermined spring force is applied to each head  120  for proper control of the heads  120  relative to the discs  110 . 
     During operation of the DSD  100 , the actuator  118  moves the heads  120  to data tracks  130  (one shown) on the surfaces  112  to write data to, and read data from the discs  110 . When the DSD  100  is deactivated, the actuator  118  moves the heads  120  to a predetermined position. For example, the actuator  118  may position the heads  120  adjacent a home position  132 , and utilize a magnetic latch  134  to constrain motion of the actuator  118  adjacent a bumper  135 . However, alternative means for restraining the actuator  118  may be employed, for example, a ramp load/unload mechanism (not shown), or a toggle latch (also not shown) may be incorporated to constrain movement of the actuator  118  during periods of inactivity of the DSD  100 . 
     During data transfer operations of the DSD  100 ; the heads  120  transfer data to and from the surfaces  112  to a printed circuit board assembly  136 . The data are processed by a preamplifier  138  and passed to the printed circuit board assembly  136  through a flex circuit  140 . Movement of the heads  120  from a first selected data track  130  to a second selected data track  130  is referred to as executing a seek operation. 
     During start-up operations of the DSD  100 ; current applied to the coil  122  imparts energy into the actuator  118  to unlatch the actuator from the magnetic latch  134 . In a preferred embodiment, the amount of energy imparted to the actuator  118  varies with the internal temperature of the HDA  106 , and the type of current (alternating current AC, or direct current DC) provided to the coil  122 . The internal temperature of the HDA  106  is determined through use of a thermistor  142  (a thermally sensitive resistor that has, according to type, a negative (NTC), or positive (PTC) resistance/temperature coefficient) mounted to the flex circuit  140 . 
     If the internal temperature of the HDA  106  is determined to be above an empirically determined temperature threshold, a level of direct current (DC) is preferably selected and applied for an initial predetermined interval of time across the coil  122 . Both the current level and the interval of time for application of the DC current level to the coil  122  may be sequentially altered during the unlatch process. If both a maximum DC current level and a maximum time interval for application of the DC current to the coil  122  has been attained, and the process of unlatching the magnetic latch has been unsuccessful, or if the internal temperature of the HDA  106  is determined to be below the empirically determined temperature threshold an alternating current (AC) unlatch process is initiated. 
     The AC current employed by the AC current unlatch process is not limited to a sinusoidal waveform, i.e., a saw-tooth, square, or combination waveform may be selected for use as the waveform of choice during execution of the AC current unlatch process. Preferably, the AC current unlatch process commences with an application of an AC current level applied across the coil  122  for a predetermined period of time. If the actuator  118  has not been successfully unlatched, the AC current level, used by the AC current unlatch process, is sequentially increased in an attempt to unlatch the magnetic latch  134  and free the actuator  118 . If a maximum current level for use during the AC current unlatch process has been achieved, and the actuator  118  remains latched, an actuator unlatch failure is reported. 
     Both the AC current unlatch process and the DC current unlatch process employ monitoring of the Back EMF (Bemf) response of the coil to determine whether or not the magnetic latch  134  has released the actuator  118 . In each case, the supply of the current to the coil  122  is suspended to float the actuator  118 , while the voltage level of the coil  122  resulting from the Bemf is sensed. 
     In executing either the AC or DC current unlatch process it has been found useful to establish a maximum current level and the duration that may be employed to overcome the magnetic force of the magnetic latch  134 , without incurring a “slam” (an undesirable and disadvantageous excessively forceful interaction of the actuator  118  with a crash stop (not shown)). 
     Preferably, for the DC current unlatch process, the maximum current level and the duration of its application that may be employed to overcome the magnetic force of the magnetic latch  134 , without incurring a “slam,” is empirically determined for each DSD  100  of interest. The maximum current determination is based on the lowest magnetic latch force specified for the DSD  100  of interest, and a maximum unlatch time limit specified for the DSD  100  of interest. For example, measurements are made to find a minimum drive current that will result in a slam of the actuator  118  of the DSD  100  of interest, when that minimum drive current is applied for a maximum time within the specified maximum unlatch time. 
     However, as one skilled in the art will recognize, variations on the desired amount of time to be used for executing the DC current unlatch process will result in a different minimum current level that will result in a slam. In other words, the shorter the duration of time made available for unlatching the actuator  118  from the magnetic latch  134 , the greater will be the level of DC current needed to induce a slam. The longer the duration of time made available for unlatching the actuator  118  from the magnetic latch  134 , the less will be the level of DC current needed to induce a slam. For purposes of disclosure and enhancing an understanding of the present invention, and not by way of imposing limitations on the present invention, the minimum drive current that will result in a slam of the actuator  118  of the DSD  100  of interest, when applied for a maximum time within the specified maximum unlatch time will be employed in describing a preferred embodiment of the present invention. 
     Turning to  FIG. 2 , the term “servoing,” also referred to as position-controlling, as used herein means maintaining control of the head  120  (of  FIG. 1 ) relative to the selected data track  130  (of  FIG. 1 ) during operation of the DSD  100 , either during seek operations or track following operations. When servoing to or on the data track  130 , the head  120  is controllably positioned by the VCM  124 . Position-controlling of the head  120  is provided by the actuator  118  (of  FIG. 1 ) responding to the VCM  124  operating under the control of a servo control circuit  146  programmed with servo control code, which forms the servo control loop. The servo control circuit  146  includes a control processor (controller)  148 , a demodulator (demod)  150 , may include an application specific integrated circuit (ASIC) hardware-based servo controller (“servo engine”)  152 , may include a digital signal processor (DSP)  154 , and includes volatile memory (VM)  156 , a digital to analog converter (DAC)  158 , and a motor driver circuit  160 . Optionally, the functions of the servo engine  152 , the DSP  154 , and the VM  156  may all be contained within the controller  148 . The components of the servo control circuit  146  are utilized to facilitate track following algorithms for the actuator  118 , and more specifically for controlling the VCM  124  in position-controlling the heads  120  relative to the selected data track  130 . 
     The demod  150  conditions head position control information transduced from the surfaces  112  (of  FIG. 1 ) to provide position information of the head  120  relative to the data track  130 . The servo engine  152  generates servo control loop values used by the controller  148  in generating command signals such as seek signals used by the VCM  124  in executing seek commands, and to maintain a predetermined position of the actuator  118  during data transfer operations. The command signals generated by the controller  148  are converted by the DAC  158  to analog control signals for use by the motor driver circuit  160  in position-controlling the head  120  relative to the selected data track  130 , for track following, and relative to the surfaces  112  for track to track seek functions. 
     In a preferred embodiment, during spin-up of the surfaces  112 , the controller  148  determines an internal temperature of the HDA  106  (of  FIG. 1 ) by referencing a resistance of the thermistor  142 . Based on the determined internal temperature, the controller  148  instructs the motor driver circuit  160  to apply either a DC unlatch current waveform, or an AC unlatch current waveform to the VCM  124  to unlatch the actuator  118  from the magnetic latch  134 . The temperature value at which the controller  148  determines which unlatch current waveform will be employed is a predetermined temperature threshold value. 
     The temperature threshold value for each type of DSD  100  may vary from DSD type to DSD type. However, what has been found is that the amount of force needed to unlatch the actuator  118  from its magnetic latch  134  decreases as the temperature within the HDA  106  increases, and that the response of the magnetic latch  134  to variations in temperature is substantially common between HDAs  106  within each type of DSD  100 . 
     In a preferred embodiment, the temperature threshold value for a DSD  100  of interest is empirically determined by finding the minimum drive current for the DC unlatch current waveform (min. current) that, when applied for a maximum time within the specified maximum unlatch time (max. interval), will result in a slam of the actuator  118  when the internal temperature of the HDA  106  is in a temperature range of about 20–23° C. While holding the current level to the min. current for the max. interval, the internal temperature of the HDA  106  is sequentially reduced to a point that the min. current at the max. interval is ineffective in unlatching the actuator  118 . 
     When the temperature has been identified at which the min. current supplied to the coil  122  (of  FIG. 1 ) over the max. interval is ineffective in unlatching the actuator  118 , a margin of a few degrees (2–4° C.) is added to the identified temperature to establish a threshold temperature. The internal temperature of the HDA  106  is moved to be substantially the threshold temperature, and the resistance value of the thermistor  142  is measured to establish the temperature threshold value. 
       FIG. 3  shows a DC current profile  162 ; of a preferred embodiment of the DC current unlatch process. A minimum current applied for a maximum time interval to effectively unlatch the actuator  118  (of  FIG. 1 ), i.e., profile portion  164 , is arrived at following a sequence of prior applied increasing levels of current. Each level of current is applied for a predetermined period of time (shown by profile portions  166 ,  168 ,  170  and  172 ). In a preferred embodiment, the drive current for each of the profile portions has been applied to the coil  122  for substantially the same time interval. Once a maximum level for the minimum current (MLMC) is arrived at (profile portion  172 ), the interval of time used for application of the MLMC to the coil  122  is sequentially increased to a max. interval. 
     It is noted that each profile portion of the DC current profile  162  is separated by a Bemf measurement period  174 , during which the Bemf of the coil  122  is measured to detect a VCM sense voltage response  176 . If the actuator  118  fails to become unlatched from the magnetic latch  134  once the MLMC has been applied for the maximum time interval, the DC current unlatch process is halted and an AC current unlatch process is initiated. 
       FIG. 4  shows an AC current profile  178 ; of a preferred embodiment of the AC current unlatch process. In applying current to the coil  122  (of  FIG. 1 ), a maximum current applied for a predetermined time interval to effectively unlatch the actuator  118  (of  FIG. 1 ), profile portion  180 , is arrived at following a sequence of prior applied increasing levels of current (shown by profile portions  182 ,  184 ,  186 ,  188  and  190 ). Each of the increasing current levels is applied for the predetermined period of time. In a preferred embodiment, the drive current for each of the profile portions has been applied to the coil  122  for substantially the same time interval. If following application of a maximum current to the coil  122  (profile portion  180 ) the actuator  118  is not unlatched from the magnetic latch  134  (of  FIG. 1 ), a “failure to unlatch error” is reported. 
     It is noted that each profile portion of the AC current profile  178  is separated by a Bemf measurement period  192 , during which the Bemf of the coil  122  is measured to detect a VCM sense voltage response  194 . If the actuator  118  fails to become unlatched from the magnetic latch  134  following an application of the maximum current across the coil  122  for the predetermined period of time, the VCM sense voltage response  194  will not occur and the AC current unlatch process will report a “failure to unlatch error.” 
     It is also noted that each profile portion ( 180  through  190 ) of the AC current profile  178  includes an acceleration state  196  and a deceleration state  198 . It is further noted that in a preferred embodiment, both the DC current profile  162  (of  FIG. 3 ) and the AC current profile  178  (of  FIG. 4 ) are driven at a voltage level of substantially 6.65 volts. Because the AC current profile  178  includes both the acceleration state  196  and the deceleration state  198 , the current to the coil  122  (of  FIG. 1 ) is applied in an alternating direction to simulate an empirically determined resonance frequency to excite the bumper  135  (of  FIG. 1 ). By simulating the empirically determined resonance frequency with the AC current profile  178  during the AC current unlatching process, the actuator  118  (of  FIG. 1 ) is more easily moved from the latched position as opposed to movement of the actuator  118  from the latched position through the use of the DC current unlatching process. The level of current used during the DC current unlatching process is substantially the same level of current used during the AC current unlatching process. However, the current applied during DC current unlatching process is applied in one direction only as opposed to the bidirectional application of the current during the AC current unlatching process. It is noted, that use of the simulated AC current during the AC current unlatching process produces one source of acoustical noise alleviated through use of the DC current unlatching process. 
     In a preferred embodiment, because more energy is imparted to the actuator  118  when executing the AC current unlatching process than is imparted to the actuator  118  when executing the DC current unlatching process, the AC current unlatching process is able to unlatch the actuator  118  from the magnetic latch  134  when the DC current unlatching process is unable to unlatch the actuator  118  from the magnetic latch  134 . However (as can be seen by a comparison of the DC current unlatch process acoustical noise response graphs of  FIGS. 5 and 6 , to the AC current unlatch process acoustical noise response graphs of  FIGS. 7 and 8 ), the acoustical noise response of the DSD  100  to an execution of the DC current unlatch process is lower than the acoustical noise response of the DSD  100  to an execution of the AC current unlatch process. In a preferred embodiment, a reduction in acoustical noise of substantially 20% has been found between application of the DC current profile  162  at substantially 6.65 volts, and application of the AC current profile  178  at substantially 6.65 volts across the coil  122  of the same DSD  100 . 
       FIG. 5  shows an acoustical noise graph  200  resulting from an execution of the DC current profile  162  (of  FIG. 3 ) across the coil  122  (of  FIG. 1 ) at substantially 6.65 volts.  FIG. 6  shows the same acoustical noise graph  200  at a resolution 10 times higher than the resolution of the acoustical noise graph  200  of  FIG. 5 . 
       FIG. 7  shows an acoustical noise graph  202  resulting from an execution of the AC current profile  178  (of  FIG. 4 ) across the coil  122  (of  FIG. 1 ) at substantially 6.65 volts, while  FIG. 8  shows the same acoustical noise graph  202  at a resolution 10 times higher than the resolution of the acoustical noise graph  202  of  FIG. 7 . 
     A comparison between  FIGS. 6 and 8  shows that although the waveform of the noise graph  200  of  FIG. 6  has a substantially similar waveform to the noise graph  202  of  FIG. 8 , the energy under the curve of the noise graph  200  is about 80% of the energy under the curve of the noise graph  202 . In other words, the acoustical noise level that results from an execution of the DC current unlatch process is about 20% lower than the acoustical noise level that results from an execution of the AC current unlatch process. 
       FIG. 9  shows a DC current unlatch process  210  commencing at start DC unlatch process step  212  and continuing at process step  214 . At process step  214 , both an initial DC current and an initial time interval are selected. The initial DC current level applied over the initial time interval drives a coil (such as  122 ). In a preferred embodiment, both the initial time interval and the initial DC current are selected to be below their respective maximum current and maximum time interval levels. At process step  216 , the initial DC current level is applied across the coil for the initial time interval. At process step  218 , the back EMF (Bemf) voltage function of the coil is measured. At process step  220 , if the measured Bemf voltage is a VCM sense voltage response (such as  176 ), the DC current unlatch process  210  has been successful, and the process concludes at DC unlatch successful end process step  222 . 
     However, if at process step  220  the measured Bemf voltage function of the coil does not display the VCM sense voltage response, the DC current unlatch process  210  continues at process step  224 . At process step  224 , if the predetermined maximum current level has not been achieved, the DC current applied across the coil is increased at process step  226  and the process reverts to process step  216  and continues through the process to process step  220 . At process step  220 , if the unlatching process is still unsuccessful the process returns again to process step  224 . 
     At process step  224 , if a determination is made that the maximum current level has been attained, the process proceeds to process step  228 . At process step  228 , confirmation is made whether or not the maximum time interval for application of the maximum current level has been achieved. If the maximum time interval has not been achieved at process step  228 , the process continues to process step  230 . At process step  230 , the time interval for application of the maximum current is increased, and the process reverts to process step  216 . If the DC unlatch process is still unsuccessful, with the DC current set at its maximum level and applied across the coil for the maximum time interval, the DC current unlatch process  210  continues to DC unlatch failed process step  232 , and then proceeds to AC unlatch process step  234 . 
       FIG. 10  shows an AC current unlatch process  240  commencing at start AC unlatch process step  242  and continuing at process step  244 . At process step  244 , an initial AC current for use in driving a coil (such as  122 ) over a predetermined time interval is selected. In a preferred embodiment, the initial AC current is set below a maximum current level. The predetermined time interval remains substantially constant during execution of the AC unlatch process  240 . At process step  246 , the initial AC current level is applied across the coil for the predetermined time interval. At process step  248 , the back EMF (Bemf) voltage function of the coil is measured. At process step  250 , if the measured Bemf voltage is a VCM sense voltage response (such as  194 ), the AC current unlatch process  240  has been successful, and the process concludes at AC unlatch successful end process step  252 . 
     However, if at process step  250  the measured Bemf voltage function of the coil does not display the VCM sense voltage response, the AC current unlatch process  240  continues at process step  254 . At process step  254 , if the predetermined maximum current level has not been achieved, the AC current applied across the coil is increased at process step  256  and the process reverts to process step  246  and continues through the process to process step  250 . At process step  250 , if the unlatching process is still unsuccessful the process returns again to process step  254 . 
     At process step  254 , if a determination is made that the maximum current level has been attained, the process proceeds to process step  258 . At process step  258 , the AC current unlatch process  240  reports the failure of the unlatch process and concludes at end process step  260 . 
     The flowchart of  FIG. 11  shows steps of an unlatch procedure  270  programmed into a controller (such as  148 ) commencing at start step  272  and continuing at process step  274 . At process step  274 , an internal temperature of a HDA (such as  106 ) is determined by reading a resistance value of a thermistor (such as  142 ). At process step  276 , a determination of whether or not the resistance value of the thermistor is greater than a predetermined temperature threshold value. If the resistance value of the thermistor is greater than the predetermined temperature threshold value, the unlatch procedure  270  proceeds to process step  278 . 
     At process step  278 , a DC current unlatch process (such as  210 ) is selected and executed. At process step  280 , a determination is made of whether or not the DC unlatch process had been successful in unlatching an actuator (such as  118 ) from a magnetic latch (such as  134 ). If the DC unlatch process had been successful, the unlatch procedure  270  concludes at end process step  282 . 
     However, if at process step  276  the resistance value of the thermistor is less than the predetermined temperature threshold value, or if at process step  280  the DC unlatch process is determined to have been unsuccessful, the unlatch procedure  270  proceeds to process step  284 . At process step  284 , an AC unlatch process (such as  240 ) is selected and executed. 
     At process step  286 , a determination is made of whether or not the AC unlatch process had been successful in unlatching the actuator from the magnetic latch. If the AC unlatch process had been successful, the unlatch procedure  270  concludes at end process step  282 . If at process step  286  the AC unlatch process is determined to have been unsuccessful, the unlatch procedure  270  proceeds to process step  288 , reports an unlatch failure and proceeds to end process step  282 . 
     The many features and advantages of the present invention are apparent from the written description. It is intended by the appended claims to cover all such features and advantages of the invention. As numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation as illustrated and described. Hence, all suitable modifications and equivalents may be resorted to as falling within the scope of the invention.