Abstract:
A circuit and method are presented for detecting a fault in a magneto-resistive head ( 18 ). The circuit includes a bias circuit ( 50 ) to produce a bias voltage across the head ( 18 ) and a pair of resistors ( 68,70 ) in series with the head ( 18 ) connected to the bias circuit ( 50 ) to carry a current (I VMR ) from the bias circuit ( 50 ) in common with the head. A circuit ( 102,102′ ) is provided to determine a ratio of a voltage across the head ( 18 ) with respect to a voltage across the head ( 18 ) and the pair of resistors ( 68,70 ), and a circuit ( 104,106,104′,106′ ) is provided for indicating a fault if the ratio falls outside a predetermined range.

Description:
BACKGROUND OF INVENTION 
     1. Field of Invention 
     This invention relates to improvements in methods and apparatuses for dynamic information storage or retrieval, and more particularly to improvements in methods and circuitry for detecting electrical resistance in electronic components, especially for detecting open and short faults in magneto-resistive read heads of mass data storage devices, hard disk drives, or the like. 
     2. Relevant Background 
     Mass data storage devices include tape drives, as well as hard disk drives that have one or more spinning magnetic disks or platters onto which data is recorded for storage and subsequent retrieval. Hard disk drives may be used in many applications, including personal computers, set top boxes, video and television applications, audio applications, or some mix thereof. Applications for hard disk drives are still being developed, and are expected to further increase in the future. 
     Typically, mass data storage devices include a data transducer, or head, that is used to read data from and write data to a rotating magnetic media, usually in the form of a disk or platter on which a material containing orientable magnetic domains is carried. The present invention pertains especially to magneto-resistive data transducers, or heads, which change in resistivity in the presence of magnetic fields adjacent the disk produced by selectively oriented magnetic domains in the magnetic material on the disk. The typical resistance of a magneto-resistive head is in the range of between about 16 and 150 ohms. 
     Sometimes, however, the head mechanism experiences faults, the faults of interest herein being open and short faults. Efforts have been made to detect such open and short faults; however, such efforts have required that the read head be biased by electrical current, not voltage, and that both the open and short detection thresholds for head resistance vary over bias level. 
     Typically in the initial setup of a mass data storage device, and, hard disk drive in particular, the drive is provided with a serial port through which the user may program various user programmable parameters of the device. In the case of a voltage mode operated MR head circuit, to which the circuit and method of the invention best address, oftentimes the user is enabled to program the operating bias voltage at which the MR head is to be operated. In such situations, if the head has a “short” condition, the bias circuit associated with the head must supply relatively large currents to the head to maintain the preprogrammed bias voltage across the head. This may result in a saturation of the driving transistor. In the past, such saturation has frustrated the accurate detection of the short condition. 
     Moreover, in such voltage bias embodiments, the bias circuit typically operates to maintain a constant voltage across the MR head. Because the bias circuit usually employs a servo circuit to maintain the constant head voltage which requires a finite amount of time to restore the voltage across the head if a change of voltage bias level or a switch from one MR head and disk to another occurs. In the past, during this settling time, detection of head faults was difficult and inaccurate. 
     What is needed, therefore, is a relatively simple and reliable circuit and method for detecting open and short conditions in a circuit element, such as a magneto-resistive data transducer or head, in which the open and short fault conditions can be detected independently of the head bias for voltage bias preamplifiers. 
     SUMMARY OF INVENTION 
     In light of the above, therefore, it is an object of the invention to provide a circuit and method for detecting open and short fault conditions in a circuit element, such as a magneto-resistive (MR) data transducer, head, or the like. 
     One of the advantages of the circuit used in accordance with a preferred embodiment of the invention described below is that an open or short head condition can be detected independently of the voltage or current bias level on the head. 
     Another advantage of the invention is that the circuit of the invention has no component saturation limit during short detection for voltage biased preamplifiers. 
     Yet another advantage of the invention is that the fault detection circuit has fast fault detection and does not require that the servo loop of the biasing circuit settle to a steady state operating mode before fault detection can be performed. This advantage has significant value due to the frequent occurrence, in practice of normal usage, of MR head switching among disks or platters and of voltage or current bias switching under the same MR head and disk. 
     Still another advantage of the invention is that the fault detection circuit responds rapidly to fault changes, and, particularly to changes in the circuit without regard to the speed at which the servo circuit responds to maintain a constant voltage up across the head. 
     Still yet another advantage of the invention is that the fault detection circuit works for both voltage and current modes of operation of a head bias preamplifier. 
     These and other objects, features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read in conjunction with the accompanying drawings and appended claims. 
     The present invention is based upon the observation that the voltage dropped across the MR head forms a ratio with the voltage dropped across the series combination of the resistors in the circuit containing the MR head. If a deviation from a normal ratio drops an indication is given that the voltage across the MR head is high, indicating an “open” fault. On the other hand, if the ratio becomes very small, an indication is given that the voltage across the MR head is very small, indicating that a “short” fault. The voltage ratio is essentially independent of the state of the servo circuit. Even though the servo circuit may have not settled to a final value, the ratio of the series combination of resistors remains substantially constant, and, therefore, faults of the head can be determined without regard to the condition servo loop. 
     Thus, through use of the circuit and methods of the invention, faults of read MR. head in hard disk drive can be reliably and promptly detected for both voltage- and current bias preamplifier chips, and the detection threshold is independent of head bias voltage or current. This invention is simple in circuit implementation, and takes advantage of using the existing MR head biasing circuit. Moreover, fast MR open or short detection can be made before the voltage bias loop settles down. The detection is independent of bias modes (voltage or currents), and the threshold is independent of voltage or current bias level. The simplicity of circuit implementation results in little or no impact on the thermal noise, power supply noise rejection, read signal path bandwidth, head-to-head switching, or read head circuit performance. 
     According to a broad aspect of the invention, a circuit is presented for detecting a fault in a magneto-resistive head. The circuit includes a bias circuit to produce a bias voltage across the head and at least one resistor in series with the head connected to the bias circuit to carry a current from the bias circuit in common with the head. In practice, this is often part of the existing MR head bias circuitry. A circuit is provided to determine a ratio of a voltage across the head with respect to a voltage across the head and the at least one resistor, and a circuit is provided for indicating a fault if the ratio falls outside a predetermined range. The circuit may be used in conjunction with bias circuits in either voltage or current biasing mode. The fault indicating circuit may include a differential comparator having two differential inputs. A voltage related to the voltage across the head and the at least one resistor less a voltage offset is applied to the differential inputs, and a circuit is provided for biasing the differential inputs with a voltage related to the voltage across the head. A circuit is connected to the differential comparator to produce a fault indicating output if a current in one side of the differential comparator is of magnitude that is outside a predetermined range. 
     According to another broad aspect of the invention, a circuit is provided for detecting a fault in a magneto-resistive head. The circuit includes a servo bias circuit to produce a constant bias voltage across the head and a pair of resistors in series with respective opposite sides of the head connected to the bias circuit to carry a current from the servo bias circuit in common with the head. A circuit is provided for determining a ratio of a voltage across the head with respect to a voltage across the head and the pair of resistors, and a circuit is provided for indicating a fault if the ratio falls outside a predetermined range. The circuit for indicating a fault may include a differential comparator having a differential inputs, wherein a voltage related to the voltage across the head and the at least one resistor less a voltage offset is applied to the differential inputs. A circuit is provided for biasing the differential inputs with a voltage related to the voltage across the head, and a circuit is provided for producing a fault indicating output if a current in one side of the differential comparator is of magnitude that is outside a predetermined range. The circuit for producing a fault comprises a circuit for producing an “open” fault if the magnitude is above a predetermined value and a “short” fault if the magnitude is below a predetermined value. 
     According to yet another broad aspect of the invention, a mass data storage device is presented. The mass data storage device includes a magneto-resistive head and a bias circuit to produce a bias voltage across the head. At least one resistor is connected in series with the head to carry a current from the bias circuit in common with the head. A circuit determines a ratio of a voltage across the head with respect to a voltage across the head and the at least one resistor, and a fault indicating circuit indicates a fault if the ratio falls outside a predetermined range. 
     According to still another broad aspect of the invention, a circuit is presented for detecting a fault in a magneto-resistive head for detecting magnetic fields in a data storage device. The circuit includes means for determining a ratio of a head voltage with respect to a voltage of a series of resistors in a current path in common with the head and means for triggering a fault indicating output signal if the ratio falls outside a predetermined range. The means for triggering a fault indicating output signal if the ratio falls outside a predetermined range may include means for triggering an open fault indicating signal if the ratio exceeds a first predetermined ratio and means for triggering a short fault indicating signal if the ratio falls below a second predetermined ratio. 
     According to still yet another broad aspect of the invention, a method is presented for detecting a fault in a magneto-resistive head for detecting magnetic fields in a data storage device. The method includes determining a ratio of a head voltage with respect to a voltage of a series of resistors in a current path in common with the head and triggering a fault indicating output signal if the ratio falls outside a predetermined range. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention is illustrated in the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a generic disk drive system, illustrating the general environment in which the invention may be practiced. 
     FIG. 2 is an electrical schematic diagram of a voltage mode MR head biasing circuit, in accordance with the prior art. 
     FIG. 3 is an electrical schematic diagram of an “open” fault detector circuit, in accordance with a preferred embodiment of the invention, for use in conjunction with the voltage mode MR head biasing circuit if FIG.  2 . 
     FIG. 4 is an electrical schematic diagram of a “short” fault detector circuit, in accordance with a preferred embodiment of the invention, for use in conjunction with the voltage mode MR head biasing circuit if FIG.  2 . 
     FIG. 5 is an electrical schematic diagram of a circuit for providing a user an indication of the analog buffered head voltage across an MR head, in accordance with the prior art. 
     And FIG. 6 is an electrical schematic diagram of a circuit used in conjunction with the circuit of FIG. 5 to provide a current for use in the circuits of FIGS.  3  and  4  representing the MR head voltage, in accordance with a preferred embodiment of the invention. 
    
    
     In the various figures of the drawing, like reference numerals are used to denote the same, like, or similar parts. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is illustrated in the accompanying drawings to which reference is now made. FIG. 1 is a block diagram of a generic disk drive system  10 , which represents one general environment in which the invention may be practiced. The system  10  includes a magnetic media disk  12  that is rotated by a spindle motor  14  and spindle driver circuit  16 . 
     A data read/write transducer or head  18  is locatable along selectable radial tracks (not shown) of the disk  12  by a voice coil motor  22 . Preferably the data read/write transducer or head  18  is a magneto-resistive (MR) head, which changes in resistivity in the presence of a magnetic field. 
     The radial tracks may contain magnetic states that contain information about the tracks, such as track identification data, location information, synchronization data, as well as user data, and so forth. The head  18  may be used both to read user data back from the disk  12 , as well as to detect signals that identify the tracks and sectors at which data is written, and to detect servo bursts that enable the head  18  to be properly laterally aligned with the tracks of the disk  12 . 
     Analog electrical signals that are generated by the head  18  in response to the magnetic signals recorded on the disk  12  are preamplified by a preamplifier  24  for delivery to read channel circuitry  26 . Servo signals, below described in detail, are detected and demodulated by one or more servo demodulator circuits  28  and processed by a digital signal processor (DSP)  30  to control the position of the head  18  via the positioning driver circuit  32 . The servo data that is read and processed may be analog data that is interpreted by the DSP  30  for positioning the head  18 . 
     A microcontroller  34  is typically provided to control the DSP  30 , as well as an interface controller  36  to enable data to be passed to and from a host interface (not shown) in known manner. A data memory  38  may be provided, if desired, to buffer data being written to and read from the disk  12 . Typically an “H” bridge writer is used to drive the signals from the interface controller  36 , read channel  26  and preamplifier  24  to be written to the head  18 . 
     According to a preferred embodiment of the invention, open and short faults of the write head  18  can be reliably detected to alert an operator or machine that a malfunction is occurring in the head. The term “open” is used herein to indicate a fault condition in which the resistance of the MR head in question exceeds a predetermined resistance. It does not necessarily require that the resistance be at or substantially at infinity. Likewise, the term “short” is used herein to indicate a fault condition in which the resistance of the MR head in question falls below a predetermined resistance. It does not necessarily require that the resistance be at or substantially at zero bias voltage. 
     A typical prior art MR head driving circuit  50  is shown in FIG. 2, to which reference is now additionally made. The head voltage bias circuit  50  provides one environment in which the circuit and method of the invention may be employed. The circuit  50  includes a transconductance amplifier  52  that has one set of inputs across which the MR head  18  is connected. The transconductance amplifier  52  may be a full differential comparator, with the voltage across the MR head  18  providing one voltage input, and a reference voltage providing another voltage input. The reference voltage input may be established by a DAC, and may conveniently be set at a value of IVMR*587 Ohms. A resistance-matching resistor  54  may be provided in parallel with the MR head  18 , as shown. 
     A capacitor  56  from which current can be supplied or sunk in the operation of the circuit  50  is connected between the output of the transconductance amplifier  52  and ground. The circuit  50  operates essentially as a servo loop to maintain a fixed predetermined as voltage on the MR head  18 , in a manner below described in detail. 
     The circuit  50  includes two current paths  58  and  60 . The current the path  60  provides a reference voltage to the transistors  62  and  64  to control the current in the current path  58 . More particularly, the first current path  58  includes an npn transistor  62  in series with a resistor  68  on the top side of the MR head  18 , connecting the MR head  18  to the Vcc supply line,  69 . Similarly, the bottom side of the MR head  18  is connected by a resistor  70  and current source  72  to a reference or ground potential  74 . The second current path  60  includes two current sources  76  and  78  connected in series with a resistor  80  between Vcc  69  and ground  74 . The node  82  between the resistor  80  and current source  78  is connected to the base of the second npn transistor  64 . The emitter of the second npn transistor  64  is connected to a node  84  between the resistor  70  and current source  72 . A capacitor  88  is connected between the respective bases of transistors  62  and  64 , as shown. 
     The current sources  72 ,  76 , and  78  are adjustable by the current supplied by the output of the transconductance amplifier  52 , as denoted by the dashed line  90 , to maintain the voltage on output node  92  at an essentially constant value. As a result, the circuit  50  operates to maintain a constant voltage across MR head  18 . It should be also noted that because the circuit  50  serves as a servo circuit, a finite amount of time is required if a change occurs to restore the voltage across the MR head  18 . 
     In operation, as the voltage on node  92  rises, current from the transconductance amplifier  52  tends to charge the capacitor  56 . This has the effect of reducing the current IVMR flowing through the MR head  18 , which, in turn, reduces the output current to the capacitor  56  from the transconductance amplifier  52 . This, reduce the voltage on node  92  to the constant voltage. On the other hand, if the voltage on output node  92  begins to fall, current is sunk from the capacitor  56 . This has the effect of increasing the current IVMR flowing through the MR head  18 , which, in turn, increases the output current from the transconductance amplifier  52  to the capacitor  56 , returning the output node  92  to its normal fixed voltage. 
     According to a preferred embodiment of the invention, fault detection circuitry which is shown in FIGS. 3 and 4, to which reference is now additionally made, is connected essentially in parallel with a portion of the servo circuit  50  of FIG. 2 at the common connection points indicated by letters A, B, C, and D. The circuit  95  of FIG. 3 may be used to detect “open” faults of a magneto resistive head, the faults being indicated on output line  44 . The circuit  95 ′ of FIG. 4 may be used to detect “short” faults of a magneto resistive head, the faults being indicated on output line  44 ′. The circuits  95  and  95 ′ are substantially the same, except for the value of the offset resistors  96  and  96 ′, and the addition of a second inverter  100  at the output of the short detection circuit  95 ′. 
     More particularly, with reference first to FIG. 3, a differential comparator  102  is connected between the supply voltage rail, V cc ,  69 , and the reference potential or ground line  74 . The output from the differential comparator  102  is developed at the source of a MOSFET device  104 , which is connected in series with a current source  106  between the supply voltage rail  69  and the ground line  74 . A diode  105  is connected to the ground rail from the source of the PMOS transistor  104 . The diode  105  insures that the output node  107  stays above ground. 
     The differential comparator  102  has two npn transistors  108  and  110 , with respective MOSFET load transistors  112  and  114 . A current source  116  is connected from the emitters of the npn transistors  108  and  110  to the ground rail  74 . The current source  116  may be of current sourcing capability, for example, of twice the current sourcing capability of the current source  106 . 
     The inputs to the differential comparator  102  are provided by similar npn transistor circuits  118  and  120 . The npn transistor circuit  118  includes an npn transistor  122  and resistor  96  connected between the supply rail  69  and the base of the npn transistor  108 . A current source  123  is connected between the base of the transistor  108  and the ground rail  74 . Similarly, an npn transistor  124  is connected in series with a resistor  126  between the supply rail  69  and the base of the npn transistor  110 . A current source  128  is connected between the emitter of transistor  124  and the ground rail  74 . 
     The current sources  123  and  128  source a current of value I 2 , which is derived from the voltage across the MR head  18  by circuitry described below in detail with reference to FIGS. 5 and 6. Thus, the voltages applied to the bases of transistors  108  and  110  include a head voltage component to enable the voltage ratio to be developed to detect the fault condition of the MR head, also as below described. 
     The inputs to the transistors  122  and  124  are provided by the voltage that is dropped across the input resistor  80 , which is connected between the supply rail  69  and ground rail  74  in series with variable current sources  76  and  78 . The capacitor  88  is connected in parallel with the resistor  80 ; consequently, the input voltage to the transistors  122  and  124  also appears across the capacitor  88 . (The current sources  76  and  78 , the resistor  80  and capacitor  88  are the same components described in the servo circuit of FIG. 2 above.) 
     As mentioned, the invention is based upon the observation that the voltage dropped across the MR head  18  forms a ratio with the voltage dropped across the series combination of the resistor  68 , MR head  18 , and resistor  70 . If, for example, if a normal ratio of the MR/V  68 ,  18 ,  70  is one third, if the ratio drops to, for example, 1/1, an indication is given that the voltage across the MR head  18  is high, from this is can be concluded that an “open” fault exists in the MR head  18 . On the other hand, if the ratio becomes very small, for example, on the order of 1/20, the voltage across the MR head  18  is very small. From this, it can be concluded that a “short” fault exists in the MR head  18 . It should be observed that the voltage ratio as described is essentially independent of the state of the servo circuit  50 . Thus, even though the servo circuit  50  may have not settled to a final value, the ratio of the series combination of resistors  68 ,  18 , and  70  would remain substantially constant, and, therefore, faults of the head can be determined without regard to the condition servo loop  50 . 
     In operation, the differential comparator  102  of the circuit  95  divides the current of the current source  116  between the npn transistors  108  and  110 . When the resistance of the MR head  18  is normal, the ratio of VMR/VR 68 ,R 18 ,R 70  is established at a “normal” ratio, for example, 1/3. (The value of the resistor  126  may be set to a very small balancing value, since it conducts only the base current of transistor  110 .) At the desired “normal” ratio, the voltage at the output node  107  is set to zero by selection of an appropriate resistance value of the offset resistor  96 . At the “normal” ratio, the current conducts primarily through the left npn transistor  108 , so that the voltage applied to the gate of the PMOS device  104  is low. This produces a normally high state on the output node  107 , which, in turn produces a normally low state on the output line  44 . 
     If the resistance of the MR head increases, in order for the voltage value across the MR head to remain constant, a smaller current is produced by the servo circuit  50  through the series combination of resistor  68 , MR head  18 , and resistor  70 . The voltage drop across the series combination of resistor  68 , MR head  18 , and resistor  70  therefore decreases. Thus, the ratio of the voltage across the MR head  18  to the series combination of resistor  68 , MR head  18 , and resistor  70  decreases. The decreased voltage ratio changes the current distribution in the differential comparator  102 , increasing the voltage on the gate of the PMOS transistor  114 . This causes the voltage on the normally high output node  107  to fall. When the voltage on the output node  107  falls below the threshold of the output inverter  128  (which may conveniently be a comparator, or similar circuit), the output on line  44  changes from low to high, indicating an open fault of the MR head  18 . 
     To establish the “normal” ratio, the value of resistor  96  is selected to produce the desired voltage on the output node  107 . Thus, in the embodiment shown, the value of the resistor is selected to drop a voltage of K 1 ×VMR, where K 1  is a ratio of the voltage across the MR head  18  to the series combination of resistor  68 , MR head  18 , and resistor  70 , and VMR is the voltage across the head. Typical values for K are shown in Table 1 below. As will become apparent, the values of K in Table 1 represent a fraction, typically ⅕, of the ratio of the voltage across the MR head  18  to the series combination of resistor  68 , MR head  18 , and resistor  70 . This is because the normal operation of the circuit that is typically provided by manufacturers to customers to generate the voltage value across the MR head for monitoring the voltage divides the head voltage by five. The monitoring circuit is shown and described in FIG. 5 below. Thus, according to Table 1, a value of K to produce an open threshold value may be, for example, 1.28 for an MR head resistance of 100 ohms. This sets the open fault threshold for the inverter  108  to correspond to a head resistance of 100 ohms. 
     The operation of the short fault detection circuit  95 ′ shown in FIG. 4 is similar. However, as mentioned, the value of the offset resistor  96 ′ is set differently at a value of K 2  times the voltage across the MR head. Since the circuit detects a “short” value, the K 2  selected may be selected to be  11 , which corresponds to a head resistance of 10 ohms. This sets the short fault threshold for the inverter  108 ′ to correspond to a head resistance of 10 ohms. 
     In order to invert the output signal so that both the open and short fault conditions produce a change from normally low to a high state, a second inverter  100  is provided in the short fault detecting circuit  95 ′ to produce the low to high transitioning fault indicating signal on output line  46 . 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 R MR  (ohms) 
                 K 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 350 (open) 
                 0.509 
               
               
                   
                 300 
                 0.560 
               
               
                   
                 277 
                 0.593 
               
               
                   
                 250 
                 0.632 
               
               
                   
                 200 
                 0.740 
               
               
                   
                 150 
                 0.920 
               
               
                   
                 100 
                 1.28 
               
               
                   
                  50 
                 2.36 
               
               
                   
                  30 
                 3.80 
               
               
                   
                  25 
                 4.70 
               
               
                   
                  20 
                 5.60 
               
               
                   
                  15 
                 7.40 
               
               
                   
                  12 
                 9.20 
               
               
                   
                  10 
                 11.0 
               
               
                   
                  8 
                 13.7 
               
               
                   
                  6 (short) 
                 18.2 
               
               
                   
                   
               
             
          
         
       
     
     In order to develop a reference that may be used to represent the voltage across the MR head, a circuit, such as the circuit  150  shown in FIG. 6 may be used. The circuit  150  of FIG. 6 develops a current I 2  that may be mirrored to provide currents in current sources  128 ,  123 ,  128 ′ and  123 ′ in the circuits of FIGS. 3 and 4. 
     More particularly, typically manufacturers of mass data storage devices provide a circuit that can be used by users that indicates an analog buffered head voltage (AMHV), which is developed from the MR head voltage. A typical such circuit  130  is shown in FIG. 5, to which reference is now additionally made. The circuit  130  includes a differential comparator  131  that receives the MR head voltage across its input terminals  132  and  132  via resistors  136  and  138 . The output from the differential comparator  131  is developed between an output pad  133  and ground. Feedback resistors  140  and  142  connect the respective outputs and inputs to scale the output voltage. In the embodiment shown, the output voltage is  ⅕ the input voltage.    
     The circuit  150  of FIG. 6 has two npn transistors  152  and  154  that may be used in conjunction with the circuit of FIG. 5, and, more particularly, in series with the output line from the comparator  131  and pad  133 . A resistor  158  is connected between the emitter of transistor  152  and ground, and a current source  160  is connected between the emitter of transistor  154  and V ee . The feedback resistor  140  is connected back to the input of the comparator  131  in the manner shown in FIG.  5 . The ABHV output on pad  133  is developed on the emitter of emitter-follower connected transistor  154 . The current  12  that flows through transistor  152 , which is controlled by the voltage ABHV, is related to the MR head voltage, and can be mirrored, as mentioned, to the circuits of FIGS. 3 and 4. 
     It should be noted that although the embodiment of the invention described is best suited for use with a voltage mode biased MR head, it can be also be advantageously employed with a current mode biased MR head. Since in the current mode head biasing circuits the current flowing through the MR head are known and controllable, however, the problems of determining head faults may not be as difficult to address in current mode systems. 
     Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention, as hereinafter claimed.