Abstract:
A circuit ( 50 ) and method are provided for generating a signal ( 84 ) indicating an open impedance fault of a coil ( 56 ) of a write head ( 18 ) of a mass data storage device ( 10 ). The circuit ( 10 ) includes a differential amplifier ( 51 ) with the electrical component ( 56 ) being connected as a load in a first leg thereof. An impedance element ( 64 ) is provided in a second leg of the differential amplifier ( 50 ), and a pair of bipolar transistors ( 52,54 ) are respectively connected in the first and second legs. The pair of bipolar transistors ( 52,54 ) have a constant bias voltage (Bias) applied to inputs thereof, and an output circuit is connected to an output node of the first leg of the differential amplifier ( 51 ).

Description:
BACKGROUND OF INVENTION 
     1. Field of Ivention 
     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 faults in a write head of mass data storage devices, hard disk drives, or the like, particularly when the write head has an open fault while its electrical resistance is still relatively small. 
     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. Many applications are still being developed. Applications for hard disk drives are increasing in number, 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 write head portion of the head mechanism of the type to which the present invention pertains has a coil through which write currents are passed to create a magnetic field adjacent the disk to selectively orient the magnetic domains of the magnetic material on the disk. The impedance of the coil is generally in the range of between about 15 and 30 ohms. 
     Sometimes, however, the head mechanisms experience faults, the faults of interest herein being an open fault and a short fault. Efforts have been made to detect such open and short faults; however, such efforts have required relatively complicated circuitry, and generally have required a considerable impedance change in the open direction before the fault could be detected. 
     In the past, circuits used to detect an open condition of the coil of the head included a circuit having first and second current paths with the coil connected between the control elements, for example, the bases, of the current controlling devices in each path. The output was applied to control the differential currents in a differential amplifier, which produced an output fault—indicating signal if the currents became unbalanced beyond a predetermined limit. The complicated circuit used generally required a detection threshold for an open head fault of over 3000 ohms. When an open head fault appeared with a head resistance between about 150 to 3000 ohms, the pre-existing technique could not properly detect the fault. 
     In the case of a short to ground, the coil was connected in series with a pair of central transistors that produce currents that are mirrored in respective outer current mirror transistors. When the coil was shorted to ground, the current in the central transistors became large, which was mirrored in the outer mirror transistors to provide an indication of the short. 
     What is needed, therefore, is a relatively simple circuit and method for detecting open and short conditions in a circuit element, such as a data transducer or head, in which the open fault condition can be detected at a selectable detection threshold slightly above the normal impedance of the head. 
     SUMMARY OF INVENTION 
     In light of the above, therefore, it is an object of the invention to provide an improved circuit and method for detecting an open condition in a circuit element, such as a data transducer or head, or the like. 
     One of the advantages of the circuit used in accordance with a preferred embodiment of the invention is that an open head condition can be selectively established at a lower detection threshold than that of circuits used heretofore. 
     Another advantage of the circuit used in accordance with a preferred embodiment of the invention is that a fewer part count is required to implement the circuit. 
     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. 
     According to a broad aspect of the invention, a circuit is provided for generating a signal indicating an impedance fault of an electrical component. The circuit includes a differential amplifier with the electrical component being connected as a load in a first leg thereof. An impedance element is provided in a second leg of the differential amplifier, and a pair of bipolar transistors are respectively connected in the first and second legs. The pair of bipolar transistors have a constant bias voltage applied to inputs thereof, and an output circuit is connected to an output node of the first leg of the differential amplifier. 
     According to another broad aspect of the invention, a circuit is provided for generating a signal indicating an impedance fault of an electrical component. The circuit includes means for providing differential amplification in first and second differential current legs, the electrical component being connected as a load in the first differential current leg. Means are also included for providing a matching impedance in the second differential current leg. Active amplifier means having a constant bias applied to inputs thereof are provided in the first and second differential current legs for providing differential amplification of currents in the first and second differential current legs. Output circuit means are connected to an output node of the first differential current leg for providing the signal indicating an impedance fault of the electrical component. 
     According to yet another broad aspect of the invention, a method is presented for generating a signal indicating an impedance fault of an electrical component connected as a load in a first current leg. The method includes differentially amplifying currents in respective the first current leg and a second current leg with a fixed amplification value. A matching impedance is provided for the electrical component in the second current leg, and an output amplifier is provided at an output node of the first current leg for providing the signal indicating an impedance fault of the electrical component. 
    
    
     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 differential preamplifier circuit for use in detecting an open fault condition of a head transducer of a mass data storage device in conjunction with an “H-bridge” write head writer circuit, in accordance with a preferred embodiment of the invention. 
    
    
     In the various figures of the drawing, like reference numerals are used to denote like or similar parts. 
     DETAILED DESCRIPTION 
     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 . 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  is used both to record user data to and 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 faults of the write head  18  can be reliably detected during operating modes other than a write mode of operation. Thus, when head resistance, which is normally in the range of about 15 to 30 ohms becomes larger than a predetermined resistance, such as 150 ohms, a fault indication signal is generated. 
     A schematic diagram of a circuit  50 , according to a preferred embodiment of the invention, for detecting open write head faults, is shown in FIG. 2, to which reference is now additionally made. The circuit  50 , which may be included as a part of the preamplifier circuitry  24  shown in FIG. 1, is relatively simple in implementation, reliable in performance, and is flexible to enable the open head detection threshold to be selectively adjusted. The circuit uses relatively few components and consumes little power. 
     The circuit  50  includes a differential amplifier  51 , which includes NPN transistors  52  and  54 , 54 ′, each having a constant bias voltage applied to their bases on line  55 . The coil  56  of the write head  18  is connected between pads  58  and  60  in a first leg of the differential amplifier  51 , with a biasing current source  62  connected from one side of the write head coil to ground. A resistor  64  is connected in the second leg of the differential amplifier  51  to the biasing current source  62  to balance the impedance of the write head coil  56 . Diodes  66  and  68 , 68 ′ are also connected in the respective legs of the differential amplifier  51  to isolate the coil from the transistors  52  and  54 , 54 ′, and current sourcing PMOS transistors  70  and  72 , 72 ′ are provided on the topside of the differential transistors  52  and  54 , 54 ′ to provide supply currents to the respective first and second current legs. 
     The drive signals are provided to the coil  56  of the write head  18  by an H-bridge writer  74 , of known construction. Impedance matching resistors  76  and  78  are connected in series between respective opposite legs of the H-bridge writer  74  and opposite ends of the coil  56  of the write head  18 . Thus, write currents are applied to the coil  56  by the H-bridge writer  74  in normal operation, such currents being isolated from the transistors  52  and  54 , 54 ′ by the diodes  66  and  68 , 68 ′ as mentioned above. 
     The fault indicating output of the circuit  50  is taken from the collector of the differential NPN transistors  54 , 54 ′, to be detected, inverted, and amplified by a PMOS transistor  78 . The drain of the PMOS transistor  78  is biased by a current source  80 , and the fault output detected thereupon is applied to an inverter  82  for delivery on output line  84 . In the circuit shown, the inverter  82  provides a digitized output signal on line  84  indicating the presence or absence of an open fault condition. 
     In operation, the value of the current that flows through the left leg of the differential amplifier  51  is established only by the bias voltage applied to the base of the NPN transistor  52 . The voltage on the collector of the NPN transistor  52  is then applied to the gates of PMOS transistors  72  and  72 ′ to provide a differential current in the right leg of the differential amplifier  51 . This is established such that the bias applied to the gate of the PMOS transistor  78  normally holds the PMOS transistor in a conducting state. 
     If the coil  56  of the head  18  opens, or experiences a detectable increase in impedance, a lower differential current flows in the right leg of the differential amplifier  51 , with most of the current flowing in the left leg. This raises the voltage on the gate of the PMOS transistor  78 , causing it to turn off. This decreases the voltage on the input of the inverter  82 , which is inverted at its output on line  84 , to produce a high state signal thereat, signaling the open head condition. 
     It will be appreciated that by careful selection of the biasing of the various circuit components, and selection of the value of the resistor  84  the voltages at the input to the inverter  82  can be made to switch the output from its normal low voltage to a high value to indicate the open condition of the coil  56  of the head  18 . It should be noted that although the term “open” is used, it does not necessarily mean that the impedance of the coil  56  is infinite. Thus, if the normal impedance of the coil  56  is in the range of about 15 ohms to about 30 ohms, the circuit  50  can be biased by selection of the value of resistor  84  to decrease the input voltage to the inverter  82  to a predetermined threshold voltage at a relatively low voltage level, compared to previous techniques. It has been found, for example, that a change in impedance of the coil  56  to only 150 ohms or more can be detected. Also, it will be appreciated by those skilled in the art that the circuit and method of this invention can also be applied to other fields where resistance detection is needed. 
     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.