Abstract:
Shorts are detected in terminals fed current by a driver connected to the terminals. To detect terminal-to-ground shorts, at least one virtual driver is connected to the driver for processing an output of the driver, and a detector detects if a short has occurred, based on an output of the virtual driver, by detecting when an output of the virtual driver exceeds a first threshold. If the detector detects that the output of the virtual driver has exceeded the first threshold, this indicates a terminal-to-ground short. To detect terminal-to-terminals shorts, a detector detects when an output of the driver drops below a second threshold. If the detector detects that the output of the driver has dropped below the second threshold, this indicates a terminal-to-terminal short. Shorts may be detected in terminals in one or more write heads. The driver, virtual driver, and detector may be included in a preamplifier. Detecting shorts in this manner avoids the loss of data without impeding performance of the write driver.

Description:
BACKGROUND 
     This invention relates generally to a system and method for detecting terminal shorts. More specifically, this invention relates to a system and method for detecting a terminal-to-terminal short and/or a terminal-to-ground short in a write head. 
     In conventional rotating disk data storage devices, one or more read/write heads, typically magneto resistive and inductive heads, respectively, including positive and negative terminals, are used to write data on and read data from an associated disk media surface. Such an arrangement is shown in FIG. 1 in which a disk assembly  100  is connected to a host system, a memory, e.g., a RAM  160 , and a CPU  170  via an interface adaptor  150 . The host system supplies control signals and data to be written to the disk assembly  100  and receives read data from the disk assembly  100  via the interface adaptor  150 . The CPU  170  controls, e.g., basic disk drive functions, based on information stored in the RAM  160 . 
     The disk assembly  100  includes a preamplifier  30  supplied voltage by a positive voltage supply and connected to the heads  20  via flexible attachments  25 . The preamplifier  30  receives from an associated channel device  40  both data signals to be written onto a disk  10  during a write operation, as well as control signals used to specify the individual read/write head  20  to be selected for a read or write operation. Data is supplied to the channel device  40  via a controller  50  connected to the interface adaptor  150 . The preamplifier  30  also typically supplies analog data signals read from the head  20  to the associated channel device  40 . A spindle motor  80  rotates the disks  10  under the control of a spindle motor controller  90 . An actuator motor, e.g., a voice coil motor  60 , moves the heads  20  to tracks at different radial positions on the disks  10  under the control of a position system  70  in response to control signals from the channel device  40 . 
     The manufacture of disk drive assemblies is becoming more complex as additional read/write heads and corresponding additional disks are being included. Chances for assembly error are high since the heads and flexible attachments are often supplied by multiple sources in an effort to meet critical production quotas and schedules. Defects can occur during assembly due to, e.g., soldering across terminals, bent flexible attachments, etc. These defects can result in terminal-to-terminal shorts that can result in lost data or can cause terminal-to-ground shorts that can cause excessive currents that can damage the sensitive read/write heads as well as the preamplifier and will also result in lost data. Disk drive yield and manufacturing expenses can be improved if circuits for detecting head assembly defects can be included in the preamplifier. 
     Fault detection devices are known. For example, U.S. Pat. No. 5,081,404 discloses a motor driver fault detection circuit that measures a motor short-to-ground or short-to-supply voltage by directly monitoring the motor terminal voltages. 
     Devices are also known for detecting excessive currents in drivers when a short-to-ground occurs. For example, U.S. Pat. No. 5,773,991 discloses a motor current sensing circuit that includes a mirrored H-bridge driver that mirrors current from the actual driver. Current is sensed by measuring the voltage across a resistor that takes the place of the motor in the mirrored driver. U.S. Pat. No. 5,483,404 describes a method for over current detection in an H-bridge motor driver. This circuit senses a differential voltage across a resistor in series between a positive supply and an H-bridge driver. 
     If conventional designs such as these were employed for sensing a short-to-ground in a write head, a high current output from the positive voltage supply connected to the driver might cause the driver to saturate, though the current may not be due to a short but may, instead, be merely a high write current. Thus, using such designs may impede driver performance. 
     There is thus a need for a technique for detecting write head terminal-to-terminal shorts and terminal-to-ground shorts to avoid the loss of data, without impeding the performance of a write head driver. 
     SUMMARY 
     It is therefore an object of this invention to provide a system and method for detecting write head terminal-to-ground shorts and terminal-to-terminal shorts that will avoid the loss of data without impeding the performance of the write head driver. 
     According to exemplary embodiment, these and other objects are met by a system and method for detecting shorts in terminals fed current by a driver connected to the terminals. 
     According to a first embodiment, at least one virtual driver is connected to the driver for processing an output of the driver. A detector detects if a short has occurred, based on an output of the virtual driver. For this purpose, the detector detects when an output of the virtual driver exceeds a first threshold. If the detector detects that the output of the virtual driver has exceeded the first threshold, this indicates a terminal-to-ground short. The shorts may be detected in terminals in one or more write heads. The driver, virtual driver, and detector may be included in a preamplifier. Detecting shorts in this manner avoids saturation of the driver due to high write current and avoids the loss of data. 
     According to a second embodiment, a detector detects if a short has occurred, based on an output of the driver. For this purpose, the detector detects when an output of the driver drops below a second threshold. If the detector detects that the output of the driver has dropped below the second threshold, this indicates a terminal-to-terminal short. These shorts may be detected in terminals in one or more write heads. The driver and detector may be included in a preamplifier. Detecting shorts in this manner avoids the loss of data. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, objects, and advantages of the invention will become apparent by reading this description in conjunction with the accompanying drawings, in which like reference numeral refer to like elements and in which: 
     FIG. 1 illustrates a conventional read/write head arrangement; 
     FIG. 2 illustrates an arrangement in which the invention may be implemented; 
     FIG. 3 illustrates an exemplary system for terminal-to-ground short detection according to a first embodiment; 
     FIG. 4 illustrates an exemplary system for terminal-to-terminal short detection according to a second embodiment; 
     FIG. 5 illustrates a method for terminal-to-ground short detection according to the first embodiment; and 
     FIG. 6 illustrates a method for terminal-to-terminal short detection according to the second embodiment. 
    
    
     DETAILED DESCRIPTION 
     For illustrative purposes, the following description is directed to a write head terminal short detection system. It should be appreciated, however, that the invention is not so limited. 
     According to exemplary embodiments, critical write head terminals shorts can be detected in a preamplifier before and during normal write operations. A block diagram of an arrangement in which the invention may be implemented is shown in FIG.  2 . 
     The system includes N inductive read heads, N inductive write heads, and a preamplifier  105 . Each write head includes a negative terminal Wrn and a positive terminal Wrp. Each read head includes a negative terminal Rrn and a positive terminal Rrp. The preamplifier  105  includes N driver units  110  and N readers  120 . One driver unit and one reader is allocated to each pair of write and read heads. Data is supplied to the write heads from lines Dp and Dn via the drivers  110 . The write heads are enabled by a signal delivered along the Write Enable line. A signal on the Head Select line selects one of the heads to write. The lines Dp, Dn, Write Enable and Head Select may be connected to a channel device such as that shown in FIG.  1 . 
     According to exemplary embodiments, the driver unit  110  outputs signals used for detecting terminal shorts. A signal WS is used for detecting a terminal-to-ground short. Output signals WSP, WSN and Iws are used for detecting a terminal-to-terminal short. These signals are delivered to short detection circuits  200 . 
     The short detection circuits  200  may include two circuits, one of which detects shorts across the positive and negative write terminals, and one of which detects a write terminal-to-ground short. Signals WSG and WTS are fault signals that represent a write terminal-to-short and a write terminal-to-ground short, respectively. For example, when the current WS is high, it causes the short detection circuits  200  to output a high signal WSG, indicating a terminal-to-ground short. When the signals WSP and the signal WSN are low, the short detection circuits  200  output a high voltage WTS, indicating a terminal-to-terminal short. These signals are output to, e.g., a register, drive an LED, a signal channel chip, etc. If a terminal-to-ground short is detected, the writer current may be quickly disabled to avoid damage to the inductive transducer and the preamplifier. 
     FIG. 3 illustrates a write head terminal-to-ground short detection system according to a first embodiment. An H-bridge driver and a virtual driver are connected to a terminal-to-ground short detection circuit  250 . The H-bridge driver and the virtual driver are included in the driver unit  110 , and the terminal-to-ground short detection circuit  250  is included in the short detection circuits  200 . The H-bridge driver includes transistors Q 1  and Q 2 , MOSFETs M 1  and M 2 , diodes D 1  and D 2  and current mirrors Iwc. When an input data signal Dp is high, transistor Q 2  and MOSFET M 1  conduct current in one direction through Head  0 . When the input data signal Dp is low, Q 1  and M 2  conduct current in the reverse direction through Head  0 . 
     The virtual driver is compatible with the H-bridge driver and includes transistors Q 3  and Q 4 . The transistors Q 3  and Q 4  are sized to pull about 0.01 Iwc through a resistor R 1  as each one conducts, alternately. 
     The terminal-to-ground short detection circuit  250  is connected to all of the drivers and includes resistors R 1 , R 2 , R 3 , R 4 , and R 5 , transistor Q 5 , capacitors C 1  and C 2 , inverters I 1  and I 2 , diodes D 3 , D 4 , and D 5 , a D-latch. Under normal operation, the current Iws output from the driver through the resistor R 1  is very small, so the transistor Q 5  is in the cutoff region. However, if either the positive write terminal Wrp or the negative write terminal Wm is shorted to ground, the current Iws through the resistor R 1  increases dramatically, and the transistor Q 5  enters the forward active region. Resistor R 3  is sized to limit the collector current of the transistor Q 5  as the base-emitter voltage of Q 5  becomes more negative. Diodes D 3 -D 5  keep the transistor Q 5  from saturating as the voltage across the resistor R 4  increases. The invertors I 1  and I 2 , the capacitor C 2 , and the resistor R 5  provide delay so that the fault will be latched correctly. The capacitor C 1  and the resistor R 2  form a low pass filter that attenuates the high frequency noise generated by the large write current switching at high speed. The signal WSG eventually goes high as the collector voltage of transistor Q 5  is latched. The D-latch holds the fault signal WSG which may be used to disable the writer current, Iwc. Alternately, or in addition, the signal WSG may be processed by, e.g., logic devices (not shown), and an appropriate action may be taken to indicate the short to the user, e.g., powering of LED&#39;s. In exemplary embodiments, the D-latch is reset when a Head Select signal is enabled, indicating that a different head is selected. 
     FIG. 4 illustrates a write head terminal-to-terminal short detection system according to a second embodiment. Similar to the system of FIG. 3, an H-bridge driver is connected to a terminal-to-terminal short detection circuit  275 . The H-bridge driver is included in the driver unit  100 , and the terminal-to-ground short detection circuit  275  is included in the short detection circuits  200 . The H-bridge driver includes transistors Q 1  and Q 2 , MOSFETs M 1  and M 2 , diodes D 1 , and D 2  and current mirrors Iwc. Transistors Q 3 ′, Q 4 ′ resistors R 3 ′, R 4 ′, and current sink Iws form a differential amplifier that senses the voltage across the selected write head terminals. Diodes D 3 ′ and D 4 ′ and resistors R 1 ′ and R 2 ′ ensure that the base voltages of the transistors Q 3 ′ and Q 4 ′ are never more than one base-emitter voltage drop below ground when a non-driven terminal swings to a negative voltage. 
     The differential pair of transistors Q 3 ′ and Q 4 ′ is designed to operate in two ways. In a normal write mode, the input signals WSN and WSP output from the write terminals are large enough to steer Iws from one side of the differential pair to the other. If the terminals are shorted, the input signals WSN and WSP are very small, so the differential pair will operate in the linear active mode. In this case, Q 5 ′ and Q 6 ′ level-shift and drive the outputs. 
     Resistors R 3 ′ and R 4 ′ and Iws are sized so that the transistors Q 3 ′ and Q 4 ′ are never saturated during normal performance. In addition, the current Iws must be large enough to drive the parasitic capacitances of the transistors Q 3 ′ and Q 4 ′ collector lines that are routed around the chip to the other drivers. 
     The terminal-to-terminal short detection circuit  275  includes transistors Q 5 ′, Q 6 , Q 7 , Q 8 , Q 9  and Q 10 , resistors R 3 ′, R 4 ′, R 5 ′, R 6 , and R 7 , capacitor C 1 ′, diodes D 5 ′ and D 6 , and current sources. The current sources have to be high enough to drive the next stage. Resistor R 5 ′ and capacitor C 1 ′ filter the common emitter voltage. Transistors Q 7  and Q 8 , resistor R 5 ′, and capacitor C 1 ′ form a peak detector circuit. The detector output varies about 500 mV depending on normal or shorted conditions. The peak detector is triggered based on whether the circuit is acting in a linear or a non-linear mode. For example, if the circuit is operating in a normal, non-linear mode, the peak detector output is high, e.g., 3.2 volts. If the circuit is operating in a linear mode, under shorted conditions, then the peak detector output drops, e.g., by 300 mV to 2.9 volts. 
     Transistors Q 9  and Q 10  and resistor R 6  comprise a comparator with a threshold set by diodes D 5  and D 6 , resistor R 7  and It. The threshold is set so that during normal operation, there is a large difference between the voltages, i.e., there is a big voltage swing. If the comparator output is less than the threshold, the output WTS is high, e.g., Vdd=5 v, indicating a terminal-to-terminal short. Otherwise, the output WTS is low, e.g., Vdd−0.8 v=4.2 v, indicating no short. The signal WTS is processed, e.g., by logic devices (not shown), to a range of 0 to Vdd, and/or an appropriate action may be taken to indicate the short to a user, e.g., powering of LED&#39;s. 
     FIG. 5 illustrates an exemplary method for detecting a terminal-to-ground short. The method begins at step  500  at which a current output Iws from the virtual driver through the resistor R 1  is monitored. A determination is made at step  510  whether the voltage across R 1  is greater than a first threshold, e.g., whether the voltage across R 1  is significant enough that transistor Q 5  enters the forward region. If not, there is not a short, and at step  520  the circuit continues in normal operation. If the voltage across R 1  is greater than the threshold, a signal, e.g., WSG, is output to disable the writer current Iwc at step  530 . 
     FIG. 6 illustrates an exemplary method for detecting a terminal-to-terminal short. The method begins at step  610  at which a voltage output from the driver is monitored, e.g., by a peak detector. The output of the peak detector is compared to a second threshold at step  610 . At step  610 , a determination is made whether the output is less than the second threshold. If not, there is not a short, and at step  620 , the circuit continues in a normal write operation. Otherwise, a high signal WTS, indicating a short, is output at step  630 . The circuit may still continue in a normal write operation, until forced to idle mode by some external means. 
     While FIGS. 3 and 5 and FIGS. 4 and 6 are described separately, it should be appreciated that the concepts illustrated in these figures may be combined, e.g., into a single system or method detecting both terminal-to-ground and terminal-to-terminal shorts. 
     Although described in the context of a write head system, it will be appreciated by those of ordinary skill in the art that this invention can be embodied in other specific forms without departing from its essential character. The embodiments described above should therefore be considered in all respects to be illustrative and not restrictive.