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 xe2x80x9cshortxe2x80x9d 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.
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 xe2x80x9copenxe2x80x9d 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 xe2x80x9cshortxe2x80x9d 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 xe2x80x9copenxe2x80x9d fault if the magnitude is above a predetermined value and a xe2x80x9cshortxe2x80x9d 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.