Disk Drive Shock and Vibration Correction
with an HDA Mounted Angular Accelerometer

Pioneering work by Dr. Sidman 
with contemporary application 
to high TPI disk products

United States Patent 5,426,545:  Sidman et al.
Active Disturbance Compensation System for Disk Drives

ABSTRACT
The present invention senses angular acceleration of a Head Disk Assembly ("HDA") in a disk drive servo system and provides an acceleration feedback signal. A filter network filters the acceleration feedback signal and supplies a filtered signal for nullifying tracking error caused by shock, vibration and windup disturbances to the system.

FIELD OF THE INVENTION
The present invention generally relates to disk drive servo systems. In particular, this invention relates to methods and apparatus for providing active disturbance rejection of shock and vibration disturbances using measured angular acceleration of a Head Disk Assembly ("HDA") to provide an acceleration feedback signal to a disk drive head positioning servo system.

BACKGROUND OF THE INVENTION
In order to reliably read data from or write data onto a disk media surface of a disk drive, a read/write transducer or head must be positioned precisely over a track of the media surface from which data is read or on which data is written. Failure to accurately position the read/write head over the desired track during a read operation results in unreliable data retrieval. If the read/write head is improperly positioned during a write operation to the disk, not only may the written data be lost, but data on adjacent tracks may be written over and destroyed. The accurate placement of the read/write head is therefore crucial to the utility of disk systems.

One method used for head position control is a disk drive servo system. The disk drive servo system typically utilizes a head positioning actuator assembly for precisely positioning the head over the desired data track. The head actuator assembly includes an actuator motor which operates to position the head under the control of a head actuator servo loop. The head actuator control servo loop both initially positions the read/write heads over the desired data track and maintains the heads in that position.

The head actuator assembly and disks are typically housed in a Head Disk Assembly ("HDA"). The HDA is typically mounted to a drive chassis with the use of a plurality of shock mounts typically fabricated of rubber or other suitable compliant material. The stiffness of the shock mounts and other physical characteristics such as the inertia of the HDA determine the absorption or attenuation of shock forces applied to the HDA and to the drive chassis. This absorption or attenuation is practically limited due to geometrical constraints on HDA and shock mount size.

The application of physical vibration or shock to the drive chassis or internally generated vibration or shock to the HDA itself tends to cause positioning errors in the servo system. These disturbances may be introduced by, for example, spindle imbalance forces, external shock and vibration, as well as a self-induced shock mount/HDA resonance excitation known to those skilled in the art as windup. For example, reaction forces applied to the HDA by the actuator motor during seek activity may cause windup.

In known systems, a tradeoff exists between sensitivity to external shock and vibration on the one hand and to windup on the other. Higher compliance shock mounts may provide less sensitivity to chassis vibration but lower the HDA/shock mount resonant frequency and increase sensitivity to windup. The opposite is usually true also.

Known disk drive head positioning servo systems typically use a linear actuator assembly. This type of assembly utilizes a guided carriage arm assembly having a plurality of heads supported at one end and a voice coil at the other end with supporting bearings positioned between the voice coil and the heads. Other disk drive head positioning servo systems use an unbalanced rotary actuator. In these systems, the unbalanced rotary actuator may be a center-of-percussion type. In both such configurations, linear acceleration component sensing provides compensation for shock and vibration disturbances.

In the case of servo systems with a balanced rotary actuator, however, translational or linear HDA acceleration measurement accomplished with a single linear accelerometer cannot sense angular acceleration of the HDA without being sensitive to other HDA vibration components. Such components include both pure translational and other angular vibration components. Accordingly, known compensation or disturbance rejection systems, while performing satisfactorily for applications using linear or unbalanced rotary actuators, fail to address the problems of spindle imbalance forces, external shock or vibration and windup in systems having a balanced rotary actuator. This failure is due to the fact that only angular acceleration of the HDA in the direction of actuator rotation substantially causes positioning errors in systems that utilize a balanced rotary actuator. Other HDA acceleration components do not substantially induce such errors. Sensing these other components to provide compensation to the head positioning servo system typically induces positioning errors and, in fact, increases the sensitivity of the servo system to shock, vibration and windup.

SUMMARY OF THE INVENTION
The present invention overcomes the deficiencies in prior art systems with a Head Disk Assembly ("HDA") having a balanced rotary actuator head positioning assembly. A servo disturbance compensation or rejection system includes an angular acceleration sensor for direct, broadband measurement of HDA angular acceleration in the direction of actuator rotation. The angular acceleration sensor provides an acceleration feedback signal which augments existing feedback control signals for determining actuator motor current. In this way, the servo disturbance compensation system modifies the actuator assembly response to HDA shock and vibration during seek or track positioning operations for minimizing tracking errors.

A standard disk drive head positioning servo system provides feedback control signals related to the relative angular velocity or position error of the actuator compared to that of the HDA. The head positioning servo system provides an error signal indicative of relative angular position or relative angular velocity.

The present invention augments the position error signal with an angular acceleration feedback signal generated by an angular accelerometer mounted to the HDA. The angular accelerometer senses angular acceleration of the HDA caused by spindle imbalance forces, external shock and vibration, and self-induced HDA/shock mount resonance excitation typically caused by seek activity. The angular accelerometer has its sensitive axis oriented in the direction that induces positioning errors-that is, in the direction of balanced rotary actuator rotation relative to the HDA. The angular accelerometer provides an output acceleration signal to an amplifier having a preselected gain for the servo disturbance rejection system.

The amplifier provides an acceleration feedback signal with an amplitude which is optimal to minimize tracking errors in the head positioning servo system due to these disturbances.

The servo disturbance compensation system includes a gain-stabilization filter network that receives the acceleration feedback signal and provides a filtered, bandlimited acceleration feedback signal indicative of angular acceleration of the HDA. In this way, the servo system nullifies tracking error caused by such disturbances. The gain-stabilization network passes the filtered acceleration feedback signal to a summer which also receives the feedback control signals from the existing servo system. The summer provides a difference signal to a power amplifier which thereafter passes a motor current control signal to the actuator assembly.

FIG. 1 is a perspective view of a Head Disk Assembly ("HDA") with an angular acceleration sensor according to the present invention.

FIG. 2 is a simplified plan view and phantom view of the HDA of FIG. 1 showing the effect of rotation of the HDA on positioning error in a disk drive with a balanced rotary actuator assembly.

FIG. 3 is a block diagram representation of an active disturbance rejection system used in the HDA of FIGS. 1 and 2 according to the present invention.

FIG. 4 is a block diagram representation of a pair of linear accelerometers arranged in a push-pull configuration for measuring angular acceleration in the active disturbance rejection system of FIG. 3.

FIG. 5 is a graphical representation of track position error and actuator motor current during and after a seek operation without the use of acceleration feedback according to the present invention shown as a function of time.

FIG. 6 is a graphical representation of track position error and actuator motor current during and after a seek operation with the use of acceleration feedback according to the present invention shown as a function of time.

FIG. 7 is a graphical representation of a comparison of track position error upon receipt of a calibrated impulse to the HDA in both an uncompensated system and a compensated system as a function of time.

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