International Business Machines Corporation

William John Case

Christopher N Wallis

Robert Duncan Commander

Servo records provided at intervals on a record medium are used in a sampled servo system to define data tracks on the medium. Asperities in the medium, or problems arising from the servo writing process can in some instances lead to mis-alignment of a servo path 33 defined by a servo sample with the corresponding desired data track. This causes non-zero position error signals to be produced even when the track following head is accurately positioned over the data track. The problem is solved by measuring the position error signals derived from each servo sample with the head constrained in the correct on-track position. The value of each of these position error signals is then stored as a digital number forming part of a correcting byte in the data section immediately preceding the associated faulty sample. The correction byte also includes a flag bit which is set to demark the next servo sample if the error is so large as to be deemed uncorrectable. Thereafter, in use if the flag bit is unset, the servo system reads the digital number from the correction byte, and uses it to dynamically correct the position error signal from the associated faulty servo sample. If the flag bit is set, the position error signal from the previous sample is carried forward and used instead of the position error signal from the current sample.

Sampled servo position control system

Positioning and motion control of two objects, for example, of a transducer and a disk record medium having tracks including pre-recorded servo information, is achieved in two modes of operation in the following manner: During a track following mode the transducer is connected in a closed servo loop and position error signals derived by the transducer are used to energize an actuator mechanism coupled to the transducer so as to maintain the transducer in the on-track position. During a track seek mode, the actuator is energized to cause the transducer to follow a desired velocity profile, at least a part of which is achieved with the transducer connected in a closed servo loop. The loop is fed with position error signals derived by the transducer sampled at times calculated to coincide with the on-track positions of a transducer following the desired velocity profile. The servo system responds as in track following mode to energize the actuator mechanism so as to reduce the instantaneous transducer position error from the true on-track positions during this part of the access operation with the overall effect of causing the transducer to follow the desired velocity profile more closely.

Motion control system

Data track positioning information for data tracks on a stack of recording disks is provided by servo information pre-recorded on one surface of the stack. In addition, each data track on the remaining surfaces of the stack has servo information relative to that track pre-recorded in sectors around the track. Track following operations are controlled in a closed loop servo system using on-track position information derived from the sectored servo tracks associated with the track being followed. The bandwidth of the position error signal derived from the sectored servo information is increased by adding high frequency components obtained from the continuous servo information on the servo surface. Track access operations are controlled using track crossing information derived from the pre-recorded servo information on the servo surface.

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Winchester, United Kingdom

William John Case