Data processing: measuring – calibrating – or testing – Measurement system – Speed
Reexamination Certificate
1999-09-24
2002-10-22
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system
Speed
C702S033000, C702S038000, C702S044000, C702S079000, C702S105000, C702S145000
Reexamination Certificate
active
06470291
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a technique for measuring the rotational speed or velocity of a motor shaft, such as the drive shaft of a tape reel in a data tape cartridge system, and providing feedback for servo control of the motor.
BACKGROUND
Computers utilize a variety of magnetic media devices for the storage of software programs and data. Information recorded on the magnetic medium takes the form of flux transitions that represent the binary “1's” and “0's” that form the digital information. Tape cartridges, such as single-reel tape cartridges, are commonly used in library or other archival data storage applications. In such applications, a drive mechanism typically provides bi-directional tape motion during read/write and locate/rewind operations. The single-reel cartridge design uses a take-up reel located inside the drive. A coupler grabs a leader pin at the start of the tape and draws it out of the cartridge and around the tape head to the take-up reel in the drive. After the leader pin is secured in the take-up reel, the take-up reel rotates and pulls the tape through the tape path. A gear built into the cartridge reel and a gear coupled to the drive reel motor form a clutch enabling the motor to drive the rotation of the tape reel within the cartridge. A separate motor drives the take-up reel. By selective operations of the drive motors, the tape drive can selectively withdraw tape from the cartridge and wind tape back into the cartridge. A drive intended for a two-reel (reel-to-reel) cartridge includes two clutches for engaging the tape reels within the cartridge. Separate motors drive the clutches, to rotate the reels first in one direction and later in the opposite direction, to wind the tape back and forth between the two reels.
The environment of such digital cassette tape drives imposes a number of requirements on the tape transport. For example, in many cases, the linear velocity of the tape at the record/playback head must be constant. The drive may actually read data from the tape during transport past the head(s) during transport in both directions. Different speeds are necessary for read/write operations and scanning operations to rapidly find a desired location on the tape. A number of techniques have been developed to determine and control tape speed based on measurement of the rotational speed of one or more of the reels.
In general, tape drives include elements to detect the speed of the tape from the rotational speed of the take-up reel and control the tape transfer by a feedback control system. For example, the shaft speed of one of the reels is detected and used to calculate the tape speed. Logic circuitry or programming serves to compare the tape speed with a tape speed command or desired setting. If the tape speed too low, the circuitry increases the drive current to a take-up reel motor to increase the torque of the take-up reel and accelerate the take-up reel and the tape. If the detected tape speed is too large, the current is adjusted to reduce the torque of the take-up reel motor to decelerate the motor and the tape.
For example, U.S. Pat. No. 4,125,881 to Eige et al. describes a reel-to-reel magnetic tape drive, which moves tape from one reel to another past a read/write head. A fine-line tachometer, mounted on one reel shaft, provides a fine-line reading in the form of a number of pulses per revolution. A second tachometer on the second reel shaft provides a single pulse per revolution of the second reel. The single pulse is used to gate the counting of fine-line tachometer pulses for each revolution of the second reel. Motor acceleration currents of a magnitude corresponding to the reel radii are generated to drive the reel motors. A servo algorithm uses the gated per-revolution fine-line tachometer count to determine the reel radii based upon the actual length and thickness of the magnetic tape, for common control of servo drivers for both the source and take-up reel motors.
U.S. Pat. No. 4,739,950 to Goker et al. discloses a system that moves magnetic tape past the read/write head at a constant velocity by separately servo controlling the source reel motor and the take-up reel motor. Separate fine encoders associated with each reel provide multiple pulses for each revolution of the respective reel. Two radius calculation circuits each receive pulses from a respective one of the encoders and each calculate therefrom the radius values of tape on both of the reels. A radius information selector then selects, for use in controlling the motors, that set of radius values that is calculated by the calculation circuit which is receiving encoder pulses at the greater sampling rate. Separate servo circuits drive the two motors at respective angular velocities determined by utilizing the selected set of radius values. In each drive circuit, the associated tape radius is multiplied by the actual reel angular velocity, as indicated by the time duration between consecutive pulses from the encoder associated with that reel. Each servo maintains this product equal to a preselected tape linear velocity value, thereby causing the separate motors to rotate the reels so as to establish the desired constant linear tape velocity.
U.S. Pat. No. 5,576,905 to Garcia et al. discloses a bi-directional, reel-to-reel tape transport in which magnetic tape can be moved in either of two opposing directions for data recording thereon. Control of tape position is implemented in a servo algorithm that uses tachometer inputs to determine parameter values for generating reel motor drive currents. A fine-line tachometer is mounted on each of two reels in the tape drive and multiplexing selects the fine-line output from the tachometer on the reel supplying tape for use in the servo algorithm.
U.S. Pat. No. 5,642,461 to Lewis discloses a wide range speed control system for a brushless DC motor, which uses a magneto resistive encoder. The encoder is coupled to a series of filters, which remove DC level and harmonic distortions from the resulting encoder signal. These filtered signals are then applied to the motor to control motor rotation speed. The speed control algorithm may be implemented in a microcontroller.
All of these tape speed control techniques rely on an accurate tachometer measurement to determine the motor or reel shaft speed. In most cases, the tachometer measurement utilizes an optical encoder associated with the shaft. The optical encoder generates a certain number of tachometer pulses during each revolution of the motor shaft. For example, a typical fine tachometer may generate a thousand pulses per revolution. In most modern servo systems, the speed measurement entails counting the optical tachometer pulses during a predetermined measurement period defined by the sampling rate of the motor control servo loop. The sample rate is defined based on the servo bandwidth and the computation power of the control processor.
Typically, the sample rate is constant, and the number of pulses counted changes with the speed of the motor. The higher sampling rate enables the servo system to make corrective changes more often but results in fewer pulses per sample, thereby reducing accuracy. As desired speeds increase, the number of tachometer pulses counted in an interval of the sampling clock also increases. If the sample time is long enough and the motor rate is high enough, there will be large count values, which produce satisfactory measurement accuracy. However, as the sample time period increases, the achievable servo bandwidth decreases, causing performance problems.
In many cases, a tape drive must operate at variable speeds, over a broad range. If the sample interval is set for accurate control at the high end of the range, the minimum speed may be so slow that there may not be enough counts to provide adequate servo measurement resolution. In some cases, there may be no tachometer pulses at all during some sample intervals.
It is possible to operate with different sample intervals, for different ranges of motor speeds. However, this r
Goker Turguy
Patrick Edward H.
Tang Stanley S.
Hoff Marc S.
McDermott & Will & Emery
Seagate Removable Storage Solutions LLC
Tsai Carol S W
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