Method and circuit for providing velocity-controlled head...

Details Method and circuit for providing velocity-controlled head... Method and circuit for providing velocity-controlled head...

2001-06-22

2004-02-24

Hudspeth, David (Department: 2651)

Dynamic magnetic information storage or retrieval

Automatic control of a recorder mechanism

Controlling the head

C360S061000, C360S078060

Reexamination Certificate

active

06697207

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to data storage devices having actuators, and more specifically, to integrated circuit technology and a method for providing closed loop self-contained control of head load/unload velocity in rotating media mass storage applications.
BACKGROUND OF THE INVENTION
Data storage devices, and in particular, data storage devices of the type that accept a removable cartridge containing a disk-shaped storage medium, usually employ either a linear actuator mechanism or a rotary arm actuator mechanism for positioning the read/write head(s) of the disk drive over successive tracks of the disk-shaped storage medium. In most disk drives, and particularly in those that receive removable disk cartridges, the linear or rotary arm actuators are moved to a retracted, or parked position when the disk drive is not in use. In such a retracted position, the read/write heads of the disk drive are moved off and away from the surface(s) of the storage medium in order to prevent damage to the head(s) and storage medium. In order to resume use of the disk drive, the read/write heads must once again be loaded onto the surface(s) of the storage medium so that the data transfer can begin. It is important that the head loading operation be carried out in a controlled manner to prevent damage to the read/write heads.
Some magnetic storage devices support a head loading velocity control mechanism for a disk drive that measures the back EMF voltage across the actuator of the disk drive to obtain an indication of the velocity of the actuator. The measured back EMF voltage is then employed in a control scheme to control the velocity of the actuator during a head loading operation. Unfortunately, the circuitry needed to measure the back EMF voltage across the actuator increases the cost and complexity of the disk drive. Furthermore, this technique provides only a rough control of the actuator velocity, which may not be acceptable in many applications.
Comparatively, other magnetic storage devices utilize a velocity control technique for a disk drive actuator that employ thermal measurements to estimate the velocity of the actuator. Again, however, the circuitry necessary to obtain accurate thermal measurements unduly increases the cost of the disk drive, and this technique is susceptible to inaccuracies.
Further yet, some devices employ high-precision glass scales affixed to a disk drive actuator for obtaining accurate position and track counting information during track seek operations. See, e.g., Thanos et al., U.S. Pat. No. 5,084,791. Unfortunately, the cost and complexity of the high-precision glass scales and associated optical circuitry make them disadvantageous. Certain products in the “BETA” line of Bernoulli disk drives manufactured by Iomega Corporation, the assignee of the present invention, employ an optical sensor and a gray-scale pattern affixed to a linear drive actuator to obtain an indication of the linear position of the actuator. However, these products do not, and are not capable of, deriving or controlling the velocity of the actuator using the position information generated with the gray-scale pattern and optical sensor.
U.S. Pat. No. 5,615,064, to Blank et al. discloses a digital storage system in which a flying read/write head is loaded onto the surface of moving storage media with controlled velocity to avoid contact with the surface of the storage media. Head load velocity is detected by measuring the back EMF generated by the head arm actuator. Improved control and accuracy is obtained by breaking up the head arm actuator drive power into a series of pulses and measuring the back EMF induced into the low impedance voice coil of the head arm actuator in between pulses but only after the actuator current has been reduced to substantially zero in order to avoid interference by actuator current induced voltages.
FIG. 1
is a schematic diagram of the voice coil motor driving circuit and a block diagram of the control, as taught by Blank et al. The current is driven through the voice coil
119
by transistors pairs
111
-
113
to move the actuator arm in one direction, and by
115
-
117
to move the arm in the other direction. When transistors
111
and
113
are turned on, current flows from the positive terminal of the power supply, down through transistor
111
, through the voice coil in a first direction from terminal
127
to terminal
129
, and out through transistor
113
to the negative terminal of the power supply. When transistors
115
and
117
are turned on, current flows from the positive terminal of the power supply, down through transistor
115
, through the voice coil in a second direction from terminal
129
to terminal
127
, and out through transistor
117
to the negative terminal of the power supply. In this way, current can be made to flow in either direction through the voice coil, and move the actuator arm in either direction. When all four transistors
111
-
117
are driven such that the current in the coil decays to zero, no voltage drops occur across the coil due to resistance.
The only significant voltage across the coil
119
is due to the back EMF generated by motion of the coil through the field magnet of the motor, although there may be some voltage due to leakage currents from the drive amplifiers. This back EMF is proportional to the velocity of the motion of the arm. The EMF is amplified by amplifier
121
, and fed to the control processor
123
. Control processor
123
includes an analog to digital (A/D) converter for converting the analog amplified EMF from amplifier
121
to digital signals for processing according to programmed instructions in a program memory. When low EMF signals are present, the processor
123
determines that the arm
27
is moving at a low velocity, and processor
123
signals the drive circuits
125
to which processor
123
is connected to once again drive current through the coil
119
in the direction to increase the velocity of arm
27
. After a calculated on-time of this drive current, the processor
123
again signals the drive circuits
125
to turn off the current to coil
119
so that a clear EMF signal can thereafter be measured, to thereby determine the velocity of arm
27
after the calculated on-time of the above-mentioned drive current. The circuit of
FIG. 1
continues to operate until a signal is received in the read head from the disk that indicates that the head has been loaded onto the disk. If the head is being unloaded from the disk, the current is driven through the voice coil in the opposite direction until the arm comes to rest in the detent
19
causing the EMF to go to zero.
Referring now to
FIGS. 2A and 2B
, further operation of the circuit of Blank et al. is described. The drive current waveform is shown on top in
FIG. 2A
, and the combined drive voltage and back EMF waveform is shown below in FIG.
2
B. These waveforms are generated by the control processor
123
using a simple threshold algorithm. In this regard, Blank et al. teaches to measure the actuator arm velocity using back EMF induced in the low impedance voice coil at times when drive current is not being applied. Because the drive voltage is several orders of magnitude larger than the back EMF, the voltage scale of the lower portion of the waveform of
FIG. 2B
is broken in the middle.
The first drive current pulse, which starts at time zero and continues until time two of
FIG. 2A
, provides torque at the voice coil to initiate movement of the actuator arm from its rest position. This drive current pulse is about five hundred microseconds. At time two, the current is turned off and the current is allowed to decay for about 200 microseconds so that back EMF can be measured without interference by the drive current voltage drop. As seen in the top part of the lower waveform, the back EMF has not yet reached the threshold
211
and, in fact, goes to zero just as EMF is measured at sample
217
. As soon as the EMF is measured and found to be below threshold
211
, the next current pulse sta

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