Patent Grant
6893329
The present invention relates to a polishing apparatus and a method for polishing an object under polish, which has a very thin surface to be polished, using an abrasive tape, and a manufacturing method for a magnetic disk utilizing them.
For a magnetic disk, which is used as an information record medium in a computer, etc., a requirement of the high recording density is becoming greater in recent years; accordingly films formed on surfaces of the magnetic disk, such as magnetic layers and protective films, are becoming thinner.
In a manufacturing process of the magnetic disk, undercoating layers with non-magnetic metal, undercoating layers with metal, the magnetic layers, the protective films, etc. are formed on surfaces of a disk substrate. Then, in order to remove small protrusions generated during these membrane forming processes and in order to clean up the surfaces of the magnetic disk, the tape cleaning is carried out on the surfaces of the magnetic disk by a polishing apparatus. The tape cleaning is to polish the surfaces of the magnetic disk by pressing tape like abrasives against the surfaces of the magnetic disk while the disk is rotating.
In this tape cleaning process, an air pressure or the spring force as described in the Japanese Patent Laid-Open 1990-106264, for example, was conventionally employed for pressing abrasive tapes against the surfaces of the magnetic disk. In an apparatus employing the spring force as described in the Japanese Patent Laid-Open 1990-106264, for example, a pressure for pressing the abrasive tape against the surface of the magnetic disk was approximately 50-75 g. With regard to the polishing apparatus carrying out the tape cleaning, there are also the Japanese Patent Laid-Open 2001-67655 and the Japanese Patent Laid-Open 2001-71249. The Japanese Patent Laid-Open 2001-67655 has a description of “the pressing force is usually 30-200 g, preferably 50-150 g, more preferably 50-100 g”. The Japanese Patent Laid-Open 2001-71249 has a description of “10 g, for example”.
The thinner the protective film, etc. becomes due to the high recording density, the lower the pressure for pressing the abrasive tape against the surface of the magnetic disk needs to be in order to prevent the damage on the polished protective film, etc. Moreover, a surface position of the magnetic disk moves during a polish due to many factors, such as deformations or waves on the surface of the magnetic disk, a deflection of the surface when the magnetic disk is rotating, assembly alignment errors of the polishing apparatus and a vibration of a spindle that rotates the magnetic disk. In the conventional polishing apparatus employing the air pressure or the spring force, when the surface position of the magnetic disk moves, the pressure for pressing the abrasive tape against the surface of the magnetic disk fluctuates, so that it becomes difficult to polish the surface of the magnetic disk uniformly.
Furthermore, the damage occurs due to the shock when the abrasive tape touches the surface of the magnetic disk, even if the pressure for pressing the abrasive tape against the surface of the magnetic disk is made small in order to prevent the damage on the polished protective film, etc. This is also becoming a problem.
The present invention is made in view of above-mentioned issues. The purpose of the present invention is to press the abrasive tape against the surface of an object under polish with a desired low pressure.
Another purpose of the present invention is to make a fluctuation of the pressure for pressing the abrasive tape against the surface of the object under polish small, and to polish the surface of the object under polish uniformly.
Another purpose of the present invention is to polish the surface of the object under polish uniformly, even if the surface of the object under polish deflects while polishing with a low pressure for pressing the abrasive tape against the surface of the object under polish.
Another purpose of the present invention is to prevent the damage generated when the abrasive tape touches the surface of the object under polish.
A feature of the present invention is rotating the object under polish, supplying and taking-up the abrasive tape to/from a tape head, and pressing the abrasive tape against the surface of the object under polish by pressuring the tape head using the electromagnetic force. For example, a voice coil motor is utilized in a tape head pressuring unit, which pressures the tape head. Since the tape head pressuring unit generates a pressuring force for pressuring the tape head using the electromagnetic force, it is able to set a minute pressuring force by controlling a drive signal, and to obtain the fine adjustment of the pressuring force easily by controlling the electric signal. Therefore, it becomes possible to press the abrasive tape against the surface of the object under polish with a desired low pressure.
Moreover, the pressuring force generated by the electromagnetic force is constant when the drive signal is fixed, and it does not depend on a position of the tape head or a surface position of the object under polish. The tape head stops at a point where the pressuring force for pressuring the tape head, the reactive force from the surface of the object under polish and the reactive force due to the elasticity of the tape head are balanced. When the surface position of the object under polish will move, the tape head will follow it and stop at a newly balanced point. Therefore, a movement of the surface position of the object under polish will be absorbed, so that it becomes possible to make the fluctuation of the pressure, with which the tape head presses the abrasive tape against the surface of the object under polish, small, and to polish the surface of the object under polish uniformly.
Another feature of the present invention is rotating the object under polish, supplying the abrasive tape to a tape head, driving a voice coil motor by generating a signal indicating a target pressuring force so as to pressure the tape head by the voice coil motor, detecting a pressuring force of the voice coil motor, and pressing the abrasive tape against the surface of the object under polish by controlling the voice coil motor with a pressure detection signal fed back to the signal indicating the target pressuring force. For example, a load cell is mounted between the voice coil motor and the tape head for detecting the pressuring force of the voice coil motor. Since the voice coil motor is controlled by feeding the pressure detection signal back to the signal indicating the target pressure, even if the surface of the object under polish deflects, the pressuring force of the voice coil motor is finely adjusted in response to a deflection by the feedback control. Therefore, it becomes possible to polish the surface of the object under polish uniformly.
Another feature of the present invention is rotating the object under polish, supplying the abrasive tape to a tape head, driving a voice coil motor by generating a signal indicating the first target position so as to move the tape head by the voice coil motor, detecting a position of the tape head, moving the tape head toward the surface of the object under polish and stopping it at a point, which is close to the surface of the object under polish, by controlling the voice coil motor with a position detection signal fed back to the signal indicating the first target position, driving the voice coil motor by generating a signal indicating the second target position so as to move the tape head by the voice coil motor, detecting the position of the tape head, making the abrasive tape to touch the surface of the object under polish by controlling the voice coil motor with the position detection signal fed back to the signal indicating the second target position, driving the voice coil motor by generating a signal indicating a target pressuring force so as to pressure the tape head by the voice coil motor, detecting a pressuring force of the voice coil motor, and pressing the abrasive tape against the surface of the object under polish by controlling the voice coil motor with a pressure detection signal fed back to the signal indicating the target pressuring force. Since the tape head is once stopped at the point, which is close to the surface of the object under polish, and the contact of the abrasive tape and the magnetic disk is carried out softly, it becomes possible to prevent the damage generated when the abrasive tape touches the surface of the object under polish.
Further details are explained below with the help of examples illustrated in the attached drawings.
In
In
In
The arms 63 are connected to movable portions 62a of the voice coil motors 62. The arms 63 are supported movably by the bearings 65, and ends of the arms 63 contact the axes 5a of the tape heads 5. When the VCM drive circuit 90 supplies drive currents to the voice coil motors 62, the movable portions 62a move due to the electromagnetic force and the arms 63 push the tape heads 5, so that the tape heads 5 press the abrasive tapes 3 against the surfaces of the magnetic disk 2.
Since the voice coil motors 62 generate pressuring forces for pressuring the tape heads 5 using the electromagnetic force, they are able to set minute pressuring forces by controlling the drive currents, and to obtain the fine adjustment of the pressuring forces easily by controlling the electric signals. Therefore, it becomes possible to press the abrasive tapes 3 against the surfaces of the magnetic disk 2 with desired low pressures.
In
Moreover, in
Furthermore, according to this example, since the abrasive tape 3 is pressed against the surface of the magnetic disk 2 by the tape head 5 that consists of a roller, the tape head 5 helps the abrasive tape 3 to run, and it becomes easy to supply the abrasive tape 3.
Furthermore, according to this example, since the magnetic disk 2 is supported by the spindle 22 such that the surface to be polished are arranged vertically, polish wastes generated from the surface to be polished drop from there, and it becomes possible to prevent the deposition of the polish wastes on the surface to be polished.
Furthermore, according to this example, since the swing arm 61 balances the tape head 5 by means of gravity such that the tape head 5 is supported parallel to the surface of the magnetic disk 2, and the tape head 5 is moved in the direction of pressing the abrasive tape 3 against the surface of the magnetic disk 2 when the swing arm 61 rotates, it becomes possible to support the tape head 5 movably by a simple component as the swing arm 61. Although, the arm 63 pushes the axis 5a of the tape head 5 in this example, other portions of the tape head 5 or the swing arm 61 may be pushed.
According to this embodiment, since the tape head 5 is connected to the movable portion 66a of the linear-type voice coil motor 66, the swing arm and the like is unnecessary, so that the structure becomes simple.
According to this embodiment, since the tape head 5 is connected to the movable portion 67a of the rotary-type voice coil motor 67, the swing arm and the like is unnecessary, so that the structure becomes simple, and the equipment becomes small comparing with the equipment utilizing the linear-type voice coil motor.
The polish wastes adhere to the abrasive tapes 3 after polish. If the abrasive tapes 3 are recovered above the magnetic disk 2, the polish wastes removed from the abrasive tapes 3 will float in the air near the surfaces to be polished. However, according to this embodiment, since the abrasive tapes 3 are recovered below the magnetic disk 2 by the recovery reels 7, it becomes possible to prevent the flotation of the polish wastes removed from the abrasive tapes 3 in the air near the surfaces to be polished. Although both the tape supply units and the tape take-up units are located below the magnetic disk 2 in this example, the tape supply units may be located above the magnetic disk 2 and only the tape take-up units may be located below the magnetic disk 2.
In the polishing apparatuses according to the embodiments explained above, it is required to rotate the magnetic disk 2 at high speed in order to improve the throughput. However, when a high-speed rotation of the magnetic disk 2 will be carried out to some extent, the voice coil motors will resonate to vibrations caused by many factors, such as deflections of the surfaces of the magnetic disk 2, etc., and mechanical vibrations will occur in the voice coil motors. Once the mechanical vibrations occur in the voice coil motors, the pressures, with which the tape heads 5 press the abrasive tapes 3 against the surfaces of the magnetic disk 2, will fluctuate.
Inside the voice coil motor 62, as shown in
According to this embodiment, the oscillation energy of the voice coil motor 62 can be consumed as the heat, and the mechanical vibration can be attenuated. Therefore, it becomes possible to stabilize the pressure, with which the tape head 5 presses the abrasive tape 3 against the surface of the magnetic disk 2, and to polish the magnetic disk 2 while rotating it at high speed.
The control circuit 91 sets the pressuring force of the voice coil motor 62 with a gain G1 of a setting circuit 93 and supplies an electric signal 101 to the voice coil motor 62 through a drive amplifier 94. The electric signal 101 causes the voice coil motor 62 to generate a certain pressuring force, and it is a current in this example. On the other hand, the current sensor 81 measures the current that flows into the coil of the voice coil motor 62. When the mechanical vibration occurs in the voice coil motor 62, a detection signal 102 from the current sensor 81 includes the information showing the amplitude, frequency, etc. of the vibration. Therefore, the current sensor 81 detects the vibration of the voice coil motor 62 by measuring the current that flows into the coil of the voice coil motor 62.
The detection signal 102 from the current sensor 81 is fed back to the control circuit 91, and the electric signal 101 supplied to the voice coil motor 62 is adjusted depending on the detection signal 102. In this example, the detection signal 102 fed back to the control circuit 91 is integrated and amplified with a gain G2 in an adjustment circuit 95, and a speed element 103 is obtained. This speed element 103 plays a role of attenuating the mechanical vibration of the voice coil motor 62 by negating a part of the output from the setting circuit 93.
According to this embodiment, it becomes possible to attenuate the mechanical vibration of the voice coil motor 62 by detecting the vibration of the voice coil motor 62 and feeding them back to the electric signal 101 that causes the pressuring force. Therefore, it becomes possible to stabilize the pressure, with which the tape head 5 presses the abrasive tape 3 against the surface of the magnetic disk 2, and to polish the magnetic disk 2 while rotating it at high speed. Moreover, comparing with the example shown in
According to this embodiment, since the high frequency signal is included in the electric signal 101, the pressuring force generated by the voice coil motor 62 includes a high frequency element, and the pressure, with which the tape head 5 presses the abrasive tape 3 against the surface of the magnetic disk 2, changes at high frequency, so that the polish performance improves.
In
When the control circuit 110 supplies drive currents to the voice coil motors 62, the movable portions 62a move due to the electromagnetic force and the arms 63 push the tape heads 5, so that the tape heads 5 bring the abrasive tapes 3 close to the surfaces of the magnetic disk 2. At this time, the linear displacement sensors 66 detect the positions of the tape heads 5, and position detection signals from the linear displacement sensors 66 are input to the control circuit 110. The control circuit 110 carries out the feedback control depending on the position detection signals from the linear displacement sensors 66 and adjusts the drive currents supplied to the voice coil motors 62, so that the voice coil motors 62 make the abrasive tapes 3 to touch the surfaces of the magnetic disk 2.
If the voice coil motors 62 are driven with linear ramp currents (or linear ramp voltages) when making the abrasive tapes 3 to touch the surfaces of the magnetic disk 2, there will be a high risk of damaging the magnetic disk 2 due to the inertia of the tape heads 5 since fixed pressures are applied to the tape heads 5. Moreover, since the tape heads 5 and the magnetic disk 2 have the inertia, it is difficult to adjust shock pressures when the abrasive tapes 3 touch the rotating magnetic disk 2 only by adjusting waveforms of the drive signals. For this reason, in this example, the tape heads 5 are stopped once just before the magnetic disk 2, then the tape heads 5 are positioned such that the abrasive tapes 3 touch the surfaces of the magnetic disk 2.
When the control circuit 110 further supplies the drive currents to the voice coil motors 62, the movable portions 62a move due to the electromagnetic force and the arms 63 push the tape heads 5, so that the tape heads 5 press the abrasive tapes 3 against the surfaces of the magnetic disk 2. At this time, the load cells 64 detect pressuring forces of the voice coil motors 64, and pressure detection signals from the load cells 64 are input to the control circuit 110. The control circuit 110 carries out the feedback control depending on the pressure detection signals from the load cells 64 and adjusts the drive currents supplied to the voice coil motors 62, so that the voice coil motors 62 gradually raise the pressuring forces and keep them after they become target pressures. Therefore, it becomes possible to stably carry out the fine adjustment of the pressuring forces of the voice coil motors 62, in other words, the load control for the magnetic disk 2.
The control circuit 110 comprises a logic control circuit 111, a load control circuit 120, a head position control circuit 130 and a detection circuit 140. The load control circuit 120 has a D/A converter 121, a differential amplifier 122, a phase compensation circuit 123, a selector 124 and a VCM drive circuit 125. The head position control circuit 130 has a D/A converter 131, a differential amplifier 132, a phase compensation circuit 133, the selector 124 and the VCM drive circuit 125. The selector 124 and the VCM drive circuit 125 are shared in the load control circuit 120 and the position control circuit 130. The detection circuit 140 has a selector 141, which receives detection signals from the load cell 64 and the linear displacement sensor 66, and an A/D converter 142, which converts the detection signal selected by the selector 141 into the digital data.
The logic control circuit 111 consists of a so-called gate array or a programmable logic device having a microprocessor unit. The logic control circuit 111 switches the load control circuit 120 and the head position control circuit 130 alternatively by generating selection signals, inputs the detection signal detected by each sensor and converted into the digital data from the detection circuit 140, and makes the VCM drive circuit 125 to supply a certain drive current according to the sequence shown in
An operation of the control circuit 110 will be hereafter explained according to the sequence shown in FIG. 11. First, the control circuit 110 carries out the bias control, in which the tape head 5 is moved from a starting point 0 and positioned at a point HP. Next, the control circuit 110 carries out the positioning control, in which the tape head 5 is moved from the point HP and stopped at a point NP, which is close to the surface of the magnetic disk 2. Then, the control circuit 110 carries out the soft contact control. In the soft contact control, the tape head 5 is moved to a point CP first, so that the abrasive tape 3 touches the surface of the magnetic disk 2. Then, the control circuit 110 turns into the load feedback control when the tape head 5 reaches the point CP, and gradually raises a load up to a final target load. When the load becomes the final target load, the control circuit 110 carries out the target load control and keeps the load. At last, after finishing a polish, the control circuit carries out the shunting control, in which the tape head 5 is positioned at the starting point O and shunted.
In the soft contact control, there are two methods in making the abrasive tape 3 to touch the surface of the magnetic disk 2. One is to position the tape head 5 at a predetermined position, so that the abrasive tape 3 is considered to contact the surface of the magnetic disk 2. Another one is to check a contact of the abrasive tape 3 and the magnetic disk 2 by actually detecting a contact pressure of approximately 50 mN using the load cell 64. The former is taken here for an example and each control will be explained hereafter.
First, in the bias control, the logic control circuit 111 generates selection signals S1, S2 for positioning. The selection signal S1 is a signal that switches the selector 124 from the load control circuit 120 to the head position control circuit 130. The selector 124 selects a signal in the load control circuit 120 when the selection signal S1 is not supplied, and it selects a signal in the head position control circuit 130 when the selection signal S1 is supplied. The selection signal S2 is a signal that switches the selector 141 from the load cell 64 to the linear displacement sensor 66. The selector 141 selects a signal from the load cell 64 when the selection signal S2 is not supplied, and it selects a signal from the linear displacement sensor 66 when the selection signal S2 is supplied.
While generating the selection signals S1, S2, the logic control circuit 111 generates the position data of the point HP as the target position signal. The control circuit 110 becomes a feedback control circuit and generates the drive current that makes a position of the tape head 5 equal to a target position. The target position signal from the logic control circuit 111 is supplied to the VCM drive circuit 125 through the D/A converter 131, the differential amplifier 132, the phase compensation circuit 133 and the selector 124, and the drive current is supplied to the voice coil motor 62 from the VCM drive circuit 125. At this time, the differential amplifier 132 generates a differential signal depending on the difference between the position detection signal from the linear displacement sensor 66 and the target position signal converted by the D/A converter 131. The position detection signal from the linear displacement sensor 66 is input to the logic control circuit 111 through the selector 141 and the A/D converter 142, and monitored. The tape head 5 stops when reaching the point HP.
In the positioning control, the logic control circuit 111 generates the drive signal data of a trapezoid wave as the target position signal while generating the selection signals S1, S2. This target position signal is supplied to the VCM drive circuit 125 through the D/A converter 131, the differential amplifier 132, the phase compensation circuit 133 and the selector 124, and the drive current is supplied to the voice coil motor 62 from the VCM drive circuit 125. At this time, the differential amplifier 132 generates the large differential signal, and the tape head 5 is moved toward the point NP, which is close to the surface of the magnetic disk 2, at high speed. The position detection signal from the linear displacement sensor 66 is input to the logic control circuit 111 through the selector 141 and the A/D converter 142, and monitored. The logic control circuit 111 carries out the stopping control when the tape head 5 reaches the point NP and makes the tape head 5 to once stop at the point NP or a close point beyond it.
In the soft contact control, the logic control circuit 111 first generates the position data of the point CP as the target position signal while generating the selection signals S1, S2. This target position signal is supplied to the VCM drive circuit 125 through the D/A converter 131, the differential amplifier 132, the phase compensation circuit 133 and the selector 124, and the drive current is supplied to the voice coil motor 62 from the VCM drive circuit 125. At this time, the differential amplifier 132 generates the differential signal depending on the difference between the position detection signal from the linear displacement sensor 66 and the target position signal converted by the D/A converter 131. The position detection signal from the linear displacement sensor 66 is input to the logic control circuit 111 through the selector 141 and the A/D converter 142, and monitored.
Here, when the logic control circuit 111 generates the position data of points, which gradually approach the point CP, by many steps instead of the position data of the point CP, the contact becomes softer. However, even if the logic control circuit 111 generates the position data of the point CP and moves the tape head 5 directly to the point CP, the contact can be soft since the distance from the point NP to the point CP is short and the tape head 5 has been once stopped.
Next, the logic control circuit 111 stops generating the selection signals S1, S2 when the tape head 5 reaches the point CP. By this, the selector 124 is switched from the head position control circuit 130 to the load control circuit 120, and the selector 141 is switched from the linear displacement sensor 66 to the load cell 64. A load detection signal from the load cell 64 is input to the logic control circuit 111 through the selector 141 and the A/D converter 142.
The logic control circuit 111 generates the load data, which rises gradually up to the final target load, as the target load signal depending on the load detection signal from the load cell 64. The control circuit 110 becomes a feedback control circuit and generates the drive current that makes the pressuring force of the voice coil motor 62 equal to a target load. The target load signal from the logic control circuit 111 is supplied to the VCM drive circuit 125 through the D/A converter 121, the differential amplifier 122, the phase compensation circuit 123 and a selector 124, and the drive current is supplied to the voice coil motor 62 from the VCM drive circuit 125. At this time, the differential amplifier 122 generates a differential signal depending on the difference between the load detection signal from the load cell 64 and the target load signal converted by the D/A converter 121. And when the pressuring force reaches the target load, the control circuit 110 carries out the target load control and keeps the pressuring force equal to the target load while polishing the magnetic disk 2.
In order to check the contact of the abrasive tape 3 and the magnetic disk 2 by actually detecting the contact pressure using the load cell 64, as mentioned above, the selector 141 should be time division controlled and both the position detection signal from the linear displacement sensor 66 and the load detection signal from the load cell 64 should be input to the logic control circuit 111. Then, the head position control circuit 130 and the load control circuit 120 should operate in parallel, so that the soft contact control and the load control are carried out simultaneously.
Even if such time division control is not carried out, the contact of the abrasive tape 3 and the magnetic disk can be checked by actually detecting the contact pressure using the load cell 64, and the load control can be carried out by monitoring the detection signal from each sensor independently, without employing the selectors 141, 124, and integrating the phase compensation circuits 123, 133. In this case, the contact pressure to be detected will be approximately dozens to ten dozens mN.
The phase compensation circuit 123 mainly consists of a lead/lag filter circuit, which carries out the phase compensation when feeding the detection signal back during the load control. The phase compensation circuit 133 mainly consists of a lead/lag filter circuit, which carries out the phase compensation when feeding the detection signal back during the positioning control.
In the shunting control, the logic control circuit 111 generates the selection signals S1, S2 again and generates the drive signal data of the trapezoid wave for returning to the starting point 0 as the target position signal. This target position signal is supplied to the VCM drive circuit 125 through the D/A converter 131, the differential amplifier 132, the phase compensation circuit 133 and the selector 124, and the drive current is supplied to the voice coil motor 62 from the VCM drive circuit 125. At this time, the differential amplifier 132 generates the large differential signal, and the tape head 5 is moved toward the starting point O at high speed. The position detection signal from the linear displacement sensor 66 is input to the logic control circuit 111 through the selector 141 and the A/D converter 142, and monitored. The logic control circuit 111 carries out the stopping control when the tape head 5 reaches the starting point 0, and makes the tape head 5 to stop at the starting point O or a close point beyond it.
According to this embodiment, since the voice coil motor 62 is driven by generating the target load signal and controlled by feeding the load detection signal from the load cell 64 back to the target load signal, even if the surface of the magnetic disk 2 deflects, the pressuring force of the voice coil motor 62 is finely adjusted in response to a deflection by the feedback control. Therefore, it becomes possible to polish the surface of the magnetic disk 2 uniformly.
Furthermore, according to this embodiment, since the voice coil motor 62 is driven by generating the target load signal, which rises gradually up to the final target load, depending on the load detection signal from the load cell 64 and controlled by generating the target load signal indicating the final target load after that, it becomes possible to prevent the damage generated when the abrasive tape 3 touches the surface of the magnetic disk 2.
Furthermore, according to this embodiment, since the tape head 5 is once stopped at the point, which is close to the surface of the magnetic disk 2, and the contact of the abrasive tape and the magnetic disk is carried out softly, it becomes possible to prevent the damage generated when the abrasive tape 3 touches the surface of the magnetic disk 2.
The sensors for detecting the positions of the tape heads 5 in the present invention are not limited to the linear displacement sensor. Although the voice coil motor is driven forward and backward in this example, the feedback control can be carried out even if the voice coil motor is driven forward only since it receives the repulsion from the magnetic disk in practice. Moreover, although the D/A converter and the differential amplifier are provided in the load control circuit 120 and the head position control circuit 130 respectively in this example, the D/A converter and the differential amplifier may be used in common.
Although the voice coil motor is utilized in the tape head pressuring unit in the examples explained above, the present invention is not limited to this and what is necessary is to generate the pressuring force using the electromagnetic force.
The polishing apparatus and the polishing method according to the present invention are applicable to the polishing process (Step 220), the mirror-polishing process (Step 230) and the tape cleaning (Step 280). However, an object under polish is not limited to the magnetic disk, and the present invention is generally applicable to many things that tend to get the damage during a polish.
| Number | Date | Country | Kind |
|---|---|---|---|
| P 2003-071613 | Mar 2003 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3971163 | Trombley et al. | Jul 1976 | A |
| 5018311 | Malagrino et al. | May 1991 | A |
| 6129612 | Reynen et al. | Oct 2000 | A |
| 6283838 | Blake et al. | Sep 2001 | B1 |
| Number | Date | Country |
|---|---|---|
| 02-106264 | Apr 1990 | JP |
| 2001-067655 | Mar 2001 | JP |
| 2001-071249 | Mar 2001 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20040198181 A1 | Oct 2004 | US |
