Sensors and Actuators 81 Ž2000. 313–316 www.elsevier.nlrlocatersna
Nanoscale positioning for magnetic recording Lianna He ) , Frank Zhigang Wang, Desmond J. Mapps, Paul Robinson, David Jenkins, Warwick W. Clegg Centre for Research in Information Storage Technology, SECEE, UniÕersity of Plymouth, Drake Circus, Plymouth, DeÕon PL4 8AA, UK
Abstract A nanoscale positioning system for magnetic recording purpose has been designed and fabricated. A two-dimensional random-access of the sample is performed by inclining the two parallelograms with the aid of two multi-layer piezoelectric ceramic actuators. A so-called ‘displacement contracting’ effect is used to enhance the positioning resolution. A positioning resolution of 1.6 nm with this stage has been achieved. This high-resolution, linearized and temperature-compensated system has the potential to address fundamental issues associated with recording at densities much greater than 10 Gbitsrin2 Ž1.5 Gbitsrcm2 .. q 2000 Elsevier Science S.A. All rights reserved.
1. Introduction The computer industry is developing rapidly towards more and more powerful systems requiring the storage of large amounts of data onto hard disk. The dramatic increase in areal density in magnetic recording has been made possible through numerous developments, including the reduction in fly heights and decreased head dimensions. Taken to an extreme, one can imagine a ‘head’, which has nanometer-scale dimensions, moving in contact or near-contact over the surface of a disk. Correspondingly, better servoing will be required to ensure acceptable error performance. To meet this need, in this paper, we will give a description of the design and development of a nanoscale positioning system for magnetic recording purposes.
2. Design principles Fig. 1 is a schematic of a one-axial moving parallelspring stage. This stage comprises three major parts: Ža. Supporting part A 1; Žb. Two rotating parts A 2 ; Žc. Parallel moving part A 3 . These form 4 hinges by reducing the end thickness of the A 2 members near A 1 and A 3 . Applying a force F horizontally along the geometric center G3 of A 3 will cause the A 2 s’ rotation around the two lower hinges
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thus resulting in a parallel displacement of A 3 . That is to say, a sample Žeither a hard disk platter or a magnetic tape coupon., mounted along the geometric center G2 of the A 2 s, will execute a horizontal displacement which is proportional to the force strength. Therefore, when F moves A 3 by a displacement of D x 1 , if the vertical distance between G3 and the lower hinge is l 1 and the vertical distance between G2 and the lower hinge is l 2 , the corresponding displacement D x 2 of the sample will result in: D x 2 s Ž l 2rl 1 . D x 1. Since l 1 ) l 2 , the designed parallel-spring stage can also be used as a displacement contractor for an actuator at the same time. For example, twice the displacement of the parallel moving part A 3 just moves the sample by a distance of one. This is a so-called ‘displacement contracting’ effect. Therefore, an enhanced resolution, compared with the position accuracy of the actuator itself, can be achieved in the sample. Next we will confine ourselves to the frequency response of this stage deformation in order to decide what kind of design issues we should follow to reach a high resonant frequency. This is the basic requirement for a high scanning rate of the sample. By solving the Lagrange’s equation of mechanics w1x in terms of the Lagrangian function, we obtain the resonant frequency as below:
4K 2 I q Ml12
where M stands for the mass of A 3 , I rotation moment of A 2 and K the elastic coefficient of the hinge. According
0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 0 9 9 - 0
L. He et al.r Sensors and Actuators 81 (2000) 313–316
Fig. 1. Schematic of a one-axial parallel-spring stage. It is also simultaneously used as a displacement contractor. See text for details.
to the formula Ž1. about the resonant frequency,a low-mass, high-stiffness stage has a high resonant frequency, which ultimately limits the scanning rate. There are 4 possible ways to achieve high resonant frequency: 1. To enlarge the elastic coefficient of the hinges; 2. To reduce the rotation moment of the rotating parts; 3. To reduce the length of the rotating parts; 4. To reduce the mass of the parallel-moving part.
3. Fabrication and characterization Based on the principles stated in the above section, a one-body two-axial parallel-spring stage has been designed and fabricated, as shown in Fig. 2. The overall size of the stage is 105 mm = 95 mm = 10 mm Žthickness.. The stage is made of phosphor bronze with low temperature coefficient and shaped by spark erosion. An ‘L’ shaped peripheral supporting part with three screw holes is used as the
Fig. 2. Photo of the one-body two-axial parallel-spring stage. An ‘L’ shaped peripheral part is the supporting part. Two ‘parallelograms’, one of which has 4 hinges, have been integrated into this stage. The sample will be mounted on the center of the small parallelogram with the aids of the four screws. Two piezoelectric actuators have been used, PZT1 is for x movement and PZT2 for y movement. See text for details.
basis of the whole stage. Two ‘parallelograms’ in a body, one of which has 4 hinges, have been integrated into this stage. The sample-containing small parallelogram, responsible for y-axial movement, is inserted inside the big one for x-axial movement. A two-dimensional random-access of the sample Žeither a hard disk platter or a magnetic tape coupon. is performed by inclining these two parallelograms with the aid of two multi-layer piezoelectric ceramic actuators. Note that the driving points of the two actuators are put on the parallel moving parts of the parallelograms, rather than on the rotating parts, to obtain an enhanced positioning resolution as demonstrated in Section 2. Multi-layer piezoelectric ceramic actuators, provided by Morgan Matroc, for generating movements in micrometer range are used here w2x. The amount of deformation of the actuator is proportional to the applied electric field and the d 33 coefficient of the material. That is to say, the displacement D x s nVd 33 , where n is the number of active layers, d 33 the strain constant, and V is the applied voltage ŽF 150 V.. The size of the actuator for x-axial movement ŽPZT1 in Fig. 2. is 4.5 mm = 5.0 mm = 11.5 mm, the maximum generated force is 1000 N, the capacitance is 1500 nF; the size of the actuator for y-axial movement ŽPZT2 in Fig. 2. is 4.5 mm = 5.0 mm = 21.5 mm, the force 1000 N, the capacitance 500 nF. The positions of the actuators can be slightly adjusted by M4 screws at the ends. This stage is controlled by a personal computer. As shown in Fig. 3, this computer produces a series of pulses through its IrO port, then counting the number of the pulses by a integrator and eventually converting them to a analog signal Ž0–1 V. by a 12 bit DrA converter. This analog signal is applied to a voltage booster ŽFig. 4.. The above stage has no scale to indicate the displacement. In advance of the practical application, we need to modify the displacement against a certain number of pulses. This is especially true when considering the hysteresis and nonlinearity in the piezoelectric ceramic actuator. That is to say, the displacement of the stage is not linear with the number of the pulses. The Optical Beam Deflection Detection ŽOBDD. technique and optical microscopy have been used to study the displacement property and the frequency response. During the OBDD measurement, a mirror is put on the sample position. To reduce the nonlinearity we use a simple method w3x to insert a capacitor in series with the
Fig. 3. Pulse signal and driving voltage.
L. He et al.r Sensors and Actuators 81 (2000) 313–316
Fig. 4. Diagram of the voltage booster.
piezoelectric actuator in the driving circuitry. The capacitance C of that capacitor is selected: for PZT1, C s 1300 nF; for PZT2, C s 550 nF. It is expected to compensate, at least, the nonlinearity due to the capacitance of the actuator. However, since this inserted capacitor shares about half of the booster’s output voltage, we have to enlarge the amplification of the booster correspondingly. In Fig. 4, R5, R6 and R7 are just used for this purpose. Fig. 5 shows the displacement of the stage along the x-direction against the driving voltage. Ža. and Žb. are the comparison between the case without compensation capacitor and that with capacitor. It is obvious that the compensation capacitor plays a very important role in reducing the nonlinearity of the piezoelectric ceramic actuators. ŽSimilar results has also been obtained for the y-direction.. Furthermore, the positioning resolution of the stage, i.e., the minimum displacement for one pulse, is 1.6 nm in the case of a random-access range of 6.5 mm = 6.5 mm. Fig. 6 is the frequency response of the stage deformation against a sinusoidal exciting driving voltage, whose voltage amplitude is 1 V and whose frequency is ranged from 5 Hz to 2 kHz. The vertical axis is its power. The 50 Hz peak due to
Fig. 5. The x-direction displacement property of the parallel-spring stage.
Fig. 6. Frequency response of the parallel-spring stage.
the mains should be ignored. So, a 208 Hz minimum resonant frequency along the x-direction has been confirmed. Similarly, a 381 Hz resonant frequency along the y-direction has also been confirmed. In this stage design, the x-direction parallel movement contains all the y movement components Žthe parallel movement mass is larger than that along the y-direction. and the rotation moment along the x-direction is also larger than that along the y-direction, so the resonant frequency along the y-direction is about twice of that along the x-direction, as interpreted according to Eq. Ž1..
4. System applications This nanoscale positioning parallel-spring stage has a wide range of practical applications. Fig. 7 is one example of applying the stage into use. The parallel-spring stage is mounted on a coarse stage, driven by two linear motors. The combined fine-positioning parallel-spring stage and coarse-positioning stage constitutes a two-stage positioning system. It can reach a centimeter order random-access range with nanometer resolution. In this system, a com-
Fig. 7. Photo of the nanoscale positioning system for magnetic recording.
L. He et al.r Sensors and Actuators 81 (2000) 313–316
mercial 9-channel thin-film inductivermagnetoresistance ŽMR. head is used to recordrdetect information on a medium sample. The slider is in physical contact with the sample. The velocity-independent-sensitivity of MR sensors make it possible to evaluate recording media and the MR head itself without causing thermal noise due to friction at room temperature. In order to realize a closedloop position control, one MR channel is used to provide servo signal for the entire head array. A PC is used to control the stage motion as well as acquire MR voltage through its interface. Reasonably good results have been obtained through this system w4,5x.
w3x K. Takata, T. Hasegawa, Appl. Phys. Lett. 55 Ž1989. 1718. w4x L.N. He, Z.G. Wang, B. Liu, D.J. Mapps, P. Robinson, Estimation of Track Misregistration by Using Dual-Stripe MR Heads, IEEE Trans. on Magnetics, Vol. 34, No. 4, July Ž1998. 2348–2355. w5x L.N. He, Z.G. Wang, D.J. Mapps, Direct-access probe data storage, 7th International Conference on Magnetic Recording Media, Maastricht, The Netherlands, 1998.
Biographies Lianna He is now Research Assistant of University of Plymouth. Dr. Lianna He has been engaged in magnetic recording by both theoretical and experimental approach.
Acknowledgements This work was performed under Bristish Government EPSRC GRrK87395. The authors thank Mr. P.J. Brown for his fabrication of the parallel-spring stage, Mr. S.C. Warner for his assistance in the coarse stage adjustments, and Mr. C. Chilumbu for his assistance in the OBDD measurements.
Frank Zhigang Wang is now with University of Plymouth as a Research Fellow. Dr. Wang has worked in numerous areas in magnetics and electronics fields. In 1995 Dr. Wang invented the Spin-Tunneling Random Access Memory ŽSTram..
Desmond J. Mapps is Professor of Information Storage Engineering at the University of Plymouth.
Paul Robinson is lecturer at the University of Plymouth.
References w1x T.L. Chow, Classical Mechanics, Wiley, New York, 1995. w2x Piezoelectric Multi-layer Actuators Technical Data Sheet, Morgan Matroc, 1997.
DaÕid Jenkins is lecturer at the University of Plymouth.
Warwick W. Clegg is Professor at the University of Plymouth.