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Precision positioning using MEMS based microactuator
Mechatronics 12 (2002) 1213–1223
Precision positioning using MEMS based microactuator Nitin Afzulpurkar *, Yossawee Weerakamhaeng Industrial Systems Engineering Program, Asian Institute of Technology, Thailand
Abstract The critical factor for the hard disk manufacturers when designing the new generation of high capacity hard disks is ‘areal density’. Although linear density has increased dramatically increases in track density are limited by servo bandwidths, which are controlled by the actuator resonance. An approach, which is currently being investigated by the researchers, is the use of a second high bandwidth actuator at the end of the conventional voice-coil actuator. After theoretical analysis and discussions with industry electrostatic design in which the microactuator will sit on the slider between the suspension and the read/write head is selected. The design factors are: higher servo bandwidth, higher resonant frequency and low voltage supply. To microfabricate such microactuator we propose to use the latest technology called microelectromechanical systems technology. After schematic drawing layout design is made, it has been tested for electrical connections in the electrical layout model and the optimized design parameters are obtained. Finally it is shown that the design will satisfy the required specifications. Ó 2002 Published by Elsevier Science Ltd.
1. Introduction The magnetic hard disk drive (HDD) has been maintaining significantly lower cost per megabit in comparison to solid-state memory technology. The areal recording density of hard disk drives has been increasing at a rate of 60% per year and the track density has been increasing at a rate of 30% per year [5]. The most profitable way that hard disks have been increased in capacity and speed is by storing more and more information in the same physical space Fig. 1. The areal density is
*
Corresponding author. E-mail address: [email protected] (N. Afzulpurkar).
0957-4158/02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd. PII: S 0 9 5 7 - 4 1 5 8 ( 0 2 ) 0 0 0 2 5 - 9
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Fig. 1. HDD capacity evolution.
defined as number of tracks per inch (TPI) times density of each track (BPI, bits per inch per track). When more data is packed in the same space, the size of each data bit is reduced requiring increased accuracy in positioning the read–write heads over the disk.
2. Servo control and frequency considerations The current HDD design employs a voice coil motor (VCM) which rotates the actuator about the pivot (cf. Fig. 2) and positions the head on the disk. An ideal actuator will maintain the perfect displacement (u) and rotational input (h) relationship ðu ¼ r hÞ because the natural frequencies and resonance effects are ignored. In the actual design the mechanical structure between VCM and head, which consists of E block (housing for head gimbal assembly), suspension and gimbal, is flexible and has characteristic resonance frequencies. The actuators have a very small stiffness about the bearing because of the flex cable and bearing which results in a vibrational mode in which the entire actuator oscillates about the pivot. Frequency (fp ) of this mode is about 100 Hz depending on the drive. To follow a data track, the
Fig. 2. Top view of the HDD.
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servo must correct off-track disturbances over a broad band of frequencies. This can result in off-track head motion and instabilities; which can cause the head to go offtrack [2]. The servo gain cannot be increased to reject the off-track disturbances near frequency (fp ). Tracking accuracy and speed of response increases as the servo bandwidth is increased. The head positioning servo systems of currently available commercial drives is limited to 500–700 Hz.
3. Development of dual-stage actuators To satisfy the accuracy, speed and frequency requirements caused by everincreasing track densities development of dual stage actuators is being carried out at many research laboratories. For description refer to [3,4,6]. In such two stage systems, a secondary actuator placed between suspension and slider can provide precision head positioning over the data track. Motion of the head results when voltage is applied to the actuator. The controller sends head movement commands to VCM for coarse, low frequency positioning used for seeking and simultaneously to the microactuator for fine, accurate and high frequency positioning. A secondary actuator placed between suspension and slider can provide precision head positioning over the data track. The second stage actuator should increase the closed loop bandwidth significantly resulting in at least an order of magnitude improvement in track-misregistration. 3.1. Types of microactuators Many different types of microactuators have been reported in the research work [1]. These can be classified into six types: electrostatic actuators, magnetic actuators, piezoelectric actuators, thermal actuators, hydraulic actuators, microsimulators. Based on the objectives for the HDD application, electrostatic and piezoelectric actuators have been used in research on the dual stage actuation systems so far. Electrostatic actuator can be linear type or rotary type. The linear actuator has many interdigitated fingers. When voltage is applied an attractive force is developed between the fingers which move together. Actuation dynamics can be described by a mass-spring-damper model [1]. Piezoelectric actuator uses piezoelectric elements which expand and contract in lengthwise direction when voltage is applied across their thickness. The top surfaces of the two piezo elements are connected by a wire stitch across the gap. One end of the bottom surface of each piezoelectric (PZT) element is adhesively to the part of load beam which is fixed to the base plate. The
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other end is adhesively bonded to the movable portion of the load beam. The PZT elements are oppositely polarized so that application of voltage makes one to expand and other to contract [3]. Operations of piezoelectric piggyback and shear-mode microactuators are described in [6,7]. Electrostatic actuator offers high accuracy and the actuation dynamics are described by simple second order linear model. Stiction and backlash is not present in these devices. On the other side the structure of the actuator has low stiffness springs which limits the maximum resonant frequency one can achieve. Piezoelectric material has high stiffness and can generate large force when used as actuator. This results in higher achievable resonant frequencies. The main disadvantage is that position accuracy is not as high as electrostatic actuator. Another disadvantage is that it cannot be made as an integral part of the head. 4. Microelectromechanical system microactuator One of the main goals of the development of microelectromechanical systems (MEMS) based devices is to be able to integrate microelectronics circuitry into micromachined structures with moving parts. This will result in completely integrated systems which will have advantages of low cost, reliability and small size as silicon chips. Using fabrication techniques and materials similar to microelectronics MEMS processes construct electrical components to provide computational capabilities and mechanical components which sense, control and provide actuation. Photolithography is the basic technique used to define the shape of the micromachined structures. Silicon macromachining is a widely used technique for MEMS device fabrication. MEMS fabrication requires drawing and layout tools to generate the pattern for adding or removing material. It needs a range of modeling tools such as simulators for mechanical movement, forces, electromagnetic fields, electronic device simulators and connective algorithms to reconcile and mix results from different simulators. 5. Structural design of electrostatic actuator The structural design method and layout for a linear resonant electrostatic microactuator (LREM) developed by the authors is described below. Typical microfabricated actuators exhibit very lightly damped resonance resulting in very poor settling performance if the resonant frequency is near the desired servo bandwidth. The approach used is to design the actuator to have an extremely high resonant frequency (5–10 times greater than servo bandwidth) so that actuator dynamics will have negligible effect on the servo system performance. Based on the above observation, the design criteria used for the design of LREM are: 1. The actuator should have resonant frequency of 20 KHz or higher so that the closed loop servo bandwidth can be 2 KHz.
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2. The actuator should attain displacement accuracy higher than 0:5 lm in order to be able to handle 40 KTPI HDD density. Based on these criteria, the design of LREM is carried out as described below. The actuator illustrated in Fig. 4 consists of a translating central plate, which is anchored to the fixed electrostatic comb drives. Actuation is achieved by electrostatic force generated using parallel plate capacitive electrodes mounted on either side of the plate. LREM is placed between the slider and the gimbal element. The desired motion of the actuator is translational motion in the direction of radial line on the disk platter. As shown in Fig. 4, LREM consists of two folded spring elements, two comb drives and one central plate. The folded spring is used for restoring the position after the plate is actuated by the comb drive. The supporting plate of the actuator is moved in the X+ direction by applying one polarity of voltage to the left hand comb drive, the plate is moved to the central position by the restoring force from the folded spring elements. The motion in X-direction takes place when opposite polarity voltage is applied to the right hand comb drive The schematic and layout representation of the electromechanical circuit of LREM is carried out using the Tanner Tools MEMS Pro software which is part of multichipmodule design system from Tanner research. We used the hierarchical layout editor L-edit which allows the MEMS design. The mechanical and electrical model of linear electrostatic comb drive is shown in Fig. 5. Based on the models of the elements the schematic for LREM is developed.
Fig. 3. Parametric illustration for folded spring elements.
Fig. 4. LREM: schematic representation.
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Fig. 5. Mechanical and electrical model of comb drive.
Fig. 6. Parameter illustration of LREM comb drive element.
After the schematic the graphical layout of the components is generated using MEMS library. This layout represents the actuator elements with a range of parameters. Parameter illustration of LREM comb drive element and folded spring element is shown in Figs. 6 and 3 respectively.
6. Optimization of the parameters The main criteria, which can be used to optimize the dimensions and performance of LREM, are as follows: Devices with small area are preferred for cost reduction. Smaller operating voltages are preferred for integrated voltages.
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The resonant frequency should be as high as possible. Large amplitudes of oscillations are required for better sensing and actuating capabilities. Dimension of the plate needs to be smaller than dimensions of a typical slider. The overall dimension of the microactuator should confirm with the effective area on the gimbal. Based on this the overall dimension of the microactuator is fixed as 3:113 mm 2:602 mm and the plate width and length are fixed at 0.4 mm each. From the parameter illustrations shown in Figs. 3 and 6, the variables, which can be optimized, are: width, length, inner gap and outer gap for folded springs, active rotor comb width (arwidth) and supply voltage. LREM behavior is simulated by varying the following parameters: spring length, active rotor comb width, voltage supply to comb drive. Sine wave voltages are applied to the comb drive to generate the electrostatic forces and DC voltage is applied as bias voltage to folded springs to preserve the electrostatic spring property. Spring length is varied between 150 and 350 lm in the step size of 10 lm. Active rotor comb width is varied from 510 to 1821 lm in the steps of 69 lm. The voltage supplied to comb drive is varied from 1 to 20 V in the steps of 1 V. By varying these parameters one at a time the displacement values at right node (rtm) and the resonant frequencies are calculated. The schematic representation shown in Fig. 4 is used to find relations between: 1. Length of folded spring, displacement of node rtm and resonant frequency. 2. Effective area of comb drive (arwidth), displacement of node rtm and resonant frequency. 3. Voltage supplied to comb drive, displacement of node rtm and resonant frequency.
7. Conclusion The advantages of dual-stage actuator for HDD over single stage actuator are identified. As areal densities increase, the track density needs to be increased. A system with high bandwidth secondary actuator placed between the suspension and the slider can be realized by MEMS technology. We calculated the optimum dimensions of the electrostatic MEMS actuator by constructing a model. Layout and mask design was done using Tanner MEMS Pro software. The relationship between parameters resonant frequency and rtm displacement against folded spring length is shown in Figs. 7 and 8. The relationship between
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Fig. 7. Relation between resonant frequency and spring length.
Fig. 8. Relationship between rtm displacement and spring length.
parameters resonant frequency and rtm displacement against arwidth is shown in Figs. 9 and 10. The relationship between parameters resonant frequency and rtm displacement against comb drive supply voltage is shown in Figs. 11 and 12. We can conclude from these figures that increase in folded spring length will reduce the resonant frequency but it is not much influenced by arwidth and comb drive voltage
Fig. 9. Relation between rtm displacement and arwidth.
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Fig. 10. Relationship between resonant frequency and bandwidth.
Fig. 11. Relation between resonant frequency and comb drive supply voltage.
Fig. 12. Relation between rtm displacement and comb drive supply voltage.
supply value. rtm displacement increases linearly with the arwidth and comb drive supply voltage. In the future work we plan to verify the resonant frequency of the actuator assembly using finite element analysis.
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Table 1 Optimized LREM component dimensions Element
Parameter list
Abbreviation
Value
Folded spring
Spring width Spring length Spring inner gap Spring outer gap Plate width Plate length
Width Length Inner gap Outer gap Width Length
5e-6 m 220e-6 m 5e-6 m 5e-6 m 400e-6 m 400e-6 m
Active rotor comb width Rotor yoke width Stator yoke width Length of comb fingers Width of comb fingers Air gap between fingers Stator-rotor finger overlab
Arwidth Rywidth Sywidth Flength Fwidth Airgap Rsoverlap
1545e-6 m 29e-6 m 50e-6 m 250e-6 m 29e-6 m 20e-6 m 100e-6 m 2 Vp
Plate Linear electrostatic comb drive
Voltage supplied
Fig. 13. Rtm displacement magnitude and phase relationship with frequency.
Optimized parameters for the LREM obtained by above analysis are shown in Table 1. The magnitude and phase of displacement values are plotted against frequency in Fig. 13. From the figure the resonant frequency is observed as 21.94 kHz which satisfies the design criteria of 20 kHz frequency. Acknowledgements The authors wish to acknowledge the valuable support and insight given by Mr. Brent Bargmann, Seagate Technologies and Mr. Chumnarn Punyasai, Dr. Itti Rittaporn of NECTEC, Thailand. References [1] Bank D, 1999. Introduction to Microengineering, MEMs Micromachine MST http://www.dbanks. demon.co.uk.
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[2] Evans RB, Griesbach JS, Messener WC. Extending bandwidth with dual-stage suspensions. Data Storage 1999:43–6. [3] Evans RB, Griesbach JS, Messener WC. Piezoelectric Microactuator for Dual Stage Control. IEEE Trans Magn 1999;35(2):977–82. [4] Fan L-S, Hirano T, Hong J, Webb PR, Juan WH, Lee WY, et al. Electrostatic microactuator and design considerations for HDD applications. IEEE Trans Magn 1999;35(2):1000–5. [5] Horsley A, Wongkomet, Horowitz, Pisano P. Precision positioning using a microfabricated electrostatic actuator. IEEE Trans Magn 1999;35(2):993–9. [6] Koganezawa S, Uematsu Y, Yamada T. Dual-stage actuator system for magnetic disk drives using a shear mode piezoelectric microactuator. IEEE Trans Magn 1999;35(2):988–91. [7] Yoshikazu S, Shinji I, Takamitsu T, Sato Y, Sato I. Piezoelectric PiggyBack microactuator for hard disk drive. IEEE Trans Magn 1999;35(2):983–7.
Precision positioning using MEMS based microactuator Nitin Afzulpurkar *, Yossawee Weerakamhaeng Industrial Systems Engineering Program, Asian Institute of Technology, Thailand
Abstract The critical factor for the hard disk manufacturers when designing the new generation of high capacity hard disks is ‘areal density’. Although linear density has increased dramatically increases in track density are limited by servo bandwidths, which are controlled by the actuator resonance. An approach, which is currently being investigated by the researchers, is the use of a second high bandwidth actuator at the end of the conventional voice-coil actuator. After theoretical analysis and discussions with industry electrostatic design in which the microactuator will sit on the slider between the suspension and the read/write head is selected. The design factors are: higher servo bandwidth, higher resonant frequency and low voltage supply. To microfabricate such microactuator we propose to use the latest technology called microelectromechanical systems technology. After schematic drawing layout design is made, it has been tested for electrical connections in the electrical layout model and the optimized design parameters are obtained. Finally it is shown that the design will satisfy the required specifications. Ó 2002 Published by Elsevier Science Ltd.
1. Introduction The magnetic hard disk drive (HDD) has been maintaining significantly lower cost per megabit in comparison to solid-state memory technology. The areal recording density of hard disk drives has been increasing at a rate of 60% per year and the track density has been increasing at a rate of 30% per year [5]. The most profitable way that hard disks have been increased in capacity and speed is by storing more and more information in the same physical space Fig. 1. The areal density is
*
Corresponding author. E-mail address: [email protected] (N. Afzulpurkar).
0957-4158/02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd. PII: S 0 9 5 7 - 4 1 5 8 ( 0 2 ) 0 0 0 2 5 - 9
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Fig. 1. HDD capacity evolution.
defined as number of tracks per inch (TPI) times density of each track (BPI, bits per inch per track). When more data is packed in the same space, the size of each data bit is reduced requiring increased accuracy in positioning the read–write heads over the disk.
2. Servo control and frequency considerations The current HDD design employs a voice coil motor (VCM) which rotates the actuator about the pivot (cf. Fig. 2) and positions the head on the disk. An ideal actuator will maintain the perfect displacement (u) and rotational input (h) relationship ðu ¼ r hÞ because the natural frequencies and resonance effects are ignored. In the actual design the mechanical structure between VCM and head, which consists of E block (housing for head gimbal assembly), suspension and gimbal, is flexible and has characteristic resonance frequencies. The actuators have a very small stiffness about the bearing because of the flex cable and bearing which results in a vibrational mode in which the entire actuator oscillates about the pivot. Frequency (fp ) of this mode is about 100 Hz depending on the drive. To follow a data track, the
Fig. 2. Top view of the HDD.
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servo must correct off-track disturbances over a broad band of frequencies. This can result in off-track head motion and instabilities; which can cause the head to go offtrack [2]. The servo gain cannot be increased to reject the off-track disturbances near frequency (fp ). Tracking accuracy and speed of response increases as the servo bandwidth is increased. The head positioning servo systems of currently available commercial drives is limited to 500–700 Hz.
3. Development of dual-stage actuators To satisfy the accuracy, speed and frequency requirements caused by everincreasing track densities development of dual stage actuators is being carried out at many research laboratories. For description refer to [3,4,6]. In such two stage systems, a secondary actuator placed between suspension and slider can provide precision head positioning over the data track. Motion of the head results when voltage is applied to the actuator. The controller sends head movement commands to VCM for coarse, low frequency positioning used for seeking and simultaneously to the microactuator for fine, accurate and high frequency positioning. A secondary actuator placed between suspension and slider can provide precision head positioning over the data track. The second stage actuator should increase the closed loop bandwidth significantly resulting in at least an order of magnitude improvement in track-misregistration. 3.1. Types of microactuators Many different types of microactuators have been reported in the research work [1]. These can be classified into six types: electrostatic actuators, magnetic actuators, piezoelectric actuators, thermal actuators, hydraulic actuators, microsimulators. Based on the objectives for the HDD application, electrostatic and piezoelectric actuators have been used in research on the dual stage actuation systems so far. Electrostatic actuator can be linear type or rotary type. The linear actuator has many interdigitated fingers. When voltage is applied an attractive force is developed between the fingers which move together. Actuation dynamics can be described by a mass-spring-damper model [1]. Piezoelectric actuator uses piezoelectric elements which expand and contract in lengthwise direction when voltage is applied across their thickness. The top surfaces of the two piezo elements are connected by a wire stitch across the gap. One end of the bottom surface of each piezoelectric (PZT) element is adhesively to the part of load beam which is fixed to the base plate. The
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other end is adhesively bonded to the movable portion of the load beam. The PZT elements are oppositely polarized so that application of voltage makes one to expand and other to contract [3]. Operations of piezoelectric piggyback and shear-mode microactuators are described in [6,7]. Electrostatic actuator offers high accuracy and the actuation dynamics are described by simple second order linear model. Stiction and backlash is not present in these devices. On the other side the structure of the actuator has low stiffness springs which limits the maximum resonant frequency one can achieve. Piezoelectric material has high stiffness and can generate large force when used as actuator. This results in higher achievable resonant frequencies. The main disadvantage is that position accuracy is not as high as electrostatic actuator. Another disadvantage is that it cannot be made as an integral part of the head. 4. Microelectromechanical system microactuator One of the main goals of the development of microelectromechanical systems (MEMS) based devices is to be able to integrate microelectronics circuitry into micromachined structures with moving parts. This will result in completely integrated systems which will have advantages of low cost, reliability and small size as silicon chips. Using fabrication techniques and materials similar to microelectronics MEMS processes construct electrical components to provide computational capabilities and mechanical components which sense, control and provide actuation. Photolithography is the basic technique used to define the shape of the micromachined structures. Silicon macromachining is a widely used technique for MEMS device fabrication. MEMS fabrication requires drawing and layout tools to generate the pattern for adding or removing material. It needs a range of modeling tools such as simulators for mechanical movement, forces, electromagnetic fields, electronic device simulators and connective algorithms to reconcile and mix results from different simulators. 5. Structural design of electrostatic actuator The structural design method and layout for a linear resonant electrostatic microactuator (LREM) developed by the authors is described below. Typical microfabricated actuators exhibit very lightly damped resonance resulting in very poor settling performance if the resonant frequency is near the desired servo bandwidth. The approach used is to design the actuator to have an extremely high resonant frequency (5–10 times greater than servo bandwidth) so that actuator dynamics will have negligible effect on the servo system performance. Based on the above observation, the design criteria used for the design of LREM are: 1. The actuator should have resonant frequency of 20 KHz or higher so that the closed loop servo bandwidth can be 2 KHz.
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2. The actuator should attain displacement accuracy higher than 0:5 lm in order to be able to handle 40 KTPI HDD density. Based on these criteria, the design of LREM is carried out as described below. The actuator illustrated in Fig. 4 consists of a translating central plate, which is anchored to the fixed electrostatic comb drives. Actuation is achieved by electrostatic force generated using parallel plate capacitive electrodes mounted on either side of the plate. LREM is placed between the slider and the gimbal element. The desired motion of the actuator is translational motion in the direction of radial line on the disk platter. As shown in Fig. 4, LREM consists of two folded spring elements, two comb drives and one central plate. The folded spring is used for restoring the position after the plate is actuated by the comb drive. The supporting plate of the actuator is moved in the X+ direction by applying one polarity of voltage to the left hand comb drive, the plate is moved to the central position by the restoring force from the folded spring elements. The motion in X-direction takes place when opposite polarity voltage is applied to the right hand comb drive The schematic and layout representation of the electromechanical circuit of LREM is carried out using the Tanner Tools MEMS Pro software which is part of multichipmodule design system from Tanner research. We used the hierarchical layout editor L-edit which allows the MEMS design. The mechanical and electrical model of linear electrostatic comb drive is shown in Fig. 5. Based on the models of the elements the schematic for LREM is developed.
Fig. 3. Parametric illustration for folded spring elements.
Fig. 4. LREM: schematic representation.
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Fig. 5. Mechanical and electrical model of comb drive.
Fig. 6. Parameter illustration of LREM comb drive element.
After the schematic the graphical layout of the components is generated using MEMS library. This layout represents the actuator elements with a range of parameters. Parameter illustration of LREM comb drive element and folded spring element is shown in Figs. 6 and 3 respectively.
6. Optimization of the parameters The main criteria, which can be used to optimize the dimensions and performance of LREM, are as follows: Devices with small area are preferred for cost reduction. Smaller operating voltages are preferred for integrated voltages.
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The resonant frequency should be as high as possible. Large amplitudes of oscillations are required for better sensing and actuating capabilities. Dimension of the plate needs to be smaller than dimensions of a typical slider. The overall dimension of the microactuator should confirm with the effective area on the gimbal. Based on this the overall dimension of the microactuator is fixed as 3:113 mm 2:602 mm and the plate width and length are fixed at 0.4 mm each. From the parameter illustrations shown in Figs. 3 and 6, the variables, which can be optimized, are: width, length, inner gap and outer gap for folded springs, active rotor comb width (arwidth) and supply voltage. LREM behavior is simulated by varying the following parameters: spring length, active rotor comb width, voltage supply to comb drive. Sine wave voltages are applied to the comb drive to generate the electrostatic forces and DC voltage is applied as bias voltage to folded springs to preserve the electrostatic spring property. Spring length is varied between 150 and 350 lm in the step size of 10 lm. Active rotor comb width is varied from 510 to 1821 lm in the steps of 69 lm. The voltage supplied to comb drive is varied from 1 to 20 V in the steps of 1 V. By varying these parameters one at a time the displacement values at right node (rtm) and the resonant frequencies are calculated. The schematic representation shown in Fig. 4 is used to find relations between: 1. Length of folded spring, displacement of node rtm and resonant frequency. 2. Effective area of comb drive (arwidth), displacement of node rtm and resonant frequency. 3. Voltage supplied to comb drive, displacement of node rtm and resonant frequency.
7. Conclusion The advantages of dual-stage actuator for HDD over single stage actuator are identified. As areal densities increase, the track density needs to be increased. A system with high bandwidth secondary actuator placed between the suspension and the slider can be realized by MEMS technology. We calculated the optimum dimensions of the electrostatic MEMS actuator by constructing a model. Layout and mask design was done using Tanner MEMS Pro software. The relationship between parameters resonant frequency and rtm displacement against folded spring length is shown in Figs. 7 and 8. The relationship between
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Fig. 7. Relation between resonant frequency and spring length.
Fig. 8. Relationship between rtm displacement and spring length.
parameters resonant frequency and rtm displacement against arwidth is shown in Figs. 9 and 10. The relationship between parameters resonant frequency and rtm displacement against comb drive supply voltage is shown in Figs. 11 and 12. We can conclude from these figures that increase in folded spring length will reduce the resonant frequency but it is not much influenced by arwidth and comb drive voltage
Fig. 9. Relation between rtm displacement and arwidth.
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Fig. 10. Relationship between resonant frequency and bandwidth.
Fig. 11. Relation between resonant frequency and comb drive supply voltage.
Fig. 12. Relation between rtm displacement and comb drive supply voltage.
supply value. rtm displacement increases linearly with the arwidth and comb drive supply voltage. In the future work we plan to verify the resonant frequency of the actuator assembly using finite element analysis.
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Table 1 Optimized LREM component dimensions Element
Parameter list
Abbreviation
Value
Folded spring
Spring width Spring length Spring inner gap Spring outer gap Plate width Plate length
Width Length Inner gap Outer gap Width Length
5e-6 m 220e-6 m 5e-6 m 5e-6 m 400e-6 m 400e-6 m
Active rotor comb width Rotor yoke width Stator yoke width Length of comb fingers Width of comb fingers Air gap between fingers Stator-rotor finger overlab
Arwidth Rywidth Sywidth Flength Fwidth Airgap Rsoverlap
1545e-6 m 29e-6 m 50e-6 m 250e-6 m 29e-6 m 20e-6 m 100e-6 m 2 Vp
Plate Linear electrostatic comb drive
Voltage supplied
Fig. 13. Rtm displacement magnitude and phase relationship with frequency.
Optimized parameters for the LREM obtained by above analysis are shown in Table 1. The magnitude and phase of displacement values are plotted against frequency in Fig. 13. From the figure the resonant frequency is observed as 21.94 kHz which satisfies the design criteria of 20 kHz frequency. Acknowledgements The authors wish to acknowledge the valuable support and insight given by Mr. Brent Bargmann, Seagate Technologies and Mr. Chumnarn Punyasai, Dr. Itti Rittaporn of NECTEC, Thailand. References [1] Bank D, 1999. Introduction to Microengineering, MEMs Micromachine MST http://www.dbanks. demon.co.uk.
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[2] Evans RB, Griesbach JS, Messener WC. Extending bandwidth with dual-stage suspensions. Data Storage 1999:43–6. [3] Evans RB, Griesbach JS, Messener WC. Piezoelectric Microactuator for Dual Stage Control. IEEE Trans Magn 1999;35(2):977–82. [4] Fan L-S, Hirano T, Hong J, Webb PR, Juan WH, Lee WY, et al. Electrostatic microactuator and design considerations for HDD applications. IEEE Trans Magn 1999;35(2):1000–5. [5] Horsley A, Wongkomet, Horowitz, Pisano P. Precision positioning using a microfabricated electrostatic actuator. IEEE Trans Magn 1999;35(2):993–9. [6] Koganezawa S, Uematsu Y, Yamada T. Dual-stage actuator system for magnetic disk drives using a shear mode piezoelectric microactuator. IEEE Trans Magn 1999;35(2):988–91. [7] Yoshikazu S, Shinji I, Takamitsu T, Sato Y, Sato I. Piezoelectric PiggyBack microactuator for hard disk drive. IEEE Trans Magn 1999;35(2):983–7.