Active magnetic inertia latch for hard disk drives

Microsyst Technol (2011) 17:127 132 DOI 10.1007/s00542-010-1168-8 TECHNICAL PAPER Active magnetic inertia latch for hard disk drives Bu Hyun Shin Kyung-Ho Kim Seung-Yop Lee Received: 2 August 2010 / Accepted: 5 November 2010 / Published online: 21 November 2010 Ó Springer-Verlag 2010 Abstract In this paper, a new hybrid inertia latch is proposed to improve the shock resistance and the loading time for hard disk drives. The new latch mechanism combines the inertia effect and the magnetic force by additional yoke and magnets, reducing the time required for loading the head to media that is the mechanical limitation of the widely used pawl inertia latch. The operating mechanism of the proposed latch is optimally designed using an electromagnetic simulation, and it is verified by experiments using a prototype. The new latch system reduces the time required for loading the head to media up to 8 ms. 1 Introduction Recently, advances in recording density of high density hard disk drives (HDDs) are required to meet the demand for huge data storages. Load/unload mechanism has been widely used in high-density HDDs because it can increase the data-storing area of disks. The other advantages of load/unload type, compared to conventional contact start stop (CSS) mechanism, are reliable shock response, low power consumption and easy assembly (Albrecht and Sai 1999). In general, a latch design is one of important parts for reliable load/unload mechanism. When HDD is unplugged B. H. Shin S.-Y. Lee (&) Department of Mechanical Engineering, Sogang University, Shinsu-Dong, Mapo-Gu, Seoul, Korea e-mail: sylee@sogang.ac.kr K.-H. Kim Samsung Electronics Corporation, 415 Maetan-3Dong, Yeongtong-Gu, Suwon-city, Korea or is unintentionally stopped, the load/unload mechanism moves the head out of media. Various types of latch for HDDs using load/unload mechanism have been designed such as inertia latch, magnetic induction latch, air flow latch, and solenoid latch to protect undesired motions of a voice coil motor (VCM) actuator at sudden disturbances. Various inertia-type latches have been preferred because the locking performance is better than that of other types of latch. However, there has been a limitation in pure inertiatype latches because the mechanical tolerance and temperature-dependent friction does not guarantee perfect latching and unlatching operations. Therefore, most mobile HDDs, requiring the perfect shock protection, use a hybrid inertia latch combining the inertia effect and magnetic bias force. However, the combined inertia latch also has a chance to fail under the specific shock conditions. 2 Pawl inertia latches In general, the widely used inertia latch mechanism has merits of simple structure and low power consumption, but it does not guarantee reliable motions to low-level shock or complicated shock sequences. Recently some suggestions are proposed to overcome the weakness, such as impact rebound (IR) bi-directional single inertia latch (Byun et al. 2002) and pawl inertia latch (Chang et al. 2005). As shown in Fig. 1, a typical pawl latch design combines the inertia effect and magnetic bias force. While the hybrid inertia latch works at high-level shock, the magnetic bias force holds actuator at low-level one. At middle-level shock, both mechanisms work together (Liu et al. 2009). A VCM bias pin located in the actuator body is made of steel. It is attracted by VCM magnet and creates a clockwise torque around the actuator pivot. By the clockwise

128 Microsyst Technol (2011) 17:127 132 Fig. 2 Structure of the proposed magnetic latch mechanism Fig. 1 Typical pawl inertia latch design without top yoke torque, the actuator body pushes the latch in order for the latch to locate in the closed position. The latch also has a bias pin. VCM magnet pulls the latch bias pin and creates a clockwise torque to the latch around its pivot. Since the magnetic bias torque of the actuator is much bigger than the bias torque of the latch, the latch position is not affected by the latch bias pin in non-operating state. However, in operating state, the actuator rotates in the counter-clockwise direction and it takes away from the latch. The latch rotates in the clockwise direction by the latch bias pin, and it locates the open position to clear the actuator load path. When the actuator body is apart from the latch, the latch will be the open position. Since the clockwise torque of the pawl latch induced by the latch bias pin is small, the moving speed of the latch is very low. Therefore, in loading moment, the maximum speed of the actuator with conventional pawl inertia is limited because the actuator moves only after the latch is released. Furthermore, the pawl inertia latch using the latch bias pin does not guarantee the reliable locking under the specific shock conditions such as actuator-loading or rebound shock between the actuator and latch. 3 New active magnetic latch 3.1 Working mechanism In order to overcome the weakness of the pawl inertia latch combining the inertia effect and magnetic bias force, a new active magnetic latch is developed. The proposed latch uses the electromagnetic force between latch magnet and VCM actuator. As shown in Fig. 2, the new latch Fig. 3 Cross-sectional view of the proposed latch and VCM actuator mechanism does not use a latch bias pin. Instead of that, there are additional yoke and magnets in the latch. The cross-sectional view of the new latch and VCM actuator is shown in Fig. 3. The geometries of yokes and magnets of VCM actuator have been also modified. Small parts of the actuator yokes are cut out to extend the moving space of magnets and yoke of the latch. The actuator coil, latch yoke and magnets compose a moving-magnet-type VCM actuator. When currents flow through the coil of the VCM actuator, the actuator generates torque in a counter-clockwise direction and the active magnetic latch induces a clockwise torque by Fleming s left-hand rule. When currents don t flow through the actuator coil, the active magnetic latch generates torque in a counter-clockwise direction, keeping its closed position regardless of the position of the VCM actuator. Arrows in Fig. 3 indicate the directions of magnetic flux of the actuator system with the new latch. Unlike other inertia latches, the proposed latch mechanism can control the position of the latch by electrical current applied to the actuator. Therefore, the new latch mechanism is called as an active magnetic latch. While the passive pawl latch generates constant torque in the clockwise direction by the latch bias bin in non-operating state, the active latch induces a counter-clockwise torque by

Microsyst Technol (2011) 17:127 132 129 Fig. 4 Simulation result on magnetic flux density of the new latch and VCM. The unit is tesla (T) additional yoke and magnet of the latch. The passive pawl latch should have sufficient magnetic bias torque of actuator in order to overcome the latch bias torque for reliable protection against low or middle level shock, whereas the active latch can minimize the actuator bias torque required to hold the actuator at the parking position. Therefore the new active latch overcomes the weakness of the passive pawl latch under complicated shock conditions such as actuator-loading or rebound shock between the actuator and latch. In non-operating state, only magnetic force is applied in the new latch design. The magnetized direction of magnets of the latch is the same as VCM magnets. There are two counter forces between the active magnet latch and VCM actuator. There exists a repulsive force between the magnets of the latch and VCM actuator. However, attractive forces are induced between the magnets of the latch and the VCM yokes, and also between the magnets of the VCM actuator and the yoke of the latch. When the repulsive force is larger than the attractive one, the active magnetic latch generates a counter-clockwise torque that makes the latch lock the actuator. It is noted that the torque is independent of the actuator position. When the VCM actuator body takes apart from the latch, the conventional latch using latch bias pin creates a clockwise torque, but the active latch generates a counter-clockwise torque. In loading state, where heads are loaded from the ramp to media, currents flow through the coil of actuator and the electromagnetic force is applied. Because the magnets and yoke of latch and the coil of actuator compose a moving magnet type VCM, this electromagnetic force creates a clockwise torque to the active magnetic latch. The electromagnetic torque of the latch will rotate its hook in a clockwise direction in order to load the actuator. Since the moving speed of the new latch induced by the clockwise torque in loading state is higher than that of the pawl inertia latch, the loading time of the actuator is dramatically reduced. The torque of the new actuator satisfies the conditions on the direction and magnitude for operating and nonoperating states. The size of yoke and magnets of the latch should be designed properly to make sure that the torque of the latch has a counter-clockwise direction in non-operating state and the latch moves fast in loading state. 3.2 Simulation using FEM The yokes and magnets of VCM actuator have been modified in order to design the new active magnetic latch. Since this modification affects the performance of VCM actuator, we have calculated the torque constant according to the rotation angle of VCM using a commercial FEM program J-MAG. Figures 4 and 5 show the simulation results on the magnetic flux density and the torque constant of the VCM actuator, respectively. The materials of yokes and magnets are SPCC and rare earth magnet (N48H), respectively. The simulation results show that the torque constant of the modified VCM actuator is similar to that of original VCM actuator before the modification of the yokes and magnets of VCM actuator. While the averaged torque constant of the modified and original VCM actuators are 0.01753 and 0.01782, respectively. Additionally, the linearity of the torque constant must be ensured for the control performance. Using the Eq. 1, the linearity of the torque constant is calculated. The linearity values of the modified and original VCM actuators are 1.93 and 1.76%,

130 Microsyst Technol (2011) 17:127 132 Fig. 5 Torque constant of the original and modified VCM actuators respectively. Although the linearity value of the modified VCM actuator is bigger than that of the original one, it is below 4% that is the error tolerance of linearity. Based on the simulation results, it is concluded that the modification of the VCM actuator for the new latch satisfies the required values of the resultant torque by VCM. Linearity ð% Þ ¼ K t Average K t Min 100 ð1þ K t Average The resultant torque by the active magnet latch has been also calculated for both non-operating and loading states. The materials of yoke and magnet of the latch is C1010 and rare earth magnet (N48), respectively. At initial position, the latch yoke is aligned to the yoke of VCM actuator. In non-operating state, the direction of torque must be clockwise in order for the latch to lock the actuator. The FEM simulation of the torque was conducted as the angle of the latch increases in a clockwise direction. The simulation results ensure that the latch creates counter-clockwise torque at all the angles. In loading state, 450 ma currents flow through the coil of actuator. Simulation results show that the latch generates a clockwise torque. The electromagnetic torque in loading state is larger than the magnetic force in non-operating state. Based on the simulation results, the sizes of yoke and magnet are selected to maximize torque in non-operating state for shock protection. 4 Experiment 4.1 Prototype We have manufactured a prototype of the proposed active magnetic latch as shown in Fig. 6. A commercial 2.5 inch Fig. 6 Prototype of the active magnetic latch HDD by Samsung Electronics is used for experiments. This commercial disk drive has the pawl inertia latch with actuator bias pin and latch bias pin. The yokes and magnets of the actuator are modified for the active magnetic latch. The inertia bias pin is eliminated, and additional yoke and magnets are added to the side of latch by epoxy glue. In order to improve performance, the latch yoke should be aligned vertically to the yoke of the actuator. In nonoperating state, the latch locks the actuator as shown in Fig. 6 because the actuator magnetic bias holds the actuator on ramp and pushes the latch. 4.2 Torque measurement The load cell CAS PW4 M-300 is used to measure the torque of the latch. The hard disk drive is put on a 1-axis stage and the tip of the load cell is in contact with the yoke of the latch. We have measured the force of the latch moving the stage. The moving distance of the stage is converted into the moving angle of the latch. The moving distance divided by the distance between the contact point of the load cell tip and the pivot of the latch becomes the moving angle of the latch. The torque of the latch is also calculated by multiplying the measured force by the moving distance. In non-operating state, it is observed that the torque of the latch is in the counter-clock direction. The measured torque of the latch in non-operating state is shown in Fig. 7.

Microsyst Technol (2011) 17:127 132 131 Torque (mn-mm) 90 80 70 60 50 40 30 20 10 0 0 2 4 6 8 10 Rotation Angle ( ) Fig. 7 Latch torque in non-operating state Torque (mn-mm) 0-20 -40-60 -80-100 -120-140 0 1 2 3 4 5 6 7 8 Rotation Angle ( ) Fig. 8 Latch torque in loading state For the loading experiment, we apply current of 450 ma through the coil of the actuator. In order to measure the torque of the latch is measured, we hold the actuator system in stationary position. The experimental result on the torque of the latch in loading state is compared in Fig. 8. The minus value in Fig. 8 represents that the torque is in the clockwise direction. loading period by the pawl inertia latch is over 30 ms (Liu et al. 2009). The proposed active magnetic latch rotates in the clockwise direction when the electromagnetic force is induced by the currents of the actuator coil. The torque of the latch is controlled by the currents of the actuator coil. The resultant torques of the latch and the actuator increase with increasing current of the coil. In non-operating state, the torque is always counter-clockwise. As we mentioned in Sect. 3, the torque in loading state should be larger than that in non-operating one. Low currents of the coil make fail to load the actuator because the low latch torque cannot make the latch rotate to be released before the actuator loads the head. We repeat the loading test of the actuator as the voltage of the actuator is increased. When the step input below 3 V is applied, the latch fails to release the actuator. However, over 3 V the actuator can load the head to media successfully. The resistance of the coil is 11.3 X, and the current of 265 ma flows through the coil. It is found that current over 265 ma is required for the latch to release the actuator. In order to make a close observation of the motion of the latch during loading, we use a high speed camera. Figure 9 shows the photo images of the high speed camera when the voltage of 4 V is applied. Unlike the case of the conventional inertia latch, the actuator speed can be increased over a limit, reducing the loading period. In experiment, the loading period is measured less than 8 ms, which is much faster than 30 ms of the conventional pawl latch. The shortened loading time by the new latch can improve the dynamic performance of HDD. 4.4 Shock protection When we implement various shock tests, experimental results show that the active magnetic inertia latch guarantees shock protection at all levels. For the counter-clockwise shock of the base, both the actuator and the latch will 4.3 Loading test The conventional pawl latch rotates in the clockwise direction when the magnetic force is induced by the latch bias pin. In loading moment, the maximum speed of the actuator with conventional pawl inertia is limited because the actuator moves only after the latch is released. The Fig. 9 Loading test of active magnetic latch at 4 V

132 Microsyst Technol (2011) 17:127 132 proposed latch rotates to the closed position in the counterclockwise direction by the yokes and magnet of the latch. Figure 10 shows the high speed camera images of the latch when the actuator experiences a rebound motion at input voltage of 3 V applied to the coil. The latch keeps locking the actuator successfully for the rebound motion induced by high level shock. Experiments confirm that the proposed latch is more reliable for shock protection than the conventional inertia latch. 5 Conclusion Fig. 10 Shock response of the proposed latch prototype during impact and rebound rotate in the clockwise direction. Then the actuator will push the latch to the closed position in order to rotate in the opposite direction for loading. When a counter-clockwise direction shock is applied to the actuator that means the clockwise shock of the base, the latch will keeps locking the actuator by the torque of the latch. It is because the latch torque is in the same direction of the shock and it blocks the loading path of the actuator. Then the actuator rotates in the counter-clockwise direction and collides with the latch. There is a chance that the high level shock makes a rebound motion of the actuator under a clockwise shock. This may cause a critical problem in the inertia latch design. After the collision with the actuator and the latch, the conventional inertia latch rotates to the open position in the clockwise direction by the latch bias pin. However, the A new active magnetic latch design is developed to improve the shock resistance and locking performance. In non-operating state, the magnetic force between the magnets and yoke of latch and the magnets and yokes of VCM makes the latch lock the actuator at all levels of shock. In loading state, the moving magnet type VCM, consisting of the magnets and yoke of the latch and the VCM coil, generates electromagnetic force in order for the latch to be released. The new latch system reduces the time required for loading the head to media up to 8 ms. The experimental results using a prototype confirm that the active magnetic latch design guarantees reliable latching/unlatching motions in both non-operating and loading states. It is also shown that the latch keep locking the actuator for all the levels of shock. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (2009-0077354) and a grant from the Seoul Research and Business Development Program (10816). References Albrecht TR, Sai F (1999) Load/unload technology for disk drives. IEEE Trans Magn 35(2):857 862 Byun Y et al (2002) Impact rebound type inertia latch for load/unload technology. Microsyst Technol 8:37 40 Chang J et al (2005) Pawl latch mechanism design and control for load/unload technology. Microsyst Technol 11:747 750 Liu Y et al (2009) Inertia magnetic latch design considering actuator load unload. Microsyst Technol 15:703 712