Expected Force Profile for HDD Bearing Installation Machine

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1 Expected Force Profile for HDD Bearing Installation Machine 91 Expected Force Profile for HDD Bearing Installation Machine Settha Tangkawanit 1 and Surachet Kanprachar 2, Non-members ABSTRACT In HDD industry, bearing installation machine is used to assembly bearing, T-ring, and HDD arm together. The required goal of this machine is to put all components together with a precise position needed. To get such precise position, the supplied force is needed. The force is one of a key factor to get the required assembly. The force and height profiles collected from the industry are used as raw data and analyzed. Two types of assembly are considered; that is, 2.5 and 3.5 HDD. The expected force profile to get the needed position in assembling the components together is achieved. The result of this paper can be used as a guideline for designing a precise bearing installation machine in order to reduce the rejection rate of the assembled products along the HDD production line. Keywords: Hard-disk drive, HGA, Bearing, T-Ring, Load cell, LVDT, Sensor, Force control. 1. INTRODUCTION Recently, the HDD (Hard Disk Drive) industry has been clearly shown to be one of the fastest growth industries in the world, especially in Thailand. Mostly, all kinds of HDD parts are produced by manufacturers located in Thailand. To get good HDD components and certainly the HDD itself, processes in HDD production line have to be improved. This can eventually increase the strength of the country for this kind of industry. The improvement can be done by doing research and development on every section of the production line. There have been much research done for improving the quality of HDD and reducing the cost in implementing HDD. For examples, in [1], it was shown that the HDD manufacturing costs due to the repeatable and nonrepeatable disturbances were lessened using a systematic combination of the internalmodel-base repetitive controller with a loop shaping filter. The performance improvement in terms of exploiting all the potentialities of HDD can be achieved by using electrostatic micro-actuator as presented in Manuscript received on March 31, 2007 ; revised on May 15, ,2 The authors are with the Department of Electrical and Computer Engineering, Faculty of Engineering, Naresuan University, Muang, Phitsanulok THAILAND, s: surachetka@nu.ac.th [2, 3]. The designed actuator offers high bandwidth and eases the assembly procedure of the slider in HDD. The improvement of the HDD assembly processes have also been developed using Shape Memory Alloy (SMA) as shown in [4]. This technique has been shown to be a promising technique to be used in the assembly of the spindle with record medium and the assembly of the driving arm with read/write head. These examples are just a few examples of the developments and research done in this industry. In this paper, the assembly of one of the key parts in HDD industries; that is, the HDD header, is studied. To get a usable HDD header, the assembling process has to be very accurate. Any mismatch or unsatisfied parts are not acceptable. The HDD header rejection could be from many reasons; for example, the parts to be put together are not precisely matched or some parts are too flexible because of the material problems. However, one of the reasons in the rejection of the HDD header is from the inaccurate position of the assembly between bearing, T-ring, and HDD arm after passing the components through a bearing installation machine. The bearing installation machine is a machine supplying force to push the bearing, T-ring, and HDD arm together as a whole. The force supplied is controlled by the height needed for a particular module of HDD arm. If the force is not supplied appropriately along the assembling process, the achieved height will not be exactly correct as needed by the specification. This then results in a rejection, which certainly causes losses in the production line. The force supplied in bearing installation machine is analyzed. The force and height as a function of time are collected from the industry. The data are gathered from almost a hundred of assembled parts. These assembled parts are accepted for being used in the next step since they are all passed through the inspection process. Using the collected force and height profiles, the expected force profile in order to get the required accurate assembly from the bearing installation machine is studied. 2. EXPERIMENTAL METHOD As mentioned, there are three parts to be combined in the bearing installation machine; that is, bearing, T-ring, and HDD arm. Bearing is the part for balancing the HDD header in order to allow the header to move around circularly. T-ring is the part acting as a

92 ECTI TRANSACTIONS ON ELECTRICAL ENG., ELECTRONICS, AND COMMUNICATIONS VOL.5, NO.2 August 2007 medium attaching bearing and HDD arm.

These three parts are shown in Fig. 1. using load cell and LVDT (linear variable differential transducer), respectively [5].

The complete assembly and the entire process described are shown in Fig. 2 and 3, respectively. (a)conventional converter Fig.2: Complete HDD arm assembly.

These three parts shown in Fig.1 have to be put together using bearing installation machine.

Then, the bearing is placed above the HDD arm within the bearing installation machine. And, the required height is assigned to the machine.

After getting the required height data, the microcontroller in the machine will drive the stepping motor to push the parts (i.e., bearing and HDD arm) together.

2 92 ECTI TRANSACTIONS ON ELECTRICAL ENG., ELECTRONICS, AND COMMUNICATIONS VOL.5, NO.2 August 2007 medium attaching bearing and HDD arm. And, the HDD arm is the part allowing the header to move to any particular sector or cylinder in the HDD to read and write the data. These three parts are shown in Fig. 1. using load cell and LVDT (linear variable differential transducer), respectively [5]. If the achieved height is at the assigned one, the microcontroller will send a command to stop the stepping motor and the process is completed. The complete assembly and the entire process described are shown in Fig. 2 and 3, respectively. (a)conventional converter Fig.2: Complete HDD arm assembly. (b)t-ring Fig.3: Diagram showing processes in the bearing installation machine. (c)hdd arm Fig.1: Three HDD components used in the bearing installation machine. These three parts shown in Fig.1 have to be put together using bearing installation machine. The process is done by first putting T-ring into the circular hole of the HDD arm. Then, the bearing is placed above the HDD arm within the bearing installation machine. And, the required height is assigned to the machine. Note that there are many sizes of HDD arms used; hence, the height assigned has to be changed accordingly. After getting the required height data, the microcontroller in the machine will drive the stepping motor to push the parts (i.e., bearing and HDD arm) together. During this process, the supplied force and the achieved height are detected The data in terms of force and height curves of accepted assembly are collected from industry. Here, there are two types of assembly considered; that is, the assembly for 2.5 HDD and the assembly for 3.5 HDD. For each of these two types, the data from 30 assembly parts are collected. Examples of force and height curves for 2.5 and 3.5 HDD assembly parts are shown in Fig.4 (a) and (b), respectively.

3 Expected Force Profile for HDD Bearing Installation Machine 93 F mov avg (t n ) = 1 M + 1 M F avg (t n k ) (2) k=0 where F mov avg is the moving average force at the discrete time t n M is the number of preceding samples used in the calculation. Fig.4: Examples of collected supplied force and achieved height curves for two types of HDD. Considering Fig. 4, these data were gathered from the use of load cell and LVDT attached to the machine. The vertical axis on the left describes the supplied force in lb (for 2.5 HDD) and in kg (for 3.5 HDD). The vertical axis on the right describes the achieved height in mm. The supplied force and achieved height were collected as a function of time. It is seen that the maximum achieved heights from both types are different; that is, the height for 2.5 HDD is approximately mm and the height for 3.5 HDD is approximately mm. According to the specification, the required heights for 2.5 and 3.5 HDD are mm and 0.31 mm, respectively. These required heights are the height of the bearing measuring from the lower HDD arm plane. It is seen that the lower circular plane of the bearing is slightly located below the lower HDD arm plane. This then allows the HDD arm to put over other HDD component without touching. For each type of HDD, the data from 30 good samples are collected. The force profiles from 30 samples are then averaged; that is, F avg (t) = 1 N N F i (t) (1) i=1 where F i (t) is the force data of the i th sample at time t. F avg (t) is the average force at time t. N is the number of collected samples which is set to be 30. t is the time in the installation process. Since the collected samples are the accepted samples, the average force profile achieved is then the expected force profile to be used. However, as we shall see in the next section that the average force profile is not quite smooth and might not be practically to be used in the design process. To get a useful expected force profile, a technique called moving average is adopted here [6, 7]; that is, 3. RESULTS AND DISCUSSIONS The collected data for both 2.5 and 3.5 HDD are shown in Fig. 5. For each case, there are 4 curves shown. These curves are shown to describe that the force profiles to get the required height are not identical. Force unit is in lb in both cases. Considering Fig. 5, it is seen that the time periods required for both types of HDD to get the required height (see Fig. 4) are different; that is, 4 s. and 6 s. for 2.5 and 3.5 HDD, respectively. Also note that, the units of the force shown in both graphs in Fig. 5 are different since the data achieved from the industry are in these units. Fig.5: Collected supplied force profiles (4 samples shown) for two types of HDD.

4 94 ECTI TRANSACTIONS ON ELECTRICAL ENG., ELECTRONICS, AND COMMUNICATIONS VOL.5, NO.2 August 2007 It is seen that even though for each type of HDD, these four curves are not identical but they are aligned in a similar profile. Hence, if the number of collected samples increases, the expected force profile will be clearly viewed. And, certainly, the determination of such expected force profile is feasible. In this research, as mentioned, 30 good force profiles for each type of HDD are collected. These profiles then allow us to determine the expected force profile using (1). The results of averaging 30 collected force profiles are shown in Fig. 6. ties in designing the force control system. Applying (2) with M = 3 to the average force profile found in Fig. 6, the moving average force profile is achieved. The moving average force profiles for both types of HDD are shown in Fig. 7. Considering the moving average force profile shown in Fig. 7, it is clearly seen that the ripples or unsmooth points found in the average profile are disappeared. This is the result of applying the moving average technique to the average force profile. The moving average technique can be viewed as a lowpass filter. Hence, any sudden changes (i.e., ripples or unsmooth points) in time, which result in high frequency parts of the signal (profile), will be filtered out. These moving average profiles can be then used in the design of the force control system in order to put the bearing and the HDD arm in the required position (i.e., the required height). Therefore, the moving average force profiles found in Fig. 7 are then the expected force profile. Fig.6: Collected supplied force and average force profiles for two types of HDD. From Fig. 6, it is seen that the average force profile for both cases are located in between 4 sampled force profiles shown. Note that the average force profile shown is not just from these 4 sampled force profiles but from 30 sampled force profiles. This average force profile should be used in the design of the force control system. However, from the average profile itself, it is seen it is not smooth and have some tiny ripples. For example, for 2.5 HDD, it is seen that at t = 0.5s, there is small ripple. Or, for 3.5 HDD, there are some unsmooth points at t = 0.6, 1.9, 4.9, and 5.2 s. These ripples or unsmooth points can cause difficul- Fig.7: Average and moving average (expected) force profiles for two types of HDD. These expected force profiles are different since they relate to different types of HDD. For 2.5 HDD, the expected force profile has an almost constant

5 Expected Force Profile for HDD Bearing Installation Machine 95 slope of approximately 15 lbs/s for t = 0 to 3.2 s. And, as t increases, the slope decreases and reaches zero at t = 3.5 s; that is, the maximum force is achieved (approximately 60 lbs). And, after that, the required force is slightly reduced to 57 lbs. For 3.5 HDD, the expected force profile has average slope of 13 lbs/s. However, the profile is not identical to that of 2.5 HDD since, for 3.5 HDD, there are zero-slope or almost zero-slope points at t = 1.5, 3.2, and 5.5 s. It is seen that for both expected force profiles, there are different points in time at which the force has to be carefully controlled. These points should then be used in the design of the force control system. It should also note that the expected force profiles found in Fig. 7 are from the averaging calculation. There is another important parameter called standard deviation of the force, which tells the information about the deviation of the force at a particular time. From the collected data, it is found that the force standard deviation becomes larger as the time increases. This can be seen by considering variation of the force profiles in Fig. 5 as time t increases. 4. CONCLUSIONS The supplied force is an important parameter in the assembly of bearing, T-ring, and HDD arm in the bearing installation machine. If the supplied force is not proper, the achieved height will not meet the required height in the specification. In this paper, the supplied force and achieved height profiles gathered from manufacturers for 2.5 and 3.5 HDD are analyzed. The expected force profile for both types of HDD is achieved. This expected force profile is useful since it can be used in designing a system to be used in the bearing installation machine to control the supplied force in order to put bearing, T-ring, and HDD arm in the required position. 5. ACKNOWLEDGEMENT This work was supported by Hard Disk Drive (HDD) Cluster, National Electronics and Computer Technology Center (NECTEC), Thailand. ference 2002, August 27-29, 2002, pp. WE-P WE-P [4] W. M. Huang, Q. Y. Liu, and L. F. Ong, Assembly of Hard Disk Drive Using Shape Memory Alloys, Asia-Pacific Magnetic Recording Conference 2002, August 27-29, 2002, pp. TU-P TU-P [5] Edwin A. Erlbacher, Force Control Basics, available at 20Papers/Force-Control-Basics.pdf [6] John G. Proakis and Dimitris G. Manolakis, Digital Signal Processing: Principles, Algorithms and Applications, 3 rd edition, Prentice Hall, Inc., [7] Sanit K. Mitra, Digital Signal Processing: A Computer-Based Approach, the McGraw Hill Company, Inc., Settha Tangkawanit received B.Eng. (Honors) in Computer Engineering from Naresuan University, Phitsanulok, Thailand in He is currently a Master student in Electrical Engineering program at Faculty of Engineering, Naresuan University, Phitsanulok, Thailand. His research interests are computer programming, control system, and robotics. Surachet Kanprachar received B.Eng. (First class honors) in Electrical Engineering from Chulalongkorn University, Bangkok, Thailand in 1996 and received M.Sc. and Ph.D. in Electrical Engineering from Virginia Tech, Virginia, USA, in 1999 and 2003, respectively. He is currently an Assistant Professor at the Department of Electrical and Computer Engineering, Faculty of Engineering, Naresuan University, Phitsanulok, Thailand. His research interests are optical communications, wireless communications, and coding theory applications. References [1] Lin Guo, Reducing the Manufacturing Costs Associated with Hard Disk Drives - A New Disturbance Rejection Control Scheme, IEEE/ASME Transactions on Mechatronics, vol. 2, no. 2, pp , June [2] S. Sassoline, M. Sarto, and L. Baldo, Electrostatic Microactuator for Future Hard Disk Drive, Asia-Pacific Magnetic Recording Conference 2002, August 27-29, 2002, pp. WE-P WE-P [3] B. Chen, H. Zhu, and J. Miao, A New Design of Electrostatic Microactuator for Hard Disk Drives, Asia-Pacific Magnetic Recording Con-

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