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In keeping with consumers’ preferences for electronic products of ever smaller size and enhanced functionality, it is necessary to reduce the profile of the auto-focusing actuators used in camera phones without sacrificing their performance. Accordingly, this study modifies the Voice Coil Motor (VCM) actuator proposed by the current group in a previous study (C. S. Liu and P. D. Lin, Opt. Express, 16, 2533–2540, 2008) to accomplish a miniaturized auto-focusing actuator for cell phone camera modules with minimal power consumption. The proposed device comprises a VCM, a closed-loop position control system, a magnetoconductive plate, and a lens support structure to drive the lens to the optimal focusing position. The experimental results show that the actuator has a zero holding current when maintaining the lens in the specified focusing position. Overall, it is shown that compared to existing VCM actuators, the proposed actuator has both a higher power efficiency and an improved positioning repeatability.
©2009 Optical Society of America
1. Introduction
In recent years, auto-focusing camera modules have been deployed in an increasing variety of portable electronic devices, including cell phones, personal digital assistants (PDAs), laptops, and so forth. The literature contains many investigations into various aspects of in-phone camera modules, including the use of voice coil motors (VCMs) [1–10] or piezoelectric motors [11–13] for auto-focusing purposes, and enhanced crystal lens [14–20] or liquid lens [21–27] imaging systems. VCM auto-focusing schemes yield an excellent image quality (see Fig. 1) and are characterized by a low cost, a low power consumption, a rapid response, a high repeatability, and a small size [4,28]. Accordingly, VCMs are ideally suited to the actuation of miniature cell phone camera modules capable of satisfying the response, repeatability and power efficiency specifications laid down in the Standard Mobile Imaging Architecture (SMIA) standard [29].
To avoid the requirement for frequent recharging, it is necessary to minimize the power consumed by the VCM actuator in order to prolong the battery life. However, to the best of the current authors’ knowledge, none of the VCM actuators proposed in the literature have a zero holding current at the focusing position [1–10,28]. It has been reported that commercial auto-focusing VCM actuators have a holding current of around I=80 mA when maintaining the moving part of the actuator at the maximum displacement position of around δ=0.3 mm [30,31]. However, in a previous study [28], the current group developed a VCM actuator in which the holding current was reduced to 17 mA ±2 mA by using a closed-loop control scheme driven by a feedback signal generated by a Hall detector. In the current study, the VCM actuator presented in [28] is further developed via the addition of a magnetoconductive plate to the fixed base of the actuator structure such that the lens can be maintained in the specified focusing position without the need for a holding current.
Fig. 1. Image quality of 2 M pixel lens module (a) without and (b) with auto-focusing function enabled, respectively.
2. Overview of VCM actuator proposed in [28]
In the VCM actuator proposed in [28], the lens is driven to the focusing position by the Lorentz force FVCM generated by the interaction between the electric field induced by two current-carrying coils on the fixed part of the structure and the magnetic field produced by two permanent magnets on the lens support structure. Figure 2 presents the free-body force diagram of the lens support structure when the VCM actuator is in a vertical position (i.e. the orientation associated with a maximum operational load). According to the principles of force equilibrium, FVCM=FW+FF, where FW is the weight of the support structure and FF is the friction force generated between the support structure and the fixed part of the actuator. In the ideal case where the friction force is neglected, the force equilibrium equation is given simply by FVCM=FW. In other words, the Lorentz force required to drive the lens module to any given vertical position is equal to the weight of the support structure. In [28], it was shown experimentally that the holding current required to maintain the lens holder in the maximum focusing position (corresponding to a vertical displacement of δ=0.3 mm) was of the order of I=17 mA±2 mA.
3. Modified structure of proposed actuator
Figure 3 presents a schematic illustration of the modified VCM actuator proposed in this study. As shown, the moving part comprises the lens module, a lens holder and two permanent magnets, while the fixed part comprises a magnetoconductive plate, a Hall element, a PCB, two coils and two vertical guide rods attached to a fixed base. Figure 4 illustrates the forces acting on the moving structure of the actuator, namely the Lorentz force FVCM and the magnetic attraction force FMA. The Lorentz force FVCM acts in the direction of the guide rods, and provides the driving force required to move the lens holder during the auto-focusing operation. Meanwhile, the magnetic attraction force FMA between the permanent magnets on the moving structure and the magnetoconductive plate on the fixed structure acts in the normal direction to the guide rods and induces a friction force FF between the lens holder and the rods. Clearly, when µFMA≥FW, where µ is the friction coefficient, the friction force FF is sufficient to hold the moving part of the VCM actuator in a stationary position without the need for an additional holding force.
As stated in Section 2, the maximum operational load occurs when the VCM actuator is positioned in a vertical orientation. Figure 5(a) presents the free-body force diagram for the moving part of the VCM actuator when being driven in the upward direction. The force equilibrium equations in the vertical and horizontal directions are given respectively by
and
Substituting Eq. (2) into Eq. (1), it is shown that the Lorentz force FVCM required to drive the lens module to the focusing position is given by
Note that Eq. (3) is obtained under force equilibrium conditions and applies to the specific case in which the VCM actuator has a vertical posture. From Eq. (3), it is shown that the maximum required Lorentz force FVCM to drive the lens module to its focusing position is FW+µFMA.
Figure 5(b) presents the free-body force diagram for the case where the moving part of the VCM actuator is held in a stationary position by the friction force FF while in a vertical orientation. The force equilibrium equations in the vertical and horizontal directions are given respectively by
and
Since the friction force FF is given by FF=µFN substituting Eq. (5) into Eq. (4), it can be shown that:
Equation (6) infers that if the value of µFMA is greater than FW, the moving part of the VCM actuator can be held at any vertical position with zero holding current, i.e. with no drain on the battery resources.
4. Detailed design specification of modified VCM actuator
Table 1. Design parameters of modified VCM actuator.
A series of finite element simulations were performed to determine the optimal values of the major actuator design parameters subject to the constraint of achieving a zero holding current. Table 1 summarizes the resulting values for each design parameter under the assumption that the moving part of the actuator has a total weight of 200 mgw. Figure 6 illustrates the numerical results obtained for the magnetic flux distribution within the proposed VCM actuator. It can be seen that the magnetic flux induced by the two permanent magnets on the moving part of the actuator passes through the magnetoconductive plate on the stationary part. As a result, the magnetoconductive plate is magnetized and a magnetic flux is produced which creates an attraction force FMA in a direction perpendicular to the direction of movement of the lens module. As discussed in the previous section, the resulting normal force FN generates a friction force between the moving part of the VCM actuator and the guide rods, which effectively maintains the lens holder in a stationary position without the need for a holding current.
Figure 7 shows the simulation results obtained for the variation of the Lorentz force FVCM and the magnetic attraction force FMA as a function of the displacement δ of the moving part of the VCM actuator. It can be seen that the magnitudes of both forces are essentially insensitive to the displacement δ. Moreover, it is observed that the Lorentz force FVCM F has a magnitude of approximately 2000 mgw and is therefore far higher than the driving force calculated in Eq. (3), i.e. FW+0.3 FMA = 200+0.3×1660=698 mgw. In other words, the Lorentz force FVCM developed in the proposed design is more than adequate to drive the moving part of the actuator for auto-focusing purposes. Figure 7 shows that the magnetic attraction force has a value of around FMA=1660 mgw. Thus, the induced friction force, i.e. 0.3 FMA=0.3×1660=498 mgw, is greater than the weight of the moving part of the actuator, i.e. FW=200 mgw. In other words, the friction force developed between the moving part of the actuator and the guide rods is sufficient to hold the lens in place without the need for a holding current.
Fig. 7. Simulation results obtained for variation of maximal Lorentz force FVCM and magnetic attraction force with lens holder displacement.
5. Experimental evaluation
The validity of the proposed VCM actuator was verified by constructing a laboratory-built prototype (see Fig. 8). The performance of the actuator was then characterized using the experimental setup shown in block diagram form in Fig. 9. The working voltage of 3.3 V (equivalent to the voltage of a cell phone battery) was provided by an Agilent E3648A power supply, while the I2C (inter-integrated circuit) input control command signal was generated by a single chip. The displacement of the moving part of the actuator was measured using a non-contacting laser displacement meter (Keyence, LC-2440).
Fig. 10. Variation of measured output displacement with input displacement command (from δ=0.1mm to δ=0.3mm for three different actuator postures.
In the characterization trials, the required displacement of the moving part of the actuator was specified in the form of an input displacement command. Once the actuator had moved to the required position, the power supply was turned off and the final displacement of the moving structure was measured. Figure 10 illustrates the variation of the output displacement δ of the moving structure with the input displacement command for three different postures of the VCM actuator, namely vertical, horizontal and 45°. Note that the output displacement results for each posture represent the five measurement result obtained for five identical input displacement commands. It is seen that a good agreement is obtained between the measured value of the output displacement and the input displacement command for all three postures. In addition, the positioning repeatability is found to be less than 2 µm. By contrast, the positioning repeatability of the closed-loop VCM actuator presented by the current group in a previous study [28,32] was of the order of 5 µm. The improved positioning performance of the current device is thought to be the result of the magnetic attraction force FMA between the permanent magnets and the magnetoconductive plate, which creates a preload between the lens holder and the two guide rods and therefore results in a smoother motion of the moving structure. As compared to the previous VCM actuator with closed-loop control [28,32], the proposed VCM actuator can improve the positioning repeatability by 60% of the previous value. It can be concluded that the proposed design can successfully play its role in auto-focusing purpose.
6. Conclusion
This study has presented an enhanced performance VCM actuator for auto-focusing applications in mobile phone camera modules. In the proposed design, the position of the lens holder is regulated using a closed-loop control system based on a feedback signal generated by a Hall element. Having moved to the required position, the lens is held in place by the magnetic force developed between two permanent magnets located on the lens holder and a magnetoconductive plate attached to the fixed base of the actuator. The experimental results have shown that compared to existing VCM actuators for mobile phone cameras, the proposed device has both a lower power consumption (i.e. a zero holding current) and an improved positioning repeatability.
Acknowledgments
This study was supported by the Ministry of Economic Affairs of Taiwan. The authors would like to express their particular thanks to Mrs. Po-Heng Lin, Shun-Sheng Ke, Yu-Hsiu Chang, and Dr. Ji-Bin Horng of the Laser Application Technology Center, Industrial Technology Research Institute, for their technological assistance throughout the course of this study.
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