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In keeping with consumer preferences for ever smaller electronic products, a requirement exists for compact, high-performance auto-focusing actuators for the camera modules deployed in cell phones. Accordingly, the present study proposes a miniaturized electromagnetic-based actuator comprising a voice coil motor (VCM) and a closed-loop position control system in which an auto-focusing capability is achieved by using a position feedback signal generated by a Hall element to dynamically adjust the position of the lens module. The experimental results show that the holding current required to maintain a lens module weighing 200 mgw in the vertical position is 17 mA ±2 mA. Compared to conventional VCM actuators deployed in cell phone camera applications, the actuator presented in this study has a smaller size (6.5 mm×6.5 mm×4 mm) and an improved power efficiency. In particular, the miniaturized actuator reduces the holding current required to maintain the lens module in the focusing position by around 75% of that required in a traditional actuator.
©2008 Optical Society of America
1. Introduction
Focus adjustment is one of the most basic functionalities in all imaging and scanning systems, and generally requires the mechanical motion of one or more lenses or mirrors within the system [1–5]. In recent years, auto-focusing camera modules have been increasingly deployed in a variety of portable electronic devices such as cell phones, personal digital assistants, laptops, and so forth. Of these devices, the mobile camera phone market has experienced a particularly strong growth, with the integration of high-end camera functions becoming commonplace even within lower- to middle-of-the-range models [6,7]. Generally speaking, the auto-focusing function in such cameras is achieved using a VCM actuator regulated using some form of open-loop position control mechanism. In designing these actuators, the aim is typically to improve the response, reduce the size, minimize the power consumption, and enhance the structural integrity so as to improve the resistance of the device to the wear and tear of daily use [8]. Of these factors, previous studies have focused primarily on improving the dynamic response of the VCM actuator [9–11]. However, optimizing the power efficiency is also essential in order to conserve the finite battery resources of the cell phone [12]. Accordingly, the current study presents a novel miniaturized VCM actuator for auto-focusing cell phone camera modules in which the position of the lens is dynamically adjusted in accordance with a position feedback signal provided by a Hall element. The experimental results show that compared to existing VCM actuators for cell phone devices, the proposed actuator is both smaller and more power efficient.
2. Conventional VCM actuator
Fig. 1. (a) Structure of conventional VCM actuator; (b) forces acting on conventional VCM actuator in vertical posture.
Figure 1(a) illustrates the structure of a conventional VCM actuator used in the auto-focusing camera module of a cell phone [13]. As shown, the fixed part of the actuator comprises two permanent magnets, a yoke and a fixed base, while the moving part, comprises a lens module, a lens-holder and a coil. In operation, the Lorentz force, FVCM, acting on the moving part of the VCM actuator is counteracted by a resistive force generated by two spring plates mounted on the upper and lower surfaces of the actuator body, respectively. As shown in Fig. 1(b), the maximum operational load occurs when the VCM actuator is located at its maximum vertical position. Under this condition, the force acting on the VCM actuator is equal to the sum of the weight Fw of the moving part of the actuator and the restoring resilience force, Fs=kδ, developed by the spring plates, where k and δ denote the spring constant of the spring plates and the vertical displacement of the moving part, respectively. According to the Lorentz law, the magnitude of FVCM is directly proportional to the intensity of the holding current I flowing in the coil. In other words, the holding current increases with an increasing displacement δ of the moving part of the actuator. For a typical commercial VCM actuator used in the auto-focusing module of a cell phone camera, the holding current required to maintain the moving part of the actuator at the maximum focusing position (corresponding to δ=0.3 mm) is around I=80 mA [14].
3. Structure of proposed actuator
Figure 2(a) illustrates the basic structure of the compact, power-efficient VCM actuator proposed in the current study. The moving part of the actuator comprises the lens module, a lens-holder and two permanent magnets (Fig. 2(b)), while the fixed part comprises a Hall element, two coils and two vertical guide rods attached to a fixed base (Fig. 2(c)). In the auto-focusing operation, a current of an appropriate intensity is passed through the coils and generates an electrical field. The interaction between this electrical field and the magnetic field induced by the permanent magnets creates an actuation force which drives the lens-holder and lens module in the upward direction. The Hall element, mounted on the side of one of the permanent magnets, senses the displacement of the moving part of the actuator and outputs an analog signal with a corresponding intensity. The analog signal is processed and an appropriate value of the coil holding current computed in order to dynamically adjust the position of the lens module such that it remains at the required focusing position. This miniaturized VCM actuator will be used in mobile camera phones. Therefore, it has to follow the Standard Mobile Imaging Architecture (SMIA standard) [15].
4. Detailed design specification of miniaturized VCM actuator
Having established a basic design structure for the miniaturized VCM actuator (see Fig. 2), a series of finite element simulations were performed to determine the optimal values of the major design parameters (i.e. the dimensions, material and magnetic energy product (BH)max of the permanent magnets, the dimensions and electrical resistance of the coils, the width of the air gap between the coil and permanent magnet, and so on) required to satisfy the dual design objectives of minimizing the physical size of the device, whilst maximizing its power efficiency. Table 1 summarizes the selected values of each design parameter for the case where the lens module has a weight of 200 mgw and the moving part of the actuator has a total weight of 320 mgw. Figure 3 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 permanent magnets passes through the coils, and the magnetic flux distribution passing through the coils has a bilateral symmetry. Therefore, the Lorentz force FVCM acting on the movable part of the actuator is aligned in the vertical direction and prompts an upward displacement of the lens module.
Table 1. Design parameters of proposed VCM actuator.
Fig. 4. Simulation results obtained for variation of Lorentz force FVCM with actuator displacement as function of holding current.
Figure 4 shows the simulation results obtained for the variation of FVCM with the displacement δ for various holding currents I. It is observed that for a constant holding current, the magnitude of FVCM is essentially insensitive to the displacement δ. Furthermore, it can be seen that FVCM increases approximately linearly with an increasing holding current I. Finally, it is noted that a holding current of I=15mA is required to generate an actuation force FVCM equivalent to the weight Fw of the moving part of the actuator, i.e. 320 mgw. Figure 5 presents a schematic illustration of the electromagnetic characteristics of the VCM actuator designed in accordance with the parameter settings indicated in Table 1. It is observed that the magnetic force lines are perpendicular to the direction of the coil currents, and thus it is confirmed that the selected parameter settings optimize the power efficiency of the proposed device. Comparing Figs. 1 and 2, it is apparent that a major difference between the current device and the conventional VCM actuator is that in the current case, the displacement of the movable part is not resisted by a spring force. Accordingly, the force equilibrium equation in the vertical direction is given simply by FVCM=Fw+FN, where FN is the friction force generated between the lens holder and the two guide rods when the moving part of the actuator is displaced in the upward direction. In the ideal case where FN=0, the force equilibrium equation is given simply by FVCM=Fw. In other words, it can be inferred that: (1) the maximum actuation force FVCM required to drive the lens module to its point of maximum displacement is equal to the weight of the moving part of the actuator, i.e. Fw=320 mgw; (2) at any displacement δ, the actuator consumes the maximum power when the moving part of the system is maintained at its vertical posture; and (3) the current required to drive the moving part to the focusing position is constant and has a value of ~15 mA (see Fig. 4). Note that the absence of a spring resistive force in the proposed design reduces the power required to displace the lens holder in the vertical direction and therefore yields a significantly improved power efficiency compared to that of a conventional VCM actuator.
Fig. 5. Electromagnetic design of proposed VCM actuator: (a) two permanent magnets and coils; (b) cross-sectional view.
5. Prototype fabrication and experimental evaluation
The validity of the proposed VCM actuator was verified by constructing a laboratory-built prototype (see Fig. 6). From inspection, the device is found to have dimensions of 6.5 mm×6.5 mm×4 mm, and is therefore consistent with the specification laid down in the Standard Mobile Imaging Architecture (SMIA standard) for the critical size of auto-focusing devices in mobile phone camera modules [15]. By comparison, the VCM actuators used in commercial auto-focusing cell phone camera modules have typical dimensions of 10 mm×10 mm×4.6 mm [14]. Hence, it is clear that the proposed design represents a significant size saving compared to existing auto-focusing actuator designs. The performance of the proposed device was characterized using the experimental setup shown in block diagram form in Fig. 7. During the experiments, the working voltage of 3.6 V required to power the VCM driver (equivalent to the voltage of a cell phone battery) was provided by an Agilent E3648A power supply. The I2C (inter-integrated circuit) input control command signal used to drive the VCM actuator was generated by a conventional desktop PC. During the characterization trials, the displacement of the moving part of the actuator was measured using a non-contacting laser displacement meter (Keyence, LC-2440) while the holding current was monitored using a Tektronix TCPA300 current meter.
Fig. 8. Experimental results obtained for variation of measured holding current with actuator displacement.
The holding current was evaluated under a background noise of 0~1 mA and was sampled five times every minute. Figure 8 illustrates the variation of the holding current I with the displacement δ of the moving part of the actuator. From inspection, it is determined that the average holding current is17 mA ±2 mA. It is apparent that this value is slightly higher than the theoretical value of 15 mA (see Fig. 4). The discrepancy between the two values can be attributed to two principal factors: (1) the friction force between the lens holder and the two guiding rods is neglected in the theoretical analysis, but has a small finite value in practice, and (2) the experimental value of FVCM is generally 0~10% lower than the computed value due to manufacturing and assembly tolerances, and thus a higher value of the holding current is required to achieve the designated actuator displacement. Significantly, the results presented in Fig. 8 indicate that in the proposed VCM actuator, the holding current required to maintain the moving part of the actuator at the focusing position is around 75% of that required in a conventional VCM actuator with an open-loop control mechanism.
Fig. 9. (a) Experimental setup used for testing auto-focusing performance of proposed VCM actuator; (b) experimental results obtained without (upper) and with (lower) auto-focusing function enabled, respectively.
To assess the auto-focusing performance of the proposed actuator, a camera module comprising the laboratory-built VCM actuator prototype, an image sensor and an electric circuit was assembled and integrated with an image processing system (see Fig. 9(a)). Figure 9(b) shows two images captured with and without the auto-focusing function enabled, respectively. It is apparent that the image quality is significantly improved when the auto-focusing function is activated, thus confirming the potential of the proposed VCM actuator for commercial auto-focusing applications.
6. Conclusion
This study has presented a miniaturized low-power VCM actuator for auto-focusing applications in mobile phone camera modules. In the proposed approach, the position of the lens module is dynamically adjusted using a closed-loop position feedback mechanism driven by a signal generated by a Hall displacement detection system. The device has dimensions of 6.5 mm×6.5 mm×4 mm, and is therefore consistent with the specification laid down for auto-focusing devices in the SMIA standard [15]. Moreover, the value of the holding current required to maintain the lens module in the focusing position is found to be I=(17±2) mA, which is around 75% of the holding current required in a conventional open-loop VCM actuator. Overall, therefore, the results show that the proposed device is both smaller and more power efficient than existing VCM actuators for mobile phone camera modules.
Acknowledgments
This study was supported by the Ministry of Economic Affairs of Taiwan under Grant 6327HB2220.
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