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A new tunable-focus liquid lens system is proposed that consists of a liquid-filled PDMS (polydimethylsiloxane) membrane, special liquid injection mechanism based on a cam structure, antagonistic winding-type SMA(shape memory alloys) actuator for changing the surface curvature of the membrane, and liquid injection control system, including a digital signal processing board and actuator driver board. The focal length of the liquid lens is adjusted by changing the radius of the curvature of the liquid lens through redistributing the liquid using an injection mechanism. In the case of liquid lens systems using conventional injection mechanisms, the nonlinear relationship between the focal length change and the actuator displacement makes it difficult to control the focal length of the lens system, as there is only a narrow control range for adjusting the focal length over a wide range. In addition, miniaturization of the lens system is difficult due to the requirement of bulky and heavy actuators, such as an electrical motor and pump[7]. Thus, a relatively light and small SMA actuator is proposed for a compact lens system. This paper then provides a detailed description of the proposed tunable-focus liquid lens system, and an experimental system is also implemented. Finally, the focusing performance of the proposed liquid lens system is analyzed, and its usefulness and effectiveness verified through a series of experiments.
©2009 Optical Society of America
1. Introduction
Vision systems are now widely applied to robotic systems, security platforms, mobile terminals, and personnel computers for the purpose of monitoring and recognizing environments and communicating with others, and these fields of application are also rapidly expanding. While most vision systems require a tunable focus function for the accurate perception and recognition of interesting objects, conventional lens systems are bulky and heavy due to the use of complex mechanisms and actuation systems to move a set of lenses forward or backward in a lens barrel to adjust the focal length. Thus, to solve these problems, researchers have been developing tunable focus lenses [1]–[5].
Tunable focus lens systems can essentially be classified into electro-wetting lenses [6], gel-type lenses [5], and liquid lenses [3] in terms of the driving method. Electro-wetting lenses are composed of a droplet of liquid on the surface and a power system providing a high voltage. As such, the surface tension and contact angle of the droplet can be continuously and rapidly changed according to the supplied voltage. However, this requires a high voltage and it is difficult to fabricate lens systems with a large aperture and lens diameter. In contrast, gel-type lenses are relatively resistant to shock and vibration, yet when the lens diameter increases for long focus tuning or decreases for short focus tuning, precise control of the focal length is very difficult due to inaccurate displacement of the lens thickness. Finally, liquid lenses are also resistant to shock and vibration, and allow more accurate focus control than gel-type lens. Therefore, among the three types of lens system, this paper focuses on liquid tunable-focus lenses.
Recently, a miniaturized liquid lens with a variable focus function [4] was developed based on MEMS technology and a liquid filling technique. The shape of the water lens between two elastic membranes is controlled by adjusting the pressure applied to the inlet. Yet, in spite of the simple concept, this technique is not applicable to a large lens system.
Generally, when the lens diameter is increased, the lens system has many optical advantages, such as a fast optical system, high numerical aperture, high resolution system, and large image size. However, in the case of liquid tunable-focus lenses, when the lens diameter is increased, this requires a bulky and heavy actuator, such as an electrical motor and pump for a higher pressure or torque generation.
Furthermore, in a liquid tunable-focus lens system, the focal length of the liquid lens is changed using a liquid redistributing mechanism that is nonlinear to the control input. In particular, since the control range for adjusting the focal length in a wide range is narrow and restricted to a specific region, precise control of the focal length becomes difficult.
Accordingly, to overcome these problems, this paper proposes a tunable-focus lens system with a simple modified injection mechanism controlled by an antagonistic winding-type SMA actuator, allowing the system to be small in size, lightweight, and have a high power density. A thin transparent PDMS membrane is also used to change the radius of the curvature of the lens. The mechanical performance of the proposed liquid lens system is proved experimentally based on a comparison of measurements and simulation results, while the dynamic tunable-focus response of the proposed lens system is demonstrated through experiments with a focus target and real scenes.
2. Proposed liquid lens system and SMA actuator
2.1. Basic Concept and Device Structure
The proposed tunable-focus liquid lens system has a PDMS membrane with a changeable radius for the curvature. The membrane curvature of the lens system is changed by redistributing the volume of the injected liquid, whereas the focal length of the lens system is determined by changing the radius of the curvature of the lens membrane. In conventional liquid lens systems, two methods are used to redistribute the liquid volume in the lens: pressing a specific part of the lens[1] and injecting liquid into the lens from an external storage[3]. However, the liquid injection mechanisms require a bulky and heavy actuator, such as electrical motors and pumps. Plus, in the case of a liquid lens, membrane deformation induces a nonlinear characteristic between the actuator displacement and the focal length change, and since the control range to adjust the focal length in a specific region is narrow, this makes focus control difficult. Thus, extended linearization of the focal length is required to expand the control range. Therefore, to overcome these problems, a small and light SMA actuator and specially designed injection mechanism are proposed. Plus, to linearize the focal length change, a modified cylindrical cam structure is applied that changes the linear rotation movement to a nonlinear directional movement on the optical axis, as shown in Fig. 3. The injection mechanism is driven by a compact, lightweight SMA actuator with a high power density. A conceptual block diagram of the proposed focus tuning lens system is shown in Fig. 1(a), along with the overall liquid lens system in Fig. 1(b).
Fig. 1. Conceptual block diagram of proposed tunable focus lens system: (a) process of focal length tuning and (b) tunable focus lens system.
2.2 Mathematical System Modeling
The change in the radius of the curvature can be basically modeled based on the condition that the liquid volume injected by the injection mechanism is equal to the volume of the generated spherical lens. Figure 2 shows the simple modeling of the shape of the spherical lens and volume of the injected liquid. The relationship between the radius of the curvature of the lens membrane and the liquid volume change can be derived using Eqs. (1)–(3).
Fig. 2. Conceptual model of lens system: (a) conceptual drawing of uniconvex lens and (b) variation of radius of curvature according to injected depth of actuator.
where R, V 1, V 2, h, a, and H are the radius of the curvature, liquid volume redistributed by the injection mechanism, volume of the uniconvex lens, lens height, lens diameter, and injection depth of the piston, respectively. The injected volume in the lens can be derived according to the restricted mechanical condition that the redistributed liquid volume V 1 in Eq.(1) and volume of the uniconvex lens V 2 in Eq.(3) are equal.
To expand the control range and create a linear focal length, this paper proposes a new injection mechanism modified from a cylindrical cam structure, as shown in Fig. 3. The displacement of the injection depth can be calculated by substituting the focal length in Eq.(4) into Eqs.(1)–(3). Meanwhile, the injection displacement of the injection mechanism induced by the rotation displacement of the cylindrical cam structure can be derived using Eq.(5).
where f, n, k, L, and d are the focal length of the uniconvex lens, reflective index, designed proportional factor for the rotational displacement and focal length change, rotational displacement, and initial focal length, respectively. The relationship between the rotational displacement, L, and the injection depth of the piston, H, represented in Eq. (5) is implemented in the cam profile. The simulated injection depth when varying the rotational displacement for the implemented lens system is shown in Fig. 7 in section 2.3.
2.3 Implementation of Liquid Lens Device
The proposed tunable-focus liquid lens system consists of a lens body, piston body, piston bar, rotation ring, and lens cover, as shown in Fig. 4(a). The components of the liquid lens system were designed using the 3-D design tool CATIA(computer aided three dimensional interactive application), while the real parts were fabricated using a RP(rapid prototyping) manufacturing method, as shown in Fig. 4(b).
Fig. 4. Components of proposed liquid lens system: (a) Designed using 3-D design tool CATIA and (b) manufactured using RP(rapid prototyping) method.
A rubber ring and plate glass are used to prevent any leakage of the liquid. The thin, transparent, and elastic qualities of the lens membrane allow the radius of the curvature to change when redistributing the liquid volume. The fabrication conditions of the PDMS membrane were determined based on repetitive experimental trials, while the fabrication process was follows: first, the two materials from a Sylgard@ 184 elastomer kit were mixed in a ratio of 10 to 0.9, and avoiding bubbles used to coat a silicon wafer at 1000rpm for 3sec. A 100um-thick, transparent lens membrane was then fabricated as the coated silicon wafer was heated at 90°C for 40sec. The assembled liquid lens system is shown in Fig. 5, and the detailed specifications given in Table 1.
Fig. 5. Proposed tunable-focus liquid lens system: (a) Assembled system using CATIA and (b) real assembly of liquid lens system.
Table 1. Detailed specifications of proposed liquid lens system.
When the rotation ring rotates clockwise or counterclockwise, the piston bar is injected or ejected, as shown in Fig. 6, thereby increasing or decreasing the radius of the curvature of the PDMS membrane, respectively.
Fig. 6. Basic driving principle of proposed liquid lens system: (a) injection motion and (b) ejection motion.
The injected depth of the piston bar in terms of the linear actuation of the rotational actuator was measured to verify the reliability of the proposed liquid lens system, and the measured and simulated results matched well, as shown in Fig. 7.
Fig. 7. Simulated and measured depth of injection mechanism when varying rotational length displacement.
2.4 Design of winding-type SMA actuator and control system.
As conventional liquid lens systems use bulky and heavy actuators, such as an electrical motor or pump, for a high driving torque, this makes it difficult to make miniature and lightweight liquid lens systems. Thus, to overcome this problem, the proposed liquid lens system uses an SMA actuator as an alternative.
Basically, an SMA expands or contracts when heated or cooled, respectively, and its advantages include being compact, lightweight, and having a high power density. However, the disadvantages of an SMA include a small ratio of deformation and slow response time [8]. Thus, a new mechanical design is needed to overcome these disadvantages. SMA actuators can be classified into two types, bias spring and antagonistic, according to the driving method, and three types, linear, zigzag, and winding, according to their mechanical shape[9]. In this paper, an antagonistic winding-type SMA actuator is proposed to provide a larger deformation and faster response time than conventional bias spring-type ones comparatively within the restricted volume of the proposed lens system[10]. Whereas a conventional winding SMA actuator has a high friction due to the winding structure, in the proposed SMA actuator, the SMA wire is wound around small-sized ceramic rollers, as shown in Fig. 8. The proposed SMA actuator is also designed as an antagonistic type to provide bidirectional control of the focal length.
The control system of the proposed tunable-focus liquid lens system consists of a main controller board(TMS320F2812), SMA driving board, and position sensor interface, as shown in Fig. 9. A block diagram of the overall control system is shown in Fig. 10. The two PWM channels from the DSP board are inserted into the FET’s gate on the SMA driving board. The SMA driver board then transmits the proper voltage to drive the SMA actuator.
3. Experiments and results
3.1 Experimental setup
The reliable operation of the manufactured new liquid lens system and linearity of the focal length are verified through a series of experiments. The focal length was measured using Korean industrial standards[KS B 5408], and the experimental setup for the focal length measurement was set up as shown in Fig. 11.
The experimental setup consisted of a halogen lamp, parallel ray barrel, proposed tunable-focus liquid lens system, CCD camera sensor module, and control system.
Fig. 12. Experimental setup for focal length measurement based on Korean industrial standards [KS B 5408].
As shown in Fig. 12, the halogen light passed sequentially through a narrow bandpass interference filter(central bandwidth=547nm, FWHM=10nm), 1mm precision pinhole, collimating lens (focal length of achromatic lens = 250mm), and tunable-focus liquid lens, then the resulting halogen light was finally focused on a CCD camera.
The CCD camera used in the experiment was an HVR-2300C, where the size of one pixel was 3.2um by 3.2um and the resolution was 2048(H) by 1536(V). The images were then transferred to a host PC for further image processing. The focal length of the liquid lens was calculated using Eq. (6).
where f, f 0, y, y' are the focal length of the liquid lens, focal length of the collimating lens, size of the subject, and size of the image formed on the CCD camera, respectively.
3.2 Results
First of all, the variation of the focal length when displaced by the antagonistic winding-type SMA actuator was measured as shown in Fig. 13, and the range of the focal length was from 40mm to 65mm. As the displacement of the SMA actuator increased from 0mm to 10mm, the measured focal length at an arbitrary position of the SMA actuator matched well with the simulated results for the focal length. Also, as expected, a linear relationship between the focal length and the displacement of the SMA actuator was observed throughout the entire range of the system focal length, as shown in the figure.
Secondly, the three test images in Fig. 14 show the focusing process of the proposed liquid lens system from a defocused image (a) at an arbitrary focal length to a focused clean image (c) through the halfway (b), where the focal length was identical to 55mm for the image test target. The autofocus procedures can be implemented by checking focus measure value in a region of interest in DSP controller board.
Fig. 14. Test images: (a) defocused image at arbitrary position (initial f = 55 mm), (b) intermediate state of focused image with f = 51.5 mm, and (c) final focused image with f = 48 mm.
Thirdly, to show applications of the proposed system, multi objects with different distances from imaging part are tested. Among multi object in Fig. 15(a), an interesting target at the desired position can be optionally user-selected by using the focal length tuning. As experimental results, the object2 which is far from the lens system is firstly focused in Fig. 15(b), and then the object1 which is close to lens system is finally focused as shown in Fig. 15(c).
Fig. 15. Multi-objects focusing test: (a) Experimental setup, (b) test image (f = 50.6 mm), and (c) test image (f = 55.1 mm).
Finally, an additional experiment was also performed to check the response time of the proposed liquid lens system with an SMA actuator. As shown in Fig. 16, from the initial position, elongation of SMA actuator = 0mm and focal length = 65mm, it took about 2.5sec to reach the desired position, elongation of SMA actuator = 6.31mm and focal length = 48.8mm. After reaching the desired position, the position of the liquid lens system was kept in a steady state. To improve this slow response problem, our current research issues are focused on this problem. It can be modified by adopting the displacement amplifying mechanism or developing new SMA materials.
4. Conclusion
A new tunable-focus liquid lens system that uses a thin elastic PDMS membrane, simple injection mechanism, and antagonistic winding-type SMA actuator was proposed that can also be applied to a large lens system with a large aperture. Creating a miniaturized and lightweight liquid lens system is not easy, as conventional liquid lens systems require a bulky and heavy actuator, also the control range of the focal length is narrow due to the nonlinear characteristics of the focal length vs. the control input variable. Thus, to solve these problems, a special antagonistic winding-type SMA actuator was proposed that is small in size, lightweight, and has a high power density. In addition, to provide linear characteristics for the focal length when varying the control input variable, a new injection mechanism was designed based on a cylindrical cam structure. The proposed system was then implemented with control system hardware and software. To demonstrate the effectiveness of the proposed system, experimental measurements and simulations were performed, and the focal length variations of the liquid lens system according to the rotational displacement were found to match well. Plus, the focusing function of the tunable-focus lens was verified using focus tuned images. Finally, the response time to reach to the desired position (focal length = 48.8mm) was measured from the initial position (focal length = 65mm). Further research is currently focused on developing a smaller and lighter tunable-focus lens system design with a miniaturized mechanism, along with an improved lens system with a fast response time.
Acknowledgements
This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean Government (MEST) (NO. R01-2007-000-20630-0)
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