Rapid and Precise Zoom Lens Design Based on Voice Coil Motors with Tunnel Magnetoresistance Sensors

Abstract

In response to the zooming delay issue during the transition from a wide-area search to high-resolution target identification in high-magnification zoom lenses, we propose a drive technology based on voice coil motors. The linear motion of the motor is directly converted into the linear movement of the zoom lens group, significantly enhancing the zoom speed. Additionally, we introduce a high-precision closed-loop control technology utilizing a magnetic scale to achieve the rapid and precise positioning of the zoom lens group. The magnetic scale detection technology achieves precise positioning by detecting periodic changes in the magnetic field, working in conjunction with tunnel magnetoresistance sensors. Demonstrated with a 40× zoom lens example, this study elaborates on the motion trajectory planning and structural dimension design process of a voice coil motor, culminating in the assembly of a physical prototype. Practical validation experiments show that the full zoom time of the lens utilizing our technology is less than 0.3 s, where the full zoom time refers to the time required for the lens to zoom from the wide-angle end to the telephoto end. In positioning accuracy test experiments, lenses using our technology achieved a positioning deviation of less than 5 μm.

1. Introduction

Continuous zoom optical lenses [1,2,3,4], which support seamless adjustments in focal length, are crucial in maintaining the original image resolution throughout the zoom process. This feature makes them extremely valuable for applications in optical security, police enforcement, and unmanned aerial surveillance [5,6]. However, lenses with a large magnification range often require considerable time to switch focus, presenting a significant zoom delay issue. Such delays can result in target loss during the zooming process, adversely affecting the efficiency of information gathering and timely data processing. High zoom speeds and accuracy significantly impact applications such as unmanned aerial surveillance and optical security. In unmanned aerial surveillance, faster zooming brings more time to respond to fast-moving targets, enabling operators to capture and identify targets more quickly and reducing the probability of losing sight of the target. In the field of optical security, high-precision zooming provides clearer images and more accurate target localization, which enhances the effectiveness and reliability of surveillance. Additionally, the ability to zoom quickly and accurately in complex and dynamic environments ensures more stable monitoring, especially in scenarios that require frequent zoom adjustments.

The issue of zoom delay is primarily caused by two factors: Firstly, zoom lenses with a large magnification ratio often require a long travel distance for the moving lens group, which extends the overall zooming time. Secondly, the traditional drive methods for zoom lenses convert the motor’s rotational motion into the linear motion of the lens group, which limits the speed of the lens group’s movement. Efforts to reduce the travel distance of the moving lens group have succeeded in increasing the zoom speed but sometimes lead to increased optical tolerance sensitivity and degraded image quality. Therefore, to effectively address the delay issues in high-magnification zoom lenses, improvements in their drive mechanisms are needed. A voice coil motor (VCM), a linear drive motor operating on the principle of the Lorentz force, which is the force exerted on a charged particle moving through a magnetic field, has the advantages of a simple structure, lack of transmission mechanisms, and fast response time. Traditional zoom drive methods, such as cams and stepper motors, convert the rotational motion of the motor into the linear movement of the lens group. This conversion process inherently slows down the driving speed. Unlike conventional systems that convert rotational motor motion into the linear movement of the lens group, VCMs can directly drive lens groups to perform linear movement. This simplification reduces the mechanical complexity and significantly enhances the zoom speed and positioning accuracy. The rapid response time of VCMs brings much faster zoom operations. They have been widely used in short-travel focusing applications [7,8,9,10,11,12,13] such as smartphones and DSLR cameras. However, research on VCMs for long-travel zoom applications is relatively scarce, primarily due to numerous engineering challenges, such as the control of the zoom precision and energy consumption management.

In recent years, rapid drive technology based on VCMs has attracted much interest in the field. C. Liu et al. [12] introduced a miniature closed-loop VCM for autofocus in mobile phone cameras. Compared to traditional open-loop miniature VCMs, this drive motor, with a smaller size, higher positioning repeatability (less than 5 μm), faster response time (less than 10 ms), and higher power efficiency (maintaining a current below 20 mA), is an ideal solution for mobile camera autofocus. C. Hsieh et al. [7] proposed a compact VCM for optical zoom modules in smartphone cameras, featuring longer displacement (±5.25 mm), more stable performance (deviation of less than 5.57%), and a shorter response time (up to 50 ms). H. Yu et al. [9] proposed a new design concept for a VCM in digital video camera focusing systems, meeting the strict requirements of low power consumption, high efficiency, and fast zoom. By varying the diameter of the winding coil, the thickness of the magnet, and the winding space in the VCM in order to reduce the battery loss caused by VCM focusing, the performance of the digital camera achieves lower battery consumption, higher efficiency, and a shorter focusing time. C. Hou et al. [14] proposed a thin optical zoom system using Alvarez freeform lenses, developing a 3× optical zoom system with two pairs of Alvarez lenses, where the movable components are driven by VCMs. Two voice coil actuators control the lateral movement of the Alvarez lenses, while the rear focusing lens group can be moved along the optical axis by another VCM, achieving autofocus. J. S. Choi et al. [15] proposed a conceptual design for a small actuator for camera lens zoom or focusing systems in camera phones using traditional VCMs due to their small size, simplicity, and reliability. They applied a topology optimization method to design the structure, maximizing the driving force without changing the input power and coil.

Although commercial VCM products are widely used in industry, their resolution is typically limited to the micron level. Furthermore, research on positioning platforms with higher accuracy and a greater travel range is currently being carried out. M. S. Shewale et al. [16] designed a VCM for high-precision applications with accuracy of less than 5 μm at a low speed and an amplitude of 500 μm. The error increased with the increasing amplitude and operating frequency. J. Yao et al. [17] designed a control system based on VCMs for talbot lithography. A double closed-loop control strategy including a position loop and a current loop was utilized, in which the current loop was attained through a driver in order to achieve the fast following of the system current; the position loop was attained using a digital signal processor (DSP) and the position feedback was realized through a high-precision linear scale. Feedforward control and position feedback proportional–integral–derivative (PID) control were used to compensate for the dynamic hysteresis and improve the response speed of the system. Q. Xu [18] proposed a VCM design that used a PID controller to control a high-precision positioning stage with a resolution of 200 nm and a travel range of 11 mm. W. Zhang et al. [19] introduced fuzzy predictive compensation following the digital control of PID. Compared with the traditional method, the experimental results show that fuzzy predictive compensation is an effective and promising method. K. Chang et al. [20] configured a VCM positioning system with a proportional–integrator observer and a discrete sliding mode controller to achieve a position error of 0.25 percent. The result for the implemented control system was superior to the 0.57% position error obtained with the PID controller. L. Zhi et al. [21] proposed an improved active disturbance rejection double closed-loop controller, which had a good suppression effect on the internal and external disturbances of the rotating VCM system and satisfied the requirements of the moving mirror control system regarding the uniformity of the scanning speed and the positional accuracy control of the optical path.

Although VCMs have been widely used in the field of zoom lenses, their applications are mainly limited to smartphones, tablets, and other portable mobile devices. In these application scenarios, the VCM’s travel range is usually short, which is sufficient for basic zoom and focus demands. Typical VCMs only have a travel range of less than 300 μm, and, although previous studies have proposed VCMs with larger travel ranges for zoom lenses, this is insufficient for certain types, especially security lenses.

In this work, we propose a long-travel zoom drive technology based on VCMs. This technology can directly convert the motor’s driving force into the linear motion of the zoom lens group, significantly enhancing the actuation speed of the zoom process. Additionally, a high-precision magnetic scale feedback closed-loop control technology is introduced to achieve high-precision positioning control during long-travel zooming. Through the design example of a 40$\times$ high-magnification zoom lens, we detail the design process of the VCM-based long-travel zoom system. The entire process, including goal establishment, structural design, control system design, prototype assembly, and performance testing, is discussed. The experimental results show that the full zoom time of lenses utilizing our technology is less than 0.3 s, significantly outperforming traditional zoom drive technologies. Furthermore, the system achieves repeatable positioning accuracy of less than 5 μm, meeting the performance requirements of optical systems. This study shows the potential application value of VCMs in high-magnification zoom lenses, providing new insights and methods for the future design of optical zoom systems.

2. System Components and Operating Principles

A rapid and precise zoom system consists of a zoom optical system, a VCM drive structure, and a high-precision positioning control system, as shown in Figure 1, which was constructed using the structural design software Pro/ENGINEER 5.0. The zoom optical system achieves optical zoom, focal plane stabilization, aberration correction, and focusing through the relative movement of multiple lens groups, ensuring high image quality across the entire zoom range. The rapid zoom function is implemented by changing the drive mechanism without altering the original optical design.

The VCM drive structure, shown in Figure 2, is the core component in achieving rapid zoom. It primarily consists of a permanent magnet, an energized coil, and a magnetic yoke. The design of the VCM considers that it should provide sufficient peak driving force within the size constraints to meet the requirements for rapid zoom. The permanent magnet generates a stable magnetic field. The magnetic yoke concentrates and increases the magnetic flux and also provides the necessary structural support to ensure the stability of the magnets and coil. The zoom lens group is fixed on the energized coil, moving together as a unit. Based on the Lorentz force law, the system adjusts the current magnitude and direction in the coil within the magnetic circuit, driving the coil and the zoom lens group to perform variable-speed reciprocating linear motions along the optical axis. This design significantly enhances the zoom speed compared to traditional transmission mechanisms. The high-precision positioning control system must meet the requirements for the axial positioning accuracy of the lens group to ensure that positioning errors do not affect the image quality.

The high-precision positioning control system ensures that the zoom lens group accurately moves to the designated position, achieving precise focal length changes during rapid zooming. As shown in Figure 3, this system mainly comprises a position-sensing element, a magnetic strip, and a control circuit. The position-sensing element is fixed to the zoom lens group and these move together, while the magnetic strip is fixed inside the lens and remains stationary. During zooming, the energized coil drives the zoom lens group and the position-sensing element to move relative to the fixed magnetic strip. The motion position-sensing element records the periodic changes in the magnetic field of the fixed magnetic strip, thereby providing real-time feedback on the zoom lens group’s movement. Through the feedback control circuit, the system achieves real-time closed-loop control, ensuring the precise positioning of the zoom lens group. The high-precision positioning control system must meet the requirements for the axial positioning accuracy of the lens group to ensure that positioning errors do not affect the image quality.

3. Rapid and Precise Zoom System Design Example

3.1. System Performance Requirements

The zoom delay issue in high-magnification zoom optical systems restricts the efficiency of target detection and identification. Rapid and precise zoom technology can effectively address this problem. In the field of drone-based defense, high-magnification zoom optical systems are required to provide wide-area search and target identification capabilities. However, traditional high-magnification zoom optical systems suffer from zoom delays, leading to target loss during the transition between short and long focal lengths. For example, in the 40× zoom optical system designed in this study, the traditional zoom drive method requires 20 s to complete a full zoom cycle. Utilizing rapid zoom technology, this lens can achieve wide-area surveillance in the short-focus state and can quickly switch to a long-focus state, enabling high-resolution identification in the long-focus mode.

Based on the functional requirements of the system, the performance specifications for the VCM-based rapid zoom optical system are as follows.

• Focal length: 7.5 mm to 290 mm (40×);
• F-number: 1.8 @7.5 mm to 6.5 @290 mm;
• Full zoom travel time: 0.3 s;
• Maximum zoom group weight: 30 g;
• Maximum zoom group travel distance: 38 mm;
• Zoom group control precision: 5 μm;
• Maximum power limit: 18 W.

Additionally, to ensure compatibility with existing zoom optical lenses, the dimensions of the lens based on rapid zoom technology should not exceed those of traditional zoom technology lenses of the same specifications. Specifically, the radial dimensions of the VCM-based rapid zoom system should not exceed 68 mm × 68 mm. The optical layout is shown in Figure 4.

3.2. Movement Trajectory Planning

For the optical system shown in Figure 4, the maximum travel distances of the two zoom lens groups are 38 mm and 26 mm, respectively, with weights of 30 g and 16 g. In this context, it is necessary to plan the trajectories for both zoom lens groups individually. The trajectory planning design principles emphasize smooth acceleration and deceleration throughout the motion process, with no sudden changes in acceleration and maintaining a certain stabilization time.

The zoom lens group with the maximum travel distance and the heaviest load is presented as an example to illustrate the movement trajectory planning design process. The current travel distance of Zoom Group 1 is 38 mm, and it is required to stabilize within 0.3 s. To ensure system stability, a third-order symmetrical S-curve is used for the trajectory planning of the zoom group. It controls the rate of change of the acceleration and velocity, achieving smooth acceleration transitions [22]. S-curve trajectory planning avoids sudden changes in acceleration, reducing mechanical shocks and vibrations, thereby enhancing the system’s stability.

The design involves the following:

• Increasing acceleration phase for 0.05 s;
• Decreasing acceleration phase for 0.05 s;
• Increasing deceleration phase for 0.05 s;
• Decreasing deceleration phase for 0.05 s.

During the motion process, the rate of change of the acceleration remains constant. Let the absolute value be J₀; then, in each motion phase, the rates of change of acceleration J(t), acceleration a(t), velocity v(t), and displacement s(t) over time are as given by Equations (1)–(4).

Increasing acceleration phase, 0 ≤ t ≤ 0.05 s:

$$\left\{\begin{array}{c}\begin{array}{c}J\left(t\right)=J_0\\ a\left(t\right)=J_{0}t\end{array}\\ \begin{array}{c}v\left(t\right)={\displaystyle \frac{1}{2}}{J}_0{t}^2\\ s\left(t\right)={\displaystyle \frac{1}{6}}{J}_0{t}^3\end{array}\end{array}\right.$$

(1)

Decreasing acceleration phase, 0.05 s ≤ t ≤ 0.1 s:

$$\left\{\begin{array}{c}\begin{array}{c}J\left(t\right)=-J_0\\ a\left(t\right)=0.05J_0-J_0(t-0.05)\end{array}\\ \begin{array}{c}v\left(t\right)={\displaystyle \frac{1}{2}}{J}_0\times { 0.05}^2+0.05\times J_0(t-0.05)-{\displaystyle \frac{1}{2}}{J}_0{(t-0.05)}^2\\ s\left(t\right)={\displaystyle \frac{1}{6}}{J}_0\times { 0.05}^3+{\displaystyle \frac{1}{2}}{J}_0\times { 0.05}^2\times (t-0.05)+{\displaystyle \frac{1}{2}}{J}_0\times 0.05\times {(t-0.05)}^2-{\displaystyle \frac{1}{6}}{J}_0{(t-0.05)}^3\end{array}\end{array}\right.$$

(2)

Increasing deceleration phase, 0.1 s ≤ t ≤ 0.15 s:

$$\left\{\begin{array}{c}\begin{array}{c}J\left(t\right)=-J_0\\ a\left(t\right)=-J_0(t-0.1)\end{array}\\ \begin{array}{c}v\left(t\right)=J_0\times { 0.05}^2-{\displaystyle \frac{1}{2}}{J}_0{(t-0.1)}^2\\ s\left(t\right)=J_0\times { 0.05}^3+J_0\times { 0.05}^2\times (t-0.1)-{\displaystyle \frac{1}{6}}{J}_0{(t-0.1)}^3\end{array}\end{array}\right.$$

(3)

Decreasing deceleration phase, 0.15 s ≤ t ≤ 0.2 s:

$$\left\{\begin{array}{c}\begin{array}{c}J\left(t\right)=J_0\\ a\left(t\right)=-J_0\times 0.05+J_0(t-0.15)\end{array}\\ \begin{array}{c}v\left(t\right)={\displaystyle \frac{1}{2}}{J}_0\times { 0.05}^2-0.05\times J_0\times (t-0.15)+{\displaystyle \frac{1}{2}}{J}_0{(t-0.15)}^2\\ s\left(t\right)= {\displaystyle \frac{5}{6}}{J}_0\times { 0.05}^3+{\displaystyle \frac{1}{2}}{J}_0\times { 0.05}^2\times (t-0.15)-{\displaystyle \frac{1}{2}}{J}_0\times 0.05\times {(t-0.15)}^2+{\displaystyle \frac{1}{6}}{J}_0{(t-0.15)}^3\end{array}\end{array}\right.$$

(4)

In each motion phase, the parameters change over time, as shown in Figure 5. From Figure 5, it can be seen that the acceleration reaches its maximum absolute value of 7.6 m/s² at 0.05 s and 0.15 s, and the velocity reaches its maximum value of 0.38 m/s at 0.1 s. The travel distance for Zoom Group 2 is 26 mm. Using the same method for trajectory planning for Zoom Group 2, the maximum acceleration and velocity are found to be 5.2 m/s² and 0.26 m/s, respectively.

3.3. VCM Structural Design

To meet the requirements of the zoom lens group’s motion trajectory and the total zoom travel time, it is essential to design the VCM structural parameters appropriately [23,24] with the limitations in the lens size and power. The basic principle is that the electromagnetic force generated by the VCM should exceed the required maximum driving force, and VCM structural design includes the structural layout design, magnetic circuit parameter design, and coil parameter design. The structural composition of the VCM is shown in Figure 6.

3.3.1. Magnetic Circuit Design

According to the principle of the Lorentz force, the motor driving force F_d that the VCM can output is given in Equation (5). In this equation, B represents the magnetic field strength, I represents the current through the energized coil, and L represents the total length of the coil in the magnetic field.

F_d = BIL

(5)

According to Equation (5), under the constraint of the motor power, the peak current of the motor is also restricted. To meet the driving force requirements under the maximum acceleration conditions, it is necessary to optimize the dimensions of the magnets and the yoke within the limited spatial constraints to achieve the maximum magnetic field strength, thereby maximizing the electromagnetic driving force. However, for systems with enough space, the magnetic flux density B is greater with larger magnets and yokes, and a greater driving force can be provided. In this study, Zoom Group 1 is the heaviest lens group and has the longest travel distance of the two zoom groups discussed. Therefore, the design method of the VCM for Zoom Group 1 is used as a case study.

The primary function of the permanent magnet is to generate a stable magnetic field, which interacts with the energized coil to produce an electromagnetic force, thereby driving the movement of the coil and the zoom group. The residual magnetic flux density B_r, maximum energy product BH_MAX, and coercive force H_cb of the magnetic material directly affect the efficiency and power output of the VCM. Higher values of these parameters indicate that the magnetic material can provide a stronger magnetic field and better magnetic stability for the same volume. Considering these characteristics, along with the operational temperature tolerance and cost, neodymium iron boron (NdFeB) 45SH was selected as the magnetic material for this study. The yoke primarily serves to guide and concentrate the magnetic field. By utilizing high-permeability materials, the yoke effectively enhances the magnetic effect, reduces magnetic field leakage, and provides structural support, thereby improving the efficiency and performance of the VCM while protecting the external environment from magnetic interference. In selecting the yoke material, both the magnetic permeability and hardness must be considered. Taking the cost into account, low-carbon steel was chosen as the yoke material.

The design of the dimensions of the magnetic circuit includes the width of the magnets and yoke (W_m and W_y), the thickness of the magnets and yoke (T_m and T_y), and the length of the magnets and yoke (L_m and L_y), as shown in Figure 7. Theoretically, the larger the width (W_m and W_y) and thickness (T_m and T_y) of the magnets and yoke, the stronger the magnetic field that they generate. However, in practical optical systems, the sizes of these components are often constrained by the available space within the lens and cannot be increased indefinitely.

To avoid magnetic leakage, the widths of the magnets and the yoke should be consistent. Theoretically, a larger magnet width can provide a stronger magnetic field. However, due to the radial constraints of the lens, the maximum allowable width (W_m and W_y) for both the magnets and the yoke should be less than 21 mm.

The primary goal in designing the thickness of the magnets and yoke is to achieve high magnetic field strength and low magnetic leakage while considering the space constraints and mechanical strength requirements. Increasing the thickness of the magnets and yoke can indeed enhance the magnetic field strength, but the marginal gain in the magnetic field strength diminishes when the thickness is beyond a certain value. Additionally, when the yoke thickness equals the magnet thickness, magnetic leakage is minimized. Due to the internal space constraints of the lens, the magnet thickness in this case is set to 3 mm, and the yoke thickness is set to 2.5 mm. A gap of 3 mm is maintained between the magnets and the yoke to accommodate a 2-mm-thick energized coil, ensuring 0.5 mm clearance for coil movement relative to the magnets and yoke.

Theoretically, the length of the magnet should at least cover the entire zoom travel distance, the height of the coil (H_c), and the travel allowance. The travel allowance is used mainly to reduce the efficiency losses caused by the rapid changes in the magnetic field at the edges of the magnet. Given the fixed values for the zoom travel distance and travel allowance based on the optical zoom parameters, in this case, the zoom travel distance is 38 mm and the travel allowance is 1 mm. At the same time, the height of the coil should ensure that the zoom lens group has a sufficient support length within the coil to prevent tilting during installation. Considering the dimensions of Zoom Group 1, the coil height should be larger than 12 mm. Thus, the maximum allowable axial space for the zoom system is 52 mm.

3.3.2. Coil Design

The dimensions of the energized coil should closely match the dimensions of the magnets and the yoke in order to minimize the coil weight and reduce the driving load weight. Therefore, the structural dimensions of the coil are essentially determined in the magnetic circuit design. The coil height (H_c) is 12 mm, the coil thickness (T_c) is 2 mm, and the long side of the inner coil (W_c) is 21 mm, which is consistent with the width of the magnets and the yoke. The short side width (L_c) of the inner coil is 3.5 mm, which includes a 0.5 mm gap between the coil and the yoke both above and below. The dimensional parameters are shown in Figure 8.

In this case study, we used low-resistivity copper wire for the energized coil. Given the core diameter d of the energized wire, the total length of the energized coil (L_total) was determined as shown in Equation (6).

$$L_{total}={\displaystyle \frac{H_c}{d}} \times {\displaystyle \frac{T_c}{d}} \times 2\left(W_c+L_c\right)$$

(6)

The cross-sectional area S_c of a single wire is shown in Equation (7).

$$S_c=\mathsf{\pi }{\left({\displaystyle \frac{d}{2}}\right)}^2$$

(7)

The resistance of the coil is given in Equation (8), where η represents the electrical conductivity of the wire. In this study, we selected copper wire with resistivity of 1.68 × 10⁻⁸ Ω·m.

$$R_c=\mathsf{\eta }{\displaystyle \frac{L_{total}}{S_c}}$$

(8)

Thus, the current in the coil is given in Equation (9), where U represents the operating voltage.

$$I={\displaystyle \frac{U}{R_c}}$$

(9)

By substituting the parameters from Equations (6)–(9) into Equation (5), the driving force F_drive that the VCM can provide is as given in Equation (10).

$$F_{drive}={\displaystyle \frac{BU{W}_c\mathsf{\pi }{d}^2}{8\mathsf{\eta }\left(W_c+L_c\right)}}$$

(10)

According to Equation (10), given fixed parameters for the magnetic field strength, wire resistivity, and coil dimensions, the motor driving force is proportional to U and d². Due to the system’s peak power limitation (less than 18 W) and the use of a standard 12 V power supply, the wire diameter d can be calculated as 0.24 mm using Equations (6)–(9).

According to Equation (10), the driving force F_drive is independent of the coil thickness T_c and height H_c. In fact, the weight of the coil increases with increasing T_c and H_c, which in turn increases the required driving force and makes zooming more difficult. However, in order to ensure the sufficient structural strength of the coil, T_c and H_c should not be below a certain value.

3.4. Driving Force Simulation Analysis

Whether the VCM can meet the driving demands for rapid zooming primarily depends on whether the peak driving force provided by the VCM exceeds the required force under the maximum acceleration conditions. For the analysis case of Zoom Group 1 in this study, the peak acceleration a_max is 7.6 m/s², as calculated in Section 3.2. The load to be driven during the motion includes the weight of the Zoom Group 1 lens assembly, the positioning sensor, and the weight of the copper coil.

According to the calculations in Section 3.3.2, the weight of the coil m_c is 12.2 g. Additionally, the weight of the positioning sensor m_a is 2.3 g, and the weight of the Zoom Group 1 lens assembly m_z1 is 30 g. The peak required driving force (F_r) is given by Equation (11).

F_r = (m_z1 + m_a + m_c) a_max = 0.338 N

(11)

According to the structural parameters of the VCM in Section 3.3.1, the magnetic field strength of the magnets and yoke is simulated in ANSYS. The simulation results are shown in Figure 9. The results indicate that throughout the entire travel range, the minimum magnetic field strength is greater than 0.15 Tesla.

By substituting the dimensions of the coil and the voltage parameters into Equation (10), we obtain a driving force of $F_{\mathrm{drive}}=2.076 N$ for the VCM. This value exceeds the peak driving force requirement, indicating that the VCM is capable of meeting the system’s minimum zoom time requirement.

3.5. Magnetic-Scale Closed-Loop Control System

The position sensor operates based on the tunnel magnetoresistance effect [25,26]. The TMR sensor mainly includes a thin-film element consisting of two ferromagnetic layers, the free layer and the fixed layer, with a thin insulating tunnel layer between them. The magnetic field direction of the fixed layer remains constant, whereas the magnetic field direction of the free layer changes in response to the external magnetic field. When the magnetic field directions of the fixed and free layers are parallel, the resistance is minimized, and the tunnel current is maximized. Conversely, when the directions are antiparallel, the resistance reaches its maximum, and the tunnel barrier current is minimized. Its working mechanism is shown in Figure 10a.

When the TMR sensor mounted on the zoom lens group moves relative to the fixed magnetic strip, the magnetic field of the strip changes and acts on the TMR sensor [27,28,29], causing the magnetization direction of the free layer to change with the external magnetic field. These changes cause the resistance of the tunnel junction to vary periodically. By measuring these periodic changes in resistance, the control system can record and adjust the position of the zoom lens group in real time, achieving precise position detection and control. The working mechanism is illustrated in Figure 10b.

The working principle of the high-precision magnetic scale closed-loop control system involves a high-precision magnetic-scale position sensor to detect the magnetic field changes of the magnetic scale strip. This technology utilizes high-precision magnetic-scale and TMR sensors to continuously monitor the position changes of the zoom lens group, providing accurate position signals. The control system then adjusts the position of the group in real time based on these signals, achieving micron-level precise positioning control through closed-loop feedback.

As shown in Figure 11, the closed-loop control process functions as follows:

• The position sensor detects the position signal;
• The control chip outputs a control signal;
• The driver chip outputs driving pulses, causing the zoom group to move in real time.

This cycle repeats continuously, enabling high-precision closed-loop control. The control accuracy of the system depends on the size of a single grating unit. In this case, a single grating unit measures 2.5 μm, ensuring that the positioning control accuracy of the zoom group meets the 5 μm requirement.

4. Experimental Verification

The rapid zoom lens prototype is assembled after completing the VCM structural design and magnetic-scale closed-loop control system, a photograph of which is shown in Figure 12. The core component of the VCM is shown in the red dashed box, where the zoom group, energized coil, permanent magnet, and magnetic yoke are also visible. Rapid zooming and precise positioning were our goals; thus, experiments to measure the zoom time and positioning accuracy were performed. In the zoom time test experiment, the full-range zoom time from the wide-angle end to the telephoto end was measured to evaluate the response speed of the rapid zoom. In the positioning accuracy test experiment, the deviation between the actual position of the lens group and the target position was repeatedly measured to evaluate the system’s accuracy and stability.

In the zoom experiment, as presented in Figure 13, we utilized a VCM and a traditional step motor to drive the 40× zoom optical system. In the zoom speed test experiment, zoom lenses with the same magnification were driven from the wide-angle end to the telephoto end. As shown in Figure 13a and Supplementary Movie S1, clear high-magnification images were obtained using a rapid zoom lens with VCMs at t = 0.1 s, and it took less than 0.2 s to complete the zooming process. As a comparison, a slow and continuous zoom was achieved using a traditional zoom lens with a step motor from t = 0 s to t = 20 s, as shown in Figure 13b and Supplementary Movie S2. The striking contrast demonstrates that the rapid zoom lens with the VCMs effectively increases the zooming speed and decreases the time required.

Additionally, experiments to measure the positioning accuracy were performed under ISO 230-2-1997 [30], and the results are listed in

Table 1. Measurement results for positioning accuracy under ISO 230-2-1997.

Test Points		Target Position 1 (0 mm)		Target Position 2 (38 mm)
		↓ *	↑	↓	↑
Positioning Deviation (mm) for Cycles j = 1, 2, 3, 4, 5	1	−0.001500	−0.000100	−0.000900	0
	2	0	0	−0.001000	0
	3	0	0	−0.001900	0.000200
	4	−0.004500	−0.004800	0.000300	0.000100
	5	0	0	0.002300	0.000300

* Movement direction.

. Two target positions (i = 1, 2) were set at 0 mm and 38 mm, respectively, and five cycles (j = 1, 2, 3, 4, 5) were carried out to obtain the positioning deviation on a bi-directional axis. Herein, ↓ and ↑ refer to different movement directions when a zoom lens group approaches a target position in either direction along the axis. For j = 4, the maximum positioning deviation was −0.0048 mm at Target Position 1. Therefore, it can be seen that our positioning control system achieves high positioning repeatability with a positioning deviation of less than 5 μm.

5. Discussion and Conclusions

It is of great practical significance to integrate VCMs and TMR sensors into zoom lens systems to realize rapid zooming and precise positioning. In particular, the proposed designs solve the time delay problem of high-magnification zoom lenses and are suitable for applications in zoom lenses that move from a wide-area search to high-resolution target identification. The entire process of realizing a novel zoom lens with VCMs and TMR sensors was undertaken, including goal establishment, structural design, control system design, prototype assembly, and performance testing.

Rapid zooming and precise positioning were our main goals, and this system exhibits potential universality for other large-travel zoom lenses if the functions are implemented by changing the drive mechanism without altering the original optical design. Furthermore, system performance requirements like the zoom travel distance and time were established through practical applications and can be customized on demand. In movement trajectory planning, the design principles emphasize smooth acceleration and deceleration throughout the motion process, with no sudden changes in acceleration and maintaining a certain stabilization time. Notably, the total zoom travel time (0.2 s) of our design is less than the required time (0.3 s) because it is necessary to leave enough time for signal transmission and processing within the closed-loop control at a later point. In addition, the VCM structure needed to be as compact as possible in order to ensure its functionality, which guided our design of the magnetic circuit and energized coil. The driving force F_drive = 2.076 N for the VCM was obtained from the designed coil and magnetic field strength simulation results, which far exceeded the peak driving force requirement (0.338 N), indicating that the VCM is capable of meeting the minimum zoom time requirement. The redundant gap between the simulated and required driving forces can meet the demands of other usage scenarios, such as the vertical motion of the zoom group during gravitational acceleration, even when the driving elements are heavy. Moreover, harsh environments should be considered because the temperature may affect the resistance of the coil, thus affecting the driving force. Significantly, precise positioning is realized through a high-precision magnetic-scale closed-loop control system, in which a single magnetic grating unit measures about 2.5 μm, ensuring that the positioning control accuracy of the zoom group meets the 5 μm requirement. In order to test and verify the reliability of our design, a rapid zoom lens prototype was assembled and experiments to measure the zoom time and positioning accuracy were performed. The striking contrast of the zoom times between large-ratio zoom lenses with VCMs and those with traditional step motors demonstrates that our design using VCMs effectively decreases the zoom time to 0.2 s. Additionally, experiments to measure the positioning accuracy under ISO 230-2-1997 indicate that the maximum positioning deviation of our positioning control system is −0.0048 mm (≤5 μm), with high positioning repeatability.

In conclusion, we propose a novel design for the integration of VCMs and TMR sensors into high-magnification zoom lenses, with the advantages of rapid zooming and high-precision positioning. This design directly converts the linear motion of the motor into the linear movement of the zoom lens group, significantly enhancing the zoom speed and positioning accuracy. Our design example ensures a sufficient margin for the zoom time and driving force to cope with various usage scenarios and environmental challenges, such as temperature variations, lens posture changes, and control errors. In these situations, the zoom time remains within 0.3 s. In the future, we will focus on precise multi-physics simulation analyses to further optimize and miniaturize the permanent magnet and magnetic yoke. Furthermore, exploring the integration of advanced control algorithms to enhance the precision and speed of the zoom groups is also crucial.