IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005 3979

### Design of a Voice Coil Motor Used in the Focusing

### System of a Digital Video Camera

### Hsing-Cheng Yu

1### , Tzung-Yuan Lee

2### , Shyh-Jier Wang

1### , Mei-Lin Lai

1### , Jau-Jiu Ju

1### , Der-Ray Huang

1### , and

### Shir-Kuan Lin

2OES/ITRI, Hsinchu 310, Taiwan, R.O.C.

Department of Electrical and Control Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C.

**This paper raises a valid method to design a voice coil motor (VCM) used in the focusing system of a digital video camera (DVC).**
**A better VCM performance, such as lower battery consumption, higher efficiency, and shorter focusing time, can be achieved by turning**
**the diameter of a winding coil, the thickness of the magnet, and the winding spaces in a VCM.**

**Index Terms—Battery consumption, digital video camera, focusing, voice coil motor.**

I. INTRODUCTION

**R**

ECENTLY, digital video cameras (DVCs) have been very
popular, and better performance of focusing is always
asked. In order to attract more customers, the conventional
focusing actuators, steppers, in a DVC are gradually replaced
with a voice coil motor (VCM), whose focusing time is shorter.
Most works concerning VCMs considered mainly the dynamic
response [1]. However, both the battery consumption and the
efficiency of a VCM are also important factors to be taken into
account in a DVC system, since DVCs are portable apparatuses.
This paper tries to introduce a design procedure for a VCM that
meets the strict requirements of low power consumption, high
efficiency, and fast focusing.
II. PROBLEMATICFORMULATIONS

A typical VCM used in a DVC consists of a permanent magnet, a moving coil, a yoke, and a steel plate as shown in Fig. 1. The design problem of a VCM restricted in a limited

space can be formulated to find the diameter of

one coil in the windings and the thickness ratio

(1) where is the thickness of the permanent magnet and is the thickness of the windings. Indeed, the magnetic field in the VCM is constructed by determining . Furthermore, the number of turns of the moving coil can be estimated as

(2) where is the width of the windings and is a function of and .

The design philosophy for a high-quality VCM used in a DVC is to define three performance indexes, namely: 1) the rising time ; 2) the battery consumption energy ; and 3) the ef-ficiency . The rinsing time is a measure of the focusing time of a DVC, while the battery consumption energy is that of the

Digital Object Identifier 10.1109/TMAG.2005.855161

Fig. 1. Structures of the VCM in DVC.

battery duration. The efficiency is used to estimate the copper loss of the VCM. These indexes are formulized as

(3)

(4)

(5) where is the maximum stroke of the VCM, is the speed of the VCM moving part, is the current of the windings per turn, is the terminal voltage, and is the coil resistance. Equations (3), (4) and (5) show that , , and are all functions of and . The design goals are to decrease and and to increase to obtain a better VCM.

It is known [2] that the dynamic equations of the VCM can be written as

(6)

(7) where is the coil inductance, is the voltage constant, is the mass of the VCM moving part, is the damping constant, is the electric force, is the loading force, and is the

force constant. If , , , , , , , , and are

3980 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005

Fig. 2. Plots oft related to and .

known, and can be obtained by solving the two differ-ential equations (6) and (7). Thus, the indexes , , and can be evaluated by (3)–(5).

can be written as

(8) where can be calculated by the Maxwell stress method [3]. It was pointed out [4] that the value of is equal to in the MKS unit. The coil inductance can be calculated by the pertur-bation of the energy stored in the magnetic field as [5]

(9) where is the magnetic energy and is the perturbation coil input current. In the design procedure, , , and can then be predicted by the three-dimensional (3-D) finite element method.

From the Ohm’s law, the coil resistance can be estimated as (10) where is the resistivity of the windings and is the length of one turn in the windings. It should be remarked that , , , and are all functions of and , i.e., they vary with and .

III. DESIGNPROCEDURE

Given that is 2.0 g, the maxima of and are 2.97 V and 30 mA, respectively, is 0.005 Nt/(m/s), is 5.21 mm, and is 0.05 gw. As and vary, the finite element method and (8)–(10) are used to obtain , , , and . The values are substituted into (6) and (7) to solve for and , which allow us to evaluate , , and by (3)–(5). The relations of , , and to and are then obtained and shown in Figs. 2, 3, and 4, respectively.

Fig. 2 reveals that decreases with the decrease of and is affected little by . It follows from Fig. 3 that decreases with the increases of both and with the exception of . On the contrary, increases with the increase of both and

. The maximum efficiency is when and

.

The requirement of low and and high forces us to

choose and , since makes

too high to be acceptable. This design choice has ,

, and . The rising time is only

1/6th as short as a conventional stepper.

Fig. 3. Plots ofE related to and .

Fig. 4. Plots of related to and .

Fig. 5. VCM manufactured by OES. (a) Computer-aided design (CAD)/ computer-aided manufacturing (CAM) simulation. (b) Materialization.

IV. IMPLEMENTATION ANDEXPERIMENTS

A VCM with , , , and

was manufactured and its photo is shown in Fig. 5. We assembled a lens holder for carrying an optical focusing lens and the coils of the VCM together. Two photo-interrupts (PIs)with

the constant interval are set to detect of

the VCM. Furthermore, a magnetic strip with a 0.8-mm polar pitch is mounted on the lens holder and the magnetoresistive (MR) sensor is used to pick up the magnetic signals from the magnetic strip, so that the moving positions of the VCM can be obtained.

In the first step, we measured the impedance of the VCM by

using an – – meter to obtain and .

As regards the measurement of and , we know that (11)

*YU et al.: DESIGN OF A VOICE COIL MOTOR USED IN THE FOCUSING SYSTEM OF A DIGITAL VIDEO CAMERA* 3981

Fig. 6. Experimental system to measureK .

Fig. 7. Characteristics of the VCM pulling force.

where is the net pulling force of the VCM. The experimental system is shown in Fig. 6, in which the force gauge and the lens holder are bound together with a thin wire. The force gauge mea-sured for several different direct current (dc) levels of 0, 5, 10, 15, and 30 mA applied to the VCM. The results are plotted in Fig. 7. It is worth mentioning that we should fix the position to measure the pulling force, so that can be regarded as a con-stant. Therefore, a laser displacement meter was also installed in the experiment system to help us adjust the measurement posi-tion. Fig. 7 shows that is indeed a linear function of , and the results are consistent with (11). Consequently, it follows from (11) and Fig. 7 that and are 42.3 gw/A and 0.04 gw, respectively.

The output signals of the two PIs provide a way to measure . Note that both PIs are disabled by a shelter piece that is inserted to the lens holder while the VCM is moving, and the shelter piece is fixed on the moving part of the VCM and moves when current is applied to the VCM. As the shelter piece crosses over a PI, the output signal of the PI will change, i.e., PI1 from “Low” to “High” level and PI2 from “High” to “Low” level. In addition to these two signals, the input voltage and the exciting cur-rent are also measured at the same time and recorded as shown in Fig. 8. The time between the rising edge of PI1 signal to the falling edge of PI2 signal is exactly the rising time . Channels 1 and 2 in Fig. 8 indicate that . Notice that chan-nels 3 and 4 in Fig. 8 show that and are maintained almost

constant. Finally, we obtain and

by substituting into (4) and (5).

The comparison of the experimental results with the design

values for and is listed in Table I. It

can be seen that the differences between them are small. This verifies the proposed design method.

Fig. 8. Rising timet measurement. TABLE I

COMPARISONWITH THESIMULATED ANDEXPERIMENTALRESULTS

V. CONCLUSION

This paper proposes a new design philosophy for a voice coil motor (VCM) used in the focusing system of a digital video camera (DVC). Three performance indexes are defined for the design goals of low power consumption, high efficiency, and fast focusing. A design procedure is proposed to achieve these goals by adjusting these three indexes. An example of designing and manufacturing VCM is also presented. Experimental results show that the measured performance indexes match the design values very well.

ACKNOWLEDGMENT

This work was supported by the Nano Technology Research Center at the Industrial Technology Research Institute.

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