Fig. 4.2. Experimental setup for the lens module servo control system.
4.2. Implementation of the VCM Servo Control IC
Fig. 4.3 shows the interface and functional block diagram of the constructed servo control IC for the VCM lens module. This IC allows user to set the controller parameter according to the load (camera lens) and response time. All modules including current controller, position filter, position controller, velocity estimator and velocity controller share the same ALU unit by time-sharing scheme for arithmetic operation. The time-sharing scheme is mixed with 40 kHz and 200 kHz sampling frequency as shown in Fig. 4.4.
Fig. 4.5 shows the detail block diagram of the ALU unit, which supports both of add, minus and single cycle operation for arithmetic operation a×b+c.
Current Controller
Position Filter Position Controller
Velocity Estimator
Velocity Controller
5µ
25µ
5µ 5µ 5µ 5µ
time (sec.)
Fig. 4.4. Timing analysis of the ALU sharing effort.
Multiplier
1 0
Reformation
Adder Shifter
2’s complement 1 0 mult_a [11:0]
mult_b [11:0]
mult_insigned
mult_o [23:0]
add_o [23:0]
sub_add_a [11:0]
mac_en shift_ref sub_add_b[11:0]
sub_en shift_add [3:0]
Fig. 4.5. Block diagram of the ALU.
4.3. Experimental Results and Analysis
To verify the design of the digital control IC, the current loop step response is first tested.
In order to capture the waveform of current command and current response at the same time, the two digital signals are sent to D/A converter and the voltage signals are shown in Fig. 4.6.
The maximum current is limited at 120 mA and the current commands are 25%, 50%, and 75% of the maximum current, respectively. The transient of current loop step response is shown in Fig. 4.7. It can be seen that the period of the current ripple is 5 µs. For unipolar voltage switching scheme, the switching frequency is 100 kHz. The rise time tr is defined as the time required for the current changes from 10% to 90% of its final value. The current loop bandwidth BWc≅0.35/tr=10 kHz, which is consistent with the simulation results in section 3.1.
Fig. 4.8 illustrates the trapezoidal position response of the VCM with the designed digital servo drive IC, in which pcmd and pfbk are position command and position feedback, respectively. The transverse distance is 0.54 mm with a feed rate of 20 µm/ms and can achieve a position control resolution of 30 µm. The magnitude of holding current at the lower position is 10 mA while that at the higher position is 40 mA. The magnitude of the peak current during the period is 60 mA, which is 50% of the maximum current. Compare the position and current response of Fig. 4.8 with that of Fig. 3.11, the experimental and simulation results are closed to each other. Next, a smaller change in position command is given. The slope of the ramp command is 20 µm/ms, the same as before. The magnitude of the position command is 0.09 mm, i.e. 15% of the full stroke. The magnitude of the peak current during one period is about 80 mA, which is 67% of the maximum current. Compare the position and current response of Fig. 4.9 with that of Fig. 3.12, the experimental and simulation results are consistent. Experimental results verify the designed digital servo drive IC can meet the design specifications of an auto-focus module used for high-performance slim-type digital camera applications.
icmd
ifbk
(a)
icmd
ifbk
(b)
icmd
ifbk
(c)
Fig. 4.6. Experimental results of the current loop step response: (a) 25%, (b) 50%, (c) 75%
of the maximum current.
tr= 35 µs
(a)
tr= 35 µs
(b)
tr= 35 µs
(c)
i
op
fbk(a)
pcmd
pfbk
pfbk
io io
pfbk
pcmd pfbk
(b)
Fig. 4.8. Experimental results of the position and current response, 90% of the full stroke:
(a) the lens module moves up-and-down, (b) the transient response.
i
op
fbk(a)
io pfbk
pcmd pfbk
io pfbk
pcmd pfbk
Ch 2 DC offset -1V Ch 2 DC offset -1V
(b)
Fig. 4.9. Experimental results of the position and current response, 15% of the full stroke:
(a) the lens module moves up-and-down, (b) the transient response.
Chapter 5
Conclusions
The VCM used for the auto-focusing of a high-performance slim-type mobile phone must meet the requirements of small size, high accuracy, fast response, and energy saving.
The purpose of the magnetic circuit design is to make the VCM achieve maximum force constant under the constraints of limited volume and available current. This thesis proposed a systematic method in searching for the maximum value of force constant of the VCM with given design constraints by using an electromagnetic software, the Maxwell 2D of Ansoft.
The nonlinear characteristics of the force constant can be derived from the 2D electromagnetic model and be used for the synthesis of its servo controller. The mathematical model of the VCM has been developed and represented by a block diagram with characterized nonlinear elements. A slim-type auto-focusing module with a transverse distance of 0.6 mm has been designed and constructed by using the designed VCM. Simulation results with experimental verification are given to illustrate the proposed design procedure can achieve satisfactory performance.
An FPGA-based servo control IC for the closed-loop control of VCM used in an auto-focus module of a mobile phone has been developed. This thesis also proposed a fully digital cascaded loops control scheme. On the aspect of the current loop design, the effects of switching, sampling and calculation delay are considered and analyzed to decide the current loop bandwidth. The design is verified by simulation and experiment. The velocity loop compensates the mechanical pole and fastens position response. The outer position loop is designed to meet the specifications such as response time and steady-state error. In the hardware realization, by using time-sharing scheme, the ALU is shared to each component of the controller to save resources. The simulation and experimental results show that the position response time meets the specification and no stick-slip limit cycle oscillation occurs, thus the solution is realizable.
In summary, this thesis provides a system design procedure from the magnetic circuit design of the VCM to the design and implementation of the dedicated servo control IC. The mathematical model of the lens module has been constructed and verified with experiments so that the digital controller can be well designed according to the model.
References
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Vita
Jhih-Da Hsu was born in Taichung, Taiwan, R.O.C., in 1982. He received the B.S. degree in electrical and control engineering from National Chiao Tung University, Hsinchu, Taiwan, in 2005 and is currently pursuing the M.S. degree in electrical and control engineering at National Chiao Tung University, Hsinchu, Taiwan. His research interests are in the areas of magnetic circuit design and brushless dc motor control.
姓名: 許智達
性別: 男
生日: 中華民國71年11月19日
論文題目: 中文:用於自動對焦數位相機之音圈馬達及其伺服控制IC之分析與設計
英文: Design and Analysis of a Voice Coil Motor with the Servo Control IC for Auto-Focusing Digital Cameras
學歷:
2005.9~2007.7 國立交通大學電機與控制工程研究所
2001.9~2005.6 國立交通大學電機與控制工程學系