機(jī)械專業(yè)畢業(yè)論文外文翻譯3

1、<p>　　Development of a 3-axis Desktop Milling Machine and a CNC System Using Advanced Modern Control Algorithms</p><p>　　1. Introduction</p><p>　　As new fields such as IT(Information Techno

2、logy), BT(Bio Technology) and NT(Nano Technology) emerge as a driving force in the industry, the interests in micro-factory system have been growing. The micro-factory is a miniaturized flexiblemanufacturing system which

3、 consumes minimal space and energy compared to theconventional one, and it is desired to produce micro/meso size mechanical components necessary for IT, BT and NT applications. </p><p>　　Major technical unit

4、s contributing to micro mechanical machining systems are,to name a few, high speed spindlesystems, micro high precision feeding systems, control systems to generate coordinated motions, tooling and chucking systems, fram

5、e design and module allocationschemes based upon optimization for high stiffness. Researchers have been trying to put micro technologies together to build micro-factory systems which make micro/mesosize precision parts t

6、o meet the needs from the manufacturing </p><p>　　In this paper, we present a miniaturized 3-axis milling machine and a dedicated CNC system for the machine. The 3-axis milling machine is constructed as one

7、of micro-factory module and designed to produce high precision micro parts. It has a desktop size of 200×300×200 mm3 and is serving as our testbed machine.From finite element analysis and an impact hammer test,

8、 we have verified that ithas a good structural stiffness and high natural frequencies. A high speed air turbine spindle on the horizo</p><p>　　A CNC system was developed for operation of the 3-axis desktop m

9、illing machine. The CNC system includes a G-code interpreter which can process a basicset of G-codes and M-codes in real-time. The CNC system consists of two parts.The one is agraphical user interface which runs under Mi

10、crosoft Windows, and</p><p>　　the other is a DSP program which interpolates commands and executes a real-time servo control. Two parts communicate each other through a dual port RAM(Random Access Memory). Jo

11、b assignments for the two parts are discussed in detailin this paper. To improve the performance of the CNC system for the 3-axis milling machine beyond the traditional PID-type control, different control schemes have be

12、en tested including H∞ control, input shaping control, disturbance observer and cross-coupled control o</p><p>　　The rest of this paper is organized as follows. Sec. 2 presents the design of the 3-axis milli

13、ng machine. The results of the finite element analysis and the natural frequencies obtained from the impact hammer test are given in this section. In Sec 3, a PC-based CNC system developed for the 3-axis milling machine

14、is discussed. Several modern control schemes including H∞ control design, input shaping control, disturbance observer, and cross-coupled control are discussed with their experimental res</p><p>　　2. Design o

15、f a 3-axis Milling Machine</p><p>　　Micro machine tools are required to have high machining accuracy while providing enough stiffness. To estimate basic machining performance and stiffness of a micro machine

16、 tool, a miniaturized 3-axis milling machine was built and used as a testbed. Fig. 1 shows the 3-axis milling machine and its specifications. It has a mini-desktop size of 200×300×200 mm3 and its cut-ting volum

17、e is 20×20×20 mm3. The vertically installed XY stage is driven by voice-coil motors, and for the z-axis, a magnetically</p><p>　　preloaded air bearing and a linear motor are used. The air spindle r

18、uns at up to 160,000 rpm and it is fast enough for high precision machining. A weight balancer using an air bearing cylinder is installed to counteract the gravity force acting on the XY stage in the y-direction. A small

19、 cutting force dynamometer is also installed underneath the work table to monitor the cutting process. Fig. 2 shows a picture of the 3-axis milling machine.</p><p>　　2.1 Static and Dynamic Analysis</p>

20、<p>　　Finite element analysis was done to investigate the static and dynamic characteristics of the designed 3-axis milling machine using a finite element model as shown in Fig. 3. The computational results showed

21、 that the deflection due to its own weight was negligible. When a 10N force was located at the machining position in z-direction, the numerical results showed that the displacement change at the work table would be about

22、 0.07 ., and the back fame would undergo less than 0.02 . deflection in z</p><p>　　The modal analysis revealed many important dynamic modes of the 3-axis milling machine. We used the impact hammer test to ve

23、rify the computed natural frequencies. The measured natural frequencies do not exactly match the computed ones, but the indicated frequency range from the finite element modal analysis was similar to that from the impact

24、 hammer test. Fig. 4 and Table 1 show the measured natural frequencies and corresponding frequency response function of the 3-axis milling machine. It can b</p><p>　　3. A CNC System</p><p>　　3.1

25、 Graphical User Interface Program</p><p>　　A PC-based CNC system was developed for the 3-axis milling machine. The developed CNC system has two parts, a graphical user interface program in the PC part and a

26、DSP program in the DSP part. The PC part runs on MS-Windows and processes user inputs. The DSP part receives thousands timer interrupts per second and interprets commands in real-time for each axis of a machine and execu

27、tes servo control loops. Two parts share a dual port RAM and communicate each other through it. </p><p>　　Fig. 5 shows the graphical user interface of the developed CNC system and its brief explanations. One

28、 of the major features of the user interface program is a 3D plot window at the bottom right corner in Fig. 5. It displays the tool path described in G-codes when the user interface program reads in a G-code file. The cu

29、rrent tool position also appears as a small red dot on the screen so that CNCusers can easily identify where the machining process goes in the G-code file. Users can also use cont</p><p>　　When a user click

30、the Open G-code button, a whole G-code file is read in and saved in a memory area, and then the G-codes appear at the bottom left list box. When the Start G-code button is clicked, the user interface program takes out a

31、line from the memory and checks its syntax and identifies all the meaningful tokens. During preprocessing a G-code line, the user interface program is supposed to compute, a motion plane, a driving axis, maximum allowabl

32、e velocity and acceleration, the starting </p><p>　　3.2 DSP Program</p><p>　　The DSP program interpolates the preprocessed G-codes in real-time and generates position commands for multiple axes

33、to follow. It also closes servo control loops. Generally a sampling rate is set to be ten times larger than the bandwidth of a plant to be controlled. The developed CNC system adopted a sampling rate of 2,000 Hz for the

34、servo loops.</p><p>　　The DSP program takes out a G-code line from the circularbuffer and computes the angle between two successive G-code lines. If the angle is less than a certain (predefined) degree and t

35、he contouring is on, it sets a flag so that the tool path does not reduce its velocity when it enters into the next segment. When a timer interrupt occurs, the DSP program computes the desired velocity and position of ea

36、ch axis and generates commands for the servo control loop. The computed velocity should be less</p><p>　　After generating real-time commands for each axis the DSP program drives the servo control loops of th

37、e 3-axis. The errors which are differences between the commands and the actual feedback positions are fed into a control algorithm such as PID and the control signals for the motor drives are computed.</p><p&g

38、t;　　4. Control System Design</p><p>　　To improve the performance of servo control for the 3-axis milling machine, several control algorithms have been tested on the 3-axis milling machine. They include PID,

39、H∞ control, input shaping control, disturbance observer, and cross-coupled control. These control schemes were digitally implemented on a Daytona DSP board from Spectrumsignal Co. The DSP board has two TI320C6701 chips o

40、n it and a sampling rate of 2,000 Hz has been used. The design procedure and experimental results from each con</p><p>　　4.1 H∞ Optimal Control Design</p><p>　　Using a conventional PID controlle

41、r for the z-axis which has a linear motor and air-bearing, it seemed that high gain PID easily started oscillations. As an alternative, an H∞ controller was designed and applied to the z-axis and performance of hand tune

42、d PID and H∞ control is compared. An open-loop plant model for control design was obtained from experimental frequency response data. The frequency responses were measured with a dynamic signal analyzer using a swept sin

43、e method that generates fi</p><p>　　Fig. 6 shows the averaged frequency response and a nominal open-loop plant model. A second order plant model was obtained from the curve fit. The identified open-loop plan

44、t model G(s) forthe z-axis was</p><p>　　We can see that the z-axis has a complex pole pair at around 4.5Hz. When a PID-type controller in a typical digital form of</p><p>　　where u(k) is control

45、ler output, e(k) is error signal, T is samplingperiod, and z is a delay, is applied to the plant, it turns out that a high gain PID can easily excite the oscillatory mode of the plant. To avoid so-called derivative kick,

46、 the derivative gain Kd was forced to act on the derivative of the actual position, not on the derivative of the position error, i.e. Kd (1-z-1)/T is multiplied by the negative position feedback, –y(k) instead of e(k) at

47、 Eq. (2). For the z-axis, using the der</p><p>　　where S(s) is the sensitivity function, T(s) is the complementary sensitivity function, K(s) is the desired H∞ controller, 1/|wp(s)|, 1/|wt(s)| and 1/|wu(s)|

48、put upper bounds on the magnitude of S(s) (for performance), T(s) (for noise attenuation) and K(s) S(s) (to penalize large inputs), respectively. The H∞ optimal controller was obtained by solving the problem2</p>

49、<p>　　Fig. 7 shows other design parameters used in the z-axis control design and the final sensitivity function from the computed H∞ controller. The final sensitivity function S(s) clearly shows that the H∞ controll

50、er has double integral action in low frequency range as intended with the shape of 1/|wp(s)|. The designed H∞ controller was converted to a discrete-time controller K(z) for a 2 kHz sample and hold rate and implemented o

51、n a DSP board for tests. The final H∞ controller K(z) was a 5th order c</p><p>　　The classical feedback sensitivity function S(s) is the transfer function from the reference signal r(t) to the control error

52、signal e(t), i.e. e(t) = S(s)?r(t). To compare tracking performance between the designed H∞ controller and PID controller, a fixed-amplitude sine wave of varying frequencies was injected as a command signal and the corre

53、sponding error signal was measured and the ratio of their magnitude versus frequency was plotted in Fig. 8. Thus it is an empirical sensitivity function p</p><p>　　A circular reference trajectory of 7.5 mm i

54、n radius in y-z plane was given to the y- and z-axis servo as a command with a feedrate of 25 mm/sec and its contour errors are compared in Fig. 9. Note that the contour errors are different from the tracking errors. A t

55、racking controller attempts to minimize the difference between the reference trajectory, which is specified as a function of time, and the output of the controlled plant. On the other hand, a contouring controller attemp

56、ts to minimize th</p><p>　　In Fig. 9, the y-axis servo motion is drawn horizontally and the z-axis servo motion is drawn vertically. The blue circle in the middle of the figure represents the 0 μm error line

57、, i.e. The controlled plant output exactly matches the spatial reference trajectory. When a PID controller is applied to the y-axis, it shows approximately 30 μm error at around 0 degree, but 50 μm error appears from the

58、 H∞ controller at the same position. This error is caused by the air cylinder counteracting gravity </p><p>　　A feedforward controller such as ZPETC (Zero-Phase Error Tracking Control) takes approximately an

59、 inversion of plant dynamics and it requires an accurate plant model. In the y-axis, the feedforward controller was inserted to reduce the error peak at around 0 degree, where the air cylinder counteracting gravity force

60、 change its moving direction. It seems that the plant model of y-axis did not capture well the nonlinear characteristics of the air cylinder especially when the y-axis changes directi</p><p>　　4.2 Effect of

61、Input Shaper The input shaper involves real-time shaping or time-delay</p><p>　　filtering of the reference command to stable systems with the objective of minimizing the residual vibration. It is natural tha

62、t giving the system an impulse will cause it to vibrate. If a second impulse is given to the system with appropriate amplitude when the system undergoes a half of vibration cycle from the first impulse, it is possible to

63、 cancel out the vibration induced by the first impulse with the opposite phase vibration by the second impulse. This is the main idea behind the input sh</p><p>　　If the impulse sequence given at Eq. (8) is

64、convolved with any desired command signal and the convolution product is then used as the command to the system, the convolution product will also cause no vibration. The convolution can easily be implemented as an FIR f

65、ilter designed from Eq. (8) and (9).</p><p>　　An H∞ controller was designed for the x-axis similarly as above described. Step responses from an experiment and simulation are compared in Fig. 10. Though the s

66、imulation does not show any overshoot, the real system showed 9% overshoot for a step command with a magnitude of 1 mm. Reflecting that some overshoot may be arguably desirable to get prompt responses from control system

67、, the step response from H∞ control was considered acceptable.</p><p>　　When step commands with bigger than 1 mm steps are given, more oscillatory behavior could be observed as shown in Fig. 11. To cope with

68、 the oscillations while radical acceleration, an input</p><p>　　shaper was considered being inserted in the inner feedback control loop. After giving large step commands, the x-axis’ damping ratio ζ was esti

69、mated to be 0.3035 and natural frequency ω to be 43.96 rad/sec. From Eq. (8) and (9), an input shaper was constructed as an FIR filter and the designed input shaper was put in the servo loop to pre-shape the command befo

70、re the H∞ controller. The experimental results with a 3 mm step command are shown in Fig. 11. The input shaper could not cancel out the o</p><p>　　It is found that the input shaper can improve the dynamic be

71、havior when acceleration or deceleration is large, but it may hurt vector coordinated motion between multiple axes because it reshapes the spatial reference trajectories which are supposed to be synchronized. Thus, the i

72、nput shaper may be safely used when an axis is controlled to move with high acceleration but two or more axes are needed to generate a coordinated motion path in synchronization, the input shaper of each axis should be d

73、</p><p>　　4.3 Disturbance Observer</p><p>　　In direct-drive systems there are no gear reduction effects to help attenuate disturbance forces at the load, and thus a disturbance observer provides

74、 a convenient method for enhancing the disturbance rejection. The above H∞ controller for the z-axis has high gain in low frequency range and shows aggressive responses to the reference command. When disturbance comes in

75、to the plant, the controller shows some oscillations and takes a little bit longer time than PID until it completely rejects the d</p><p>　　The inverse of a nominal plant model, 1/ ( ) n G z is obtained from

76、 ZPETC(Zero Phase Error Tracking Control)5 and a Butterworth lowpass filter with a cutoff frequency of 50 Hz is designed for Q(z) and they are used in our experiments. Fig. 13 compares the recovering time and displacemen

77、ts of H∞ controller with and without a disturbance observer when an 1 Volt plant input disturbance is injected at 0.1 second with 0.01 second duration. We can see that the disturbance observer helps the servo cont</p&

78、gt;<p>　　4.4 Cross-coupled Control</p><p>　　In machining applications, one of the most important issues is the elimination of machining error to ensure the quality of the final product. The dimensiona

79、l accuracy of the final product is one important measure of product quality. Contouring error directly describes the dimensional accuracy. Two major approaches have been adopted to reduce contouring errors. In the first

80、approach, reduction of axis of tracking errors can reduce contour errors indirectly. The applications of feedforward control</p><p>　　In Fig. 16, we compare contour errors with and without the cross-coupled

81、control. It clearly shows that cross-coupled control added to H∞ controller reduces the peak of the contour error occurred at around 0 degree, which is caused by friction of the air cylinder in the y-axis. The maximum pe

82、ak of the contour error diminishes from 50 μm to 20 μm. In our 3-axis milling machine, feedforward control does not show any significant improvement in reducing the large peak of the contour error but on the</p>&

83、lt;p>　　5. Conclusions</p><p>　　In the paper, a 3-axis desktop size milling machine and a CNC system are presented. The milling machine utilizes two voice-coil motors for XY stage and a linear motor for th

84、e z-axis and a 160,000 rpm air turbine spindle. Through finite element analysis and impact hammer test, we verified that the 3-axis milling machine has highstructural stiffness and high natural frequencies. A CNC system,

85、 which was developed for the 3-axis milling machine, was also discussed. The CNC system consists of two pro</p><p>　　We presented and compared a few control techniques to improve control performance of the d

86、esktop size 3-axis milling machine. The H∞ controller intentionally designed with double integrators showed better performance than PID control in terms of tracking error and stiffness for the z-axis, which is equipped w

87、ith a linear motor and air bearing. But H∞ control and PID control are similar when they are applied for voice coil motor driven x- and yaxes. The input shaping control is useful when high a</p><p>　　The dis

88、turbance observer showed its usefulness in counteracting disturbances. The closed-loop servo system recovered from sudden impulse-like disturbances comparably fast when it has a disturbance observer on top of a feedback

89、controller. The disturbance observer may help preserving machining accuracy when large cutting forces apply to the tool stage. Cross-coupled control appears to be very effective to improve not only contour error but also

90、 tracking error when a significant friction exists. I</p><p>　　三軸工作臺(tái)銑床及運(yùn)用先進(jìn)現(xiàn)代控制算法</p><p><b>　　的數(shù)控系統(tǒng)的研發(fā)</b></p><p><b>　　1. 簡(jiǎn)介</b></p><p>　　隨著新的領(lǐng)域

91、如IT(信息技術(shù))、BT(生物技術(shù)) 和NT(納米技術(shù)) 的出現(xiàn)及其對(duì)工業(yè)發(fā)展的帶動(dòng)，人們對(duì)微型工廠系統(tǒng)的興趣與日俱增。 微型工廠是一個(gè)小型柔性制造系統(tǒng)，它所用的空間和能量相對(duì)傳統(tǒng)工廠要小得多，而且它適宜生產(chǎn)IT、 BT 和NT產(chǎn)業(yè)中所需的微型/中型尺寸的機(jī)械部件。</p><p>　　用于微型機(jī)械加工系統(tǒng)主要的技術(shù)單元有高速主軸系統(tǒng)、高精度進(jìn)給系統(tǒng)、協(xié)調(diào)運(yùn)動(dòng)控制系統(tǒng)、工裝和chucking系統(tǒng)、框架設(shè)計(jì)以及基于

92、高剛度優(yōu)化的模塊分配系統(tǒng)。研究人員一直在試圖將微型技術(shù)與構(gòu)建生產(chǎn)微型/中型精準(zhǔn)件的微型工廠系統(tǒng)結(jié)合起來滿足制造業(yè)的需求。</p><p>　　本文中,我們描述了一種微型三軸銑床及其數(shù)控系統(tǒng)。這個(gè)三軸銑床作為一個(gè)微型工廠模塊用來生產(chǎn)高精度零件。它的尺寸為200×300×200 mm3，作為我們的測(cè)試機(jī)床。從有限元分析和沖擊錘測(cè)試中，我們已經(jīng)證實(shí)了它有很好的結(jié)構(gòu)剛度和高的固有頻率。水平Z軸方向上的

93、高速空氣渦輪主軸的轉(zhuǎn)速可達(dá)到160,000rpm。這臺(tái)三軸銑床在真實(shí)加工試驗(yàn)中成功的現(xiàn)實(shí)了其加工性能。</p><p>　　數(shù)控系統(tǒng)用于操作三軸工作臺(tái)銑床，它包括一個(gè)G指令譯碼器，能實(shí)時(shí)編輯基本的G指令和M指令。該數(shù)控系統(tǒng)由兩部分組成，一個(gè)是在Windows系統(tǒng)下的用戶界面，另一個(gè)是可加入指令并執(zhí)行實(shí)時(shí)伺服控制的DSP 程序。這兩部分通過一個(gè)雙向RAM（隨機(jī)讀取存儲(chǔ)器）傳遞信息，兩部分的工作分配在這里進(jìn)行詳細(xì)的討

94、論。 為了提高三軸銑床數(shù)控系統(tǒng)優(yōu)于傳統(tǒng)PID型控制方式的性能，我們對(duì)三軸銑床進(jìn)行了不同的控制方法的研究，包括H∞控制、輸入成型控制、擾動(dòng)觀察器和非門控制器。</p><p>　　本文其余內(nèi)容如下：第二部分介紹了三軸銑床的設(shè)計(jì)。這部分給出了通過沖擊錘測(cè)試得到的有限元分析和固有頻率的結(jié)果。在第三部分中，我們討論了基于PC的用于三軸銑床的數(shù)控系統(tǒng)。第四部分討論了一些現(xiàn)代控制方法如H∞控制、成型控制、擾動(dòng)觀測(cè)器和非門控制

95、的優(yōu)缺點(diǎn)。第五部分給出結(jié)論。</p><p><b>　　2. 三軸銑床設(shè)計(jì)</b></p><p>　　微型機(jī)床要求具有很高的加工精度，同時(shí)要提供足夠的剛度。為了估計(jì)微型機(jī)床基本的機(jī)械加工性能和剛度,我們?cè)O(shè)計(jì)了一個(gè)微型三軸銑床并將其作為測(cè)試機(jī)床。圖1顯示了該三軸銑床及其規(guī)格。它的工作臺(tái)體積為200×300×200 mm3，加工范圍為20×

96、;20×20 mm3。垂直方向的XY軸由電機(jī)驅(qū)動(dòng)，Z軸由磁力預(yù)緊空氣軸承和直流電機(jī)控制，空氣主軸轉(zhuǎn)速可達(dá)160,000rpm，足以用于高精度加工。水平工作臺(tái)安裝了運(yùn)用空氣軸承的平衡塊來抵消Y方向上的重力的作用，工作臺(tái)下邊還安裝了小切削力功率計(jì)用以監(jiān)測(cè)加工過程。圖2為三軸銑床的圖片。</p><p>　　圖1 三軸銑床及其參數(shù)</p><p><b>　　圖2 三軸銑床圖

97、</b></p><p>　　2.1靜態(tài)和動(dòng)態(tài)分析</p><p>　　本文運(yùn)用了有限元模型對(duì)設(shè)計(jì)的三軸銑床進(jìn)行了有限元分析用以研究其靜態(tài)和動(dòng)態(tài)性能，如圖3所示。計(jì)算結(jié)果表明,由于其自身重量撓度微不足道。當(dāng)把10N的力加載到Z方向的加工位置處時(shí)，數(shù)值計(jì)算結(jié)果表明,工作臺(tái)處位移改變量大約為0.07,而背面的Z方向偏差小于0.02。它表明三軸銑床具有良好的剛度與其良好的結(jié)構(gòu)設(shè)計(jì)和一