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Vibration Control of Relative Tool-Spindle Displacement for Computer Numerically Controlled Lathe With Pipe Frame Structure Yoshitaka Morimoto Mem. ASME Professor Director of Advanced Materials Processing Research Laboratory, Kanazawa Institute of Technology, 3-1 Yatsukaho, Hakusan, Ishikawa 924-0838, Japan; e-mail: mosandb1neptune.kanazawa-it.ac.jp Naohiko Suzuki Takamatsu Machinery Co., Ltd., 3-1 Asahigaoka, Hakusan, Ishikawa 924-8558, Japan e-mail: suzukitakamaz.co.jp Yoshiyuki Kaneko Takamatsu Machinery Co., Ltd., 3-1 Asahigaoka, Hakusan, Ishikawa 924-8558, Japan e-mail: kanekotakamaz.co.jp Minoru Isobe Takamatsu Machinery Co., Ltd., 3-1 Asahigaoka, Hakusan, Ishikawa 924-8558, Japan e-mail: kanekotakamaz.co.jp A new computer numerically controlled (CNC) lathe with a pipe frame bed has been developed. This structure is expected to have enough space between the truss bars to solve the space problem and have enough rigidity for machine tools. Therefore, a CNC lathe whose frame consists of pipes, joints, and diagonal braces has been developed with enough rigidity and space utility for chip evacuation. From the viewpoint of machine tool usage, real-time vibration control theory is applied to control the relative displace- ment between the tool post and the spindle to suppress specific relative vibration modes. DOI: 10.1115/1.4027594 Introduction Conventional beds or frames for machine tools generally con- sist of cast iron or welded steel plates. These structures are suita- ble from the viewpoint of the machine tools dynamic characteristics. Although the demand for downsizing machine tools (such as in desktop machines) is increasing, there is a strict volumetric limitation to obtain enough space for sufficient chip evacuation. In this case, there are problems of heat conduction and heat transfer. To solve these problems, we propose a tradi- tional but new frame structure for machine tools that consists of pipes and connecting parts. We looked at the flexibility of the frame design using a pipe frame structure. The frame structure can be arranged with standardized pipe (diameter and length) and a connecting block. In the case of a lathe, this structure allows for a change in size based on the spindle unit and X-axis table, which are designed as standardized units. For developing a CNC lathe using this structure 1, the follow- ing problems are assumed: (1) structural vibration generated by the spindle motor unbal- ance and the cutting force disturbance, (2) thermal deformation due to the low heat capacity, (3) insufficient rigidity between the spindle and the X- and Z axis tables. In this report, we focus on the vibration problem. There have been several prior studies 27 on the applications of vibration control for machine tools. In these research papers, the control methods have been applied to the tool post of the lathe machine. These results demonstrated the superiority of vibration control of the tool post displacement but do not consider the structural vibra- tion modes that appear in the low-frequency range compared with the local vibration modes, such as the boring bar and tool post. Developed CNC Lathe A unique CNC lathe has been developed based on the idea of a pipe frame structure. The specifications are shown in Table 1, and a schematic view is shown in Fig. 1. A maximum spindle speed of 10,000 min C01 can be obtained. The smallest resolution for the X and Z axes is set to 0.1lm. The lathe weighs only 525 kg (including the covers) and fits within a space of 440 WC21450 LC21050 H mm 3 , which is sufficiently compact. The CNC controller (Mitsubishi Electric Co., Ltd., M700V) is set beside the machine body. Vibration Control Method The most important vibratory motion for the CNC lathe is the relative displacement between the spindle and the tool post. The machined workpiece is often influenced by the antiphase motion between the spindle and the tool; this motion is generated by the resonant frequency. In this case, the antiphase motion is the most important behavior for the workpieces geometric error. This motion can be evaluated by measuring the relative vibration between the spindle and the tool. Although this displacement should be measured directly by a displacement sensor during cut- ting, it is very difficult to position the sensor because of the space constraints, coolant, and chips. Figure 2 shows the typical compli- ance transfer function between the spindle head and the represen- tative frame-connected block. There are several peaks. When we try to apply vibration control using this data, the control point is hardly determined. Then, two small acceleration sensors are used to detect the relative vibration mode. One is set on the spindle unit case and is measured as the signal x s ; the other is set on the tool post of the X-axis table and is measured as the signal x t . The relative vibration x is expressed as follows: x x s C0 x t (1) Then, the difference between the two acceleration sensor meas- urements in Eq. (1) is calculated and used to extract the antiphase motion between them. The result of the calculated transfer func- tion is shown in Fig. 3. A frequency peak at 45 Hz can be observed, and it can be considered a simple one-degree-of-free- dom system. We can use this differential signal as the output of the vibration response. Then, modal analysis can be used to obtain the vibration modes, to decide the target vibration mode, and to determine where to set the actuator for the vibration control. The synthesized mode shape vector at 45 Hz is shown in Fig. 4. The antiphase mode between the spindle and the tool post is actually seen from the vector Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received February 20, 2013; final manuscript received April 11, 2014; published online May 21, 2014. Assoc. Editor: Tony Schmitz. Journal of Manufacturing Science and Engineering AUGUST 2014, Vol. 136 / 044502-1 CopyrightV C 2014 by ASME Downloaded From: http:/manufacturingscience.asmedigitalcollection.asme.org/ on 05/26/2014 Terms of Use: http:/asme.org/terms figure. The mode vectors at the head stock for the spindle ( and fi) are so small that the ends of the two actuators can be set at these points. Based on this result, the other ends of the actuators are set on connecting blocks below the X-axis table to control the antiphase vibration mode, as shown in Fig. 1. These two actuators generate the control forces to suppress the vibratory motion by the active vibration control method. The relative displacement, velocity, and acceleration are defined as x, _x, and x, respectively. Assuming that x d and _x d denote the desired relative displacement and the relative velocity, the relative displacement and the relative velocity are defined as x d C0 x;_x d C0 _x The controlled force can be expressed as follows: K v _x d C0 _xK d x d C0 x (2) where the weight coefficients K d and K v are used. The equation of motion is described as follows when a harmonic force f a e jxt acts on this system: mxc_xkx f a e jxt K v _x d C0 _xK d x d C0 x (3) Because the desired relative displacement x d and relative velocity _x d are to be zero, Eq. (3) is described as follows: mx cK v _x k K d x f a e jxt (4) In Eq. (2), substituting f f a e jxt , the following equation can be derived: x _x C26C27 C0c=m C0k=m 10 C20C21 _x x C26C27 1=m 0 C20C21 f (5) This equation of motion is rewritten as the following state equation: _ X AXBF (6) In the case of applying a control force that is proportional to the state vector X, the control force is described as F C0KX.Then, substituting this into Eq. (6), the following state equation is obtained: _ X AC0BKX (7) Once this equation can be obtained, the only thing to do is calculate the feedback gain K using MATLAB (MathWorks MATLAB and Simulink). To calculate this coefficient K, the modal parameters m, c, and k must first be determined. We calculated these values using the simple “mass response method” because the vibratory motion can be assumed to be one-degree-of-freedom from the result in Fig. 3. Once the known mass is set on the tool Fig. 1 Schematic view of developed CNC lathe Fig. 2 Typical measurement result of compliance transfer function by conventional impulse excitation method on a repre- sentative point Fig. 3 Transfer function between input force and relative dis- placement (tool post and spindle) Fig. 4 Mode shape vector (top view) Table 2 Identified modal parameter Items Value Unit Mass, m 214 kg Damping factor, c 8.01 N/(m/s) Spring constant, k 1.62C210 07 N/m K v 3.66C210 03 N/m/s K d 5.73C210 02 N/(m/s) Table 1 Specifications of CNC lathe Items Specifications Head stock Chuck Collet chuck Maximum speed 10000 min C01 Tool post type Horizontal linear Minimum resolution 0.0001 mm Size 440 WC21450 LC21050 H mm Machine weight 525 kg CNC controller Mitsubishi Electric Co., Ltd. M700V 044502-2 / Vol. 136, AUGUST 2014 Transactions of the ASME Downloaded From: http:/manufacturingscience.asmedigitalcollection.asme.org/ on 05/26/2014 Terms of Use: http:/asme.org/terms post, the change in the resonant frequency is measured, and the modal parameters are obtained, as shown in Table 2. These are substituted into Eq. (7), and the feedback gain K is calculated according to linear-quadratic (LQ) control theory. Vibration Control System Lead zirconate titanate (PZT) devices are used in the vibration control system as the actuator source. Structural vibration control and motion control for machine tools use this device because of its large generated force and high-frequency response 811. We developed an actuator that consists of a PZT, a fluid cham- ber with two different cross-sectional areas, and connecting rods, as shown in Fig. 5. The fluid chamber is used to transform the generated force from the PZT. Although the longitudinal displace- ment of the PZT is short enough, the displacement is reduced by the chamber area. This effect makes the generated displacement less than a submicron order. Two actuators are set between the connecting blocks as brace bars. The dynamic characteristics are evaluated using the impulse response method, and the result is shown in Fig. 6. This actuator can be used below 200 Hz. The effects of the vibration control are evaluated in Effects of Active Vibration Control section. Although the thermal elongation of the brace bars has to be considered when the long run operation, we do not consider the thermal deformation of the frame because of the short-time machining. Effects of Active Vibration Control The relative vibration between the spindle and the tool post is controlled by two actuators. In this case, according to LQ control theory, we have developed the control system to realize the control method represented by the block diagram in Fig. 7 and in Table 3. This system runs with a sampling speed of 3000 Hz. The relative vibration is generated by the unbalance of the spindle unit. Since the maximum displacement is observed at around 2725 min C01 (45.4 Hz), the target frequency is set to 45 Hz. The experimental procedure has been executed during air cut- ting and during cutting. From the experimental result and fre- quency response of the actuator, the target-controlled frequency range is set from 10 to 100 Hz. A band-pass real-time filter is adapted to obtain the vibration signal and to suppress the disturb- ance noise and high-frequency signals. To evaluate the effect of the active vibration control, two accel- eration sensors due to the limitation of the installation space are used for the control. The relative vibration is numerically inte- grated to obtain the relative velocity and the relative displace- ment. The relative displacement itself during motion is measured by a laser displacement sensor that is set on the tool post and detects the relative displacement of the spindle unit. The control result during finish cutting is shown in Fig. 8. The control parameters have been determined previously during air cutting. This condition is not always applicable to the condition during cutting because the workpiece and the tool contact each other, and the cutting force acts on the contact point. Although the modal parameters are not always constant, our control system is applied using the same conditions as air cutting, and the effect of the vibration control is evaluated during cutting. When the control is started, the relative displacement decreases its amplitude. FFT analysis is used to determine the difference in the frequency element between with control and without control, as shown in Fig. 9. The solid line shows the result with control, and the dotted line shows the result without control. Almost 50% of the vibration amplitude around the target frequency of 45 Hz is suppressed by our developed system. The effect of an increase in rigidity by the actuator installed as a brace can be seen in the lower frequency range. On the other hand, the effect above 100 Hz cannot be seen because of the filter effect of the control Fig. 5 Schematic diagram of PZTactuator for vibration control Fig. 6 Transfer function between input voltage of actuator and table displacement by impulse response Fig. 7 Block diagram for vibration control Table 3 Experimental setup for vibration control Items Value Unit Spindle speed 2700 min C01 Target frequency 45 Hz Sampling frequency 3000 Hz Depth of cut 0.1 Mm Feed 0.02 mm/rev Workpiece material C3604BD Fig. 8 Comparison of relative motion during cutting Journal of Manufacturing Science and Engineering AUGUST 2014, Vol. 136 / 044502-3 Downloaded From: http:/manufacturingscience.asmedigitalcollection.asme.org/ on 05/26/2014 Terms of Use: http:/asme.org/terms system. These results demonstrate that this vibration control sys- tem used during finish cutting is suitable for our pipe frame CNC lathe. To improve the effect of the vibration control, the layout (including the direction of the actuator) should be considered, and further improvement of this actuator system is needed to suppress the vibration amplitude more efficiently. Evaluation of Workpiece by Vibration Control Since it has been confirmed that the vibration control is effec- tive even during cutting, we evaluated the accuracy of the machined workpiece. Because the target control frequency was set to 45 Hz, the effect of the control result appears as surface waviness. Then, the surface profile was measured by the surface roughness tester without a high-pass filter. Figure 10 shows the measured results. The upper figure shows the profile without vibration control, and the lower figure shows the result with vibra- tion control. The surface waviness is remarkably improved. The improvement of the waviness at this frequency range is related to the relative vibration mode that appears in this frequency range. Figure 11 shows the harmonic analysis of the measured round- ness result. Since the target frequency is set to the spindle rota- tional frequency, the effect of the vibration control generally appears on the roundness with two undulations per round. The measured result shows this tendency well. The amplitude of the undulations per round decreased by 50% from the amplitude with- out control. These results show the effectiveness of our control system at suppressing the relative vibration mode between the spindle and the tool post and the improvement in the accuracy of the machined workpiece. Conclusions In this paper, the effectiveness of the real-time vibration control for the developed CNC lathe by a single degrees of freedom model has been evaluated using the differential signal between the spindle and the tool post. The main results obtained as follows: (1) Almost 50% of the vibration amplitude around the target frequency of 45 Hz is suppressed by our developed system. (2) The roundness amplitude of the undulations per round decreased by 50% from the amplitude without control. (3) Results obtained show the effectiveness of the applied con- trol system at suppressing the relative vibration mode between the spindle and the tool post. Further investigations are required to improve the vibration control system, including the layout (the direction of the actuator) and actuator performance to suppress the vibration amplitude more efficiently. Acknowledgment The authors express their gratitude for the assistance of Kazu- hiro Shimizu, who has supported our experiments. This study was partially supported by grant funding from the New Energy and Industrial Technology Development Organization. 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Fig. 9 Comparison of frequency analysis of relative vibration between tool and spindle during cutting Fig. 10 Comparison of axial profile of machined workpiece measured by surface roughness tester without low-pass filter Fig. 11 Comparison of harmonic analysis of measured roundness 044502-4 / Vol. 136, AUGUST 2014 Transactions of the ASME Downloaded From: http:/manufacturingscience.asmedigitalcollection.asme.org/ on 05/26/2014 Terms of Use: http:/asme.org/terms