Experimental Study on Vertical Active Vibration Isolation System for Ultra-precision Machine Tools

(a) Passive isolation
(b) Active Vibration Isolation Figure 1 Isolation Mode Equivalent Model 1 Introduction In the process of ultra-precision machining, environmental vibration has a great influence on machining accuracy and surface quality. The interference of environmental vibration will not only cause the vibration of the machine body, but more importantly it will cause the position change between the cutting tool and the machined part. The latter will directly reflect the quality of the machined surface of the machined part. Therefore, it is necessary to set an excellent vibration isolation device. Vibration isolation is divided into passive vibration isolation and active vibration isolation, the equivalent model shown in Figure 1. Passive vibration isolation, as shown in Figure 1a. This vibration isolation method does not require an energy device other than the system to support the vibration isolation device. Traditional vibration isolation is generally a passive vibration isolation using vibration isolation elements with a certain stiffness k and damping x. Active vibration isolation, as shown in Figure 1b. This vibration isolation method requires energy devices outside the system to support the vibration isolation device. The principle is to precisely detect the vibration signal to be eliminated through the sensor, and then through the feedback system to process the measured signal, output the feedback quantity k1 and control the actuator so that the generated excitation force F cancels the vibration interference. To eliminate the purpose of vibration. In this paper, an active vibration isolation experiment was conducted on an ultra-precision machine tool developed by Harbin Institute of Technology with an air spring as a vibration isolation element. 2 vibration isolation theory analysis of passive vibration isolation passive vibration isolation system model shown in Figure 1a, by Newton's second law has mx" + x (x'-y') + k (xy) = 0 (1) ,x,x',x" are the vibration displacement, velocity and acceleration of the vibration isolation table: y, y' are the environmental vibration displacement and velocity, respectively. From (1), the absolute transmission coefficient of a passive vibration isolation system can be obtained. T is not = x(w) = {1+(2ew/wn)2} 1⁄2 y(w) (1-w2/wn2)2+ (2ew/wn)2 (2) where wn is the natural frequency and wn=(k/m)1⁄2:e is the damping ratio, e=x/(2mwn). Discussed by formula (2): When < 1: When w>>wn, T none = (2ewn/w) <1. Active vibration isolation active vibration isolation system shown in Figure 1b, take the feedback control force F = -k1x ", then the Newton second law has F = mx" + x (x'-y') + k (xy) (3) According to the formula (2) derivation process, the absolute transmission coefficient of the active vibration isolation system T is T = x(w) = {1+(2e'w/w'n)2} 1⁄2 y(w) ( 1-w2/w'n2)2+(2e'w/w'n)2 (4) where w'n is the active vibration isolation resonant frequency and w'n=[k/(k1+m)] 1⁄2
Figure 2 The relationship between T and w in an active vibration isolation system is discussed by equation (4): When w< T none = {[1+(2e)2]1⁄2/(2e)} When 1>>w'n, T has =(2e'w'n/w)<1. From the above discussion, draw the curve of the relationship between T and w, and no comparison with T, as shown in Figure 2. As can be seen from Figure 2, the active control is characterized by the reduction of the resonant frequency, but due to the influence of the frequency response of the active isolators (actuators), the amplitude at the resonance peak is greater than the passive control at the natural frequency. The peak at the point, that is to say the introduction of active control will give a certain improvement in low frequency vibration. According to this principle, an active vibration isolation system in the vertical direction of an ultra-precision machine tool is established, as shown in Fig.3. The control idea of ​​this system is: the table surface vibration signal is picked up by the sensor, enters the computer after the amplifier, the filter, the analog/digital conversion, processes the negative feedback signal by the number 2 mode conversion through the computer software to enlarge after the power amplifier makes the exciter vibration. When the table vibrates upwards, the exciter pulls the table downwards: When the table vibrates downward, the exciter pushes the table upwards, thereby achieving the purpose of counteracting the tabletop vibration.
Figure 3 Active Vibration Isolation System 3 Active Vibration Isolation Experiment Study of Active Vibration Isolation Table Features Adjust the parameters of the control system to the optimal state and run the program so that the excitation force acts on the bed. To verify the effectiveness of the system, spectrum analyzers were used to obtain the frequency domain characteristic curve and the absolute transmission system change curve before and after the introduction of the control system. The results are shown in Figure 4 and Figure 5. It can be seen from Fig. 4a that the main frequency of the ambient interference vibration is approximately 15 Hz and 3.5 Hz, and the passive isolation table itself has effectively isolated the high frequency vibration above 20 Hz. Therefore, the system only considers the vibration of less than 20 Hz. It can be seen from Fig. 4b that after the active vibration isolation, the vibrations near the main vibration frequency of the environment at 15 Hz and 3.5 Hz are significantly reduced, especially in the frequency range of 3 to 5 Hz. As can be seen from Fig. 5, the absolute transmission coefficient of the frequency above 10 Hz and below 0.6 Hz after the introduction of active control is basically unchanged: the resonance peak frequency is reduced, from the original 4.25 Hz to 1.75 Hz, and the peak value is increased. Accord with active vibration isolation theory. Passive vibration isolation after the frequency greater than 5Hz peak value is less than 0dB (20lgT), that T is not less than 1, began to have vibration isolation effect: from Figure 5b visible, the frequency is greater than 3Hz after the peak is less than 0dB, that is, active vibration to make frequency The effect of vibration isolation begins to be greater than 3 Hz, and the vibration isolation effect of 3 to 5 Hz is significantly improved, which is consistent with the characteristics of the frequency domain. But at the same time, as can be seen from Figure 4b and Figure 5b, the frequency band 5 ~ 10Hz vibration than originally increased, after repeated testing of the control system found that the increase in vibration in this area is entirely due to the frequency response of the exciter.
(a) Passive isolation frequency domain characteristics
(b) Frequency domain characteristics curve of active vibration isolation Figure 4 Comparison of frequency domain characteristics of vibration isolation platform before and after the introduction of the control system
(a) Passive vibration isolation absolute transfer coefficient curve
(b) Absolute transmission coefficient curve of active vibration isolation Figure 5 Absolute transmission coefficient variation curve of vibration isolation table before and after introduction of control system Comparison of the effect of active vibration isolation on machining surface quality The influence of the quality is specially cut with aluminum as the workpiece material. The experimental conditions are as follows: the spindle speed is 1000 r/min, the feed rate is 2 μm/r, and the back blade quantity is 3 μm. After testing, after the introduction of active control, the surface roughness (Ra) of the workpiece has decreased from 18.97 nm to 15.64 nm, indicating that the vibration isolation system has certain practicality. 4 Conclusions In this paper, we have established a vertical vibration isolation system for ultra-precision machine tools through theoretical analysis of the two vibration isolation theories and conducted an experimental study. The following conclusions have been drawn: After the introduction of active control, the stage is subject to environmental disturbances and main vibration isolation. Ability to increase. With the introduction of active control, the resonant frequency decreases and the peak value increases. This is in line with the active vibration isolation theory: Passive vibration isolation has a vibration isolation effect when the frequency is greater than 5 Hz and the vibration isolation effect is reduced to more than 3 Hz. The low-frequency vibration isolation condition is obtained. A certain degree of improvement, especially 3 ~ 5Hz effect is more obvious. The use of active vibration isolation improves the surface quality of the workpiece.