Extraction of current characteristic value Since the detected current has many interference signals, it is necessary to filter the current signal collected in real time. The sampling frequency is 1 kHz, the cutoff frequency is 350 Hz, and the signal is subjected to 4th-order low-pass filtering to obtain a filtering formula. Since the current signal is a physical quantity of instantaneous fluctuation, its instantaneous value does not have a direct correspondence with the processing load. Therefore, the current signal must be extracted. After repeated trials and theoretical analysis, the spindle motor current is single-phase instantaneous. The amount of timed integral value can be used as the characteristic value of the current signal.
In the formula, I is the current characteristic value T is the spindle current signal period i is the directly filtered current value 3 data fitting during the machining process, setting the spindle speed, cutting depth and other processing parameters of the machine tool are unchanged, The feed rate is continuously changed, the current values ​​at various feed rates are sequentially detected, and the relationship between them is fitted by a polynomial of 3 times. In the formula, f is the feed rate, and h is the current characteristic value I1, I2, ..., In corresponding to the known n feed speeds. The value is 0=1, 1=f, 2=f2, 3= F3, fitting the third-order polynomial by the method of least squares curve fitting, and setting the vector a=(a0, a1, a2, a3)T, I=(I0, I1,..., In)T, then the system of equations For the ATAa=ATI formula, since 0, 1, 2, 3 are linearly independent, the unique solution a0=a0*, a1=a1*, a2=a2*, a3=a3* can be obtained to obtain the function I(f). )=a0*+a1*f+a2*f2+a3*f3 This is the relationship between the current characteristic value and the feed rate when the cutting depth is constant.
Under the same processing conditions, changing the depth of cut and repeating the above process, a series of curves between current and feed can be obtained at different depths of cut. For example, under the processing conditions of spindle speed n=1000r/min and cutting depth h=1.2mm, I(f)=1.4612+0.872f+0.4277f2+0.0944f34 control strategy can be obtained from the experiment. The spindle motor current indirectly monitors the cutting load, adjusts the feed amount in real time, and keeps the cutting load constant to a certain extent, so as to control the deformation during machining, so that the machining deformation remains basically unchanged, thereby improving the machining accuracy of the CNC machine tool. .
The cutting process experiment is carried out under the condition that the spindle speed of the machine tool and other processing conditions are the same. After repeated verification and comparison, the maximum load that the machine tool can meet the optimal processing conditions under the condition is obtained, that is, the optimal processing condition. The current characteristic value I0, the relationship between the current and the feed with different depths and above is shown in the figure as follows: Under certain machining conditions, the machine tool performs the workpiece at a given feed speed. Processing, the system detects the spindle current of the machine tool in real time, and calculates its characteristic value, compares it with I0, and obtains the feed speed value that satisfies the optimal machining condition under the condition. As shown in the figure, the feed rate is real-time as shown in the figure. The command is sent to the processing system to keep the current characteristic value substantially at I0. If the detected current changes, the above control process is repeated to achieve constant load control of the CNC machine tool cutting process. The specific control process is as follows: (1) Real-time detection of the machine tool spindle current Ic, calculation of its characteristic value I, the current cutting depth h from the current feed rate f0; (2) from the known current-feed relationship curve According to the current cutting depth h, the current-feed curve under the current machining conditions is determined.
(3) The optimum feed rate value under this condition is obtained from the current machining condition optimum current characteristic value I0, that is, the abscissa at the intersection of the curve and the straight line I0 in the figure, and the feed rate value thus obtained is Adjusted feedrate ft. (4) The obtained ft is sent to the processing system, and a feedback command word is sent back by the processing system to realize real-time online compensation of the force deformation error. (5) Continue to continuously detect the machine current, and if the obtained characteristic value changes, repeat the above process.
The machining control example contains the workpiece with complex curved surface. This is the upper surface of the mobile phone mold which is processed by rough processing and semi-finishing on the basis of a square material. The workpiece used for rough machining is traditional processing. The left workpiece is processed by constant load control. The same tool and initial machining parameters are used in the machining process. Compared with the traditional machining method, the machining accuracy is greatly improved and the machining efficiency is also improved under the constant load control machining mode.
There are many kinds of constant load control algorithms. The control algorithm is verified on the DM4600 CNC milling process with the Chinese medieval star numerical control system platform, and the control effect is achieved. However, the real-time detection of load signals and the real-time transmission of control commands have a crucial impact on the accuracy of control. How to shorten the response time and improve real-time performance is the direction that needs to be improved and improved.
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