In the cutting process, CNC machine tools often cause cutting overload due to programming parameters and the use of instructions. The detection and control methods of existing NC machine tool cutting overloads are: (1) Using the overcurrent characteristics of the spindle and the feed motor to cut off the strong power supply: (2) The CNC device collects the spindle and the feed motor over-current signal to send a drive alarm to make Spindle and feed motor stop running. Although these two methods can protect the CNC machine tool, the tool is often stuck in the workpiece and it is easy to damage the workpiece, tool and spindle. In order to solve the above problems, a new method of on-line monitoring of cutting overload is proposed in this paper. That is, the cutting torque is calculated by detecting the power and angular velocity of the main shaft and the feed motor during the cutting process. Then according to the cutting torque limit value, determine whether the cutting is overloaded. When the CNC device receives the cutting overload signal, it controls the tool to exit the cutting overload zone according to a certain path. 1 The working principle of online monitoring In the cutting process, cutting overload will cause the change of cutting torque, and the change of cutting torque will cause the change of motor output power. Therefore, the detection of motor power can determine whether the cutting overload, CNC machine receives the cutting overload signal control spindle and feed motor, issued a cutting overload alarm, the tool out of the cutting overload zone, thus ensuring the workpiece machining accuracy and machine safety. When the machine tool is cut, the cutting output power of the main motor and feed motor is P0=Pi-P1 where: P0——motor output power when cutting Pi - instantaneous value of motor active power P1 - the motor itself and the machine drive system Power consumption According to the cutting theory, the cutting torque of the spindle or the feed motor is M=P0/w. Medium: M—cutting torque of the spindle or feed motor P0—the output power of the spindle or feed motor during cutting. ——The angular speed of the spindle or feed motor is set as the cutting overload limit value. When M>Ms, the cutting overload is considered. The above analysis shows that if the cutting overload is determined, it is actually converted to Pi, P1, and W values. According to the principle of electrotechnics, the instantaneous power of the three-phase motor is P=VACiA+VBCiB. In the formula: VAC—A, C The voltage between two lines, VBC—B, C The voltage between two lines, iA and iB—are A, respectively. The current on the B fire line also knows P = Pi + Pw, Pw is the motor reactive power. So Pi = VACiA + VBCiB - Pw can be obtained by measuring the motor voltage, current and reactive power. Reactive power Pw can be eliminated with a digital band-rejection filter with adjustable frequency. P1 consists of fixed loss power and variable loss power. The fixed loss power does not change with external factors, and the variable loss power is measured when the motor is started without cutting. The angular speed w can be obtained by directly measuring the motor speed. Let the current position of the tool be (X0, Y0, Z0), and the starting point of the current machining path is (Xs, Ys, Zs). Then the tool reverse direction displacement is DX=Xs-X0, DY=Ys-Y0, DZ=Zs-Z0. If the current machining trajectory is a straight line, the CNC device executes the following NC program and exits the cutting overload zone. G01 G91 XDX YDY ZDZ F1500 M05 M02 If the current machining path is an arc (G02/G03), the CNC device executes the following NC program exit (where X, Y, Z are selected according to the interpolation plane, Rs is the radius value of the arc). ). G03/G02 G91 XDX YDY ZDZ RRs F1500 M05 M02
Fig. 1 Block diagram of the online monitoring system hardware
Figure 2 cutting overload detection software
Fig. 3 The CNC machine handles cutting 2 Hardware composition of the online monitoring system The hardware block diagram of the online monitoring system is shown in Fig. 1. The hardware circuit consists of CNC and cutting overload detection. The cutting overload signal is input through the interrupt source input of the CNC device. The detection part uses the 8031 ​​microcontroller, which is controlled by the CNC device computer. The current sensor consists of a Hall element and an amplifier. The voltage sensor consists of a transformer with a high ratio. The low-pass filter filters out high-frequency components in the voltage signal. w The measurement circuit is composed of a proximity switch, a level conversion circuit, and a pulse shaping circuit. Its pulse output is connected to the count of the 8031 ​​microcontroller. The measured voltage and current values ​​are converted into 8031 ​​single-chip microcomputer after A/D conversion, and the cutting torque is obtained. Then the cutting overload is determined according to the cutting torque limit value. 3 Online monitoring software The cutting overload monitoring software is divided into two parts: one is the cutting overload detection software (Figure 2): The other part is the CNC device processing cutting overload interrupt service program (Figure 3). The working process of the detection part is: After the monitoring system is powered on, the detection circuit collects the motor voltage signal. If there is voltage, it means that the motor has worked. After the motor is working, calculate the loss power P1 value of the motor and drive system. Run the current and voltage subroutines to calculate the instantaneous active power Pi and the output power P0 of the motor. Start the microcontroller timer/counter, find the value of w, and finally calculate the cutting torque M. If the M value does not exceed the cutting overload limit value Ms, the machining completion signal provided by the CNC device is detected. If the machining is completed, the cutting overload detection is completed. If no machining is completed, the overload detection is continued. If the M value exceeds Ms, a cutting overload signal is output and the CNC device waits for a response. After the completion of the CNC response, the online overload interrupt service routine detection ends. The working process of the CNC device in response to the interruption processing of the cutting overload is shown in Fig. 3. 4 Experimental verification The on-line monitoring experiment of cutting overload was performed on the X feed axis of the XH0825 mini vertical machining center jointly developed by Nanjing Institute of Technology and Nanjing No. 2 Machine Tool Plant. The X feed axis uses Panasonic servo drive unit MQMA082A. Rated power is 750W, rated current is 2.2A. The voltage detection adopts the transformer mode, and the secondary voltage is rectified and filtered into the A/D converter. w The measuring circuit is detected by the proximity switch LJ12A3-4-Z/B1X and fed to the counting end of the 8031 ​​microcontroller. In the experimental process, the cutting overload torque is set to 1.15 times the rated torque, that is, when the cutting overload is 15%, the CNC device responds and the X-axis is moved in the opposite direction to the original cutting direction, and the tool exits the overload zone. . Experiments show that the online monitoring of the cutting overload is feasible. 5 Concluding remarks The online monitoring method of cutting overload proposed in this paper provides a guarantee for the reliability of CNC machining. CNC device manufacturers can use the online monitoring principles, system hardware and software design methods described in this article to integrate cutting overload online monitoring hardware and software into CNC devices to greatly improve the performance of CNC machine tools.
Fig. 1 Block diagram of the online monitoring system hardware
Figure 2 cutting overload detection software
Fig. 3 The CNC machine handles cutting 2 Hardware composition of the online monitoring system The hardware block diagram of the online monitoring system is shown in Fig. 1. The hardware circuit consists of CNC and cutting overload detection. The cutting overload signal is input through the interrupt source input of the CNC device. The detection part uses the 8031 ​​microcontroller, which is controlled by the CNC device computer. The current sensor consists of a Hall element and an amplifier. The voltage sensor consists of a transformer with a high ratio. The low-pass filter filters out high-frequency components in the voltage signal. w The measurement circuit is composed of a proximity switch, a level conversion circuit, and a pulse shaping circuit. Its pulse output is connected to the count of the 8031 ​​microcontroller. The measured voltage and current values ​​are converted into 8031 ​​single-chip microcomputer after A/D conversion, and the cutting torque is obtained. Then the cutting overload is determined according to the cutting torque limit value. 3 Online monitoring software The cutting overload monitoring software is divided into two parts: one is the cutting overload detection software (Figure 2): The other part is the CNC device processing cutting overload interrupt service program (Figure 3). The working process of the detection part is: After the monitoring system is powered on, the detection circuit collects the motor voltage signal. If there is voltage, it means that the motor has worked. After the motor is working, calculate the loss power P1 value of the motor and drive system. Run the current and voltage subroutines to calculate the instantaneous active power Pi and the output power P0 of the motor. Start the microcontroller timer/counter, find the value of w, and finally calculate the cutting torque M. If the M value does not exceed the cutting overload limit value Ms, the machining completion signal provided by the CNC device is detected. If the machining is completed, the cutting overload detection is completed. If no machining is completed, the overload detection is continued. If the M value exceeds Ms, a cutting overload signal is output and the CNC device waits for a response. After the completion of the CNC response, the online overload interrupt service routine detection ends. The working process of the CNC device in response to the interruption processing of the cutting overload is shown in Fig. 3. 4 Experimental verification The on-line monitoring experiment of cutting overload was performed on the X feed axis of the XH0825 mini vertical machining center jointly developed by Nanjing Institute of Technology and Nanjing No. 2 Machine Tool Plant. The X feed axis uses Panasonic servo drive unit MQMA082A. Rated power is 750W, rated current is 2.2A. The voltage detection adopts the transformer mode, and the secondary voltage is rectified and filtered into the A/D converter. w The measuring circuit is detected by the proximity switch LJ12A3-4-Z/B1X and fed to the counting end of the 8031 ​​microcontroller. In the experimental process, the cutting overload torque is set to 1.15 times the rated torque, that is, when the cutting overload is 15%, the CNC device responds and the X-axis is moved in the opposite direction to the original cutting direction, and the tool exits the overload zone. . Experiments show that the online monitoring of the cutting overload is feasible. 5 Concluding remarks The online monitoring method of cutting overload proposed in this paper provides a guarantee for the reliability of CNC machining. CNC device manufacturers can use the online monitoring principles, system hardware and software design methods described in this article to integrate cutting overload online monitoring hardware and software into CNC devices to greatly improve the performance of CNC machine tools.
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