2 the structure and working process of CNC machine tools
The numerical control machine tool in this example is composed of input and output devices, numerical control devices, programmable controllers, servo systems, detection and feedback devices, and the machine tool host, as shown in figure 1.
Figure 1 Structure Diagram of CNC Machine Tool
The input device can transmit different machining information to the computer. Early CNC machine tools, the input device is punched paper tape, and now it has been eliminated; At present, the use of keyboards, disks, etc. It greatly facilitates information input. Output refers to the output of internal working parameters (including original parameters and fault diagnosis parameters under normal and ideal working conditions of machine tools). In general, these parameters need to be output when the machine tool is just working, so as to be recorded and saved. After working for a period of time, comparing and contrasting the output with the original data is helpful to judge whether the machine tool works normally. The numerical control device is the core and leading part of the numerical control machine tool, which completes the processing and calculation of all processing data and finally realizes the instructions of various functions of the numerical control machine tool. It includes microcomputer circuit, various interface circuits, CRT display and other hardware and corresponding software. The programmable controller controls the spindle unit and processes the speed instruction in the program to control the spindle speed; Manage tool magazine, automatic tool change, tool selection mode, cumulative use times of tools, tool remaining life and tool sharpening times; Control the positive and negative rotation and stop of the spindle, quasi-stop, cutting fluid switch, chuck clamping and loosening, manipulator taking and feeding, etc. It also controls the external switches of the machine tool (travel switch, pressure switch, temperature control switch, etc.). ); Control output signals (tool magazine, manipulator, turntable, etc.). ). The detection feedback device is composed of detection elements and corresponding circuits, which mainly detects the speed and displacement and feeds the information back to the numerical control device to realize closed-loop control and ensure the machining accuracy of the numerical control machine tool. The working process of CNC machine tools is shown in Figure 2.
Fig. 2 Block diagram of working process of CNC machine tool
The preparation process of NC machining is complex, which includes many contents, including the structural understanding of parts, process analysis, formulation of process plan, preparation of machining program, selection and use of tools. The adjustment of the machine tool mainly includes tool naming, transfer to tool magazine, workpiece installation, tool alignment, tool position measurement, and the state of each part of the machine tool. Program debugging is mainly to check and adjust the logical problems of the program itself and the rationality of its design. Trial cutting is a dynamic investigation of the design scheme of parts processing, and the whole process needs to be evaluated after the results of the previous step are realized. After the trial cutting is successful, the parts can be formally processed and the processed parts can be inspected. The first three steps are standby time. In order to improve work efficiency, it is hoped that the shorter the standby time, the better, which is more conducive to the rational use of machine tools. This index directly affects the evaluation of the utilization rate of machine tools (that is, the real movement rate of machine tools).
3 Several Problems to be Solved in CNC System of Machine Tool
This machine tool consists of mechanical and electrical parts. When designing the overall scheme, we should consider the realization scheme of various functions of the machine tool from both mechanical and electrical aspects. The mechanical requirements of CNC machine tools and the functions of CNC system are very complicated, so it is more important to communicate with each other by mechanical and electrical means. The selection, assembly, programming and operation of machine tool control system should be reasonable, and the accuracy and stability must meet the use requirements. At the same time, in order to facilitate debugging and maintenance, all operations are equipped with manual functions, such as manual fast and slow movement of each shaft, high and low speed rotation of the spindle, cutting fluid and lubrication switch. PLC acts in sequence according to logical conditions or sequence, and also controls interlocking protection actions according to logical relationship, which has nothing to do with sequence and sequence. PLC has developed into the main product to replace relay circuit and sequence control, and is widely used in the electrical control of machine tools.
How to improve the positioning speed and ensure the positioning accuracy in actual control is a problem that needs serious consideration and practical solution. Accuracy is a performance index that machine tools must ensure. The position accuracy of position servo control system largely determines the machining accuracy of CNC machine tools. Therefore, position accuracy is an extremely important index. In order to ensure sufficient position accuracy, on the one hand, it is necessary to correctly select the open-loop magnification in the system, on the other hand, it is necessary to put forward the accuracy requirements for the position detection elements. Because in the closed-loop control system, it is difficult to distinguish the error of the detection element itself from the deviation of the detected quantity, the accuracy of the feedback detection element often plays a decisive role in the accuracy of the system. High-precision control system must have high-precision detection elements as a guarantee. When the field conditions change, some control parameters of the system must be modified accordingly. In order to meet the continuity of production, it is required that the variable parameters of the control system should be modified online. Although the original setting parameters can be changed conveniently and quickly by using the programmer, the programmer cannot be used by the field operators. Therefore, we should consider developing other simple and effective methods to realize online modification of PLC variable control parameters. In addition, in order to prevent the PLC from being damaged by excessive voltage, a rheostat is added at the power input. In order to prevent overheating, PLC is not allowed to be installed directly above heating elements such as transformers, and the frequency converter keeps a certain distance from PLC and servo drivers. Leave a proper gap between components for heat dissipation, and install a fan on the distribution box to cool down. In addition, in order to ensure the safe and stable operation of the control system, we should also solve the safety protection problems of the control system, such as the travel protection of the system and the automatic detection of faulty components.
Analysis of positioning servo control system of Yonghong FBs series PLC numerical control machine tool
CNC machine tool is a kind of high precision and high efficiency automation equipment, so it is particularly important to improve the reliability of CNC machine tool. Reliability is one of the main quantitative indicators to evaluate reliability, and reliability is defined as the probability that a product will complete the specified function within the specified conditions and time. For CNC machine tools, the functions here mainly refer to the functions of CNC machine tools, such as various functions and servo performance of CNC machine tools. The functional components of CNC machine tools play an extremely important role in expanding the functions and improving the performance of machine tools. Therefore, it is different from the choice of general accessories and accessories. It should not only be coordinated with the overall structure of CNC machine tools, but also have the best matching performance when it is integrated into the whole system, and can also well reflect the personalized characteristics of CNC machine tools. The high-speed numerical control system can not only improve the data processing ability, but also have the function of thermal error compensation unit and advanced control technologies such as speed feedforward control, position loop feedforward control, acceleration and deceleration smooth control. Therefore, it is necessary to choose a stable and reliable control unit to ensure the normal and efficient operation of CNC machine tools.
In view of the above requirements, the author adopts FBs-44MN PLC of Taiwan Province Yonghong Electric Appliance Co., Ltd. as the main control unit of the machine tool, which has high cost performance, small volume, convenient use and simple wiring. Its programming software WinProladder is known as the master of ladder diagram, which is easy to learn, easy to use and powerful. Editing, monitoring and debugging are very smooth. Keyboard and mouse are used together, and online real-time instructions are used to query and operate instructions, which doubles the editing and input efficiency. At the same time, it is equipped with a man-machine interface, which can modify and set program parameters and display and monitor the running state. The content of the man-machine interface is programmable. The control system has the characteristics of high reliability, low price and compact structure, which is very suitable for the control requirements of machine tools. The specific control idea is shown in Figure 3.
Fig. 3 electrical control system diagram of yonghong PLC FBs-44MN CNC machine tool positioning.
Programmable logic controller (PLC) is the logic control center of various functions of machine tools, which is integrated in numerical control system, mainly referring to the integration of control software, while PLC hardware often adopts distributed structure in large-scale systems. As can be seen from Figure 3, the system control center is controlled by Yonghong PLC FBs-44MN, and is equipped with a man-machine interface, which can modify and set program parameters, display and monitor the running state, and the contents of the man-machine interface can be programmed. The three axes are all digital AC servo systems, and the servo motor of each axis drives the ball screw to move the workbench and the spindle milling head with linear guide rail through the coupling, which has accurate positioning and high speed. The spindle milling head is controlled by frequency converter. According to the cutter, workpiece and feed, set a reasonable spindle speed, and set its start and stop in the program. Each shaft is equipped with limit sensors at both ends and origin sensors, and the cooling and lubrication are also detected abnormally. The alarm information is displayed on the alarm lamp and man-machine interface. The actual position feedback signal measured at any time by position detection devices such as grating and inductosyn is compared with the given value, and the difference between them is amplified and transformed, and the actuator is driven to move in the direction of eliminating deviation at a given speed until the difference between the given position and the actual position of feedback is equal to zero. Compared with the open-loop feed system, the closed-loop feed system has complex structure, high cost and strict requirements for ambient room temperature. Design and debugging are more difficult than open-loop systems. However, it can achieve higher accuracy, faster speed and greater driving power than the open-loop feed system. In the early days, the motor was generally used as the positioning control, which was enough for necessary occasions because of its low speed or low precision requirements. When the running speed of machinery is accelerated in order to obtain efficiency, and when the requirements of product quality and accuracy are getting higher and higher, the control of motor stop position is beyond the reach of general motor force. The best way to solve this problem is to use CNC positioning control combined with stepping or servo motor for positioning control. But in the past, because of its high price, its popularity was limited. In recent years, due to the development of technology and the reduction of cost, its price has been accepted by users, and the number of users is increasing. In order to cope with this trend, Yonghong PLC FBs series integrates the functions of special CNC positioning controller in the market into the SoC chip of PLC, which not only eliminates the complicated data exchange and connection procedures between PLC and special CNC positioning controller, but also greatly reduces the overall cost, providing users with a cheap, simple and convenient solution for integrating CNC positioning control with PLC. The SoC chip inside Yonghong PLC FBs-44MN includes multi-axis high-speed pulse output and high-speed hardware counter, and provides easy-to-use and designed positioning program editing, which is more handy for this application. In the numerical control closed-loop control system composed of PLC and servo driver, PLC is responsible for sending high-speed pulse commands to servo driver. Besides the displacement detection signal installed in the servo motor is directly fed back to the servo driver, an additional displacement detector is installed behind the transmission mechanism to truly reflect the actual displacement, and this signal is fed back to the high-speed hardware counter inside the PLC, so as to carry out more accurate control and avoid the above-mentioned semi-closed loop shortcomings. The positioning function of Yonghong PLC FBs series integrates the special numerical control positioning controller in the market into PLC, so that PLC and numerical control controller can share the same data area without complicated work such as data exchange and synchronous control between the two systems, but the commonly used numerical control positioning control instructions (such as DRV, SPD…, etc.) can still be used. PLC controls 4-axis positioning, which can be used for multi-axis simultaneous control. It not only provides point-to-point positioning speed control, but also provides linear interpolation between axes. When the system application exceeds 4 axes, more positioning motion control can be realized by using CPU LINK function of Yonghong PLC. The servo performance required by CNC machine tools for position system includes: positioning speed and contour cutting feed speed; Positioning accuracy and contour cutting accuracy; Surface roughness of finishing; Stability under external interference. These requirements mainly depend on the static and dynamic characteristics of the servo system. For a closed-loop system, it is always expected that the system has high dynamic accuracy, that is, when the system has a small position error, the moving parts of the machine tool will respond quickly. In the machining of CNC machine tools, in order to meet the requirements of high-speed and fast movement and single-step inching at the same time, the feed drive is required to have a wide speed range.
As an auxiliary working mode, single-step point action is often used to adjust the workbench. The selection of the highest speed of the servo system should consider the mechanical allowable limit of the machine tool and the actual processing requirements. Although high speed can improve productivity, it also requires higher driving. In addition, from the point of view of system control, there is also a problem of detection and feedback, especially in computer control systems, whether the software processing time is enough must be considered. The full closed-loop servo system is to place the position detection element on the moving part at the end of the measured coordinate axis, detect the transmission error caused by pitch error, clearance and various interferences in the mechanical transmission chain, and carry out feedback compensation control, thus improving the position control accuracy of the machine tool. In the full closed-loop servo control system, the selection of position detection elements and feedback elements is very important. Inductive synchronizer has the advantages of high precision, good repeatability, strong anti-interference ability, oil resistance, pollution resistance and simple maintenance, and is especially suitable for high-precision fully closed-loop CNC machine tools. CNC machine tools require stability, rapidity and accuracy. However, the mechanical transmission of large CNC machine tools has large moment of inertia and low natural frequency, so it is difficult to make it much higher than the cut-off frequency of the system. The full closed loop contains almost all unstable nonlinear factors of the feed system shaft, and improper adjustment is easy to cause machine tool jitter.
Therefore, on the basis of ensuring rapidity, the fully closed-loop servo system of CNC machine tools mainly solves the stability of machine tool feeding motion and obtains higher position accuracy than the semi-closed-loop servo system. The encoder of the servo motor feeds back the displacement detection signal to the servo driver, and the driver processes the pulse frequency and pulse number of the input signal and the frequency and pulse number of the feedback signal through the internal deviation counter and frequency-voltage circuit to obtain the pulse deviation value and rotation speed error value, so as to control the servo motor to realize a high-speed and high-precision speed and position closed-loop processing system. The rotational speed of the servo motor is proportional to the pulse frequency of the input signal, and the movement of the motor is determined by the number of pulses. Fig. 4 is a schematic diagram of a servo motor under the control of PLC.
Fig. 4 schematic diagram of servo motor of NC machine tool
5 related program design and operation
PLC inputs programs through a programmer to achieve control purposes. Because the working process of PLC is cyclic, the program execution speed is very fast. In addition, based on the hardware design, the software fault detection design uses software to detect the state of the external travel switch. When the travel switch fails, the operation of the machine tool is stopped by program control, which effectively reduces the accidents caused by the failure of machine tool components.
Fig. 5 is an instruction diagram for editing the parameter setting of the positioning program by using the programming software WinProladder, and fig. 6 is a detailed operation processing program diagram.
Fig. 5 Illustration diagram of positioning program parameter setting
Fig. 6 Processing program diagram
6 concluding remarks
China is a big country in machine tool production and application, but the application level of CNC technology is not high, which seriously restricts the improvement of China's manufacturing level. The related international development plan poses a severe challenge to the development of numerical control technology in China, and also brings opportunities. Only by choosing a suitable PLC can the positioning achieve the expected effect. The numerical control positioning function of Yonghong FBs series PLC plays an important role in the design of machine tool numerical control system. The long-term operation of the machine tool shows that the whole system has reasonable design, high control precision and reliable operation, which improves the automation level of production and reduces the labor intensity of operators.
Because of PLC control, the anti-interference ability of electrical parts is increased, and the reliability of machine tool operation is improved, thus increasing the flexibility of equipment and improving the use efficiency of equipment.