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1 Introduction
In recent years, PLC has become more and more widely used in the field of industrial automation control. Its comprehensive advantages in terms of control performance, assembly cycle and hardware cost are unmatched by other industrial control products. With the development of PLC technology, it has more and more applications in position control, process control, data processing and so on. In the actual design and production process of the machine tool, in order to improve the precision of CNC machine tool processing, the selection of its positioning control device is particularly important. The NC positioning function of Yonghong FBs series PLC is more accurate than other PLCs, and the design and debugging of the program is quite convenient. This paper puts forward how to apply the NC positioning control of Yonghong PLC to realize the control function of CNC machine system to meet the control requirements, which is feasible in actual operation. The whole machine control system has clear programming ideas, simple and practical hardware circuit, high reliability, strong anti-interference ability, good performance and price ratio, etc. The design ideas of software and hardware can be used to design and transform related CNC machine tools of industrial and mining enterprises. Learn from.
2 CNC machine tool structure and working process
The numerical control machine tool of this example consists of input and output devices, numerical control devices, programmable controllers, servo systems, detection feedback devices and machine tool hosts.
The input device can transfer different processing information to the computer. In the early stage of the production of CNC machine tools, the input device was a perforated paper tape, which has now been eliminated. At present, the use of keyboards, disks, etc., greatly facilitates information input. The output refers to the internal working parameters of the output (including the normal parameters of the machine, the original parameters under the ideal working condition, the fault diagnosis parameters, etc.). Generally, these parameters are required to be recorded and saved in the working state of the machine. After working for a period of time, the output is compared with the original. The data is compared and compared to help determine whether the machine tool is working properly. The numerical control device is the core and leading of the numerical control machine tool, completes the processing and calculation of all processing data, and finally realizes the command work of the functions of the numerical control machine tool. It contains the circuit of the microcomputer, various interface circuits, hardware such as CRT display and corresponding software. The programmable controller controls the spindle unit, processes the speed command in the program to control the spindle speed, manages the tool magazine, performs automatic tool exchange, tool selection mode, cumulative tool usage, tool remaining life, and tool sharpening times. Management; control the spindle forward and reverse rotation, quasi-stop, cutting fluid switch, chuck clamping release, robot picking knife and other actions; also control the machine's external switch (stroke switch, pressure switch, temperature control switch, etc.) Control the output signals (tool magazine, robot, rotary table, etc.). The detection feedback device is composed of the detecting component and the corresponding circuit, mainly detecting the speed and displacement, and feeding back the information to the numerical control device to realize the closed-loop control to ensure the machining precision of the numerical control machine tool.
The preparation process of CNC machining is complicated and contains many contents, including structural understanding of parts, process analysis, formulation of process plans, preparation of machining programs, selection of tooling and methods of use. The adjustment of the machine tool mainly includes the tool naming, transfer into the tool magazine, workpiece installation, tool setting, measuring the position of the tool, and the status of each part of the machine tool. Program debugging is mainly to check and adjust the logic of the program itself and its design rationality. The trial cutting process is a dynamic investigation of the part processing design plan, and the whole process needs to be evaluated after the previous step to achieve the results. After successful trial cutting, the parts can be formally processed and the processed parts can be tested for results. The first three steps are standby time. In order to improve work efficiency, it is desirable that the standby time is as short as possible, which is more conducive to the reasonable use of the machine tool. This indicator directly affects the evaluation of machine tool utilization (ie machine tooling rate).
3 Several problems need to be solved in machine tool numerical control system
The machine tool is composed of two parts: mechanical and electrical. In the overall design of the machine, the implementation of various functions of the machine tool should be considered from the aspects of electromechanical. The mechanical requirements of the numerical control machine tool and the functions of the numerical control system are very complicated, so it is more important to communicate with each other. . Machine tool control system options, assembly, programming and operation should be reasonable, accuracy and stability must meet the requirements of use. At the same time, in order to facilitate debugging and maintenance, various operations are provided with manual functions, such as manual movement of each axis, spindle high and low speed rotation, cutting fluid and lubrication switch. The PLC performs sequential actions according to logical conditions or acts according to timing. In addition, there is control of interlocking protection actions according to logical relationship irrespective of sequence and timing. PLC has become the main product to replace relay lines and perform sequential control. Applications in electrical control are also common.
How to improve the positioning speed in actual control, while ensuring the positioning accuracy is a problem that needs to be seriously considered and effectively solved. Accuracy is a performance metric that machine tools must guarantee. The positional accuracy of the position servo control system largely determines the machining accuracy of the CNC machine. Therefore, positional accuracy is an extremely important indicator. In order to ensure sufficient positional accuracy, on the one hand, the size of the open-loop amplification factor in the system is correctly selected, and on the other hand, the accuracy of the position detecting component is required. Because in the closed-loop control system, it is difficult to distinguish between the error of the detection element itself and the deviation of the detected quantity, and the accuracy of the feedback detection element often plays a decisive role in the accuracy of the system. High-precision control systems must be guaranteed with high-precision sensing components. When the site conditions change, some control parameters of the system must be modified accordingly. To meet the continuity of production, the modification of the variable parameters of the control system should be performed online. Although the programmer can easily and quickly change the original setting parameters, the programmer can not be used by the field operator; therefore, other simple and effective methods should be considered to realize the online modification of the variable control parameters of the PLC. In addition, in order to prevent the PLC from being damaged by excessive voltage, a varistor is added to the power input terminal. In order to prevent overheating, the PLC must not be installed directly above the heating element such as the transformer, and the inverter maintains a certain distance from the PLC and servo drive. Leave appropriate gaps between the 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, the safety protection of the control system should also be solved, such as the stroke protection of the system and the automatic detection of faulty components.
4 Analysis of NC Machine Positioning Servo Control System of Yonghong FBs Series PLC
CNC machine tools are a high-precision, high-efficiency automation device, which is especially important to improve the reliability of CNC machine tools. Reliability is one of the main quantitative indicators for evaluating reliability. It is defined as the probability that a product will perform a specified function under specified conditions and within a specified time. For CNC machine tools, the functions here mainly refer to the use of machine tools, such as the various functions of CNC machine tools, servo performance. The functional components of CNC machine tools play an extremely important role in the function expansion and performance improvement of the machine tool. Therefore, it is different from the selection of general accessories and accessories, and must not only be harmonious with the overall structure of the CNC machine tool, but also integrated into the whole system. It has the best matching performance, and it also shows the personalized characteristics of the CNC machine. The numerical control system for high speed can not only improve the data processing capability, but should have the thermal error compensation unit and the advanced control technology such as speed forward control, position loop feedforward control and 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 the CNC machine.
In view of the above requirements, the author uses the FBs-44MN PLC of Taiwan Yonghong Motor Co., Ltd. as the main unit of the machine tool control. The machine has high cost performance, small size, convenient use and simple wiring. Its programming software WinProladder is known as the ladder master, easy to learn and use and powerful, editing, monitoring, debugging and other operations are very convenient, button, mouse and online instant command function query and operation instructions, so that editing and input efficiency doubled. At the same time, it is equipped with man-machine interface for program parameter modification, setting and running status display monitoring, and programmable man-machine interface content can be programmed. The control system has the characteristics of high reliability, low price and compact structure, and is very suitable for the control requirements of the machine tool and the specific control ideas.
The programmable logic controller is the logical control center of the various functions of the machine tool. It is integrated in the numerical control system, mainly refers to the integration of control software, while the PLC hardware often adopts a distributed structure in large-scale systems. It can be seen from Fig. 3 that the system control center is controlled by Yonghong PLC FBs-44MN, and equipped with man-machine interface for program parameter modification, setting, and running status display monitoring, and programmable man-machine interface content. The three axes are all digital AC servo systems. Each axis servo motor drives the ball screw through the coupling to move the table with the linear guide and the spindle milling head. The positioning is accurate and the speed is fast. The spindle milling head is controlled by the inverter. According to the tool and the workpiece and the feed amount, the spindle is set to a reasonable speed and its start and stop is set in the program. Each axis is equipped with a two-terminal limit sensor and an origin sensor. The cooling and lubrication also have an abnormality detection. The actual position feedback signal measured by the position detecting device such as a grating and an inductive synchronizer is displayed at the alarm light and the human-machine interface. At any time, compared with the given value, the difference between the two is amplified and transformed, and the actuator is driven to move at a given speed toward the direction of eliminating the deviation until the difference between the given position and the actual position of the feedback is equal to zero. The closed-loop feed system is more complex in structure than the open-loop feed system, and the cost is also high, and the environmental room temperature requirements are strict. Design and commissioning are more difficult than open loop systems. However, it is possible to obtain higher accuracy, faster speed, and higher power consumption characteristics than the open loop feed system. Early use of general motor as positioning control, because the speed is not fast, or the accuracy requirements are not high, so it is enough to deal with the occasion; when the mechanical operation speeds up in order to obtain efficiency, when the product quality and accuracy requirements are higher and higher, the motor The control of the stop position is not what the general motor can achieve. The best way to solve this problem is to use NC positioning control with stepper or servo motor for positioning control. However, in the past, due to its high price, it limited the universality of its use. 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 uses is increasing. In order to cope with this trend, Yonghong PLC FBs series integrates the special NC positioning controller function currently available on the PLC into the internal SoC chip of PLC, except for eliminating the complicated data exchange and connection procedure between PLC and dedicated NC positioning controller. , to significantly reduce the overall cost, to provide users with a cheap, simple and convenient PLC integrated NC positioning control program. The internal SoC chip of Yonghong PLC FBs-44MN contains multi-axis high-speed pulse output and high-speed hardware counter, and provides easy-to-use and designed positioning program editing. For this application, it is even more powerful and easy to use. In the NC closed-loop control system composed of PLC and servo drive, PLC is responsible for sending high-speed pulse command to servo drive. Except that the displacement detection signal installed in the servo motor is directly fed back to the servo drive, the external displacement detector is installed behind the transmission mechanism. It truly reflects the actual displacement and feeds this signal back to the high-speed hardware counter inside the PLC, which allows for more precise control and avoids the shortcomings of the above semi-closed loop. The positioning function of Yonghong PLC FBs series integrates the special NC positioning controller on the market into PLC, so that PLC and NC controller can share the same data area without data exchange and synchronization control between two systems. Complex work, but still commonly used NC positioning control commands (such as DRV, SPD...etc.). The PLC controls the positioning of the 4-axis and can be used for multi-axis simultaneous control. In addition to providing point-to-point positioning speed control, it also provides linear interpolation between the axes. When the system is applied more than 4 axes, you can also use the CPU LINK function of Yonghong PLC to achieve more positioning motion control. The servo performance required by the CNC machine tool for the position system includes: positioning speed and contour cutting feed rate; positioning accuracy and contour cutting accuracy; surface roughness of finishing; stability under external disturbance. These requirements mainly depend on the static and dynamic characteristics of the servo system. For closed-loop systems, it is always desirable to have a higher dynamic accuracy, that is, when the system has a small position error, the moving parts of the machine will react quickly. In the machining of CNC machine tools, the servo system requires a sufficiently wide speed range for the feed drive in order to simultaneously satisfy high-speed rapid shift and single-step jog.
Single-step action is an auxiliary work method that is often used in the adjustment of the workbench. The maximum speed of the servo system is chosen taking into account the machine's mechanical permissible limits and actual machining requirements. High speeds can increase productivity, but drive requirements are higher. In addition, there is a problem of detection and feedback from the perspective of system control, especially in computer control systems, it must be considered whether the time of software processing is sufficient. The full-closed-loop servo system places the position detecting component on the terminal moving part of the measured coordinate axis to detect the transmission error caused by the pitch error, the gap and various disturbances in the mechanical transmission chain, and performs feedback compensation control to improve the machine tool. The position control accuracy. In a full-closed servo control system, the choice of position sensing and feedback components is critical. Inductive synchronizer has the advantages of high precision, good repeatability, strong anti-interference ability, oil and stain resistance and simple maintenance. It is especially suitable for high-precision full-closed CNC machine tools. CNC machine tools require stability, speed and accuracy. The mechanical transmission of large CNC machine tools has large moment of inertia and low natural frequency. It is difficult to make it much higher than the system cut-off frequency. The full-closed loop includes the feed system. Almost all unstable nonlinear factors on the shaft can be easily shaken by the machine if it is not properly adjusted.
Therefore, the full-closed-loop servo system of the CNC machine tool is mainly to solve the stability of the feed motion of the machine tool on the basis of ensuring the rapidity and obtain higher positional 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 obtains the pulse deviation value after the pulse frequency and the pulse number of the input signal and the frequency and pulse number of the feedback signal are processed by the internal deviation counter and the frequency conversion voltage circuit. With the speed error value, this enables the servo motor to achieve a high speed, precise speed and position closed loop processing system. The speed of the servo motor is proportional to the pulse frequency of the input signal, and the amount of movement of the motor is determined by the number of pulses. Figure 4 is a schematic diagram of the operation of the servo motor under PLC control.
5 related program design and operation
The PLC inputs the program through the programmer for control purposes. Since the PLC working process is a loop, the program execution speed is very fast. In addition, the software fault detection design uses software to detect the external travel switch state based on the hardware design. When the travel switch fails, the machine control is stopped by the program control, which effectively reduces the accident caused by the component failure.
6 Conclusion
China is a big country in the production and application of machine tools, but the application level of numerical control technology is still not high, which seriously restricts the improvement of China's manufacturing industry. The relevant international development plan poses a severe challenge to the development of China's numerical control technology, and it also brings opportunities. Only by selecting the right PLC can the positioning achieve the desired effect. The NC 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 operation. The labor intensity of the personnel.
Due to the use of PLC control, the anti-interference ability of the electric part is increased, and the operational reliability of the machine tool is improved, thereby increasing the flexibility of the device and improving the use efficiency of the device.
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