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The application of PLC in CNC system of machine tools (Part 1)

1 introduction in recent years, PLC has been more and more widely used in the field of industrial automatic control. Its comprehensive advantages in control performance, assembly cycle and hardware cost are unmatched by other industrial control products. With the development of PLC technology, it is more and more used 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 machining accuracy of the NC machine tool, the selection of its positioning control device is particularly important. The NC positioning function of Yonghong FBS series PLC is more accurate than that of other PLCs, and the program design and debugging are quite convenient. This paper presents how to use the NC positioning control of Yonghong PLC to realize the control function of machine tool NC system to meet the control requirements, which is feasible in practical operation. The whole machine control system has obvious advantages such as clear program design idea, simple and practical hardware circuit, high reliability, strong anti-interference ability, and good performance price ratio. The design idea of its software and hardware can be used for reference for the design and transformation of relevant NC machine tools in industrial and mining enterprises

2 composition and working process of NC machine tool

this example of NC machine tool is composed of input, output device, NC device, programmable controller, servo system, detection feedback device and machine host


input device can transfer different processing information to the computer. At the initial stage of the production of CNC machine tools, the input device was perforated paper tape, which is now tending to be eliminated; At present, the use of keyboard, disk, etc., greatly facilitates the information input work. Output refers to the output of internal working parameters (including the original parameters under normal and ideal working conditions of the machine tool, fault diagnosis parameters, etc.). Generally, these parameters need to be output in the just working state of the machine tool for recording and saving. After working for a period of time, the output is compared and compared with the original data to help judge whether the machine tool is working normally. The numerical control device is the core and leading of the numerical control machine tool, which completes the processing and calculation of all processing data, and finally realizes the command 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 the program 2 Some universal testing machines for rubber materials have not been used for a long time, and the U-shaped leather ring in the oil cylinder has dried up. This hardness tester (rubber hardness tester) has just been widely used in the hardness measurement of rubber and plastics. The leather ring cannot play a role when it is used, and the pressure does not go up. It needs to be soaked for a period of time or the rotation speed command under replacement to control the rotation speed of the main shaft; Manage the tool magazine, including automatic tool exchange, tool selection mode, cumulative tool usage times, tool remaining life and tool grinding times; Control the spindle power battery to reduce the price by a large margin, such as forward and reverse rotation, stop, quasi stop, cutting fluid switch, chuck clamping and loosening, manipulator tool taking and feeding, etc; It also controls the external switches (travel switch, pressure switch, temperature control switch, etc.) of the machine tool; Control the output signals (tool magazine, manipulator, rotary table, etc.). The detection feedback device is composed of detection elements and corresponding circuits. It mainly detects the speed and displacement, and feeds back the information to the NC device to realize closed-loop control to ensure the machining accuracy of the NC machine tool

the preparation process of NC machining is complex and contains many contents, including the understanding of part structure, process analysis, formulation of process scheme, preparation of machining program, selection of tooling and use methods. The adjustment of the machine tool mainly includes tool naming, tool magazine adjustment, workpiece installation, tool setting, tool position measurement, status of various parts of the machine tool, etc. Program debugging is mainly to check and adjust the logic problems of the program itself and its design rationality. Trial cutting is a dynamic investigation of the part processing design scheme, and the whole process needs to be carried out after the evaluation of the results of the previous step. After the trial cutting is successful, the parts can be formally processed, and the results of the processed parts can be tested. 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 the machine tool. This index has a direct impact on the evaluation of the utilization rate of machine tools (i.e. the actual movement rate of machine tools)

3 several problems to be solved in the NC system of machine tool

the machine tool is composed of mechanical and electrical parts. In the design of the overall scheme, the implementation scheme of various functions of the machine tool should be considered from the mechanical and electrical aspects. The mechanical requirements of the NC machine tool and the functions of the NC system are very complex, so it is more necessary to communicate with the mechanical and electrical parts to develop strengths and avoid weaknesses. The options, assembly, programming and operation of the 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 commissioning and maintenance, all operations are set with manual functions, such as manual fast and slow movement of each axis, high and low speed rotation of main axis meeting pcl2.1 specification, cutting fluid and lubrication switch, etc. PLC acts in sequence or in sequence according to logic conditions. In addition, it controls interlocking protection action according to logic relationship independent of sequence and sequence. PLC has developed into the main product to replace relay circuit and carry out sequence control, and it is also widely used in the electrical control of machine tools

how to improve the positioning speed and ensure the positioning accuracy in the actual control is a problem that needs to be seriously considered and practically solved. Accuracy is a performance index that machine tools must guarantee. The position accuracy of position servo control system determines the machining accuracy of CNC machine tools to a great extent. Therefore, the position accuracy is a very important index. In order to ensure sufficient position accuracy, on the one hand, it is to correctly select the open-loop magnification in the system, and on the other hand, it is 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, and the accuracy of the feedback detection element often plays a decisive role in the accuracy of the system. A high-precision control system must be guaranteed by high-precision detection elements. When the site conditions change, some control parameters of the system must be modified accordingly. In order to meet the continuity of production, it is required to modify the variable parameters of the control system. Although the original setting parameters can be easily and quickly changed by using the programmer, the programmer can not be handed over to the field operators generally; Therefore, it should be considered to develop other simple and effective methods to modify the variable control parameters of PLC. In addition, in order to prevent PLC from being damaged by high voltage, a varistor is added to the power input. In order to prevent overheating, the PLC shall not be installed directly above the transformer and other heating elements, and the frequency converter shall be kept at a certain distance from the PLC and servo driver. Appropriate clearance shall be reserved between components for heat dissipation, and a fan shall be installed on the distribution box for cooling. In addition, in order to ensure the safe and stable operation of the control system, the safety protection problems of the control system should also be solved, such as the travel protection of the system, the automatic detection of faulty components, etc

4 analysis of NC machine tool positioning servo control system of Yonghong FBS series PLC

NC machine tool is a kind of high-precision and high-efficiency automation equipment, so it is particularly important to improve the reliability of NC machine tool. Reliability is one of the main quantitative indicators to evaluate reliability. It is defined as the probability that a product will complete a specified function under specified conditions and within a specified time. For CNC machine tools, the functions here mainly control the use functions of the machine tools, such as various functions of CNC machine tools, servo performance, etc. The functional components of the CNC machine tool play a very 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. It must not only be harmonious with the overall structure of the CNC machine tool, but also be integrated into the whole machine system to have the best matching performance, and can well show the personalized characteristics of the CNC machine tool. The high-speed CNC system can not only improve the data processing ability, but also have the functions of thermal error compensation unit and advanced control technologies such as speed forward-looking control, position loop feedforward control and acceleration/deceleration stable control. Therefore, a stable and reliable control unit must be selected to ensure the normal and efficient operation of CNC machine tools

in view of the above requirements, the author uses fbs-44mn PLC of Taiwan Yonghong Motor Co., Ltd. as the main control unit of the machine tool. This machine has high cost performance, small volume, very convenient to use and simple wiring. Its programming software winproladder is known as the master of ladder diagram. It is easy to learn and use and has powerful functions. It is very easy to operate, monitor and debug. It can be used simultaneously with buttons, mouse and real-time command functions to query and operate instructions, so that the input efficiency is doubled. At the same time, it is equipped with a man-machine interface for program parameter modification, setting and operation status display monitoring, and the contents of the man-machine interface can be programmed. The control system has the characteristics of high reliability, low price and compact structure. It is very suitable for the control requirements of machine tools and specific control ideas

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