Discussion on electrical maintenance technology of

Discussion on electrical maintenance technology of CNC machine tools (II)

position feedback (see "position measurement" in the previous section) has three situations: one is that there is no position measurement element, and there is no position feedback when setting open-loop control for position and measuring speed, acceleration, vibration and other parameters, and generally, the stepping motor drive is open-loop; One is semi closed-loop control, that is, the position measuring element is not on the final moving part of the coordinate axis, that is, there are some transmission links outside the position closed-loop control. This situation requires that the transmission part outside the ring should have a considerable amount of customers can freely choose the required electromechanical protection level and the transmission stiffness and transmission accuracy of the cooling mode. After adding the reverse clearance compensation and pitch error compensation, a very high position control accuracy can be obtained; The third is full closed-loop control, that is, the position measuring element is installed on the final moving part of the coordinate axis. Theoretically, the position accuracy of this control is the best, but it has higher requirements for the whole mechanical transmission system rather than low. Otherwise, it will seriously affect the dynamic accuracy of the two coordinates, and make the machine tool work only when the speed loop and position accuracy are reduced. Another important problem that affects the accuracy of full closed-loop control is the accurate installation of measuring elements, which should not be underestimated

(4) feedforward control is opposite to feedback. It is a regulating circuit that pre adds the command value to the back. Its main function is to reduce the tracking error to improve the dynamic response characteristics and improve the position control accuracy. Because most machine tools do not have this function, this article will not detail it, just note that the feed-forward must be added after the best commissioning of the above three control loops

3. About preventive maintenance

the purpose of preventive maintenance is to reduce the failure rate. Its work mainly includes the following aspects

(1) personnel arrangement assign special operators, technicians and maintenance personnel to each CNC machine tool, and all personnel should constantly strive to improve their business and technical level

(2) establish rules and files, formulate operating rules for the specific performance and processing objects of each machine tool, and establish work and maintenance files. Managers should often check, summarize and improve

(3) daily maintenance a daily maintenance plan should be established for each CNC machine tool, Including the maintenance contents (such as the lubrication and wear of the coordinate shaft transmission system, the lubrication of the main shaft, the oil, water and air circuits, various temperature controls, the balance system, the cooling system, the tightness of the transmission belt, the cleaning of the relay and contactor contacts, whether the plugs and terminals are loose, the ventilation condition of the electric gas holder, etc.) and the maintenance cycle of all functional components and components (daily, monthly, half a year or irregular)

(4) improve the utilization rate. If the NC machine tool is idle for a long time, when it needs to be used, first of all, the static and dynamic transmission performance of the machine tool will be affected due to grease solidification, dust and even rust, which will reduce the accuracy of the machine tool, and the blockage of the oil system is a big trouble; From the electrical point of view, because the hardware of the entire electrical control system of a numerical control machine tool is composed of tens of thousands of electronic components, their performance and life are very discrete. From a macro point of view, it is divided into three stages: basically in the so-called "running in" stage within one year. At this stage, the failure rate shows a downward trend. If the machine tool is continuously started during this period, the "running in" task will be completed faster, and the one-year maintenance period can also be fully utilized; The second stage is the effective life stage, that is, the stage of giving full play to efficiency. Under the conditions of reasonable use and good daily maintenance, the normal operation of the machine tool can be at least five years; The third stage is the aging stage of system life. Electrical hardware failures will gradually increase, and the service life of CNC system is about 8 ~ 10 years on average

therefore, in a period of time without machining tasks, it is best to run the machine tool idly at a low speed, and at least always power on the CNC system, even every day

III. maintenance and troubleshooting technology

1 Classification of common electrical faults

electrical faults of CNC machine tools can be classified according to the nature, appearance, cause or consequence of the fault

(1) according to the location of the fault, it is divided into hardware fault and software fault. Hardware failure refers to the abnormal state or even damage of electronics, electrical components, printed circuit boards, wires and cables, connectors, etc., which can be eliminated only after repair or even replacement. The software fault generally refers to the fault generated in the PLC logic control program, which can be eliminated only by inputting or modifying some data or even modifying the PLC program. Part processing program failure also belongs to software failure. The most serious software failure is the defect or even loss of CNC system software, which can only be solved by contacting the manufacturer or its service organization

(2) according to whether there is indication when the fault occurs, it is divided into the fault with diagnostic indication and the fault without diagnostic indication. Today's CNC systems are designed with perfect self diagnosis programs to monitor the software and hardware performance of the whole system in real time. Once a fault is found, it will immediately alarm or a brief text description will be displayed on the screen. Combined with the diagnostic manual equipped with the system, we can not only find the cause and location of the fault, but also the troubleshooting method tips. Machine tool manufacturers will also design relevant fault instructions and diagnostic instructions for specific machine tools. The faults with diagnostic indications in the above two parts, together with various indicator lights on each electrical device, make it easier to eliminate most electrical faults. Part of the fault without diagnostic indication is caused by the incompleteness of the above two diagnostic procedures (such as the switch is not closed, the connector is loose, etc.). This kind of fault depends on the working process and fault phenomenon and consequences before the fault occurs, and depends on the familiarity and technical level of the maintenance personnel with the machine tool, hoping to bring help to everyone, and the technical level can be analyzed and eliminated

(3) according to whether the fault is destructive when it occurs, it can be divided into destructive fault and non-destructive fault. It is not allowed to repeat the destructive fault, the fault that damages the workpiece or even the machine tool during maintenance. At this time, the corresponding inspection and analysis can only be carried out according to the phenomenon when the fault occurs, which is technically difficult and has certain risks. If