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The Experience of Self-diagnosis of Faults in the CNC Miling Center

(1) Power on self diagnosis

The so-called startup self diagnosis refers to the diagnosis automatically executed by the internal diagnosis program of the system when the NC system is powered on. It is similar to the startup diagnosis of the computer. Power on self diagnosis can automatically check the key hardware in the system, such as CPU, memory, I / O unit, MDI / CRT unit, paper tape reader, floppy drive and other devices; Determine the installation, connection status and performance of the specified equipment; Some systems can also diagnose some important chips, such as RAM, ROM, special LSI, etc.

The self diagnosis of the NC system is carried out when the machine is started. It can enter the normal operation state only after all items are confirmed to be correct. The time of diagnosis depends on the CNC system. It usually takes only a few seconds, but some take a few minutes.

(2) Online monitoring

Online monitoring can be divided into two forms: internal program monitoring of CNC and monitoring through external equipment. CNC internal program monitoring is a method to automatically diagnose, check and monitor the status of each part through the internal program of the system. The online monitoring scope includes the CNC itself and the servo unit, servo motor, spindle servo unit, spindle motor and external equipment connected with the CNC. Online monitoring is always effective during the working process of the system. The internal program monitoring of NC system includes interface signal display, internal status display and fault display.

(3) Interface signal display

It can display the current status of all interface signals between CNC and PLC, CNC and machine tool. Indicates the on-off condition of digital I / O signal to help analyze the fault. During maintenance, you must understand the significance of signals between CNC and PLC, CNC and machine tool, as well as various conditions for signal generation and cancellation, so as to carry out corresponding inspection. The “function manual”, “connection manual” provided by the manufacturer of CNC system and the “electrical schematic diagram of machine tool” provided by the manufacturer of machining center are the technical guidelines for the above state inspection.

(4) Internal status display

Generally speaking, the following aspects can be displayed by using the internal status display function: the external reasons for the non execution of circular instructions (machining programs)
For example, whether the CNC system is in “in place inspection”; Whether it is in the “machine tool locked” state; Whether the “waiting speed arrival” signal is on; Whether to wait for the measurement signal of “position encoder” when programming the feed of each rotation of the spindle; Whether it is waiting for “spindle 1 turn signal” during thread cutting; Whether the feed rate is set to 0, etc.

(5) Fault information display

In CNC system, fault information is generally displayed on CRT in the form of “alarm display”. The contents of alarm display are different according to different CNC systems. Most of these information appears in the form of “alarm number” and text. The specific contents and troubleshooting methods can be consulted in the “maintenance manual” provided by the manufacturer of CNC system. Monitoring through external equipment refers to a method of automatically diagnosing, checking and monitoring the state of each part of NC machine tool by using computer, PLC programmer and other equipment.
For example, through the computer and PLC programmer to dynamically detect the PLC program in the form of ladder diagram and function diagram, it can read and check the PLC program when the machine tool manufacturer does not provide the PLC program, so as to speed up the maintenance progress of NC machine tool.

(6) Offline test

Offline test, also known as “offline diagnosis”, is the test and inspection of the NC system itself after separating the NC system from the machine tool. The off-line test can further locate the fault of the system and try to minimize the fault range. The off-line test of NC system needs special diagnosis software or special test device. Therefore, it can only be carried out in the manufacturer or special maintenance department of NC system.

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