Our accelerator (Siemens, Germany) has a long time to start up every day, and the accelerator has been used for nearly 7 years. Therefore, the mechanical, optical, dose, and circuit systems of the whole machine are beginning to appear aging. Accelerator failures also range from simple to simple component-level failovers, making maintenance more difficult. 1 Fault phenomenon One afternoon, during the process of the intensity-modulated operation of the accelerator, the multi-leaf grating (MLC) suddenly appeared to show that the blade was stuck, and the next treatment subfield could not be automatically changed, and the screen showed a red flashing alarm, and the accelerator could not continue to work. 2 fault check After observation, it was found that: 1 the technician workstation screen has a red flashing alarm on the screen. The twisting device reset key cannot be reset, enter the machine maintenance mode, the software resets, the fault remains; 2, after entering the machine room, the control manual box function controls the blade group X1 respectively. X2 work, X1, X2 blade group except 29# Two blades can not move, other blades can move, but the positions of the blades are not correct; 3 separately operate the 29# blades of X1 and X2 groups. The 29# blade of the X2 group can be controlled to move, but cannot be moved when the 29# blade of the X1 group is controlled. The preliminary conclusion is that the MLC is basically normal. It may be that the X1 blade group 29# single blade spokes overshoot, and the limit of 10 cm after the midline is over, causing the entire MLC position to be disordered, resulting in failure. 　 3 fault repair (1) Re-calibrate the MLC blade position. On-site inspection did not find other abnormalities in the accelerator. It was only possible to try to focus on the MLC blade position based on past experience. After calibration according to the standard, the fault disappeared, the machine resumed normal operation, and the radiotherapy work was completed that day. But the next day, the patient was treated with intensity-enhancing, and the above faults reappeared. Observing the MLC blade, the X1 blade group 29# has a overshoot at 10 cm from the midline. The MLC blade position is re-calibrated according to the standard, and the power-on test is normal. But in the next few days, this fault repeated many times. (2) Circuit analysis. The basic working principle of the MLC circuit: the MLC blade position is programmed by the MLC control cabinet, and the motor drive signal is given to drive the motor to determine the position of each blade. The rotation of the blade motor drives the rotation of the position potentiometer and the position code potentiometer, 2 The feedback signal generated by the potentiometer is processed and sent to the SENSOR board for comparison. The comparison result signal is sent back to the MLC control cabinet SLAVE1# board to compare with the actual setting signal to determine whether the position of the blade is correct, and then adjust the blade motion to Set the location. Therefore, after careful analysis by the MLC circuit diagram, the possibility judgment is obtained: 1 controlling the X1 driving circuit board of the MLC to fail and sending an incorrect control signal; 2 the blade driving 29# the motor itself is faulty or the position potentiometer and the position decoder are damaged, Unable to send valid feedback comparison signal; 3 SENSOR board failure, actual position signal and coding position signal contrast error, give error result signal; 4 SLAVE1# board failure, give error judgment signal, return error control motor drive board. (3) Fault handling: using the segmentation method, with the 29# drive motor as the mark, divided into two sections before and after the drive and feedback; using the replacement method, the 29# drive motor group and the adjacent 15# drive motor group are replaced as a whole. The test failure phenomenon still occurs and the faulty blade position is not transferred. The #29 motor unit can be eliminated. Restore the original position of the two units, in order to facilitate the observation of the fault phenomenon, without covering the outer casing, Recalibrating the MLC location took 2 days and the failure did not reappear. Careful analysis of the maintenance process: 1 In order to judge this fault, only the X1 blade group 29# drive motor group was replaced, and no other work was done; 2 it was easy to observe the fault, the cover of the accelerator head was not covered, and the position of the SENSOR board was found through the position check. In the outermost layer of the machine head, it can be seen directly; 3 After nearly one week of tracking observation, it is found that the fault occurrence time is basically around 16:00~19:00 in the afternoon, and the fault occurrence interval is shorter and shorter, and the fault occurs. The frequency is getting higher and higher; 4 After the fault occurs, after correcting the position of the MLC blade, it can basically continue to use normally; 5 To verify the idea, the accelerator head is open for 1 week, and the same fault does not occur in the accelerator. Experiments show that it is basically judged that the accelerator is overheated due to long-term operation, and the thermal stability of a component on the SENSOR board is deteriorated. The POTENTIOMETER return signal and the ENCODER return signal of the X1 blade group 29# are on the SENSOR board. Compare the error and issue an error message, causing this failure to occur. (4) Troubleshooting: The problem is concentrated on the SENSOR I/F#1 board. By analyzing the SENSOR I/F#1 circuit, it is found that all the circuits work around the CPU#1 and CPU#2 chips (P80C31BH). CPU #1 and CPU #2 are active plug-in chips, indicating that these two chips are fragile components. Because there is a correlation between the CPU#1 and CPU#2 chips, it is decided to try again with the replacement method. So I purchased 2 P80C31BH chips (purchase price 118.00 yuan) and replaced the CPU#1 and CPU# on the SENSOR I/F#1 board. 2 chip, re-calibrate the MLC blade position, try to open the shell state, the fault does not appear. After 3 days, restore the head casing for another 1 week and troubleshoot. Up to now, the accelerator has been used for more than half a year, and similar faults have not appeared. 4 Summary Make full use of the self-test procedures and powerful conditions provided by large equipment, and patiently track down the fault location to minimize the fault range. Do not blindly repair, should analyze the circuit diagram, targeted work, do more with less. Segment the problem and further narrow down the fault. Be sure to carefully observe the malfunction of the equipment. For suspected parts that are easy to be damaged and components to be replaced, an experimental verification is required to eliminate the possibility of failure.
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