Trouble analysis and elimination of returning to r

2022-08-06
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Fault analysis and troubleshooting of NC machine tool back to reference point

1. Overview

NC machine tool back to reference point can be divided into absolute pulse encoder mode and incremental pulse encoder mode according to different detection elements. The system using absolute pulse encoder as feedback element. After the installation and commissioning of the machine tool, in the normal use process, as long as the backup battery of absolute pulse encoder is effective, every subsequent startup, There is no need to go back to the reference point. In the system using incremental pulse encoder, the machine tool must first return to the reference point after each startup to determine the coordinate origin of the machine tool. Finding the reference point is mainly related to the zero point switch, encoder or grating ruler zero point pulse. Generally, there are two ways

1) it moves rapidly in the direction of axial preset point. After the stop presses the zero switch, it decelerates and continues to move forward until the stop disengages from the zero switch. The CNC system starts to look for the zero point. When the first zero pulse is received, it can determine the position of the reference point. At present, the machine tool equipped with FANUC system and Beijing knd system generally adopts this zeroing mode

2) the axis moves rapidly in the predetermined direction. After the stop is pressed to the zero switch, it decelerates in the opposite direction. When it disengages from the zero switch again, the axis changes its direction and moves to the reference point. When the stop is pressed down again, the CNC system starts to look for the zero point. When the first zero pulse is received, it can determine the position of the reference point. Machine tools equipped with Siemens, American AB system and central China system generally adopt this zeroing mode

the system determines the mode or how to move through the programming of PLC and the parameter setting of the NC system. The speed of the axis is also set in the machine parameter. The process of returning to the reference point of the NC machine tool is completed by the cooperation of the PLC system and the NC system. The NC system gives the command of returning to zero, and then the axis moves in the predetermined direction. After pressing the zero switch (or breaking away from the zero switch), PLC sends a deceleration signal to the CNC system. The CNC system decelerates in the predetermined direction. The measurement system receives the zero pulse. After receiving the first pulse, the coordinate value is designed. After finding the reference point for all axes, the process of returning to the reference point is completed

the common faults of NC machine tools that cannot return to the reference point are as follows: first, the zero switch has problems; Second, the encoder has problems; Third, the system measurement board has problems; Fourth, the zero switch is too close to the hard (soft) limit position; Fifth, the system parameters are lost. The following describes the maintenance process with several examples I have encountered in my work

2. Maintenance example

example 1) XH714 machining center starts up and returns to the reference point, and the X axis moves back to the opposite direction of the reference

the machine is equipped with a Siemens 810D numerical control system, which adopts a semi closed loop control mode and uses an incremental pulse encoder as a detection feedback element

analysis: the action process of turning the x-axis back to the reference point when the machine tool is started is as follows: the axis back to the reference point moves rapidly first. When the zero switch is pressed by the stop, the i32.2 signal at the PLC input point changes from 1 to 0. After receiving the jump signal, the CNC outputs the deceleration command to make the x-axis move in the opposite direction at low speed after braking. When the stop releases the zero switch, the i32.2 signal jumps from 0 to 1. After braking, the x-axis changes direction, Move to the reference point at the speed of returning to the reference point. When the zero switch is pressed down by the block again, the i32.2 signal changes from 1 to 0. At this time, when the CNC receives the zero mark pulse I0 sent by the incremental pulse encoder, the x-axis continues to run to the distance set by the parameter and stops. The reference point is established and the process of returning to the reference point is over

this way of returning to the reference point can avoid the harm to the machining center caused by the abnormal operation of returning to the reference point at the reference point. When the x-axis of the machining center is already at the reference point and the operation of returning to the reference point is carried out, the initial signal of i32.2 is zero. After the CNC detects this state, it sends a movement command in the opposite direction to the reference point. After the zero switch is released, i.e. i32.2 is 1, the x-axis brake changes the direction and moves to the reference point at the speed of returning to the reference point to carry out the above process of returning to the reference point

according to the fault phenomenon, it is suspected that after the zero switch is pressed, although axis X has left the reference point, the switch cannot be reset. Use PLC diagnostic check to confirm correct judgment

ask the operator that each axis is in the middle position when the machine tool is started up, and eliminate the fault cause of switch spring fatigue failure due to stop at the reference point for deceleration and the stop continuously pressing the zero switch. It also indicates that the deceleration switch has failed before shutdown

carefully observe the processing process and find that the processing center stops at the reference point after each processing cycle. This greatly increases the possibility of zero switch failure and the probability of failure. This may be the real cause of this fault

since the NC code generated by CAM software programming is mostly in the format of G28 back reference point before the end of the program (M30), it is recommended that NC programmers add back the G code command of the middle point of each axis before the end of the program (M30) when preparing the part processing program, and remove the G28 command to reduce the occurrence of such failures

example 2) the xh713/4 machining center returns to the reference point and gives an over travel alarm

the machining center is equipped with fanuc-omd control system. The semi closed loop control mode is adopted. The incremental pulse encoder is used as the detection feedback element. The back reference point adopts the stop pressure zero switch. It slows down and moves away from the zero switch to start looking for the zero point

the above is the relevant introduction of the composition of the fatigue testing machine and the basic management in normal use

the parameters were lost due to the failure of the backup battery of CNC. After the backup parameters are reloaded by the computer, when the reference point is returned, the soft limit overtravel alarm occurs at the middle position of the travel range for each axis. At this time, move each axis manually. Even if the mechanical position is in the middle of the travel range, the CRT also displays the soft limit overtravel alarm for the position coordinates of each axis

this is because after the backup battery fails, the CNC recognizes the mechanical position at this time as the reference point position when the battery is reinstalled and started

the solution is to first set the positive soft limit value of each axis to the maximum value, then make the three-axis return reference point, establish the correct zero point of the machine tool, and then change the three-axis soft limit to the original value. The specific steps are as follows:

1) under the offset menu, set pwe=1

2) set a set of no.700, 702 and 704 (x, y and z) of CNC parameter 4 servo control oil source (including inlet proportional servo valve) as the maximum value respectively

3) manually move XYZ away from the mechanical origin for a certain distance

4) in the reference point return to zero mode, each axis returns to the reference point manually

5) carefully observe whether each axis is at the back reference point, especially the Z axis related to ATC. If the position is not accurate, repeat steps 3 to 4 until it is accurate

6) change the parameters changed in the second step back

the large-scale zigzag stress corrosion testing machine adopts the electronic loading condition. 7) reset PWE to zero

in this way, the problem of over range alarm at the reference point will be solved

example 3) during the use of V560 machining center, the z-axis return reference point has a soft limit over travel alarm

the machining center is equipped with fanuc-oima control system, which adopts the semi closed loop control mode. The reference point adopts the stop pressing zero switch, decelerates forward, disengages from the zero switch, and starts to find the zero point

observe that the z-axis display on the CRT is 6.01, which is a soft limit overtravel. The test confirms that when the alarm occurs, the manual return to the reference point has not been completed.

observe that the PMC signal dgnx 9.3 input by the deceleration switch changes normally when the manual return to the reference point, indicating that the deceleration switch has no problem. Set the CNC parameter No.704 (Z-axis soft limit) to the maximum value, and the manual return to the reference point is normal. No.704 was reset to 6000, and the over range alarm occurred again at the reference point

analysis: since there is no problem with the deceleration switch, the process of returning to the reference point has not been completed, and the soft limit overtravel occurs. It indicates that the stopper is not loosened, which may be due to the loosening of the position of the deceleration switch

the inspection found that the position of the deceleration switch was indeed relaxed. After readjusting the position of the deceleration switch and tightening the fixed gongs and nails, the problem was solved

however, it should be noted that once the position of the deceleration switch is loosened, the lead screw pitch compensation parameters originally set when the machine tool leaves the factory will be inaccurate. It is necessary to re measure the screw pitch compensation parameters of the machine tool with a laser measuring instrument before setting

example 4) when a CNC milling machine equipped with Beijing KND-100M is started up and returned to the reference point, the two XZ axes are normal, but when the Y axis returns to the reference point, 222 "Y-direction servo preparation failure alarm" appears

analysis: carry out targeted inspection according to the fault phenomenon. When the servo drive module is inspected, it is found that there is No. 23 servo alarm. At this time, check the fault manual and there are the following explanations:

1) the motion resistance of the ball screw is too large or there is a problem with the ball screw itself. However, no problems were found during manual movement inspection

2) the servo motor is damaged. By measuring the winding resistance servo, no problems were found

3) the load carrying capacity of the servo drive module was insufficient or damaged, and the control panel had problems, resulting in an error alarm

check the servo drive module. Replace the XY axis servo drive module of the same model and eliminate the fault. It can be seen that the fault is the unstable performance or contact of the y-axis servo drive module. However, a few days later, the fault occurred again. When the x-axis returned to the reference point, the 212x servo preparation failure alarm occurred again. According to the previous experience, when the servo drive module is checked, it is found that there is No. 23 (servo preparation not thread) servo alarm. It seems easy to conclude that the servo drive module of the original Y-axis (now replaced to the x-axis) has been completely damaged. However, for further confirmation, the fault still exists after replacing the XY axis servo drive module of the same model again, indicating that this fault is not related to the servo drive module

it was found through inspection that the stop of the x-axis forward limit switch had moved towards the stop of the deceleration switch, resulting in that when the x-axis returned to the reference point, the hard limit switch had been stopped before the action of returning to the reference point was completed, thus causing the CNC to generate the above alarm

after readjusting the position of the hard limit switch and tightening the fixed screws, the machine tool returns to the reference point and returns to normal

3. Summary

the fault that the NC machine tool cannot return to the reference point is one of the common faults in the NC machine tool. This kind of fault is generally caused by the loosening of the stop, the failure of the deceleration switch, the loss of parameters, the inaccurate setting of the soft limit and other factors. Of course, the damage of encoder or grating ruler and the problem of zero pulse of encoder or grating ruler will also cause the failure of not returning to the reference point, but the encoder and grating ruler are relatively reliable and have a low probability of failure. As long as we master the relevant working principle of the reference point of the CNC machine tool and the mechanical structure of the equipment, understand its operation method and action sequence, and fully investigate and analyze the fault phenomenon, we will be able to find the cause of the fault, check and repair, eliminate the fault, and finally make the machine return to normal. (end)

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