1. Overview When the numerical control machine tool returns to the reference point, the absolute pulse encoder and the incremental pulse encoder are two different methods according to the detection elements. The absolute pulse encoder is used as the feedback element system. After the machine tool is installed and debugged, during normal use, only The backup battery of the absolute pulse encoder is valid, and it is not necessary to return to the reference point for each subsequent power-on. In a system using an incremental pulse encoder, the machine tool must first perform a reference point return after every power-on to determine the coordinate origin of the machine tool. The search reference point is mainly related to the zero pulse of the zero switch, encoder, or scale. There are generally two ways.
1) Rapid movement in the direction of the axial predetermined point, and the deceleration forward movement continues after the block depresses the zero point switch. After the block disengages the zero point switch, the CNC system starts searching for the zero point. When the first zero point pulse is received, it is determined. Reference point location. Machines equipped with the FANUC system and the Beijing KND system currently use such a zero return method.
2) The axis rapidly moves in the predetermined direction. After the block presses the zero point switch, it reverses the deceleration movement. When it leaves the zero point switch again, the axis changes direction and moves toward the reference point. When the block presses the zero point switch again, The CNC starts to search for zeros. When the first zero pulse is received, the position of the reference point is determined. Machines equipped with SIEMENS, American AB systems, and Central China systems generally use this zero return method.
Which method or how to move, the system is determined by the PLC programming and CNC machine parameter settings, the axis speed is also set in the machine parameters, CNC machine tool reference point is the process of the PLC system In conjunction with the numerical control system, the zero return command is given by the numerical control system, and then the axis moves in the predetermined direction, and after pressing to the zero point switch (or leaving the zero point switch), the PLC sends a deceleration signal to the numerical control system, and the numerical control system decelerates the movement in the predetermined direction. The zero point pulse is received by the measurement system. After receiving the first pulse, the coordinate value is designed. After all the axes have found the reference point, the process of returning to the reference point ends.
The failure of the CNC machine tool to return to the reference point is generally the following situations: First, there is a problem with the zero point switch; Second, there is a problem with the encoder; Third, there is a problem with the system measurement board; Fourth, the zero switch and the hard (soft) limit position are too Near; fifth is the loss of system parameters and so on. The following describes the maintenance process with several examples that I encountered in my work.
2, maintenance examples Example 1) The XH714 machining center is powered back to the reference point and the X axis moves back in the opposite direction of the reference.
The machine is equipped with a SIEMENS 810D numerical control system and adopts a semi-closed-loop control method. An incremental pulse encoder is used as a detection feedback component.
Analysis: The process of the X axis of the machine restarting to the reference point is: the reference point axis moves rapidly. When the zero point switch is pressed by the limiter, the PLC input point I32.2 signal changes from 1 to 0, and the CNC receives this. The deceleration command is output after the transition signal, so that the X-axis brakes and moves in the reverse direction at a low speed. When the limit switch releases the zero point switch, the I32.2 signal changes from 0 to 1 and the X-axis changes direction after braking. The reference point speed is moved toward the reference point. When the zero point switch is pressed again by the dog, the I32.2 signal changes from 1 to 0. At this time, the CNC receives the zero mark pulse I0 issued by the incremental pulse encoder. When the X axis continues to run until the distance set by the parameter stops, the reference point is established and the process of returning to the reference point ends.
This way of reference point back can avoid the harm caused to the machining center by the abnormal operation such as returning the reference point to the reference point. When the X-axis of the machining center is already at the reference point and the reference point is back, the initial signal of I32.2 is zero. After the CNC detects this state, it sends a movement instruction in the opposite direction to the reference point. After the zero switch is released, that is, I32.2 is 1, the X-axis changes direction after braking, and moves back to the reference point at the reference speed to perform the above-mentioned referencing.
According to the fault phenomenon, after the zero switch is pressed, the switch cannot be reset although the X axis has left the reference point. Use PLC diagnostic checks to confirm the correctness.
Asked the operator, when the machine is turned on, each axis is in the middle position, which eliminates the cause of the failure of the switch spring due to the fact that the stop deceleration at the reference position and the stop continuously presses the zero point switch. It also shows that the deceleration switch has failed before shutting down.
After careful observation of the machining process, it was found that after each machining cycle was completed, the machining center stopped at the reference point. This greatly increases the possibility of zero point switch failure and increases the probability of failure. This may be the real reason for this failure.
Due to the NC code generated by CAM software programming, before the end of the program (M30), most of them refer to the G28 reference point format. Therefore, it is recommended that the NC programmers add back to each before the end of the program (M30) when compiling the part program. The G code instruction at the middle point of the axis, and remove the G28 instruction to reduce the occurrence of such failures.
Example 2) An overtravel alarm occurs when the XH713/4 machining center returns to the reference point.
The machining center is equipped with a FANUC-OMD control system, uses a semi-closed-loop control method, uses an incremental pulse encoder as a detection feedback element, uses a block zero pressure point switch at the reference point, decelerates ahead, and is free from a zero point switch to start searching for a zero point. the way.
Loss of parameters due to the failure of the backup battery of the CNC. When the computer backs up the backup parameters and then returns to the reference point, a software limit overtravel alarm occurs at the middle of the stroke range of each axis. At this time, the axes are manually moved to make the mechanical position in the middle of the stroke range. The CRT also displays the soft limit overtravel alarm for each axis position.
This is because when the backup battery fails, the CNC recognises the mechanical position at this time as the reference position when reloading the battery.
The solution is to set the positive soft limit value of each axis first to the maximum value, and then make the reference point of three axes back to establish the correct machine zero point, and then change the three axis soft limit to the original value. Specific steps are as follows:
1) Under the OFFSET menu, set PWE=1.
2) Set the three axes of the CNC parameters NO.700, 702, and 704 (X, Y, and Z) to their maximum values, respectively.
3) Manually remove the XYZ from the mechanical origin by a certain distance.
4) In the reference point return-to-zero mode, each axis manually returns to the reference point.
5) Observe carefully whether each axis is in the reference position, especially the Z axis related to ATC. If the position is inaccurate, repeat steps 3 to 4 until it is accurate.
6) Change the parameters changed in the second step back again.
7) Reset PWE to zero.
In this way, the issue of an overtravel warning at the reference point is resolved.
Example 3) When the V560 machining center is in use, a soft limit overtravel alarm appears at the Z-axis reference point.
The machining center is equipped with a FANUC-OIMA control system. It adopts a semi-closed-loop control method. The reference point is used to block the zero point switch, decelerate the front and move away from the zero point switch, and start searching for the zero point.
Observe that the Z axis on the CRT shows 6.01, which is the soft limit overtravel. The test confirms that the process of manually returning to the reference point has not been completed when this alarm occurs.
When manually returning to the reference point, observe that the deceleration switch input PMC signal DGNX 9.3 changes normally, indicating that there is no problem with the deceleration switch. Set the CNC parameter No. 704 (Z axis soft limit) to the maximum value of 99999999 and manually return the reference point to normal. NO.704 is reset to 6000, and the over-reference alarm is over.
Analysis: Since there is no problem with the deceleration switch, the process of returning to the reference point has not yet been completed, and the soft limit overtravel has occurred. Note that there is no slack in the stopper. It may be that the position of the deceleration switch is loose.
The inspection found that the position of the deceleration switch did indeed relax. After re-adjusting the position of the deceleration switch and tightening the fastening pin, the problem was solved.
However, it should be noted that once the position of the deceleration switch is loosened, the screw pitch compensation parameters originally set at the time of the delivery of the machine are inaccurate. Need to use the laser measuring instrument to re-measure the screw pitch compensation parameters of the machine before setting.
Example 4) A CNC milling machine equipped with a Beijing KND-100M has two XZ axes that are normal when the machine is powered back to the reference point, but when the Y axis returns to the reference point, 222 “Y servo servo is not ready”.
Analysis: According to the phenomenon of failure to conduct targeted inspection, in the inspection of the servo drive module, found 23 servo alarm. Check the fault manual at this time, as explained below:
1) The ball screw has excessive movement resistance or the ball screw itself has a problem. However, the manual move check did not find the problem.
2) The servo motor is damaged. No problem found by measuring its winding servo
3) The servo drive module's carrying capacity is not enough or damaged, and the control panel has a problem and generates an error alarm.
Check the servo drive module and replace the same type of XY axis servo drive module to eliminate the fault. It can be seen that the fault is the unstable performance or contact instability of the Y-axis servo drive module. However, after a few days, a fault occurred again. When the X axis returns to the reference point, 212X servo-ready alarms appear again. According to the previous experience, when the servo drive module was checked, it was found that there was a servo alarm on the 23rd (the servo is not ready). It seems easy to conclude that the servo drive module misjudged the original Y axis (now changed to the X axis) has been completely damaged. However, in order to further confirm, the fault still exists after swapping the same type of XY axis servo drive module once again, indicating that this fault has nothing to do with the servo drive module.
Originally, after inspection, it was found that the stop of the X-axis positive limit switch has moved toward the stop of the deceleration switch, causing the X-axis to return to the reference point, and the back-reference position action has been blocked to the hard limit switch. Cause the CNC to generate the above alarm.
After re-adjusting the position of the hardware limit switch and tightening the fixing screw, the reference point of the machine tool returns to normal.
3, total The failure of the CNC machine tool to return to the reference point is one of the more common failures in CNC machine tools. This kind of fault is generally caused by factors such as looseness of the stopper, failure of the deceleration switch, loss of parameters, and inaccurate setting of the software limit. Of course, the damage of the encoder or grating scale and the problem of the zero pulse of the encoder or scale may also cause failures of the reference point. However, the encoder and scale are relatively reliable and have a failure. The probability is relatively low. 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, sequence of actions, and fully investigate and analyze the fault phenomenon, we can find the cause of the fault, check and repair, and eliminate the fault. Ultimately, the machine is back to normal.
