Analysis and Elimination Methods for Reference Point Return Faults of CNC Machine Tools
Abstract: This paper deeply analyzes the principle of the CNC machine tool returning to the reference point, covering closed – loop, semi – closed – loop and open – loop systems. Through specific examples, various forms of reference point return faults of CNC machine tools are discussed in detail, including fault diagnosis, analysis methods and elimination strategies, and improvement suggestions are put forward for the tool change point of the machining center machine tool.
I. Introduction
The manual reference point return operation is the prerequisite for establishing the machine tool coordinate system. The first action of most CNC machine tools after startup is to manually operate the reference point return. Reference point return faults will prevent program processing from being carried out, and inaccurate reference point positions will also affect the machining accuracy and even cause a collision accident. Therefore, it is very important to analyze and eliminate reference point return faults.
II. Principles of CNC Machine Tools Returning to the Reference Point
(A) System classification
Closed – loop CNC system: Equipped with a feedback device for detecting the final linear displacement.
Semi – closed – loop CNC system: The position measuring device is installed on the rotating shaft of the servo motor or at the end of the lead screw, and the feedback signal is taken from the angular displacement.
Open – loop CNC system: Without a position detection feedback device.
(B) Reference point return methods
Grid method for reference point return
Absolute grid method: Use an absolute pulse encoder or a grating ruler to return to the reference point. During machine tool debugging, the reference point is determined through parameter setting and machine tool zero return operation. As long as the backup battery of the detection feedback element is effective, the reference point position information is recorded every time the machine is started, and there is no need to perform the reference point return operation again.
Incremental grid method: Use an incremental encoder or a grating ruler to return to the reference point, and the reference point return operation is required every time the machine is started. Taking a certain CNC milling machine (using the FANUC 0i system) as an example, the principle and process of its incremental grid method for returning to the zero point are as follows:
Switch the mode switch to the “reference point return” gear, select the axis for reference point return, and press the positive jog button of the axis. The axis moves towards the reference point at a fast moving speed.
When the deceleration block moving together with the worktable presses down the contact of the deceleration switch, the deceleration signal changes from on (ON) to off (OFF). The worktable feed decelerates and continues to move at the slow feed speed set by the parameters.
After the deceleration block releases the deceleration switch and the contact state changes from off to on, the CNC system waits for the appearance of the first grid signal (also known as one – revolution signal PCZ) on the encoder. As soon as this signal appears, the worktable movement stops immediately. At the same time, the CNC system sends out a reference point return completion signal, and the reference point lamp lights up, indicating that the machine tool axis has successfully returned to the reference point.
Magnetic switch method for reference point return
The open – loop system usually uses a magnetic induction switch for reference point return positioning. Taking a certain CNC lathe as an example, the principle and process of its magnetic switch method for returning to the reference point are as follows:
The first two steps are the same as the operation steps of the grid method for reference point return.
After the deceleration block releases the deceleration switch and the contact state changes from off to on, the CNC system waits for the appearance of the induction switch signal. As soon as this signal appears, the worktable movement stops immediately. At the same time, the CNC system sends out a reference point return completion signal, and the reference point lamp lights up, indicating that the machine tool has successfully returned to the reference point of the axis.
III. Fault Diagnosis and Analysis of CNC Machine Tools Returning to the Reference Point
When a fault occurs in the reference point return of a CNC machine tool, a comprehensive inspection should be carried out according to the principle from simple to complex.
(A) Faults without alarm
Deviation from a fixed grid distance
Fault phenomenon: When the machine tool is started and the reference point is manually returned for the first time, it deviates from the reference point by one or several grid distances, and the subsequent deviation distances are fixed each time.
Cause analysis: Usually, the position of the deceleration block is incorrect, the length of the deceleration block is too short, or the position of the proximity switch used for the reference point is improper. This kind of fault generally occurs after the machine tool is installed and debugged for the first time or after a major overhaul.
Solution: The position of the deceleration block or the proximity switch can be adjusted, and the fast feed speed and fast feed time constant for reference point return can also be adjusted.
Deviation from a random position or a small offset
Fault phenomenon: Deviate from any position of the reference point, the deviation value is random or small, and the deviation distance is not equal each time the reference point return operation is performed.
Cause analysis:
External interference, such as poor grounding of the cable shielding layer, and the signal line of the pulse encoder is too close to the high – voltage cable.
The power supply voltage used by the pulse encoder or the grating ruler is too low (lower than 4.75V) or there is a fault.
The control board of the speed control unit is defective.
The coupling between the feed axis and the servo motor is loose.
The cable connector has poor contact or the cable is damaged.
Solution: Corresponding measures should be taken according to different reasons, such as improving the grounding, checking the power supply, replacing the control board, tightening the coupling, and checking the cable.
(B) Faults with alarm
Over – travel alarm caused by no deceleration action
Fault phenomenon: When the machine tool returns to the reference point, there is no deceleration action, and it keeps moving until it touches the limit switch and stops due to over – travel. The green light for reference point return does not light up, and the CNC system shows a “NOT READY” state.
Cause analysis: The deceleration switch for reference point return fails, the switch contact cannot be reset after being pressed down, or the deceleration block is loose and displaced, resulting in the zero – point pulse not working when the machine tool returns to the reference point, and the deceleration signal cannot be input into the CNC system.
Solution: Use the “over – travel release” function button to release the coordinate over – travel of the machine tool, move the machine tool back within the travel range, and then check whether the deceleration switch for reference point return is loose and whether the corresponding travel switch deceleration signal line has a short circuit or an open circuit.
Alarm caused by not finding the reference point after deceleration
Fault phenomenon: There is deceleration during the reference point return process, but it stops until it touches the limit switch and alarms, and the reference point is not found, and the reference point return operation fails.
Cause analysis:
The encoder (or grating ruler) does not send out the zero flag signal indicating that the reference point has been returned during the reference point return operation.
The zero mark position of the reference point return fails.
The zero flag signal of the reference point return is lost during transmission or processing.
There is a hardware failure in the measurement system, and the zero flag signal of the reference point return is not recognized.
Solution: Use the signal tracking method and use an oscilloscope to check the zero flag signal of the encoder’s reference point return to judge the cause of the fault and carry out corresponding processing.
Alarm caused by inaccurate reference point position
Fault phenomenon: There is deceleration during the reference point return process, and the zero flag signal of the reference point return appears, and there is also a process of braking to zero, but the position of the reference point is inaccurate, and the reference point return operation fails.
Cause analysis:
The zero flag signal of the reference point return has been missed, and the measurement system can find this signal and stop only after the pulse encoder rotates one more revolution, so that the worktable stops at a position at a selected distance from the reference point.
The deceleration block is too close to the reference point position, and the coordinate axis stops when it has not moved to the specified distance and touches the limit switch.
Due to factors such as signal interference, loose block, and too low voltage of the zero flag signal of the reference point return, the position where the worktable stops is inaccurate and has no regularity.
Solution: Process according to different reasons, such as adjusting the position of the deceleration block, eliminating signal interference, tightening the block, and checking the signal voltage.
Alarm caused by not returning to the reference point due to parameter changes
Fault phenomenon: When the machine tool returns to the reference point, it sends out an “not returned to the reference point” alarm, and the machine tool does not execute the reference point return action.
Cause analysis: It may be caused by changing the set parameters, such as the command magnification ratio (CMR), the detection magnification ratio (DMR), the fast feed speed for reference point return, the deceleration speed near the origin are set to zero, or the fast magnification switch and the feed magnification switch on the machine tool operation panel are set to 0%.
Solution: Check and correct the relevant parameters.
IV. Conclusion
The reference point return faults of CNC machine tools mainly include two situations: reference point return failure with alarm and reference point drift without alarm. For faults with alarm, the CNC system will not execute the machining program, which can avoid the production of a large number of waste products; while the reference point drift fault without alarm is easy to be ignored, which may lead to waste products of processed parts or even a large number of waste products.
For machining center machines, since many machines use the coordinate axis reference point as the tool change point, reference point return faults are easy to occur during long – term operation, especially the non – alarm reference point drift faults. Therefore, it is recommended to set a second reference point and use the G30 X0 Y0 Z0 instruction with a position at a certain distance from the reference point. Although this brings some difficulties to the design of the tool magazine and the manipulator, it can greatly reduce the reference point return failure rate and the automatic tool change failure rate of the machine tool, and only one reference point return is required when the machine tool is started.
Abstract: This paper deeply analyzes the principle of the CNC machine tool returning to the reference point, covering closed – loop, semi – closed – loop and open – loop systems. Through specific examples, various forms of reference point return faults of CNC machine tools are discussed in detail, including fault diagnosis, analysis methods and elimination strategies, and improvement suggestions are put forward for the tool change point of the machining center machine tool.
I. Introduction
The manual reference point return operation is the prerequisite for establishing the machine tool coordinate system. The first action of most CNC machine tools after startup is to manually operate the reference point return. Reference point return faults will prevent program processing from being carried out, and inaccurate reference point positions will also affect the machining accuracy and even cause a collision accident. Therefore, it is very important to analyze and eliminate reference point return faults.
II. Principles of CNC Machine Tools Returning to the Reference Point
(A) System classification
Closed – loop CNC system: Equipped with a feedback device for detecting the final linear displacement.
Semi – closed – loop CNC system: The position measuring device is installed on the rotating shaft of the servo motor or at the end of the lead screw, and the feedback signal is taken from the angular displacement.
Open – loop CNC system: Without a position detection feedback device.
(B) Reference point return methods
Grid method for reference point return
Absolute grid method: Use an absolute pulse encoder or a grating ruler to return to the reference point. During machine tool debugging, the reference point is determined through parameter setting and machine tool zero return operation. As long as the backup battery of the detection feedback element is effective, the reference point position information is recorded every time the machine is started, and there is no need to perform the reference point return operation again.
Incremental grid method: Use an incremental encoder or a grating ruler to return to the reference point, and the reference point return operation is required every time the machine is started. Taking a certain CNC milling machine (using the FANUC 0i system) as an example, the principle and process of its incremental grid method for returning to the zero point are as follows:
Switch the mode switch to the “reference point return” gear, select the axis for reference point return, and press the positive jog button of the axis. The axis moves towards the reference point at a fast moving speed.
When the deceleration block moving together with the worktable presses down the contact of the deceleration switch, the deceleration signal changes from on (ON) to off (OFF). The worktable feed decelerates and continues to move at the slow feed speed set by the parameters.
After the deceleration block releases the deceleration switch and the contact state changes from off to on, the CNC system waits for the appearance of the first grid signal (also known as one – revolution signal PCZ) on the encoder. As soon as this signal appears, the worktable movement stops immediately. At the same time, the CNC system sends out a reference point return completion signal, and the reference point lamp lights up, indicating that the machine tool axis has successfully returned to the reference point.
Magnetic switch method for reference point return
The open – loop system usually uses a magnetic induction switch for reference point return positioning. Taking a certain CNC lathe as an example, the principle and process of its magnetic switch method for returning to the reference point are as follows:
The first two steps are the same as the operation steps of the grid method for reference point return.
After the deceleration block releases the deceleration switch and the contact state changes from off to on, the CNC system waits for the appearance of the induction switch signal. As soon as this signal appears, the worktable movement stops immediately. At the same time, the CNC system sends out a reference point return completion signal, and the reference point lamp lights up, indicating that the machine tool has successfully returned to the reference point of the axis.
III. Fault Diagnosis and Analysis of CNC Machine Tools Returning to the Reference Point
When a fault occurs in the reference point return of a CNC machine tool, a comprehensive inspection should be carried out according to the principle from simple to complex.
(A) Faults without alarm
Deviation from a fixed grid distance
Fault phenomenon: When the machine tool is started and the reference point is manually returned for the first time, it deviates from the reference point by one or several grid distances, and the subsequent deviation distances are fixed each time.
Cause analysis: Usually, the position of the deceleration block is incorrect, the length of the deceleration block is too short, or the position of the proximity switch used for the reference point is improper. This kind of fault generally occurs after the machine tool is installed and debugged for the first time or after a major overhaul.
Solution: The position of the deceleration block or the proximity switch can be adjusted, and the fast feed speed and fast feed time constant for reference point return can also be adjusted.
Deviation from a random position or a small offset
Fault phenomenon: Deviate from any position of the reference point, the deviation value is random or small, and the deviation distance is not equal each time the reference point return operation is performed.
Cause analysis:
External interference, such as poor grounding of the cable shielding layer, and the signal line of the pulse encoder is too close to the high – voltage cable.
The power supply voltage used by the pulse encoder or the grating ruler is too low (lower than 4.75V) or there is a fault.
The control board of the speed control unit is defective.
The coupling between the feed axis and the servo motor is loose.
The cable connector has poor contact or the cable is damaged.
Solution: Corresponding measures should be taken according to different reasons, such as improving the grounding, checking the power supply, replacing the control board, tightening the coupling, and checking the cable.
(B) Faults with alarm
Over – travel alarm caused by no deceleration action
Fault phenomenon: When the machine tool returns to the reference point, there is no deceleration action, and it keeps moving until it touches the limit switch and stops due to over – travel. The green light for reference point return does not light up, and the CNC system shows a “NOT READY” state.
Cause analysis: The deceleration switch for reference point return fails, the switch contact cannot be reset after being pressed down, or the deceleration block is loose and displaced, resulting in the zero – point pulse not working when the machine tool returns to the reference point, and the deceleration signal cannot be input into the CNC system.
Solution: Use the “over – travel release” function button to release the coordinate over – travel of the machine tool, move the machine tool back within the travel range, and then check whether the deceleration switch for reference point return is loose and whether the corresponding travel switch deceleration signal line has a short circuit or an open circuit.
Alarm caused by not finding the reference point after deceleration
Fault phenomenon: There is deceleration during the reference point return process, but it stops until it touches the limit switch and alarms, and the reference point is not found, and the reference point return operation fails.
Cause analysis:
The encoder (or grating ruler) does not send out the zero flag signal indicating that the reference point has been returned during the reference point return operation.
The zero mark position of the reference point return fails.
The zero flag signal of the reference point return is lost during transmission or processing.
There is a hardware failure in the measurement system, and the zero flag signal of the reference point return is not recognized.
Solution: Use the signal tracking method and use an oscilloscope to check the zero flag signal of the encoder’s reference point return to judge the cause of the fault and carry out corresponding processing.
Alarm caused by inaccurate reference point position
Fault phenomenon: There is deceleration during the reference point return process, and the zero flag signal of the reference point return appears, and there is also a process of braking to zero, but the position of the reference point is inaccurate, and the reference point return operation fails.
Cause analysis:
The zero flag signal of the reference point return has been missed, and the measurement system can find this signal and stop only after the pulse encoder rotates one more revolution, so that the worktable stops at a position at a selected distance from the reference point.
The deceleration block is too close to the reference point position, and the coordinate axis stops when it has not moved to the specified distance and touches the limit switch.
Due to factors such as signal interference, loose block, and too low voltage of the zero flag signal of the reference point return, the position where the worktable stops is inaccurate and has no regularity.
Solution: Process according to different reasons, such as adjusting the position of the deceleration block, eliminating signal interference, tightening the block, and checking the signal voltage.
Alarm caused by not returning to the reference point due to parameter changes
Fault phenomenon: When the machine tool returns to the reference point, it sends out an “not returned to the reference point” alarm, and the machine tool does not execute the reference point return action.
Cause analysis: It may be caused by changing the set parameters, such as the command magnification ratio (CMR), the detection magnification ratio (DMR), the fast feed speed for reference point return, the deceleration speed near the origin are set to zero, or the fast magnification switch and the feed magnification switch on the machine tool operation panel are set to 0%.
Solution: Check and correct the relevant parameters.
IV. Conclusion
The reference point return faults of CNC machine tools mainly include two situations: reference point return failure with alarm and reference point drift without alarm. For faults with alarm, the CNC system will not execute the machining program, which can avoid the production of a large number of waste products; while the reference point drift fault without alarm is easy to be ignored, which may lead to waste products of processed parts or even a large number of waste products.
For machining center machines, since many machines use the coordinate axis reference point as the tool change point, reference point return faults are easy to occur during long – term operation, especially the non – alarm reference point drift faults. Therefore, it is recommended to set a second reference point and use the G30 X0 Y0 Z0 instruction with a position at a certain distance from the reference point. Although this brings some difficulties to the design of the tool magazine and the manipulator, it can greatly reduce the reference point return failure rate and the automatic tool change failure rate of the machine tool, and only one reference point return is required when the machine tool is started.