The measurement feedback control device of the CNC machine tool detects the speed and displacement of the actuator (such as the motor, the tool post, the worktable, etc.) through the detection element. In the servo system (automatic control system with mechanical position or angle as the control object), the detection element plays an important role. It feeds the measured signal back to the numerical control device through the circuit to form a semi-closed or closed loop system to compensate The movement error of the actuator can achieve the purpose of improving the movement accuracy.
To some extent, it can be said that the working accuracy of the machine tool mainly depends on the accuracy of the detection elements in the closed-loop control system.
Example 1 When the horizontal machining center of Siemens 8M system is running normally, the machine tool suddenly stops working, and the CRT displays NC alarm 104. The operator turns off the power to start, the alarm is eliminated, and normal operation resumes. After tens of minutes, the fault recurs.
Query NC104 alarm, which means: X-axis measurement closed-loop cable is broken and short-circuited, signal is lost, incorrect threshold signal, incorrect frequency signal. The X, Y, and Z axes of this machine tool adopt grating rulers to detect the position of the machine tool displacement and carry out feedback control to form a closed loop system.
According to experience, if the detection element is contaminated by dust and oil, it will send out wrong signals. Check that the scanning head and scale are not contaminated by oil and dust. Then check the differential amplifier and measurement circuit board. No undesirable phenomena were found, after these work. We focused on the feedback cable, measured the feedback terminal, and found that the voltage of line 13 was unstable. After the power failure, the measurement found that the swing resistance of the cable changed greatly. The inspection found that this line seemed to be connected along the X-axis along the rail. Non-connection will cause the feedback value to be unstable and cause the motor to lose step. After wiring, the fault will be eliminated.
Example 2 A vertical machining center equipped with FAGOR8030. The position of the reference point is unstable when returning to the reference point. Fault with poor reference point positioning accuracy.
According to experience, the main reason for the synchronization error of the pulse encoder is the poor zero pulse of the encoder or the low speed of returning to the reference point. Since checking the reference point zero pulse requires an oscilloscope, you can generally check the setting of the reference point speed and position gain first, and confirm that the position following error of the system is above 1281xm.
If the parameter settings are correct, the possible cause is a bad “zero pulse” signal. Since the signal pulse width of the zero pulse is narrow, it is very sensitive to interference, so the following aspects must be checked:
The first is that the supply voltage of the encoder must be within the range of 5V to 0.2V. When it is less than 4.75V, it will cause “zero pulse” output interference. Secondly, the shielded wire for encoder feedback must be reliably connected, and the position feedback cable should be kept as far away as possible from interference sources and power lines. In addition, the “zero pulse” output of the encoder itself must be correct to meet the system’s requirements for zero pulse.
After checking that the machine tool works normally in manual mode, the reference point deceleration speed and position loop gain are set correctly, the 5V voltage of the measuring encoder is normal, and the action process of returning to the reference point is correct. It is preliminarily determined that the fault is caused by the disturbance of the encoder zero pulse. The inspection found that the shielding wire of the shaft encoder connection cable fell off, and after connection, the positioning accuracy reached the requirements of the original machine tool.
Beginners often ask, why does a CNC machine tool need to return to the reference point? Can it not return to the reference point? Simply put, the purpose of returning to the reference point is to establish a unique coordinate system on the machine every time it is powered on. Because the power is turned off after the machine tool is processed. The CNC system loses the memory of each coordinate position. After turning on the power, the coordinates must be returned to a fixed position of the machine tool, that is, the zero point or origin of the coordinate system, which is also called the datum point or the machine reference point. The operation of returning to the reference point will directly affect the normal operation of the CNC machine tool.