What are the faults of CNC machine tools caused by the detection components?

The CNC machine tool measurement feedback control device detects the speed and displacement of the actuator (such as motor, tool holder, table, etc.) through the detection component. In the servo system (automatic control system with mechanical position or angle as the control object), the detecting component plays an important role, and it feeds the measured signal back to the numerical control device through the corresponding circuit to form a semi-closed loop or closed loop system (again It is called feedback control system) to compensate the motion error of the actuator and achieve the purpose of improving the motion precision.

To some extent, it can be said that the accuracy of the machine tool depends mainly 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 appears NC.

Alarm 104, the operator turns off the power and restarts, the alarm is eliminated, and normal operation resumes. After several tens of minutes, the fault reappears.

Query NC104 alarm, which is expressed as: X-axis measurement closed-loop cable break short circuit, signal loss, incorrect threshold signal, incorrect frequency signal. X, Y, Z of this machine

The three-axis uses a grating scale to position the machine displacement and performs feedback control to form a closed-loop system.

As a rule of thumb, the test element emits an erroneous signal if it is contaminated by dust and oil. Check that the readhead and scale are not contaminated by oil and dust. Then check the differential amplifier and the measurement board. No bad phenomena have been found after these work. We focused on the feedback cable and measured the feedback terminal. We found that the voltage on line 13 was unstable. After the power failure, the measurement found that the cable had a large change in the swing resistance. It was found that the line was in the X-axis with the movement of the guide rail. If the connection is not connected, the feedback value is unstable, causing the motor to lose synchronization. After rewiring, the fault is eliminated.

Example 2 A vertical machining center with a FAGOR 8030. The reference point position is unstable when the reference point is returned. A fault with poor reference positioning accuracy.

As a rule of thumb, the main cause of pulse encoder synchronization errors is the encoder zero pulse error or the reference point speed is too low. Since an oscilloscope is required to check the zero point pulse of the reference point, it is generally possible to check the setting of the reference point speed and the position gain first, and confirm that the position following error value of the system is above 1281xm.

If the parameter settings are correct, the possible cause is a "zero pulse" signal. Since the signal pulse width of the zero pulse is narrow, it is very sensitive to interference, so it must be checked for the following aspects:

First, the supply voltage of the encoder must be in the range of +5V+O. 2V. When less than 4

At .75V, it will cause "zero pulse" output interference. Secondly, the shielded wire fed back by the encoder must be reliably connected and the position feedback cable should be kept away from the interference source and the power line as much as possible. In addition, the encoder itself"

The zero-pulse output must be correct to meet the system's requirements for zero pulse.

After checking that the machine works normally in manual mode, the reference point deceleration speed and position loop gain are set correctly, the measuring encoder +5V voltage is normal, and the action of returning to the reference point is correct. The initial determination of the fault is caused by the interference of the encoder zero pulse. The inspection found that the shield wire of the shaft encoder connection cable fell off, and after reconnection, the positioning accuracy reached the original machine tool requirements.

There are often beginners asking why CNC machine tools need to return to the reference point?

Can't you go back to the reference point? Simply put, the purpose of the reference point is to create a unique coordinate system on the machine after each power-on. Because after the machine has finished turning off the power. The CNC system loses its memory for each coordinate position. After the power is turned back on, it is necessary to return each coordinate to a fixed position of the machine tool, that is, the zero point or the origin of the coordinate system, also called the reference point or the machine reference point. Referencing point operation will directly affect the normal operation of the CNC machine.

Example 3 The static geometric accuracy of the BTM-4000 CNC copy milling machine caused the X-axis to run unstable. The specific performance is that when the x axis stops at a certain position according to the instruction, it can't stop.

BTM-4000 is a CNC copy milling machine imported from Italy. The system adopts the Italian FEDIA CNCl0 system. The servo uses Siemens products.

After the machine has been in use for a period of time, the position lock of the X axis has drifted. When the Z axis is stopped at a certain position, the motion does not stop, and approximately ± 0 appears.

.0007m amplitude deviation. The frequency of this vibration is low, and it can be seen intuitively that the screw is rotating back and forth. In view of this situation, it is preliminarily concluded that this is not the self-oscillation of the control loop, and may be a fixed length (magnetic scale) and a dynamic ruler (

There is an error between the reading heads). After adjusting the fixed length and the movable ruler to match the gap, the situation has greatly improved, and then the static geometric accuracy of the machine tool has been adjusted to eliminate the fault.

Example 4 A horizontal machining center with SINU-MERIK 840D system with EXE grating measuring device. Alarm No. 114 appears during operation, accompanied by alarm No. 113.

From the cause of the alarm, due to the 114th alarm. Cause 113

No. alarm, the fault location is located in the position measuring device. There are two possibilities for alarm No. 114: one is cable disconnection or grounding; the other is signal loss. The former can be diagnosed by visual inspection and measurement. For the latter, the signal is mainly missed. If the amplitude of the sinusoidal signal output from the scale is reduced for some reason, in the signal processing process, the position of the zero crossing of the processed signal is affected, and the output pulse is squeezed together in severe cases. Causes loss. Because the signal generated by the photocell is proportional to the intensity of the light, the amplitude of the signal is reduced simply because the brightness of the source is degraded or the optical system is dirty. The scanning unit is taken out from the ruler, and after disassembly, it is seen that the surface of the lens under the bulb is frosted glass, indicating that the grating surface also has a mist, and the dirt is also present on the bulb and the photocell, and the dirt causes the light source to emit light. The falling and output signals are reduced and eliminated by cleaning the grating.

As long as the electronic components are not damaged, the probability of failure of the measuring device is small, so the general measuring device alarms, mainly due to signal loss, that is, "missing reading". The measurement signal is prone to loss in the process of generating the transformation. There is a problem with the detection component. Do not blindly disassemble it. For example, when the scale grating or the indicator grating has dirt, it should be carefully removed. Before cleaning, check the scale surface and the surrounding area for any hard objects such as chips. If it should be cleaned, wipe it with absorbent cotton and high-purity alcohol. Or generally wipe the cloth to avoid causing human error.