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Why does the CNC system produce vibration crawling

The oscillation failure of the machine tool usually occurs in the mechanical part and the feed servo system. There are many reasons for the oscillation. In addition to the inevitable transmission gap, elastic deformation, friction resistance and many other factors in the mechanical aspect, the influence of the relevant parameters of the servo system is also an important aspect. Servo system is divided into AC and DC. This article mainly discusses the oscillation of DC servo system due to the influence of parameters. Most CNC machine tools adopt a fully closed loop method.
There are roughly four reasons for the vibration of the servo system: a. Poor position loop and instability of the output voltage; b. Vibration caused by a poor speed loop; c. The servo system's adjustable positioner is too large and the voltage output is distorted; d. The screw clearance is too large. Distortion of the output parameters of these control loops or too large gaps in mechanical transmission devices are the main factors that cause vibration. They can all be optimized through the servo control system.
  How to eliminate oscillation?
1. Oscillation caused by closed-loop servo system: Some CNC servo systems use semi-closed loop devices, and the full-closed-loop servo system must adjust parameters under the premise that the local semi-closed loop system does not oscillate. Therefore, the two are similar. Discuss the parameter optimization method in the case of full closed loop.
2. Reduce the gain of the position loop: There are reference standard values ​​in the servo system, for example, the FANUC0-C series is 3000, and the Siemens 3 system is 1666. If oscillation occurs, the gain can be reduced appropriately, but not too much, because the stability of the system must be ensured. State error.
  3. Reduce the load inertia ratio: The load inertia ratio is generally set at about 70% of the parameter shown when vibration occurs. If the fault cannot be eliminated, it is not advisable to continue to reduce the parameter value.
4. Add proportional calculus (PID): the proportional calculus is a multi-function controller, which can not only effectively gain proportional gain to the current and voltage signals, but also adjust the output signal to lag into the leading problem. Oscillation failure is sometimes caused by The output current and voltage are lagging and leading, and the output current and voltage phase can be adjusted by PID at this time.
5. Adopt high-frequency suppression function: The above discussion is about the parameter optimization method for low-frequency oscillation, and sometimes the CNC system will generate high-frequency harmonics in the feedback signal due to some oscillation reasons on the machine, which makes the output torque not constant , Resulting in vibration. For this kind of high-frequency oscillation, a first-order low-pass filter link can be added to the speed loop, which is a torque filter.
   The speed command and the speed feedback signal are converted into a torque signal by the speed controller. The torque signal passes through the first-order filtering link to cut off the high frequency components, thereby obtaining an effective torque control signal. By adjusting the parameters, the frequency above 100 Hz generated by the machine can be cut off, so as to achieve the effect of eliminating high-frequency oscillation. In summary, the use of dual-position feedback enables the system to operate in both full-closed loop and semi-closed loop modes, which greatly increases the adjustment range of the system and also increases the adjustment parameters of the system. It can be seen from the time constant that the system can perform full closed loop error adjustment in the stopped state, and semi-closed loop adjustment in the transition state. Now take FANUC0-C as an example to briefly introduce the specific parameter adjustment process. First, set the parameter P8411# (DPFB) to 1, that is, to select the dual position feedback function; P8499 is the maximum amplitude of the position feedback, generally set to 0; P8478 (numerator) and P8479 (denominator) are the constant settings of the mid-position conversion link. It can be set according to requirements; P8480 is the parameter setting code of the first-order delay link, and its setting range is: 10~300mS, generally set to about 100mS; P8481 is the zero amplitude, generally 0, but it can be adjusted appropriately due to oscillation higher. The dual-position feedback function is a more flexible error correction method, which has a good parameter optimization and system stability function during system debugging.
   The oscillation phenomenon of the numerical control system has become a common problem of the numerical control full-closed loop system. When the system oscillates, it will cause the machine tool to have crawling and vibration failures, especially in the horizontal axis with a column and the axis of the rotating CNC table, the system has a higher frequency of oscillation. This problem has become one of the important factors affecting the normal use of CNC equipment.

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