Oscillation failures of machine tools generally occur in the mechanical part and the feed servo system. There are many causes of oscillation. In addition to the inevitable mechanical play of the transmission, elastic deformation, friction resistance and many other factors, the influence of the relevant parameters of the servo system is also an important aspect. Servo systems are divided into AC servo systems and DC servo systems. This article mainly deals with the oscillation of DC servo system due to the influence of parameters. Most CNC machine tools use a fully enclosed approach.
There are approximately four reasons for servo system vibration: a. Bad position loop and unstable output voltage; c. The adjustable positioner of the servo system is too large and the voltage output is distorted; Excessive screw play. Distortion of the output parameters of these control loops or excessive play of mechanical transmission devices are the main factors causing vibrations. They can all be optimized via servo control systems.
How to eliminate oscillations?
1. Oscillation caused by a closed loop servo system: Some CNC servo systems use semi-closed loop devices. Full closed-loop servo systems must adjust parameters assuming that the local semi-closed-loop system does not oscillate. So the two are similar. Discuss the method of optimizing parameters in the case of a fully closed loop.
2. Reduce the position loop gain: There are reference standard values in the servo system, such as 3000 for FANUC0-C series and 1666 for Siemens 3 series. In case of oscillation, the gain can be reduced appropriately, but not too much, because the stability of the system must be ensured. Status error.
3. Reduce the load inertia rate: The load inertia rate is usually set to about 70% of the displayed setting when vibration occurs. If the fault cannot be eliminated, it is not recommended to continue reducing the parameter value.
4. Add proportional calculation (PID): Proportional calculation is a multi-functional controller that can not only effectively obtain proportional gain for current and voltage signals, but also adjust the output signal to lag by relation to the thread. Oscillation faults are sometimes caused by the delay and lead of the output current and voltage. At this point, the output current and voltage phases can be adjusted via PID.
5. Use high frequency suppression function: The above discussion is the method of optimizing low frequency oscillation parameters. Sometimes the CNC system generates high frequency harmonics in the feedback signal due to some oscillation reasons on the machine tool, which makes the CNC system perform better. unstable output torque and causes vibration. For this high-frequency oscillation, a first-order low-pass filter link, i.e., a torque filter, can be added to the velocity loop.
The speed command and speed feedback signals are converted into torque signals by the speed controller. The torque signal passes through the first-order filter link to filter out 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, eliminating high frequency oscillations. To summarize, the use of dual position feedback allows the system to operate in fully closed loop and semi-closed loop modes, which greatly increases the system tuning range and also increases the system tuning parameters.
The time constant shows that the system can perform full closed-loop error adjustment in the stopped state and half-closed-loop adjustment in the transition state. Now, taking FANUC0-C as an example, we will briefly introduce the specific parameter setting process. First, set parameter P8411# (DPFB) to 1, that is, select the dual position feedback function; P8499 is the maximum position feedback amplitude, usually set to 0 and P8479 (denominator) are constant parameters for the ; intermediate position conversion link. It can be adjusted as needed; P8480 is the parameter setting code of the first-order delay link, its setting range is: 10~300mS, usually set to about 100mS, the amplitude is zero, usually 0, but due to the strong oscillation; , this may be an appropriate adjustment. The dual position feedback function is a more flexible error correction method.
The oscillation phenomenon of CNC systems has become a common problem in fully closed-loop CNC systems. When the system oscillates, it will cause crawling and vibration failures in the machine tool. Especially on the horizontal axis with the columns and axis of the rotating CNC worktable, the oscillation frequency of the system is high. This problem has become one of the important factors affecting the normal use of CNC equipment.
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