In principle, the servo system of CNC machine tools should include all parts from the position command pulse to the actual position output, including position control, speed control, drive motor, detection components, mechanical transmission components and so on. However, in many systems, in order to facilitate manufacturing, the position control part of the servo system is usually integrated with the CNC device. Therefore, the servo feed system of the machine tool is generally referred to as the speed control of the servo feed system. Unit, servo motor, detection component part, not including position control part.
The servo feed system used in CNC machine tools can be divided into open loop control, closed loop control, semi closed loop control, and hybrid control according to the structure of the control system. According to the type of servo motor used in the servo feed system, the servo feed system commonly used in semi-closed loop and closed-loop CNC machine tools can be divided into DC servo drive and AC servo drive. In the CNC machine tools from the 1970s to the 1980s, DC servo drives were generally used; from the middle and late 1980s, AC servo drives were used on CNC machine tools.
DC servo drive system is generally divided into "thyristor speed control mode" (referred to as SCR speed control system) and "transistor pulse width modulation speed control mode" (referred to as PWM speed control system) according to the power amplification component type adopted by its main circuit. ) two categories. Analog control or digital control can be used for control, and therefore, in some cases, it can be divided into analog and digital types.
AC servo system generally adopts PWM modulation signal to control the main circuit of power transistor for driving amplification, and is divided into analog servo and digital servo according to its command signal and control form. The initial AC servo system is generally an analog servo system, and the AC servo system currently used is usually a digitally controlled all-digital AC servo system.
In the years of maintenance work, I have dealt with many faults related to the servo system of CNC machine tools. I have summarized the common faults of the feed servo system. Common feed servo faults are as follows:
1, overtravel
When the feed motion exceeds the software limit or the hardware limit switch position set by the software, an overtravel alarm will occur, and the alarm content will be displayed on the display of the CNC system. According to the manual of the CNC system and the electrical schematic diagram, You can eliminate and cancel the alarm. Note: If an over-travel alarm occurs when one of the axes of the machine is not moved to the end position, it is usually because the limit switch is broken or the limit switch is stuck when the machine is running.
2, overload
Usually, when the load of the feed motion is too large, frequent positive and negative movements, poor lubrication of the transmission chain, or damage to the inclined iron, grounding of the motor power line, etc., the servo motor current is high, the motor temperature is too high or the motor Overload alarm. Sometimes the drive control unit, drive components, and the motor itself may malfunction during the operation of the machine. Alarm information such as servo motor overload, overheating or overcurrent is usually displayed on the display of the CNC system. At the same time, on the feed drive unit in the power cabinet, the indicator light or the digital tube will prompt the drive unit to overload, over current and other information.
The model of the large equipment manufacturing plant is TK42160C numerically controlled boring machine, which adopts the numerical control system of SIEMENS 840D. The X axis has caused the overload alarm due to the grounding of the servo motor power line in the drag chain. After replacing the new motor power cable, the machine tool is normal.
3, crawling
When it occurs in the starting acceleration section or low-speed feed, it is generally caused by factors such as poor lubrication state of the feed transmission chain, low servo system gain, and excessive load. In particular, the coupling between the servo motor and the ball screw is not synchronized with the rotation of the servo motor due to loose coupling or defects in the coupling itself, such as cracks, so that the feed is made. The movement is slow and slow, causing creep.
The model of the large-scale equipment manufacturing branch is 2A637гФ1 CNC boring machine. The X-axis has a phenomenon of low-speed feed instability (ie crawling phenomenon). After inspection, it was found that the X-axis oblique iron was slightly damaged due to the slippery oil. After treatment, The X-axis low-speed feed is normal.
4, vibration
During feed (especially at low speed), the vibration phenomenon of a certain axis of the machine tool is usually caused by the instability of the speed measurement signal, such as the speed measuring device failure, the speed feedback signal interference, etc.; the speed control signal is unstable or interfered; the terminal is in poor contact, such as Loose screws, etc. When the vibration occurs in the commutation moment of the forward direction and the reverse motion, it is generally caused by the backlash of the feed chain or the servo system gain being too large. When the machine is running at high speed, vibration may occur and an overcurrent alarm will occur. Machine vibration problems are generally speed problems, so you should look for the speed loop; and the entire adjustment process of the machine speed is done by the speed regulator, that is, any speed-related problems, you should look for the speed regulator, so the vibration problem Look for the speed regulator. Mainly from the given signal, feedback signal and speed regulator itself to find faults.
5, the servo motor does not turn
In addition to the speed control signal, the numerical control system to the feed drive unit also has an enable control signal, which is generally DC+24V DC voltage. The servo motor does not rotate. Commonly used diagnostic methods are: check whether the CNC system has a speed control signal output; check whether the enable signal is turned on. Observe the I/O status through the display of the numerical control system, analyze the plc ladder diagram (or flow chart) of the machine tool to determine whether the starting condition of the feed axis, such as lubrication and cooling, is satisfied; for the servo motor with electromagnetic brake, check it. Whether the electromagnetic brake is released; check if the feed drive unit is faulty and the servo motor is faulty.
The large-scale equipment manufacturing plant used the SIEMENS 840D numerical control system model XK2425/2 Wuhan CNC gantry milling machine had a failure of the Z-axis servo motor. The fault phenomenon is that the Z axis does not move regardless of the positive and negative Z axis (Z axis motor does not rotate), but there is no alarm. By observing the "Diagnostics" - "Service Display" - "Drive Adjustment" menu displayed on the display of the CNC system, the pulse enable signal of the axis is found to be absent. The enable signal of the shaft is given by the control board of the drive unit of the shaft, so it is preliminarily determined that the control panel of the shaft is faulty or the control board is disconnected from the enablement signal of the numerical control unit. First, replace the Z-axis drive control board with the Y-axis drive control board, and find that the Z-axis is normal at this time, and the same failure phenomenon as the Z-axis occurs when the Y-axis is opened. The fault can be determined that the Z-axis drive control board is faulty, and the Z-axis is replaced after replacing the new drive control board.
6, position error
When the servo axis moves beyond the position tolerance range, the numerical control system will generate an alarm with excessive position error, including following error, contour error and positioning error. The main reasons are: the allowable range of the system setting is small; the gain of the servo system is improperly set; the position detection device is contaminated or improperly adjusted; the cumulative error of the feed drive chain is too large: the balance device (such as the balance hydraulic cylinder) when the spindle box moves vertically Unstable.
7, drift
Drift means that when the given command value is zero, the axis still moves. It is eliminated by the CNC system error compensation and the zero speed adjustment of the drive unit.
8. Clearance and looseness of mechanical transmission components
In the feed chain of CNC machine tools, the transmission is often damaged due to the gap between the keyway and the key of the transmission element. Therefore, in addition to careful selection of the key coupling mechanism during design, the machining and assembly must be strictly investigated. The preload of the bearing should be checked when assembling the ball screw to prevent axial movement of the ball screw, as clearance is another cause of significant drive clearance.
The feed system used in our CNC machine tools is divided into two categories: DC speed control system and AC servo system. At present, the AC servo system is widely used, mainly including AC servo systems produced by FAGOR, SIEMENS, NUM, Yaskawa and other manufacturers. For different servo systems, although the display status and alarm number of the system are different when it is faulty, the working principle is basically the same, and the maintenance methods have many similarities. As a equipment maintenance worker, you should learn more, read more, and often summarize, so that scientific methods and effective measures can be used to quickly identify the cause of the failure, solve the problems at any time, and ensure that the CNC machine tools are safe and reliable. Run to increase device usage.
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