Continuing with the content of the previous chapter, we will continue to introduce cnc milling related technologies today
Eight. Feed servo axis control
The feed of the machine tool table (including the turntable) is driven by a servo mechanism, which is currently electrified and driven by a servo motor, and most of them use a synchronous motor. The motor is directly connected to the ball screw (as shown in the figure below), so that due to the short transmission chain, the loss of motion (lost motion) is small, and the response is fast, so high precision can be obtained.
The feed servo of the machine tool belongs to the position control servo system. As shown in the figure below, the input terminal receives the position pulse serially output from the CNC interpolator in each interpolation cycle. The number of pulses indicates the amount of movement of the position (usually a pulse is 1 μm---that is, the resolution of the system is 1 μm); the frequency of the pulse (that is, the number of pulses output per unit time) indicates the speed of feed ; The symbol of the pulse indicates the feed direction of the axis, usually the pulse is directly sent to the command input ports of different servo axes.
The figure below only shows one feed axis. The actual machine tool has several axes, but the control principle is the same. When several axes receive interpolation commands in the same interpolation period, due to the different feed amounts, feed speeds, and motion directions at the same time, the resulting motion is a curve, and the tool moves according to this curve. The workpiece contour required by the program can be processed.
The requirements for feed servo are not only static characteristics, such as: positioning accuracy and stability when stopping. More importantly, the feed servo has good rigidity, fast response, good motion stability and high resolution. In this way, high-quality workpieces with smooth surfaces can be processed at high speed and high precision.
1. The structure type of the servo system
The servo system is divided into open-loop and closed-loop structures.
open loop:
The so-called open loop is a servo system without position feedback. The electrical system of this structure is driven by a stepper motor. Since there is no feedback of speed and position, the following accuracy is poor and the responsiveness is poor, so the machining accuracy is poor and the efficiency is low.
closed loop:
A closed loop is a servo system with position feedback of the controlled element. The composition of the system includes: executive element------servo motor (generally directly connected with the ball screw); speed controller and position controller, the position controller receives the output command of the CNC interpolator
2. Synchronous motor
The electrical system of the closed-loop servo structure is currently driven by an AC servo motor, and most of them use a permanent magnet synchronous motor.
The structure of the permanent magnet synchronous motor is shown in the figure below. The rotor is a magnetic pole made of permanent magnetic steel with high magnetic permeability, with a motor shaft in the middle, and the two ends of the shaft are supported by bearings and fixed on the casing. The stator is a magnetic conductor made of silicon steel sheets. The inner surface of the magnetic conductor has tooth grooves, and the three-phase winding coils wound with wires are embedded. In addition, an encoder is installed at the rear end of the shaft.
When the three-phase winding of the stator is supplied with three-phase alternating current, the generated space rotating magnetic field will attract the magnetic poles on the rotor to rotate synchronously. The speed control and electric power supply of synchronous motors use inverters. In the inverters, the power drive circuit elements that change from DC to three-phase AC need to commutate in real time according to the position of the rotor magnetic field, which is very similar to that of DC motors. The commutation of the rotor winding current with the position of the stator field. Therefore, in order to detect the position of the rotor magnetic field of the synchronous motor in real time, an encoder (optical code disc ------ 11 in the figure) is installed on the motor shaft (rear end). Thanks to the optical code disc, no matter the speed of the motor is fast or slow, the position of the magnetic pole magnetic field on the rotor can be actually measured along with the rotation of the motor shaft, and the position value can be sent to the control circuit so that the controller can The commutation of the power components of the inverter is controlled in real time, and the self-controlled commutation of the servo drive is realized. Therefore, some people call the drive controller and motor of this synchronous motor a self-commutating synchronous motor; in addition, because its control characteristics are similar to a DC motor, it is also called a commutator-free DC motor.
Linear motor: In order to increase the movement speed of the worktable, increase the acceleration, simplify the transmission chain, and thereby improve the transmission accuracy, a linear motor has recently appeared. This kind of motor is a direct-connected motor, that is, it is directly installed on the linear motion table.
3. Position sensor and speed sensor
Photoelectric encoder: The encoder is a measuring element for rotary motion, usually installed on the motor shaft or ball screw, and the physical quantity it directly measures is the angle at which the motor or screw rotates. There are two types of encoders: incremental measurement or absolute measurement.
Linear encoders: Currently, linear encoders are used to measure the position or displacement of linear moving parts. There are transmission scales using glass and reflection scales using metal substrates. The working principle is similar to that of a photoelectric encoder. The transmissive grating ruler is easy to install and is directly mounted on the side of the workbench, so it is used more often.
4. Servo drive
The amplifier (driver) that controls the operation of the synchronous servo motor is an AC inverter.
FANUC divides the amplifier into two modules: rectifier power supply module (PSM) and servo inverter module (SVM)
Nine. Spindle drive control
1. control block diagram
Spindle control is mainly speed and motor speed control. In the program, use the command: S and the five-digit value to command the rotation number of the spindle.
For example: S1200; means that the spindle is required to rotate at 1200 rpm. The commands for forward and reverse rotation are M03 (forward rotation); M04 (reverse rotation). In order to detect the rotation speed of the spindle, a speed sensor is installed on the spindle or the spindle motor.
2. Spindle speed sensor and position sensor
Only speed control without position feedback loop. The speed measurement and feedback of the spindle motor uses a magnetic sensor mounted on the spindle motor shaft. As shown below. As the spindle motor turns, the sensor sends out 128, 256, 384 or 512 pulses (depending on the model of the motor) per revolution to count the number of revolutions of the spindle motor. If the motor and the main shaft are not 1:1 coupled, a position encoder must be installed on the main shaft, and the number of rotations of the main shaft is measured with a one-turn signal from the encoder. Usually this kind of encoder is photoelectric, and it sends out 1024 pulses per turn, and also sends out a one-turn signal. This encoder can be used to realize thread processing and rigid tapping and spindle orientation during tool change of machining centers
3. Spindle motor
There are two types of motors used for spindle drive. Type: asynchronous motor and synchronous motor. The asynchronous machine is easy to manufacture, has high reliability, and has good high-speed operation performance, so FANUC uses an asynchronous machine. The synchronous machine has good low-speed performance, good controllability, high torque at low speed, and a wide range of constant power speed regulation is easy to achieve. Generally, asynchronous motors are generally used for processing aluminum and light metal parts with high spindle speed. The spindle speed for processing cast iron or steel parts is low, and some processing methods (such as Cs axis method) also require large torque at low speeds, so synchronous motors tend to be used. Especially recently, in order to improve the machining accuracy, the mechanical design makes the drive motor directly connected to the main shaft of the machine tool, so the development of synchronous machine as the drive of the main shaft, especially the built-in main shaft motor, often uses the synchronous machine. If you need more learning materials, the 373600976 group can help you
4. Spindle Motor Drive Amplifier
The control of the spindle motor is similar to the servo motor described above. But usually there is only speed control, so there is no need for a position loop. The figure below is a block diagram of the FANUC spindle motor driver. Divided into two modules: PSM and SPM. PSM is a power supply module, which, like the feed servo drive, converts the input AC power supply into a DC power supply to supply power to the inverter. SPM is the inverter part, which converts the DC power into three-phase AC to supply power to the stator of the motor.
ten. Control of external axes
Use servo motors to control the actions of auxiliary mechanical equipment on or outside the machine tool, such as: tool changing manipulator, changing worktable, loading/unloading, workpiece or blank handling.
Power Mate i is a position motion controller, which controls the independent movement of each axis or coordinated movement in time, so that an axis moves to a certain position or a certain distance at a certain speed. However, there is no positional dependency between them, that is, the system does not need to have the function of a positional interpolator. Of course, FANUC's Power Mate i D has a two-axis linkage interpolator, which can be used according to actual needs.
Dongguan Leda Metal Co.,Limited
