DC servo motor has been widely used in CNC feed servo system

Posted by airconditioning on February 20th, 2020

DC servo motor has been widely used in CNC feed servo system. It has good speed regulation and torque characteristics, but it has a complicated structure, high manufacturing cost and large volume. Moreover, the brushes of the motor are easy to wear and change direction. The generator will generate sparks, which will limit the capacity and application of the DC servo motor. AC servo motors do not have structural shortcomings such as brushes and commutators; and with the development of new power switching devices, ASICs, computer technology and control algorithms, the development of AC drive circuits has been promoted, making AC servo driven The speed regulation characteristics can better meet the requirements of the feed servo system of CNC machine tools. Modern CNC machine tools tend to use AC servo drive. AC servo drive has a tendency to replace DC servo drive. 1. Structure of AC servo motor AC motor is divided into AC induction motor and AC synchronous motor. The AC induction motor has a simple structure, large capacity, and low price, and is generally used as a driving motor for main motion. Permanent magnet synchronous AC servo motor is used as the driving motor for feed motion, and its structure is shown in Figure 1. The motor consists of a stator, a rotor, and a detection element. The stator is formed by stacking punching sheets, and its shape is polygonal, without a base, which is good for heat dissipation. A three-phase winding with a certain number of poles is embedded in the stator cogging. The rotor is also formed by stacking punched sheets, and a permanent magnet is installed in the rotor. The number of pole pairs is the same as that of the stator. Permanent magnets include aluminum-nickel-cobalt alloys, iron-oxygen alloys, and neodymium-iron-boron alloys, which are rare earth permanent magnet alloys. The performance of rare earth permanent magnet alloys is the best. The detection components are generally pulse encoders, or rotary transformers and speed measuring generators can be used to detect the angular position, displacement and rotational speed of the motor, so as to provide absolute position information, position feedback and speed feedback of the rotor of the permanent magnet AC synchronous motor. the amount. 2.

The relationship between the rotation speed n of the AC servo motor's variable frequency speed-regulated AC motor and the AC power frequency f, the number of motor pole pairs p, and the speed slip ratio s is (1) for asynchronous motor s \u0026 ne; 0, for synchronous motor Then s = 0. It can be known from formula (1) that when the frequency f of the power source is changed, the rotation speed n of the motor changes in proportion to f. The back-EMF of the stator winding of the motor is E = 4.44fWkw \u0026 Phi; If the impedance voltage drop of the stator is omitted, the stator phase voltage U \u0026 asymp; E = 4.44fWkw \u0026 Phi; The above formula shows that kw is constant. As the frequency f increases, the air gap magnetic flux \u0026 Phi; will decrease. It can also be seen from the torque formula ， that the value of \u0026 Phi; decreases and the induced current I2 of the motor rotor decreases accordingly, which will inevitably lead to the reduction of the allowable output torque M of the motor. In addition, if the phase voltage U is constant, as f decreases, the air gap magnetic flux \u0026 Phi; will increase, which will saturate the magnetic circuit, increase the excitation current and cause a sharp increase in iron loss, and the power factor will decrease. Therefore, when changing the frequency f for speed regulation, the phase voltage U of the stator needs to be changed at the same time to maintain the value of \u0026 Phi; close to constant, so that M is also close to constant. It can be seen that the key problem of frequency conversion and speed regulation of AC servo motor is to obtain AC power with frequency and voltage regulation.There are many types of FM voltage regulators. The AC-DC-AC conversion circuit is usually used to achieve this.

The main component of this circuit is a three-phase current inverter. Figure 2 shows the principle circuit diagram of the most widely used voltage type power transistor (GTR) three-phase inverter. The AC-DC conversion diode rectifier circuit obtains a constant DC voltage Ud. The power transistor switching elements T1, T4, T3, T6, T5, and T2 form a three-phase PWM inverter. The capacitor C tries to maintain the inverter input. The DC voltage Ud is constant, so this line is called a voltage inverter. Inverter switching elements T1, T2, and T3 are controlled by triangle wave 1 and sine wave 2 with a certain frequency and voltage amplitude generated according to the speed control control requirements. The waveforms 1 and 2 are used to generate equal-amplitude and equal-distance signals. The rectangular pulses 3 of unequal width are used as control signals to control their on and off. As a result, three sets of rectangular pulses similar to the control waveform 3 are obtained at the output of the inverter. This waveform is equivalent High-end waterproof motor factory to the three-phase sinusoidal voltage 4 when driving the motor. As can be seen from the above discussion, the key to the frequency conversion and voltage regulation of the inverter is to obtain the required control waveform 3 from the inverter control end. The realization of the control waveform (that is, the motor speed control method) is now widely adopted as vector transformation control. Figure 3 is an example of the schematic diagram of an AC servo speed control system. The system consists of a power converter and a control platform. The power converter is composed of a rectifier and an inverter. The role of the rectifier is to convert the input three-phase AC power into DC power, as shown in the upper left part of Figure 3. The inverter converts the DC power into the DC power according to the requirements of the control signal. For the required three-phase AC power, high-performance inverters often use new IPM power modules with higher switching frequencies, as shown in the upper right of Figure 3. The controller platform adopts the DSP + FPGA solution on the hardware, as shown in the lower part of Figure 3. The main function of FPGA (field programmable gate array) device and DSP (digital signal processor) is to work with software to realize the scheduling of all control tasks, the processing of input and output signals, the generation of inverter control signals, and other controls. Features, etc. The single-chip microcomputer AT89C52 realizes the management of the display digital tube, keyboard (for debugging and parameter setting) and serial port. Due to space limitations, the detailed role of each module will not be discussed in detail here.