Dc servo motor servo motor in nc feed servo system was widely used, it has good speed and torque characteristics, but its complex structure, high manufacturing costs, big volume, and motor brush is easy to wear and tear, commutator produce sparks, the capacity of the dc servo motor and using occasions restricted. Ac servo motor without brush and commutator and other structural defects; And the new type of power switch device, application-specific integrated circuit, the development of computer technology and the control algorithm and so on, to promote the development of ac drive circuit, makes the speed regulation characteristic of ac servo driver can adapt to the requirements of the nc machine tool feed servo system. Modern nc machine tools are tend to be driven by ac servo, ac servo drive to replace dc servo drive. 1. The structure of the ac servo motor ac motor with ac induction motor and ac synchronous motor. Ac induction motor has simple structure, large capacity, low prices, generally use background movement of the drive motor. Permanent magnet synchronous ac servo motor is used as the drive motor feed movement and its structure schematic is shown in figure 1. Motor consists of stator and rotor and detecting element. The stator by the folded plate, its appearance is a polygon, no base, so that is conducive to heat dissipation. Embedded in the stator tooth a logarithm of three-phase winding. Rotor by the folded plate, and in which is equipped with a permanent magnet, logarithmic and stator pole of logarithmic same. Permanent magnets are: alnico, ferrite and the rare earth permanent magnet ndfeb alloy, alloy with rare earth permanent magnet alloy performance is best. Detecting element with pulse encoder, also can use rotating transformer tachogenerator, used to detect corner position, displacement and rotating speed of the motor, in order to provide the absolute position of permanent magnet synchronous ac motor rotor position information, feedback and speed feedback amount. 2. Ac servo motor frequency control of ac motor speed n, very logarithmic p with ac power frequency f, the motor and the relationship between the transfer speed skating rate s (1)For asynchronous motor s≠ S = 0, 0, for synchronous motor. By type (1), change the power frequency f, the motor speed changes as a direct proportion of n and f. Motor stator winding of the electric potential of E = 4. 44 fwkwΦIf you omit the stator impedance voltage drop, the stator phase voltage U≈ E = 4。 44 fwkwΦOn type, kw is constant, if the phase voltage U is changeless, then with the increase of frequency f, the air gap flux & Phi; Will decrease. And can be seen from the torque equation, & Phi; Value decreases, and the motor rotor induced current I2 also decreases accordingly, will inevitably lead to allow the output torque of the motor M down. In addition, if the phase voltage U the same, with the decrease of the f, air gap magnetic flux & Phi; Will increase, which will make the magnetic circuit saturation, excitation surge current rising cause iron loss, the power factor. So change frequency f's speed, need to change the stator phase voltage U at the same time, in order to maintain & Phi; Value is close to the same, so that M is nearly the same. Visible ac servo motor frequency control of motor speed is the key problem to obtain the frequency modulation of ac power voltage regulator. There are many kinds of FM source. Usually the communication -Dc -Communication transform circuit, the circuit of three phase current is the main part of inverter. As shown in figure 2 is the most widely used type voltage power transistor (GTR)Three-phase inverter main circuit principle diagram. By the ac -Diode rectifier circuit dc transform to obtain a constant dc voltage Ud, power transistor switching element T1, T4, T3, T6, T5, T2 of three-phase PWM inverter, capacitance C is trying to maintain the input dc voltage inverter Ud as a constant value, therefore, this line is called the voltage type inverter. Inverter switching element T1, T2, T3 is controlled by the triangular wave 1 and generated according to the requirements of speed regulation control has a certain frequency and voltage amplitude of the sine wave 2, through the comparison of the wave 1 and 2 to generate continuous, 3, isometric and wide range of rectangular pulse as control the on-off control signals. Thus won three groups in the output of the inverter with 3 similar rectangular pulse waveform, the waveform in the drive motor, its action is equivalent to 4 three-phase sine voltage. From the above discussion, the inverter is the key to realize frequency conversion regulating inverter control end achieve the required control waveform 3. The realization of waveform control methods (The motor speed control mode), now widely adopted by the vector transformation control. Figure 3 is an instance of the ac servo control system diagram, the system consists of two parts, power converter and control platform. Power converter and consisting of rectifier and inverter, the role of the rectifier is the input of three-phase alternating current into direct current (dc), as shown in figure 3 upper left; Inverter is to direct current (dc) into required according to the requirement of the control signal of three-phase alternating current (ac), now often USES the new type of high performance inverter switching frequency high power module IPM, as shown in figure 3 upper. The controller platform on the hardware of the scheme of DSP + FPGA as shown in figure 3 as shown in the lower part. The FPGA (Field programmable gate array)Devices and DSP (Digital signal processor)'s main function is, together with the software implementation of all control the task scheduling, the processing of input and output signal, the inverter control signal generation, and other control functions, etc. Single-chip microcomputer AT89C52 to realize the digital tube display, keyboard,Used for debugging and parameter Settings)As well as the management of the serial port. Limited space, detailed function of each module, here no longer is discussed in detail.
