With the rapid development of modern motor technology, modern power electronics technology, microelectronics technology, permanent magnet material technology, AC adjustable speed regulation technology and control technology, permanent magnet AC servo technology has great development. The performance of permanent magnet AC servo system is improving day by day, and the price tends to be reasonable, which makes permanent magnet AC servo system replace DC servo system, especially in the field of high precision, high performance requirements of servo drive has become a development trend of modern electric servo drive system.
Permanent magnet AC servo system has the following advantages:
Motor no brush and commutator, reliable work, simple maintenance and maintenance;
Stator winding heat dissipation fast;
Small inertia, easy to improve the rapidity of the system;
Suitable for high speed and large torque working state;
Under the same power, smaller volume and weight, widely used in machine tools, mechanical equipment, handling mechanism, printing equipment, assembly robot, processing machinery, high-speed winding machine, textile machinery and other occasions, to meet the development needs of the transmission field.
After the development of analog and hybrid mode, the driver of permanent magnet AC servo system has entered the digital era. The full digital servo drive not only overcomes the large dispersion, zero drift, low reliability and other determinations of analog servo, but also gives full play to the advantages of digital control in control precision and flexible control method, making the servo drive not only simple structure, but also more reliable performance. Now, the high performance servo system, most of the permanent magnet AC servo system including permanent magnet synchronous AC servo motor and full digital AC permanent magnet synchronous servo driver two parts.
The servo drive consists of two parts: the drive hardware and the control algorithm. Control algorithm is one of the key technologies to determine the performance of AC servo system, which is the main part of foreign AC servo technology blockade and the core of technology monopoly.
The basic structure of AC permanent magnet servo system
Ac permanent magnet synchronous servo driver is mainly composed of servo control unit, power drive unit, communication interface unit, servo motor and corresponding feedback detection device. Its structure is shown in Figure 1. The servo control unit includes position controller, speed controller, torque and current controller and so on. Our AC permanent magnet synchronous driver integrates advanced control technology and control strategy, so that it is very suitable for high precision, high performance requirements of the servo drive field, but also reflects the powerful intelligence, flexibility is incomparable to the traditional drive system.
At present, the mainstream servo driver adopts digital signal processor (dsp) as the control core. Its advantage is that it can realize more complex control algorithm, and matters are digitized, networked and intelligent. Power devices generally use intelligent power module (ipm) as the core design of the drive circuit, ipm internal integrated drive circuit, and has overvoltage, overcurrent, overheating, undervoltage and other fault detection protection circuit, in the main circuit also added soft start circuit, in order to reduce the impact of the start process on the driver.
Servo driver can be divided into two modules, power board and control board. As shown in Figure 2, the power plate (drive plate) is a strong electric department, which includes two units. One is the power drive unit ipm used to drive the motor, and the other is the switching power supply unit to provide digital and analog power for the whole system.
The control board is the weak current part, the control core of the motor and the running carrier of the core control algorithm of the servo driver technology. The control board outputs pwm signal through the corresponding algorithm, which is used as the driving signal of the driving circuit to change the output power of the inverter, so as to achieve the purpose of controlling the three-phase permanent magnet synchronous AC servo motor.
Power drive unit
The power driving unit first rectifies the input three-phase or mains power through the three-phase full-bridge rectifier circuit to obtain the corresponding direct current. The three-phase permanent magnet synchronous AC servo motor is driven by the three-phase sinusoidal pwm voltage frequency converter after good rectification. The whole process of power drive unit can be simply described as ac-dc-ac process. The main topological circuit of ac-dc is the three - phase full - bridge uncontrolled rectifier circuit.
The inverter part (dc-ac) adopts the intelligent power module (ipm) which integrates the drive circuit, the protection circuit and the power switch. The main topology is the three-phase inverter circuit schematic diagram shown in Figure 3. Using the technique of pulse width modulation (pwm), the pulse width modulation (PWM) changes the frequency of the output waveform of the inverter by changing the alternating on-off time of the power transistor, and changes the on-off time ratio of the transistor in every half cycle. That is to say, by changing the pulse width to change the inverter output voltage auxiliary value to achieve the purpose of power regulation.
vt1 ~ vt6 in Figure 3 are six power switch tubes, s1, s2 and s3 respectively represent three bridge arms. The switch state of each bridge arm is specified as follows: when the switch tube of the upper bridge arm is in the "on" state (the switch tube of the lower bridge arm must be in the "off" state at this time), the switch state is 1; When the lower bridge arm switch tube is in the "on" state (then the lower bridge arm switch tube must be in the "off" state), the switch state is 0. The three bridge arms only have two states of "0" and "1", so s1, s2 and s3 form eight switching tube modes of 000, 001, 010, 011, 100, 101 and 111, among which the 000 and 111 switching modes make the inverter output voltage zero, so this switching mode is called zero state. The output line voltage is uab, ubc and uca, and the phase voltage is ua, ub and uc, where udc is the DC power supply voltage. The attached table analysis can be obtained according to the above.
Control unit
The control unit is the core of the whole AC servo system, realizing the system position control, speed control, torque and current controller. The digital signal processor (dsp) not only has fast data processing capability, but also integrates rich ASIC for motor control, such as a/d converter, pwm generator, timing counter circuit, asynchronous communication circuit, can bus transceiver and high speed programmable static ram and large capacity program memory. The servo driver realizes vector control (vc) by adopting the control principle of magnetic field orientation (foc) and coordinate transformation, and controls the motor by combining sinusoidal pulse width modulation (spwm) control mode. The vector control of permanent magnet synchronous motor generally controls the stator current or voltage by detecting or estimating the position and amplitude of the rotor flux of the motor. In this way, the torque of the motor is only related to the flux and current, which is similar to the control method of the DC motor and can obtain high control performance. For permanent magnet synchronous motor, the flux position of the rotor is the same as the mechanical position of the rotor. In this way, the flux position of the rotor of the motor can be known by detecting the actual position of the rotor, so that the vector control of permanent magnet synchronous motor is simplified compared with that of the asynchronous motor.
Servo driver controlled AC permanent magnet Servo Motor (pmsm)
When the servo driver controls the AC permanent magnet servo motor, it can work under the current (torque), speed and position control mode respectively. The control structure block diagram of the system is shown in Figure 4. Since AC permanent magnet servo motor (pmsm) uses permanent magnet excitation, its magnetic field can be regarded as constant. At the same time, the motor speed of AC permanent magnet servo motor is synchronous speed, that is, its revolution is zero. These conditions greatly reduce the complexity of the mathematical model of the AC servo driver driving the AC permanent magnet servo motor. As can be seen from Figure 4, the system is based on measuring the two-phase current feedback (ia, ib) of the motor and the motor position. By combining the measured phase current (ia, ib) with the position information, the id and iq components were obtained through the change of coordinates (from the coordinate system a, b, c to the rotor coordinate system d, q), and then entered into their respective current regulators. The output of the current regulator goes through the reverse coordinate change (from d, q coordinate system to a, b, c coordinate system) to obtain the three-phase voltage instruction. The control chip, through the three-phase voltage instruction, after reverse and delay, gets 6 pwm waves output to the power device to control the motor operation. In the system under different instruction input mode, instruction and feedback through the corresponding control regulator, get the next level of reference instruction. In the current loop, the torque current component (iq) of the d, q axes is the output or external given by the speed control regulator. In general, the flux component is zero (id=0), but when the velocity is greater than the limit value, a higher velocity value can be obtained through magnetic weakening (id "0").
The transformation from a, b, c coordinate system to d, q coordinate system is realized by clarke and park transformation; The transformation from d q to a, b, c coordinates is realized with the contravariant transformation of Clark and Parker.