A fully digital slip vector control system supported by IGBT and MCS96 series CPU chips is designed. The system has many remarkable advantages, such as high precision, wide speed range, fast dynamic response, remarkable energy-saving effect, high reliability, small harmonic current, little pollution to the power grid, safety and reliability, and complete protection functions. It is not only used for spindle servo drive of CNC machine tools, but also widely used for speed control of common machine tools, fans, pumps and other transmission machinery.
2 control strategy
Vector transformation control is developed on the basis of motor unification theory, electromechanical energy conversion and coordinate transformation theory, and has the characteristics of advanced, novel and practical. Vector control is to control the AC motor by simulating it as a DC motor. The stator current vector of AC motor is decomposed into two DC components oriented according to the rotor magnetic field by coordinate transformation, and these two components are controlled. The mathematical model of asynchronous motor based on vector control is the voltage equation of asynchronous motor oriented by rotor magnetic field [3]:
Based on this, the basic equation of vector control can be deduced as follows
M, t coordinates rotate with synchronous angular velocity ωs, m axis coincides with rotor flux ψ r, θM is rotor flux orientation angle, which changes with time, that is.
After coordinate transformation, the stator three-phase currents iA, iB and iC get two current components iM and iT that rotate synchronously in m and t coordinates. The conversion relationship between them is as follows
According to the above principle, the block diagram of vector control system is shown in figure 1.
In the figure, the inverter adopts current-tracking PWM inverter, the given excitation φ*r is obtained by ωr/φr function generator, the given torque T*m is given by speed regulator St, the vector controller calculates the given excitation current i*T and the given slip angle frequency ω*f according to φ*r and t * m, and the ω* f is added with the motor speed ωr to obtain the stator angular velocity ωs, and then the phase signal θM of rotor flux linkage is obtained by integration.
Three-phase currents iA, iB and iC are converted by vector converters to obtain field-oriented current components iM and iT. IM and iT are compared with i*M and i*T respectively, and their deviations Δ im and Δ it are output by the three-stage hysteretic controller. DM, dT and θM form a data word, and the corresponding voltage vector is selected through the switch control table, and a group of switching pulses s a, SB and SC are generated at the same time, so that the inverter can be controlled timely and accurately, and excellent speed regulation performance can be obtained.
3 system hardware design
The inverter adopts AC -DC- AC voltage structure and SPWM flux vector control mode, and the main loop is mainly composed of rectifier circuit, filter circuit and inverter circuit. The inverter circuit consists of IGBT module, and the control part takes 80C 196 dual CPU as the core, which constitutes a fully functional digital slip vector control system. The system adopts a general modular structure, and all the hardware is as follows:
The system hardware block diagram is as follows:
CPU 1# mainly completes the work of speed loop, and completes the functions of speed detection, A/D sampling at a given speed, keyboard input, parameter modification, status display, protection function and fault self-diagnosis. The most important task of CPU 1# is to realize digital speed regulator and slip regulator, and provide instruction signals i*M, i*T and ω*f to CPU2#.
CPU2# mainly completes the work of current loop, obtains the three-phase current and voltage signal values through 12 bit A/D, and then obtains the important system parameters such as the instruction values provided by CPU 1# from * * shared RAM. According to the principle of vector transformation, vector transformation operation is carried out, which consists of torque angle generator, rotor flux position synthesizer, MT/ABC rotary converter, voltage vector optimizer, zero vector action time determiner and current hysteresis comparator to complete the main work of vector control. The output control voltage vector enters the interlocking drive signal circuit composed of 8255 and enters the IGBT-based drive circuit through the photoelectric coupler.
4 system software design
In this dual-computer system, CPU 1# mainly completes the adjustment of speed outer loop and provides PWM resources for CPU2#, while CPU2# mainly completes the adjustment of current inner loop, and both of them * * * enjoy the resources of 8 155 internal RAM. The real-time requirement of the system is very strong. In view of this, the system software uses the unsigned number of the original code in assembly language for operation, and is compiled in a modular way as follows:
Because the system controls the torque and speed of the motor by controlling the instantaneous position and amplitude of the stator current, the current following characteristic is the key to realize the scheme. By adopting advanced CPU2# chip and improving program design, the running speed of the system is greatly improved.
The three-phase current hysteresis comparison scheme is adopted here, that is, the given value of three-phase current is obtained by CPU2#, and the actual three-phase current is directly measured by three Hall elements, and the two are compared as the input of the current hysteresis comparator to obtain the output voltage vector control signal of each phase. In addition, it can also prevent the system operation error or even error caused by the uncertainty of the actual current value sampled in the A/D link. Each output signal controls the turn-on and turn-off of the single-phase bridge transistor, so that the current deviation of each phase is controlled within the hysteresis width. The smaller the hysteresis width, the higher the switching frequency and the closer the phase current is to sine wave. However, the frequency is also limited by the switching frequency limiting ability of the switching elements. In order to reduce the switching frequency as much as possible, the output voltage vector is optimized and the zero voltage vector is inserted flexibly. In actual operation, it can be seen that it effectively reduces the higher harmonic components in motor operation, obviously improves the current waveform, improves the stability of the system, and obviously reduces the heat loss of the main circuit. 5 protection circuit design
As we all know, the detection and protection circuit is the lifeline of the inverter, and a well-designed and fully functional detection and protection circuit has always been crucial. There are six kinds of signals detected by the control panel from the main loop, which are used to complete the vector control algorithm and various protection functions.
5. 1 current detection and overcurrent protection circuit
The current detection signal comes from the Hall element at the U and V two-phase output terminals of the inverter, and the Hall element obtains the 15V power supply through the socket CN2. U and V two-phase current detection signals are amplified by 20 times by primary operational amplifiers A6 and A5 and then sent to secondary operational amplifiers A8 and A7 (as shown in Figure 3).
The action value of overcurrent protection can be determined by adjusting the amplification factor of two-stage operational amplifier. U and V phase currents are superimposed by an inverting adder A9 to obtain a W phase current signal. U, V and W phase currents are sent to the positive and negative inputs of two comparators at the same time. The reference voltages of the positive and negative input terminals of the comparator are+10V and-10V respectively. When the three-phase current is normal, its corresponding voltage is between 10V, and the input of six comparators is 1. This signal is inverted by triode and sent to monostable trigger composed of multivibrator D4528. The output of -Q is 0, and the output signals of comparators A 17 and A 18 should also be 0, so the protection circuit does not operate. Once overcurrent occurs, the phase and output signal of the comparator are 0, and the input signal (pin 12) of D4528 is 1, and its output becomes 1 after monostable delay. After amplification by transistor N2, the driving signal of GTR is turned off, and the CPU is notified to send out an overcurrent alarm signal. The function of monostable trigger is to avoid the misoperation of protection circuit caused by some interference signals or instantaneous peak current.
5.2 Overvoltage and Undervoltage Protection Circuit
DC voltage detection collects signals from both ends of the intermediate DC loop (as shown in Figure 4). DC high voltage (about 600V) is divided by R6 1 and R62 and sent to the positive input terminals of four comparators A 1 ~ A4, which are compared with the four reference voltages A, B, C and D to complete overvoltage and undervoltage protection, and notify the CPU to send corresponding alarm signals.
The reference voltage of the comparator is taken from the voltage divider composed of resistors R5 1 ~ R57. After the standard voltage of 10V is divided by resistors, four different reference voltages are taken out and sent to the inverting input terminals of four comparators respectively. The output signal of the comparator is isolated by optocoupler, filtered by resistor and capacitor, and sent to CPU for processing through Schmidt inverter. The peripheral resistance parameters of the other three comparators are the same.
Under normal circumstances, the voltage sampling value (about 3V) is between point B and point C. Comparators A 1 and A2 output 1, and A3 and A4 output 0. The voltage range between b and c is very wide. When the power supply voltage changes between 300 ~ 460 V, the frequency converter works normally. The voltage range between a, b, c and d is very small. Once the voltage exceeds this range, the inverter will send out an overvoltage or undervoltage warning signal and stop according to the predetermined control sequence.
5.3 Overheating Detection Circuit
The four heat sinks in the cabin are respectively equipped with a heat-sensitive element PTH5~PTH8, and the four heat-sensitive elements are connected in series to the optical coupler P4. Its schematic diagram is shown in Figure 5. Under normal circumstances, the thermal element is a normally closed contact, and the output signal of the optocoupler is 0; When the heat sink is overheated, the heat-sensitive element is disconnected, and the output signal of the optocoupler is 1. After RC filtering, the driving signal of GTR is turned off, and the CPU is informed to send out an overheating alarm signal.
5.4 Ground Fault Detection Circuit
Grounding fault detection is realized by detecting the three-phase current balance through the coil sleeved on the main circuit, and its schematic diagram is shown in Figure 6. Under normal circumstances, the cutoff output of the optocoupler is 1. When the leakage current is relative to the ground, the three-phase current is unbalanced, and the potential induced by the detection coil turns on the optocoupler P5 12, sending out a fault signal.
5.5 Fuse Blow Detection Circuit
Fuse detection takes the voltage signal at both ends of F, and its schematic diagram is shown in Figure 6. When fast melting is normal, the voltage at both ends is extremely small, and the protection circuit does not act. When the fuse is blown due to overcurrent, the voltage at both ends becomes high, and the optocoupler turns on to send out a fault signal, which is driven by two Schmidt inverters and sent to CPU.
5.6 Speed sampling
This system requires accurate speed sampling signal, which is difficult for general speed measuring devices to meet, so we adopt 1024 high resolution rotary encoder. Two-phase signals A and B are isolated and input into digital speed measuring unit through optical coupler.
6 experimental results and conclusions
The main technical indexes of the frequency converter with a capacity of 22kVA are given here: the output current is 3 1A, the basic speed is 1500r/min, the speed regulation range is 50 ~ 4800r/min, and the speed regulation accuracy is less than 0.2% of the maximum speed (10% ~1).
Fig. 7 shows the output voltage and current waveforms of three-current hysteretic control and optimized voltage vector control inverters at 10Hz. It can be seen from the actual waveform that the higher harmonic component in the current is very small.
Figure 8 shows the dynamic response curve of the system. When the system enters the steady state, the load is suddenly added (TL = 50n·m), and the maximum dynamic speed decreases by only 9r/min, and the recovery time is also very short. The system has no steady-state error, fast dynamic response, no speed overshoot and strong anti-interference ability under no-load and load conditions.
In a word, the overall design of the inverter is thorough, the structure is complex, the performance is excellent, and the protection function is complete, which has achieved good popularization and application effects.