Frequency Converter Knowledge: Control

Control

DSP -based digital controller implementation in general frequency converter

introduction

The key to the variable frequency speed regulation system is to design a reasonable frequency converter, and its core is the digital controller of the variable frequency speed regulation system. The digital controller of the frequency converter includes signal detection, filtering, shaping, real-time completion of the core algorithm , generation of drive signals, system monitoring, protection and other functions.

The hardware part of the inverter digital control system includes microprocessor, interface circuit and peripheral equipment. The microprocessor is the control core of the system. It processes the data input from the input interface through the internal control program, completes the control calculation and other tasks, and sends various control signals to the periphery through the output interface circuit. In addition to detection components and actuators, the peripheral equipment also includes various operation, display and communication equipment.

This paper uses TI's TMS320F240 to design a digital controller for high-speed motor speed control system. The frequency can be given digitally or analogly through the keyboard. At the same time, a brief analysis of its functions and technologies is made, and the output waveform of the controller when the motor is running steadily at 18000r/min is given.

1 Hardware structure diagram and working principle of digital controller

The hardware of the digital controller uses TMS320F240 fixed-point DSP as CPU and CY7C199 as external data and program memory, with 32K of data and program memory each; 16 analog/digital input channels, one of which can be used for analog frequency setting; an 8-bit digital I/O port, which can be used to give digital frequency through the keyboard; 4 12-bit digital/analog conversion channels for motor output signal control; RS232 and SPI series compatible interfaces, in which SPI is used as an LED display of motor frequency during variable frequency speed regulation , and the SCI port is expanded into an RS232 interface. Its functional layout block diagram is shown in Figure 1.

Figure 1: Schematic diagram of the hardware structure of the digital controller.

The operating frequency of the motor or inverter is given by the keyboard. At the same time, its frequency display is echoed on the LED through the display program inside the DSP . When the run key is pressed, the keyboard design frequency is sent to the SV PWM processing subroutine that generates the space voltage vector. The generated SVPWM waveform is output after being protected by the GAL device. At the same time, the real-time dynamic frequency of the motor or inverter is displayed through the LED. The orthogonal encoding pulse can be connected to the motor's photoelectric encoder to form a speed loop feedback for the system. The A/D module can be connected to the motor's current loop. As for the protection interrupt source of the variable frequency speed regulation system, it is provided by the DSP pin PDPINT, which is mainly overvoltage, overcurrent, control voltage undervoltage, overheating and other interrupt sources. The speed of the motor or the output frequency of the inverter can be changed by the keyboard.

2 Hardware Design

Digital signal processor is the core part of digital controller, and also the core part of digital controller's functions such as signal detection, filtering, shaping, real-time completion of core algorithm, generation of drive signal, system monitoring and protection. The functional module design of digital controller is as follows.

2.1 Design of data and program memory

DSP is a high-speed access device, which has high requirements for peripheral interface chips. Although DSP itself can provide 0~7 wait states in software to match the speed of off-chip access devices, in order not to affect the control and simulation functions of the entire system, a memory with a relatively high access speed is generally used as the off-chip data and program memory of DSP. This article uses CY7C199 memory, which has an access time of 15ns. It does not need to provide software wait states or add hardware wait circuits. Because CY7C199 is a 32K 8-bit memory, 4 pieces of this memory are used to form a 32K 16-bit memory RAM, with 32K for data and program respectively.

2.2 DSP reset and clock circuit design

In order for the system to be correctly initialized by the reset signal, there must be certain requirements for the pulse width of the reset signal. For TMS320F240, the reset signal must be at least 1ms. However, after power-on, it takes 20ms or even longer for the system's oscillator to reach a stable working state. Generally speaking, it is more appropriate to set a low-level pulse of 100~200ms on the reset pin when powering on. According to this principle, the integrated microprocessor monitoring reset circuit of MAXIM is used to complete it. This article uses MAX705. Compared with the traditional microcomputer monitoring circuit composed of discrete components, the MAX705 monitoring chip has high reliability, good dynamic response, low power consumption, simple design, and small size. It has been widely used in electronic product design.

In design, the clock is often not given enough attention. In fact, the clock is a very important part of circuit design. The DSP clock can be provided by an external source or by an on-board oscillator. Since the DSP and other chips work based on the clock, if the clock quality is not high, the reliability and stability of the system will be difficult to guarantee. This article uses an external clock input, and an active crystal oscillator generates a 10MHz pulse. The copper cladding and LC filter circuit are used to suppress external interference, ensuring the stable operation of the system.

2.3 RS232 serial port circuit design

RS232 is a serial communication interface standard released by the American Electronics Industries Association in 1960. Currently, RS232C and RS232D are widely used.

The standard connection of RS232C is DB25. However, in actual applications, non-standard DB9 connection is used. In actual applications, the defined pins are selected according to needs. The biggest feature of RS232C electrical characteristics is the use of negative logic. The level of logic 1 is -3V~-15V, and the level of logic 0 is +3V~+15V. Therefore, there is a problem of level conversion interface in use. This article uses the self-boosting integrated chip MAX232C to form it. It is only powered by a +5V power supply . The ±10V power supply required for level conversion is generated by the on-chip charge pump. After the controller is completed, the serial communication interface (SCI) of the computer is tested. The data communication is normal and can work stably.

2.4 Design of D/A output function block

In digital control systems, D/A and A/D circuits are indispensable. Depending on the various application scenarios, the system has different speed requirements for D/A and A/D. This article uses the parallel input D/A chip DAC 7625, which is a D/A converter with 12-bit data parallel input and 4 analog outputs. Its settling time is 10μs, power consumption is 20mW, and the power supply can be a single power supply +5V and dual power supply ±5V. It is widely used in motor control and data acquisition. The data input of the digital-to-analog converter DAC comes from the high 12 bits of the DSP and is sent to the data end of the DAC7625 through the 74LS245. It uses a single power supply of +5V. The reference voltage VHEFH uses the +2.5V provided by a precision voltage regulator, and VHEFL is an analog ground. Its output is

It is amplified by the operational amplifier TLCH2272 and the output range is 0~+5V.

2.5 Keyboard input interface circuit and LED display circuit design

The keyboard and seven-segment LED display are the most commonly used input and output devices in microcomputer systems. They are the main channels for information exchange between humans and machines. The function of the keyboard is to convert the data and commands that people want to process into binary codes that can be recognized by computers, that is, symbols that can be recognized by computers; the seven-segment LED display converts the computer's calculation results, status and other codes into symbols that people can recognize and display them. The keyboard is the main input device of the computer system, especially in microprocessors, and keyboard design becomes inevitable. This article takes into account the rapidity of DSP processing speed when designing, and adopts a hardware delay circuit for the keyboard de-jitter link. The specific circuit is shown in Figure 2.

Figure 2: DSP keyboard input interface circuit in digital controller.

The seven-segment LED display has two connection modes: static display and dynamic display. The dynamic scanning mode saves hardware. There are two commonly used BCD seven-segment decoding drive and dynamic scanning drive circuits, such as Intel 8279, Max 7219, etc. The controller uses the MAX7219 chip. DSP has a serial interface SPl for dealing with peripherals , which provides convenience for serial door display. MAX7219 is a serial common cathode LED digital display driver . It has multiple control and data registers. Its working mode can be flexibly designed through programming. It is a small, powerful, flexible and convenient serial interface. The problem that needs to be paid attention to in the application is that the MAX7219 has poor EMI resistance. Relatively speaking, it is more reliable to use MAX7221. Another problem is that although the manual says that the register can use any number, for example, the upper 4 bits in the data format are represented by XXXX, but in actual applications, it is best to use non-zero bits. This article uses 1111 to represent it, which can increase the anti-interference ability. In addition, appropriate capacitance must be added to the serial data line and power supply to improve anti-interference capability. Special attention should be paid to the power supply. If the fluctuation is large, the MAX7219 will be easily damaged.

2.6 Design of SVPWM pulse output module

The space voltage vector SVPWM pulse output is a key part of the digital controller. The motor speed regulation or the inverter frequency is controlled by the SVPWM waveform. In order to prevent the upper and lower bridge arms of the inverter from being directly connected, although dead zones can be added in the DSP internal programming, the SVPWM pulses generated by the microprocessor may cause control confusion due to program runaway. For safety reasons, GAL devices are used to make interlock protection circuits to prevent the upper and lower devices of the same bridge arm of the inverter from being directly connected. The digital controller uses Lattice 's GAL16V8.

3 Software Design

As the inverter products continue to mature, its functions are constantly enriched and its reliability is constantly improved, which leads to the complexity and difficulty of its program compilation. The variable frequency speed regulation system designed in this paper is for laboratory bearingless high-frequency motors. It mainly completes some basic functions, such as frequency setting and display, torque compensation function at low speed, etc. The program is not particularly complicated. The design program is nearly 2000 lines. The program has been tested and proved to run well. The entire program in the variable frequency speed regulation system in this paper is mainly composed of the main program, keyboard program, display program, PWM program, fault protection interrupt program, etc.

3.1 Main program and fault protection interrupt program

The main program is the most important part of the whole program. It completes the main functions of the inverter. Its flowchart is shown in Figure 3 (a). The program initialization part mainly includes: initialization of I/O port, initialization of waveform generator, initialization of timer counter, initialization of SPl, initialization of MAX7219, etc. Reading data to internal register means reading commonly used data into internal register, shortening DSP processing time and better realizing the implementation. The process is to judge the value given by the key and determine which of the set frequencies is the final target frequency. The frequency display part is to display the final target frequency through LED in thousands, hundreds, tens and units as usual. Operation control is to decide whether to start the motor according to the RUN key. In terms of hardware design, Fuji's third-generation intelligent power module IPM is used . It integrates the output alarm functions of overvoltage, overcurrent, overheating, control voltage undervoltage, short circuit, etc., which are sent to the external interrupt source pin PDPINT of DSP through optical coupling isolation to complete the corresponding protection function. The specific flow chart is shown in Figure 3 (b).

Figure 3: Main program and protection program flow.

3.2 SVPWM interrupt subroutine

The PWM interrupt subroutine is the key program for the whole controller. The completion of the space voltage vector modulation is realized by it. The specific flow chart is shown in Figure 4. The PWM generation program mainly completes the following functions: dynamic display of the frequency when the motor is running. According to the target frequency given in the main program, the angular velocity ω can be obtained. After the integral operation of ω, the angle θ of usref can be obtained. Then the projections usα and usβ of usref on the α and β axes of the two-phase stationary coordinate system are calculated. With θ, the sector/N where the reference voltage vector is located can be calculated at the same time. According to the known quantity, the action time T1, T2 and T0 of the two adjacent voltage vectors are obtained from the common value, and then the three full comparison registers CMPRx (x=1, 2, 3) inside the DSP are assigned to generate the corresponding 5VPWM waveform.

Figure 4: SVPWN interrupt subroutine flow chart.

4 Experimental Results

According to the system hardware circuit and software control algorithm introduced above, the prototype was experimentally studied. The no-load steady-state operation of the asynchronous motor was tested to detect the feasibility of the prototype. The experimental results were recorded in waveform. The PWM control waveform and the measured line voltage waveform of the asynchronous motor during 300 Hz steady-state operation are shown in Figure 5.

Figure 5: Control waveform output by the controller and measured motor line voltage waveform at 300Hz.

The parameters of the high-frequency motor used in the experiment are as follows:

Rated voltage Un=220V, rated current In=1.5A, rated frequency f=400Hz, number of pole pairs of asynchronous motor=1, rated power Pe=800W, rated no-load current 0.75A.

5 Conclusion

The digital controller with TMS320F240 digital signal processor as the core is a signal processing system, which can complete the detection, filtering, shaping of signals, the real-time completion of core algorithms, the generation of drive signals, system monitoring, protection and other functions. Compared with the system composed of general single-chip microcomputers , it has fast processing speed, better real-time performance, and is easy to select and cooperate. At the same time, it integrates measurement , monitoring and protection, can communicate with the host computer, and has high use value.