1 Introduction
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In recent years, digital signal processor (DSP) chips have been widely used in the fields of automatic control, image processing, communication technology, network equipment, instrumentation and home appliances; DSP provides an efficient and reliable hardware foundation for digital signal processing. At present, the most widely used DSP chip is the product of TI (Texas Instruments), and the TMS320C2000 series is a DSP suitable for digital control. This series of DSP chips has perfect performance and integrates flash memory, high-speed A/D converter, high-performance CAN module, etc., so it can reduce development difficulty, shorten product development time, and effectively reduce development cost. The price/performance ratio. The TMS320LF2407 chip is the latest, most integrated and highest performance motion control DSP chip in the TMS320C2000 series.
Natural gas as a power source is an ideal substitute for traditional fuels. Dedicated electronic control systems for various natural gas engines are constantly emerging, and the performance of natural gas engines such as power, economy, and emissions is continuously improved. The natural gas-specific engine can have lower emissions than the equivalent gasoline engine and diesel engine through good control, and the power performance is greatly improved. Therefore, it is very important to design a natural gas engine control system by using a high-performance DSP chip.
2 TMS320LF2407 chip introduction
The DSP chip is a microprocessor with a special structure. The chip's internal Harvard architecture with separate programs and data, with a dedicated hardware multiplier, extensive pipeline operation, provides special instructions that can be used to quickly implement a variety of digital signal processing algorithms.
The TMS320x24x family of digital signal processors is a new programmable DSP chip developed by TI (Texas Instruments, USA) for digital motor control, embedded control systems and digital control systems. The TMS320LF240x is a new addition to the x24x family of DSP controllers designed to meet control applications. The on-chip program memory of the LF240x is Flash EEPROM, including the LF2402, LF2406 and LF2407 models. The LF2407 is a x240x series DSP controller* with the strongest and most complete on-chip facilities. It is widely used in code development, system simulation and practical systems. Its main features:
(1) Using high-performance static CMOS technology, the supply voltage is reduced to 3.3V, reducing the power consumption of the controller; the execution speed of 30MIPS shortens the instruction cycle to 33ns (30MHz), thereby improving the real-time control capability of the controller. .
(2) Two event manager modules EVA and EVB, each including: two 16-bit general purpose timers; eight 16-bit pulse width modulation (PWM) channels. Suitable for controlling AC induction motors, brushless DC motors, stepper motors and inverters.
(3) The 10-bit A/D converter has a minimum conversion time of 500 ns. It is optional to have two event managers to trigger two 8-channel input A/D converters or one 16-bit channel input A/D converter.
3 TMS320LF2407 chip in natural gas engine control system
The traditional control system is controlled by single-chip microcomputer. First, its running speed is not high, it can not achieve good real-time control, and can not effectively achieve the effect of online detection and control. Second, the integration degree of single-chip microcomputer is low, and there is no on-chip. The dedicated peripherals required by the motion control system require more components, increasing the complexity of the system circuitry and reducing the reliability of the system. However, TI (Texas Instruments)'s x240x series DSP chips are used, and dedicated motion control peripheral circuits are integrated on-chip, and its internal DSP core can provide higher computing speed, operational precision and the ability to process large amounts of data operations. The DSP chip can better meet the real-time requirements of the system and can easily implement complex control algorithms.
3.1 hardware design of the control system
The main function of the control system is to control the ignition time of each cylinder through two speed sensors and one temperature sensor, one speed sensor is measured, the other speed sensor measures the signal to calculate the speed, and the temperature sensor measures the temperature of the exhaust gas. The measured engine speed is determined by the measured speed and exhaust gas temperature, and the solenoid valve is adjusted according to some control rules to control the intake air volume of the natural gas and the air, so that the engine speed is constant to improve the dynamic performance of the natural gas engine.
The control system uses the DSP chip of TMS320LF2407. The peripheral components of the system are few and the circuit structure is very simple. The schematic diagram of the system structure is shown in Figure 1:
Figure 1 system structure diagram
1) Design of the human interface part:
The main design is 8-bit LED display and 20-key keyboard. One is for the display of speed and exhaust temperature, and the other is for the input and adjustment of the ignition angle. The intelligent control chip HD7279A with a keyboard and a digital tube is connected to the DSP chip, and only four I/O ports of the DSP can be used, which can effectively reduce the hardware circuit and improve the system reliability. Since the I/O port output of the TMS320LF2407 is 3.3V, the clamp circuit can be added when connected to the HD7279A pin.
HD7279A is a smart display driver chip with serial interface. It can drive 8-bit common cathode digital tube and 64-key keyboard matrix at the same time. It can directly drive LED without external components, and can complete LED display and keyboard interface with single chip. All features. The circuit is shown in Figure 2:
The HD7279A should be connected to a common-negative digital tube. The keyboard and digital tube that are not needed in the application can be disconnected. If the keyboard is not used, the 8 10K resistors and 8 100K pull-down resistors connected to the keyboard in Figure 2 can be omitted. If a keyboard is used, the eight 100K pull-down resistors in the circuit must not be omitted. If the digital tube is not used, the 8 0.2K resistors connected to the DP and SA-SG connections in Figure 2 can be omitted, otherwise they cannot be omitted. In practice, 8 pull-down resistors and 8 keyboard connection bit selection lines DIG0-DIG7 8 bit selection resistors should follow a certain proportional relationship, the pull-down resistor should be greater than 5 times the bit selection resistance and less than 50 times, the typical value is 10 times.
The HD7279A requires an external RC oscillator circuit for the system to work. The typical values ​​are R=1.5K and C=15pF. If the chip does not work properly, check the oscillator circuit first.
Figure 2 keyboard and LED display
2) Design of the signal acquisition part
The control system mainly collects the rotational speed and exhaust temperature of the natural gas engine.
The signal collected by the speed sensor is that the pulse signal is amplified by the LF356, and then shaped by the LM339 comparator, and then sent to the I/O port of the DSP through photoelectric isolation, and the speed value is calculated by the number of the pulse signals obtained.
The exhaust temperature is about 800 degrees, so the temperature signal is obtained by using a K-type thermocouple. The temperature of the thermocouple cold junction is compensated by AD590. After being amplified by OP07, it is sent to the analog signal port of the DSP. The circuit block diagram is shown in Figure 3:
Figure 3 Speed ​​and temperature signal acquisition block diagram
The AD590 is a constant current source temperature sensor. When a certain operating voltage is applied to both ends, the output current changes linearly with temperature. The output current changes by 1μA for every 1°C change in temperature. The temperature compensation of the thermocouple is carried out by using AD590. According to the Seebeck coefficient of different thermocouples, the appropriate resistance is selected, and the voltage change of the reading change is offset with the cold end electromotive force of the thermocouple at this time, thereby achieving temperature compensation.
3.2 software design
The software is written in assembly language, using TI's CC2000 development environment, the main program is mainly to achieve display functions and data acquisition functions. The main program flow chart is shown in Figure 4:
Figure 4 main program flow chart
In the control program, the calculated rotational speed and temperature are compared with the expected value to obtain the error and error change rate, and then the gas and air solenoid valves are adjusted according to the established control rules, thereby changing the intake air amount of the gas and the air. In order to output a constant speed. The DSP chip has a dedicated hardware mechanism for motor control, which can directly generate PWM waveforms, which can be directly used to control gas and air solenoid valves.
4 Conclusion
The control system based on the DSP chip is actually a single-chip system, because the various functions required for the entire control can be realized by the DSP chip. Therefore, the volume of the target system can be reduced, the number of external components can be reduced, and the reliability of the system can be increased. For those applications where performance and accuracy are high, real-time is strong, and volume is small, the DSP based chip to form the control system is a very high-performance price-performance method.
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