Design of Air Conditioning Control System Based on Tms320f241 Digital Signal Processor

Intelligent power module of variable frequency air conditioner

At present, traditional air conditioners still occupy the main position in the air conditioner market. The start and stop control mode is determined by the room temperature, and the cage electromechanical control compressor is used to adjust the cold and warm air. However, due to the constant speed of the compressor and the simple control mode, the traditional air conditioner has the disadvantages of poor temperature regulation ability and low operation efficiency. Therefore, we use DSP technology, AC permanent magnet motor, space field oriented control technology and space vector pulse width modulation (SVPWM) to develop a new control system. Compared with the control system of traditional air conditioning, it has the characteristics of good operation performance, higher efficiency, low noise and remarkable energy-saving effect.

In the new control system, tms320f241 is designed for variable frequency air conditioning

DSP provides a programmable product development platform. Based on this platform, users can develop AC, DC and one driven multi system, and further upgrade the products. Moreover, the advanced motor control algorithm will help users solve some technical bottlenecks in the design of variable frequency air conditioning, such as reducing the energy consumption and noise of the system. DSP has high-performance computing ability and is suitable for different types of digital control devices. It can replace the previous expensive sensors and external components, so as to reduce the system cost and greatly shorten the manufacturer's R & D cycle. In the new generation of DSP products, confidentiality function will be provided to prevent software piracy and protect users' core technology. This paper presents an air conditioning control system based on tms320f241dsp, which makes full use of its peripherals for motor control, so that the structure of the control system is simpler and the performance price ratio is higher.

1 system control principle

The control system adopts space magnetic field oriented control strategy. In order to realize the effective control of electromagnetic torque, the stator current vector is decomposed into two components in the synchronous rotating coordinate system: one component coincides with the electrode magnetomotive force, which is called torque current component, i.e. q-axis current component; The other component coincides with the excitation magnetic field, which is called excitation current, i.e. d-axis current component. The decoupling control of magnetic field and torque is realized by controlling the phase and amplitude of stator current space vector, that is, controlling the phase and amplitude of torque current component and excitation current component. In this way, the AC permanent magnet motor can be modular into a separately excited DC motor, so as to obtain the same speed regulation performance as the DC motor.

SVPWM control signal is a digital signal generated by DSP using its internal hardware. From the working state of the inverter, there are eight ways to turn on the power devices. The upper bridge arm device is generally represented by "1" and the lower bridge arm device is generally represented by "0". As shown in Figure 1, six effective vectors (V1 V6) and two zero vectors (V0 & V7) at the origin form the basic voltage space vector. Using their linear combination, more new and more voltage space vectors with different phases from the basic voltage vector can be obtained, and finally form a group of voltage space vectors with equal amplitude and different phases, so as to form a rotating magnetic field as close to a circle as possible. In this way, there are more than six switching states of the inverter in a cycle, and some switching states will appear repeatedly, so the output voltage of the inverter is a series of pulse waves with equal amplitude and unequal width. This forms a voltage space vector controlled PWM inverter.

From the definition of voltage space vector:

Where T4 and T6 are the off time of the 4th and 6th power devices in one cycle.

When T4 and T6 are insufficient, insert zero vector supplement, which is generally:

2. Structural features of tms320f241

Tms320f241 is a DSP chip suitable for motor control introduced by TI company in the United States. The execution rate of the chip is very fast. The internal Harvard structure of multi bus is adopted. Under the action of pipelining, the instruction cycle is only 50ns at the internal clock frequency of 20MHz [1]. The CPU has a 32-bit central arithmetic logic unit and a special hardware multiplier, which can complete a multiplication operation of 16 bits multiplied by 16 bits in one instruction cycle. The memory has 8K on-chip flash memory. The rich event manager includes two 16 bit general timers, five comparators and three capture units, two of which have the function of quadrature encoder pulse interface. There are also eight comparison / pulse width modulation (PWM) channels, 8-Channel 10 bit analog-to-digital converter (ADC), serial communication interface (SCI) and serial external device interface (SPI). Compared with TMS320F240, it has the advantages of small volume, complete functions and less resources to complete the control scheme.

3 composition of variable frequency air conditioning control system

Block diagram of the whole air conditioning control system. The system is composed of permanent magnet air conditioning compressor, system board with tms320f241 digital signal processor as the core, stator current detection link and intelligent power module pm10csj060. The system board is composed of tms320f241, external SRAM and control signal driver chip. All control and adjustment of the system are completed by tms320f241 controller with software, which can directly output SVPWM signal, which is isolated by optocoupler and connected to intelligent power module to drive air conditioning compressor. Because the IP system uses permanent magnet materials and large-scale integrated circuits and other electronic components, it not only saves energy and raw materials, but also improves product quality, prolongs service life and reduces failure rate.

3.1 permanent magnet air conditioning compressor

The system uses permanent magnet synchronous motor as the actuator of air conditioning compressor. The motor is made of neodymium iron boron (NdFeB) permanent magnet material, which provides a constant excitation magnetic field, which reduces its volume, light weight and less heating, which is more conducive to the long-term operation of the compressor. It has the advantages of simple structure, no mechanical commutation, little maintenance, simple control compared with cage motor, and easy to realize high-performance excellent control.

3.2 rate detection

The actuator of the control system adopts permanent magnet synchronous motor. When the quadrature decoding pulse circuit (QEP circuit) of tms320f241 is enabled, pins cap1 / qep0 and CAP2 / qep2 receive the photoelectric encoder to generate quadrature pulse signals. Through logical detection of each edge (rising edge and falling edge) of the two signals, a frequency quadrupling signal and a direction signal are generated. The quadruple frequency signal is used as the counting pulse, and the direction signal determines the counting direction of the internal counter of tms320f241 to make the counter count continuously up / down. In this way, the speed and rotation direction of the motor can be obtained from the calculated value and counting direction of the counter.

3.3 stator current detection

In this system, Hall current sensor is used to detect the A and B phase currents in the stator current, and the C phase current is obtained by calculation. It is a phase a stator current detection circuit. The current feedback signal output by the sensor is voltage taken through a 25 Ω resistance, and then summed with the 2.5V analog offset after passing through the voltage follower to become a signal in the range of 0 5V. At this time, the obtained signal can be sent to the A / D conversion module integrated on tms320f241 through pins adcin6 and adcin3, so as to obtain the stator current feedback signal.

3.4 drive and protection

In the main circuit of the air conditioning control system, the third generation intelligent power module pm10csj060 of Mitsubishi company is used as the inverter, and the six channel space vector signal SVPWM output by tms320f241 is isolated by the driving circuit and optocoupler as the driving control signal of the intelligent power module. Pw10csj060 is an integrated component that integrates six IGBTs and their driving circuit and protection circuit in the same package. The high-efficiency driving circuit is integrated with IGBT, which shortens the product design and development cycle and further improves the reliability; A current sensor is integrated in the module to detect over-current and short-circuit current; Each IGBT has an independent protection circuit to make the module work more reliable; Very small switching loss and high frequency make the inverter achieve silent operation. The dead time can be generated by the hardware circuit in the module, but the user can also be generated by software by setting the dead time control unit register provided by tms320f241 chip.

The protection circuit is used for overheating, overload, short circuit, undervoltage and other fault protection of the main circuit. The fault output signal is isolated by the optocoupler and connected to the protection terminal pin pdpint of tms320f241. When the internal logic circuit of the chip fails, the SVPWM output signal is blocked, so as to realize the driving protection of the air conditioning compressor.

4 software design

The space magnetic field orientation control strategy is completed by tms320f241 software. The software control first initializes the program and defines the variable constant. Then enter the user module program, cycle and wait for the current SVPWM underflow interrupt to occur, communicate with the remote control receiver during the waiting time, and the user can adjust the indoor temperature setting value. When the underflow interrupt generator generates the next SVPWM signal, the spatial magnetic field oriented control algorithm is completed in the SVPWM interrupt service subroutine, which is the same cycle as SVPWM.

Block diagram of interrupt service subroutine for SVPWM. When the underflow interrupt occurs, it is the beginning of the next SVPWM cycle. The IA and IB current feedback signals are changed into data through the ADC module to judge whether the stator current space vector is orthogonal to the d-axis. When it is not orthogonal, the QEP circuit processes the encoder pulse and calculates the magnetic pole position, motor speed and rotation direction; In case of quadrature, after assigning values to variables such as magnetic pole position, it directly enters the current vector transformation control loop, namely DQ axis current PI regulator. When the current loop circulation count value variable NSP is equal to the given value nspr, the program enters the speed regulation loop, and the current regulation is carried out after determining the given values of IDR and IQR current [2].

5 experiment

The experiment adopts AC permanent magnet motor, rated torque: 0.96nm, rated speed: 3000rpm, rated power: 300W, magnet material: NdFeB. SVPWM control signals generated for six PWM full comparators of tms320f241. It is the SVPWM waveform for continuous switching of space vector sectors in the motor, so as to realize the frequency conversion control of air conditioning motor.

Experiments show that the digital SVPWM can reduce the switching loss of power devices, improve the utilization of voltage, and make the inherent hardware circuit of tms320f241 play a more effective role.

The full digital frequency conversion air conditioning control system based on tms320f241 makes full use of the super real-time computing power of the chip and the rich integrated devices in the chip, which makes the system simple in structure, short in product development cycle and strong in reliability. Therefore, the control system composed of this chip has a very broad practical application value.

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