Application of digital signal processor TMS320F241 in variable frequency air conditioners

    Abstract: A control system based on TMS320F241 digital signal processor (DSP) is proposed, which can realize full digital speed regulation of variable frequency air conditioners. The system makes full use of the high-performance processing capabilities and advanced control technology of the DSP chip, and uses intelligent power modules to drive the air-conditioning compressor, making it simple in structure, good in operation, low in noise, and highly reliable. The experimental results show the feasibility of the solution and the superiority of DSP applied to variable frequency air conditioning control systems.

    Keywords: digital signal processor, variable frequency air conditioner, intelligent power module

At present, traditional air conditioners still occupy a dominant position in the air conditioner market. It determines the start and stop control method based on the room temperature, and uses a cage-type electromechanical control compressor to regulate air conditioning and heating. However, due to the constant speed of the compressor and the simple control method, traditional air conditioners have shortcomings such as poor temperature regulation capabilities and low operating efficiency. Therefore, we developed a new control system using DSP technology, AC permanent magnet motor, space magnetic field oriented control technology and space vector pulse width modulation technology (SVPWM). Compared with the traditional air conditioning control system, it has good operating performance and It has the characteristics of higher efficiency, low noise, and very significant energy saving effect.

In the new control system, the TMS320F241 DSP designed for variable frequency air conditioners provides a programmable product development platform. Users can develop AC, DC and one-to-multiple systems based on this platform, and further upgrade the products. Moreover, advanced motor control algorithms will help users solve some technical bottlenecks in the design of inverter air conditioners, such as reducing system energy consumption and noise. DSP has high-performance computing power and is suitable for different types of digital control devices. It can replace expensive sensors and external components in the past, thereby reducing system costs and greatly shortening the manufacturer's research and development cycle. The new generation of DSP products will provide confidentiality functions to prevent software piracy and protect users' core technologies. This article proposes an air conditioning control system based on TMS320F241DSP, making full use of its peripherals for motor control to make the control system structure simpler and more cost-effective.

1 System control principle

The control system adopts a spatial magnetic field oriented control strategy. In order to achieve 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 and is called the torque current component, that is, the q-axis current component; the other component coincides with the excitation magnetic field and is called the excitation current, that is, the d-axis current component. By controlling the phase and amplitude of the stator current space vector, that is, controlling the phase and amplitude of the torque current component and the excitation current component, decoupling control of the magnetic field and torque is achieved. In this way, the AC permanent magnet motor can be converted into a separately excited DC motor to obtain the same speed regulation performance as the DC motor.

The SVPWM control signal is a digital signal generated by DSP using its internal hardware. Judging from the working status of the inverter, there are eight ways for the power device to be turned on. The conduction of the upper arm device is represented by "1", and the conduction of the lower arm device is represented by "0". As shown in Figure 1, six effective vectors (V1~V6) and two zero vectors (V0&V7) at the origin constitute the basic voltage space vector. By using their linear combination, more new and more voltage space vectors with different phases from the basic voltage vector can be obtained, ultimately forming a set of voltage space vectors with equal amplitude and different phases, thereby forming a rotating magnetic field that is as close to a circle as possible. In this way, in one cycle, the inverter will have more than six switching states, and some switching states will repeat multiple times, so the output voltage of the inverter is a series of pulse waves of equal amplitude and unequal width. This forms a voltage space vector controlled PWM inverter.

For example, in sector 3 of Figure 1, according to the parallelogram rule:

It can be solved from the definition of voltage space vector:

In the formula, T4 and T6 are the switching times of the 4th and 6th power devices in one cycle.

When T4 and T6 are insufficient, a zero vector is inserted to make up for it, generally as follows:

2 Structural features of TMS320F241

TMS320F241 is a DSP chip suitable for motor control launched by the American TI Company. The execution speed of this chip is very fast. It adopts multi-bus Harvard structure internally and has pipeline function. At the internal clock frequency of 20MHz, the instruction cycle is only 50ns[1]. The CPU has a 32-bit central arithmetic logic unit and a dedicated hardware multiplier, which can complete a 16-bit by 16-bit multiplication operation in one instruction cycle. The memory has 8K on-chip flash memory. The rich event manager includes two 16-bit general-purpose timers, five comparators, and three capture units, two of which have quadrature encoder pulse interface functions. 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 peripheral interface (SPI), etc. Compared with TMS320F240, it has the advantages of small size, complete functions, and can use less resources to complete the control scheme.

3. Composition of variable frequency air conditioning control system

The block diagram of the entire air conditioning control system is shown in Figure 2. The system consists of a permanent magnet air conditioning compressor, a system board with TMS320F241 digital signal processor as the core, a stator current detection link and an intelligent power module PM10CSJ060. The system board consists of TMS320F241, external SRAM and control signal driver chip. All control and adjustment of the system are completed by the TMS320F241 controller using software, which can directly output the SVPWM signal, and then connected to the intelligent power module to drive the air-conditioning compressor after optical coupling isolation. Because the system uses electronic components such as permanent magnet materials and large-scale integrated circuits, ip not only saves energy and raw materials, but also improves product quality, extends life, and reduces failure rates.

3.1 Permanent magnet air conditioning compressor

The system uses a permanent magnet synchronous motor as the actuator of the air conditioning compressor. The motor is made of neodymium iron boron (NdFeB) permanent magnet material to provide a constant excitation magnetic field, which makes it smaller in size, lighter in weight and generates less heat, which is more conducive to the long-term operation of the compressor. It has a simple structure, no mechanical commutation, does not require much maintenance, is relatively simple to control compared to cage motors, and is easy to achieve high-performance and excellent control.

3.2 Rate detection

The executive components of the control system adopt permanent magnet synchronous motors. When the quadrature decoding pulse circuit (QEP circuit) of TMS320F241 is enabled, the pins CAP1/QEP0 and CAP2/QEP2 receive the photoelectric encoder to generate orthogonal pulse signals. By analyzing each edge (rising edge and Falling edge) logic detection generates a quadruple frequency signal and a direction signal. The quadruple frequency signal is used as a counting pulse, and the direction signal determines the counting direction of the internal counter of TMS320F241, allowing the counter to 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, the Hall current sensor is used to detect the A and B phase currents in the stator current, and the C phase current is obtained through calculation. Figure 3 shows the A-phase stator current detection circuit. The current feedback signal output by the sensor takes the voltage through a 25 Ω resistor, and then is summed with the 2.5V analog offset after passing through the voltage follower to become a signal in the range of 0 to 5V. At this time, the obtained signal can be sent to the A/D conversion module integrated on TMS320F241 through pins ADCIN6 and ADCIN3, thereby obtaining the stator current feedback signal.

3.4 Driver and protection

In the main circuit of the air conditioning control system, Mitsubishi's third-generation intelligent power module PM10CSJ060 is used as the inverter. The six-channel space vector signal SVPWM output by TMS320F241 is isolated by the drive circuit and optocoupler as the drive control signal of the intelligent power module. PW10CSJ060 is an integrated component that integrates six IGBTs, their drive circuits and protection circuits in the same package. The high-efficiency drive circuit is integrated with the IGBT, which shortens the product design and development cycle and further improves 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 ensure The module works more reliably; very small switching losses and higher frequency enable the inverter to achieve silent operation. The dead time can be generated by the hardware circuit in the module, but the user can also generate it by software by setting the dead zone control unit register provided by the TMS320F241 chip.

The protection circuit is used to protect the main circuit from overheating, overload, short circuit, undervoltage and other faults. The fault output signal is isolated by an optocoupler and connected to the protection pin PDPINT of TMS320F241. When the internal logic circuit of the chip fails, the SVPWM output signal is blocked, thereby achieving drive protection for the air conditioning compressor.

4 Software design

The spatial magnetic field orientation control strategy is all completed by the TMS320F241 software. The software control first initializes the program and defines variable constants. Then enter the user module program, wait in a loop 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 set value. When the overflow interrupt generator generates the next SVPWM signal, the spatial magnetic field oriented control algorithm is completed in the SVPWM interrupt service subroutine, which is in the same cycle as SVPWM.

Figure 4 is the SVPWM interrupt service subroutine block diagram. When the underflow interrupt occurs, it is the beginning of the next SVPWM cycle. The Ia and Ib current feedback signals are converted into data quantities through the ADC module to determine whether the stator current space vector is orthogonal to the d-axis. When it is not orthogonal, the QEP circuit processes the encoder pulses and calculates the magnetic pole position, motor speed and rotation direction; when it is orthogonal, after assigning values ​​to variables such as the magnetic pole position, it directly enters the current vector transformation control loop, that is, the dq-axis current PI regulator . When the current loop count value variable nsp is equal to the given value nspr, the program enters the speed adjustment loop. After determining the idr and iqr current given values, the current is adjusted [2].

5 experiments

The experiment uses an AC permanent magnet motor, rated torque: 0.96Nm, rated speed: 3000rpm, rated power: 300W, magnet material: NdFeB. Figure 5 shows the SVPWM control signals generated by the six PWM full comparators of TMS320F241. Figure 6 shows the SVPWM waveform of continuous switching of space vector sectors in the motor, thereby achieving frequency conversion control of the air conditioner motor.

Experiments have proven that the SVPWM implemented digitally in this system can reduce the switching loss of the power device, improve the voltage utilization, and enable the inherent hardware circuit of TMS320F241 to function more effectively.

The fully digital variable frequency air conditioning control system based on the TMS320F241 chip makes full use of the chip's super real-time computing power and the rich integrated devices on the chip, making the system simple in structure, short in product development cycle, and highly reliable. Therefore, the control system composed of this chip has extremely broad practical application value.