Embedded microcontroller for controlling permanent magnet brushless DC motors

1. Overview

The brushless DC motor is composed of a motor body and a driver, and is a typical mechatronics product. The stator winding of the motor is mostly made into a three-phase symmetrical star connection, which is very similar to a three-phase asynchronous motor. The rotor of the motor is attached with a magnetized permanent magnet. In order to detect the polarity of the motor rotor, a position sensor is installed in the motor. The driver is composed of power electronic devices and integrated circuits, etc. Its functions are: receiving the start, stop, and brake signals of the motor to control the start, stop, and brake of the motor; receiving the position sensor signal and the forward and reverse signals to control the on and off of each power tube of the inverter bridge to generate continuous torque; receiving the speed command and speed feedback signal to control and adjust the speed; providing protection and display, etc.

Permanent Magnet Brushless DC Motor (hereinafter referred to as PMBLDC) is widely used in household consumer products (air conditioners, refrigerators, washing machines) and IT peripheral products (printers, floppy drives, hard drives) due to its inherent characteristics. The performance of permanent magnet brushless DC motor depends not only on the design of the motor itself, but also on the drive technology and the drive board that implements this drive technology. The driver board of permanent magnet brushless DC motor is also a bridge connecting the upper control MCU and the power electronic converter: it receives the command of the upper computer and achieves the predetermined control target through a certain control method; at the same time, it returns the current motor state and some basic parameters to the upper control MCU. From the above functions that the PMBLDC driver board should achieve, it can be seen that the core device of this type of driver board should be an embedded microcontroller with multiple (more than 6) PWM outputs. Since the application of INTEL MCS-51 series microcontrollers in China has a broad foundation, we selected the Infineon C504 embedded microcontroller that fully adopts the 8031 ​​core and has a maximum clock frequency of 40MHz.

2. Hardware driver mode of C504

C504, in addition to adding peripheral interface function circuits such as A/D conversion, also adds a 7-way all-digital PWM output unit CCU (Capture/Compare Unit) that is very suitable for motor control. The functional block diagram of CCU is shown in Figure 1. When C504 works in hardware drive mode, cout3 can modulate cc0, cout0, cc1, cout1, cc2, cout2, and its output timing is controlled by INT0, INT1, INT2 coding.

In Figure 1, fosc/2 on the left is the time base for the entire unit to operate;

The CTRAP terminal can control the state of the cc0-cout2 output terminals. When in the CTRAP state, the state of cc0~cout2 can be defined by software;

The cc0-cout2 terminals are 3-phase 6-channel 16-bit PWM outputs;

ccx and coutx (x=1,1,2) are complementary signals, and their dead time can be controlled by setting the value of Offset RegiSTer;

cout3 is a single 10-bit PWM output terminal, while INT0~INT2 are externally controlled. The output order of cc0~cout2 is controlled by the encoding of INT0~INT2.

Figure 2 shows the current waveform of C504 when it is used in an outer rotor permanent magnet brushless DC motor as the drive motor of an electric bicycle working in hardware mode.

This prototype has now been put into production by Hangzhou Guangyang Electromechanical Technology Co., Ltd. When the permanent magnet brushless DC motor used as an electric vehicle drive is controlled by an embedded microcontroller, a series of functions that cannot be achieved by ordinary analog controllers, such as smooth starting, can be achieved.

3. Software drive mode of C504

In addition to the hardware drive mode, C504 also provides a software drive mode that can arbitrarily combine PWM outputs, as shown in Figure 3.

The term "arbitrary combination" has two meanings:

1) The number of phases of the motor can be selected, and different ccx and coutx (x=0, 1, 2) terminals can be used;

IV. Conclusion

The CCU unit of InfinEON's embedded microcontroller C504 has improved functions that are very suitable for PMBLDC motor control. By selecting hardware and software modes, the required control purpose can be easily achieved; and C504 is low-priced and has strong anti-interference performance, making it suitable as a control chip embedded in the middle component between power converters and computers such as driver boards. At present, we use C504 as the underlying embedded control chip of the multi-axis control system and are preparing to apply it to industries such as CNC machinery and printing.

2) The control method can be general PWM or SPWM control, which can be applied to the quasi-sinusoidal current control of PMBLDC.

Figure 4 is the current waveform of a DCDD (DC Direct Drive) washing machine. It can be seen from the figure that the current waveform is very close to a sine wave, achieving a good control effect.