The electric dental chair adopts the rare-earth permanent magnet brushless DC motor drive system, which combines the brushless DC motor that has developed rapidly in recent years with DSP control technology, and selects a DSP controller to complete the operation of the two rare-earth permanent magnet brushless DC motors. control with low cost and high performance.
The rare earth permanent magnet brushless motor has the characteristics of high efficiency, good performance, simple structure, simple heat dissipation and low noise, which can meet the special requirements of the medical industry for all aspects of medical equipment. Therefore, replacing the brush motor drive mechanism and the single-phase AC asynchronous motor drive mechanism with a linear drive mechanism driven by a rare earth permanent magnet brushless motor has outstanding social meaning and broad market prospects.
The power dental chair has two degrees of freedom, pitch and lift. This servo system selects two rare earth permanent magnet brushless DC motors as the main components of motion control, which are driven separately for the two degrees of freedom.
In the motor control part, only one DSP (digital signal processor) chip TMS320LF2407A is used to complete the control of two brushless motors, making full use of the rich on-chip resources of DSP, and selecting DSP for the PWM input of the two motors, eliminating the need for logic The chip improves the reliability of the system. The memory chip is expanded through the SPI on-chip serial peripheral, and the FRAM (non-volatile memory) is selected to have the function of power-down maintenance, and to complete the memory of the position of the electric seat when the power is turned off.
motor
The extra power of the motor is 55W, the extra voltage is 24V, the working condition is short-term operation, and the extra speed is 1800rpm. The slot shape is trapezoidal mouth sector slot, the winding mode is three-phase star-shaped six-state, the slot full rate is 52%, and the rotor magnet is made of bonded NdFeB magnetized radially. The detailed parameters of the magnetic steel are as follows: Br>6000Gs, Hc>5000Oe, [BH]max>8MG·Oe, Hjc>10000Oe. In the finite element analysis, the actual electromagnetic torque of the motor is 0.2394N m.
controller
The main control unit of the controller uses TI's DSP chip TMS320LF2407A. The main control unit needs to detect the current signal and rotor orientation signal of the motor, and provide six PWM drive signals for the drive board. Therefore, to control a brushless DC motor, six PWM output, one ADC conversion, three capture units.
Regarding the TMS320LF2407A processor, because it has two event managers (EVA and EVB), each event manager has 3 capture units and can output 6 channels of PWM, so the chip can directly work together for two brushless DC motors supply control signals. Because the peripheral circuit of the control chip is more sophisticated, but it has a large program memory, therefore, in terms of hardware, to achieve flexible control of the motor, focus on starting from the software.
drive
The drive circuit of the motor control system adopts a full-bridge structure, and modulates the voltage and current waveforms of the motor by controlling the opening and closing of the six power components. Complete the PWM modulation of the motor voltage by using four independent 16-bit general-purpose timers and six full comparators contained in the TMS320LF2407A. TMS320LF2407A contains programmable dead zone control, any one of the six full comparators is used to generate a pair of PWM outputs with programmable dead zone and output polarity together with a general-purpose timer and a dead zone control unit. There are twelve such PWM outputs. Every six outputs can be used to control a brushless DC motor, completing the DSP control of two motors.
IRF540 power tube is selected. The maximum operating voltage of this equipment is 100V, the maximum operating current is 28A, and the maximum power loss is 150W, which completely meets the power requirements of the system; the freewheeling diodes D1~D6 are fast recovery diodes; capacitors C1~C3 are used to absorb the DC bus. The peak voltage on the voltage is to avoid the breakdown of the power tube due to the excessive bus voltage; the resistor R2 is used to detect the current on the DC bus of the motor to avoid the motor overcurrent failure. The overcurrent signal Isence is processed and input to the front stage of the power bridge In the driver chip and DSP processor, the gate drive signal can be blocked in time when overcurrent occurs.
The pre-driver of the power tube adopts the motor driver chip IR2130 of IR company, which integrates the functions of driving, dead zone, over-current maintenance and so on. It can complete the driving of six power tubes with a single power supply. It is very convenient to use, especially Suitable for low-power drive.
software
According to the defined functions, the software is divided into the following major modules:
Initialization module: main program initialization module (_c_int0), EVA initialization module (EVA_INIT), EVB initialization module (EVB_INIT);
Motor starting module: motor A starting module (MOTORA_START), motor B starting module (MOTORB_START);
Commutation module: motor A commutation module (COMMUTATION_A), motor B commutation module (COMMUTATION_B);
Hall signal capture module: Hall signal capture module of motor A (CAPIN_A), Hall signal capture module of motor B (CAPIN_B);
Keyboard analysis module: the keyboard input analysis module of motor A (KEY_ANALYSE_A), the keyboard input analysis module of motor B (KEY_ANALYSE_B);
PID adjustment module (_PID), orientation memory module (REM).
The combination of rare earth permanent magnet brushless DC motor and DSP control technology is an important development direction of brushless motor.
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