The electric dental chair uses a rare-earth permanent magnet brushless DC motor drive system. This system combines the rapidly developing brushless DC motor with DSP control technology. It uses a single DSP controller to control two rare-earth permanent magnet brushless DC motors, and features low cost and high performance.
Rare-earth permanent magnet brushless motors, with their high efficiency, superior performance, simple structure, easy heat dissipation, and low noise, meet the specific requirements of the medical industry for various aspects of medical devices. Therefore, replacing brushed motor drives and single-phase AC asynchronous motor drives with linear drive systems driven by rare-earth permanent magnet brushless motors has significant social implications and broad market prospects.
The electric dental chair has two degrees of freedom: pitch and height. This servo system uses two rare-earth permanent magnet brushless DC motors as the main motion control components, driving each of the two degrees of freedom separately.
The motor control section uses only one DSP (Digital Signal Processor) chip, TMS320LF2407A, to control two brushless motors. This fully utilizes the abundant on-chip resources of the DSP, providing all PWM inputs for both motors directly to the DSP, eliminating the need for logic chips and improving system reliability. The memory chip is expanded via SPI on-chip serial peripherals, and FRAM (Non-Volatile RAM) with power-down protection is used to remember the position of the electric seat even when power is off.
motor
The motor has an additional power of 55W, an additional voltage of 24V, operates under short-term running conditions, and an additional speed of 1800rpm. It is designed using a square wave configuration and has twelve slots with two pole pairs. The slot shape is trapezoidal-shaped and fan-shaped. The winding configuration is a three-phase star connection with six states, and the slot fill factor is 52%. The rotor magnets are made of bonded neodymium iron boron and are radially magnetized. The detailed parameters of the magnets 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 0.2394 N·m.
controller
The main control unit of this controller uses the TI DSP chip TMS320LF2407A. The main control unit needs to detect the motor's current signal and rotor position signal, and provide six PWM drive signals to the drive board. Therefore, controlling a brushless DC motor requires 6 PWM outputs, one ADC conversion, and three capture units.
Regarding the TMS320LF2407A processor, because it has two task managers (EVA and EVB), each with three capture units and capable of outputting six PWM signals, this chip can simultaneously provide control signals to two brushless DC motors. While the peripheral circuitry of the control chip is relatively mature, it possesses a large program memory. Therefore, to achieve flexible motor control, the focus is on the software aspect, rather than the hardware.
drive
The drive circuit of this motor control system adopts a full-bridge structure. By controlling the on/off state of six power components, the motor voltage and current waveforms are modulated. PWM modulation of the motor voltage is achieved using four independent 16-bit general-purpose timers and six full comparators contained within the TMS320LF2407A. The TMS320LF2407A includes programmable dead-time control; any one of the six full comparators, along with the general-purpose timers and dead-time control unit, is used to generate a pair of PWM outputs with programmable dead-time and output polarity. There are twelve such PWM outputs in total. Six outputs can be used to control a brushless DC motor, enabling DSP control of two motors.
The IRF540 power transistor is selected. This device has a maximum operating voltage of 100V, a maximum operating current of 28A, and a maximum power loss of 150W, which fully meets the power requirements of this system. The freewheeling diodes D1 to D6 are fast recovery diodes. The capacitors C1 to C3 are used to absorb the peak voltage on the DC bus to prevent the power transistor from being damaged by excessively high bus voltage. The resistor R2 is used to detect the current on the DC bus of the motor to prevent the motor from malfunctioning due to overcurrent. This overcurrent signal (Isence) is processed and input to the pre-stage driver chip and DSP processor of the power bridge, which can promptly block the gate drive signal in case of overcurrent.
The power transistor preamplifier uses the IR2130 motor driver chip from IR Corporation. This chip integrates driving, dead time, and overcurrent protection functions into one, and can drive six power transistors with a single power supply. It is extremely convenient to use and is especially suitable for low-power drive applications.
software
Based on the defined functions, the software is divided into the following major modules:
Initialization modules: main program initialization module (_c_int0), EVA initialization module (EVA_INIT), EVB initialization module (EVB_INIT);
Motor starter modules: Motor A starter module (MOTORA_START), Motor B starter module (MOTORB_START);
Commutation modules: Motor A commutation module (COMMUTATION_A), Motor B commutation module (COMMUTATION_B);
Hall signal capture modules: Hall signal capture module for motor A (CAPIN_A), Hall signal capture module for motor B (CAPIN_B);
Keyboard analysis modules: Keyboard input analysis module for motor A (KEY_ANALYSE_A), Keyboard input analysis module for motor B (KEY_ANALYSE_B);
PID control module (_PID), orientation memory module (REM).
The main software adopts a modular structure commonly used in engineering, and Hall signal acquisition and PID regulation use a stop control method. System initialization completes the configuration of DSP system registers, I/O ports, and system stop settings. The operating status of the two motors is obtained by the keyboard analysis module, which determines whether the motor is running, started, or stopped based on the obtained status. The motor operating status is indicated by the global variables RUN_STATE_A and RUN_STATE_B: 088H indicates the motor is running, 00H indicates the motor is started, and 0FFH indicates the motor is stopped.
The combination of rare-earth permanent magnet brushless DC motors and DSP control technology is an important development direction for brushless motors.
