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, which 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 drive mechanisms and single-phase AC asynchronous motor drive mechanisms with linear drive mechanisms driven by rare-earth permanent magnet brushless motors has significant social implications and broad market prospects.
System Principles and Components
Figure 1 System schematic diagram
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. As shown in Figure 1, motor A is the pitch motor, and motor B is the height motor.
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, 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 electric seat's position even when power is off.
motor
The motor has a rated power of 55W, a rated voltage of 24V, operates under short-time conditions, and a rated speed of 1800rpm. It is designed as a square wave motor with twelve slots and two pole pairs. The slots are trapezoidal and fan-shaped, and the winding configuration is a three-phase star connection with six states. The slot fill factor is 52%. The rotor magnets are made of bonded neodymium iron boron with radial magnetization. Specific magnet parameters are as follows: Br > 6000Gs, Hc > 5000Oe, [BH]max > 8MG·Oe, Hjc > 10000Oe.
The overall assembly drawing of the motor is shown in Figure 2:
Figure 2. Motor assembly diagram
In the finite element analysis, the actual electromagnetic torque of the motor is 0.2394 N·m. The distribution of parameters such as the motor's magnetic flux and magnetic induction intensity is shown in Figures 3 and 4. As can be seen from Figure 4, the distribution of the air gap magnetic induction intensity of the motor is basically a square wave distribution, and the average value is basically consistent with the electromagnetic scheme design results.
Figure 3. Magnetic flux distribution of the motor
Figure 4. Air gap magnetic induction intensity of the motor
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, to control a brushless DC motor, six PWM outputs, one ADC conversion, and three capture units are required.
The TMS320LF2407A processor, with its two event managers (EVA and EVB), each with three capture units and capable of outputting six PWM signals, can simultaneously provide control signals directly to two brushless DC motors, as shown in Figure 1. While the peripheral circuitry of the control chip is relatively mature, its large program memory necessitates a software-centric approach to achieving flexible motor control, rather than focusing solely on hardware.
drive
The drive circuit of this motor control system adopts a full-bridge structure, modulating the motor voltage and current waveforms by controlling the switching on and off of six power components. PWM modulation of the motor voltage is achieved using four independent 16-bit general-purpose timers and six full comparators integrated into 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.
Figure 5 Power Main Circuit
The main power circuit of the control system is shown in Figure 5. An IRF540 power transistor is selected, which has a maximum operating voltage of 100V, a maximum operating current of 28A, and a maximum power loss of 150W, fully meeting the power requirements of the system. Fast recovery diodes are used for freewheeling diodes D1-D6. Capacitors C1-C3 are used to absorb voltage spikes on the DC bus to prevent excessive bus voltage from damaging the power transistor. Resistor R2 is used to detect the current on the DC bus of the motor to prevent overcurrent failure. This overcurrent signal (Isence) is processed and input to the pre-amplifier chip and DSP processor of the power bridge, allowing for timely blocking of the gate drive signal during overcurrent.
The pre-amplifier for the power transistors uses the IR2130 motor driver chip from IR Corporation. This chip integrates functions such as driving, dead time, and overcurrent protection, and can drive six power transistors using a single power supply. It is extremely convenient to use and is particularly suitable for low-power drive applications.
software
Based on the defined functions, the software is divided into the following main modules:
Initialization modules: main program initialization module (_c_int0), EVA initialization module (EVA_INIT), EVB initialization module (EVB_INIT);
Motor starting modules: Motor A starting module (MOTORA_START), Motor B starting 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), position memory module (REM).
Figure 6 Software Main Flowchart
The main software adopts a modular sequential structure popular in engineering, and Hall signal acquisition and PID regulation use interrupt control. Figure 6 is the system software flowchart. Figure 6(a) is the main program flowchart, which mainly realizes system initialization and the reading and setting of the operating status of the two motors. After system initialization, the DSP system registers, I/O ports and system interrupts are set. The operating status of the two motors is obtained by the keyboard analysis module. Based on the obtained status, it is determined whether the motor is in the running, starting or stopping state. The motor operating status is represented by the global variables RUN_STATE_A and RUN_STATE_B. 088H indicates that the motor is in the running state, 00H indicates that the motor is starting, and 0FFH indicates that the motor is stopping.
In the software, both motor commutation and PID speed regulation employ interrupt control. Figure 6(b) is the motor inverter commutation flowchart. Each module of this interrupt subroutine performs three main functions: capturing Hall position signals, inverter commutation, and speed calculation, ensuring normal motor operation and providing speed feedback values for PID speed regulation. Figure 6(c) is the motor PID regulation interrupt flowchart; the interrupt signal for PID interrupt regulation is generated by internal software.
The combination of rare-earth permanent magnet brushless DC motors and DSP control technology is an important development direction for brushless motors.
