The current Wheelchair Brushless Controller design can be roughly divided into two types: one is to use the front passive wheel and the rear wheel drive method, because it is the front passive wheel and the rear drive wheel method, so it cannot be used in places with a narrow turning radius. The other is based on the omni-directional movement method that you want to be able to move freely front, rear, left, and right. This method is to install the driving wheel in the area near the center of the car body, which can adapt to a certain narrow environment, but because the wheel and the car body cannot Relative motion occurs, so no matter how hard you work on the drive configuration, you can't make the radius of gyration zero.
Aiming at the shortcomings of the above two wheelchair control schemes, an omnidirectional wheelchair control system is designed and implemented. It is an electric wheelchair control system with a chip as the control core. It adopts electric wheel PID control and two-way electric wheel speed difference coordinated control. Scheme, the main circuit structure is given, the design scheme is realized through the actual circuit, and finally the feasibility of the design scheme is proved through the experimental results.
1. Overall system design
It mainly uses a single digital signal processor with two brushless DC motors, which directly controls the two motors through the digital signal processor, and sends the current position feedback signals of the two brushless DC motors to the digital signal processor. The processor calculates the feedback signal obtained by sampling and the given position signal, and sends out the corresponding pulse width modulation wave, controls the three-phase inverter, and then controls the action of the motor.
In the system, the two brushless DC motors are a centralized whole for the digital signal processor. The digital signal processor obtains and feeds back the data. In the system, the processing of the two brushless DC motors is sometimes independent of each other. , So it has relatively independent software and hardware systems. In addition, the control performance of each channel is almost the same, so it can be interchanged without affecting the control performance.
The button keyboard and display components are directly linked to the digital signal processor, there is no signal interference problem during the transmission process, and the various stages of the system can be directly controlled by the button keyboard, and the display components can be intuitive reflect.
The hall sensor of the motor sends the rotor position of the motor to the capture unit of the digital signal processor with three high and low signals, and then compares with the commutation table to control the power on and off of the three-phase inverter to make it Match the position of the rotor.
In addition, the system uses the sampled phase voltage signal as a power protection signal and sends it directly to the power protection pin of the digital signal processor after level conversion. When a low level appears, it will trigger the power protection interrupt of the digital signal processor. The signal processor will block the pulse width modulation wave of the corresponding channel.
The pulse width modulated wave from the digital signal processor is isolated by the optocoupler and sent to the power drive chip. After power amplification, it is sent between the gate and source of the three-phase inverter power tube to control the switching action of the power tube.
2. System control strategy
The design of the main control strategy includes the following two aspects.
2.1 Single-channel control strategy
The single-circuit motor uses PID closed-loop control to adjust the speed. The closed-loop performance of the system position can directly determine the dynamic and static performance of the motor system, which is a key part of system control.
The PID algorithm has the characteristics of simple structure and convenient debugging. Although the amount of change is fixed during adjustment, the frequency generated by the closed loop is very high, so when the amount of change is set to a smaller amount, the system performance requirements can also be met. By debugging the actual circuit parameters, a good overshoot can be achieved while basically satisfying the rapidity of the system, thereby improving system stability.
2.1 Two-way motor control strategy,
The design contains two independent driving electric wheel control systems. Since the two motors are installed on the shafts of the two driving wheels, when the internal parameters of the two-wheel drive device and the external road conditions are not equal, even if the two motors are input Similarly, there is no guarantee that the speeds of the two driving wheels are the same. Here, a compensation system is used to eliminate the speed difference between the two electric wheels during the operation of the system.
In linear running, the speed difference coordination system is used to adjust the speed of the two electric wheels. Set a driving wheel so that the speed of the driven wheel follows the driving wheel, so that the speeds of the two electric wheels are as consistent as possible. When turning, the inner and outer motors decelerate at the same time. The reduction of the inner motor is much greater than that of the outer motor. Use their own reference speeds to perform independent PID calculations to ensure that the wheelchair turns when the road conditions are not very good.
3. System hardware design
According to the functional requirements of the Wheelchair Brushless Controller , the design is mainly composed of a digital signal processor and two brushless DC motor drive circuits, as well as necessary level conversion circuits. The digital signal processor is responsible for the design and control of the entire system, and the collection of all signals And processing is completed by the digital signal processor, and also generates the required pulse width modulation signal and fault protection interrupt request.
The drive circuit mainly uses the drive chip as the core drive to control the speed and steering of the motor. To prevent the voltage across the bootstrap capacitor from discharging, the diode should be a high-frequency recovery diode. On the other hand, in order to prevent the voltage at both ends of the bootstrap capacitor from being discharged below the threshold voltage of the undervoltage protection action, the value of the capacitor should be sufficiently larger.