Summary
In this paper, aiming at the electromagnetic energy-saving group of the 12th National College Students' Smart Car Competition, a brushless DC motor (BLDC) speed control system is designed with a single power supply of super capacitors . Under the premise of realizing the full discharge of the supercapacitor and the maximum utilization of electric energy, the device realizes the closed-loop stable operation of the control system and can output PWM for speed regulation through the MCU , which greatly reduces the complexity of the driving algorithm.
Keywords: super capacitor BLDC speed control system PWM
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Chapter 1 System overall scheme design
According to the relevant regulations of the competition rules, the smart car system adopts the 32 -bit microcontroller K60P100 of NXP (formerly Freescale) as the core controller, and the software is developed in the IAR development environment. The motor speed control adopts "voltage feedforward + PID " composite control, uses the input capture function of the K60 single- chip FTM module to calculate the speed and distance of pulse counting, and adjusts the motor speed through the PWM control drive circuit to complete the closed-loop control of the speed of the smart car.
The system consists of three modules: microcontroller module, power module, brushless motor drive module. The functions of each module are as follows:
1. The 32 -bit microcontroller k60 module, as the control center of the whole system, can collect the voltage signal at both ends of the supercapacitor and the pulse signal fed back by the drive module in real time , make control decisions according to the control algorithm, and output a pulse width modulation signal (PWM) To drive the brushless DC motor (BLDC) to complete the control.
2. The power supply module provides a stable and appropriate power supply for the hardware system of the entire smart car.
3. The brushless motor drive module drives the brushless DC motor to adjust the speed according to the control signal given by the microcontroller , and at the same time, the brushless motor feeds back the speed pulse to the microcontroller .
Chapter 2 Hardware Circuit Design Part
2.1 Power supply circuit design part
For the system we designed, we designed the circuit shown in Figure 2.1
Figure 2.1 Schematic diagram of the power module circuit
Use 3.3V to power the K60 microcontroller, using LDO . The input end is connected to the output end of TPS63070 .
2.2 BLDC selection and transmission design part
2.2.1 Motor selection
According to the relevant regulations of the competition rules, the motors of the electromagnetic energy-saving group can be selected by themselves. When the smart car is running on the track, the motor is the main power-consuming device, so we select brushed DC motor ( RS540 motor), brushless DC motor , and hollow cup motor ( 716 motor) with more suitable parameters . The relevant parameters are compared, and the more energy-saving motor is selected.
Table 1 Comparison of three types of motor-related parameters
|
RS540 motor |
Brushless Motor |
Hollow Cup Motor 716 |
Rated voltage |
4.8-9.6V |
7.4-11.1V |
3.7V |
Rated current |
9.55A |
7A |
0.8A |
rated power |
64.9W |
55W (7.4V), 80W (11.1V) |
46.3W |
Rated torque |
31.0mN · m |
117.5mN·m |
13mN·m |
Rated speed |
20040 |
4470 |
34000 |
Motor weight |
160g |
22.5g |
3g |
From Table 1 , from the perspective of rated current, it is not difficult to find that the hollow cup motor is the most power-efficient, the brushless motor is the second, and the brushed DC motor consumes the most power; however, considering the low output torque of the hollow cup motor, and the planetary gear motor Although The torque can be greatly improved, but it involves a complex mechanical gear structure, and friction between the gears produces energy loss during operation, so a brushless motor is finally selected.
2.2.2 Transmission Design
For the sake of energy saving, we did not choose gear transmission, but a belt transmission structure. The gear ratio between the motor and the rear axle is 3 : 5 . The belt drive mechanism has a great influence on the driving ability of the car model. Improper installation of the belt transmission part will increase the load on the rear wheel driven by the motor; if the belt is too loose, it is easy to idle, and if the belt is too tight, it will increase the transmission resistance. Therefore, we try to keep the drive belt shaft parallel during the motor installation process, and the transmission part is easy and smooth, and there is no jamming or hysteresis. The noise is very small, and there is no collision noise. The rear wheel deceleration belt mechanism is basically adjusted, and the power transmission is very smooth.
2.3 BLDC drive circuit design part
2.3.1 BLDC basic drive circuit
For the BLDC motor we selected , we began to design its drive circuit. In the process of analyzing the driving principle, we read the relevant information and found the basic driving circuit, that is, the three-phase bridge full-control driving circuit.
However, it is obvious that this fully controlled circuit using discrete components requires a high level of algorithm complexity, is not easy to operate, and consumes power, so we decided to switch to a BLDC driver chip with integrated MOSFET .
2.3.2 Selection of BLDC driver chip
At present, there are many BLDC driver chips on the market. The companies with excellent performance in this respect include TI and Microchip in the United States . Therefore, we have retrieved the two most suitable chips for this system from the official websites of these two companies. They are Texas Instruments' DRV11873 and Microchip's MTD6501D, both chips are sensorless brushless DC motor drivers.
Table 3 Comparison of performance parameters of driver chips
|
DRV11873 |
MTD6501D |
Input voltage range |
5V-16V |
2V-14V |
Overcurrent Protection Limits |
Adjustable with external resistor, max 2A |
Built-in, not adjustable, 0.8A max |
PWM input frequency |
7k~10kHz |
0.02k~100kHz |
PWM high level input voltage |
2.7V~5.5V |
3.2V ~ 3.6V |
Trigger commutation conduction mode |
150 ° turn-on |
180 ° turn-on |
From Table 3 , it can be seen that both have a wide voltage input range, which is in line with the characteristics of the supercapacitor supply voltage gradually decreasing; both support PWM speed regulation, but MTD6501D only supports 3.2V~3.6V as the high level. PWM wave, the high level output by the k60 single-chip microcomputer we selected is about 5V , which will trigger the overvoltage protection of this chip.
DRV11873 adopts three-phase four-wire system, and adds a COM line on the basis of the original three-phase three-wire system , which can adapt to three-phase balanced drive of three-phase unbalanced load; but MTD6501D does not adopt this method.
Studies have shown that the MTD6501D consumes much more power than the DRV11873 during startup and stall .
In addition, the literature [1] compares and analyzes the influence of commutation methods on torque fluctuation. When the winding inductance of the brushless DC motor cannot be ignored, the optimal commutation lead angle is selected and the influence of the commutation freewheeling process is considered. Appropriate 150 ° 12-beat commutation can be used to reduce torque fluctuations when running at high speed or low speed .
To sum up, we choose DRV11873 as the driver chip of BLDC in this system.
2.3.3 PCB Layout
Referring to the technical documentation of the chip, a reliable circuit board has been designed, and the test results are excellent.
Chapter 3 Speed closed-loop control algorithm part
3.1 Traditional digital PID control algorithm
The figure shows the structure diagram of the traditional digital PID control algorithm. The core formula of PID operation is as follows:
U (k) = U (k-1)+ KP [E (k)-E (k-1)]+ KI*E (k)+ KD [E (k)-2*E (k-1)+ E (k-2)]
However, we found in the actual debugging that the output speed of the system decreases with the decrease of the capacitor voltage. The reason is that there is an approximate linear relationship between the output speed of the motor driver chip and the power supply voltage, so we introduce the voltage disturbance before the input. Feed control.
3.2 Voltage feedforward + PID control algorithm
The system selects the digital PID with voltage feedforward as the control algorithm of the system closed-loop. The principle of voltage feedforward control: when the capacitor voltage decreases , it is adjusted in time to keep the output speed unchanged. The block diagram of the digital PID control system with voltage feedforward is shown in the figure. The expression of the PID digital control algorithm is
U (k) = U (k-1)+ KP [E (k)-E (k-1)]+ KI*E (k)+ KD [E (k)-2*E (k-1)+ E (k-2)]
In the formula , KP, tI, tD are the set values, KI is the integral coefficient, and KD is the differential coefficient.
KI, KD are calculated by KI= KPT/ tI, KD= KPtD/ T. T is the sampling period.
The core formula of PID operation is also very simple, as follows.
U (k) = U (k-1)+ Kp*[(E (k)- E (k-1)]+ KI*E (k)+ KD*[E (k)- 2*E (k- 1)+ E (k-2)];
U(k-1)= U(k);
E (k-2) = E (k-1);
E (k-1) = E (k);
Uout = U (k);
In the program, you only need to set the input control parameters KP, KI, KD, and the given pulse can be set by yourself.
After testing, when the output is stable, the error between the actual and the given is within ±5%, and the system runs well.
references
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thanks
I would like to thank the School of Electrical Engineering of Yanshan University for its support to the Smart Car Competition, I would like to thank this third-level project for giving me this opportunity to express my views here, and I would also like to thank the two teammates for their support and encouragement over the past year.