Implementation of a smart car navigation system with photoelectric sensing and path memory

The birth of the smart car provides a new means of transportation for people to alleviate urban traffic congestion and improve vehicle safety. Among the many smart car navigation solutions, visual navigation is one of the hotspots of smart car research because it is the closest to human driving. At present, visual navigation methods have achieved initial success in the highway environment. Such environments are highly structured, road curvature is limited, and road conditions are relatively simple. However, with the rise of urban environmental smart car research, visual navigation has faced new challenges. There are not only many types of roads in urban environments, but also a large turning radius, which often leads to road tracking failure due to limited vision. This paper will introduce the overall scheme of the smart car, path identification scheme selection, steering and drive control and path memory algorithm.

Smart car overall plan

The smart car system is based on Freescale's MC68S912DP256 and consists of a power module, a sensor module, a DC motor drive module, a steering motor control module, a control parameter selection module, and a single-chip module, as shown in Figure 1. The working voltage of the smart car system is composed of three systems of +1.6V, +5V and 7.2V, of which 7.2V is used to supply power to the drive motor and the steering gear, and 5V is used to supply power to the vehicle speed sensor, MCU and photoelectric sensor receiving tube, 1.6V. Power the LED. In order to facilitate the adjustment of the online control parameters, a control parameter selection module is also provided, and different programs or control parameters can be called through several button settings to adapt to different site conditions.

Figure 1 Overall structure of the smart car

The working mode of the smart car is: the photoelectric sensor detects the track information, the speed sensor detects the current speed, the battery voltage monitoring circuit detects the battery voltage, and the information is input into the single chip for processing. The control algorithm is used to issue control commands to the car, and the steering trajectory and speed of the car are controlled in real time through the steering servo and the drive motor.

Path identification scheme selection and circuit design

The path identification scheme is the first to be determined, and there are mainly the following problems.

* Photoelectric recognition or camera recognition;

* How are the sensors arranged? How large is the interval, shape, single row or double row;

* The distance the sensor can detect forward;

* Whether the sensor signal is digital or analog;

* How to implement on the circuit.

Because the photoelectric recognition scheme is simple and reliable, this paper adopts the photoelectric recognition scheme.

Digital photoelectric recognition and analog photoelectric recognition

The photoelectric sensor is a sensor in which a photovoltaic device is used as a conversion element. It can be used to detect non-electricity directly causing changes in light quantity, such as light intensity, illuminance, radiation temperature measurement, gas composition analysis, etc. It can also be used to detect other non-electric quantities that can be converted into light quantity changes, such as part diameter, surface roughness, strain. , displacement, vibration, velocity, acceleration, and the shape of the object, the identification of the working state, and so on. Photoelectric sensors are widely used in industrial automation devices and robots because of their non-contact, fast response and reliable performance. In recent years, new optoelectronic devices have emerged, especially the birth of CCD image sensors, which has opened a new page for the further application of photoelectric sensors.

The analog photoelectric sensor can theoretically greatly improve the path detection accuracy. The illuminating and receiving of the analog photoelectric sensor are conical spaces with a certain taper angle. The voltage is proportional to the horizontal distance of the sensor from the black path marking line: the closer the black line is, the lower the voltage is, the farther away from the black line , the higher the voltage (the specific correspondence is related to the photoelectric tube model and the height from the ground), as shown in Figure 2.

Figure 2 Schematic diagram of sensor voltage and offset distance

Therefore, as long as the sensor voltage-offset distance characteristic relationship is grasped, the distance between each sensor and the black mark line can be determined according to the magnitude of the sensor voltage (instead of merely judging whether the sensor is on the line), thereby obtaining the relative path mark of the longitudinal axis of the vehicle body. The position of the line gives the continuously distributed path information.

According to the actual vehicle test, the accuracy of the path detection can be improved to 1mm. The information collected by the sensor can ensure that the MCU can obtain accurate track information, thus ensuring the precise control of the car.

Double row arrangement and forward looking design

In this paper, the smart car performance simulation platform [2] is developed, and the layout of the sensor is deeply studied [3]. Since the steering gear, the motor and the car are all high-order inertia delay links, it takes a certain time from input to output. The sooner the information about the road ahead is known, the more the hysteresis from input to output can be reduced. The track that detects a certain distance in front of the car is called forward-looking. In a certain prospective range, the larger the forward-looking sensor solution, the higher the limit speed will be, and the accuracy of the guide line in the high-speed driving process is relatively high. The overall response performance of the system is good. Therefore, the path identification module is designed to be raised to form an angle with the ground, the front row sensor is used for forward looking, the rear row sensor identifies the starting point of the track, and calculates the deviation slope of the longitudinal axis of the vehicle from the center line of the track to facilitate better Adjust the posture of the vehicle.

In order to ensure that the photoelectric sensor still has sufficient luminous intensity when the ground clearance is as large as possible, this paper adopts the control mode of large current pulse triggering illumination.

According to the experimental test, the current passing through the LED is about 0.5A. If 15 sensors are used, the instantaneous current is 7.5A. Such a large current will definitely cause a certain impact on the battery voltage, which is not conducive to the whole system. normal operation. Therefore, the illumination time of the front and rear rows of sensors is staggered, and the illumination is controlled by two sets of trigger circuits. This effectively reduces the impact on the battery voltage when the infrared light-emitting tube emits light.

Steering and Drive Control and Path Memory Algorithm

Drive motor control

In this paper, a toothed disc is added to the output shaft of the motor, and the rotation of the output shaft of the motor drives the rotation of the toothed disc. Place the pair of light emitters and the receiving tube on both sides of the code wheel. When the code wheel rotates, the light transmitted by the teeth on the code wheel passes through the light-emitting tube, which hinders the light from propagating. Therefore, the resistance across the receiving tube will vary greatly, so that the voltage across the sampling resistor will vary greatly in the circuit.

Using the pulse capture port on the processor to collect the number of voltage pulses per unit time, the motor speed is obtained and the vehicle speed is obtained.

The motor drive adopts Freescale's MC33886. The difference is that this paper uses three MC33886 parallels, which can reduce the on-resistance and improve the motor drive capability, and the heating condition of MC33886 has also been greatly improved; On the other hand, reduce the impact of the overcurrent protection circuit inside the MC33886 on motor starting and braking.

The motor adopts PID closed-loop control, which can adjust the duty cycle of PWM in time according to different load conditions, so that the vehicle can quickly track the target speed.

In order to increase the speed of the vehicle as much as possible, the maximum target speed is set on the straight road, the fixed speed control is adopted, and the speed is reduced near the curve. When the curve is officially turned into the curve, the speed is adjusted to the extreme speed of the cornering, and the speed will be accelerated in advance when the curve is to be exited. .

Steering control

According to the layout of the double-row analog photoelectric sensor currently used, the offset of the longitudinal axis of the vehicle body from the center line of the track can be obtained, and the slope of the center line relative to the longitudinal axis of the vehicle body can be obtained, thereby knowing the posture of the vehicle body in the current state. Further, steering control is performed.

Here, the rotation angle obtained from the front row sensor signal is θ1, and the rotation angle obtained from the longitudinal axis slope information obtained from the front and rear row sensor signals is θ2, and the final steering angle is determined as follows:


By adopting such a control strategy, weighted control of the actual attitude of the vehicle can be realized, the cornering speed is greatly improved, and the cumulative error of decision due to the detection accuracy problem is reduced. In addition, the combination of large forward and double row achieves the characteristics of early turn in normal corners and delayed steering in S-curve.

In order to make the steering gear better respond to a given corner value, PID adjustment is used to adjust the parameters through road test, so that the vehicle maintains high stability at high speed.

Path memory algorithm

Since the competition rules require the vehicle to travel two times on the runway, the first lap of the vehicle records the number of pulses collected by the rotational speed sensor and the steering angle of the steering gear to determine the distinction between straight, curved, S-curved and cornering. Information such as direction and turning radius. According to the data information recorded on the first lap, each road point of the second circle can be segmented. The highest speed is used on the straight road, and the speed is decelerated before entering the curve, which is reduced to the maximum speed of the corner. For different corners, choose different speeds. The advantage of the path memory algorithm is that for complex S-curves, the effect achieved by a CCD-like probe can be achieved, with a small steering angle being used, which can greatly shorten the time. See [4] for specific algorithms.

Experience and conclusion

The development of the smart car in this paper has evolved into a pulse-illuminated, large-fronted, double-row, and analog sensor solution from the initial small forward-looking single-row digital sensor through 6 rounds of development and development; the control strategy is upgraded from simple PID control to path. Memory control makes the navigation performance of the vehicle greatly improved. Through the smart car development process, some experience is gained.

* At the beginning of development, actual detection of photoelectric sensor characteristics, steering gear characteristics, drive motor characteristics, vehicle mechanical performance, steering side slip characteristics, and battery characteristics are required.

* According to the automobile theory, the vehicle is structurally adjusted within the allowable range of the rule to achieve better mechanical performance.

* The organizing committee developed a simulation platform, which should make full use of the simulation tool to study the path identification scheme based on photoelectric sensor, and combine the hardware selection and its own experience in control and electronics to determine the path identification scheme. A forward-looking solution helps to increase the speed of the vehicle.

* Vehicle control can meet the requirements by PID, and the setting of parameters needs to be combined with road test. The speed and speed of the car should not be too drastic, and smooth control can also achieve good results.

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