Smart electric vehicles and drive system circuit design strategy

　　basic requirements

　　① The electric vehicle starts from the starting line (the vehicle body must not exceed the starting line) and reaches point B along the 2cm wide black guide line. Under the white paper laid in the "straight area", 1 to 3 thin iron sheets with a width of 15cm and different lengths are buried along the guide line. When the electric vehicle detects the thin iron sheet, it immediately emits sound and light indication information, and stores and displays the number of thin iron sheets detected in the "straight area" in real time.

　　② After reaching point B, the electric vehicle enters the "curve area" and reaches point C along the arc guide line (it can also leave the arc guide line to reach point C). There is a square thin iron sheet with a side length of 15 cm buried under point C. The electric vehicle is required to stop at point C for 5 seconds after reaching point C and detecting the thin iron sheet, and emit intermittent sound and light information during the parking period.

　　③ Under the guidance of the light source, the electric vehicle passes through the obstacle area, enters the parking area and reaches the garage. The electric vehicle must pass between two obstacles and must not touch them.

　　④ The electric vehicle shall stop immediately after completing the above tasks. The whole journey shall not exceed 90 seconds. It shall stop automatically immediately when the driving time reaches 90 seconds.

　　Figure 2 System overall block diagram

　　Line tracking circuit

　　Solution 1: Use a CCD monochrome camera, a computer motherboard and an image acquisition card. The recognition of black lines against a white background is relatively mature and has good results. However, it is expensive and difficult to find a suitable carrier.

　　Solution 2: Use color sensors. Currently, color sensors are used more and more widely, and the effect is acceptable. However, they cost several hundred yuan, have relatively complex processing circuits, and require a light source, so they are not a good choice.

　　Solution 3: Use two infrared transmitting and receiving tubes, one on the left and one on the right. This sensor is not only cheap and easy to buy, but also has a simple and easy processing circuit (as shown in Figure 3). It has a good actual use effect and can smoothly guide the car to point C.

　　In this circuit, the purpose of adding comparator LM311 is to convert analog quantity into switch quantity for easy processing. In order to ensure a certain power of transmission, the transmission circuit requires a current of no less than 20mA.

　　According to , R1=150Ω can be selected.

　　When starting, the car straddles the black line. Two infrared transmitting and receiving tubes are installed in the white area on both sides of the black line. The output is low voltage. When it deviates and is on the black line, the output is high voltage. Because the black line is narrow (2cm), in order to adjust the direction of the car in time, the comparator threshold is selected to be 2.5v, that is, the position of black and white, and the adjustment begins. Experiments show that the effect is ideal.

　　Figure 3 Infrared transmitter and receiver processing circuit
 

　　Obstacle avoidance circuit

　　Solution 1: Use laser sensor to measure distance. It can measure the distance between the car and the obstacle very accurately, but the price is high and the processing is complicated, which does not meet our requirements.

　　Solution 2: Use ultrasonic sensors. Imported ultrasonic sensors have thin transducers and processing circuits. They output analog signals proportional to distance. Through AD conversion, distance information can be obtained. They are expensive. There are also some simpler ultrasonic sensors and processing circuits that can output switch information. They are not expensive and are a good choice. However, since I didn’t buy a ready-made processing circuit and I have never made such a circuit before, I didn’t use it because I was short on time.

　　Solution 3: Use two infrared sensors on the left and right. Infrared sensor is a commonly used obstacle avoidance sensor. Its processing circuit is shown in Figure 4. By adjusting the two potentiometers R23 and R24, the detection distance of the two infrared sensors can be adjusted to 10-80cm. The switch output (TTL level) is simple and reliable. We use this circuit to reliably detect obstacles in the left front, right front, and front, which provides a guarantee for successful obstacle avoidance.

　　Figure 4 Infrared transmission and reception processing circuit

　　Light source detection circuit

　　In order to detect the intensity of light, we added two photosensitive sensors, i.e. photoresistors, to the left and right front of the car. The circuit is shown in Figure 5. The resistance of the photosensitive sensor changes according to the intensity of the light shining on it, and the output voltage changes accordingly. After passing through ADC0809, a digital value corresponding to the light intensity is obtained, thereby guiding the car to approach the light source. Different types of photoresistors have different dark resistance and light resistance, and different voltage divider resistors need to be selected according to the different parameters of the photoresistors.

　　Figure 5 Light source detection circuit

　　Metal detection circuit

　　It uses an eddy current iron metal detection sensor, model: LJ18A3-8-Z/BX. The reliable detection distance is less than 8cm.

　　Motor drive circuit

　　The commutation and drive circuits of the electric car are quite rough, and the characteristics of the motor are also very bad, and the speed cannot be adjusted. When the voltage is low, the speed is slow, the driving torque is small, and it cannot move; when the voltage is high (just when the battery is replaced), the speed is very fast and difficult to adjust. A lot of time was spent on this, and the effect was very poor. Finally, it was decided to replace the motor and drive circuit of the car. The rear wheel uses a pair of reduction DC motors, and its drive circuit is shown in Figure 6. Using PWM control, the motor speed can be adjusted more conveniently.

　　Figure 6 Motor drive circuit

　　The simple intelligent electric car is transformed from an electric toy car. The control part of the system is based on a single-chip microcomputer. By collecting and processing various sensor signals in the forward channel, it can better realize the motion control of the rear channel drive and steering motor and the processing, display and sound and light alarm of related information.