Design of intelligent toy car based on optical sensor

Intelligent robots are increasingly used in today's society. From ordinary toy robots to industrial control robots, from robots that can cook to robots that can conduct space exploration, it is foreseeable that the application of intelligent robots will be more extensive in the future. Everyone is familiar with ordinary wireless remote control cars, and everyone thinks Nintendo's video game WII is amazing. The familiar ones are not fun, and the amazing ones are unaffordable. This may be a common problem encountered by many people. This design starts from a new perspective of thinking and makes a smart car that can be played in daily life to share with readers who share the same hobby.

Overall system design

The principle of the smart car system is to install a three-dimensional coordinate sensor on the car. The car will have intelligent perception function and will move along with the target object. The system mainly has three components: one is the three-dimensional coordinate light sensor (ETOMS-ET21X111), which is used to collect the moving coordinates of the target object. The sensor is very simple to use; the second is the MCU (EMC-EM78P156), which reads the sensor data to control the motor rotation. EM78P156 is a common MCU on the market, which is easy to use and cheap; the third is the motor. The motor can be an ordinary DC motor, which uses PWM control. The overall framework of the system is shown in Figure 1.

Figure 1 System overall framework

The overall function of this design can be simply summarized as: allowing the car to follow people (or objects). Separately, the following three small functions need to be realized: the sensor can correctly read X,

The Y and Z coordinate values ​​are the first condition. The MCU can correctly judge the size changes of the X and Z coordinate values, which is the key. Some people may wonder why the Y coordinate change is not judged? That is because the car cannot jump up and down (up and down are the Y axis). The MCU controls the motor direction and the motor PWM time according to the size changes of the coordinate values, which is the result.

Hardware system design

1 Sensor peripheral circuit design

ETOMS-ET21X111 is a high-performance light sensor with X, Y, and Z coordinate data output functions. It has the following features: high-speed data output, up to 75 frames of coordinate data per second; low voltage operation, voltage range 2.7 ~ 3.5V; standard RS232 serial data output format to output coordinate values; use external crystal oscillator, range 0.5 ~ 12MHz, usually 3.58MHz; with controllable exposure interface EO4 ~ EO7.

The four interfaces EO4 to EO7 are used for exposure control. They can be controlled by software or hardware. You can choose the appropriate one according to your needs. This design uses hardware to set all four interfaces to high level.

The detailed interface circuit around the sensor is shown in Figure 2. From Figure 2, we can see that EO4~EO7 are high. This is the exposure setting that is pulled high by hardware. It can also be set in software. When the IC is working normally, the coordinate data is output by the RS232 port. Note that the 4 LEDs in Figure 2 are infrared LEDs. The IC operating voltage is 3.3V, and the system is powered by 5V. The IC uses a 3.58MHz external crystal oscillator, and it can work normally after power-on automatic reset.

Figure 2 Sensor interface circuit [page]

2 MCU interface circuit design

The detailed design of the MCU peripheral control circuit is shown in Figure 3. In Figure 3, L and L+ control the left motor PWM, and R and R+ control the right motor PWM. RS232 receives the sensor coordinate data input. The IC works at 3.3V voltage and automatically resets after power-on. The system clock uses a 4MHz external crystal oscillator.

Figure 3 MCU interface circuit

3 Left motor control circuit

The left motor control circuit is shown in Figure 4. The right motor control circuit is the same as the left one. In the figure, Q3 and Q4 use PNP tubes, and L and L+ cannot be LOW at the same time to avoid short circuit.

Figure 4 Left motor control circuit [page]

Software system design

After the system is powered on, it is first initialized, the registers of EMC78P156 are set, the interrupt flag register is enabled, and the interrupt is waited. Figure 5 is the main program flow chart.

Figure 5 Main program flow chart

When an interrupt occurs, the interrupt handling subroutine is entered. First, the interrupt flag must be turned off and the scene must be protected. Then the XYZ coordinate values ​​are read and analyzed, which can be divided into the following situations.

(1) Determine the change in the X-axis. If the X value is greater than 14 and less than or equal to 17, the motor does not rotate left or right. Then determine the change in the Z-axis coordinate value. If the Z value is also greater than 14 and less than or equal to 17, the motor does not rotate forward or backward.

(2) If the X-axis coordinate value is greater than 17, determine the Z-axis coordinate. If the Z value is greater than 17, reverse the right motor and then turn the left and right motors backward. If the Z value is less than 14, rotate the left motor forward and then turn the left and right motors forward. Otherwise, the motors do not rotate.

(3) If the X-axis coordinate value is less than 14, determine the Z-axis coordinate. If the Z value is greater than 17, reverse the left motor, and then the left and right motors rotate backward; if the Z value is less than 14, rotate the right motor forward, and then the left and right motors rotate forward; otherwise, the motors do not rotate. The flow of the interrupt handling subroutine is shown in Figure 6.

Figure 6 Interrupt handling subroutine flow[page]

Design Tips

1 Sensor design tips

ET21X111 has the best spectral response to infrared rays, but natural light contains a large amount of infrared rays, so strong natural light will affect the sensor data, causing a large deviation between the output coordinates and the actual coordinates. The solution is to add a filter, but this can only have an attenuation effect. The specific application depends on the situation.

2 Motor Control Circuit Design Techniques

When designing a circuit to control the forward and reverse rotation of a motor, please note that: when the MCU is powered on, the state of the I/O is uncertain, so at the beginning of the program, the two I/Os of Q3 and Q4 must be set to HI (Q3 and Q4 are PNP tubes, if they are NPN tubes, the I/O is set to LOW) to prevent both I/Os from being LOW when powered on, causing Q3 and Q4 to be turned on and form a short circuit. In addition, it should be noted that only one of Q3 and Q4 can be turned on at the same time.

3 MCU Design Tips

When the brushless DC motor starts or rotates, it will generate large power burrs, which is very detrimental to the operation of the MCU, so this LC π-type filter circuit is added, as shown in Figure 7.

Figure 7 Filter circuit

4 Programming skills

When the smart car is running, it needs to read the coordinate data from the sensor while controlling the PWM output of the motor. The sensor will output coordinate data every 12ms, so the best way is to use interrupts to read the sensor data and perform PWM output when there is no data output.