Solution for automobile reversing obstacle detection system

The ultrasonic sensor technology used in the reversing obstacle

detection system can detect nearby obstacles and provide the driver with reversing warnings and assisted parking functions. The principle is to use ultrasonic waves to detect any obstacles on or near the reversing path and issue warnings in time. The designed detection system can provide both audible and visual warnings, and the warning indicates that the distance and direction of the obstacle in the blind spot have been detected. In this way, whether parking or driving in narrow places, with the help of the reversing obstacle alarm detection system, the driver's psychological pressure will be reduced, and he can take necessary actions with ease. This PIC l8F8490 microcontroller and ultrasonic sensor are very cheap and can be used in many models. So what is a reversing obstacle detection system based

on ultrasonic sensors? To this end, you should first understand the relevant technical issues of ultrasonic sensors.

The composition and working program of the ultrasonic sensor system (see Figure 1)

It consists of a transmitting sensor (or ultrasonic transmitter), a receiving sensor (or ultrasonic receiver), a control part and a power supply part. The transmitter sensor consists of a transmitter and a ceramic vibrator transducer with a diameter of about 15mm. The function of the transducer is to convert the electrical vibration energy of the ceramic vibrator into ultrasonic energy and radiate it into the air; while the receiving sensor consists of a ceramic vibrator transducer and an amplifier circuit. The transducer receives ultrasonic waves to generate mechanical vibrations, converts them into electrical energy, and uses them as the output of the sensor receiver to detect the transmitted ultrasonic waves. In actual use, the ceramic vibrator of the transmitting sensor can also be used as the ceramic vibrator of the receiver sensor. The control part mainly controls the pulse chain frequency, duty cycle, sparse modulation, counting, and detection distance emitted by the transmitter. The power supply (or signal source) of the ultrasonic sensor can be DC12V±10% or 24V±10%.

If a 40KHz high-frequency voltage is applied to a piezoelectric ceramic piece (dual crystal oscillator) with a resonance frequency of 40KHz in the transmitting sensor, the piezoelectric ceramic piece will stretch and shrink according to the polarity of the applied high-frequency voltage, thus transmitting an ultrasonic wave of 40KHz frequency, which is transmitted in a sparse and dense form (the sparse and dense degree can be modulated by the control circuit). The ultrasonic wave waveform is shown in Figure 1 and transmitted to the ultrasonic receiver. The receiver uses the principle of piezoelectric effect, that is, applying pressure on the piezoelectric element to cause strain on the piezoelectric element, and then a 40KHz sinusoidal voltage with one side as the "+" pole and the other side as the "-" pole is generated. Because the amplitude of the high-frequency voltage is small, it must be amplified.

According to the equivalent circuit and impedance characteristics of ultrasonic sensors, for the transmitting sensor, it works in series resonance, that is, the impedance Zr is the lowest at the resonance frequency fr, so it can supply the maximum power and can use a larger vibration sensor; while for the receiving sensor, it works in parallel resonance, that is, the impedance Zα is the highest at the resonance frequency fα, and it is difficult to supply high power, but a high impedance Zα can get a larger amplitude signal, so the sensor at fα has high sensitivity.

The working mode of the ultrasonic transducer of the reverse obstacle detection system is reflective, that is, the transmitting sensor transducer emits ultrasonic waves with a frequency of 40KHz, which are reflected by the receiving sensor transducer after hitting the obstacle and converted into electrical signals, as shown in Figure 2. Its propagation medium is air.

Design scheme of reversing obstacle detection system based on ultrasonic microcontroller technology

Figure 3 is a block diagram of the design scheme of automobile reversing obstacle detection system. The design scheme includes: IC1 main controller - using Microchip's PIC l8F8490 microcontroller as the main controller of the automobile reversing obstacle detection system; sending part (i.e. sending ultrasonic sensor) and receiving part (i.e. receiving ultrasonic sensor); temperature sensor (TC1047A), communication interface RS-232 driver and peripheral circuits such as LCD or LED display.

Using a microcontroller as the main controller of the detection system [page]

The microcontroller is the core of the reversing detection system. The Microchip PIC l8F8490 microcontroller is very suitable for applications such as automotive body control. Because it is a flash memory, power management microcontroller with an on-chip LCD driver control module function, that is, it has an operating rate of 10MIPS-10 million instructions per second (MIPS), 16KB flash memory, 768 bytes of RAM, an LCD controller, two PWMs, two comparators and four timers, as shown in the middle IC1 of Figure 3. Therefore, it is the main control part of the highly integrated solution for the reversing detection ultrasonic sensor application. The microcontroller uses nanowatt technology to implement power management functions, which can significantly improve power efficiency and system reliability, and can meet low-power design requirements including driving LCD displays in sleep mode. Its series of products can provide up to 192-segment LCD drivers for different embedded control applications, with various package sizes and integrated features.

Control of the transmitter part

The transmitter sends a 40KHz pulse square wave at a rate of 4-5 times per second during 1mS. In order to generate a transmission pulse chain, a driver can be used to drive the ultrasonic transducer. The driver is Microchip's TCl428 MOSFET driver as shown on the left side of Figure 3. Since the standard frequency of the actual ultrasonic sensor characteristics is 40KHz, the 40KHz pulse square wave chain sent by the transmitter is a universal transmission frequency, but it is not a fixed ultrasonic frequency. It can be selected according to the blind area range and the distance of the obstacle. The higher the standard frequency (or center frequency), the shorter the distance measurement and the higher the resolution. Common ultrasonic sensor standard frequencies are 30KHz, 4KHz, 75KHz, etc.

When the transmitter sends the first pulse, timer 1 starts counting. Since the voltage amplitude of the pulse chain is required to be higher than the system voltage (+5V) (this is determined by the signal source required by the transducer), this transmitter part should contain a boost circuit. Here, a simple and effective way to increase the voltage is to use Microchip's MCPl650 boost DC/DC controller, which only needs a capacitor, an inductor and two resistors to easily select the required output voltage. The function of PWM1 (pulse width modulator) in the main controller IC1 is to adjust the output voltage of the boost circuit to a constant value. The

control of the receiver part

The receiver part includes an ultrasonic receiving transducer, an amplifier, a filter and a comparator as shown in the lower left side of Figure 3. The output of the receiving transducer is a low-amplitude sine wave with the same frequency as the transmitting pulse chain frequency. In order to amplify the signal output by the transducer, Microchip's MCP6293 operational amplifier can be used. The characteristics of this operational amplifier are that although the package is small, it has a bandwidth of 10MHz and a pin-selectable low-power mode. The output signal can be transmitted to an LC bandpass filter circuit (also known as an amplitude detection circuit), the center frequency (standard frequency) of the bandpass filter is the same as the standard frequency of the receiver 40KHz. The function of this amplitude detection circuit is to convert the received pulse into a smooth and complete waveform, and its high-frequency noise is filtered out, so that a detected signal is formed. Then the signal is sent to the comparator to be compared with the attenuation voltage. It should be noted that the reference voltage of the comparator is an attenuation voltage (generated by the RC resistor-capacitor circuit), so that the attenuation reference will continuously attenuate the received signal over time. Until it is less than a preset distance value, timer 1 stops counting. The

selection and parameters of ultrasonic sensors

can be selected from domestic TC40-167R series or MA40S2S (transmitting sensor), MA40S2R (receiving sensor) ultrasonic sensors. Its main parameters are (taking TC40-167R as an example): standard frequency 40±0.1KHz; sensitivity (dB) ≥-68; sound pressure (dB) ≥114; direction angle (degrees) 60; electrostatic capacitance/pf-2500; working temperature (℃) -20℃--+70℃; effective distance ≥15M, and the effective distance of reflection reception is 4M-7M.

The detection distance calculation and display of the reversing obstacle

detection system warns the driver when an obstacle is detected. The new automobile reversing obstacle detection system can use multiple sets of sending and receiving ultrasonic sensors to expand the detection range according to the actual use environment, thereby improving the accuracy of obstacles, that is, detecting and displaying the nearest obstacle. Therefore, the number displayed on the LCD or LED display of the reversing obstacle detection system is the distance of the nearest obstacle.

Calculation method of ultrasonic distance measurement

Ultrasonic waves are sent out in the form of pulse trains, and the pulse frequency is the center frequency (or standard frequency). Timer 1 of the PIC l8F8490 main controller (as shown in IC1 in Figure 3) starts counting from the rising edge of the first pulse until the main controller receives a continuously attenuated received signal less than the set value (that is, when the distance is less than a preset value). Timer 1 stops counting. Therefore, the measured time interval (the time difference between the transmitted signal and the received reflected signal) △t multiplied by the speed of sound is equal to twice the measured distance L (2△L). If the propagation speed of ultrasound at temperature T is V, the microcontroller can calculate the distance between the car and the obstacle. The formula for calculating the measured distance is:

To this end, as long as the distance is less than a preset value, not only will there be a display but also an audible and visual alarm signal will be issued.

Solutions to improve the performance and accuracy of the reversing detection system

Transducer selection and installation

Depending on the required accuracy, distance and system cost, there are several different implementation options. The higher the frequency and power of the transducer, the higher the accuracy. The higher the frequency of the transducer, the smaller its volume, which makes the system easier to install on the car. Low-frequency transducers also have the advantage of a larger detection range and easier detection of objects around the transducer. An inexpensive technique to reduce interference is to add a 3 cm tube around the receiving transducer, so that the effective signal can be received and the directionality can be increased.

An important factor affecting the degradation of system performance is the crosstalk between the transmitter and the speaker. The received signal can only be detected if the effect of the transmitted pulse in the receiving transducer is completely eliminated. It is important to minimize the mechanical coupling between the two components. A technique that can be used is to mount each transducer on a different PCB. If they share a substrate, a thin piece of foam plastic can be placed behind the transducer. If a single transducer solution is used for transmit-receive, a sufficiently long delay should be set by software after the transmit and before the receive part is enabled. Any protective coating on the transducer should be avoided. All of these techniques can improve the performance of the ultrasonic system.

Regarding the improvement of system accuracy,

the temperature sensor eliminates this error.

Since the propagation speed of ultrasonic waves changes when the ambient temperature changes, this will cause ranging errors. The error can be eliminated by using the temperature sensor TCl047A to measure the air temperature and then send it to the A/D converter in the main controller for temperature compensation.

Improving the receiver LC bandpass filter effect can increase signal fidelity and system accuracy. The additional gain stage of the reflected signal is also helpful to increase the detection range and improve accuracy.

Conclusion

The above-mentioned type of automobile reversing obstacle detection system design is an application technology combining microcontrollers and ultrasonic sensors, and is a solution to improve driving safety and experience. Because the system can identify obstacles in the driving blind spot, the driver will be more comfortable driving.

Through the comparison of schemes, the reverse obstacle detection system based on microcontroller technology is much more convenient and flexible than the detection system composed of hardware circuit system (such as the detection system composed of LM1812 chip with built-in transmitting and receiving circuit and peripheral circuit), because it can give full play to the advantages of software technology, and can add functions according to the needs of operation and parking environment and replace many hardware circuits, making the reverse obstacle detection system more reliable and accurate.

References:
1. Microchip Technology Incorporated 2004 Product Catalog.
2. Sensor Technology and Its Applications, edited by He Xicai. Published by Beijing University of Aeronautics and Astronautics Press in April 2005 (end)