With the rapid development of China's automobile industry and highway industry, the development and development of vehicle collision avoidance systems based on cost-effective ultrasonic ranging technology has important social and economic value.
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The vehicle collision avoidance system has the function of automatically detecting obstacles in front, automatically decelerating or braking, and is a necessary safe driving aid for advanced cars and heavy vehicles in the future. Major automobile companies such as Japan, the United States and Europe have invested considerable human and material resources to develop anti-collision and safety warning systems for use in advanced vehicles, including millimeter-wave radars, CCD cameras, GPS and high-end microcomputers. According to overseas media reports, DaimlerChrysler has successfully developed an electronic brake system for commercial vehicles (especially trucks) that uses a vehicle-mounted forward-looking radar sensor to detect the front scene and the vehicle controller handles the sensory information. A virtual scene is formed, thereby judging whether the current road condition needs to start the automatic braking device. The new brake system will be available in the next two or three years, with an expected price of 3,745 euros [1]. Obviously, in the case of ordinary cars, the automatic electronic brake device is too expensive.
Ultrasonic ranging sensors are inexpensive, and their performance is virtually unaffected by light, dust, smoke, electromagnetic interference, and toxic gases, and is easy to use. However, the common ultrasonic range finder has a short acting distance, generally less than or equal to 10 m, which limits its performance in high-speed driving. The range of the ultrasonic range finder is not only dependent on the high performance ultrasonic probe, but also on the electromechanical energy conversion efficiency of the ultrasonic transmitting and receiving circuits. This paper mainly studies a high-efficiency ultrasonic transducer transceiver circuit to increase the range of the ultrasonic range finder, so that it can be applied in the future domestic car active collision avoidance system.
1 Ultrasonic ranging principle
Acoustic waves with a resonant frequency higher than 20 kHz are called ultrasonic waves. Ultrasonic waves are linear propagation modes. The higher the frequency, the weaker the diffraction ability, but the stronger the reflection capability. Utilizing this property of ultrasonic waves, an ultrasonic sensor, or ultrasonic transducer, can be fabricated, which is a device or device that converts electrical energy into acoustic energy and converts acoustic energy into electrical energy. The transducer can convert electrical energy into mechanical energy and send ultrasonic waves outward under the excitation of electric pulse. Conversely, when the transducer is in the receiving state, it can convert acoustic energy (mechanical energy) into electrical energy.
The most common method of ultrasonic ranging is echo detection. The working principle is that the transducer emits an acoustic pulse to the medium, and the sound wave (echo) that must be reflected back after the sound wave encounters the object (target) is applied to the transducer. If the sound velocity of the medium is known to be c, the time difference between the arrival time of the first echo and the time of the transmission pulse is t, then the distance between the transducer and the target can be calculated according to the formula s=ct/2, as shown in Fig. 1. Shown. Taking into account the cost of the sensor and the ease of installation, the ultrasonic probe with both transceivers is used, that is, the actual distance d=s.
The speed c of the sound wave is related to the temperature T [2]. If the ambient temperature changes significantly, the temperature compensation problem must be considered. The relationship between the speed of sound in air and temperature can be expressed as:
2 drive circuit design
The ultrasonic frequency driving power supply shown in Fig. 2 is used to excite the ultrasonic transducer to transmit ultrasonic waves outward, and the ultrasonic frequency power supply and the ultrasonic transducer device constitute an ultrasonic generator.
2.1 Design of FET power amplifier circuit
Here, a class B push-pull amplifier circuit that is used more on the ultrasonic generator is used. The characteristic is that when there is no excitation signal, the quiescent current of the two power tubes IRF120 is zero; when there is an excitation signal, the two power tubes work alternately, and the output half-wave signals are combined to form a complete waveform.
SN75732 is a dual-channel NAND gate TTL/MOS dedicated interface device, in which pin 2 is the enable input of two NAND gates (active high), pin 1/7, pin 3/6 respectively It is the input/output terminal of two NAND gates; the pin 4 is digital ground; the pin 8 is connected to the 5V DC power supply, and the pin 5 is connected to the DC power supply VDD. With this interface circuit, the MOSFET power transistor can be directly driven by the TTL level. As long as the resistor R1 is properly selected, the gate-source voltage VGS of the MOSFET power transistor IRF120 can be determined, thereby determining the drain current ID when the power transistor is turned on; R is used to limit the magnitude of the drain current ID, so as to prevent the power transistor from being turned on instantaneously. Large current surges. When the strobe signal is low, the two NAND gates of the SN75732 output low level, the power tube IRF120 is turned off, the transmitting circuit does not work, and the relay J is in the on state (contact with SIG1 and SIG2); when the strobe signal is When the level is high, the ultrasonic frequency pulse signal is logically transformed by the NAND gate HC00, so that the two NAND gates of the SN75732 alternately output a high level, and the two power tubes IRF120 are alternately turned on and off (push-pull amplification). The pulse transformer boosts the output of a high-amplitude sine wave, and the transducer radiates the energy as acoustic energy. At this time, the relay J is in a normally closed state (the transducer is connected to the output of the drive circuit).
To make the nonlinear distortion not obvious, its power is the largest and the load should be fixed. Therefore another function of the transformer is to couple the actual load RL' to the desired value RL to achieve impedance matching. As shown in Figure 3, AB and BQ represent the ID and (VDD-VDSS), respectively, so the area of ​​ΔABQ represents the output power of the complementary symmetrical circuit operating in Class B. The larger the area of ​​ΔABQ, the larger the output power Po is. IDm is the maximum current flowing through the power tube, corresponding to the load line AQ in the figure, the power triangle area is the largest, and the nonlinear distortion is not obvious. Therefore, the maximum power load resistance should be RL = (VDD - VDSS) / IDm.
The FET IRF120 adopts a voltage driving method, which has nothing to do with the load current and the safe working area, and the circuit design is relatively simple; for the switching speed, it can greatly improve the switching speed and the temperature influence is small compared with the bipolar device; It is only limited by the power consumption of the tube, and has no effect of secondary breakdown, thus replacing the power transistor as a power amplifier component.
2.2 Transformer design
The pulse transformer is the most important device in the ultrasonic transducer drive circuit. Its purpose is to raise the pulse voltage signal and match the output impedance of the power amplifier with the load impedance of the transducer. Generally, the pulse transformer determines the size and ratio of the transformer by the power of the transformer and the amplitude of the original secondary voltage signal [3]; while the ultrasonic transducer drive transformer mainly determines the transformer by the power and the primary and secondary inductance and impedance matching. Size and ratio.
2.2.1 Determination of transformer operating frequency and input voltage pulse width
The operating frequency of the pulse transformer depends on the operating frequency of the ultrasonic transducer. Here, a transducer with fr=30 kHz is selected, and the corresponding resonant circuit equivalent impedance RL'=450 Ω. Then the voltage pulse width in a half cycle is:
Where T is the duty cycle of the pulse transformer. D is an important parameter in the design of the circuit, it has a great influence on the main switching element, output transformer and converter efficiency. Here, D = 0.9, then Ton = 15 μs.
2.2.2 Determination of Transformer Ratio and Calculation of Power Load
It can be seen from the previous derivation that the power amplifier efficiency is the highest when RL=(VDD-VDSS)/IDm, taking VDD=12V. Considering that the maximum current that the battery used by the vehicle can provide is limited, take IDm=5A, and the power tube operating characteristic curve can be checked. VDSS=2V, and RL=(12-2)/5=2Ω, so the transformer ratio is:
Where, Pout is the operating power of the transducer; η is the efficiency of the transformer, η = 0.95; VAm is the voltage amplitude on the equivalent load RL. Substituting the known value into equation (4) gives Pout ≈ 24W.
2.2.3 Selection of transformer core
The core is an important part of the pulse transformer. The main indicators such as the volume and quality of the pulse transformer are determined by the core. Commonly used core materials are electrical steel, soft magnetic alloy, soft ferrite, amorphous alloy, and the like. Among them, the ferrite core has good processability, low price, and high electrical resistivity, ensuring high effective pulse permeability in the case of narrow pulse, which is more than ten times higher than that of cold rolled electrical steel. For push-pull circuits, the core size can be selected as follows:
Where S is the effective cross-sectional area of ​​the core; Q is the window cross-sectional area of ​​the core, and only the sum of the cross-sectional areas of the windings is smaller than the window area of ​​the core, so that the core window can be wound around all the windings; Bm is the maximum working flux of the core Density, here select the material is 3E25 core, from the BH characteristic curve of the material to find Bm=250mT; KT is the filling factor of the core, for the ferrite core, KT=1; Ku is the utilization factor of the core window It is related to the winding wire diameter and the winding process level, generally taking 0.1~0.5; J is the allowable current density of the wire, generally taking 3~5A/mm2.
Substituting the known value into equation (5), the calculated SQ = 1403.5 mm2. According to the reference [4], the core of the E25/10/6 model manufactured by Philips can be selected, and its effective area is S=38.4 mm2.
2.2.4 Calculation of the number of turns of the primary and secondary windings of the transformer
The number of turns N1 of the primary winding of the transformer is determined by:
Substituting the known amount, N1 = 10. Thus, the transformer's secondary ratio N can be used to determine the number of turns in the secondary winding of the transformer, namely:
N2 = N1 × N = 10 × 15 = 150.
The diameter of each winding wire can be calculated by:
Where, Ii is the effective value of the current flowing through the winding. Finally verify that the transformer can bypass the required number of turns.
Many parameters of the pulse transformer are mutually influential, so when making the transformer, it is necessary to repeatedly debug to achieve the best impedance matching and high efficiency. The output of the transformer (ie the magnitude of the voltage applied to the ultrasonic transducer) affects the range and accuracy of the system. The no-load voltage of the secondary side of the transformer used in this system can reach 300V.
3 receiving circuit design
Since this circuit is used in a car collision avoidance system, generally only a positive power source is provided on the vehicle, so the design of the receiving circuit uses a single power source. It consists of a preamplifier circuit, a bandpass filter circuit and a post amplifier circuit.
3.1 preamplifier circuit
Considering that the output resistance of the ultrasonic transducer is relatively large, the preamplifier must have a sufficiently large input impedance. The preamplifier circuit is a differential amplifier consisting of a precision, high input impedance instrumentation amplifier AD623. Since the transceiver sensor is used, interference between the transceiving signals may occur, and a large transmission signal energy may directly enter the receiving circuit, which is much larger than the echo, so the preamplifier will be saturated and the circuit operation is unstable. To this end, the input of the receive signal amplifier is connected to a pair of mutually inverted diodes for clamping to protect the subsequent amplifier circuit.
3.2 Bandpass filter
Here, an infinite-gain multi-path feedback filter circuit is used, which is a filter circuit composed of an operational amplifier with theoretically infinite gain imparted with multi-path feedback. Figure 4 shows the basic structure of an infinite-gain multiplexed second-order band-pass filter circuit composed of a single operational amplifier.
The filter parameters are:
The infinite gain multi-path feedback filter circuit has high stability because there is no positive feedback. For the convenience of calculation, C1=C2=680pF, Ap=6, and Q=3 can be selected first, and the above equations are combined: R3=47kΩ, R1=47kΩ, R2=2kΩ. Since the single supply is used, it is raised to a level at the anode of the amplifier. The MC7805 is used here to convert the supply voltage to 5V to provide bias. The output of the filter is amplified by A level and then connected to the acquisition card for A/D sampling.
4 Experimental results and conclusions
Ultrasonic ranging experiments were performed on the circuits previously designed. This experiment used NI data acquisition card 6024E to collect data. The 6024E is a high-performance multifunction board with analog, digital, and clock I/O ports, using a PCI bus. The maximum acquisition rate is 200kHz, using the DAQ-STC counter chip. Includes three timer groups that control the analog input, analog output, and general purpose count/timing functions. Used for the general count/timing function are two 24-bit counters [5]. The clock 1 is used to generate a control signal, and the clock 0 generates a 30 kHz pulse as an input signal to the drive circuit.
The number of pulses to be transmitted should be selected appropriately. When the number of pulses is large, the transmitting transducer can overcome its vibration inertia to obtain sufficient vibration. The other acoustic modes have less influence, and the transmitted ultrasonic pulse energy is large. However, the dead zone of the ranging is also at this time. Large (ranging dead zone refers to the smallest distance that can be measured), generally consists of 10 to 20 pulses.
The system software is programmed with LabView. Figure 5 shows the comparison of the measurement results of the two circuits in the same environment.
5(a) is the measurement result of the previous circuit at 6 meters, the amplitude is small, there is a power failure phenomenon during the measurement process, and the power tube is seriously heated, which indicates that the power consumption is relatively large. 5(b) is the measurement result of this circuit. It can be seen that the measurement distance of the circuit is obviously improved, and the tube has substantially no heat generation, and the power supply remains stable. It can be seen that the design of the circuit is rigorously deduced, the device selection is reasonable, the parameters are optimized, the impedance matching between the transducer and the power amplifier is improved, and the power amplifier efficiency and the electromechanical conversion efficiency are significantly improved. The circuit control is convenient and the performance is good. The clear echo can still be obtained at a distance of 9.5m, which makes a wide range of ultrasonic ranging possible.
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