Design of Ultrasonic Distance Measurement System Bad on
Microcontroller
Zhihong Xiao 1, a, Siyu Wu2, b and Qiyuan An 3, c
1 School of Electronic Engineering, Xi'an Shiyou University, Xi'an, China
2 School of Instrument Science and Opto-electronics Engineering, Hefei University of Technology,
Hefei, China
3 School of Software & Microelectronics, Northwest Polytechnic University, Xi'an, China
a , , c Keywords: ultrasonic nsor, microcontroller, distance measurement, high precision
Abstract. The principle of an ultrasonic distance measurement system is introduced. The system adopts non-contact measurement method and it consists of signal generator, echo receiver circuit, microcontroller, time quential logic control circuit, target distance detection circuit,temperature measu
rement circuit, RS-485 rial interface circuit, ultrasonic nsor and its driver circuit, etc. The results show that the system has high precision within measuring range from 20cm to 15m. Introduction
The ultrasonic wave has features of high frequency, short wavelength, good directivity, less diffraction and better penetration. It can be transmitted along a straight line with slow propagation speed. It could produce reflected waves while encountering impurity or an interface. Due to all the characters, the ultrasonic wave has been ud more and more widely in measurement [1,2]. Principle of ultrasonic distance measurement.
The method of ultrasonic distance measurement, as a way of non-contact measurement, could not be influenced by such factors like lights, the colors of object, etc. And it has the capacity of adapting to rigor environments, such as the darkness, the interference from smoke and vapor, and electromagnetic interferences, etc. Therefore it can be ud in material level measuring, vehicles auto navigation, distinguishing and locating objects, vehicles assistant safety-driving system topography detection and many other fields. etc[3,4].
This method is emitting ultrasonic wave by ultrasonic transmitter at a certain direction, when the sound wave meets the object, the reflection wave will transmit to the ultrasonic receiver. At this moment, the distance between the emitter and object can be described in (1)
2r
Ct
L= (1)
Where C is the velocity of ultrasonic wave in the medium (m/s),
r
素性t is the time difference between ultrasonic nding and receiving (s), L is the measuring distance (m).
美国乡村音乐Design of the system.
The system consists of signal generator, echo receiver circuit, microcontroller, time quential logic control circuit, target distance detection circuit,temperature measurement circuit, RS-485 rial interface circuit, ultrasonic nsor and its driver circuit, etc. AT89C51 of ATMEL Company is ud
as microprocessor. The hardware block diagram of the system is shown in Fig.1.
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幸福反义词
Fig.1. The block diagram of hardware system
The ultrasonic nsor. The ultrasonic nsor, as known as ultrasonic transducer or ultrasonic probe, has a lot of species, at prent, the piezoelectric ultrasonic transducer is ud most frequently. It is mainly compod of piezoelectric chips and can both emit and receive ultrasonic waves. The piezoelectric effect is reversible. The piezoelectric crystal plate can deform and vibrate to produce ultrasonic waves when the plate is applied voltage changing periodically. On the contrary, the piezoelectric chip can receive ultrasonic waves, becau after being pressured, it can accumulate electric charges and generate voltage [5].
The combined ultrasonic transducer could both emit and receive ultrasonic wave. After the transducer emits a short vibrating signal, it switches to receiver mode. The model of this system’s censor is CUSS200, with the resonance center frequency at 75 kHz
坟籍Transmitting circuit. The emitting of ultrasonic wave is accomplished by both transducer and its driver circuit. Under the excitation of high-voltage electrical pul, the transducer could produce a homologous acoustic pul. There is a direct connection between transmit power and distance
remaining to be measured. The driver circuit is shown in Fig.2.
Fig.2. Driver circuit Fig.3. The equivalent circuit of transducer
In Fig.2, the FET IFR830 works as a switch. When IFR830 is in off mode, the voltage of 150V will charge the capacitanceR 2, R 4 and L E . The time constant of charging should be less than the minimum of pul repetition period of transmitting signal, otherwi the energy storage capacitance won’t get enough energy and the energy of transmitting acoustic pul will decline. When the trigger pul makes the switch IFR830 on, the energy storage capacitance will discharge by R 4, L E and transducer. The equivalent circuit of transducer is shown in Fig.3. The L E and the static capacitance C 0 of transducer work as a parallel resonant circuit. Its resonant frequency equals to 75 kHz, which is the resonant frequency of transducer. Due to the fact that each transducer’s resonant frequency is not quite the same, L E should be a ries connection of a static and a adjustable inductances. R 4, as a damp resistance, is mainly ud to reduce the width of transmitting
acoustic pul and enhance resolution.
However, the transmitting acoustic power will decrea all together with the reduced width of transmitting acoustic pul. Ergo we need to take a comprehensive consideration while choosing the damp resistance. The high-voltage diode 2CK27D’s function is keeping conducting when the transducer is transmitting acoustic wave; turning off while the transducer is receiving echoes; and parate transducer and signal generator in order to prevent noi from transmitting circuit entering receiver circuit.
实践论Echoes receiver circuit. The receiver circuit consists of ultra-low noi preamplifier filter, band-pass filter, AGC, and delay integrator.
The preamplifier filter, as the forefront of the whole signal-receiving circuit, requires a high S/N ratio and a wide dynamic range (>100dB). The model of operational amplifier is OP37, which has features as low noi and high input resistance. To reduce electromagnetic interference, the preamplifier should be placed in a shield box and near the transducer. The isolation resistance and a pair of rever
clamping diodes ttled ahead of the preamplifier are ud to prevent high-frequency strong pul enters into the amplifier. In the process of receiving the echo, the reflected ultrasonic wave reach the surface of the object, some of the wave has been absorbed or scattered, the echo signal are extremely faint, the clamping effect become invalid and the signal can directly enters the preamplifier.
After passing the preamplifier, the signal enters band-pass filter, which is a kind of high quality factor amplifier, who function is amplifying effective signal and sift out noi.
The attenuation of echo signal varies a lot, when the varieties of objects, the distance or angle between object and transducer change; let alone the existence of various kinds of background nois. Therefore, the effective signal accounts for a small proportion of the whole dynamic range. Thus, we need to modify the gain of amplifier accordingly by designing AGC, which adopts logarithmic amplifier, as dynamic-range compressor, so as to remove the noi signal but not influence effective signal.
The time-quential logic control circuit and object distance measuring circuit are mainly compod by shaping circuit, detector, comparator, etc. For excluding accidental errors, 8 quential echoes obtained can be considered as effective signal.
Temperature measurement circuit. The speed of ultrasonic wave can be described in (2). µ
γλT C = (2) Where γis the specific heat of air,λ is gas constant; µis Molar mass of gas, T is thermodynamic temperature.
The speed of ultrasonic wave is not a constant in the air. If the temperature increas 1˚C, the speed of ultrasonic will increas 0.6m/s [6,7]. The speed can be described in (3).
T C 607.04.331+= (3) The temperature measurement circuit adopts censor AD590, as a current output type device, which enjoys a high level of linearity and the accuracy won’t change as the length of wire.
The modified formula of the distance measurement
Becau of the great quantity of the data storage and data calculation of the system, the project adopts the method of lower computer controlled by upper computer. A lot of the calculations are performed by the PC. The main functions of the system software include producing all kinds of controlling signals, storing data, processing signals and communications between upper computer and lower computer, etc.
After the upper computer get the time data nt by microcontroller, the target distance can be calculated by a ries of processing (considering temperature) like filtering, taking an average, etc. However, the deviation is very huge when calculating by formula (1). A modified formula is obtained after a large quantity of experiments. The modified formula can be described in (4)
)2
12(21T T n n t C L r ∆−+−= (4) Where T ∆is the total delay time caud by voltage/acoustic wave and acoustic wave/voltage conversion performed by transducer; n is the circuit delay coefficient; T 1is the period of oscillation of the transducer;
The circuit delay coefficient is the correction factor obtained by comparing and fitting a great quantity of measured data with the real distance. In this method, n approximately equals to 0.8. Conclusions
We did a large number of experiments under divergent circumstances. The results indicate: distance
between 0.2m to15m, deviation is less than 2%, which means the accuracy is very well. Repeating the measurement many times for each point, we could e that the repetitive error is less than 1%.The results also illustrates that the waveform of echoes won’t change when the type of target objects are deferent. If the threshold of comparator of object-distance measurement circuit is suitable, the change of the measured target won’t exert much influence on the results. The test results are shown in table 1.
Table 1. The test results Standard
values (cm)
Measured values (cm) Error Standard values (cm) Measured values (cm) Error 20
20.3 1.5% 600 602.4 0.4% 40
40.5 1.2% 800 804.8 0.6% 80
-80.8 -1.0% 1200 1214.4
1.2% 100
100.5 0.5% 1400 1421.1 1.5% 300
300.9 0.3% 1500 1530.2 2.0%
Acknowledgements
The authors would like to express their gratitude to Professor Jiatian Zhang and Xiaofeng Wu for their valuable discussions and suggestions. Thanks are also to all colleagues who previously provide technical support.
References
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Design of Ultrasonic Distance Measurement System Bad on Microcontroller 10.4028//AMM.333-335.296
