Views:1 Author:Site Editor Publish Time: 2020-10-12 Origin:Site
1.The influence of ultrasonic propagation speed on ranging
Stable and accurate ultrasonic propagation velocity is a necessary condition to ensure measurement accuracy. The propagation speed of a wave depends on the characteristics of the propagation medium. The temperature, pressure, and density of the propagation medium will all have a direct effect on the speed of sound. For distance measurement, the main cause of the change in sound velocity is the change in the temperature of the medium, which is one of the main sources of errors in ultrasonic distance measurement sensor. Therefore, in the ranging process, the ultrasonic velocity must be corrected. The relationship between the ultrasonic propagation velocity in the air and the temperature can be expressed as c=331.4×1+t/273u33114+01607t (m/s), where t is the ambient temperature. Therefore, using the ultrasonic velocity of 341m/s at normal temperature to calculate the distance of ultrasonic ranging under different temperature environments has a large error. In order to improve the accuracy of distance measurement, it is necessary to perform temperature compensation on the ultrasonic speed, and use temperature sensors and other temperature measuring devices to measure the value of the environment temperature, thereby obtaining the ultrasonic speed in the environment. It is also possible to use a combination of sound speed preset and temperature compensation to correct the sound speed, which will more effectively reduce the error caused by temperature changes.
2. Factors affecting the determination of echo time t and methods to reduce errors
In the measurement process, in order to prevent the interference of other signals and improve the reliability of the measurement, when the single-chip computer starts to count, the ultrasonic sensor often transmits a pulse train composed of multiple square waves (such as 5-9 pulses as a train) as the measurement. If the threshold voltage of the comparator in the receiving circuit of ultrasonic transducer distance measurement is a certain value, due to the influence of dust and other substances, the actual measurement may not be necessarily be the zero-crossing trigger of the first echo. Through the observation and analysis of the ultrasonic receiving echo, it is found that after the received echo is detected by the envelope, the front of the envelope curve is an exponentially rising curve, approximately at the peak of the ninth wave to the envelope, and the third The wave is approximately 75% of the peak. Therefore, the receiving circuit is often designed to stop counting when the third echo is received. Therefore, the final measured time is 3 pulses longer than the actual distance corresponding to the sending time, which causes the measurement error of the echo time t.
In order to improve the timing accuracy, it is necessary to accurately detect the arrival time of the ultrasonic transducer sensor. A single comparator with a fixed threshold is used to detect the echo. Due to the absorption attenuation and diffusion loss of the sound wave during transmission, the sound intensity decays exponentially as the distance of the target increases. Within the range, the distance between the nearest target and the farthest target The large difference in echo amplitude may cause the time of crossing the threshold to move back and forth, thereby affecting the accuracy of timing.
The method to solve this problem: Method one is to use a dual-comparator shaping circuit, which can more accurately determine the arrival time of the echo front. As shown in Figure 2, vm is the peak voltage, let v1 be the threshold voltage of comparator 1, v2 is the threshold voltage of comparator 2, (where (v2>v1, its value is set by experiment), when the ultrasonic sensor emits ultrasonic .When the timer t1 and t0 of the single-chip microcomputer start timing at the same time, when the comparator 1 flips, t0 stops timing. At this time, the time counted by t0 is t1. When the comparator 2 flips, t1 stops timing. At this time, the time counted by t1 Is t2, obviously t2>t1, t is the propagation time corresponding to the front edge of the echo, then the distance calculated by t is more accurate than t1 and t2.
The second method is to serially connect the automatic gain control circuit (agc) in the echo receiving circuit, so that during the receiving time of the amplifying circuit, the voltage amplification factor increases exponentially with the increasing of the measuring distance to compensate for the absorption attenuation and Diffusion loss keeps the amplitude of the received echo constant or only changes in a small range to meet the requirements of the shaping circuit, and then output through the shaping circuit, which can greatly improve the accuracy of ranging. Of course, because the aGC circuit (including the amplifier itself) has a lag in the step response of the signal, the instantaneous tracking may not be very good, and the echo signal is just explosive, so there is a certain error, but this is negligible .
The third method is to design a circuit that gradually decreases the threshold voltage as time increases during the measurement time, and generates a threshold signal that increases at any time and decreases exponentially and is added to the comparator. This will compensate for the return caused by increasing in the measurement distance. The wave amplitude is reduced to improve the accuracy and repeatability of the measurement.Using programmable amplifiers and digital potentiometers and other devices, through the combination of software and hardware, a variety of such circuits can be designed. It is also possible to combine an operational amplifier and a field effect tube to form a controlled amplifier. The field effect tube is used as a voltage-controlled resistor to form a feedback regulation loop. But the followability of this circuit is not as good as the above-mentioned digital circuit.
3. The influence of the incident angle of the ultrasonic beam on the detection target on the ranging.If the system is used to measure the distance between the surface and the point, when the incident angle of the ultrasonic wave (or the angle of the reflected wave incident on the receiving transducer) is less than 90b, the distance measured by the system is the measured point (object) and the transducer. Rather than the vertical distance d between the measurement plane and the measuring object, this will cause measurement errors. The way to solve this problem is to use the relevant knowledge of triangles to calculate and correct.
4. Dead zone
During distance measurement, the high frequency ultrasonic transducer uses a series of ultrasonic waves as the measurement carrier for a period of time, so the reception can only be started after the transmission is completed. Set the time of sending the beam to t, then the signal reflected from the object within t time cannot be captured. In addition, the ultrasonic sensor has a certain inertia, that is, there is a process from forced vibration to balanced vibration to damped vibration. Therefore, there will be a certain after vibration after the transmission is completed. This after vibration also generates a voltage signal through the transducer. The signal is superimposed on the echo signal, so that the circuit cannot identify the true echo, which disturbs the system's work of capturing the return signal. Therefore, the system cannot be activated for echo reception before the after vibration disappears. The above two reasons cause the ultrasonic sensor to have a certain measurement range, that is, there is a so-called blind zone.
In addition, there are many other causes of measurement errors, such as the command operation takes a certain amount of time, which makes the measuring data too large, the stability and accuracy of the time base pulse frequency, and other material interference in the field environment.