Error analysis of ultrasonic sensor on mine excavator
This paper introduces the principle of distance measurement by ultrasonic sensor and the influence of environment on the accuracy of measurement, and analyzes the reason of the error of the sensor used to measure the liquid level in the centralized lubrication system of mining excavator. It is pointed out that the selection of ultrasonic sensor should pay attention to the influence of environment, blind area and other factors on the measurement accuracy, improve the measurement accuracy, make the instrument reach the design index and meet the actual needs of the working condition.
Introduction
Mining excavators are widely used in the geotechnical stripping and ore mining of large open pit mines, mainly for the loading of materials transported by mining trucks, semi-continuous mining and other equipment, and are one of the important mining equipment. The centralized lubrication system is an important part of the mining excavator. It can be controlled by the system program, and the grease can be transported to the rotating parts, rolling bearings and open gears through the pipeline at a distance, quantitatively and regularly for automatic lubrication, which reduces the workload of manual lubrication and saves maintenance time. Thus, the production efficiency of the equipment is improved. However, the liquid level detection of the lubricating oil tank is realized by the maintenance personnel using the traditional liquid level gauge to visually measure the oil level, resulting in time consuming, low efficiency and low automation of the equipment. Therefore, ultrasonic sensors are used to measure liquid level in the optimization process of mine excavator function. By cooperating with the electrical control system, the automatic function of liquid level detection is realized.
1 Ultrasonic characteristics
Ultrasonic waves are mechanical waves that are inaudible to the human ear and have frequencies ranging from tens of kilohertz to tens of megahertz. Ultrasonic wave is generated by the vibration of the energy exchange chip under the excitation of voltage, and its wavelength is short and its frequency is high. Ultrasonic rays can be as reflective and directional as light, and the higher the frequency, the stronger the directional. When the ultrasonic wave is at a certain frequency, it attenuates greatly when propagating in gas due to the absorption of energy, but it attenuates less in solid and liquid. The higher the vibration frequency, the stronger the energy transmitted than ordinary sound waves, so the propagation in solids and liquids has a strong penetration ability, so it is widely used in solids and liquids. Ultrasonic can work in the working environment that ordinary sensors are not capable of, providing a contactless distance detection scheme, and with its long and wide detection range, helping to solve many problems in practical applications. Ultrasound is particularly suitable for the detection of objects with good acoustic reflection characteristics: solids, liquids and small particulate matter.
2. Working principle and composition of ultrasonic liquid level sensor
Ultrasonic sensors are widely used in harsh and complex industrial environments with excellent performance, and can be used to detect some materials with surface reflection characteristics or occasions that require accurate measurement. There are usually many ways to measure liquid level, but compared with other liquid level measurement methods, using ultrasonic sensors to measure liquid level has the following advantages: it can be easily measured in real time, and the system response time is short and not long
There is hysteresis; The accuracy of the data obtained is high. Easy to install, flexible, small overall size, easy to arrange in a small space. Reliable and stable performance, especially for the chemical and physical properties of liquid (medium) adaptability is very strong, can achieve non-contact measurement.
2.1 Principle of ultrasonic sensor to measure liquid level
Ultrasonic sensor to measure the basic principle of the liquid level: ultrasonic pulse signal is emitted by the emission sensor (ultrasonic probe), the signal is transmitted in the liquid and the interface between the liquid and the air is reflected, the sensor receives the reflected ultrasonic echo signal, the control system can get a time from the transmission to the reception. According to its propagation speed and propagation time, the propagation distance is calculated, so as to obtain the liquid level height. The schematic diagram is shown in Figure 1.
Figure 1 Ultrasonic ranging principle
2.2 Composition of ultrasonic sensor
The ultrasonic sensor uses the piezoelectric effect and electrostriction effect of the piezoelectric crystal to convert mechanical energy and electrical energy into each other. It consists of the following four parts. Sending sensor (transmitting probe) : the high-frequency pulse electrical signal input by the transmitting circuit is vibrated by the piezoelectric crystal due to deformation, and the ultrasonic wave is emitted; Receiving sensor (receiving probe) : The mechanical vibration generated by the received echo is converted by the piezoelectric crystal into electrical energy, which passes through the receiving circuit as the output of the receiver. Transmit and receive sensors, which can be the same part in actual use; Control part: Use the integrated circuit to control the transmission of ultrasonic waves, and judge the signal reception of the receiver. The main control of the ultrasonic pulse frequency and detection distance; Power supply part: The ultrasonic sensor is usually powered by external DC power supply, and the sensor is supplied by the internal voltage regulator circuit.
3 Error analysis
The propagation speed of ultrasonic wave in the medium is easily affected by the temperature, pressure, density and other media and environment. Secondly, the blind area of the sensor itself can easily lead to the instability of the measurement data. In order to make the measurement data more accurate, the measurement method and related parameters must be adjusted appropriately to eliminate the external factors that produce errors.
3.1 Impact of environment on measurement
Due to the physical characteristics of sound wave propagation, the range and speed of the ultrasonic beam depend on: air temperature, relative humidity, and air pressure.
As can be seen from Figure 2, when the temperature changes from 20 ℃ to 40 ℃, the sound velocity will also change correspondingly. Therefore, the calculated sound velocity must be corrected according to the ambient temperature of the sensor application, in order to improve the accuracy of the measurement data. In the process of measuring distance, the ultrasonic sensor must be equipped with temperature compensation, so that most of the influence of temperature on the output of the ultrasonic sensor can be eliminated. At the same time, the heating of the sensor's own circuit will also create additional temperature errors of the sensor, which should also be considered.
FIG. 2 Theoretical relationship between air temperature, air pressure and sound velocity
As can be seen from Figure 3, the detection range of the ultrasonic sensor increases significantly under low temperature conditions and is not affected by relative humidity. However, the detection range of ultrasonic sensors is reduced under high temperature conditions and is greatly affected by relative humidity. Therefore, when selecting the ultrasonic sensor, it is necessary to fully consider this special product, select a reasonable liquid level sensor that is suitable for the type of working conditions used on site, and reduce the measurement error.
3.2 Blind area of the sensor
The area between the ultrasonic transducer and the starting point of the detection range is called the blind zone. Afterwave interference causes a blind spot of the sensor, because the transducer emits ultrasonic waves with a high-voltage pulse, after the pulse is over, the transducer will have a long residual vibration. After the transducer enters the receiving state from the transmitting state, the generated residual vibration signal is first received, and the residual vibration signal has not attenuated to a certain time, and the residual vibration signal is superimposed after the transducer receives the ultrasonic echo signal, so that the real ultrasonic echo can not be identified by the circuit, resulting in measurement data errors. The blind area of ultrasonic sensor is related to the measuring range, and the general measuring range is large, the blind area is large; On the contrary, the blind area is small. Therefore, the sensor selection should be based on the actual needs to choose the appropriate range of the sensor. Moreover, in the design of the highest level of the tank, the blind area of the sensor can not be ignored to avoid the distance between the highest level and the sensor probe in the blind area of the sensor, resulting in measurement errors.
4 Conclusion
Through analysis, it can be seen that the main reason for the error of the sensor used to measure the liquid level in the centralized lubrication system of the mining excavator is that the severe vibration of the measured liquid level during the use of the equipment leads to large waves, and when the liquid contains bubbles, it is easy to cause confusion in the reflection of sound waves, resulting in large measurement errors. At the same time, there are errors caused by the change of the air in the tank (barrel) with the ambient temperature, and the artificial oil level is too high when refueling, and the oil sticks to the sensor and affects the measurement accuracy of the sensor.
Therefore, the above factors are fully considered in the design and selection, the liquid level sensor that is reasonable and suitable for the type of working conditions used in the field is selected, and the measurement parameters can be adjusted through software to reduce the measurement error. Make the instrument reach the design index, meet the needs of the actual function, so as to improve the automation level of the system.
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