Non-contact air distance measurement method based on ultrasonic sensor

Abstract: In order to overcome the defects of some traditional distance measurement methods that cannot be measured in some special occasions, the design uses P89LPC932 as the core and uses ultrasonic sensors to realize non-contact air distance measurement. The close relationship between sound speed and temperature is fully considered, and temperature compensation is performed to further obtain a distance measurement of up to about 700 cm, with an optimal accuracy of 1%. The design has strong anti-interference ability, simple installation, small size, low power consumption, and is easy to embed into other systems.

Keywords: P89LPC932; ultrasonic; distance meter; non-contact air distance measurement

introduction

Some traditional distance measurement methods have insurmountable defects in certain special occasions. For example, liquid level measurement is a kind of distance measurement. The traditional electrode method uses differential distribution electrodes to detect the liquid level by applying electricity or pulses. The electrodes are easily corroded and electrolyzed when immersed in water or other liquids for a long time, and lose sensitivity. Using ultrasonic distance measurement can solve these problems.

The ultrasonic rangefinder designed in this paper uses three range-measuring modes to select jumper J1 (short distance, medium distance, and adjustable distance). The overall solution is that when the measurement key is pressed, the probe sends ultrasonic waves, and when the ultrasonic waves encounter obstacles, an echo signal will be generated; the system amplifies the echo signal received by the probe and sends it to the controller; the temperature measurement circuit measures the temperature, calculates the measured distance, and displays it on the digital display, with the last 4 digits showing the distance and the first 2 digits showing the temperature.

1 Basic Principle of Ultrasonic Rangefinder

The principle of using ultrasound to measure distance is shown in Figure 1. It can be simply described as follows: the regularly transmitted ultrasound waves are reflected when encountering obstacles. The reflected waves are received by the receiver and converted into electrical signals. In this way, the distance can be calculated by measuring the time difference △t between transmission and reception, and then according to formula (1):

Where: C is the propagation speed of ultrasound in air, C is 331 m/s at 0℃, C is 347 m/s at 25℃, and its relationship with ambient temperature T (unit: ℃) is as shown in formula (2):

It can be seen that the speed of sound is closely related to temperature. In applications, if the temperature does not change much and there is no special accuracy requirement, the speed of sound can be considered to be basically unchanged. Otherwise, temperature compensation must be performed. The temperature compensation method is to first calculate the speed of sound C at that time according to formula (2) and then calculate the distance according to formula (1).

In addition, it can be seen from Figure 1 that since ultrasound uses the received reflected waves to calculate the distance, there is inevitably an angle between the emission and the reflection, and its size is 2a. When a is very small, the distance can be directly calculated according to formula (1); when a is large, the distance must be corrected, and the correction formula is as shown in formula (3):

2 System Hardware

The ultrasonic rangefinder mainly includes: temperature detection circuit, ultrasonic emission and control circuit, ultrasonic reception and signal processing circuit, display circuit, microprocessor and its auxiliary circuit and RS 232 communication interface circuit. Its structural block diagram is shown in Figure 2.