High precision ultrasonic measurement distance system design and implementation (1)

Introduction

In engineering practice, ultrasonic because links to strong, energy consumption is slow and in the media of distance, usually used to measure distance.

It is mainly applied to reversing radar, rangefinder, level measuring instrument, the development of mobile robot, construction site, and some of the industrial field, for example: distance, liquid level, well depth, pipe length, velocity, etc. Use of ultrasonic testing is often more quickly, easily, and calculate the simple and easy to do real-time control, measurement accuracy also meets the requirements of industrial utility, has been widely used.

The basic principle of ultrasonic range finder

Ultrasonic generator at some point give ultrasonic signal, encountered the measured object is reflected by ultrasonic receiver receives.

Just calculate the ultrasonic signal from the transmitter to the received echo time, know the propagation speed of the media, they can be calculated from the measured distance of objects:

d=s/2=(vt)/2 （1）

Where d is measured by the distance between the Finder, s for ultrasonic roundtrip through distance, v for ultrasonic propagation speed of the media, t-ultrasonic from launch to receive.

In order to improve accuracy, consider the different temperature ultrasonic velocity of propagation in the air with temperature variations:

v=331.4+0.61T （2）

Type, t is the actual temperature (℃), the unit for v m/s.

Piezoelectric ultrasonic sensor principle

At present, the ultrasonic sensor can roughly be divided into two categories: one category is used for ultrasonic electrical way, a class is mechanically produced by ultrasound.

Electrical methods including piezoelectric, magnetostrictive-and electric-type; mechanical means there Garba Commission flute, whistle and pneumatic Rotary liquid flute, etc. They produce ultrasonic frequency, power and acoustic characteristics, and thus uses varies. In the project, now more commonly used is a piezoelectric ultrasonic sensor.

Piezoelectric ultrasonic sensor actually utilize the resonator piezoelectric crystals to work.

Piezoelectric ultrasonic generator inside two piezoelectric chip and a resonance Board. When it's two poles plus pulse signal, and its frequency equal to piezoelectric bimorph of natural oscillation frequency, piezoelectric bimorph resonance will happen, and drive the resonance plate ultrasonic vibration occurs. On the other hand, if two electrode is not applied voltage, when resonance Board received ultrasonic, the oppression of piezoelectric chip for vibration, mechanical energy into electrical signals, i.e. for ultrasonic receiver.

Reflective ultrasonic range finder hardware circuit design

The system hardware circuit consists of a single minimum system and temperature compensation circuit, ultrasonic transmitter, ultrasonic receiver circuit, show circuit, as shown in Figure 1.

This ultrasonic distance meter of concrete works as follows, in single-chip produces a reset signal by MC9S12DG128B produces a control signal, control peripheral circuits produce ultrasonic 40kHz, the plastic is enlarged to the ultrasonic transducer emits ultrasonic frequency 40kHz.

At the same time, the internal timer count MC9S12DG128B, measurement of ultrasonic signal received from the issue of the time, and to ultrasonic transducer R received ultrasonic signal amplification, filtering, shaping, and as the received signal to start the timer input capture features, complete a ultrasonic distance measurement of time. At the same time, the temperature sensor DS18B20 measured current environment temperature, read into the SCM, and then after its processing, the LCD screen displays the corresponding measuring values as well as the current temperature.

Micro-controller MC9S12DG128B

MC9S12DG128B is Freescale company launched the S12 controller in a 16-bit microcontrollers.

Its high integration, on-chip resources rich interface module including SCI, SPI, I2C, A/D, PWM, etc. in FLASH storage control and strong encryption.

MC9S12DG128B microcontrollers using enhanced 16-bit CPU, S12-chip bus clock frequency up to 25MHz; on-chip resources including 8kB RAM, FLASH, EEPROM 128kB 2kB, SCI, SPI, PWM serial interface module; PWM module can be set to 4-channel 8-bit or 2-channel 16-bit, wide range select clock frequency; it also provides two 8-channel 10-bit resolution a/d converters, controller area network CAN and enhanced capture timer, and supports the background debug mode (BDM).

Ultrasonic transmitter

[1] [2] [3]