Ultrasonic Tx/Rx Tutorial

  

Ultrasound Wave

Sound waves are mechanical oscillations which comprise positive and negative-phase pressure-impulse changes which our composition gives a complete wave. In general we can classify them into three categories, acoustic sounds, infrasound and ultrasound. The first are the audio zone between 16-16000 cycles per second, and the sounds can perceive a normal person. Then we have the infrasound beneath the preceding and covering the range from 1 to 16 cycles per second and is extremely harmful. Finally we ultrasounds which are placed beyond the audio band, ie. Of 20kHz, and they are to be dealt with then.

 

ultrasonic spectrum diagram

 

The ultrasonic energy propagates in the medium in the form of longitudinal waves. Unlike the transverse waves in the longitudinal particles making up the transmission medium oscillate about their equilibrium position during the propagation direction of the wave and the energy propagates in a direction parallel to that of oscillation. This creates curds and dilutions, without having material transfer, according to the theory behind waves.

At this point we can refer to the ways in which an ultrasound behaves and interacts with matter.

 

Attenuation

The attenuation of sound is an inevitable phenomenon that occurs in any transmission medium, and is reducing the intensity of the sound wave as it propagates in space. This reduction obeys the law of exponential decay that is:

I (x) = I0 e-μx

While a more representational form is the following:

 

volume attenuation

 

The attenuation is due to the fact that the oscillation of the medium particles requires energy, which is pumped by the energy carrying waves. This causes the sound a reduction in tension of the form

heat. The rate of this attenuation is representative of the spreading agent and customary be called attenuation coefficient.

 

Wave Reflections

Ultrasound waves as reflected in their incidence on a surface. The following scheme may give us an overview of the phenomenon in which a p denoted waves displayed.

 

Ultrasound waves reflected

 

The reflection, however, depends on the acoustic impedance Z of the two media, which is defined as:

 

Z = ρ * c

 

Where as, denoted p the density of the medium and c is the propagation speed of sound in that medium. As mentioned previously the sound wave will weaken, so the reflected and the traveling wave will have a reduced width in relation to the incident. That is why we have set the reflectance, the mathematical expression of which is as follows:

 

(p2/p1) = (Z1-Z2) / (Z1+Z2)

 

As the above reason tends to zero so we tend to have a full roll, and if this ratio tends to drive tend to have a complete reflection.

Closing for ultrasound should say that they find wide application in various branches of science such as physics, engineering, industry and medicine. Ultrasound began to be applied in medicine, the Doppler method is the most commonly used imaging method, and its use is increasing in recent years. Even we can meet them in the SONAR technology, inkjet type printers in the integrated circuit industry and end in similar applications to ours, such as the distance measurement.

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Ultrasonic Sensors

At this point we will make a description of the instrument that allows us to make the desired measurement. The instrument called the sensor and defined as a device used for measuring a physical quantity by converting an electrical signal , or it means a voltage or a current . The sensors are shown in a wide range depending on the desired us measured physical quantity, such as temperature, the displacement of an object, the liquid level, the speed and the acceleration, radiation and many others.

At this point we can mention the inverters , which is a device that absorbs energy of a system and turns it into another form of energy. Example converter is a resistor, which receives electrical power (electrical energy) and converts it to a temperature (thermal energy). Scoring systems used those transducers which convert into electrical energy, other forms of energy, as this allows the processing of the measurement. For the above reason the terms sensor and converter usually acquire the same importance in the metering industry.

The sensors can be divided into two main categories, active and passive. Criterion of this separation is the need to supply an external voltage in order to operate. In active sensors have to convert a form of energy into electricity. Such sensors provide their output voltage or current proportional to the value of the measurand, without requiring external power. Some examples of such sensors are thermocouples and piezoelectric sensors, which we use in the case of our system. The other category as mentioned above are passive sensors, whose operation is based on a change in an electrical characteristic, resistance or inductance, for example, by varying the metered quantity. But in this case it requires external power in order to produce the corresponding electrical signal, and some examples of this class of sensors is the photoresistors. Typically the physical size causes a change of the dimensions (e.g., length, width) or electric-magnetic properties (e.g., dielectric constant, magnetic permeability).

Below are sensors used for a variety of physical measurements such as temperature, pressure or radiation, and as will be seen, the area occupied may differ by several orders of magnitude depending on our needs or manufacturing costs.

First, we see one of the most simple and widely used sensors, a temperature sensor.

 

temperature sensor

 

This sensor is applicable to electronic thermometers and air conditioners.

Another sensor for measuring optical radiation, is the CCD type sensor, which finds wide application in digital cameras.

 

ccd sensor

 

ccd application

 

Find important application in medicine and the science of chemistry to various measurements and chemical sensors, where an example is shown below.

 

chemical sensor chemical sensor

 

But there are sensors that occupy a much larger volume than the above. Thus we arrive at the sensor-detector which is used in much debated the CERN experiment to detect subatomic particles and shown below.

 

subatomic particles detector

 

Features of Ultrasonic Sensors

We will continue the description of the sensors with a reference to their basic characteristics.

  • Transfer function or characteristic curve: The one sensor transfer function is the relationship between the electric current in the sensor output with the value of the physical magnitude measured and is usually given by the manufacturer with a mathematical relationship, which is attributed to a chart such as the following for the case that we have linear element.

     

    function transfer chart

 

  • Sensibility: The sensitivity is particularly important feature which in some cases is critical for the realization of an accurate measurement. Defined by the following mathematical relationship:  S = dV/ dX where dXis defined as the change of the measurand and as dVis the corresponding change in the sensor output signal.

  • Measuring range: The measurement ranges define the limits within which the appliance and operates reliably. Usually the sensors mentioned two measurement regions, one for input and one for the outlet. This feature is also important and should be considered by the person concerned as it may result in distortion or even the non-occurrence of measurement.

  • hysteresis: Hysteresis is called the phenomenon in which differences are observed in the output of a sensor when the change of the input direction reversed, and typically occurs in the magnetic material or mechanical systems, and results in deterioration of measurement accuracy. A typical case of this phenomenon is shown in the diagram below.

     

    Hysteresis graph

     

  • Calibration: It is the process of determining the transfer function of a sensor. So by measuring the value of the sensor output signal is possible to calculate the value of the measured magnitude. As but calibration set and the process of comparing an unknown precision measuring instrument with corresponding known precision instrument to determine, or to optimize the accuracy. The latter also called calibration.

  • Drift: It is likely due to factors such as operating temperature or humidity, to provide a change in the sensor output signal, without being noticed by the corresponding change in the measured physical quantity. The above phenomenon is called drift. But there is also the long-term drift due to factors such as the deterioration of the sensor parts, contamination or aging of materials.
  • Dead Zone: Dead zone is called the measurement range for which the sensor is not responsive to changes in the measured quantity, and this region is common to is around zero.
  •  
  • dead band resolution
  • Sensor Resolution: Finally we will refer to one of the most important features of sensors, their resolution. This size gives the smallest change in the input can be sensed by the sensor. The definition alone we can understand that the resolution is a feature that the user of the sensor should definitely take into consideration.

 

The sensor used in the system that describes the work belongs to the piezoelectric transducers. Principle of operation of such sensors is the piezoelectric effect , which was discovered by Pierre and Jacques Curie in 1880. According to this phenomenon, the single crystal deformation due to mechanical stress in this causes the load reorientation and corresponding movement of equal positive and negative loads on opposite sides of the crystal. The result of the above processes is the voltage developing across the proportional deflection crystal that caused it.

 

piezoelectric effect

 

The piezoelectric effect occurs in certain types of crystals such as crystals and Quartz Rochelle or in some ceramic materials such as BaTiO3.

An important property of the phenomenon is the reversibility, namely that the application of voltage across the crystal is liable to cause distortion. The inverse phenomenon finds broadest application in the design of actuators (actuators). The measurement of the piezoelectric crystal deforming can be effected either by measuring the load developed either the resulting voltage across the load. However, the phenomenon is observed due to leakage of charge through the crystal (self-discharge) even if the distortion is constant, the voltage to follow the discharge curve and tends asymptotically to zero, making use of crystals mainly for measuring dynamical deformations, pressures and forces.

The sensor used exploits this phenomenon, only the mechanical deformation of the crystal leads to the generation of harmonic frequency waves lies in the ultrasonic range. The electrical equivalent of a sensor of the type described is shown below.

 

piezoelectric sensor circuit

 

And the corresponding frequency response:

 

frequency response sensor

 

Wherein the yellow region is one in which the sensor is operating normally, while the top is formed to the right indicates the mechanical resonance of the crystal.

For a system like that we developed was the need for a transmitter and a receiver respectively. These functions assumed by the piezoelectric crystal that analyzed above. It functions as a transmitter of as we apply a voltage and generates an analog ultrasonic wave in space. And then another identical crystal receives this mechanical wave and converts it into a voltage. For carrying out the measurement is based on the reflectance method, the mechanical wave that is emitted to impinge on the sensor on a vibrating surface and will be reflected. As the receiver detects the reflected, converts it into an electric signal (voltage) which is then guided for processing. If two sensors are used, as in our case, use a single functioning as a transmitter (TX) and one that acts as a receiver (RX). But there is a second category, characterized by the fact that they act as transmitters and receivers.

 

Principle of Operating Measurement with Ultrasound

The realization of measurements with ultrasound initially based on a very simple physical function, that of the ultrasound reflection, since that is a wave. The procedure is as follows: sending ultrasound to the surface to be measured, via our transmitter, and is then reflected back and received by our receiver. Ultrasound is to be received is exploited in order to derive a measurement. There are two predominant methods used in measurements using ultrasound. The first method is the TOF (Time Of Flight), while the second method uses the phenomenon DOPPLER and namesake. Then we will refer to the way in which each method and in cases that can be applied.

 

phenomenon of reflection

 

array sensor surface

 

Method for Time Of Flight (TOF)

Feature of this methodology is its simplicity. Customary finds application in distance measuring system or liquid level in depths and sonar measurements. The operating principle of the method is the following. The ultrasound emitted by the system, while a clock starts counting. Once the ultrasound taken then the clock stops counting, and the system taking into account the ultrasound propagation speed and measurement time can output an estimate of the distance you have chosen. As shown it is a rather simple methodology, but sufficiently reliable.

 

doppler method

 

DOPPLER Method

Another method finds application in the field of measurement using ultrasonic is DOPPLER method. The method exploits the homonym phenomenon studied by physics and is the method by which we chose to make our measurements. For this reason there will be a brief reference to it. The DOPPLER phenomenon named by Christian Doppler natural Austria, who suggested in 1842. This phenomenon pragmateftai varying the frequency of a wave for an observer who moves at a speed relative to the speed of the source which produces the wave. A typical example of the phenomenon of our daily life is a passing vehicle which carries siren. The frequency accept is higher compared to the transmitted as the vehicle approaches us, and upon removal of the frequency received is lower, always compared to the transmitted.

 

moving source wavelength

 

The mathematical description of this phenomenon is given by the following relationship:

 

f = ( c ± ur / c ± us ) f0

 

Where:

  • c is the speed of sound in the medium.

  • ur with the observer's speed, and is positive if the observer moves towards the source.

  • us and the spring rate, which is considered positive if the source is removed from the observer.

  • and fis the transmit frequency of the source.

 

From the above we can understand that by sending an ultrasound to a surface, the reflected wave that we will take, it is quite easy by measuring the frequency shift to extract a vibration speed value of this surface. Also If we use the TOF method we can simultaneously detect the correct position. 

Applications encountered the phenomenon is enough, and in a broad scientific field. Starting medicine, where used in imaging or blood flow measurements, are continuing in radar technology, to measure the speed of objects. While used in astronomy wherein the effect is used to measure the speed with which the stars and galaxies are approaching or moving away from the earth.