Senzor Ultrasonic

8/3/2019 Senzor Ultrasonic

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ULTRASONIC SENSOR

Ultrasonic sensors are commonly used for a wide variety of noncontact presence,proximity, or distance measuring applications. These devices typically transmit a short burst ofultrasonic sound toward a target, which reflects the sound back to the sensor. The system then

measures the time for the echo to return to the sensor and computes the distance to the targetusing the speed of sound in the medium.

The wide variety of sensors currently on the market differ from one another in their

mounting configurations, environmental sealing, and electronic features. Acoustically, they

operate at different frequencies and have different radiation patterns. It is usually not difficult to

select a sensor that best meets the environmental and mechanical requirements for a particularapplication, or to evaluate the electronic features available with different models. Still, many

users may not be aware of the acoustic subtleties that can have major effects on ultrasonic sensor

operation and the measurements being made with them.

The overall intent of this article is to help the user select an ultrasonic sensor with the bestacoustical properties, such as frequency and beam pattern, for a particular application, and how to

obtain an optimum measurement from the sensor. The first step in this process is to gain a better

understanding of how variations in the acoustical parameters of both the environment and the

target affect the operation of the sensor. Specifically, the following variables will be discussed: Variation in the speed of sound as a function of both temperature and the composition of the

transmission medium, usually air, and how these variations affect sensor measurement accuracyand resolution

Variation in the wavelength of sound as a function of both sound speed and frequency, and

how this affects the resolution, accuracy, minimum target size, and the minimum and maximum

target distances of an ultrasonic sensor

Variation in the attenuation of sound as a function of both frequency and humidity, and howthis affects the maximum target distance for an ultrasonic sensor in air

Variation of the amplitude of background noise as a function of frequency, and how thisaffects the maximum target distance and minimum target size for an ultrasonic sensor

Variation in the sound radiation pattern (beam angle) of both the ultrasonic transducer and the

complete sensor system, and how this affects the maximum target distance and helps eliminate

extraneous targets

Variation in the amplitude of the return echo as a function of the target distance, geometry,

surface, and size, and how this affects the maximum target distance attainable with an ultrasonic

sensor

Fundamental Ultrasonic PropertiesUltrasonic sound is a vibration at a frequency above the range of human hearing, usually

>20 kHz. The microphones and loudspeakers used to receive and transmit the ultrasonic sound

are called transducers. Most ultrasonic sensors use a single transducer to both transmit the sound

pulse and receive the reflected echo, typically operating at frequencies between 40 kHz and 250

kHz. A variety of different types of transducers are used in these systems [4]. The following

sections provide an overview of how the sound pulse is affected by some of the fundamental

ultrasonic properties of the medium in which the sound travels.


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Speed of Sound in Air As a

In an echo ranging sysand its return to the receiver i

the speed of sound in the trandistance measurement is direc

calculation. The actual speed

the medium through which th

The speed of sound in

where:

c(T) = speed of sound in air a

T = temperature of the air i

The speed of sound in

gas, and is affected by both th

sound for various gases at 0C

Wavelength of Sound

The wavelength of sou

frequency, as shown by the ex

where:

= wavelength

c = speed of lightf = frequency

2

unction of Temperatureem, the elapsed time between the emission omeasured. The range distance to the target i

mission medium, which is usually air. The atly proportional to the accuracy of the speed

f sound is a function of both the compositio

sound travels (see Figure 1).

air varies as a function of temperature by the

a function of temperature in inches per seco

n C

different gaseous media is a function of the

chemical composition and temperature. Ta

[6].

s a Function of Sound Speed and Frequency

nd changes as a function of both the speed o

pression:

f the ultrasonic pulsethen computed using

curacy of the targetof sound used in the

and temperature of

relationship [5]:

(1)

nd

ulk modulus of the

le 1 gives the speed of

sound and the

(2)


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Figure 2 is a plot of th

temperature in air.

Attenuation of Sound As a F

As the sound travels, t

in the transmission medium.

in determining the maximum

frequency, and at any given frof humidity that produces the

125 kHz, for example, the ma

attenuation occurs at 50% RHSince an ultrasonic sen

range calculations should use

attenuation in air at room tem

by:

(f) = 0.01 f

where:

(f) = maximum attenuation i

f = frequency of sound in k

Between 50 kHz and 3

(f) = 0.022 f 0.6

3

wavelength of sound as a function of frequ

unction of Frequency and Humiditye amplitude of the sound pressure is reduce

nowing the value of this absorption loss, or

ange of a sensor. The attenuation of sound i

quency the attenuation varies as a functionmaximum attenuation is not the same for all

imum attenuation occurs at 100% RH; at 40

.sor usually is required to operate at all possi

he largest value of attenuation. A good esti

erature over all humidities for frequencies u

dB/ft

Hz

00 kHz, the maximum attenuation over all h

ncy at room

due to friction losses

ttenuation, is crucial

air increases with the

f humidity. The valuerequencies [7]. Above

kHz, maximum

le humidities, target

ate for the maximum

p to 50 kHz is given

(3a)

midities is:

(3b)


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Figure 3 and Figure 4 i

humidity.

Figure 3. The maximum attenuation of soun

temperature can beplotted as a function of f

humidities for frequencies between 40 kHz

Background Noise

The level of backgroureason is that less noise at the

that is produced is greatly atte

Effects of Frequency, Distan

Pressure

In an ultrasonic sensor

the sound pressure generated

pressures are typically exprespressure is usually measured iusually 12 in. (30 cm). The so

(//) 1Pa as follows:

SPL(R0) = 20 log(p)

where:

SPL(R0) = sound pressure lev

p = sound pressure at d

As the sound travels thdue to both absorption (attenu

radiating beam as the sound p

transducer is given by:

SPL(R) = SPL(R0) - 20 log

4

llustrate the attenuation of sound as a functi

d in air at room

requency over all

nd 250 kHz.

Figure 4. This family of curves s

attenuation of sound in air at roo

humidity for frequencies betwee

d ultrasonic noise diminishes as the frequenhigher frequencies is produced in the environ

nuated as it travels through the air.

ce, and the Transmission Medium on the

, the transducer produces a short pulse of sou

ill vary from one type of sensor to another.

ed in decibels because of their large dynamin micropascals (Pa) at a reference distance,und pressure level (SPL) at R0 is then conver

l at distance R0 in dB//1Pa

istance R0 in Pa

rough the medium, the magnitude of the souation) and spreading loss caused by the expa

lse travels from the transducer. The SPL at

R/R0) - (f) R

n of frequency and

hows the variations in the

m temperature as a function of

40 kHz and 200 kHz.

y increases. Thement, and the noise

agnitude of Sound

nd. The magnitude of

In acoustics, sound

ranges. SoundR0, from the sensor,ted to dB referenced to

(4)

d pressure is reducedding surface of the

distance R from the

(5)


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where:

SPL(R) = sound pressure levSPL(R0) = sound pressure lev

(f) = attenuation coeffici

Relative Echo Levels From

Different Ultrasonic Freque

If the sound pulse is re

surface, then the entire beam iThis total beam reflection is e

source at twice the distance. T

loss for the sound reflected fr

equal to 20 log (2R), and the

2R. For this to hold, it is imp

surface be both larger than theensure total reflection, and pe

beam.Equation (5) can be us

effect of varying the sound fre

distances from the sensor. In

range of 1 ft.

Figure 6. The relaplotted against ra

5

l at distance R in dB//1Pal at distance R0 in dB//1Pa

ent in dB/unit distance at frequency f

Flat Surface for

ciesflected from a large flat

s reflected (see Figure 5).uivalent to a virtual

herefore, the spreading

m a large flat surface is

bsorption loss is equal to

ortant that the reflecting

entire sound beam topendicular to the sound

d to compute the relative

quency on echoes produced from a flat refle

igure 6, it is assumed that each sensor produ

tive echo levels from a flat reflecting target at varying distances age for different frequencies.

Figure 5. A sound beam r

equivalent to the sound asat an equal range behind t

tor at different

ced the same SPL at a

re

flected from a flat surface is

generated from a virtual transducere reflecting plate.


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Therefore, the variations in EL are only a function of the varying attenuations due to the

different frequencies of sound. The maximum attenuation for all humidities was used for the

value of for each frequency.

SummaryPart 1 of this article has provided an overview of the some of the fundamental acoustical

parameters that affect the operation of an ultrasonic sensor. Part 2 will address the use of these

acoustical data to optimize the selection of an ultrasonic sensor for a particular measurement.

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