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- Sound is transmitted through the propagation of pressure in the air.- The speed of sound in the air is normally 331m/sec at 0oC.- Two of the important characteristics of sound waves are frequency and intensity.- The human ear can hear the frequencies in the range of 20 Hz -20 kHz.
Sonic Distance Sensors
Introduction
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 1
- The human ear can hear the frequencies in the range of 20 Hz -20 kHz.- The frequencies above this rang is called Ultrasonic frequencies.
Theory of operation
Sonic distance sensors measure the distance of an object by measuring the time period between the sent ultrasonic sound pulse and the echoed pulse.
The sensor head sends ultrasonic sound pulses at high frequency and measures the time period between the sent pulses and the echoed pulses.
Example
It may send a short pulse of 200kHz frequency and fixed intensity every 10msec, at which the time period of the sent pulse is only a few milliseconds before a new pulse is sent out . The time instant of the reflected pulse is measured.
This process continuous periodically, i.e. every 10msec
A digital signal processor embedded in the sensor can calculate the distance
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 2
A digital signal processor embedded in the sensor can calculate the distance of the object
tx sound ∆= ⋅����
Where is known, is measured is calculated sound���� t∆ x
-1m.s 15.2730457.20 += TCair
where is the temperature in degrees Celsius (�C).T
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 3
Ultrasonic Sensors
Mainly used in area of inspection and testing, especially for non destructive testing.
The waves have frequencies higher than the audible frequency of 20Hz to 20kHz..
The penetrative quality of ultrasonic waves makes them useful for noninvasive measurements in environments that are radioactive, explosive or difficult to access.
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 4
Used for sensing distance, level and speed. For medical imaging devices, dimensional gauging and robotics applications
Ultrasonic transducer emits a pulse of an ultrasonic wave and then receives the echo from the object targeted.
Receiver
Transmitter
Processing
unit Ultrasonic Transducer
The transducer produces ultrasonic waves in the frequency of 30 – 100 kHz.
Whenever an ultrasonic beam is incident on a surface, one portion of the beam is absorbed by the medium, another portion is reflected. and a third portion is transmitted through the medium.
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 5
In proximity sensing applications
The ultrasonic beam is projected on the target, and the time it takes for the beam to echo from the surface is measured
If there is a relative movement between the source and the reflector, the Doppler effect is employed
Techniques of Ultrasonic
As an example; Ultrasonic automotive vehicle detection systems are based on two techniques:-
-� Pulse Technique
-The detector measures the time spent between transmission and reception of an ultrasonic signal to determine the distance between the transmitter/receiver and the object
t∆
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 6
-� Doppler Shift Technique.
-The frequency of the received ultrasonic signal changes in relation to the emitted frequency depending on the velocity V of the object.
-If the object is approaching the detector, then the frequency of the signal received increases in relation to the emitted frequency.
-If the frequency is reduced, then the object is moving away from the detector
Ultrasonic Generation
a- Piezoelectric
- The ultrasonic waves can be generated by the movement of a surface that creates compression and expansion of the medium.
- Piezoelectric transducers, are the excitation devices most commonly used for surface movement.
- When an input voltage is applied to a piezoelectric element, it causes the element
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 7
- When an input voltage is applied to a piezoelectric element, it causes the element to flex and generate ultrasonic waves.
- This effect is reversible, the element generates a voltage whenever it is subjected to vibrations, such as the incoming ultrasonic waves.
- The typical operating frequency of the transmitting ultrasonic element is close to 32kHz
- If the ultrasonic instrument operates in the pulsed mode, then the same piezoelectric crystals are utilized for transmitting and receiving purposes.
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 8
-The input voltage applied to the ceramic element causes it to flex and transmit ultrasonic waves. -Because piezoelectricity is a reversible phenomenon, the ceramic generates voltage when incoming ultrasonic waves flex it. -In other words, the element may work as both the transmitter and the receiver.
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 9
b - Magnetostrictive
Magnetostrictive is a property of ferromagnetic materials, as they deform when subjected to magnetic fields. Oscillators generated by this principle can produce ultrasonic waves.
c – Metal film capacitor
Metal film capacitor, can be used to generate ultra sound by applying a high frequency voltage to the metal film capacitor
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 10
Methods for Employing Ultrasound in a Sensor
a - Intrinsic Method
-intrinsic method, this method is mainly used in the ultrasonic probe sensor for medical applications.
-The ultrasound signal changes as it passes through an object, due to acoustic impedance and absorption characteristic of the object.
-The resulting signal is an image determining the properties of the object
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 11
b - Extrinsic Method
-The flight time of an ultrasound burst from its source to an object and then back to receiver is measured
-The ultrasound burst is projected at the target object.
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 12
-The time if the received ECHO is clocked.
-The signal processor computes the position of the target
2ct
x =
X = distance between the ultrasound generator and targetC = speed of soundt = time of flight from transmitter and back to receiver
transmitter / receiver are positioned close to each other as compared with the distance to the object
Ultrasonic Distance Sensing
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 13
is the distance to the object,
is the speed of the ultrasonic waves to travel to the object and back to the receiver
2cosθct
x =
xc
transmitter/receiver are not positioned close to each other as compared with the distance to the object
Ultrasonic Stress Sensing
Ultrasonic beams may be used for stress measurement
Ultrasonic Sensor Applications
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 14
The system consists of an ultrasonic probe that is placed in contact with the specimen.
The ultrasonic probe consists of an ultrasonic driver, a receiver and a control device to change the electrical signal to vibrations, and vice versa.
Operation
-When in contact with the specimen
-The ultrasonic transmitter caused wave to travel across the specimen.
-These waves are received by the receiver and converted to an electrical signal
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 15
-These waves are received by the receiver and converted to an electrical signal
-The principle relies on changes in propagation of sound in a specimen causing stress changes.
-The probe is moved around the specimen to map out the stress field distribution.
-By rotating the probe it is possible to determine the direction of the stress
Magnetostrictive Displacement
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 16
-The sensor head generate an interrogation current pulse, which travels along the Magnetostrictive wire
-This pulse interacts with the magnetic field of the permanent magnet and generates an ultrasound pulse.
-This pulse is received at the sensor head.
-The time of flight is proportional to the distance of the magnet from the sensor head.
- If the target object is attached to the magnet of the sensor its position X can be determined using the time of flight
ctx =
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 17
Process Monitoring Sensors
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 18
Self Parking Cars
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 19
(A) Ultrasonic Transducer
(B) Directional diagram
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 20
• Park Distance Control (PDC) uses sonar sound waves and high-tech electronics to for park guiding .• An electronic source unit generates a signal.• The signal is transmitted, bounces off an object and returns to the source.• Based on the "echo", the amount of time it takes for the signal to return, the sonar takes a picture of nearby objects.
Flow Meter Sensor
-The principle is the detection of frequency or phase shift caused by flowing
-This is based on the Doppler effect or on the detection of the increase or decrease in effective ultrasound velocity in the medium.
-The effective velocity of sound in a moving medium is equal to the velocity of sound relative to the medium plus the velocity of the medium with respect to the source of the sound.
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 21
- A sound wave propagating upstream will have a smaller effective velocity, andthe sound propagating downstream will have a higher effective velocity.
-Because the difference between the two velocities is exactly twice the velocity of the medium,
-Two ultrasonic generators positioned on opposite sides of a tube of flow. Piezoelectric
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 22
-Two ultrasonic generators positioned on opposite sides of a tube of flow. Piezoelectric crystals are usually employed for that purpose.
-Each crystal can be used for either the generation of the ultrasonic waves (motor mode) or for receiving the ultrasonic waves (generator mode).
-The same crystal, when needed, acts as a "speaker" or a "microphone."
- The two crystals are separated by distance X and positioned at angle with respect to flow.
-It is possible to place small crystals right inside the tube along the flow. That case corresponds to = 0. The transit time of sound between two transducersA and B can be found through the average fluid velocity Vc:
θ
θ
θcoscVCx
T±
=
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 23
-where c is the velocity of sound in the fluid. The plus and minus signs refer to the downstream and upstream directions, respectively.
-The velocity Vc is the flow velocity averaged along the path of the ultrasound.
-By taking the difference between the downstream and upstream velocities, we find
222
cos2coscos2
CxV
VCxV
T c
c
c θθ
θ ≈+
=∆
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 24
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 25
-To improve the signal to-noise ratio, the transit time is often measured for both upstream and downstream directions;
-That is, each piezoelectric crystal works as a transmitter at one time and as a receiver at the other time.
-This can be accomplished by a selector
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 26
-Sinusoidal ultrasonic waves (about 3 MHz) are transmitted as bursts with the sameslow clock rate (400 Hz).
-A received sinusoidal burst is delayed from the transmitted one by time T , which is modulated by the flow
-The time T is detected by a transit-time detector; then, the time difference in both directions is recovered by a synchronous detector.
-Such a system can achieve quite good accuracy.
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 27
-An alternative way of measuring flow with ultrasonic sensors is to detect a phasedifference in transmitted and received pulses in the upstream and downstream directions.
The phase differential can be derived as
2
cos �4CxVf
f c θ=∆
-where f is the ultrasonic frequency.
-The sensitivity is better with the increase in the frequency; however, at higher frequencies, we should expect stronger sound attenuation in the system, which may cause a reduction in the signal-to-noise ratio.
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 28
For the Doppler flow measurements, continuous ultrasonic waves can be used
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 29
A flow meter with a transmitter–receiver assembly positioned inside the flowing stream
-As in a Doppler radio receiver, transmitted and received frequencies are mixed in a nonlinear circuit (a mixer).
-The output low-frequency differential harmonics are selected by a bandpass filter.
-That differential is defined as
Cvf
fff srs
2±≈−=∆
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 30
� indicate different directions of flow
fs = the transmitting frequency
fr = the received frequency
The differential frequency is directly proportional to the flow velocity
Liquid level sensing Two dimension sensing – box height and width
Sensor A
Sen
sor
B
Sensor B
Sensor
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 31
Liquid
Box
Sen
sor
B
Sensor B
One dimension sensing –box height
Web Slack sensing
ELCT903, Sensor TechnologyDr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department 32
Roll diameter sensing