Sonic Distance Sensors

Transcription

1 Sonic Distance Sensors Introduction - Sound is transmitted through the propagation of pressure in the air. - The speed of sound in the air is normally 331m/sec at 0 o C. - 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. - 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. 1

2 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 of the object x = sound t t Where sound is known, is measured x is calculated C air = T m.s where T is the temperature in degrees Celsius (C). 2

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4 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. 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 4

5 The transducer produces ultrasonic waves in the frequency of 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. 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 5

6 Techniques of Ultrasonic As an example; Ultrasonic automotive vehicle detection systems are based on two techniques:- - Pulse Technique t -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 - 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 6

7 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 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. 7

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. 8

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10 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 10

11 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 b - Extrinsic Method -The flight time of an ultrasound burst from its source to an object and then back to receiver is measured 11

12 -The ultrasound burst is projected at the target object. -The time if the received ECHO is clocked. -The signal processor computes the position of the target x = ct 2 transmitter / receiver are positioned close to each other as compared with the distance to the object X = distance between the ultrasound generator and target C = speed of sound t = time of flight from transmitter and back to receiver 12

13 Ultrasonic Distance Sensing x c is the distance to the object, is the speed of the ultrasonic waves to travel to the object and back to the receiver x = ct cosθ 2 transmitter/receiver are not positioned close to each other as compared with the distance to the object 13

14 Ultrasonic Sensor Applications Ultrasonic Stress Sensing Ultrasonic beams may be used for stress measurement 14

15 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 -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 15

16 Magnetostrictive Displacement -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. 16

17 -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 x = ct 17

18 Process Monitoring Sensors 18

19 Self Parking Cars (A) Ultrasonic Transducer (B) Directional diagram 19

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. 20

21 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. - A sound wave propagating upstream will have a smaller effective velocity, and the sound propagating downstream will have a higher effective velocity. -Because the difference between the two velocities is exactly twice the velocity of the medium, 21

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." 22

23 - 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 θ = 0. The transit time of sound between two transducers A and B can be found through the average fluid velocity V c : θ T = C ± x V c cosθ -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 V c is the flow velocity averaged along the path of the ultrasound. 23

24 -By taking the difference between the downstream and upstream velocities, we find T = 2xVc 2 C + V c cosθ 2 cos θ 2xVc cosθ 2 C 24

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26 -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 26

27 -Sinusoidal ultrasonic waves (about 3 MHz) are transmitted as bursts with the same slow 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. -An alternative way of measuring flow with ultrasonic sensors is to detect a phase difference in transmitted and received pulses in the upstream and downstream directions. 27

28 The phase differential can be derived as 4 f xvc cosθ f = 2 C -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. 28

29 For the Doppler flow measurements, continuous ultrasonic waves can be used A flow meter with a transmitter receiver assembly positioned inside the flowing stream 29

30 -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 f = f s f r ± 2 f C s v f s = the transmitting frequency f r = the received frequency indicate different directions of flow The differential frequency is directly proportional to the flow velocity 30

31 Liquid level sensing Sensor Two dimension sensing box height and width Sensor A Liquid Senso or B Box Sens sor B 31

32 One dimension sensing box height Web Slack sensing Roll diameter sensing 32