Ultrasound
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Summary
Introduction
A sonar device is a good way to measure distances across a short range in a non-contact environment. Ulrasonic waves are acoustic propogations whose frequency range is above that which can be detected by human ears.
The most common way that ultrasound is generated is as a direct conversion from electrical energy. This is known as motor mode. This is rather simply accomplished by taking advantage of the piezoelectric effect; an oscillatory electrical signal is applied to a crystal attached to a mounting. The charge causes the crystal to expand and contract with the voltage which has the effect of generating acoustic propogation. The acoustic waves are detected by a piezoelectric receiver, which converts the mechanically-transduced waves back into voltage.
The signal may also be generated by consumer electronics products, but great care must be taken to ensure that the signal is not attenuated in this range. Speakers typically have filter circuits to prevent ultrasonic propogation, and the frequency response of many microphones roll off in this range. This is partly because of the amount of ultrasound present in our daily life; percussive sounds and metallic ringing both contain ultrasounic frequencies.
There are several different ways in which ultrasonic propogation can be implemented as a sensing device. Whenever an ultrasonic sensing system is implemented, the angle at which the system propogates must be taken into consideration. Most transmitters have a narrow angle of operation. It is necessary to ensure that other ultrasonic sources do not propagate towards the receiver.
The first method looks for a doppler shift in the received waveform. In this sense, an ultrasound wave is transmitted, reflected off of an object, and then reacquired at the receiver. If the object reflecting the wave is stationary, there will be no change in the frequency of the received waveform. If the device moves, however, the frequency will deviate, and from this deviation the speed of the object can be determined.
Another method is as an echo locator. In this method, an ultrasonic signal of very short duration is pulsed from the transmitter, reflected off of an object and received at the receiver. Given that the sound travels at a constant rate (the speed of sound, for a given medium), the amount of time the wave takes to return can be used to determine how far away the reflecting object is. Care must be taken in this implementation to ensure that the duration of the pulse is less than the minimum echo time to prevent overlap. It is suggested that this system be calibrated each time it is used in a different environmental conditions. Without calibration, large shifts in temperature and humidity may give erroneous results due to the variation in the speed of sound under these conditions.
In still yet another way, ultrasound can be directly propogated from transmitter to receiver. Here, the amount of attenuation that the signal experiences is related to the distance of propogation. This method is less accurate than the echo time method due to the variations in amplitude with transceiver alignment angle and the nonlinear relation between distance and amplitude.
Devices
Source Country Price Jameco (http://www.jameco.com/) USA US$ 6.95
[edit]40TR16F / 40TR16P
Ultrasonic Sensor
Variants:
Datasheet: 40TR16F.pdf (http://www.jameco.com/wcsstore/Jameco/Products/ProdDS/136653.PDF) (52 KB)
Resources:
Notes:
Media
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External Links & References
- Raes, Godfried-Willem, 1999/2004. Gesture Controlled Musical Instruments - A Practical Report (http://www.logosfoundation.org/ii/gesture-instrument.html)
- wikipedia:Ultrasound_flow_meter
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