Understanding Ultrasonic object detection.

D

Denzel

Senior Member
Im just trying to get my head round the signals / amplitudes that an ultrasonic reciever picks up based on the defects or objects in its path.
im new to this and im trying to understand...
the ultrasonic pair:
one transmits ultrasonic pulses and the other recieves them

Is this correct?
the closer the object is to the sensor the bigger the amplitude of the signal in the reciever
or
the closer an object is the closer the signals are together on a time graph
or
the size of the object/ defect decides how big the amplitude is. bigger = more ultrasonic reflecting off therefore higher amplitude and vica versa

im just confused... this diagramme I made below shows a walking robot with ultrasonic pair moving towards an object. the objects size remains the same so does the amplitude increase as the robot gets closer or does the time between each signal decrease, or both.


http://i2.tinypic.com/6jlrvbk.jpg

Sorry about the questions im just confused.
 
W

womai

Senior Member
The standard ultrasonic range detectors send out a short, focussed pulse (or train of pulses) on one side, and the other one receives the echo. The microcontroller then measures the time delay between those two events, which is proportional to the distance of the object. The time delay is simply

delay = 2 * distance / speed_of_sound

The factor 2 comes because the sound has to travel all the way to the object and then the reflected sound has to travel back. The speed of sound depends on the temperature of the air and the elevation as well as current air pressure, but the total variation isn't too bad if you don't need super-high accuracy.

Relying on the size of the reflection for distance measurement won't work well because that depends on far too many parameters - distance of the object, size of the object, reflectivity of the object, angle between object surface and sound direction (you get a far stronger echo back when the surface is perpendicular to your sound direction), etc. etc. In a practical setup many of those parameters will be unknown or highly variable.

Wolfgang
 
M

Michael 2727

Senior Member
^ ^ ^ What Womai said ^ ^ ^

Speed of sound (Sea Level, 20 DegC on a Wednesday) = 343 Meters/Second.
So the sound will travel 34.3cm or 1.12 feet every millisecond.
But Ultrasonic Detection uses the echo back which will be 2 x the object
distance away. The amplitude may be 100s times more at 10cm/4in than
4 meters but that does not matter here. Only the time difference will give
you an accurate distance measurement.

The amplitude only matters when it comes to the maximum range of
the device. If the amplification used can no longer detect the echo
comming back then you have reached the MAX limit of the device.
And as Womai said, the angle of the surface and ability to reflect
the echo back will also determine the MAX range.

There is a lot more to it in practice but these are the basics.
 
R

Rickharris

Senior Member
The SRF module only gives the time difference between the trigger and the received pulse. No amplitude information al all.
 
You must log in or register to reply here.
Top