ultrasonic distance sensor question
Steve12
Posts: 8
Hi,
I'm just starting to learn about the ultrasonic distance sensor.
The manual I'm using is, "Smart Sensors and Applications".
I wonder if someone can tell me if the distance measurement
to an object varies according to the softness or hardness of
the object? The manual mentions this, but isn't really clear about
whether the distance calculation changes.
I apologize if this information is already here and I missed it.
Thanks,
Maryann
I'm just starting to learn about the ultrasonic distance sensor.
The manual I'm using is, "Smart Sensors and Applications".
I wonder if someone can tell me if the distance measurement
to an object varies according to the softness or hardness of
the object? The manual mentions this, but isn't really clear about
whether the distance calculation changes.
I apologize if this information is already here and I missed it.
Thanks,
Maryann
Comments
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- Stephen
It seems to me that I can have two objects which are the same distance
from the sensor,·and get two different distance measurements, depending
on the relative hardness of the objects. Maybe the sensor isn't meant to be
accurate to the degree I first thought it was.
Mary.
I would assume a 'softer' object would 'smear' the echo, as well as returning less of it. Thus, it becomes a question of when the echo signal crosses the 'detection threshold' for the sensor. And this could give you some plus-and-minus uncertainty about the "real" distance being measured.
Also, the amount of echo is also determined by the 'angle' at which it hits a surface. For example, bouncing sound off a wall at 90 degrees would be optimal. If bounced at 45 degrees, you might not get any echo back, or the echo you DO get back may have 'bounced' more than once. A 'soft' or rumpled surface may have little of its reflecting surface at 90 degrees to the sensor.
Bouncing off something like a can or coffee cup it would seem you're guaranteed to have SOME part of the survace at right angles. Bouncing off something like a fuzzy teddy bear would be more difficult.
The setup displays a continuous·180 degree "sweep" forward of the bot and plots a colored pixel for each "hit" or return echo·(green; OK, yellow: Close, red: Too Close). It also displays text below the sweep area indicating the distance (in cm) to the center of the closest·object·and the angle to the closest object in brads (currently) and the direction (<, >, ^).
Because of the excellent resolution and speed of these displays, I've noticed a number of interesting characteristics of the PING))) sensor. What follows is not an exaustive list, but might help some who are not able to get real-time visual feedback on the sensor's performance.
1. The texture, shape, density, reflective qualities of objects do not effect the distance accuracy of the sensor. The speed of sound through air of a given density does not change based on the properties of the object it's reflected from. The above factors do greatly effect the strength of the echo signal returned to the sensor.
2. Round, firm objects (hardwood, metal or glass cylidrical objects) seem to work best. The claim that I read somewhere that the sensor can "see" a broom handle at 3 m is true (unless it's hiding behind a soup can
3. Because the PING))) sensor uses reflected sound, objects within range can be hidden by other objects. This can happen either·because of line-of-sight·situations or because a highly reflective object near a low-reflectivity object produces a much stronger return and masks the latter object.
4. The sensor does not seem to be bothered by noise within the human audible range. I've tested in very noisy invironments without any noticable effect on the sensor's performance.
5. The sensor can return strange and confusing data if it's doing a sweep while moving or if an object in the sweep area is moving at more than a slow crawl (tracking the cat doesn't work very well).
I hope this helps those that are experimenting with the PING))) sensor. If·anyone has·questions regarding specific test scenarios, I'll be glad to let you know about my results or I'll try to duplicate the scenario and·let you know the results.
Regards,· Steve
The distance measurement is not, as Steve said, affected by the density of the object
from which the echo is returned.
A soft object will absorb (depending on how soft it is) a part of the sound wave's
energy, so the strength of the sound returned will be weaker than the one sent.
On a scope, we would see that the amplitude of the wave would be
smaller on the return trip.
The less dense the target object, then the more energy absorbed and the less amplitude
(weaker the signal) being returned.
But, the speed of the return signal would always be the same as the signal sent, unless
there is a change in the medium through which it must pass.
I hope I got that right.... thanks again,
Maryann