Sonar speed useage

AIman

How fast can something be moving and sonar still be effective?
I want to add sonar to my RC plane and car. The car goes upto 45 and the plane 60+. How long will sonar work before speed affects it and if there are problems can I get around them? Will the range of the sonar affect things?

For example Ping))) has a 3 meter range and the SRF10 has a 6 meter range. Does range matter when it comes to speed?

Beau Schwabe

Sound travels at about 1050ft per second, but varies depending where you are in relation to sea level, and the temperature.

given that, if your car goes 45mph, your creating an "ultrasonic head wind" if you will of about 66ft per second... So your introducing about a 6% error.

with your plane going 60mph, (88ft per second) your error increases to about 8%


ft per second = mph * 1.4667

%error = 1 - [noparse][[/noparse] (Speed of sound in feet) / [noparse][[/noparse] (Speed of sound in feet) + (ft per second) ] ]


Note: This does not take into account any vibration capable of introducing error.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Bruce Bates

Gents -

You're considering the speed to be constant I suspect, and I'd doubt that would be the reality in the field.

Regards,

Bruce Bates

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Beau Schwabe

Bruce,

With the exception of vibration playing a significant part, I was expressing a worst case scenario with just speed alone.
Let me know if I'm off base with my assumption.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 10/5/2006 3:05:17 PM GMT

pjv

Hi All;

At (considerably) higher speeds the Doppler effect of course will need to be considered. The effective frequency goes up, but the resonant frequency of the sonic elements remain unchanged. At velocities below 50ish feet per second this distortion probably still lands within the receiver's resonant pass band, but at over 100ish feet per second it will at least reduce the sensitivity.

But then these speeds are not likely to be considered....... I just bring the point up for interest.

Cheers,

Peter (pjv)

SteveW

Bear in mind that the wind will be whistling around the sensors, with substantial energy at 40KHz. You might want to move to solid aluminium sensors to solve that, but they're rather less sensitive, and generally wors in all ways than the open sensors. Ping is unlikely to work very well with a simple substitution.

Also, what information do you think you'll get from ultrasound? At the speeds you're discussing, you're not going to 'see' things far enough out to avoid them.

Steve

AIman

For the airplane I want to be able to detect the ground. Landing is a problem for me and I have a bad tendency to come in to steep, so I was thinking of mounting a sonar on the bottom of the plane to aid in 'crash avoidance'. For the car its a similar concept, except that it would be on the sides looking outward.

All the aircraft needs to do is pull up, not distinguish objects and the car is difficult to turn at top speed unless you take wide turns (actually its a T-max truck). Both·R/C vehicles are gas powered so moving the extra weight is a simple matter.

The goal is·for the airplane simply avoid the ground and for the truck avoid an object within its saftey zone.

For that matter, if sonar won't work then what would?

SteveW

Ah, the plane might well work out OK. Sounds like it would be worth building a test rig and sticking it out of a car window, before messing with the plane. This would let you tweak the mechanics of shielding the sensors much more easily. Do you fly off grass or hard surfaces?
One thing (of many) to ponder is that ultrasound won't detect objects at an angle nearly as well as it will flat-on. Expect the ground to vanish from time to time, and for the distance to vary more with angle than it does with height, and write your software to survive these events.

As for the truck avoiding obstacles at speed, well, I'd do that second...

Steve