IR Versus Ultrasonic rangefinder on grass

Hi, I'm pretty much new to robotic, even though I've worked with kit like fischertechnik and Lego NXT.

I know the base of it, and i'm good with electronics.


but here it is i want to build plane that coud fly by itself using rangefinder (Ir or ultrasonic) to keep a distance from the ground, but i've read that IR sensor aren't good outside and that ultrasonic doesn't work good on absorbant material (Grass).

Since it's a plane I also have to consider weight in my choice.

Wich sensor would be the best for this task ?

I am afraid that the 2

I am afraid that the 2 sensors will be too unsteady for this; Too short range, the plane moving too fast etc.

The one with the longest measuring distance is the SRF05 / Ultrasonic. In principle it can detect something on 3 meters… But you do not get anything acurate before 2 meters. 2 Meters is nothing, no time to react for a plane in flight.

The only thing you could IMHO is to add something on the nose that would kill thepropellar or something - like an airbag the last 1/10 second before a crash.

You would have to use the extremely expensive laser, if you where to meaure distance from a plane to ground.


Not so sure

Frits is definitely right about the range, but I can’t se the relevance of the forward speed.

The IR wouldn’t work: the grass would reflect it all over the place.

The Ultrasonic would probably work. Yes, teh grass would disperse it a bit, but the receiver would pick it up OKay. the thing is, unless it’s an automated landing system, you’d probably want your aircraft to be higher than 2m.

The other problem with measuring the distance to the ground is bank and roll. If you’re in a banked turn and the sensor is pointing down relative to the aircraft, then it wil ACTUALLY be pointing out sideways, making the ground appear further away than it really is.

Don’t aircraft altimeters work on air pressure? I don’t know what the air pressure variation is at low altitudes, but I’d like to find out…

this is no expensive RC airplane

The plane i’ll use is a Air hogs defender, it’s a stryofoam plane for kids, it’s made to be crashed so no probleme for this and it’s a very slow flying plane and very simple, the faster the motor spins the higher the plane goes and for direction you just slow down one of the two motors.

For the landing it’s simple, you just slowly crash it :slight_smile:

No way an Air hogs defender

No way an Air hogs defender will carry the extra weight!

And yes - there is the "roll"-problem, but there IS however also the speed to consider. Trust me :slight_smile: You will get results after things has changed. It takes up to 20ms for a ping to return. And you will get a lot of noise, so you would have to do several sonic pings to get an average… an average of something that by then is changed.

I do not mind if you do not trust me :slight_smile: But I know what I am talking about, I am just trying to save you time & trouble :slight_smile:

Make a quite easy test, compared to trying to make it fly: Put your equipment (SRF05 or IR) on a broomstick, and wave it around, while having cables down to your hand holding it… See if you get reliable data fast enough to have a plane react (it does take time from the plane reacts to the direction is changed)… or just an enormous amount of jumping numbers…

Making a skidding / sliding Wall Racer is nothing compared to navigating a plane.

Of course there are (fun) ways to make "robots" control planes (in fact I am on a project with a guy right now) - but mounting an SRF05 or SHARP IR on an Air hogs defender… is NOT going to give good results :slight_smile:

Carry-on luggage restrictions
It’s true. The compressed air plane would hardly carry the battery.

Ah, finally, a question I am

Ah, finally, a question I am qualified to answer :slight_smile: Yeah, aviation altimeters work on air pressure, but they only tell you altitude above sea level, not above ground level. It’s up to the pilot to be aware of how high the ground is below them. Pressure altimeters also need to be adjusted regularly to match the current pressure at sea level. As the day progresses and the air gets warmer (or as you fly to a different place with different weather), the air pressure changes and the altimeter needs to be adjusted to match, or else it will provide a false reading. The current air pressure is one of the bits of information included in any airport’s ATIS broadcast, and it’s standard practice to listen to those broadcasts as you make your way across the country and continually adjust your altimeter to reflect the current conditions.

The accuracy of pressure altimeters isn’t really fine enough for scale-size flying at low altitudes. The FAA requirements for certification only require an accuracy of +/- 20 feet at low altitudes (below 1000’ above sea level), and greater error is allowed above that, up to +/- 205 feet at an altitude of 35,000 ft.

And to answer your last statement, air pressure decreases by about 1 inch of mercury for every 1000 feet of altitude.

Since military aircraft are usually more concerned with height above GROUND level (doing low-level flying, etc), they usually have radar altimeters that just shoot a radar beam downwards. I don’t imagine that’s feasible for the average robot-building hobbyist.

GPS modules are quite cheap nowadays – SparkFun has several tiny ones for well under $100. It still only gives you altitude above sea level, so it wouldn’t be too useful for this project, but it’s something I’ve been meaning to incorporate in a robot sometime.


ok, It’s not a good idea for a first robot

I’ll try build a “first robot” first, maybe I’ll try this project later. when I’ll be better with programming.

I do believe you, but I take this as a challenge :stuck_out_tongue:

Btw, ultrasound take 20 milisec to bounce back, but in theory IR goes a the speed of light, am I right ?

It is a small RC plane, not
It is a small RC plane, not a compressed air plane.

It does sound like a fun

It does sound like a fun project, and a foam plane shouldn’t cause too much damage when everything goes horribly, horribly wrong :slight_smile: I agree that it wouldn’t be a good first robot project, but you should definitely try it as a third or fourth robot. I’m actually tempted to give it a try myself once I finish my current project… :


Foaming Airplains
If it’s an airhog it is definitely too small to carry the extra electronics. There are a few options, however, that works incredibly well and are pretty cheap overall:

* GWS Slow Stick
* GWS E-Starter
* GWS Formosa

No, I’m not a rep for GWS but their planes are really inexpensive and will handle a lot of abuse. I’ve broken the fuselage in half on the E-Starter and with some 5 minute epoxy and 20 minutes later I was flying again. The wing loading is pretty well and they’ll hold a bunch of stuff. You can mount cameras and other crazy things. The slow stick has the best lift to weight ratio, but will be blown around in a breezy location, unless you chop the wing a little bit.

There are a lot of people who add altimeters, GPS and other instrumentation to their planes which work quite well. One place you should find a lot of help is, they’ve helped me a lot and could point you in the right direction.

Ah, sorry. The airhogs I
Ah, sorry. The airhogs I know of were a compressed air single-cylinder engine.

Wow - good tips! GWS Slow
Wow - good tips! GWS Slow Stick is now on whish-list!

seems nice
I’ll try with de air hogs first then when i’ll fail i’ll get one of those.