low altitude (height) sensor

[dareaper]

hi people, any idea where i can get an affordable altitude, or rather height sensor that can have accuracy up to 1~5 mm? i would need the sensor to have a sensing range from 0 to around 10 to 15 cm above the ground, and to be able to work on both hard ground and water. i know i am probably looking at laser or ultra sonic sensors, but was hoping someone here will be able to point me to some other lower cost alternatives. thanks alot! :)

[ElectroLawndart]

dareaper,
Take a look at the Sharp GP2D02 IR distance sensors. They're easy to interface to and work great. I've used them in my ground robotics projects. They use trianglelation rather than time of flight like laser range finders and sonars.
I'd be curious as to how much filtering you would have to do over water because of the ripple effect. Are you looking to "fly" something in ground effect like a hydroplane or float something like a hovercraft?

Dart

[dareaper]

dareaper,
Take a look at the Sharp GP2D02 IR distance sensors. They're easy to interface to and work great. I've used them in my ground robotics projects. They use trianglelation rather than time of flight like laser range finders and sonars.
I'd be curious as to how much filtering you would have to do over water because of the ripple effect. Are you looking to "fly" something in ground effect like a hydroplane or float something like a hovercraft?

Dart

hi ElectroLawndart, yep, i need to build a uav ground effect plane for my sch project, so i need a height control system for it. thanks for the heads up, will check it out :)

edit: checked out the gp2d02, unfortunately i dont think it will suit my purpose because i need the sensor to be able to detect a minimum height of at least 2~3 cm... :(

[ElectroLawndart]

Hi dareaper,

Bummer about the Sharp sensor. One nice thing about wings in ground effect is that once you trim out in GE it become self correcting. Fly higher and ground effect becomes less and the aircraft mushes back down into ground effect. Did this with a Piper Warrior on a dry lakebed. Trimed out with flaps, kept the power setting the same then put slight back pressure on the yoke. The aircraft would balloon up then settle back down into ground effect.

Dart

[dareaper]

Hi dareaper,

Bummer about the Sharp sensor. One nice thing about wings in ground effect is that once you trim out in GE it become self correcting. Fly higher and ground effect becomes less and the aircraft mushes back down into ground effect. Did this with a Piper Warrior on a dry lakebed. Trimed out with flaps, kept the power setting the same then put slight back pressure on the yoke. The aircraft would balloon up then settle back down into ground effect.

Dart

yep, a GE plane would be self correcting, but because my model is based on a commercial GE plane, and one of the purpose of the project is to prove the viability of the actual craft, thus the rc model have to be able to fly smoothly at the optimal cruising height, rather than pitching up and down...

guess i will have to search for a viable ultrasonic sensor then, thanks alot ElectroLawndart :)

[JayFrancis]

For the GP2D02 - it is common practice to mount it 10cm away from the edge of your "platform". i.e. in my ground robots, I mount it so 10cm is at the edge of my robot. That way I can avoid the "odd behavior" in the 0 to 10cm range. If you have a high wing airplane, you could probably mount it inside the wing so that (unless you've already whacked the wingtip into the ground) you end up beyond the dead zone.

Make sure you understand the latency in the Sharp IR sensors before using them in something that moves too fast... All of them (including the analog output ones) update about every 85ms. Their outputs also vary with the reflectivity of the target (don't know how this would work over water). Finally, I've found that they have a device to device offset that you need to calibrate out.

--Jay

[kd7ost]

Here's a few US sensor modules that are ready to interface you controller to.

Dan

http://www.lynxmotion.com/Product.aspx?productID=59&CategoryID=8
http://www.parallax.com/detail.asp?product_id=28015

[ElectroLawndart]

thus the rc model have to be able to fly smoothly at the optimal cruising height, rather than pitching up and down...


Optimal height would be a function of angle of attack and airspeed. Rear stab controls AOA and once it's set there shouldn't need much adjustments. If I remember correctly, if you want to go faster, you want to fly lower with less AOA and more airspeed. There shouldn't be much pitching up and down provided you're flying at a high enough reynolds number just like the difference between light parklyers and larger R/C aircraft. Might not be a problem since your wing loading would be higher than a similar plane that was meant to higher out of GE. Make sure to keep your rear stabilizer high enough to be out of GE otherwise your height will have major effects on pitch.

Dart