dknguyen

Hi. I was wondering how you determine whether or not you are at constant velocity in an airfact. THe IMU I'm getting comes with accelerometers which I never really planned a use for, but they're there so might as well use them how I can. I was thinking about using them to measure the inclination for the compass whenever the plane is at constant velocity but I am not sure how to determine that.

THe only way I could think of was the measring airspeed with the pitot tube but this is dependent on whether the wind is changing or not since if the wind is slowing down as fast as your plane is speed up you still measure zero speed when you are actually accelerating. Do you just assume the wind is essentially at constant speed (or changing slowly enough to assume that?) Or does it just fall apart?

On that note, my main source of resetting the roll/pitch gyro bias errors were IR horizon sensors that would reset the roll/pitch to zero whenever they detected such. But they'd have to be matched and thermopiles are far too expensive to buy a bunch and try and match a few, except I am not quite sure how to match them either (in analog) due to their different offsets and gains without a LOT of trial and error. It's another reason I'd like to use the accelerometers somehow (to allow roll/pitch reset whenever the plane is cruising rather than just whenever the roll/pitch crosses zero).

I suppose GPS could also be used to determine constant velocity, though I don't know how accurate it would be for that and it'd have to be fairly accurate to not distort inclination readings from the accelerometer. I've never quite trusted GPS and would at least like to augment it with something.

Let's also ignore the case of the wind accelerating in an axis where the pitot tube doesn't measure airspeed for now...

Blueteeth

I wouldn't have thought GPS would be accurate enough for measuring constant velocity...unless you take measurements with quite a long period, then average them. In which case it would take a while (10 minutes?) before you get an actual reading of acceleration (or lack there-of).

The accelerometer sounds like the best choice. Plenty of dev boards available for it, easily interfaced with a microcontroller, but its 3D. Theorectically of course, it should indicate 'no acceleration' regardless of the planes orientation. All you should have to do is check whether or not it is accelerating on any axis, and take the appropirate action. What that action is, is up to you...I imagine doing trig on a micro could be tough.

Also, I would have thought you would need some form of low pass filtering, as planes, regardless of size, vibrate, so you'd get constant readings of acceleration. Averaging readings taken every 100ms or so should do. You'll have to buy one and do some tests.

Blueteeth.

Ps, I have NO experience in RC planes or their controls..just thought I'd add something.

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3v0

I have zero experiance with accelerometers. Determining if the plane is at constant speed would seem to be the ideal application for them.

It confuses me that you are looking at GPS and airspeed.

What am I missing ?

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Hero999

It's a known fact that you can't get the speed of a moving body without referencing it to something else. Whether it be a GPS satellite, the earth (using an accelerometer) or wind speed.

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dknguyen

Yeah, I was fuddling around in my head trying to think about whether the acceleration of the plane and how it is affected by referencing it to the wind or the ground.

My issue is being able to detect a condition (whatever that may be) when the only forces on the accelerometer is the force of gravity and not the acceleration of the plane. I am having trouble figuring out the reference though whether it be the wind or ground. THe pitot tube obviously measures the airspeed with respect to the wind and not the ground.

Like in space if you accelerate the vehicle, an accelerometer will still detect acceleration, but what is this acceleration being referenced to? It's an increase in velocity and velocity has to be referenced to something else. Or must you pick an arbitrary point in space to do it? Forces also need a reference right? On the surface I wouldn't think they do, but then I think that forces are directly related to acceleration which does need a reference.

If a plane's ground speed (in the absence of wind) equals the wind speed (referenced to the ground) and are in opposite directions, the true ground speed of the plane would appear to be zero since it's stationary overhead. But if the no-wind ground speed and the windspeed increased equally in opposite directions such that the ground speed is still zero, would you still detect an acceleration aboard the plane? Because airspeed is increasing, but ground-speed is not. It's fairly easy to figure out what happens if the two speeds change unequally since there will be both an acceleraiton observed in the airspeed and from the ground, but this is the problem I can't seem to resolve in my head. What is the accelerometer using as a reference? Because you don't just flick a switch inside it and say "reference to the ground" or "reference to that star".

Blueteeth

Why do you need a reference? From your first post I was under the impression you needed to know whether the plane was at a constant velocity or not...which is relative, not absolute (which would need an external reference). You didn't mention you needed to measure the velocity.

With an accelerometer, you're looking for 'lack' of acceleration, on any axis. Look at the datasheet for one for an explaination of its operation...think about it, does your 'wii remote' use a reference?...

I tihnk I've over simplified, adn therefore completely misunderstood your problem. As for measuring absolute velocity, I guess GPS would be handy, and of course use a 'pitot tube' for speed (not velocity).

Blueteeth

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dknguyen

You mean to measure the acceleration of gravity before take-off and then whenever the magnitude of acceleration matches that assume constant velocity? (Of course, ignoring the fact that you can get the same reading by accelerating 2x as fast as gravity in the opposite directon). I've been trying to tip-toe around doing this because you can get the same magnitude reading for more than one condition. It doesn't happen if there's no gravity since everything is symmetric about zero...and about zero that's only one point.

No, I don't need to measure velocity. I realized it was a bit confusing so that's why in my previous post I reworded the question to:
"My issue is being able to detect a condition (whatever that may be) when the only forces on the accelerometer is the force of gravity and not the acceleration of the plane."

Doesn't acceleration need to also have a frame of reference? Since If I am on a bus, everyone around me doesn't appear to accelerating with respect to me until they get up and start walking around the bus. I mean...if velocity needs a frame of reference and acceleration is a change in velocity, shouldn't acceleration also need a frame of reference (and in the same way shouldn't forces also require a frame of reference?).

EDIT: I thought this was interesting.
http://www.physicsforums.com/archive...p/t-11408.html

I thought the 2nd post was the most interesting. Something about kinematics vs kinetics and how velocity isn't really dependent on anything so it's easy. But acceleration is dependent on force and how you measure them. If you and I are standing next to each other and I measure your acceleration then it is zero since we are both stationary to each other. BUt we know we're accelerating since we can feel the force on our feet, except that we aren't accelerating in the sense of kinetics kinetics since our velocities (relative to each other and the earth) are still zero even though gravity is exerting a force on us because the ground's normal force produces zero net force. So confusing. Bah!

Gravity does weird things. Took me forever a year ago to figure out why an accelerometer measures zero acceleration in free fall even though the sensor is accelerating 9.81m/s^2 toward the ground when falling (gravity acts equal on the sensor frame as well as the sensor beam and accelerates them both equally in the absence of a normal force to support the sensor frame).

Blueteeth

I think we are both over complicating it now An accelerometer device (as far as I know anyway) uses capacitive measurement for an element that is free to move under springs. As you accelerate, forward say, the element moves back. At constant velocity (in a given direction remember, not speed) the element experiences no force other than gravity (no interia). As I said, its relative, gravity does not change so whether you are on the ground, in the air or what it won't make a difference.

Now this sentence:

"My issue is being able to detect a condition (whatever that may be) when the only forces on the accelerometer is the force of gravity and not the acceleration of the plane."

I did read that, well worded. It still confused me Gravity isn't a force btw, its an acceleration. So you wish to isolate the accelerometer (fed up with typing that word) from the forces experienced by a plane of changing velocity?

Ultimately, this is what I thought you meant, and is whats going on in my mind. Whether it answers your question or I've missed the point completely.
You wish to create a device that will detect a condition of 'constant velocity'. Which...is zero acceleration. Since the accelerometer works on all three dimensions/axis, it could detect up/down/left/right/forward/backward change in velocity.

I guess a good question, which I really should have asked before is... What is this for? What would this condition control?

Blueteeth

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dknguyen

I kind of see where the confusion is coming from.

Yes, that is what I mean.

The use of this is to know when I can depend on the accelerometer's readings to measure gravity (and only gravity) in order to determine the inclination of the plane. THis would be used as an additional drift-free absolute measurement along with the horizon sensors to periodically correct gyroscope readings. THe secondary use for this would be to know when I can a 2-D compass is level with the ground and has a valid reading or to provide an inclination measurement for a 3D compass.

Whether that be airspeed, ground speed (which I can't measure due to size and cost), or GPS (except let's forget about GPS for now). To detect a moment of zero acceleration for all axises (except for that caused by gravity) is another way I would word it.

Blueteeth

Ahhhhh lol

Ok so I was a bit off the mark. So you're looking for a reference to determine the 'inclination of the plane'. I get ya. You were way ahead of me all the time..it seems this is something you need, to correct something else...as a sort of 'back up'. Well unfortunately an accelerometer will not detect the angle of gravity (ie: the planes orientation is space).

If you wish to know 'which way is down', and that is the point, please say just to clear up confusion I've made for myself. (curse my over thinking things).

I feel I am out of my depth here, as all i can provide is 'random ideas' about how to determine 'which way is down'. like...

GPS will tell you absolute position..so over time, you can have two points, A and B, work out the change in distance, height and direction, then you know which way your plane is heading (and therefore if its facing down or up and by how much). Then again, wind will affect that.

Apologies for spamming the topic with a misunderstanding.

Blueteeth

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jpanhalt

It is not clear to me what your question is. Are you trying to maintain constant velocity of an aircraft relative to the ground? That is very difficult to do even with a pilot on board and lots of equipment. In fact, in strong up-drafts and down-drafts from storms, it may be both impossible from the standpoint of aircraft performance and dangerous from the standpoint of aircraft design strength.

One usually tries to fly at constant speed relative to the relative wind, i.e., indicated airspeed. Thus, with an up-draft, you can let the airplane rise, which makes passengers and air traffic control nervous, or keep the nose down and reduce power to maintain a constant indicated airspeed. In reality, the pilot does a little of both.

As for ground speed, GPS is probably the most common today. DME, which is a ground-based older system with some similarities to GPS, and Loran (most stations over land have been deactivated now with GPS) have also been used. Celestial navigation can be used, but is pretty uncommon today. The big aircraft also use inertial systems. Last but not least, there is the method of pilotage or dead reckoning, a lost art. John

3v0

Yah we all know that
But the question was
No memtion of finding out what the velocity is.

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dknguyen

I am not trying to mainstain constant velocity. I am just trying to detect when I am in it so a valid accelerometer-based inclination reading can be taken. Like on a car, you could either stop a car and take a reading or assume no slip and take the reading whenever the wheel RPM is constant.

Obviously, I can only use the second method on a plane since I can't just stop the plane. But I am not sure how to do the same thing on a plane (assuming I can measure 3-dimensional ground speed and 3-dimensional airspeed) since constant velocity is a relative thing and I am not sure if I am supposed to use airspeed or ground speed (since there is ground speed vs airspeed and a constant airspeed doesn't mean a constant ground speed).

If it's ground speed, it's too big and expensive for me to measure unless I use (inaccurate) GPS since radar is out of the question. If it's airspeed it might be feasible if I make a 3 DOF pitot tube and am flying fast enough. I just can't figure out whether or not the constant velocity (or zero acceleration except for gravity) requirement for the accurate accelerometer inclination reading is constant ground speed or constant airspeed

The only method I know that would work right now is to calibrate for the force of gravity before takeoff and whenever the magnitude of acceleration is equal to this reading, take the inclination measurement, then use the gyros (assuming they don't ever have a chance to drift 180 degrees) to determine the whether or not the plane is inverted to filter out the case where the plane is accelerating more than 1g away from the surface of the earth which would also give the same reading.

Worded another way...
I was told by the manufacturer of tilt-compensated compasses that the readings aboard an aircraft will be valid (ie. the inclinometer will be valid) if the aircraft is under conditions of no acceleration. Okay. No acceleration- but I can't figure out if that's no acceleration with respect to the ground, or the surrounding air.

Ambient

Is this project for an R/C airplane or full size? If it is R/C this will be a little tricky, cause any wind will constantly be making it accelerate, even if you don't notice it visually. Airspeed is the only thing you can measure without GPS, and it is more important anyways.

3v0

The reference will be the ground below.

Air moving around the craft create forces that act to move it. Think what would happen if the craft was suddenly caught by the wind and accelerated. If you were sitting in that craft you would feel that acceleration.

EDIT: Think of an invisiable line between the plane and the center of the earth. Ignore the rotation of the earth, wind, everythings else. As long as the plane does not accelerate in any of the three axis the accelerometers will show no acceleration execpt for that of gravity.

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