accelerometer, gyro or infrared?
Hey everyone, I'm new to this site. I'm starting on a uav project and I had a question that googling hasn't sufficiently answered.
I've noticed that, for tilt sensing, many designs that I've seen are using an infrared setup to detect the difference in temperature between ground and sky. Is there an advantage to this setup over digital gyro's or accelerometers? Possibly price or sensitivity?
Also, after reading up on the subject, I'm not sure I understand the practical distinction between gyro's (like this) and accelerometers (like this). It seems that both sense tilt on a set number of axes, with the accelerometers also being able to measure acceleration.
In addition to automated flight, I'm hoping to hoping to create an augmented reality overlay for a wireless camera feed from the uav. Some of my ideas for features would be, a 3d, very large cylinder overlaid on the video feed to simulate a visual fence that sits on the earth that would demarcate the distance that the uav can travel before leaving radio range. Also, a blue line suspended in space showing the path that the uav has traveled, for instance, after taking off, if you banked the plane long enough to see the point of take-off there would be a line starting on the ground reaching into the air. Also, arrows hovering over points of interest, like way-points or the take-off and landing area.
To do this would require, having an updated 3d point in real space. Hopefully a gps module will fill this need, though from what I've read, altitude sensing for a gps sensor may not be accurate enough. And an accurate heading and yaw/tilt sensing. I've written a program using processing that takes input from a video source and can overlay the features that I mentioned earlier using made-up positioning data. So for this setup, does it seem like a gps unit, (for lat,long and altitude) and an accelerometer for yaw and tilt would work? If so, the only thing I'd be missing would be the heading. I wondered though, since the plane is always moving, and I have constant lat and long measurements, wouldn't it be possible to calculate the heading of the uav? Or would that not be accurate enough and require a magnetometer?
Thank you for reading,
falldeaf
I've noticed that, for tilt sensing, many designs that I've seen are using an infrared setup to detect the difference in temperature between ground and sky. Is there an advantage to this setup over digital gyro's or accelerometers? Possibly price or sensitivity?
Also, after reading up on the subject, I'm not sure I understand the practical distinction between gyro's (like this) and accelerometers (like this). It seems that both sense tilt on a set number of axes, with the accelerometers also being able to measure acceleration.
In addition to automated flight, I'm hoping to hoping to create an augmented reality overlay for a wireless camera feed from the uav. Some of my ideas for features would be, a 3d, very large cylinder overlaid on the video feed to simulate a visual fence that sits on the earth that would demarcate the distance that the uav can travel before leaving radio range. Also, a blue line suspended in space showing the path that the uav has traveled, for instance, after taking off, if you banked the plane long enough to see the point of take-off there would be a line starting on the ground reaching into the air. Also, arrows hovering over points of interest, like way-points or the take-off and landing area.
To do this would require, having an updated 3d point in real space. Hopefully a gps module will fill this need, though from what I've read, altitude sensing for a gps sensor may not be accurate enough. And an accurate heading and yaw/tilt sensing. I've written a program using processing that takes input from a video source and can overlay the features that I mentioned earlier using made-up positioning data. So for this setup, does it seem like a gps unit, (for lat,long and altitude) and an accelerometer for yaw and tilt would work? If so, the only thing I'd be missing would be the heading. I wondered though, since the plane is always moving, and I have constant lat and long measurements, wouldn't it be possible to calculate the heading of the uav? Or would that not be accurate enough and require a magnetometer?
Thank you for reading,
falldeaf
Replies to This Discussion
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ModeratorPermalink Reply by Chris Anderson on -
We should set up a FAQ, as this gets asked nearly every week. Gyros do not measure tilt, they measure rotation. And acceleration and tilt are the same thing, since gravity is just an acceleration.
You need both because gyros drift (long-term errors) and accelerometers are noisy (short-term errors). You have to combine them to correct the errors.
There is much debate over whether inertial measurement is better than IR. Check out our interview with Dean Goedde for more.
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Permalink Reply by falldeaf on -
Sorry for increasing the noise in the signal/noise ratio. I quickly perused the previous forum entries and didn't see the question or find it in a faq. I've mostly got a strict programming background and only a few sophisticated electronics projects under my belt. The physics and electronic sensors are totally new.. I'm doing my best to pick it up quickly. ;P Thank you for the link to the interview, that was definitely an interesting read.
Oh, also, It seems like you're an admin, here. If so, thank you for the site. It's an amazing and well organized resource.
.f
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Permalink Reply by Marc Ramsey on
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An accelerometer measures acceleration. A 3-axis accelerometer will tell you the orientation of a stationary platform relative to earths surface, once that platform starts moving, however, things get more complicated. If the platform is in free-fall, it will show zero acceleration. If it is accelerating in a particular direction, that acceleration will simply be added to whatever acceleration is being provided by gravity, and you will not be able to distinguish. A 3-axis accelerometer in an aircraft in a properly coordinated turn with a 60 degree angle of bank, for instance, will show 2 G "vertical" acceleration in the aircraft, despite the fact that the aircraft is tilted 60 degrees relative to the horizon. So, accelerometers alone can't be used to keep in an aircraft in a particular orientation.
A gyro measures rate of rotation around a particular axis. If a gyro is used to measure the rate of rotation around the aircraft roll axis, it will measure a non-zero value as long as the aircraft is rolling, but measure zero if the roll stops. So, a roll gyro in an aircraft in a coordinated turn with a 60 degree bank will be measure a rate of zero, same as an aircraft flying straight and level. You can approximate the current roll angle by integrating the roll rate over time, but you can't do so without some error creeping in. Just to make life more interesting, gyros drift with time, so additional error will accumulate over a period of minutes or even seconds, and eventually, you'll have a totally inaccurate idea of your current roll angle relative to the horizon. So, gyros alone can't be used to keep in an aircraft in a particular orientation.
GPS has a relatively slow update rate (1 to 10 Hz) and is subject to short term errors. It is possible to use GPS alone to keep a very stable and slow flying airframe on a particular ground track on a calm day.
An inertial measurement unit (IMU) combines (fuses) information from two or more sensors, such as gyros, accelerometers, magnetometer, and/or GPS, to determine orientation and velocity vector relative to the earth. The computations are fairly complex, and special filtering is often required to eliminate the measurement noise these silicon devices are subject to, so a "low cost" off-the-shelf IMU with decent specs can easily cost $1000 to $5000 US.
Infra-red horizon sensing "autopilots" are inexpensive and work fairly well as long as they have a clear view of the horizon. Unfortunately, mountains, clouds, haze, buildings, etc., will confuse them.
In the end, the techniques used to stabilize a UAV will be very much dependent on the intended use, budget, and how comfortable one might be working with sensor fusion, Kalman filters, etc.
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ModeratorPermalink Reply by Chris Anderson on -
Marc,
That was brilliantly concise and clear. I'll turn that into a FAQ that we can put on the home page.
Many thanks!
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Permalink Reply by falldeaf on
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Holy crap, that was incredibly informative. Thank you very much for taking the time to post that. Besides having a better understanding of gyro's and accelerometers, the terms you've used have been a very good spring-board for more google research. After doing more research and going back to my original post I've already seen some more inconsistencies in my understanding of flight mechanics... This is a fascinating subject, but a complicated one. Anyway, thank you again for your response, it's helped.
.f
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Permalink Reply by hypergolic on -
Yes I agree, great post. I am actually working on the sensor fusion part myself and it's proving to be quite a challenge.
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Permalink Reply by shai on -
Hello falldeaf,
I think you have a mistake.
A 3-axis accelerometers will show 0 G on every axis when stationary or at a constant speed.
In the event of a free fall parallel to the Z axis, the value on this axis will show 1 G while the X and Y axis will remain 0.
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Permalink Reply by falldeaf on
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shai,
You're addressing the wrong person. I did not make the assertion that an accelerometer would would output zero on the z-axis during a free fall. That was Marc Ramsey's post. I'm only just learning about accelerometers, but I would guess that it is Marc that is right, not you. As you said, its acceleration will be zero when stationary or at a constant speed. Acceleration is the rate of change in velocity, so assuming that an object has reached terminal velocity it's acceleration would be zero.
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Permalink Reply by Curt Olson on -
Marc is correct. If you set a z axis accelerometer on a table, it will read -9.8m/s^2 acceleration rate. If you turn it upside down on it's head, it will read +9.8m/s^2 acceleration.
If there is no acceleration when you are stationary on the surface of the earth, then standing on your head versus standing on your feet shouldn't really feel any different, right?
Only in a free fall without any drag, it will read 0 acceleration.
Once you reach terminal velocity, I believe it will be back to reading -9.8m/s^2 ... I think ... (?)
Maybe I should convince my brother to strap on a uav next time he goes sky diving so we can inject some real free fall data into this discussion. :-)
Curt.
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Permalink Reply by falldeaf on
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As usual, sparkfun.com to the rescue ;P
http://www.sparkfun.com/commerce/news.php?id=194
On the other hand, getting your hands on the data yourself would be more fun, so here's a good present idea for your brother.
http://www.sparkfun.com/commerce/product_info.php?products_id=8725
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Permalink Reply by Adam on -
Shai,
The MEMS-cantilever type accelerometers truly measure the acceleration AND gravity they are experiencing. Even when stationary, a three axis accelerometer would have non-zero readings. I have seen some single axis guys (drop-testing rigs) setup to subtract the known value of gravity so that the output of the accelerometer is proportional to the external acceleration ("rate change of velocity"), but this is technically not what the accelerometer measures.
I believe (I will be corrected if wrong, I hope) that "free-fall" is when a body is falling with "rate change of velocity" equal to the gravitational acceleration. In this case, the accelerometer would measure zero. Terminal velocity is reached when the drag force equals (and opposes) the force applied by gravity. The velocity is constant and the accelerometer would then only measure the gravity vector (a nonzero reading, for all intensive purposes, identical to that measured at rest on the earth's surface).
These misconceptions are very common, even within our community.
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ModeratorPermalink Reply by Chris Anderson on -
Sigh. This isn't a matter of speculation. Just try it and see. The force of gravity is an acceleration: 9.8m/second^2. So a stationary 3-axis accelerometer will show 1G on the z and 0 on the X&Y. In ideal free-fall it would show 0 on all three axis.
Kids do this experiment with Lego Mindstorms sensors in 4th grade, so I really don't know why adults are still confused by this.
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