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
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When I said: "astronauts in the space shuttle" I didn't mean 100 miles above earth.
I meant thousands of miles away from earth gravitational field.
Than and only than you are truly weightless.
To sum it up,
When people say they "feal weightless" they are actually accelerating.
An acceleration sensor calibrated for measuring "true acceleration" (non inertial frame) will show 0 m/s^2 when stationar and 9.8 m/s^2 when starting to free-fall.
If a sensor shows 9.8 m/s^2 when stationar and 0 m/s^2 when free-falling, than it's calibrated for an inertial frame and won't measure true acceleration.
Sparkfun.com has the 6 degrees of freedom accelerometer/gyro combo. Mix this with the propeller chip and some khalman filtering and you will have complete atitude information for your cameras. We were looking to do the same. We did the research but never implemented it. The 6DOF gave us (only if you have altitude data of the terrain) the point on the ground we were looking at. and the footprint of the camera field of view. If you are using a pan and tilt you will need to have the position data of that. Also, altitude will have to be barometric to be accurate enough. GPS altitude is to jumpy and can my off over 100 meters at times.
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.
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.
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.
Replies
When I said: "astronauts in the space shuttle" I didn't mean 100 miles above earth.
I meant thousands of miles away from earth gravitational field.
Than and only than you are truly weightless.
To sum it up,
When people say they "feal weightless" they are actually accelerating.
An acceleration sensor calibrated for measuring "true acceleration" (non inertial frame) will show 0 m/s^2 when stationar and 9.8 m/s^2 when starting to free-fall.
If a sensor shows 9.8 m/s^2 when stationar and 0 m/s^2 when free-falling, than it's calibrated for an inertial frame and won't measure true acceleration.
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.
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.
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.