Hi every body,I'm going to build a quad copter and right now I'm studying the control alternatives.I'm going to build the control system (micro controlled) but i have some doubts:- Is really a difference between a Gyroscope and a 3 axis accelerometer? Due to my ignorance I think that is enough with an accelerometer readings to stabilize and control the motors speeds to do the movement that the controller wants to do.- I thinking to include a magnetic compass, i have seen magnetic field sensing chips to implement the compass, with this sensor can I calculate as well the position respect the earth? or isn't precise enoughAt last but not least, I thinking to mount a little wireless access point (the fonera, that works with an embedded Linux easy to program) to read the telemetry data and send complex orders to the controller (in future stages of the project), what is the best / commonly used solution to link the UAV to a computer? What range provides it?Thank you very much,Regards,Chema
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The only way I could see the accels actually functioning is if we're reaching steady state a lot sooner than you think. Experimentally, giving a quad a 5 degree tilt seems to have it going at a pretty steady speed after the initial acceleration. I do realize how the forces involved work but perhaps since only a small portion of the thrust is going to lateral movement, and there's so much turbulence it can steady out quite low.
You're right. It's counter-intuitive (at least for me), but the accels do always cancel out. At least so long as you're maintaining altitude.
Odd that so many projects use accels and no one I've read about has noticed that it shows nothing. Makes me feel like we're still missing something, but I'll admit your logic does add up.
Note that, near hover, the accelerometer is basically useless for correcting drift in your gyros; this is because changes in angle produce lateral accelerations which cancel out the gravity vector in your x and y axes. Accels will help only when you have reached a steady-state velocity. Near hover, the accelerometer can only be used to measure thrust reliably.
A lot of people have been using accels to cancel drift in rotorcraft state estimation, but if you work through the math you will find that it doesn't work like they think it does. Essentially what they have is a gyro integrator that slowly pulls pitch and roll state estimates to zero (because near hover, the accels always think that pitch and roll are close to zero!) It happens to work well enough because, near hover, pitch and roll are typically not far from zero, and they are zero on average.
SO... in short, if you are just planning on operating near hover, you won't gain anything by using accelerometers.
Gyros give good short term readings but drift over time. Accels give useless short term readings, but can be filtered to give a reading that doesnt drift over time. The accels are used to correct for the gyros. Gyros only is an option, but only if you plan to be flying manually, and compensate for the sensor drift yourself.
Magnetometers can run into problems with all the current going to the motors. As they rev up and down you might see as much as a 15 or 20 degree shift in angle. There may be solutions to this...
Check out Xbee for wireless data. Their modules are pretty cheap and the better ones can go a mile or more.
Thanks for the help (and sorry for not reading the FAQ), and after reading your answer and the FAQ I also think that I must use the gyros...
I'll take a look to the TTL modems, but my idea of use the AP is because I can write a C program in the fonera that uses The serial port or even an I2C bus, and control from my PC with a web browser. I shouldn't deal with the TCP/IP directly...
What range can offer the TTL modems?
Thanks!
Regards!
Firstly you are going to need those gyros I think.
Secondly magnetometer is a great idea, they are actually surprisingly accurate I think you will find.
Lastly look into some TTL based serial modems.
Wifi = TCP/IP = difficult in comparison to analogue serial stream.
Replies
You can use a precise (+-2cm) Indoor "GPS": http://marvelmind.com/.
Here is a couple of demos:
Just a thought...
You're right. It's counter-intuitive (at least for me), but the accels do always cancel out. At least so long as you're maintaining altitude.
Odd that so many projects use accels and no one I've read about has noticed that it shows nothing. Makes me feel like we're still missing something, but I'll admit your logic does add up.
A lot of people have been using accels to cancel drift in rotorcraft state estimation, but if you work through the math you will find that it doesn't work like they think it does. Essentially what they have is a gyro integrator that slowly pulls pitch and roll state estimates to zero (because near hover, the accels always think that pitch and roll are close to zero!) It happens to work well enough because, near hover, pitch and roll are typically not far from zero, and they are zero on average.
SO... in short, if you are just planning on operating near hover, you won't gain anything by using accelerometers.
Magnetometers can run into problems with all the current going to the motors. As they rev up and down you might see as much as a 15 or 20 degree shift in angle. There may be solutions to this...
Check out Xbee for wireless data. Their modules are pretty cheap and the better ones can go a mile or more.
I'll take a look to the TTL modems, but my idea of use the AP is because I can write a C program in the fonera that uses The serial port or even an I2C bus, and control from my PC with a web browser. I shouldn't deal with the TCP/IP directly...
What range can offer the TTL modems?
Thanks!
Regards!
Secondly magnetometer is a great idea, they are actually surprisingly accurate I think you will find.
Lastly look into some TTL based serial modems.
Wifi = TCP/IP = difficult in comparison to analogue serial stream.