[Sparks333]

Ooh boy. Inertial sensor topic. This ought to be good...

Alan Anderson's comment regarding accelerometers and tilt is dead on, with the aforementioned caveat that accelerometers are 'easily confused' - when an accelerometer attempting to measure attitude, it is actually measuring is the gravity vector. Since inertial sensors don't have a way to tell the difference between acceleration due to gravity and acceleration due to motion, once the robot starts moving, it's going to skew the acceleration vector off the gravity norm. They are very good for finding which way is down in a scenario with little motion - if you want to get clever, integrating gyroscopes (or, more properly, angular rate sensors) into the system with either a Kalman or an complimentary filter can give a fair approximation of which way is down in the presence of motion. But, it sounds like the bigger question is not which way is down, but what direction am I pointing. This is where the encoders coupled with gyros might help you out - again, with some form of sensor fusion, with the assumption that the gyros will drift if left to their own devices, and the difference in encoder ticks (left and right) won't be able to capture the motion of a turn at high speeds. Efoote868 is right in that with a very carefully tuned inertial loop relying on gyros alone can give you a fair approximation for the 15 seconds of autonomous, but assuming you want a heading lock to last longer than that, either Ether's heading reset button or locking onto a known field landmark (like a vision target) and resetting the heading angle is required. Traditional INSes knock out heading errors with magnetic field sensors (digital compasses), but there are too many stray mag fields in and around a robot to be trusted. One method I'd always wanted to try was to put two ultrasonic sensors on the side of a robot, as far apart as I could, and try to range to the wall to get an idea of angle, within reason.

Generally speaking, if you want to heading-lock your frisbee shooter (or measure the azimuth, if we're going to be proper about this), I'd use a vision system first and foremost, and a complimentary-filtered gyro-encoder scheme for when it went out of view. If you want to know the vertical angle of the shooter (inclination), unless your robot is really strange, I'd recommend just using a potentiometer and assuming the robot is level - it's simple, it's direct, and it doesn't drift. Dunno if that covers your question, but figured I've give my $0.02 and then some.

Sparks