Why is my encoder causing my pendulum angle to drift?

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The discussion focuses on an issue with a rotary encoder causing angle drift in a pneumatic inverted pendulum system. Despite using an Arduino to read the encoder's quadrature output, the pendulum's angle drifts several degrees after a few seconds of operation, even when manually shaken. The encoder model used is the S1-1250 by US Digital, and the user has tested different Arduino models and encoders without resolving the issue. Recommendations include using an external counter to monitor interrupts and simplifying the program further, but the drift persists regardless of these adjustments. The problem may stem from missing interrupts or asymmetric processing times, as the drift tends to occur in a consistent direction.
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So I am reading a rotary encoder through an an arduino using interrupt pins for the quadrature output. I'm not an expert, but the system seems to work fine, i.e. gives correct output readings. It is for a pneumatic inverted pendulum, which can jitter, and change the cart velocity very suddenly. I have noticed that after running for several seconds, the angle drifts. When I realign the pendulum with gravity, the angle has drifted several degrees. Even shaking it with my hand (pneumatics turned off) causes the angle to drift.

Encoder is a S1-1250 by US Digital. 1250 counts per revolution, with standard, ground, power, ChA, and ChB outputs. (Unused index channel). Have tried an arduino mega as well as duemilanove, and two separate encoders (same model number). Someone recommended getting an "encoder counter" because the pulses are probably too high for the arduino.

However, a previous student did the exact same project without problems. The only differences are that my rail friction is much lower (3X lower), and my pendulum is a carbon fiber rod with a weight on the end, while his was just an aluminum shaft.
See http://dasl.mem.drexel.edu/~jiyueHe/pneumatic_inverted_pendulum/ for the tutorial I'm following

Thanks,
-Josh
 
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I would place a small bet on missing interrupts. If the drift is always in the same direction you might have asymmetric processing times on one input.

You could try an external counter to compare the number of interrupts you get, or simplify your program somehow to keep an accurate count.
 
Drift is in both directions, but usually CW. The program is simplified to the point where only the encoder related code is present. I also directly downloaded a simple encoder program from the arduino website and the problem still remains. I also eliminated serial output and used an LED to indicate where the supposed 0 point is. It always still drifts when I shake it about.
 

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