Position and velocity from accelerometer

Started by Jim Fee November 8, 2007
Jerry Avins <""jya\"@ieee,org"> writes:
> [...] > The way incremental encoders are connected by the unwary, the "CW" and > "CCW" pulses don't occur at the same shaft angle, but are offset by > half a quantized angle increment.
Are you referring to what used to be called "optical shaft encoders?" The one I used some 27 years ago had two signals, A and B. Both are square waves when the shaft is rotated at a constant angular velocity. The phase of B leads A by 90 degrees in one direction and lags A by 90 degrees in the other direction. My up/dn detector was a D flip-flop with the clock being the A signal and the B feeding the D input.
> For reciprocating motion, that can > lead to a slow drift.
I think I see this - if the shaft is in just the wrong position, you'll get a positive edge on A for rotation of X radians and a negative edge for a rotation of -X radians. If you're using positive-edge triggering based on the A signal, you'll miss the -X rotations. (X is less than one angular quantization step).
> For an unfortunate small-amplitude oscillation, it can lead to a > disastrous false indication of continuous motion.
You mean if the A signal was glitching? Yeah, I see that.
> My circuit (and a more complicated state machine) give CW and CCW > pulses at the same shaft position. The difference is the direction > from which that position is approached. That solves most of the > annoying difficulties that seem small enough to let slide until they > become large ones when the brass show up for a demo.
I couldn't decode your state table in the adjacent post. For one thing, what's the difference between "CW" and "cw"? -- % Randy Yates % "She tells me that she likes me very much, %% Fuquay-Varina, NC % but when I try to touch, she makes it %%% 919-577-9882 % all too clear." %%%% <> % 'Yours Truly, 2095', *Time*, ELO