Re: Position and velocity from accelerometer

On Nov 9, 9:29 am, Jerry Avins <""jya\"@ieee,org"> wrote:
Peter Nachtwey wrote:
"Jim Fee" <f...@xxxxxxxxxxxxxxxxxx> wrote in message
news:RYSdnXnD2Mlsxq7anZ2dnUVZ_oKhnZ2d@xxxxxxxxxxxxxxx
We have a "cybex" type machine, designed to apply velocity, acceleration
Where w and n represent noise but how do I use two transducers to correct
the noise in each other?

One accelerometer must be mounted at the end of the leg. The other must be
mounted at the knee and must rotate with the leg. This allows one to
subtract the forces or acceleration due to gravity. The encoder should help
take care of the offsets in the accelerometer or between the two
accelerometers.

How much information do you think subtracting one large number from
another and twice integrating the result will provide?
I don't see where
the encoder fits into that scheme. What does it do?
Provide position feed back, what else? Why does one need to integrate
twice when there is position feed back? Integrating once will do for
the velocity and that can be 'averaged' with the velocity calculated
from the position feed back. When the encoder detects the position
isn't changing then the accelerometer offsets can be zeroed out. The
OP is correct about multiple feed back devices providing a better
estimate of the current state. A steady state Kalman filter with
both position and acceleration feed back input will do.

We have made use of two accelerometers and a position feedback to do
motion control for those very challenging systems with a low natural
frequency and low damping factor. Higher order feed back can make a
big difference the ability to control these challenging systems.

I don't understand why noise should be a big problem. One should be
able to get very good acceleration values out of the A to D converter
after applying and offset and scale. In our case +/- 5g is mapped to
+/- 32767 A to D counts. That is plenty of resolution and a count or
two of noise will not hurt. Encoders shouldn't never be noisy. The
main problem with encoders is the quantizing.

Peter Nachtwey


.

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