Hi Dave, On Tue, Mar 4, 2008 at 3:41 PM, Dave Tweed wrote: > OK, I'm confused. Why are we having this conversation? > > On one hand, you seem to already have sources for the level of knowledge > you seek. It sounds like you could quickly put together such an app note > yourself. > With some research, I could put together such an app note. It's not so much for me to get this info, it is more to see a whole bunch of info that I've found in various places put together in one place for my reference and for other's education :) > On the other hand, you seem to be a bit confused about gravity and > acceleration. > > First of all, gravity does not have units of Newtons. It is indeed, an > acceleration field, with its magnitude measured in units of length per > time squared. You are right, I was being sloppy with units. > > Yes, any real accelerometer measures its own acceleration *relative* to > the local gravity field. As Einstein showed, there are no *absolute* > references. Gravity in one frame of reference is in no way distinguishable > from a constant linear acceleration in another, so I'm not sure what point > you're trying to make. Perhaps we're in violent agreement. > Except that a constant linear acceleration from almost any other source (e.g. a rocket engine) WOULD produce an output on the accelerometer but gravity would not. If you are out in space and being attracted by the gravity of a planet nearby, accelerating at 1 m/s^2, the accelerometer would read the same thing as it would if the planet were not there. If you are in empty space and fire a rocket motor and accelerate at 1 m/s^2, your accelerometer WOULD register 1 m/s^2. > When I set my accelerometer on a table, it reports that it is accelerating > at 9.8 m/s^2 upward. It is my interpretation, based on the lack of motion > in my larger external reference frame, that attributes this acceleration > to the gravity field and not to actual movement. If I lift it up, the > reading momentarily increases, and if I lower it, the reading decreases. > The accelerometer cannot by itself separate which part of the reading is > due to gravity and which is due to my movements. It's a fundamental system > startup issue -- actual movement is indistinguishable from sensor bias and > scale factor errors. An independent source of information is always needed. > I would put it this way: when it is sitting on the table, it is reading the normal force. When you pick it up in your hand, it is reading the force your hand is putting on it (holding it up). When you accelerate it upward, it will still read the force your hand is putting on it (which is now slightly higher). Sean > > > -- Dave Tweed > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist