question about Memsic 2125 Dual-axis Accelerometer and force reading

James Armstrong

I want to use the 2125 to detect when a wheelchair bumps into something. From playing with it the X and Y outputs always show the tilt. The demo will indicate a constant gforce if I hold it at an angle. What is the best way to determine what I would call instantaneous gforce or a bump. Would you take the force reading and subtract it from the previous reading to get a difference that would indicate the direction it was bumped from and how hard?

- James

David B

Yes, that sounds right. You're looking for the change of force, so you'd get it by detecting the change in the measurement. The technical term for that is "jerk" (seriously!) You might want to get a little sophisticated and keep a running average of the force value, so you can distinguish a real jerk from a noise spike.

See http://math.ucr.edu/home/baez/physics/General/jerk.html for a little discussion on force, acceleration, jerk, etc.

David

Beau Schwabe

James Armstrong,

You said...
Would you take the force reading and subtract it from the previous reading to get a difference that would indicate the direction it was bumped from and how hard?

This is essentially how the "normalizing" routine works. Basically you move the accelerometer to a position that you agree to be normal or level. The returned readings
are anything that deviates from the normalized setting.


See:
http://forums.parallax.com/showthread.php?p=600561
http://forums.parallax.com/showthread.php?p=605325

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe

IC Layout Engineer
Parallax, Inc.