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I was given a task to put a accelerometer inside this block diagram. I've done tons of research but still couldn't find any way. Ignore those chinese words. The left and the right is the motor of the car. Straight profile gen meaning they are running in a linear acceleration. Alignspeed is the infared sensor.

PD loop and gyroscope

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    \$\begingroup\$ You don't use an acceleromter as a speedometer (integrating acceleration collects too much error to give speed), at most you use it to improve the interpretation of other sensors. \$\endgroup\$ – Chris Stratton Jul 26 '13 at 2:59
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    \$\begingroup\$ @ChrisStratton, that depends how you are using the accelerometer. If you put it on the wheel, you can use it to count revolutions of the tire, then use the radial velocity with known the radius of the wheel to get a reasonably accurate estimation of speed. \$\endgroup\$ – travisbartley Jul 26 '13 at 3:11
  • \$\begingroup\$ @ChrisStratton Thanks for the opinion guys. That's great information. \$\endgroup\$ – user26782 Jul 26 '13 at 6:41
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You can't directly use an accelerometer as a speedometer. You have to integrate the acceleration over time to get the change of velocity; if you accelerate from a known velocity (eg at rest - no velocity) at a known acceleration for a known period of time, you can calculate the velocity you will be travelling at.

If you don't have a known starting velocity, you can only determine how the velocity has changed since you started to measure the acceleration.

It's not particularly accurate and prone to drift over time. Even precision accelerometer systems (such as those used for inertial navigation systems) will drift over time and need to be re-referenced either by starting again from a know state (eg still and facing a known direction) or supplemented by a different measurement system (eg GPS).

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