Detecting Horizontal Movement

[Bruce]

As Darell already mentioned, the accelerometer is all you'll need. What you're looking to do is what accelerometers are made for....;o)

[JetPack Spartan]

There are a couple companies making autopilot devices for autonomous helicopters, which use a camera looking down to maintain position in much the same way a mouse does.

[Atom058]

Bruce - So, if an accelerometer is attached to a moving object and the object is moving at a constant speed, I will get some feedback that the object is moving? Even over a smooth surface?

JetPack - That sounds interesting (it is working much like an optical mouse), but I think it is getting out of my price range - otherise, I'd consider the GPS route, too...

Thanks Guys!

[JetPack Spartan]

When the accelerometer detects acceleration you can determine the speed by the amount of acceleration and the amount of time spent accelerating. Then, if there is no further acceleration or deceleration, you know that you are still moving at that speed. So you won't get feedback that you are in motion, the lack of feedback tells you that your speed hasn't changed.

[amgen]

an accelerometer has a base setting which is derived from "freefall, a no acceleration , no forces base point", so on earth, which is rotating, the reading is an offset from the base point and motion is an offset from that ofset ???

[Darrel Taylor]

Darrel - Thanks for your response, but don't accelerometers just detect changes in speed? I would think that once speed was constant, there would be no signal until the speed changed again (sped up or slowed down). Jeff

Bruce - So, if an accelerometer is attached to a moving object and the object is moving at a constant speed, I will get some feedback that the object is moving? Even over a smooth surface?

That's what the "Integration" does. It converts Acceleration over time into speed.

Sorry, it took me awhile to make something that would show how it works ... but here goes.

"Integration" is the "I" in PID loop.
By itself, it converts change in a value over time, to an approximated absolute value.
In other words, acceleration over time can show speed.
Speed over time can show distance.

I made a simulation that shows a simple acceleration to speed integration.
The PIC on the left simulates a single axis analog accelerometer, using HPWM to generate a DC voltage. Although they are ideal waveforms that you will never see from a real accelerometer, it was the easiest way to get exactly the same acceleration as deceleration profiles. Nature balances it out in real life, taking the exact amount of deceleration to stop an object that was accelerated.

The yellow trace is the accelerometer, and its output only registers the Change in speed.
The PIC on the right is running the very short "Integrater.pbp" program (attached).
The output (blue trace) of the integration is sent to the HPWM to be able to see on the scope.
The red trace is a digital output indicating "Forward Motion".

You can see that the output (blue) is a constant speed when there's no change in acceleration.
http://support.melabs.com/DT/Accel_Integrate.jpg



The integrated Speed could then be integrated again to produce distance traveled.

Disclaimers:
The simulation is not scaled to any particular Accelerometer, acceleration range or maximum speed range.
It is only intended to show how "Integration" works.

In real life, motion in other axis would affect the output, so multiple axis should be monitored.
Additional sensors (Gyros and Magnatometers) can be combined for greater accuracy and stability.

Products like the UM6-LT can provide all the sensor data required for all 3 axis, plus combined orientation information.

[Ioannis]

Darrel, either moving at a constant speed-constant direction or at complete stop, the result of the Integratio won't be the same?

Ioannis

[Acetronics2]

Hi,

The only problem of that method is small errors ( < 1 LSB measurements for simple explanation ... ) are not taken into account and final result is then false.

Try this into an elevator ( or an automated train ) and I promise you will stay a moment in it calling for help ... !!!

Alain

[Darrel Taylor]

Hi,

The only problem of that method is small errors ( < 1 LSB measurements for simple explanation ... ) are not taken into account and final result is then false.

Try this into an elevator ( or an automated train ) and I promise you will stay a moment in it calling for help ... !!!

Alain

Apparently, you didn't read the whole post.