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[on:]

Folks,

I have a new project idea and want to do some ground work first.

I have seen this:
http://www.toysdownunder.com/arduino/acellerometer-lis302dl.html
But what is the difference between one of these and a "Gyro"?

What I am wanting to do - prototyping at this stage - is get a 3 axis (or more?) accellerometer and have a 3 axis detector.  On the "top" of the gymble thingy, there will be an arrow.

When working, if I tilt the unit, it will keep the "top" level.
If I roll it, it will keep the "top" level.
If I twist/yaw it, the arrow will keep pointing in the same direction.

This should be possible, but is how I explained it correct and are those devices the things I will need?

The other question if the motors.  Stepper or Servo.   

I have read the differnce, but for the sake of simplicity at this stage.....

Thanks.

[Reply #1 on:]

If you're building a gimbal and just need to know the tilt of the .. whatever, then you're looking for an accelerometer. It will always tell you what direction is "down" (gravity) but will get confused for short periods of time if it's being shaken a lot.

A gyroscope tells you the rate of rotation of an object. If your object was spinning you could compensate for that spin, but lacking an external point of reference it would only take a few seconds for your object to have no idea what direction was "up" anymore. In that respect gyros are typically combined with accelerometers for a stable platform that are more immune to shaking.

A stepper is more precise than a servo (assuming you mean an RC servo) and is capable of continuous rotation but requires an external driver for the high amounts of current used to energize its coils. A servo typically only allows 180 degrees of rotation, is less expensive, and requires no external driver as it is self-contained in the servo.

One comment on that LIS302DL; that's an older technology accelerometer. It's not very accurate (8-bit resolution) and its 3V signaling requirements mean you'll need a "level converter" so an Arduino running at 5V will not blow it up when it's trying to talk to it.

[Reply #2 on:]

Folks,

I have a new project idea and want to do some ground work first.

I have seen this:
http://www.toysdownunder.com/arduino/acellerometer-lis302dl.html
But what is the difference between one of these and a "Gyro"?

Accelerometers measure accelerations. Gyros measure changes in orientation.

There's a constant 1g acceleration for objects on the surface of the Earth so an accelerometer can usually tell you which way is 'down' - good for measuring the roll/pitch of a boat.

What an accelerometer can't measure is rotations around the direction of acceleration. e.g. you can't use it to measure changes in a boat's heading (because 'down' doesn't change when the boat turns left/right). For that you need a gyro.

Some chips have both types of sensor in them.

[Reply #3 on:]

you can't use it to measure changes in a boat's heading (because 'down' doesn't change when the boat turns left/right).

Are you sure about that?

[Reply #4 on:]

you can't use it to measure changes in a boat's heading (because 'down' doesn't change when the boat turns left/right).

Are you sure about that?

Yep.

I've got one on the table in front of me...

[Reply #5 on:]

You've got a boat on a table?

[Reply #6 on:]

Yep.

When you rotate an object at a constant angular speed, which of the following has to be true:

1) there is force on the object (thus it can be sensed by an accelerometer);
2) there is no force on the object;
3) all of the above;
4) none of the above.

[Reply #7 on:]

Yep.

When you rotate an object at a constant angular speed, which of the following has to be true:

1) there is force on the object (thus it can be sensed by an accelerometer);
2) there is no force on the object;
3) all of the above;
4) none of the above.

I'll pick A.



Thanks folks for the clarification.

[Reply #8 on:]

I'll pick A.

You would be correct.

[Reply #9 on:]

(Blush)

Phew, good to know some parts of the brain still work.

[Reply #10 on:]

1) there is force on the object (thus it can be sensed by an accelerometer);

If you're using an accelerometer to sense rotation, you're assuming:
1) The accelerometer is not at the center of axis of rotation.
2) The distance from the center of rotation to the accelerometer is known.
3) A second accelerometer is being used so tilt can be differentiated from spin.
4) All of the above.

But to answer your question, if an object is rotating at a constant angular velocity then there is no force being applied to it. Acceleration is a change in velocity.

[Reply #11 on:]

I'll pick A.

You would be correct.

Ummm....Newtons first law disagrees with you.

[Reply #12 on:]

I'll pick A.

Thanks folks for the clarification.

You're going to be be disappointed.

Don't blame me though, I only own one of the things and have seen it with my own eyes.

[Reply #13 on:]

if an object is rotating at a constant angular velocity then there is no force being applied to it.

Then what's keeping the earth from falling into the sun?

Acceleration is a change in velocity

Q1: You are going east at 1km/h, and then  you go west at 1km/h. Is there a change in velocity?

Q2: what does that mean in the context of the boat?

[Reply #14 on:]

have seen it with my own eyes.

Millions of people have seen a flat earth.

If we all believed in our own eyes, we would still be in the dark age.