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Using Arduino => General Electronics => Topic started by: lost_and_confused on Oct 23, 2012, 05:07 am

Title: Accelerometer vs gyros
Post by: lost_and_confused on Oct 23, 2012, 05:07 am
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.
Title: Re: Accelerometer vs gyros
Post by: Chagrin on Oct 23, 2012, 10:15 am
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.
Title: Re: Accelerometer vs gyros
Post by: fungus on Oct 23, 2012, 12:32 pm

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.

Title: Re: Accelerometer vs gyros
Post by: dhenry on Oct 23, 2012, 08:27 pm
Quote
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?
Title: Re: Accelerometer vs gyros
Post by: fungus on Oct 23, 2012, 08:51 pm

Quote
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...
Title: Re: Accelerometer vs gyros
Post by: AWOL on Oct 23, 2012, 08:58 pm
You've got a boat on a table?
Title: Re: Accelerometer vs gyros
Post by: dhenry on Oct 23, 2012, 09:58 pm
Quote
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.
Title: Re: Accelerometer vs gyros
Post by: lost_and_confused on Oct 23, 2012, 10:19 pm

Quote
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.

Title: Re: Accelerometer vs gyros
Post by: dhenry on Oct 23, 2012, 10:35 pm
Quote
I'll pick A.


You would be correct.
Title: Re: Accelerometer vs gyros
Post by: lost_and_confused on Oct 23, 2012, 10:47 pm
(Blush)

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

Title: Re: Accelerometer vs gyros
Post by: Chagrin on Oct 24, 2012, 03:38 am

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.
Title: Re: Accelerometer vs gyros
Post by: fungus on Oct 24, 2012, 11:34 am

Quote
I'll pick A.


You would be correct.



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

Title: Re: Accelerometer vs gyros
Post by: fungus on Oct 24, 2012, 11:37 am

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.

Title: Re: Accelerometer vs gyros
Post by: dhenry on Oct 24, 2012, 12:51 pm
Quote
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?

Quote
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?
Title: Re: Accelerometer vs gyros
Post by: dhenry on Oct 24, 2012, 12:52 pm
Quote
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.
Title: Re: Accelerometer vs gyros
Post by: Groove on Oct 24, 2012, 12:54 pm
Quote
Then what's keeping the earth from falling into the sun?

A big stack of turtles.
Title: Re: Accelerometer vs gyros
Post by: fungus on Oct 24, 2012, 01:01 pm

Quote
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?


I'm not sure you know what rotation is.

According to you, these are completely unnecessary: http://www.ebay.com/sch/i.html?_nkw=mpu6050

Yet they exist... how do you explain that?
Title: Re: Accelerometer vs gyros
Post by: dhenry on Oct 24, 2012, 01:15 pm
Quote
According to you, these are completely unnecessary


You are mistaken.
Title: Re: Accelerometer vs gyros
Post by: fungus on Oct 24, 2012, 02:28 pm

Quote
According to you, these are completely unnecessary


You are mistaken.



...says the person who doesn't understand Newton's First Law Of Motion.
Title: Re: Accelerometer vs gyros
Post by: codlink on Oct 24, 2012, 03:07 pm

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


Yes.  The object has to stop to go in the opposite direction.

Dhenry, you are making yourself look worse and worse with every post you make.  If you have any dignity left, before your next post, do some research on the subject.
Title: Re: Accelerometer vs gyros
Post by: Groove on Oct 24, 2012, 03:21 pm
Quote
Q1: You are going east at 1km/h, and then  you go west at 1km/h. Is there a change in velocity?
Is this one of those "what colour was the bear?" type questions?
Of course there is a change of velocity, because velocity is a vector quantity, so specifies a rate of displacement and direction.

What was it the OP was asking about?
Title: Re: Accelerometer vs gyros
Post by: dhenry on Oct 24, 2012, 06:57 pm
To save you some time, the radial acceleration for an object orbiting at speed v and radius r is v^2 / r.

I guess this thread shows why we need better teachers.
Title: Re: Accelerometer vs gyros
Post by: fungus on Oct 24, 2012, 07:20 pm

To save you some time, the radial acceleration for an object orbiting at speed v and radius r is v^2 / r.

I guess this thread shows why we need better teachers.



What does that have to do with measuring yaw using an accelerometer?

Title: Re: Accelerometer vs gyros
Post by: Coding Badly on Oct 25, 2012, 03:43 am

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

A big stack of turtles.


I thought it was one big turtle and four elephants?
Title: Re: Accelerometer vs gyros
Post by: codlink on Oct 25, 2012, 03:44 am

What does that have to do with measuring yaw using an accelerometer?


Copy and paste featuring Google makes everyone sound smart.