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right turn

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right turn

Postby baffled7 » 24 Jul 2012, 19:38

Hi I am new to the NXT and I'm having a bit of a problem getting my robot to do an accurate right turn. The NXT 2.0 programme has a selection of pre- written programmes on a menu where it says for a right turn the standard robot (the one suggested in the education packs) needs to turn 170 degrees, which gives me a quite accurate right turn for my robot. However, when I have done the measurements and the maths behind the right turn, where the robots wheels are moving in opposite directions and pivoting around the centre point between the wheels giving a circle diameter of 110mm and wheel circumference of 175.9mm, I worked out it would need 0.49 rotations/ 176.4 degrees. This gives me a a right turn which overshoots.

Any suggestions as to how they have arrived at 170 degrees? Is it just a case of error in the motors and a difference in rounding or something more substantial than this?
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Re: right turn

Postby hassenplug » 25 Jul 2012, 13:35

baffled7 wrote:Hi I am new to the NXT and I'm having a bit of a problem getting my robot to do an accurate right turn. The NXT 2.0 programme has a selection of pre- written programmes on a menu where it says for a right turn the standard robot (the one suggested in the education packs) needs to turn 170 degrees, which gives me a quite accurate right turn for my robot. However, when I have done the measurements and the maths behind the right turn, where the robots wheels are moving in opposite directions and pivoting around the centre point between the wheels giving a circle diameter of 110mm and wheel circumference of 175.9mm, I worked out it would need 0.49 rotations/ 176.4 degrees. This gives me a a right turn which overshoots.

Any suggestions as to how they have arrived at 170 degrees? Is it just a case of error in the motors and a difference in rounding or something more substantial than this?


Sounds like you've done a very good job of trying to work out the math.

Given the robot works exactly as claimed, I'd say the most suspect item is the measurements you've made. Those are (A) Wheel circumference and (B) distance between wheels, right?

It travels farther than you expect, meaning (A) the wheels are BIGGER than you think, or (B) they're closer together than you think.

(A) For the wheels to be bigger than measured, you may need to compress them as you measure them, more than they are normally compressed while in use.

(B) Measuring the distance between wheels could be harder to do, because most wheels have a "contact patch" where they touch the ground, and that patch is not a line, it's more of a rectangle. Meaning when the robot turns, the outside of the wheel covers more distance than the inside of the wheel. So, calculating the diameter of the circle it not so exact.

I would suggest you run the calculations backwards, and figure out what the diameter SHOULD be for a 170 deg turn. Then, see if that makes sense on that robot.

Math. Fun stuff.

Steve
- in Theory there is no difference between Theory and Practice. In Practice, there is.
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Re: right turn

Postby jwiger » 28 Jul 2012, 15:29

Did you measure the width at the outer surfaces of the tires or in the center line of each tire?
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