Problems and Issues with Three Wheel Driving Mechanism

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shaiqbashir
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Problems and Issues with Three Wheel Driving Mechanism

Post by shaiqbashir » Wed Dec 27, 2006 9:01 am

Hi Guys!

Hope u would be fine out there!

Well! im planning to make a robot. The drive mechanism that i have choosen for that robot is a Tri-Cyclic mechanism. That is two Big Wheels at the back, both of them will be connected with one single motor. SO there will be one motor driving two back wheels. Just like in a car.

There will be a third wheel in the front portion of the robot that will be used for steering purposes. Now i want to ask you that what are the known issues and problems with this type of drive mechanism?

WHat motor should i use for steering? Should it be Servo or a Stepper?

Is the steering motor needs to be powerful enough??

What could be different problems i can face in this mechanism?

Please tell me as soon as possible!

Take carez!

Good BYe!

Aarnat
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Problems and Issues with Three Wheel Driving Mechanism

Post by Aarnat » Sat Nov 17, 2007 5:54 am

Hi shaiqbashir,
I know this post is a little old but, I will still reply. I am also sorry for the length of the response too.

You may encounter some problems with driving the rear wheels with one motor on a deferential. If you are making this from scratch you may have to do some research on how a differential works before making your own.

September 2007 Servo Magazine show a robot on the Vex platform with a rear drive axle like the one your trying to make, but with two wheels to steer in the front. The flaw in the design is hinted towards in the end of the article but is later revealed two months later in November’s edition. Basically there were two problems: 1) a solid rear axle which turned both wheels at the same rate (I'll talk more about this later) and 2) bad steering geometry.

If you have a solid drive rear axle to drive with you will be putting an equal force on the two wheels which turns them at the same rate. This is good for driving in a straight line, but bad for turning. The cool thing about the deferential is that it allows the two wheels to rotate at two different speeds while being powered. The reason why they have to turn different speeds is because the outer wheel has a larger distance/ radius to turn and the inner wheel has a smaller distance/ radius to turn. The outer wheel of the turn has to spin faster to keep up with the inner wheel; otherwise the robot skids/ hops/ makes a bunch of noise as it turns.

The steering geometry of the robot is important too. The front wheels need to be able to somehow turn at the same radius as the back two wheels. If you have ever witnessed a car turning in the snow or rain, you will see all four tracks temporarily shown of all four tires instead two tracks for the right and the left wheels. They all have different tracks because the front tries have to turn/angle themselves in order to allow the rear wheels to turn at their different rates, while being Y amount away from the rear wheels and X away from the midpoint of the two rear drive wheels.

You may have to do some research to make a deferential yourself or go hunting online for a pre-made axle.

Hopefully that will brig the problem into the light,
Aarnat

P.S.: Again sorry about the long response

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slamer
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What Does this drive train need to do?

Post by slamer » Sun Nov 18, 2007 9:05 am

The drive train of a robot determines how well it will perform. A single motor drive turning two rear wheels using a Diferencial (a must have) will work. But it's manuverability will be limited in many situations. Basically it will have a wide turn radius that can make turning in tight spaces difficult.
If you need to manuver in tight spaces it's like trying to parallel park a car.
A zero turn radius or skid steer drive train will give a lot more capability and better performance. I've built both types of drive trains for robot's with several variations of each. 2WD or 4WD Skid steer has proven to be a lot better solution. Want to build a good high performance Drive train. Think BobCat skid steer loader. You will like this a lot more.

Aarnat
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What des this drive train need to do?

Post by Aarnat » Sun Nov 18, 2007 5:50 pm

I will agree with slamer on this, a 2WD or 4WD is a much better drive train in most cases because it's much better manuverability. It does't take much to make a 2WD or a 4WD robot but if you want to challenge yourself with the mechanics of a differential, go for it. I have a VEX system myself and have built several robots on it. Almost all of them have been 4WD and I perfer 4WD. Once I tried 2WD on a 4 Wheeled robot and the robot had a horible turn radius, wich will be even worse with a differential because the two wheels will be turning the same direction instead of same and/or oposite directions. Also, is this robot going to be a remote controlled robot or autonomous robot? That will make a difference when programming.

Aarnat

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