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Unread 20-06-2002, 15:50
Chris Hibner's Avatar Unsung FIRST Hero
Chris Hibner Chris Hibner is offline
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Drive train suggestions

As stated earlier, drawing too much current while driving is a result of too much friction in the drive train while turning. At first, it seems like a pretty tough problem to solve since you want as much friction as possible to push things with, yet you want much less friction for turning.

While the problem seems tough at first, the solution is fairly simple. A little static moment analysis of your drive system should make things very clear.

There are three basic methods of keeping a lot of friction for pushing a having much less for turning. Two methods I like (we use them every year on our robot), and the other method I don't care for. Here they are:

1. Use two drive wheels in the front or rear and use caster wheels on the other end. This is very simple, but I don't really like this method since not all of your robot weight is on driven wheels which will ultimately mean you will have less pushing force than you could potentially have.

2. For the parts of your drive train that contact the floor, design them such that they have much more friction in one direction than they have in the perpendicular direction. That is, make your wheels / treads grip in one direction and slip in the perpendicular direction. There are two good examples of this: snow skis and ice skates. Both slide very easily back-and-forth, but are very sharp and grip well side-to-side. You can design your wheels / treads this same way - we've been doing is for years with great results.

Another method to achieve this (but a lot more complicated) is to use the holonomic wheels that a lot of teams have been using. This way you have low turning friction like casters yet all of your wheels are driven.

3. The last method is to do some static moment analysis on your drive train. In short, you will find that if your wheel base is longer than your track width, you won't turn very well (if you will turn at all). You should always have a shorter wheel base than your track width - especially if you don't do #2 above.

Our team always does a combination of #2 and #3 above. We've always had great drive trains that have great traction and are easy to steer and control.

Two things to remember:

a) If you do a static moment analysis of a 4-wheel drive tank steer system, you'll see that there are two ratios affecting how easily your robot turns. They are: 1. the ratio of the track width to wheel base (higher ratio => easier turning); and 2. the ratio of the longitudinal friction coefficient to lateral friction coefficient (higher ratio => easier turning).

b) Some turning friction is GOOD!!!! If you eliminate all of your turning friction, the drive train becomes very difficult to control. This is because when you let go of the stick(s), the robot continues to turn until friction stops it. For the robot that is easiest to control, you ideally want the robot to stop turning as soon as you release the sticks. Therefore, you should never have a goal to eliminate all of the friction - the goal should be a good balance between keeping the motor current low and being able to stop the robot from turning once the sticks are released.

I hope this helps,

Chris
 


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