Problems with the stretcher?

Posted by Chris Orimoto at 1/20/2001 10:57 PM EST

Student on team #368, Kika Mana, from McKinley High School and Nasa Ames/Hawaiian Electric/Weinberg Foundation.

Is anyone else having problems pushing the
stretcher (with weight) on/over the bridge? I think
we are having some MAJOR traction problems
here. Can anyone provide help or compliance
here?

Posted by nick237 at 1/20/2001 11:08 PM EST

Engineer on team #237, sie h2o bots, from Watertown high school ct and sieman co.

In Reply to: Problems with the stretcher?
Posted by Chris Orimoto on 1/20/2001 10:57 PM EST:

Physics should tell you that its easier to pull up a hill rather than push, thats why truck drivers sit in the front and pull their loads, or is it so they can see?
Try to pull rather than push, you should have better traction and control. It should be the same with the goal too.
nick237

: Is anyone else having problems pushing the
: stretcher (with weight) on/over the bridge? I think
: we are having some MAJOR traction problems
: here. Can anyone provide help or compliance
: here?

Posted by Raul at 1/21/2001 12:09 PM EST

Engineer on team #111, Wildstang, from Rolling Meadows & Wheeling HS and Motorola.

In Reply to: Re: Problems with the stretcher?
Posted by nick237 on 1/20/2001 11:08 PM EST:

I am not convinced that the robot will get better traction pulling the stretcher or goals over the bridge. With the proper design, you can get as much or more consistent traction when pushing.

Some factors to consider:
There is a transition from horizontal to sloped. This means if you are not careful in your design, you could either greatly reduce the normal force on the wheels nearest the strecher or lift them off the ground altogther. So, if you are relying on your rear wheels for traction when pulling the strecher, you better use momentum to get you through that transition.

Control issues:
It is much easier to control what you are hauling if you pull it. When you pull it,it is directionally stable. In other words the payload just follows you. But it does require a wide turn to make sure it it lined up straight behind you.
When you push, you have to direct its direction, which does take more effort in controlling. However, if you design it to grab the stretcher so it cannot jack-knife on you then, it is much easier to line it up with the bridge in a tight radius and control its direction. This becomes even easier if you use crab (aka swerve, aka omni-directional) steering on the wheels farthest from your payload.

Raul

Posted by JVN at 1/21/2001 12:38 AM EST

Student on team #250, GE Dynamos - Capital District Robotics Team, from Shenendehowa High School and General Electric.

In Reply to: Problems with the stretcher?
Posted by Chris Orimoto on 1/20/2001 10:57 PM EST:

hey-

If you lift the goal(s)/stretcher you add their weight onto the weight of your own robot, thereby
increasing the normal force exerted and greatly increasing traction. This method isn’t very
practical if your trying to move the stretcher, but it works VERY well with the goals.

Other than that there are a couple of things we do to get better traction. Full track (tank treads)
seem to work pretty good. Also the drill-motors have decent gearboxes. If you are commited to
power/traction you might want to design custom gearboxes, just drop the ratios until you get the
amount of torque you want. ( i forget our ratios).

We haven’t been having trouble moving the stretcher/goals… maybe some of that stuff will
help, good luck

~John