pic: steering methods

[JesseK]

For the last two drawings (linkage drives), I believe there is a more practical way to link them when considering an FRC drive train. In order to keep the wheel base at its maximum, regardless of which way the wheel modules are oriented, the motor must always stay on the inside and the wheel on the outside perimeter of the bot. The easiest way to achieve this is to link diagonal wheel modules together rather than the modules on a side.

Great drawings though!

[DonRotolo]

Nice drawings. We are investigating if car-type steering will have any advantages with our chassis this year.

Originally Posted by GeoffP

left/right (tank) control will not work well this year since friction is so low. remember that sliding (dynamic) friction is roughly half of not-sliding / rolling (static) friction.

Several threads (and my own tests) have shown that the 2:1 ratio of transverse to lateral friction is a myth, reality shows it to be about 6:5.

[GeoffP]

Originally Posted by Don Rotolo

Nice drawings. We are investigating if car-type steering will have any advantages with our chassis this year.

Several threads (and my own tests) have shown that the 2:1 ratio of transverse to lateral friction is a myth, reality shows it to be about 6:5.

the 2:1 ration i am referring to is static vs dynamic friction coefficients. a simple test would be to fix 3 wheels leaning towards each other like a tripod. then incline the field material and measure the angle they start sliding at. while they are sliding, decrease the angle until they stop and measure that angle. the ratio of the sine of the angles should approximately match the ratio of the coefficient of frictions static vs dynamic. it will also be your increase in acceleration if you can prevent your wheels from sliding either laterally or transversely.
static friction is intermolecular bonding between 2 stationary surfaces (even if one is rolling). dynamic friction is trying make those bonds while the surfaces are sliding plus includes momentum. they should be very different.
that being said, the ratio is much closer with some materials and especially at low friction so your testing may be correct. thanks for posting that.

[GeoffP]

if skid steering is used (2 left wheels in-line and driven 1 speed, 2 right wheels in-line and driven another speed), and the side robot is up against the wall where wheels on one side are on carpet and the wheels on the other are on the slippery field, then will the robot be able to away from the wall?another method would be flywheel steering where the one side is pushed up or down to turn the bot with the forces from the torque produced from trying to tilt the axis. that may not get you away from the wall either though.
i think articulated steering is necessary.

[gblake]

Originally Posted by GeoffP

it will also be your increase in acceleration if you can prevent your wheels from sliding either laterally or transversely.
static friction is intermolecular bonding between 2 stationary surfaces (even if one is rolling). dynamic friction is trying make those bonds while the surfaces are sliding plus includes momentum. they should be very different.
that being said, the ratio is much closer with some materials and especially at low friction so your testing may be correct. thanks for posting that.

All of this is nice for a machine that is rolling in a straight line (no changes in the orientation of its velocity vector, just changes in the magnitude); but since most of these discussions center around the notion of using "traction control" during turns...

Just as soon as one of these slippery wheels is rotated (about a vertical axis) to try to turn a bot, I assert that the wheel has to begin slipping sideways; and further I assert that it will continue to slip sideways until the dynamic (NOT static) friction forces between the wheel and the floor wipe out the bot's forward velocity.

If I am right, then no amount of traction control attempts made after a wheel is turned will put the wheels back into the static friction domain until the bot's forward (its pre-turn direction) motion is gone. Once that forward motion is gone and the wheels are no longer skidding sideways, traction control can then help move the bot in its new direction.

Alternatively, traction control could be used to slow a bot before a turn is attempted. Doing that would minimize the time spent slipping sideways after a turn is begun, but it wouldn't do much to help you run away from a pursuing bot if you are trying to do some evasive maneuvering.

Am I overlooking something? If traction control isn't very valuable, I would hate to see teams invest more into it than is appropriate.

Blake

[GeoffP]

Great analysis Blake. I agree with the scenario you describe and your bleak outlook on the difficulties we will have with turning. Your arguement also suggests that skid-steering would be even more futile(?). at least articulated steering gets the thrust vectors in the right direction. simple traction control such as just limiting the accel and decel of the motors may be helpful in a straight or in the middle of a turn. if we try to stay just under the static friction force then the bot may sporadically "grab" and respond better to our drive commands. i forsee a lot of jacknife steering though with 5 other bots and 120 balls on the floor.