What is crab steering??

[Jeff Rodriguez]

ANother question:

How did you guys set up the controls for it, i know team 48 used the two joysticks as explained before, but how did other teams do it
So far the teams that i know have had crab steering are:
team 48 in 2002
team 111 in 2002
team 226 in 2002
and i remember 47 having it in 2000

[Matt Reiland]

Our Control System:

The CODE is posted here and on the CODEX

We tried to keep operation as simple as possible for the operator, since we knew it was going to be hard to control because of its speed (right around 12ft/s) so we stayed with only one joystick.

In normal mode if the operator pressed no buttons it locked all four wheels straight ahead and steered in a tank setup. The speed defaulted to somewhere around 50% of maximum.

The top button activated our 4 wheel steer, such that when it is held down the robot will keep its orientation but the drive will follow the joystick in any direction. Again another plug for LeoM, he came up with a really sweet algorythm for the steering angle in one line that works flawlessly since PBasic couldn't do it.

The trigger then kicked everything into full speed, we usually came off the line in turbo 4 wheel steer to try and hit the center goal no matter where we were lined up at.

As a last helper for the driver, our operator interface board switched the orientation from front to back with a switch so that made figuring out the inputs when the robot is reversed no longer necessary.

Now that we have tried it somewhat successfully I would probably go to the mechanical setup more like Team 111 with independent front rear steering to give you more options. I doubt we will have to push like last year so the high speed setup might work this year. Last year we didn't have the right ratio or traction to rotate a goal about the center of the robot, so we had to rotate around the goal then take off with it. A system like Team 111 where the front/rear oppose each other in direction would probably have worked better since we scrub off so much power to use tank steering.

I think the hardest part of using the single joystick was the inverse trig functions in pbasic and now those are available to all teams.

[Paul Copioli]

Team 217 used a steering wheel and a joystick for throttle. We tried the 1 joystick method and even used LeoM's algorithm (thanks LeoM and Matt R.), but our drivers just had information overload. They had to shift (many ratios, not just one gear), switch between crab and monster truck steer all with one joystick. It was just too much. Our students love using the steering wheel, so we had 2 modes: Crab, and monster truck. In crab mode, the steering wheel turned all 4 wheels in the same direction from -90 to +90 degrees (0 is straight ahead). In monster truck mode (4 wheel steering), we turned the front wheels 3x as much as the rear wheels in opposite directions. Buses also use this steering. We could not use skid steer, because we had too much traction on all 4 wheels (we placed a priority on traction). The joystick was just used for throttle and switching gears. We had 2 gearboxes and 2 steering motors. The front wheels were combined in steering and shared the same gearbox. The rear wheels were combined in steering and shared the same gearbox.

We have been using the steering wheel for the last 2 years and love it. The steering wheel works great for skid steer too.

-Paul

[Al Skierkiewicz]

One way to think of the response is that the center of motion for each drive wheel is the steering axis of the drive module not the steering center of the robot. With that said, the limiting factors on response are the momentum of the robot (plus weight of the robot and whatever it is attached to) and the friction with the floor, the speed of the steering motors and of course the reaction time of the driver. Since drivers are about the same average age and practice, we can assume the reaction time is under a second and far less than the other factors. Since your scenario generally does not take place at sustained maximum speed, the other factors are dominated by the speed of the steering mechanism and that is very fast in the robots I have seen. Low center of gravity is essential, of course, as these moves would generally lift the drive wheels off the floor or tip the robot over. The Michigan Tech Team had a unique method for crab steering involving three drive wheels (the wheels were the special design to grab in one direction and roll sideways) and they achieved steering by applying different power to each motor. It was very impressive to watch and a very simple design.