7th Wheel
Robot agility was perceived to be an important attribute for the Lunacy competition. Agility is the key to defense and expected to be necessary (but not sufficient) for effective offense as well.
A novel 7th Wheel is provided to enable 2-mode driving to enhance agility.
In default 6wd mode, the 7th Wheel is raised above the playing surface and inactive. The robot drives and steers in normal tank mode. Pilot/Driver control is tank also, via left and right joysticks.
The Pilot/Driver can activate the 7th Wheel by depressing the trigger on either joystick. This lowers the 7th Wheel, unweighting the rear four main wheels in the process. If the robot is moving when the 7th Wheel is lowered, it acts as a brake (effective since the rover wheel transverse coefficient of friction is higher than the in-line). The three weighted, driven wheels provide an excellent pivot drive with little or no transverse wheel drag (excluding the trailer).
When the 7th Wheel is triggered, joystick control changes to arcade control, with the triggering joystick being the active one. Either Pilot/Driver joystick may be used to activate the 7th Wheel. Releasing the joystick trigger returns the robot the default 6wd mode.
Development
This idea grew out of: 1. Team 1565’s 2008 “Twitch” design. This was a bi-axial drive-train which changed orientation from x to y orientation using a pneumatic cylinder. Team 1640 prototyped and tested the “Twitch” drive-train in half-scale during the summer of 2008. 2. The finding that rover wheel friction coefficients are higher in the transverse direction than in-line.
Key design considerations were:
- Sufficient torque – the Globe is adequate in this regard
- Similar velocity to main drive
- Rigidity of the pivoting arm – The arm is exposed to considerable torsional stress when the 7th Wheel is engaged with the robot in motion. To hold up to this stress, the pivoting arm is a double-beam flanking the wheel on either side. Beam elements are (1) 2” angle and (1) 80/20 1020 (1” x 2”) profile. The wheel is supported by both beams.
- Actuation is provided by a 1½” diameter x ½” stroke pneumatic cylinder.
category:DEWBOT V
