DEWBOT X Drive Train
Issues with the 2013 pivot design
Our 2013 robot, DEWBOT IX, participated in more competitions, demos and driver practice than any robot before it (we said this last year, but it is true again this year). That said, the 2013 pivots performed extremely well, but the following issues were observed:
- One Pivot tube/Pivot Top weld failed in 2013 (in off-season competition), resulting in the total constructive loss of the pivot module
- One miter gear fell off its coaxial drive shaft (off-season). Repairable but not repaired.
- One Igus bushing came loose from the module top plate (off-season). Repaired.
- Occasional loss of calibration was experienced due to slippage of the angle sensors' fiberglass flex couplings' set screws.
- Driving straight during autonomous was not possible without at least two (catecorner) pivots equipped with tachometers - 2013 pivots were tachometer-ready, but only tachometer-equipped for development trials.
Developments on the 2013 pivot designGary Deaver led the sensor development while Patrick D led software development.
The following design changes were made:
- All pivot modules are tachometer-equipped
- All motor power wires terminate in 4-conductor 40-amp Anderson Connectors (no more disconnecting at the motor controllers)
- All sensor wires terminate at dual PWM connectors
- Ruland FRC12-4-4-A (FIRST Choice) clamp-type 6-beam couplings replace the old fiberglass set-screw couplings
- 2024 AL replaces 6061 AL for the pivot module bottom plate
- 3/8"-16 x 4" steel axle studs replace 2013's cut-down 4.5" axles
In lieu of encoders, which provide more data than needed, it was decided that we would develop tachometers (similar to those used for bicycles) to measure pivot drive speed and distance.
Six (6) neodymium magnets were secured equally spaced around the lower rim of the 56T coaxial drive pulley. A coil pickup is mounted on the pivot module top plate in a position directly below the arc of the magnets. All pivots are tachometer-equipped.
The angular distance between magnets corresponds to 0.785 in of robot travel.
Each Autumn, we conduct a value engineering exercise on our pivot drive design to understand what changes we would make if we were to select pivot drive again. The results of this exercise are summarized in the accompanying Table.
This year, between the Tachometer, Anderson Connectors and our conservative design approach, the pivot modules put on a little weight. Careful buying (and FIRST CHOICE) yielded some small dollar savings despite the utility improvements.
Pivot Module CAD Design and Bill of Materials
Calibration - no change from 2012
Chassis & Wheel-base
Chassis was once again welded 6061 Al. Mostly 1" x 2" x 0.130" U-Channel. Our 2011 chassis design served as the starting-off point for the design.
Wheelbase is 21" (nominally wide) x 22.25" (nominally long). Chassis exterior dimensions are 27.75" x 27.75". All these values are identical to 2013's robot
Driving straight in autonomous
We finally managed this trick in time for MAR Championship. A gyroscope provided the necessary input to a proportional loop which steered the front wheel pivots. A tremendous success.