DEWBOT VIII Drive-train Team Page
Student Lead: Molly
Head Mentor: Ben Kellom
Pivot drive Steering Motor selection
The BaneBots RS540 motors used in 2011 are not available for the 2012 robot. Alternatives are BaneBots RS395 and BaneBots RS550 motors. After analysis, BaneBots RS395 motors with 132:1 reduction were selected, primarily based on their lower contribution to robot mass.
Motor/Gearbox for Barrier crossing
Early prototype testing using DEWBOT VI demonstrated that raised, 8" diameter KitBot wheels are incapable of allowing a pivot drive chassis being driven over the mid-field barrier. An alternative is the use of driven, 8" diameter pneumatic wheels. What motors & gearboxes are needed to drive these wheels? Analysis indicates that FisherPrice motors and Gearbox's will do the job (one should alone, in fact, if the power transmission to the two wheels is managed).
FRC Team 1717's awesome pivots
Team 1640 is generally considered a competent practitioner of pivot (a.k.a swerve, unicorn,...) drive. I was, however, humbled when I saw team 1717's (D'Penguineers) pivot module at St. Louis. More so afterwards when I watched videos of how they move. If we use Pivot again, we've got a new performance target. My photos from Championships are below (original 4368 x 2912 pixel photos are available upon request).
While the 1717 pivot module design is mechanically impeccable (albeit at 9 lbm a bit heavier than ours), the videos make it crystal clear that their primary benefit comes from control and software, not mechanics.
From the photos, I deduce that 1717 has discrete left & right pivot modules. Top module plates (at least) appear to be machined left-right specific.
There appears to be seven (7) discrete vertical axles in the module (supporting two drive trains - wheel drive & steering). Four (4) axles are exclusively for the steering and two (2) are exclusively for drive; the last is for the pivot itself which coaxially incorporates both steering (outside) and drive (inside). There are two parallel horizontal axles in the pivot (wheel drive). Two vertical module axles contain coaxial elements (the gear shift and the pivot itself). Instrument take-offs are provided for CIM motor speed (not reduced) and pivot angle.
Steering reduction is done in three stages via spur gears. Angle is measured at the third stage (output) axle. This axle also mounts a synchronous pulley which drives the steering of the actual pivot (1:1).
Gear shift with independent pivot drive is very cool and probably useful under many conditions. By taking much of the drive reduction with gears, a smaller difference between drive & driven pulleys are needed.
1717's Awesome Pivots' History
Thanks be to Jake in response to Siri. (CD)
- 2009 - Coupled Drive, CIM+Belt & 545s
- This was the first year that we decided to use a swerve drivetrain. Our drivetrain was split down the middle of the robot. On the left side, both of the drive wheels were paired together and powered by 1 CIM motor through a single speed belt reduction that used timing pulleys. The left wheels were also paired together for turning. The turning was powered by a Banebots RS-545 though a Banebots planetary transmission.
- The right side of the robot was a mirror image of the left side.
- 2010 - Coupled Drive, CIM+Supershifter & 545s
- The drive for the wheels were again coupled together in pairs. Both of the left wheels were driven together and both of the right wheels were driven together. To power this drive, each pair of wheels was powered by two CIM motors through a two-speed transmission that was a re-packaged AM Supershifter gear-set with some custom modifications.
- For turning, the front left and back right wheels were paired together and the front right and back left wheels were paired together. This enabled both translational motion in any direction and rotation of the robot about its origin when it was not translating. Each turning pair was powered by a single fisher price motor through our custom gearbox with purchased gears.
- 2011 - Coupled Drive, CIM+Custom Shifters & 775s
- The wheel pairings were the same as in 2010, but the turning motors were Banebots RS-775 motors.
- Our drive transmissions were custom two-speed transmissions that were optimized for our swerve system. The turning transmissions were custom as well. Also, we cut all of our gears in house and they were made from steel.
- 2012 - Unicorn Drive, CIM+Custom Shifters & 550s
- In 2012, our drivetrain consisted of four independent wheel modules. Each wheel module’s drive was powered by one CIM motor with a two-speed custom gearbox. Our wheel modules' turning was powered by a single Banebot RS-550 with a custom transmission. All of our gears were made from aluminum and they were cut in house.
1717's dynamic driving
- 1717 utilities dynamic steering in their 2012 drive train. This is similar to the mode described in Crab with a Twist as Twist 2, shown schematically to the right and in this paper's Figure 3. Their driver controls are field-centric (rather than chassis-centric). We (Clem and Siri so far) and they believe that this can make driving control much more intuitive. (To prove their point, they and won their first event--not to mention their second--with an all-rookie team and less than 10 hours of driving practice.) Like us, they also allow alternative modes for each game, including a what is essentially a very useful basketball pivot foot this year.
- 2012 Gyro
- The 2012 gyro is a SparkFun #9094, but they're unsatisfied with performance: major asymmetry and drift. This year, they simply reset when square against the fender (rather than using a magnetometer, etc to account for drift).
- Code (2011ish)
- Though they haven't published their 2012 code (we certainly can't blame them!), they have a public Sourceforge page with custom DS, and their 2011 pre-season code is posted as well (C++).