Provisional Dewbot V Chassis
Based on what the team learned during prototype testing in summer of 2008, and subsequent drive-train analysis, a Provisional 6wd drive-train and chassis was designed during the Fall with the expectation of using this for DEWBOT V. A great deal of thought and analysis went into this design (as the urgencies of build season were absent). In the end, this design was not built owing to the unique attributes of FRC's Lunacy competition. The effort was not wasted, however, as the thought processes and analyses employed in this design were swiftly put to good use in designing DEWBOT V's chassis and drive-train. Many of the design features described here found a home in DEWBOT V.
Design is a traditionally-oriented (long) 6wd FRC Chassis fitting the typical FRC starting envelope (38" x 28").
Key objectives in this design were:
- A maneuverable, stable, fast, reliable, high-traction and high-power chassis
- Provide logical, protected & servicable space for electronic and pneumatic systems
- Provide a clear platform for scoring systems
- Build servicability into the design from the start (make it front-of-mind)
- Keep high-precision custom machined components to a minimum
- Manage weight
- Did I already say reliable? A robot with an unreliable drive-train spends a lot of very embarasing time sitting still in competition. Been there. Done that.
View the pdf format chassis drawings.
Open the Microsoft Excel Drive-train Model.
See the Microsoft Powerpoint design presentation.
Drive-train is 6wd Tank. Middle wheels are directly driven by the gearbox. Fore and aft wheels are slave-chain driven from the middle wheels. This arrangement provides reliability, as the loss of any one drive chain (a common problem with DEWBOT III and DEWBOT IV, but so far never an issue with DEWBOT V) would have a minimal impact on robot mobility.
Fore and aft wheels are anchored in Bearing Block Assemblies bolted into 80/20 1010 channel. The Bearing Block Assemblies contain ½" ID flangeless ball bearing assemblies. Bearing Block Covers retain the bearing races. By mounting the Bearing Block Assemblies on 80/80 1010 profile, adjusting wheel position to maintain chain tension is straightforward. View pdf format bearing block drawings.
Middle wheels have no bearing blocks. Their drive shafts are cantalievered from the gearboxes.
Type 35 steel chain is used. Each of the (4) chain circuits has 102 links. 22-tooth aluminum sprockets drive (and are driven by) the chains.
AndyMark 8" Performance Wheels (am-0105) were used on DEWBOT IV. Traction treads were secured to the wheels via pop-rivets. We did not consider drilling pop-rivet holes in any specific fixed pattern. As a result, changing treads required drilling out old rivets and then drilling new holes for new rivets. A God-aweful job and an excellent learning experience.
For the new chassis, we really liked AndyMark's new 6" Plaction Wheels (am-0199). These wheels are lighter and cheaper and best of all, they split in the middle to allow change of worn treads.
We realized quickly that putting bolts through the wheels to secure spockets and hubs would negate this very nice feature, so we modified the wheels. We designed countersinks in the wheel interior so that easy splitting of the wheel was retained. View the pdf format Plaction wheel mod drawings.
The first pass through this chassis utilized a conventianal AndyMark AM Shifter Gen 2 (am-0076) with a fully redundant chain drive (4 circuits per side). this provided high reliability (the loss of any one chain would not impact performance), but at a serious weight penalty and with motors and gearboxes mounted high and potentially in the way of scoring systems.
So the AndyMark AM Shifter was resdesigned for direct-drive. The existing gear sets were retained, but new face plates designed which accomodated a third axle. The two S35-12H 12-tooth sprockets (am-0019) were removed from the 2nd axle and replaced with a single 20-tooth drive gear (am-0186). A third axle is added having a modified 45-tooth gear (am-0185), providing an additional reduction of 45:20 (9:4), thereby enabling direct drive. Gear reduction ratios are 9.375:1 (high gear) and 24:1 (low gear). Maximum robot velocity is 14.8 ft/s and 5.8 ft/s in high and low gear, respectively.
View the pdf format gearbox drawings.
Attention to Servicability
A great deal of attention was paid to servicability in this design.
- The choice of wheels and mods to the wheel design were based on ease of replacing treads
- Left and right drivetrain assemblies (motors, gearboxed, rails, chains, wheels,...) were identical and could be changed-out. A complete replacement drive-train as a spare part was feasible.
- Tool access holes are drilled in the chassis frame to facilitate service
- Bolt/nut access holes are cut in one side of all square tubes. Nut access holes are sized for socket wrench clearance.
- Polycarbonate panels are cut to provide easy access to cRIO modules
- Outboard 80/20 rails to serve as ergonomic lift points
- Battery position provides both security and fast, easy changes
Legacy in DEWBOT V
While this chassis was never built, the design work proved beneficial to the design of DEWBOT V in several ways.
First, quite a lot of rigorous analysis went into the design and tools were developed to aid in this analysis. As an example, a Microsoft Excel based Drive-train Model was developed simulating a full-power robot drag-race, allowing us to know just how fast and far we could go how quickly. All KitBOT motor curves were put into an Excel worksheet as well. These tools and preparation were all ready for us to use in meeting the Lunacy challenge.
Second, many specific features and good ideas in the design were incorporated (generally modified) into DEWBOT V. These include:
- The basic drive-train arrangement: a direct-driven middle wheel with slave-chain driven fore and aft drive wheels.
- Basically the same wheel-sprocket-chain arrangement used
- The direct-drive gearbox was simplified (for 1 CIM) and used
- DEWBOT V Wheel bearing blocks are slightly modified version of the bearing block in this design
- DEWBOT V mounts the bearing blocks in 80/20 1010 profile for easy chain tension adjustment
Third, the attention to servicability and reliability was retained.