DEWBOT XI Drive Train
With the loss of Ben Kellom as mentor and Edgetech as sponsor, we redesigned our swerve module to reduce the machine time requirements and to bring as much of the machining in-house as practical.
New sponsor AMS Filling Systems provided the parts we could not produce in-house.
During his four years as a 1640 mentor, Ben Kellom made enormous contributions to the team; especially (but not limited to) our machining processes, quality, equipment, workflow, and (most importantly) how we think about both machining and quality as we design our robots. In his role, Ben was a key driver in the development of our swerve drive technology. Ben's employer, Edgetech, graciously sponsored the team by machining the pivot (and other) parts.
With Ben's departure, we revised the pivot design with an objective of reducing the machine time required to produce these, making the following changes:
- Pivot side plates are 2-D (0.250" thk)
- Pivot side plates were cut via water jet (old plates had to be CNC machines and started with 0.500" thk plate)
- Pivot Braces replaced by 7075 Al stand-off; these stand-offs were manufactured in-house
- Pivot Tubes were manufactured in-house
- Pivot Module top and bottom plates were cut via water jet
Aggregate sponsor machine time was reduced by more than 50%.
Sensors6127V1A360L.5FS. Cost is up a bit ($12.32 each). These sensors have performed well for us since our adoption of them in 2012.
Sensors were coupled to the steering drive shaft via clamp-type flexible helical beam couplings (Ruland FRC12-4-4-A from FIRST Choice). The couplings reduced the potential for damaging the sensors due to misalignment. Clamp couplings eliminate the slippage problems experienced with set-screw couplings.
Tachometer - For 2014 we used a latching Hal effect sensor and 6 magnets mounted underneath the large timing pulley. The Hal sensors are cheap the magnets are not. Worked well but we needed better resolution and simpler sensor. We used the Pololu QTR-1A reflectance sensor and strips of 3MM retroflectance tape on the large pulley. $4.25 for 2. While this sensor is an analog output, the digital input of the Roborio chopped and digitized the sensor. Worked well.
Printed Sensor Mounts
Both the Angle encoder and the photosensor were mounted using 3-D printed mounts, a first for Sab-BOT-age. In the past, we had machined polycarbonate mounts.
Use of Mini-CIMs in lieu of CIM MotorsModeling showed that Mini-CIMs could serve as drive motors without difficulty.
The team's value engineering efforts in 2015 focused almost exclusively on reducing sponsor machine time.
The cost increase and weight reduction are both primarily the result of selecting Mini-CIMs for the drive motors in lieu of traditional CIMs. CIMs would have been covered by KOP/FIRST Choice, and therefore free. Mini-CIMs (VexPro) were $24.99 ea. Weight savings by using the Mini-CIM vis-à-vis CIM were 0.63 lbm. Most of the balance would be due to losing the magnets which had been used for the 2014 speed encoders. A comparison of 2013 (no magnets), 2014 (magnets) and 2015 (no magnets - Mini-CIMs) weights supports this.
In water-jet cut parts, most of the critical holes were machine milled for accuracy. The exception to this was the main pivot bushing hole in the module's top plate, which was left as a water-jet cut hole. This turned out to be tight and created some friction/maintenance problems. Fortunately, this joint squeaked before it seized, so there were readily apparent symptoms on which to base service.
Other than this, these swerve modules (writing post FRC Championship) must be rated as our best to-date.