DEWBOT III Drive Train
Dewbot III possesses a drive-train and chassis having good performance and reliability. Drive-train is four-wheel-drive (4wd) Tank with Traction Wheels forward and Omni Wheels aft.
We used AndyMark Shiftboxes with (1) CIM motor per side. Taking into account 12-tooth drive sprockets and 42-tooth driven, overall gear reduction ratios were 14.6:1 and 37.3:1. Maximum robot speeds (@ max efficiency) was 11.4 and 4.4 ft/s (in high & low gears, respectively). Servo motors were used for gear shifting. We found these to be unsatisfactory and switched to pneumatic actuators on DEWBOT IV.
In practice (competition), we seldom used high gear, therefore seldom shifted gears.
Type 35 polymer chain was used for power transmission to save weight. Polymer chain weighs 0.0105 lb/in, as opposed to steel chain at 0.0175 lb/in. This can represent a significant weight savings (although this weight savings was not needed on Dewbot III). We experienced a lot of chain breaks with the polymer chain, so we are reluctanct to use this in new critcal applications. Fortunately, each wheel has its own drive chain so the failure of any one chain during competition did not have a large impact on out ability to compete successfully.
Front wheels are 8" diameter x 2" wide IFI Traction Wheels (IFI-WHEEL-820-50) with Roughtop treads (µ = 1.3).
Rear wheels are 8" diameter "Coolie Dualie" (am-0104) Omni Wheels. Coefficients of friction are:
- 1.05 - Radial, static
- 1.00 - Radial, kinetic
- 0.29 - Transverse, static
- 0.21 - Transverse, kinetic
The Omni Wheels make this robot steerable, but also leave it vulnerable to side pushes.
A copper heat sink was installed on each of the CIM motors.
The chassis frame is aluminum, brazed (welded??) together with zinc by Ed Faller. This worked out very well.
Ed Faller was Head Mechanical Mentor and personnally lead the Drive-Train effort.
Drivetrain Strengths:
- Overall drive-train performance was quite good
- 4wd with independent chains provided high reliability despite the tendency to break chains
- Traction - Omni Wheel combination provides good traction with okay steering
- Able to climb ramps for bonus points
Drivetrain Weaknesses:
- There is a distict tendency to oversteer (hysteresis)
- Vulnerable to side pushing at rear (omni wheel end)
- Polymer chains broke frequently (but with insignificant consequence)
- Servos do not provide adequate force for reliable gear shifting
- Robot quite light, so not a pusher
- Weight was available for (2) more CIMs and steel chains - we'd have been better off with these
DEWBOT III had the last Ed Faller drive-train and it was was clearly far superior to DEWBOT IV's. Ed was missed upon his retirement.
Note: We tend to steer by the drive-stop-turn-drive method. This is very visible in video. Not at all sure that the drive-train makes us do this. This is an inefficient means of steering. It wastes time.
