DEWBOT XIII Drive Train
FRC 1640's traditional 8.30:1 reduction from CIM to 4" wheel has provided power, control and agility, but not speed. Frankly, we've been a slow robot; growing relatively slower as teams adopt more aggressive tank drive trains. Then along comes SteamWorks, with a critical need to run gears across the field quickly. Our old swerve drive is just not up to this!
Fortunately, for the past two years, the team has worked to develop the employment of continuously variable transmission (CVT) as a means of employing variable gear reduction without excessive weight. By relying on pulleys, CVT fits 1640's swerve design philosophy far better than gear-based shifting. In fact, the fit is natural.
In past swerve systems, reduction had been managed in two stages: a 3.11:1 1st stage reduction via HTD5 pulleys (18T & 56T) from CIM to coaxial drive shaft; and a 2.67:1 2nd stage reduction from the miter gear to wheel via sprockets and chain.
The constant velocity pulley comprises two v-pulley flanges forced together by spring loading. Under low belt tension, the pulley flanges remain together and the v-belt rides in the high-diameter position. As belt tension is increased, the pulley flanges are pushed apart, bringing the belt into a lower-diameter path. Since the drive pulley is constant velocity, increasing the belt tension increases the reduction ratio.
The V-belt is tensioned variably using a servo which controls the positions of a pair of tensioning pulleys. The servo and tensioning pulleys are mounted on an upward extension of the coaxial drive shaft.
A CIM motor provides the drive power for the wheels.
Our old standard steering motor, the BaneBots RS540, is no longer on the FRC approved list. It was replaced by the AndyMark 9015 motor (am-0912) at a weight penalty of 0.16 lbm. This motor is mounted on a BaneBots P60S-555-5 132:1 reduction gearbox (same gearbox used since 2013).
The belt tensioning servo is a Rev Robotics Smart Robot Servo (REV 41-1097). Its metal gears provide good durability.
- In 2013, the very heavy 1" top flanged bearing (McMaster 6384K373)
- There's a new encoder scheme: a Hall-Effect sensor employed as a tooth sensor under the 3.5" pulley. This is not satisfactory.