DEWBOT XIII Drive Train

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1640 Swerve 2017.png
One drawback of swerve drive vis-à-vis tank drive is the relative penalty paid for incorporating gear shifting. Tank drive, with two independent powerplants, requires two gear shifting mechanisms; Swerve, with four independent powerplants, requires four. This is a serious design hurdle for a drive train which is already a little avoirdupois.

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.

CVT Reduction

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.

1640 Swerve 2017 - CVT.png
The current system replaces the HTD5 pulley reduction with a spring-loaded constant velocity pulley (Torque Transmission VPS-10 and a V-belt pulley (Torque Transmission PO-3.5-3/8) connected with a V-belt (Gates 2L 180). The constant velocity pulley had a pitch diameter which varies between 1.375 and 2.75 inches; the other pulley has a fixed 3.4 inch pitch diameter. These provide a 1st stage reduction which can be varied between 2.47:1 and 1.24:1. In combination with the old 2nd stage reduction, the overall reduction can be varied over the range 6.59:1 to 3.30:1.

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.

Motors

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.

Other Developments

  • In 2013, the very heavy 1" top flanged bearing (McMaster 6384K373)
    25mm 6805-2Z bearing and printed holder
    was replaced with an Igus polymer bushing and a bronze sleeve. This arrangement occasionally failed either in having the Igus bushing become unseated from the module top plate or through increasing rotational friction. In 2017, this was replaced once more with a bearing, but a much lighter, 25mm bearing (6805-2Z) with a printed bearing retainer. The top of the steering tube is turned down accordingly and a boring head was used to assure the bearing's proper fit in the modules' top plates. So far, this has been a positive change with no issues observed. The original 1" top flanged bearing (McMaster 6384K373) remains in service in the bottom position. This replacement of Igus & bronze bushings with a 25mm bearing race proved to be entirely positive.
  • There's a new encoder scheme: a Hall-Effect sensor employed as a tooth sensor under the 3.5" pulley. This is not satisfactory.