DEWBOT XVI Drive-Train

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2020 Swerve Module - standoffs not shown to provide visibility of mechanical components
Our 2020 Swerve modules are redesigned from the ground up. Key benefits are reduced mass, low profiles and improved reliability of the Continuously Variable Transmission (CVT) drive. This is the output of a team development project conducted over the past two years.

The reduced height of chassis connection from 6.54 to 3.88 inches allows for a simpler and lighter chassis frame. The E-Panel can now be connected directly to the lower surface of the chassis perimeter frame, rather than to a drop-down from this perimeter frame. The reduction in overall module height clears space in the robot corners for scoring mechanisms.

A major change is in the CVT reduction is achieved. The same CVT pulley is used as in previous models, but in lieu using a pair of servo-driven "pinch pulleys" which tension the CVT's V-belt in order to affect change in reduction (by spreading the CVT pulley halves), the C-C distance of the CVT belt loop axles is changed by shifting the position of the drive motor/CVT pulley axle. This position shift is driven by a servo. The "pinch pulley" mechanism was fragile and subject to failure.

Mounting and overall heights of 1640's 2019 and 2020 Swerve Modules
The shorter swerve modules are also better protected by the robot's bumpers.

During the COVID-19 interregnum, further development was done on the basic design, including a conversion from CVT back to fixed gearing.

Structural Changes

In previous 1640 pivots, rotational support was provided by a pair of 1" bearings on 2" spacing. Drive transmission passed through the 1" tube supported by these bearings, and hence to the wheel. The new design replaces this with a single 3½ inch x-cross bearing. The design relies upon a single large radius bearing on a single plane which is much closer to the load point (the floor) to maintain rotational axis integrity.

Drive power is transmitted through this larger diameter rotating element via an off-axis bevel gear. No more miter gears, transfer axle and chain. Eliminating this rotational axis reduces height.

The bottom plate, which bears the weight of the robot and shock loads, is ¼" G-10 fiberglass composite. The top plate still exists but no longer plays a structural role. Rotational elements are printed from PETG (polyethylene terephthalate glycol-modified). Wheel stanchions are re-enforced with a pair of screws threaded through top to bottom on either side of the wheel axis (basically rebar for the PETG).

Important Note: Driving the wheel via bevel gears requires very accurate and consistent wheel registry on the axle. No axial slop can be tolerated. Spacers must be installed to hold the wheel in the right position.

Power Plants

In the original 2020 design, NEO motors were used for both Drive and Steering. In 2021, Steering power was changed to a NEO 550.

Drive Transmission

The 2020 drive transmission comprised:
2020 swerve speed test

  1. ) A nominally 0.9375" - 1.875" diameter CVT pulley driving a 2" diameter pulley via a v-belt - 2.03:1 to 1.07:1 reduction
  2. ) An 18T spur gear driving a 26T spur gear (20 DP; 14° contact angle) - 1.44:1 reduction
  3. ) A 15T bevel gear driving a 60T bevel gear bolted to a 4" diameter wheel - 4:1 reduction

Overall drive reduction range is 12.33:1 to 6.16:1.

We discovered that NEO motors demand steel gears. First used printed plastic spur gears and then 7074 AL spur gears in the drive transmission and these were both chewed up in short order. Bevel gears were always steel.

In 2021, the drive transmission was modified to a fixed gear ratio and the CVT pulley and v-belt eliminated. The 2021 transmission comprised:

  1. ) A 12T pinion driving a 30T spur gear - 2.5:1 reduction
  2. ) A 24T spur gear driving a 20T spur gear - 0.83:1 reduction
  3. ) A 15T bevel gear driving a 60T bevel gear bolted to a 4" diameter wheel - 4:1 reduction

Overall drive reduction is 8.33:1.

Steering Transmission & Sensing

Multistage planetary gearboxes are gone.

Steering is reduced 43.6:1 in 2 stages:

  1. ) A 6T Pinion drives a 48T spur gear (20 DP; 14° contact angle). The pinion is steel, while the 48T gear is printed PETG.
  2. ) Synchronous pulley (HTD5) reduction 11:60

Focus is on minimizing steering backlash.

Steering angle is measured off the intermediate shaft, which is geared 11:60 to the Hall-Effect angle sensor. This 11:60 angle measurement reduction is separate and independent from the 11:60 steering reduction. Separate but equivalent. This is a little awkward, but works.

Manufacturing

To control costs, FRC 1640 purchases hubbed 60T steel bevel gears for the wheels and machines these as needed. The results are very nice, but it's a lot of machining. After and before photos are shown below.