Difference between revisions of "4-Wheel Pivot Drive"

Revision as of 23:58, 18 October 2009

4-Wheel Independent Pivot-Wheel Drive describes a 4wd drive-train in which each of the (4) wheels are independently driven and may be independently pivoted for steering purposes. The design offers the potential for excellent drive-train performance and a solution to conventional (tank) drive-train design constraints. The design also brings some clear design and control challenges.

This arrangement provides the possibility to operate in several Drive/Control different modes:

1. Crab mode – pivots all wheels together and at common speed to steer the robot in any direction on the 2-d playing surface (true 2-d drive). The mode does not control chassis orientation.
2. Snake mode – pivots front and rear wheels in opposite directions to guide the robot through a turn
   1. X-bias – drive direction aligned with the long-axis of the robot
   2. Y-bias – drive direction aligned with the short-axis of the robot
3. Automobile mode – pivots front wheels only to guide robot through a turn
   1. X-bias
   2. Y-bias
4. Tank mode – Does not use pivots to steer (but can use pivots to change drive orientation ala Twitch). Steering accomplished by differential L & R drive speeds.
   1. X-bias
   2. Y-bias

Overview

Inventor view of VEX Pivot Prototype

During the Team 1640 2009 Summer Program, a Vex-scale 4-Wheel Pivot prototype was designed, built, programmed and tested. While Vex-scale, the prototype was equipped with an FRC cRIO controller. This provided realistic (FRC-like) control of the prototype, allowed an accurate assessment of control capabilities and problems, and also left us with transportable code and useful programming experience.

Our tests with this prototype were very satisfactory. Our test-driver, Carly, was particularly impressed with the robot's maneuverability in Crab and Snake modes (Automobile and Tank modes were much less interesting), and by the easy, intuitive control.

Based on our prototype trials, we decided to develop the design of a full-scale hypothetical Multi-Mode Pivot-Drive Chassis.

Vex-Scale Prototype

The Vex-scale prototype possessed some limitations for our purposes. We understood and accepted these limitations, but they are important and so are cited here.

The chief limitation was the limited Pivot angle. In the interest of simplicity, coaxial wheel drive was not employed. In addition, potentiometers were selected to measure pivot angle. These potentiometers provided an absolute angle measurement, but were limited to 270° of rotation.

A second limitation was the lack of an on-board battery. This meant that we had to operate teathered to a 12V FRC battery. Control, however, was wireless via the current FRC equipment.

The last limitastion was speed. The prototype used Vex motors and small wheels - ungeared.

Hypothetical FRC-scale Chassis