SRC2 - Explicit Steering - Wheel Orientation for Turning

SRC2 Rover

The first post in this series explained I'm currently involved with the qualifying round of the NASA Space Robotics Challenge - Phase 2 (SRC2). The competition requires controlling the rover to the right. It uses Explicit Four Wheel Steering which allows the orientation of the wheels to be independently changed, This provides multiple ways for the rover to move, e.g. straight, crab, turn, pivot.

The challenge is there is no controller for the rover in the Robot Operating System (ROS) because the rover wheels are controlled by effort, not the typical speed control.

This article address the geometry of controlling the rover when it is turning. The diagram below illustrates the rover making a counter-clockwise turn around the Instantaneous Center of Rotation (ICR). The arrows represent the wheel orientation. The dotted box is drawn proportional to the wheel base and track of the rover. Note the orientation of the X/Y axis which is ROS standard for robots.

Explicit Steering - Rover Turning

Term	Description
ICR	Instantaneous Center of Rotation
Rr	Radius from ICR to center of rover
Ri, Ro	Radius of  rover's inner (i) and outer (o) sides through ICR
Wb, Wt	Wheel base and wheel track of rover. Lengths are representative of actual size.
WRi, WRo	Radius of inner(i) and outer (o) wheels
δi, δo	Steering angle for inner (i) and outer (o) wheels

NASA Space Robotics Challenge - Phase 2

Another NASA Centennial Challenge began earlier this year. It will be the 3rd I've entered. I also entered the 2019 ARIAC competition which makes 4 competitions. The current competition is the Space Robotics Challenge - Phase 2 (SRC2). In this competition robotic mining rovers explore the Moon to detect volatiles and collect them, detect a low orbiting cube sat, and position one rover aligned with a fiducial on a processing station.

Links to Follow-on Posts

• SRC2 - Explicit Steering - Wheel Orientation for Turning
• SRC2 - Explicit Steering - Crab, Straight, and Pivot Movements
• SRC2 - Explicit Steering - Wheel Speed

In the following posts I'll explain what I can about this topic. There are a number of research papers available however my posts will provide a simplified explanation. 

Explicit Steering

The biggest challenge in this competition is controlling the rover. It is the size of a small SUV with Explicit Four Wheel Steering, i.e. each of the four wheels is steered separately. If you've seen the Mars rover it is a similar design. That's the base rover, above.

Explicit steering allows flexibility in movement of the rover. It can turn all four wheels to the same angle to move sideways in a crab movement. This also provides straight forward movement. By orienting the wheels at different angles the rover can turn. An extreme example of this is pivoting in place.

Robot Operating System

The base software for the competition is the Robot Operating System (ROS) which consists of the fundamentals for communicating amongst software nodes and a large number of packages that provide useful capabilities. Unfortunately there isn't one for controlling the SRC2 rover.

There are two ways of controlling the wheels for locomotion. The predominant one is issuing a speed command. A number of ROS controller packages provide this capability. Another specified the effort or torque. There are effort controllers but none directly apply to the SRC2 rover.

The location of the rover on the surface is required for reporting the position of volatiles, moving to them, and generally controlling the rover's movement. This requires deriving odometry from wheel movement, vision processing via stereo cameras and an inertial measurement unit (IMU). Again, this is not provided. It is especially challenging due to the low friction between the wheels and simulated Moon surface which allows slippage of the wheels.