SRC2 - Explicit Steering - Crab, Straight, and Pivot Movements

SRC2 Rover

This is the third in a series of post about my involvement with the qualifying round of the NASA Space Robotics Challenge - Phase 2 (SRC2). The first post introduced the basics of the competition. One aspect of the challenge is there is no controller for the rover depicted 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 second post explored the geometry on positioning the wheels for a turn. This post will address pivoting in place and crab movement, i.e. moving sideways. It also addresses the trivial crab case of moving straight forward or back. 

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.

The Autonomous Roboticist

Since September 2016 I've been competing in the NASA Space Robotics Centennial Challenge (SRC). The challenge had a qualifying period and the final competition. I was one of the twenty teams from an international pool who qualified for the final competition. In mid-June the competitors ran their entries on a simulation in the cloud. The last few days, June 28 -30th capped the competition with a celebration at Space Center Houston, an education and entertainment facility next to the NASA Johnson Space Center.

On Thursday, the 29th, teams were invited to give presentations to the other teams, the NASA people who organized the challenge, and others. I used the opportunity to speak about my approach to the competition but also to raise the question of how an amateur roboticist, like myself, can make a meaningful contribution to robotics. 

Two ways are through competitions like this and by contributing software to the Robot Operating System (ROS). There aren't always competitions to work and ROS contributions don't fulfill my desire. In part, ROS misses the mark because before adding a new, usable package, I need to develop something new and useful. Now perhaps there are existing topics that need software and ROS packaging but how do I learn about them? 

And underlying issue for the amateur is knowing the state of the art in academia and industry. Often current academic material is behind paywalls. The amateur is also lacking in the background that lead to the current work. 

One of the reasons for this entry, and a possible new blog with the title, is to see if a third way can be found or created.

Moving Onto Linux

A brief note to set the stage for some posts I'll be making: I have switched to Linux. With the expiration of XP support it was time to switch and I wanted to get off the Windows treadmill. I also wanted to switch to start using Robot Operating System (ROS) with OpenCV for my projects.

I bought a bare-bones desktop machine with quad-core I7 @ 3.6 GHz with 8 Gb of memory. Just a TB of disk since I don't store a lot of video, games, etc. I add a System 76 laptop also with quad-core I7 but not as fast, also with 8 Gb memory. I went with high end machines but didn't pay for the premium processors which would have added $$200-300 to the cost. My thought was to get machines that will last for 5-6 years if they don't break.

Both have Ubuntu 14.04 LTS but I could not stand the Unity interface. Sorry, but these machines shouldn't look like giant smartphones. I installed Gnome to get a menuing interface.

I've been quite happy and have gotten used to Linux. There are still a few places where the Linux and program developers need to be sat down and beaten with rubber hoses until they fix problems. The main one is you should be able to position windows and have them start up in the same place. My understanding is both groups point the finger at the other one saying, "It's your responsibility." It probably needs support from both sides so just fix it, dang it.