Competition Year: 2017
University: University of Southampton
Team Members: George Alexopoulos (Team Leader), Radovan Gallo (2017), Aniket Chopda (2017), Jun Yuan Tan (2017), Neophytos Demetriou (2017), Andreas Hadjiraftis (2017), Rahul Nair (2017)
Faculty Advisers: Dr Mohamed Torbati
Email Address: M.M.Torbati@soton.ac.uk
The Pegasus Mars Rover
Figure 1: The Pegasus Rover
Humans have been searching for life in space for many years and now in our generation, scientists and engineers are looking into how human life could be sustained in foreign planets such as Mars. To find life but to also sustain life in Mars autonomous robotics systems will play a key role. Rovers need to be able to carry numerous tasks such as extreme retrieval and delivery of objects, perform scientific tasks, equipment servicing tasks and to assist astronauts in emergency situations.
Figure 2: Taking the Rover for a spin
Team Pegasus, from the University of Southampton, is the first team from the UK to participate in the University Rover Challenge. We are a group of eight final year Mechanical Engineering students equipped with passion, willingness, experience, and are united by the same endeavor which is to travel to the US and compete with teams from all around the world. Our team decided to design and build the next generation Mars Rover that will one day work alongside human explorers in the field.
The overall mechanical design of the Pegasus rover is based on our three key attributes – robustness, reliability, and simplicity. Our team has adopted an iterative design process that has enabled us to develop and test new design concepts rapidly. We have managed to assemble a preliminary testing prototype of the rover and its robotic arm in December, which has enabled us to evaluate the design thoroughly and apply lessons learned to produce an improved system.
The suspension is perhaps the most crucial part of the entire project as it will ultimately determine the ability of the rover to handle difficult terrain and directly affect the performance of the entire system. It was decided to use the Rocker-Bogie suspension design as it was specifically developed for slow-moving mobile robots that have to overcome difficult terrain. Due to its suitability for the task, it is also used on all of the current and planned Mars rovers.
Figure 3 – (Left) Assembly of the rocker and boogie joint. (Right) Assembly of the suspension.
The suspension consists of 6 wheels, with one motor in each wheel. The motors should be positioned in between the two suspension plates to protect them from getting damaged. Torque transmission from the motor to the wheels at a reasonable speed could only be accomplished through the use of bevel gears.
Figure 4 – Assembly of the wheel on the suspension
The robotic arm is a 5 DoF articulated system consisting of a rotating base, two rotary joints powered by linear actuators, and a 2 DoF wrist mechanism. The robotic arm is also designed to be easily removable depending on the nature of the task. The workspace of the arm was derived according to the needs of the Equipment Servicing and Extreme Retrieval Tasks where several design iterations were required to identify the most optimal geometry of the joints and actuators. The design also incorporates an interchangeable end effector which connects to the wrist mechanism allowing it to be changed depending on the nature of the task.
The wrist mechanism, actuated by 2 DC motors and a set of bevel gears, enable the gripper to have additional dexterity making it even easier to adapt to difficult situations such as in the case of the Equipment Servicing tasks.
Figure 5 – (Left) Prototype of Robotic Arm lifting a 7.5kg dumbbell. (Right) – Final CAD of Robotic Arm Design
A prototype has been built and tested to validate the range of motion and lifting capabilities of the robotic arm, and modifications have been made to reduce its weight. The robotic arm is in the final stages of manufacturing and simulations of different competition scenarios will be run to train the robotic arm operator.
Figure 6 – Robotic arm and gripper
A four-bar linkage configuration was chosen and was implemented with two distinct actuation mechanisms. The final gripper design consists of a DC motor that drives a worm gear which in turn rotates a pair of worm wheels each of which are attached to the fingers. This arrangement prevents backlash and the fingers are interchangeable to fit the requirements of the task.
Figure 7 - Final Gripper Design
The seamless interaction of software and Rover mechanics is the key to assuring the rover’s success. The software team has continually strived to maintain this harmony between the sub-systems. At all stages, the code was kept as simple as possible to avoid complication and thus ensuring good coding practices. We decided to use National Instrument’s software; LabVIEW. This was decided to accelerate software development and deployment, by avoiding the complexity of text-based programming. This required all the team members to be trained in LabVIEW FPGA and Real - Time by the team leader who is the most experienced LabVIEW user.
Due to the complexity of the system, we decided to follow a modular approach in developing the Rovers software. More specically we separated the functionality of the main program into independent modules so that each one of them could execute only a specic aspect of desired functionality. This allowed us to create modular code for each separate electronic component. Once the component had been tested and the functionality validated, it would be included in the main program. Another benefit of following a modular programming approach was that any improvements or changes
that needed to be carried in one module, would have a minimal impact on the overall application running on the myRIO.
The use of the myRIO is ideal for such a robotic application; as it has a Real-Time processor and an FPGA chip onboard. Taking advantage of these aspects we were able to efficiently distribute our application into different layers; FPGA, Real Time and the host PC in our base station. As depicted on the left part of the figure below, the FPGA layer is deterministically generating PWM signals to drive the motors but also is acquiring raw signals from the humidity and temperature sensors which are then transferred to the Real Time processor for further processing. By generating the PWM signals and acquiring the raw signals in the FPGA this frees up resources in the Real-Time processor to handle other aspects of the application.
Figure 8 - myRIO with electronic components connected
On the right-hand side of the schematic, we can see the functionality that the Real-Time processor is handling. Vital information is obtained from the compass and IMU through I2C communication whilst also obtaining GPS data through UART communication. Using Network Published Shared Variables the communication between the embedded device and our base station is facilitated by the Wireless Ethernet network that has been implemented. This means we can send the various sensor measurements to the host PC located in our base station. In the meantime, with the xBOX controller connected to the host PC via USB, the user is able to remotely control the robot, since the values generated from the controller are transferred to the myRIO.
National Instruments Hardware and Software:
sbRIO - 9626
LabVIEW Robotics Module 2016
Real - Time Module
Other Hardware and Software:
8 x Planetary DC Geared Motors
5 x Dual Channel 10A DC Motor Driver
2 x Linear Actuators
2 x Ubiquity Rocket M2 2.4 GHz Radios
1 x Ubiquity AirMax 2G10 2.4GHz 10dBi Omni Antenna
1x Ubiquity Airmax 2G15 - 120 15dBi Sector Antenna
1 x Tycon Power 12v-14v DC-DC Converter and PoE Injector
1 x Drill
1 x xBox Controller
Various Sensors (Temperature, Humidity, GPS, IMU, Compass)
Nominate Your Professor
We would like to nominate Dr Mohamed Torbati who is acting as the supervisor of this project. We would like to thank him for all the support and guidance that he has provided us with.
Critical Design Review of the Rover
Figure 6 - The Pegasus Team, from left to right: R. Galo, G. Alexopoulos, N. Demetriou, A. Chopda, R. Nair, J. Tan, A. Hadjiraftis (top)