Trials were carried out on a half-scale model of ExoMars known as ExoTeR (ExoMars Testing Rover). During two days of testing, the vehicle used its ‘AutoNav’ software to successfully navigate around ESTEC’s 9 x 9 m ‘Planetary Utilisation Testbed’, commonly referred to as the ‘Mars Yard’ by ESA’s robotic engineers.
Mast-mounted stereo cameras allow the rover to digitally map the elevation of its surroundings, with the navigation software calculating the safest and most efficient route to a target point. Communications delays of between four and 24 minutes mean target points can be sent from Earth, but the rover will navigate the route to those points autonomously.
“Rather than sending complete hazard-free trajectories for the rover to follow, autonomous navigation allows us to send it only a target point,” said ESA robotics engineer Luc Joudrier.
“The rover creates a digital map of its vicinity and calculates how best to reach that target point. Looking at the map it tries to place the rover in all these adjacent locations to work out if the rover would be safe in every one of these positions – or if the rocks are too high or terrain too steep.”
While ExoMars will cover just 100m per Martian day, ExoTer traversed the Mars Yard at a relatively brisk 2m per minute. According to Joudrier, the way in which the software makes decisions is comparable to how the human brain works when we are walking, with constant feedback between vision, processing and movement working in harmony.
“Working from the local navigation map, the rover computes the safe path toward the goal and begins to move along a segment of the calculated path, at the end of the segment it repeats the same mapping process to progress,” he said.
“It is similar to a human walking. We look ahead to decide where we are going but as we walk we peer down at our feet and if necessary change course to avoid obstacles. Once we have chosen a path without obstacles, we make sure we follow that path to remain safe.”
When ExoMars launches in 2020, it will actually carry two sets of autonomous navigation software, with another developed by Airbus in Stevenage.
“The combination should give the rover added flexibility,” Joudrier explained. “The idea is that one might turn out to perform better in more difficult terrain, while the other could move faster along easier ground.”
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