Exploring the final frontier: robots in space

NASA's Jet Propulsion Laboratory is designing the next generation of space robotics to explore other planets. E&T visits the labs in Pasadena.

Science-fiction has ensured that robots are firmly established as part of space exploration, and from the moment the robot arm on Voyager unfurled itself on the surface of Mars in 1976, robots have been a practical reality in orbit.

Designed to operate in hazardous conditions and in extreme temperatures, these machines have evolved to such an extent that many modern space missions would not be viable without their presence.

Since I was introduced to the spider-like robots in Arthur C Clarke's 'Rendezvous with Rama' in the early 1970s, they have held a fascination for me. However, a visit to NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, soon dispels the glamorous notion of space robots. Perched in the foothills of the San Gabriel Mountains, north-east of Los Angeles, the sprawling facility is home to NASA's technology research centre, which includes robotics among its remit.

The latest robotic creation from JPL is the Axel rover system, a family of platforms aimed at providing versatile mobility for scientific access and human-oriented exploration of planetary surfaces in the solar system.

My host for the day is Issa Nesnas, a NASA engineer and principal investigator on the Axel project. "There are a number of things that set space robotics apart from terrestrial robotics," Nesnas explains. "You are very energy constrained and you have limited computation. The environment in space is much harsher than the environment on Earth and that is very much a function of where you end up going. Your computation has to be radiation hardened to operate in a space environment.

"There are also thermal challenges where the computational platform and the robot itself could experience temperature extremes, from very hot to very cold. On top of this is the dusty and dirty environment."

Nesnas has been with NASA for over 12 years. Prior to that he worked in the robotic industry where he was exposed to car automation and small part assembly. The biggest change he has seen since arriving at JPL is the drive to bring more intelligence to the robots that are put in space.

"Robotics today is still far from what humans can do," Nesnas says. "There are a lot of applications in space where it makes sense to send robots to prepare and do precursor type missions for humans.

"I also think there is the spirit of exploration that we, as humans, will always have and we will always expand our reach within the Earth and the solar system."

Axel rover design

A primary goal of the Axel system design is minimal complexity. Therefore, the basic Axel rover uses a symmetrical design, with only three actuators to control its wheels and a trailing link. The link serves several purposes: it provides a reaction lever arm against wheel thrust, it adjusts the rover's pitch for pointing its stereo cameras, and it provides redundancy if one of the wheel actuators fails.

"That is exactly the approach that we wanted to take," Nesnas says. "There have been several ideas about how to access challenging terrain. The fundamental idea of access is that we want to go with a very simple approach but make it robust."

Any long-haul trip in space is mass constrained so the plan was always to keep the robot as small as possible. With the target areas likely to be high-risk there was also a requirement to send multiple vehicles. "We are already constrained by the mass that can land on the surface, so we tried to make it as minimal mass as possible," Nesnas continues.

"The other reason to keep it small and simple is as it is going into challenging terrains, it has to be very robust and the more complex the system, the more likely it is to fail. We have actually retrofitted a robot that was designed for a completely different purpose."

The basic concept of Axel is a yo-yo that operates on a tether which is simply rolled over the surface. "From a robotic stand point, it may not be the most interesting robot, but in terms of space exploration we want the simplest machine that can accomplish the task well."

It can traverse steep and rocky terrains, and tolerate strong impacts during landing or driving. Additionally, Axel is designed with co-location of its sensors, actuators, electronics, power, and payload inside the central cylinder. This configuration provides compactness for launch, and robustness against environmental extremes in planetary missions.

"It has three actuators and controllers that control these actuators, and it also has a computing platform," Nesnas explains. "There is also one stereo camera pair - two cameras in a stereo configuration, which can take images and generate three-dimensional information about the robot's surroundings, which is critical.

"It also has sensors inside the body that tell it how it is tilted. There is an inertial measurement system which has both gyroscopes and accelerometers, that tells us whether it is upside down or right side up.

"The reason it is symmetric around the cylindrical body is because it can flip over while coming down a rough surface and we want it to be able to operate upside down or right side up. So there is no such thing as right side up for the Axel.

"Also, imagine a surface where you have a promontory or an overhang, as the Axel gets lowered at some point it is going to be hanging in the air with no surface. The yo-yo allows it to go up and down on a thread without any surface: it just unreels its tether and goes down, and reels in its tether and goes back up."

Independent control

Nesnas explains that there is one motor that turns the left wheel and one motor that turns the right wheel. By controlling them independently, the device can go forward or backward. It can also do a turn in place by turning one wheel one way and the other wheel the other way. By controlling the different speeds it can do arcs.

"So with these two motors we can do all the different motions that a regular rover can do, but it can do more," Nesnas continues. "The trailing link is rotating relative to the body, so we can adjust the pitch of the body without moving the axel. So I can move the two wheels forward and the trailing link backwards and the axel doesn't move; all that I have done is pitched the cameras which are mounted on the body and I can look forward, up, down, and behind me. So that is how we play the three motors together."

Another major advantage of the single Axel design is its potential use as a part of a larger system. One concept employs a single Axel deployed by tether from a larger rover for access to steep terrain.

Alternatively, Axel rovers can be arranged in a family of configurations to carry larger payload modules. With planned in-field docking capability, Axels can be used for a variety of exploration purposes before and after payloads are deployed.

"In 1999, there was a call from NASA that they were looking for a rover that can self repair and be tolerant to failures," Nesnas says. "Knowing all the different challenges in building robots for space, there are different approaches where you can actually build self-reconfigure and self-repair mobile robots. Our approach was much simpler and was to take a robot and separate what we call the mobility from the payload."

The payload is what carries the science instrumentation and the mobility is what carries the payload. "The mobility elements have more moving parts and so are more likely to fail," Nesnas continues. "The payload elements are the instruments and cameras which are less likely to fail. By separating these two and having one carry the other, we can build a mission that is made out of ten mobility platforms and ten payload modules. These could have different instrumentations, different sensors, drill mechanisms, arms, whatever you want to put in them.

"You land a mission with ten of each along with these Axel modules, which are very simple mobility modules that can dock on either side of a rectangular science payload module and carry them to where you want to do the investigation. If one of the motors fails, you eject it, separate the failed mobility platform, and have another Axel dock in place."

The robot was demonstrated earlier this year from a mock-up of NASA's Phoenix Lander. It descended down a steep incline and collected several soil samples by ploughing the end of its robot arm deep into the soil.

Power problems

Currently, the robot runs on battery power, which limits its life to just over an hour; a problem that the team are working on. "Axel doesn't have a very large surface area, so using solar power will limit its capabilities," Nesnas says.

There are a number of options to be explored. One of them is providing power through the tether. If it is attached to a lander or some kind of mass on the surface, it could use solar power provided by a large array to charge the batteries on board Axel. The team are also looking at longer-life batteries. "For now it is battery operated and we need to recharge the batteries. How we do this is one of the things to be explored in the context of the mission, depending on other constraints."

At present NASA does not have a mission earmarked for Axel, but Nesnas is convinced that his tethered marsupial is ready for any challenge that is thrown his way.

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