The Curiosity rover took its first scoop of Martian soil last month. This achievement, like nearly everything the rover has done since its dramatic landing on Mars, was made possible by 31 actuators. Each actuator consists of an encoder, a brushless DC motor, a planetary gearbox, a brake, and a resolver.
The actuators, made by Aeroflex Corp., are located throughout the rover, and are responsible for most of its moving parts, including its wheels, robotic arm, and remote sensing mast (the rover’s “head”), among others.
Today, these actuators are driving Curiosity around on the surface of Mars. But engineering difficulties with the actuators caused the mission’s launch to be delayed by two years (the first such launch-date slip since the Viking program in the 1970s), and put the program at risk of cancellation by Congress.
Click on the image below to see images of the rover on Earth and on Mars.
This engineering drawing shows the location of the arm on NASA's Curiosity rover, in addition to the arm's turret, which holds two instruments and three tools. The arm places and holds turret-mounted tools on rock and soil targets. It also manipulates the sample-processing mechanisms on the 66-lb (30-kg) turret.
The arm has five degrees of freedom of movement provided by rotary actuators known as the shoulder azimuth joint, shoulder elevation joint, elbow joint, wrist joint, and turret joint. (Source: NASA/JPL-Caltech)
I spoke with Rob Manning, NASA’s chief engineer for the Mars Science Laboratory (the official name for the Curiosity rover), about these difficulties, and how his team successfully overcame them. I started my conversation with Mr. Manning by mentioning that many Design News readers daydream about the kind of work he and his teammates do.
“Sometimes we daydream about having a normal life,” he replied. Managing a high-profile space mission can be a “rocky road,” but that’s “typical when building things of this complexity.”
“Our foibles are highly visible,” Manning said. “They’re watched by Congress and the public with a lot of scrutiny.” On the other hand, the sense of accomplishment when everything comes together is indescribable. He spoke with obvious excitement about the challenges of producing “fantastically complex designs” and making them work, millions of miles from earth.
Manning is starting his 33rd year at NASA. The Mars Science Laboratory is his 14th project, and his third as chief engineer. Before Curiosity, Manning was the chief engineer for NASA’s previous Mars rover missions, including the Spirit and Opportunity rovers in 2004, and Mars Pathfinder, the vehicle that landed the diminutive Sojourner rover in 1997.
Sojourner was designed for a seven-day mission, although it survived for a total of 83 days. Spirit and Opportunity were each designed for 90-day missions. Spirit survived for six years. Opportunity is still active today, but must hibernate during the Martian winter, which lasts more than four months. During this season, temperatures drop as low as -120C (-184F). The demise of the Spirit rover has been attributed to these extremely cold temperatures.
In contrast, Curiosity was designed to operate throughout the Martian year. This means that its actuators must be capable of functioning even during the cold of the Martian winter. As the temperature decreases, the viscosity of liquid lubricants increases exponentially. To put this into perspective, a grease that, at room temperature, has a base oil viscosity similar to SAE 20 motor oil, at -120C would have a base oil viscosity higher than that of molasses.
That's what it takes. The problems in space systems are generally very difficult becuase the conditions do not occur, in general, on earth. In addition, except in a limited number of cases, once a system is launched, that's it.
Generally, the budgets are a lot bigger than for terrestial based systems. They are not unlimited, but sometimes they seem to be. The comment about missing the date is instructive. In a normal engineering situation, you might actually abandon a product development if you miss a key date. This could be a model year or a selling season (e.g., XMAS). When RIM announced that their Blackberry 10 phones would not come out until late January 2013, their stock fell. This is equivalent to the constant threat of Congress cancelling or delaying a program. It can be really frustrating for the engineers involved.
Great article, Dave. The obvious emotion among the engineers in image #6 is a reminder of the comments from NASA engineers in the late '60s and 70s, who used to go outside and look at the moon for inspiration during late night work sessions. As you point out, this is a dream job for most engineers.
A really interesting article on what happens behind the scenes to design a better machine by learning from what didn't work in the past, and also having the patience to see it through. That it's about one of the most interesting and well-known robots to be created in the last decade also makes it a worthwhile read. Thanks for the blog post, Dave.
Mechanical, electrical and power management issues are common to many designs, but not many engineers have such a hard deadline as a launch window to contend with. The men and women who work on these extra-terrestrial projects have a lot to contend with and my hat's off to them and the managment team that make these projects happen - despite the long odds for success. Thanks for the post.
Good article, Dave. One detail that caught my attention was the description of the thermoelectric generator used to charge (and repeatedly re-charge) lithium batteries. I didn't realize the Curiosity was stocked with multiple Li-Ion cells, and knowing that this type of chemistry has a relatively finite life span of charge/discharge cycles (about 500-600 times) that definitely seems to dictate a finite life span. Of course, that span could be measured in decades if one cycle were several weeks long. Is that the case-?
@JimT: I don't have a definite answer on the battery life, but this status report says that "the batteries are expected to go through multiple charge-discharge cycles per Martian day." The rover is supposed to last at least one Martian year, or about 669 Martian days. So it sounds like the batteries are expected to last well over a thousand charge-discharge cycles.
The Spirit and Opportunity rovers, which have similar batteries, survived for several Martian years, and as the article notes, Opportunity is still alive.
Dave; thanks for that. Yardley website indicates the Lith-Ion cells are good to 2100 deep cycles. That's unheard-of in the consumer electronics industry. I had been making vague assumption's that, while the Consumer Market and the Aerospace industries have vastly different needs, that the basic physics and chemistry of the batteries would be a common denominator. Certainly NOT the case at 2100 cycles. That's approximately 3-4 times better than the high-end expectations for Consumer portables at about 600 cycles. I'm book-marking that Yardley site, for sure. Thanks!
Rob Manning asked me to convey that Howard Eisen and his team were mainly responsible for the successful resolution of the issues described in the article. Eisen is the Deputy Flight System Manager, and like Manning, has been closely involved in all of NASA's Mars rover missions, from Sojourner in the late '90s to the Curiosity rover today.
Of course, the success of the Curiosity rover is the product of the work of literally thousands of people, at NASA and at their suppliers (including Aeroflex and Yardney, among others) and other partners.
Excellent post Dave. It's amazing to me how dedicated engineers always find a way to make it work. Also its obvious NASA appointed the right individual to lead this team. I wonder if Congress would be so accommodating today if delays of this type presented themselves for comparable programs.
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