May 2021

32 n MOTORS May 2021 www.drivesncontrols.com Motors help NASA helicopter to take to the Martian skies L ast month, NASA's Ingenuity helicopter made history when it took off for a short test flight above the surface of Mars, thus becoming the first device to be flown in a controlled manner on another planet. It climbed at about 1m/s to a height of some 3m above the Martian surface, where it hovered for about 20s before returning to the surface and landing on its four legs. The flight was the culmination of six years of development by engineers and scientists at NASA’s Jet Propulsion Laboratory (JPL), working in collaboration with civilian firms. The helicopter, which weighed 1.8kg on Earth, was about 0.68kg in the Martian gravity that is about 38% of the strength of that on Earth. The craft is just under 50cm tall, and is powered by carbon fibre blades arranged as two 1.2m-long contra-rotating rotors that spin at roughly 2,400 rpm – about five times faster than those on Earth. It is powered by a photovoltaic array on top of the rotor system that charges six lithium-ion batteries – sufficient to allow flights of up to 90 seconds at heights of up to 5m over distances of up to 50m. Ingenuity is a “technology demonstration” – a project that seeks to test new capabilities for the first time. It does not carry any scientific instruments, other than two cameras, and its aim is to demonstrate rotor- powered flight in the extremely thin, carbon- dioxide-filled atmosphere of Mars – which has a density about 1% of that on Earth. This is equivalent to flying about 30km above the Earth’s surface – something that no terrestrial helicopter can do. Because of this, Ingenuity had to be extremely light, with rotor blades that are much larger and spin much faster than what would be needed for a helicopter of a similar mass on Earth. During initial tests to spin the two rotors at high speed on the Martian surface, Ingenuity’s onboard computer shut down. The JPL team adjusted the commands that it sends to start the rotors, and the tests were then completed as planned. As well as having to be small and lightweight, the helicopter’s components – including its vital electric motors – had to be stress-tested to ensure they could survive the intense cold and radiation in deep space and on Mars, where night-time temperatures can drop to –90°C. Tests were performed in a vacuum chamber containing low-pressure CO 2 . To simulate Mars’ low gravity, the test craft was suspended from a thin line attached to a motor and a precision torque sensor. This “gravity offload” system produced a controlled lifting force to counteract Earth’s more powerful gravitational pull. The carbon-fibre rotor blades are powered by lightweight, high-efficiency, propulsion motors developed by AeroViroment, a Californian company that specialises in unmanned aircraft. It is perhaps best-known for its solar-powered flying wings that are designed to fly for long periods at altitudes of 20km or more as a platform for telecommunications equipment. AeroVironment also developed and built Ingenuity’s rotors, control mechanisms, power electronics, landing gear, load-bearing structures, and the thermal enclosure to protect NASA/JPL’s avionics, sensors, and software systems. The helicopter is also equipped with six brushed DC motors modified by their manufacturer, Switzerland’s maxon motor, for their arduous duties. The 10mm-diameter motors (based on maxon’s DCX drives) control the pitch of the rotor blades and thus the flightpath of the helicopter. “The biggest challenge in developing the motors was the extreme weight requirement,” recalls maxon design engineer, Aiko Stenzel. “Every tenth of a gramme had to be saved to make the helicopter fly. What’s great is that, despite the weight savings, we found a drive solution that has enough power to adjust the rotor blades. And this in the face of high vibrations and temperature fluctuations.” maxon drives can also be found on the Perseverance rover vehicle, which the helicopter was attached to when it landed on Mars on 18 February. Ten BLDC motors and a special gearbox are being used, among other things, to handle soil samples onboard the rover. One of them lowered the helicopter safely onto the Martian surface. The technologies being tested on Ingenuity could lead to flying vehicles for future robotic and human missions to Mars. These could offer a unique viewpoint not possible using spacecraft orbiting high overhead, or rovers and landers on the surface. They could perform reconnaissance tasks, and access terrain that is difficult for rovers. Future helicopters could even carry light payloads from one site to another. n Last month’s pioneering flight of a NASA helicopter above the Martian surface was made possible by an array of lightweight electric motors. Ingenuity on the surface of Mars, showing how dust has accumulated on its solar panel, and some of the vital electric motors