Miniature DC motors are on a mission to help tiny satellites reach their big potential
Space research isn’t just for the big space agencies anymore. In fact, anyone wishing to make observations from space should not only think small but also inside-the-box. Or more accurately, inside the cube. That’s because in recent years academia has been creating and deploying very small satellites called CubeSats — sometimes referred to as nano satellites — that are increasingly making space study more viable to more organizations.
A discrete CubeSat unit (U) measures no more than 10 cm x 10 cm x 10 cm with a mass of 1.33 kg and can be scaled up with additional units. In addition to their small size, CubeSats are simpler and much less expensive to design, build and deploy than their much larger counterparts. According to ClydeSpace, a Glasgow, Scotland-based firm that specializes in small satellite products, those conveniences are made possible by a standard interface between the launch vehicle and the spacecraft so developers can share their resources for launches. Most of the components are typically commercial-off-the-shelf (COTS).
Ready to launch
Since their first proposals in 1999 by professors at Stanford University and California Polytechnic professors as a low-cost, hands-on vehicle to provide space education to students, CubeSats have spread to commercial applications and even to hobbyists. In addition to making space research accessible to students, CubeSats are used for earth imaging, weather study, and to test electronics for use in space, among other uses. There are also several crowd-funded CubeSat-based applications being planned and even a proposal called “Outernet” by a New York-based company to beam long-range WiFi from space.
On June 19, 2014, UKube-1, the UK’s first CubeSat mission and Scotland’s first satellite will launch from the Baikonur Cosmodrrome, Kazakhstan onboard a Russian Soyuz-2 rocket. According to Clyde Space, the 3U satellite’s payloads will include a include GPS device aimed at measuring plasmaspheric space weather; a camera that will take images of the Earth, and test the effect of radiation on space hardware, using a new generation of imaging sensor; an experiment to demonstrate the feasibility of using cosmic radiation to improve the security of communications satellites and to flight-test lower cost electronic systems; and an advanced mission interface computer.
The right attitude is important
Attitude control is essential to a cubesat’s ability to carry out its mission. The satellite must be precisely pointed in the right direction. Without attitude control, propulsion and communication with Earth may be compromised, and the satellite can lose its ability to gather the data it needs when pointed the wrong way. Further, imprecise orientation can present thermal management issues if the CubeSat’s exposure to sunlight and cooling are not taken into account. Attitude control in CubeSats is particularly difficult because their very small size offer very limited room for the essential hardware.
An attitude control system (ACS) requires gyroscopes to measure the satellite’s orientation and actuators to control the torque necessary to maintain the proper orientation. MEMs gyroscopes have the tiny size and very low power requirements to fit into a miniature satellite’s control system. A cubesat ACS may also include solar sails to make small adjustments to the velocity, and magnetic torquors – permanent magnetcs that react against a local magnetic field to provide the torque necessary for stabilization.
The UKube-1’s ACS’ torque reaction position system is based on a reaction/momentum flywheel driven by an off-the shelf, brushless DC motor. ClydeSpace chose a 20-mm brushless DC motor from maxon motor uk, which specializes in producing very small motors. Maxon worked with Clyde Space to develop a custom solution to fit the size constraints for the design and to stand up to the rigors of space deployment.
The system is comprised of three motors, with three corresponding axes. The motors are spun at a fixed speed. When that fixed speed is modified, the flywheel turns in the opposite direction than the direction needed to re-position the satellite. “By adjusting the speed relative to the original speed,” says Paul Williams (title) of maxon motor, “If you run at, for instance, 4000 rpm and you start to decelerate that motor by maybe 100 rpm, there’s a reaction to that.” This allows the satellite to move slowly. Three axes in the ACS will provide the control necessary to correctly position the UKube-1.
From standard product to a space-ready motor
While CubeSats allow more entities to build and deploy satellites, they’re not exactly cheap, either. CubeSats are built in low volumes, so companies that make them need to find high quality parts that can be modified fairly easily for the demands of space exploration. When modifying its 20-mm DC motor, maxon had three criteria. It had to withstand the high vibration that occurs during launch, it had to use materials with low outgassing properties because vacuum environments cause some materials to outgas heavily and affect instruments, and the motor’s built-in electronics, such as the Hall-effect sensors, have to be immune to the effects of radiation in space. The company also integrated an inertial disk which Williams describes as a disk within the motor itself, essentially making it part of the motor.
Still, finding a vendor that can take off-the-shelf parts and customize them for low-volume applications in harsh environments is key to a successful implementation. “It’s not like building a one-off motor,” says Williams, “If you went to some of the other manufacturers, they’ll probably say ‘look, if you want us to get involved, 40 to 50 motors whatever, a couple hundred motors a year, is just too much technical challenge, too much difficulty doing this.’” Ultimately, the successful project will depend on partnering with a vendor that has the right combination of tooling, data, experience and a complete system of build. This combination could literally help take your design to new heights.