Even though it has been done many times before, the docking maneuver between two objects in space is always a delicate and potentially dangerous procedure. The speed is extremely high (about 28,000 km/h in the case of the International Space Station, or ISS), and corrections are difficult. When the two objects are about to meet, maneuvering thrusters can no longer be used since their exhaust plumes can cause damage. To prevent harm, cargo transporters are caught by a robotic arm installed in the ISS and berthed manually. Manned spacecraft, on the other hand, dock directly in a computer-controlled process.
This type of docking maneuver is going to become easier and safer in the future, so the European Space Agency (ESA) has commissioned its industry partners to design a new docking system called the International Berthing and Docking Mechanism (IBDM). This conforms to the International Docking System Standard (IDSS), a standard on which the leading space agencies worldwide have agreed. The system will therefore be compatible with the ISS and most other spacecraft. The mechanism's first missions will be with the Dream Chaser, a craft being developed by Sierra Nevada Corporation that looks like a compact version of the Space Shuttle and that will soon perform cargo flights to the ISS.
Docking Energy is Absorbed
The IBDM is an androgynous coupling system, meaning that the connector is identical on both sides. It consists of a hard, inner ring (Hard Capture System) and a soft outer ring (Soft Capture System) that has six degrees of freedom and force sensors. The outer ring first absorbs the docking energy. Then the final airtight connection is made and secured by mechanical hooks that pull the two spacecraft tightly together.
SENER is developing and installing the Hard Capture System. The company is currently working on the qualification model, which is due for testing in 2020. “Then, the IBDM needs to be used as quickly as possible on a supply flight for the ISS,” said SENER's Gabriel Ybarra. One of the next steps would be to use it in NASA's Lunar Space Station, which is planned to go into orbit around the Moon and could serve as a launch point for manned missions to Mars in the future.
Dual Systems for Maximum Safety
This is a challenging project for the engineers at SENER. “We first needed to fully understand all the requirements set by ESA and NASA and figure out how to fulfill these requirements. And especially with regard to safety, because the docking mechanism can also cope with manned flights,” Ybarra explained. As well as being lightweight and delivering the required torque, the electrical drives that are used must also be extremely reliable. This is why SENER has been working with drive specialist maxon for several years.
maxon's engineers developed two drives for SENER that can be used to execute a huge variety of functions. This first drive consists of two brushless EC-4pole motors and a GPX UP gearhead. Twelve of these actuators power the locking hooks in the IBDM docking mechanism. The second drive combines a flat motor with a planetary gearhead. It is used in 11 places to manage the plug-in connections and the retaining eyes as well as other ancillary functions.
As the IBDM docking mechanism is a flight-critical application, redundant drive systems are required. The backup must function even if the primary drive fails. This is often solved by means of a backup motor that can take over in an emergency. This is the approach used for the locking hook actuator. For the other drive system, however, maxon engineers found a different, unconventional solution: an additional stator is used instead of an extra motor. The flat motor, therefore, has two stators and two windings, each of which is capable of independently driving the rotor — a solution that guarantees safety while saving space.
“The team understands our requirements and is very quick with design modifications,” Ybarra said about maxon. Moreover, both partners have a passion for mechatronic systems. “It feels great to be involved in the entire cycle, from design to production and testing. This makes it extremely interesting. And when the system moves for the first time, it's like watching your children take their first steps.”
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