The European Robotic Arm (ERA) is a robotic arm that is attached to the Russian Orbital Segment (ROS) of the International Space Station. Launched to the ISS in July 2021; it is the first robotic arm that is able to work on the Russian Segment of the station. The arm supplements the two Russian Strela cargo cranes that were originally installed on the Pirs module, but were later moved to the docking compartment Poisk and Zarya module.
The ERA was developed for the European Space Agency (ESA) by a number of European space companies. Airbus Defence and Space Netherlands (formerly Dutch Space) designed and assembled the arm and was the prime contractor; it worked along with subcontractors in 8 countries. In 2010, a spare elbow joint for the arm and ERA's Portable Workpost was launched preemptively, attached to Rassvet or Mini Research Module 1(MRM-1). The Nauka Module and Prichal module serves as home base for ERA; originally, the arm was going to be attached to the canceled Russian Research Module and later to the also canceled Science Power Platform.
Technical characteristics The European Robotic arm consists of two 'limbs' that are symmetrical sections made primarily of carbon fibre reinforced plastic, and are approximately 5 m (16 ft) long. At either end of the arm are identical gripper mechanisms called End Effectors (EES), which the arm uses to hold onto the station, grab onto payloads, and to assist in EVA activities. In total there are seven motorized joints, three are located at each of the wrists, and allow for roll, yaw, and pitch movements, giving 7 degrees of freedom of movement. The elbow consists of one motorized joint, which is limited to pitch movements. The arm is controlled by the ERA Control Computer (ECC), or On Board Computer (OBC) which is located towards the elbow. The two limb design of the arm gives it the ability to 'walk' around the exterior of the Russian segment of the station under its own control, moving hand-over-hand between the pre-fixed base points.
When fully extended the arm has a total length of 11.3 m (37 ft), and can reach and has 7 degrees of freedom. It has a mass of 630 kilograms (1,390 lb) and can hold a total payload mass of 8,000 kilograms (18,000 lb). The arm's tip can reach a speed of 0.1 metres per second (0.33 ft/s) and has an accuracy of 5 mm (0.20 in). The European Robotics Arm itself was designed to be serviceable with spare parts kept outside Rassvet module and allow for the exchange of large parts via EVA in case of failure or an emergency situation. These parts are called EVA Replaceable Units (ERUs), also referred to as Orbital Replacement Units (ORUs), which can all be replaced in-orbit; the ERA is composed of three ERU sections. The arm is wrapped with beta cloth blankets for thermal protection.
On average the arm uses 475 watts of power to operate while having a peak operational power consumption of 800 watts. When on standby the arm only consumes 420 watts, and 250 watts while in hibernation.
End effectors
Located at either end of the arm are the End Effector Subsystems (EES). The EES is designed to attach specifically with the base points located on the Russian segment of the station. Since the ROS uses different base points than the US Orbital Segment, the arm is unable to operate and access the other parts of the station. The EES consists of two Basic End Effectors (BEE) that connect to Base Points (BP) and Grapple Fixtures (GF). During normal operations, one end effector is connected to a base point on the station, this provides a sufficiently stiff connection between the station and ERA, as well as enabling the transmission of power, data and video signals for operation of the ERA. At the same time, the second end effector provides mechanical and electrical power to the grappled objects, and allows for data transfer. Both of the end effectors are also capable of measuring torques/forces as well as contain a Camera and Lighting Unit (CLU) which aids in the control of the arm; an additional two CLUs are located on both limbs.
Integrated Servicing Tool Located within each of the end effectors' lower compartment is an integrated servicing tool. It is used to provide torque for a grappled object, acting like a wrench. It is capable of screwing and unscrewing bolts. The IST head starts with rotating slowly at first upon system comment, until popping into the IST head receptacle, which allows it to provide the mechanical power to the screw interface.
Hardware and software The ERA is controlled from a central computer called the ERA control computer. Control is assisted by the additional microprocessors located in the different subsystems.
ERA control computer (ECC) The ECC is located towards the elbow of the ERA, and composes three main components; the Application Layer (AL), the Service Layer (SL), and the operating system (OS). The application layer contains the high level command, mission plan, and event handling; the service layer contains the main robot motion control loops, motion related checks and image processing; together the AL and SL constitute the Application software (ASW). Together the OS and ASW provide the main control the arm.
Located with in the ECC's RISC subsystem is the ERA's main central processing unit (CPU). The main CPU is a Temic designed ERC32 with 10 MHz clock, addressing 1.5 megabytes (Mb) of memory, as well as 32 kilobytes (Kb) of ROM.
The ECC's I/O subsystem includes an Intel 80C186 with 11 MHz clock addressing 16 kB ROM. It handles the communications between the RISC processor and the other subsystems of the ERA.
The ECC's housekeeping subsystem includes an Intel 80C31 with 8 kB ROM.
Joint subsystem Located in each wrist and in the elbow, there is one microprocessor that takes care of the bus communication, local inhibits, and safety checks, as well as general housekeeping.
The communications of the Joint subsystem are handled by the Joint I/O which includes an Intel 80C31 with 8 MHz clock, and 24kB of ROM, as well as communication through dual ported RAM.
The control of the subsystem was handled by an Intel 80C32 with 27 MHz clock, addressing 16 kB ROM and 8 kB DPRAM. The processor ran high speed digital control loops.
End effector subsystem While the ECC handles primary control of the end effectors, the EES still includes its own micro-processor to aid in control.
Using a torque/force sensor inside the end effectors, the ECC actively controls the arm's pose to keep the contact forces within tight limits. To handle the drift and calibration, the torque force sensor pre-processes the data locally, with its own microprocessor. The processor located within the sensor is an Intel 80C86 with 7.2 MHz clock, addressing 64kB of memory and 32 kB ROM.
A separate processor in the end effector control electronics takes care of the bus communication, grapple mechanism control, and general housekeeping. The processor also controls the Integrated Service Tool. The processor is Intel 80C31 with 12 MHz clock, addressing 32 kB RAM and 48 kB ROM.
Control Astronauts can control the ERA from both inside and outside the space station. The ERA is also the only robotic arm on the station to not include controllers, but is instead controlled through several different interfaces. While possessing multiple control modes, it is primarily used in autotrajectory mode, but manually selectable single-joint modes are also available. In addition to being able to be controlled from the station, ground operators can control the arm, as well.
EVA control Control from outside the space station uses a specially designed interface that can be controlled while in a spacesuit known as the Extra Vehicular Activity-Man Machine Interface (EVA-MMI or EMMI). The EVA-MII has 16 LED displays of eight characters each; displaying essential status data, command verification data and emergency caution and warning messages. It also provides redundancy for critical status data through redundant LEDs.
The EVA-MMI allows for manual control of the arm in emergencies during an EVA. During manual control, only one degree of freedom at a time in a joint space, or one degree of freedom in Cartesian Space is able to be used. Also on the console is the emergency stop button which, when pressed, will activate brakes in all joints and at the same time send a command to the Russian Segment to switch the whole arm off. Whenever the ERA is active, there will always be two astronauts with an EVA-MMI, at two different locations. One has full control of the arm, while the other can at times issue an emergency stop command, as well as take over control of the arm. The EVA-MMI can survive long periods of space exposure, totaling up to 18 months.
Onboard control Control from inside the space station is done by the Intra Vehicular Activity-Man Machine Interface (IVA-MMI), using a laptop, which shows a computer generated model of the ERA and its surroundings. As well on several monitors, video feeds from the arm are also available. In comparison to the EVA-MMI, the IVA-MMI provides more data from the arm, including the intended position of the arm. The IVA-MMI is used during servicing missions. The control systems finished development in 2001.
Capabilities The ERA has the ability to operate under manual control, as well as automatically or semi-automatically. The autonomy of the arm allows for astronauts to focus on other tasks, due to being freed up from others that the ERA can cover.
Specific tasks of the ERA include: Installation, removal or replacement of experiment payloads Transferring payloads in and out of the Station through the Russian airlock Inspection of the Station with the use of cameras Handling of (external) payloads Support of astronauts during space walks Freeing cosmonauts to do other work during spacewalks Before the cancelation of the Science Power Platform, the first task of the ERA included the assembly of solar arrays on the Russian Orbital Segment, as well as replacement of said solar arrays. Many of the arms early tasks were associated with the platform.
Development and service history
Early design Design on the European Robotic Arm started in 1985, with Fokker Space performing a study for ESA on a 10-meter (33 ft.) manipulator arm for the Hermes Space Plane, known as the Hermes Robotic Arm (HERA). It functioned similarly to the Canadarm on the American Space Shuttle; being located within the cargo bay and controlled from the cockpit. The arm was attached to the Hermes at the shoulder. Due to design problems, the Hermes cargo bay had to become a closed structure, with HERA being relocated an end structure called the Hermes Resource Module.
After the decision to cancel the Hermes program, ESA and the Russian Space Agency, in 1992, agreed to use the HERA technology to service the Mir-2 Space Station. The name was then changed to the European Robotic Arm (ERA). It was here that a control computer was added to the arm's design, as well as two new operational consoles were designed, one for a cosmonaut in EVA, and one for a cosmonaut inside the station.
The following year in 1993, Russia agreed to join in the merging of the Space Station Alpha with the Mir-2 into one International Space Station. Following this, the ERA elements of Mir-2 were adapted to the Russian segment of the station, and would act as the main assembly and servicing toll for the segment.
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