A number of external robotic systems are used to move items and space walkers around the outside of the station for assembly, maintenance and logistics. Most are fixed in place to modules.

Two systems in the U.S. Segment, however, have an advantage of being able to be moved around the station. These are:-

1. Space Station Remote Manipulator System [SSRMS]
2. Special Purpose Dextrous Manipulator [SPDM]

References: Canadian Space Agency,  Wikipedia

Canadarm2 is large multi-jointed robot 'arm' that can be attached to various dedicated point around the outside of the station.

It can also be used in conjunction with the Mobile Servicing System [MSS].

References: NASA - Canadarm2

The Space Station Remote Manipulator System [SSRMS], called "Canadarm2" is a bigger, smarter version of the space shuttle's robotic arm. Both arms were built for NASA by the Canadian Space Agency (CSA), one of the partner nations in the ISS project.

Canadarm2 is designed to remain in space attached to special attachment points on the outside of the ISS. It can also become part of the Mobile Servicing System (MSS) described below.

Canadarm2 is operated remotely by the Station crew from a Robotic Work Stations (RWS) in the U.S. Destiny Laboratory or the Cupola. The RWS has two sets of control joysticks; one Rotational Hand Controller (RHC) and one Translational Hand Controller (THC). In addition to this is the Display and Control Panel (DCP) and the Portable Computer System (PCS) lap top.

Video cameras and sensors inform the operator of the exact position of each of the arms joints. Operations are also monitored by staff on Earth at the Mission Control Center (NASA), or from the Canadian Space Agency (CSA).

Canadarm2 can relocate itself on the Station by using its Latching End Effectors (LEE). These can attach to either a base or manipulator allowing the arm to "walk" from one attachment point to another around the Station. The attachment points are called Power and Data Grapple Fixtures (PDGF).

In addition to moving itself around the station, Canadarm2 can move any object with a grapple fixture.

In construction of the station the arm was used to move large segments into place. It can also be used to capture un-piloted ships like the SpaceX Dragon, Orbital ATK Cygnus and the Japanese H-II Transfer Vehicle (HTV). These are equipped with a standard grapple fixture which the Canadarm2 uses to capture and dock the spacecraft. The arm is also used to un-dock and release the spacecraft after use.

Canadarm2 was transported to the ISS and installed in April 2001 during the U.S. Space Shuttle STS-100 mission [Flight 6A].

It was carried on a Spacelab Pallet in the shuttle's cargo bay. To fit the arm on the pallet both booms were hinged in the middle and folded for transport. During installation the booms were un-folded until straight and bolted permanently into this position.

Canadarm2 was initially located on a Power and Data Grapple Fixture (PGDF) on the side of the U.S. Laboratory module (Destiny). It was later moved to the Mobile Transporter (MT) to become part of the Mobile Servicing System (MSS).

The Special Purpose Dextrous Manipulator [SPDM], called 'Dextre' is a smaller two-armed robot capable of handling delicate assembly tasks to reduce the workload of astronauts on space walks.

It has a dual-arm design providing added flexibility and can remove and replace smaller components where precise handling is required. It is also equipped with lights, video equipment, a tool platform and holders.

The SPDM can perform dextrous tasks by sensing various forces and moments on the pay load. In response, it can automatically compensate its movements to ensure the pay load is manipulated smoothly.

It can either be attached to the end of Canadarm2 or ride independently on the Mobile Base System [MB].

Canadarm2 can then deliver equipment to it for servicing. Dextre is controlled remotely by crew inside the Station.