F
R

1
1
-
2
0
0
1

MANIPULATOR ARMS FOR SPACE SHUTTLE AND ISS

By Matthias Gründer

The requirement for installation of a manipulator (grasping arm, derived from the Latin "manus" for hand) in the payload bay of the Orbiter was identified as far back as the preliminary stages of planning for the US Space Shuttle transport system. Only with the aid of such a mobile gripping device, at the time still referred to as a "crane", would it be possible to complete the planned tasks in orbit, i.e. the release and retrieval of satellites and also the provision of support for repair and assembly work.

It was not long before the functional requirements for such a Payload Deployment and Retrieval System (PDRS) were defined in more detail and a corresponding invitation to tender was issued. Despite all endeavours, none of the US companies active in the industry was able to even approach satisfying the conditions, so that the contract to develop the crane arm went to Canada, to Spar Aerospace Ltd. of Brampton, Ontario. This resulted in what was (and still is) a unique situation, in which one of the most critical subsystems of the US Space Shuttle system originated not from the USA but from another country.

A Space Robotics Division was established at Spar Aerospace especially for the project, and the company's engineers created a masterpiece in only a relatively short development and trial period. They created a 16-part manipulator system, henceforth better known under the name of Canadarm, that actually resembles an arm, with shoulder, upper arm, elbow, forearm, wrist and hand. The entire carbon fibre structure is 15.2m long, has a diameter of 38cm and, in its original form, was able to move loads of up to 29,484kg.

The Canadarm is attached at the "shoulder", on the left-hand side of the Shuttle Orbiter's loading bay. Two TV cameras, one at the elbow and one at the wrist joint, enable the responsible mission specialist to follow every movement and control the manipulator from the upper rear flight deck.

The manipulator in turn is controlled by means of two joysticks. The joystick commands are converted into commands by the general-purpose Orbiter computer, and feedback in response to control inputs is received back within 80ms. In this way the arm can be moved 6cm per second with a load and 60cm per second without.

The Canadarm's "hand" is not actually very hand-like even though the original idea had been to implement a grasping device that would be similar to a hand. However, it transpired during trials that, for example, it would not have been possible to securely grasp and move satellites weighing several tonnes with such a device. Finally, after several long series of tests, an "end effector" was developed, an ingenious device that yet functions in a quite simple manner.

The open end of the effector is placed over a grapple probe (knobbed pin) that is bolted onto the payload. When the probe comes into contact with a contact plate, three motor-driven steel cables draw together in the opposite direction so that the object is enclosed and thus locked in position. Release of the load entails the opposite procedure. For certain tasks, however, a special effector can also be mounted. For example, this might transmit electrical power to the target object.

Five Canadarms have been built and installed in the NASA Shuttles. Of these, one was lost in the tragic Challenger accident in January 1986. All the others have performed flawlessly since they were first deployed from the second Shuttle mission in November 1981, and have never even developed a single fault. In the meantime all the arms have been upgraded and can now convey loads of up to 266,000kg in zero-gravity.

The excellent experience that NASA astronauts have had using Canadarm has logically flown into the design of the successor system, which is to be installed on the International Space Station, under the name of "Canadarm 2". In this three-part manipulator, the Canadian engineers are delivering a veritable masterpiece.

Consisting of the Mobile Servicer Base System (MSBS), Remote Manipulator System (RMS) and Special Purpose Dextrous Manipulator (SPDM) subsystems, the multi-function arm will in future be able to move on rails in a direction transverse to the direction of travel, along the grid structure of the station, and thus to also reach points some way away.

However, the MSBS rail car will probably not be delivered to the station until ISS Assembly Flight UF-2 in April 2002, whereas the RMS arm has meanwhile already been installed on the US Destiny laboratory module, in April 2001. Finally, the SPDM is scheduled for October 2003 during mission UF-4, following which the manipulator system will be fully operational.

The designers attached particular importance to improving the manipulator on the future SPDM still further. As well as an improved end effector, this manipulator has additional claws enabling small objects to be held, plus two colour television cameras which take pictures directly of the site being worked on. Two additional cameras on the elbow of the RMS transmit pictures of the environment to the operator of the arm, who, once the station has been fully assembled, will have his workstation in one of the domes on the US Node 3 module.

This will enable the duty astronaut to carry out complicated assembly work even in locations which he cannot see directly. Thus, in the truest sense, Canadarm 2 is the extended arm of the station crew for the transport of large payloads, even if this is "only" along the grid structure at whose ends the enormous solar-powered booms of the station are attached.

From there, the 17.6m long arm can reach virtually every point of the station that it needs to reach, but not quite all. For this reason it will be supplemented by several small manipulators, such as the Russian Strela, the European Robotic Arm (ERA) and one American and one Japanese device. Each of these has a particular job to perform, but none of them is a patch on the Canadarm.

From page 46 of FLUG REVUE 11/2001