The structure of Canadarm

The Canadarm, now retired, was a remote-controlled, payload-handling device made up of a number of components:

Joints

Canadarm joint. This basic element is called a joint-one-degree-of-freedom or JOD. (Photo: MDA)

The Canadarm joint. This basic element is called a joint-one-degree-of-freedom or JOD.
Photo: MacDonald, Dettwiler and Associates Ltd. (MDA)

Each subassembly component of the SRMS (i.e. the shoulder, elbow or wrist) was made up of a basic element called a joint one-degree-of-freedom or JOD. JODs are motor-driven gearboxes that allow the basic structure of the arm to bend and turn much like a human arm.

There were two JODs in the shoulder joint, which allowed the whole arm to pitch (an up-and-down motion) and yaw (a side-to-side motion). There was another JOD in the elbow joint to allow the lower arm to pitch. Three more JODs in the wrist joint allowed the tip of the arm to pitch, yaw and roll (a rotating motion).

The SRMS could move even more freely than a human arm to accomplish very complex manoeuvres. The JOD motors had their own brakes and joint motor speed control. Each JOD also incorporated a device called an encoder, which accurately measures joint angles.

Booms

Between the shoulder, elbow and wrist joints were upper and lower booms constructed of graphite-epoxy. The upper arm boom was about 5 metres long and 33 centimetres in diameter. It was made up of 16 plies of graphite-epoxy (each ply is 0.013 centimetres thick) and weighed about 23 kilograms. The lower arm boom was about 5.8 metres long by 33 centimetres in diameter, made up of 11 layers of graphite-epoxy and weighed about 22.7 kilograms. Each boom was protected with a Kevlar bumper (the same material used in bulletproof vests) to prevent dents or scratches on the carbon composite.

Canadian Space Agency astronaut Chris Hadfield pilots Canadarm during mission STS-74. (Photo: NASA)

Canadian Space Agency Astronaut Chris Hadfield pilots the Canadarm during Mission STS-74.
Photo: NASA

Wiring harness

The wiring harness provided electrical power to all the joints and the end effector (the mechanical hand) as well as data and feedback information from each of the joints. This link went from the SRMS in the payload bay and continued back into the cabin of the Space Shuttle where astronauts would control the actions of the arm remotely.

End effector

Canadarm, on the floor at the Spar Aerospace facility in Weston, Ontario.

The Canadarm, on the floor at the Spar Aerospace facility in Weston, Ontario. Photo taken in the 1980s

In place of a hand, the Canadarm had a cylindrical end effector that allowed the arm to capture stationary or free-flying payloads. It provided a large capture envelope and a mechanism capable of soft docking and holding payloads rigid.

For instance, when an International Space Station (ISS) component, such as a 17-tonne Integrated Truss Structure that contained a pair of giant solar arrays had to be deployed out from inside the shuttle's cargo bay, the astronaut who operated the Canadarm would first position the robot arm so that the wire noose inside its end effector would tighten and pull the fixture and the large truss structure attached to it snug against the end effector's rim.

Closed-circuit televisions

A closed-circuit television system of cameras and lights on the arm and in the cargo bay transmitted pictures to television monitors inside the orbiter to assist the astronauts.

SRMS control system

The Space Shuttle's general-purpose computer (GPC) controlled the movement of the SRMS. The astronaut's hand controllers would relay to the computer what the astronauts would like the arm to do. Built-in software studied the astronaut's commands and calculated which joints to move, what direction to move them in, how fast to move them and what angle to move to.

While the computer was issuing commands to each of the joints, it monitored each joint every 80 milliseconds. Any changes inputted by the astronauts to the initial trajectory commanded were re-examined and recalculated by the GPC and updated commands were then sent out to each of the joints.

The SRMS control system continuously monitored itself every 80 milliseconds. Should a failure occur, the GPC would automatically apply the brakes to all joints and notify the astronaut of a failure condition. The control system also provided a continuous display of joint rates and speeds, which were displayed on monitors located on the flight deck of the Shuttle. As with any control system, the GPC could be overridden and the astronaut could operate the joints individually from the flight deck.

Thermal protection system

A white and close-fitting insulated blanket worked with thermostatically controlled heaters to keep the Canadarm at acceptable temperatures in the vacuum of space, protecting its components from the intense heat of the sun's rays, or from extreme cold when the arm was in shadow.

Thermal protection system
Length 15.2 m (50 ft.)
Diameter 38 cm (15 in.)
Weight on Earth 410 kg (905 lbs.)
Speed of movement - unloaded: 60 cm a second
- loaded: 6 cm a second
Upper and lower arm booms Carbon composite material
Wrist joint Three degrees of movement
(pitch/yaw/roll)
Elbow joint One degree of movement (pitch)
Shoulder joint Two degrees of movement (pitch/yaw)
Translational hand controller Right, up, down forward, and backward movement of the arm
Rotational hand controller Controls the pitch, roll, and yaw of the arm