Canadarm2

NASA. Public domain (NASA / US government work).

Overview

Canadarm2, formally the Space Station Remote Manipulator System (SSRMS), is the primary manipulator of the International Space Station. It is one of four elements of the Mobile Servicing System, alongside the Mobile Base System, the Mobile Transporter and the Special Purpose Dexterous Manipulator [2]. It shares its heritage with the Shuttle Remote Manipulator System (SRMS) [7] but was redesigned around three requirements the SRMS did not carry: a 30 year on-orbit life against a two-week Shuttle mission with between-flight refurbishment, payloads to the mass of an orbiter, and on-orbit maintainability [5].

Specifications

Parameter	Value	Source
Length	17.6 m (57.3 ft)	[1]
Boom exterior diameter	0.35 m, composite	[13]
Dry mass	1497 kg	[13]
Degrees of freedom	7, all identical rotary joints	[1], [2]
Design payload requirement	116,000 kg	[1]
Design life on orbit	30 years	[1]
End effectors	2 identical Latching End Effectors, either usable as base	[1]
Power transfer through the end effector	1800 W average, 2500 W peak	[1]
Data buses	2 x MIL-STD-1553B	[1]
Video	up to 3 simultaneous composite NTSC channels	[1]
Cameras	4, one fixed at each end and one on a pan and tilt at each elbow side	[2]
Lights	4, integrated with the cameras	[2]
Positioning accuracy for berthing	5 to 10 cm	[2]

Mission profile

Parameter	Value	Source
Host	International Space Station	[12]
Station orbital replacement units designated for robotic servicing	about 250	[2]

Kinematics

Seven identical joints give a range of travel of about plus or minus 270 degrees per joint [1], [2], extended to plus or minus 281 degrees at the hard stops [1]. Six joints are sufficient to place an end effector at an arbitrary pose; the seventh makes the arm kinematically redundant, so a given end-effector pose corresponds to a continuous family of joint configurations and the arm can be routed around obstructions and through singularity-free paths [6]. Redundancy resolution is performed by the control system rather than commanded by the operator.

Both ends carry identical Latching End Effectors and either can act as base. The arm relocates by grappling a Power Data Grapple Fixture with the free end, releasing the old base, and inverting its kinematic model. The base and tip end effectors are of identical design, which is what gives the arm the operational flexibility to use either end as tip or base and so to relocate itself on the station [1]. For travel along the truss the arm rides that base on the Mobile Transporter [2]. The transporter concept behind it was traded in 1987 against a station design life of 30 years, payloads to 120,000 kg and limited on-orbit servicing capability [5].

Joints and actuators

Each joint uses a brushless permanent-magnet DC motor through a two-stage speed reducer with an overall ratio of 1845:1, with the joint parameters below [1].

Joint parameter	Value
Gear reduction	1845:1, two stage
Motor	brushless permanent magnet DC
Servo torque output	1044 N m minimum
Brake torque, both joint motor modules engaged	1630 N m minimum
Maximum joint angular velocity	5.0 deg/s
Range of travel	about +/-270 deg, hard stops at +/-281 deg

Each joint carries two identical joint motor modules, only one driven at a time, with the brakes of both engaging and disengaging together [1]. That arrangement, not a single fault-tolerant actuator, supports the 30 year life requirement: a failed module is switched out rather than repaired. Redundancy is carried through the whole arm as primary and secondary strings in power, video and control, with a back-up command string giving three control paths [2].

Joints are line-replaceable on orbit: the joint drive module and the end effector snare and rigidize mechanisms are all on-orbit replaceable units [1]. Joint components are qualified over wide temperature limits, with four levels specified:

Component	Tightest specified range	Qualification range
Gears and bearings	-25 to 135 C	-50 to 155 C
Motor	-25 to 180 C	-50 to 200 C
Electronics	-25 to 99 C	-50 to 120 C
Cables and connectors	-70 to 135 C	-86 to 151 C (max)

Source: [1].

Those ranges have to be held across roughly 34,000 eclipse cycles per 5.8 years of exposure, the rate measured on LDEF, where thermal cycling produced microcracking in composites and delamination of some coatings [8]. Gearbox lubricant must hold its film at 1e-4 to 1e-7 torr, where vapor pressure rather than viscosity governs the choice of fluid: a multiply alkylated cyclopentane sits at 2.3e-3 Pa at 150 C against 3.5e-6 Pa for a perfluoropolyalkylether [11].

Latching End Effector

Capture is a three-stage sequence. Snare cables inside the end effector rotate to draw the grapple fixture probe onto the axis, a rigid ring carriage then rigidizes the interface, and four externally mounted latches engage to add preload and to allow umbilical connectors to mate for power, data and video pass-through [1], [2].

Parameter	Value	Source
Force-moment sensing	6 axis, at the end effector	[2]
Snare	3 s	[1]
Rigidize	25 s	[1]
Latch	60 s	[1]
Umbilical mate	60 s	[1]
Snare plus rigidize, fast capture mode for free flyers	within 30 s	[1]

The capture envelope accommodates 0.1 m of axial and plus or minus 0.1 m of radial misalignment of the grapple probe, with plus or minus 15 degrees in roll and plus or minus 10 degrees in pitch and yaw [1]. Interface loads are limited by design to 950 N m of bending moment when snared and rigidized with no separation, rising to 3120 N m about any axis at 3 degrees of separation. Structural margins are a yield factor of 1.1, an ultimate factor of 1.5 and a fatigue scatter factor of 4 [1].

The latches carry the load path for large payloads; the snare alone does not [1]. A visiting vehicle is therefore captured in fast mode, which completes snare and rigidize within 30 s against the 148 s of the full snare, rigidize, latch and umbilical mate sequence, and is latched afterwards, because the vehicle holds its relative position under its own control only for a bounded window.

Payload handling performance

Condition	Tip velocity	Tip rate	Stopping distance
Unloaded arm	0.37 m/s	4.0 deg/s	4.0 m
20,900 kg payload	0.022 m/s	0.24 deg/s	0.61 m
116,000 kg payload	0.012 m/s	0.04 deg/s	1.09 m

Source: [1].

Tip speed falls by a factor of about 30 between the unloaded arm and the heaviest design payload, and stopping distance is not monotonic in payload mass: the unloaded arm coasts 4.0 m because it is fast, while a 116,000 kg payload coasts 1.09 m because it is slow [1]. Both numbers set the clearance that must be planned around structure the payload passes.

Force sensing and vision

Force-moment sensors at the end effectors measure contact loads in six axes [2]. They are the operator’s primary quantitative feedback, because camera views give poor depth resolution at the scale of a 20 tonne payload, and the sensor bounds the interface loads listed above [1]. Precise position is computed photogrammetrically by the Space Vision System from targets placed on the payload [2].

Operator interface

The arm is flown from Robotic Work Stations on orbit [2]. The workstation control layout was settled by evaluation at NASA JSC Building 9 from 18 to 28 January 1994 with nine NASA astronauts and one CSA astronaut, comparing hardware switches, software controls, keypad entry and on-screen controls for each function [4]. Emergency stop, brake on and off, camera pan, tilt and zoom, backup drive and backup Latching End Effector release were all required to remain hardware controls rather than software ones.

Ground control

Round-trip communication delay to the station is 5 to 6 seconds or more [3]. That rules out continuous force-reflecting teleoperation and drives a move-and-wait strategy in which the task is planned on the ground and a bounded segment is executed on orbit. The MARCO ground control demonstration flew a two-layer architecture against exactly this constraint: a task-directed planning layer deciding what to do, and a sensor-based autonomous execution layer deciding how, with visual servoing on PDGF markers closing the position loop on orbit rather than from the ground.

The same delay is why free-flyer capture is planned around a defined capture window rather than flown reactively: the vehicle holds a fixed relative position and the arm closes on it.

Environment

External hardware on the station is exposed to the LEO environment described on the microgravity environment page: a meteoroid population with an average impact speed of 19 km/s [9], a trapped-particle single event effects environment that governs avionics design at 400 km and 51.6 degrees [10], and atomic oxygen erosion on ram-facing surfaces.

Successor

Canadarm3 is planned for the lunar Gateway. Round-trip light time at lunar distance permits supervision but not the direct hand-controller operation used on the ISS, so Canadarm3 is designed for greater onboard autonomy.