Chinese Space Station Remote Manipulator System

Qiu Xin’an et al., Chinese Journal of Space Science 44(5):939 (2024) [3]; CC BY 4.0.

Overview

The Chinese Space Station Remote Manipulator System is a pair of seven-joint arms on the Tiangong space station. The core module manipulator, mounted on Tianhe and launched with it on 29 April 2021, is the larger of the two [3], [8]. The experiment module manipulator is mounted on Wentian and is used for finer work on exposed payloads [4], [5]. Both arms are of SSRMS configuration: three rotational joints at the shoulder, one at the elbow, three at the wrist, with link offsets at shoulder and wrist [5]. Both have identical end effectors at each end, so either end can act as the base and the arm can relocate itself across the station exterior by re-grappling target adapters [1], [8]. The two can be joined end to end into a single fourteen-degree-of-freedom chain [3], [9].

The core module manipulator is described by its developers as the first large space robotic system developed independently in China for engineering application, and as one of the four key technologies of the space station program [3]. It is the third seven-joint relocatable station arm to fly, after Canadarm2 and the European Robotic Arm, and the first Chinese system of the class [1].

Specifications

Capabilities of the machines. Values marked CMM are the core module manipulator, EMM the experiment module manipulator [2], [8].

Parameter	CMM	EMM	Source
Degrees of freedom	7	7	[2], [5]
Joint layout	3 shoulder, 1 elbow, 3 wrist	3 shoulder, 1 elbow, 3 wrist	[5]
Deployed length	10.2 m	5 m	[8]
Mass	738 kg	not published	[8]
Rated payload	25,000 kg	3,000 kg	[2]
Tip position accuracy	45 mm	10 mm	[2]
Tip orientation accuracy	1 degree	1 degree	[2]
Tip linear velocity, unloaded	0.3 m/s	0.2 m/s	[1], [2]
Tip linear velocity, loaded	0.02 m/s	0.03 m/s	[1], [2]
Tip angular velocity, unloaded	3 deg/s	3 deg/s	[1]
Tip angular velocity, loaded	0.15 deg/s	0.15 deg/s	[1]
End effectors	2, identical	2, identical	[1], [4]
Six-axis force/torque sensors	2	2	[4]
Combined chain reach, CMM plus EMM	over 15 m		[1]
Combined chain degrees of freedom	14		[3]

A separate figure of 0.6 m/s is given by CNSA for the maximum linear speed of motion of the core module manipulator [8]; the 0.3 m/s unloaded tip velocity in the table is the value tabulated in the reviewed literature [2].

The published EMM link dimensions run from base to tip.

Member	Length, mm
Base to shoulder	716.1
Shoulder offsets	430, 430
Lower boom	2080
Elbow offset	387
Upper boom	2080
Wrist offsets	430, 430
Wrist to tip	716.1

Source: [5].

The shoulder and wrist offsets are what make the inverse kinematics of this configuration analytically awkward, since the offset pose destabilizes solutions obtained by Denavit-Hartenberg methods [5], [6].

Mission profile

Event	Date	Note
Launch of CMM with Tianhe	2021-04-29	[3]
First on-orbit key-technology verification runs	2021	[3]
Tianzhou-2 transposition test	2022-01-06	47 minutes, started 06:12 Beijing time [7]
Launch of EMM with Wentian	2022-07-24	[2]
Combined dual-arm operation	from 2022	[3], [9]

In the Tianzhou-2 test the arm unlocked and separated the docked cargo spacecraft from Tianhe, moved it to a predetermined position, then reversed the sequence and returned it for re-docking and locking [7]. The China Manned Space Agency stated that this was the first time the arm had moved a large spacecraft in orbit, and that the run verified the feasibility of robotic module transposition ahead of the assembly of the station.

On-orbit tasks performed with the operating subsystem to 2024 comprise extravehicular crew support, relocation identification runs, exterior inspection, and the autonomous transposition of the cargo spacecraft [3]. The task set the arms were designed for is given as eight classes: module transposition and assisted docking, spacecraft capture and assisted docking, extravehicular activity support including carrying a crew member, exterior cargo handling, exterior inspection, maintenance of large exterior equipment, payload installation and retrieval, and status checking [3], [8].

Architecture

Each arm is a symmetric kinematic chain: end effector, wrist three-axis cluster, boom, elbow joint, boom, shoulder three-axis cluster, end effector [1], [5]. Symmetry is the design decision that produces the relocation capability, because the arm has no distinguished base [1]. In the operating subsystem the base is a software state rather than a mechanical one: the console identifies which end effector is the fixed end from the online status of the remote terminals on the bus and assigns the base accordingly [3].

The core module manipulator does not use a claw. Capture is made against a set of target adapters distributed over the station exterior and over the payloads to be handled, which the end effector engages in the manner of a plug [9]. The same adapters are the anchor points for relocation, so the population of adapters on the hull sets the reachable envelope of both arms [8], [9].

A six-axis force/torque sensor sits between each end effector and the modular interface of the arm [4]. The EMM sensor is a monolithic symmetric elastic element machined from Ti-6Al-4V, instrumented with resistance strain gauges in Wheatstone bridges, with the acquisition electronics, temperature channels and analogue reference channels housed inside the sensor body and reporting over RS-422 to the end effector. Its purpose is to supply the measurement on which impedance or compliance force control of the arm depends; without it the arm’s capacity to interact with the environment is described as greatly weakened.

Relocation

Relocation is the mobility mechanism of both arms. With the free end effector engaged on a target adapter, the current base end effector releases, and the arm walks end over end across the exterior [1], [8]. CNSA describes the motion as inchworm-like [8]. The capability is a property of the symmetric topology rather than of any dedicated mechanism [1].

The EMM can also work from the tip of the CMM instead of from a station anchor point, which extends tip reachability past 15 m [1]. In that configuration the pair form a fourteen-degree-of-freedom chain, and the operating subsystem supports both time-shared and coordinated control of the two arms in the combined state [3].

Power and thermal

Both arms are powered from the station bus [3]. No generation or storage figures are published for either arm in the reviewed literature [1], [2].

Thermal control of the EMM end effector, which carries the force/torque sensor, is a combination of passive control and an active heater circuit. The primary heater loop is controlled by a temperature relay; the backup loop is controlled by a temperature relay together with a thermistor. All thermistor sensors are arranged on the end effector [4]. The sensor also carries temperature channels on its own acquisition chain, both for the elastic element and for the circuit board, which are digitized alongside the force channels. Operating and survival temperature ranges are not published.

Compute and avionics

The arms are commanded through the space manipulator operating subsystem, which sits inside the core module and is built on a multi-level, multi-bus architecture combining Ethernet, MIL-STD-1553B and RS-422 [3]. The console controller communicates over 1553B with the station data management computer, the guidance navigation and control computer, the arm end effectors and the central controller, sending commands and collecting status; it also passes the six-axis force data from the fixed end of the large arm to the GNC computer. Ethernet carries simulation parameters, console telemetry and display parameters to the tracking and control network and drives the virtual simulation platform. RS-422 carries the operation panel, the translational hand controller and the rotational hand controller.

The console controller is a dual-computer cold standby pair [3]. To shorten the closed loop when the large arm is working at radius, and to cut the number of hops between the large and small arms, the system uses a dual-bus mechanism in which bus controller authority is switched between buses. In ground operation mode the console forwards its own telemetry to the data management bus on a fixed 1 s period [3]. At the far end of that bus tree the end effector aggregates its own force, torque and temperature channels over RS-422 from the sensor body [4].

Radiation tolerance of the EMM force/torque sensor electronics is handled by shielding plus part-level measures. The sensor electronics sit inside a 4 mm titanium alloy housing, and the internal dose is computed from that shielding thickness [4].

Autonomy

Both arms can be operated in an automated mode running stored sequences, or under remote control from the station or from the ground [1].

Path planning and reachability rest on the inverse kinematics of the seven-joint offset configuration, which admits infinitely many solutions and must be reduced before it can be solved [5], [6]. The published treatment of the Wentian arm fixes one joint angle and decouples the remainder, giving closed-form solutions when joints 1 and 7 or joints 2 and 6 are fixed and numerical solutions when joints 3, 4 or 5 are fixed; against a 1e-4 accuracy threshold the method succeeded on 99.79 percent of sampled poses, with the failures concentrated near singularities [5]. Joint limits have to be applied to the full solution set to discard over-limit branches.

Communications

Arm command and telemetry ride the station’s internal buses rather than a dedicated radio link. Regular arm telemetry is forwarded by the console to the data management computer for ground monitoring, and separately over the Ethernet interface to the tracking and control network [3]. Ground command is the normal operating path for both arms, which are described in the reviewed literature as operable in automated mode or under remote control from the station or from a ground station [1]. In ground operation mode the console acts as a data forwarder and all console keys and hand controllers except the mode-change and emergency keys are inhibited [3].

Sensing and cameras

Each arm carries cameras rather than science instruments. The core module manipulator has an elbow camera and end-mounted cameras [9]. The crew display arrangement is 1 plus 4: a large lower-left screen showing the single primary target monitoring camera, an upper-left screen showing four exterior camera feeds in quadrants, and a third screen to the astronaut’s right showing arm parameter instrumentation [3]. All three are liquid crystal touch displays. The console hand controllers also drive the elbow camera pan and tilt head and the end effector directly, as separate single-joint operations.

The force/torque sensors are the other sensing channel, and are the input to compliant operations at the end effector [4].

Modes of operation

The operating subsystem defines three modes of the human interface, switchable either by ground command or by the crew at the console, and separately distinguishes single-joint motion from end-pose motion [3]; the arms themselves also distinguish automated sequence execution from manual rate control [1].

Mode	Behavior
Ground operation	Default. Console forwards data only; ground issues commands and monitors. All console keys and hand controllers inhibited except mode change and emergency [3]
On-orbit operation	Console interprets key, touchscreen and hand controller actions, generates arm commands and issues them to the arm subsystem, while continuing to forward telemetry to the data management computer [3]
Simulation operation	Console generates commands but sends none to the arm; commands go over Ethernet to the virtual simulation platform, which drives a three-dimensional model. Used for on-orbit rehearsal and training [3]

The virtual simulation platform models the assembled station body, the large arm, the small arm, both experiment modules, the crew member and the foot restraints [3]. During on-orbit operation it displays the planned path of the arm ahead of the motion. Rehearsal in simulation carries the weight it does because ground testing of a manipulator this size is itself constrained: most large space manipulators cannot even lift themselves in 1 g, and each available method, suspension, air bearing support, neutral buoyancy and hardware in the loop, carries its own limitations [1].

Ground operations

The arms are operated from the ground by default and from the core module console when a crew task requires it [3]. The console comprises a controller, an operation panel, a translational hand controller and a rotational hand controller. Single-joint motion, elbow camera pointing and end effector actuation are commanded from the panel and controllers; end pose is commanded by the two hand controllers together, translation on one and orientation on the other.

The hand controllers are force-feedback devices built on a modified delta parallel mechanism, with active links, follower links, a moving platform and a fixed platform connected through revolute joints, chosen over a modified 3-RRR spherical parallel mechanism because the delta suits translational degrees of freedom where the spherical suits rotational ones [3]. Feedback torque is computed by a haptic rendering model under impedance control and applied to the operator’s hand through the handle motors, closed around handle position and motor current. Four force terms are summed: a repulsive term that grows as the arm tip approaches an obstacle and is zero beyond a set radius, a sinusoidal situation-awareness term whose frequency encodes slave-side state, a viscous term proportional to handle velocity that warns during arm motion, and a restoring term with a dead zone that removes handle jitter and returns the handle to null.

Ground verification of the subsystem ran through hardware-in-the-loop simulation, desktop soft-connection testing, a two-dimensional air-bearing table, and a three-dimensional suspension platform [3]. Ground-based test and validation of contact operations under gravity is a standing challenge for space manipulator systems generally, not a gap peculiar to this program [1].

Technologies developed

The transferable results reported by the program are the multi-bus console architecture with switchable bus controller authority, adopted to shorten the closed-loop period when the large arm works at radius [3]; the multimodal human-machine interface combining visual, auditory and haptic channels with a delta-mechanism force feedback handle and a four-term haptic rendering model; the on-orbit virtual simulation platform used for pre-task rehearsal in the same console loop that flies the arm; and the titanium six-axis force/torque sensor qualified for vibration, thermal vacuum and radiation for use at the end effector of a station arm [4].

Source limitations

The open literature on this system is thin. Capability numbers here come from two reviews [1], [2] and from Chinese government pages [8], [9]; the only primary engineering papers retrieved are on the operating subsystem [3], the end effector force sensor [4], and the kinematics of the small arm [5]. Joint-level design, actuator specification, structural material, power draw, thermal ranges and end effector capture envelope are not published in any source obtained.