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.

The problem

A station manipulator has to move modules and visiting vehicles of tens of tonnes, support crew on extravehicular activity, and inspect and maintain exterior equipment, all from a base that is itself a moving structure [1], [40]. The load range is wide: the Chinese core arm is rated for 25,000 kg and moves at 0.02 m/s loaded, while the same machine moves at 0.3 m/s unloaded [1], [2]. The environment removes the support that terrestrial arms take for granted. Most large space manipulators cannot lift themselves in 1 g, so full-system ground verification is not possible and each substitute (suspension, air bearing support, neutral buoyancy, hardware in the loop) removes some dynamics and keeps others [1]. The arm also has to reach hardware spread over the whole station exterior. The design answer for the seven-joint class is a symmetric chain with an end effector at each end, so that the arm can walk across target adapters on the hull instead of relying on a fixed base [1], [40].

What has flown

The Chinese Space Station Remote Manipulator System is a pair of seven-joint arms on the Tiangong station. Both are of the SSRMS configuration: three rotational joints at the shoulder, one at the elbow and three at the wrist, with link offsets at shoulder and wrist [5]. The two can be joined end to end into a single fourteen-degree-of-freedom chain [3], [41].

The core module manipulator launched with the Tianhe module on 29 April 2021 [3], [40]. Its developers describe it as the first large space robotic system developed independently in China for engineering application [3]. Published figures for it are a 10.2 m deployed length, 738 kg mass, 25 t payload and 0.6 m/s maximum tip speed, all as agency statements with no stated measurement basis [40]. Tasks performed with the operating subsystem to 2024 comprise extravehicular crew support, relocation identification runs, exterior inspection and the transposition of a cargo spacecraft [3].

The first large-payload operation was the Tianzhou-2 transposition test on 6 January 2022 [39]. The arm unlocked and separated the docked cargo craft from Tianhe, moved it to a predetermined position, reversed the sequence and returned it for re-docking and locking, in 47 minutes from a 06:12 Beijing time start [39]. The China Manned Space Agency called it the first time the arm had operated a large spacecraft in orbit. The report gives no arm loading, accuracy or re-docking alignment data [39].

The experiment module manipulator launched with Wentian on 24 July 2022 and is the shorter arm, 5 m deployed and rated for 3,000 kg, used for finer work on exposed payloads [2], [40]. Its end effector carries a six-axis force/torque sensor machined from Ti-6Al-4V, with acquisition electronics inside a 4 mm titanium housing and a heater circuit for thermal control [4]. Operating and survival temperature ranges are not published [4].

Both arms are commanded through the space manipulator operating subsystem in the core module, which combines Ethernet, MIL-STD-1553B and RS-422 buses [3], [1]. The console controller is a dual-computer cold standby pair, and bus controller authority switches between buses to shorten the control loop when the large arm works at radius [3]. The arms accept automated command sequences and manual rate control, and the operating subsystem defaults to ground control [1]. Its modes are ground operation (console forwarding data only), on-orbit operation by the crew, and simulation operation, in which commands drive a virtual platform instead of the arm [3]. The crew console uses force-feedback hand controllers built on a modified delta parallel mechanism, with a haptic rendering model that sums repulsive, situation-awareness, viscous and restoring force terms [3].

Relocation is the mobility mechanism. With the free end effector engaged on a target adapter, the base end effector releases and the arm walks end over end, described by CNSA as inchworm-like [40], [41]. The base is a software state: the console identifies the fixed end from the online status of the remote terminals on the bus [3]. The experiment arm can also work from the tip of the core arm, extending reach past 15 m [1].

What is being tried now

None of the approaches below has a published on-orbit result on the Chinese arms. The status of each is stated per family.

Inverse kinematics for the offset seven-joint arm. The shoulder and wrist offsets destabilize Denavit-Hartenberg solutions, and the redundancy gives infinitely many solutions [5], [6]. A 2026 treatment of the Wentian arm fixes one joint angle: closed form when joints 1 and 7 or 2 and 6 are fixed, numerical when 3, 4 or 5 are fixed. In simulation it solved 99.79 percent of 10,000 sampled poses at a 1e-4 threshold in about 1.1 ms, against 85.44 percent and 499.1 ms for a plain Jacobian baseline [5]. An alignment-constraint solution for the same arm type was tested separately [6]. The two success rates use different baselines and thresholds and are not comparable [5]. Status: simulation only.

Operator interface and teleoperation under delay. The flown interface is the haptic console above [3]. Learning-based planning for delayed teleoperation is a proposal: a Soft Actor-Critic planner with state augmentation held a 0.94 task success rate at a delay of one simulation step on a free-floating base with a rotating target, where a mapping method fell to 0.41 at four steps [35]. The environment is MuJoCo with the authors’ own arm model [35]. Status: simulation only.

Visual servoing for capture and approach. A 2024 survey classifies position-based, image-based and hybrid visual servoing for servicing arms and finds most validation in simulation or laboratory setups, with a few runs on the CSA testbed [34]. The CSSRMS adapter-based capture is cooperative and does not require this class of method [41]. Status: laboratory and simulation for non-cooperative targets.

Uncertainty-aware goal selection. NASA Langley propagates joint covariance through the Jacobian and rejects goal states whose 99 percent ellipsoid is not enclosed in the capture envelope [38]. The case shown is a commercial arm with encoder error only, in a ground demonstration with no flight data [38]. Status: ground.

Learning and AI-enabled operations. A 2026 survey of 52 studies organizes simulation environments, datasets, planning and onboard deployment for space robots, and reports no first-party measurements [33]. A review of space autonomy states that spacecraft remain largely reliant on ground-in-the-loop planning with pre-scripted sequences [25]. Status: proposals and simulation.

Ground emulation of microgravity. Air-bearing floors give two translations and one rotation: the ESA ORBIT floor, and NASA floors of 21.3 by 29.9 m at Johnson and 13.4 by 26.2 m at Marshall [2]. Hardware-in-the-loop facilities pair a dynamics simulation with industrial arms; one 2023 facility reported only qualitative tracking deviation and had its 6 m rail still in procurement [36]. A 2026 underwater method time-scales a mission plan so a BlueROV2 shares the disturbance robustness degree of the space plan, shown on one run per mission [37]. The CSSRMS operating subsystem was verified through hardware-in-the-loop simulation, desktop soft-connection testing, a two-dimensional air-bearing table and a three-dimensional suspension platform [3]. Status: ground test.

The numbers

Values marked CMM are the core module manipulator, EMM the experiment module manipulator.

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	[40]
Mass	738 kg	not published	[40]
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]
Six-axis force/torque sensors	2	2	[4]
Combined chain reach	over 15 m		[1]
Combined chain degrees of freedom	14		[3]

CNSA gives 0.6 m/s as the maximum linear speed of the core arm [40]; the 0.3 m/s unloaded value in the table is the one tabulated in the reviewed literature [2].

EMM link dimensions, base to tip, in millimeters [5]:

Member	Length
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

Event	Date	Source
Launch of CMM with Tianhe	2021-04-29	[3]
Tianzhou-2 transposition test, 47 minutes	2022-01-06	[39]
Launch of EMM with Wentian	2022-07-24	[2]

Inverse kinematics comparison on the Wentian arm, 10,000 simulated poses [5]:

Method	Success rate	Mean solve time
Fixed joint angle decoupling	99.79 percent	1.1 ms
Plain Jacobian iteration	85.44 percent	499.1 ms

What is not established

Capability numbers come from two reviews [1], [2] and from Chinese government pages [40], [41]. The agency figures carry no test basis, and the reviews disagree with the agency on core arm length, 10.5 m [1] against 10.2 m [40]. Primary engineering papers exist only for the operating subsystem [3], the end effector force sensor [4] and the kinematics of the small arm [5], [6].

Joint-level design, actuator specification, structural material, power draw, thermal ranges and the end effector capture envelope are not published in any source obtained. The kinematics results are simulation only; the Wentian solver is conditional rather than a general closed form for the seven-joint system, and its baseline is a weak Jacobian iteration [5]. The Tianzhou-2 report gives duration but no loading, accuracy or alignment [39]. No equivalent of the contact-force accommodation results published for Western station arms exists for the Chinese arms, although the force/torque sensors supply the needed measurement [4]. No CSSRMS-specific measurement of base disturbance is published, and none of the ground emulation methods above has been reported as applied to the arms beyond the list in the operating-subsystem paper [3], [36].