# Tutorial Review on Space Manipulators for Space Debris Mitigation
## 2 On-Orbit Space Manipulators  
Space manipulator robotics has played a significant role on the ISS, which has installed on it three manipulator systems: the Canadian Mobile Servicing System (MSS), the Japanese Experiment Module Remote Manipulator System (JEMRMS), and the European Robotic Arm (ERA) [5]. The MSS includes the 17 m long 7 degree-of-freedom Space Station Remote Manipulator System (SSRMS) with its relocatable base (Table 1), which is comparable to the 11 m long 7 degree-of-freedom ERA with its relocatable base in contrast to the 10 m long 6 degree-of-freedom JEMRMS fixed to the JEM. Both SSRMS and ERA are symmetric about their elbows, with latching end effectors at the end of each three degree-of-freedom wrist enabling hand-over-hand relocatability. Both were designed for assembly and servicing, while JEMRMS was designed for experiment payload manipulation from a fixed location. If we compare SSRMS with its predecessor the Shuttle remote manipulator system (SRMS) (Table 2), the improvement in performance is evident. The special purpose dextrous manipulator (SPDM) is a bolt-on dual-arm fine manipulation system to the SSRMS comprising two 3.4 m long 7 degree-of-freedom arms with a positioning accuracy of 2 mm. Their joints are similarly configured with three shoulder joints (roll, pitch and yaw), an elbow pitch joint and three wrist joints (roll, pitch and yaw). Lighted cameras are mounted onto each forearm boom and at each end effector. These robot manipulators and their predecessor, the Shuttle RMS (SRMS), have been deployed for human servicing tasks in providing mobile but stable footings for astronauts and, latterly, more sophisticated manipulation tasks, primarily orbital replacement unit (ORU) exchange on the ISS. \begin{table}
\begin{tabular}{c c c c c} \hline \hline Link i & \(\theta_{i}\) (\({}^{\circ}\)) & \(\alpha_{i}\) (\({}^{\circ}\)) & \(\mathbf{a_{i}}\) (m) & \(\mathbf{d_{i}}\) (m) \\ \hline
1 & 90 & 90 & 0 & \(\mathrm{d_{1}}\) \\
2 & 90 & 90 & 0 & \(\mathrm{d_{2}}\) \\
3 & 0 & 0 & \(\mathrm{a_{3}}\) & 0 \\
4 & 0 & 0 & \(\mathrm{a_{4}}\) & 0 \\
5 & 180 & 90 & 0 & 0 \\
6 & \(-\)90 & 90 & 0 & \(\mathrm{d_{6}}\) \\
7 & 180 & 90 & 0 & \(\mathrm{d_{7}}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Denavit-Hartenburg kinematic parameters for the SSRMS manipulator (adapted from Ref. [5]). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Model** & **DOF** & \begin{tabular}{c} **Length** \\ **(m)** \\ \end{tabular} & \begin{tabular}{c} **Mass** \\ **(kg)** \\ \end{tabular} & \begin{tabular}{c} **Payload** \\ **(kg)** \\ \end{tabular} & \
\begin{tabular}{c} **Position/Orientation** \\ **Accuracy (cm/\({}^{\circ}\))** \\ \end{tabular} &
\begin{tabular}{c} **End Effector Velocity** \\ **(m/s) Unloaded/Loaded** \\ \end{tabular} \\ \hline SRMS & 6 & 15 & 480 & 30,000 & 5/1 & 0.6/0.06 \\ SSRMS & 7 & 17 & 1700 & 116,000 & 6.5/0.7 & 0.4/0.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: SRMS and SSRMS physical and performance parameters (adapted from Ref. [5]). One of the most sophisticated space robot platforms is the 150 kg Robonaut, which was delivered to the ISS as an intravehicular manipulator system in 2011 [6]. It is a teleoperated anthropomorphic design to impart astronaut extravehicular activity (EVA)-equivalence in a package that is similar in size to an EVA-suited astronaut with a reach of 0.7 m. It comprises two 7 degree-of-freedom arms with shoulder, elbow and wrist which mount two five-fingered hands, a head mounted onto a pan-tilt-verge orientable neck, a torso and a 6 degree of freedom (DOF) grappling leg. Robonaut's hand comprises 14 DOF including a forearm, a 2 DOF wrist and a 12 DOF hand (\(2\times 3\) DOF fingers, \(1\times 3\) DOF thumb, \(2\times 1\) DOF fingers plus a 1 DOF palm) mounted onto a 5 DOF arm. Each finger possesses a six-axis force sensor and force sensors at each joint to detect applied forces. Robonaut implements 16 sensors per joint and the forearm mounts 14 motors/harmonic drives, 12 circuit boards and the wiring harness.