# Tutorial Review on Space Manipulators for Space Debris Mitigation
## 5 Space Manipulator Operations--Evolution from Teleoperation to Autonomy
Careful attention must be paid to human-computer interfacing (HCI) technologies for robotic servicer missions [83]. HCI defines the nature of ground station support to the servicer and is critical to the function of the robotic spacecraft. In teleoperative mode, the ground operator controls the remote robot directly through bilateral reflection of operator movements, e.g., Lunakhods 1 and 2 on the moon. End-effector (egocentric) referenced control gives enhanced performance over fixed world (exocentric) referenced control [84]. A minimum time of 20 min is required to capture a tumbling target--this will require more than one ground station for LEO operations which typically has an 8-min ground contact window. Alternatively, a GEO relay such as TDRSS may be used, but this introduces significant signal time delays. There are several challenges to teleoperative on-orbit servicing missions--time delay and jitter [85]. Time delays in teleoperation of a slave manipulator at a remote site by a human operator (master) invoke move-and-wait behaviours in which human input is followed by waiting for the response before initiating further corrective inputs. This is inefficient. PID controllers are sensitive to time delays due to the integral term as it reacts too quickly to errors. The Smith predictor is a model-based predictive controller [86]. It feeds back the predicted response immediately and the prediction error once the real response has been measured after the time delay. There are two feedback loops--a normal outer feedback loop that feeds the output of the plant back to the input subject to time delay and an inner feedback loop which predicts the current unobserved output of the plant. The Smith predictor at the master (operator) side in relation to the slave (remote) side is given by:  
\[G_{m}(s)=\hat{G}_{s}(s)e^{-s.\Delta t}\text{= }\frac{K}{1+\tau s}e^{-s \Delta t} \tag{7}\]  
where \(\Delta t\) = two-way time delay, \(G_{m}(s)\) = delayed plant, \(\hat{G}_{s}(s)\) = predicted plant model without delay, \(\tau\) = plant time constant. Stability requires an accurate model of dynamics to minimise prediction errors and stability is not guaranteed unless model errors are zero. The Smith predictor compensates for known constant time delays of short duration. It is sensitive to mismatches between the actual \(G_{s}(s)\) and estimated \(\hat{G}_{s}(s)\). A Kalman filter may be employed to reduce the estimation error. The most common approach to overcoming the problem of time delays is to employ predictive graphical displays [87; 88]. Predictive displays involve graphical models of the slave robot and its environment which project its predicted behaviour to compensate for time delays. The higher the fidelity of the model to reality the better the prediction but this has computational costs. It has been demonstrated with the ETS-VII satellite that 7 s time delays in the ground teleoperation control loop can be accommodated readily through predictive graphics based on environmental models [89]. However, this assumes an environment that exhibits only very low-speed dynamic behaviour and is highly predictable. ETS-VII's environment was a controlled target, but this will not be so for active debris removal. Reduction of the 5-7 s time delay in GEO-based teleoperation may be achieved through dedicated high-capacity lines from the ground station directly connected to the antenna (rather than across traditional internet routing and protocol overheads), which can reduce round trip time delays to GEO to \(<\)1 s. There will still be time delays however for LEO operation and beyond through TDRSS. Time delays between the transmission of the commanding signal to the remote robot and the return of feedback to the operator beyond 1-2 s introduce stability issues. The implementation of force/tactile/haptic feedback control introduces further significant instability difficulties inherent in such time delays. Direct bilateral force teleoperation is based on haptic feedback from the remote robot (slave) to the human operator (master) [90].