# Detecting and Controlling Slip through Estimation and Control of the Sliding Velocity
## 6 Experiments
This time, the block rested horizontally on the picking desk (Figure 12a), and the robot did not grasp it in its CoG so that a gravitational torque was applied on the fingertips. The results are shown in Figure 13. Once again, as soon as the lift began, the observer estimated a slippage velocity (bottom plot), and the controller responded by increasing the grasping force (top plot, blue line). Even if the small black was lighter than the big one, after the lift (Figure 12b), the grasping force reached similar values (5 N). This was because this time, the torsional moment (top plot, magenta line) also had to be sustained. The effects of the torsional load were also evident in the estimated CoR position \(\bar{c}\). In the previous experiment, it was saturated to 1.5, but now, the CoR estimation algorithm computed a much lower value, i.e., \(\bar{c}=0.4\). This means that, if the grasping force decreased, the slider would be more prone to a rotation than a translation. Additionally, in this experiment, the control algorithm was stressed at \(t=17\) s by applying disturbances (Figure 12b). Even in this different configuration, the results were analogous to the previous experiment. As soon as the disturbances were applied, the observer detected slippage velocity peaks, and the controller responded by increasing the grasping force to avoid the slippage. Figure 11: First experiment: lifting the big resin block. **Top** plot: grasping force. **Middle** plot: slippage indicator (blue) and estimated \(\bar{c}\) (red). **Bottom** plot: estimated slippage velocity around the CoR \(\bar{\omega}\). Figure 12: Snapshots of the second experiment: **(a)** grasp configuration; **(b)** after lifting; (c) disturbance applied by a human operator. The third experiment shows how the manipulation ability developed in this work can be used in a collaborative scenario. The robot had to grasp a plastic bottle that was knocked over the picking desk (Figure 14a) and had to hand it over to a human partner in a vertical orientation. The results are shown in Figure 15. The first phase was similar to the previous experiment. The robot grasped the bottle far from its CoG so that the slippage controller had to also counteract the gravitational torque. This time, after the lift (\(t=6\,\mathrm{s}\), Figure 14b), the measured torque (top plot, magenta line) increased up to \(15\,\mathrm{N}\mathrm{m}\mathrm{m}\), and the slippage controller automatically modulated the grasping force to \(16\,\mathrm{N}\) (top plot, blue line). In this configuration, the estimated CoR position was very low (\(\tilde{c}=0.3\), bottom plot). Thanks to this information, we can tell in advance that, if the grasping force is decreased, the manipulated object would start to rotationally slip about the grasp axis, and the translational slippage would be negligible. This means that it is possible to activate the pivoting control modality to reorient the object in hand to the vertical configuration depicted in Figure 14c. At \(t=11.5\,\mathrm{s}\) the pivoting began, the grasp force exponentially decreased, and the bottle started to rotate. Since the object was moving, the viscous friction caused an initial increment of the torsional torque, but, as the object rotated toward the vertical configuration, the gravitational torque decreased. While the bottle rotated, the CoR increased, and as soon as \(\tilde{c}>1\) (at \(t=16\,\mathrm{s}\)), the pivoting maneuver was considered completed and the grasp force stopped its descent since the object had reached the configuration in Figure 14c and can be handed over to the human partner. The final handover phase (Figure 14d) and the control of the forces exchanged between the robot and the human was carried out with the algorithm in [31]. The fourth experiment demonstrates the slipping avoidance algorithm's ability to automatically choose the grasp force when handling objects of very different weights. The objective of this experiment was to hand the aluminum can over to a human partner.