# Detecting and Controlling Slip through Estimation and Control of the Sliding Velocity
## 6 Experiments  
This section presents a series of experiments performed on a real system to evaluate the control strategy presented in Section 5. The experiments were carried out in a lab-scale in-store logistic scenario [28; 29]. The experimental setup for the manipulation experiments is depicted in Figure 8.  
Figure 7: Normalized LS with the corresponding values of \(\tilde{c}\). The grasping device was a commercial 1 DOF WSG50 parallel gripper by Weiss Robotics, DE, equipped with the SUNT Touch force/tactile sensor [8] designed and produced in our laboratory. The sensor has an accuracy of 0.1 N and 0.002 Nm in the measurement of forces and torsional moments, respectively. This end effector was mounted on a LBR iiwa robot by Kuka, DE, and the whole system was controlled via a ROS network. The gripper was commanded at 50 Hz, the fingertips provided the tactile data at 500 Hz, and the iiwa robot was controlled via the FRI interface at 1 kHz. Four objects were selected (Figure 9): two resin blocks (a big and a small one), a plastic bottle, and an aluminum can. We considered the objects rigid so that all the parameters, except \(\mu\) and \(\beta_{A}\), estimated in Section 4 could also be used with these objects. Obviously, not all the objects in Figure 9 were perfectly rigid, but we could approximate them as rigid compared to the sensor soft pad. The two remaining friction coefficients \(\mu\) and \(\beta_{A}\) could be roughly estimated online by rubbing the fingertips on the object and comparing the measured force with the robot velocity with a procedure similar to the one described in [30]. The object-dependent parameters are reported in Table 2. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Big Resin Block** & **Small Resin Block** & **Plastic Bottle** & **Aluminum Can** \\ \hline \(\mu\) & 0.85 & 0.7 & 0.85 & 0.75 \\ \(\beta_{A}\) (Ns/m) & \(1.24\cdot 10^{7}\) & \(1.825\cdot 10^{7}\) & \(1.963\cdot 10^{7}\) & \(2.121\cdot 10^{7}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Friction parameters estimated for the manipulated objects. Figure 8: Manipulated objects. From left to right: big resin block, small resin block, plastic bottle, aluminum can. The first experiment involved the big resin block. At the beginning, the block rested vertically on the picking desk, and the robot was commanded to grasp it above its CoG (Figure 10a) and lift it. As soon as the gripper grasped the object, the slipping avoidance algorithm was activated. The results are shown in Figure 11. At \(t=1\,\)s, the lift began, the sudden increment of the external load due to the object weight caused the velocity observer to detect a sliding velocity. Thanks to the dynamic contribution, the controller responded by increasing the grasp force up to \(5\,\)N. Since the object was grasped above the CoG and almost no gravitational torque was applied to the fingertip, after the lift (Figure 10b), the estimated normalized CoR position \(\tilde{c}\) was high (middle plot red line). The CoR should be theoretically infinity in this condition; however, to avoid numerical issues, we saturated the normalized CoR to an upper limit of \(1.5\). When the lift was completed and all the dynamic effects vanished, the static contribution stabilizes our slippage indicator \(\mathcal{S}\) (middle plot, blue line) to the reference value \(\tilde{\mathcal{S}}_{a}=0.8\). To test and stress our algorithm, at \(t=14\,\)s, a human operator applied various disturbance forces by hands on the resin block (Figure 10c,d). The observer detected such events as slippage velocity peaks that were counteracted by the dynamic contribution. At the same time, as soon as the disturbances were applied, the slippage indicator increased, and the static controller also helped to regulate the force to higher values. Figure 10: Snapshots of the first experiment: (**a**) grasp configuration; (**b**) after lifting; (**c**,**d**) disturbance applied by a human operator. Figure 9: Experimental setup. The second experiment was carried out with the lighter small resin block.