# A Human-Inspired Control Strategy for Improving Seamless Robot-To-Human Handovers
## 3 Results and Discussion
### Evaluation of the Robotic Impedance Control Based on the HRH  
This section proposes how to achieve the appropriate impedance force control based on the performance evaluations of the robotic dynamic response in the human-to-robot handovers. According to the test procedure, a set of 20 participants has undertaken three repetition sets of each handover test under the required variable conditions. They are defined as a receiver agent and have to grasp the object using one hand and twisting the object is not allowed. The object was fabricated as a bottle-shaped object with 60 mm diameter, 270 mm length and a total mass of 1.25 N. The HRH experimental apparatus is shown in Figure 8. The HSR is required to grasp an object and move facing to the object handover location, while the human performs standstill in comfortable positions on opposite sides of the robot. Once the object is manipulated to the transfer location the human is then allowed to grab the object and start naturally pulling the object before taking the responsibility for the object finally. In the meantime, interactive force (\(F_{int}\)) profiles against time (\(t\)) presenting how the robot given regulates the bilateral force before releasing the object to be transferred to the receiver was measured in real-time. The circumstances affecting the robot dynamic behaviour compose of spring stiffness (\(K_{d}\)) and damping factor (\(B_{d}\)), whereas \(M_{d}\) was strictly specified as a constant parameter of 0.25 Kg. Initially, a set of preliminary tests was undertaken to establish the relationship between the interactive force and handover time under various spring stiffness (\(K_{d}\)) and damping factor (\(B_{d}\)) values. The goals of the experiments are to understand how the influential factors affect the interactive forces in the HRH system and to determine the ranges of \(B_{d}\) and \(K_{d}\) used in the substantive HRH. The test outcomes were demonstrated in Table 1. According to human active comfort, the impedance control system should  
Figure 7: (a) Robot displacement profile in the HRH test and (b) Bode magnitude and phase plot. have lower stiffness and damping coefficient values. Conflictingly, in terms of active safety, it should have lower stiffness but a higher damping coefficient. Based on the recommendations of the preliminary study, as the humans generally felt comfortable in the natural physical HRH, \(B_{d}\) should be set as 50 and 75 \((\text{NmS/rad})\) and the range of \(K_{d}\) should be varied from 0 to 400 \((\text{Nm/rad})\), respectively. The substantive HRH experiments were successfully carried out by the 20 selected subjects. A comparison of the average interactive force and handover time under the several conditions based on the HRH tasks is given in Table 2. After careful observation with regard to the HRH test results, it was complicated to select the set of optimized impedance parameters as its nature necessitates human judgments. After asking the participants, two desirable choices of the impedance parameters were selected based on the recommendations, which consist of Set 1: \(B_{d}(50)\), \(K_{d}\) (200) and \(M_{d}(0.25)\) and Set 2: \(B_{d}(75)\), \(K_{d}(50)\) and \(M_{d}(0.25)\), as highlighted in Table 2. A questionnaire technique was subsequently employed to gather information from the human participants. Rating scale questions were introduced and specified with the five-point rating scale from 1 (very dissatisfied) to 5 (very satisfied). The two questions were used to ask the participants after participating in the HHH and HRH based on both impedance parameter sets as follows:  
1. How do you compare the qualitative performance of the HHH to that of the HRH using the parameter Set 1?