# Lessons Learned from Investigating Robotics-Based, Human-like Testing of an Upper-Body Exoskeleton
## 3 Methods and Results
### Approach A: Direct Placement
#### 3.1.2 Implementation  
Initially, we attempted to leverage the anthropomorphic kinematic chain of the Panda to position its \"upper arm\" (composed of links 2 and 3) into the arm shell of the exoskeleton, as depicted in Figure 5 (left). This configuration would be advantageous because the anthropomorphic structure of the robot arm could be coupled with the exoskeleton while retaining sufficient degrees of freedom to perform tasks with the environment. Motion generation would benefit from this setup since there is a limited yet interpretable mapping from human motion to the kinematic chain. From the interface perspective, the contact would be modeled as rigid but capable of sliding and rotating inside the arm shell and identified as useful in the problem analysis. Moreover, the support of the exoskeleton would be meaningfully applied to the robot chain, allowing for reasonable interpretation of joint torques from the joints contributing to the lifting motion.  
However, the dimensions of the robot's links do not align with the dimensions of the arm shell. Additionally, finding a suitable position to place the robot inside the exoskeleton while ensuring a sufficiently large workspace for reasonable motion creation without collisions, especially with the exoskeleton itself, proved to be difficult. Another issue with the setup in Figure 5 (left) is that the robot only has joints 1 and 2 to move the exoskeleton. This limitation hinders human-like motion generation, as it can only create a spherical elbow motion and does not account for the complex motion generated by the human shoulder. This limitation is inherent in anthropomorphic standard manipulators, as additional degrees of freedom at the base link would be required to replicate the more versatile motions of the human shoulder.  
Figure 5: Placing a robot directly inside the exoskeleton presents challenges (see text). **Left**: positioning upper arm links 2 and 3 into the arm shell of the exoskeleton. **Middle** and **right**: placing the lower arm (link 5) into the arm shell.  
An alternative approach would be to deviate from exploiting the anthropomorphic structure of the robot and to place any other link inside the arm shell, establishing contact with the robotic arm. Figure 5 (middle and right) illustrates the idea of placing the \"lower arm\" (link 5) into the arm shell. While this approach retains the benefit of allowing contact in the interface to slide and move in the arm shell, the workspace is severely limited, making reasonable motion ranges impractical. The middle and right illustrations in Figure 5 show configurations with the lowest and highest possible lifted positions of the exoskeleton, highlighting the constrained workspace.  
Similar challenges arise with different placements of the robot, as additional environmental constraints, such as collision and singularity avoidance, need to be considered. Moreover, interpreting joint torque measurements becomes more challenging, as the topology of the applied support force undergoes significant changes. Consequently, the ability to leverage the anthropomorphic design of the robot for measurement interpretation is compromised.