**4. Discussion**

The results showed that it is possible to use our test bench to measure the performance of industrial exoskeletons under repeatable conditions. The test bench is open access, it mostly requires commercial off-the-shelf components, and the parts that need some custom fabrication are simple, making it low cost (less than 800 USD), with easy implementation.

In the tests, it was found that dynamic measurements were increased in torque when compared with static measurements. That was expected since, in dynamic measurements, inertia and other forces also contribute to the actual measured values. This is relevant because it is not possible to compare two tests carried out under different dynamic conditions, while our device can provide the same testing conditions.

There were small variations in the torques measured in both tests. It is possible that those variations are due to the sliding of the biceps strap during movement. This underscores one of the contributions of this study: the torques effectively applied on the user's arm were not necessarily the torques measured directly in the exoskeleton because of the relative movement of the equipment to the trunk and the biceps.

The main advantage of using a standardized test that considers effectively transmitted torques for the exoskeletons is the possibility of comparison under the same conditions. In this way, it is possible to obtain the characteristics of each exoskeleton and the specific

angular positions in which it provides the higher torque levels. This information is critical to adapt the exoskeleton to the task and the user. For instance, if the task requires a certain shoulder angle, and the exoskeleton is not sufficiently assisting the user, it is possible to increase the torque level. It is also possible that the exoskeleton is providing an excessive torque in a certain position that demands extra user effort to maintain that position. Finally, the exoskeleton angle-torque curves provided by our test bench can be very useful to plan the introduction of exoskeletons in precision tasks: if the torque changes abruptly between two angular positions, this would be perceived by the user as a perturbation that may hinder precision performance.

One of the main aspects in the design of a test bench for industrial exoskeletons is the generalization of the attachment, because there are several exoskeletons in the market, and each one has its unique characteristics. In this study, we used MATE (v1.0, Comau, Turin, Italy) to validate the test bench. This exoskeleton fitted very easily for the test bench, which was designed to simulate a human arm. However, it is possible that other exoskeleton models may need further adaptations of the test bench.

It must be noted that the results for the static and dynamic tests show a compression force between 0◦ and −20◦. However, when using MATE, it is possible to feel that the spring changed its actuation direction, which should have been detected by the test bench by a transition to positive values. The reason for this behavior within the machine is believed to be due to the way the exoskeleton was fixed for testing. In the human body, that range of movement is followed by a slight torsion of the trunk and slight sliding of the exoskeleton, which was not reproduced in the machine. Thus, from 0◦ to −20◦, the exoskeleton started to offer resistance to the movement, which was registered as a compression force in that region, not as a traction force, as expected.

It is important to keep in mind that the goal of this research was to evaluate the test bench, and not the exoskeleton itself. Thus, there might be differences between the values presented here and other studies about the same exoskeleton, due to differences in the testing conditions. In this work, the shoulder translation joint was at one extreme of its range of motion. This underscores the need for testing devices that provide homogeneous testing conditions for benchmarking.

Actuation can be further improved to allow better dynamic response, and additional sensors, e.g., inertial or pressure sensors, could provide more information about the tests, such as the vibration during exoskeleton use or the pressure at the contact surfaces between the exoskeleton and the user.

It is also possible to include additional fixation points to measure a variety of exoskeletons with different sizes while avoiding extreme joint positions along with improving the beam load cell g remodeling of the connections with the machine.

Finally, we presented the design and validation of a test bench for industrial exoskeletons. This device allows for the implementation of standardized tests that can be useful to assess objectively one exoskeleton allowing the comparison between different exoskeletons. Moreover, the results from the test bench combined with measurements with subjects, considering physiology, biomechanics, and user's perception, would allow designing and/or choosing the best exoskeleton for a certain user and a given task.

**Author Contributions:** Conceptualization, A.F.-C., V.N.H. and D.d.M.R.; methodology, V.N.H. and D.d.M.R.; software, V.N.H.; validation, V.N.H. and D.d.M.R.; formal analysis, A.F.-C.; investigation, A.F.-C.; resources, A.F.-C.; data curation, A.F.-C.; writing—original draft preparation, C.T.; writing—review and editing, C.T.; visualization, C.T.; supervision, A.F.-C.; project administration, A.F.-C.; funding acquisition, A.F.-C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors thank Pedro Parik Americano and Victor Bartholomeu for the technical support with the electronics, and João Pedro Pinho, for providing the Myotrace for these measurements. We would like to thank Mercedes-Benz do Brasil for lending the exoskeletons for the tests.

**Conflicts of Interest:** The authors declare no conflict of interest.
