# Design and Experimental Evaluation of an Electrorheological
## 4 Conclusions  
This article presents a closed-loop haptic module based on the tunable viscosity of ER fluids. By combining an ER haptic actuator with a bending sensor, a method for controlling the desired actuation force over the actuator's stroke is possible. In our experiments, we characterized our custom bending sensor and found it to have a nearly linear response with respect to bend angle, making it suitable for sensing the state of the actuator's indentation. In subsequent studies, we tested the sensor's static and dynamic response when embedded in the actuator. We investigated the haptic module's ability to give controlled outputs with respect to indentation depth. Specifically, we examined the system's ability to present tactile pulses, kinesthetic step-on, and mapping the frequency sensation to the pressed depth. The results of these studies indicate that the module can offer controllable haptic sensations. This work may lead to a haptic interface that can render both tactile and kinesthetic sensations while maintaining a compact form factor. We envision this being used to produce numerous effects (for example, rendering the stiffness and tactile properties of different deformable materials) in haptic applications, such as games, mobile devices, and simulations. To achieve haptic sensations beyond the simple outputs presented in this study, a control scheme that better accounts for the device's nonlinearity together with the unpredictable behaviors of real users is necessary [66; 67; 68; 69]. Additionally, we acknowledge that our sensor design is one of many techniques that may be used to detect the actuator's state, such as pressure-sensitive films directly integrated into the membrane's PDMS [70; 71] or flow sensors within the device itself [72; 73]. In addition to the proposed strain sensor, which tracks the interface's input, i.e., pressed depth, future work may incorporate a stress sensor to also monitor the device's force output in real-time. Traditionally, within the field of haptics, providing simultaneous kinesthetic and tactile feedback would require combining two different actuators, such as a motor or linear actuator and a linear resonant actuator, eccentric rotating mass, or piezoelectric motor. The  
Figure 12: Results of the mapped frequency experiment: (**a**) programmed frequency with respect to indentation depth; (**b**) force–depth curve a 4 kV square wave with depth-dependent frequency; (**c**) isolated force due to the ER effect. fast response time and controllable resistive force offered by smart fluids enable both modes of feedback in a single package, such as the ER actuator presented in this article. We believe that smart materials and structures may continue to contribute to the future of haptic interfaces, but future work will require additional engineering to further miniaturize the footprint of control electronics to enable more mobile applications. Additionally, drawing from advances in soft electronics and sensors may lead to more wearable use cases. Conceptualization, [PERSON]-H.K. and T.-H.Y. ; methodology, A.M. and [PERSON]-H.K. ; software, A.M.; validation, T.M. and S.-Y.W. ; formal analysis, A.M. and [PERSON]-H.K. ; investigation, A.M., [PERSON]-H.K., T.M., S.-Y.W. and T.-H.Y. ; resources, [PERSON]-H.K., S.-Y.W. and T.-H.Y. ; data curation, A.M.; writing--original draft preparation, A.M.; writing--review and editing, A.M. and [PERSON]-H.K. ; visualization, A.M. and T.M. ; supervision, [PERSON]-H.K. and T.-H.Y. All authors have read and agreed to the published version of the manuscript. This work was supported by Institute for Information and Communications Technology Promotion (IITP) grant funded by the Korea government (MSIT, Grant No. 2020-0-00003, Development of high piezoelectric coefficient composite and ultra-low power multilayered piezoelectric sensor/actuator multi-functional module). Not applicable. The authors would like to thank the Ohio Space Grant Consortium for providing a fellowship to a student author. The authors declare no conflict of interest.