# Design and Experimental Evaluation of an Electrorheological
## 1 Introduction
Often these MR-based devices were designed to demonstrate the potential of MR fluid-based designs but would be challenging to scale down for personal electronics due to the limiting size of the electromagnetic coil needed to generate feelable forces. [PERSON] et al. [41; 42; 43] addressed this challenge by engineering a button type device for providing congruent tactile and kinesthetic feedback in a sufficiently small form-factor for mobile devices. Similarly, [PERSON] et al. [44] designed a miniature haptic knob based on MR fluids for providing torque feedbacks when twisted. Unfortunately, despite modeling and optimization, designing MR actuators still requires precise manufacturing to produce the miniature coil. Electrorheological (ER) fluid has a field-dependent viscosity, much like MR fluid, but instead responds to applied electric fields, rather than magnetic fields. Similar to MR fluid, ER fluid-based devices feature fast response times, low power consumption, and stability. However, ER fluid relies on simpler electrical control; only two electrodes spaced approximately 1 mm apart are required. When compared to a coil, the electrode form factor enables thinner, more mobile designs [45; 46; 47]. Past works have investigated ER fluid for its ability to convey haptic feedback [48; 49; 50; 51]. However, previous studies have rarely focused on device miniaturization or providing combined tactile and kinesthetic feedback. [PERSON] et al. [52] demonstrated a thin, button-type actuator based on ER fluid. Designing button-type actuators has long been studied in human-computer interaction, but often requires combining separate actuators for each sensation or convincing the user of displacement through haptic illusions [53; 54; 55]. In contrast, using smart fluids enables a single mechanism for providing both sensations. Subsequent studies improved the design by activating the fluid in both squeeze and flow modes, resulting in a wide range of forces (2.4-6.2 N at maximum depth) and frequencies across the spectrum of human perception (0-300 Hz) [56; 57]. Furthermore, the device measured only 6 mm thick, making it a feasible prototype for mobile designs. To demonstrate this actuator in real-world applications, such as haptic rendering of compliant objects, a method of sensing the actuator's state is necessary to incorporate control of the sensation with respect to pressed depth [58]. In other words, incorporating a sensing capability of the ER actuator to measure the displacement of the actuator's contact surface is critically important to control the actuator's force output with respect to the user's finger position. Thus, the primary goals of this study are to embed a bending-sensitive sensor within a haptic button actuator based on ER fluid, where sensations are generated from fluid flow through charged electrodes due to the user's press, and to experimentally evaluate the ER haptic module with the embedded sensor. In the following sections, we first detail the design of the proposed haptic module consisting of an electrorheological haptic actuator with an embedded depth sensor to provide controllable feedback during indentation, creating a tunable button. We evaluate the performance of the module by first characterizing the sensor's bend-sensitive resistance. Then, we mechanically test the module's combined sensing and actuation performance and assess the module's ability to provide various haptic force profiles.