# Design and Experimental Evaluation of an Electrorheological
## 1 Introduction  
The recent rise in accessible, consumer-grade virtual reality has increased demand for haptic devices that can enhance user experience. Haptic feedback provides a more immersive experience by stimulating users through an additional and often underutilized sensory channel: touch. The ability to feel and manipulate objects in an environment (real or virtual), such as feeling the stiffness of squeezing a tennis ball in one's hand or feeling the opposing force of pushing open a heavy door, gives users a more realistic sensory experience. This is especially true when combined with the more common visual and auditory feedback [1, 2]. Therefore, haptic feedback can benefit a variety of applications, including not only virtual reality, but also machine teleoperation, training tools and simulators, gaming, accessibility, and entertainment [3, 4, 5, 6, 7, 8]. For example, commercial touchscreen devices use haptic actuators to provide vibrations to create eyes-free feedback that complements user input, such as vibrations during typing to emulate the feeling of a physical keyboard [9, 10]. Vibrating actuators are capable of notifying users and even invoking haptic illusions [11, 12, 13], but they fall short of delivering truly realistic haptic feedback. To truly provide realistic haptic sensations, devices must be able to take full advantage of the wide sensory range afforded by our sense of touch. To do so, the combination of two feedback modalities is necessary: (1) tactile and (2) kinesthetic [14]. Tactile feedback is sensed by mechanoreceptors in the skin and provides information related to vibration, surface roughness, and textures [15, 16]. Kinesthetic feedback is perceived as forces and torques by muscles, tendons, and joints,and provides information about position, movement, and force [17; 18]. Therefore, when designing interfaces and interactions based on touch, both modes of haptic feedback should be present to convey realistic haptic sensations by covering the entire spectrum of human haptic perception. Unfortunately, much of the research on new haptic devices focuses on presenting either kinesthetic or tactile feedback, rather than devices to provide both at once. The primary reason for this divide is that while miniature tactile actuators have been widely studied and commercialized, the development of miniature kinesthetic actuators has proven challenging because of physical limitations. Most of the kinesthetic devices proposed so far have adopted bulky commercial motors or pneumatic pumps to generate force feedback [19; 20; 21; 22; 23]. However, kinesthetic-scale motors and pumps are limited to tethered applications due to their large size and power draw. To overcome the limitations of traditional actuators, actuators based on smart materials have been identified as an attractive alternative because of their tunable properties, mechanical simplicity, and conformability [24; 25; 26; 27; 28; 29; 30]. For example, piezoelectric actuators have been used to generate vibrotactile feedback [31; 32]; however, the forces produced are too small to invoke kinesthetic sensations. Shape memory alloys, another smart material, can provide kinesthetic forces by bending and pulling, but their refresh time is not sufficiently fast to generate high frequency tactile sensations [33; 34; 35]. Additionally, shape memory alloys often introduce unwanted heat into the system, which may interfere with the intended haptic sensation. Thus, we identify a gap between these two materials' properties: to achieve concurrent tactile and kinesthetic sensations, the ideal material must be capable of generating high forces at fast rates, as well as easy to control with a subtle stimulus. Under these criteria, smart fluids are an attractive candidate for their rapid, stable response and ability to generate high yield stresses through their tunable viscosity. Magnetorheological (MR) fluid, one such smart fluid, has been the basis for numerous haptic devices [36; 37; 38; 39; 40].