# A Tiny Haptic Knob Based on Magnetorheological Fluids
## 1 Introduction  
Currently, in many consumer electronic devices, a knob is commonly used to handle and manipulate several functions. Because a knob interface does not have start and end positions (it has only a direction of rotation), users can relatively identify the values or items listed in order. For this reason, knob interfaces facilitate a more accurate control compared with other input interfaces, such as buttons or switches, which are often used to perform \"on/off\" functions. One of the most important factors in designing input devices is delivering a timely and useful response to users through haptic sensation. Let us consider the case where a driver manipulates a navigation menu consisting of 10 submenus using a haptic knob during driving. If the haptic knob creates a haptic pulse every time the cursor meets a new submenu in the navigation menu, the driver can control the menu more intuitively, with less reliance on sight. Furthermore, if the proposed knob provides different resistive torque depending on the targets, the knob can be a better input interface. Several studies regarding haptic knobs have been conducted. [PERSON] et al. proposed a rotary haptic knob using a DC motor and brake [1]. [PERSON] et al. suggested a haptic dial system for multimodal prototyping using a DC motor [2]. They also developed a remote controller based on the haptic knob to intuitively interact with a target device [3]. [PERSON] et al. presented a haptic knob using an ultrasonic motor and a power clutch [4]. [PERSON] et al. established a design principle for handheld haptic devices, developed a haptic knob, and implemented a portable handheld device based on the haptic knob [5]. [PERSON] et al. proposed a low-cost one-degree-of-freedom (1-DOF) haptic knob that can be used for rehabilitation or virtual training [6]. Although the devices in these studies were designed to provide a variety of torque feedback, they were too large to be inserted into tiny consumer electronic devices. One possible approach to construct a tiny haptic knob (or rotary-type haptic actuator) is using magnetorheological (MR) fluids. MR fluid-based haptic actuators have several advantages. One advantage is that the actuators based on MR fluids are operated passively; hence, there is no instability problem in the actuators [7]. Another advantage is that MR fluid-based actuators can be developed not only in free form but can also generate high resistive force/torque under low input voltage in real time [8; 9; 10]. For these reasons, many researchers have focused on the development of rotary-type haptic actuators using MR fluids. [PERSON] et al. developed a hybrid rotary-type haptic actuator consisting of a DC motor and MR fluid to achieve a high torque density [11]. Furthermore, they designed a powerful MR actuator with a relatively high dynamic range to develop a multifinger interface [12]. [PERSON] et al. proposed the design of an MR fluid-based bidirectional haptic actuator with two coils placed directly on each side of the housing [13]. [PERSON] et al. designed an MR fluid-based haptic actuator that can solve the stiction problem [14]. [PERSON] et al. developed linear and rotary actuators and applied them to a 3-DOF spherical force feedback system [15]. Although these modules showed the feasibility of MR fluids for tiny rotary-type haptic knobs, they are still too large to be embedded into small consumer electronic devices. MR fluids have three operating modes: flow mode, shear mode, and squeeze mode [16]. In the flow mode, MR fluids flow between two stationary plates. In the shear mode, MR fluids flow between two plates moving relative to one another. In the squeeze mode, MR fluids resist the normal directional displacement of the pole owing to a perpendicular external normal force. To maximize the resistive torque generated by MR fluids in a tiny-size device, we must consider a new structure in which multiple operating modes of MR fluids contribute to the actuation simultaneously.