# Magnetorheological Fluid-Based Haptic Feedback Damper
## 1 Introduction  
Haptic feedback is a crucial perceptual modality, with its technology extensively employed in remote-sensing robotic systems, tele-surgery, and other fields [1]. Traditional haptic feedback systems primarily utilize motor drives or other actuating mechanisms, which are often considered as dampers. In current engineering and scientific research fields, traditional damping systems often suffer from problems such as limited adjustment range and slow response speed [2; 3]; therefore, the design and construction of a damper that can overcome these difficulties is of great significance to the development of haptic feedback technology. A magnetorheological fluid comprises magnetic particles, carrier fluids, and stabilizers [4], whose flow properties change significantly under the influence of a magnetic field. Specifically, in the absence of an external magnetic field, an MRF behaves as a low-viscosity Newtonian fluid [5]. However, under the influence of a strong magnetic field, an MRF is transformed into a Bingham fluid [6] characterized by high viscosity and low fluidity. The increase in viscosity imparts solid-like properties to the fluid, and this occurrence is termed the magnetorheological effect [7; 8]. Utilizing this effect, magnetorheological fluids (MRFs) have many applications in various fields, such as in the industrial sector, where external magnetic fields can be manipulated to regulate local flow resistance and pressure, thus maintaining bearing clearance under varying loads and enhancing bearing work stability [9; 10]. In the healthcare domain, MRFs can be employed for drug delivery or improvement of the tactile feedback experience for distant surgery [11; 12; 13]. In finishing processes, the incorporation of abrasive particles within an MRF results in a magnetorheological polishing fluid thatforms regular micro-abrasive heads under gradient magnetic fields that remove minute surface material from workpieces [14]. Thanks to its controllable and rapidly adjustable characteristics, applying the magnetorheological fluid to dampers may be able to provide an effective solution to the above problem [15; 16]. For example, in vehicle suspension systems, magnetorheological dampers can adjust the damping force in real time according to road conditions and vehicle speed to improve the driving stability and comfort of the driver and passengers [17]. In the vibration control of cable-stayed bridges, magnetorheological dampers can effectively suppress the vibration of the cable-stayed cables to improve the safety and service life of the bridges [18]. However, although research on magnetorheological fluids (MRFs) has achieved certain results, applying them to dampers still encounters many challenges and difficulties, such as the difficulty of precise control of the magnetic field applied to the MRFs, the influence of the MRF viscosity on the overall temperature, and the difficulty of decreasing the fabrication cost [19]. For this reason, this study explored the application of magnetorheological fluids in the field of dampers using a keyboard as an example. A keyboard is a common input device in which each key can be considered as a damper; thus, a keyboard was a capable vehicle for testing the magnetorheological fluid damper designed in this study. Among the traditional keyboards, membrane keyboards operate with low noise but are not good enough in terms of haptic response [20]; mechanical keyboards can provide a lot of haptic feedback through key displacement but add noise and monotonous tactile sensation. Moreover, the key damping force of both types of keyboards is fixed at the factory and cannot be adjusted. To address the limitations of traditional keyboards, we used magnetorheological fluids to change their damping force to increase the haptic interaction between humans and machines. In this study, a keyboard designed based on the magnetorheological fluid damper used a permanent magnet as the output unit of the magnetic field.