# Design of a New Bilayer Multipole Electromagnetic Brake System for a Haptic Interface
## 1 Introduction  
Haptic devices have become an integral part of modern human-computer interfaces, where they simulate real environments by providing realistic sensations back to a distant human operator. Recently, many useful haptic devices have spanned various applications, not limited to the automation [1], robotics [2] and medical [3] fields. Transparency and stability are the main requirements in the design of a haptic device that guarantee its high-fidelity feedback display to correctly simulate the physical environment [4]. Haptic interfaces mainly consist of active [5] and/or passive [6] actuators; those have the tendency to provide counter sensations as a result of human interaction. The key considerations in designing these actuators for haptic applications are the output forces/torques/vibrations, response time, power consumption, volume, etc. The active actuators, like electric motors though, have a fast response but they can only display a limited range of virtual impedance due to their relatively low torque to volume ratios. Their active nature also makes them vulnerable to the instabilities when interacting with the discretized components of the haptic rendering loop. Passive actuators, because of their energy-dissipating-only characteristics, are inherently stable. Their comparatively higher torque to volume ratios can display wide ranges of virtual impedance from the environment. Electromagnetism-based passive brakes are popular choices in many haptic applications. Various designs of these brakes have been proposed in the literature that exploit the generation of eddy currents proportional to rotational velocity [7; 8], or the change in shear viscosity of either electrorheological (ER) fluids [9] or magnetorheological (MR) fluids [10] as a function of electric or magnetic potential, respectively. The eddy current-based brakes work for high input powers and are not suitable for low velocity applications. The rheological fluid-based brakes can provide high braking torques proportional to the sheer stress values of corresponding working fluids. In order tomaximize the controllable braking torques in a given volume, some special designs have been proposed by researches. Those include cylindrical-shaped radial-arrangements of multiple electromagnetic poles in the form of layers [11; 12]. The superposition principle of magnetic flux across the poles and layers magnifies the range of braking torque. However, these brakes tend to achieve material saturation easily that limits their operation to low ranges of input power. As the rotor is submerged in the working fluid, the operator constantly feels viscous forces at zero-input; thus, distorting the transparency of the haptic display. The unavoidable wear losses due to shear contact between fluid particles and the seal surface lead to a potential risk of fluid leakage, restricting their use in the direct human-contact applications, such as invasive surgery, rehabilitation devices, etc. [13]. Such devices require one to follow the additional safety measures mentioned as Means of Operator Protection (MOOP) in IEC-6061 standards [14]. The Coulomb friction-based electromagnetic brakes, on the other hand, utilize the variable reluctance principle to generate attraction forces on the spring-connected rotor and push it against a friction surface to apply braking. The initial isolation of rotor and stator allows free motion at zero-input. A commercially available disk-shaped design of the electromagnetic brake provides friction torque for single degree of freedom (DOF) rotation [15]. A compact EM actuator with a dual piston presses a rotor section against a conformable stator shell to provide friction braking [16]. A spherical joint-like EM actuator implements Coulomb friction-based braking in a 3-dof haptic device [17]. In these brakes, the design allows limited controllability of available braking, and the single thick solenoid coil can potentially increase the inductive time constant.