# Effect of Pole Head Profile on Torque Performance of a Disc-Type Multipole Magnetorheological Brake
## 1 Introduction  
Magnetorheological fluids are intelligent smart materials that are controllable, adjustable and vary in their rheological properties (elasticity, viscosity or plasticity) in the application of a magnetic field supplied by a coil current. These fluids can also exhibit fast, strong and reversible changes [1]. Magnetorheological fluids are suspensions of miniature particles, made up of carbonyl iron or powdered iron or iron-cobalt alloy, that are transformed into a solid gel and line up to form chain-like structures in the direction of the magnetic flux paths after a few milliseconds when the magnetic field is applied [2]. Magnetorheological fluids have characteristic properties that enable a quick response time and stiffness, which are directly proportional to the applied magnetic field and also can achieve a wide range of apparent viscosity, which is controllable since the strength of the magnetic field can be varied with the help of an input current to the electromagnet [3, 4]. MR fluid holds great potential in electromechanical interfaces such as brakes, where the operative function is in direct shear mode. These smart fluids are confined in a small gapbetween a rotor and a stator, which are perpendicular to the rotational axis [5]. The magnetic field lines are radially directed to penetrate the MR fluid gap to generate controllable shear stress that causes friction on the rotating disk surfaces, thus generating a retarding torque [6].Giving priority to ensuring magnetorheological brake rapid response, a wide range of research has been carried out by many researchers over the last decades to enhance output braking torque when the increased magnetic field is limited to magnetic saturation in the working area of the fluid. The magnetorheological brake was initially designed in 2003 by [PERSON] and [PERSON] with a single disc configuration model, indicating a progressive increase in brake torque with an increase in magnetic field strength [7]. The torque produced in a disc-type brake depends on various parameters such as disc radius, disc thickness and the composition of MR fluid. These parameters affect the shear stress generated that sources friction in the surfaces of the rotating disk while braking due to the change in viscosity under the applied magnetic field [8]. A dual disc design was proposed to achieve high torque density when fluid was inserted between the rotating and fixed discs. [PERSON] et al. studies revealed that MR fluid gains fast braking responses due to exerted shear force acting perpendicularly to the dual rotating discs [9]. [PERSON] et al. experimented and proved that the multidisc concept can satisfy the requirement of maximum braking torque by showing a good linear relationship under the condition of magnetic saturation [10]. Further, these studies were extended to single pole multiple discs concept in which a model proposed by [PERSON] et al. exhibited advantages of rapid response, magnetic medium characteristics and arrangement structure of the magnetic flux flow [11]. To increase the active area of the MR fluid, the multiple electromagnetic poles concept was introduced in MR brakes after a single electromagnetic pole reached the limits of torque enhancement due to the restriction of the magnetic field's strength. In 2013, [PERSON] et al. proposed this novel design and outcomes that explain the increased magnetic flux flow through pole heads [12]. Later on, [PERSON] et al. introduced a model of six electromagnetic poles with bilayer MR fluid by merging concepts of the multipole and multilayering to study the magnetic field superposition's influence on the braking torque, and this study revealed that, due to geometric dimensional parameters at each pole head, the working area of the fluid increases and exhibits great enhancement in braking torque and TVR [13]. [PERSON] et al.