In this paper we proposed a magneto-rheological fluid (MRF) brake manipulated by moving magnetic shield in which magnetic field is generated by permanent magnet (PM). With this movable shield mechanism as a switch, the magnetic field and the corresponding braking torque can be controlled continuously within small space. Furthermore, by changing the edge angle of the shield, the toque-displacement response can be modified. The size of the device is 270mm in diameter and 100mm in length. The estimated torque is 300Nm when excited by high strength NdFeB PMs, thus is suitable for rotary table of machining tool. A MRF is a smart material that consists of micro-sized magnetic particles dispersed in a carrier fluid such as oil. Without external magnetic field it appears as normal viscous fluid. With external magnetic field the magnetized particles link as chains that restrict the movement of the fluid, hence make the MRF resistive to shear force. This reaction is reversible and responses in several tens of millisecond [1-2]. Many devices such as damper, brake, etc. employed the MRF effect such that their properties can be controlled by changing magnetic fields [3-5]. In MRF brake/clutch the MRF fills the working gap between the fixed stator and the movable rotor, thus the shear stress formed between them and the resulting braking torque can be manipulated by changing magnetic fields [6]. The major two ways to produce magnetic field are electromagnet (EM) and PM. Excited by EM, relation of braking torque and magnetic field with MRF is explored [7] leading to automobile applications [8]. Although EM is easier for controlling the devices, it requires larger space and continuous electric supplement than PM to provide the same magnetic field strength, worsen by high power consumption leading to heat generation and fail-safe power failure issues. On the other hand, although PM is of high magnetic field and compact in size, and no heat and power consumption, the magnetic field control is much difficult. A PM excited MRF clutch in which the transmitted torque can be switched by moving the magnet is studied in [9-10]. In this paper a MRF brake module with mechanical movable shields as magnetic field switches is proposed. This device is designed for brake module of a rotary table for machining tool system. By moving blocks of ferromagnetism steel as magnetic shields, the magnetic field of PMs can be continuously controlled, similar to magnetic stands in which magnetic flux is confined within an iron case when the knob is turned off and vice versa. In this research we adopted this principle by using steel structure as shields. As shown in Fig.1 a pair of PMs is on the both sides of a typical disk type MRF brake module. Each magnet is encircled by two fixed steel shields and two movable ones. The 2D magnetic field of cross section of the device is simulated by Ansoft with 3D model built by this view revolving about the center axis. When the movable shields are closed, the magnets are encircled and thus no outward magnetic flux. When the shields are opened magnetic flux emits and magnetizes the MRF. Then braking torque is produced gradually in response to the opening displacement of the movable shields. Base on the resulting magnetic field the shear force can be obtain, and the braking torque is then calculated by integrating the shear force about its revolving axis. The outer diameter of the brake module is set to be 270mm for dimension restriction, the inner/outer diameters of PMs are 174/218mm and thickness is 24mm. A shield structure of 5mm in thickness is sufficient for magnetic shield purpose, and a displacement of 7mm between on and off states is enough for magnetic switch function. To separate the movable and fixed shields, an optimal edge angle is expected to alleviate the magnetic force between the movable and fixed shields such that the shields could be slip-out by smaller shear force rather than pull out directly by stronger normal force. Relationship of braking torque vs. displacement of movable shields for different edge angles is studied with results shown in Fig.2. The torque-displacement response varies by angle variation. When the edge angels are 0 ~ 45°, the curves increase rapidly and different angles result in similar effects. At angles above 45° the torque response rates decrease and become nearly linear to the displacement at the angle between 75 ~ 80° within the designed displacement range. For angles more than 80° the rates further decrease and when close to extreme value 90°, where almost no torque is observed for displacements less than 2mm, thus is not suitable for this design. The results indicated that braking torque can be manipulated by giving shield displacement as input and the torque response can be specified by shield edge angle. For manufacture consideration we adopted 60° for the first prototype. An infinite displacement of the moving shield results in a braking torque of 337.4Nm. When opening to 7mm, the resulting torque is 299.5Nm. This indicates that 90% of torque range can be controlled by moving these shields within 7mm. After proper mechanism designed for controlling the movable shields, the proposed MRF brake module will be manufactured and its performance result will be evaluated.
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