Issue 32

N. Golinelli et alii, Frattura ed Integrità Strutturale, 32 (2015) 13-23; DOI: 10.3221/IGF-ESIS.32.02 18 (a) (b) (c) (d) (e) (f) Figure 6 : Custom components. Piston head (a) , flange (b, c) , rod (d) and bottom-rod with end plug (e) . Coupling between rod and flange (f) . Design of the magnetic circuit The aim in the design of a magnetic circuit is to determine the necessary amp-turn (NI) able to develop the required magnetic field and therefore the required damping forces. An optimal design requires to reach the desired magnetic field induction in the fluid gap while minimize the energy lost in steel flux conduit and region of non-working area. The entire circuit should have low reluctance, so soft iron or high permeability steel should be used. For an MR liquid, the permeability may be quite low, as shown in Fig. 4c. At high flux density, the iron may saturate, and be a limiting factor, so the cross-section of the iron must be adequate all around the magnetic circuit. The total flux in the circuit is the same at all sections around the circuit, so the critical point of the iron is the part with the lower cross-sectional area. The number of coils may vary to meet system requirements. Fig. 7a shows the most common configuration in which the flux lines flow around a single coil. The last two configurations in Fig. 7 have multiple coils and similar characteristics, except for the polarity of the magnetic field. In configuration Fig. 7b, the magnetic flux lines have all one only direction from the center of the solenoid. In configuration Fig. 7c, there is a trade-off between the different coils, which also affects the circuit length. The main advantage of this solution is a decrease of the overall inductance of the circuit that allows, compared to other, less response time of the same device. (a) (b) (c) Figure 7 : Coil configurations. Single coil (a) , coherent multiple coils (b) , incoherent multiple coils (c) . The typical design process for a magnetic circuit can be summarized as follow [14]:  Determine the magnetic induction B mrf in the MR fluid to give the desired yield stress τ B .  Determine the magnetic field intensity H mrf by using the B-H relationship of the fluid.  The magnetic induction flux is given by   mrf mrf B A , in which A mrf is the effective area of activation of the fluid. Since the magnetic induction flux remains constant through all the circuit length, calculate the magnetic induction in the steel B steel :