Electromagnetic Shock Absorbers

Automobiles and trucks have shock absorbers to damp out the vibration experienced due to roughness of the roads. However, energy in conventional shock absorbers gets dissipated as heat and not used in any way. Regenerative electromagnetic shock absorbers provide means for recovering the energy dissipated in shock absorbers. Electromagnetic shock absorbers for potential use in vehicles are fabricated and tested for their performance. NOMENCLATURE Bi = Magnetic flux in tesla f = Frequency in Hz F = Force in N h = Height of pole ring in mm I = Current in amp K = Constant (nhBi) in volt-s/m L = Length in mm n = Number of turns / mm P = Power generated in watts Rc = Total resistance of coils in ohms Rl = Resistance of external load in ohms v = Velocity in m/s V = Voltage in volt INTRODUCTION Goldner et. al [1] proposed electromagnetic shock absorbers to transform the energy dissipated in shock absorbers into electrical power. Gupta [2] has studied the available energy from shock absorbers as cars and trucks are driven over various types of roads. Graves et. al [3] studied electromagnetic regenerative damping. They mention that energy regeneration is small and may be relevant only for electric vehicles. They also propose ways to amplify the motion of the shock in order to increase recoverable energy which on the other hand may have a negative effect on vehicle dynamics. Another interesting observation made by them is that device output voltage must be large enough to overcome the barrier potential of the storage device. Suda and Shiba [4] studied a hybrid suspension system where active control is adopted at low frequency and passive control by energy regenerative damper is adopted at high frequency. Fodor and Redfield [5] tried to design a regenerative damper. However, they came across the design limitation of amplifying mechanical devices input force which is necessary because available energy is low and a threshold for energy storage exists. Karnopp [6] studied the electromagnetics involved in designing permanent magnet linear motors used as variable mechanical dampers. However, until now no practical electromagnetic shock absorbers have been designed for automotive or truck usage. EM SHOCK An EM Shock has been fabricated. The shock consists of three assemblies: the permanent magnet assembly, the coil assembly, and the case assembly. Voltage is induced in the shock windings when the coil assembly moves relative to the magnet assemblies. The case assembly aligns and enables the piston-like motion between the coil and magnet assemblies. Generator Design The magnet assembly consists of an inner magnet stack surrounded concentrically by a larger diameter outer magnet stack. Each stack consists of three axially magnetized ring magnets separated by two iron-pole rings and two additional pole rings located at the ends of the stack. Sintered anisotropic NdFeB permanent magnets are used. The polarity of the magnets is chosen such that radial magnetic flux emanates from both sides of each iron pole and the flux of the inner pole rings adds to that of the outer rings. Note that the radial direction of the flux from the pole rings is opposite at opposite ends of each magnet ring. Also, the flux through the two end pole rings is about half that in the interior pole rings. For purposes of estimating performance, a 1 Tesla (T) radial flux density is assumed to emanate from the interior pole rings and 0.5 T from the end rings. The coil assembly consists of an inner coil surrounded concentrically by a larger diameter outer coil. Each coil consists of four continuously wound layers of #25 magnet wire with approximately 800 turns. However, each coil is broken into four sections, separated by insulators. In assembly, each coil section is centered on a different iron pole ring. The winding direction is reversed in adjacent section of each coil to accommodate the reversal in radial flux of adjacent pole rings. In other words, the induced voltage in each section of the coil has the same polarity. Voltage Generation To first order, the magnetic flux from the magnet assembly radially penetrates each coil section over the height of the pole ring, h = 10 mm. Thus, for coils with n = 8.26 turns/mm moving axially with a velocity v past a stationary pole emanating flux density, Bi, a voltage,