ELECTROMAGNETIC SHOCK ABSORBERS

Dr. Abhijit Gupta Dr. T. M. Mulcahy and Northern Illinois University Dr. J. R. Hull Mechanical Engineering Department Argonne National Laboratory DeKalb, IL 60115 Energy Technology Division Argonne, IL 60439

ABSTRACT 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,

V = n h v Bi, (1) is generated in each section of the coil. Assuming Bi = 1 T and the coil is at 0.01 m/s, then each middle section of the outer coil will generate an open-circuit voltage of 0.169 volts. Each middle section of the inner coil will produce, in proportion to its smaller diameter, a smaller voltage of 0.062 volts. The bottom and top sections of each coil will generate only half these voltages, since their Bi = 0.5 T. For both coils the total voltage is

V = K v(m/s) = 68.9v = 0.69 volts. (2) Damping Force When a straight wire of length L(m) conducts a current, I(A), and is subject to a magnetic field, Bi(T), normal to the wire, a force, F(N), is exerted on the wire of magnitude

F = I L Bi (3)

The direction of the force is normal to the wire and field. The damping force developed on the coil assembly of the EM shock is the sum of the forces exerted on each section, i, of the coil, and (3) is applicable because of the coil geometry and the radial directions of the flux. Already, L = n h and Bi = 0.5 T or 1 T are known for each section of the coils. The current I will be the same in all sections of the coils, but its magnitude depends on the impedance of the coil and the external load. For the frequency range of interest, 0<f<100 Hz, the inductive reactances of the EM coils are negligible in comparison to its resistance. The resistance of the inner coil is 9 ohms and the outer coil has a resistance of 22 ohms, for a total of Rc = 31 ohms. By combining (2) and (3) for every section of the coils, the total damping force is

F = K2 v / (Rl + Rc) (4)

where the impedance of the external load is assumed to be entirely resistive, Rl. The power developed in the shock coil is given by

P = K2 v2 Rc / (Rl + Rc)2 (5) The maximum damping force is developed when the external load is zero, Rl = 0. Maximum power occurs at the external load when Rl = Rc, and is equal to the power that occurs at the coil of the EM shock. Results EM shock fabricated at ANL was tested on a 300 lb electrodynamic shaker. The base of the shock was supported from a stand and the moving rod was attached to a stinger through an impedance head as shown in Fig 1. The shaker was run using sine dwell at certain frequencies. One end of the inner coil and one end of the outer coil were connected such that combined voltage can be measured. The other ends were connected with various resistances (0.1 Ω, 30 Ω, 50 Ω or open circuit). The EM shock was excited at two different levels 0.5g and 1 g at frequencies ranging from 10 Hz to 100 Hz.

Fig. 1 Shaker set up

The EM shock was tested at 1 g level with a 33-ohm external resistance (close to optimum resistance) and results are shown in Table 1. All values are RMS values.

Frequency in Hz

Velocity in mm/sec

Displacement in mm

Voltage across 33 ohm in volt

Power Generated in watts

10 110.38 1.757 3.082 0.2878 11 100.35 1.452 2.276 0.1570 12 91.99 1.22 2.09 0.1324 15 73.59 0.781 1.47 0.0655 20 55.19 0.439 1.333 0.0538 30 36.79 0.195 0.883 0.0236 40 27.60 0.11 0.673 0.0137 50 22.08 0.07 0.553 0.0093 60 18.40 0.049 0.475 0.0068 70 15.77 0.036 0.417 0.0053 80 13.80 0.027 0.372 0.0042 90 12.26 0.022 0.34 0.0035

100 11.04 0.018 0.31 0.0029

Table 1. Results for 33-ohm external resistance

Figure 2 shows power as function of velocity for the 33-ohm case.

Velocity vs power for 33 ohm case

0.00000.05000.10000.15000.20000.25000.30000.3500

0.00 20.00 40.00 60.00 80.00 100.00 120.00Velocity in mm/sec

Pow

er in

wat

ts

Figure 2. Velocity and power for 33-ohm external resistance

Measurements of open-circuit voltage were made for comparison to predictions shown in Table 2.

Velocity in m/s

Predicted Voltage

Measured Voltage

0.01 0.69 0.707 0.05 3.44 2.52

Table 2. Open circuit voltage (at 33 Ω)

Next, power generated was measured, by measuring the voltages across a known resistance (in this case 33 Ω) and compared with predictions. See Table 3.

Velocity in m/s

Predicted Power in watts

Measured Power in watts

0.01 0.0038 0.003 0.05 0.095 0.054

Table 3. Power generated with optimum resistance

(at 33 Ω) Conclusion The fabricated electromagnetic shock performed as expected. A larger magnetic field will be necessary if more power needs to be generated. Acknowledgment This work was supported by the Summer Faculty Research Participation Program of Argonne National Laboratory and by the U.S. Department of Energy, Office of Heavy Vehicle Technologies and Office of Advanced Automotive Technologies, under Contract W-31-109-Eng-38.

References 1. Goldner, R.B., Zerigian, P., and Hull, J.R., “A Preliminary

Study of Energy Recovery in Vehicles by Using Regenerative Magnetic Shock Absorbers,” SAE Transactions – J. Commercial Vehicles, Vol. 110. 2001, pp. 53-59.

2. Gupta, A., Various internal communications with Argonne

National Laboratory. 3. Graves, K.E., Iovenitti, P.G., and Toneich, D., “Electronic

Regenerative Damping in Vehicle Suspension Systems,” International Journal of Vehicle Designs, Vol. 24, Nos. 2/3, 2000, pp. 182-197.

4. Suda, Y. and Shiba, T., “A New Hybrid Suspension

System with Active Control and Energy Regeneration,” Vehicle System Dynamics Supplement, Vol. 25, 1996, pp. 641-654.

5. Fodor, M.G., and Redfield, R., “The Variable Linear

Transmissions for Regenerative Damping in Vehicle Suspension Control,” Vehicle System Dynamics, Vol. 22, 1993, pp. 1-20.

6. Karnopp, D., “Permanent Magnet Linear Motors Used as

Variable Mechanical Dampers for Vehicle Suspensions,” Vehicle System Dynamics, Vol. 18, 1989, pp. 187-200.