PHENOMENOLOGICAL MODEL FOR MAGNETORHEOLOGICAL DAMPERS By B. F. Spencer Jr., l Member, ASCE, S. J. Dyke,z Associate Member, ASCE, M. K. Sain,3 Member, ASCE, and J. D. Carlson 4 ABSTRACT: Semiactive control devices have received significant attention in recent years because they offer the adaptability of active control devices without requiring the associated large power sources. Magnetorheo- logical (MR) dampers are semiactive control devices that use MR fluids to produce controllable dampers. They potentially offer highly reliable operation and can be viewed as fail-safe in that they become passive dampers should the control hardware malfunction. To develop control algorithms that take full advantage of the unique features of the MR damper, models must be developed that can adequately characterize the damper's intrinsic nonlinear behavior. Following a review of several idealized mechanical models for controllable fluid dampers, a new model is proposed that can effectively portray the behavior of a typical MR damper. Comparison with experimental results for a prototype damper indicates that the model is accurate over a wide range of operating conditions and is adequate for control design and analysis. INTRODUCTION Passive and active control systems represent the two ends of the spectrum in the use of supplemental damping strategies for response reduction in civil engineering structures subjected to strong earthquakes and severe winds [see, for example, Soong (1990); Soong et al. (1991); Housner and Masri (1990, 1993); Housner et al. (1994)]. On the other hand, semiactive control systems combine the best features of both approaches, offering the reliability of passive devices, yet maintaining the versatility and adaptability of fully active systems. According to presently accepted definitions, a semiactive control device is one that has properties that can be adjusted in real time but cannot input energy into the system being controlled. Such devices typically have very low power requirements, which is particularly critical during seismic events when the main power source to the structure may fail. Moreover, because many active control systems for civil engineering applications operate primarily to modify structural damping, preliminary studies indicate that semiactive control strategies can poten- tially achieve the majority of the performance of fully active systems. Various semiactive devices have been proposed that utilize forces generated by surface friction or viscous fluids to dissipate vibratory energy in a structural system. Examples of such devices that have been considered for civil engineering applications include variable orifice dampers [e.g., Shinozuka et al. (1992); Kawashima et al. (1992); Mizuno et al. (1992); Constantinou et al. (1993); Symans et al. (1994); Sack and Patten (1994); Sack et al. (1994); Patten et al. (1994); Kurata et al. (1994)], friction controllable braces [e.g., Akbay and Aktan (1990, 1991); Dowdell and Cherry (1994); Cherry (1994)], friction controllable isolators [e.g., Feng and Shino- zuka (1990); Kawashima et al. (1992)] and variable stiffness devices (Kobori et al. 1993; Inaudi and Kelly 1994). Another class of semiactive devices uses controllable fluids. The essential characteristic of controllable fluids is their ability 'Prof., Dept. of Civ. Engrg. and Geological Sci., Univ. of Notre Dame, Notre Dame, IN 46556. 2Asst Prof., Dept. of Civ. Engrg., Washington Univ., St. Louis, MO 63130. 'Freimann Prof., Dept. of Electr. Engrg., Univ. of Notre Dame, Notre Dame, IN. 4Engrg. Fellow, Lord Corp., Mech. Products Div., Thomas Lord Res. Ctr., 405 Gregson Dr., Cary, NC 27511-7900. Note. Associate Editor: Sarni F. Masri. Discussion open until August I, 1997. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on October 10, 1995. This paper is part of the JourlUll of Engineering Mechanics, Vol. 123, No.3, March, 1997. ©ASCE, ISSN 0733-9399/97/0003-0230- 0238/$4.00 + $4.00 + $.50 per page. Paper No. 11799. 230/ JOURNAL OF ENGINEERING MECHANICS / MARCH 1997 to reversibly change from a free-flowing, linear viscous fluid to a semisolid with a controllable yield strength in millisec- onds when exposed to an electric or magnetic field. Two fluids that are viable contenders for development of controllable dampers are electrorheological (ER) fluids and magnetorheo- logical (MR) fluids. Although the discovery of both ER and MR fluids dates back to the late 1940s [U.S. Patent No. 2,417,850 (1947); Winslow (1949); Rabinow (1948)], research programs have primarily concentrated on ER fluids. A number of researchers have considered ER fluid dampers for civil en- gineering applications [e.g., Ehrgott and Masri (1992, 1994); Gavin and Hanson (1994); Gavin et al. (I 994a,b); Gavin (1994); Makris et al. (1995, 1996); Gavin et al. (1996a,b)]. Recently developed MR fluids appear to be an attractive alternative to ER fluids for use in controllable fluid dampers (Carlson 1994; Carlson and Weiss 1994; Carlson et al. 1995; also refer to http://www.rheonetic.com/mrfluid/). MR fluids are the magnetic analogs of ER fluids and typically consist of micron-sized, magnetically polarizable particles dispersed in a carrier medium such as mineral or silicone oil. When a mag- netic field is applied to the fluids, particle chains form, and the fluid becomes a semisolid and exhibits viscoplastic be- havior similar to that of ER fluids. Transition to rheological equilibrium can be achieved in a few milliseconds, allowing construction of devices with high bandwidth. Additionally, Carlson and Weiss (1994) indicated that the achievable yield stress of an MR fluid is an order of magnitude greater than its ER counterpart and that MR fluids can operate over a wide range of temperatures with only slight variations in the yield stress. Moreover, MR fluids are not sensitive to impurities such as are commonly encountered during manufacturing and us- age, and little particle/carrier fluid separation takes place in MR fluids under common flow conditions. Further, a wider choice of additives (surfactants, dispersants, friction modifiers, antiwear agents, etc.) can generally be used with MR fluids to enhance stability, seal life, bearing life, etc., since electro- chemistry does not affect the magneto-polarization mecha- nism. The MR fluid can be readily controlled with a low volt- age (e.g., V), current-driven power supply outputting only 1- 2 A. Recognizing the significant potential of devices based on MR fluids, a number of researchers have recently undertaken their study [see, for example, Hossis and Lemaire (1991); Carlson (1994); Carlson and Weiss (1994); Carlson et al. (1995); Demchuk (1993); Dyke et al. (1996a,b); Fedorov (1992); Grasselli et al. (1993); Kabakov and Pabat (1990); Kashevskii (1990); Kordonsky (1993a,b); Kordonsky et al. (1990, 1993); Lemaire et al. (1994); Minagawa et al. (1994); Pabat (1990); Savost'yanov (1992); Shulman et aI. (1986, 1989); Spencer et al. (1996a,b)]. To evaluate the usefulness of MR devices in response re- J. Eng. Mech. 1997.123:230-238. Downloaded from ascelibrary.org by Indian Inst of Technology - Roorkee on 09/24/15. Copyright ASCE. For personal use only; all rights reserved.