1 INTRODUCTION

Starting in the mid 1990s, a number of investigatorshave explored the possibility of using semi-activevibration control devices in civil engineering struc-tures to mitigate the effects of undesirable motionsdue to hazardous wind or seismic loads. Semi-activedevices combine the best features of both passiveand active control devices. Semi-active devices of-fer most of the adaptability of full active systemswhile retaining much of the simplicity, reliabilityand cost effectiveness of passive systems. Becauseof their high force capacity, low power requirementand overall mechanical simplicity, magnetorheologi-cal (MR) fluid dampers have been identified as aparticularly attractive means of enabling semi-activecontrol in buildings and bridges. Recent laboratorystudies have shown that MR fluid dampers can bescaled to sizes appropriate for civil engineering ap-plications.

That MR fluids are suitable for achieving semi-active control beyond the laboratory has been dem-onstrated (Carlson et al. 1996, Mehri 2000, Conte2000, Jones 2000). Beginning with the commer-cialization of MR fluid rotary brakes for use in aero-bic exercise equipment in 1995 (Design News1995), application of MR fluid technology in real-world systems has grown steadily. The past twoyears have witnessed a rapid expansion of MR fluidtechnology into commercial applications well be-yond the development laboratory. MR fluid tech-nology has been embraced by a number of manu-

facturers for inclusion into a diverse spectrum ofproducts. To illustrate the present state of MR fluidtechnology, five recent applications wherein MRfluids have been fully developed into commercialproducts are reviewed. The MR fluid systems to bedescribed include: real-time, semi-active vibration control system

for heavy duty truck seating of which there areover 5000 systems presently in use (Design News1995),

adjustable MR fluid shock absorber for NASCARracing automobiles (Carrera 1998),

rotary MR brake that provides real-time force-feedback in a steer-by-wire system for industrialforklifts (Newton 1999),

rotary and linear MR dampers that are coupledwith industrial pneumatic actuators to enable low-cost position and velocity control (Mehri 2000),and

MR fluid damper used in an advanced prostheticknee to enable real-time motion control such thatgait automatically adapts to any condition (Jones2000).

These applications represent more than just a dem-onstration of basic MR fluid functionality. Each ofthese applications represents a fully field-provenMR device that embodies all of the necessary re-finements required to make it reliable, robust andlong-lived in actual service in real-world environ-ments that are quite demanding. Historical concernsabout sedimentation, abrasiveness and fluid durabil-

Magnetorheological fluids ready for real-time motion control

J. David CarlsonLord Corporation, Materials Division, Cary, North Carolina, USA

ABSTRACT: The past two years have witnessed a blossoming of the commercialization magnetorheologicalfluid technology. Magnetorheological fluids have now been embraced by a number of manufacturers for in-clusion into a diverse spectrum of products. To illustrate the present state of magnetorheological fluid tech-nology, this paper reviews five recent applications wherein the technology has been fully developed intocommercial products. As such, these applications represent more than just a laboratory demonstration of ba-sic functionality. Each of these applications fully embodies all of the necessary developments and refine-ments required to make these magnetorheological fluid devices long-lived in actual service in the field, ame-nable to mass production techniques, and available at a cost commensurate with the value perceived by theend user.

ity have been solved. These MR devices are alsoamenable to standard mass production techniquesand are commercially available today at pricescommensurate with the value perceived by the user.Certainly, much work remains before MR fluidbased semi-active control systems could be acceptedfor protecting civil engineering structural systemsagainst wind and seismic events. However, theproven success of MR fluids in other motion controlapplications indicates that the possibility of suchsystems is real.

2 SEMI-ACTIVE MR DAMPERS FOR SEATS

In early 1998 a real-time, semi-active vibration con-trol system became available for use in the seats ofClass 8 (eighteen wheeler) trucks (Lord 1998).Manufacturers of premium large trucks such asWestern Star and Freightliner offer the MR fluidbased Motion Master system as a standard option. Acomplete semi-active vibration control system in-cluding MR fluid damper, sensor, microprocessor,current driver and ancillary cables as shown in Fig-ure 1 is now offered as a retrofit kit to truck opera-tors and is available to anyone over the www for un-der $300 (Conte 2000). Today, over 5000 MR fluidbased, semi-active vibration control systems are inuse in heavy-duty, over-the-highway trucks in theUnited States. These systems receive high praisefrom the drivers who experience them. They rou-tinely deliver many hundreds of thousands of kilo-meters of service. Their robustness is illustrated bythe fact that there have been no failures in the field.

Figure 1. MR fluid based semi-active vibration control systemfor heavy-duty truck seats.

Figure 2. MR fluid based controllable shock absorber forNASCAR racing automobiles.

3 MR FLUID SHOCKS FOR RACING CARS

Late in 1998, MR fluid based, adjustable shock ab-sorbers for oval and dirt track racing automobileswere introduced by Carrera (Anderson 1998). AMagneShock by Carrera is shown in Figure 2.

Race-car operators are today able to purchaseMagneShock cockpit controlled, primary suspensiondampers for about $500. A set of four of these con-trollable MR fluid dampers eliminate the necessityof maintaining the usual stable of perhaps dozens ofdifferent passive hydraulic shock absorbers. Car op-erators no longer need to physically change shockabsorbers to optimize their vehicle for the conditionsof the track, weather or tire condition. Rather, theyneed only perform a few test laps while adjusting thecockpit controls for the MR fluid dampers. A singleset of four MR shocks may replace dozens of pas-sive hydraulic dampers. The driver can easily andquickly optimize the left-to-right and fore-to-aftdamping in order to optimize car performance on thetrack. Improved track times by as much as 2 secondsper mile have been realized (Leroy 2000).

4 STEER-BY-WIRE

This is an example of industrial haptics wherein asmall MR fluid rotary brake is being used to providereal-time sensory feedback to the operator of an in-dustrial forklift vehicle that is equipped with a steer-by-wire system.

The R14 vehicle, shown in Figure 3, is an allelectric forklift introduced by Linde earlier this year(Marjoram 2000). It is intended for close maneu-vering and manipulation in confined, clean-spacessuch as food handling warehouses with large drive-in freezers. There is no mechanical connection be-tween the steering wheel and the ground wheels.Steering is accomplished entirely by electrical con-trol. Rotation of the steering wheel turns an opticalencoder which supplies an electrical signal that istransmitted to the drive ground wheel and causes amotor to orient them in the desired direction. Thesteering wheel and the optical encoder are bothmounted the shaft of a MR brake. The brake pro-vides a variable amount of rotational resistance de-pending on the instantaneous vehicular motion andorientation of the ground wheels. Such tactile feed-back to the operator is necessary to insure stable op-eration. The MR brake and magnetic rotary encoderare packaged into a common package as shown inFigure 4 and mount directly to the dashboard of theforklift.

Figure 3. MR brake provides real-time force-feedback to theoperator in conjunction with steer-by-wire operation.

Hall effectrotary encoder

MR brake

Driveshaft forsteering wheel

Mounting studs

Figure 4. MR brake and rotary position encoder used in thesteer-by-wire application.

Pneumatic actuator

MR fluid damper

MR fluid valve

Figure 5. Linear pneumatic actuator coupled with MR fluiddamper having an external bypass valve to enable precise posi-tion and velocity control.

5 INDUSTRIAL PNEUMATICS

MR fluid rotary brakes and linear dampers are beingused in conjunction with pneumatic actuators to en-able open and closed-loop position and velocitycontrol with a precision not normally available withpneumatic systems. MR technology is able to con-tribute significant added value to pneumatic actua-tors. The inclusion of a simple MR fluid device isable to add stiffness to an otherwise compliant sys-tem overcoming the inherent compressibility of air.

Four different manufacturers have recently intro-duced pneumatic systems with an MR control op-tion: Parker, phd , Tol-O-Matic and Turn-Act. Fig-ures 5 and 6 show the Turn-Act and Parkerembodiments for MR control of rotary position ac-tuators.

MR rotary brake

Pneumaticrotary actuator

Figure 6. Rotary pneumatic actuator coupled with MR brake toenable precise position and velocity control.

6 ADVANCED PROSTHETICS

Perhaps the most exciting of all of the new MR fluiddevelopments is that of real-time controlled dampersfor use in advanced prosthetic devices. The ideahere is to use a small MR fluid damper to control, inreal-time, the motion of a prosthetic knee. Thebenefit is a more natural gate that can automaticallyadapt to virtually any condition.

The HIP or High Intelligence Prosthesis is anabove the knee prosthesis that has been introducedby Biedermann Motech this year (Matthis 2000,Carlson 2000, Jones 2000). The basic elements ofthe system are shown schematically in Figure 7. Agroup of sensors determines the instantaneous stateof the knee: knee angle, swing velocity, axial forceand moment. A microprocessor-based controllerdetermines the current needed to be applied to theMR fluid damper to allow for proper motion orlocking of the artificial knee based on the instanta-neous action being carried out by the user. Oncecalibrated to a specific user the system automaticallyadapts in real-time to the users walking speed, stairsor inclination. The HIP system allows the user tomove without having to consciously control what isgoing on with the prosthesis.

As shown in Figure 8, all of the electronics areintegrated into the mechanical structure of the pros-thetic knee along with the MR damper and batterypack. The entire system operates from a recharge-able battery that provides about two-days of opera-tion between charges. The HIP system with a MRfluid damper provides superior gait control over awider range of activity than does servo-motor con-trolled systems costing many times as much.

MR fluiddamper

Force andmomentsensors

Microprocessor

Battery

Positionand speedsensor

Figure 7. Above-knee prosthesis with real-time gait controlprovided by MR fluid damper.

MotionMasterMR Fluid Damper

Force and MomentSensors

Batteries

ControllerAnglular Position &Rate Sensor

Figure 8. Details of the Prolite Smart Magnetix knee prosthe-sis.

7 SUMMARY

Five MR fluid-based devices which exemplify thecurrent state of magnetorheological fluid technologyhave been described. These new applications arefully developed products that have traversed the fullcourse of product development and are today com-mercially products available to the consumer. All ofthe cited example applications are based on MRfluid. It is a commentary on the present state ofcontrollable fluid technology that no comparable ex-amples based on electrorheological fluid could beidentified.

REFERENCES

Anderson, R. 1998. Carrera MagneShock, Atlanta: Arre In-dustries.

Carlson, J.D. 2000. 1st Int. Conf. on Advanced Prosthetics,Newport Beach: Flex Foot.

Carlson, J.D. et al. 1996. Commercial Magneto-RheologicalFluid Devices. In W. A. Bullough (ed), Proc. 5th Int. Conf.on ER Fluids, MR Fluids and Assoc. Tech.: 20-28, Singa-pore: World Scientific.

Conte, F.R. 2000. Lords Motion Master. Owner Operator,Jan/Feb 2000: 39-40.

Design Engineering 1999. A Smoother Ride for Vehicle Seats.Design Engineering, Nov. 1999: 54-55.

Design News 1995. Brake Cuts Exercise-Equipment Cost, De-sign News, vol. 50(25): 28.

Jones, M. 2000. Lord Corp Brings New Life to Knee. TheCary News, Oct 28, 2000: 1B-4B.

Leroy, D.A. 2000,Lord Corp., pers. comm.Lord Corp. 1998. Motion Master Ride Management System.

pub no. PB8008a. Cary: Lord Corp.Matthis, W. 2000, Biedermann Motech Corp.., pers. comm.Mehri, D. 2000. Economical Actuator Provides Simple Precise

Motion Control. Design News, vol. 55(20): 63-65.Newton, S. 1999, Lansing Linde Corp., pers. comm.