seminar code
09-08-2014, 04:32 PM
Properties and Applications of Commercial Magnetorheological Fluids
[attachment=66826]
ABSTRACT
The rheological and magnetic properties of several commercial magnetorheological (MR) fluids are
presented and discussed. These fluids are compared using appropriate figures of merit based on
conventional design paradigms. Some contemporary applications of MR fluids are discussed. These
applications illustrate how various material properties may be balanced to provide optimal performance.
INTRODUCTION
Magnetorheological (MR) fluids are materials that respond to an applied magnetic field with a
change in rheological behavior. Typically, this change is manifested by the development of a yield stress
that monotonically increases with applied field. Interest in magnetorheological fluids derives from their
ability to provide simple, quiet, rapid-response interfaces between electronic controls and mechanical
systems. That magnetorheological fluids have the potential to radically change the way
electromechanical devices are designed and operated has long been recognized.
MR fluids are considerably less well known than their electrorheological (ER) fluid analogs. Both
fluids are non-colloidal suspensions of polarizable particles having a size on the order of a few microns.
The initial discovery and development of MR fluids and devices can be credited to Jacob Rabinow at the
US National Bureau of Standards (Rabinow, 1948a, 1948b, 1951) in the late 1940s. Interestingly, this
work was almost concurrent with Winslow's ER fluid work. The late 1940s and early 1950s actually
saw more patents and publications relating to MR than to ER fluids. Except for a flurry of interest after
their initial discovery, there has been scant information published about MR fluids. Only recently has a
resurgence in interest in MR fluids been seen (Shtarkman, 1991; Kordonsky, 1993; Weiss et al., 1993;
Carlson et al.,1994; Carlson, 1994; Carlson and Weiss, 1994). While the commercial success of ER
fluids has remained elusive, MR fluids have enjoyed recent commercial success. A number of MR
fluids and various MR fluid-based systems have been commercialized including an MR fluid brake for
use in the exercise industry (Anon., 1995; Chase, 1996), a controllable MR fluid damper for use in truck
seat suspensions (Carlson, Catanzarite and St.Clair, 1995; Lord, 1997) and an MR fluid shock absorber
for oval track automobile racing.
Active Fluid Volume and Device Aspect Ratio
While Equations (3) and (4) are certainly useful in the design of controllable fluid devices, they
often do not provide the best insight into the significance of the various parameters. It is often useful to
algebraically manipulate the above equations to provide a minimum active fluid volume (Duclos, 1988;
Carlson et al., 1994):
V = k η
τ2
λWm (5)
where k is a constant and V = Lwg can be regarded as the necessary active fluid volume in order to
achieve the desired control ratio λ at a required controllable mechanical power level Wm. For pressure
driven flow: k=12/c2
, λ=∆Pτ/∆Pη and Wm=Q ∆Pτ. For direct shear: k=1, λ=Fτ/Fη and Wm=Fτ S. It is
important to note that in both cases the minimum active fluid volume is proportional to the product of
three terms: a term that is a function of fluid material properties (η/τ2); the desired control ratio or
dynamic range λ; and the controlled mechanical power dissipation Wm sought.
Rheological Properties
The rheological properties of controllable fluids depend on concentration and density of particles,
particle size and shape distribution, properties of the carrier fluid, additional additives, applied field,
temperature, and other factors. The interdependency of all these factors is very complex, yet is
important in establishing methodologies to optimize the performance of these fluids for particular
applications.
The magnetorheological effect of the four MR fluids was measured on a custom rheometer using a
46 mm diameter parallel plate geometry set at a 1 mm gap. In the parallel plate geometry, shear rate
varies linearly across the fluid sample with the maximum shear rate occurring at the outer radius. The
rheometer is capable of applying greater than 1 Tesla through the fluid sample. Figure 2 shows the shear
stress in the MR fluids as a function of flux density at a maximum shear rate of 26 s-1. At such a low
shear rate, this shear stress data is approximately equivalent to the fluid yield stress as defined in Eq. (1).
At low flux densities, the fluid stress can be seen to exhibit a power law behavior. The approximate
power law index of 1.75 lies in the range of low to intermediate field behavior predicted by
contemporary models of magnetorheology. Both linear models and models accounting for nonlinear
magnetic effects such as particle saturation (Ginder, Davis and Elie, 1995; Jolly, Carlson and Muñoz,
1996) predict quadratic behavior at very low flux densities. The non-linear model proposed by Ginder,
Davis and Elie (1995) predicts a power law index of 1.5 at intermediate fields. Beyond flux densities of
about 0.2-0.3 Tesla, the effects of magnetic saturation are revealed as a departure from power law
behavior. The stress response ultimately plateaus as the MR fluids approach complete magnetic
saturation. As can be seen, the flux density at which this saturation occurs increases as the iron volume
fraction in the fluid increases.
In all MR fluid formulations optimized for a specific application or class of applications, the fluid
viscosity in the absence of a field is most significantly a function of the carrier oil, suspension agents,
and particle loading. The figures-of-merit described earlier are benefited by a low fluid viscosity, but
must be balanced with other fluid requirements such as temperature range and particle resuspendability.
Because of both the addition of suspension agents and changes in magnetic particle microstructure
during shear, most MR fluids exhibit significant shear thinning.
MR Fluid Figures of Merit
Figure 5 shows the figure of merit F1= τ2
/η as a function of shear rate for the four MR fluids. This
figure of merit reflects the dynamic range (or control ratio) of an MR device as well as required MR
fluid volume and power consumption. It is seen that as shear rate increases, the four fluids exhibit
significant improvements in F1 due to the substantial shear thinning character of these fluids. A broad
range of F1 is observed amongst the MR fluids resulting from the acute dependence of F1 on viscosity.
Figure 6 shows the figure of merit F2= τ2
/ηρ as a function of shear rate. This figure of merit is closely
related to F1 except that it further penalizes fluid density. With the exception of vertical shifts in the
data, F2 demonstrates much the same character as F1. It is interesting to note that, as measured by F2,
MRX-126PD is particularly well suited for low shear rate applications and MRX-242AS is better suited
for high shear rate applications. Both F1 and F2 illustrate that MR fluids in general exhibit better
rheological behavior at higher shear rates due to the shear thinning character.
Heavy Duty Vehicle Seat Suspensions
Recently a small, monotube MR fluid-based damper (shown in Fig. 8) has been commercialized for
use in a semi-active seat suspension system for large on- and off-highway vehicles (Carlson, Catanzarite
and St.Clair, 1995; Lord, 1997). In this application the MR damper represents enabling technology for a
variety of semi-active control schemes. This damper has also served as a testbed for developing
phenomenological device models (Spencer, et al., 1996; Pang et al., 1998).
“Seal-Less” Vibration Damper
A small, controllable MR fluid vibration damper that is being used for real-time, active-control of
damping in industrial applications is shown in Figure 11 (Carlson, Catanzarite and St.Clair, 1995). The
damper functions by moving a small steel disk or baffle in a chamber of MR fluid. Primary controlled
motion is axial although secondary lateral and flexing motions may also be accommodated. Damping
forces of 0 to ± 125 N are produced in the primary direction. This damper may be also be used as a
locking device. This damper does not require dynamic, sliding seals. The relatively small amplitudes
encountered (± 3 mm) allow the use of elastomeric rubber elements instead. Typically these dampers are
used as controllable dashpots in parallel with separate spring elements. Stiffness may also be added to
the elastomeric elements so that the damper functions as a controllable mount
CONCLUSIONS
The rheological, magnetic, and material properties of four commercial MR fluids have been
presented. Several figures of merit for MR fluids based on these properties were also computed and
presented. The examples presented illustrate that the properties and attributes of commercially available
MR fluids are wide-ranging. Some of these attributes are summarized in Table 5. It is evident that MR
fluids have evolved from laboratory curiosity to true engineering materials that involve engineering
trade-offs. The formulation of MR fluids involves the optimal balancing of properties for particular
applications or class of applications. Several applications were discussed to illustrate how various
material properties may be balanced
[attachment=66826]
ABSTRACT
The rheological and magnetic properties of several commercial magnetorheological (MR) fluids are
presented and discussed. These fluids are compared using appropriate figures of merit based on
conventional design paradigms. Some contemporary applications of MR fluids are discussed. These
applications illustrate how various material properties may be balanced to provide optimal performance.
INTRODUCTION
Magnetorheological (MR) fluids are materials that respond to an applied magnetic field with a
change in rheological behavior. Typically, this change is manifested by the development of a yield stress
that monotonically increases with applied field. Interest in magnetorheological fluids derives from their
ability to provide simple, quiet, rapid-response interfaces between electronic controls and mechanical
systems. That magnetorheological fluids have the potential to radically change the way
electromechanical devices are designed and operated has long been recognized.
MR fluids are considerably less well known than their electrorheological (ER) fluid analogs. Both
fluids are non-colloidal suspensions of polarizable particles having a size on the order of a few microns.
The initial discovery and development of MR fluids and devices can be credited to Jacob Rabinow at the
US National Bureau of Standards (Rabinow, 1948a, 1948b, 1951) in the late 1940s. Interestingly, this
work was almost concurrent with Winslow's ER fluid work. The late 1940s and early 1950s actually
saw more patents and publications relating to MR than to ER fluids. Except for a flurry of interest after
their initial discovery, there has been scant information published about MR fluids. Only recently has a
resurgence in interest in MR fluids been seen (Shtarkman, 1991; Kordonsky, 1993; Weiss et al., 1993;
Carlson et al.,1994; Carlson, 1994; Carlson and Weiss, 1994). While the commercial success of ER
fluids has remained elusive, MR fluids have enjoyed recent commercial success. A number of MR
fluids and various MR fluid-based systems have been commercialized including an MR fluid brake for
use in the exercise industry (Anon., 1995; Chase, 1996), a controllable MR fluid damper for use in truck
seat suspensions (Carlson, Catanzarite and St.Clair, 1995; Lord, 1997) and an MR fluid shock absorber
for oval track automobile racing.
Active Fluid Volume and Device Aspect Ratio
While Equations (3) and (4) are certainly useful in the design of controllable fluid devices, they
often do not provide the best insight into the significance of the various parameters. It is often useful to
algebraically manipulate the above equations to provide a minimum active fluid volume (Duclos, 1988;
Carlson et al., 1994):
V = k η
τ2
λWm (5)
where k is a constant and V = Lwg can be regarded as the necessary active fluid volume in order to
achieve the desired control ratio λ at a required controllable mechanical power level Wm. For pressure
driven flow: k=12/c2
, λ=∆Pτ/∆Pη and Wm=Q ∆Pτ. For direct shear: k=1, λ=Fτ/Fη and Wm=Fτ S. It is
important to note that in both cases the minimum active fluid volume is proportional to the product of
three terms: a term that is a function of fluid material properties (η/τ2); the desired control ratio or
dynamic range λ; and the controlled mechanical power dissipation Wm sought.
Rheological Properties
The rheological properties of controllable fluids depend on concentration and density of particles,
particle size and shape distribution, properties of the carrier fluid, additional additives, applied field,
temperature, and other factors. The interdependency of all these factors is very complex, yet is
important in establishing methodologies to optimize the performance of these fluids for particular
applications.
The magnetorheological effect of the four MR fluids was measured on a custom rheometer using a
46 mm diameter parallel plate geometry set at a 1 mm gap. In the parallel plate geometry, shear rate
varies linearly across the fluid sample with the maximum shear rate occurring at the outer radius. The
rheometer is capable of applying greater than 1 Tesla through the fluid sample. Figure 2 shows the shear
stress in the MR fluids as a function of flux density at a maximum shear rate of 26 s-1. At such a low
shear rate, this shear stress data is approximately equivalent to the fluid yield stress as defined in Eq. (1).
At low flux densities, the fluid stress can be seen to exhibit a power law behavior. The approximate
power law index of 1.75 lies in the range of low to intermediate field behavior predicted by
contemporary models of magnetorheology. Both linear models and models accounting for nonlinear
magnetic effects such as particle saturation (Ginder, Davis and Elie, 1995; Jolly, Carlson and Muñoz,
1996) predict quadratic behavior at very low flux densities. The non-linear model proposed by Ginder,
Davis and Elie (1995) predicts a power law index of 1.5 at intermediate fields. Beyond flux densities of
about 0.2-0.3 Tesla, the effects of magnetic saturation are revealed as a departure from power law
behavior. The stress response ultimately plateaus as the MR fluids approach complete magnetic
saturation. As can be seen, the flux density at which this saturation occurs increases as the iron volume
fraction in the fluid increases.
In all MR fluid formulations optimized for a specific application or class of applications, the fluid
viscosity in the absence of a field is most significantly a function of the carrier oil, suspension agents,
and particle loading. The figures-of-merit described earlier are benefited by a low fluid viscosity, but
must be balanced with other fluid requirements such as temperature range and particle resuspendability.
Because of both the addition of suspension agents and changes in magnetic particle microstructure
during shear, most MR fluids exhibit significant shear thinning.
MR Fluid Figures of Merit
Figure 5 shows the figure of merit F1= τ2
/η as a function of shear rate for the four MR fluids. This
figure of merit reflects the dynamic range (or control ratio) of an MR device as well as required MR
fluid volume and power consumption. It is seen that as shear rate increases, the four fluids exhibit
significant improvements in F1 due to the substantial shear thinning character of these fluids. A broad
range of F1 is observed amongst the MR fluids resulting from the acute dependence of F1 on viscosity.
Figure 6 shows the figure of merit F2= τ2
/ηρ as a function of shear rate. This figure of merit is closely
related to F1 except that it further penalizes fluid density. With the exception of vertical shifts in the
data, F2 demonstrates much the same character as F1. It is interesting to note that, as measured by F2,
MRX-126PD is particularly well suited for low shear rate applications and MRX-242AS is better suited
for high shear rate applications. Both F1 and F2 illustrate that MR fluids in general exhibit better
rheological behavior at higher shear rates due to the shear thinning character.
Heavy Duty Vehicle Seat Suspensions
Recently a small, monotube MR fluid-based damper (shown in Fig. 8) has been commercialized for
use in a semi-active seat suspension system for large on- and off-highway vehicles (Carlson, Catanzarite
and St.Clair, 1995; Lord, 1997). In this application the MR damper represents enabling technology for a
variety of semi-active control schemes. This damper has also served as a testbed for developing
phenomenological device models (Spencer, et al., 1996; Pang et al., 1998).
“Seal-Less” Vibration Damper
A small, controllable MR fluid vibration damper that is being used for real-time, active-control of
damping in industrial applications is shown in Figure 11 (Carlson, Catanzarite and St.Clair, 1995). The
damper functions by moving a small steel disk or baffle in a chamber of MR fluid. Primary controlled
motion is axial although secondary lateral and flexing motions may also be accommodated. Damping
forces of 0 to ± 125 N are produced in the primary direction. This damper may be also be used as a
locking device. This damper does not require dynamic, sliding seals. The relatively small amplitudes
encountered (± 3 mm) allow the use of elastomeric rubber elements instead. Typically these dampers are
used as controllable dashpots in parallel with separate spring elements. Stiffness may also be added to
the elastomeric elements so that the damper functions as a controllable mount
CONCLUSIONS
The rheological, magnetic, and material properties of four commercial MR fluids have been
presented. Several figures of merit for MR fluids based on these properties were also computed and
presented. The examples presented illustrate that the properties and attributes of commercially available
MR fluids are wide-ranging. Some of these attributes are summarized in Table 5. It is evident that MR
fluids have evolved from laboratory curiosity to true engineering materials that involve engineering
trade-offs. The formulation of MR fluids involves the optimal balancing of properties for particular
applications or class of applications. Several applications were discussed to illustrate how various
material properties may be balanced