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by Hib Halverson
Page 3
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The History of Corvette Electronic Ride Control Systems
(cont.)
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Hit MRF with a magnetic field and the CIP
line-up into fibrous structures. If you’re really into the science of rheology,
know that the magnetic field changes the properties of the MRF from those of a
"Newtonian" fluid to those of a "Bingham" plastic fluid. The first has shear
stress that is proportional to the shear rate. The second exhibits a yield
stress that must be exceeded before flow begins, after which it exhibits a
rate-of-shear vs shear-stress curve that is linear.
Image: Delphi Corporation.
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MRF reacts to changes in magnetic field strength almost
instantaneously. The speed depends on how quickly the field can be altered. A MR
shock absorber can do that in a couple of milliseconds. The overall, system
response is slower but, still, noticeably quicker than any other ride-adaptive
shock system in the world. Other hydro-mechanical systems may have a potentially
wide range of damping authority but, in practice, can’t switch rapidly or
accurately enough to make such a range effective during high-frequency wheel
movement.
Magnetorheological fluid was discovered more than
half-a-century ago by the late Jacob Rainbow, a prolific scientist and inventor
working for the U.S. National Bureau of Standards. It wasn’t until about 1990,
once digital signal processor controls became fast enough and cheap enough, that
MRF in automotive suspension dampers became practical. . |
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After that, it took Lord and Delphi over a decade to take MRF from a product around which scientists
would stand, fingering their pocket protectors, pondering, "Uh yeah, dude,
this stuff is sweet. It could work in, like...a shock absorber on a car." to
where Mike Neal and I could drive a 50th Anniversary Corvette 120 mph over the
worst dip at Milford. |
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There were countless challenges in getting the fluid to
perform well and be durable in a shock absorber. The three most significant were
developing it: 1) to have consistent rheological properties over the life of the
shock absorber, 2) so its iron particles are not abrasive, and 3) so CIP would
not quickly settle out of the fluid when the car was at-rest.
During its commercialization of magnetorheological fluid,
Lord discovered the "In-Use-Thickening" (IUT) phenomena. Over a moderate period
of time, the surfaces of the iron particles, a mix of iron oxides, carbides and
nitrides, would flake away. The debris would suspend in the carrier fluid and,
as they accumulated, the fluid would thicken. If the off-state fluid gets
thicker; off- and low-state damping become more aggressive but high-state
damping doesn’t change, so the range of damping is reduced
In their untreated form, the abrasive CIP would grind away the insides of the
shock absorber and themselves in short order.
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The Lord people have this cool little "Rheonetic Fluid" demo device.
It’s a couple of syringes full of water-based MRF joined end-to-end. A small
round magnet is included. Work the syringe pistons back and forth then bring the
magnet close. All of a sudden you feel resistance. Bring the magnet closer and
the syringes lock up. Image: author. |
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Iron is heavier than the fluid carrier, so untreated CIP
would quickly settle to the bottom of the shock as soon as the vehicle’s
suspension stopped moving. As they settled, they would "clump" together making
redistribution of the particles difficult and degrading damping once the vehicle
resumed service.
Exactly how Lord solved these problems, particularly IUT, is
a mystery. When asked about specifics, Lord Corporation’s Marketing and Sales
Manager, Dr. Lynn Yanyo, continued the company’s policy of not commenting,
saying those aspects of its product are proprietary, then referred us to some
public domain information.
Processes used by the oil refining industry make abrasive
substances suspended in liquids nonabrasive. Additives are introduced into MRF
which we believe either coat or lubricate, or both, the CIP such that they
become nonabrasive. The settling problem is also addressed with additives,
though we’re not sure whether they form a coating that makes CIP more buoyant or
whether the additive is, are you ready...a "thixotropic agent", which thickens
the fluid while it’s at rest such that it resists settling of the particles, but
as soon as the fluid moves, causes it to reliquify. Dr. Yanyo is fond of making
analogies to food. "Ketchup is thixotropic," she told us, "and, until
sheared, it won't flow. Once it’s sheared (shaken loose in the bottle) it
flows easily."
In any event, IUT and abrasion are virtually nonexistent.
Lord’s enigmatic, Rheonetic Fluid is, also resistant, but not totally immune, to
settling. GM’s durability testing showed that if the car is at rest for a very
long period of time, say six months or longer, there can be some settling of the
CIP, however, it takes only a few strokes of the shock to agitate the MRF enough
that the particles are redistributed and the shock works as intended.
MR Shock Absorber Hardware
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Our hosts for the "Milford MR Show", Darin and Mike, showing off the
hardware. Dellinger (right) points out the heart of an MR shock, the
magnetic piston assembly. Image: author. |
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There are no valves in a Delphi MagneRide shock
absorber. They are replaced by an electromagnet inside the piston assembly
of each damper. This design results in a 40% reduction each shock’s parts
count.
The lack of valves also results in smooth, laminar flow of the MRF through
the piston. Where traditional shocks are prone to noisy, turbulent oil flow,
this laminar flow actually reduces suspension noise. Laminar flow is, also,
more predictable, so MR dampers meet design specifications to within a more
narrow, 2-3% tolerance. |
The rest of MSRC shock architecture is similar to that of a traditional,
gas-charged, mono-tube unit. The shock tube bolts to the suspension. The piston
rod bolts to the chassis. The rod ends in a piston which moves up-and-down in
the tube. A piston seal prevents MRF from bypassing the piston and a piston rod
seal keeps fluid from leaking out. At the bottom of the tube, a floating divider
piston and seal separates the magnetorheological fluid from a high-pressure gas
chamber which pressurizes the fluid to prevent foaming.
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From the outside, this MagneRide shock, on the left front of an ‘03
Anniversary car, looks just like most other shock absorbers. It’s basic
architecture is similar, too. Image: author |
Each piston/electromagnet assembly has four channels machined into it. As
the piston moves up and down in the shock tube, magnetorheological fluid
flows through these channels. When the magnetic field is weak, it flows
freely. As the magnetic field gets stronger, the fluid’s CIP, attracted to
the magnet’s poles and each other, form fibrous structures altering the
yield stress or the "rheology" of the fluid.
This change in yield stress is localized inside the piston channels. As MRF
moves into the channels, the CIP align in matrices and, as the fluid exits
the channels, the matrices dissipate. The altered-rheology fluid inside the
channels restricts flow and dissipates the kinetic energy of the piston’s
motion thus absorbing the "shock" of suspension movement.
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Our friends at GM took a rear MR shock and made this cutaway, demo unit.
For clarity, the floating divider piston which normally divides the
volume beneath the shock piston has been removed. Image: author. |
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Lord’s Rheonetic Fluid allows a wide variance in restriction available
giving the shocks their wide damping bandwidth. The speed with which the
fluid can vary its state makes that wide bandwidth practical. |
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For this close-up of a MR shock piston, we added a rubber band to
simulate a section of the flow of MRF through one of the four channels
in the piston. The areas within the red circles are where the magnetic
lines of flux pass through the MRF and cause the fibrous matrices to
form. Image: author. |
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Once the fibrous structures form in the MRF, resistance to flow, or
damping, is accomplished by the non-laminar flow though the piston
channel. The center "plug" of MRF has not sustained enough shear to
exceed its yield strength. This reduces flow velocity close to the
center of the flow and that resists flow. Image: Delphi Corporation. |
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It’s been widely stated, even in some GM service data, that
the magnetic field causes a change in the MRF’s viscosity. In reality, there is
no change in viscosity. It is the localized restriction to flow posed by the
"plug" of altered- yield - stress, MRF which causes the damping.
Ok. We’ve got shocks full of Star Wars MRF,
so what’s next?
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The "Sky Hook" Algorithm |
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Sponsored by:
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