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by Hib Halverson
Page 3

The History of Corvette Electronic Ride Control Systems (cont.)

Hit MRF with a magnetic field and the CIP line-up into fibrous structures. If youíre 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.

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, canít 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 wasnít until about 1990, once digital signal processor controls became fast enough and cheap enough, that MRF in automotive suspension dampers became practical. .

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.


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 doesnít 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.


The Lord people have this cool little "Rheonetic Fluid" demo device. Itís 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.

Iron is heavier than the fluid carrier, so untreated CIP would quickly settle to the bottom of the shock as soon as the vehicleís 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 Corporationís Marketing and Sales Manager, Dr. Lynn Yanyo, continued the companyís 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 weíre 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 itís 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 itís sheared (shaken loose in the bottle) it flows easily."

In any event, IUT and abrasion are virtually nonexistent. Lordís enigmatic, Rheonetic Fluid is, also resistant, but not totally immune, to settling. GMís 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

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.
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 shockís 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.

From the outside, this MagneRide shock, on the left front of an Ď03 Anniversary car, looks just like most other shock absorbers. Itís 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 fluidís CIP, attracted to the magnetís 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 pistonís motion thus absorbing the "shock" of suspension movement.

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.
 
Lordís 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.
 


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.

 


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.

 

Itís been widely stated, even in some GM service data, that the magnetic field causes a change in the MRFís 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. Weíve got shocks full of Star Wars MRF,
so whatís next?

 

 The "Sky Hook" Algorithm

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