Magnetic Dampers

This article was first published in 2001.

It has a complex name but it involves a concept that's quite simple to understand.

The name?

Magneto-rheological dampers.

The concept?

Use oil within dampers that contains lots of small iron particles. When you want stiff dampers, apply a magnetic field and watch the fluid instantaneously increase its effective viscosity. And when you want soft damping? Take away (or reduce) the magnetic field. And to allow easy control, use an electro magnet to produce the magnetism.

It's both a brilliantly simple technology and one that has enormous potential in improving suspension systems.

It:

• Can be easily introduced to cars without changing the current basic design of automotive suspension systems
• Is able to be controlled electronically
• Can be easily interfaced with other automotive electronic systems such as ABS and Automatic Stability Control.

In addition, magneto-rheological (MR) dampers are quiet, and can provide - on paper at least - very substantial ride/handling advantages over conventional suspension systems.

Intrigued? Well, you can buy MR dampers right now for specialised racing applications, and the first car with MR dampers as Original Equipment - a Cadillac - is tipped to appear in 2003.

Current Suspension Systems

As with the combustion engine itself, current car suspension systems show the result of a very long development. However, fundamentally, suspension systems remain very crude. Steel springs support the body, with a variety of linkages used to maintain the wheels in a helpful geometry during their movement. To prevent the springs simply oscillating once they have been deflected, dampers ("shock absorbers") are used to stop the spring continuing to bounce. The resistance to motion that the wheel has is therefore very much dependent on the spring rate, and the speed and stroke of damper movement.

As a result, there are frequent trade-offs required in the design of the suspension. For example, soft springing and little damping can gain excellent vibration isolation. However, such a system will bottom-out frequently on real world bumps. On the other hand, good shock reduction is achieved by a system with a relatively stiff, highly-damped system. And on smooth roads, the stiffer the suspension is, often the better the resulting handling - but such an approach is unacceptable on cars that must negotiate bumpy roads.

While active suspensions - where the entire car's mass is borne by hydraulic or pneumatic rams - were once seen as the answer, the complexity, fail-safe requirements, power consumption and weight of such systems has seen their development slowed to a crawl.

MR dampers look to be the new answer to systems that can actively change in their damping abilities - quick enough that dive and squat, roll and pitch can be compensated for, in addition to being able to alter their behaviour to suit the road and driving style.

MR Fluid Dampers

MR fluid is composed of a liquid (normally a mineral oil but also possibly a water- or silicone-based fluid) in which varying percentages of very small iron particles are suspended. The iron particles are coated with an anti-coagulant (presumably so that they when they touch one another they don't clump) and will settle out of the liquid over time. However, given that they are in constant or near constant movement in the applications in which they are employed, this settling characteristic doesn't cause too much of a problem.

The discovery of MR fluid (and its close relative electro-rheological - ER - fluid) goes back to the 1940s. However, despite many scientific papers lauding the potential of the material, little has been achieved in practical applications. This now appears about to change.

When exposed to a magnetic field, the micron-sized particles dispersed throughout the MR fluid align themselves along the magnetic lines of force. Just like the iron filings that the science teacher used to shows lines of force around a bar magnet, the iron particles within the fluid form a pattern. The electro-magnets are arranged so that the lines of force extend across the fluid, with the particles forming strings like pearls on a necklace. If, in order to flow, the 'necklaces' need to be broken, the fluid acts as if its viscosity has increased. (In fact the carrier fluid - let's call it just oil for simplicity - hasn't increased in viscosity at all, but the combination of the iron and the oil behaves in a more viscous manner.)

This characteristic of MR fluid can be used in three distinct ways.

If a very thin layer of MR fluid (eg 0.005 - 0.015 inches) is placed between two electro-magnetic plates - and then the plates are slid relative to one another - the MR fluid is said to be operating in 'shear mode'. Such an approach can be used for the production of clutches and brakes.

A similar set-up - but this time with the plates being pushed towards one another - is termed 'squeeze mode'. This approach can be used for damping very small movements.

But of most interest to us is 'valve mode'. This is where there is a flow of MR fluid from one reservoir to another, with the connection being through a small diameter passage. If the passage is subjected to a variable intensity magnetic field, the amount of fluid that is able to flow will be altered. In effect, the fluid will act as if the opening changes in size.

It's the last approach which is used in MR fluid damper design.

In damper design, the areas where the fluid is subjected to magnetic fields are called 'choking zones' - because that's where the fluid flow is choked! As with conventional damper designs, MR dampers can be constructed in either mono or twin tube forms.

MR dampers have three distinct advantages over traditional dampers:

1. Low damping levels when not subjected to the magnetic field. This allows the damper to exert very little resistance to motion in conditions where that is advantageous.
2. High damping levels as required, even at low shaft velocities and travel.
3. Fast response time - MR dampers can switch between conditions in milliseconds.

Major components in a MR damper suspension system consist of a soft spring (basically to support the weight of the car), MR dampers with low 'off' damping levels but with a high enough damping ability to control travel even under the highest anticipated loads, and an electronic controller. Some proposed MR dampers incorporate a gas spring within them, allowing the removal even of the traditional metal spring from the suspension system.

Real Life Uses

So it all sounds great in theory - so why aren't we driving around on MR dampers in all of our cars? That's a question that will be better answered over the next few years, but right here and now there are many people sitting on seats suspended by MR dampers. Most of them are driving buses and trucks...

The Lord Corporation in the US is the primary driver behind the uptake of current MR technologies. They have developed and marketed a number of MR items, with the most successful at this stage being Motion Master® suspension seat mechanism. Designed to be retro-fitted to the suspension seats widely used in trucks and buses, the Motion Master® uses a controllable damper filled with MR fluid, a sensor arm that measures the position of the seat, and a controller that adjusts the damping force in response to changes in seat position. The system adjusts the damping force 500 times a second and includes a ride mode switch selectable between firm, medium and soft settings.

The damper used in the suspension seat system is dubbed by the company Rheonetic® and has a diameter of 35mm. It has a stroke of about 50mm and can dissipate mechanical energy at the rate of about 600 watts. It can develop about 2000 Newtons force, but requires only 4 watts of electrical power to achieve this maximum damping rate.

The variation in damping rate which is achievable with the different electrical current input is shown here.

Carrera, another US company, is now marketing the MagneShock^TM, a MR fluid damper produced under license from the Lord Corporation. The first generation maintained a 50:50 bump/rebound ratio over a wide range of electrically-adjustable damping rates, while the company's second generation design allows bump and rebound to be separately adjusted for each damper. The dampers have no valves or other small moving parts within them. They cost US$975 each, while a digital controller that can handle four dampers adds another $US1500.

However, probably the most important use of MR dampers will occur in 2003, when GM in the US launches a Cadillac equipped not just with MR dampers, but with a complete ride control system based around them. The Magneride system includes the following input sensors:

• Steering angle
• Vehicle yaw
• Vehicle speed
• Four damper position sensors
• Lateral accelerometer

Outputs comprise the four MR dampers and also an air compressor that will drive a self-levelling system (implying that the springing will be very soft indeed!). Rear dampers will feature a 46mm bore and be used together with front struts (36mm bore), with both front and rear dampers packaged within existing suspension architectures. The system will have a 15-millisecond response time.

Delphi Automotive Systems - the branch of GM doing the development - claim that the Magneride system will have the following benefits:

• Simple monotube damper design with no electro-mechanical valves or small moving parts.

• Improved performance and reliability over valve-based competitive systems.

• Full software tuneable damping characteristics provide excellent low-frequency body control without excessive harshness at high velocities.

• Excellent roll control during transient steering and evasive manoeuvres.

• Wide range of force control and high band-width for fast response.

• Low power requirements (20 watts per damper max).

• Flatter ride - by controlling vehicle body motions.

• Greater sense of safety and security - due to improved road holding capabilities.

• Enhanced handling - by controlling the lateral and longitudinal load transfer characteristics of the suspension during transient movements.

• Road isolation - by reducing the transmission of high-frequency road disturbances through the dampers.

• Greater vehicle dynamics control when combined with unified chassis control (eg auto stability control)

Conclusion

MR dampers use a technology that is very attractive in its apparent simplicity. Despite the fact that the Lord Corporation is currently charging US$595 a litre for MR fluid, the small amounts actually required in dampers and the economies of scale that will come from the widespread use of the material would see actual cost penalties per car fall considerably. That Delphi are prepared to release so many details of a car suspension system at least two years away from launch implies a complete confidence in the system, along with an expectation that it will be taken up by other car companies...

And there's absolutely no reason why dedicated controllers and MR dampers couldn't be a huge success in the aftermarket... After all, it's one of the very few breakthrough technologies easily able to be retrofitted.

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