FLUID POWER

Variable fluids smooth ride and steering

Electro rheological fluids, which stiffen in response to applied voltage, show great promise for improved variable dampers and power steering systems. Tom Shelley reports

Novel designs of shock absorbers, based on the advanced use of electro-rheological (ER) fluids. can change their damping parameters in thousandth of a second. And new power steering systems can respond at 500 Hz thanks to the same technology.

Using no conventional valves, the dampers, steering pumps, and actuators use electric voltage to locally control the viscosity of (ER) fluids. (see panel)

The fluids have been known for some time, but only now are practicable hydraulic systems using them reaching the point where they can go into cars. Prototype devices have been developed by Messrs Carl Schenck, based in Darmstadt, Germany using Bayer's ER fluid Rheobay.

The Car Dynamics Institute at the Technical High School in Darmstadt have just completed a series of tests of a variable rate damper under the direction of Professor Dr-Ing Bert Breuer. The variable rate is achieved by connecting the upper and lower parts of the damper cylinder through a special valve. In the valve, voltage is applied across the annular space between an internal cylindrical electrode and the valve casing.

Professor Breuer is director of automotive engineering at Darmstadt University. Schenck's Dr Horst Rosenfeldt, design and development manager, Fluid Mechatronics, says that reaction times are now down to 1ms, which is 10 times faster than variable rate hydraulic dampers based on conventional proportional electro-hydraulic valve technology.

Damping force can be varied up to 30 times at low velocities, or by up to twice at high velocities.Development efforts are now concentrating or reducing asymmetry effects --- differences in damping behaviour between expansion and compression movements.

The steering pump uses a rotor with a series of ducts in its casing. The fluid flow is generated by the rotor, while specific sections of the ducts are temporarily sealed from time to time by applying voltage to the ER fluid.

During the suction phase in each pump element, low viscosity fluid enters through the inlet port while voltage is applied to the outlet port, blocking it. During the discharge phase, the outlet port is opened by turning off the applied voltage, and the inlet port is blocked by applying voltage. In use, the pump runs continuously. Fluid pressure and volume flow are controlled by the magnitude and timing of the voltages supplied to the valves.

Along with the pump, a corresponding actuator has also been developed with four valves. European Automotive Design is at present, unable to reveal further details of this in order to protect patent applications.

Fast, sensitive response

The response of the pump plus actuator is up to 500Hz, well above what is possible with conventional electro hydraulics. The system therefore gives much quicker, more precise and more sensitive response, making it suitable for drive by wire systems as opposed to power assisted manual steering. (Drive by wire simplifies vehicle design and make four wheel steering feasible. Four wheel steering improves vehicle manoeuvrability at low speeds and handling at high speeds).

Prototype steering pumps and actuators are expected to be ready about now. Production dampers and power steering systems should, if all goes well, become available in the next few years.

Other applications investigated by Bayer include small clutches for 2 Nm torque transmissions, and vibration dampers for engine and machine bearings. Much study has been undertaken of squeeze mode dampers, in which fluid is drawn into and expelled from a space between two flat electrodes. In another damper configuration, voltage is applied across the gap between a piston and an insulated electrode round the inside of the damper cylinder. High response servo hydraulic applications are also thought appropriate.

Carl Schenck

ER fluids key to fast control

Electrorheological (ER) and magneto rheological (MR) fluids have been known since the 1940s. Bayer's ER fluid called Rheobay, is based on soft, cross linked polyurethane particles in silicone oil. The particles are on average, 5 microns in diameter and contain dissolved metal ions. The metal ions are necessary for fast polarisation and the electro rheological effect.

While many ER fluids contain hard, inorganic particles, which make them abrasive, Rheobay is non abrasive and so suitable for use in otherwise conventional hydraulic systems.

Imposition of an electric field in the range 1,000 to 4,000 V/mm polarises the particles, so that they have an induced charge, and makes them line up into chains. The transition is gradual, and so can be used to control fluid viscosity, hence its attractiveness for variable rate dampers.

Alternatively, imposition of full field strengths of 3,000 V/mm or above can be used to turn fluid into a gelatinous plug. Relaxation of the field allows it to flow again. Shear stresses at any particular imposed field strength are dependent on temperature. The two fluids currently offered: AI 3565 and AI 3566 are, however, designed to give a flat response over the ranges 20 to 60 degC and 60 to 120degC respectively.

Voltage gradients may be high, but electric current consumption is microscopic - typically a few millionths of an amp per square centimetre. Power consumption is therefore only thousandths of a watt per square centimetre of electrode surface area.

Zero field viscosity is typically around 50mPa/s, and is unaffected by shearing rate. The fluids show now signs of degradation over long test periods. Conventional paper element fluid filtration is inappropriate because of the heterogeneous nature of the fluids, but magnetic filtration to remove metallic particles shows no deleterious effects.

Rheobay products are commercially available in 5kg and 25kg containers. Larger containers are available on request.

Bayer

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