GB2330891A - Electro-rheological fluid cell - Google Patents

Abstract

An electro-rheological fluid cell, eg for a shock absorber, has a reservoir 20 containing a electro-rheological fluid 25 whose viscosity varies with temperature. A first electrode 40 receives a supply voltage, and second terminal 50 drains current from the first electrode via an aperture 70 containing the fluid. A bimetallic strip 60 provides a variable current drain from the first electrode in dependence upon the temperature of the fluid, such that its viscosity remains constant with temperature.

Description

ET,ECTRO-RHEOLOGICAL FLUID CEIL AND METHOD Field of the Invention This invention relates to Electro-Rheological fluid cells, and particularly but not exclusively to temperature compensation for such cells.

Background of the Invention Electro-Rheological (ER) fluids are being used increasingly in the automotive industry, for damping applications such as shock absorbers, engine mounts and the like. By varying the voltage applied across an ER fluid in a cell, the viscosity of the fluid is varied, thereby providing a variable, controllable damping arrangement.

A problem with this arrangement is the current density of a typical ER fluid changes dramatically with temperature, and this has an effect on the time taken to respond to a change in voltage. For the situation in which an initial high voltage is applied and then removed, a fluid typically takes milliseconds to return to its low voltage viscosity when at high temperatures. However, at low temperatures this figure is more like seconds. Whilst a millisecond response time represents an acceptable delay time, many applications would find a time in the order of seconds unacceptable.

It is known to use a Direct Current (DC) power supply with an active pull down transistor (i.e. the capability to sink current as well as source current). However, this requires complex and relatively expensive circuitry, since an active sink requires active devices that can function at 4-5kV. A single silicon transistor, such as a field-effect transistor (FET) can only work to 1-1. 1.5kV.

It is also known to use an Alternating Current (AC) power supply.

However ER fluids will only work at relatively low AC frequencies (typically to 50-60Hz). This means that either the high voltage must be fed via a low frequency step-up transformer (which is very bulky, heavy and expensive) or the AC must be generated by pulse width modulation from a 4-5kV DC source which again requires high voltage semiconductors. Due to the low current drain these must drive the output in a "push-pull" fashion (i.e. have both active pull up and active pull down devices).

This invention seeks to provide an ER fluid cell and method which mitigates the above mentioned disadvantages.

Summarv of the Invention According to a first aspect of the present invention there is provided an Electro-Rheological fluid cell comprising: a reservoir arranged for containing an Electro-Rheological fluid; a first electrode, disposed in the reservoir and coupled to receive a supply voltage; a second electrode, disposed in the reservoir and coupled to drain current from the first electrode via an aperture containing the Electro-Rheological fluid; and, a supplementary drain circuit having a temperature dependent component disposed in the reservoir, the circuit being coupled to provide a variable current drain from the first electrode in dependence upon the temperature of the Electro-Rheological fluid.

Preferably the supplementary drain circuit comprises a bimetallic strip and a contact surface, the bimetallic strip being the temperature dependent component. The supplementary drain circuit alternatively preferably comprises a memory metal strip and a contact surface, the memory metal strip being the temperature dependent component.

Preferably the contact surface is arranged define a temperature dependent variable aperture with the strip. The temperature dependent component of the supplementary drain circuit is alternatively a thermistor.

According to a second aspect of the present invention there is provided a method for providing temperature compensation in an Electro-Rheological fluid cell, comprising the steps of: providing voltage to a first terminal, such that current flows between the first terminal and a second terminal via an aperture containing Electro Rheological fluid; and providing a supplementary drain path having a temperature dependent component, the path being coupled to variably drain current from the first terminal in dependence upon the temperature of the Electro-Rheological fluid.

In this way an Electro-Rheological fluid cell is provided, which is temperature compensated such that the decay time of the first electrode and hence the response of the remains substantially constant with temperature.

Brief Description of the Drawing An exemplary embodiment of the invention will now be described with reference to the single figure drawing which shows a preferred embodiment of an Electro-Rheological fluid cell in accordance with the invention.

Detailed Description of a Preferred Embodiment Referring to the single figure drawing, there is shown an ER fluid cell 10, comprising a reservoir 20 containing ER fluid 25, source and drain electrodes 40 and 50 respectively, and a temperature compensating arrangement comprising a bimetallic strip 60 and a contact surface 80.

The cell 10 has an upper piston 30 and a lower piston 35, at least one of which will typically be coupled to provide a damping action for a component of a vehicle or machine (not shown). The ER fluid 25 may be any typical ER fluid, such as Bayer Rheobay TP AI 3566 fluid.

The source electrode 40 is coupled to a supply voltage via a supply terminal 45, in order to receive a varying supply voltage in the order of kV.

The drain electrode 50 is coupled to a ground terminal 55. Between the source electrode 40 and the drain electrode 50 is an aperture 70. If the upper piston 30 is moved up (or down), the ER fluid 25 flows through the aperture and causes the lower piston 35 to moved up (or down).

Alternatively the aperture 70 may be formed in the upper piston 30, in which case the piston 35 will be removed, and the resistance will be provided as the upper piston moves through the ER fluid 25.

The viscosity of the ER fluid 25 is dependent upon the voltage applied between the source electrode 40 and the drain electrode 50 across the aperture 70. This is provided by the supply voltage coupled to the supply terminal 45. In this way the speed of the flow of ER fluid 30 through the aperture 70, and therefore the speed of movement of the upper and lower pistons 30 and 35 respectively may be varied by varying the voltage applied across the aperture 70.

The response of the ER cell 10 to a change in applied voltage at the supply voltage terminal 45 is partly dependent upon the capacity of the ER fluid 25 to carry current, and this in turn is dependent on the temperature of the ER fluid 25.

Bayer Rheobay TP AI 3566 has the following maximum current densities at 3kV/mm: 0.6uA/cm2 at 25"C 9. 6uAlcm2 at 60"C 48.8uA/cm2 at 90"C 117.3uA/cm2 at 1200C Without the temperature compensation arrangement, assuming that current density is substantially constant with respect to voltage, this means that for an initial voltage of 3kV applied at the supply terminal 45 where the aperture 70 has 1 cm2 of ER fluid 25 and a capacitance of 1nF, the time taken for the charge on the source electrode 40 to decay to zero will range from 5 seconds at 25"C to 26ms at 1200C.

The bimetallic strip 60 is disposed within the ER fluid 25, and has one end secured to a sidewall of the reservoir and electrically connected to the supply terminal 45, in order to receive therefrom the varying supply voltage. The contact surface 80 is mounted on an opposite sidewall to that on which the bimetallic strip 60 is secured. The contact surface 80 has a suitably shaped profile facing the bimetallic strip 60, and is electrically connected to the ground terminal 55. A variable aperture 90 exists between the contact surface 80 and the bimetallic strip 60.

When the ER fluid 25 is at low temperatures, the bimetallic strip 60 will be substantially flat, and will be in close proximity to the contact surface 80, resulting in a small gap at the variable aperture 90, so providing a high leakage current between the supply terminal 45 and the ground terminal 55 via the ER fluid 25 in the aperture 90. This high leakage current assists in the discharge of the source electrode 40, such that the voltage across the aperture 70 returns to zero within a matter of milliseconds.

As the temperature of the ER fluid 25 increases, the bimetallic strip 60 bends, thus increasing the gap at the variable aperture 90 and reducing the contact surface area so reducing the leakage current through the aperture 90.

In this way, a supplementary current drain path is provided, which varies with temperature so as to ensure that the voltage across the aperture 70 returns to zero within a matter of milliseconds, at any given temperature.

This will ensure that the time constant (the response time) of the ER cell 10 does not change significantly with temperature changes.

It is also possible to use this technique to monitor the fluid temperature, by monitoring the leakage current flowing across the variable aperture 90.

This is useful as other fluid characteristics (such as shear stress) also change with temperature.

It will be appreciated that alternative embodiments to the one described above are possible. For example, the bimetallic strip 60 may be replaced by a 'memory metal' strip, namely a material that'remembers' two distinct shapes at two distinct temperatures.

Moreover the bimetallic strip 60 and contact surface 80 could be replaced by an alternative temperature dependent arrangement for providing a current path, such as a thermistor disposed in the ER fluid 30.

The pistons described above could be replaced by diaphrams or membranes, which by changing shape produce a pressure on the ER fluid 30.

Also, other arrangements are possible, such as a rotation axle disposed in the fluid, the axle having blades or fins arranged to move through the ER fluid by rotating about the axle. In this arrangement electrodes could be disposed in the fins.

Claims (1)

1. Claims 1. An Electro-Rheological fluid cell, comprising: a reservoir arranged for containing an Electro-Rheological fluid; a first electrode, disposed in the reservoir and coupled to receive a supply voltage; a second electrode, disposed in the reservoir and coupled to drain current from the first electrode via an aperture containing the Electro-Rheological fluid; and, a supplementary drain circuit having a temperature dependent component disposed in the reservoir, the circuit being coupled to provide a variable current drain from the first electrode in dependence upon the temperature of the Electro-Rheological fluid.

   2. The Electro-Rheological fluid cell of claim 1 wherein the supplementary drain circuit comprises a bimetallic strip and a contact surface, the bimetallic strip being the temperature dependent component.

   3. The Electro-Rheological fluid cell of claim 1 wherein the supplementary drain circuit comprises a memory metal strip and a contact surface, the memory metal strip being the temperature dependent component.

   4. The Electro-Rheological fluid cell of claim 2 or claim 3 wherein the contact surface is arranged define a temperature dependent variable aperture with the strip.

   5. The Electro-Rheological fluid cell of claim 1 wherein the temperature dependent component of the supplementary drain circuit is a thermistor.

   6. A method for providing temperature compensation in an Electro Rheological fluid cell, comprising the steps of: providing voltage to a first terminal, such that current flows between the first terminal and a second terminal via an aperture containing Electro Rheological fluid; and providing a supplementary drain path having a temperature dependent component, the path being coupled to variably drain current from the first terminal in dependence upon the temperature of the Electro-Rheological fluid.

   8. An Electro-Rheological fluid cell substantially as hereinbefore described and with reference to the single figure drawing.