GB2267140A - Electro-Rheological fluid decelerator - Google Patents

Abstract

An electrorheological (ER) fluid decelerator comprises a body 1 having a piston assembly 2 fitted with a stop 6 to decelerate an impacting load. The load 1 contains an ER fluid which, upon moving of the piston assembly 2 down the body, passes through a valve 8 and returns to the back of the piston assembly 2 via return line 20. The valve 8 is normally held open by springs g, but outflowing fluid tends to close it, thus pushing electrodes 10 and 11 closer together. A voltage applied between the electrodes (controlled by a computer, Fig. 2) can thicken the ER fluid to resist this, and thus holds the valve in a desired position. A linear deceleration or any other preprogrammed deceleration can thus be attained. The body may contain conventional hydraulic fluid, with ER fluid only used around the electrodes, the fluids being kept apart by suitable seals. <IMAGE>

Description

ELECTRO-RHEOLOGICAL FLUIDS DECELERATORS This invention relates to devices of the hydraulic type whereby motion can be controlled and/or programmed in a linear or a non-linear manner. In particular this invention relates to the use of electro-rheological fluids to control the motion.

ER fluids were discovered by Willis M Winslow in 1939. He conducted extensive work on fluids and devices; numerous patents were also granted (see US patents 2,417,850, 2,661,596 and 3,046,507). They are suspensions of micron size polymer particles in a non-conducting oil. The fluids exhibit the remarkable ability of changing their physical state from liquid to solid when subjected to an electric field. Once the field is removed they revert to their original liquid state. The effect is extremely rapid, occurring within a few milliseconds. The degree of solidification of the fluid can be controlled by varying the strength of the electric field.

These features of an ER fluid makes them desirable for use wherever fluid type materials require control by electrical means. US patent 2,561,596 indicates a number of typical applications. ER fluids are especially useful in valves, as per US patent 4,840,112, where internally generated pressures are created, in dampers, as per UK patent application 2,111,171, and clutches, UK patent application 2,125,230 and US patent 4,444,298, for control of transmitted shear stresses. Other typical ER fluid applications cited in patent literature are anti-vibration mounts, US patent 4,361,006, DE patent application 3,910,447 and UK patent application 2,2O5,920. Shock absorbers are also commonly cited inventionS e.g. US patent 4,858,733 and UK patent application 2,216,225.The unique behaviour of ER fluids allows a designer to combine several functions within a single device or to offer performance characteristics which are unavailable with conventional techniques.

Backaround Information Every moving object possesses kinetic energy. If the object is required to change direction or stop, its kinetic energy must be dissipated. Unless the energy dissipation is controlled, shock forces will cause damage to the structure of the object or to associated equipment. The simplest form of energy dissipator is a spring or rubber bumper. However, these devices only store energy which has to be dissipated elsewhere in the system. A dashpot is slightly better since it provides a means of energy conversion. Unfortunately, this energy conversion occurs chiefly at the start of the stroke and imposes a significant initial shock load.

Linear decelerators are an attempt to overcome the problem.

They are designed to dissipate energy linearly over the entire stroke. They are however, complex multiport hydraulic units with a limited temperature range. The deceleration is notably better than with a dashpot but actually occurs as a series of peaks when each port is closed successively. Linear decelerators are precision machined devices and are therefore costly to manufacture.

In comparison, an elementary ER dissipation device is mechanically extremely simple It would comprises a piston in a cylindrical sleeve. ER fluid is forced to flow past the piston during motion. The flow path can be elctriid and when this occurs, the fluid solidifies and presents a restriction to motion. The degree of solidification can be controlled electronically and a true linear deceleration can be achieved.

By programming the control electronics, any required deceleration profile can be provided for the device. However, when detailed calculations are conducted the energy absorbtion of an ER decelerator according to this principle is limited and consequently has no advantage over an existing conventional device. To overcome this limitation and to utilise the advantages of ER fluids some other method of controlling the device is required.

According to the present invention a flow control device for use in ER decelerators is described whereby the flow characteristics are controlled by the ER fluid in compression.

A valve member is constructed so that it can be retained in the open position during flow by te introduction of a force which opposes the flow forces tending to close the valve. The opposing force is generated directly from an electrical input by using the compressive characteristic of the electrically stressed fluid between the electrodes.

It well known that electro-rheological fluids respond to various forms of electric field, for example, d.c., a.c., and pulsed d.c. The use of the term electric field is taken to cover all forms of applied electric field and the invention described herein can be used with such fields.

It is possible for the invention to be used as a component in an hydraulic system that uses only ER fluid or in an hydraulic systems that use both conventlcnal hydraulic fluids and ER fluids. In the latter case the two fluids, conventional hydraulic and ER fluid, are kept separate by means of suitable seals. Such a device can be used as an element in conventional hydraulic systems and does not require the entire system to use ER fluid.

The invention will now be described by way of example only by reference to the accompanying drawings.

Figure 1 is a cross sectional view through the device and Figure 2 a schematic of the operation of the device to achieve constant internal pressure.

Referring to Figure 1, the device has a body 1 which contains a piston assembly 2 with suitable seals 3.

The piston assembly is supported by a suitable arrangement of seals 4 and bearings 5. A load impacting on stop 6 at the end of the piston assembly causes the piston to move down the body thereby causing ER fluid 7 to flow through valve 8 held in the open position by light springs 9 and to be returned to the rear of piston 2 via return line 20. Under the flow forces generated by the motion of the piston, this valve 8 would close. Such operation is well known in conventional hydraulic device design. In this embodiment with valve 8 closed, the device would be in a state of hydraulic lock. However, by applying a voltage between the two electrodes 10 and 11, ER fluid therebetween solidifies and the valve 8 can be held open. The pressure drop generated through valve 8 can be calculated using the expressions for the laminar flow of a fluid through a radial gap. The pressure drop through the valve is a function of its geometry and the opening of the valve. Consequently the kinetic energy of the impacting load can be dissipated by controlling the pressure drop across the valve and will result in the load being arrested. As the load is decelerated, the flow rate through valve 8 decreases and hence the pressure drop across the valve decreases. This can be compensated by reducing the valve opening. This is achieved by reducing the voltage applied between electrodes 10 and 11. Hence by controlling the compressive stress characteristics of the ER fluid the opening of valve 8 can be controlled and controlled deceleration of the load can be achieved. The fluid inertial forces due to the impact are significant and the valve 8 needs to be shielded from them. This is achieved by diverting the fluid away from the valve by a diverter 12.Other features to account for are the fluid displaced by the piston rod. This is stored in an accumulator 13 during compression and released during the extension of the device. The accumulator vs typically a compliant foam material. Extension is aided using a spring 14.

The pressure is monitored by a suitable pressure transducer and a control circuit shown in rigure 2 is used to process the signal and control the closure of valve 8. The valve 8 although shown as a simple poppet valve can be more complex. For example,it may have bleed holes to prevent total closure in the event of an electrical failure. The valve, rather than closing on a large orifice, may cover a number of smaller orifices. These may be either radially spaced around the valve or positioned on a tapered valve assembly. There are numerous variants to the valve element each giving its own particular control characteristic.

It is in the nature of this invention that all such characteristics can be controlled by the ER fluid in compression.

The control circuit can be described by way of Figure 2. For linear deceleration constant internal pressure is required. A typical control strategy is outlined in Figure 2. Upon impact of the load the piston is moved causing an increase in internal pressure. This pressure characteristic can be compared with a characteristic either stored in the memory of a microcomputer or generated by the microcomputer from measurements of displacement against time. Hence it is possible to compute an error signal in relation to the pressure required by the decelerator. The error signal can then be processed and an output voltage generated to control the high voltage power supply. The signal processing is such that the non-linear closure characteristics of valve 8 are offset. The nature of either the stored characteristic of the generated one can be modified as required for specific purposes.

Hence deceleration profile of a non-linear nature can be envisaged.

The foregoing relates to a preferred exemplary embodiment of the invention. Understandably other variants and embodiments and changes could be made in construction without departing from the scope of the invention and thereof are possible within the spirit and scope of the invention. It is intended that all matter contained in the above descripticn or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims (7)

1. A flow control device for use in an ER decelerator whereby the flow characteristics are controlled by the ER fluid in compression, and including a valve member constructed so that it can be retained in the open position during flow by the introduction of a force which opposes the flow forces tending to close the valve, the opposing force being generated directly from an electrical input by using the compressive characteristics of the electrically stressed fluid between electrodes.

2. A device as claimed in claim 1, wherein one of the electrodes is movable with the valve member.

3. A device as claimed in claim 1 or 2, when used as a component in an hydraulic system that uses only ER fluid.

4. A device as claimed in claim 1 or 2, when used as a component in an hydraulic system that uses both conventional hydraulic fluid and ER fluid which are kept separate by suitable seals.

5. A device as claimed in claim 1, substantially as hereinbefore described with reference to the accompanying drawings.

6. An ER decelerator comprising a body containing ER fluid, a flow control device as claimed in claim 1 or 2, and a piston assembly, the piston assembly having an end provided with a stop against which a load to be decelerated impacts in use to move the piston assembly down the body so as to cause the ER fluid to flow through the flow control device and to be returned to the rear of the piston assembly via a return line.

7. An ER fluid decelerator as claimed in claim 5, substantially as hereinbefore described with reference to the accompanying drawings.