GB2255150A - Motion control using electro-rheological fluids - Google Patents

Abstract

A device of the hydraulic type whereby motion can be controlled and/or programmed in a linear or a non-linear manner by the use of electro-rheological fluids, incorporates five sub-components, ie. the mechanical element, the ER fluid, a high voltage source, controlling logic circuit, and sensors. The device can include a gas chamber 8 to provide an outward spring bias. <IMAGE>

Description

NOTION CONTROL USING ELECTRO-RHEOLOGICAL FLUIDS 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). The effect was studied extensively in America during the 1960s and was termed the electroviscous effect. This description implied that the field changed the viscosity of the suspension. Current terminology, electro-rheology, was proposed by a Russian, Deinega, in 1970. This implied that the field changed the rheology of the suspension and the term has now been generally adopted.

ER fluids possess the ability to change from a liquid to a solid when subjected to an electric field and back again to a fluid when the field is removed. Typically they consist of micron sized particles - usual-ly polymers - suspended in an oil. The reversible change occurs in a few milliseconds. The main structural property of ER fluids is termed its yield strength or stress, which is the ability to withstand an externally applied force (shear stress) without rupture.

These features of an ER fluid makes them desirable for use wherever fluid type materials require control by electrical means. US patent 2,661,596 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 4,861,006, DE patent application 3,910,447 and UK patent application 2,205,920. Shock absorbers are also commonly cited invention, 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.

Background Information Every moving object possesses kinetic energy. If the object is required to change direction or stop, its kinetic energy must be controlled. 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 controller 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.

An ER device has five basic elements as indicated in figure 1 which shows in block diagram form a common arrangement of: 1. mechanical components 2. ER fluid 3. high voltage source 4. controlling logic circuit 5. feedback elements to provide control signals.

Two other usual components are the load to be controlled, item 6, and additional sensors, item 7. The load, shown as a mass in figure 1, is taken to be any controlled variable, e.g velocity, acceleration, or position. It is the integration of these five elements that produces a practical ER device. The advantages of such an invention over conventional solutions are lower costs, faster operation, more precise control, and higher reliability.

The present invention will be described by way of example with reference to the five basic elements above together with reference to the accompanying drawing and figures. The drawing is a sectional elevation of a-motion control device constructed in accordance with the invention, to be actuated with the use of an electro-rheological fluid. The invention relates to the application of electro-rheological (ER) fluids to controlled motion in a deceleration device. It incorporates the direct interface of mechanics with electronics.

Mechanical Elements One embodiment of the mechanical components are shown in figure 2. This consists of an outer insulating body assembly, 1, to which an inner conducting body assembly, 2, is attached. This composite assembly is frequently found in the mechanical elements of ER systems. The important requirements of the insulating material are that it can withstand the electric field and that it has adequate mechanical strength. Located concentrically within the outer body is a piston arrangement, 3. Between the inner electrode, 2, and the piston, 3, is a flow path, 10, for the ER fluid to move through. The piston, 3, can be constructed of a conducting material or it can be constructed of a composite structure similar to the outer body. Holes, 7, can be drilled into the piston to reduce weight and inertial forces. This requires and endcap, 17, to be fitted to the piston.This can be of a compliant material and thus serve a secondary purpose, that of a bump stop. The endcap is held in place by bolts, 19, and if required the ends can be sealed, 20, with a suitable sealing material. A voltage is applied to one of the electrodes, 2, which increases the apparent viscosity of the ER fluid. The piston, 3, serves as the other electrode. This is commonly taken to ground for reasons of safety and ease of construction. It is important to maintain the separation of the electrode, 2 and 3, and in this construction a bearing, 11, and spigot, 5, arrangement is utilised. The spigot, 5, is of an insulating material and it held in place by bolts, 16. As the insulating material is a poor bearing surface it is, in this arrangement, provided with a metal sleeve, 6.The piston is supported at the other end by a bearing, 12, which is fitted into another endcap, 4, bolted, 15, to the outer body assembly, 1. The endcap is, in this embodiment, an insulating material. To prevent escape of the ER fluid a seal, 14, is fitted in the endcap, 4. Another seal, 13, is fitted to the endcap, 17, which is attached to the piston, 3. This prevents ER fluid from passing into the air space, 8, in the piston, 3.

The air space, 8, can be closed as shown or open to atmosphere.

If the air space is closed the entrapped air will, under compression, act as an air spring.

The operation of the mechanical element is detailed next.

If a force is applied to the piston assembly, 3, it will be moved in the direction indicated by the arrow A, and the ER fluid in chamber 21 will pass through the flow path, 10, into chamber 22.

With no electric field applied the device will act in a fashion similar to a simple dashpot, in which case the reaction force will be a function of the velocity of the piston. However, by applying an electric field between the electrode, 2, and the piston, 3, the apparent viscosity of the ER fluid can be increased thereby increasing the reaction force on the piston.

By means of a suitable sensor, for example, a linear measuring device, the motion of the piston can be controlled to any desired profile independent of the loading conditions. During motion the air or gas trapped in the void, 8, will be compressed. When the load and the applied field are removed the piston will be forced back, by the compressed gas, to its equilibrium position. If the electric field is maintained the piston can be held in place despite the compressed gas. By reducing the electric field the return stroke, in the direction marked by the arrow B, can be controlled, thus bi-directional control of motion can be achieved. In the situation where the air space, 8, is open to atmosphere a spring can be fitted. This can be internal located, for example in the air space, or external fitted by suitable means to the endcap, 4, and the piston, 3.If only unidirectional control is required and a return spring is incorporated it is advantageous to fit a suitable non-return valve can into the piston assembly thereby facilitating the return stroke of the device.

Electro-Rheoloqical Fluid The general characteristics of an ER fluid are shown in figures 3 to 6. The data is for aqueous based fluids, as typified by UK patents 1,570,234 and 2,153,372, evaluated in a Couette rheometer. ER fluids based on the non-aqueous type, as typified by US patent applications 4,744,914 and 3,984,339 are also applicable to this invention.

A typical fluid response to the applied external field is shown in figure 3. In practice, fields of 0.5 to 3 kV/mm are generally used although the maximum applied field can exceed 6 kV/mm.

The application of an electric field causes a current to flow in the fluid and this behaviour is shown in figure 4. The current is non-linear with the field as shown. The current is also temperature dependent and increases as the temperature is increased.

A further feature of ER fluids is the ability to maintain shear stress developed under the field as the fluid is sheared.

This behaviour is shown in figure 5. At 0 kV/mm the slope of the plot is the Newtonian viscosity of the fluid.

In figure 6 the Newtonian component has been subtracted from figure 5. The remaining data are termed the excess shear stress.

Note that these vary with the shear rate.

High Voltaae Electrical Source A high voltage (HV) electric field is required to effect the changes in an ER fluid. Experience has shows that voltages of 500 to 3,000 volts are required across spacings of 0.25 to 5.0 mm.

Such voltages are greater than those commonly found in electronic circuits and consequently specially designed circuits are required. Applied voltages can be direct current (dc), alternating current (ac), or pulsed dc (see UK patent application 2,125,230). Pulsed dc is of particular interest. Control can be exercised by varying the pulse width, and hence the time of engagement, of the electric field. This type is commonly known as pulsed width modulation (PWM). The HV generator can be a separate component, integral to the device, or of a self generating type. In the later case this can be either piezoelectric, as per patent application 2,205,920, or generated using integral winding in the body of the device.This mode is advantageous in motion control devices as kinetic energy is transferred to electrical energy, which then controls the apparent viscosity of ER fluid in the device. This gives a completely self contained device.

Electronic Control Unit An electronic control unit (ECU) is used to provide control signals to the HV generator. The ECU and the HV generator can be separate items or combined into a single package. The HV generator applies the electric field to the electrodes and thereby to the electro-rheological fluid in the device. The ECU includes a processor, memory and communication interfaces. These control the magnitude of the electrical field so that the apparent viscosity of the ER fluid varies from a fluid condition to virtually a solid condition. The ECU is also connected to a power source such as a battery, solar source, alternator, mains derived, self generated, or any acceptable combination of these.

An electrical field of predetermined magnitude is applied to the ER fluid in the device to achieve the desired fluid condition as defined either by the memory or the controlling logic in the ECU.

The ECU varies the applied electrical field according to the received signals from one or more sensors. For example, the sensor may be of the position or acceleration type. Under certain condition a signal is communicated to the ECU when motion is detected above a prescribed limit. The ECU calculates the appropriate electrical field to bring the motion back to the prescribed limit. In this case the applied electric field is increased and thereby increases the apparent viscosity of the fluid. Obviously more than one sensor can be employed to detect system and environmental conditions and then signal to the ECU.

The ECU can be programmed to respond to inputs from sensors, then evaluate the required performance from the ER fluid, and apply the required electrical field. Thus, the ECU could be programmed to control the motion of the device as a function of both environmental conditions and imposed loads on the device.

Feedback and Sensor Elements Sensors are required to control motion and to accommodate variations in the environment and the loading on the device. Any sensor used will be selected according to feedback and control logic integral to the ECU. For example, velocity applied to the piston of the device could be measured by a linear variable differential transformer, acceleration by an accelerometer, or displacement by a linear position resistance transducer. If required, the ECU can prescribe and control a load. In this case the sensors could be of a form suitable to measure internal pressure. As an alternative, a direct measurement of the load could be made using, for example a load cell.

Clearly the sensors described above relate in the main to linear motion. What is equally apparent is that with appropriate linkage or by mechanical arrangement, rotary motion can be similarly controlled in a precise and smooth fashion. The transducer or sensors can in this case be of a rotary type, for example a rotary encoder.

Experience also shows that temperature fluctuation can have significant effect on the performance of both hydraulic systems and ER fluid systems. Thus incorporation of some form of temperature sensors will enable the electronic control unit to take appropriate compensatory action and thereby maintain the desired motion profile.

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 description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims (20)

WHAT IS CLAIMED IS:

1 An electrically controlled linear motion device utilising ER fluids to control the motion in an essentially linear fashion.

2 An electrically controlled linear motion device utilising ER fluids to control the motion in an essentially non linear fashion.

3 An electrically controlled linear motion device according to claims 1 and 2 whereby the motion profile is pre programmed.

4 An electrically controlled linear motion device according to claims 1 and 2 whereby the motion profile is adaptive.

5 A linear motion control device according to claims 1 to 4 where uni-directional control of motion is achieved.

6 A linear motion control device according to claims 1 to 4 where bi-directional control of motion is achieved.

7 A linear motion control device according to claims 1 to 4 where uni-directional control of velocity is achieved.

8 A linear motion control device according to claims 1 to 4 where bi-directional control of velocity is achieved.

9 A linear motion control device according to claims 1 to 4 where uni-directional control of acceleration is achieved.

10 A linear motion control device according to claims 1 to 4 where bi-directional control of acceleration is achieved.

11 A linear motion control device according to claims 1 to 4 where uni-directional control of load is achieved.

12 A linear motion control device according to claims 1 to 4 where bi-directional control of load is achieved.

13 A linear motion control device according to claims 1 to 12 where environmental fluctuations can be compensated.

14 A linear motion control device according to any of the claims 1 to 13 where the applied electric field is of a direct type.

15 A linear motion control device according to any of the claims 1 to 13 where the applied electric field is of an alternating type.

16 A linear motion control device according to any of the claims 1 to 13 where the applied electric field is a pulsed type.

17 A linear motion control device according to any of the claims 1 to 13 where the applied electric field is of a pulsed width modulated type.

18 A linear motion control device according to any of the claims 1 to 13 where the applied electric field is self generated.

19 A linear motion control device according to any of the claims 1 to 18 where the ER fluid is of the aqueous type.

20 A linear motion control device according to any of the claims 1 to 18 where the ER fluid is of the non-aqueous type.