The invention relates to an actuator, comprising a chamber filled with a gas and with carbon fibres, light generating means, the light of which may enter the chamber for heating the carbon fibres and the gas in the chamber, and expansion means connected to the chamber for generating a force and/or a movement. An actuator of this kind is known from US 5218666. The known, very small actuator is intended for opening and closing an extremely miniaturised pair of tweezers. For this known application, the optical power needed for heating the gas in the very small chamber is small.

The present invention is more in particular intended for actuators with relatively large chambers, where relatively large forces are to be generated. For this type of applications, the necessary optical power becomes a critical factor in the sense of availability and affordability . For that reason, the chamber must be designed such that

substantially all optical radiation entering the chamber shall be absorbed by the carbon fibres. According to an aspect of the invention the actuator is thereto character- ised in that an inner wall of the chamber is provided with reflection means, for reflecting light emitted by the light generating means. The reflection means may comprise the polished walls of a metal chamber, but one may also use other materials, for example a metallised plastic film for cladding the inner wall of the chamber. Dependent upon the type of application of the actuator, the expansion means preferably comprise bellows, a plunger or a membrane.

A favourable embodiment of the inventive actuator is characterised in that the light generating means comprise a LED or a LED array, consisting of a tightly packed array of LED chips. With both a LED and a LED array a high power density light beam can be realised, which means that only a small opening in the reflecting chamber is needed and a loss of light via this opening will be minimised.

A very favourable embodiment is characterised in that the chamber also contains fibres, substantially transparent to light emitted by the LED or LED array. These fibres, for example glass fibres, will deflect and reflect the light inside the chamber, therewith decreasing the number of reflections on the wall of the chamber and the losses due to a finite reflection coefficient of the wall. An

additional advantage is that the glass fibres support the carbon fibres, so that carbon fibres can be distributed more uniformly throughout the chamber. A further advantage is that the glass fibres mutually separate the carbon fibres, which tend to stick together in a longitudinal direction. A still further advantage is that the glass fibres reduce a flow of heat towards the wall.

Another favourable embodiment is characterised in that the light generating means comprise a current source and first switching means, for connecting the LED or LED array to the current source. Preferably, the switching means are

arranged for generating a pulsed current having an

adjustable duty cycle, for realising a desired force or movement .

Every time the LED or LED array of the actuator is switched on, in particular the carbon fibres close to the LED or LED array will suddenly heat up, together with the surrounding gas. This generates a pressure wave, powerful enough to damage components of a LED array, more in particular the interconnections of LED chips in the array. A favourable embodiment in which damage to a LED or LED array is

prevented, is characterised in that the actuator comprises a window, located between the chamber and the LED or LED array. An additional advantage is that the chamber and the window together form a hermetically sealed unit that can be filled with a gas with desired properties. From a logistics point of view this is desirable, as these units can be simply stored and combined with a selected LED or LED array so as to realise an actuator with specified properties.

When a window is placed between the chamber and the LED or LED array, a pressure wave will be generated in the chamber next to the window, every time the LED or LED array is switched on. This pressure wave will shake the carbon fibres inside the chamber, which may improve the heat transfer from the carbon fibres to the gas. A favourable embodiment which advantageously uses this effect is

characterised in that a frequency of the pulsed current is selected for optimising the heating of the gas inside the chamber . A further very favourable embodiment is characterised in that the chamber has an at least substantially spherical or ellipsoidal shape, therewith reducing optical and thermal losses at the wall of the chamber by substantially

optimising the ratio between the volume of the chamber and the surface area of the chamber.

A further very favourable embodiment is characterised in that the chamber consists of a first chamber halve and a second chamber halve, with the expansion means connecting the edges of the first chamber halve and the second chamber halve. Due to the large surface area that can be obtained in this way, a powerful and yet compact actuator can be obtained. When the LED or LED array is switched on, both halves form a substantially closed chamber, so that

practically no light will be lost. As a result, the

pressure will rapidly build up. For an actuator according to the invention, comprising a chamber that is homogeneously filled with fibres, the best performance will be obtained when the light intensity from the light generating means is at least substantially homogeneous throughout the chamber. If the intensity is lower than average at some point, this point will

contribute less to the pressure build-up. If the intensity is higher at some point, this point may contribute more to the pressure build-up, but also the carbon fibres will heat up more, therewith introducing radiation losses in the form of long wavelength infrared radiation. A favourable

embodiment for distributing the light intensity more homogeneously throughout the chamber is characterised in that the light generating means comprise at least a first LED or LED array connected to the first chamber halve and a second LED or LED array connected to the second chamber halve .

A further favourable embodiment is characterised in that the actuator is provided with second switching means, for connecting the first LED or LED array and the second LED or LED array to the current source.

If the light intensity from the light generating means is homogeneously distributed throughout the chamber, the pressure will rise uniformly throughout the chamber. This implies that there will be substantially no flow of gas inside the chamber. Some gas flow might be favourable, as this gas flow will carry away heat from the carbon fibres. A favourable embodiment which may realise a gas flow inside the chamber is characterised in that the second switching means are arranged for alternately connecting the first LED or LED array and the second LED or LED array to the current source .

A favourable embodiment in which the performance of the actuator can be optimised is characterised in that a switch frequency of the second switching means is selected for optimising a heating of the gas inside the chamber.

The invention also relates to an arrangement provided with an actuator as disclosed in one of the previous paragraphs.

The invention will now be further explained with a

reference to the following figures, in which:

Fig. 1A schematically shows in cross-section a possible embodiment of an actuator according to the invention;

Fig. IB schematically shows in cross-section an

alternative embodiment of an actuator according to the invention;

Fig. 2A schematically shows in cross-section a possible embodiment of an actuator provided with a

separate absorption chamber;

Fig. 2B schematically shows in cross-section an embodiment with a separate absorption chamber and a filler body;

Fig. 3A schematically shows in cross-section an absorption chamber with a cylinder and a plunger connected to it;

Fig. 3B schematically shows in cross-section an absorption chamber with a membrane welded on top of it;

Fig. 4A schematically shows in cross-section an actuator comprising two chamber halves;

Fig. 4B schematically shows in cross-section an alternative embodiment of an actuator comprising two chamber halves;

Fig. 5 schematically shows in cross-section a further alternative embodiment of an actuator comprising two chamber halves. Fig. 1A schematically shows in cross-section a possible embodiment of an actuator 1 according to the invention. Actuator 1 consists of metal bellows 2, on a top end closed with a metal lid 3 and on a bottom end closed with a bottom plate 4, for example a printed circuit board on which a LED 5 is mounted which can be controlled with the aid of an electronic circuit. The space enclosed by bellows 2, top lid 3 and bottom plate 4 is filled with a gas and with carbon fibres 6. The insides of bellows 2, lid 3 and bottom plate 4 are well polished, so that they will almost

completely reflect light transmitted by LED 5. When LED 5 is switched on, the carbon fibres 6 will be heated by the radiation emitted by LED 5, which in turn will heat the surrounding gas, as a result of which the pressure of the gas is increased and bellows 2 will become longer. For a quick response, thin carbon fibres are to be used for heating up quickly and for quickly transfer the heat to the surrounding gas. Fig. IB schematically shows in cross-section an alternative embodiment of an actuator according to the invention, where an inside of bellows 2 is provided with a reflective layer 7, for example a metallized plastic film, for reflecting light transmitted by LED 5. It is possible then to

manufacture bellows 2 from rubber or from a suitable plastic, for example with the aid of a 3-dimensional printer, in which case bellows 2 may be directly printed onto for example a carrier for a printed circuit. Fig. 2A schematically shows in cross-section a possible embodiment of an actuator 1 provided with a separate absorption chamber 8. Absorption chamber 8 is manufactured from plastic and is provided with a reflective layer 7, for example a thin metallization layer, and a glass window 9 which connects to a commercial available 100W 450nm LED array 10, for example a JX-100PBYB from Justar Electronic Company. Absorption chamber 8 is filled with a gas and with carbon fibres 6 or a mixture of carbon fibres 6 and glass fibres 11, the latter of which support and mutually

separate carbon fibres 6. When LED array 10 is switched on, carbon fibres 6 will be heated by the radiation emitted by LED array 10 and in turn heat the surrounding gas, as a result of which the pressure of the gas increases and bellows 2 become longer. Actuator 1 has also be tested using a 100W 660nm LED array 10, for example a JX- 100R10X10G, giving substantially the same results when the wall plug efficiency of both LED arrays is taken into account. This indicates that the heating of the carbon fibres is substantially independent of the wavelength, at least within the visible region.

Fig. 2B schematically shows in cross-section a further embodiment, with a metal absorption chamber 8, of which the inner wall is polished, so as to make it highly reflective for light emitted by LED array 10. Absorption chamber 8 is provided with an internal, curved glass window 12, which distributes the light emitted by LED array 10 more

uniformly inside absorption chamber 8. Bellows 2 are provided with a filler body 13 for minimising dead space and thus for maximising a pressure increase. Filler body 13 is made of a hard plastic foam with a closed cell structure and is provided with a reflective layer 7, for reflecting light emitted by LED array 10. The gas inside absorption chamber 8 may be for example air or nitrogen, but also a gas with a low thermal conductivity may be used, like xenon, in order to reduce the heat flow towards the wall of absorption chamber 8. One may also use a gas with a high thermal conductivity like helium if the recovery time of the actuator is important. Fig. 3A schematically shows in cross-section a metal absorption chamber 8 connected to a plunger 14 and a cylinder 15. Plunger 14 is for example made graphite and is provided with a reflective layer 7, for reflecting light emitted by LED array 10. Cylinder 15 is for example made of glass. This combination is. known for realising minimal friction. The sealing of plunger 14 will not be perfect, as a result of which it will be in the shown position when the actuator is not activated. When LED array 10 is switched on, the expanding gas in absorption chamber 8 will push plunger 14 upwards.

Fig. 3B schematically shows in cross-section a metal absorption chamber 8 with a metal membrane 16 welded on top of it. The insides of both the absorption chamber 8 and membrane 16 are polished, so as to make them highly

reflective for light emitted by LED arrays 10. When LED arrays 10 are switched on, membrane 16 will puff up and will exert a force onto an object mounted above it, for example a break shoe or onto a liquid or gas in an

additional chamber, the two chambers plus two valves forming a pump. LED arrays 10 may be connected to a pulsed current source, for adjusting a force of the break shoe or for generating a pump action.

Fig. 4A schematically shows in cross-section an actuator 1 where a metal absorption chamber consists of a first chamber halve 8a and a second chamber halve 8b. Second chamber halve 8b is provided with a glass window 9, which connects to a commercial available LED array 10. For both chamber halves 8a, 8b the inner wall is polished, so as to make them highly reflective for light emitted by LED array 10. Around the interface between first chamber halve 8a and a second chamber halve 8b, rubber bellows 2 are fixed.

Absorption chamber 8a, 8b is filled with a gas and with carbon fibres 6 or a mixture of carbon fibres 6 and glass fibres 11, the latter of which support and mutually

separate carbon fibres 6. When LED array 10 is switched on, carbon fibres 6 will be heated by the radiation emitted by LED array 10 and in turn heat the surrounding gas, as a result of which the pressure of the gas increases and chamber halve 8a is pushed upwards. LED array 10 is

preferably operated via a current source 17 and first switching means 18, for generating current pulses with a frequency chosen for optimising the performance of actuator 1 by employing the effect of the pressure wave which is generated every time the LED array is switched on. The duty cycle can be selected via an input 19 for obtaining a desired force and/or a movement.

Fig. 4B schematically also shows in cross-section an actuator 1 where the metal absorption chamber consists of a first chamber halve 8a and a second chamber halve 8b. In this embodiment, second chamber halve 8b is provided with an internal, curved glass window 20, which distributes the light emitted by LED array 10 more uniformly inside

absorption chamber 8. Around the interface between first chamber halve 8a and a second chamber halve 8a metal bellows 2 are fixed, so as to enable chamber halve 8b to be pushed upwards. With metal bellows, a gas like xenon may be used, giving a better performance thanks to its low thermal conductivity. Alternatively a gas like helium may be used, shortening the recovery time of the actuator after LED array 10 is switched off, thanks to its high thermal conductivity .

Fig. 5 schematically shows in cross-section an actuator 1 where the absorption chamber consists of a first chamber halve 8a and a second chamber halve 8b made of thin-walled stainless steel. First chamber halve is provided with a glass window 9a and a LED array 10a, while second chamber halve 8b is provided with a glass window 9b and a LED array 10b. For both chamber halves 8a, 8b the inner walls are polished, so as to make them highly reflective for light emitted by LED arrays 10a, 10b. Around the interface between first chamber halve 8a and a second chamber halve 8b, bellows 2 are fixed. Absorption chamber 8a, 8b is filled with a gas and with a mixture of carbon fibres 6 and glass fibres 11, the latter of which support and mutually

separate carbon fibres 6. The amount of carbon fibres is chosen such that the light from LED arrays 10a, 10b is at least substantially uniformly distributed within chamber 8a, 8b. The amount of glass fibres is chosen such that the mixture completely fills chamber 8a, 8b, so that no dead volumes will be present. When LED arrays 10a, 10b are switched on, carbon fibres 6 will be heated by the

radiation emitted by the LED arrays and in turn heat the surrounding gas, as a result of which the pressure of the gas increases and chamber halves 8a, 8b are pushed apart. A first prototype of this embodiment with a chamber diameter of 12 centimetres and a weight of 200 grams (not including the two 100W LED arrays which each weigh 30 grams) ,

generate a force of 250N, using atmospheric air and about 1 gram of a 30%-70% mixture of carbon fibres and glass fibres inside the chamber. LED arrays 10a, 10b can be switched on and off simultaneously via a current source 17 and second switching means 21, but it is also possible to use

switching means 21 for switching LED arrays 10a, 10b on and of alternately. This will improve the heat transfer from the carbon fibres to the surrounding gas, as every time a LED array is switched on, the surrounding gas will expand and cause a gas flow toward the opposite side of chamber 8. Many arrangements may use the inventive actuator

advantageously because of its low weight and/or its high MTBF, for example robots, aircraft, vehicles and more in particular space vehicles.