WO2017144873A1

WO2017144873A1 - Rf tuning device, actuator and method

Info

Publication number: WO2017144873A1
Application number: PCT/GB2017/050454
Authority: WO
Inventors: Anthony Hooley
Original assignee: Anthony Hooley
Priority date: 2016-02-23
Filing date: 2017-02-22
Publication date: 2017-08-31
Also published as: GB201700279D0; GB201700278D0; GB201603081D0; GB201700281D0; GB201700280D0

Abstract

A tuning device for an RF cavity or RF antenna has an actuator (7) mechanically coupled (8) to a tuning element (14), such as a tuning pin of an RF cavity. The actuator is an SMA-wire actuator capable of linear movement connected via a mechanical link to an RF tuning pin such that the tuning pin is moveable by the actuator in and out of the RF-cavity. The actuator may comprise a straight SMA wire or a bowstring SMA wire coupled to a return spring, or it may be an SMA-wire actuator comprising a pair of opposed bowstring-actuators (1,2) eliminating the requirement for a return spring and with a design methodology that allows a wide range of performance specifications and envelope sizes.

Description

RF Tuning Device, Actuator and Method

The invention relates to an RF (radio frequency) tuning device, an actuator for use in an RF tuning device, and associated methods. In particular the actuator is suitable for accurate positioning over small displacements, and may find application in tuning RF devices such as RF cavities or RF antennas.

Introduction and Prior Art:

Shape Memory Alloy (SMA) is a well known actuator material and its most common form is a wire made of NiTi (Nickel-Titanium) alloy in roughly 50:50 proportions and possibly with other trace additives. Apart from being a tough and corrosion resistant alloy, NiTi wire (after suitable treatment during manufacture) has the unusual property of contracting by between ~3% to ~10% when heated through a narrow temperature range (e.g. say between ~80C and ~1 10C) the actual temperature range being a function both of the precise alloy composition, and the stress in the wire.

Making a practical actuator from NiTi SMA wire with a defined stroke S

(displacement), force output F, and speed of operation T, involves a series of design choices and compromises.

The more force F required then: the thicker that the wire must be which in turn leads to a slower (de)activation if the wire is naturally cooled; or, the more parallelled strands that must be used. Note that arbitrarily fast activation (contraction) can be achieved by applying sufficient heating power, with care taken not to overheat the wire. The wire may be heated in any practical way but the most useful and simplest is usually Joule-heating by the application of an electric current through the wire.

Once a sufficiently thick wire (bundle) has been chosen to achieve at least the required output force, then the length of wire used needs to be chosen. At worst- case, a contraction of ~3% to 4% can be relied upon, then in its simplest form (a straight wire pulling the load) the wire length L must be: L ≥ S/(0.03) where S is the required stroke. If the value of L so derived exceeds the space constraints for the application then some more complex form of actuator needs to be designed, generally involving leverage, to increase the useful stroke from a shorter wire. There are very many ways to achieve this described in the art, one of which is the bow-string actuator. Once the force and displacement has been achieved by the design, the speed constraint needs to be addressed. If the chosen wire thickness can natural- convection-cool adequately quickly then a practical design has been achieved. If too slow, then splitting the wire into several parallel thinner strands will often achieve suitably fast operation. If not, then some form of active cooling is required which might be for example forced-air- or water- cooling.

Finally, some form of reverse-actuation must be provided, since SMA wire, once heated and contracted, will generally retain its shortened form until mechanically pulled back to its original cold-length. Most frequently, as is known in the art, this return-stroke function is achieved using a mechanical spring, whose

force/displacement, and initial-offset characteristics, are chosen to optimise the operation of the actuator. In some applications where the actuator lifts a load against the force of gravity, no additional mechanical return-actuation mechanism is required, as gravity can fulfil that purpose. The SMA bowstring actuator is known in the art, and is formed by attaching both ends of an SMA-wire to a nominally fixed base or frame, the ends being slightly closer together than the length of the wire so that there is a bit of slack. The load is then attached to the mid-point of the wire, and the actuation force is in the direction orthogonal to the line through the wire ends. As with other SMA actuators, some means of provision of return force is needed to stretch the SMA wire once it has been heated and contracted, and as described above, a spring may be used for this purpose. Alternatively a second SMA actuator may be used, and this second actuator may also be a bowstring actuator. This configuration comprising two opposing bowstring actuators is hereby referred to as a double bowstring actuator, and the form is known in the art (see e.g. US2012/0104292 Active Drain Plug for High Voltage Battery Applications, Kollar, C.A. et al). The control of the actuator is achieved by heating and cooling the SMA wire. As described and as known in the art, this is most conveniently done by Joule-heating of the wire by the passage of a controlled electric current through the SMA wire. For simple low-precision applications, a suitable electric current can simply be turned on, to heat and thus shorten the wire and therefore drive the actuator, and then turned off again to allow the wire to cool back to ambient temperature naturally. Where precision position-control is required by the application, some sort of actuator position estimation is most usefully employed. While open-loop control of SMA wire actuators can be useful, the variation of length of the SMA wire with temperature, and thus with electric Joule-heating current, is not only non-linear, but also highly hysteretic, as well as being a function of ambient temperature and of mechanical load force. Position-feedback is thus essential in most if not all precision positioner applications. One good proxy for SMA wire-length, and thus for actuator position, is SMA wire-resistance. A simple measurement of the electrical resistance of the SMA wire of the actuator can be used to derive a good estimate of the wire-length, and thus the output position of the actuator. This technique which is known in the art, provides reasonable precision position-estimation without the addition of a separate position-sensor, and is thus cheap and simple to implement, as well as being compact. Summary of Invention

The invention provides an RF-cavity tuning device, an RF tuning device, a method for tuning an RF cavity, a method for RF tuning, and a method for developing an actuator as defined in the appended independent claims, to which reference should now be made. Preferred or advantageous features of the invention are set out in dependent subclaims. Tuning an RF cavity requires rapid, accurate positional control of a small, low-mass tuning pin or similar apparatus. As the skilled person would appreciate, the same requirements apply to the tuning of other RF devices such as RF antennas, and the description herein relating to RF cavities therefore also applies to such devices. Shape memory alloy (SMA) actuators are known, as described above, but the skilled person's conventional wisdom is that SMA actuators are only suitable for applying relatively high forces in a relatively poorly controlled way. Thus, for example, SMA actuators are known to be used for "on-off" switching purposes, or for opening and closing valves or taps. However, the inventor has appreciated that despite the apparently poor matching of the requirements for tuning an RF cavity and the actuator properties of an SMA actuator, in fact a carefully-designed SMA actuator can, surprisingly, be effectively used for tuning an RF cavity.

When the temperature Top of the SMA-wire of an SMA-wire actuator is increased between the temperature limits Tmin to Tmax (both specific to the particular formulation of the SMA-wire used, but typically ~80C < Tmin < Tmax < 1 10C, or for some types of SMA wire even < 140C or < 160C), the SMA-wire shortens and is able to apply a force to a mechanical load. When Top of the SMA-wire is decreased between Tmin and Tmax, the SMA-wire may lengthen but in general it will only do so if an external tension force is applied to it. Thus in order to cycle an SMA-wire actuator through a lengthening phase followed by a shortening phase, at least two separate force producing components are required: one of these is the SMA-wire of the actuator, and the other can be any convenient force producing device, including but not exhaustively, a gravity-pulled weight, a mechanical spring that has previously been stretched by the shortening of the SMA-wire, or a second SMA-wire actuator separately controlled from the first, such that the second SMA-wire actuator has its wire temperature increased between the limits Tmin and Tmax, when the first SMA- wire's wire-temperature is being decreased. This part of the cycle of the first SMA- actuator is called the "return-stroke".

It will be known to those skilled in the art that for SMA wires in common use, Tmax may vary over a wide range and may be as high as 140C or even 160C or more. The present invention seeks to address some or all of the following problems: first, SMA wire is commercially available in only a relatively few standard diameters (and thus a relatively few maximum-pulling-force values); secondly, if the displacement required from the actuator is sufficiently large, and the overall compactness required of the actuator, are such that a standard straight-wire actuator will not fit within the design envelope, then some sort of mechanical leverage system is needed to achieve the specified displacement; thirdly, if the operating power available is sufficiently small that the use of a thicker wire to provide more initial force prior to the lever arrangement (which increases mechanical displacement but reduces

mechanical force output available) is not a design option; fourthly, if there is insufficient force available from the SMA wire actuation to both move the

mechanical-load, and to pull against a suitably strong mechanical return-spring (for the return stroke).

Hereinafter "cold length" means the length (of an SMA component) when the SMA material is completely un-actuated, i.e. for NiTi when all the material of the SMA component is in the Martensite state, and when the stress on the material is at least half the maximum safe working stress, and preferably for NiTi around 300MPa, to ensure that the SMA component is at its full "natural" length.

So in one aspect of the present invention, a first SMA wire, Wirel , of cold length 2Lc, may be rigidly mechanically mounted to a fixed base structure (Base) at both ends of Wirel , such that the two wire ends are a distance 2Y apart, with 2Y < 2Lc, or equivalently, Y < Lc. The two mechanical mounts also incorporate electrical connections to the ends of Wirel , these being denoted as terminals T1 1 and T12. The electrical portions of T1 1 and T12 are electrically insulated from each other. This may be achieved either by using electrically conductive terminal/mounting material attached to an electrically insulating Base, or by mounting the electrically conductive portions of T1 1 and T12 to the Base via insulating portions of the terminals, in which case the Base may be electrically conductive, e.g. metal; any other suitable mechanical and electrical arrangement that achieves the same ends as may be devised by those skilled in the art may be substituted without loss of generalization and is included herein. One preferred arrangement is that the terminals T1 1 and T12 are metal e.g. phosphor-bronze components mechanically crimped to the SMA wire to provide good low-resistance electrical connection and strong mechanical connection without damage to the SMA wire, insulatedly mounted to the Base.

A second SMA wire, Wire2, of cold length 2Lc, is similarly (to Wirel ) mechanically and electrically mounted to the Base at both ends of Wire2, with terminals denoted as T21 and T22, such that the two ends of Wire2 are a distance 2Y apart, with Y < Lc as before.

It will be apparent that it is merely desirable and probably convenient but not necessary that the T21 to T22 distance is the same as the T1 1 to T12 distance. In fact these separations could be quite different in size. Similarly, it may be desirable or convenient for the cold lengths of the two wires to be the same, but as the skilled person would understand, these lengths could be different, whether or not the separations between the mechanical mounts for the wires are the same or different.

The four terminals may be arranged such that a line through T1 1 and T12 is parallel to a line through T21 and T22, and also so that a line through T1 1 and T21 is parallel to a line through T12 and T22. In that case, the four terminals are positioned at the corners of a rectangle. As already noted the lengths of each of one pair of parallel sides of the rectangle are of length 2Y. Let the length of each of the other two parallel sides of the rectangle be 2Z. When Wirel is heated to its maximum actuated temperature Tmax then Wirel contracts to or near to its minimum length 2Lh < 2Lc. A further constraint on the dimensions is that 2Y < 2 Lh, or equivalently Y <Lh.

It will also be apparent that again it is merely desirable and probably convenient but not necessary that the four terminals lie in a plane, and similar there is no necessity for T1 1 -T12 to be strictly parallel (nor even approximately so) to T21 -T22; however, design and analysis is simpler if this is so, although perfectly adequately functioning actuators may be designed without these conveniences. To simplify the presentation of the analysis we shall assume that planarity and parallelism are present but those skilled in the art will easily see that less symmetrical arrangements are perfectly in order and functional. So preferably all four terminals lie in a plane, and preferably a line through T1 1 and T12 is parallel to a line through T21 and T22.

A control system is provided capable of precision measurement of the resistance of at least one of the SMA wires, and preferably of both wires, and this control system continually or continuously estimates the length of the at least one wire from the resistance measurements whereupon using the known geometry of the actuator and preferably a measurement of the present ambient temperature around the actuator, the controller calculates the output position of the actuator and also controls the heating current in each wire to continuously maintain the output position of the actuator at any desired position or progressive sequence of positions within the stroke range of the actuator, as commanded by input signals to the controller.

In a preferred embodiment the midpoints of Wirel and of Wire2 are mechanically linked by a Link which is electrically and thermally insulating, but mechanically stiff. When Wirel is heated to its maximum actuated temperature Tmax and thus Wirel is at or near its minimum length 2Lh, and simultaneously Wire2 is unheated, at or near ambient temperature Tamb and thus has stretched length ~2Lc, the length 2Q of the Link between the two wire midpoints is such as to not quite draw both Wirel and Wire2 taut, or to draw the wires slightly taut, or to a low tension. By symmetry, when Wirel is cold (at or near Tamb) and simultaneously Wire2 is heated to ~Tmax and thus is contracted to its minimum length 2Lh, then the mechanical Link again has the effect of not quite drawing taut Wirel and Wire2. Geometrically, the following relationship holds for the various dimensions:

B² = Lh²-Y²;

A² = Lc²-Y²;

Y < Lh < Lc;

and either

2Q > 2Z - A - or

A+B+2Z > 2Q;

These latter two variants of the configuration correspond to the two actuator mechanical/geometric layouts where the two bow-strings either point towards each other or away from each other. They behave similarly as far as actuation is concerned and either may be used to best suit the mechanical application. The constraints on Q are approximate, since as described below, making Q much bigger or smaller than these values results in more or less minimum tension in the wire.

Thus this actuator configuration, which can be seen in a preferred embodiment to be comprised of a coupled symmetrical pair of opposing bowstring actuators, is capable of moving the Link between two limit positions, in either direction (along the line of the Link, or the line of motion of the Link), and obviates the need for a return spring for reverse actuation, each of the two bowstring actuators acting as the reverse mechanism for the other.

Further by suitable choice of dimensions Lc, Y and Q, it can be shown that the mechanical displacement output achieved (i.e. the movement of the Link, which preferably is the mechanical output element of the actuator) may be tailored to the required actuator specification and in particular can be greater than the

displacement achievable by a similar straight wire SMA actuator within the same size actuator-device envelope. This effectively provides mechanical leverage, or mechanical gain G. Because this arrangement does not require the provision of a mechanical return-spring, advantageously none of the available wire displacement force is used internally to overcome the force of such a return spring.

The bowstring-actuator in this preferred embodiment has mechanical gain G

(leverage) given by the following expression:

G = L/sqrt(L²-Y²) where G is the leverage or mechanical gain, sqrt is square root, L is the SMA-wire length at angle c to the dimension Y which in turn is orthogonal to actuation direction. Angle c is given by: c = arccos(Y/Lc)

When Y is ~0 the gain G ~1 .0 and the configuration is more or less equivalent to a straight-wire actuator. When Y~L the gain G becomes close to infinite. In practice a wide range of useful gains, 1 <G<10 can be achieved. Of course when G is high the mechanical force output will be appropriately reduced by a factor ~1 /G times that of the SMA-wire alone. In practice, the maximum mechanical output force Fmax oi the actuator is given by

Fmax = 2Fwmax*Sin( c ); where Fwmax is the maximum allowable tension in the SMA-wire, and as before c is the angle between the SMA-wire direction and the direction orthogonal to the movement of the Link. Clearly G will be large only when L is close to Y in length, in which case angle c is quite close to Odeg, and the force output will be a small fraction of the wire's inherent pulling capability. Note also that in fact the mechanical gain G is not a constant but varies slightly with actuator output stroke, since L varies with the temperature of the SMA-wire. This is a small effect and may usually be neglected.

Thus in one preferred embodiment of the Invention, a pair of similar or identical bowstring SMA-wire actuators are mechanically coupled, preferably by a Link, at their wire-centres so as to act in opposition to each other. It will be apparent to those skilled in the art that a pair of dissimilar bowstring SMA- wire actuators mechanically coupled at or near their wire-centres so as to act in opposition to each other will also function perfectly well, but are merely less convenient to analyse and control.

Preferably, the two bowstring actuators are arranged to lie in the same plane, although useful configurations are possible where this is not the case, and such configurations are included herein. Preferably the mechanical coupling is thermally insulating, and preferably the mechanical coupling is electrically insulating.

Preferably the two bowstring actuators are arranged parallel to each other.

Preferably the two bowstring actuators are arranged so that their actuation directions lie along the same line. Preferably, the two bowstring actuators have similar or identical dimensions to each other. Preferably the two bowstring actuators have mirror symmetry. Preferably, the two bowstring actuators are controlled in such a manner that when one SMA-wire is fully heated to maximum working temperature Tmax and thus actuated (contracted), the other wire is minimally heated and capable of reaching full or near to full cold length at low stress, and vice versa. Preferably, where one SMA-wire is partially heated to a temperature below Tmax, heating of the other wire is controlled such as to maintain the stress in both wires safely below their maximum safe working stress. Preferably, the actuator controller is arranged to provide controlled heating to one or the other (or both) SMA-wires so as to allow, zero, partial or full actuation in either direction.

In any of the above embodiments of this aspect of the invention a control system may be provided capable of measuring the position of the actuator output (i.e. that part of the actuator connected to its load, to be moved by the actuator) by the use of suitable position sensor means. In a preferred embodiment the position sensor means are implemented by electrical resistance measurement of one or more of the SMA-wires of the actuator embodiments, and from the measurement(s) of resistance said control system using a first algorithm estimates the instantaneous length(s) of the SMA-wire(s), and from these length estimates together with knowledge of the specific actuator mechanical configuration and geometry of the controller is capable of estimating the output position of the actuator and thus is able to accurately position the external load mechanically driven by the actuator. In a preferred embodiment the controller continually or continuously uses the position-sensor data to estimate the position of the actuator output. In another preferred embodiment the controller uses temperature sensor means to sense the ambient temperature around the actuator and its wires, and in a more preferred embodiment temperature sensor means are implemented by one or more of the SMA-wires of the actuator and the controller uses a second algorithm to estimate the ambient temperature from internal parameters and electrical measurements of one or more of the SMA wires. The controller in all embodiments described controls the electrical heating current to the one or more SMA wires. In preferred embodiments the controller continually or continuously maintains the output position of the actuator at precise position or positions within the stroke range of the actuator, which said positions may be commanded from time to time by input signals to the controller from an external system. A related aspect of the present invention may advantageously provide a design method or methodology for the design of a precision continuously positioning SMA- wire actuator of the present invention which given an actuator-specification determines the dimensions and geometry of the actuator, including the wire size, said method being described in detail below, complete with relevant design equations.

So in another preferred aspect of the invention, an SMA-wire actuator is connected via a mechanical link to an RF tuning pin causing the tuning pin to move in and out of an RF cavity so as to adjust the electromagnetic tuning of the cavity. Preferably the mechanical link is electrically insulated. Preferably the SMA-wire actuator is capable of precision continuous positioning to any point throughout its stroke.

In a preferred embodiment of this aspect of the invention the SMA-wire actuator comprises a straight section of SMA-wire connected in series to a mechanical spring which provides the return force for the return-stroke of the SMA-wire actuator cycle. In a more preferred embodiment of this aspect of the invention the SMA-wire actuator comprises two separate straight-wire SMA actuators mechanically connected in series, each actuator capable of providing the return force for the other during the other's return-stroke of its SMA-wire actuator cycle. In an even more preferred embodiment the two separate straight-wire actuators have similar or identical dimensions, and the wires of each are preferably aligned along the same line or direction.

In a second preferred embodiment of this aspect of the invention, the SMA-wire actuator is a bowstring-actuator with a mechanical spring to provide return force for the return stroke of the SMA-wire bowstring-actuator cycle.

In a second more preferred embodiment of this aspect of the invention, the SMA- wire actuator comprises a pair of opposed bowstring-actuators mechanically connected in opposition, each bowstring-actuator acting to produce the return force for the other bowstring-actuator's return stroke of that other bowstring-actuator's cycle. In a more preferred embodiment the two bowstring actuators have similar or identical dimensions. In an even more preferred embodiment the two bowstring actuators are positioned with their actuation directions lying along the same direction line. A first preferred mechanical configuration is the IOB (Inwards Opposed

Bowstrings) configuration. A more compact preferred mechanical configuration is the OOB (Outwards Opposed Bowstrings) configuration. A most compact preferred mechanical configuration is the AOB (Alternate Overlapping Bowstrings)

configuration.

In any of the above embodiments of the invention a control system may be provided capable of measuring the position of the load by the use of suitable position sensing. In a preferred embodiment the position sensing is implemented by electrical resistance measurement of one or more of the SMA-wires of the actuator

embodiments, and from the measurement(s) of resistance said control system estimates the instantaneous length(s) of the SMA-wire(s), and from these length estimates together with knowledge of the specific actuator mechanical configuration is capable of estimating the output position of the actuator and is thus able to accurately position the external load mechanically driven by the actuator. All of the actuators and associated technology described herein for the tuning of RF cavities, are equally suitable for the tuning of RF antennas. In another aspect of the invention an RF antenna tuning device is comprised of any one or more of the SMA- actuators herein described connected via a mechanical link to a moveable tuning element such that the element moves relative to the other components of the antenna tuning device, in so doing changing the optimum frequency of the antenna tuner.

Brief Description of the Drawings

Fig.1 shows a plan view schematically illustrating an actuator embodying the present invention.

Fig.2 shows the same plan view of the actuator as Fig.1 but in the oppositely actuated state.

Fig. 3 is to be interpreted as a geometrical layout representation of the same actuator as shown in Figs.1 and 2.

Fig.4 illustrates an alternative geometry of an actuator embodying the present invention.

Fig.5 illustrates a most compact geometry of an actuator embodying the present invention.

Fig.6 shows a schematic plan view of an example of a physical embodiment of the present invention.

Fig.7 is a perspective view of a complete double bowstring actuator with an RF tuning pin attached to its output port, according to an embodiment of the invention.

Fig.8 illustrates the attachment of the actuator in Fig.7 to an RF tuning cavity. Fig.9 is a close-up of cut-away portion of the RF cavity of Fig.8 and the actuator and tuning pin of Fig.6 in place. Detailed Description of the Drawings

A further description and examples will now be given with reference to the Drawings. Fig. 1 shows a plan view of an actuator according to a first embodiment of the present invention. There is a Base 7 supporting four terminals T1 1 at 3, T12 at 4, T21 at 5 and T22 at 6. A first SMA wire 1 is mechanically attached and electrically terminated at its two ends to terminals 3 and 4, and a second SMA wire 2 is mechanically attached and electrically terminated at its two ends to terminals 5 and 6. A rigid electrically-insulating link 8 is attached to the midpoint of wire 1 at 9 and to the midpoint of wire 2 at 10. Other than these two attachments to the wires, link 8 is free to move relative to the Base 7, although its movement may be guided in a direction along a line between the wire centres 9 and 10 by any suitable mechanical means, e.g. a slot in the Base 7. The mechanical configuration shown in Fig.1 with the "midpoints" 9, 10 of the bowstrings 1, 2 facing inwards towards each other without the bowstrings overlapping is designated Inward Opposed Bowstrings or IOB.

This example illustrates a symmetrical and planar embodiment.

In the view shown in Fig. 1 , wire 1 is actuated (heated to Tmax and thus maximally contracted) and has total length 2Lh (the sum of length from 3 to 9 of length Lh, and from 9 to 4 also of length Lh), while wire 2 remains at or close to ambient

temperature and has length close to its maximal cold (unheated) length 2Lc (the sum of length from 5 to 10 of length Lc, and from 10 to 6 also of length Lc). It will be seen that in this configuration link 8 is off left of centre of the actuator.

Fig. 2 shows the same actuator as Fig.1 but now with wire 1 unheated and close to ambient temperature and of total length close to its maximum value 2Lc, and wire 2 heated to Tmax and actuated and thus contracted close to its shortest length 2Lh. It will be seen that the nett effect of these changes is move the link 8 from left of centre (in Fig.1 ) to right of centre (in Fig.2) by an amount S (not shown), along the line of centres 9 and 10 of the wires 1 and 2.

Fig. 3 is to be interpreted as a geometrical representation of the same actuator as shown in Figs.1 and 2, where the capital letters (A, B, Q, Y, Z) are linear dimensions, and lowercase letters (a and b) are angles. The following equations can immediately be written down:

Lc² = Y²+A² (1 )

Lh² = Y²+B² (2)

A+2Q+B = 2Z (3)

S = A - B (4)

This last equation (4) says that the total output displacement S of the actuator is simply the difference between dimension A and dimension B, an argument by symmetry.

Tan(a) = A/Y (5)

Tan(b) = B/Y (6)

Fig.4 illustrates an alternative geometry where the relevant equation for A, B, Z and Q is:

A+B+2Z > 2Q;

The mechanical configuration shown in Fig.4 with the midpoints 9, 10 of the

Bowstrings 1, 2 facing away from each other is designated Outwards Opposed Bowstrings or OOB.

If the two SMA-wires 1, 2 (see Fig. 5) forming the bowstrings of the opposed bowstring actuators can adequately be kept mechanically and electrically separated (e.g. by the insertion of a thin smooth insulating sheet between the wires, or alternatively by using smooth insulation-coated SMA-wires, or by mounting SMA- wires 1 and 2 above opposite faces of base 7) then the bowstrings or SMA-wires 1, 2 may overlap each other— this arrangement makes for a more compact actuator as can be seen from the smaller Base 7 (shorter in the Z-direction but by working within the design constraints can also be shorter in the Y-direction too). An example of this form is shown in Fig.5. In this case the A, B, Q, Z relationship is:

2Q < A+B-2Z

and note now that in this arrangement Z is more properly defined as the separation of the terminals T1 1 and T21 (and of T12 and T22). The constraint on the length 2Q of Link 7 is now given by:

2Q < A+B-2Z, Q > 0.

The mechanical configuration illustrated in Fig.5 is designated Alternate

Overlapping Bowstrings or AOB. In this AOB configuration, the actuator envelope size 2Ze in the Z direction is given by

2Ze =2A-2Z. We next illustrate how to design the actuator for a specific application. The application specification is as follows:

Output stroke: S [mm]

Output load force: Fop [newton]

Maximum actuator dimension in direction of actuation: 2Zmax [mm]

Maximum actuator dimension in direction orthogonal to actuation: 2Ymax [mm] Maximum full-stroke operation time: t [sec]

Useful stroke available directly from SMA-wire: k [mm/mm] (a wire property) For standardly available SMA actuator wires k is generally in the range from ~3% to ~8% ( a 100mm wire might be expected to reliably and repeatedly contract by at least 4mm when utilised optimally).

First in the design process one attempts to satisfy the application specification with a simple straight-wire actuator: in that case the maximum length of wire in the direction of actuation is 2Zmax and so the available direct-stroke is 2/cZmax. If this is >S then a simple straight-wire actuator will suffice; it then remains to choose the SMA-wire gauge, and in this case the smallest available wire gauge with maximum allowable wire tension Fwmax > Fop is chosen. However, it very often occurs that the overall size constraints do not allow adequate stroke from a straight SMA wire, and in this case when 2/cZmax < S, then the actuator of the present invention may be an adequate solution instead.

Now we may also make use of the maximum dimension 2Ymax orthogonal to stroke direction to fit a longer piece of SMA wire (two pieces in fact, one for each of the two opposed-bowstrings).

In the IOB configuration (Inwards Opposing Bowstrings, Fig.1 ) the dimension 2Q (the length of the Link) is a free variable and can be any convenient length that allows adequate mechanical separation of the two SMA-wire centres, together with proper mechanical coupling to the mechanical load to be moved by the actuator. Importantly, Q can usually be small compared with Zmax. The actuator envelope size 2Ze in the Z direction is given by

2Ze > A+B+2Q. In the OOB configuration (Outwards Opposing Bowstrings, Fig.4), where now 2Z is the separation of the terminals T1 1 and T21 (and also of T12 and T22) the approximate constraint on Q is of the form

2Q < A+B+2Z If Q is much bigger than this then there is never a wire-temperature condition where at least 1 wire is fully slack. However, if Q is much smaller than this then there will be too much slack in the system and stroke will be reduced. In the OOB configuration the actuator envelope size 2Ze in the Z direction is given by

2Ze > 2Q.

In the AOB configuration (Alternate Overlapping Bowstrings, Fig.5), where 2Z is the separation of the terminals T1 1 and T21 (and also of T12 and T22) the approximate constraint on Q is of the form

2Q < A+B-2Z In the AOB configuration the actuator envelope size 2Ze in the Z direction is given by

2Ze = 2A-2Z.

Next the wire gauge is chosen. The smallest available wire gauge with maximum allowable wire-tension Fwmax > Fop is first chosen

Next we compute the minimum wire angle b (see Fig.3) which will give the required output force Fop as

b_min = Asin(Fop/(2^*Fwmax)

Knowing b_min we can compute the dimension B and Lh the hot-length (actuated length) of a half wire as

B = Ymax^* Tan(b_min)

Lh =Ymax/Cos(b_min)

Knowing Lh and the wire factor e we can compute the wire cold-length Lc as

Lc = Lh/(1 - f)

Knowing the length Lc of SMA-wire to be used, the wire actuation constant k, we can compute the actuator mechanical gain G using

G = L/sqrt(L²-Y²)

so the gain Gh at the hot-end of the stroke is

Gh = Lh/sqrt(Lh²-Ymax²)

and the gain Gc at the cold-end of the stroke is

Gc = Lc/sqrt(Lc²-Ymax²)

The dimension A is calculated as

A = sqrt(Lc²-Ymax²) and so finally we can compute the maximum stroke Smax as

Smax = A-B

which for this actuator to be a viable solution to the specification needs to satisfy Smax > S.

If Smax < S then the next bigger available wire gauge is chosen, and the method thereafter repeated.

We next further illustrate the utility of actuators embodying present invention by examining a real application, and design a double-opposed-bowstring actuator example of the present invention to satisfy the real application's specification.

The example application is to move a tuning pin for an RF cavity or similar RF device. The application specification is as follows: Output stroke: S>1 [mm]

Output load force: Fop>50 [mN]

Maximum actuator dimension in direction of actuation: 2Zmax=21 [mm]

Maximum actuator dimension in direction orthogonal to actuation: 2Ymax=21 [mm] Maximum full-stroke operation time: tmax<5 [sec]

Maximum (100% duty cycle) power consumption Pmax: 500 [mW]

Useful stroke available directly from SMA-wire: k = 4%[mm/mm]

so Zmax=10.5mm and Ymax=10.5mm.

An SMA wire of 25micrometres diameter available as standard is capable of a load of Fwmax~8.9g ~0.087N = 87mN and so has adequate force capability before the stroke is increased by a leverage factor G (to be determined).

Using the above design equations:

b_min = Asin(Fop/(2*)Fwmax) = 0.29rad = 16.6deg

B = Ymax* Tan(b_min) = 2.99mm

Lh =Ymax/Cos(b_min) = 10.4mm

Lc = Lh/(1 - f) = 10.9mm Gh = Lh/sqrt(Lh²-Ymax²) = 3.49 (the mechanical gain Gh of the actuator when the wire is fully heated to Tmax)

Gc = Lc/sqrt(Lc²-Ymax²) = 2.55 (the mechanical gain Gc of the actuator when the wire is at or below Tmin)

A = sqrt(Lc²-Ymax²) = 4.27mm

a_min = 0.40rad = 23.1 deg

Smax = A-B = 1 .28mm

Fopmax = 2Fwmax*Sin(a_min) = 68.5mN

Fopmin = 2Fwmax*Sin(b_min) = 50.0mN (by design)

So we can see that this design solution satisfies the minimum output force specification of Fop=50mN and provides ~28% excess output stroke Smax = 1 .28mm versus the spec of 1 mm. A 25micrometres SMA wire-diameter actuator of these dimensions can be expected to consume well under 100mW of power at 100% duty cycle (calculated as ~61 mW for this specific design), and have a cycle-time of <0.3sec, both figures very well within specification.

In an IOB configuration, with a Link 7 length 2Q=7.25mm this results in a maximum actuator envelope width of Ze = 15.1 mm and a maximum actuator depth 2Ymax of 20mm, well within the specified envelope of 22x22mm. The height of the actuator could be <1 mm, so an actuator envelope of 15.1 x20x1 mm is possible.

Using the OOB configuration with a Link 7 length 2Q=8.25mm, results in a maximum actuator envelope width of Ze = 8.25mm and a maximum actuator depth 2Ymax of 20mm, which is more compact. The height of the actuator remains unchanged at typically <1 mm, so an actuator envelope of 8.25x20x1 mm is possible.

Finally using the AOB configuration, with a Link 7 length 2Q=2mm results in a maximum actuator envelope width of Ze = 5.25mm and a maximum actuator depth 2Ymax of 20mm, which is significantly more compact still. Again, the height of the actuator remains unchanged at typically <1 mm, so an actuator envelope of

5.25x20x1 mm is possible. A differently conservative design could increase angles a_min to 0.45rad and b_min to 0.35rad somewhat to reduce the output stroke to 1 .14mm and increase the minimum output force to ~59.9mN, with actuator envelope size for the three configurations IOB, OOB and AOB of 17.4x20x1 mm, 9.5x20x1 mm, 6.5x20x1 mm respectively.

Note that these designs are based on an SMA-wire k factor of 4% which is conservative. With a k factor of 5% the former design produces a stroke of 1 .56mm and a minimum output force of 50mN, and the latter design produces a stroke of 1 .41 mm and a minimum output force of 59.9mN, again all performance figures well above specification, and in envelope sizes for the former design of 15.6x20x1 mm, 8.6x20x1 mm and 5.6x20x1 mm for the IOB, OOB and AOB configurations

respectively, and for the latter design of 18.0x20x1 mm, 9.8x20x1 mm and

6.8x20x1 mm for the IOB, OOB and AOB configurations respectively.

As a further variant, and still assuming a k factor of 5%, we can reduce the actuator total Y dimension to say 15mm when the design procedure results in, for example, a minimum force of 50mN with minimum stroke of 1 .17mm in actuator envelope sizes for the IOB, OOB and AOB configurations respectively of 1 1 .9x15x1 mm,

6.7x15x1 mm and 3.7x15x1 mm— alternatively, for example, a minimum force of 59.9mN and minimum stroke of 1 .05mm, and actuator envelope sizes for the IOB, OOB and AOB configurations respectively of 13.6x15x1 mm, 7.6x15x1 mm and 4.6x15x1 mm.

Using the different geometries shown in Fig.1 (IOB), Fig.4 (OOB) and Fig.5 (AOB) allows actuators with similar performances but which are variously compact. Also, varying the Link length 2Q within the allowable design constraints as shown and defined by the equations given, will also allow the modification and tuning of the actuator envelope size. Clearly all manner of variants between these limits, and others above these limits, are possible, and still produce working actuators capable of operating within the application design specification. A practical implementation of these actuators can be achieved with a simple single- layer printed circuit board (PCB) as the Base 7, holding soldered-in crimp-terminals made of suitable metal e.g. phosphor bronze, or stainless steel, as the Terminals T1 1 ... T22, in which case the actuator control electronics can easily and preferably be mounted on the same PCB. SMA-wires of the correct (designed) length are then crimped between the terminals - alternatively the wire may be pre-crimped between the terminals, before they are added to the Base 7. In this case if the terminals are soldered into the Base then care must be taken to avoid overheating the SMA-wire crimped into the terminals. Another alternative is to use a metal Base 7 fitted with insulated mounted terminals T1 1 ... T22, in which case a separate small PCB can be used to carry the control electronics with this then wired to the terminals, before or after terminal-insertion, as described above.

The link can be fabricated from any suitable insulating material such as PTFE, although much cheaper polymer alternatives able to withstand the maximum wire temperature of <1 10C (or HOC or 160C, depending on the SMA material used) are available and known to those skilled in the art. A low thermal conductivity material for the Link 8 is preferred, and also a material with low thermal capacity, in order to minimise heat loss from a heated SMA wire. In practice, for the RF tuning pin application the material forming the Link 8 would most usefully be extended beyond the wire-centre point at one end of the link, and have the tuning pin directly attached at its far end, eliminating any mechanical bearings or levers or other rotary

components, and providing direct actuation of the tuning pin.

Fig.6 shows a plan view of an example of an actual embodiment of the present invention, and is based on one of the design examples described herein above, which satisfies the real-application design specification. This is an approximately-to- scale drawing where the top "9mm" and left "20mm" legends and dimension lines are dimension scales in mm. It can be seen that the overall actuator size envelope is approximately 20x9mm. The Base 7 is a PCB and the four terminals 3, 4, 5, 6 are phosphor-bronze crimp-terminals, soldered into the Base PCB 7. The two SMA-wires 1 and 2 are crimped into the terminal-pairs 3 & 4, and 5 & 6, respectively. There is an electronic actuator control chip 13 at bottom-right of the PCB, and chip 13 is electrically connected to the four terminals via PCB traces 15. The control chip is connected by further PCB traces to an edge-connector 12 for connection to the external system. The midpoints 9 and 10 of wires 1 and 2 are mechanically connected together by electrically insulated Link 8. Link 8 extends to the right of the actuator where it is directly mechanically connected to the tuning pin 14 of the RF- cavity (not shown) to be tuned.

Thus this novel double-opposed bowstring actuator in its various configurations and the design technique of the present invention provides a flexible and versatile actuator which can be tailored to various loads, strokes and mechanical layouts for a vast range of applications. It is particularly suited to the controlled movement of RF- cavity tuning pins, providing a compact, low power reliable actuator with no bearings and no moving parts to wear (only shape-changing components); other similar SMA- wire actuators have shown to be capable of reliable operation with over 500,000 to 5million complete cycles.

Fig.7 is a perspective schematic view of an embodiment of the present invention which is a double bowstring SMA-wire actuator connected to a tuning pin 130 for tuning an RF cavity (not shown). 1 is an SMA wire, Wirel, of a first bowstring actuator, with its ends crimped at 20, 21 into mechanical and electrical terminals. 2 is the SMA-wire Wire2 of a second, opposed, bowstring actuator. Wirel engages with a first load attach pin 23 mounted in a push-rod 3 which is free to slide within the body 4, 4A, 4B of the actuator, along a line orthogonal to 20 to 21. Wire2 similarly engages with a second load attach pin also mounted in the same push-rod 3, the bowstrings thus can jointly move the push-rod in either direction along its length, taking with it the tuning pin 130 attached to its end by bolt 25.

Fig.8 is a perspective schematic view of the actuator 143 of Fig.7 attached to the outside body of an RF tuning cavity 140 with RF connecting ports 141, 142 at either end. Cavity 140 is shown cut away around the location of the RF tuning pin 130 which passes through a clearance hole in the cavity wall, preferably with electrical grounding means therearound to tightly couple the conductive portion of pin 130 with the conductive inner surface of cavity 140. It will be seen that by operating the actuator 143 the pin 130 will change the internal (electrical) geometry of the RF cavity and thus affect its resonances, as desired by this application.

Fig.9 is a close-up of the actuator 143 and the cut-away section of cavity 140 and showing the inside 146 of the cavity and the cut-away walls 145.

In other preferred aspects of the invention, any of the SMA-wire actuators described above may be connected via an electrically insulated mechanical link to an RF tuning pin causing the tuning pin to move in and out of an RF cavity so as to adjust the electromagnetic tuning of the cavity. Examples include the SMA-wire actuator comprising a straight section of SMA-wire connected in series to a mechanical spring which provides the return force for the return-stroke of the SMA-wire actuator cycle, or comprising two separate straight-wire SMA actuators mechanically connected in series, each actuator capable of providing the return force for the other during the other's return-stroke of its SMA-wire actuator cycle. In an even more preferred embodiment the two separate straight-wire actuators have similar or identical dimensions, and the wires of each are preferably aligned along the same line of direction. In another preferred embodiment, the SMA-wire actuator is a bowstring- actuator with a mechanical spring to provide return force for the return stroke of the SMA-wire bowstring-actuator cycle, or more preferably comprises a pair of opposed bowstring-actuators mechanically connected in opposition. In any of the above embodiments of the invention a control system may be provided capable of measuring the resistance of each of one or two of the SMA-wires of the actuator embodiments, and from the measurement(s) of resistance said control system estimates the instantaneous length(s) of the SMA-wire(s), and from these length estimates together with knowledge of the specific actuator mechanical configuration is capable of estimating the output position of the actuator and thus able to accurately position the external load mechanically driven by the actuator.

Claims

1 . An RF-cavity tuning device comprising:

an SMA-wire actuator capable of linear movement connected via a

mechanical link to an RF tuning pin such that the tuning pin moves in and out of the RF-cavity.

2. An RF-cavity tuning device according to Claim 1 wherein the SMA-wire actuator is comprised of a straight section of SMA-wire connected in series to a mechanical spring which provides the return force for the return-stroke of the SMA-wire actuator cycle.

3. An RF-cavity tuning device according to Claim 1 wherein the SMA-wire actuator is comprised of two separate straight-wire SMA actuators mechanically connected in series, each actuator capable of providing the return force for the other during the other's return-stroke of its SMA-wire actuator cycle.

4. An RF-cavity tuning device according to Claim 3 wherein the two separate straight-wire actuators have similar or identical dimensions.

5. An RF-cavity tuning device according to Claim 4 wherein the wires of each actuator are preferably aligned along the same line of direction.

6. An RF-cavity tuning device according to Claim 1 wherein the SMA-wire actuator is a bowstring-actuator with a mechanical spring to provide return force for the return stroke of the SMA-wire bowstring-actuator cycle.

7. An RF-cavity tuning device according to Claim 1 wherein the SMA-wire actuator is comprised of a pair of opposed bowstring-actuators mechanically connected in opposition, each bowstring-actuator acting to produce the return force for the other bowstring-actuator's return stroke of that other bowstring-actuator's cycle.

8. An RF-cavity tuning device according to Claim 7 wherein the two bowstring actuators have similar or identical dimensions.

9. An RF-cavity tuning device according to Claims 7 or 8 wherein the two bowstring actuators are positioned with their actuation directions lying along the same direction line.

10. An RF-cavity tuning device according to Claims 7, 8 or 9 wherein the mechanical configuration is the IOB configuration.

1 1 . An RF-cavity tuning device according to Claims 7, 8 or 9 wherein the mechanical configuration is the OOB configuration.

12. An RF-cavity tuning device according to Claims 7, 8 or 9 wherein the mechanical configuration is the AOB configuration.

13. An RF-cavity tuning device according to any of Claims 1 to 12 also comprising a control system capable of measuring the resistance of each of one or two of the SMA-wires of the actuator embodiments.

14. An RF-cavity tuning device according to Claim 13 wherein said control system uses the measurement(s) of resistance to estimate the instantaneous length(s) of the SMA-wire(s).

15. An RF-cavity tuning device according to Claim 14 wherein said control system uses said length estimates together with knowledge of the specific actuator mechanical configuration to estimate the output position of the actuator.

16. An RF-cavity tuning device according to any preceding Claim wherein said mechanical link is an electrically insulated mechanical link.

17. A tuning device for an RF apparatus or device, such as an RF cavity or an RF antenna, comprising:

an SMA-wire actuator capable of linear movement connected via a

mechanical link to a tuning element of the RF apparatus or device such as a tuning pin.

18. A tuning device according to claim 17, in which the SMA-wire actuator is as defined in any of claims 2 to 16.

19. A method for tuning an RF cavity, comprising the steps of;

coupling a tuning pin of the RF cavity via a mechanical link to an SMA-wire actuator capable of linear movement, and controlling the SMA-wire actuator such that the tuning pin moves in and out of the RF-cavity to tune the RF cavity.

20. A method for tuning an RF cavity according to Claim 19, in which the SMA-wire actuator comprises two separate SMA wires, each capable of providing a return force for the other during the other's return-stroke of its SMA-wire actuator cycle, and controlling the temperatures of both wires to control the position of the tuning pin.

21 . A method for tuning an RF cavity according to Claim 20, in which each SMA wire is a bowstring of a pair of opposed bowstring-actuators mechanically connected in opposition, each bowstring-actuator acting to produce the return force for the other bowstring-actuator's return stroke of that other bowstring-actuator's cycle.

22. A method for tuning an RF cavity according to claim 19, 20 or 21 , comprising the step of measuring the resistance of each of the SMA wires of the actuator embodiments and using the estimated instantaneous length(s) of the SMA wire(s) as estimated from the measurement(s) of resistance as an input to the control of the movement of the tuning pin.

23. A method for tuning an RF cavity according to claim 22, wherein said control system uses said length estimates together with knowledge of the specific actuator mechanical configuration to estimate the output position of the actuator and the tuning pin.

24. A method for tuning an RF apparatus or device such as an RF cavity or an RF antenna, comprising the steps of;

coupling a tuning element of the RF apparatus or device via a mechanical link to an SMA-wire actuator capable of linear movement, and controlling the SMA-wire actuator such that the tuning element moves to tune the RF apparatus.

25. A method for tuning an RF apparatus or device according to claim 24, comprising method steps as defined in claims 19 to 23.

26. A method for designing an SMA actuator for actuating a predetermined load through a predetermined distance range at a predetermined speed or in a

predetermined time, comprising the steps of;

1 ) assessing the following parameters;

Available package size for the SMA actuator;

Required actuator output stroke: S

Required actuator output load force: Fop

Maximum actuator dimension in direction of actuation: 2Zmax

Maximum actuator dimension in direction orthogonal to actuation: 2Ymax

Maximum full-stroke operation time: t

Useful stroke available directly from SMA-wire: k (a wire property)

2) attempt to satisfy the application specification with a straight-wire actuator: in which case the maximum length of wire in the direction of actuation is 2Zmax and so the available direct-stroke is 2/cZmax; if this is >S then specify a simple straight-wire actuator; but if the overall size constraints do not allow adequate stroke, such that 2/cZmax < S, then:

3) specify an opposed double bowstring actuator having dimensions calculated as follows:

minimum wire angle b which will give the required output force Fop as

b_min = Asin(Fop/(2^*Fwmax);

knowing b_min compute the dimension B and Lh the hot-length (actuated length) of a half wire as

B = Ymax^* Tan(b_min)

Lh =Ymax/Cos(b_min); knowing Lh and the wire factor k compute the wire cold-length Lc as

Lc = Lh/(1 - f); knowing the length Lc of SMA-wire to be used, the wire actuation constant k, compute the actuator mechanical gain G using

G = L/sqrt(L²-Y²)

so the gain Gh at the hot-end of the stroke is

Gh = Lh/sqrt(Lh²-Ymax²)

and the gain Gc at the cold-end of the stroke is

Gc = Lc/sqrt(Lc²-Ymax²); and calculate the dimension A as

A = sqrt(Lc²-Ymax²)

giving maximum stroke Smax as

Smax = A-B

which for this actuator to be a solution to the specification needs to satisfy

Smax > S.

27. A method according to claim 26 wherein step 2) further comprises choosing the smallest available wire gauge with maximum allowable wire tension Fwmax > Fop before attempting to satisfy the application specification with a straight wire actuator; and wherein an additional step 4) after step 3) is:

4) if Smax < S then choose the next biggest available wire gauge, and repeat step 3) onwards.

28. A method for designing an SMA actuator as defined in claim 26 or 27, in which the double bowstring actuator is either: an IOB configuration with the dimension 2Q (the length of the Link) being any convenient length that allows adequate mechanical separation of the two SMA-wire centres, together with mechanical coupling to the load to be moved by the actuator, the actuator envelope size 2Ze in the Z direction being given by

2Ze > A+B+2Q; or an OOB configuration, where now 2Z is the separation of the terminals T1 1 & T21 (and also of T12 and T22) and the constraint on Q is of the form 2Q < A+B+2Z and the actuator envelope size 2Ze in the Z direction is given by

2Ze = 2Q; or an AOB configuration, where 2Z is the separation of the terminals T1 1 & T21 (and also of T12 and T22) and the constraint on Q is of the form 2Q < A+B-2Z and the the actuator envelope size 2Ze in the Z direction is given by

2Ze = 2A-2Z.