Abstract:
A fluid transmission that employs a fluid to transmit a force, comprising a conduit for the fluid made from heat shrink polymer tubing, wherein at least a portion of the heat shrink polymer tubing is shrunken, whereby the force can be transmitted by the fluid from a first or proximal end of the conduit to a second or distal end of the conduit. Also, an actuator and methods for manufacturing the transmission and actuator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Phase Application of PCT International Application No. PCT/AU2006/001294, entitled “A FLUID TRANSMISSION”, International Filing Date Sep. 4, 2006, published on Mar. 8, 2007 as International Publication No. WO 2007/7025353, which in turn claims priority from Australian Patent Application No. 2005904837, filed Sep. 2, 2005, both of which are incorporated herein by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a fluid transmission for the transmission of force, of particular use in hydraulic or pneumatic actuators. 
     FIELD OF THE INVENTION 
     Transmission of an actuating force by the movement of fluid through pipes is employed where smooth and linear motion is required. The most common method uses a cylinder enclosing a piston at the driven end, and a fluid pump (which may also comprise a piston and cylinder) at the driver end. 
     Pneumatic systems use an actuating fluid in the form of a gas such as air, so leakage of the actuating fluid is a lesser problem than where hydraulic oils are employed. However, hydraulic systems (where the actuating fluid is in the form of a liquid such as water or oil) can produce greater force and, as liquids are effectively incompressible, greater precision and linearity of motion. 
     Both pneumatic and hydraulic systems have well defined areas of application. Their most common embodiments require precision cylinder bores and pistons. They also rely on the maintenance of fluid seals, typically in the form of which are generally elastomer “o”-rings. Systems that do not require a sliding seal exist (e.g. the pneumatic bellows systems of a pianola) but are not in widespread use. 
     Electromagnetic linear drives that employ linear motors or leadscrews and piezoelectric linear actuators (e.g. Burleigh inchworm drives) are widely used but are complex. Pressure operated linear actuator systems are generally less expensive. 
     Hydraulic (or pneumatic) drivers and actuators can also be made from impermeable flexible bags or sacks connected by flexible pipes. The bags or sacks can be made from elastomeric polymers or from inelastic but flexible material; the latter can be made from a more general class of material than the former. In both cases, the expansion of the bag under pneumatic or hydraulic action can be used to exert a force where desired. 
     Such systems can be versatile and potentially of low cost. They are not widely used, however, possibly because they are not easily made. In particular, the fabrication of small examples can be difficult and ensuring that the seals do not leak can be time consuming. 
     Another feature of certain fluid actuating systems is the manner in which the conveniently obtainable output power/force scales as the size is reduced. For example, the maximum force able to be exerted by an electromagnet is proportional to the volume of the magnetic material of which it is composed (which scales as the cube of its linear dimensions.) Hence, reducing the size of a electromagnetic solenoid or electric (magnetic) motor by a factor of 10 reduces force or power output by a factor of 1000. This inverse cube power law also applies to piezo and many other motors. Currently, the smallest readily available electromagnetic motor is 1.8 mm in diameter and 44 mm long, but costs around AU$1,000 with the required gearbox to produce reasonable torque/force. 
     In the case of electrostatic motors, the force available to drive the motor is proportional to the square of the linear dimensions, that is, the area of the two attracting plates in an electrostatic motor. Reduction in size of such systems to a tenth reduces the force or power to 1/100, a factor of 10 better than an electromagnetic motor. For this reason electrostatic actuating is almost universally employed in nanomotors. These nanomotors are generally in the form of vibrating resonant “comb drives” formed by photolithography and deep etching from silicon wafers. The silicon torsion bridge suspension is strong and highly elastic, so quite high amplitude vibration can be achieved. However, the amplitudes of the vibrations are ultimately limited by the torque produced by the electrostatic forces—which is small—and are only maximized if the waveform of the drive voltage is applied at the resonant frequency. 
     SUMMARY OF THE INVENTION 
     According to a first broad aspect of the invention, the present invention provides a fluid transmission that employs a fluid to transmit a force, comprising a conduit for the fluid made from heat shrink polymer tubing, wherein at least a portion of the heat shrink polymer tubing is shrunken, whereby the force can be transmitted by the fluid from a first or proximal end of the conduit to a second or distal end of the conduit. 
     The conduit may additionally include (at the proximal and/or distal end) one or more portions of unshrunk or semishrunk heat shrink polymer tubing, either integral with the shrunken portion or comprising separate portions of heat shrink polymer tubing. 
     In particular, the transmission may include a driver section formed from unshrunk or semishrunk heat shrink polymer tubing and located at the proximal end. The transmission may include one or more driven section formed from unshrunk or semishrunk heat shrink polymer tubing and located at the distal end. 
     Thus, driver section is analogous with a master cylinder in a hydraulic system, and the driven section is analogous with a slave cylinder in a hydraulic system. The flow of the fluid (whether hydraulic or pneumatic) between the driver section and the driven section may be modified by other components located between the driver section and the driven section of the transmission or located elsewhere in the transmission. Such components may be internal to the heat shrink polymer tubing (and acting within shrunken or semishrunken sections of tubing), or external to the heat shrink polymer tubing (and acting on unshrunk, semishrunken or shrunken sections of tubing). 
     As with electrostatic motors, the force transmitted by the transmission is proportional to the square of the linear dimensions, that is, the area of the driven section&#39;s opposing walls that are pushed apart by the pressurised fluid. Hence, reduction of the size of the transmission by a factor of 10 reduces the force or power by a factor of 100. 
     In one embodiment, the transmission includes a spring mechanically coupled to either a driver section or a driven section of the transmission so as to react against expansion of the driver or driven section. 
     The heatshrink process may be carried out, in order to shrink or partially shrink the heat shrink polymer tubing, by means of a hot air gun or other source of hot gas (including by placing the polymer tubing in an oven). It may also be carried out by radiant heat or by contact with a hot object. 
     The thermal gradients employed for the heatshrink process may be arranged so that the deformation of the polymer tubing leaves it in a shape adapted for the intended application. For example a portion of polymer tubing that it is desired remain unshrunk may be protected from the hot air used for shrinking. This can be done, for example, by locating that portion in a slot or other constraining cavity (and performed either cold or after prior heating of that section of polymer tubing), or holding the desired portion between the jaws of a pair of pliers or the like. The shrunken tube when in its hot pliable state may also be formed into a desired shape in a jig or loom to facilitate subsequent assembly processes. 
     In one embodiment, the conduit is a first conduit and the fluid transmission includes one or more additional like conduits. 
     According to another broad aspect, the present invention provides a method of manufacturing a fluid transmission, comprising: forming a conduit for the fluid from heat shrink polymer tubing; and heat shrinking at least a portion of the heat shrink polymer tubing; whereby a force can be transmitted by the fluid from a first or proximal end of the conduit to a second or distal end of the conduit. 
     In one embodiment, the method includes forming at least one integral driver section comprising unshrunken or semishrunken heat shrink polymer tubing. In some embodiments, the method includes forming at least one integral driven section comprising unshrunken or semishrunken heat shrink polymer tubing. 
     The invention also provides various devices for achieving certain desired mechanical effects and employing a fluid transmission as described above, as will be apparent from the description of various embodiments. 
     According to a further aspect of the invention there is provided an actuator, comprising:
         a plurality of pivotably connected members;   at least one expandable bag located between a pair of said members; and   a fluid conduit in fluid communication with said expandable bag for expanding said bag by transmitting a fluid to said bag, said fluid conduit comprising heat shrink polymer tubing at least a portion of which is shrunken;   wherein expansion of said bag urges said pair of members apart.       

     In one particular embodiment, the actuator includes four members connected as a quadrilateral. The quadrilateral may be, for example, a parallelogram or a trapezium. 
     A plurality of such actuators can be coupled according to the present invention to form a complex or compound actuator. 
     According to a further aspect of the invention there is provided a device comprising an actuator as described above. The device may be, for example, a toy in which the actuator is used to actuate movement of a portion of the toy (such as a limb). In other examples, the device is a camera, a robot, a microscope or a mobile telephone. 
     According to a further aspect of the invention there is provided a method for manufacturing a fluid transmission, comprising:
         selectively masking a length of heat shrink polymer tubing; and   heating said heat shrink polymer tubing to shrink a portion or portions of said heat shrink polymer tubing that is not masked;   whereby at least two unshrunken sections and at least one shrunken section are formed, to provide a driver bag and a driven bag with a fluid conduit therebetween.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In order that the invention may be more clearly ascertained, embodiments will now be described, by way of example, with reference to the accompanying drawing, in which: 
         FIG. 1  is a view of a fluid transmission according to an embodiment of the present invention; 
         FIG. 2  is a view of a fluid transmission according to another embodiment of the present invention; 
         FIGS. 3   a ,  3   b ,  3   c  and  3   d  are views of a fluid transmission according to another embodiment of the present invention; 
         FIG. 4  is a view of a fluid transmission according to another embodiment of the present invention; 
         FIG. 5  is a view of a flow restriction device within a length of conduit according to another embodiment of the present invention; 
         FIG. 6  is a view of a fluid transmission according to another embodiment of the present invention; 
         FIG. 7  is a cross-sectional view of a one-way valve encased in a shrunken section of heat shrink polymer tubing according to an embodiment of the invention; 
         FIG. 8  is a view of a fluid transmission according to another embodiment of the present invention; 
         FIG. 9  is a view of a fluid transmission according to another embodiment of the present invention; 
         FIG. 10  is a view of a fluid transmission according to another embodiment of the present invention; 
         FIG. 11  is a view of a double acting fluid transmission according to another embodiment of the present invention; 
         FIGS. 12   a ,  12   b ,  12   c  and  12   d  are successive views of a fluid transmission manufacturing process according to an embodiment of the present invention; and 
         FIG. 13  is a view of a fluid transmission according to another embodiment of the present invention; 
         FIG. 14  is a view of a device employing a fluid transmission according to another embodiment of the present invention; 
         FIGS. 15   a  and  15   b  are views of a system for providing large amplitude motion according to another embodiment of the present invention; 
         FIGS. 16A and 16B  are schematic views of a trapezoidal actuator device according to another embodiment of the present invention; 
         FIGS. 17A and 17B  are schematic views of a parallelogram actuator device according to another embodiment of the present invention; 
         FIGS. 18A and 18B  are schematic views of a flatpack actuator device according to another embodiment of the present invention; 
         FIG. 19  is an isometric view of a rhomboid actuator device according to another embodiment of the present invention; 
         FIG. 20  is schematic view of a tableaux of moveable manikins according to another embodiment of the present invention; 
         FIG. 21  is schematic view of a doll according to another embodiment of the present invention; 
         FIG. 22  is schematic view of a doll according to another embodiment of the present invention; 
         FIG. 23  is a view of novelty greeting card according to another embodiment of the present invention; 
         FIG. 24  is a cross-sectional view of the novelty greeting card of  FIG. 23 ; 
         FIG. 25  is a cross-sectional view of an actuator parallelogram according to another embodiment of the present invention; 
         FIGS. 26A and 26B  are schematic views illustrating the manufacture of an actuator device according to another embodiment of the present invention; 
         FIGS. 27A and 27B  are schematic views illustrating the manufacture of another actuator device according to another embodiment of the present invention; 
         FIGS. 28A to 28D  are schematic views illustrating the manufacture of still another actuator device according to another embodiment of the present invention; 
         FIG. 29  is a view of a bi-stable actuator according to another embodiment of the present invention; 
         FIG. 30  is a schematic view of an armature provided with an actuator according to a further embodiment of the present invention 
         FIG. 31  is a view of a fabrication apparatus according to an embodiment of the present invention for producing heat shrink tube and bags; and 
         FIG. 32  is a view of a fabrication apparatus according to another embodiment of the present invention for producing heat shrink tube and bags. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a view of a simple fluid transmission  10  according to an embodiment of the present invention. The transmission includes an unshrunk driver section  11  of heat shrink polymer tubing connected by a shrunk section  12  to another unshrunk driven section  13 ; these three sections are integral with one another. The transmission  10  is filled with a suitable fluid, which might in many applications be water, air or oil. However, the fluid can be selected according to intended use, compatibility with the material of the polymer tubing and likely environmental conditions in which it will be used. 
     Pressure applied to driver section  10  by finger  14  forces fluid along shrunk section  12  and expands driven section  13 , thereby raising weight  15 . 
     The transmission  10  includes shrunken sections  16  and  17  that form seals (to prevent the escape of the hydraulic or pneumatic fluid) by means of plugs or crimps  18  and  19 . These ends may be sealed by various means, including shrinking the end down onto a short section of rod, heat sealing or melting the end, and—as illustrated in FIG.  1 —providing an external crimping device. This last option was found to be the best. A U-shaped or e-shaped piece of metal strip was used. Shrinking onto the tubing was found to be useful to change between tubing sizes and to allow the incorporation of other fluid devices. 
       FIG. 2  is a view of a fluid transmission  20  according to another embodiment of the invention, in which a force applied at unshrunken driver bag  21  can move the fluid along integral shrunken pipe  22  and to produce motion of a plurality of integral unshrunken bag sections  23 ,  24 ,  25 . (Plates  26  and  27  are provided above and below driver bag  21 , respectively, to distribute the force applied to the driver bag  21 .) 
     Clearly the actuated (i.e. driven) sections  23 ,  24  and  25  can be widely separated from one another. The volume of fluid that can be provided by the compression of driver bag  21  is at least as great as the volume required to actuate sections  23 ,  24  and  25 . 
       FIG. 3   a  is a view of a fluid transmission  30  according to another embodiment of the present invention. The fluid transmission  30  includes a spring  31  in the form of a folded metal sheet that partially encloses a driven hydraulic bag  32 . When pressure is released from the driver bag  34  (such as by the lifting of the pressure of finger  33 ) the spring  31  forces fluid in the transmission  30  back to the driver bag  34 , connected integrally to the driver bag  34 , and is thereby inflated. 
       FIGS. 3   b  and  3   c  are cross-sectional views of spring  31  and driven bag  32 . In these views, the spring  31  and driven bag  32  are shown, respectively, compressed and expanded (or relaxed).  FIG. 3   d  is an isometric view of spring  31  and driven bag  32 , shown expanded. 
       FIG. 4  is a view of a fluid transmission  40  according to another embodiment of the invention. Fluid transmission  40  includes a driver bag  41  connected by integral shrunken polymer tubing  42  to three remote driven bags  43 ,  44  and  45 ; the driven bags are located in respective spring clips  46 ,  47  and  48 . Driver bag  41  is arranged for actuating driven bags  43 ,  44  and  45  and hence clips  46 ,  47  and  48 . As will be appreciated, if the spring constants of the clips  46 ,  47  and  48  differ, or if the lengths of the driven bags differ, it is possible to produce a sequence of operation of movements of the three driven bags. For example, if the driven bags have identical lengths, but the clips increase in stiffness in the order  46 ,  47 ,  48 , the driven bags will be actuated in the sequence  45 ,  44 ,  43 . Deflation of these driven bags—once the force is released from driver bag  41 —will reversed and hence  43 ,  44 ,  45  (an effect that may be referred to as FILO: first in, first out). 
     In the various embodiments described herein, fluid flow within the conduit of the fluid transmission can be modified or controlled by locating constriction elements or valves in the conduit. During manufacture, shrinkage of the heat shrink polymer tubing can be employed to form or to enclose such devices. These devices may be used to produce a variant of effects. 
     For example,  FIG. 5  is a view of a flow restriction device according to an embodiment of the invention in situ within a length of conduit, generally at  50 . The restriction device  52  comprises a short rod with a small bore  54  passing axially along the length of the rod, and can be heat sealed in position inside the length of conduit  56 . This flow restriction device is considerably more convenient and reproducible than an externally located flow restriction device. 
     Thus,  FIG. 6  is a view of a fluid transmission  60  according to another embodiment of the invention that includes a flow restriction device. The transmission  60  includes a driver bag  61  integrally connected to three driven bags  63 ,  64  and  65  by means of integral shrunken polymer tubing  62 . The driven bags  63 ,  64  and  65  are located in respective spring clips  66 ,  67  and  68  (of identical spring constant). In the shrunken polymer tubing  62  are located three flow restriction device: a first flow restriction device  69   a  between driver bag  61  and driven bag  63 , a second flow restriction device  69   b  between driven bag  63  and driven bag  64 , and a third flow restriction device  69   c  between driven bag  64  and driven bag  65 . 
     Compression of bag  61  pumps fluid into the driven bags  63 ,  64 ,  65  but the sequence of operation is  63 ,  64 ,  65  owing to the restriction of flow. The deflation sequence is also  63 ,  64 ,  65 . 
       FIG. 7  is a cross-sectional view  70  of a one-way valve encased in a shrunken section of heat shrink polymer tubing according to an embodiment of the invention, which may be regarded as a hydraulic analogue of a diode. A short, rigid tube  71  (constituting the valve body) is encased in heatshrink  72 . One end of the interior of this tube is enlarged to form a valve seat  73 . A ball  74  is positioned in this expanded section. A spring  75  may be held in a position to press the ball back into the valve seat. 
     Fluid can flow with minimal resistance in the direction shown by arrow  76 . Fluid flow in the opposite direction encounters considerable resistance, but it may be desirable not to block it completely. 
     It may also be desirable to produce one way valves in which a part of the valve permits a pre-determined back flow rate. This could be effected, for example, by providing the tube  71  with an axial bore for allowing back flow, in which the diameter of the bore is selected to set the back flow rate. It will also be appreciated that mushroom valves, poppet valves, flap valves could be employed. 
       FIG. 8  is a view of a fluid transmission  80  according to an embodiment of the invention that includes a one-way valve. Transmission  80  could be used to lift a lid quickly but then lower it slowly. When the driver bag  81  is compressed (such as by a finger  82 ), the fluid in the transmission—which may be water—passes with minimal resistance in the forward direction through the one-way valve  83  and along tube  84 . 
     The fluid then passes into the driven bag  85  which expands against the spring  86 , thereby raising, for example, a lid (not shown) in direction  87 . 
     When the force is removed from driver bag  81 , the fluid is able to flow back through the higher reverse resistance of valve  83  and into the driver bag  81 , slowly lowering (for example) the lid. 
       FIG. 9  is a view of a fluid transmission  90  according to another embodiment of the invention, which is similar to that of  FIG. 8  but with extra components to provide a still more controlled and uniform raising of the lid. 
     These components also act to protect the transmission from accidental excess digital force overload. 
     The transmission  90  is essentially identical in its components and operation with that shown in  FIG. 8  with the addition of a further driven bag (the hydraulic analogue of a capacitor) between the one-way valve  92  (cf. one-way valve  82  in  FIG. 8 ) and driven bag  96  (cf. driven bag  86  in  FIG. 8 ). Fluid from driver bag  91  flows through one-way valve  92  under finger pressure and expands further driven bag  93  against the pressure of further spring  94 . The fluid from the further driven bag  93  moves along heat shrink conduit portion  95  to actuate the required motion by expanding driven bag  96 . Optionally, a flow restrictor may be located—if desired—in the conduit  95  at  97  to control the activation rate. 
       FIG. 10  is a view of a fluid transmission  100  according to still another embodiment of the invention, which is similar to that of  FIG. 9  but with a further one-way valve and a fluid reservoir. This allows multiple pump stroke actuation, which could be desirable for certain applications. 
     Referring to  FIG. 10 , a fluid reservoir  101  in the form of an expanded bag section of unshrunken heat shrink is connected to the driver bag  102  via one-way valve  103 . Pressure on driver bag  102  pumps fluid through to the pressure maintaining further driven bag  104  with spring  105 . A spring  106  compresses the fluid in reservoir  101  and ensures that driver bag  102  is refilled for the next stroke. For the successful operation of ultimate driven bag  107  and spring  108 , the sequence of spring strengths (more accurately spring constant/bag length) is graduated such that spring  105  is stronger than spring  108 , which is stronger than spring  106 . Driver bag  102  is provided either without a spring (as illustrated) or, optionally, with a spring weaker than all other springs  105 ,  106 ,  108 . 
     Hydrostatic pressure has not been found to be important in tests carried out to date, but could conceivably need to be taken into consideration in some applications. 
       FIG. 11  is a view of a double acting fluid transmission  110  according to an embodiment of the invention. This transmission can provide greater force in each stroke direction than single driver bag transmissions acting against a spring return. Fluid transmission  110  includes two conduits  111 ,  112  of heat shrink polymer tubing, each with shrunken portions (tubes  113 ,  114  respectively), unshrunken driver bags ( 115 ,  116  respectively) and unshrunken driven bags ( 117 ,  118  respectively). 
     The driver bags  115 ,  116  are located on opposite sides of a lever  119  provided to facilitate manual operation and pivoted at  120 . Motion of the lever  119  in direction  121  or  122  squeezes driver bag  115  or  116  respectively against stationary support structure  123  or stationary support structure  124  respectively. 
     The excess fluid resulting from the compression of either driver bag  115  or driver bag  116  flows along tube  113  or  114  respectively into driven bag  117  or  118  respectively. This causes movement of lever  125  (pivoted at  126 ) in either direction  127  or  128  respectively. Stationary support structures  129 ,  130  are provided adjacent to respective driven bags  117 ,  118  on the remote side in each case of lever  125  to stop the driven bags  117 ,  118  expanding in an unwanted direction. 
     In such a system the forward and reverse movements have a symmetrical feel which makes this system suited for a joystick control. A more complex joystick control could employ two further hydraulic bags in a plane perpendicular to that shown in  FIG. 11 . 
     Another embodiment of the invention provides a convenient fluid transmission manufacturing method. Heat shrink tubing is readily flattened out; a convenient method of forming unshrunk sections, therefore, is to flatten the required section(s) of the tubing and place these flattened sections into one or more slots of appropriate length. Referring to  FIG. 12   a , a portion of heat shrink polymer tubing  140  is located in a slot  142  in a work piece  144 .  FIG. 12   b  is a view of the tubing  140  located in the slot  142 .  FIG. 12   c  is a view of the tubing  140  located in the slot  142  while the tubing  140  is heated by means of heat gun  146 . The slot  142  shields the portion of tubing in the slot  142  from the hot air from the heat gun  146  (or other heat source) being used to shrink the exposed portions  148   a ,  148   b . Hence, the portion in the slot  142  remain unshrunken. 
     Referring to  FIG. 12   d , once the tubing  140  has been removed from the slot  142 , the transition between the circular shrunken portions  148   a ,  148   b  and the flat unshrunken central portion  150  causes the central portion  150  to be thermally set in a form comparable to that of a hot water bottle, where the main body of the central portion  150  is held flat by the shoulders  152  formed at the junction with the shrunken portions  148   a ,  148   b . This shape is particularly convenient for the design and the installation of the hydraulic member or “loom” in devices in which it is to be used. 
     It is also possible to shield a portion of heat shrink polymer tubing from being shrunken by gripping that portion with a pair of articulating jaws such as those of a pair of pliers. The method is readily applicable to small volume production or to large scale manufacture. 
     The shrunken sections outside the slot or jaws generally assume a circular cross section with increased wall thickness. Both these characteristics minimise volume changes in the conducting tube when fluid pressure is increased. Also, while the shrunken section remains hot, it is possible to extend its length by pulling its ends. 
     It is also possible to arrange the heat shrink polymer tubing in a jig so that, once cooled, the shrunken sections will be set in a way that will make assembly or operation of the ultimate transmission more convenient. 
       FIG. 13  is a view of a fluid transmission  160  according to still another embodiment of the invention, including an adjustment device for adjusting a steady position component. In  FIG. 13  rotation of screw  161  produces a motion of plate  162  that compresses a hydraulic driver bag  163  against a fixed plate  164 . The fluid displaced moves along shrunken tube section  165  into driven bag  166  and makes it expand. This transmission could be of value where precise adjustment of static loads is required in applications such as micromanipulators, micro-dissectors, tilt adjusters microscope stage focussing and levelling of objects. 
     Another device employing a fluid transmission according to an embodiment of the invention is shown generally at  170  in  FIG. 14 . In device  170 , compression of driver bag  171  produces expansion of large driven bag  172  in a volume  173  defined by opposed plates  174  and  175 . A number of other secondary driven bags  176 ,  177 ,  178  and  179  are also disposed in the volume defined by plates  174  and  175 , between large driven bag  172  and one of the plates  174 . The expansion of the large driven bag  172  compresses the secondary driven bags  176 ,  177 ,  178  and  179  causing expansion of the tertiary driven bags  180 ,  181 ,  182  and  183 . 
     It may be desired to operate these tertiary driven bags sequentially using graded springs. If, however, it is intended for them to operate simultaneously it may be desirable to interpose a right plate between secondary driven bags  176 ,  177 ,  178  and  179  and the large driven bag  172 . 
     Large amplitude motions can be achieved by systems using the bending of an unshrunken section of the heat shrink tubing.  FIGS. 15   a  and  15   b  are views of a system  190  according to another embodiment of the invention, that includes a fluid transmission  191  and in which 140° of movement is obtained by providing a crease line or fold  192  in driven bag  193  (arranged vertically). When fluid enters driven bag  193 , the bag opens out from the bent configuration shown in  FIG. 15   a  to the straightened configuration shown in  FIG. 15   b.    
     EXAMPLE 
     Experiments were carried out with standard 2 mm diameter heat shrink. A driven bag of dimensions 2.5 mm×8 mm was used to lift a mass of 2 kg, raising it by over 1 mm. 
     A more precise set of experiments was carried out using Zeus Sub-Lite-Wall brand PTFE Heat Shrinkable tubing. (PTFE heat shrink tubing remains highly flexible even when shrunk, and can have an external diameter of as little as ˜125 μm when shrunk, so is particularly advantageous in the embodiments described herein.) A driven bag was formed from this material which had the dimensions 0.9 mm×3.0 mm. The driven bag lifted a mass of 120 g to a height of approximately 0.5 mm. The wall thickness of this tube is given by the manufacturer as 0.051 mm. This means that the stroke of this motion is 5 times the collapsed wall thickness, which is very large compared with other miniature actuators such as piezo elements and the like. 
     The driven bag was tested with excess pressure to destruction. The irreversible stretching and bursting pressure of the unsupported bag was found to be in the region of 40 to 60 kPa. 
     If the driven bag were supported, it is estimated that the bag could raise over one kilogram with a stroke of 0.2 to 0.3 mm. 
     A variety of heat shrink tubing has been successfully used to construct hydraulic systems according to the present invention, including:
     i) Zeus brand PTFE heat shrink 4:1, in a wide range of tube sizes;   ii) Sumitomo Corporation “Sumitube C” brand polyolefin tube (which has a shrink temperature of 90° C.), in several sizes and in both clear and pigmented varieties;   iii) Flame retardant polyolefin; and   iv) Tyco Raychem brand PVC heat shrink tube.   

     As an alternative to heat shrink, the systems of the present invention may also be constructed with blow expanded tubing. Zeus brand PTFE tube was successfully expanded and tested. Further, it is envisaged that blow moulding could also be used to construct the bags and tubing. Though not tested, it is envisaged that a wide range of thermoplastics would be suitable, if generally less convenient than heat shrink. 
     Another type of device employing a fluid transmission according to an embodiment of the invention is shown schematically at  200  in  FIGS. 16A and 16B . The device  200 —which constitutes an actuator—comprises four straight, essentially rigid members  202 ,  204 ,  206 ,  208  that are pivotably coupled to one another by four pins  210  and define a trapezoidal shaped space  212 . The pins that couple the base member  202  to side members  204 ,  204  are spaced more widely than the pins that couple the side members  204 ,  204  to top member  208 . In addition, top member  208 —though terminating at the point at which it is coupled to one side member  206 , extends beyond side member  204 . 
     The device includes, within trapezoidal shaped space  212 , a driven bag  214  (coupled by a conduit for admitting a fluid, which conduit is—for simplicity—omitted from these figures). 
     When a fluid is driven into the driven bag  214  (whether by a driver bag of the type described above or otherwise), driven bag  214  expands to a greater volume, as depicted in  FIG. 16B . (For the purposes of comparison, the initial shape and volume of driven bag  214  is shown with dotted curve  216 .) The expansion of driven bag  214  forces side members  204 ,  206  upwards. In addition, owing to the closer spacing of the pins coupling these side members to the top member  208 , the top member  208 —though initially parallel to base member  202 , is progressively rotated until one end  218   a  is considerably higher than the other  218   b.    
     The device  200  thus acts as a hydraulic actuator. As will be appreciated, in a practical device the members may be in the form of plates and the pins may be replaced with any other suitable coupling mechanism, including hinges, magnets, flexible members (such as nylon thread), ball/socket joints, and combinations of these. 
     A device  220  comparable to that of  FIGS. 16A and 16B  according to another embodiment is shown schematically in  FIGS. 17A and 17B . Referring to  FIG. 17A , device  220  comprises four rigid members  222 ,  224 ,  226 ,  228 , in this embodiment coupled by four flexible hinges  230  to form an enclosure  232  for a hydraulic driven bag (not shown). 
     Base rigid member  228  is coupled to a fixed base  234 , while one or more of the other rigid members (in this example, load member  226 ) is connected to whatever load  236  that it is desired be moved. 
       FIG. 17B  shows device  220  after hydraulic driven bag  238  has been inflated through tube  240 . This causes that member  226  most remote from base member  228 , as well as the load  236 , to move upwardly in an arc  242 . The enclosure  232  defined by rigid members  222 ,  224 ,  226 ,  228  is now parallelogram in shape. 
     Another embodiment comparable to device  220  of  FIGS. 17A and 17A  is shown schematically at  250  in  FIG. 18A and 18B , and like reference numerals have been used to indicate like features. As in device  220 , the combined lengths of members  228  and  224  equals that of members  222  and  226  (referred to herein as the “flatpack” criterion), but base member  228  is longer than load member  226  and member  230  is correspondingly shorter than member  222 . 
     Accordingly, when driven bag  238  is expanded, load  236  is rotated relative to the base  234 , as well as being moved through arc  244 . 
       FIG. 19  is an isometric view of a hydraulic unit  260  according to another embodiment, comprising a rhombus  262  with four sides  264 ,  266 ,  268 ,  270  of equal size, with adjacent sides joined by respective hinges (not shown). The rhombus  262  defines an interior volume in which a hydraulic bag  272  is located oriented transverse to the rhombus  262 . When a fluid is driven into hydraulic bag  272  through tube  274 , hydraulic bag  272  and hence rhombus  262  is expanded in the manner illustrated in  FIG. 17B . 
     The hydraulically actuated devices of  FIGS. 16A to 19  have numerous applications. One example is shown schematically in  FIG. 20 , which depicts a tableaux  280  of moveable manikins  282 ,  284 . Each  FIG. 282 ,  284  has legs comprising pairs of parallelogram-shaped segments, those of manikin  282  reversed relative to those of manikin  284 ; each segment encloses a hydraulically driven bag  286 . The bags  286  are coupled in series by tube  288  to a driver bag  290 . The depression of the driver bag  290  by a finger  292  forces fluid along tube  288  into the ankle of manikin  282  and into the bags  286 . The bags  286  of manikin  282  expand and activate the parallelogram-shaped segments, causing manikin  282  to bob up. The fluid continues to move along tube  288  and enters the ankle of manikin  284 , expanding the bags in that manikin. This activates the parallelogram-shaped segments of manikin  284 , which causes manikin  284  to bob down. 
       FIG. 21  is a schematic view of a hydraulically actuated manikin or doll  300  according to another embodiment. Doll  300  is similar to manikin  282  of  FIG. 20  (and like reference numerals have been used to indicate like features), but its upper and lower limbs  302 ,  304  are attached to the trunk  306  of the doll  300  by magnets  308 . This allows an increased range of static poses of the doll  300 . Limbs  302 ,  304  are tipped with small pieces of iron  310 , and the trunk  306  has complementary pieces of iron  312 ; magnets  308  attract the respective pieces of iron to hold the limbs  302 ,  304  to the trunk  306 . Alternatively, each magnet  308  may attract a piece of iron on one side of each joint and be glued to the other. Doll  300  has further magnets  314  on the soles of the shoes  316  of the doll  300 , for attracting the feet of the doll  300  to a magnetic floor  318 . Suitable strong compact rare earth magnets are available in disc form, as depicted (enlarged) at  320 . 
       FIG. 22  is a schematic view of a hydraulically actuated manikin or doll  330 , according to another embodiment, which a further degree of freedom of static pose is provided. This is done by including U shaped pieces of soft iron sheet between separate active units or between other components where an articulated joint is desired. Referring to  FIG. 22 , the legs  332 ,  334  of doll  330  are articulated to trunk  336  of doll  330 . At each hip joint  338 , a piece of flat iron  340  is attached to the top of the leg and held tight by a flat magnet  342 . The other side of magnet  342  holds fast to a U shaped piece of soft iron  344 . Iron  344  (formed by folding a flat piece into a U shape) is shown edge-on. The other side of the U shaped piece of iron  344  is held by a further magnet  346 , whose other pole holds fast to a lower iron portion  348  of trunk  336 . The two pieces of iron  344  are generally identical, except that one (on the left in the figure) is close in shape to a V. These pieces of iron  344  can also be rotated to give a full range of static ball joint positions. 
       FIG. 23  is a view of another embodiment, a greeting or good luck card  350 . Card  350  has a fold  352  at its upper edge, and includes a concealed actuated bladder  354  behind the face  356  that is exposed once the card has been opened (as depicted in this figure). An actuator bladder  358  is located behind the opposite face  360  and connected to the first bladder  354  by tube  362 . Pressure on actuator bladder  358  by the hand of the recipient of the card  350  causes a fluid held within the bladders and tube to be forced out of the actuator bladder and into actuated bladder  354 ; actuated bladder  354  is coupled to a exposed, cardboard movable part  364  of face  356  (in this example, a hinged paw of a cat design), such that the expansion of actuated bladder  354  causes movable part  364  to move. 
       FIG. 24  is a cross-sectional view—not to scale—of card  350  (along line A-A in  FIG. 23 ). Card  350  has a slot  366  through which the movable part  364  projects. The lower, concealed portion  368  of movable part  364  is folded into a parallelogram  370  with paper hinges at each vertex (not shown). Parallelogram  370  is glued at  372  to itself, and at  374  to the rear of face  356 . Actuated bladder  354  is located inside parallelogram  370 . 
     The parallelograms and trapezoids of the devices described above may be constructed of many materials, including many that are inexpensive such as paper and cardboard. For example,  FIG. 25  is a cross-sectional view of an actuator parallelogram  380  formed from a piece of Kraft paper (comprising corrugated cardboard  382  between paper skins  384   a ,  384   b ). The external skin  384   a  forms the hinges  386 . The integrity of the parallelogram  380  is maintained by gluing at  388 . 
       FIG. 26A  depicts an alternative approach, comprising a strip  390  of metal, plastic, paper or cardboard. The strip  390  has four holes  392 , and is formed into a parallelogram (as shown in  FIG. 26B ) by being bent at these holes. The material at the sides of the holes provides the hinges at  394 ,  396 ,  398 ,  400 . The ends of strip  390  are glued or otherwise fastened together at  402 . 
       FIG. 27A  depicts a still further approach, comprising a strip  410 —again of metal, plastic, paper or cardboard—in which sections  412  have been weakened by abrasion or erosion so that the strip  410  can be bent into a parallelogram  414 . The weakened abraded or eroded sections  412  provide the hinges  414 ,  416 ,  418 ,  420 . The ends of strip  410  are fastened at  422 . 
       FIGS. 28A ,  28 B,  28 C and  28 D are successive views of the fabrication of a parallelogram  430  according to still another embodiment, and formed by stamping and folding a sheet  432  of material such as sheet metal. Referring to the plan and perspective views of  FIGS. 28A and 28B , four neck portions  434  are provided to act, ultimately, as hinges. Referring to  FIG. 28C , side tabs  436  of sheet  432  are folded upwardly and downwardly respectively. 
     The final, folded configuration is shown in  FIGS. 28D  (with one end portion, which would be fastened to the other end portion  438 , omitted for clarity). 
     The embodiments of  FIGS. 16A to 28D  may also optionally include a mechanism for providing a restoring force to urge the bladder—after actuation—back to a collapsed condition and ready for re-activation. This may be done in a number of ways. 
     For example, the hinges may be made of resilient metal strip bent to shape at the appropriate positions to form a flattened parallelogram. This may conveniently be achieved by making the entire perimeter of the parallelogram from one single piece of resilient strip and attaching rigid pieces to the strip at appropriate sections to form the unbending sides of the parallelogram. 
     Alternatively, a restoring force could be provided by independently positioned pieces of resilient wire that push together opposing sides of the parallelogram. The resilient wire would be of similar shape to the spring used in conventional clothes pegs. 
     Another approach employs rubber bands. These could be positioned around the parallelogram, acting to restore the flattened position of the parallelogram. 
     Still further, the force of gravity could be exploited, acting on a weight.  FIG. 29  is a view of such a system  440 . The inertia of the weight W is used to cause a parallelogram  442  to act in a flip-flop manner. The system  440  includes a hydraulic mechanism, comprising actuated bladder  444  inside parallelogram  442 , actuator bladder  446  and connecting tube  448 . When this hydraulic mechanism is operated to produce a fast motion, the inertia of the moving weight W causes the weight W to overshoot, traversing an arc  450  from the initial illustrated position to a new stable, rest position shown dashed at  452 . Hence, a bi-stable motion is produced. 
       FIG. 30  is a schematic view of an armature  460  provided with an actuator according to a further embodiment of the present invention. The armature  460  could be used in many applications, including in load bearing structures, but in the illustrated embodiment it is adapted for use as the arm of a boxer figurine, so is fitted with a miniature boxing glove  462 . 
     Armature  460  principally comprises a pantograph-like framework of pivotally connected rods. A first pair of rods  464  are pivotally connected to a base  466  (attached to or forming the shoulder of the boxer figurine), pivotally connected to second pair of rods  468 . The second pair of rods  468  are pivotally coupled to a terminating element  470 , to which is attached the boxing glove  462 . A first actuated bag  472  is located between first pair of rods  464 , and a second actuated bag  474  is located between second pair of rods  468 . The armature  460  includes tubing (not shown) for conducting fluid to these bags. When these bags  472 ,  474  are expanded, the respective pairs of rods are urged apart, which results in the whole armature extending laterally from base  466 . 
     The armature  460  also includes a releasable magnetic latch in the form of permanent magnet  476   a  and piece of iron  476   b . Magnet  476   a  and iron  476   b  are located opposite each other on the upper rod of each pair of rods  464 ,  468 . In a minimally extended arrangement, magnet  476   a  and iron  476   b  are in contact and latch the armature in that configuration. When the bags  472 ,  474  are expanded, the armature  460  initially will not respond, as the attraction between magnet  476   a  and iron  476   b  will initially exceed the force of the bags urging the magnet and iron apart. When the force of the bags becomes sufficient to break the attraction, the armature  460  and boxing glove  462  extend rapidly, simulating what in physiology is termed a ballistic movement. 
     It will be noted that the rods  464 ,  468  of armature  460  define—at the “elbow”  478 —an additional parallelogram. This additional parallelogram does not have a bag in it (though in some embodiments it may), but links the motions of the two parallelograms defined by first rods  464 , second rods  468 , base  466  and terminating element  470 . This is advantageous in some applications, such as where variable loads are encountered. 
     In one variation on this arrangement a pair of flexible plastic “fridge” magnets is employed. The magnetic poles on such magnets are arranged in a series of parallel lines (viz. N-S-N-S-N etc); if two such magnets are slid against one another (moving at right angles to the pole lines) a jerky periodic motion results, which can make the motion of a doll more realistic and add interest. 
     The tube/bag combinations of the above-described embodiments can be made by any suitable technique, but certain techniques adapted for mass production are described below.  FIG. 31  is a view of one fabrication apparatus  480  for producing heat shrink tube and bags. Apparatus  480  comprises a framework  484  that includes a barrel  486  with flat exterior panels  488  distributed about the barrel  486  to support the tube  482 . The barrel is rotatably mounted on a shaft  490 . The framework  484  also includes two protective bars  492 , which rotate with the barrel  486  and protect portions of heat shrink tube  482  from the hot air used to shrink the tube  482 . Protective bars  492  that cooperate with two of the exterior panels  488  to clamp the tube  482 , thereby defining unprotected lengths  494 ,  496 ,  498 ,  500  of heat shrink tube  482 . 
     Apparatus  480  also includes a hot air gun  502  for directing hot air  504  towards heat shrink tube  482 . The hot air  506  shrinks the unprotected lengths  494 ,  496 ,  498 ,  500  of heat shrink tube  482  to form the non-expandable tube sections of a hydraulic system. The protected sections of the heat-shrink tube  482  form the bladders or bags of that hydraulic system. 
       FIG. 32  is a view of another fabrication apparatus  510  for producing heat shrink tube and bags. Apparatus  510  comprises two clamps  512 ,  514  (each comprising a pair of blocks) for retaining five lengths  516  of heat shrink tube. Hot air gun  518  directs hot air  520  towards the lengths  516  of heat shrink tube, shrinking the unprotected portions of lengths  516  to form the non-expandable tube sections of a hydraulic system, but leaving the clamped and hence protected portions of lengths  516  to for the bladders of the hydraulic system. 
     It can be seen, therefore, that the various embodiments of the present invention provide a wide range of possible actuators for use in many devices, with the actuators constructed of a variety of inexpensive materials and having simple hinges that may be integral with the quadrilateral component. It will also be appreciated that the actuators could be based on other polygons. 
     Other arrangements, however, comprise an actuated bag located between a pair of hinged elements. Still other actuators employ more than one actuated bag. 
     Possible applications include, in addition to those described above, the provision of facial movement in dolls and the like, animated books (particularly for children), industrial robotics, lens focussing mechanism (such as for mobile telephone cameras or other digital cameras), other electronic equipment where mechanical and electromechanical actions are employed, slow release lids and covers, micro/nanotechnology devices, and scientific instrumentation (such as microscopy or endoscopy stages). 
     Conclusion 
     The miniature fluid transmissions made possible according to the present invention are particularly suited to slow uniform linear motion where substantial force is required and a high degree of damping is a desirable feature. A further advantageous feature of the described embodiments is the high mechanical work efficiency given by these transmissions compared with cylinder/piston hydraulic systems. As the size of the latter decreases the proportion of the stroke energy taken up by sliding friction of the seals increases. The transmissions described above, however, are estimated to have greater than 90% efficiency for bore sizes of less than 1 mm 2 . 
     Modifications within the scope of the invention may be readily effected by those skilled in the art. For example, a flat coil spiral of unshrunken heat shrink will unwind when compressed fluid is fed into it. This may be employed as a device or actuator. The coil characteristics may be improved by heating it while constrained. Another actuator device can be formed by a section of the heat shrink material being formed into a concertina structure by enclosing a coil spring in the lumen of the tube before the heat shrink process is done. An internal folded metal strip can also be used. It is to be understood, therefore, that this invention is not limited to the particular embodiments described by way of example hereinabove. 
     In the preceding description of the invention, except where the context requires otherwise owing to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 
     Further, any reference herein to prior art is not intended to imply that such prior art forms or formed a part of the common general knowledge.