Axially contractable actuator
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation-in-part of my co-pending U.S. patent application Ser. No. 06/553,530 filed Nov. 21, 1983, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to an axially contractable actuator particularly suited for robotics applications. 
     Robotics technology is frequently presented with the problem of mimicking the function of human hands and arms. Mechanical analogies to hands and arms clearly must include some replacement for the many human muscles used to flex and move the human fingers, hands and arms. When fluid power, either hydraulic or pneumatic, is used in robotics, a fluid cylinder appears to be a likely substitute for human muscles. However, high pressure fluid requirements due to limited fluid cylinder size and space and positioning problems complicate the use of fluid cylinders or make their use impossible for some applications such as in self-propelling walking robots. 
     Fluid cylinders are also not entirely suitable as actuators in the food and drug industries. Restrainers must be used to contain dripping caused by leaking seals and misaligned cylinder rods. 
     SUMMARY OF THE INVENTION 
     According to the invention, an actuator has first connection means and second connection means at opposite first and second ends of the actuator and is contractable along an axis extending between the connection means. The actuator comprises at least one hollow, enclosure having an opening for admitting pressurized fluid. A constraining means cooperates with the enclosure for converting radial expansion of the actuator into axial contraction when pressurized fluid is admitted into the enclosure. 
     The enclosure may be of an elastomeric material. 
     The constraining means may comprise a network of non-stretchable, flexible tension links. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of an actuator, according to an embodiment of the invention, in a pre-installation state; 
     FIG. 2 is a side view of the actuator of FIG. 1 in an axially uncontracted state after installation on a hinged arm; 
     FIG. 3 is a fragmentary view of an alternative embodiment having a network with six-sided meshes; 
     FIG. 4 is a side view of the actuator of FIGS. 1 and 2 in an axially contracted position after installation; 
     FIG. 5 is a diagrammatic side view illustrating in simplified form the function of the network of the actuator of FIGS. 1-4; 
     FIGS. 6 to 8 are diagrammatic perspective views illustrating in simplified form the function of the actuator of FIGS. 1-4; 
     FIG. 9 is a perspective view, partly broken away, of an actuator according to another embodimment of the invention; 
     FIG. 10 is an exploded perspective view of the actuator of FIG. 9; 
     FIG. 11 is a perspective view of the friction reducing layer of the actuator of FIGS. 9 and 10; 
     FIG. 12 is a perspective view of the elastomeric enclosure of the actuator of FIGS. 9 to 11; 
     FIG. 13 is a sectional view along line 13--13 of FIG. 11; 
     FIG. 14 is a side view of an actuator according to a further embodiment of the invention in an axially uncontracted state; 
     FIG. 15 is a side view of the actuator of FIG. 14 in an axially contracted state; 
     FIG. 16 is a sectional view along line 16--16 of FIG. 15; and 
     FIG. 17 is a sectional view along line 17--17 of FIG. 14. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an actuator 1 according to an embodiment of the invention. The actuator has a hollow enclosure 2, in this case of an elastomeric material. In other embodiments a plurality of enclosures could be used together in parallel. The enclosure may be made of rubber, synthetic rubber or a suitable elastomeric plastic material. The enclosure is closed at a first end 3 where it is bonded about a threaded stud or bolt 4. The stud 4 provides connection means for connecting the actuator to a mounting bracket 8 as seen in FIG. 2. The stud 4 is connected to the bracket by a pin 10. The mounting bracket is connected to an articulated arm 39 by a bolt and nut combination 14. 
     The enclosure has a second end 16 which is open in that it is bonded about an open ended nipple 18. The nipple has a threaded outer end 22 adapted to engage a fitting 24 of a hose 26 as shown in FIG. 4. In this manner pressurized fluid, such as hydraulic fluid or pressurized air, can be admitted into the open end of the enclosure. The nipple is connected to a bracket 23 by a nut 25 and thereby comprises a second connection means of the actuator. Bracket 23 is mounted on the arm 39 by a nut and bolt combination 27. The bracket and arm serve as an example only of means actuated by the actuator. 
     The actuator has a network 28 of non-stretchable, flexible tension links 30 extending about the enclosure. The links may be, for example, flexible braided wire covered with plastic. A plurality of such wires are connected together at nodes 32 to form the essentially tubular network. Alternatively the links may be of other materials such as nylon twine. The network has a first end 34. Similarly, the network has a second end 36. At end 36 the wires comprising the network pass through a plurality of apertures 35 extending through a ring 20 and extending circumferentially about the ring. The ring fits over nipple 18 and butts against end 16 of the enclosure. Knots 37 are formed on the ends of the wires to retain the ends of the wires on the ring. In a similar manner, end 34 of the network 28 is connected to ring 6 fitted over stud 4. 
     In referring to the actuator, axial dimensions and directions extend along longitudinal axis 38 of FIG. 1 extending between the ends of the enclosure. Transverse dimensions and directions are perpendicular to this axis. 
     FIG. 1 illustrates the actuator in its pre-installation or off-the-shelf condition. The enclosure 2 is unstretched and the network 28 fits loosely about the enclousre in a bag-like manner. As may be observed, there is considerable space between the network and the enclosure except at the ends 34 and 36. It may also be observed from FIGS. 1 and 4 that the network has meshes which are larger near the center of the network to fit the shape of the expanded enclosure. The meshes are progressively smaller towards the two ends of the enclosure. 
     FIG. 2 illustrates the actuator in an extended, initial condition. In this case, the enclosure has been axially stretched until the network fits closely about the enclosure. This is the axially uncontracted state of the actuator after installation on the arm 39 with a hinged or articulated joint 41. The initial tension required to maintain this uncontracted state is provided by a weight 43 connected to a bolt 45 on the end of the arm. 
     In order to contract the actuator axially, pressurized fluid is admitted into the enclosure by hose 26 as illustrated in FIG. 4. The pressurized fluid admitted into the enclosure causes radial expansion as shown in FIG. 4 where the enclosure bulges most prominently at the midpoint between its two ends. The network acts as constraining means which is, at the same time, radially expandable, but axially contractable. The wires or other tension links comprising the network are essentially non-stretchable. Consequently the radial expansion of the network, caused by the radial expansion of the enclosure, must be accompanied by axial contraction of the actuator as may be observed by comparing FIGS. 2 and 4. 
     Since the entire surface area of the actuator is employed in a function analogous to a piston in a fluid cylinder, the resultant axial pulling force is several times larger than the total force exerted by the pressurized fluid acting on a piston inside a fluid cylinder of the same diameter as the actuator. 
     In the above embodiment there is a tendency for the network to contract axially faster than the enclosure, resulting in buckling of the enclosure near its two ends as pressurized fluid is introduced. For this reason, the network has the loose pre-installation state shown in FIG. 1. Providing the initial stretch to the enclosure upon installation, as illustrated in FIG. 2, prevents this buckling. 
     FIGS. 1,2 and 4 illustrate a network comprising four sided meshes. FIG. 3 illustrates an alternative embodiment wherein the netowork 28a has meshes with six sides when open. 
     The theory of operation of the actuator 1 is explained with reference to FIGS. 5 to 8. The network is represented by a line 40 of length L in FIG. 5. At one end, the line is attached to a fixed mount 42. At the opposite end, the line is attached to a load 44 slidably resting on a surface 46. 
     In FIG. 5, a small force FL has been applied perpendicular to the line 40. The small force FL produces a tension force FT which is many times larger than the force FL for a small angle a. At the same time, the load is moved a distance D. The relationships are defined by the following equations: ##EQU1## 
     FIG. 6 shows an elastomeric tube or enclsure 3 of length L. The tube is sealed at both ends, but has a port 5 for admitting a pressurized fluid. The tube is surrounded by eight non-stretchable, flexible tension links 7, only three of which can be seen from the illustrated side. At their first ends 9, the links are connected to mount 42. At their second ends 11, the links are connected to load 44 slidably resting on a surface 46. 
     When pressurized fluid is introduced through port 5, a pulling force F is created due to radial expansion only of the enclosure surrounded by the links. Referring to FIG. 7, the load 44 has travelled a distance D1 due to the radial expansion. Distance D1 is greater than the distance D of FIG. 5. This is because the links are now deformed into the arc shape of FIG. 7 rather than the sharp bend of FIG. 5. ##EQU2## where P=pressure inside enclosure 
     S=surface area of enclosure 
     a=angle between centre axis 38 and tangent 41 to a point on enclosure surface. 
     If the links are interconnected at regular intervals to form a network 13, as seen in FIG. 8, and the enclosure is inflated, a two-fold application of the case of FIG. 5 occurs. Firstly, the pressurized fluid inside the enclosure provides a force along the tube&#39;s meridians as seen in FIG. 7. Secondly, tension forces TF along equators of the tube produce pulling forces PF along the links of the network. 
     An axially contractable actuator according to the invention offers significant advantages over hydraulic or pneumatic cylinders. The actuator is easier to manufacture and could be considerably less expensive than a cylinder. No sealing or leakage problems are likely to occur because no sliding seals are required as in the case of cylinders. Thus it would be very attractive for installation where fluid leakage is of great concern. The actuator is uneffected by side forces unlike fluid cylinders which cannot tolerate side forces. At the same time, the actuator can be installed more tightly than hydraulic cylinders, allowing more sophisticated robotic arms and hands to be designed. 
     FIGS. 9 to 12 illustrate an alternative actuator 1.1 which is generally similar to actuator 1. Corresponding parts are numbered the same with the additional designation &#34;.1&#34;. 
     Actuator 1.1 has an enclosure 2.1 which is spindle-shaped in the pre-installation state of FIG. 12. This allows even wall thickness after expansion of the enclosure. 
     Actuator 1.1 also has a network 28.1 of non-stretchable, flexible tension links 30.1 which are embedded in a layer 50 of flexible material extending about the enclosure. The layer may be of a suitable flexible plastic, for example. The layer of material is loose and bulges outwardly at 52 between the meshes in the pre-installation state. This permits the layer 50 to readily stretch to the uncontracted state even though the material needn&#39;t be elastomeric. This also provides a minimal resistance by the layer 50 against transverse expansion to the axially contracted state. At end 36.1 wires comprising the network are placed and bonded inside semicircular channels 35.1 extending along a cylinder 20.1. The channels are arranged circumferentially about the cylinder. The cylinder fits over a nipple 18.1 and is bonded to it. Wire 37.1 is wound about cylinder 20.1 and bonded to retain wires of the network on the cylinder. In a similar manner, end 34.1 of the network 28.1 is connected to cylinder 6.1 fitted over stud 4.1. 
     Actuator 1.1 also has a perforated friction reducing layer 54 in the nature of a thin resilient sheet-like tube between the layer 50 and the enclosure 2.1. Layer 54 reduces resistance to expansion caused by friction between the network 28.1 and the enclosure 2.1 in conjuction with layer 50. The perforations 80 eliminate the vacuum that may be created between layers. A suitable lubricant such as an oil, grease or petroleum jelly is applied between layer 54 and the enclosure 2.1 to further reduce friction. The lubricant may also be applied between layers 50 and 54. Layer 54 has a first end 58 and second end 59. At first end 58 it is fitted over and bonded to a first end 3.1 of elastomeric enclosure 2.1. Similarly, at second end 59 it is fitted over and bonded to second end 16.1 of the elastomeric enclosure. 
     Actuator 1.1 may have a longer expected life than actuator 1 due to the reduced friction and consequent reduced wear on the enclosure. 
     FIGS. 14-17 show an actuator 1.2 according to a further embodiment of the invention. This embodiment employs a combined enclosure and network 60. The walls 62 are of an elastomeric material, such as rubber and serve as the enclosure. A network 63 of non-stretchable, flexible links 64, such as braided wire, are embedded in walls 62. A second network 66 of similar or lighter wire, for example, extends across each of the meshes 68 of the network 63. This second network stops undue outward bulging of enclosure 62 between the wires of network 63. 
     The actuator 1.2 is similar to previous embodiments, having a port 70 for connecting a hose for supplying a pressurized fluid. Rings 74 and 76 provide connection means at opposite ends of the actuator. Wires or links 64 extend about the rings for added strength as may be seen in FIG. 17. Rings 74 and 76 and links 64 are encapsulated in suitable rigid plastic bodies 75 and 77 at each end of the actuator. 
     Although an elastomeric material is preferred for the enclosure, other sheet-like, flexible, non-permeable materials of plastic, for example, can be used. Referring to FIG. 9, the entire actuator may comprise the network 28.1 embedded in the non-elastomeric layer 50 which serves as the enclosure. The connecting means could be of either the form shown in FIG. 9 or the form shown in FIG. 14. The material is oversized and tends to bulge outwardly between the links of the network. This accommodates the necessary expansion and distortion of the enclosure without the need of elastomeric qualities. As used herein the term &#34;network-shaped structure&#34; refers to embodiments where the network is part of the enclosure, as disclosed in this paragraph, or is embedded in a separate layer of flexible material, or is a separate component, as disclosed elsewhere herein.