Soft actuators

Abstract

An actuator includes first and second ends defining an axis there between, and at least four inflatable chambers. Each inflatable chamber is resiliently deformable, elongate, and extends axially between the first and second ends and circumferentially about a central core defined between the ends and by the inflatable chambers. A first pair of the four inflatable chambers is contra rotatory about the core to a second pair of the four inflatable chambers. A pressure change in one or more of the inflatable chambers causes motion of the first end relative to the second end. The actuators can be employed in robots or robotic arms.

Claims

1. An actuator comprising: a first end and a second end defining an axis there between; and at least four inflatable chambers; wherein each one of the at least four inflatable chambers is resiliently deformable, elongate, and extends axially between the first end and the second end and circumferentially about a central core defined between the first end and the second end; wherein a first pair of the at least four inflatable chambers is contra rotatory about the central core to a second pair of the four inflatable chambers; such that a pressure change in one or more of the at least four inflatable chambers causes motion of the first end relative to the second end; and the at least four inflatable chambers being distributed symmetrically about the central core, wherein each of the at least four inflatable chambers is directly attached to the central core, the central core being made from a resiliently deformable material.

2. The actuator of claim 1 wherein the central core is hollow.

3. The actuator of claim 1 wherein the central core is generally cylindrical in form.

4. The actuator of claim 1 wherein each of the at least four inflatable chambers extends into direct operational contact with both the first and second ends.

5. The actuator of claim 1 wherein the at least four inflatable chambers alternate circumferentially, one from the first pair then one from the second pair, so as to be distributed in a zig zag fashion about the central core.

6. The actuator of claim 1 wherein each of the at least four inflatable chambers is attached to the central core of a resiliently deformable material and the first and second ends are of a stiffer material than that of the central core.

7. The actuator of claim 1 wherein the central core is hollow and the first and second ends of the actuator are each provided with an aperture extending there through in an axial direction and in communication with the central core.

8. The actuator of claim 1 further comprising a pneumatic circuit or a hydraulic circuit for selectively increasing and/or decreasing the pressure within each of the at least four inflatable chambers by supply of a fluid.

9. The actuator of claim 1 wherein the at least four inflatable chambers are self-supporting at least when partially inflated and the central core is a space between them.

10. The actuator of claim 1 further comprising a control system for controlling pressure changes within the at least four inflatable chambers.

11. A robot or a robotic arm comprising a plurality of actuators according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1a Shows an actuator in perspective view; FIG. 1b shows the same actuator in a top (plan) view; and FIG. 1c shows a front elevation view;

(2) FIG. 2a shows mould components for an actuator; FIG. 2b shows the mould part assembled; and FIG. 2c shows the mould fully assembled;

(3) FIG. 3 shows a secondary moulding process;

(4) FIGS. 4a and 4b show first and second ends of an actuator;

(5) FIG. 5 shows an actuator of the invention in perspective view;

(6) FIGS. 6a, 6b and 6c show an actuator in perspective views to illustrate forces applied; and FIGS. 6d and 6e show the same actuator bending;

(7) FIG. 7 shows graphically bending properties of an actuator;

(8) FIG. 8 shows graphically bending properties of an actuator;

(9) FIGS. 9a, 9b and 9c show an actuator in perspective, top and perspective views respectively, to illustrate forces applied for twisting (rotational) motion; FIG. 9d shows top (plan) views of the same actuator twisting;

(10) FIG. 10 shows graphically, twisting (rotation) properties of an actuator;

(11) FIG. 11 shows elevation views of an actuator extending in length; and

(12) FIG. 12 shows graphically extension properties of an actuator.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO SOME EMBODIMENTS

(13) As described below, as an embodiment of the invention, the inventors have designed a hollow soft pneumatic actuator (HOSE), made of silicone rubber Eco-Flex® 00-30 with the top and bottom sections (first and second ends) reinforced by 3D printed parts. This novel approach allows the actuator to bend in two directions, extend and twist along each axis, with a total 4 DOFs and high dexterity in its movements. The manufacturing process is described herein and the performance in terms of range of motion and applied pressure, described and discussed.

(14) The first and second ends of the actuator are reinforced by using 3D printed parts to avoid deformation of the ends. Different activation combinations of the 4 chambers result in different movements of the actuator. The chambers are constructed from thin walled (0.5 mm), Ecoflex® 00-30 super soft silicon rubber enabling the actuator to perform the movements with a maximal pressure of 4.2 psi.

(15) In the exemplary embodiment, discussed below, the invention provides a hollow soft pneumatic actuator with an external diameter of 30 mm, internal diameter of 5 mm, overall length of 40 mm, and a total weight of circa 9.4 grams.

(16) The actuator of the exemplary embodiment can extend more than 300% of its original length, bend up to ±90° along X axis, ±115° along Y axis, and twist with a total range of ±30°, along the Z axis.

(17) Referring to FIG. 1a, 1b, 1c, the actuator 1 comprises four inflatable chambers C1, C2, C3 and C4. The curved chambers have a rectangular section, surrounding a central hollow cylinder 2 which constitutes a central core of the device. The chambers (C1, C2, C3, C4) extend between a first end 4 and a second end 6 and circumferentially about the cylinder 2. The ends 4,6 have apertures 7 providing access to the hollow centre of cylinder 2, thus providing an aperture 8 through the centre of the actuator 1. FIG. 1a includes an illustration of the orthogonal X, Y and Z axes, the Z axis direction being defined by the first and second ends.

(18) The chambers and the cylinder 2 are of a resiliently deformable silicone rubber. The chamber and cylinder may be a single, one piece moulding as in this example, or the chambers may be applied (e.g. bonded) to cylinder 2. The first and second ends 4,6 are more rigid, of a relatively hard plastics material and are bonded to the cylinder 2 and ends of the chambers 4,6.

(19) Two of the four inflatable chambers spiral upwards about the cylinder 2 in a clockwise direction when viewed from above (first pair C1 and C3) and the other two in an anti-clockwise direction (second pair C2 and C4).

(20) The four inflatable chambers alternate circumferentially: one from the first pair (C1); then one from the second pair (C2); then the second from the first pair (C3); then the second from the second pair (C4) so as to be distributed in a zig zag fashion around cylinder 2. Thus, as shown in FIG. 1 a generally alternating ‘V’ then ‘A’ arrangement is provided. (In the orientation of FIG. 1a: an A between C1-C2; a V between C2-C3; an A between C3-C4; and a V between C4-C1.

(21) When inflated, the curved chambers produce a force applied on the surface of the hollow cylinder 2, which can be decoupled in two components. A non-predominant (NP) force, is defined as the component on the plane X-Y, and a predominant force, perpendicular to the NP force in the Z direction. The NP forces play an important role for the dexterity of the actuator, as described in more detail below and with reference to FIGS. 5 to 12.

(22) The combined activation of two adjacent chambers (Cs) cancels part of the NP components, whilst activating diagonally opposed chambers doubles the effect of the NP components. When adjacent chambers are activated (C1-C2, C3-C4, C1-C4, C2-C3), part of the NP components have opposite direction and are absorbed by the actuator's structure. This results in a rotation along the X axis (C1-C2 positive, C3-C4, negative) or Y axis (C1-C4 positive, C2-C3 negative). (Positive in this context refers to an increase of pressure, negative a decrease in pressure, of the chambers referred to). When diagonal chambers are activated (C2-C4, C1-C3), the NP components have opposite direction but different application point. This produces a torque applied to the central hollow ring, and ends 4,6, resulting in twisting in a positive i.e. clockwise direction of end 6 relative to end 4 (C2-C4) or in a negative i.e. anti-clockwise direction (C1-C3). Thus a rotation about the z axis is achieved. To avoid collapse of the inner hollow cylinder 2 (top and lower section) due to the softness of the material, the two stiffer end parts 4,6 reinforce the top and the bottom of the actuator. This stiffening of, the top and bottom sections of the actuator, produces a rotation avoiding squeezing of the internal hollow section of the actuator. The central aperture 8 is kept open.

(23) The angle between the chambers C1, C2, C3 and C4 and the vertical Z axis, named a, (FIGS. 6 and 9), changes with the extension of the chamber, resulting in a different behaviour of the actuator 1 during its motion, as explained in further detail below.

(24) An example of a manufacturing process, the range of motion of the actuator and effect of the curved chamber on its dexterity will now be described.

(25) Mould Construction

(26) Silicone rubber Ecoflex® 00-30 (shore hardness=00-30, tensile strength=200 psi/1.38 MPa, 100% modulus=10 psi/68.95 kPa, elongation at break of 900%), was selected due to its high stretchability and low tensile modulus. The actuator was produced by using a 3D printed mould as shown in FIG. 2a. The mould is modular and composed of 13+1 parts in order to simplify the removal of the silicon actuator after the material is cured. Four chambers are made by using the moulds M2, M3, M4, M5, connected to the bottom and top part of the chambers, M12, M13. The top part is connected by 4 pins, in order to align the chambers (M2, M3, M4, M5) and keep four holes in the cast, for the insertion of tubes. The lower part of each chamber, is connected with the bottom part of the mould by using a central pin (M1), leaving four open sections, which requires a secondary process. Additional 4 parts of the mould, (M6, M7, M10, M11) are used to define the outer shape of the actuator. Half cylinder parts M8, M9, are used to close the outer surface part of the mould. They are made of transparent material to verify the alignment of the internal part of the mould. FIG. 2b shows the assembled mould, apart from M8, M9. FIG. 2c the complete mould ready for filling.

(27) Casting

(28) Ecoflex® 00-30 parts A and B are mixed with in a 1:1 ratio, and degassed under vacuum for 10 minutes, to remove any air and avoid bubble formation in the final cast. The mixture is then poured into the assembled mould (FIG. 2-c) and cured, either at room temperature for 4 hours or, the process can be speeded up by heating in an oven at 60 degrees Celsius for 30 minutes. After curing, the lateral, top and bottom parts of the mould are removed (M8, M9, M12, M13, M6, M7, M10, M11). The moulds inside the chambers are removed (M2, M3, M4, M5) leaving the actuator with only the inner cylinder (M1), needed to keep the central part stiff and to facilitate its manipulation for the subsequent stage. An additional mould (M14) is used to close (seal) the lower (in this view) part of the actuator, again using Ecoflex 00-30, as shown in FIG. 3. At the same time, four silicon tubes 10 with an external diameter of 2.1 mm are glued into the upper (in this view) holes of the actuator as shown in FIG. 3. The first and second ends 4,6 are stiffer parts (see FIG. 4) and are employed to reinforce the structure and keep the end surfaces plane, as well as to increase the rotation along Z axis. In this example they have an external diameter of 30 mm and a wall thickness of 0.5 mm, and are 3D printed by using VeroClear-RGB10 polymer (tensile strength 7,250-9,450 psi/50-65 MPa, modulus of elasticity 290,000-435,000/2,000-3,000 Mpa, elongation at break 10-25%).

(29) Both ends 4,6 are provided with a central aperture 7. Two further apertures 12 (open sections) at the first end 4 are needed to allow silicon tubes 10 to be connected to the chambers C1 to C4. Both, the first and second ends 4,6 are glued to the soft part of the actuator using superglue (cyanoacrylate adhesive). The prototype of this process is shown in FIG. 5 which is inverted with respect to FIG. 3.

(30) Control

(31) The control was designed to be affordable and ease of implementation, by using off-the-shelf components. A total of 8 electro-valves (EV), 4 to inflate (IVs) and 4 to deflate (DVs), are used to control the 4 chambers. The chambers are naturally deflated, opening the DVs and by using the internal pressure. Each chamber has an analogue pressure sensor (Honeywell model ASDXAVX015PGAA5), used to monitor the chambers pressure and to limit the maximal pressure applied to each chamber, in order to avoid any damage. An external compressor (GS AS18) provides the air for inflation of the chambers.

(32) Both valves and sensors are connected to an Arduino board, which functions as a bridge interface (BI) and implements a low-level control (LLC). The role of the LLC is to activate-deactivate the EV, and acquire the 4 analogue pressure sensors. The role of the BI, is for data exchange with the external interface (EI) through a serial communication port. The high-level control (HLC) is implemented in Matlab® Simulink and includes a serial communication interface to the LLC and the Xbox One Wireless Controller. The control architecture aims to perform simple movements of the actuator by using an open-loop control and visual feedback by the operator.

(33) Experiments and Discussion

(34) The proposed actuator was tested by measuring the pressure and the range of motion for each DOF, described in detail in the following sections. Each test has been performed by replicating each DOF, 3 times and increasing the air volume at each step.

(35) Bending

(36) The actuator 1 is able to bend along X (C1-C2, C3-C4) and Y (C1-C4, C2-C3) axis, by activation of adjacent inflatable chambers. The central cylinder 2 works as a fulcrum about which the actuator can bend.

(37) The behaviour of the actuator may be considered in terms of pressurising (activating) a pair of inflatable chambers that are adjacent when moving round the circumference of cylinder 2.

(38) In this consideration the V shape is defined as the part of the actuator where the ends of two adjacent chambers contact at an end, with A shape being the opposite, where the two adjacent ends are spaced apart at an end, as indicated in FIG. 5.

(39) When two adjacent chambers are activated, the V shape connection at an end behaves differently from the A shape connection at an end. Both, V and A shape connections, produce a force and a torque reaction from the cylinder 2, resulting in a bending and extension of the actuator.

(40) FIGS. 6a to 6c illustrate a simplified representation of the forces configuration in both V and A shape connections. The forces acting on end 6 are labelled F.sub.1 to F.sub.4, corresponding to the chambers C1 to C4 respectively (chamber C1 and corresponding force F.sub.1 not visible in this view). The two adjacent chambers' non-predominant (NP) force components are in the same plane: one component, along the X or Y axis, is equal but with opposite direction, hence absorbed by the structure; the other one, along Y or X axis, is equal but with the same direction, hence, producing the rotation of the actuator. When the actuator extends, α.sub.i decreases in the V shape, and increases in the A shape, hence the NP forces decreases (FIGS. 6b, 6c).

(41) In the A shape connection the central ring receives a force applied at a distance d2 (C1-C4 or C2-C3), lower than d1 in the V shape (C1-C2 or C3-C4). The angle α.sub.i in the A shape, is higher than the V shape, and >3/2 π, therefore producing a force in the opposite direction. This results in a greater bending of the central ring in the A shape, than the V shape.

(42) FIGS. 6d and 6e show the actuator bending in A shape (6-d), and V shape (6-e). Starting from the rest positions shown in the in the central view, inflation of the adjacent chambers causes the second end 6 to bend to the left or to the right until at approximately at right angles to the first end 4.

(43) FIG. 7 shows the graph of angle vs pressure when chamber C1-C2 (positive angle) and C3-C4 (negative angle) are activated (inflated to higher pressure).

(44) FIG. 8 shows the graph of angle vs pressure when chamber C1-C4 (positive angle) and C2-C3 (negative angle) are activated.

(45) All the graphs show four phases: i) activation, ii) motion—decreasing pressure, iii) motion—increasing pressure, and iv) saturation. The activation phase shows discontinuous pressure, resulting in a negligible movement of the tip of the actuator. This is considered as the activation pressure limit, which is the minimum value required to induce movement in the actuator. The phase ii), where the actuator starts to bend, results in a reduction of the pressure when the bending increases. This is related to two effects.

(46) The first one a) is that the angle α.sub.i (FIG. 6) changes when the actuator starts to bend, reducing the NP components, hence having all the forces produced by the actuator in line with Z axis. This increases the torque applied to the tip of the actuator and therefore the bending. The second factor b) is that the central hollow is bent and become floppy, reducing its resistance, hence the pressure inside the chambers.

(47) In phase iii), the pressure increases again till phase iv). In phase iv), the saturation of the actuator results in an increasing pressure with a negligible effect on bending. The range of motion along X axis is ±90° at 2.2 psi, and along Y axis ±115° at 2.3 psi.

(48) Twisting (Rotation)

(49) By activating diagonally opposed chambers, the actuator can twist and extend, as shown in FIG. 9, achieving a total range of ±30° at 2.1 psi. FIG. 9a shows the actuator in perspective with a datum line 12 marked on the top of second end 6 to show twisting more clearly. FIG. 9b shows a top view and 9c a front view. The activation of a pair of diagonal chambers (e.g. C2, C4), produces diagonal forces (F.sub.2, F.sub.4) composed of a predominant components along Z axis, and a part of the NP component along X (C1-C3), or Y (C2-C4). The NP component of each force (F.sub.2, F.sub.4) is composed of two forces. One is compensated by the activation of the other chamber, the other one is involved in the twisting of the actuator (F.sub.2x and F.sub.4x in this example).

(50) The NP components involved in the rotation, are opposite in direction and applied at a distance of 2d1, producing a torque along the central hollow ring, positive (C2-C4) or negative (C1-C3), causing the rotation of the actuator in two directions. The NP components along the X-Y axis, are higher at the activation when α.sub.i is higher, and they decrease when the actuator extends, consisting in a saturation as the rotational angle. FIG. 9d shows the twisting (rotation about the Z axis) viewed from above. From the central, at rest, position clockwise or anti clockwise rotation can be achieved.

(51) FIG. 10 outlines the twisting performance. The rotation is focused in a small range of pressure. Higher pressures result in an extension of the actuator after this preliminary phase.

(52) Extension

(53) Activating all the chambers C1-C2-C3-C4 together, as shown in FIG. 11, extends the actuator by up to 300% or more of the initial length, at 4.2 psi. From the left hand at rest front elevation view, the first and second ends 4,6 move away from each other to the fully extended right hand view. Radial expansion occurs with the same percent as the extension. FIG. 12 shows the extension performance of the actuators. After the activation phase, the air volume increases, although the pressure decreases, as discussed for the other DOF motions.

(54) Positive and negative values are shown in all graphs (FIG. 7, 8, 10), in both bending and twisting experiments, they are not perfectly symmetric. This phenomenon is due to the non-symmetry distribution of wall thickness in all the chambers, caused by the moulding process. All the reported graphs show a pressure that decreases after the first activation phase ii), and then increases, phase iii). This implies that the actuator may not be controlled using a pressure sensory feedback. Hence, a closed-loop control is desired and can be implemented by using the orientation of the top part of the actuator, (e.g. by using an inertial sensory unit, or a tracking system) and the volume of the inflated air, as the control parameters. Pressure sensors, can be used as a safety control to monitor the stress of the material.

(55) Summary of Prototype Testing

(56) The invention provides a Hollow Soft Pneumatic Actuator (HOSE) with 4 DOFs and a manufacturing process therefor. Performance of the actuator was evaluated in terms of range of motion versus the applied pressure. The control components are off-the-shelf and easy to implement. The actuator performance has shown to exhibit dexterity along each DOF with a low input pressure, less than 2 psi. The hollow central part may be used for cabling the actuation tubes and electrical cables to embed any electronic board on the top part of the actuator for implementing more complex tasks.

(57) The range of rotation is related to the overall length of the actuator, the curved shape of the chamber and the wall thickness. The control system has been implemented to perform simple movements by using an Xbox® One Wireless Controller. A closed-loop control may be provided, by using additional sensors to detect the position and orientation of the top part of the actuator.

(58) As will be appreciated by the skilled person, a modular actuator based on the present invention may further comprise onboard electronics and embedded small valves, to control actuator function. The invention may further extend to a complex manipulator comprising one or more modular actuators as described above. For example, a robot or a robotic arm. The simple manufacturing process allows the present invention to be miniaturized and be used for several applications with a limited operative space. The actuator's dexterity of movement can be an advantage for applications such as manipulation of surgical instrument end effectors or positioning endoscopic cameras for inspection of body lumens.

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