Low strain pneumatic networks for soft robots
1. An actuator, comprising:
- a base comprising a strain limiting layer;
a plurality of unsupported chambers secured to the strain limiting layer, each chamber having interior side walls, exterior side walls, and a top wall, wherein the unsupported chambers do not share a common wall along a portion of the interior side walls;
a channel that fluidically interconnects the plurality of chambers; and
an inlet port for receiving a pressurizing fluid, wherein the interior side walls expand a greater amount relative to the exterior side walls and the top wall upon pressurization of the chambers to cause bending of the actuator around the strain limiting layer and cause said portion of the interior walls to touch each other.
An actuator includes a plurality of chambers comprised of an extensible material, the chambers having interior side walls and exterior walls, wherein at least a portion of the interior side wall is separated from an interior side wall of an adjacent chamber; and a strain limiting base; and a channel that fluidically interconnects the plurality of chambers, wherein the interior walls are configured to be more compliant than the exterior walls.
Patent #US 20030110938A1
Current AssigneeSeiko Epson Corporation
Sponsoring EntitySeiko Epson Corporation
|Pump-integrated flexible actuator|
Patent #US 20020157388A1
Current AssigneeSeiko Epson Corporation
Sponsoring EntitySeiko Epson Corporation
|Method and system employing strings of opposed gaseous-fluid inflatable tension actuators in jointed arms, legs, beams and columns for controlling their movements|
Patent #US 4,784,042 A
Current AssigneeJoseph M. Juarez
Sponsoring EntityNathaniel A. Hardin, Henry M. Paynter
|FLUIDIC ACTUATOR AND METHOD OF MANUFACTURE|
Patent #US 20120216672A1
Current AssigneeCarlo Menon, John Berring
Sponsoring EntityCarlo Menon, John Berring
|SOFT ROBOTIC ACTUATORS|
Patent #US 20140109560A1
Current AssigneePresident and Fellows of Harvard College
Sponsoring EntityPresident and Fellows of Harvard College
- 1. An actuator, comprising:
a base comprising a strain limiting layer; a plurality of unsupported chambers secured to the strain limiting layer, each chamber having interior side walls, exterior side walls, and a top wall, wherein the unsupported chambers do not share a common wall along a portion of the interior side walls; a channel that fluidically interconnects the plurality of chambers; and an inlet port for receiving a pressurizing fluid, wherein the interior side walls expand a greater amount relative to the exterior side walls and the top wall upon pressurization of the chambers to cause bending of the actuator around the strain limiting layer and cause said portion of the interior walls to touch each other.
- View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32)
This application claims benefit of the earlier priority date of U.S. Provisional Patent Application No. 61/867,845, entitled “LOW STRAIN PNEUMATIC NETWORKS FOR SOFT ROBOTS,” filed on Aug. 20, 2013, which is hereby incorporated by reference herein in its entirety.
The present invention was made with United States government support under DARPA Grant No. W911NF-11-1-0094. United States government may have certain rights in this invention.
All patents, patent applications and publications cited herein are hereby incorporated by reference in their entirety in order to more fully describe the state of the art as known to those skilled therein as of the date of the invention described herein.
This technology relates generally to soft robotics. In particular, this invention relates to a low strain soft robotics.
A robot is an example of a controlled programmable device capable of executing mechanical or electrical tasks. Robots have replaced human in performing those repetitive and dangerous tasks which humans prefer not to do, or are unable to do due to size limitations or the extreme environments. Robotics deals with the design, construction, operation, and application of robots, as well as computer systems for their control, sensory feedback, and information processing.
Soft robotic technologies are appealing for locomotion, and for manipulation of fragile or irregularly shaped objects; their potential to bring new capabilities to the field of robotics stems from the compliant materials of which they are fabricated, the simplicity of their actuation, and their potential for low cost. Soft actuators have been fabricated from a variety of materials (polymers, elastomers, hydrogels, granules) and operate with several different modes of actuation, e.g., pneumatic, hydraulic, electric and chemical modes of actuation.
Pneumatically powered elastomeric actuators are appealing because these structures are light weight, inexpensive, easily fabricated, and provide linear or non-linear motion with simple inputs. One type of composite soft robotic actuators is based on a pneumatic network design that comprises mm-scale channels embedded in an extensible elastomeric layer with an inextensible layer made of either a stiffer elastomer or the same elastomer embedded with a fabric. See International Application No. PCT/US2011/061720, filed Nov. 21, 2011. Upon pressurization of a single inlet, the pneumatic network actuator bends, and provides motion analogous to several hard actuators connected in series. Bending of the serially connected pneumatic network actuators occur by the straining of the top wall of the chamber, allowing the inside walls to deflect away from each other at the interface of the inextensible layer (similar to a hinge).
Pneumatic actuators powered by compressed-air can generate complex motions, however, most of these motions have, thus far, been slow (on the order of seconds) to achieve their maximum amplitude. Rates of actuation are limited by the large changes in internal volume required to achieve the full range of motion of the actuator, and therefore by the rate at which low-pressure gas can be transported through the tube connecting the actuator to a gas source. Pneumatic networks (networks of small channels embedded in elastomeric structures that can be inflated with air) usually require significant changes in volume (ΔV/V>3) to achieve their full range of bending. This requirement for large ΔV/V limits the performance of soft actuators that use pneumatic networks in three ways: i) it requires the transfer of large volumes of gas for actuation, (and as a result, limits rates of actuation to low values), ii) it generates a change of volume of the actuator that is significant (and as a result, requires that the system have large volumes in which to operate), and iii) it imposes high strains on the material of which the pneumatic network is fabricated (and, as a result, shortens the operating lifespan of the pneumatic networks).
Improvements to soft robotic design are therefore needed.
Pneumatic network actuators for use as soft robots are described.
In one aspect, an actuator includes a plurality of chambers comprised of an extensible material, the chambers having interior side walls and exterior walls, wherein at least a portion of the interior side wall is separated from an interior side wall of an adjacent chamber; a strain limiting base; and a channel that fluidically interconnects the plurality of chambers, wherein the interior walls are configured to be more compliant than the exterior walls.
In one or more embodiments, he exterior walls include exterior side walls having a thickness that is greater than the thickness of the interior side walls.
In one or more embodiments the exterior walls include top walls having a thickness that is greater than a thickness of the interior side walls.
In one or more embodiments, the exterior walls are made from materials having a greater stiffness than the interior side walls.
In any of the preceding embodiments, the base can include an inextensible, flexible layer.
In any of the preceding embodiments, the base can include an inextensible layer embedded in a layer of flexible polymer, and for example, the inextensible layer can be paper, fabric or polymer film, and for example, the paper or fabric is provided as a matted sheet, woven sheet or fibers aligned along the length of the base.
In one or more embodiments, the base comprises an extensible, flexible material that is configured and arranged to be stiffer than the exterior walls of the chambers.
In any of the preceding embodiments, the plurality of chambers is made up of an elastomer.
In any of the preceding embodiments, the actuator is powered by pressure provided by a gas or liquid source.
In any of the preceding embodiments, the pneumatic or hydraulic channel is positioned between the interior side wall of the plurality of chambers, or the pneumatic or hydraulic channel is located in the strain limiting base and is positioned and arranged to fluidically connect the plurality of chambers.
In any of the preceding embodiments, the actuator further includes an inlet port for receiving a pressurizing fluid.
In another aspect a method of actuation includes providing an actuator according to any the embodiments noted herein; and pressurizing the actuator to cause the actuator to move from a first resting position to a second actuated position.
In one or more embodiments, pressurizing causes the interior side walls to expand preferentially.
In any of the preceding embodiments, the actuation occurs in less than 1 second, or the actuation occurs in less than 250 msec, or the actuation occurs in less than 100 msec.
In any of the preceding embodiments, the actuation is greater than 1 m/sec.
In any of the preceding embodiments, the actuator undergoes a circumferential-expansion of less than 3% during actuation
In any of the preceding embodiments, the actuator undergoes an energy loss of less than 10% between the actuated and unactuated state
In any of the preceding embodiments, the actuator can be cycled more than 10000 times without failure, or the actuator can be cycled more than 200000 times without failure, or actuator can cycle more than one million times without failure.
In any of the preceding embodiments, the frequency of actuation is greater than 1 Hz, or the frequency of actuation is greater than 2 Hz.
In any of the preceding embodiments, the actuator bends along a roughly circular trajectory below a threshold actuation rate and the tip of the actuator bends preferentially, causing the actuator to curl on itself above a threshold actuation rate.
In another aspect, a method of making an actuator includes molding an extensible layer comprising a plurality of chambers comprised of an extensible material, the chambers having interior side walls and exterior walls, wherein at least a portion of the interior side wall is separated from an interior side wall of an adjacent chamber; molding a strain limiting layer comprises of a material having a greater stiffness than the extensible layer; and joining the extensible layer to the strain limiting layer at predetermined locations to define chambers having a pneumatic channel that fluidically interconnects the plurality of chambers.
Pneumatic network actuators having freestanding chambers can achieve high actuation speeds (>1 m/s) and reliable long-term operation. Pneumatic network actuators having freestanding chambers have improved actuation efficiency at a given pressure by almost 10× in terms of bending angle, by 50% in terms of force, 30× in terms of lost energy, and over 1000× the number of actuation cycles as compared to comparably dimensioned pneumatic network actuators having embedded chambers. Pneumatic chambers having freestanding chambers exhibit lower fatigue of the component polymer materials that allows for a longer lifetime of the actuator.
The advantages of these actuators are that they can easily have multiple bending points which actuate by pressurization or passively bend by an external force; these properties can be useful for positioning sensitive interlocking objects in an assembly line. In addition, these materials are soft, and resistant to damage by impact or pressure. Because they are also inexpensive, they can be routinely replaced, or used in high numbers. The design improves speed, force, energy consumption, and fatigue, all of which are factors for commercial applications of soft robotic actuators. This rapid motion and dependability is useful for many applications such as those in automated assembly lines, and for mobile and agile robots for search and rescue operations.
These and other aspects and embodiments of the disclosure are illustrated and described below.
The invention is described with reference to the following figures, which are presented for the purpose of illustration only and are not intended to be limiting.
Soft robots capable of rapid pneumatic activation and having low fatigue are described. A simple actuator can bend from a linear shape to a quasi-circular shape in 50 milliseconds when pressurized with a pressure of ΔP=345 kPa. When inflated fully, the channels and chambers of this new design for a pneumatic network experience only one-tenth the change in volume of that required for a motion of equal amplitude using a design involving embedded chambers. This small change in volume requires comparably low levels of strain in the material at maximum amplitudes of actuation, and this has low rates of fatigue and failure. This actuator can operate over a million cycles without significant degradation of performance. This design for actuators for soft robots combines high rates of actuation with high reliability of the actuator, and opens new areas of application for them.
The pneumatic network design decreases the amount of strain on the material required for full amplitude motion of these actuators. In one or more embodiments, the pneumatic network contains separate walls for every chamber of the soft bodied device to control the expansion of the elastomeric walls and to thereby improve the efficiency of actuation. The efficiency of bending is improved by reducing the stiffness of the freestanding interior walls of the chamber relative to the external walls, thereby requiring less pressure to achieve full motion of the actuator. It has been surprisingly determined that when the walls of the chambers are all uniform, they experience higher fatigue and are susceptible to failure due to excessive strain experiences during expansion of the chambers. By providing interior side walls that are thinner than the exterior walls of the pneumatic chambers, the amount of strain experienced by the device during activation can be reduced, thereby reducing fatigue and corresponding failure, as well as increasing the speed of actuation.
The low strain, rapid actuation pneumatic network includes an extensible top layer and inextensible bottom layer. The soft robots include a plurality of unsupported chamber or “freestanding” chambers secured to a common strain limiting layer along one face. The unsupported chamber does not share a common wall along the at least a portion of the length of the side wall. The chambers can be “detached”, that is, the chamber does not share any portion of its side wall with its neighboring chambers, or “semi-detached”, that is, a chamber shares a portion of its side wall with its neighboring chambers. In other embodiments, the pneumatic network can be described as being a ‘double-contoured’ pneumatic network, because the contour of the upper surface and lower surface are different. The chambers are fluidically connected, for example, through a pneumatic conduit located in the base containing the strain limiting layer or in the side walls of the chambers. Soft robots possessing a plurality of detached, semi-detached or freestanding chambers secured to a common strain limiting layer along one face can be pressurized to provide large and versatile actuation to soft elastomers. The interior side walls are designed to be more compliant than the exterior walls. In one or more embodiments, the exterior walls are thicker than the interior side walls. In one or more embodiments, the exterior walls are made of a stiffer material than the interior side wall to obtain a more compliant interior side walls. Therefore, an increase in the internal pressure preferentially expands the inside walls and minimizes the strain that occurs on the other exterior walls. In addition, the close proximity of two neighboring chambers causes the expanding inside walls to push against each other, and results in a preferential elongation of the extensible layer, with only small changes (<1%) in the height of this layer. While described below for use with pneumatic activation, it is recognized that the device may be used with hydraulic activation. The response times may be longer for hydraulic activation.
The principle of pneumatic actuation according to one or more embodiments is illustrated in
The extensible layer is connected to a strain limiting base 150 including a stiffer, yet still pliable backing layer 160. The stiffer backing layer may be a material having a higher elastic modulus than the chambers. The high modulus material can be used for the base where inflation is undesirable, while a low elastic modulus material can be used for the chambers where extensibility is needed. Alternatively, the stiffer backing layer may include an inextensible backing material, such as paper or fabric, embedded in the base. In such cases, the base and chambers can be made of the same or different materials.
Upon pressurization of the chambers through the pneumatic conduit 150, the soft-elastomer network expands as shown in
In comparison, in a pneumatic network in which the channels are embedded in a single elastomeric block in which the chambers share interior walls, the expansion force is applied to a great extent to the top wall, as is illustrated in
The actuation resulting from pressurizing the interconnected channels in the freestanding chambers pneumatic network according to one or more embodiments is additive in the direction of the repeated sequence. Repeated channels need not be of the same geometry, and a network of channels comprising a number of channels with different geometries and orientations can therefore be used to create a complex movement, or change of the shape of a membrane upon pressurization.
As used herein, “stiffness” refers to the resistance of the elastic body to extension by an applied force. In general, elastic modulus is related to, but not the same as, stiffness. Elastic modulus is a property of the constituent material; stiffness is a property of a structure. That is, the elastic modulus is an intensive property of the material; stiffness, on the other hand, is an extensive property of the network and is dependent on the material modulus and the shape and boundary conditions. Because stiffness is a function of the Young'"'"'s modulus, the material modulus can be used as a comparative measure of the relative stiffness of the channels walls and a predictor of deflection upon pressurization of the channel networks.
Strain is a description of deformation in terms of relative displacement of a body. A deformation results from a stress induced by applied forces, in the case here, for example, by the pressurizing force. Because materials of lower stiffness or smaller elastic modulus will deform to a greater degree than the higher elastic modulus materials, the low stiffness materials experience strain or deformation first. As a result, the strain in the material of higher stiffness or greater elastic modulus is smaller or “limited.” As used herein, the layer or wall of the channel that is stiffer, e.g., has a higher elastic modulus, is referred herein to the “strain limiting” layer or wall or membrane.
The size and shape of the chambers can be varied. Variable that can be varied include chamber height, thickness of the interior side walls, thickness of the top and exterior side walls, and number of chambers in the pneumatic actuator. Generally, increasing the number of chambers increases the curvature of actuation at a given pressure (due to additive effect of actuation of each individual chamber). Increasing chamber height reduces curvature of actuation at a given pressure. The chamber features can be curved, rounded or rectilinear.
The side walls may be fully detached, meaning that adjacent chambers do not share any portion of the side walls. In other embodiments, the side walls can be semi-detached, namely, a portion of the side walls from adjacent chambers are connect and a portion is detached. Note that some portion of the side wall can be used to form the conduit joining adjacent chambers,
Fabrication of Low Strain Pneumatic Networks
Low strain pneumatic networks are made using a molding process. First, molds are made using a 3D printer based on a computer-aided-design (CAD) model. The actuator requires three molds: an interior and exterior mold for the top extensible layer, and a third mold for the bottom inextensible layer (
The features of the exterior mold as shown in
The list of materials that can be used with this technique is extensive and encompasses elastomers such as latex, polyurethanes, silicones, vulcanized rubber for the extensible materials, and fabrics such as paper, Kevlar©, cotton, nylon, etc. for the strain limiting layer. Typically two types of elastomers, Ecoflex 30 and Elastosil M4601 are used; both are two-component silicone rubbers that vulcanize at room temperature. Ecoflex 30 is extremely soft (shore value of 00-30), while Elastosil M4601 is relatively stiff (shore A30.) In one observation, it was found that stiffer materials are less energy efficient but provide faster actuation. Paper, fibers mats and fabric may be used as the inextensible material embedded in the base to create the strain limiting layer. The choice of materials, coupled with the design of the chambers, determines the response of the device to pressure. The pressure necessary to achieve a particular amplitude of actuation scales with the stiffness of the materials. Each combination provides a different behavior in bending, upon actuation: for the same chamber geometry, the bending increases with increasing difference in elastic modulus between the elastomer and the strain limiting fabric (or layer).
Inlet holes are made using a biopsy punch at one end of the actuator. The size of the biopsy punch depends on the desired tubing to be inserted into the actuator, but typically ranges from 1-3 mm in diameter. Tubing should be of a slightly larger diameter than the inlet hole so an adequate seal will be made by the elastic force of the elastomer. In other embodiments, the soft robotic body is injection molded, in which case, inlets, outlets and conduits can be integrated into the soft robotic body.
Actuators are typically pressurized by compressed air or nitrogen supplied by gas tanks or a compressor. Hydraulic actuation is also performed using water supplied by a syringe pump. Alternatively, vacuum can be applied to bend the actuator slightly in the opposite direction.
Performance Characterization of Low Strain Pneumatic Networks
A variety of chamber design parameters can be modified in order to change the actuation response in the pneumatic network. Exemplary parameters that can be varied to control the performance of the freestanding actuator include the number, height, and wall thickness of the chambers. The effect of these parameters on the performance of the actuator was evaluated by measuring the radius of curvature at 4 PSI and the pressure required to actuate the low strain pneumatic network to its full range of motion. The results are shown in
As is apparent from the results reported in
Increasing the heights of the chambers appears to have a plateau effect and therefore eventually taller chambers would only increase the actuator size, weight, and amount of material used without increasing the actuator'"'"'s performance.
Comparison of Low Strain Pneumatic Networks to Embedded Chamber Pneumatic Networks
An advantage of soft robots, over conventional hard robots, is that rapid prototyping allows new designs to be fabricated and tested quickly, in many cases, within a single day. Despite the advantages brought by rapid prototyping, a “trial and error” approach does not provide all the information necessary to make optimal design choices.
In designing this class of actuator, it is desirable to be able to predict the details of their non-linear response to pressure. A computer software generated finite element method (FEM) of the pneumatic network can help to understand the mechanisms controlling the behavior of these actuators, and can also be used as a tool for developing more efficient actuator designs.
For the actuator having unsupported (freestanding) chambers, preferential expansion of the chambers in the axis of bending is achieved by molding gaps into the exterior of the actuator so the chambers no longer share inside walls, and are only connected at the base of each expanding chamber (
The performance of an actuator having unsupported chambers differs in several ways from an actuator having embedded chambers having otherwise similar properties, e.g., similar number of chambers, height of chamber, and wall thickness: i) the actuator having unsupported chambers bends more at a given pressure, ii) the actuator having unsupported chambers expands less when at its full range of motion, iii) the actuator having unsupported chambers exerts more force for a given pressure, iv) the actuator having unsupported chambers fatigues/fails less often.
The reason for these differences can be rationalized by considering the strain experienced by the actuator when pressurized. To estimate the strain experienced by both embedded actuators and actuators having unsupported chambers, a FEM using Abaqus 6.11-Simulia (Dassault Systems was employed). The FEM results show significantly more (˜2.5×) strain (maximum principal or non-deformational) is experienced by the embedded chamber actuator as compared to the actuator having freestanding chambers for equivalent amplitude of motion. See,
For the FEM model, the most critical aspect is properly modeling the non-linear elastomeric behavior of the silicone materials used (Elastosil M4601). Stress-strain curves were measured using pieces of cured Elastosil M4601 in the standard geometry (ASTM D638-IV) at a rate of 500 mm/min for tensile and compression testing. Compression and tensile data were plotted on one graph and fitted to the Yeoh hyperelastic material model in Matlab. Equation 1 is the fitting equation:
where, n=3, I1 is the first strain invariant, J is the volume ratio, and constants Ci0 and Di are the coefficients fitted. Ci0 describes the shear behavior of the material, and Di introduces compressibility. Samples were assumed incompressible so Di was set to 0. The Yeoh model provided a good match to the experimental data.
The elastomeric components of the actuators are categorized within the FEM as solid elastomers, with the inextensible paper layers categorized as “shells”. The embedded chamber actuator is comprised of 19,826 quadratic solid tetrahedral elements. The inextensible paper layer has a total of 738 quadratic triangular elements. The entire embedded chamber actuator is modeled using 31,622 nodes. Similarly, for the actuator having freestanding chambers, a total of 26,593 quadratic solid tetrahedral elements are used. The inextensible paper layer has a total of 845 quadratic triangular elements. This entire actuator is modeled using 44,637 nodes. The coefficients for the hyperelastic material properties are c1=0.11 and c2=0.02. An isotropic material type for the Elastosil was given a density of 1130 Kg/m3.
For the inextensible component (paper) a density of 750 Kg/m3, a Young'"'"'s Modulus of 6.5 GPa, and a Poisson'"'"'s ratio of 0.2 were defined. In both FE models, the gravitational forces are accounted for and the fluid pressure is set to act on all internal faces of the cavities. The model was used to simulate the strain experienced by both the embedded and freestanding chambers in a pneumatic network upon pressurization, which are shown in
There are two implications that suggest the freestanding chamber actuator is a better design than the embedded chamber actuator when intended for high-amplitude, rapid motion; i) The material of the freestanding chamber actuator will be less fatigued than that of the embedded actuator, since the freestanding chamber undergoes less strain per actuation; and ii) more rapid actuation can be achieved for a given flow rate of pressurized gas, since less internal pressure is required.
To confirm these results, the amount of pressure required to bend both an embedded chamber and a freestanding chamber actuator was determined. Both actuators were secured at one end and allowed to hang freely at the other. The actuator having freestanding chambers is much more compliant than the actuator having embedded chambers, since the individual expanding chambers are not held together by elastomer between the interior walls. This is illustrated in
The speed by which a pneumatic network bends depends on its rate of pressurization (Pa·s−1) and the net forces acting on it. The velocity of the tip of both low strain pneumatic networks (having freestanding chambers) and embedded chamber pneumatic networks was measured when pressurized to 72 kPa (10.44 psi), which is the pressure required to bend the actuators having freestanding chambers fully. The trajectory of the free end of the actuator was tracked using images taken from a camera and plotted them using a graph that allowed comparison of observed and FEM predicted positions. From both
The reduction in volume that the freestanding chambers pneumatic network undergoes during inflation is advantageous for potential applications in space-limiting settings such as search-and-rescue and medical procedures (such as, minimally invasive surgery). The embedded chambers pneumatic network requires outward expansion of its extensible layer to bend, and any restrictions on the space available for its expansion could prevent proper function. The expansion of the extensible layer for the freestanding chambers pneumatic network, however, occurs largely within the space between each of its chambers, and thus allows it to fit into tighter spaces.
Control of the movement of the actuator is another important parameter characterizing performance. The embedded pneumatic network follows a non-linear relationship between pressure and volume, and is similar to a balloon in that volume increases but pressure is relatively stable after a threshold pressure (
This same measurement was performed on actuators made out of a stiffer elastomer for the base (Elastosil M4601 Young'"'"'s modulus of ˜7 MPa, Shore A hardness 28) for the extensible layer. An actuator having freestanding chambers made from Elastosil M4601 as compared to the Ecoflex 30 (Young'"'"'s modulus ˜0.1 MPa, Shore A hardness 00-30) requires ˜8× more pressure to fully bend, loses ˜4× more energy for one cycle of bending and unbending, and requires ˜1.5× less volume of water to fully bend. Compare,
One metric for the performance of elastomeric actuators is the life-span of the device in use; the life-span is limited by fatigue in the material. The number of full actuation cycles (each cycle consisting of nearly full bending and relaxing) that each type of actuator would withstand before bursting was measured. The same freestanding chambers actuator made out of Elastosil M4601 was tested for fatigue after being actuated up to 1 million times. The actuator was refilled with air, fixed at one end in air, and connected to a computer-controlled solenoid valve. Each cycle composed of a 100 ms pressurization period that fully bends the actuator (using a 15 psi (103 kPa) head pressure) and 400 ms of venting that was sufficient for the actuator to return to its original position.
Fatigue of the material was assessed by measuring pressure-volume (PV) hysteresis curves for three separate freestanding chambers pneumatic networks, before and after subjecting them to cyclic testing (
In contrast, the embedded actuator, tested in the same manner (actuated at 0.33 Hz), failed after ˜46 cycles.
Rapid Actuation of Freestanding Chamber Pneumatic Networks
All previous experiments were purposefully performed with the actuators moving at relatively slow rates in order to better capture their movement using normal capture rates of our video camera. The speed of the actuators depends on the rate pressurized gas molecules can be transferred into the actuator.
To obtain higher flow rates of pressurized gas, the actuator having freestanding chambers was pressurized with a short pulse of high-pressure air (40 psi). The motion of the free end of the actuator moved at ˜1 m/s over a distance of ˜7 cm (measured using Image J software). Under similar conditions, the embedded chamber actuator burst. The resilience of the actuator having freestanding chambers according to one or more embodiments is due to the low strain needed for the actuator to bend.
To compare how rapidly the embedded and freestanding actuators bend, each actuator was pressurized with a miniature compressor (BTC-IIS, Parker, Holliss, N.H.) that delivers compressed air at pressures up to 193 kPa, at a maximum flow rate of ˜11 L/min (Movie S2 and S3). Using this compressor, on average, the embedded chamber bends fully in 3.3 seconds (standard deviation of 0.54 s for N=7 actuations) and the freestanding chambers actuator bends fully in 130 ms (standard deviation of 8 ms for N=7 actuations).
To determine the maximum frequency the freestanding chambers actuator could bend, a computer-controlled solenoid valve was used to pulse highly pressurized gas for short durations, and then vented the system to equilibrate with atmospheric pressure. The head pressure (that is, the pressure set by the regulator) was adjusted to achieve full bending of the actuator. The highest frequency of full bending and relaxing achieved by the freestanding chambers actuator, using a 50% duty cycle (equal durations of pressurizing and venting periods), was 2 Hz using 76 kPa head pressure.
The freestanding chambers actuator showed an interesting bi-modal behavior; the change between modes occurs above ˜200 kPa head pressure. For slower rates of actuation, the chambers of the pneumatic network inflate relatively evenly, and the actuator bends along a roughly circular trajectory. Above the threshold rate, however, the tip of the actuator bends preferentially, and causes the actuator to curl on itself. The embedded chambers actuator, in contrast, was only observed to provide the second mode of bending (i.e. it curls on itself) since each chamber always undergoes a snap-through instability. The actuation pathway of the same actuator under two different pressures (ΔP=207 kPA and 345 kPA) is shown in the time lapse photo series of
Force Exerted by Actuators Having Freestanding Chambers
Using a transducer (Nano 17, 6 axis F/T sensor, ATI industries), the force exerted by the tip of the actuator was measured in order to understand how much force these actuators exert for a given pressure. The actuator having freestanding chambers was secured at one end and the transducer was placed beneath the tip of the opposite end. The data plotted in
The actuator having freestanding chambers is able to apply a force of ˜1.4N for 72 kPa (pressure that provides full range bending for the actuator having freestanding chambers). At 72 kPa, the embedded chambers actuator applies a force of ˜1N; that is, 40% less than that of the actuator having freestanding chambers. Compare,
In one or more embodiments, the range of exerted forces of the actuator, however, can be tuned by selecting elastomers of different stiffness. Both silicones and polyurethanes are commercially available with a large range of material properties. Therefore a low strain pneumatic network can be made with higher modulus materials in order to achieve higher or lower forces as desired.
Keyboard Playing Low Strain Pneumatic Networks
At least one class of applications for soft robots will require high dexterity and good control of pressure, yet use minimal controls. As an example of such a task, we built a set of actuator fingers to play an electronic keyboard. To play the keyboard in a way that mimics a human hand, the actuators require sufficient speed to maintain a desired tempo, and sufficient force (0.65 N) to depress the keys. The portion of the actuator that contacts the key weighs ˜4.5 g and, therefore, its gravitational weight can only produce a maximum of 0.045 N of force. Therefore the remaining force (0.605 N) required to depress a key must come from the bending force of the actuator which comprise the internal pressure acting on the actively bending region of the actuator and the acceleration of that bending region.
A solenoid valve was used to control whether the actuator was pressurized with compressed air (15 psi was the maximum pressure of our control system) or vented to atmosphere. Each solenoid valve was connected to an actuator via tubing (21 cm long and 3.5 mm in diameter) and a needle (16 gauge). The actuator hit the key ˜100 ms after opening the valve; to do so, its tip traveled a distance of ˜2.6 cm, yielding a momentum of ˜13×10−4 N−s and a force of ˜0.005N.
To demonstrate the precision and speed of the low strain pneumatic networks according to one or more embodiments, we used four actuators to play “Mary Had a Little Lamb” on the keyboard, which is shown in
The performance of freestanding chamber pneumatic networks has been demonstrated in several aspects: i) speed achieved for a given rate of inflation, ii) force exerted for a given pressure, iii) change in volume required for a given degree of bending, iv) number of actuation cycles before failing, and v) correlation between the pressure in the pneumatic network and its degree of bending without a load.
Specifically, the improved speed (25×) and force (1.4×) of the freestanding chamber pneumatic network is useful for creating soft robots that move quickly and can exert higher forces. Alternatively, one could actuate a freestanding chamber pneumatic networks with a smaller compressor and obtain similar performance than a larger compressor, the advantage being a reduction in the overall size and power-consumption of the robot (an option particularly useful for untethered applications).
The reduced change in volume (8× smaller than the embedded chambers pneumatic network) minimizes fatiguing of the material, and thus improves the durability of the freestanding chamber pneumatic network to such a level that the actuator does not fail within a million cycles of full bending. This high durability could allow its use in long-term applications, and when high margins of safety are required for high-consequence uses (such as in medical procedures). Furthermore, since the freestanding chamber pneumatic network expands into the space between the chambers, the actuator assumes a smaller volume upon inflation potentially allowing it to enter smaller spaces; this is an attribute beneficial for applications in search-and-rescue and minimally invasive surgery.
In addition to improving the performance (i.e., speed and force) of the freestanding chamber pneumatic network, the rate of actuation (as opposed to the geometry of the actuator) could determine the motion of a pneumatic network. The bi-modal bending behavior that occurs at high speeds may provide interesting opportunities for control schemes by allowing at least two types of motion from a single design, simply by changing the rate of pressurization.
It will be appreciated that while a particular sequence of steps has been shown and described for purposes of explanation, the sequence may be varied in certain respects, or the steps may be combined, while still obtaining the desired configuration. Additionally, modifications to the disclosed embodiment and the invention as claimed are possible and within the scope of this disclosed invention.