DIELECTRIC POLYMER ACTUATOR

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application claims the benefits of U.S. provisional patent application No. 60/695,001 filed June 30, 2005, which is hereby incorporated by reference.

TECHNICAL FIELD

[0002] The present invention relates to a dielectric polymer actuator. More specifically, the present invention relates to a dielectric polymer actuator for use with an active prosthesis.

BACKGROUND

[0003] Various mechanisms such as electric, hydraulic or pneumatic devices have been used as actuators. Generally, actuators are used in apparatuses requiring a controllable movement such as printers, industrial automated devices or motorized prostheses.

[0004] As early as 1950, a pneumatic muscle actuator, named the

"McKibben pneumatic artificial muscle", was developed as a part of an orthotic limb system. Nowadays, in the context of motorized prostheses design, electric actuators have been the preferred technology. The primary reason for selecting electric actuators over other technologies is fixed portiond on the coupling efficiency of about 90%, the coupling efficiency being the ratio of mechanical work over electrical work. Furthermore, electric actuators use a well known technology and are readily-available in both rotational and linear configurations.

[0005] The main drawbacks of the electric actuator technology are the weight of the device and the noise related to mechanical transmission.

SUMMARY

[0006] The present invention relates to a dielectric polymer actuator, comprising:

a first portion; a second portion; a dielectric polymer roll composed of at least two dielectric polymer sheets each having a negative side and a positive side; and a positive electrode terminal electrically connected to the positive side of the dielectric polymer sheets and a negative electrode terminal electrically connected to the negative side of the dielectric polymer sheets; the dielectric polymer roll being positioned relative to the first and second portions such that, when a voltage is applied between the positive and negative electrode terminals, a size of the dielectric polymer roll increases and provides a displacement of the second portion with respect to the first portion.

[0007] The present invention also relates to a dielectric polymer actuator as described above, wherein the at least two dielectric polymer sheets composing the dielectric polymer roll are rolled in such a way that the negative sides do not enter in contact with the positive sides.

[0008] The present invention further relates to a dielectric polymer actuator as described above, wherein the at least two dielectric polymer sheets are radially and/or axially pre-strained.

[0009] Further still, the present invention related to a dielectric polymer actuator comprising: a first, second and third portions; a first movement transmission means connected to the second portion and exiting from an opening in the third portion;

a first and second dielectric polymer roll each composed of at least two dielectric polymer sheets having a negative and a positive sides; a first positive electrode terminal electrically connected to the positive side of the at least two dielectric polymer sheets of the first dielectric polymer roll and a first negative electrode terminal electrically connected to the negative side of the at least two dielectric polymer sheets of the first dielectric polymer roll; and a second positive electrode terminal electrically connected to the positive side of the at least two dielectric polymer sheets of the second dielectric polymer roll and a second negative electrode terminal electrically connected to the negative side of the at least two dielectric polymer sheets of the second dielectric polymer roll; the first dielectric polymer roll being positioned relative to the first and second portions such that, when a first voltage is applied between the first positive and negative electrode terminals, a size of the first dielectric polymer roll increases and provides a displacement of the second portion with respect to the first portion; the second dielectric polymer roll being positioned relative to the second and third portions such that, when a second voltage is applied between the second positive and negative electrode terminals, a size of the second dielectric polymer roll increases and provides a displacement of the second portion with respect to the third portion.

[0010] The present invention further relates to a dielectric polymer actuator as described above, further comprising a second movement transmission means connected to the mobile portion and exiting from an opening in the first portion.

[0011] Additionally, the present invention relates to a method of forming a dielectric polymer roll assembly for use in a dielectric polymer actuator, the method comprising the steps of: a. providing at least two dielectric polymer sheets, the dielectric polymer sheets having a negative side and a positive side; b. partially coating the negative side and the positive sides of the at least two dielectric polymer sheets with an electrically conductive medium;

c. orienting the at least two dielectric polymer sheets such that their top side alternates between the negative side and the positive side; d. providing electrical contacts to the top side of the at least two dielectric polymer sheets; e. superimposing the at least two dielectric polymer sheets; f. rolling the superimposed dielectric polymer sheets over a partially compressed resilient support member such that the negative sides do not enter in contact with the positive sides; and g. securing the rolled dielectric polymer sheets onto the partially compressed resilient support member.

BRIEF DESCRIPTION OF THE FIGURES

[0012] Illustrative embodiments of the invention will be described by way of examples only with reference to the accompanying drawings, in which:

[0013] Figure 1 is a cross sectional view of a dielectric polymer actuator according to the illustrative embodiment of the present invention;

[0014] Figure 2 is a perspective cross sectional view of a bellow of the dielectric polymer actuator of Figure 1 ;

[0015] Figure 3 is a flow diagram of a method used for forming a dielectric polymer roll;

[0016] Figure 4 is a perspective view of dielectric polymer sheets pre- strained in corresponding frames;

[0017] Figure 5 is a perspective view of the pre-strained dielectric polymer sheets of Figure 4 partially coated with carbon grease;

[0018] Figure 6 is a cross sectional view taken along axis Vl-Vl of Figure 5 of the dielectric polymer sheets;

[0019] Figure 7 is a perspective view of the two frames of Figure 5 in the process of being superimposed;

[0020] Figure 8 is a perspective view of the two frames of Figure 5 superimposed;

[0021] Figure 9 is a perspective view of the two pre-strained dielectric polymer sheets of Figure 5 transferred to one frame;

[0022] Figure 10 is a perspective view of the actuator rolling process;

[0023] Figure 11 a schematic view the charge alignment of two superimposed dielectric polymer sheets being rolled up;

[0024] Figure 12 is a cross sectional view of the layering of rolled-up dielectric polymer sheets;

[0025] Figure 13 are cross sectional views of a dielectric polymer actuator in three different states;

[0026] Figure 14 is a cross sectional view of a second illustrative embodiment of the dielectric polymer actuator; and

[0027] Figure 15 is a cross sectional view of a third illustrative embodiment of the dielectric polymer actuator.

DETAILED DESCRIPTION

[0028] Generally stated, the dielectric polymer actuator technology described therein addresses electrical actuators limitations such as weight and noise with its use of low density polymer and the absence of a mechanical transmission. The dielectric polymer actuator also exhibits passive properties, such as damping and elasticity, that may be exploited in the context of prosthetic applications without the addition of external mechanical component.

[0029] Referring to Figure 1 , a dielectric polymer actuator (100) according to an illustrative embodiment of the present invention includes a fixed portion (12), a mobile portion (14), which may move along a guide shaft (16) fixed to the fixed portion (12), a resilient member (17), operatively connected between the fixed portion (12) and the mobile portion (14), and a support member (18) supporting a dielectric polymer roll (20) composed of a plurality of superimposed dielectric

polymer sheets. The dielectric polymer roll (20), as well as the support member (18), which supports it, are attached to the fixed portion (12) using a fixed portion clamp (22) and to the mobile portion (14) using a mobile portion clamp (24), and are protected by a cover (28a, 28b). Positive and negative electrode terminals (25, 26), are connected to the positive and negative sides of the individual dielectric polymer sheets of the dielectric polymer roll (20), respectively. Optionally, a load cell (29) and a linear encoder sensor (30) may also be mounted to the mobile portion (14) so as to provide feedback to a controller.

[0030] The fixed portion (12) may be made, for example, of aluminum 2024-

T4 for its adequate stiffness properties.

[0031] The support member (18) may take the form of, for example, a bellow, a stack, a bow or an inflated cylinder. In the illustrative embodiment, the support member (18) takes the form of a bellow, the bellow offering low friction and high usable strain, and the resilient member (17) takes the form of a spring, both will be identified as such from hereinafter. Holes may be drilled in the bellow (18) in order to ensure that proper extension and compression is realized without air clamping.

[0032] The mobile portion (14), which may also be made of aluminum 2024-

T4, has multiple functionalities including displacement guide and instrument holder. To obtain a linear displacement along the guide shaft (16) that is exempt of rotation, a standard ball spline linear guide (not shown) such as, for example, the THK© LM system, may be included as part of the mobile portion (14). As mentioned hereinabove, the mobile portion (14) may also support an optional load cell (29) to measure the pressure applied by the dielectric polymer actuator (100) as well as an optional linear encoder sensor (30), which measures the linear displacement of the dielectric polymer actuator (100).

[0033] It is to be understood that other materials may be used for the fixed portion (12) and mobile portion (14). It is also to be understood that other means for providing a linear displacement along the guide shaft (16) may be used in lieu of the standard ball spline linear guide.

[0034] The optional linear encoder sensor (30) may be, for example, a

HEDS 9100 manufactured by USDigital©, which is a zero-friction device consisting of a lensed LED source and a monolithic detector integrated chip enclosed in a small polymer package. It is to be understood that other models of linear encoder sensors or other types of sensors may also be used.

[0035] The bellow (18) may be selected such as to offer the lowest possible resistance to the movements of dielectric polymer actuator (100). For example, the bellow (18) may be a commonly available bellow made of neoprene-nylon, having an outer diameter of 4.1 cm, an inner diameter of 2.5 cm and a retracted length of 15 cm. It is to be understood that the dimensions of the bellow (18) may vary depending on the application for which the dielectric polymer actuator (100) is to be used. It is also to be understood that other materials may be used for the bellow (18) such as, for example, silicone, butyl, Gortiflex®, Viton®, Hypalon® and Buna-n elastomer, with nylon, Kevlar or fiberglass reinforcing fabrics.

[0036] To help prevent the bellow (18) from twisting or squeezing during the coiling process, Acetal stiffening washers of 0.78 mm may be added inside every bellow convolution. Acetal offers good stiffness combined with the toughness and durability of metal in several applications. This material has also good elasticity memory which allows for an easy insertion of the washer into bellow's convolution. For proper bellow's expansion-retraction, small venting holes may also be drilled between each bellow's convolutions.

[0037] As may be seen in Figure 2, for electrical insulation against high voltage sparks, each ends of the bellow (18) may be protected by an Ultra High Molecular Weight Polyethylene (UHMW) lid (44) fitted into each bellow cuff ends (42). It is to be understood, however, that other types of electrical insulators may also be used.

[0038] Referring back to Figure 1 , the cover (28a, 28b) serves as protection for the dielectric polymer actuator (100) as well as for insulation. It is preferably lightweight and offers sufficient insulation for a 6 KV source. The cover consists of two cylinder cups (28a, 28b), one fix cup (28a) and one mobile cup (28b), which

are telescopically engageable so as to permit proper extension and contraction of the dielectric polymer actuator (100). The thin walls may be made, for example, of Polymethylpentene (TPX) rectangular film sheets that are coiled and glued with double tape to form a thin wall pipe. This material has excellent dielectric strength of 650 KV/mm, thus a cover (28a, 28b) having walls of a thickness of, for example, 0.25 mm may easily resist 6 KV.

[0039] In an alternative embodiment, the cover (28a, 28b) may be mechanically designed so as to also function as a linear guide, thus permitting the omission of the guide shaft (16).

[0040] The dielectric polymer roll (20) is formed by rolling dielectric polymer sheets, for example VHB 4905 dielectric polymer sheets from 3M™, over the bellow (18) following a spiral pattern and then clamping the dielectric polymer roll (20) with plastic polymer clamps (22, 24). It is to be understood that other types of dielectric polymer sheets may also be used.

[0041] A possible method for forming the dielectric polymer roll (20) is depicted by the flow diagram shown in Figure 3, with references to Figures 4 to 10. The steps of the method are indicated by blocks 202 to 212.

[0042] The method begins at block 202 where two dielectric polymer sheets

(221 , 222) are pre-strained by affixing them in respective top and bottom rectangular frames (101 , 102) with bull-dog clips (104), as shown in Figure 4. The purpose of the pre-straining of the dielectric polymer sheets (221 , 222) is to increase their field strength properties and to maximize dielectric breakdown strength. It is to be understood that the amount of pre-straining may vary depending on the application. Then, at block 204, the pre-strained dielectric polymer sheets (221 , 222) are partially coated with an electrically conductive medium (35), such as, for example, carbon grease, on both their top and bottom surfaces, as shown in Figures 5 and 6, forming positive (31) and negative (33) conductive areas that cover a large portion of the surfaces of each dielectric polymer sheets (221 , 222), with a 2.5 cm long non-effective border (32) to prevent sparking. It should be noted that, as best shown in Figure 6, the polymer sheets

(221 , 222) are oriented such that their topside electrodes are of a different polarity. In the embodiment shown in Figures 5 and 6, the top surface of polymer sheet

(221) has a positive conductive area (31) while the top surface of polymer sheet

(222) has a negative conductive area (33). Conversely, the bottom surface of polymer sheet (221) has a negative conductive area (33) while the bottom surface of polymer sheet (222) has a positive conductive area (31).

[0043] At block 206, positive (34) and negative (36) electrode contacts made of, for example, copper foils are positioned on the top side of each polymer sheet (221 , 222). Following this, at block 208, the top rectangular frame (101) is placed on the bottom rectangular frame (102), such that both pre-strained dielectric polymer sheets (221 , 222) are in direct contact forming layered polymer sheet (224) which is transferred to the bottom frame (102), as shown in Figures 7 to 9. Advantageously, in order to help avoid the formation of air bubbles between the pre-strained dielectric polymer sheets (221 , 222), two UHMW rollers may be rolled on both side of the superimposed pre-strained dielectric polymer sheets (221 , 222), which also helps the bonding of the pre-strained dielectric polymer sheets (221 , 222) together into layered polymer sheet (224). Afterwards, the layered polymer sheet (224) transfer to the bottom frame (102) may be done manually clip (104) by clip (104).

[0044] At block 210, and as shown in Figure 10, the layered polymer sheet

(224) is rolled over the partially compressed bellow (18) following a continuous spiral pattern, unclipping the bull-dog clips (104) one by one as the bellow (18) is rolled over the frame (102) to form the dielectric polymer roll (20). Once formed, the dielectric polymer roll (20) is then clamped to the fixed portion (12) and mobile portion (14) using plastic polymer clamps (22, 24), as previously shown in Figure 1.

[0045] Finally, at block 212, the positive (34) and negative (36) electrode contacts are electrically connected to the electrode terminals (25, 26), which are shown in Figure 1.

[0046] Referring now to Figure 11 , the spiral layering of the layered polymer sheet (224) onto itself results in the positive (31) and negative (33) conductive areas being continuous throughout each layer of the spiral without coming into contact with each other. Figure 12 shows how the positive (34) and negative (36) electrode contacts may be positioned between layers of positive (31) and negative (33) conductive areas, respectively, once the layered polymer sheet (224) is enrolled around the bellow (18) to form the dielectric polymer roll (20).

[0047] It is to be understood that in an alternative embodiment, the dielectric polymer roll (20) may be formed of more than two dielectric polymer sheets.

[0048] In use, the dielectric polymer actuator (100) uses the displacement and/or the strength of the dielectric polymer roll (20) created by Maxwell force in order to provide useful work. When a voltage is applied to the electrode terminals (25, 26), an electric field is created and a compression force is generated, having for effect that the dielectric polymer roll (20) tends to decrease in thickness. By volume conservation law, the dielectric polymer roll (20) length and width are increased with proportional ratio. The dielectric polymer actuator (100) converts these dimension modifications into an axial displacement of the mobile portion (14) along the guide shaft (16) in the direction of arrow (1). When the voltage is no longer applied to the electrode terminals (25, 26), the dielectric polymer roll (20) regains its initial length and width, which translates, along with the resiliency of spring (17), into an axial displacement of the mobile portion (14) along the guide shaft (16) in the direction of arrow (2). Thus, the dielectric polymer actuator (100) may be viewed as being in a push configuration.

[0049] As previously explained, the dielectric polymer roll (20) is created by first enrolling the layered polymer sheet (224) onto the partially compressed bellow (18). Since the spring (17) is operatively connected between the fixed portion (12) and the mobile portion (14), the compression of the bellow (18), which is attached to both the fixed portion (12) and the mobile portion (14), results in the compression of the spring (17). Referring now to Figure 13, compressing the bellow (18), and thus the spring (17), before enrolling the layered polymer sheet

(224) onto the bellow (18) results in the dielectric polymer actuator (100) having an initial length I ₀₀ . When the spring (17) is released, it expands until it reaches an equilibrium point between its potential energy and the elasticity of the dielectric polymer roll (20), giving the dielectric polymer actuator (100) an actual length ^ ₀ .

[0050] When the dielectric polymer actuator (100) is free standing, i.e. no voltage is applied to it, the compressed spring (17) holds the dielectric polymer roll (20) in tension. If a voltage is applied to the electrode terminals (25, 26), the Maxwell pressure squeezes the layered polymer films (224) which tends to expand with a behavior described by the following equation: εε V ² [0051] P _Maxwell = -^- Equation 1

where: ε = dielectric permittivity; ε _o = 8.85e-12 F/m; V- voltage (V); z = sheet thickness (m).

[0052] As a result, the dielectric polymer roll (20) extends in length along the guide shaft (16) as the compressed spring (17) is released.

[0053] The stroke achieved by the dielectric polymer actuator (100) is essentially dependent upon the spring (17) used, the voltage applied and the polymer properties. The stroke may be determined by the following equation:

[0054] Ax = Jr ^Maxwe " ^~ Ir ^>oad . Equation 2 spring polymer

[0055] Thus, if the displacement of the dielectric polymer actuator (100) is equal to 0, i.e. δx = 0, the force delivered by the dielectric polymer actuator (100) is maximal and equal to the Maxwell force as per the following equation:

[0056] F _Uaxwell = vε _o εE ² x A _polymer ; Equation 3

F . where: E =

Z

Volume

[0057] A A p _p o _φ lym _m e _e r _r = = í - ^' ^ Ab _M ell _k o _m w ^■ . Equation 4

where: Volume = actuator volume (m ³ );

Abeihw = cross-sectional bellow area (m ² ).

[0058] When the voltage applied to the electrode terminals (25, 26) is cut, the spring (17) contracts until it reaches back its equilibrium point, giving the dielectric polymer actuator (100) an actual length I ₀ once again.

[0059] The dielectric polymer actuator (100), such as illustrated in Figure 1 , may be used in a variety of applications, an example of which is for powering an ankle member of an actuated leg prosthesis. For this specific application, it may be assumed that during level walking, the maximum torque at the ankle is approximately 0.7 Nm/kg. Fixed portiond on a 70 kg person, the required torque would then be around 50 Nm, thus using a 0.1 m level arm on a three-bar mechanism, a 500 N actuator would be able to produce the required torque.

[0060] To obtain those requirements, a dielectric polymer actuator (100) with a dielectric polymer roll (20) whose dielectric polymer sheets are pre-strained approximately 400% radially and 80% axially. In this particular example, the polymer length after pre-strain would be about 27 cm with an initial length of 15 cm, for an axial pre-strain of 80%, a width after pre-strain would be about 40 m with an initial width of 8 m, for a radial pre-strain of 400% and a thickness after pre-strain of 0.074 mm with an initial thickness of 0.5 mm, for a polymer volume of 770 cm ³ . Using a bellow (18) with a radius of 1.9 cm, the pre-strain polymer sheets may be rolled about 225 turns around the bellow (18), for a polymer thickness of 3.6 cm. This particular example gives a displacement of 5 cm and a force of 500 N, for a spring (17) constant of 4320 N/m and a dielectric polymer roll (20) cross section area of 28 cm ² . The power required to actuate the device would then be approximately 866 W.

[0061] In a second illustrative embodiment shown in Figure 14, the dielectric polymer actuator (200) may be used in a push-pull configuration. The dielectric polymer actuator (200) includes two fixed portions (112a, 112b), a centrally provided mobile portion (114) to which is attached a movement transmission means (116) (unlike in the dielectric polymer actuator (100) shown in Figure 1 where the shaft (16) is fixed to the fixed portion (12)), two springs (117a, 117b) operatively connected between respective fixed portions (112a, 112b) and the mobile portion (114), and two bellows (118a, 118b) supporting, respectively, two polymer rolls (120a, 120b) composed of a plurality of superimposed dielectric polymer sheets. The movement transmission means (116) may take the form of, for example, a shaft or a fluid. In the illustrative embodiment, the movement transmission means (116) takes the form of a shaft and will be identified as such from hereinafter. However, if the movement transmission means (116) was to take the form of a fluid, it is to be understood that the dielectric polymer actuator (200) would include a fluid conduit (not shown) within the bellow (118b) containing the movement transmission means (116). The dielectric polymer rolls (120a, 120b), as well as the bellows (118a, 118b), which support them, are attached to their respective fixed portions (112a, 112b) using fixed portion clamps (122a, 122b), and to the mobile portion (114) using mobile portion clamps (124a, 124b), and are protected by a cover (128). Positive and negative electrode terminals (125a, 126a, 125b, 126b), are connected to the positive and negative sides of the individual dielectric polymer sheets of the dielectric polymer rolls (120a, 120b), respectively.

[0062] In use, similarly to the dielectric polymer actuator (100) shown in

Figure 1 , the dielectric polymer actuator (200) uses the displacement and/or the strength of the dielectric polymer rolls (120a, 120b) created by Maxwell force in order to provide useful work. When a voltage is applied to the electrode terminals (125a, 126a), an electric field is created and a compression force is generated, having for effect that the dielectric polymer roll (120a) tends to decrease in thickness. By volume conservation law, the dielectric polymer roll (120a) length and width are increased with proportional ratio. The dielectric polymer actuator (200) converts these dimension modifications into an axial displacement of the

mobile portion (114) and the shaft (116) in the direction of arrow (3). Similarly, when a voltage is applied to the electrode terminals (125b, 126b), the dielectric polymer roll (120b) length and width are increased, which translates into an axial displacement of the mobile portion (114) and the guide shaft (116) in the direction of arrow (4).

[0063] It should be noted that the cover (128) of the dielectric polymer actuator (200) consists in a single fixed cylinder cup (128) unlike the cover (28a, 28b) of the dielectric polymer actuator (100) shown in Figure 1 , which consists of two cylinder cups (28a, 28b), one fix cup (28a) and one mobile cup (28b), which are telescopically engageable. This is due to the fact that the distance between the ends of the dielectric polymer actuator (200), i.e. fixed portion (112a) and fixed portion (112b), is fixed and that the mobile portion (114) moves between those ends.

[0064] In a third illustrative embodiment shown in Figure 15, the dielectric polymer actuator (300) is very similar to the dielectric polymer actuator (200) of Figure 14, therefore only the differences will be described for convenience purposes. The dielectric polymer actuator (300) includes two movement transmission means (116a, 116b) one on each side of the mobile portion (114). As it may be seen, the other components of the dielectric polymer actuator (300) are similar to those of the dielectric polymer actuator (200) shown in Figure 14, the movement transmission means (116b) having been added for more versatility.

[0065] In an alternative embodiment (not shown), one or more of support members, for example the bellows (18, 118a, 118b), may have the property of being resilient, thus making possible the elimination of the respective associated resilient members (17, 117a, 117b). In a further alternative embodiment, the resilient member (17) of the dielectric polymer actuator (100) of Figure 1 may be eliminated by using a load connected to the mobile portion (14) in order to keep the dielectric polymer roll (20) under tension. Similarly, the resilient members (117a, 117b) of the dielectric polymer actuators (200, 300) of Figures 14 and 15 may be eliminated by using the fact that the pre-straining of the dielectric polymer

rolls (12Oa ₁ 120b) are in opposite directions, i.e. dielectric polymer roll (120a) is pre-strained in the direction of arrow (3) while dielectric polymer roll (120b) is pre- strained in the direction of arrow (4).

[0066] It is to be understood that even though the various embodiments are schematically illustrated, it will be apparent to persons skilled in the art how to physically construct working embodiments of the present invention.

[0067] Although the present invention has been described by way of particular embodiments and examples thereof, it should be noted that it will be apparent to persons skilled in the art that modifications may be applied to the present particular embodiments without departing from the scope of the present invention.