GB2385196A

GB2385196A - Sagittally-amplified piezoelectric actuator

Info

Publication number: GB2385196A
Application number: GB0203122A
Authority: GB
Inventors: David Livingstone
Original assignee: 1 Ltd
Current assignee: 1 Ltd
Priority date: 2002-02-11
Filing date: 2002-02-11
Publication date: 2003-08-13
Anticipated expiration: 2022-02-11
Also published as: GB0203122D0; GB2385196B

Abstract

A mechanically amplified actuator system with one or more actuators of electro-active material and a sagittal amplifier system is described having a frame structure 11, one or more bender actuator 12 having an extended tape-like form with a proximate end mounted onto said frame structure and a distal end; and one or more tie members 13 of high stiffness capable of transmitting a pulling force linking said distal ends of said actuators and an object 14 to be moved. The moved object 14 may be the diaphragm of a loudspeaker. A variety of embodiments are described (Figs 2-6).

Description

AMPLIFIED ACTUATOR FIELD OF THE INVENTION The present invention relates to mechanically amplified actuators. More specifically, it pertains to actuators made of electro-active material. Even more specifically, the invention relates to such actuators being sagittally amplified.

BACKGROUND OF THE INVENTION Many piezoelectric or electro-active actuators of the prior art utilized various lever arm arrangements by which the lever arm was pivotally connected to a fixed support with the piezoelectric member placed in compression against the lever arm a short distance from the pivot point. The motion amplification was achieved at the end of the lever arm distal the pivot point and was based upon the ratio of the distance from the pivot point of the piezoelectric member and the end point of the lever arm away from the pivot point.

Other devices used to achieve motion amplification for piezoelectric actuators involved the use of hydraulic amplification in which the piezoelectric member was used to move a piston having a large surface area moving an hydraulic fluid. The hydraulic fluid was then used to drive a smaller area piston a greater distance. The motion amplification was directly proportional to the ratio of piston areas.

In all of the prior art devices the mass of the mechanical amplifying systems were large resulting in a limitation of the reaction time of the linear actuator.

In United States Patent No. 4,318, 023, a sagittally amplified actuator is described that reduces the mass requirements of an electroexpansive actuator in that it comprises, basically, a

fixed base support on which is hingedly mounted at least one lever arm support connected to one end of a sagittal tension member. An electroexpansive member is placed in compression against the lever support arm causing it to move the sagittal tension member longitudinally to deflect the midpoint of the sagittal tension member perpendicular to its longitudinal axis as the electroexpansive member expands and contracts.

Similarly amplified electro-active devices are described in other U. S. Patents, such as U. S. Pat. Nos. 4,929, 100 ; 4,952, 835 ; and 6,294, 859.

In view of the prior art, it is an object of the present invention to provide a new mechanically amplified electroactive actuator. Ideally, the actuator has a low inertia or moving mass and stores only small amount of energy in operations, i. e. have a high stiffness.

It is a further object of the invention to improve the known sagittally or taut band amplified actuators.

SUMMARY OF THE INVENTION In view of the above objects, the present invention provides an apparatus as claimed in the independent claims.

According to a first aspect of the invention, there is provided a mechanically amplified actuator comprising at least two first tension members adapted to exert a first pulling force on a suspended movable object and at least two second tension members adapted to exert a second pulling force on the suspended movable object. The tension members act similar to a bowstring or ligament. The two forces cause the object to move in opposite directions. All tension members are operated by one or more electro-active actuator elements, preferably by

electro-active actuator elements exhibiting a bending motion when energized. To increase the amount displacement, bimorphs or multilayered cantilevers are used.

In a first embodiment of the invention the tension members have negligible elasticity and compressible strength. Such members may be ties or tendons of minimal weight. Particularly suitable low weight materials are plastics or composite materials having a density lower than steel.

In a second embodiment, the tension members or at least a part of the tension members have both tensile and compressible strength. Thus it is possible to operate these members in pulling and pushing mode.

In another preferred embodiment of the invention, the object to be moved is exclusively held in suspension by the tension members. This reduces the amount of friction the object is subjected to when being moved.

In yet another preferred embodiment of the invention, the first tension members are operated or driven by first electro- active elements and the second tension members are operated or driven by second electro-active elements. In further variants of this embodiment, the first tension members consist of at least two tension members or groups of members each operated by a distinct electro-active element. Similarly, the second tension members may consist of at least two tension members or groups of members each operated by a distinct electro-active elements.

Electro-active materials are materials that deform or change their dimensions in response to applied electrical conditions or, vice versa, have electrical properties that change in response to applied mechanical forces. The best-known and most

preferred type of electro-active material is piezoelectric material, for example lead zirconate titanate (PZT), but other types of electro-active material include electrostrictive and piezoresistive material. The invention uses high-displacement benders consisting of a substantially flat extended tape-like structure. To increase displacement the flat tape-like structure may by bent into a curved, folded or corrugated form.

These and other aspects of inventions will be apparent from the following detailed description of non-limitative examples making reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: FIG. 1A shows a schematic perspective top view of a first example of an amplified actuator system in accordance with the invention; FIG. 1B is a perspective side of the system shown by FIG.

1A ; FIG. 1C is a diagram to illustrate certain aspects of the example of FIGs. 1A and lB ; FIG. 2 shows a schematic perspective top view of a second example of an amplified actuator system in accordance with the invention; FIGs. 3A, B show a schematic cross-section of a third example of an amplified actuator system in accordance with the invention in various stages of motion;

FIG. 4 is a schematic perspective view of a compact actuator system in accordance with the invention; FIG. 5 is a schematic perspective view of a first actuator system in accordance with the invention using force- transmitting members with compressive strength; and FIG. 6 is a schematic perspective view of a second actuator system in accordance with the invention using force- transmitting members with compressive strength.

DETAILED DESCRIPTION The amplified actuator system of the present invention is described with reference to the embodiment shown in FIG. 1.

In a simplified form, the system 10 includes an outer frame structure 11 on which there are mounted high force, low displacement actuators 12. In the example, the actuators are four piezoelectric bimorph benders 12-1 to 12-4, connected to an electrical power supply and control system (not shown).

Each actuator 12-1 to 12-4 is connected by a tie or tendon 13- 1 to 13-4 to a moveable object 14. The ties are used to suspend the object at a central location within the frame structure 11. The ties or tendons 13-1 to 13-4 are designed to combine low weight with a high tensile strength and stiffness. Suitable materials include metal and metal alloys or composite fibres such as Kevlar, high tensile carbon fibre, and selected glass fibres. The suspended object 14 could be for example a piston or diaphragm. In the example, a pair of ties 13-1,13-2 is attached to the top face of the piston 14 analogous to a bowstring. An opposing pair of ties 13-3,13-4

is attached to the bottom face of the piston. Typically, the plane of one pair of ties is orthogonal to the plane of the opposing pair for good stability of the piston, though this is not essential with regard to the principle of operation.

The operation of the bowstring actuator 10 is described with reference to FIGs. 1B and 1C. In FIG. 1B, which is a perspective side view of FIG. 1A, the piston 14 is shown moving towards the left. As the ties 13-1,13-2 are pulled by the bending actuators 12-1,12-2 outwards as indicated by the arrows A, the actuators 12-3,12-4 bend inwards in direction of the arrows B thus allowing the ties 13-3,13-4 to move with the piston 14. The travel of the piston is determined by the maximum displacement of the actuators 12-1,12-2 and the amplification provided by the ties 13-1,13-2. The actuators 12-3,12-4 move inwards by a complementary amount so as to keep the ties under tension. Conversely, when the actuators 12-3,12-4 move outwards and the actuators 12-1,12-2 move inwards, the piston 14 will move to the right. By suitable control of the actuators 12-1 to 12-4 the tension in the ties can be maintained constant over the whole range of motion of the piston 14.

As illustrated by FIG. 1C, the instantaneous velocity ratio of the amplified actuator system 10 is equal to the ration y/x.

The dimensions of the tie geometry and the piston position determine this ratio. In a practical device the ties may be fixed to the piston at some distance from the centre to prevent the piston from tilting randomly about its centre.

This feature however reduces the length of y and therefore the maximum value of y/x. To increase the length of y, a tie may cross the centre region of the suspended object to be fixed to a point close to its outer circumference. In practice, a higher force may be achieved by using more than two actuators

on each side of the suspended object. Using annular actuators that expand and contract in radial direction when energized and which are located at the top and bottom of a frame structure, a high-force variant of this invention may be envisaged similar in form to the rim (frame and actuator), spoke (ties) and hub system (piston) of a bicycle wheel.

It is also possible to stack the amplified actuators system of the present invention in a way exemplified by the device of FIG. 2. In FIG. 2, a piston rod 243 connects pistons 241,242.

In turn, each piston is suspended in an actuator system 201, 202 similar to the actuator system 10 of FIG. 1. Again bimorph benders 22-1 to 22-8 are used to provide a pulling force on the ties 23-1 to 23-8 depending on the desired direction of motion. The device of FIG. 2 is designed to offer an improved axial alignment of the moving piston.

The single tie suspension of the examples describe above can be altered into a more complex suspension system as shown in FIGs. 3A and 3B.

In FIG. 3A, a cross-section of a two-stage tie suspension system is shown in its neutral position.

Four piezoelectric benders 32-1 to 32-4 are mounted on a frame structure 31. The benders operate primary ligaments or ties 33-1 to 33-4. These primary ligaments are tied back to parts 311,312 of the frame 31. Secondary ligaments 33-5 to 33-8 join the primary ligaments 33-1 to 33-4 with the suspended object 34. As is illustrated in FIG. 3B, the displacement of the piezoelectric actuators 32-1 to 32-4 is amplified via the ties 33-1 to 33-8 causing a motion and corresponding

displacement of the suspended object 34. The displacement of the benders is parallel to the displacement of the suspended object.

The configuration of FIG. 3 can be used for a compact design as illustrated making reference to the perspective view of an actuator in FIG. 4. The frame structure 41 includes a rectangular central cage section 410 with two upwardly extending projections 411 and two identical downwardly extending projections 412 at opposite corners of the central cage. Each projection supports a piezoelectric bender 42-1 to 42-4 extending approximately parallel to one side of the cage sections. Primary ligament 43-1 to 43-4 are taut between the distal end of each bender and its nearest corner of the cage section. The secondary ligaments 43-5 to 43-8 are taut between the primary ligaments and the diaphragm supported. The displacement generated by the piezoelectric cantilevers 42-1 to 42-4 is proportional to their length. Hence, the design of FIG. 4 offers a relatively large displacement within a small volume or housing.

In the following example of the present invention, structural link elements are used. Whereas the ties or tendons of the previous examples exhibit no compressive strength, a structural link element is defined to be capable of transmitting both, pushing and pulling forces. However, the structural link elements have to resist bending or buckling, and, thus, require a certain degree of stiffness that is usually associated with increased mass. Only advanced plastics and composite materials are likely to provide high compressive strength or stiffness and low density.

Referring now to FIG. 5, a first example of a device is shown where the motion of three electro-active benders 52-1 to 52-3 located equidistantly around the circumference of the annular frame 51 is transmitted to a suspended object 54 using three vshaped struts 53-1 to 53-3.

With the struts providing sufficient support for the object 54, there is no requirement for opposing sets of ties. However, if a relatively larger force is to be transmitted, struts and piezoelectric benders are best placed on both sides of the frame 51.

In FIG. 6, the suspended object or diaphragm 64 is held in place by laterally extended struts 63-1 to 63-4 made of corrugated sheets of polycarbonate film. In order to move upwards, the two upper linkages 63-1,63-2 are pulled apart, and act in the same way as the ties in the above described examples. The lower linkages 63-3,63-4 are pushed together and act as struts. When moving the diaphragm 64 downwards, their roles reverse, and the upper linkages act as struts with the lower linkages acting as ligaments (ties). Four piezoelectric benders 62-1 to 62-4 cause the motion of the upper and lower linkages. Suitable braces 621 facilitate the transmission of force to and stabilize the sheet-like struts 63-1 to 63-4. For acoustic applications these struts act as extension of the diaphragm.

It will be appreciated by a person skilled in the art that the above describe low-mass links or joints between the movable or suspended object is particularly suitable for high frequency applications. With increasing frequency the advantages the above invention offer become more pronounces. The invention is seen to be particularly advantageous for applications operating in the acoustic frequency range, e. g. between 20,100 or 1000 and 20000 Hz, or higher.

Claims

CLAIMS 1. A mechanically amplified actuator system with one or more actuators of electro-active material and a sagittal amplifier system, comprising a frame structure; one or more electro-active bending actuators with a proximate end mounted onto said frame structure and a distal end; and one or more tie members capable of transmitting a pulling force linking said distal ends of said actuators with an object to be moved.
2. The actuator system of claim 1 having a first and a second group of actuators mounted on the frame structure, each group comprising one or more actuators, and a first and second group of tie members associated with said first and second group of actuators, wherein said first group of tie members is adapted to move the object in a first direction and said second group of tie members is adapted to move said object into a second, opposite direction.
3. The actuator system of claim 1 wherein at least one or more of the tie members are not capable of transmitting a pushing force.
4. The actuator system of claim 1 wherein at least one or more of the tie members are capable of transmitting a pushing and a pushing force from the actuators to the object.
5. The actuator system of claim 1 wherein at least one or more of the tie members comprise first and second

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sections, with said first section extending from the distal end of the actuator to the frame structure and said second section extending from said first section to the object.
6. The actuator system of claim 1 wherein the frame structure encloses a volume adapted to house the object.
7. The actuator system of claim 1 wherein the frame structure and the object are exclusively linked by the tie members.
8. The actuator system of claim 7 wherein the tie members are arranged to exclusively maintain the object suspended while being in motion.
9. The actuator system of claim 1, adapted to operate at acoustic or higher frequencies.
10. The actuator system of claim 1, wherein the object is a diaphragm.
11. The actuator system of claim 1 wherein the actuators comprise a continuous extended tape of electro-active material.
12. The actuator system of claim 1 wherein the actuators are bimorph benders.
13. The actuator system of claim 1 wherein the electro- active material is piezoelectric material.