Ti tie: Actuator

The present invention relates to an actuator.

In harmonic drive transmissions concentric inner and outer gears engage one another at diametrically opposed locations. A wave generator is rotated and imparts a continuously moving elliptical form or wave-like motion to the inner gearing element, known as a flexspline. This causes the meshing of the external teeth of the flexspline with the internal teeth of the outer gear (known as a circular spline) at their two equidistant points of engagement on the major elliptical axis to progress in a continuous rolling fashion. It also allows for full tooth disengagement at the two points opposite the minor axis.

Since the flexspline has two less teeth than the circular spline and because full teeth disengagement is made possible by the elliptical shape of the wave generator, each complete revolution of the wave generator causes a two tooth displacement of the flexspline in relation to the circular spline. This displacement is always in the opposite direction of the rotation of the wave generator e.g. if the wave generator is rotating in a CW direction, the two-tooth-per-revolution displacement of the flexspline will be in a CCW direction and vice versa

In this way, a basic three element harmonic drive assembly is capable of functioning as a speed reducer. Input from a main power source through the wave generator is at a high speed, but the two-tooth-per-revolution displacement causes the flexspline to rotate in the opposite direction of, and at a considerably slower speed than, the wave generator. The reduction ratio which results can be calculated by dividing the number of teeth on the flexspline by two (the difference between the number of teeth on the circular spline and the flexspline). If a fixed circular spline had 80 teeth and an output flexspline has 78 teeth, the ratio would be 80/(80-78)=40:1

However all drives thus far based on the original invention have used tooth engagement between the circular spline and the flexispline (which ensures that the wave generator rotates within the flexispline rather than rotating it directly and more importantly, until now the output torque provided by engagement with the rotational movement of the flexispline has only been realised as rotational movement.

The present invention seeks to provide an improved actuator.

Accordingly, the present invention provides an actuator for transmitting motion comprising: a cylindrical, outer member; a wave generating means; a flexible member rotatably mounted on said wave generating means; and cooperating helical means on said outer member and said flexible member; wherein said wave generating means is axially secured relative to said flexible member and rotatable within said flexible member to generate a deflection wave in said flexible member thereby to cause said cooperating means to move said flexible member and thus said wave generating means axially relative to said outer member.

In one preferred embodiment the helical cooperating means comprises a helical ridge formed on or attached to a radially inner surface of said outer member and a cooperating helical ridge formed on or attached to an outer surface of said flexible member.

Advantageously, at least one of said cooperating means is a screw thread.

In a further preferred form of the invention said wave generating means comprises a member with at least 2 cams positioned equidistantly around the perimeter. The wave generating means is rotatably mounted within said flexible member such that as the wave generating means rotates, the eccentric path described by the surfaces of the cams is progressively imparted to the flexible

member in a continuous wave. Alternatively, the wave generating means may be in the form of an ellipse.

Advantageously, the outer surface of the wave generating means in contact with the inner surface of the flexible member and the inner surface of the flexible member in contact with the outer surface of the wave generating means will both have a low coefficient of friction. More advantageously, the contact between these surfaces may be made by way of rolling bearings in at least 2 positions.

Advantageously, the outer surface of the flexible member in contact with the inner surface the outer member and the inner surface of the outer member in contact with the outer surface of the flexible member will both have a high coefficient of friction

The flexible member may be prevented from moving axially relative to said wave generator by radial flanges on said wave generating means.

Preferably, the actuator further comprises a drive motor coupled to said wave generating means for providing rotational drive to said wave generating means.

The drive motor may conveniently be coupled to said wave generating means by a gear means.

Advantageously, the outer member serves as a housing for the actuator and houses said drive motor.

In the preferred form of the new invention the direct output from the actuator is linear and is coaxial with the rotation of the wave generating means. The preferred embodiment of the actuator provides linear output motion with a range of forces from a rotational input source.

The term "helical ridge" as used herein means any form of integral or separate raised surface which is capable of acting as a screw thread and includes but is not limited to continuous or broken screw threads of one or more turns, of single or multiple starts and of any cross-section.

The present invention is further described hereinafter, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a sectional perspective view of a preferred form of actuator according to the present invention;

Figure 2 is a section on the line 2-2 of Figure 1 ;

Figure 3 is a vertical section through the actuator of Figure 1 on the line 3-3 of Figure 2;

Figure 4 is a view similar to that of Figure 3 on the line 4-4 of Figure 2;

Figure 5 is an enlarged view of a portion of Figure 4; and

Figure 6 is a view, similar to that of Figure 5, of a portion of Figure 3.

Referring to the drawings, these show a preferred form of actuator 10 according to the present invention for converting rotary motion into linear motion. The actuator is referred to as a helical wave actuator.

The actuator comprises an outer member 12 which conveniently also forms a housing for the device. The outer member is conveniently cylindrical in shape and has a cylindrical inner wall 14.

The housing 12 has an upper, end closure 18 and a lower, end closure 20.

The upper closure 18 supports an electric drive motor 22 which, in the illustrated embodiment, is housed coaxially within the housing 12. The drive motor 22 has an output shaft 24 which is connected to a wave generator 26, also within the housing 12, such that the motor 22 rotatably drives the wave generator 26.

Although the drive motor is shown with a direct drive connection to the wave generator 26, the drive motor may be eccentrically mounted within the housing

12. In such a case the drive shaft 24 would be coupled to the wave generator by a gearing or gear mechanism which would normally afford a step down gearing, although a step up gearing could be used.

The wave generator 26 is conveniently formed by an upper portion 31 and a lower portion 33. The lower portion 33 is in the form of a cup-shaped member 30 having a base 32 in the form of a cylindrical or circular disc with a peripheral wall 34 upstanding on the base 32. The wall 34 is elliptical in shape and has a major axis dimension which is conveniently slightly smaller than the diameter of the base 32, the effect being to provide a radially extending lower flange formed by the base which extends from the wall 34 around the periphery of the wall. The upper portion 31 is a mirror of the base 32 and is secured, coaxially to the lower portion 33, the effect again being to provide a radially extending upper flange formed by the upper portion 31 which extends from the wall 34 around the periphery of the wall. Prior to assembly of the upper and lower portions, a flexible member 40 which is generally cylindrical in its relaxed state is located around the wall 30 and coaxially with it. Once the member 40 is in place the upper portion 31 is secured to the lower portion 33 with the result that the member 40 is restrained from axial movement relative to the wave generator by the upper and lower flanges.

The maximum external dimension of the member 30 is such that it does not contact the inner wall 14 of the housing 12.

In order to reduce friction between the flexible member 40 and the wall 34 a recess or groove 37 is formed in the wall 34. This groove 37 is conveniently annular as shown in Figures 3 and 4.

The outer member (12) and the flexible member 40 have cooperating helical means which in this example are formed by a helical ridge or screw thread 16 on the inner wall of the outer member and a helical ridge or screw-thread 42 formed on the radially outer surface of the flexible member which engages with the internal thread 16.

Although the wall 34 of the above described wave generator is in the form of an elliptical cam, it will be appreciated that other suitable cam shapes may be used. For example, the wall 34 may be formed by a plurality of equi-angularly spaced projecting portions or cams each of which act in the same manner as the major axis end portions of the elliptical wall, and which as the wave generator rotates, cause the helical ridge or screw thread 42 of the flexible member to engage the helical ridge or screw thread 16 of the outer member, at a point coincident with the position of the projecting portions or cams.

The flexible member 40 has a generally uniform cross section and when located coaxially with the wave generator 36 is held in a generally elliptical shape. The thickness of the flexible member and the dimensions of the wave generator are sized such that the helical ridge or screw-thread 40 of the flexible member fully engages with the helical ridge or screw-thread 16 of the housing 12 at positions coincident with the ends of the major axis of the wave generator ellipse as it rotates within whilst the threads are disengaged at positions coincident with the minor axis of the wave generator ellipse.

An output member in the form of a pushrod 60 is coaxially connected to the wave generator and extends through an opening 62 in the lower, end closure 20 to

transmit the linear movement of the wave generator to an external device. The pushrod can also be in abutment with the wave generator where there is an external bias urging the pushrod axially towards the wave generator.

Because the circumference of the outer surface of the flexible member 40 is smaller than the circumference of the inner surface of the housing 12 each revolution of the wave generator within the flexible member 40 causes the flexible member to rotate an amount which is equal to the difference between these circumferences and in the opposite direction to the rotation of the wave generator. As the flexible member rotates, the helical engagement of the two screw threads also causes the flexible member to move axially and this axial movement is transmitted by the output member 62 to the external device being controlled by the actuator.

The rotational speed of the flexible member in relation to the speed of the wave generator is governed by the difference in circumference of the outer face of the flexible member and circumference of the inner face of the housing. However, the speed of linear motion of the flexible member 40 is also governed by the angle of the helix chosen for the thread engagement surfaces of the flexible member and housing element. Both of these parameters are variable and can be chosen to suit the characteristics of the drive motor and the required output force.

In order to prevent the flexible member 40 from rotating directly with and at the same speed as the wave generator 26, and thereby negating the effect of the gearing of the actuator, it is necessary to create a degree of friction where the screw threads of the flexible member 40 and housing engage. This can be achieved by way of high friction surfaces on one or both of the screw threads. However, shear loads are also created or supplemented at the point of contact of the two screw threads by way of the reactive axial forces which are generated during operation of the actuator (for example, from a jacking load).

The above described helical wave actuator has a number of advantages.

It provides a high speed reduction ratio. The helical wave actuator can provide high single-stage coaxial reduction ratios from 1/30 to 1/500 using a very simple three part mechanism.

It is free of backlash (lost motion). Like the harmonic gear drive the helical wave actuator can be made with very little or no backlash.

It has a small number of components and is easy to assemble. Because it comprises high reduction ratios with only three basic components and since all three components are co-axially aligned, the helical wave actuator can be easily built into component-assembled products.

It is small-sized and lightweight. Machinery/equipment can be made smaller in size and lighter in weight because the helical wave actuator provides similar levels of torque, force and speed reduction ratios as conventional gearing mechanisms at a fraction of the size.

It provides quiet, vibration-free operation. With the helical wave actuator, quiet and vibration-free operations are possible because there are no teeth in rolling contact with one another, and the circumferential speed of the engagement faces is low.

The helical wave actuator can be used for linear actuations of valves, closures, switches and displacement pumps, for jacking of static loads, positioning and referencing operations, and for venting actuation and levelling devices.

The term "screw thread" as used herein can include continuous threads and threads which are not continuous i.e. have gaps or spaces.

It will also be appreciated that the number of starts and the helical profile for each screw thread can be varied or chosen in dependence on load conditions.

Although the flexible member is described as a single member having an integral outer screw thread it will be appreciated that any suitable means may be used and screw thread on either or both of the outer member and flexible member may be formed by discrete means including, for example, a spring.