GB2136529A

GB2136529A - Linear actuator

Info

Publication number: GB2136529A
Application number: GB08406201A
Authority: GB
Inventors: Gilbert G Pitt
Original assignee: CAM SYSTEMS Ltd
Current assignee: CAM SYSTEMS Ltd
Priority date: 1983-03-11
Filing date: 1984-03-09
Publication date: 1984-09-19
Also published as: GB2136529B; GB8406201D0

Abstract

A linear actuator (1) comprises a stepping motor (4) which is operable under program control to rotate a worm gear (5) which carries a helical thread (6). The output member (10) of the actuator comprises a rack (8) having teeth (12) which mesh with the thread (6), and a free sliding rod (9). The teeth (12) form part of an imaginary internal screw thread which mates with the thread (6) so that the worm gear and rack mesh along the entire length of the worm gear. Three actuators (1,2,3) may be cascaded as shown to provide three axis position control of e.g. a tool. <IMAGE>

Description

SPECIFICATION Linear actuator This invention relates to a linearactuatorthe preferred embodiments ofthe invention being suit able for a wide range of applications where close control of linear position and/or movement is required.

Prior art linear actuators can broadly be classified as of piston-and-cylindertype; or rack-and-pinion type; orleadscrew-and-nuttype; orcam-and-camfollower type. Each of these types has advantage and disadvan tagus well recognised bythoseskilled in machine design. The present invention provides a new type of linear actuatorwhich, in many design applications, has significant advantages over any of the prior art types referred to above.

According to one feature of the present invention a Iinearactuatorcomprises: a rotary motor the output shaft of which drives a worm gearformed with at least one helically extending thread; and a linear output member including a rack having formed thereon teeth which mesh with the worm gear whereby rotation of the stepping motorcausescorresponding longitudinal movement of the linear output member.

The invention will be better understood from the following description of an embodimentthereof, given bywayofexampleonly, reference being had to the accompanying drawings wherein: Figure 1 schematically illustrates an embodiment of the invention and shows howthree linear actuators may be interconnected to provide three dimensional positional control; and Figures 2 and 3 are respectively side elevation and plan views of an embodimentofthe invention.

Referring firstly to Figure 1,three linear actuators 1,2,3 are shown interconnected to provide three- dimensional positional control, as described in more detail hereinafter. Each linear actuator comprises a stepping motor4, each stepping motor driving a respective worm gear formed with at least one helically extending thread 6. Each stepping motor4 and worm gear5 is mounted on a housing 7, and each housing slidably supports a pair of rods 8,9 which together form a linear output member 10. Each rod 8 includes a rack portion 11 formed with teeth 12 which mesh with the thread 6 ofthe associated worm gear 5.

In use, each stepping motor4 is operated under programme control to provide a desired pattern of rotary movement. This rotary movement is converted to corresponding linear movement of the associated rod 8 byway ofthe worm gear 5 and meshing rack 11.

The rod 9 is supported forfree sliding movement within the housing 7 and acts as a steady arm.

It should be appreciated that the teeth 12 are cut to mesh accurately with the helical thread 6. In effect, the rack 11 can be regarded as a portion of an imaginary cylinderwhich is formed with an internal helical screw thread which mates with thethread 6. Thus, the teeth 12 mesh with the thread 6 along the entire length of the worm gear and are machined to provide close and substantially backlash free engagement between the rack 11 and worm 6.

In the arrangement illustrated in Figure 1 the housing 7 of the actuator 1 is fixed to a suitable suppprtwhilstthe linear output member 10 of the actuator 1 is rigidly fixed to the housing 7 of the actuator 2. Similarly, the linear output member 10 of the actuator 2 is rigidly secured to the housing 7 of the actuator 3. The linear output member 10 ofthe actuator 3 carries a member, e.g. a tool holder, which isto be located by the actuators 1,2,3. It will be seen, therefore, that the actuator 1 provides for movement ofthetool holderin the direction ofthearrowA; the actuator 2 provides for movement of the tool holder in the direction ofthe arrow B; and the actuator 3 provides for movement of the tool holderinthe direction ofthe arrow C; the arrows A,B,C, being orthogonal.Thus, suitable programme control ofthe stepping motors 4will enable three dimensional position control of the tool holder.

The above described arrangement has a member of substantial advantages over prior art linear actuators.

Firstly, since linear movement is directly related to rotation ofthe stepping motors which are in turn controlled from a suitable programme, any desired pattern of linear motion can be produced. For example, the stepping motors can be programmed to produce a cyclic linear motion directly comparable with that produced by a cam and camfollower or may be programmed to produce rapid movement of the linear output member consistent with predetermined limits of inertial loading, or any other desired pattern of movement. The pattern of movement can readily be changed by changing the control programme, such change being effected without any modification to the linear actuators themselves.Further, since there is a direct correlation between output member position and stepping motor rotation from any predetermined datum point, accurate positional location can be achieved under programme control without the use of position detectors or limit switches. Further, the use of a rack having, in effect, an internal helically cut thread in association with a worm gear enables a high linear output force to be obtained with very little back-lash, and enables the device to be self-holding, i.e. with the stepping motors 4 non-energised a high axial loading can be transmitted from the rackto the worm gear without causing rotation oftheworm gear.

Referring now to Figures 2 and 3there is illustrated a practical embodiment of the invention in which the stepping motor4 is bolted directly to a housing 7 within which the worm gear is located. If desired, the housing 7 can also house a reduction gear between the stepping motor output and the worm gear. The housing 7 provides a first mounting forthe assembly.

The output member 10 includes, in addition to the rods 8,9 mounting blocks 13,14which rigidly intercon- nectthe ends of the rods 8,9 The mounting blocks 13,14 provide further mounting points for the actuator.

Although in the above described embodiment a single rack meshes with the worm gear it will be appreciated that each actuator may include a plurality of racks meshing with a common worm gear, the teeth of the respective racks being positioned relative to each other such thatthey all constitute parts of an imaginary continuous internal screwthread. If two or three racks are provided the freely sliding steady arm may be omitted. Further, whilst the above described embodiment of the invention utilizes a single thread 6 on the worm gearitwill be appreciated that a multi-startthread may be provided if desired.

Whilst in some embodiments ofthe invention the motor housing will be fixed and the linear output memberwill produce the desired output motion, it should be appreciated that the linear output member could be fixed and the desired output motion could be imported to the motor housing on the like. Such arrangements are intended to come within the scope ofthe appended claims. Further, whilst the above embodiment has been described with reference to a stepping motor it will be appreciated that any other suitable motor, e.g. a D.C. servo motor, may be used.

Claims

1. Alinearactuatorcomprising: a rotarymotorthe output of which drives a worm gearformed with at least one helically extending thread; and a linear output member including a rack having formed thereon teeth which mesh with the worm gear whereby rotation ofthe motor causes corresponding longitudinal movement of the linear output member.

2. A linear actuator according to claim 1 wherein the teeth on the rackform part of an imaginary internal helical screw thread which mates with the thread of the worm gearwherebythe worm gearthread and the rackteeth mesh along the entire length of the worm gear.

3. A linear actuator according to claim 1 or claim 2 wherein the linear output member includes, in additiontothe rack, a rod mounted forfree sliding movement and coupled to the rack.

4. A linear actuator according to claim 3 wherein the rack and rod are both mountedforsliding movement in a housing which houses the worm gear and upon which is mounted the motor.

5. A linear actuator according to claim 1 or claim 2 wherein the output member comprises a plurality of said racks, each in meshing engagement with the worm gear, the racks being coupled to each other.

6. Alinearactuatoraccordingto any preceding claim wherein the motor is a stepping motorsuitable for operation under program control ora D.C. servo motor.

7. A linear actuator according to any preceding claim wherein the actuator is self-holding when the motor is non-energised.

8. A linear actuator according to any preceding claim wherein a second said actuator is connected to the output member ofthefirst mentioned actuator and a third said actuator is connected to the output member of the second actuator,theaxesofmove- mentofthethree actuators being orthogonal.

9. A linear actuator substantially as herein de scribed with reference to the accompanying drawing.

10. Positioning apparatus comprising a linear actuator according to any preceding claim; means for mounting an item to be positioned on the output member of the actuator; and programmable control meansforoperating the motorunder program control to position an item mounted on the output member in successive positions determined by the program.