GB2583530A - Linear actuator - Google Patents

Abstract

A linear actuator comprising an electrically powered motor (24 fig. 3) driving an externally threaded 32 drive shaft 26 for rotation. A carriage 34 is axially moveable relative to the drive shaft, and a drive member 36 carried by the carriage is located within the thread formation such that rotation of the drive shaft drives the carriage for axial movement. The thread formation is of non-uniform pitch along the length of the drive shaft. The drive member may comprise a pin 36 that extends substantially tangentially of the drive shaft. The thread formation of the drive shaft may comprise a region of relatively shallow pitch 32a and a region of relatively steep pitch 32c.

Description

LINEAR ACTUATOR

This invention relates to a linear actuator, and in particular to a linear actuator in which the output thereof is variable.

There are a number of applications in which linear actuators are used to drive components for movement along a linear or substantially linear path. In many applications, the force required to move the component along the path is not uniform, but rather varies at different positions along the path. One example of such an application is in a scissor linkage. In such an arrangement, when starting from a condition in which the linkages lies substantially parallel to the path of movement followed, in use, by a driven part of the actuator, the force that must be applied to drive the linkage for movement is relatively high. As the linkage is moved away from this position, the force that must be applied to drive the linkage for movement reduces.

Typically, the motor used to operate the linear actuator in such an arrangement is of uniform output and is selected to be just able to generate a sufficiently high output to enable the scissor linkage to be moved when the scissor linkage is at its high input load requirement position. However, by using such a motor, throughout a large part of the operation of the scissor linkage the motor is over powered, and when the scissor linkage is at its high input load requirement position, the motor sounds as if it is straining or struggling to operate. To avoid such straining or struggling, an over powered motor may be provided. Electronic motor controllers may be used to address this issue.

Clearly, such an arrangement is disadvantageous as a larger, more expensive motor than is required, will often be provided, and a relatively complex, expensive electronic motor controller may be required.

It is an object of the invention to provide a linear actuator in which at least some of the disadvantages associated with known arrangements are overcome or are of reduced impact.

According to the present invention there is provided a linear actuator comprising an electrically powered motor operable to drive a drive shaft for rotation, the drive shaft having an outer periphery formed with a thread formation, a carriage axially moveable relative to the drive shaft, and a drive member carried by the carriage and located within the thread formation such that rotation of the drive shaft drives the carriage for axial movement, wherein the thread formation is of non-uniform pitch along the length of the drive shaft.

By providing a thread of non-uniform pitch, it will be appreciated that for a given speed of rotation of the motor and the drive shaft, there will be parts where the carriage moves relatively slowly and other parts where the carriage moves relatively quickly. By appropriate design of the thread, the actuator can be tuned to the application in which it is to be used so that throughout the operation of the actuator the motor is operating at its optimum load.

By way of example, where the linear actuator is used to drive a scissor linkage for movement, where the linkages extend substantially parallel to the direction of movement of the carriage, the part of the thread formation with which the drive member is in engagement is preferably of a relatively shallow pitch, the thread pitch increasing as the scissor linkage moves away from this position. If there is a desire to slow the movement of the scissor linkage as it approaches its opposite extreme position, the thread may again include a region of shallow pitch.

Preferably, the drive member comprises a pin that extends substantially tangentially of the drive shaft.

Whilst reference is made hereinbefore to the use of the linear actuator in driving a scissor linkage for movement, it will be appreciated that this represents merely one example application in which the actuator may be employed, and that the invention is not restricted in this regard. By way of example, it may be employed in driving components of downhole equipment for movement in the oil and gas technologies, or for driving the flaps or slats of an aircraft for movement.

The invention will further be described by way of example, with reference to the accompanying drawings, in which: Figures 1 to 6 are views illustrating a patient lifting device incorporating an actuator in accordance with an embodiment of the invention; and Figures 7 and 8 are views illustrating part of the actuator of the embodiment of Figure 1 in greater detail.

Referring to the accompanying drawings, a lifting device 10 is illustrated for use in lifting an individual from substantially ground level to an elevated position. By way of example, the device 10 may be used in the event that the individual has fallen, and needs to be lifted to allow the individual to be seated upon a chair, moved to a bed or other piece of furniture, or moved to a wheelchair or the like. It will be appreciated that these represent just examples of applications in which the invention may be employed, and that the invention is not restricted in this regard.

The lifting device 10 comprises a pair of side units 12, 14 which are pivotally or hingedly connected to one another. The side units 12, 14 are substantially identical to one another, and so only one of the side units 12, 14 is described herein in further detail.

The side unit 12 comprises a base 16 which, in use, rests upon the ground. The base 16 comprises an elongate base plate 18 supporting a motor housing 20 and a guide member 22 in the form of a tubular shaft of substantially circular cross sectional shape. The motor housing 20 houses an electrically operated motor 24, an output of which is connected to an elongate drive shaft 26 that is located within the guide member 22. The drive shaft 26 is supported for rotation by bearings 30. The drive shaft 26 is provided, on its outer periphery, with a helical groove defining a thread formation 32.

As best shown in Figures 7 and 8, the thread formation 32 is of non-uniform pitch along the length of the drive shaft 26, including end regions 32a, 32b of relatively shallow pitch, and a central region 32c of a steeper pitch. The pitch of the thread formation 32 varies smoothly between these positions. As can be seen from the drawings, the thread formation 32 is not only of varying pitch, but the profile of the groove defining the thread formation, when viewed from a side of the drive shaft 26, is also non-uniform as discussed below.

Supported upon the drive shaft 26 within the guide member 22 is an inner carriage 34. The inner carriage 34 carries a drive pin 36 that extends generally horizontally, substantially tangentially to the drive shaft 26, the drive pin 36 sitting within the thread formation 32. The profile of the groove defining the thread formation is chosen so that the drive pin 36, which extends substantially tangentially to the drive shaft 26, being of length greater than the diameter of the drive shaft 26 and so projecting beyond both sides thereof, closely fits within the groove, and remains so fitted throughout the length of the groove as the inner carriage 34 translates along the length of the drive shaft 26, in use. In order to achieve this, the groove must be of increased width in the steeper pitch regions, and of smaller width in the shallower pitch regions.

The groove is conveniently formed using a cutter of generally cylindrical form arranged perpendicularly to the axis of the drive shaft 26, and offset from the axis of the drive shaft 26 so as to be positioned substantially tangentially thereto.

An outer carriage 38 is supported upon the exterior of the guide member 22, the outer carriage 38 being secured to the inner carriage 34 by means of bolts 40 which extend through an elongate slot formed in the underside of the guide member 22. The outer carriage 38 is provided with an inner sleeve 42 of a low friction material, the inner sleeve 42 bearing against the outer surface of the guide member 22.

In use, operation of the motor 24 to drive the drive shaft 26 for rotation causes the inner carriage 34, and hence the outer carriage 38, to be driven axially or linearly, with the outer carriage 38 supported by and guided by the guide member 22. As the pitch of the thread formation 32 is non-uniform, it will be appreciated that for a given speed of rotation of the drive shaft 26, the inner and outer carriages 34, 38 will move relatively slowly when the drive pin 36 cooperates with a part of the thread formation 32 of relatively shallow pitch, and move more rapidly when the drive pin 36 cooperates with a part of the thread formation 32 of steeper pitch.

The side unit 12 further comprises a scissor linkage 44 in the form of a pair of link arms 46a, 46b which are pivotally connected to one another in an X-shaped configuration. A lower end of the arm 46a is pivotally connected to the base 16 adjacent the motor housing 20, and a lower end of the other arm 46b is pivotally connected to the outer carriage 38.

The side unit 12 further comprises a top bar 48 which is pivotally connected to the upper end of the arm 46b. The upper end of the other arm 46a is provided with a roller 50 which is able to travel within a channel defined by the top bar 48.

The scissor linkage 44 is of asymmetric design, the pivotal connection between the arms 46a, 46b being offset from an axis interconnecting the points at which the arm 46a is connected to the base 16 and the roller SO, and from an axis interconnecting the points at which the arm 46b is connected to the outer carriage 38 and the top bar 48.

The bases 16 of the side units 12, 14 are pivotally interconnected, and the top bars 48 of the side units 12, 14 are pivotally interconnected, the pivot axes being substantially coaxial.

A fabric or flexible plastics material seat 52 is connected to the top bars 48, extending therebetween, and a seat back 54 is attachable to the top bars 48. A brace 56 is attachable between the bases 16.

The lifting device 10 can be stowed in a relatively compact form with the scissor linkages 44 positioned such that each top bar 48 lies immediately above the respective base 16, and with the side units 12, 14 positioned adjacent one another.

In use, in the event that an individual has fallen and requires lifting, the lifting device 10 is positioned close to the individual, and the side units 12, 14 thereof are pivoted apart so that the lifting device adopts a V-shaped form. The brace 56 may be employed to secure the bases 16 to one another in this configuration, and the seat back 54 installed to secure the top bars 48 to one another. In this condition, the seat 52 will rest upon the ground between the top bars 48.

The individual can then be slid or otherwise moved onto the seat 52, and may rest against the seat back 54. Once in this position, the motors 24 are operated. Operation of the motors 24, as described hereinbefore, causes rotation of the drive shaft 26 and linear movement of the inner and outer carriages 34, 38, moving the points at which the arms 46b are pivotally connected to the outer carriages 38 closer to the points at which the arms 46a are pivotally connected to the bases 16. As a consequence of this movement, the scissor linkages 44 operate to lift the top bars 48 away from the bases 16, and so lifting the seat 52 and the individual sat thereon from the ground to a raised or elevated position.

Initially, the drive pin 36 cooperates with a part of the thread formation 32 of shallow pitch and so, for a given speed of rotation of the motor and drive shaft, the inner and outer carriages 34, 38 will move relatively slowly. This corresponds with the part of the movement of the scissor linkage 44 at which the greatest input load is required, and by having the actuator operate to drive the carriages 34, 38 for relatively slow movement during this phase in the operation, the motor is able to supply a sufficient output to drive the scissor linkage for such movement.

Subsequently, as the requirement of the scissor linkage 44 reduces, the drive pin 36 cooperates with parts of the thread formation 36 of steeper pitch and so movement of the inner and outer carriages 34, 38 accelerates. As the lifting device approaches its upper extreme position, deceleration is achieved by the drive pin 36 cooperating with another region of the thread formation of relatively shallow pitch.

At least part of the individuals weight is borne through the bearing formed by the inner sleeve 42 and is transmitted therefrom through the guide member 22 and remainder of the base 16 to the ground. Accordingly, this load is not transmitted directly to the drive shaft 26 in such a manner as to cause bending or the like thereof.

Once the lifting device 10 has been used to lift the individual to a suitable height, operation of the motors 24 may cease. Friction within the drive train is sufficient that the lifting device 10 will remain in the raised position. The individual may then be transferred to, for example, a wheelchair, a chair, a bed or the like, for example using a transfer board or the like to aid such movement. After the individual has been transferred from the lifting device 10, the device 10 may be returned to its original configuration for transportation or storage.

By providing a scissor linkage 44 of asymmetric form, it will be appreciated that the force required to commence lifting may be reduced.

The use of an actuator of the above form is advantageous in that there is no need to include a motor or excessively large form, and by appropriate design of the thread formation, straining or struggling of the motor can be avoided.

If desired, the arrangement described hereinbefore may be modified to include two or more thread formations, for example cut to different depths and/or of different widths (formed using different diameter cutters), and respective drive pins engaging therewith to allow two or more components to be driven by the rotation of a single drive shaft 26, the components being driven at different speeds to one another and/or in different directions to one another.

Whilst the description hereinbefore is of the use of the actuator of the invention in driving a patient lifting device for operation, it will be appreciated that this represents merely one example application in which the actuator of the invention may be employed and that the invention is not restricted to such use. Rather, it may be used in a wide range of applications, for example as set out hereinbefore.

It will be appreciated that whilst a specific embodiment of the invention is described herein with S reference to the accompanying drawings, a wide range of modifications and alterations may be made thereto without departing from the scope of the invention as defined by the appended claims.

Claims (4)

1. CLAIMS: 1. A linear actuator comprising an electrically powered motor operable to drive a drive shaft for rotation, the drive shaft having an outer periphery formed with a thread formation, a carriage axially moveable relative to the drive shaft, and a drive member carried by the carriage and located within the thread formation such that rotation of the drive shaft drives the carriage for axial movement, wherein the thread formation is of non-uniform pitch along the length of the drive shaft.
2. 2. An actuator according to Claim 1, wherein the drive member comprises a pin that extends substantially tangentially of the drive shaft.
3. 3. An actuator according to Claim 2, wherein the thread formation is defined by a groove formed in the drive shaft, the groove being shaped to define at least one region where the thread formation is of relatively shallow pitch and at least one region where the thread formation is of relatively steep pitch.
4. 4. A drive shaft for use in an actuator as claimed in any of the preceding claims, the drive shaft having an outer periphery formed with a thread formation, wherein the thread formation is of non-uniform pitch along the length of the drive shaft.