GB2324910A

GB2324910A - A linear actuator

Info

Publication number: GB2324910A
Application number: GB9708881A
Authority: GB
Inventors: Robert Graham Harris
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-05-02
Filing date: 1997-05-02
Publication date: 1998-11-04
Anticipated expiration: 2017-05-02
Also published as: GB2324910B; GB9708881D0

Abstract

A linear actuator comprises two solenoid arrangements, each with a coil 2 and a resiliently biased armature. An elongate member 1 extends through the armatures and each armature is formed from two separate parts 3, 4. The armature parts 3, 4 are arranged to provide a clasping grip and linear movement action upon the elongate member 1 when its associated solenoid coil 2 is energised. When the said solenoid coil 2 is de-energised the armature parts 3, 4 release their grip and are returned along the elongate member 1 by resilient means 11. The armature parts 3, 4 may comprise cylindrical and semi-cylindrical portions and non-magnetic portions 6. The non-magnetic portions are used to separate the armature parts 3, 4 to ensure a good clasping grip.

Description

A LINEAR ACTUATOR This invention relates to a linear electromagnetic actuator which, when energised, can provide continuous axial movement of an elongate member.

Linear actuators are widely used for the positioning and actuation of devices in all areas of manufacturing and processing industries and are generally of two basic types: 1. Solenoid actuators which produce reasonable output loads only over very small displacements and 2. Motorised actuators which require some form of gearing to convert rotary into linear movement. These can produce very high output forces but the use of intermediate gearing means that the device has to be powered back to its initial position. This being the case, they cannot be considered as failsafe in certain applications.

According to the present invention there is provided a linear actuator comprising two solenoid coils, two armatures associated respectively with the two solenoid coils and an elongate member extending through the two armatures, each armature being urged by resilient means in a first direction parallel to the longitudinal extent of the elongate member and comprising two parts which grip the elongate member and move the elongate mernber in a second direction opposite to said first direction when the coil associated with that armature is energised and which release their grip on the elongate member and are returned by the resilient means to their initial position when the coil associated with that armature is de-energised.

In operation, one of the coils is energised to produce a first displacement of the elongate member. The other coil is then energised before said one coil is deenergised. This locks the elongate member in position against the reaction of an external load so that when said one coil is de-energised, the elongate member undergoes a second displacement. Repeated alternate cycling of the two coils will then cause continuous movement of the elongate member.

Preferably, each armature part has inner and outer surfaces which are each semi-cylindrical over at least a major part of its longitudinal extent. In this case, advantageously, each armature part has inner and outer surfaces which are each cylindrical over a minor part of its longitudinal extent.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal section through part of one embodiment of a linear actuator according to the invention, Figure 2 is a cross section through the armature of the part shown in Figure 1 on a reduced scale, and Figure 3 shows examples of applications of a linear actuator according to the invention..

Referring firstly to Figures 1 and 2, the first part of a linear actuator comprises a body 5, a coil 2, an armature comprising two half shells 3 and 4, and an elongate member in the form of an operating rod 1 extending through the armature.

Although not shown, the actuator also has a second identical part (not shown) comprising a further body 5, a further coil 2 and a further armature 3, 4. The operating rod 1 is common to both parts of the actuator.

Each half shell 3,4 has semi-cylindrical inner and outer surfaces over most of its length and cylindrical inner and outer surfaces at one end. The cylindrical surfaces of the shell 3 are at the opposite end of the armature to the cylindrical surfaces of the shell 4.

The operating rod 1 is free to slide in the recess formed between each pair of half shells 3 and 4 when the associated coil 2 is de-energised. The half shells are magnetically in series such that the magnetic flux created by the energised coil 2 passes across the gap 8 between the half shells. Since the half shells are free to close under the action of this magnetic field, the rod 1 is trapped between the two. In practice, the gap between the half shells and rod is just sufficient to guarantee the pinch load and is, therefore, much smaller than the working gap distance 9.

The pinch load on the rod 1 is of a similar magnitude to the axial load but with suitable surface finishes on the contact surfaces and/or a change in geometry, the holding forces can be made significantly higher if the application demands.

The magnetic flux also passes across the working gap 9, causing both half shells 3,4 and the trapped rod 1 to move through the working gap distance.

The remaining path for the flux comprises the body 5 and the small air gap 7 between the body 5 and the rear of the half shell 4. This gap provides a sliding fit to allow free axial movement of the half shells.

A spring 11 resets the half shells 3,4 back to their initial position when the coil 2 is de-energised and the working gap setting 9 is maintained by the retaining plate 10.

The two half shells 3,4 are separated longitudinally, by a pair of nonmagnetic spacers 6. These ensure that the magnetic flux does not bypass the haif shell gap which could reduce or eliminate the pinch load on the rod 1.

The operating rod 1 can be of any non-magnetic material or composite provided that any ferromagnetic part or section is not permitted to significantly bypass the operating flux from its series working path.

For some applications, a ferromagnetic section may be deliberately introduced into the rod 1 to automatically reduce the axial force after a predetermined travel is achieved.

In operation, one of the two coils 2 is energised to produce a first displacement of the operating rod 1. The other coil is then energised just before the aforesaid one coil is de-energised. This locks the operating rod in position against the reactive force of an external load so that when the said one coil is de-energised, the operating rod undergoes a second displacement. Repeated alternate cycling of the two coils 2 will then cause continuous movement of the operating rod 1.

The maximum feed rate of the rod 1 is determined by the permitted power demand of the device, by the system load which dictates the length of the working gap 9 and by the maximum frequency response of the system. Within these limits, the feed rate is directly proportional to the applied supply frequency since each cycle moves the rod through the working gap distance.

This feature also provides accurate positional control of the rod with minimal backlash and hysteresis.

Figure 3a shows a linear actuator as described above pushing (or pulling) a dead weight unidirectionally.

Figure 3b shows two actuators pushing/pulling a dead weight bidirectionally.

Figure 3e shows two actuators operating in unison to push a reactive load.

Claims

1. A linear actuator comprising two solenoid coils, two armatures associated respectively with the two solenoid coils and an elongate member extending through the two armatures, each armature being urged by resilient means in a first direction parallel to the longitudinal extent of the elongate member and comprising two parts which grip the elongate member and move the elongate member in a second direction opposite to said first direction when the coil associated with that armature is energised and which release their grip on the elongate member and are returned by the resilient means to their initial position when the coil associated with that armature is deenergised.

2. A linear actuator as claimed in claim 1, wherein each armature part has inner and outer surfaces which are each semi-cylindrical over at least a major part of its longitudinal extent.

3. A linear actuator as claimed in claim 2, wherein each armature part has inner and outer surfaces which are each cylindrical over a minor part of its longitudinal extent.

4. A linear actuator substantially as hereinbefore described with reference to the accompanying drawings.