GB2405996A

GB2405996A - Tilt switch

Info

Publication number: GB2405996A
Application number: GB0321483A
Authority: GB
Inventors: Peter Simpson
Original assignee: Active Switch & Sensor Ltd
Current assignee: Active Switch & Sensor Ltd
Priority date: 2003-09-13
Filing date: 2003-09-13
Publication date: 2005-03-16
Anticipated expiration: 2023-09-13
Also published as: GB0321483D0; GB2405996B

Abstract

A tilt switch has a magnetically-operable switch 20 and an elongate guide 15 extending in close proximity to the switch 20. A magnetic mass 16 is supported on the guide 15 for sliding movement therealong, to interact with the switch 20 when in one limiting position. The guide 15 and mass 16 are configured so that as the mass slides along the guide, it is turned about an axis parallel to or coincident with the guide axis.

Description

TILT SWITCH

This invention relates to a tilt switch - that is to say, an electrical switch which operates (that is, either to complete or open) a circuit when the body of the switch is tilted through a predetermined angle from a normal or rest position.

Most simple forms of tilt switch utilise gravity acting on a mass, and as such are also susceptible to acceleration.

Tilt switches are very well known and are widely used in a variety of different circumstances. Until relatively recently, a common form of tilt switch utilised a glass vial within which is contained a bead of mercury and a pair of electrodes which the bead of mercury may bridge, when the vial is tilted in the appropriate sense. Conversely, tilting the vial in an opposite sense allows the bead of mercury to run away from the electrodes, so opening a circuit within which the electrodes are included. Environmental and human health concerns have made mercury tilt switches unacceptable for many applications, on account of the highly toxic nature of and the difficulty of recycling mercury. As such, research is under way to develop alternative fomms of tilt switches, but the manufacture of a low cost alternative having a comparable performance to a mercury tilt switch is proving to be somewhat difficult. For example, rolling ball switches are used, but they have low current carrying capabilities and are prone to corrosion leading to unreliability.

To improve on rolling ball switches, tilt switches are currently being made using reed switches in conjunction with a magnet, the switch and magnet being relatively movable under the influence of gravity when the body of the switch is tilted through more than a pre-set angle, or is subjected to more than a defined - 2 acceleration, such as a shock load. Conventionally, the reed switch is held stationary and the magnet is mounted for sliding movement on a suitable rail, between an active position where the reed switch is operated and an inactive position where the reed switch is in its normal position. Reed switches have hermetically sealed contacts and are very reliable, but when used with a sliding magnet the switching point is unfortunately somewhat indeterminate. As such, it is difficult to ensure the tilt switch will operate at the same angle of tilt relative to a datum position. There are various reasons for this, but most important appears to be the gradual reduction or increase in the magnetic field influencing 0 the reed switch as the magnet slides along its guide, though any friction and stiction between the magnet and its rail, will also be significant.

The present invention aims at providing a tilt switch having a magnetically-operable switch and sliding magnet combination, but which is able to operate more reliably and predictably at a predetermined tilt angle relative to a datum, than is easily achievable with current designs of low-cost tilt switch using a sliding magnet and reed switch.

According to the present invention, there is provided a tilt switch comprising a magnetically-operable switch having first and second states, an elongate guide extending in proximity to the magnetically-operable switch, a magnetic mass supported by the guide for sliding movement therealong between first and second positions, the mass when in the first position magnetically interacting with the magnetically-operable switch to cause the operation thereof to its first state and when in the second position allowing the switch to assume its second state, and the guide and mass being configured to - 3 co-operate so that as the mass slides along the guide, the mass is fumed about an axis parallel to the length of the guide.

With the tilt switch of this invention, the magnetic mass slides under the influence of gravity or acceleration along the length of the guide, when the tilt switch is tilted or is subjected to acceleration, in either case in excess of a pre- set value determined by the mounting of the tilt switch on some other component. Simultaneously with that sliding movement, the mass also turns about the length of the guide and in this way the magnetic field which influences the operation of the magnetically-operable switch (hereinafter referred to as a "magnetic switch") may be caused more rapidly to increase or decrease. Thus, more rapid and certain opening or closing of the magnetic switch may be achieved, at a more predictable position of the magnetic mass along the length of the guide, than can be achieved with a simple linearly-sliding magnet.

Further, there is a reduced likelihood of secondary operation of the magnetic switch, as the magnetic field is moved further away from the switch.

In a highly preferred form of this invention, the magnetic mass comprises a mass of a non-magnetic material carrying a permanent magnet suitably disposed for close juxtaposition to the magnetic switch when the magnetic mass is in its first position. For example, the mass may be of a generally cylindrical external shape, but having a permanent magnet let thereinto so that one pole of the magnet is at or adjacent the cylindrical surface of the mass. In such a case, the permanent magnet also may be cylindrical with its poles at the two ends thereof, the magnet extending in a radial bore formed in the mass.

With the preferred form of the invention, the mass (apart from the permanent magnet) may be made of a non-magnetic metal such as copper or a non-magnetic metal alloy such as brass or an appropriate grade of stainless steel. In the alternative, the mass may be of a relatively high density plastics material, advantageously also having low friction characteristics to facilitate sliding along the guide.

Most preferably, the guide has a non-circular cross-sectional shape and is twisted along its length, the mass having a central opening of essentially the same cross-sectional shape as that of the guide whereby the mass will turn as it slides along the guide. For instance, the guide may have a generally square cross-section, in which case the mass should have a central opening of the same cross-sectional shape but of slightly larger dimensions so as to be a free sliding fit on the guide. Other cross-sections could equally be used, though it is important to maintain friction as low as possible, to ensure the easy sliding of the mass, under the influence of gravity.

As an alternative to the above-described arrangement with a guide of a non-circular cross-section, the guide and the bore of the magnetic mass could both be circular, and there is an auxiliary arrangement which causes the mass to turn as it slides along the guide. One possibility is to form a helical groove along the length of the guide, the mass having a peg projecting internally of its central opening to engage the groove of the guide and so to cause the mass to turn as it slides along the guide. Similarly, an external peg could project from the surface of the mass, which peg engages in a groove formed in a further guide externally of the mass, again to impart turning movement to the mass as it slides along the guide. Other mechanisms could be provided but having the same functionality of causing the mass to turn as it slides along the guide.

In a preferred embodiment, the guide is of a low friction plastics material such as PTFE and has a generally square cross-section, the guide being formed with a helical twist typically in the range of 90 to 150 , but preferably 110 to 120 , between the first and second positions of the mass. A metal alloy mass may be employed with such a guide, the mass having a machined helical bore which is an easy sliding fit on the guide.

The magnetic switch conveniently is a simple reed switch comprising a pair of reed contacts hermetically sealed within a casing. If required, a power reed switch may be employed. The switch may be configured as a nominally open switch, a normally closed switch or a changeover switch, the state of the switch being changed by the presence of a sufficient magnetic field in close proximity thereto. In the altemative, a solid- state magnetic switch, such as a Hall-effect switch, may be employed.

The magnetic switch, magnetic mass and the guide are preferably contained within a housing of a suitable shape, with the guide extending between end faces of the housing. Those end faces may then define the first and second positions for the magnetic mass. For such an arrangement, the reed switch may be disposed within the housing with its axis parallel to that of the guide.

The tilt switch of this invention may be configured expressly to sense shock, acceleration or even vibration, instead of simple tilting. For these alternative purposes, one or more springs may be provided between the - 6 magnetic mass and a fixed part, such as an end wall of the housing within which the mass and guide are contained. For example, a spring may be provided to urge the magnetic mass to its second position where the magnetic switch is not influenced by the magnetic field, the mass then moving against the action of that spring on being subjected to acceleration having at least a component in the direction of the length of the guide. If the acceleration is sufficiently high, such as may occur with a shock load, then the magnetic mass will move sufficiently far along the guide to operate the switch.

The tilt switch may include more than one magnetic switch, for operation 0 by the magnetic mass. For instance, two such reed switches may be provided, one as has been described above and so operated when the magnetic mass is in its first position, and the other so as to be operated when the magnetic mass is in its second position. For this purpose, the magnetic mass may carry more than one permanent magnet, for operating the plurality of reed switches.

By way of example only, this invention will now be described in greater detail, reference being made to the accompanying drawings, in which: Figure 1 is a cut-away isometric view of the embodiment of tilt switch of this invention, with the magnetic mass in its first position; Figure 2 is a view similar to that of Figure 1 but with the magnetic mass in its second position; and Figure 3 is similar to Figures 1 and 2, but with the mass removed, for clarity.

The embodiment tilt switch of this invention and shown in the drawings comprises a generally rectangular housing 10 having opposed end walls 11, opposed side walls 12 and top and bottom walls 13,14. Between the end walls 11 is arranged a guide 15, formed of a plastics material such as PTFE. The guide is secured to the end walls so that there is clearance between the guide and the side, top and bottom walls of the housing. The guide may be pinned, screwed or otherwise secured in position, for example by using an adhesive.

The guide has a square cross-section, though with the corners between the faces of the guide slightly rounded. The guide is twisted along its length, typically through 110 to 120 .

Mounted on the guide 15 is a generally cylindrical magnetic mass 16, the mass having a central opening the shape of which corresponds to the crosssectional shape of the guide 15. The central opening is helically formed and the cross-sectional dimensions of the opening are slightly greater than the corresponding dimensions of the guide, so that the mass is a free sliding fit thereon. A radial bore 17 is formed in the mass 16 and a permanent magnet 18 is secured in that bore, the magnet having its poles at the two ends respectively thereof. A reed switch 20 is mounted in the two end walls 11 of the housing 10, by means of its wire tails 21, using an adhesive to secure those tails in position in openings in the end walls. The reed switch is of a wholly conventional design and has electrodes mounted internally within a hermetically sealed envelope 22 and arranged so that the electrodes change from their normal state when the switch is subjected to a sufficient magnetic field. The electrodes may be normally open and so close when there is a sufficient field, or may be normally closed and so open when there is a sufficient field. In a further design, the - 8 switch may have three electrodes and may be arranged to perform a change over function.

The switch 20 is positioned within the housing so that when the magnetic mass 16 is in a first position (shown in Figure 1) the permanent magnet 18 is immediately adjacent the envelope 22 of the reed switch. The field of that magnet thus causes the switch to change its state. On movement of the mass to its second position, (shown in Figure 2), the magnet has moved away from the reed switch, both axially of the guide 15 and also in a fuming sense such that the axis of the magnet no longer is aligned with the reed switch. In this way, a rapid and predictable switching function may be achieved, both as the mass moves from its first position to its second position and also as the mass moves in the opposite sense.

The drawings show a basic embodiment of tilt switch of this invention.

The housing could contain two (or even more) reed switches at suitable dispositions, so that one switch is operated when the mass is at its first position and the other when the mass is at its second position. If more than two such reed switches are provided, then the magnetic mass may support more than one permanent magnet. Another possibility is to replace the reed switches by magnetically-operated solid-state Hall-effect switches. In this way, a digital output may be obtained from the tilt switch assembly.

Whatever form of magnetic switch is employed, connections may be made to it internally of the housing, so that a cable comes out of the housing, ready for connection to some other equipment. Instead, the housing may be provided with a cable connector into which a plug on the end of a cable may be - 9 - inserted. A further possibility is to provide the housing with projecting pins, to facilitate the mounting of the housing on a printed circuit board, the pins being connected internally of the housing to the or each switch, as appropriate.

In order to allow the switch to sense acceleration, shock loads or vibration, a simple helical spring may surround the guide 15 and urge the mass 16 to its second position. Acceleration in the direction of the length of the guide will move the mass towards its first position and if the acceleration is sufficient, the switch will operate. The housing 10 may be elongated, in order to accommodate the spring, when compressed.

0 The housing may be fitted with printed circuit board pins to facilitate the mounting of the tilt switch to a circuit board, the pins being electrically connected to the tails of the reed switch. Another possibility is to fit the tilt switch into a relay assembly with the reed switch serving to operate the coil of the relay. The relay contacts may handle higher switch loads than the reed switch could accept and in this way the tilt switch may control the operation of higher-powered equipment. - 10

Claims

1. A tilt switch comprising: - a magnetically-operable switch having first and second states; - an elongate guide extending in proximity to the magnetically-operable switch; - a magnetic mass supported by the guide for sliding movement therealong between first and second positions, the mass when in the first position magnetically interacting with the magnetically-operable switch to cause the operation thereof to its first state and when in the second position allowing the switch to assume its second state; wherein - the guide and mass are configured to co-operate so that as the mass slides along the guide, the mass is turned about an axis parallel to the length of the guide.

2. A tilt switch according to claim 1, wherein the magnetic mass comprises a mass of a non-magnetic material carrying a permanent magnet which is closely juxtaposed to the magnetic switch when the magnetic mass is in its first position.

3. A tilt switch as claimed in claim 2, wherein the magnetic mass is of a generally cylindrical external shape, and a permanent magnet is let thereinto so that one pole of the magnet is at or adjacent the cylindrical surface of the mass.

4. A tilt switch as claimed in claim 3, wherein the permanent magnet is cylindrical with its poles at the two ends thereof, and the magnet extends in a radial bore formed in the mass. - 1 1

5. A tilt switch as claimed in any of claims 2 to 4, wherein the magnetic mass is of a non-magnetic metal or a non-magnetic metal alloy. I

6. A tilt switch as claimed in any of claims 2 to 4, wherein the magnetic I mass is of a relatively high density plastics material

7. A tilt switch as claimed in any of the preceding claims, wherein the guide has a non-circular cross-sectional shape and is twisted along its length into a helical fomm, the mass having a central opening of essentially the same cross- sectional shape as that of the guide whereby the mass will turn as it slides along the guide.

8. A tilt switch as claimed in claim 7, wherein the guide has a generally I square cross-section, and the mass has a central opening of the same cross sectional shape but of slightly larger dimensions, so as to be a free sliding fit on the guide.

9. A tilt switch as claimed in any of claims 1 to 6, wherein the magnetic mass has a central opening of circular cross-section and the guide also is of circular cross-section, there being an auxiliary guide arrangement to cause the mass to turn as it slides along the guide.

10. A tilt switch as claimed in any of claims 1 to 7, wherein the guide is I formed from a low friction plastics material and has a generally square cross section.

11. A tilt switch as claimed in any one of claims 7, 8 or 10, wherein the guide has a helical twist in the range of 90 to 150 , between the first and second positions of the mass. - 1 2

12. A tilt switch as claimed in any of the preceding claims, wherein the magnetic switch comprises a reed switch comprising at least a pair of reed contacts hermetically sealed within a casing.

13. A tilt switch as claimed in any of claims 1 to 12, wherein the magnetic switch comprises a solid-state magnetic switch.

14. A tilt switch as claimed in any of the preceding claims, wherein a housing is provided to contain the magnetic switch, magnetic mass and the guide

15. A tilt switch as claimed in claim 14, wherein the housing has a pair of end faces between which extends the guide, the end faces defining the first and second positions for the magnetic mass.

16. A tilt switch as claimed in any of the preceding claims, wherein one or more springs are arranged between the magnetic mass and a fixed part of the tilt switch, to urge the magnetic mass to a pre-defined position.

17. A tilt switch as claimed in any of the preceding claims, wherein there is provided more than one magnetic switch for operation by the magnetic mass.

18. A tilt switch as claimed in claim 17, wherein the magnetic mass carries more than one permanent magnet, for operating the plurality of reed switches.

19. A tilt switch as claimed in claim 1 and substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.