GB2339076A

GB2339076A - Movement sensing switch

Info

Publication number: GB2339076A
Application number: GB9809677A
Authority: GB
Inventors: Peter Ratcliffe; Peter Simpson
Original assignee: ASSEMTECH EUROP Ltd
Current assignee: ASSEMTECH EUROP Ltd
Priority date: 1998-05-07
Filing date: 1998-05-07
Publication date: 2000-01-12
Also published as: GB9809677D0

Description

Movement Sensing Switches This invention relates to movement sensing

switches.

The requirement for electrical devices that can detect or analyse movement (usually referred to as utilt switches") has been widespread for some time, and they are used in various kinds of electrical equipment such as anti-theft devices.

A common design of tilt switch relies on the use of liquid mercury flowing around conductive members. Though reliable in operation, mercury type switches are no longer environmentally acceptable due to the high toxicity of the mercury and the consequent health and environmental issues on disposal. Consequently other forms of tilt switch are being employed, and mostly these have a chamber, at least two contacts within the chamber and a conductive ball able to roll in the chamber to bridge the contacts.

Movement switches of this type have various disadvantages. Though rolling ball switches are environmentally acceptable, many designs are able to detect movement in only certain directions, and will not operate satisfactorily if not mounted and operated in the correct orientation. In addition many such tilt switches suffer from erratic operation due to high contact resistance, which can develop if the switch is not operated for an extended period.

In an attempt to mitigate the above-described disadvantages, according to the present invention there is provided a movement sensing switch comprising a first conductive member defining an outwardly open annular channel, a second conductive member surrounding with clearance at least the annular channel of the first member, a conductive ball free to roll within the annular channel and having a diameter greater than said clearance, and a base supporting the first and second conductive members in an electrically isolated manner, whereby movement of the switch causes the conductive ball to roll within the channel making or breaking a conductive 5 bridge between the first and second conductive members.

The switch could be formed in a number of different configurations, but for ease of manufacture the first conductive member preferably has a substantially cylindrical profile, the annular channel being formed in the curved outer surface of the cylinder. As it is necessary for the second lo conductive member closely to surround at least the annular channel in the first conductive member, it is desirable for the second conductive member to be formed in a tube-like shape which fits around the first conductive member. In practice, rather than a simple tube shape, the second conductive member may be formed from a tube with a closed end and therefore would be cup- shaped.

When the second conductive member is cup-shaped and encloses a cylindrical first conductive member, the base is preferably disc shaped and the rim of the second conductive member is fixed to the outer edge of the base, effectively closing the open end of the second conductive member. In this case, the base may have a non-conductive central portion to which the first conductive member is secured, so as to be isolated from the second conductive member. The outer portion of the base may be either conductive or non-conductive, but preferably is of a metal.

The first contact is advantageously formed with a generally axial 25 extension which passes through an aperture formed in the central region of the base and acts to secure the first conductive member to the base as well as providing a means for electrically connecting the first conductive member to a circuit. As the central portion of the base is nonconductive, the first and second conductive members are kept in electrical isolation from one another, 5 other than when bridged by the ball.

In use it is preferred that the electrical contacting surfaces are protected from corrosion or damage from the atmosphere. This is because any degradation of the conductivity of these surfaces would limit the efficiency of the conductive bridge and therefore the device as a whole.

lo Hence, it is preferred that the base and the second conductive member define a closed volume containing the first conductive member and the ball. In a particularly preferred embodiment of the invention the closed volume is hermetically sealed during manufacture, and may be filled with an inert gas prior to the hermetic sealing.

A further method of reducing corrosion of the conductive parts is to coat them with corrosion resistant materials. Naturally this coating must not impair the conductivity of these surfaces, and consequently it is preferred to coat the conductive parts with gold, and more preferably with an undercoating of silver prior to the gold layer coating. This coating has a second advantage, in that not only does it serve to protect the surface from corrosion, it also reduces the electrical contact resistance of the surfaces.

The conductive parts of the device may obviously be formed from any suitable conductive material, but preferably they are formed from metal which has both the conductive and structural properties that are required. In the alternative, the first and second conductive members and possibly also the ball could be made of non-conductors but be given conductive properties by way of a conductive coating on the respective component.

By way of example only, one specific embodiment of the present invention will now be described in more detail, with reference to the 5 accompanying drawings in which- Figure 1 is a section through the embodiment of movement sensing switch according to the present invention; and Figure 2 is bottom plan view of the embodiment of Figure 1.

Referring to Figures 1 and 2, an embodiment of switch according to the lo present invention is illustrated, and comprises a disc-shaped base 10 having a central portion 11 which is non-conductive and a outer portion 12 which is conductive. The outer portion 12 is formed from metal such as steel, and the central portion 11 is formed from a ceramic bead which is fused to the outer portion 12 during manufacture of the base.

A first conductive member 14, which defines an annular channel 15 and having an axial extension 16, is affixed to the base 10 and is formed for example from mild steel. The axial extension 16 passes through an aperture in the central portion 11 and extends beyond the outer face of the base 10. The ceramic bead is fused to the axial extension 16 during manufacture of the switch, at the same time as the bead is fused to the outer portion 12.

Surrounding the first conductive member 14 is a cup-shaped second conductive member 17 having a rim 19 which is attached to the circular outer edge 18 of the base 10 by welding, so that an hermetic but electrically conductive seal is formed therebetween. The second conductive member 17 may for example be formed from cold-rolled steel.

A conductive ball 20, preferably formed from brass, is located within the annular channel 15 and is free to roll therein. The ball 20 is prevented from completely escaping the confines of the channel 15 by the second conductive member 17.

During use, the switch may be mounted by soldering on to a circuit board (not shown), the portion of the axial extension 16 which extends beyond the outer face of the base 10 being connected to a board track. The conductive outer portion 12, which is in electrical contact with the second conductive element 17, is also electrically coupled to the circuit board. As the first conductive member 14 is enclosed, it forms the live part of the switch and the second conductive member 17 the earth or common return. This prevents unwanted activation of the switch, by accidental earthing.

When the switch is motionless, the ball 20 is stationary within the channel 15, probably bridging the first and second conductive members 14,17, but possibly only in contact with the first conductive member 14.

Movement of the switch will cause the ball 20 to roll, and this will cause it to either make or break an electric contact between the two conductive members. This change of electrical state can readily be detected by a suitable latching circuit. Any imperfections in the quality of the contact between the ball 20 and either of the conductive members 14,17 will assist the detection of the ball movement, especially if the latching circuit includes an edge detector.

Claims

1. A movement sensing switch comprising a first conductive member defining an outwardly open annular channel, a second conductive member surrounding with clearance at least the annular channel of the first member, a conductive ball free to roll within the annular channel and having a diameter greater than said clearance, and a base supporting the first and second conductive members in an electrically isolated manner, whereby movement of the switch causes the conductive ball to roll within the channel making or breaking a conductive bridge between the first and second conductive lo members.

2. A movement sensing switch as claimed in claim 1, in which the first conductive member is generally cylindrical and the annular channel is formed in the curved surface thereof.

3. A movement sensing switch as claimed in claim 1 or claim 2, in which the 15 second conductive member is generally cup-shaped and entirely encloses the first conductive member.

4. A movement sensing switch as claimed in any of claims I to 3, in which the base is disc-shaped and has a non-conductive central portion on which the first conductive member is supported.

5. A movement sensing switch as claimed in claim 3 and claim 4, in which the rim of the cup-shaped second conductive member is joined to the circular outer edge of the disc shaped base.

6. A movement sensing switch as claimed in claim 5, in which the first conductive member has an axial extension passing through an opening in the non-conductive central portion of the base and which axial extension serves to fix the first conductive member to the base without connecting electrically thereto-

7. A movement sensing switch as claimed in any of the preceding claims, in which the second conductive member and the base define an enclosed 5 volume surrounding the first conductive member and the ball.

8. A movement sensing switch as claimed in claim 7, in which the enclosed volume is hermetically sealed, from the external environment.

9. A movement sensing switch as claimed in claim 8, in which the enclosed volume contains an inert gas.

lo 10- A movement sensing switch as claimed in any of the preceding claims, in which the first conductive member, the second conductive member and the ball are coated with a corrosion resistant conducting material.

11. A movement sensing switch as claimed in claim 1 and substantially as herein described and as illustrated in the accompanying drawings.