US3444493A - Magnetically operated reed switch assembly - Google Patents

Description

May 13, 1969 (5. w. E. NIEUWHOF 3,444,493

I MAGNETICALLY OPERATED REED SWITCH ASSEMBLY Filed Aug. 29, 1966 United States Patent 01 iice 3,444,493 Patented May 13, 1969 US. Cl. 335206 1 Claim ABSTRACT OF THE DISCLOSURE A magnet operated reed switch wherein a permanent magnet on the end of a slidable rod within a ferromagnetic shield tube is arranged to close the contacts of one or other of two reed switches positioned adjacent each end of the tube, when the magnet is exposed at the end of the shield tube adjacent the appropriate reed switch. The slidable rod would be connected to the mechanism whose travel is 'to be limited e.g., a piston in a hydraulic cylinder and the length of the shield tube would be of such length as to allow the switching action of the reed switches to occur at the desired position of travel of the slidable rod.

This invention relates to magnetic switching apparatus and more particularly to a method and apparatus using magnetic reed switches as limit switches.

Limit switches for controlling moving mechanisms are in widespread use and the standard device used for this operation is the small relay or micro-switch. These devices are satisfactory for many applications but being basically mechanical devices are subject to break-down and malfunctioning. There are many applications especially in the nuclear reactor field where it would be most desirable to have a switch mechanism for controlling operating mechanism that has a much higher reliability factor than present devices especially the micro-switch.

It is therefore an object of the present invention to provide a switching mechanism that is rugged, inexpensive, and most particularly, reliable to a very high degree.

It is another object of the invention to provide a switching mechanism that has a very small reset travel distance of the switch.

It is another object of the invention to provide a switching mechanism that can be readily employed to sense quite accurately a position of a mechanical part through the wall of a pressurized system.

Applicant has found that these and other objects of the invention may be achieved by using a magnetic reed switch as the switching device and operating this reed switch by means of a magnet connected to the operating mechanism and moveable inside a shielding tube such that when the magnet is inside the shielding tube the reed switch is open and when the magnet protrudes beyond the end of the shielding tube the reed switch contacts will close. It has been found that a very small movement of the protruding magnet will result in a relatively large change in the magnetic field strength in the location of the reed switch.

In drawings which illustrate an embodiment of the invention:

FIGURE 1 is a cross section of the switching device in idealized form showing the magnet and the reed switch in the open or non-operative state,

FIGURE 2 is a cross section similar to that of FIG- URE 1 but with the magnet protruding from the shield tube and the reed switch in the closed or operative state,

FIGURE 3 is a view partially broken showing a limit switch arrangement for a hydraulic cylinder, and

FIGURE 4 is a view of the magnet operating device of FIGURE 3. 7

Referring to FIGURE 1 a permanent magnet 1 is fixed on thev end of a rod 2 which would be connected to the mechanism the motion of which it is wished to control. The magnet is positioned, e.g. cemented in position with epoxy resin, inside a shielding tube 3 having an open end 7 adjacent a magnetic reed switch 4 having reed contacts 5 and end leads 6. In the position shown the magnetic flux 8 of the magnet is contained by the shield tube 3 and little or no fiux passes through the reed switch. In FIGURE 2 the magnet protrudes beyond the end of shield tube 3 and in this case some otthe lines of magnetic flux 8 pass through the reed switch 4 causing the contacts 5 to close. End leads 6 would be suitably connected to external electro-mechanical"operating devices (not shown) for stopping the movement of rod 2. Because the reed switch is closed at a relatively precise position of the magnet 1 as it comes out of the end of the shield tube 3, it will be seen that a switch device that may be employed as a limit switch is achieved. This arrangement provides a high rate of change of magnetic field strength in the neighbourhood of the reed switch which gives the desirable small reset travel of the switch.

FIGURE 3 shows the device of FIGURES 1 and 2 adapted to an actual working device in this case a hydraulic or pneumatic cylinder 11 having end plates 12 and 13 and tie rods 14. Piston rod 15 is connected to yoke 16 which in turn would be connected toa mechanism to be operated. A bracket 17 attached between piston rod 15 and yoke 16 carries a pushrod 18 on the end of which is fixed a permanent magnet 24 (see FIGURE 4). Pushrod 18 works inside a tube 19 mounted on brackets on end plates 12 and 13. A shielding tube 20 encircles tube 19 for a predetermined portion of its length. A second tube 21 is mounted adjacent and parallel to tube 19. Reed switches 22 and 23 are mounted inside tube 21 at predetermined locations and would be connected by end leads (not shown) to external circuits and operating devices. The length of shield tube 20 and the location of the two reed switches are chosen such that the reed switches will operate at the desired position in its travel of the magnet 24, push rod 18 and thus piston rod 15.

Tubes 19 and 21 are made of non-magnetic material, for example, copper. Shielding tube 20 is made of a terromagnetic material. In an actual device built this tube was a split tube formed from a sheet of transformer plate.

The arrangement of FIGURE 3 is by way of example only. Other limit switch arrangements using the invention will readily suggest themselves. Reed switches having very reliable operating characteristics are readily available commercially. These rugged devices make the present invention adaptable to many conditions where the standard limit switches might not give the necessary performance and reliability.

The permanent magnet described above might be replaced by an electromagnetic device such as a coil wound on a core but this would require the use of a power source for energization which would probably make its use less attractive than the permanent magnet.

What is claimed is:

1. Magnetic switching apparatus for limiting the reciprocal travel between first and second relatively moving mechanical components comprising:

(a) a first magnetic reed switch having normally open contacts connected to said first component at a first location,

(b) a second magnetic reed switch connected to said first component at a second location,

3 4 (c) a shielding tube made of magnetic material fixed determined limits of reciprocal motion between the to said first component and having its open ends said components. positioned adjacent said first and second reed switches, and References Cited (d2 adpgrmanint rtnagne; i g zl 3 3 2 szfd r nz 5 UNITED STATES PATENTS S n n e mam er Sal p e I g 2,976,378 3/1961 Goddard 335-1553 x net adapted to travel within said shleldlng tube on relative motion between said components, 31253338 5/1966 Bulfnette 335-153 X (e) the location of said reed switches and the length 3,330,016 7/1967 Smth 335*205 X of said shielding tube being chosen such that the said magnet will protrude alternately from the ends of 10 BERNARD GILHEANY Exammer' the said shielding tube to cause the contacts of the R. N. ENVALL, JR., Assistant Examiner. respective switches to close abruptly at the pre-

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