GB2195711A

GB2195711A - Piston and cylinder assembly with a position indicator

Info

Publication number: GB2195711A
Application number: GB08723460A
Authority: GB
Inventors: Kurt Stoll
Original assignee: Festo SE and Co KG
Current assignee: Festo SE and Co KG
Priority date: 1986-10-07
Filing date: 1987-10-06
Publication date: 1988-04-13
Anticipated expiration: 2007-10-06
Also published as: IT8722128A0; DE3634062A1; GB8723460D0; DE3634062C2; GB2195711B; IT1222816B

Abstract

A piston and cylinder assembly has a cylinder (1) in which a piston (5) movable in the axial direction (8) is housed. A position transducer (24) is fixed to and moves with the piston. This position transducer (24) is magnetically coupled to a permanent magnet (18) guided on the outside of the cylinder (1) in the region of its circumference for movement parallel to the axial direction (8). Outside of the cylinder (1) and to the side of the traversing path of the permanent magnet (18), there is mounted a swich (19) capable of contact-free operation by the magnetic field of the permanent magnet approaching its range. <IMAGE>

Description

SPECIFICATION Piston and cylinder assembly The invention relates to a piston and cylinder assembly with a cylinder in which an axially movable piston is housed, with a permanent magnet moving with the piston in the axial direction of the cylinder, and with at least one switch mounted on the outside of the cylinder for contact-free operation by the magnetic field of the approaching permanent magnet.

During the operation of piston and cylinder assemblies of this type, switching actions may be triggered in dependence on specific positions of the piston relative to the cylinder, for instance with the object of changing the state of the piston and cylinder assembly or of driving other equipment or machines adjacent to the assembly. A preferred application is the control of electrically operated valves by switch operation, in order to stop the piston at the end of its stroke or to initiate the reversal of its direction of movement. Known pistons have been carrying permanent magnets in the region of their circumferences, which magnets are designed to move with said pistons, and the switches have been located on the circumferences of the cylinders near the traversing path of the magnet moving with the piston.If the magnet is approaching the switch as the piston reciprocates, the switch as the piston reciprocates, the switch is triggered by the field of the permanent magnet, in particular by closing two normally open contacts by magnetic force. This existing construction is relatively complex and expensive, since the piston has to be secured against rotation in order to ensure the correct alignment between permanent magnet and switch and thus the required switching accuracy. There have been proposals to provide a rotating annular magnet in the region of the circumference of the piston, but such magnets are complex and expensive to produce. Irrespective of their construction, all such assemblies produced to date have the further disadvantage that a permanent magnet which has lost its power is virtually impossible to replace, any replacement, if at all possible, involving major costs.

The object of the invention is the creation of a piston and cylinder assembly of the type referred to above, which would be easier to maintain and be simpler and more cost-effective in production.

According to the present invention there is provided a piston and cylinder assembly comprising a cylinder in which an axially movable piston is housed, a permanent magnet guided externally of the cylinder in the region of the circumference thereof for movement with the piston in the axial direction of the cylinder the piston being connected with a position transducer which is arranged in the cylinder for movement with the piston and is magnetically coupled to the permanent magnet for traversing movement thereof, and at least one switch mounted externally of the cylinder for contactfree operation by the magnetic field of the permanent magnet, the switch being mounted at the side of the traversing path of the permanent magnet when viewed in the direction of traverse.

The permanent magnet is therefore no longer located within the cylinder, but outside in the region of its circumference. All that is required within the cylinder is a position transducer capable of magnetic coupling with the permanent magnet by means of its magnetic field. This position transducer may be a metal part or a metal disk transmitting the movement of the piston to the permanent magnet, thus causing the latter to move with the former. This offers the advantage that the permanent magnet itself can be relatively small and therefore cheap without affecting the quality of the coupling with the position transducer. The switch to be operated by the permanent magnet is also mounted in the region of the circumference of the cylinder, close by the traversing path of the permanent magnet.

This ensures a highly accurate switching action, because the approaching magnetic field can act on the switch without obstruction, without being screened and thus weakened by the cylinder wall as in known assemblies. The external mounting of the permanent magnet offers the further advantage of easy access for the replacement of magnets which have lost their power. Retro-fitting of existing piston and cylinder assemblies without a switching arrangement of this type does not present any problems either, in particular if the piston of such an assembly is made of metal and can itself be used as a position transducer.

The positioning of the transducer may be such as to have the advantage that the piston does not require to be secured against rotation, because switching accuracy is not affected by the rotation of the piston. In a further development, the disk representing the position transducer is preferably embedded, for instance integrally cast into the piston interior, so that the- external dimensions of the piston can remain virtually unchanged, resulting in a very compact assembly.

A very compact construction can be achieved when both the permanent magnet and the switch are mounted in close proximity to the circumference of the cylinder or even on the cylinder itself.

A further advantage of the piston and cylinder assembly is the possibility of arranging a plurality of switches on the cylinder, all of them preferably being adjustable in the axial direction of the cylinder. By this means, several switches can be operated in each stroke of the piston, resulting in the initiation of several switching functions. The switches may be mounted on a common guide or on individual guides as required, these being arranged near the traversing path of the permanent magnet.

Embodiments of the invention will now be described by way of example only, in which: Figure 1 is a diagrammatic representation of a fist embodiment of the piston and cylinder assembly according to the invention in the form of a side view and partial axial section; Figure 2 is a cross-section of the piston and cylinder assembly along ll-ll of Fig. 1: Figure 3 is a cutout of a further embodiment of the piston and cylinder assembly in the form of an axial section; and Figure 4 is a cross-section of the piston and cylinder assembly illustrated in Fig. 3 along IV-IV of Fig. 3.

This piston and cylinder assemblies in the embodiments illustrated in the drawing comprise a cylinder 1 or 1' of any cross-section desired, an annular cross-section being illustrated. This cylinder 1, 1' essentially consists of a cylinder tube 2, 2' with a cap 3, 4, 4' on both ends. (In the present specification, similar components of the two embodiments have identical reference numbers, those of the embodiment according to Figs. 3 and 4 being further identified by means of an apostrophe.

The cylinder 1, 1' contains a piston 5, 5', which may be designed without a piston rod or with a piston rod extending through it. The embodiment described here has a piston rod 7, 7' coaxial with the piston 5, 5' and the cylinder 1, 1' at one of its end faces 6, 6'.

This extends outwards through one of the cylinder caps 3, forming a seal.

The piston rod 7, 7' together with the piston 5, 5' is movable under guidance in the axial direction 8, 8' of the cylinder 1, 1', dividing the interior 9, 9' of the cylinder into two operating chambers 10, 11 or 10', 11', which are sealed against each other and which can be supplied with a pressure medium, in particular compressed air, though a port 14, 15, 15' in one of the cylinder caps, the same port being used for venting. The direction of piston movement is determined by suitable control of the pressure medium supply and the venting of the operating chambers.In the embodiment according to Fig. 1, the operating chambers 10, 11 or 10', 11' are sealed against each other by means of a continuous sealing ring 16 arranged on the circumference of the piston 5, while in the embodiment according to Fig. 3, two sealing rings 16', 16" are arranged at an axial distance from each other on the circumference of the piston 5'.

The piston 5, 5' is connected to a position transducer 17, 17', which is mounted in the interior 9, 9' of the cylinder to move directly and instantaneously with the piston rod. This position transducer will be described in detail later. It is attracted or can be magnetised by the magnetic field of a permanent magnet as a result of consisting of a material having these properties. Preferred materials would be metals, in particular steel, and the position transducer may either be made entirely of a material of this type, or substantially of a material which is not magnetisable but has a magnetisable material, such as particles, embedded in it.

This position transducer 17, 17' is magnetically, and thus contact-free, coupled with a permanent magnet 18, 18' mounted on the outside of the cylinder 1, 1' in the region of its circumference and movable under guidance in the axial direction 8, 8' of the cylinder.

Coupling is achieved by the position transducer being in the magnetic field of the permanent magnet, which permeates the cylinder tube 2, 2' consisting of a non-magnetic and non-magnetisable material. The magnetic force is sufficient to prevent any relative movement in the axial direction 8, 8' between the permanent magnet 18, 18' and the corresponding position transducer 17, 17'. As a result of this, the permanent magnet 18, 18' copies any reciprocating movement of the piston 5, 5' by moving externally along the cylinder.

In the region of the circumference of the cylinder 1, 1', there is further provided at least one switch 19, 19', which is arranged adjacent to the traversing path of the permanent magnet 18, 18' if viewed in its direction of traverse or in the axial direction 8, 8'. In the present context, "at the side", or "adjacent" means circumferentially close to the traversing path of the magnet, if viewed in the axial direction 8, 8' as in Figs. 2 and 4. The switch 19, 19', in the embodiment described a socalled reed switch, has two normally open contacts 21, 22 or 21', 22' inside a housing 20, 20' consisting of a material such as glass.

If the permanent magnet 18, 18' now passes the switch 19, 19' as it moves along with the piston 5, 5', the switch is operated by the magnetic field of the permanent magnet 18, 18' without contact, causing the contacts 21, 22 or 21' 22' to close under the action of the magnetic force. The position of the permanent magnet 18 and the piston 5 triggering the switch 19 is indicated at 23 in Fig. 1 by a dotted line.

In this way, signals may be emitted by the switch operated in dependence on the position of the piston and transmitted to other components, devices, machines etc. not illustrated in the drawing to trigger various functions. The switch 19, 19' may, for instance, be arranged near one of the caps of the cylinder and control the valves which in turn control the pressure medium supply or venting of the operating chambers 10-11', thus stopping the piston movement at the end of the piston stroke or reversing the direction of the piston movement. A preferred arrangement, which has been incorporated in the embodiments described here, comprises a switch 19, 19' which is continuously adjustable in the axial direction 8, 8' of the cylinder, thus enabling switching even when the piston is positioned in mid-stroke between the two cylinder caps.

There is further the possibility of arranging several switches (not illustrated) along the path of the permanent magnet 18, 18' in order to produce several switch signals during one and the same stroke of the piston.

In the embodiments illustrated, the position transducer 17, 17' is mounted directly on the piston 5, 5' and has the shape of a disk 24, 24' coaxial with the piston, its circumference being at a small distance from the cylinder bore. This arrangement offers the advantage that the piston does not require to be secured against rotation, since there is always a section of the position transducer opposite the permanent magnet 18, 18' at a small radial distance relative to the axis 8, 8', irrespective of any rotation of the piston. This ensures a precise, permanent magnetic coupling effect between the two components in question.In the embodiment according to Figs. 1 and 2, the disk 24 is-preferably detachably by means of screws 25-attached to the end face of the piston 5 opposite to the piston rod 7; adhesive bonding, too, would be feasible. In the embodiment illustrated in Figs. 3 and 4, on the other hand, the disk 17' is arranged within the piston 5' being virtually embedded therein. The piston may in this case be made of a plastic material.

In a further embodiment, which is here not illustrated, the position transducer is represented by the piston itself, which is made of a magnetisable material; irrespective of these possibilities, the piston 5, 5' may still be secured against rotation in the known manner. In this case, the dimensions of the position transducer could be limited to that region of the piston circumference which is radially opposite the permanent magnet.

In order to save material, the position transducer 17, 17' as illustrated in Figs. 1 to 4 could alternatively have an annular shape.

In the embodiment according to Figs. 1 and 2, the permanent magnet 18 is guided with sliding play on a guide rail 29 extending parallel to and at a distance from the external surface of the cylinder tube 2. In the embodiment illustrated, the guide rail has the shape of a rod with circular cross-section passing through the coaxial hollow cylindrical magnet 18. The guide rail 29 extends along the entire axial dimension of the cylinder, each end being attached to the cylinder 1 by a bracket 30. In the embodiment illustrated, the switch 19 is, relative to the cylinder axis 8, arranged radially exterior to the guide rail 29 or the traversing path of the magnet 18, being mounted, by means on a mounting element 31, on a further guide rail 32 extending parallel to the guide rail 29 for continuous adjustment and locking in any position. In this way, the switching point can be set as required.

In the embodiment according to Figs. 3 and 4, the permanent magnet 18' is held in a bracket 32, which may be block-like and by means of which it is guided directly on the cylinder 1'. For this purpose, a guide 33 extending along the axial direction of the cylinder is mounted on the outside of the cylinder tube, this being a precise dovetail guide in the embodiment illustrated. The switch 19' is in this embodiment offset relative to the traversing path of the permanent magnet 18' in the circumferential direction of the cylinder, if viewed in the direction of traverse of the permanent magnet 18', having approximately the same radial distance from the axis 8' as the permanent magnet itself.This design results in a very compact construction, because there are no components protruding far enough from the cylinder to cause an obstruction, a fact which is in particular due to the mounting of the switch on the cylinder circumference itself. The switch is, similar to that of the embodiment according to Fig. 1, also attached to a mounting element 31', which is, however, in contrast to the embodiment according to Fig. 1, carried in a guide 34 provided on the cylinder wall. The two guides 33, 34 are parallel to each other, being merely offset relative to each other by a certain arc length in the circumferential direction of the cylinder.

The piston and cylinder assembly according to the invention is simple and cost-effective in design and, moreover, extremely easy to maintain, because the permanent magnet is mounted externally on the cylinder in an accessible place, resulting in easy replacement when it loses its power.

Claims

1. A piston and cylinder assembly comprising a cylinder in which an axially moveable piston is housed, a permanent magnet guided externally of the cylinder in the region of the circumference thereof for movement with the piston in the axial direction of the cylinder the piston being connected with a position transducer which is arranged in the cylinder for movement with the piston and is magnetically coupled to the permanent magnet for traversing movement thereof, and at least one switch mounted externally of the cylinder for contactfree operation by the magnetic field of the permanent magnet, the switch being mounted at the side of the traversing path of the permanent magnet when viewed in the direction of traverse.

2. A piston and cylinder assembly according to claim 1, wherein the position transducer is represented by the piston.

3. A piston and cylinder assembly according to claim 1, wherein the position transducer is mounted directly on the piston.

4. A piston and cylinder assembly according to claim 1 or 3, characterised in that the piston is secured against rotation and the po sition transducer is arranged opposite the permanent megnet in the cylinder extending over a section of the circumference of the cylinder.

5. A piston and cylinder assembly according to claim 3, wherein the position transducer is a disk coaxial with the piston, the disk circumference being in the region of the bore of the cylinder.

6. A piston and cylinder assembly according to claim 5, characterised in that the disk is located inside the piston.

7. A piston and cylinder assembly according to claim 5, wherein the disk is attached to an end face of the piston.

8. A piston and cylinder assembly according to claim 3, wherein the position transducer is a coaxial ring mounted on the piston the circumference of the ring being in the region of the bore of the cylinder.

9. A piston and cylinder assembly according to any of the preceding claims, wherein the position transducer is made of a material attracted by the magnetic field of the permanent magnet, for example a metal such as steel.

10. A piston and cylinder assembly according to any of the preceding claims, wherein the position transducer is made of a magnetisable material.

11. A piston and cylinder assembly according to any of the preceding claims, wherein the permanent magnet is guided directly on the cylinder, the guide, for example a dovetail guide, being provided on the external cylinder wall.

12. A piston and cylinder assembly according to any of claims 1 to 10, wherein the permanent magnet is guided along a guide rail which extends parallel to the cylinder and is spaced therefrom and which is connected to the cylinder.

13. A piston and cylinder assembly according to any of the preceding claims, wherein the permanent magnet is mounted on a bracket for movement relative to the cylinder.

14. A piston and cylinder assembly according to any of the preceding claims, wherein the switch is a reed switch.

15. A piston and cylinder assembly according to any of the preceding claims, wherein the switch is mounted radially outwardly of the traversing path of the permanent magnet relative to the cylinder axis, the spacing of the switch from the cylinder axis being greater than that of the permanent magnet.

16. A piston and cylinder assembly according to any of claims 1 to 14, wherein the switch is offset relative to the traversing path of the permanent magnet in the circumferential direction of the cylinder when viewed in the direction of traverse of the permanent magnet.

17. A piston and cylinder assembly according to any of the preceding claims, wherein the switch is adjustable in the axial direction of the cylinder and lockable externally of the cylinder.

18. A piston and cylinder assembly according to claim 17, wherein the switch is mounted directly on the cylinder by means of a mounting element.

19. A piston and cylinder assembly according to claim 18, wherein the switch is mounted in a dovetail guide provided on the cylinder wall, which extends parallel to the guide of the permanent magnet.

20. A piston and cylinder assembly substantially as hereinbefore described with reference to Figs. 1 and 2 or Figs. 3 and 4 of the accompanying drawings.

21. Any novel subject matter or combination including novel subject matter disclosed in the foregoing specification or claims and/or shown in the drawings, whether or not within the scope of or relating to the same invention as any of the preceding claims.