GB2171847A

GB2171847A - Piston and cylinder transducers

Info

Publication number: GB2171847A
Application number: GB08604936A
Authority: GB
Inventors: Kurt Stoll; Gerhard Hihn
Original assignee: Festo SE and Co KG
Current assignee: Festo SE and Co KG
Priority date: 1985-03-01
Filing date: 1986-02-27
Publication date: 1986-09-03
Also published as: JPH0343488B2; GB2171847B; JPS61211507A; DE3546865C2; IT8619461A0; IT1204813B; GB8604936D0

Abstract

In a piston and cylinder transducer wherein the cylinder member (1) is closed at its two ends (23 and 16) by cylinder end caps (6 and 7) and wherein the piston (3) bears a permanent magnet (38), the cylinder (1) is made of a drawn section and on its outer face has at least one integrally formed longitudinal guide (11) extending from one end of the cylinder to the other for adjustably mounting at least one sensor (10) which is actuable by the magnet (38). <IMAGE>

Description

SPECIFICATION A piston and cylinder transducer The inyention relates to a piston and cylinder transducer and more particularly to a short stroke cylinder actuator the position of whose piston is detected gy at least one sensor on the outside of the cylinder responding to a magnet on the piston.

Conventional piston and cylinder transducers of this type commonly have a cast cylinder which is machined to specification and has the sensor screwed on it. However it is more especially in the case of large production runs that piston and cylinder transducers are comparatively high in price if produced in this way since it is necessary to produce a suitable casting mold for the cylinder in complex manner and such mold may only be used once over. A further shortcoming is that the sensor may only be adjusted to a small extent and normally such sensors are so attached to the outer face of the cylinder that they only respond to the terminal positions of the piston.

One object of the presint invention is to so improve piston and cylinder transducers of the initially noted type that manufacture may be simplified and cheapened.

A further aim of the invention is to provide a functionally reliable and readily adaptable piston and cylinder transducer.

A still further object of the invention is to devise such a piston and cylinder transducer that is compact and which provides a precise indication of the position of the piston along the whole length of the cylinder. Furthermore such indication is to be able to be precisely adjustable.

In order to achieve these or other aims in the invention a piston and cylinder transducer comprises a cylinder, cylinder end caps shutting off ends of the cylinder, a piston able to slide axially in said cylinder while making sealing contact with a bore thereof and defining at least one piston space in said cylinder, a piston rod connected with said piston and extending through at least one of the cylinder end caps, a permanent magnet means mounted on said piston, at least one sensor mounted on an outer periphery of the cylinder for actuation by said magnet means, said cylinder being made of drawn section with at least one integrally formed longitudinal guide for mounting said sensor so that same may be axially adjusted along the guide.With such a construction the designer now has the advantage of being able to adjust the one or more sensors steplessly in the length direction of the cylinder and then fix it or them at the desired location. This makes it possible not only to the detect the end positions of the piston but furthermore to indicate any position of the piston, at which the sensor generates an indicating signal succeptible of further processing. There is the advantage that the longitudinal guides for the sensor or the sensors may be mounted on the cylinder on or in its outer face even during the course of producing the cylinder so that there is a substantial drop in the costs of production of the cylinder. There is for example the possibility of manufacturing the cylinders of different length simply by cutting off from a running length of the drawn cylinder section.The longitudinal guides for the one or more sensors will then hardly need any finishing operations and there is also the possibility of simple mounting and adjustment of the sensor or sensors since the longitudinal guides extend along the full length of the cylinder and open at its ends. Dependent of the selection of the cylinder section, it is possible for one or more longitudinal guides to be provided on the outer periphery of the cylinder without this entailing any marked increase in the manufacturing costs. By the use of more than one longitudinal guide it is more especially possible to have short axial distances between two sensors without the sensors interfering with each other.All these beneficial effects may be produced in compact designs of piston and cylinder transducers and such compactness is more particularly desired when it is a question of use as short stroke piston and cylinder transducers, since such cylinders are frequently employed where there is a shortage of space.

Further features of the invention are noted in the claims.

In accordance with one such further feature of the invention, the cylinder has a square of other rectangular cross section and on three of its outer faces extending in its length direction there are respective longitudinal guides for sensors while the fourth outer face is provided with connection facilities for driving fluid lines.

Such a design is strictly in line with everyday needs, and it makes it possible for a number of sensors to be mounted while keeping to a comact configuration.

In order to simplify mounting of the sensors and at the same time to guide them precisely, the one or more longitudinal guides may be in the form of a guide groove in which the sensor is fitted with a running fit, or the one or more longitudinal grooves may be in the form of a guide rail formed on the outer face of the cylinder. It is particularly the provision of a guide in the form of a groove which enables a highly compact construction, since the longitudinal guides do not increase the transverse dimensions of the cylinder.

It is possible to produce the longitudinal guide or guides directly by drawing the cylinder. This reduces the complexity of manufacture of the cylinder to obtain the longitudinal guides.

The cylinder end cap remote from the piston rod may be in the form of a stamped component permanently mounted in the end of the. cylinder. This further feature of the invention involves a simply produced and simply mounted form of cylinder cap. The breadth of this cylinder cap as measured in the axial direction may with advantage be made small so that the possible length of stroke of the piston is hardly reduced thereby. The outcome is thus a compact piston and cylinder transducer, whose cylinder is not increased in length by the presence of the end cap and whose maximum stroke length is more or less identical to the length of the cylinder itself.The compactness of this arrangement is further enhanced if the cylinder end cap remote from the piston rod is connected with the cylinder without screw means, as for example by ultrasonic, laser, or other forms of welding, or by crimping the cap edge to the cylinder. Screw or other fastening means would tend to cause a disproportionate increase in the overall size of the unit. Furthermore, the provision of a seal between the cylinder and the end cap is brought about when it is mechanically fixed in place, i.e. no separate sealing operation is needed.

The magnet means may comprise an annular magnet that is mounted on or in the radial face of the piston coaxially. This makes it unnecessary to provide separate means for preventing rotation of the piston or of the piston rod, since whatever the position of the piston about its axis in relation to the cylinder, the magnet means will also be adjacent to the sensor as it moves past it. In accordance with a still further feature of the invention, the annular magnet is secured to the piston by bonding, this representing a rapid and simple method of mounting the annular magnet, while at the same time ensuring that the magnet is securely held in place.

The annular magnet may be in the form of a disk so as to make possible a precise switching operation as produced by the magnet cooperating with the sensor, inasfar as the surface of the annular magnet adjacent of the sensor is very small in area. This offers the advantage that the switching hysteresis between two switching operations of the sensor is reduced to a very minimum.

The piston may have a sieeve-like coaxial positioning head on one of its radial faces for the magnet to be mounted on, for example by the sleeve-like head being crimped radially outwards to hold the magnet. These features again represent advantageous further developments of the invention The piston may have an annular seal on its outer face in order to contact the bore of the cylinder and make sealing engagement therewith. The annular seal may be drawn onto an annular retainer bead which is integrally formed on the outer periphery of the piston so as to extend radially outwards from it.

Such features provide for the best possible sealing action between the piston and the cylinder. The annular magnet is so designed that is outer periphery radially projects beyond the outer periphery of the piston and comes to an end just short of the bore of the cylinder. This further feature of the invention insures that there is no interference with the field of the annular magnet.

It is possible for the cylinder to be so drawn that it has an connection ridge extending along its full length and serving for connection of driving fluid lines such as flexible hose, This connection ridge may have two connection holes adjacent to the end caps for the attachment of driving fluid flexible hose or other lines and so placed that they lead into two chambers in the cylinder which are separated from each other by the piston. These features of the invention make possible a secure attachment of the driving fluid lines and at the same time make possible a substantial reduction in the weight of the piston and cylinder transducer.There is the advantage that the thickness of the cylinder wall is reduced to a minimum as commensurate with the desired strength of the piston and cylinder transducer and it is along at the connections for the lines that the material of the cylinder is made thicker in the form of the connection ridge into order to provide a sufficient length of threaded holes to receive the line connectors. The ridge is produce by suitable design of the drawing tool when the cylinder section is manufactured so that the saving in weight is also accompanied by a reduction in costs when producing the connection ridge.

In accordance with a further feature of the invention the piston is screwed onto the piston rod. This simplifies the manufacture of the piston and the piston and the piston rod and offers more especially the advantage that the piston rod may be replaced, as for example when the piston and cylinder transducer is to be used in a different location involving the use of a piston rod with a different length.

In what follows the reader will be given a more detailed account of the invention referring to one working example and a possible modification thereof as illustrated in the accompanying drawings.

Figure 1 shows the embodiment of the piston and cylinder transducer in accordance with the present invention in perspective.

Figure 2 is a longitudinal section taken through the piston and cylinder transducer on the line ll-ll of Fig. 1.

Figure 3 is a front view of the piston and cylinder transducer looking in the direction of the arrow Ill in Fig. 1.

Figure 4 iilustrates a particularly convenient, modified form of attachment of an end cap on the cylinder.

The embodiment of the piston and cylinder transducer to be seen in Fig. 1 is in the form of short stroke cylinder actuator which is very compactly dimensioned. Such short stroke pis ton and cylinder transducers are more especially utilized if the amount of space available is so small as to prohibit the use of larger piston and cylinder transducers or however the piston or piston rod only has to have a small stroke, as form example for the operation of a valve to Figs. 1 and 2, the reader will see the cylinder 1 having a bore 2, i.e.

inner face, on which a piston 3 runs longitudinally so as to divide the cylinder into two cylinder spaces 4 and 5. The space in the cylinder is shut off at the ends by cylinder end caps 6 and 7, the cap 6 having a piston rod 8 extending through it. The piston rod 8 is joined to the piston 3.

In accordance with the invention the cylinder is made of drawn section, such section being characterized by a high strength and low costs of production. Furthermore the ultimate outer form of the cylinder may be determined while it is being produced, that is to say by simply selecting the desired form of drawing tool. This possibility is particularly advantageous in connection with the mounting of longitudinally adjustable sensors 10, see more especially Fig. 1, on the outer periphery 9 of the cylinder 1. It is possible to design the drawing tool so that longitudinal slots, or more properly speaking, grooves 13, for the sensors are produced on the outer periphery 9 so that hardly any operations are required for finishing the grooves.

The form of the piston and cylinder transducer illustrated in the figures has its cylinder 1 designed with an essentially square or other rectangular cross section or outline so that generally speaking the cylinder has the form of a block, rectangular prism or parallelepiped.

On its outer surfaces 12 the cylinder 1 has respective longitudinal guides 11 in the form of guide grooves 13 (or slots ) with a preferably dovetail cross section The sensors 10, of which only one will be seen in Fig. 1, possess a mounting foot 14 that is complementary to the respective guide groove 13 and is fitted therein with running play. It is best for the longitudinal guides 11 or the guide grooves 13 to be arranged in the middle of each outer face 12 running in the direction of its length, since it is here that the cylinder wall is thinnest and therefore the distance from the piston 3 running in the cylinder 1 is least.This small distance from the piston is advantageous since the sensors 10 are operated by a magnet means which will be described below and which is mounted on the piston; the switching effect, i.e. the strength of the magnetic field decreases with an increase in the wall thickness.

It is furthermore to be noted that the form of the guide grooves 13 is generally uncritical and the important point is rather more that they should be such that at least one sensor 10 may be mounted and slid therein. Furthermore, the exact design of the longitudinal guides 11 is not critical and they do not have to be in the form of grooves but might be configured as guide rails on the outer face of the cylinder so that the sensors might be siidingly mounted thereon (not illustrated).

The structure of the piston and cylinder transducer will now in what follows be described in more detail with reference to Fig. 2.

The cylinder end cap 7 on the end of the cylinder 1 remote from the piston rod of the piston 3 is produced by stamping and is fitted into the cylinder bore 2. To make this possible the cylinder bore 2 is made with a larger diameter at the end 16 of the cylinder for a depth that is generally the same as the thickness of the cylinder end cap. There is therefore a radial shoulder 1 7 between the rated diameter of the cylinder bore 2 and the larger diameter and the end cap 7 is located on such shoulder. In the state in which the end cap is fitted in place the outer radial face 18 of the cylinder cap 7 is generally flush with the end 16 of the cylinder 1.The cylinder end cap 7 remote from the piston rod is secured in the cylinder 1 by welding, more specially by ultrasonic or laser welding, so that an annular weldment or seam 22 results which on the one hand ensures a proper seal between the cylinder end cap and on the other hand secures it mechanically in place in the cylinder.

The preferred form of the cylinder end cap 7 remote from the piston rod is that of a disk so that it is smooth and without projections both on its inner and also on its outer side.

The second cylinder end cap 6 having the piston rod 8 extending through it is mounted in a widened section 24 of the bore 2 in the cylinder at the end thereof opposite to the end 23. The external diameter of the cylinder end cap 6 is here equal to the diameter of the wider section 24 and the transition from the wider section 24 to the cylinder bore 2 is in the form of a radial shoulder 25 on which the cylinder end cap 6 is located. The depth of the wider section 24 of the cylinder 1 as measured in the axial direction exceeds the thickness of the cylinder end cap 6 in this part so that the cylinder end cap 6 is let into the space of the cylinder. The cylinder end cap 6 is secured in place by a locking ring 26 fitting into a peripheral groove 27 in the wall of the wider section 24 so as to press the cylinder end cap 6 towards the shoulder 25.

Between the cylinder end cap 6 and the shoulder 25 there is an annular gasket 28 to provide a seal between the space 4 in the cylinder and the outside.

The cylinder end cap 6 with the piston rod 8 extending through it has generally the form of a bell with a central cylindrical boss 30, on whose end 31 a disk-like annular collar 32 is formed so as to be coaxial to it and whose outer edge part 29 is received in the said wider section 24 of the space in the cylinder.

The hollow cylindrical boss 30 projects out wards beyond the end 23 of the cylinder and has a coaxial passage 33 therethrough in which a guide bushing 34, in which the piston rod 8 runs, and a packing 35 are mounted, the latter serving provide a sealing effect between the wall of the passage 33 and the outer face of the piston rod 8. In this respect the shaft packing 35 is mounted in a wider section of the passage 33 in the end part 36 of the boss 30 opposite to end part 31. The shaft packing 35 is located by means of a locking ring 39.

In order to determine the two end of stroke positions of the piston there are abutment faces 37 and 37' on the inwardly facing sides of the cylinder end caps 6 and 7 for engagement by the radial faces of the piston, the lower one being referenced 15. The piston itself is preferably produced separately from its piston rod 8 and screwed to it so that, if the occasion should arise, it is a simple matter to replace the piston rod by another one. The piston carries a permanent magnet means 38 for cooperation with the sensor or sensors 10 on the outer face of the cylinder 1. Preferably, the sensors 10 are thus in the form of reed or inductive switches, as is indicated in Fig. 2.

If during its stroke the magnet means 38 moves past a sensor 10 the magnetic field of the magnet means will be responded to by the sensor 10, which produces an electrical signal able to be used for control functions.

The magnet means 38 is fixedly joined to the piston and the switching point of the sensor 10 or sensors may be adjusted by shifting them in the longitudinal direction of the cylinder 1 in the said longitudinal grooves 11. It is thus possible to indicate and monitor the end positions of piston.

In order to provide an efficient and at the same time compact sealing arrangement between the piston 3 and the bore 2 of the cylinder the external diameter of the piston 3 is made smaller than the diameter of the cylinder bore 2 and on the outer face 42 of the piston 3 there is a retainer bead 43 extending circumferentially around the piston coaxially and projecting radially therefrom. An annular packing 44 is mounted on the bead 43 and runs along the bore 2 of the cylinder. This annular packing represents an integral configuration with two seals rings and ensures an efficient sealing action between the piston and the bore 2 in both directions of motion so that there are two separate cylinder spaces 4 and 5.

The main component of the magnet means 38 is an annular magnet 45, which is coaxially mounted on the radial piston face 46 with the piston rod 8 extending from it. For attachment of the magnet 45 the piston 3 has a coaxial short sleeve-like head 47 on which the preferably disk-like annular magnet 45 is mounted coaxially and whose axial end is crimped over in a radially outward direction so that the annular magnet 45 is gripped thereby. This excludes the possibility of accidental detachment of the annular magnet from the piston 3.The annular magnet 45 extends beyond the outer periphery 42 of the piston 3 radially and comes to an end just short of the cylinder bore 2 so that the radial distance between the magnet means and the sensor is reduced to a minimum and accordingly the switching is highly precise: owing to the disk-like design of the annular magnet as noted there is only a very small amount of switching hysteresis, something that is more especially important in the case of short stroke cylinders. Owing to the high load on the annular magnet in the axial direction, which is due to the radial overhang past the outer periphery 42 of the piston 3, the magnet is to be additionally attached to the radial piston face 46, preferably by bonding.

In order to enhance the switching effect of the magnet without increasing its axial dimension (which would increase switching hysteresis) it is best to have a second annular magnet 48 which is let into the radial piston face 46 so that it is flush with such surface. Accordingly this second annular magnet 48 contacts the piston side of the magnet 45 with it forms a further part of the magnet means 38.

Owing to the presence of the annular magnet 48 set in the piston the magnet means has high magnetic force which is conducted by the disk-like. annular magnet 45. Accordingly the magnet means has a low switching hysteresis owing to the small axial thickness of the disklike annular magnet 45. It would naturally furthermore be possible to replace the disk-like annular magnet 45 by a magnetisable metal disk in order to conduct the magnetic field of the sunken annular second magnet towards the sensors 10. It is more especially preferred to manufacture the annular magnets of oxide material.

For actuation of the piston and cylinder transducer, which is double acting, there is a screw threaded connection hole 49 and 49' leading into each of the cylinder spaces 4 and 5 so that fluid under pressure may be admitted to and released from the cylinder spaces via pressure hose or other pressure line. Since for the secure attachment of such pressure fluid lines the length of screw thread has to be greater than the wall thickness of the cylinder wall, while on the other hand the such thickness should be limited, it will be seen more especially from Fig. 1 that the fourth outer face 50 of the cylinder 1, which is free of any longitudinal guide or groove, has an integral connection ridge 51 extending in the length direction of the cylinder from one end thereof to the other so that at such ridge the wall thickness of the cylinder 1 will be substantially greater than elsewhere. The connection holes 49 and 49' are made through this connection ridge 51 into the space inside the cylinder 1. There is then as sufficient thickness pf wall material to ensure secure fixation of the pressure fluid lines without mitigating against a compact configuration of the piston and cylinder transducer of the present invention. The connection ridge 51 is preferably made when the cylinder 1 is drawn so that the expense of manufacture remains low and it is only necessary to produce the threaded holes 49 in the ridge 51. With respect to the longitudinal grooves 11 it is additionally to be noted that they naturally stretch along the full length of the cylinder 1 and are thus open at the two ends of the cylinder 1, just as the connection ridge 51 ends in the terminal planes of the cylinder.In the perspective view of Fig. 1 and in Fig. 3 showing the form of the piston and cylinder transducer it will be readily possible to see the square or other rectangular form of the cylinder 1, which on three of its outer faces 12 has lengthways grooves 11 and has the connection ridge 51 standing proud from its outer face 50. It will be clear that the attachment of the sensors 10 is possible in a very simple way and in fact it is only necessary to slip their mounting feet 14 into the respective longitudinal groove 11 at one end of the cylinder 1. In Figs. 1 and 3 there reader will be able to see threaded mounting holes 53 in the corners 52 of the cylinder 1 so that the same may be mounted in suitable machine components or the like.It will also be clear that the reason for making the cylinder 1 rectangular is that a circular cylinder outline would make it more harder to produce mounting holes in the cylinder so that it might then be necessary to use brackets in addition. Furthermore, the end of the piston rod 8 has an attachment hole 54 so that the parts to be driven by the piston and cylinder transducer may be attached to the piston rod 8.

In what follows an account will be given of a further manner of attachment of the end cap 7 remote from the piston rod in the cylinder, see Fig. 4. In this case the cylinder end cap 7 is mounted in the manner noted earlier in the wider section of the cylinder bore 2 and moved inwards until it contacts the annular shoulder 17. The end part 56 of the cylinder 1 is crimped inwards so that the cylinder end cap 7 has its edge embraced annularly and it is pressed against the annular shoulder 17.

The outer edge 55 of the cap 7 is chamfered.

It is then most expedient to weld the cap so that the crimped parts are fluid-tightly joined to the cylinder end cap 7.

Claims

1. A piston and cylinder transducer comprising a cylinder, cylinder end caps shutting off ends of the cylinder, a piston slidable axially in said cylinder while making sealing contact with a bore thereof and defining at least one piston space in the cylinder, a piston rod connected with the piston and extending through at least one of the cylinder end caps, permanent magnet means mounted on the piston, and at least one sensor mounted on an outer periphery of the cylinder for actuation by the magnet means, the cylinder being made of drawn section with at least one integrally formed longitudinal guide for mounting the sensor so that same may be axially adjusted along the guide.

2. A piston and cylinder transducer as claimed in claim 1, wherein the cylinder has the form of a rectangular prism, at least some of the outer surfaces of which, parallel to the axis of the cylinder, each have the longitudinal guide for the sensor, one of the outer surfaces being adapted for connection with driving fluid lines.

3. A piston and cylinder transducer as claimed in claim 1 or claim 2, wherein at least one of the guides is in the form of a guide groove in which the sensor is fitted with running play.

4. A piston and cylinder transducer as claimed in claim 1 or 2, wherein at least one of the longitudinal guides comprises a guide rail formed on an outer surface of the cylinder.

5. A piston and cylinder transducer as claimed in claim 3 or 4, wherein said at least one longitudinal guide is produced directly by drawing the cylinder.

6. A piston and cylinder transducer as claimed in any of the preceding claims, wherein the end cap remote from the piston rod is a stamped component permanently set in a wider end section of the cylinder bore.

7. A piston and cylinder transducer as claimed in claim 6, wherein the end cap remote from the piston rod is screwlessly connected with said cylinder.

8. A piston and cylinder transducer as claimed in claim 7, wherein the end cap remote from the piston rod is welded in place in the wider bore section.

9. A piston and cylinder transducer as claimed in claim 7, wherein the end cap remote from the piston rod is ultrasonically welded in place in the wider bore section.

10. A piston and cylinder transducer as claimed in claim 7, wherein the end cap remote from the piston rod is laser-beam welded in place in the wider bore section.

11. A piston and cylinder transducer as claimed in claim 7, wherein the end cap remote from the piston rod is crimped in place in the wider bore section.

12. A piston and cylinder transducer as claimed in any of the preceding claims, wherein the magnet means comprises an annular magnet coaxially mounted on a radial face of the piston.

13. A piston and cylinder transducer as claimed in claim 12, wherein the annular magnet is bonded to the piston.

14. A piston and cylinder transducer as claimed in claim 12 or 13, wherein the annular magnet is disk-like with a central coaxial opening and an axial thickness substantially less than the difference between its internal and external diameters.

15. A piston and cylinder transducer as claimed in any of claims 12 to 14, wherein the piston has a sleeve-like locating head formed thereon, on which the annular magnet is placed so as to surround the head.

16. A piston and cylinder transducer as claimed in claim 15, wherein the locating head overlaps the annular magnet by virtue of the locating head being crimped radially outwards onto the magnet.

17. A piston and cylinder transducer as claimed in any of claims 12 to 16, wherein the annular magnet has a larger outer diameter than the piston and extends essentially as far as a position just short of the bore.

18. A piston and cylinder transducer as claimed in any of claims 12 to 17, wherein the annular magnet is located on the radial side of the piston adjacent the piston rod.

19. A piston and cylinder transducer as claimed in any of claims 12 to 18, wherein the annular magnet is made of an oxide material.

20. A piston and cylinder transducer as claimed in any of the preceding claims, comprising an annular packing mounted on an outer peripheral face of the piston so as to make sealing contact with the bore.

21. A piston and cylinder transducer as claimed in claim 20, wherein the annular packing is fitted onto an annular bead on the outer periphery of the piston, the bead being formed integrally with the piston and projecting proud thereof towards the bore.

22. A piston and cylinder transducer as claimed in any of the preceding claims, wherein the cylinder is drawn with a longitudinal ridge formed on one side thereof to increase the wall thickness of the cylinder and to provide for the connection of pressure lines.

23. A piston and cylinder transducer as claimed in claim 22, having connection holes in the ridge leading into respective cylinder spaces separated by the piston.

24. A piston and cylinder transducer as claimed in any of the preceding claims, wherein the piston rod is detachably screwed to the piston.

25. A piston and cylinder transducer as claimed in any of the preceding claims, wherein the sensor comprises a reed switch.

26. A piston and cylinder transducer substantially as hereinbefore described with reference to the accompanying drawings.