GB2096421A - Position transducer for fluid actuated ram - Google Patents

Abstract

In a fluid actuated ram device comprising two co-operating elements, one co-operating element (the ram 3) is provided with a plurality of magnetically coded tracks (e.g. binary gray code) and the other co-operating element with means (e.g. inductive coils, 14) for reading each magnetically coded track to determine uniquely the position of one co-operating element relative to the other. <IMAGE>

Description

SPECIFICATION Position transducer for fluid actuated ram This invention relates to fluid actuated ram devices and in particular to fluid actuated ram devices with a position transducer for determining ram extension.

For the purposes of this specification a ram device is defined as comprising a pair of cooperating elements consisting of a casing defining a pressure cylinder and a piston forming a seal against the wall of the cylinder and slideably disposed therein. The piston is associated with a ram and the ram and piston may be formed integrally or may be distinct parts connected together.

In orderto exert automatic control over a fluid actuated ram it is desirable to be able to sense accurarely the longitudinal position of the ram relative to the cylinder casing at all times.

One type of position transducer known to the Applicant consists of a plurality of sensors disposed longitudinally of the cylinder which sense the passage of the ram. Such a system produces a digital output which varies in accordance with the axial position of the ram in the cylinder. However the accuracy to which the transducer is able to work, ie its resolution, depends upon the number of sensors per unit length axially of the cylinder and for high resolution a large number of sensors will be needed. The resolution of the transducer will thus be limited by the physical size of the sensors and hence the density at which they can be placed along the cylinder, but more frequently cost will be the limiting factor.

A second known type of position transducer comprises an arm secured at one end to the ram such that it moves with it longitudinally of the cylinder, the other end forming part of a potentiometerwhich can hence produce an analogue voltage proportional to the extension of the ram rod. This type of transducer is beset by several disadvantages, firstly in that it increases the overall dimensions of the ram. In many industrial applications space is at a premium and this is particularly so for fluid actuated rams where in general only relatively small ram extensions are possible even in quite large rams.

A second disadvantage is that the arm constitutes a further moving part and as such is susceptible to mechanical failures. This leads to further disadvantages in that the transducer requires frequent and possibly skilled maintenance.

The Applicant's copending UK Patent Application No 8014077 describes a position transducer for a fluid actuated ram wherein a leadscrew is rotated by the longitudinal movement of the ram to provide a variable analogue voltage to determine th extension of the ram. Although completely contained in the cylinder of the ram device and hence not appreciably increasing its size, this transducer does again rely on mechanically moving parts entailing some of the disadvantages previously discussed.

US patent No 3956973 describes a ram device having a ram comprising a piston of magnetic material with a helical groove formed on its surface and covered with a layer of non-magnetic material. The non-magnetic covering may be in the form of a shrink fitted sleeve or may be electro-deposited on to the helical groove. The helical groove provides a regular magnetic discontinuity which may be sensed by a magnetic transducer to provide an indication of the position and velocity of the ram as it moves relative to the transducer.

This provides a method of determining the position of the ram without unduly increasing the working dimensions of the ram device and furthermore without introducing any additional moving parts with corresponding mechanical failures that this may bring.

However the helical groove will now provide an absolute indication of the position of the ram as each cycle of the groove will be identical. The transducer will not be able to distinguish one part of the ram from another but will merely be able to determine howfarthe ram has moved from its last known position. Therefore the transducer must be 'zeroed' each time it is switched on.

The present invention seeks to provide a fluid actuated ram with an absolute positional transducer capable of a high degree of accuracy which does not add unduly to the overall dimensions of the ram and which is free from moving parts so as to reduce maintenance and consequentially to increase the operating life of the ram.

According to the present invention, there is provided a fluid actuated ram device as hereinbefore defined, one of the two co-operating elements having a plurality of magnetically coded tracks thereon, each track extending longitudinally in the direction of intended relative movement between the cooperating elements, the tracks being jointly coded such that each of a plurality of longitudinally spaced positions along the tracks is uniquely identified, and means carried by the other co-operating element for reading the code of each of the magnetically coded tracks so that the position of one cooperating element relative to the other cooperating element may be uniquely determined.

Normally it will be more convenient to provide the magnetically coded tracks on the ram, the means for sensing then being mounted in fixed relation to the cylinder body.

In order to contain the actuating fluid within the pressure cylinder an adequate fluid tight seal must be provided between the ram and the cylinder for all possible extensions of the ram. To facilitate this the surface of the ram must be smooth and a particularly suitable material for the surface is chromium.

Preferably the code is a binary code. The binary coding for each longitudinal position of the ram can be regarded as a series of 0's (off state) and l's (on state). The method of coding will preferably be such that only one binary digit will change when the ram moves from one longitudinal position to an adjacent longitudinal position. This is to ensure that spurious signals do not arise due to transitional states caused bytwo digits changing at slightly differenttimes as the ram moves from one position to the next For example a change from binary 101 to binary 110 may, if the last two digits do not change simultaneously, produce a transitional state of 100 or 111 which will give incorrect information about the position of the ram.If only one digit is allowed to change in any adjacent movement of the ram then no transitional states may occur. This type of coding is known as an unambiguous code.

The binary coding may represent its decimal equivalent either as natural binary or in binary coded decimal (BCD). In natural binary coding the complete decimal figure is translated into a binary number (eg 477 = 111011101 ) whereas in BCD each decimal digit is individually translated into a 4 digit binary code (eg 477 =0100,0111,0111).

Where BCD code is used the coding is preferably cyclic. A cyclic code is one in which a change from the 4 digit binary coding representing decimal 9 to that representing decimal 0 will produce a change of only one binary digit. In this way the binary coding representing the decimal values 0 to 9 may be repeatedly read without the possibility of producing spurious transitional states.

Conveniently the coding used for the magnetic track is binary Gray code or alternatively binary Gray excess - 3 code. Binary Gray excess-3 code is a cyclic code and is also unambiguous producing no transitional states.

The magnetic tracks must be recorded on to the ram in such a way as to be unaffected by and to have no adverse affect on the normal operation of the ram device.

The magnetic code can be formed in a ram of steel or other magnetic material by permanently mag netising different zones within each track with differ ent magnetic intensities.

The magnetic field used to magnetise these zones must normally be relatively strong. This is firstly due to the fact that there is normally a layer of chromium on the surface of the ram which, having relatively poor magnetic properties, will cause a dampening effect on the magnetic field reaching the magnetic material under the surface of the ram. Secondly it is necessary to use a strong magnetic field to magnetise the ram so that the magnetisation levels will remain adequate under conditions of normal operation of the ram device.

Alternatively a ram of steel or other magnetic material can have its surface relieved to differing degrees at differing zones along each track, a cylindrical surface of the ram being made up by nonmagnetic material such as chromium, the trackthus comprising zones of differing magnetic reluctance.

Parts of the ram material may be removed by photo-etching before coating the ram with chromium. The ram is subsequently ground to form the smooth surface required for efficient operation of the ram device. This produces a ram of magnetic material coated with a layer of chromium with a smooth surface but of a non-uniform depth. This provides a verying magnetic reluctance along the length of the rod which may be sensed by an appropriate reading means.

Conveniently the means for reading the code of each magnetically coded track is an inductive coil.

Preferably the coil will notcontactthe ram butwill be disposed close to the ram, so as to fall within the magnetic field of the track on the ram. Where the track is coded with zones of different magnetic intensities the coil is so orientated relative to the ram that the zones pass in succession across the coil during arm movement so as to induce in the coil a varying electromotive force. Where the track is coded with zones of different magnetic reluctance a current is passed through the coil to produce a constant magnetic field. As the ram moves relative to the coil the zones of differing magnetic reluctance pass across the coil and induce in it a varying electromotive force.

There may be situations in which the ram cannot be prevented from rotating a small amount relative to the cylinder, eg due to the looseness of the connection between the ram or cylinder and machinery which is actuated thereby. As there is a plurality of tracks, each track must be 'read' by a separate coil and care must be taken to ensure that the coils do not overlap on to adjacent magnetic tracks. This problem may be obviated by using coils which read relatively narrow track bands as compared with the width of the magnetic tracks.

Alternatively one magnetic track may be coded as a reference track which is distinguishable from all other tracks and serves to enable a sensor to computt the rotational position of the ram and hence to take account of any misalignment. The magnetic tracks must be spaced such that it is not possible for a coil to overlap two magnetic tracks as this will superimpose the code from one track on to that of another and produce eroneous readings.

Additionally there may be provided electronic processing means adapted to receive the emf's from the reading coils, analyses the emf's and to exercise control over a driving fluid system which moves the ram relative to the cylinder body. The processing means can be used to determine both the position and the velocity of the ram and hence can be used to pre-programme the ram device to perform repetitively a sequency of movements.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which, Figure 1 is a schematic sectional elevation showing a fluid actuated ram device according to the invention, Figure 2 is a sectional elevation of part of a ram device having one type of magnetic coding according to the invention, Figure 3 is a sectional elevation of part of a ram device having a second type of magnetic coding according to the invention, Figure 4 is a diagram showing a representation of a natural binary code, and Figure 5 is a diagram showing a representation of a binary Gray code.

As shown in figure 1, a hydraulic ram device comprises a pair of cooperating elements in the form of a pressure cylinder 1, and a piston 2 slideably constrained and forming a seal within the cylinder 1.

Integral with the piston 2 is a ram 3 which projects out of the cylinder 1. The piston 2 divides the cylinder into two chambers 6, 7, one chamber 6 on the same side as the ram 3 (on the right hand side of the piston as seen in figure 1) and a chamber7 on the opposite side of the piston (on the left hand side as seen in figure 1).

At the end of the cylinder through which the piston ram 3 projects there is a seal 4 which prevents fluid escaping from between the cylinder and the piston rod.

Constant high pressure fluid is directed through a line 5 into chamber 6. An Accumulater (not shown) may be connected into the line 5 to keep the high pressure in the line relatively constant.

Chamber7 in the cylinder communicates through a line 8 with a main valve 9 which is supplied with high and low pressure fluid through lines 10 end 11 respectively.

The surface of the ram 3 is coated with a thin layer of chrnmium as shown at 12. Magnetically coded tracks shown schematically at 13 are recorded under the surface of the piston rod and are reed by a plurality of reading coils 14 disposed around the circumference of the piston rod end close to the surface thereof. The reading coils 14 are securely mounted on the pressure cylinder so that as the piston rod moves relative to the cylinder the magnetic trad 13 pass close to the reading coils which hence supply a varying electrical signal to an electronic processing unit 15.

The electronic processing unit 15 processes the information from the reading coils and provides a signal to control a miniature solenoid valve 16. The valve 16 is supplied with high and low pressure fluid through lines 17, 18 respectively and provides &commat; a fluid pressure signal to the main valve 9 through fluid pressure lines 27, 28. Depending on the signal from the solenoid valve 16, main valve 9 supplies high or low pressure fluid to the chamber 7 to move the ram in or out of the pressure cylinder. The electronic processing unit 15 may be programmed to cause the ram to carry out a sequence of movements most often repetitively.

Figure 2 shows a section through a piston in which the magnetic tracks have been recorded by photoetching, the section being taken through, and showing the profile of one trad For the purpose of clear illustration, the depth of the etching is shown greatly exaggerated. The piston rod is shown generally at 20 and comprises an inner core 21 of steel or other magnetic material covered with a layer of chromium 22. The core has a surface 23 which has been photo-etched to give a varying relief in the form of a plurality of longitudinal tracks bearing a suitable code eg a cyclic binary code.The chromium layer 22 is electrodeposited on to the steel core and is then ground to give a smooth external surface 24. Thus the depth of the chromiurn layer will vary from place to place along the piston rod and therefore the magnetic reluctance of the rod will vary accordingly.

This varying magnetic reluctance is sensed by a strongly inductive coil 25 positioned close to but not contacting the surface 24, with the axis of the coil parallel to the surface. One such coil is provided for each track. A current I is passed through the coil 25 to setup a magnetic field in the region of the piston rod.

As the piston rod moves relative to the coil regions of differing magnetic reluctance will pass close to the coil 25 and induce in it a varying electro-motive force. This varying emf is fed to the electronic processing unit 15 (not shown in figure 2) and used to compute positional information about the piston rod.

Figure 3 shows an alternative embodiment of piston rod in which a uniformly cylindrical steel core 30 is covered with a uniform layer of chromium 31 (whose thickness is shown somewhat exaggerated).

The layer 31 may be most easily formed by electrodeposition and the external surface 32 subsequently ground smooth. A suitable magnetic coding may be recorded along each of a plurality of lon gitudinal tracks along each track individually a recording coil (not shown) carrying a variable but high current. This high current density produces a strong magnetic field permanently to magnetise differing regions of the piston rod to different magnetisation levels.

These coded regions of varying magnetisation levels are read by inductive coils 33, one for each traces, positioned close to but not contacting the surface 32 of the piston rod. Avarying emf will be induced in each coil 33 as the piston rod moves relative to it causing the regions of differing magnetisa- tion levels to pass adjacent the coil. ps the magnet- isation levels are permanently recorded on to the piston rod it is not necessary to passe small current through each inductive coil 33 as in the method of reading the coding in the embodiment shown in Fig ure 2.

The magnetisation levels must be high enough to ensure that they can be read through the layer of chrnmium 31 despite its poor magnetic properties.

Also the levels must not be significantly decreased by long periods of normal operation of the ram.

Alternatively periodic re-recording or boosting of the magnetisation levels may become necessary.

FReferring to Figures 4 and 5 there is shown a diag dramatic representation of two types of coding that may be recorded on to a ram. In Figures 4 and 5 there is shown the natural binary and binary Gray codes respectively dor the decimal values 450 to 511 in increasing value from right to left as shown in the Figures. Binary '1' is represented by shaded areas as shown at 50 and binary '0' is represented by unshaded areas as at 51. The shaded areas 50 may be areas of high magnetisation as permanently magnetised by a recording coil as in Figure 3 or alternatively they may be less relieved areas as in Figure 2.

The magnetic tracks run horizontally as seen in Figures 4 and 5 and Sand are such that the most signific- ant digit 52 is towards the top of the figure and the least significant digit 53 is towards the bottom. As the ram moves horizontally each track will pass across its respective stationary reading coil (not shown) and the coding presented to each coil will change. With a natural binary code there are positions (as shown for example at 54 in Figure 4) where more than one digit will change in the movement to an adjacent position. However as can be seen from Figure 5, with binary Gray code there is no point where two or more binary digits change due to movement to an adjacent position. Thus every position along the ram will have a unique coding and no ambiguity in the coding may arise.

To obtain the natural binary equivalent of a binary Gray code it is merely necessary to pass the Gray code through a bistable, most significant digit first.

It will be appreciated by the skilled reader that binary Gray code is just one cyclic and unambiguous code that may be used as positional coding for a fluid actuated ram. Other such codes may be usefully employed in fluid actuated rams without departing from the scope of the invention.

Claims (19)

1. Afluid actuated ram device as hereinbefore defined, one of the two co-operating elements having a plurality of magnetically coded tracks thereon, each track extending longitudinally in the direction of intended relative movement between the cooperating elements, the tracks being jointly coded such that each of a plurality of longitudinally spaced positions along the tracks is uniquely defined, and means carried by the other co-operating element for reading the code of each of the magnetically coded tracks so that the position of one co-operating element relative to the other co-operating elernent may be uniquely determined.

2. Afluid actuated ram device according to claim 1 wherein the magnetically coded tracks are on the ram and the means for sensing is mounted in fixed relation to the cylinder body.

3. Afluid actuated ram device according to claim 1 or claim 2 wherein the surface of the ram is formed of chromium.

4. Afluid actuated ram device according to any of claims 1 to 3 wherein the magnetically coded tracks are coded in a binary code.

5. Afluid actuated ram device according to any of claims 1 to 4 wherein the magnetically coded tracks are coded in an unambiguous code.

6. Afluid actuated ram device according to any one preceding claim wherein the magnetically coded tracks are coded in a cyclic code.

7. Afluid actuated ram device according to any one preceding claim wherein the magnetically coded tracks are coded in binary Gray code.

8. Afluid actuated ram device according to any one preceding claim wherein the magnetically coded tracks are coded in binary Gray excess-3 code.

9. Afluid actuated ram device according to any one preceding claim wherein the code on the mag netically coded tracks is formed by permanently magnetising different zones with in each track with different magnetic intensities.

10. Afluid actuated ram device according to any one of claims 1 to 8 wherein the code on the magnet ically coded tracks is formed by relieving a ram of magnetic material to differing degrees at differing zones along each track, a cylindrical surface of the co-operating element being made up by non magnetic material, the track thus comprising zones of differing magnetic reluctance.

11. A fluid actuated ram device according to claim 10 wherein the said magnetic material is steel.

12. Afluid actuated ram device according to claim 10 or claim 11 wherein the co-operating element is relieved to differing degrees by photoetching.

13. A fluid actuated ram device according to any one preceding claim wherein the means for reading the code of each magnetically coded track is an inductive coil.

14. Afluid actuated ram device according to claim 13 as appendantto claim 2 wherein the coil does not contact the ram.

15. Afluid actuated ram device according to claim 13 or claim t4as appendantto claim 10 comprising means for passing a current through the coil to produce a constant magnetic field.

16. Afluid actuated ram device according to any of claims 13 to 15 wherein each coil reads a relatively narrow track band as compared with the width of the magnetic track.

17. Afluid actuated ram device according to any one preceding claim wherein one magnetic track is coded as a reference track which is distinguishable from all other tracks whereby a sensor can compute the rotational position of the ram.

18. Afluid actuated ram device according to any one preceding claim wherein there is provided electronic processing means adapted to receive electronic signals from the reading means, analyse the electronic signals and exercise control over a driving fluid system which moves one co-operating element relative to the other co-operating element.

19. Afluid actuated ram substantially as hereinbefore described with reference to the accompanying drawings.