GB2106984A

GB2106984A - Hydraulic ram with stroke indication

Info

Publication number: GB2106984A
Application number: GB08226870A
Authority: GB
Inventors: Harry Rosenberg; Karl-Heinz Plester
Original assignee: Gewerkschaft Eisenhutte Westfalia GmbH
Current assignee: Gewerkschaft Eisenhutte Westfalia GmbH
Priority date: 1981-09-24
Filing date: 1982-09-21
Publication date: 1983-04-20
Also published as: DE3137951A1; JPS5869997A; BE894469A; PL135861B1; AU8856582A; DE3137951C2; ES515918A0; FR2513370A1; ZA826989B; ES8306520A1; GB2106984B; PL238318A1

Abstract

A hydraulic ram 11 has a piston rod 22 reciprocable relative to a cylinder. The piston rod 22 is provided with a plurality of longitudinally- spaced permanent magnets embedded therein. Two sensors 25 and 26 are provided for sensing movement of the magnets, and hence relative movement between the piston rod 22 and the cylinder. The arrangement is such that any pair of adjacent magnets are sensed by different sensors 25 and 26. The magnets may be in single or double rows. In a single row, the outermost pole of each in the rod alternates. In a double row, one may have south poles the other north poles, or both rows may have the same poles. Magnets 30, 31 denoting end positions are provided. <IMAGE>

Description

SPECIFICATION Hydraulic ram This invention relates to a hydraulic ram having a plurality of magnets embedded in its piston rod and sensing means associated with its cylinder.

Hydraulic rams of this type are particularly useful as advance rams for longwall mineral mining installations.

A known type of hydraulic ram has a plurality of permanent magnets embedded in its piston rod, and a sensor fixed to its cylinder. The sensor incorporates an induction coil or a magnetic switch, and so senses the magnets as it passes over them during extension or retraction of the ram. Such a sensor will emit a control signal whenever a magnet is sensed, and these control signals can be used to measure a working stroke of the ram. Hydraulic rams of this type are used in particular for control purposes in longwall mineral mining installations. They serve, for example, for effecting a synchronisation (or follow-up control) of hydraulic rams such as, for example, the advance rams of mine roof support units, or the advance rams of the roof bar extensions of mine roof support units.

When hydraulic rams of this type are used as advance rams for underground conveying and winning installations, the permanent magnets can also be used for measuring the distances through which the rams are extended or retracted, and therefore for measuring the advance of the installation. In this system, a row of magnets can be arranged on the piston rod, so that, when the advance stroke of the ram takes place, the magnets move in succession past the sensor (which is arranged at the end of the cylinder). In so doing, they provide electrical control signals which can be counted by an electronic counter which provides a measure of the advanced distance.

Precise measurement of distance can be carried out only when signals are generated in a reliable manner, even when the distance between the permanent magnets on the piston rod is relatively short. Moreover, counting of the control signals should take place only in the prescribed direction of travel, that is to say in the direction of advance, and not during travel of the ram in the opposite direction. Such uncontrolled movements in the opposite direction of travel occur in the case of hydraulic rams used for advancing conveying and winning installations, for example as a result of what are called "breathing" movements, that is to say as a result of the conveyor being forced backwards towards the goaf side of the working by a passing winning machine, such as a plough.

Such a "breathing" movement is needed to create a path for the passage of the winning machine.

During these "breathing" movements, and the return movements occurring after the passage of the plough, a permanent magnet, arranged on the piston rod, may pass the sensor several times, so that several control signals are emitted, and these signals falsify the measurement of distance.

Also, in the case of hydraulic rams that are used for other purposes, return strokes may occur intentionally (or unintentionally), and these falsify the result of the measurement if only one measurement of distance in one direction of travel is required.

The aim of the invention is, therefore, to provide a hydraulic ram which does not suffer from these disadvantages.

The present invention provides a hydraulic ram having a piston rod reciprocable relative to a cylinder, the piston rod being provided with a plurality of longitudinally-spaced permanent magnets embedded therein, wherein at least two sensors are provided for sensing movement of the -magnets and hence relative movement between the piston rod and the cylinder, the arrangement being such that any pair of adjacent magnets are sensed by different sensors.

Advantageously, the sensors are fixed to the cylinder, and each of the magnets has one of its poles at the cylindrical surface of the piston rod.

In a preferred embodiment, the magnets are arranged in a single row, and wherein the magnets are arranged so that south poles alternate with north poles at the cylindrical surface of the piston rod along the row. In this case, there may be two sensors, the sensors being spaced apart in the direction of the longitudinal axis of the piston rod, and wherein one of the sensors is such as to detect only south poles, and the other sensor is such as to detect only north poles.

In another preferred embodiment, the magnets are arranged in a plurality of rows which extend parallel to the axis of the piston rod, the magnets of one row being longitudinally off-set with respect to the magnets of the adjacent row(s), and wherein there are as many sensors as rows of magnets, each sensor being aligned with a respective row of magnets. Advantageously, there are two rows of magnets, in which case the magnets in each row are spaced apart by the same distance, and the magnets in one row are off-set with respect to the magnets in the other row by half said-distance. The magnets of each row may be arranged with the same pole at the cylindrical surface of the piston rod. Moreover, all the magnets may have the same pole at the cylindrical surface of the piston rod.

Where there are two rows of magnets, the magnets of one row may have south poles at the cylindrical surface of the piston rod, and the magnets of the other row may have north poles at the cylindrical surface of the piston rod.

Advantageously, the hydraulic ram further comprises an additional set of permanent magnets embedded in the piston rod, the additional set of magnets being positioned so as to define an end position for the movement of the piston rod relative to the cylinder. Preferably, the additional set of magnets is positioned to define a fullyextended position of the piston rod with respect to the cylinder.

Where the magnets are arranged in a single row, there may be two magnets in said additional set of magnets, said two magnets being longitudinally spaced and having opposite poles at the cylindrical surface of the piston rod, and wherein said two magnets are arranged in said single row of magnets.

Where there are two rows of magnets, there may be two magnets in said additional set of magnets, said two magnets being aligned respectively with said two rows of magnets. In this case, where the magnets of the two rows have opposite poles at the cylindrical surface of the piston rod, one of said two magnets may have a south pole at the cylindrical surface of the piston rod, and the other of said two magnets may have a north pole at the cylindrical surface of the piston rod, said one magnet being in the row of magnets having north poles at said cylindrical surface, and said other magnet being in the row of magnets having south poles at said cylindrical surface.

Alternatively, where the magnets of the two rows all have the same pole at the cylindrical surface of the piston rod, the two magnets in said additional set of magnets may have the same pole at the cylindrical surface of the piston rod as the magnets in said two rows of magnets.

Moreover, the hydraulic ram may then further comprise a second additional set of magnets, said second additional set of magnets being positioned to define a fully-retracted position of the piston rod with respect to the cylinder. Advantageously, there are two magnets in said second additional set of magnets, said two magnets of the second additional set being aligned respectively with said two rows of magnets.

Each sensor may incorporate an induction coil or magnetic switch, so that the sensors emit electrical control signals whenever one of the magnets is sensed.

By arranging the magnets in at least two rows, and off-setting the magnets of each row with respect to the magnets of the adjacent row(s), it is not only possible to space the magnets at greater distances apart than was possible with known hydraulic rams (which helps to prevent the magnetic fields of the adjacent magnets overlapping and thus helps accurate sensing of the magnets) but it is also possible to eliminate incorrect measurements resulting from the abovementioned "breathing" or return-stroke movements. This can be achieved by providing the hydraulic ram with an electronic counter for counting the control signals emitted by the sensor, the counter incorporating a blocking circuit which prevents the counter counting two successive signals from the same sensor.Thus, if several control signals are delivered to the counter in succession by the same sensor sensing the same magnet, as would occur, for example, during "breathing" movements of the conveyor, the incorrect signals can be suppressed by the blocking circuit, so they are not registered in the counter, and an incorrect measurement of the advance distance is not made.

Where additional magnets are provided for defining the end positions of the movement of the piston rod, the arrangement may be such that the counter is re-set whenever the sensors detect the magnets of an additional set of magnets.

Preferably, each of the magnets is a cylindrical bar magnet which is arranged in a cup-shaped bush made of stainless, acid-resistant, nonmagnetic steel, each of said bushes being held by a force-fit in a respective radial bore drilled in the piston rod, each of the bushes having a closed outer end surface which lies flush with the cylindrical surface of the piston rod.

This invention also provides a hydraulic ram having a piston rod reciprocable relative to a cylinder and differential sensing means for detecting movement of the piston rod relative to the cylinder in a direction opposed to a predetermined direction, the sensing means being constituted by a plurality of longitudinally-spaced permanent magnets embedded in the piston rod and at least two sensors for sensing movement of the magnets and hence relative movement between the piston rod and the cylinder.

Three mine roof support unit advance mechanisms, each of which includes a hydraulic advance ram constructed in accordance with the invention, will now be described, by way of example, with reference to the accompanying drawings, in which: Figs. 1 to 3 are diagrammatic plan views of the three advance mechanisms, and each shows a hydraulic ram having different arrangements of magnets provided on its piston rod; Figs. 1 a to 3a are diagrams of the magnet arrangements of the rams of Figs. 1 to 3 respectively; and Fig. 4 is a transverse cross-section of the piston rod of one of the hydraulic rams of Figs. 1 to 3.

Referring to the drawings, Figs. 1 to 3 each show a scraper-chain conveyor 10, which forms part of an underground mining installation. A plough (not shown) is driven to and fro along the face side of the conveyor 10 so as to win mineral material, such as coal. The conveyor 10 can be advanced by means of hydraulic advance rams 11 in the direction indicated by the arrow A, that is to say towards the workface (not shown). The advance rams 11 are braced against hydraulic roof support units 12, each roof support unit being associated with a respective advance ram. Each of the roof support units 12 (the floor region only of one of which is shown in Figs. 1 to 3) has a roof bar (not shown) supported above a pair of spaced floor girders 1 3 by hydraulic props 14. Each of the props 14 is mounted on a respective floor girder 13 by means of a universal joint (not shown).

Each of the roof support units 12 is connected to the conveyor 10 by means of a respective advance mechanism which includes the advance ram 11 associated with that unit. Each advance mechanism also includes a pair of resilient relay rods 1 5. The relay rods 1 5 of each advance mechanism are connected at their front (face-side) ends to a head-piece 16, which is connected to the conveyor 10 by a pivot joint 1 7. At their rear (goaf-side) ends, the relay rods 1 5 are interconnected by a cross-piece 18, which is guided, for movement in the direction A, by means of guide elements 1 9 in slideways 20 provided on the floor girders 1 3 of the associated roof support unit 12.The advance ram 1 1 of each advance mechanism is connected to the crosspiece 1 8 of that advance mechanism by means of a pivot joint 21. The piston rod 22 of that advance ram 11 is connected to a transverse yoke 23, which interconnects the front ends of the two floor girders 1 3 of the associated roof support unit 1 2.

Advance mechanisms of this type are well known.

In order to advance the conveyor 10 in the direction A of face advance, the advance rams 11 are retracted, so that the conveyor is pressed forwards by the relay rods 15. As this happens, the piston rod 22 of each advance ram 11 is braced against the associated roof support unit 12, which is anchored in the longwall working as its props 14 are fully extended. In order to advance a given roof support unit 12 to follow up the advance of the conveyor 10, the advance ram 11 of that unit is extended, so that its piston rod 22 pushes that roof support unit forward in the direction of the arrow A.

Each pair of Figs. 1 and 1a,2 and 2a, and 3 and 3a, show a piston rod 22 provided with a plurality of permanent magnets embedded therein. Each pair of figures shows a different arrangement of the magnets. Thus, Figs. 1 and 1 a show a first arrangement in which the magnets are arranged in two rows I and II. In row I, the permanent magnets are disposed with their south poles S at the cylindrical surface of the piston rod 22, whereas in row II, the poles of the permanent magnets are reversed, that is to say their north poles N lie at the cylindrical surface of the piston rod. The distance X between adjacent magnets in each of the rows I and II is equal.The magnets of row I are offset from those of row II, in the direction of the axis of the piston 22, by a distance x 2 In the circumferential direction of the piston rod 22, the magnets of the two rows I and II are offset from each other by at least 900, and preferably by 1 800. In the latter case, the magnets lie at diametrically opposite sides of the piston rod 22.

As shown in Fig. 1, a pair of sensors 25 and 26 are arranged on a guide bush 24, which is provided at the end of the cylinder of the advance ram 11. The sensors 25 and 26 are aligned respectively with the two rows I and II of magnets, and so pass along the permanent magnets of the two rows I and II when the piston rod 22 is retracted and extended. The sensors 25 and 26 may each incorporate an induction coil, a magnetic switch or the like, so that, when one of the sensors passes over a magnet, it emits an electrical control signal. These control signals are fed, via lines 27 and 28, to an electronic counter 29. Two permanent magnets 30 and 31, which indicate end positions, are embedded in the piston rod 22 at its piston-carrying end.The magnet 30 is in row I, and the magnet 31 is in row II, and the N and S poles of the magnets 30 and 31 are reversed relative to the poles of the other magnets forming the rows I and II.

When the piston rod 22 is fully extended, the two magnets 30 and 31 are sensed by the sensors 25 and 26, which emit control signals which reset the counter 29. With the counter re-set, a subsequent advance movement of the conveyor 10 can be measured, by counting the number of signals emitted by the sensors 25 and 26 during the relative movement of the cylinder and the piston rod 22, as the ram 11 is retracted. This relative movement causes the sensors 25 and 26 to slide along the magnets of the two rows I and II.

As this happens, magnets having outwardly facing north poles are sensed alternately with magnets having outwardly facing south poles. The control signals emitted by the sensors 25 and 26 are counted by the counter 29, and are analysed by summation for the purpose of measuring the distance travelled by the cylinder of the advance ram 11, that is to say the distance through which the conveyor 10 advances.

Let is be assumed that, in the course of the advance operation, the sensor 25 moves into the position P shown in Fig. 1 a, after having travelled past the permanent magnets 32 having outwardly-facing south poles "upstream" thereof.

In this position, the sensor 25 has emitted an appropriate number of control signals, and these have been counted by the counter 29. If, in this position P, the cylinder of the ram 11 is pressed back (in the direction opposite that indicated by the arrow A) so that the piston rod 22 is extended -- which movement can occur, for example, as a result of the conveyor 10 being pushed back slightly by a passing coal plough then the sensor 25 again passes over the adjacent "upstream" permanent magnet 32. Ordinarily, this would result in the sensor 25 emitting a further electrical control signal which would be counted by the counter 29, and so lead to an incorrect measurement.In order to avoid such incorrect measurements, the counter 29 is provided with a blocking circuit (not shown) which is so designed that the control signals are counted only when these signals are received alternately from the two sensors 25 and 26, that is to say when magnets having outwardly-facing north poles are alternately sensed with magnets having outwardly-facing south poles. The counter 29 is accordingly so designed that, after each control signal from the sensor 25 is counted, the next signal from that sensor is blocked until a control signal from the sensor 26 has been counted. Thus, reliable measurement of distance becomes possible, irrespective of any "breathing" movements of the conveyor 10 that might occur.

Moreover, the arrangement of the permanent magnets in the two circumferentially-offset rows I and II means that the distance X between the magnets in the individual rows can be doubled as compared with the single-row arrangements of the permanent magnets of known advance rams.

If the spacing between the magnets in the two rows I and II is 100 millimetres, for example, "breathing" distances of about 40 to 45 millimetres are possible, without incorrect measurements occurring.

The embodiment shown in Figs. 2 and 2a differs from that of Figs. 1 and 1 a in that the permanent magnets of the two rows I and II are of like polarity, for example, they have outwardly facing north poles. The permanent magnets 30 and 31, which are arranged at the end of the piston rod 22, and serve to indicate the end position of piston rod extension and to re-set the counter 29, also have outwardly-facing north poles.

The same effect is achieved with this arrangement by blocking control signals from one of the sensors 25 or 26 until a control signal has been emitted by the other sensor 26 or 25. Thus, a blocking circuit (not shown) is provided in the counter 29 which ensures that control signals are only counted when they are delivered alternately from the sensors 25 and 26. If, for example, the sensor 25 is in the position P, then a control signal that is emitted, during a reverse stroke, by that sensor passing over the previously-traversed permanent magnet 32, is not counted; since, in this position, it is possible to count only a control signal emitted by the other sensor 26.

It will be understood that, in the arrangement shown in Figs. 2 and 2a, the permanent magnets in the two rows I and II may alternatively have outwardly-facing south poles.

It is also possible to provide the piston rod 22 with a pair of permanent magnets (not shown) adjacent to the free end thereof. These magnets are embedded side-by-side in the piston rod 22 and in alignment with the magnets in the rows I and II. Thus, when the advance ram 11 is fully retracted these magnets are sensed by the sensors 25 and 26, which emit control signals.

These signals indicate the fully-retracted end position of the piston rod 22. These signals also re-set the counter 29, so that the counter can be used to measure the distance through which the piston rod 22 is subsequently extended, and hence the distance through which the associated roof support unit 12 is advanced to follow up the advance of the conveyor 10. Thus, with this arrangement, the distance of advance can be measured during ram retraction and ram extension.

Figs. 3 and 3a iliustrate an arrangement in which the permanent magnets are in only one row. The permanent magnets are arranged on the piston rod 22 with alternating polar orientations, that is to say with alternate outwardly-facing north and south poles. The two permanent magnets 30 and 31, which indicate the end position of piston rod extension and re-set the counter 29, are arranged one behind the other on the piston-carrying end of the piston rod 22. The sensors 25 and 26 are correspondingly arranged one behind the other in the direction of the axis of the piston rod 22, so that, when the advance ram 11 is fully extended, they are aligned with the magnets 30 and 31. The sensor 25 is influenced only by magnets having outwardly-facing north poles, and the sensor 26 is influenced only by magnets having outwardly-facing south poles.In order to preclude incorrect measurements due to "breathing" movements or the like, the counter 29 is provided with a blocking circuit (not shown).

This blocking circuit is such that control signals are counted only when they are delivered alternately by the sensors 25 and 26.

This arrangement of permanent magnets having alternating polar orientations and arranged in one row along the piston rod 22, permits only relatively small "breathing" distances to be accommodated, and requires fairly wide spacing of the permanent magnets on the piston rod, so that the number of magnets provided along the length of the piston rod is limited to some extent.

Fig. 4 illustrates how the permanent magnets are embedded in the piston rod 22. This figure shows how one magnet is embedded in the piston rod 22, but it will be appreciated that all the other magnets are embedded in the same manner. Thus, the magnet 37, which is a permanent cylindrical bar magnet, is arranged with its poles disposed in the required manner, and with slight axial clearance, in a cup-shaped bush 38. The bush 38 is retained by a force-fit, in a radial bore 39 drilled in the piston rod 22, the open end of the bush bearing against the closed end 40 of the bore. The closed end surface 41 of the bush 38 lies flush with the cylindrical surface 42 of the piston rod 22, and is rounded to match this cylindrical surface. The thickness of the closed end of the bush 38 is less than that of its cylindrical wall. It may be 1 to 2 millimetres, for example. The bush 38 is made of a non-magnetic stainless and acidresistant steel.

The magnet arrangements described above with reference to Figs. 1 to 3, may of course, also be provided when the advance ram 11 is arranged with its piston rod 22 connected to the conveyor 10, and with its cylinder connected to the associated roof support unit 12. In this case, the piston rod 22 is extended from the cylinder when the conveyor 10 is advanced in the direction indicated by the arrow A. The permanent magnets 30 and 31 must, in this case, be arranged near the free end of the piston rod 22.

Claims

1. A hydraulic ram having a piston rod reciprocable relative to a cylinder, the piston rod being provided with a plurality of longitudinallyspaced permanent magnets embedded therein, wherein at least two sensors are provided for sensing movement of the magnets and hence relative movement between the piston rod and the cylinder, the arrangement being such that any pair of adjacent magnets are sensed by different sensors.

2. A hydraulic ram as claimed in claim 1, wherein the sensors are fixed to the cylinder.

3. A hydraulic ram as claimed in claim 1 or claim 2, wherein each of the magnets has one of its poles at the cylindrical surface of the piston rod.

4. A hydraulic ram as claimed in claim 3, wherein the magnets are arranged in a single row, and wherein the magnets are arranged so that south poles alternate with north poles at the cylindrical surface of the piston rod along the row.

5. A hydraulic ram as claimed in claim 4, wherein there are two sensors, the sensors being spaced apart in the direction of the longitudinal axis of the piston rod, and wherein one of the sensors is such as to detect only south poles, and the other sensor is such as to detect only north poles.

6. A hydraulic ram as claimed in claim 3, wherein the magnets are arranged in a plurality of rows which extend parallel to the axis of the piston rod, the magnets of one row being longitudinally off-set with respect to the magnets of the adjacent row(s), and wherein there are as many sensors as rows of magnets, each sensor being aligned with a respective row of magnets.

7. A hydraulic ram as claimed in claim 6, wherein there are two rows of magnets.

8. A hydraulic ram as claimed in claim 7, wherein the magnets in each row are spaced apart by the same distance, and the magnets in one row are off-set with respect to the magnets in the other row by half said distance.

9. A hydraulic ram as claimed in any one of claims 6 to 8, wherein the magnets of each row are arranged with the same pole at the cylindrical surface of the piston rod.

10. A hydraulic ram as claimed in claim 9, wherein the magnets of one row have south poles at the cylindrical surface of the piston rod, and the magnets of the other row have north poles at the cylindrical surface of the piston rod.

11. A hydraulic ram as claimed in any one of claims 6 to 9, wherein all the magnets have the same pole at the cylindrical surface of the piston rod.

12. A hydraulic ram as claimed in any one of claims 1 to 11, further comprising an additional set of permanent magnets embedded in the piston rod, the additional set of magnets being positioned so as to define an end position for the movement of the piston rod relative to the cylinder.

13. A hydraulic ram as claimed in claim 12, wherein the additional set of magnets is positioned to define a fully-extended position of the piston rod with respect to the cylinder.

14. A hydraulic ram as claimed in either of claims 12 to 13 when appendant to claim 5, wherein there are two magnets in said additional set of magnets, said two magnets being longitudinally-spaced and having opposite poles at the cylindrical surface of the piston rod, and wherein said two magnets are arranged in said single row of magnets.

1 5. A hydraulic ram as claimed in either of claims 12 and 13 when appendant to claim 7, wherein there are two magnets in said additional set of magnets, said two magnets being aligned respectively with said two rows of magnets.

1 6. A hydraulic ram as claimed in claim 15 when appendant to claim 10, wherein one of said two magnets has a south pole at the cylindrical surface of the piston rod, and the other of said two magnets has a north pole at the cylindrical surface of the piston rod, said one magnet being in the row of magnets having north poles at said cylindrical surface, and said other magnet being in the row of magnets having south poles at said cylindrical surface.

1 7. A hydraulic ram as claimed in claim 15 when appendant to claim 11, wherein the two magnets in said additional set of magnets each has the same pole at the cylindrical surface of the piston rod as the magnets in said two rows of magnets.

1 8. A hydraulic ram as claimed in claim 13, or in any one of claims 1 5 to 1 7 when appendant to claim 13, further comprising a second additional set of magnets, said second additional set of magnets being positioned to define a fullyretracted position of the piston rod with respect to the cylinder.

19. A hydraulic ram as claimed in claim 18, wherein there are two magnets in said second additional set of magnets, said two magnets of the second additional set being aligned respectively with said two rows of magnets.

20. A hydraulic ram as claimed in any one of claims 1 to 19, wherein each sensor incorporates an induction coil or a magnetic switch.

21. A hydraulic ram as claimed in any one of claims 1 to 20, wherein each of the magnets is a cylindrical bar magnet which is arranged in a cupshaped bush made of stainless, acid-resistant, non-magnetic steel, each of said bushes being held by a force-fit in a respective radial bore drilled in the piston rod, each of the bushes having a closed outer end surface which lies flush with the cylindrical surface of the piston rod.

22. A hydraulic ram as claimed in any one of claims 1 to 21, further comprising an electronic counter for counting the control signals emitted by the sensors.

23. A hydraulic ram as claimed in claim 22, wherein the counter incorporates a blocking circuit which prevents the counter counting two successive signals from the same sensor.

24. A hydraulic ram as claimed in either one of claims 22 and 23 when appendant to either claim 12 or claim 18, wherein the arrangement is such that the counter is re-set whenever the sensors detect the magnets of an additional set of magnets.

25. A hydraulic ram substantially as hereinbefore described, with reference to, and as illustrated by, Figs. 1, 1 a and 4, Figs. 2, 2a and 4, or Figs. 3, 3a and 4 of the accompanying drawings.

26. A hydraulic ram having a piston rod reciprocable relative to a cylinder and differential sensing means for detecting movement of the piston rod relative to the cylinder in a direction opposed to a predetermined direction, the sensing means being constituted by a plurality of longitudinally-spaced magnets embedded in the piston rod and at least two sensors for sensing movement of the magnets and hence relative movement between the piston rod and the cylinder.