More especially, the invention is concerned with determining the position of an inner member relative to an outer member, for example a piston slidable in a cylinder, and has application to a fluid power linear drive.

A common type of fluid power linear drive is a pneumatic piston/cylinder unit in which reciprocating movement of the piston in the cylinder is employed to carry out a desired operation via a force transmission member connected to the piston.

Currently a number of position measurement systems (position feedback cylinders) are available for determining the position of a piston in a cylinder. The majority of these systems use either a linear variable differential transformer (LVDT), linear potentiometer or magnetostrictive type position measurement device fitted concentrically inside the piston.

This requires the piston to be hollow and has many disadvantages both for manufacture and use of the piston/cylinder unit.

Firstly, the available piston area is reduced, reducing the available force from the cylinder. Secondly, the strength of the piston is severely reduced. Thirdly, manufacturing a hollow piston adds time and cost to the manufacturing process-typically, due to logistical reasons, hollow pistons are gun drilled and not made from hollow stock. Fourthly, the arrangement of the sensor in the piston necessitates taking the electronic connections through the cylinder wall or end cap, increasing the complexity of sealing the cylinder and increasing the number of possible leak paths.

The combined effect of these is that a position feedback piston/cylinder unit must be sized bigger to exert a similar force to a standard piston/cylinder unit. Added to the extra cost of the manufacturing process and complexity of seals, this increases significantly the cost of the known position feedback piston/cylinder units.

Position feedback piston/cylinder units may be rodded in which the force transmission member is a rod attached to the end of the piston or rodless in which the force transmission member is a carriage connected to the piston and slidable along the cylinder.

A further problem of the known measurement systems is that they are difficult to integrate into a rodless unit. For these units (and also for many rodded units) a"piggy back"approach has been employed in which a fixed part of a position sensor is strapped onto the outside of the cylinder and a moveable part is mechanically coupled to the force transmission member, i. e. the carriage of a rodless unit or rod of a rodded unit. This is generally a messy solution and leaves a sensitive measuring device open to the atmosphere.

The present invention has been made from a consideration of the foregoing problems and disadvantages of the known position measurement systems.

Thus according to one aspect of the present invention, there is provided apparatus comprising a cylinder having a piston slidable in the cylinder, and a position measurement device to determine the position of the piston, the device being arranged in or on a wall of the cylinder and magnetically coupled to the piston through the cylinder wall.

By magnetically coupling the position measurement device to the piston, the device can be arranged on the cylinder without any mechanical connection to the piston and the piston does not have to be hollow.

Moreover, as the device is no longer inside the cylinder, there is no change in the internal dimensions and therefore force output of the cylinder. As a result, the invention can be applied to both rodded and rodless cylinders.

Preferably, the position measurement device comprises a position sensor having a fixed, elongate part running the length of the cylinder and a movable part which can move linearly relative to the fixed part.

The position sensing device may be a magnetostrictive transducer but is preferably a linear potentiometer of the type having a fixed, elongate resistive element and, a fixed, elongate conductive element extending in spaced, parallel relationship thereto, and a follower magnetically coupled to the piston so as to be movable with the piston relative to the resistive and conductive tracks.

Electrical contact between the resistive and conductive elements is effected by the follower and the resistive element has connections which, when supplied with a set voltage input, will give a varying output depending upon the position of the follower and so indicative of the position of the piston within the cylinder.

In one embodiment, the electrical contact is provided through the follower bridging the elements as it moves along the cylinder to provide an indication of the position of the piston within the cylinder.

The follower bridging the resistive and conductive tracks may be in the form of a slider or carriage magnetically coupled to the piston so that the magnetic flux passes through the wall of the cylinder and, when the piston moves in the cylinder, it pulls the follower with it. For example, the piston and the follower may each be made of or provided with one or more permanent magnets, preferably a rare earth magnet (s).

Alternatively, one may be magnetic and the other may be magnetisable Preferably, the cylinder is made of a non-ferrous material having a wall thickness that allows effective magnetic coupling between the piston and the follower. For example, the cylinder may be made of aluminium. The magnets are preferably selected such that the magnetic flux through the cylinder wall is sufficient to maintain the coupling when the piston accelerates at its maximum speed. The follower preferably also has a low mass so that inertia at maximum acceleration is not sufficient to de-couple it from the piston.

Where the follower is in the form of a slider, friction between the slider and the cylinder wall may be reduced by using good surface finishes (lubricated or unlubricated). In a preferred arrangement, the surface of the cylinder wall is PTFE (polytetrafluoroethylene) anodised aluminium.

Alternatively, a bearing element or small air gap may be provided between the slider and the cylinder wall.

In one arrangement, the slider is located in a recessed portion of the cylinder wall. For example, the cylinder wall may be formed with a longitudinal groove to receive the slider. In this way, the groove reduces the thickness of the cylinder wall separating the slider from the piston and provides a channel for the slider to run along.

Preferably, the groove is shaped so that the cylinder wall separating the slider and the piston is of uniform thickness. The guide and slider may be provided with a protective cover secured to the cylinder. The cover may be PTFE anodised aluminium.

In another arrangement, the slider and guide are located within the cylinder wall. For example, the cylinder may be formed with a longitudinal cavity such that the slider and guide can be inserted from one end. In this way, the slider and guide are encapsulated providing added protection and enhanced robustness. Preferably, the cavity is shaped so that the cylinder wall separating the slider and the piston is of reduced, uniform thickness.

Where the follower is in the form of a carriage, friction between the carriage and cylinder wall may be reduced by providing the carriage with one or more roller elements. The roller elements and/or cylinder wall may be provided with good surface finishes (lubricated or unlubricated) to further reduce friction. The surface of the cylinder wall may be PTFE (polytetrafluoroethylene) anodised aluminium. The roller elements may be made of brass.

In one arrangement, the carriage is received in a groove or slot in the cylinder wall and is provided with contacts to bridge the resistive and conductive tracks as the carriage moves along the cylinder.

In another embodiment, electrical contact between the resistive and conductive elements is provided by single point pressure acting on the conductive element by the magnetic force attracting the follower towards,

for example, the piston perpendicularly to the longitudinal axis of the potentiometer.

In one arrangement, the follower is in the form of a sphere or a cylinder whereby, in use, the follower rolls along the fixed part of the position sensing device. The movable part is either magnetic or magnetisable and is preferably spherical. For example, it may be a steel ball.

Because the follower can roll relative to the fixed resistive and conductive elements, frictional drag between the fixed and moving parts is considerably reduced. This reduces or eliminates any hysteresis effect and also means that the magnetic coupling to the follower may be less strong.

In another arrangement, the follower is in the form of a small disc or "puck"that sits flat on the conductive element (i. e. does not roll), and preferably is made of or provided with a permanent magnet, preferably a rare earth magnet. Alternatively, it may be magnetisable.

Because the follower can slide, some damping is induced into the system and reduces the effect of"sine-waving"whereby, when a ball is used, it often partially slides as well as rolling producing a sinusoidal movement above and below the absolute position of the sensor.

It will be appreciated that the shape of the follower and strength of the magnetic coupling of the piston and follower can be modified to suit the size of the cylinder whereby balancing of damping and friction are effected to minimise the effect of hysteresis.

Preferably, the piston is connected to a force transmission member for transmitting movement of the piston for a desired end use. For example as an actuator for specific operations in a given application.

The force transmission member may be a rod connected to one end of the piston to project in a fluid-tight manner from the cylinder. Alternatively, the force transmission member may be a carriage positioned on the cylinder and connected to the piston via a salable opening in the cylinder wall.

Preferably, the cylinder is pneumatically or hydraulically operated by means of a pressure fluid. The pressure fluid may be a gas such as air for pneumatic operation of the cylinder or a liquid such as oil for hydraulic operation of the cylinder.

The cylinder may be single or double acting. For example, means may be provided to admit and/or remove pressure fluid at opposite ends of the cylinder. Advantageously, the piston is mounted for reciprocating movement within the cylinder. For example, the piston may be reciprocated by alternately admitting/removing pressure fluid at opposite ends of the cylinder.

According to another aspect of the invention there is provided a method of determining the position of a piston within a cylinder comprising providing a position sensor in or on the cylinder wall and magnetically coupling a movable part of the sensor to the piston through the cylinder wall.

According to a further aspect of the invention ; there is provided a linear drive comprising an outer member having a bore therein, an inner

member received in the bore, means for alternately admitting/removing pressure fluid at opposite ends of the bore for reciprocating the inner member in the bore, and a position sensor magnetically coupled to the inner member through a wall of the outer member for movement with the inner member to enable the position of the inner member to be determined.

The inner member may be a piston and the outer member a cylinder.

According to yet another aspect of the invention, there is provided a position measurement system for a determining the position of a first member slidable in a bore of a second member, the system comprising a position sensor magnetically coupled to the first member through a wall of the second member.

The first member may be a piston and the second member a cylinder.

The invention will now be described in more detail by way of example only with reference to the accompanying drawings wherein:- Figure 1 is a perspective view of a first embodiment of a pneumatic piston/cylinder unit according to the invention; Figure 2 is a section through the unit of Figure 1 with parts omitted for clarity and showing a detail of the piston; Figure 3 is a section, similar to Figure 2, showing a modification of the first embodiment;

Figure 4 is a perspective view, partly sectional, of a second embodiment of pneumatic piston/cylinder unit according to the invention; and Figure 5 is a perspective view of an alternative follower for a position sensing device for a piston/cylinder unit according to the invention.

Referring first to Figures 1 and 2, a pneumatic piston/cylinder unit 1 according to a first embodiment of the invention is shown comprising a cylinder 2 and a piston 3 mounted for reciprocating movement within the cylinder 2.

The cylinder 2 includes a body 4 and end caps 5,6 of non-ferrous material, for example aluminium. The body 4 is of rectangular section with a cylindrical bore 7 and longitudinal grooves 8 in the outer surface of each side to reduce the wall thickness. The body 4 is an extrusion and can be cut to any desired length from a length of extruded stock.

The end caps 5,6 are secured to the body 4 in a fluid-tight manner by threaded fasteners 9 engaging tapped holes (not shown) in the ends of the body 4. Each end cap 5,6 is provided with an internally threaded hole 10,11 respectively for attaching a pneumatic line (not shown).

The piston 3 is reciprocated lengthwise of the cylinder 2 by alternately connecting opposite ends of the cylinder 2 to a source of pressurised fluid (air) and vacuum via the pneumatic lines.

The piston 3 is provided at one end with a coaxial rod 12 that projects in fluid-tight manner through end cap 5 at one end of the cylinder 2. The rod 12 provides a force transmission member responsive to linear movement of the piston 3 for any desired purpose. For example, the

piston/cylinder unit 1 may provide a linear drive for carrying out actuation of a desired operation via the rod 12.

The unit 1 further includes a position sensor 13 to detect and provide feedback on the position of the piston 3 for the intended application of the unit 1. In this embodiment, the sensor 13 is a linear potentiometer comprising a fixed part or guide 14 and a movable part or follower in the form of a slider 15.

The fixed part 14 extends lengthwise of the cylinder 2 along one side and is secured at each end to the end caps 5,6 via mounts 16. An electrical connection 17 is provided at one end of the fixed guide 14.

The movable slider 15 is slidably mounted on the guide 14 and carries a magnet 18 located in the adjacent groove 8 on the side of the cylinder 2.

The piston 3 is provided in the outer surface with a pair of magnets 19 extending around substantially the entire circumference of the piston 3.

The groove 8 locally reduces the thickness of the wall of the cylinder 2 sufficiently for the magnetic flux of the magnets 18,19 to pass through the wall and magnetically couple the slider 15 to the piston 3.

The fixed part 14 of the potentiometer comprises a fixed, elongate resistive element and a fixed, elongate conductive element extending in spaced, parallel relationship.

In this embodiment, the resistive and conductive elements are arranged in spaced, superposed relationship bridged by the slider 15 to provide electrical contact therebetween.

In a modification (not shown), the resistive and conductive elements are arranged in spaced, side-by-side relationship and again bridged by the slider to provide electrical contact therebetween.

In use, as the piston 3 moves within the cylinder 2, the slider 15 is pulled along the guide 14 by the piston 3 via the magnetic coupling therebetween. As a result, an indication of the position of the piston 3 can be obtained by means of the electrical contact between the resistive and conductive elements via the slider 15.

More particularly, the resistive element has connections which, when supplied with a set voltage input, will give a varying output depending upon the position of the slider 15 and so indicative of the position of the piston 3 within the cylinder 2.

The rod 12 is fixed to the piston 3 so that the position of the rod 12 is also known from the position of the piston 3 and movement of the rod 12 can be used for any desired purpose.

The groove 8 is configured so that the wall of the cylinder 2 separating the magnets 18,19 is of uniform thickness to enhance the magnetic coupling and reduce the risk of de-coupling when the piston accelerates at its maximum speed.

A small air gap 20 is provided between the magnet 18 of the slider 15 and the wall of the cylinder 2 so that the slider can move freely along the groove 8. In addition, the slider 15 and magnet 18 are of low mass so that their inertia at maximum acceleration of the piston 3 does not cause the slider 15 to de-couple from the piston 3.

In the above-described embodiment, the position sensor 13 is mounted externally on the outside of the cylinder 2. A cover (not shown) may be provided to fit over the position sensor 13 for protection against damage and also for preventing contamination of the sliding surfaces by ingress of dirt or detritus.

Alternatively, the position sensor may be encapsulated within the cylinder body as shown in Figure 3 where like reference numerals in the series 100 are used to indicate corresponding parts.

In this modification, the extrusion for the cylinder 102 is formed with a longitudinal cavity 125 in which the position sensor 113 is mounted. In this way, the position sensor 113 is enclosed for robustness and protected from damage.

Whilst the piston/cylinder unit described above generally functions quite adequately and to a large extent solves the problems associated with prior position measurement systems, a hysteresis problem may arise at high piston speeds.

That is to say, there may be a tendency for the slider 15 to lag slightly behind the piston 3 because of the frictional drag between the slider 15 and the guide 14 that could adversely affect the position indication.

Referring now to Figure 4 of the drawings, there is shown a piston/cylinder unit according to a second embodiment of the invention that is substantially identical to the first embodiment shown in Figs 1 and 2 save for the construction of the position sensing arrangement.

For convenience, like reference numerals in the series 200 are used to indicate corresponding parts and the following description largely omits reference to the general construction and operation of the unit itself which will be understood from the description of the first embodiment.

The piston 203 moves in the cylinder 202 of the unit 201 by means of a differential pneumatic pressure applied across ports 210 and 211.

Mounted on the external surface of the cylinder 202 are a resistive element and, superposed thereon, a spaced, conductive element 250 of a linear potentiometer as described previously. A suitable potentiometer is, for example, type SFW 22 ex Resenso.

The piston 203 carries two ring magnets 251 and 252 which are axially magnetised with like poles facing each other and which are axially spaced by a small air gap of the order of l-2mm.

The magnets 251 and 252 magnetically attract the movable element, in the form of a steel ball 253 seated on the conductive element 250 of the linear potentiometer. The steel ball 253 moves along the conductive element 250 of the linear potentiometer as the piston 203 moves within the cylinder 202.

Where the ball 253 contacts the conductive element 250, the latter is locally forced by the ball 253 into electrical contact with the resistive element of the potentiometer.

The resistive element of the linear potentiometer has connections 217 at one end which, when supplied with a set voltage input, will give a

varying output depending upon the position of the ball 253 and so indicative of the position of the piston 203 within the cylinder 202.

It will be appreciated that although the linear potentiometer is shown to be external and exposed it may be covered by a separate cover as described above with reference to Figure 3.

Alternatively a space for it may be provided within or on the cylinder wall. The cover or space, as the case may be, may form a guide for the ball 253 to move in.

In a modification (not shown), the steel ball 253 is replaced by a small disc or puck that lies flat on the conductive element and is magnetically coupled to the piston.

The puck is movable along the conductive element in response to movement of the piston and is attracted towards the piston to produce electrical contact between the resistive and conductive elements of the potentiometer and provide an indication of the position of the piston within the cylinder as previously described.

Referring now to Figure 5, there is shown an alternative follower 360 in the form of an elongate, rectangular body 361 of aluminium housing three longitudinally spaced brass rollers 362,363, 364.

The end rollers 362,364 are arranged at the bottom and the centre roller 363 is arranged at the top of the body 361 to locate and support the follower 360 for movement lengthwise of a longitudinal groove, channel or slot in the cylinder wall (not shown).

The follower 360 is magnetically coupled to the piston (not shown) by magnetic attraction between one or more permanent magnets 365,366, mounted between the rollers 362, 363,364 of the follower 360 and one or more permanent magnets (not shown) on the piston.

The follower 360 is provided at one end with an electrical contact 367 arranged to bridge between elongate, fixed resistive and conductive elements (not shown) of the linear potentiometer to provide an indication of the position of the piston within the cylinder as described previously.

As will be appreciated, the rollers 362,363, 364 help to control sliding friction between the follower 360 and the cylinder (not shown) and minimise the effects of hysteresis on the system.

It will be understood the invention is not limited to the embodiments above-described. For example, the size and/or shape of the piston and cylinder may be altered from that shown. Thus, the piston may be of circular or non-circular section with the bore in the cylinder being of complementary shape. The position sensor may be of any suitable type capable of being magnetically coupled to the piston.

In the above-described embodiments, the piston is connected to a rod that acts as the force transmission member for the unit. It will be understood, however that the invention has application to so called"rodless"units in which the piston is connected to a carriage arranged externally of the cylinder and slidable lengthwise of the cylinder to act as the force transmission member for the unit.

In the rodded unit, the piston may have a rod attached to each end. In the rodless unit, the piston may be attached to more than one carriage on the

same or different sides of the cylinder. A combination of rodded and rodless units may be provided by attaching the piston to a rod at one or both ends and to a carriage on one or more sides of the cylinder.

Furthermore, while the invention has been described with reference to a pneumatic piston/cylinder unit, it will be appreciated that the invention has application to a hydraulic piston/cylinder unit in which the actuating medium for the piston is a liquid such as oil.

Other modifications will be apparent to those skilled in the art and are deemed within the scope of the invention.