Patent Description:
It is known to use radiation-emitting sensors for monitoring piston travel, especially for high precision and/or high-pressure applications. For example, electromagnetic waves are suitable for measuring the linear stroke of a piston. The linear measurement of stroke can be determined within a hydraulic cylinder by sending an electromagnetic signal from an antenna/receiver that is mounted either on the full-bore side of the cylinder to reflect off the piston face or on the annular side of the cylinder to reflect off the rear of the piston. Electromagnetic waves are chosen which have high penetration abilities through many non-metallic or non-conductive substances such as hydraulic oil.

Examples of such an arrangement are described in <CIT> and <CIT>). Also the examples of <CIT>, <CIT> and <CIT> describe such arrangements.

Utilisation of the higher frequency ranges brings significant advantages in regard to signal quality and possible information generation. However, the higher the frequency the more severe the penetration past the piston becomes.

The invention is directed towards achieving more accurate piston position measurement.

The invention provides a fluid ram as set out in claim <NUM>.

Preferably, the barrier gap is at a taper angle to longitudinal, for example about <NUM>°.

Preferably, the radial gap between the barrier and the groove base has a radial dimension in the range of <NUM> and <NUM>.

Preferably, the barrier is around the piston adjacent an end of the piston closest to the sensor.

Preferably, the ram further comprises a seal around the piston at a location axially separate from the barrier, and a guide ring around the piston at a location axially separate from the barrier.

Preferably, the piston comprises or supports, in order in the axial direction from the sensor, the barrier, a guide ring, a seal, and a guide ring.

Preferably, the barrier is shorter in the longitudinal dimension than the groove, so that it is mounted in the groove in a manner to allow a gap on both longitudinal sides of the barrier and radially inwardly of the barrier, and the relative dimensions of the barrier and the groove are such as to allow flow of pressurized fluid in the longitudinal direction through said gaps and past the barrier to a piston seal. Optionally the barrier is shorter than the groove in the longitudinal direction by a distance in the range of <NUM> to <NUM>.

We describe various rams with a cylinder, an end cap, and a piston <NUM> on a piston rod. The ram has a sensor for very accurate measurement of longitudinal position of the piston face, the sensor having an electromagnetic wave transmitter and receiver in the end cap and facing the piston face. Accuracy of the measurements is excellent by ensuring that almost all radiation which is emitted is reflected back to the receiver. The radiation comprises HF electromagnetic waves. Excellent accuracy is achieved despite the fact that the piston wall does not touch the cylinder wall in operation. This gap avoids damage to the cylinder wall surface, avoids failure of the high-pressure seals between the piston and the cylinder, and allows for thermal expansion and contraction. These benefits are achieved, while also avoiding the potential problem of sensor electromagnetic waves bypassing the main reflective target (the piston), not completely dissipating behind it, and reflecting off other components.

This is achieved by the ram having a barrier between the piston and the cylinder wall, alongside pressure seals and guide rings. Advantageously the barrier does not affect operation of conventional components which extend around the piston, such as seals and guide rings/sleeves. Referring to <FIG> and <FIG> a ram <NUM> has a cylinder <NUM> and an end cap or head <NUM>. A piston <NUM> is on a piston rod <NUM>. This provides an annular space <NUM> around the rod <NUM> and behind the piston <NUM>, and there is a space <NUM> in front of the piston face <NUM>. A distance sensor is in this example an electromagnetic transceiver antenna <NUM> is mounted in the cylinder head <NUM>, arranged to direct HF electromagnetic waves towards the piston <NUM> face <NUM>. The antenna <NUM> comprises a metallic core, surrounded by an insulating plastics housing, and fitted with high pressure hydraulic seals. There is an annular electromagnetic wave barrier <NUM> around the piston <NUM>, in the gap between the piston and the cylinder internal wall. In the example shown, the dimensions of the cylinder and the piston are <NUM> bore cylinder with <NUM> rod.

The antenna <NUM> emits HF electromagnetic waves in the direction of the piston <NUM>, the waves are then reflected back in the opposite direction by the piston <NUM> and the barrier <NUM>.

In more detail, the piston has front and rear guide rings <NUM> of type C10 phenolic resin size <NUM> outer diameter x <NUM> inner diameter x <NUM> wide, which assist accurate travel on-axis. Between the guide rings <NUM> in the axial direction there is a high-pressure seal <NUM> of type Hallite <NUM>™ (<NUM> outer diameter x <NUM> inner diameter x <NUM> wide).

The barrier <NUM> is mounted in an annular groove <NUM> around the piston <NUM>. The barrier <NUM> is configured with an excess internal diameter so that a small gap <NUM> exists between the barrier's internal radially inwardly facing surface and the piston.

The seal <NUM> is a main pressure seal, which has the function of creating a hydraulic seal around the piston. It is of a material other than metal as it is in direct contact with the cylinder <NUM> inner surface and must not damage it and must not leak oil. The seal <NUM> may be of rubber, polyurethane, or PTFE for example as is known in the art.

In this example there are two guide rings <NUM>, however in other examples there may be only one. The function of the guide rings is to ensure that the (metal) piston <NUM> does touch the inner surface of the cylinder <NUM>. They resist all side loads that are induced into the piston <NUM> from external forces, under both low and high temperatures. The material of each guide ring <NUM> is such that it does not scratch or score the cylinder <NUM> surface, and they may for example be plastics or phenolic based materials, also as is known in the art.

The material of the barrier <NUM> is preferably a good electrical conductor. Examples are aluminium, phosphor bronze, malleable cast iron, a conductive plastics material, or a combination of any of these. Other suitable materials are Grey Cast Iron, also known as Flake Graphite Iron. In this example there is impregnated graphite to assist sliding with reduced friction against the cylinder bore. The impregnated graphite is considered to provide self-lubrication of the barrier. Aluminium is particularly preferred because of its mechanical and conductive properties. The configuration and material of the barrier is such that it will not damage the inner surface of the cylinder, and will substantially block electromagnetic waves without affecting operation of the high-pressure seal <NUM> and the guide rings <NUM>. In this example the barrier <NUM> is of GD250/EN-GJL-<NUM> to DIN EN <NUM> (Grey cast Iron) material with a tensile strength of <NUM> to <NUM> N/mm2. In general, another preferred material is an iron alloy with impregnated graphite, one example being Flake Graphite Iron.

The enlarged detail in <FIG>, Detail A, shows an annular gap <NUM> between the piston <NUM> and the inside surface of the cylinder <NUM>. It is this gap which would allow the microwaves <NUM> to travel through the space <NUM> and past the piston <NUM>, but instead they are blocked, or at least reduced to an acceptable level, by the barrier <NUM>.

The barrier <NUM> is shown in more detail in <FIG>. In this case the dimensions of the barrier <NUM> are <NUM> diameter x <NUM> wide x <NUM> deep (radial dimension). The barrier is in the form of a ring which is not closed, having a circumferential gap <NUM> at a taper angle to longitudinal. The gap is in this example <NUM> in width. The barrier <NUM> has a natural shape as illustrated with the gap <NUM> present. This outside diameter is very slightly larger than that of the cylinder bore, providing for a very small bias radially outwardly against the cylinder bore. If there are distortions arising from, for example, temperature changes, then the barrier <NUM> can contract slightly by closing the gap <NUM>. It is even possible for the outside diameter to reduce very slightly beyond closing the gap, by sliding motion of the ends upon closing the gap <NUM>. Due to the taper, these faces slide relative to each other to further reduce the outside diameter if required due to ram distortions. Another benefit of the taper direction of the gap is that there is always a barrier presented to electromagnetic waves in the axial direction.

The barrier <NUM> is mounted in the groove <NUM> such that it has its own built-in bias to remain with a diameter equal to or greater than the cylinder bore. However, the cylinder bore will vary in size due to tolerances, temperature change, and distortion among others. Therefore, the barrier <NUM> is able to change its outer diameter accordingly. There is a small available space <NUM> in the radial direction between the barrier <NUM> and the groove <NUM> to allow for this. Preferably the space is of radial dimension between <NUM> and <NUM>. This space <NUM> is very advantageous to allow the free movement of the barrier <NUM> to accommodate cylinder and piston expansion and contraction, but this feature does not allow the electromagnetic waves to pass, as they reflect off the piston <NUM> side walls.

Moreover, the gap <NUM> extends also in a preferred embodiment around one or both sides of the barrier in the longitudinal direction. This allows entry of pressurized fluid in a manner which does not allow localized build-up of pressure which might damage the barrier <NUM>. It is most preferred to that the gap extend on both longitudinal sides, thereby allowing passage of pressurized fluid past the barrier and all of the way back to the high-pressure seal <NUM>. This allows the seal to take the applied pressure, preventing damage to the barrier, and allowing it to perform its function of blocking electromagnetic waves. The pressurized fluid has a path to the high-pressure seal <NUM>, but this path does not allow any significant passage of electromagnetic waves, due to the orthogonal faces of the groove <NUM> and the barrier <NUM>. It is preferred that the gap on the longitudinal sides is in the range of <NUM> and <NUM>. For illustration purposes, the size of the gap <NUM> is exaggerated in <FIG>.

Referring to <FIG> in another example a ram <NUM> has a cylinder <NUM>, a piston <NUM>, and a piston rod <NUM>. Like parts are given the same reference numerals. A sensor <NUM> is mounted on the inner face of a gland <NUM>, emitting electromagnetic waves towards the rear side of the piston in an annular space <NUM>. The piston <NUM> has a pair of guide rings <NUM> and a high-pressure seal <NUM> similar to those of the ram <NUM>. In this case, however, the annular barrier <NUM> is in a piston circumferential groove <NUM> around the rear end of the piston <NUM>. In this position it performs the same function as the barrier <NUM> in the ram <NUM>, and the gap <NUM> is present for the same purpose, in this case allowing passage of pressurized fluid to the left as viewed in this drawing.

In other examples the barrier is longer in the axial dimension, as shown in the example of <FIG>, which is not covered by the present invention. In this example, not within the scope of the invention, a barrier <NUM> has an axial length of <NUM>, and two gaps <NUM> at <NUM>°. The material of the barrier <NUM> is in this example is an aluminium alloy, Grade 6082T6, with a tensile strength of <NUM> to <NUM> N/mm<NUM>.

The barrier <NUM> is in two pieces which interface together at the gaps <NUM> to make it easy to fit around the piston. In this case there isn't a spring effect toward a larger diameter to press against a cylinder bore, however because it is in two pieces the manufacturing tolerance can be very tight for optimum placement with an effect of brushing or rubbing along against the cylinder bore. In general, the arrangement of the barrier <NUM> (with only one gap, <NUM>), is advantageous because of the natural bias outwards. However, in the barrier <NUM> the gaps <NUM> allow movement to cater for distortion and to provide a gap between the barrier and the piston for the same purpose as the gap <NUM> of the barrier <NUM>. The fact that the gaps <NUM> are at an angle to longitudinal, <NUM>° in this case, means that electromagnetic waves can do not have a path in the longitudinal direction.

Any barrier, irrespective of its longitudinal length, may be manufactured as one piece with a single cut and stretched over the piston for fitting.

It is envisaged that in other examples the barrier may also perform the function of a guide ring, as the selected material already has bearing capabilities and load resisting properties, hence the selection of the high strength aluminium alloy grade 6082T6. A difference from conventional guide rings, however, is that there would be a gap under the rings for the same purpose as the gap <NUM>, which is not ideal for a guide ring. While this combined functionality is possible, it is preferred that the barrier ring and the guide ring are separate components due to the very different nature of their functions.

It will be appreciated that the material of the blocker or barrier may be a dynamic metallic or semi-metallic or metallic-composite suitable to block electromagnetic waves emitted by the sensor. It fulfils the function of blocking low to very high frequency microwaves, thus greatly reducing the extent of echoes re-entering the measuring space from the opposite side of the cylinder. It is estimated that the improvement in sensing accuracy is about <NUM>% to <NUM>%.

The blocker <NUM> according the present invention, as well as the blocker <NUM> from the example not covered by the present invention, also fulfils all dynamic mechanical/environmental requirements to be found inside pressurised hydraulic cylinders. It operates dynamically without affecting the performance of the hydraulic cylinder. It also allows the use of high frequency microwaves in the range S-band <NUM>-<NUM> as against medium frequency in the range L-band <NUM>-<NUM> thus increasing measuring accuracy to stroke lengths of beyond <NUM>.

It will be appreciated that the invention solves the problem of sensor signals, such as an ultrahigh frequency signal, directed towards the face of the piston will also penetrate the piston seals and enter the annular side of the cylinder. The invention prevents such a signal from being reflected by the inner wall of the gland, the cylinder rod, the cylinder wall and any other metallic features. Hence, it prevents such an errant HF from arising and hence prevents the consequent problem of such an errant signal passing back through the piston seals and creating an echo that disturbs the signal quality being received from the signal target (the piston face). It will be appreciated that this benefit is surprisingly achieved without affecting normal operation of the ram.

Claim 1:
A fluid ram (<NUM>) having a piston (<NUM>) mounted in a cylinder (<NUM>), an electromagnetic wave sensor (<NUM>, <NUM>) mounted to detect movement of the piston in its travel within the cylinder, wherein,
the piston comprises an electromagnetic wave barrier (<NUM>, <NUM>) of electrically conductive material mounted to the piston around at least part of its circumference to prevent passage of electromagnetic waves past the piston, and
the barrier is mounted in a circumferential groove (<NUM>) around the piston,
characterized in that, the barrier (<NUM>):
is annular, extending substantially fully around the piston,
has only one circumferential gap (<NUM>), and has a built-in bias to remain with a diameter greater than a cylinder bore whereby it has a position lightly pressing against a cylinder internal surface,
has a size providing a radial gap (<NUM>) between the barrier and a base of the groove, and
is of a material selected from aluminium, phosphor bronze, malleable cast iron, a metal alloy with impregnated graphite, and a conductive plastics material.