GB2106651A - Displacement indication device - Google Patents

Abstract

The displacement of a reciprocably displaceable rod (5) of a first material 6 extending through a first end 3 of a cylinder 2 is measured by providing the rod 5 with a longitudinally extending portion of a second, different, material 7 having a thickness which varies in known manner along the rod length. A thickness sensor means 9 disposed adjacent said portion 7 of said rod 5 and externally of the cylinder 2, produces a first signal corresponding to the local thickness of the portion 7, a signal processing means 11 converting said first signal to a second signal for driving a signal display means 12 indicating positional displacement of said rod. The first material is preferably steel and the second material chrome plate, although either materials may comprise a plastics material eg PTFE. The sensor 9 may measure magnetic, capacitive or ultrasonic variations. Both the rod and portion 7 may be tapered to provide a perfect cylinder, or the portion 7 may comprise a tapered insert in the rod. <IMAGE>

Description

SPECIFICATION Displacement indication device This invention relates to displacement indicator devices and in particular to the provision of such devices with means for indicating displacement or a rod e.g. a piston rod.

It is often desirable to know more or less precisely the position of the piston rod in a piston and cylinder device at various stages of its stroke. Normally direct observation is not possible and/or is inconvenient.

It has previously been proposed to provide a remote indication of the piston rod displacement and hence position during the course of its stroke by providing a piston rod-like extension projecting in the opposite direction to the piston rod through a respective end of the cylinder and provided with a sliding contact engageable with an elongate potentiometer to complete an electrical circuit therethrough and displaceable therealong as the pison is displaced, thereby producing a corresponding variation in the current through said circuit.

The current is then processed to display a signal corresponding to the instantaneous displacement or position of the piston.

Such an arrangement can however result in loss of operating power and efficiency of the piston and cylinder due to the need for an additional bore in the cylinder with its associated seals which will also involve significant additional costs. Also this design has limited reliability due to the difficulty of maintaining a good electrical contact.

It is an object of the present invention to avoid or minimize one or more of the above disadvantages.

The present invention provides a displacement indicator device for a reciprocably displaceable member comprising a said member of a first material, at least part of said member having a longitudinally extending portion of a second material which is different from said first material; a thickness sensor means disposed adjacent or in close proximity to said portion of said rod member for producing a first signal corresponding to the thickness of said portion at a point along its length opposite said sensor means; said portion having a variable thickness along its length which thickness is a function of the longitudinal separation from a predetermined point on said portion; and a signal processing means arranged for converting said first signal to a second signal substantially directly related to longitudinal separation for driving a signal display means so as to provide a substantially guantitative indication of positional displacement of said displaceable member.

With a device of the present invention it is possible to obtain more or less continuously a quantitative indication of the instantaneous position and/or displacement of the piston without modifying the basic structure of the piston and cylinder, the basic displacement indicator means being simply provided by making part of the normal piston rod structure (including any coating thereon) of a different material. The sensor means can be readily mounted immediately behind the cylinder and avoids the need for cumbersome full length full length potentiometer-type devices.

Further preferred features and advantages of the invention will appear from the following description given by way of example of some preferred embodiments illustrated with reference to the accompanying drawings in which:- Figure 1 is a generally schematic and partially sectioned, part-structural part-circuit diagram, side view of a piston and cylinder device provided with a displacement indicator means; Figure 2 is a partly sectioned elevation of a rotary actuator with the principal parts of another displacement indicator means; and Figure 3 is an end elevation of the actuator of or Fig. 2.

In Fig. 1 is shown a piston and cylinder device 1 comprising a cylinder 2 having a first end 3 and a piston 4 reciprocably movable in said cylinder 2 with a piston rod 5 extending through said first end 3 of said cylinder 2 and constituting a displaceable member.

The piston rod 5 is usually made of steel 6 which isthen plated with another metal, for example chrome plates 7. In accordance with the the present invention the steel body 6 of the piston rod 5 is made frusto-conical with a slight taper and is then plated with a thin layer of chrome 7 so as to build up the outer surface 8 of the piston road to a substantially perfect cylindrical form. It should be noted that the thickness of the coating and degree of taper shown in the drawing have been considerably exaggerated for the purposes of clarity only. Nevertheless it is of course possible to use coatings with various thickness and degrees of taper if so desired. It will also be appreciated that the variable thickness portion could be provided in other ways.Thus for example it could be in the form of a tapered longitudinally extending groove in a cylindrical piston rod surface which is then filled up to the level of the cylindrical surface (and if desired beyond that if the whole cylinder is to be plated) with a coating or filler material as appropriate, thereby producing a wedgeshaped layer of coating material in the groove.

Such an embodiment may however be less preferred if it necessitates alignment of the groove with sensor means (see below).

Although the above materials are generally the most convenient, other materials may be employed. Thus for example the coating or filler material may be a suitable plastics material such as e.g. polytetrafluoroethylene (PTFE). The main body itself could also be made of a suitable plastics material depending on the magnitude of the forces to be borne by the piston and cylinder device.

Behind the end wall 3 of the cylinder 2 there is located a thickness sensor means 9 having a probe 10 in more or less close contact with the coating 7 of the piston rod and conveniently encased in a housing 1 3 extending from the end wall 3 of the cylinder 2. Various suitable layer thickness sensor means are well known in the art and could be used depending upon various factors such as the nature of the two materials involved, the thickness of the layer, the sensitivity required etc. There could for example be used an ultasonic sensor means which is based on transmitting ultrasonic sound waves from the probe into the material and monitoring the reflection thereof from the interface between the body material and the coating material.

Alternatively there could be used a sensor means based on variations in capacitance wherein is measured the distortion of an electric field extending from the probe through the coating or filler, by said coating or filler.

Conveniently though where one of the piston rod body and coating or filler (usually the former) is a paramagnetic material and the other is non-magnetic, there is used an electro-magnetic field sensor means in.which the probe comprises spaced apart magnetic pole means having a magnetic field (induced by the passage of an electric current through a coil adjacent thereto in a suitable electrical circuit) extending from the end of said probe through said coating or filler. The presence of the layers of para-magnetic and non-magnetic material will modify the magnetic field and hence the current in the coil and electrical circuit by an amount dependent upon the thickness of the layer of the coating or filler immediately below the probe.Since this thickness is a known function of the distance between the piston 4 and the probe 10, said distance can be derived from the measurements made by the sensor means. Specifically dhanges in the current in the coil electrical circuit are detected by a suitable detector means which produces a first signal corresponding to the thickness of the coating or filler. This is then amplified and/or modified as required in a processing means (PROC) 11 to produce a second signal corresponding to longitudinal separation or displacement of the piston from a predetermined point e.g. one or other end of .the cylinder or the probe position, for driving a suitable signal display means (VDU) 12 e.g. and LED or LCD digital read-out.

Various suitable sensor means are commercially available. One example that may be mentioned is a thickness gauge such as The nature and complexity of the processing means required will depend on the nature of the function by which the coating of filler thickness is related to distance along the piston rod. Although in the above example the relation is governed by a linear function, nonlinear functions may also be employed for example where increased sensitivity is required during a particular part of the piston stroke. Naturally in such cases more complex processing means will be required to 'decode the first signals.

Fig. 2 shows a rotary actuator 20 for converting a linear motion to a rotary motion. The actuator is of generally conventional construction and comprises a housing 21 with an elongate rod member 22 having a rack 23 formed along one side and piston means 24 at each end. The rack teeth 23 engage gear teeth 25 extending around an annular gear wheel 26 which has a central aperture 27 provided with a keyway 28 for driving engagement with a shaft to be driven (not shown).

The piston means 24 are a sliding fit within respective oppositely extending cylinder means 29 provided in said housing 21, up to an adjustable end stop means 30. The cylinder means are provided at their distal ends with pressurised fluid connection means 31.

The rod member 22 is provided along one side remote from the rack 23 with a thin longitudinally extending tapered insert portion 32 of a different material. A thickness sensor means 33 is mounted in a wall 34 of the houding 21 opposite and in close proximity to the tapered insert 32 at a point approximately central with respect to the range of travel of the rod member 22. The sensor means 34 is connected by a suitable lead 35 to processor and display means in similar manner to that described with reference to the embodiment of Fig. 1.

As the rod member 22 is displaced reciprocably under the influence of the pressurised fluid supplied to the respective cylinder means 29 to act on the respective piston means 24 at each end to rotate the annular gear wheel 26 and any shaft connected thereto, the thickness of the insert 32 directly opposite the securing head of the sensor means 33 changes thereby producing a changing signal output which is converted into a substantially precise quantitative indication of the instantaneous displacement of the rod member and/or if desired gear wheel or shaft connected thereto with respect to a predetermined reference point.

Claims (9)

1. A displacement indicator device for a reciprocably displaceable member comprising a said member of a first material, at least part of said member having a longitudinally extending portion of a second material which is different from said first material; a thickness sensor means disposed adjacent or in close proximity to said portion of said rod member for producing a first signal corresponding to the thickness of said portion at a point along its length opposite said sensor means; said portion having a variable thickness along its length which thickness is a function of the longitudinal separation from a predetermined point on said portion; and a signal processing means arranged for converting said first signal to a second signal substantially directly related to longitudinal separation fro driving a signal display means so as to provide a substantially quantitative indication of positional displacement of said displacement member.

2. A device as claimed in claim 1 wherein said displaceable member is of a ferro-magnetic metal and said portion is of a nonmagnetic metal.

3. A device as claimed in claim 2 wherein the sensor means is in the form of an electromagnetic thickness sensor comprising a probe having an electrical circuit arranged for inducing a magnetic field extending generally normally from the tip of a probe, and electric current responsive means arranged for directly or indirectly generating a said first signal in response to perturbation of the magnetic field and thereby the current in said electrical circuit, by said member.

4. A device as claimed in any one of claims 1 to 3 wherein said function is a linear function.

5. A device as claimed in any one of claims 1 to 4 wherein said portion is in the form of an annular coating on said member.

6. A device as claimed in claim 5 wherein the radially outer surface of said coating is cylindrical and the radially inner surface is frusto-conical.

7. A device as claimed in any one of claims 1 to 6 wherein said member is a piston rod member connected to a piston reciprocably displaceable inside a cylinder with said piston rod extending through a first end of said cylinder.

8. A device as claimed in any one of claims 1 to 4 wherein said member is a piston rod member of a rotary actuator, said piston rod member having piston means at each end reciprocably displaceable inside respective cylinder means, a rac- along one side engaging a gear wheel, and a said longitudinally extending portion along another side remote from said rack with a said thickness sensor means disposed adjacent or in close proximity to said portion.

9. A device according to claim 1 substantially.as described hereinbefore with particular reference to the accompnying drawings.