GB2155638A

GB2155638A - Transducer for fluid operated ram

Info

Publication number: GB2155638A
Application number: GB8406161A
Authority: GB
Inventors: Peter Caleb Frederi Wolfendale
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-03-09
Filing date: 1984-03-09
Publication date: 1985-09-25
Also published as: GB2155638B; GB8406161D0

Abstract

In an hydraulic ram a position sensor rod (1) is supported at one end in a bore in ram rod (2). A capacitance (C1) exists between the sensor and ram rods (1, 2) and a capacitance (C2) exists between the sensor rod (2) and a fixed reference electrode (13). Measurement of the ratio of (C1) and (C2) which are in the same dielectric medium gives an indication of ram rod position. Alternatively the sensor rod (1) may be arranged for movement with the ram rod (2) so that the sensor rod (1) passes beyond the reference electrode (13) and into variable coupling with a further electrode. <IMAGE>

Description

SPECIFICATION Transducer for fluid operated ram This invention relates to a transducer for a fluid operated ram that enables the position of a ram rod thereof to be determined.

Broadly stated the invention provides a transducer for determining the relative position of first and second electrically conductive members that are relatively movabie in translation by the supply or withdrawal of a working fluid from a space in which said members are located one member being inside the other, a reference electrode associated with the first member in fixed position relative thereof and exposed to said working fluid and means arranged to sense the impedances between said first member and said second member and between said first member and said reference electrode and to produce a ratio output signal signifying the relative position of the first and second members.

The invention further provides a fluid operated ram comprising a piston in a cylinder, a ram rod connected to the piston for movement therewith, said rod having a bore on the piston side thereof, a non-moving sensor rod that enters the bore in the ram rod and defines a first impedance therewith, a nonmoving reference electrode coupled to the sensor rod and defining a second impedance therewith and means connected to said ram and sensor rods and to said reference electrode and operable to determine the ratio of said first and second impedances thereby to produce an output signal indicative of the position of the ram rod.

The invention also admits of a modification where the sensor rod is carried by the piston for movement therewith, said rod passing beyond a reference electrode defining the second impedance to a sensor electrode with which it defines a first impedance of variable area.

It is of essence that a ratio measurement rather than an absolute measurement is made in order for reproducible results to be obtained. The position measuring system of the invention has the advantages of simplicity and inexpensiveness combined with those of accuracy and stability.

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 fragmentary longitudinal section of a pneumatic or hydraulic ram fitted with capacitative transducer means; and Figures 2, 3 and 4 are diagrammatic circuits for measuring the impedance ratio of the transducer according to the invention.

In the drawings, an hydraulic ram has a plain sensor rod 1 electrically isolated from the casing and located so that it extends into a bore in a ram rod 2 which is also of uniform cross-section throughout its length, The rod 2 is located within a cylinder 3 that forms an outer case of the hydraulic ram. A piston 4 is attached to the ram rod 2 and forms a seal between it and the cylinder. The end of the ram is closed by means of a back plate 10 that is apertured to permit the proximal end of the sensor rod 1 to pass therethrough and into a cap 11, with a support 1 2 of insulating material between the back plate 10 and the cap 11.An insulating layer of "Araldite" or similar material is interposed between the electrode 14 and the support 1 2. The support 1 2 supports an annular reference electrode 1 3 that is coaxial with the rod 1 and an end cap electrode 14, also with an intervening "araldite" layer.

In operation, a working fluid is introduced into cavities 5, 6 to opposed sides of the piston 4 to urge the rod 1 in one or the other direction thereby to actuate an external device. The working fluid may be air, oil, water or a combination of materials. It is the object of this invention to enable the position of the rod 2 with reference to the case 3 to be determined. If the working fluid is air or a non-conductive oil or other fluid there will be no conduction between the sensor rod 1 and the ram rod 2 or the reference electrode 1 3.

As the rod 2 changes position relative to the fixed sensor rod 1 they act as a cylindrical capacitor having a fixed and movable electrode surface and whose capacity denotes rod position. But the capacitance between rod 2 and sensor rod 1 is also influenced by the dielectric constant of the working fluid which may be variable, in the case of an oil owing to changes in temperature and deterioration with time. It is therefore desirable to have a means of position measurement that is independent of the characteristics of the dielectric. The capacitance between the reference electrode 1 3 and the rod 1 is also influenced by the same dielectric medium that is located within the cavity 5 to the piston side of the ram.If measurement is made of the ratio of a capacitance C, between the sensor rod 1 and the ram rod 2 and a capacitance C2 between the reference electrode 7 and the sensor rod 1 then any variation in the characteristics of the working fluid will affect both and a more stable measurement will be obtained.

A variety of ratiometric capacitance measuring circuits are known and representative forms are diagrammatically illustrated in Figs.

2 and 3. In Fig. 2 the capacitance C1 is connected between an input voltage source 1 5 and one input of an operational amplifier 20 and the other capacitor C2 is connected between the input and the output of the operational amplifier 20. As is well known, the output of the operation amplifier 20 then depends on the ratio C1/C2. In Fig. 3 the voltage is fed to capacitors C1 and C2 which are connected in differential arrangement to an input to operation amplifier 20 which has a capacitor C3 thereacross. The output is fed via switch 21 to integrator 22 whose output is fed back at 23 to vary the proportion of the input voltage fed to capacitor C2. As is well known such a circuit can reach a balance characterstic of the ratio of C1 to C2. Inpedance measuring circuits are described, for example, in our UK Patent Specification No's 1491201, 2036982B.

If a conductive working fluid such as water is used instead of a dielectric medium then resistances between the sensor rod 1 and the ram rod 2 and reference electrode 1 3 are measured and the resistances could be measured ratiometrically for example by using the operational amplifier circuit of Fig. 4 which is the resistor analogy of Fig. 2 and is believed self-expanatory.

Claims

1. A fluid operated ram comprising a piston in a cylinder, a ram rod connected to the piston for movement therewith, said rod having a bore on the piston side thereof, a nonmoving sensor rod that enters the bore in the ram rod and defines a first impedance therewith, a non-moving reference electrode coupled to the sensor rod and defining a second impedance therewith and means connected to said ram and sensor rods and to said reference electrode and operable to determine the ratio of said first and second impedances thereby to produce an outut signal indicative of the position of the ram rod.

2. A ram according to Claim 1 wherein the working fluid of said ram is air or an oil and the ratio measuring means measures a capacitance ratio.

3. A fluid operated ram provided with ratiometric impedance position measurement means substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

4. A modification of the ram of Fig. 1 wherein the sensor rod is carried by the piston for movement therewith, said rod passing beyond a reference electrode defining the second impedance to a sensor electrode with which it defines a first impedance of variable area.

5. A transducer for determining the relative position of first and second electrically condudctive members that are relatively movable in translation by the supply or withdrawal of a working fluid from a space in which said members are located one member being inside the other, a reference electrode associated with the first member in fixed position relative tereof and exposed to said working fluid and means arranged to sense the impedances between said first member and said second member and betwen said first member and said reference electrode and to produce a ratio output signal signifying the relative position of the first and second members.

6. Apparatus for measuring the position in a space of a body that is moved by the supply of fluid to or withdrawal of fluid from such space comprising a sensor that defines via said fluid a variable impedance with said body anda fixed impedance with a reference electrode operating in said fluid, and means for measuring the ratio of said first and second impedances and supplying an output signal indicating the position of the body.