GB2206212A

GB2206212A - Determining position and/or replacement

Info

Publication number: GB2206212A
Application number: GB8814640A
Authority: GB
Inventors: Alexander Russell
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-06-24
Filing date: 1988-06-20
Publication date: 1988-12-29
Also published as: GB8814640D0; GB8714842D0

Abstract

Absolute determination of position/displacement e.g. of a piston 13 in a hydraulic ram is by means of a loading armature 19 carried by the ram piston and shiftable along a stationary elongate stator 17 having a regularly multi-tapped winding forming a series of windings each of axial length equal to half the axial length of the armature. "Coarse" determination of the armature position is effected by scanning the series of windings and for each winding measuring the impedance. Detection of low impedance shows the presence of the armature. Thereafter, a group of the windings in the vicinity of the armature is examined by means of a bridge circuit and analogue values of impedance used to determine accurately the "fine" position of the armature. A feature of the apparatus is automatic step-switching between adjacent groups of windings so that analogue values are maintained within limits corresponding to linear variations. The armature 19 is a solid metal insert e.g. of aluminium and the starter is anchored by a pressure-tight gland 18. <IMAGE>

Description

DETERMINING POSITION AND/OR DISPLACEMENT This invention relates to an apparatus and method for use in determining position and/or displacement of one object relative to another.

The invention is principally, but not exclusively, concerned with position and/or displacement in the context of linear motion such as of a piston-and-cylinder arrangement, especially of the type more specifically designated as a hydraulic ram.

According to one aspect of the present invention, there is provided apparatus for use in determining position and/or displacement of one object relative to another, comprising a stator for association with said one object and having a multi-tapped winding or a series of windings, an alternating current source for energising the windings, armature means associated with the other object and effective to load inductively windings on the stator, and electrical sensing means operable to detect and measure inductive loading of windings on the stator.

According to another aspect of the present invention, there is provided apparatus for use in determining position and/or displacement of one obje-ct relative to another, comprising a stator for association with said one object and having a multi-tapped winding or a series of windings, an alternating current source for energising windings on the stator, armature means associated with the other object and effective to load inductively windings on the stator, electrical sensing means operable to detect and measure inductive loading of windings on the stator, and switching means operable in a first mode to scan the stator windings and to connect the latter in turn to said sensing means, and operable in a second mode to provide electrical connection between the sensing means and a group of the stator windings.

Further, according to the present invention, there is provided a method for determining position and/or displacement of a loading armature relative to a stator having a multi-tapped winding or a series of windings, comprising the steps of energising the windings in turn and measuring a loading for each winding to estimate the approximate position of the armature along the stator, energising a group of the windings including said approximate position and measuring the impedance of the group to determine more accurately the position of the armature.

By using a loading armature in conjunction with a series of stator windings, the position and/or displacement of the armature with respect to a group of the windings is deduced from measurement of the impedance of that group whilst the windings thereof are energised.

By using switching means in a first mode to scan a series of windings, a "coarse" indication is obtained as to the current position of the armature on the stator, since the group of windings adjacent such said position will have a reduced impedance and so can be detected. By using the switching means in a second mode to connect the detected group of windings in a measuring circuit, the position of the armature is determined more accurately to obtain a "fine" indication.

Coupling between the armature and the stator windings is only by induction so that the apparatus for determining the position and/or displacement is free from direct electrical contacts or commutation between the relatively movable parts.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic sectional elevation of a hydraulic ram incorporating apparatus for determining position and/or displacement in accordance with the present invention; Fig. 2 is a schematic representation of the principal features of the apparatus incorporated in Fig. 1; and Fig. 3 is a schematic representation similar to Fig. 2, but showing additional detail and graphic data.

In Fig. 1 of the drawings, a hydraulic ram assembly consists of a cylinder 10 having hydraulic ports 11 and 12, and a piston 13 with a piston rod 14 provided with a - hollow bore 15 opening to the crown of the piston 13.

The head of the cylinder 10 is designated by reference numeral 16 and carries an elongate stator 17 anchored by a pressure-tight gland 18. Recessed into the crown of the piston 13, is an armature 19. The stator 17 carries a series of windings in the form of a multitapped single winding, the tapping leads being taken through the gland 18 for connection to a switching device 20 housed in a chamber 21 adjacent the cylinder head 16. Connectors 22 and 23 serve to connect the switching device 20 to other components as described hereunder. The armature 19 consists of a solid metal insert of high electrical conductivity such as aluminium or silver; and the dimensioning of the armature 19 (apart from the axial dimensions discussed hereunder) is such as to permit good conductive coupling between the armature and the windings of the stator 17.

During operation of the hydraulic ram of Fig. 1, the armature 19 moves to-and-fro along the stator 17, and the position and/or displacement of the armature 19 and hence of the piston rod 14 is determined as described hereunder on an "absolute" basis and without direct electrical contact or commutation between the relatively movable parts.

In Figs. 2 and 3 of the drawings, parts corresponding with those mentioned in relation to Fig. 1 are given the reference numerals used in Fig. 1. The stator 17 consists of a former of an insulating material having wound thereon a continuous close-coiled single winding of enamelled copper wire in cylindrical configuration.

The single winding is regularly tapped at intervals along the axis of the stator and the tapping leads, designated by reference numeral 24 in Fig. 2, are bundled as depicted in Fig. 2 and connected directly to the switching device 20. The axial length between each tapping is equal to half the length of the armature which is designated by L in Figs. 2 and 3.

The switching device 20 consists of an array of electronic switches capable of being enabled and disabled selectively by means of a switch controller using coventional co-ordinating logic. The switch controller is incorporated at 25 in Figs. 2 and 3 together with an amplifier facility and the threshold detector as mentioned hereunder. General reference numeral 26 designates the provision of main control and co-ordinating logic, and also data stores and display means for providing analogue and/or digital display of position/ displacement data. The facilities incorporated at 26 may include a programmable facility,and incorporate a coventional processor.

General reference numeral 27 designates the provision of an alternating current source for energising the stator windings, together with electrical sensing means in the form of a bridge circuit and voltage detector. The facilities at 27 are routed through a coarse/fine switch facility 28 which together with the switch controller and the switching device 20 constitutes switching means capable of operation in two different modes as described hereunder. The main control facility at 26 is also connected to monitor conventional machine controls at 29 so that the state of the machine generally is known to the position/displacement detecting equipment.

In Fig. 3, the facility designated 27 in Fig. 2 is shown more particularly to consist of an alternating current source 27A, resistive components of a bridge circuit 27B, and a voltage detector 27C. Six mutually equal-length windings of the stator 17 are identified between tappings A, B, C, D, E, F and G. The armature 19 is shown in position P representing the mid point of the armature and co-incident with tapping C.

The coarse/fine switch 28 together with the switching device 20 is shown in Fig. 3 as connecting winding pairs A/B, B/C and C/D, D/E into the bridge circuit of facility 27 respectively to balance the resistive arms 27B.

This arrangement is achieved under control from the main control facility 26, and utilises connections w, x and y of connector 23. The main control facility 26 in conjunction with the switch controller at facility 25 is also operable to set the switching components 28 and 20 to scan the stator windings in turn. Thus, winding A/B would be brought into circuit by a connection between tapping A and terminal w and a connection between B and the fourth terminal z, terminals w and z being connected to the alternating current source 27A and the voltage detector 27C the measurement of which is applied to the threshold detector included in the facility 25.

Fig. 3 incorporates a graphic representation of voltage values obtained when the switching arrangements are set to "fine". The graphic data shown consists of two graphs G1 and G2. Graph G1 represents variations in impedance obtained by small movements of the armature 19 relative to the tapping C, the corresponding voltage output being zero when the position P coincides exactly with tapping C. Graph G2, as explained further hereunder, represents the same information, but with respect tapping D.The graphic data contained in Fig. 3 includes broken lines T+ and T-, and these broken lines represent threshold values detected by the threshold detector of facility 25 in response to which the switch controller steps the connections between terminals w, x and ffi either up or down in order to examine the next group of windings and to determine accurately the position of the armature 19 without having to rely on voltage values in the vicinity of the maximum and minimum voltages where inaccuracies can occur as is depicted by the broken lines in graphs Gl and G2. In Fig.3, the graph G1 is representative of the position of the armature 19 at P and is useful for armature positions betweenP+ L/4.If excursion of the armature 19 is greater than the "quarter" distance just mentioned, then the threshold detector in facility 25 causes stepping of the switching device 20 automatically to address the next adjacent windings group. Thus, the graph G2 in Fig. 3 is representative of the group of windings between tapping B and tapping F.

Operation of the apparatus described above in general is as follows. On switching on of the machine controls 29, the main control and co-ordinating logic and other logic devices associated therewith are automatically initialised. The main control and coordinating logic detects whether or not the machine and therefore the armature is currently in motion. If motion is detected, the operation to determine the "coarse" position of the armature is inhibited. If "coarse" determination of armature position is enabled, then the coarse/fine switch 28 is set to "coarse" and terminals w and z are applied to respective mutually adjacent tappings beginning with A/B so that each winding section is energised in turn by the alternating current source 27A.

During each energising, a loading value is obtained and by the application of comparison techniques the approximate position of the armature 19 is obtained. Either of two possible groups of four windings is then connected into the bridge circuit using terminals w, x and . Thereupon, the accurate position of the armature 19 is determined from the analogue value of V lying between T- and T+ on the linear portion of the appropriate graph. In the event that the analogue value is not within the threshold limits, the controlling logic automatically adjusts the switching by means of the switching device 20 to select the more appropriate group of windings.

Once the "coarse" position of the armature 19 has been determined, the "fine" position and/or displacement may thereafter be determined on a dynamic basis, that is whilst the machine is in motion.

For practical purposes, the individual inter-tapping distances or winding axial lengths should be mutually equal as precisely as possible. However, the armature length L need not be precisely equal to twice the winding lengths. Acceptable tolerance may be determined by trialand-error.

It will be appreciated that a "dead length" in determining position and/or displacement of the hydraulic ram is equal to the length L of the armature 19. In order to reduce the armature length, the number of tappings must be increased appropriately.

Modifications of the apparatus described above within the scope of the appended claims include an alternative to the multi-tapped arrangement described, viz.a series of separate windings whereof each end is brought out to connection with a switching device. Thus, with such an alternative arrangement an armature means consisting of axially-spaced loading elements may be used and axiallyspaced groupings of windings may be addressed. However, such an arrangement increases the "dead length".

Also, in a further modification, the hollow cylindrical armature is replaced by a solid or "male" armature for travel within the windings of the stator. An alternative material for the armature is a ferrite material.

It will be appreciated that the position/ displacement-determining apparatus of the present invention may be used in conjunction with any two relatively movable objects; and that the armature/stator assembly may be used on a "stand alone" basis.

In the context of the present specification, the term "absolute" means that position data is ascertained without reference to any initial datum or basic reference point.

Claims

1. Apparatus for use in determining position and/or displacement of one object relative to another, comprising a stator for association with said one object and having a multi-tapped winding or a series of windings, an alternating current source for energising the windings, armature means associated with the other object and effective to load inductively windings on the stator, and electrical sensing means operable to detect and measure inductive loading of windings on the stator.

2. Apparatus for use in determining position and/or displacement of one object relative to another, comprising a stator for association with said one object and having a multi-tapped winding or a series of windings, an alternating current source for energising windings on the stator, armature means associated with the other object and effective to load inductively windings on the stator, electrical sensing means operable to detect and measure inductive loading of windings on the stator, and switching means operable in a first mode to scan the stator windings and to connect the latter in turn to said sensing means, and operable in a second mode to provide electrical connection between the sensing means and a group of the stator windings.

3. Apparatus according to claim 2, wherein the said sensing means is operable in said second mode to step in such a way as to provide electrical connection between the sensing means and groups of stator windings overlapping with the group first mentioned in claim 2.

4. Apparatus according to claim 2 or 3, wherein the armature means comprises a single armature capable of influencing inductively an effective length of windings along the stator, and the length of each winding is substantially half said effective length.

5. Apparatus according to claim 4, wherein the armature comprises a hollow cylinder of a material having good electrical conductivity arranged for bidirectional movement along the stator in close inductive coupling with said windings.

6. A piston-and-cylinder assembly incorporating apparatus as claimed in any one of claims 2 to 4, wherein the piston of the assembly has a hollow piston rod opening to the crown of the piston, the stator is anchored at one end thereof to a head of the cylinder and disposed in telescopic arrangement with the piston rod, and the armature means is disposed adjacent the piston crown.

7. A piston and cylinder assembly according to claim 6, wherein the cylinder head incorporates a pressure-tight gland providing sealed passage therethrough of electrical conductors from the stator windings.

8. A piston and cylinder assembly according to claim 7, including an auxiliary chamber adjacent said cylinder head and housing at least part of the said switching means.

9. A method for determining position and/or displacement of a loading armature relative to a stator having a multi-tapped winding or a series of windings, comprising the steps of energising the windings in turn and measuring a loading for each winding to estimate the approximate position of the armature along the stator, energising a group of the windings including said approximate position and measuring the impedance of the group to determine more accurately the position of the armature.

10. The method of claim 9, including the step of comparing said impedance with a predetermined value, and step-switching to a different group of the windings according to the result of the comparison.

11. Apparatus for use in determining position and/or displacement of one object relative to another, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

12. A method for determining position and/or displacement of a loading armature relative to a stator having a multi-tapped windings or a series of windings, substantially as hereinbefore described with reference to the accompanying drawings.