GB2370695A

GB2370695A - Linear potentiometer

Info

Publication number: GB2370695A
Application number: GB0200405A
Authority: GB
Inventors: Mark Warren Bentley; David James Thompson
Original assignee: Penny and Giles Controls Ltd
Current assignee: Penny and Giles Controls Ltd
Priority date: 1999-04-01
Filing date: 1999-04-01
Publication date: 2002-07-03
Anticipated expiration: 2019-04-01
Also published as: GB2370696B; GB0200405D0; GB2370696A; GB2349986A; GB0200407D0; GB2349986B; GB2370695B; GB9907709D0

Abstract

A linear potentiometer has casing 1 with end fittings 2, 3 and shaft 6 extending therethrough and mounting slider block 7 on which electrical contacts are mounted making sliding contact with wound and pickup elements. End fitting 3 is formed by two axially juxtaposed parts 3a, 3b with the axially outer one snap fitted into a recess in the casing inner wall ensuring the end fitting can be readily removed, disassembled and the seals 19, 21 serviced. The slider 7 has face (7a Fig. 5) at an acute angle to the casing axis to which phosphor bronze contact springs (24) are clamped and bend on contact with the element to remove the need for 'stroking'. The elements are snap fitted into grooves 11,12 having a sectional periphery greater than semicircular in plastics former 10. Opposing groove 30 accepts a pressure tap.

Description

POTENTIOMETER

The invention relates to a potentiometer.

Potentiometers are widely used to provide a voltage signal which is 5 proportional to physical displacement of a member and the invention is concerned with linear potentiometers where the voltage signal is proportional to the axial displacement of a shaft. In previously proposed potentiometers of this kind a wound element and a pickup extend longitudinally within a cylindrical housing, a voltage is placed across the length of the winding, the pickup has negligible electrical resistance 10 and a slider block is connected to the shaft and mounts electrical contacts which resiliently engage the wound element and the pickup, the voltage existing between the zero potential end of the wound element and the corresponding end of the pickup being proportional to the axial displacement of the slider block and thus the shaft from the zero end of the wound element.

15 Sealing the potentiometer, particularly around the shaft entry, is always a problem, seal life is generally quite low and for example, five or six seal changes may be required during the life of a potentiometer.

Forming of the resilient electrical contacts for the slider block is an expensive and highly skilled operation.

20 Mounting of the wound element and the pickup in a plastics former so that they are maintained parallel is again an expensive and skilled operation.

Mounting of electrical correction pressure taps to connect the ends of the wound element and the pickup to wires to extend out of the casing again has caused troubles in the past as has the passage of the wires through end portions of the casing.

25 According to one aspect of the invention, a linear potentiometer comprises a tubular casing with a front end fitting in one end thereof through which a shaft extends wherein the front end fitting comprises two juxtaposed members each having a shaft seal therein and with an outer one of the said members provided as a moulding which can be snap fitted into a recess in the casing to retain the end fitting within the 30 casing.

By unsnapping the outer member from the casing and removing the end fitting from the casing and the front shaft, the seals can be serviced to extend the life of the

potentiometer. Preferably the recess in the casing is a circumferential groove and the outer member includes flexible portions mounting projections which can engage in the groove and be disengaged therefrom by deflecting the flexible portions towards the axis of the casing.

5 According to another aspect of the invention, a slider block mounted on a shaft of a linear potentiometer has a face angled at an acute angle to the axis of the potentiometer, to which face are clamped electrical contact springs which make sliding electrical contact with a wound element or a pickup extending longitudinally within the potentiometer.

10 The contact springs can be clamped in a linear extending position but due to the angle of the mounting face are automatically curved to a desired curvature when engaged with the wound element or pickup thereby obviating the need for a "stroking" operation to preform a curve into the contact spring.

According to a further aspect of the invention, a groove in a plastics former of 15 a linear potentiometer, which former extends longitudinally of the potentiometer, to receive a wound element or a pickup, is provided with a cross section which is greater than a semicircle such that the wound element or pickup can be snapped into the groove and retained therein by the edges of the groove due to the form of the groove.

Twin grooves for the wound element and the pickup are advantageously 20 provided in one face of the former and in a middle portion of an opposite face a further groove is provided into which pressure taps can be clipped to make electrical contact between an electrical wire and the wound element or the pickup, the wires extending through an end fitting of the potentiometer to make external connection.

Preferably the end fitting is injection moulded in a mould through which the wires 25 extend such that they are permanently sealed therein.

The invention is diagrammatically illustrated by way of example in the accompanying drawings, in which: Figure 1 is a schematic cutaway view of a linear potentiometer according to the invention;

Figure 2 is a greatly enlarged version of the righthand end of Figure 1; Figure 3 is an end view of a former of Figure 1; Figure 4 is an enlarged view of the lefthand end of Figure 1; Figure 5 is a side view of a slider block shown in Figure 4; 5 Figure 6 is a schematic sectional end view corresponding to Figure 4; and Figures 7, 8 and 9 show a plan view, side view and end view respectively of a pressure tap of a linear potentiometer according to the invention.

Referring to the drawings, and firstly to Figure 1, a linear potentiometer generally includes a cylindrical casing 1 which at one end receives a rear end fitting i- 10 2 and at the other end receives a front end fitting 3. A front end rod 4 with a connecting eye 5 is coupled to a shaft 6 which extends through the front end fitting 3 and at its rear end is connected to a slider block 7. The rear end fitting 2 has a connection eye 8 and is sealed in the cylindrical casing 1 by means of an O-ring 9 which is provided in an O-ring groove 9a. A former 10, shown in Figure 3, extends 15 longitudinally within the casing 1, is mounted on the rear end fitting 2 and mounts in grooves 11 and 12 therein a wound element and a pickup rod respectively.

As can be seen in Figure 2, the front end fitting 3 comprises two juxtaposed members 3a and 3b each having a centre bore 13 to receive the shaft 6 and an external generally cylindrical surface to engage a counterbore 14 provided at the end 20 of the casing 1. A step IS is provided at the end of the counter bore 14 and a further . is, counter bored recess 16 is provided adjacent the extreme end of the casing 1 into which engage outwardly extending projections 17 on the end fitting member 3_. The member 3a includes an exterior groove 18a for an O-ring 18 and an interior recess 19_ to receive a quad-seal 19, of known kind. Between the members 3 and 3_ of 25 the end fitting is a washer 20 and an inner recess 21_ in the member 3b receives a felt seal 21. Transverse slots 22 in the member 3b provide flexibility for outer portions of the member 3_ which mount the projections 17 to allow the projections 17 to snap into the groove 16 or be disengaged therefrom. By means of the division of the end member 3 into the members 3_ and 3_, and as shown provided with the 30 washer 20 therebetween, and the disengageable projections 17 and the recess 16, the life of the potentiometer can be greatly extended since it is possible readily to

disengage the front end fitting 3 from the casing 1 and replace the quadseal 19 in the recess 19a and the felt seal 21 in the recess 21a. The leftward extending portion 23 of the member 3_, as viewed in Figure 2, forms a bearing for the shaft 6.

Referring to Figure 5 the slider block 7 shown therein is provided with a face 5 7_ which is at an acute angle to the axis of the casing 1 and onto which face 7_ a phosphor bronze electrical contact spring 24 is clamped to mount the spring 24 on the slider block 7. In fact the spring 24 is one of a pair of springs, one of which engages the wound element and the other of which engages the pickup rod. By providing the mounting face 7_ of the slider block 7 for the pickup springs 24 to be at an acute : 10 angle to the axis of the casing 1, a previously necessary highly skilled operation of . "stroking" the phosphor bronze spring to pre-set a curve thereinto can be avoided.

Referring to Figure 3, it can be seen that the grooves 11 and 12 for the wound element and the pickup rod respectively provided in one face of the former 10, which former 10 is a plastics extrusion, are formed with a section which basically comprises 15 more than a half circle with the mouth of each recess formed by projections which due to their shape have some flexibility. Accordingly the wound element and pickup can be snapped into the grooves 11, 12 and retained therein by the form of the grooves. Previously grooves provided to receive such members were generally of square section and glue was provided beneath the members and subsequently cooked 20 in an oven to cure it.

Figure 6 shows a pressure tap 25 to be engaged in a groove 30 provided in the former 10 on an opposite face thereof to the grooves 11 and 12.

As shown in Figures 7, 8 and 9 the pressure tap 25, which is formed of beryllium cooper sheet with a gold plate finish, is formed by pressing from a blank 25 so as to have a tongue 26 at a mid position, two almost circular portions 27 and upturned portions in a V-formation 28 spaced along the length. The pressure tap 25 can be engaged in the groove 30 provided in an opposite face of the former 10 to the grooves 11 and 12 to connect a connection wire to the wire wound element or the pickup. Projections 31 at the mouth of the groove 30 can deflect to enable the 30 circular portions 27 which are connected to contact wires to be engaged in the groove 30, the end portions of the V-formation 28 engaging the projections 31 which are

it) s restored by their own resilience to retain the circular portions 27 in the groove 30.

The arm 26 projects through a slot in the former 10 to make electrical connection with the wound element or the pickup.

The end fitting 2 is moulded in a mould through which the electrical contact 5 wires extend such that when the end fitting 2 is removed from the mould the wires extend therethrough from a position which will be within the casing 1 inboard of the O-ring slot 9 and out through a projecting portion 32 for external connection.

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Claims

1. A linear potentiometer in which a groove in a plastics former which extends longitudinally of the potentiometer, which groove receives a wound 5 element or a pickup, is provided with a cross-sectional periphery which is greater than a semicircle such that the wound element or pickup can be snapped into the groove and retained therein by the edges of the groove due to the form of the groove.

10

2. A linear potentiometer according to claim 1, in which twin grooves for the wound element and the pickup are provided in one face of the plastics former and in a middle portion of an opposite face a further groove is provided into which a pressure tap is clipped to make electrical contact between an electrical wire and the wound element or the pickup, the wire extending 15 through a rear end fitting of the potentiometer to make external connection.

3. A linear potentiometer according to claim 2, in which the end fitting is injection moulded in a mould through which the wire extends such that it is permanently sealed therein.

4. A linear potentiometer comprising a tubular casing with a front end fitting in one end thereof through which a shaft extends wherein the front end fitting comprises two members juxtaposed longitudinally of the casing and each having a shaft seal therein and with the longitudinally outer one of the 25 said members provided as a moulding which can be snap fitted into a recess in the casing to retain the end fitting within the casing.

5. A linear potentiometer according to claim 4, in which the recess in the casing is a circumferential groove and the longitudinally outer member 30 includes flexible portions mounting projections which can engage in the groove

and be disengaged therefrom by deflecting the flexible portions towards the axis of the casing.

6. A linear potentiometer in which a slider block is mounted on the shaft 5 and has a face angled at an acute angle to the axis of the potentiometer, to which face are clamped electrical contact springs which make sliding electrical contact with a wound element or a pickup extending longitudinally within the potentiometer. 10

7. A linear potentiometer according to claim 6, in which the contact springs are clamped in a linear extending position but due to the angle of the angled face are automatically curved to a curvature when engaged with the wound element or pickup.