GB2263200A - Reduction drive for shaft-driven potentiometers - Google Patents

Abstract

In a rotary potentiometer comprising a shaft (14) intended to be rotated about its longitudinal axis by more than a complete revolution when an input member (not shown), of which the angular position is to be monitored electrically, connected to the shaft moves from one limit position to the other, the shaft producing relative angular motion between a single-turn potentiometer track (4) and at least one movable member (30) in low-resistance sliding contact therewith, the shaft rotation is transmitted to the movable member (30) through a speed-reduction mechanism (eg. pinion 22 rotatably mounted on eccentric cam 20 and fixed gear wheel 26 having teeth 23 mating with teeth 24 of pinion 22) disposed in a common plane. The shaft may also be directly connected with a second potentiometer tract (4). In an alternative form of the reduction drive (Fig. 3 not shown) friction is used instead of engaging gear-teeth. <IMAGE>

Description

POTENTIOMETERS This invention relates to potentiometers, and particularly to such potentiometers intended to be driven by a shaft able to rotate in either direction.

It is usual for angular potentiometers to extend over just less than a complete revolution, and to be driven by an input member able to be rotated through more than 3600, and often through several revolutions in going from one limit position to the other. In such cases, it is known to interpose a reduction gear box in the forcetransmission path between the input member and the potentiometer slider. The disadvantages of this solution include the inherent inaccuracies in the gear box, such as those arising from backlash, whereby the rotary position of the shaft derived from the analogue signal produced by the potentiometer differs significantly from the actual position of the input member.This error signal might be quite unacceptable in some applications, such as when the potentiometer is used to indicate to a vehicle control system the rotary position of a steering wheel from the rotary motions thereof.

Another disadvantage is the size of the resultant potentiometer, with its integral gear box, giving rise to many problems as to where, in a car, lorry or other vehicle, it may be mounted most conveniently.

The present invention aims at using differential movement which virtually eliminates backlash and is sufficiently wear-resistant and compact to provide a shaft-driven potentiometer of small size and low signal error.

Accordingly the present invention provides a rotary potentiometer, comprising a shaft intended to be rotated about its longitudinal axis by more than a complete revolution when an input member, of which the angular position is to be monitored electrically, connected to rotate the shaft moves from one limit position to the other, the shaft being adapted to produce relative angular motion between a single-turn potentiometer track and at least one movable member in low-resistance sliding contact therewith, the shaft rotation being transmitted to the movable member through a speed-reduction mechanism disposed in a common plane.

In a particular preferred form of the invention, the potentiometer has two tracks and two movable members, the second movable member being directly connected to the input member to rotate therewith and being in lowresistance sliding contact with the second potentiometer track. This enables the potentiometer to be used to give both a coarse and a fine indication of the angular position of the input member.

The present invention will now be described by way of example with reference to the accompanying drawing, in which: Figure 1 is an exploded isometric view of one form of shaft-driven potentiometer of the present invention; Figure 2 is an axial cross-section of the potentiometer of Figure 1 when in its assembled position, and Figure 3 is an isometric view of an alternative form of differential drive.

In the Figures, those components which are common are given the same references.

The potentiometer shown in the exploded view of Figure 1 comprises a support 2 for a virtually-complete annulus 4 of electro-resistive material, such as of carbon.

Although in its basic form there will be only one potentiometer, in that version shown in Figure 1 an additional potentiometer is shown mounted on another support at the other end of the rotary axis 5 of the potentiometer. The two closely-adjacent terminals of the annulus 4 which forms the two arms of the potentiometer end in contacts 6, and they are in turn connected electrically to two of the external terminals 8, with the third terminal 8 being connected to the solid annular conductor 10 for reasons which will be explained below.

Extending through a cylindrical passage 12 in support 2 is a rotary shaft 14 which is adapted, at its end 16 projecting from the nearer support 2 (as viewed), to have a rotary input member (not shown) secured to it. In one application of the potentiometer of the present invention, the input member may be a steering wheel or a member movable therewith, so that the shaft 14 is driven in 1:1 synchronism with the input member.

The shaft 14 has movable with it a C-shaped member 18, and an eccentric cam 20. Rotatably mounted on the cam 20 is a pinion 22 having its outer periphery formed with conformal teeth 24. Positioned outwardly of the pinion 22 is a fixed gear wheel 26 having in it a substantiallycylindrical opening of which the face is formed with gear teeth 28 mating with teeth 24 of pinion 22. When the potentiometer is assembled together, the stationary gear wheel 26, the pinion 22 and the cam 20 lie in the same radial space between the surface of cam 20 and the housing of the potentiometer.The arrangement is such that one or more of the teeth 24 of the pinion 22 are always in meshing engagement with teeth 28, but as the number of teeth on the pinion are less than those in the gear wheel, relative rotation of the shaft 14 and gear wheel 26 causes precession of pinion 24 in a direction opposite to the direction of rotation of shaft 14.

Rotatably mounted on the shaft 14 on the side of the.

stationary gear wheel 26 remote from the pinion 22 is a substantially C-shaped slider member 30 carrying a pair of wipers 32, of which one is adapted to engage the resistive track 4, and the other the complementary conductor 10, both tracks being printed on the inner (as viewed) face of substrate 2.

Projecting from the respective face of pinion 22 are three pegs 34, of which each is received in a complementary recess 36 in slider member 30.

The arrangement is such that as the pinion 22 is caused to precess in one angular direction as the shaft 14 rotates in the opposite angular direction, the pegs 34 carry the member 30 with them, causing the wipers 32 to move while making low-resistance and low-friction contact with the resistive track 4 and conductive track 10.

The effective gearing provided by the co-planar gear train shown in Figure 1 ensures that the wipers 32 make less than one complete revolution as the shaft 14 moves from one of its angular limit positions to the other.

The actual step-down ratio is chosen in accordance with the maximum angular rotation of the intended input member. Once the step-down ratio is known, it is a relatively-simple matter for the dimensions and number of teeth of the pinion 22 and gear wheel 26 to be chosen accordingly.

In the simplest form of apparatus according to the present invention, the member 18 may be replaced by a stationary end wall of the housing of the potentiometer.

In this case, the shaft 14 is formed with the cam 20, and projecting from the cam is a stub shaft engaging a recess in the end face of the housing so that the end face and the substrate 2 provide journals for the shaft.

However, in that modified form of the invention shown in Figure 1, the end wall of the housing is replaced by another substrate 2 carrying the electro-resistive and conductive tracks and the contacts 8, etc. In this embodiment, pairs of contacts are carried by member 18, on that face which is hidden from view from the view point chosen for Figure 1, so that the wipers engage their respective track. Unlike member 30, the contacts carried by member 18 rotate at the same speed as shaft 14, and in the same direction, so that the respective contacts make several laps round the potentiometer tracks of the remote substrate 2 as viewed in the drawing, compared with the rotation of the movable member 30 of the potentiometer positioned on the near end of shaft 14 in Figure 1.

Because the edge cam 20, pinion 22 and gear wheel 26 are positioned in the same radial plane, and have substantially the same thickness measured along the axis of the shaft, there is very little space taken up between the inner face of number 18 and that face of member 30 which carries wipers 32. The present invention therefore provides a robust and compact mechanism by which analogue signals of the rotary position of shaft 14 can be derived by measuring the changes in electrical resistance between the terminals 8 at the respective end of the potentiometer housing 38.

The potentiometer at the other end of the housing 38, which is engaged by wipers 32 caused to rotate in a 1:1 relationship with the shaft 14 may be used to derive other analogue signals, such as those which count the number of revolutions of the wipers 32, and therefore of the steering wheel or other input member, for the significantly-smaller angular movement of the wipers 32 on the main potentiometer. Alternatively, circuitry connected to this potentiometer may derive a more accurate angular position for shaft 14.

The alternative form of reduction drive shown in Figure 3 uses friction in place of engaging gear-teeth. The pinion 22 of Figure 1 is replaced by an annular drive member 43, and the fixed gear-wheel 26 by a stator 45.

The member 43 is rotatably mounted on the cam 20, and its cylindrical outer surface 44 is in frictional contact with the inner face of the cylindrical aperture 46 in stator 45. This frictional contact may be ensured or enhanced by making one of the contacting surfaces of rubber or other elastomeric material, as by seating an Oring (not shown) in either member 43 or stator 45. Thus as member 43 rolls around aperture 46 without sliding, it precesses at a speed which is related to the speed of shaft 14, and therefore cam 20, by the ratios of the effective diameters of the contacting surfaces.

Claims (11)

1. A rotary potentiometer, comprising a shaft intended to be rotated about its longitudinal axis by more than a complete revolution when an input member, of which the angular position is to be monitored electrically, connected to the shaft moves from one limit position to the other, the shaft being adapted to produce relative angular motion between a single turn potentiometer track and at least one movable member in low-resistance sliding contact therewith, the shaft rotation being transmitted to the movable member through a speed-reduction mechanism disposed in a common plane.

2. A potentiometer as claimed in claim 1, in which the shaft has an eccentric edge cam movable with it, the cam having rotatably mounted on it a drive member in rolling contact with a stator, whereby the drive member precesses as the shaft rotates, the drive member being adapted to rotate at least one wiper in sliding contact with an arcuate body of electro resistive material.

3. A potentiometer as claimed in claim 2, in which the drive member and the stator are formed with co operating teeth.

4. A potentiometer as claimed in claim 2, in which the drive member and the stator have contacting surfaces of frictional material.

5. A potentiometer as claimed in claim 4, in which one of the surfaces is provided with an O-ring fast with either the drive member or the stator.

6. A potentiometer as claimed in any preceding claim, in which the drive member has a plurality of pegs projecting axially from one face thereof and working in recesses in a member rotatably mounted on the shaft and carrying the said wiper(s).

7. A potentiometer as claimed in any preceding claim, in which a stationary support carries two radially spaced arcuate tracks of electro conductive material, of which one track is of electro resistive material having its ends connected to a pair of terminals projecting from the support.

8. A potentiometer as claimed in claim 7, in which the potentiometer is in a housing from which the terminals project, the housing being adapted to have part of the shaft of the potentiometer projecting therefrom so that a rotary input member may be connected thereto.

9. A potentiometer as claimed in any preceding claim, in which axially spaced from the first support for the potentiometer track is a second like support having contact wipers there for connected to rotate with the shaft in a 1:1 relationship.

10. A potentiometer as claimed in claim 9. in which the supports have their tracks facing towards each other, and with their respective contact terminals projecting from the same face of the housing.

11. A rotary potentiometer substantially as described herein with reference to, and as shown in, Figures 1 and 2 or Figure 3 of the accompanying drawings.