GB2228835A - Improvements in or relating to potentiometers - Google Patents

Abstract

A rotary potentiometer is disclosed comprising a pair of spaced continuous paths one of which is of an electrically resistive material and is intended to have a voltage applied across circumferentially spaced points. Bridging means 4 displaceable with respect to the resistive path electrically connects the latter to the other path to apply a voltage which is a function of the displacement of the bridging means. With uniform concentric rings each with a pair of terminals displaced 180 DEG from each other and the pairs of terminals angularly displaced by 90 DEG and with a voltage applied across the terminals of the resistive ring, the whole of the other ring will be at a potential which is a function of the angular displacement of the bridging means from one of the terminals of the resistive ring. This can provide a measure of such displacement. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO POTENTIOMETERS This invention relates to improvements in potentiometers.

Rotary potentiometers generally are of the toroidally wound, conductive plastic, graphite or hybrid types. A voltage is applied across the ends of the generally circular resistive path and tapped by at angularly displaceable members to provide an output voltage which is a function of the angular displacement of the tapping point about the centre of the circular resistive path with respect to a datum position. Such known types of rotary potentiometers in which the voltage is applied across the ends of the resistive path has such ends spaced with respect to each other so that the resistive path subtends an angle less than 3600 to the centre.

It is an object of the present invention to provide an improved rotary potentiometer in which the resistive path subtends an angle of 3600 to the centre and the tapping point is capable of being angularly displaced about the centre through an angle of 3600 or more.

According to the present invention, there is provided a rotary potentiometer comprising a first continuous path of an electrically resistive material adapted to have a voltage applied across two circumferentially spaced points, a second continuous electrical path, means displaceable with respect to the first path electrically to connect the first path to the second path and apply to the second path a voltage which is a function of the displacement of the displaceable means with respect to the first path and means associated with the second path to derive a measure of such applied voltage.

Preferably, the first and second paths are spaced rings of substantially uniform resistivity along their length.

With advantage, the second path is electrically conductive.

Desirably, the first and second paths are spaced and separated by a normally electrically resistive material which becomes electrically conducting when exposed to light and the displaceable means comprises means for displacing a light beam along said normally resistive material to span the distance between the first and second paths.

Alternatively, the first and second paths are spanned by a displaceable slider electrically to connect one path to the other.

The first and second paths are preferably formed as uniform concentric rings which can be radially or axially spaced from each other or can be both radially and axially spaced.

Additional continuous paths may be associated with the second paths, such additional paths having associated angularly displaceable bridging means for electrically connecting one of the first and second paths thereto.

In a preferred construction, the first path is provided with a first pair of contact points circumferentially spaced by substantially 1800 for the application of the applied voltage and the means associated with the second path includes a second pair of contact points circumferentially spaced with respect to the second path by 1800 and the second pair of contact points are angularly displaced with respect to the first pair of contact points by 90o.

Embodiments of the invention will be described by way of example, reference being made to the accompanying drawings in which: Figure 1 is a schematic representation of a known toroidally wound rotary potentiometer, Figure 2 is a schematic representation of a known rotary potentiometer having a resistive path formed of a conductive plastics material, Figure 3 is a view similar to Figure 2 illustrating a known modification, Figure 4 is a schematic view of a rotary potentiometer according to the present invention, Figure 5 shows an electric circuit representing the parameters of the Figure 4 embodiment, Figure 6 is a graph illustrating the relationship of the voltage at the tapping point on the potentiometer of Figure 4 in relation to its angular displacement relative to a datum position, and Figure 7 illustrates an alternative embodiment of the present invention.

The known form of rotary potentiometers illustrated in Figure 1 comprises a resistance wire 1 wound on a toroid 2 of electrically insulating material with the ends of the wire 1 connected to terminals 3 across which an input voltage Vi is applied. The wound resistance wire 1 subtends an angle of less than 3600 to the centre. A wiper 4 contacts the wire 1 and is angularly displaceable about the central axis by a shaft 5 to tap the winding and provide an output voltage Vo which can be collected by slip ring associated with the shaft 5 and which is a function of the angular displacement of the wiper 4 from a datum position determined by either one of the terminals 3 with respect to which it is measured. The winding of the wire 1 is not necessarily uniform, in which case the voltage Vo will not be a linear function of its angular displacement.

It is clear that the two ends of the wound resistance wire cannot come into electrical contact and although the wiper 4 can be arranged to rotate over the gap between the ends of the wire (e.g. by dropp-ing the ends of the winding so that the wiper does not electrically bridge the gap), the electrical angle of the potentiometer must always be less than 3600.

The rotary potentiometer shown in Figure 2 is similar to that described with reference to Figure 1 except that the wire wound toroid is replaced by an electrically resistive track 6 of plastics material, the track 6 being in the form of a discontinous ring on a substrate of electrically insulating material. The output voltage Vo is taken from a slip ring (not shown) associated with the central shaft 5 or, as illustrated in Figure 3, the shaft 5 may be replaced by a central ring 7 of low resistance electrically conductive material connected to an output terminal 8. The bridging wiper 4 can be mechanical or it can be opto-electrical.In the latter case, the space between the outer discontinous ring or track 6 and the central ring 7 comprises a layer 8 of photoresistive material the resistance of which changes from being normally highly resistive to being highly conductive when exposed to light. The wiper 4 would be replaced by a spot of light rotatable about the centre and bridging the track 6 and the central ring 7.

Figure 4 illustrates an embodiment of the invention. In this embodiment, the rotary potentiometer comprises two spaced uniform concentric rings 10 and 11. Each ring is continuous to provide first and second continuous paths electrically isolated from each other apart from the bridging wiper 12. The rings can be formed of electrically conducting plastic, they can be wire wound, provided by a carbon track or any other known construction or a hybrid thereof. In this case, the outer ring 10 is electrically resistive and the inner ring 11 is electrically conductive. The outer ring 10 has a pair of terminals 13 and 14 across which an input voltage can be applied. These terminals 13 and 14 are spaced 1800 apart about a central axis 15 about which the wiper 12 is angularly displaceable.The inner ring 11 has a pair of terminals 16 and 17 which terminals are 1800 apart about the central axis 15 and which pair of terminals is 90o displaced with respect to the pair of terminals 13, 14. No electrical connections other than the terminals 13, 14 and 16, 17 are required.

Figure 5 shows the electrical analogue circuit where R1 is the resistance from terminal 16 to the wiper 12 on the inner track.

R2 is the resistance from terminal 17 to the wiper 12 on the inner track, R3 is the resistance from terminal 13 to the wiper 12 on the outer track, R4 is the resistance from terminal 14 to the wiper 12 on the outer track, and R5 is the resistance of the wiper 12.

The reference 18 identifies an operational amplifier.

If an input voltage Vi is applied across the terminals 13, 14 of the outer ring 10, then the whole of the inner ring 11 will be at an electrical potential determined by the angular displacement of the wiper 12 from either one of the terminals 13, 14. There will be two angular positions of the wiper 12 for a given potential of the inner ring 11 other than Vi and zero volts.

Figure 6 shows the potential Vii on the inner ring as a function of the angular displacement 6 of the wiper 12.

The output voltage at terminals 16 and 17 is equal and is symmetrical about the 1800 angular displacement point of the wiper 12. The output voltage Vii is shown with reference to the zero volt connection on terminal 14.

For an angular displacement e of the wiper 12 from terminal 14 in the range of O to 1800, the value of resistance R2 is always greater than R1 as the path from the wiper 12 to terminal 17 is greater than that from the wiper 12 to terminal 16. Consequently, the output Vop from the operational amplifier 18 will be in a low state representing logic 0. Conversely, for an angular displacement of the wiper 12 from terminal 14 in the range 180 to 3600 the output from the operational amplifier 18 will be in a high state representing logic 1. This output can be used directly in a digital system as a sign parameter to indicate plus or minus 1800 angular displacement. It can also be used directly as the most significant bit in any analogue to digital conversion.

As previously described, the wiper 12 need not be mechanical but could be provided by a continuous ring of light sensitive material disposed between the inner and outer rings 11 and 10 and linking the rings all the way round and an angularly displaceable light source which changes the light sensitive material from a normally high electrical resistance to a low resistance. The light source can be provided by a rotatable slit in an otherwise opaque disc or a preformed rotatable light guide or optical fibre so that the actual source of light could be stationary. Alternatively, the source of light can rotate.

Figure -7 illustrates another embodiment of the invention in which the rings 10 and 11 are concentric and axially spaced on the inside or outside of an electrically insulating bar or tube 19.

It should be noted that once the concentric ring principle has been established, and while the bridge between the rings 10 and 11 has been signified as resistance R5, the system applies also to any semiconductor bridge. The embodiments described have two concentric rings with two terminals on each ring 1800 apart, and displaced by 90o from the terminals on the other ring. It therefore follows that an array of light sensitive diodes or transistors placed symmetrically around the rings and electrically isolated from each other would give the same local conductive path from outer to inner as described above. Similarly, an array of magneto-resistive or Hall effect elements would react locally to a magnet.

The embodiments of the invention described have the advantages 1. Realisation of a liner 3600 potentiometer without the normal discontinuity associated with conventional construction.

2. The availability of a derived digital output equivalent to the most significant bit (MSB).

3. The analogue output of the potentiometer is linear over 0O to 1800 and reflected from 1800 to 3600.

4. Slip-rings are not required as the rotating element is not a tap to pick off the output voltage. The rotating element is a simple bridge allowing greater mechanical flexibility in design e.g. to make the shaft the stationary part and rotate the outer case, or to make an annular potentiometer with a large hole in the centre.

5. The potentiometer can be non-contacting operating from d.c. supplies.

It will be appreciated that the main feature of the invention is the provision of a rotary potentiometer having two spaced uniform concentric rings each with a pair of terminals 1800 displaced from each other with the pairs of terminals angularly displaced by 900 the one from the other.

It will be apparent, however, that additional rings may be provided and electrically bridged with respect to the first two rings to provide other voltages as parameters of angular displacement. Also, where the rings are not uniform, an output voltage can be obtained which is a predetermined non-linear function of the angular displacement of the wiper. Also, by departing from the terminal configuration of 1800 and/or 900 described, the durations of the logic 0 and logic 1 outputs as a function of the angular displacement of the wiper can be varied in a predetermined manner.

Claims (12)

CLAIMS:

1. A rotary potentiometer comprising a first continuous path of an electrically resistive material adapted to have a voltage applied across two circumferentially spaced points, a second continuous electrical path, means displaceable with respect to the first path electrically to connect the first path to the second path and apply to the second path a voltage which is a function of the displacement of the displaceable means with respect to the first path and means associated with the second path to derive a measure of such applied voltage.

2. A potentiometer according to claim 1 in which the first and second paths are spaced rings of substantially uniform resistivity along their length.

3. A potentiometer according to claim 2 in which the second path is electrically conductive.

4. A potentiometer according to claim 2 or 3 in which the first and second paths are spaced and separated by a normally electrically resistive material which becomes electrically conducting when exposed to light and the displaceable means comprises means for displacing a light beam along said normally resistive material to span the distance between the first and second paths.

5. A potentiometer according to claim 2 or 3 in which the first and second paths are spanned by a displaceable slider electrically to connect one path to the other.

6. A potentiometer according to any one of claims 2 to 5 in which the first and second paths are formed as concentric rings.

7. A potentiometer according to claim 6 in which the first and second paths are radially spaced the one from the other.

8. A potentiometer according to claim 6 or 7 in which the first and second paths are axially spaced with respect to each other.

9. A potentiometer according to any one of the preceding claims in which the first path is provided with a first pair of contact points circumferentially spaced by substantially 1800 for the application of the applied voltage and the means associated with the second path includes a second pair of contact points circumferentially spaced with respect to the second path by 1800.

10. A potentiometer according to claim 9 in which the second pair of contact points are angularly displaced with respect to the first pair of contact points by 900.

11. A potentiometer according to claim 1 in which the first and second continuous paths are provided by two uniform concentric rings spaced from each other and each ring is provided with a pair of terminals 1800 apart, the pair of terminals of the first path being angularly displaced with respect to the terminals of the second path by 900.

12. A rotary potentiometer constructed, arranged and adapted to operate substantially as herein described with reference to Figure 4 or Figure 7 of the accompanying drawings.