The hybrid potentiometer element can be represented diagrammatically by a ladder network of resistors:Where: -Rwis the resistance of each turn of resistance wireRpis the resistance of the polymer per unit length (each unit being the wire pitch)Riis the resistance at the interface between the wire and the polymerConsideration of the current distribution in this ladder network shows that there is an initial increase in the current levels in thepolymer film (see fig.2). Because i Fig.1 Fig.2
RwRiRp i
2 i 2 i
2+ i 2 + i i
3 3 i
i 2 2+ + i i
3 3+ + i i
4 4 i
i 2 2 i
i 3 3 i
4 i 4 i
1 i 1 i
i 1 1- - i 3 i
( 3 (~ ≈ i 1 ) i
1)~ i
i total total 1 is at least three orders greater than i
2 or i
3 , the current flowing in the wirewound element can be consideredvirtually constant and thus the voltage increments in each turn of wire are constant, giving linearly increasing voltages at eachpoint of contact between the wire and the polymer film. Kirchoff's first law makes it evident that the voltage generated in the polymer film increases from the low applied voltage end at a non-linear rate until the potential across the interface betweenpolymer and wire is zero. At this point, the current flow in the polymer remains sensibly constant, and the voltage increaseslinearly with track length until the mirror image of the effect just described is encountered at the high applied voltage end ofthe track. These end-effects obviously give rise to a non-linearity of output, but occur only over about 1% of the track length ateach end.One of the great strengths of the hybrid track is the unique way in which it is self compensating for any non-uniformity ofcarbon distribution within the plastic matrix, or for changes in the resistivity of the polymer. Non-uniformity of carbondistribution is a function of manufacture: the actual resistivity of the polymer varies approximately by ±3%, due to the practicaldifficulties in mixing so as to produce a perfectly uniform distribution of carbon particles within the plastic matrix. Changes in the resistivity of the polymer can be caused by wear and/or temperature and humidity variations whilst in service.Contact wear reduces the cross sectional area of the track, temperature variations cause further polymerisation of theplastic/carbon mix, reducing its resistivity. Water absorption by the basic plastic matrix causes a physical expansion of thepolymer, reducing the density of carbon and thus increasing resistance.The self-compensating effect works in the same way as the end-effects already described - for instance, non-uniformity ofcarbon particle distribution and hence resistivity will give rise to non linear changes of voltage along the polymer track and hence to potential differences across the plastic/wire interfaces. These potential differences will cause current to flow into orout of the polymer, depending on the polarity of the potential. However, as explained earlier, by virtue of Kirchoff's first lawthese currents will always tend to reduce to zero within a few turns of wire, and consequently the voltage picked off at the polymer film will be constantly corrected to that of the wirewound element, which we have already seen to vary sensibly linearlywith track length. No other type of potentiometer has this self-compensating feature - it is unique to hybrid technology.If the sheet resistivity of the polymer film changes for any reason, the apparent resistance of the hybrid element will not beappreciably affected due to the very small changes in current flow (remembering that the resistance of the polymer is at leastthree orders higher than that of the wirewound element), nor will the output law drift because of the voltage regulating characteristic of the current distribution. The polymer film on a hybrid track element is at least twice the thickness of a normalplastic film potentiometer and, with similar wear rates, it follows that over the lifetime of the respective potentiometers the voltage output from the hybrid element will remain unchanged, whilst that of the normal film element inevitably changes as the thickness of the element changes with wear and the worn area increases in resistance. The effects of temperature and humidity changes are much greater on the stability of resistance and voltage output of a plastic film track than that of a hybrid track.Hybrid tracks are manufactured on specialist automated machinery which produce linear track lengths which are thenprocessed and packaged into the various models in the SLS range of linear position sensors. A special forming process can be introduced during manufacture to produce circular tracks, and these are used in the SRS range of rotary position sensors.

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