The Design of Precision Wire-wound Potentiometers
The Design of Precision Wire-wound Potentiometers
Many modern electronic devices call for transducers in the form of precision potentiometers actuated by extremely low operating forces. For the windings of such potentiometers certain of the platinum alloys provide the required combination of properties, and this article discusses both the choice of resistance material and the design and construction of these potentiometers.
The great increase in recent years in the use of analogue devices in computing and control circuitry has drawn attention to the enormous possibilities of precision potentiometers designed for extremely light operating forces.
A potentiometer is an electro-mechanical device consisting of a resistance element in contact with a movable slider. A diagram of a simple potentiometer is shown in Fig. 1, in this case the resistance element being a slide wire of uniform cross-section. If a battery B is connected across the ends of the slide wire then the open circuit voltage V appearing between the slider and one end of the slide wire P will be proportional to the displacement 1 of the slider from P.
Fig. 2 shows how this slide wire potentiometer can be used to compare two unknown e.m.f’s, E1 and E2. The battery B, whose e.m.f. is greater than either E1 or E2, passes a current I through the slide wire, the resistance R having the purpose of limiting this current. Points S1 and S2 on the slide wire can be found for both E1 and E2 where, with the switch K closed, no current flows through the galvanometer G. The ratio of the length of slide wire PS1: PS2 equals the ratio of the e.m.f’s under comparison, E1: E2.
This elementary form of potentiometer was first modified by R. E. Crompton and is still in common use, though later versions make provision internally for calibrating the potentiometer against a standard cell of known voltage enabling direct readings to be made of unknown e.m.f’s.
Types of Potentiometer
For laboratory work the potentiometers used are generally of the enclosed box type comprising four or more decades of which the last may be in the form of a slide wire and the rest in the form of fixed resistors that can be switched in or out of circuit. With such potentiometers low voltage D.C. measurements can be made with an accuracy as high as one part in 100,000.
The most widely known form of potentiometer is that used in electronics engineering and known to the circuit engineer as a “pot”. This normally consists of a resistance element of circular form with the slider mounted on an arm revolving on a central shaft. This type of component is small and inexpensive although its accuracy is low. A common use of such a potentiometer is as a radio volume control, where the resistance element is normally of composition material, or as an adjustment in other types of electronic apparatus.
A more recently developed type of potentiometer comprises an accurately wound resistance element, often having miniature dimensions, used in conjunction with mechanical sensing elements to provide information, in terms of voltage, on such physical changes as displacement, temperature, pressure or acceleration. Transducers incorporating such potentiometers are used for data transmission in telemetry systems and as control elements in automatic processes in both of which applications accurate measurement of these values is essential. Typical transducer elements are shown in the accompanying illustrations.
In the simple transducer shown in Fig. 3 a linear potentiometer is actuated by a pushrod; the axial displacement of this rod can be measured in terms of the output from the potentiometer. The low pressure transducer illustrated in Fig. 4 comprises a potentiometer similarly actuated by a bellows pressure chamber so that when the potentiometer is energised the slider potential varies in proportion to an applied steady or pulsating pressure. Fig. 5 shows an accelerometer incorporating twin transducer elements. Here the acceleration of a spring-loaded mass in the base of the instrument controls the output of the unit. An entirely different type of construction is shown in Fig. 6. Here the winding also consists of a length of wire in the form of a helix but with a brush replacing the slider of the conventional potentiometer. This lifts the wire from the support as it rotates, giving an output of infinite resolution. A precision potentiometer with a circular winding and rotating slider is shown in Fig. 7. The output from the slider wires is fed out from the potentiometer through the slip ring and brushes near the centre.
The design and construction of this general group of precision potentiometers will now be considered in greater detail.
Selection of Resistance Wire
The resistance element consists of precision drawn fine wire close wound on to a former whose shape is often determined by the equipment into which the potentiometer is to be fitted. Generally speaking, the main requirements of the alloy used for the wire are relatively high resistivity, freedom from surface films and a low rate of wear. The last two requirements are to some extent complementary; freedom from surface films permits a low brush pressure to be used and this in turn contributes to the long life of the winding.
Certain of the platinum group alloys are ideally suited for this purpose. They combine freedom from surface films – in many cases at elevated temperatures – with the electrical and mechanical requirements sought by the designer.
There are, however, many other factors that govern the choice of material for resistance wires. In certain applications a low temperature coefficient of resistance is desirable. As a general rule it is found that as the resistivity increases the temperature coefficient decreases and this may present a design problem if a low resistance element is required with a low temperature coefficient.
A high tensile strength is also necessary in order that very fine wires shall be able to withstand the stresses involved in winding without fracture. Miniature precision resistance elements are wound with resistance wires as small as 0.0004 inch diameter, and there is a natural tendency to take advantage of the high tensile strength of the hard drawn wires to case winding problems. In the hard drawn condition the platinum-bearing alloys have realised an ultimate tensile strength of 100 tons per square inch, but it should be remembered that alloys in a severe state of cold work tend to relax over long periods even at ordinary temperatures. Generally the resistivity of an alloy is affected by cold work and the use of an extremely hard drawn wire may give rise to instability of resistance with time. In certain cases therefore it may be desirable to use a properly annealed wire and to accept the greater difficulties involved in handling and winding.
Another factor which is not always obvious to the instrument designer tempted by reading of exceptionally high resistivities and tensile strengths combined with low coefficients is that an alloy chosen for use in a precision potentiometer should not be unduly complex in constitution nor critical in its heat treatment. In either case the manufacture of such a material may well give rise to unacceptable variations in resistivity from batch to batch, or even along a given length of wire, while instability of resistance with time may again be encountered. Moreover, an alloy of great hardness and tensile strength is difficult to draw into fine wire and may yield wire of non-uniform cross section.
There is no limit to the number of compositions that might satisfy these sometimes conflicting requirements in some degree, but it is obviously desirable to confine the choice to a range of materials which have been developed and used commercially over long periods and found to be satisfactory. The table on page 78 lists such a range of alloys and their properties, and while special-purpose materials may sometimes be called for, this range should provide for quite wide variations in design requirements.
The 10 per cent rhodium-platinum alloy, formerly used as a potentiometer wire, is not now employed to any great extent, its temperature coefficient being too high for precision resistance measurements. The principal value of this alloy as a resistance material is its retention of strength at high temperatures in heating applications.
The two iridium-platinum alloys provide useful combinations of moderately high resistivity with a low temperature coefficient but the 20 per cent alloy has the greater mechanical strength. In the same bracket as the latter material is the ternary ruthenium-rhodium-platinum alloy which is now being employed extensively in the newer types of sensitive potentiometer. Alternatively, the straight ruthenium-platinum alloy provides an appreciably higher resistivity with a lower temperature coefficient although its mechanical strength is a little less.
For higher resistance values the 5 per cent molybdenum-platinum alloy offers a good balance of properties, possessing a slightly higher resistivity and a lower temperature coefficient than the tungsten-platinum alloy.
Where the lowest possible temperature coefficient is required the 40 per cent silver-palladium alloy has a great deal to commend it. It has a moderately high resistivity and is widely used in potentiometer work, but it is inferior to the platinum alloys in terms of wear resistance and mechanical strength.
Naturally the successful use of any one of these materials necessitates an unusual degree of precision in wire drawing and adherence by the manufacturer to close tolerances on the uniformity of resistance per unit length of wire. In order to be certain of securing the required properties it is desirable to specify resistance values per yard or per metre rather than diameter of wire, and provided that the requirements are made clear to the supplier in this way a maximum deviation of ± 3 per cent from the specified value can be obtained. Fig. 8 shows an operator threading one of the multiple-die wire drawing machines employed in the final stages of the production of precision resistance wires. The nomogram shown on page 80 will be found useful in arriving quickly at the most appropriate alloy and nominal diameter of wire to meet a particular design problem.
The use of close-wound potentiometers demands, of course, that the wire used should be insulated and in the interests of overall accuracy it is essential that the insulating coating should be as uniform as possible. This requirement is very adequately fulfilled by enamels based upon epoxy-resins such as Diamel.
Types of Former
The former is equally important in the manufacture of precision potentiometers. The material must be dimensionally stable, non-hygroscopic and capable of being produced with a smooth finish on the winding surface. A surface finish of 10 micro-inches is not uncommon for circular section formers but any figure for surface finish must be considered in relation to the actual resistance wire diameter. Ebonite and resin-bonded paper laminates are suitable as former materials provided the ambient operating temperature is low. At ambient temperatures of 100°C and above the high coefficient of linear expansion will increase the former dimensions and may result in permanently distorted windings.
Flat card-type formers are also used in linear potentiometers and for sine-cosine units, while other non-linear potentiometers are usually based on a tapered form of the card. Special treatment must be given to card-type formers to produce a rounded surface on the edge of the card since a rough surface will cause the wire to be displaced during winding, resulting in high and low turns with non-uniform spacing. Such defects will seriously affect the accuracy of the potentiometer. Where an operating angle of 360° is required toroidal formers are used, although these are far more difficult to wind.
The choice of slider material is governed by the need to maintain a low and constant resistance between the slider and the winding and to ensure that their rates of wear are sufficiently low to give the potentiometer an adequate operational life.
When a noble metal winding is used, a noble metal alloy is also favoured for the slider since it has a similar degree of freedom from tarnish films. Gold bearing alloys are in common use as slider materials. Some of these alloys can be work-hardened, either during rolling or drawing or during the forming process, to give mechanical properties similar to those of phosphor bronze, while other alloys are now available that can be heat treated after forming to increase their hardness, and consequently their wear resistance, as well as to improve their mechanical properties.
In the transducer elements illustrated in Figs. 3, 4 and 5 the sliders are made from a gold-bearing alloy having a relatively low modulus of elasticity and this enables a slider pressure not exceeding one gramme to be obtained at the point of contact with the winding. If the environmental conditions in which the instrument must operate include severe vibrations of varying force and frequency then it is desirable to use a slider of high transverse stiffness to increase its natural frequency, but this in turn makes it difficult to obtain a low contact pressure. This can be partially overcome by correct slider design, but usually some compromise is made depending on the permissible sensitivity to static and dynamic acceleration forces.
The resolution of a potentiometer is, in effect, a measure of its ability to distinguish between small differences in slider position and it may be considered as the resistance of one turn of the winding expressed as a ratio of the total resistance of the winding. The resolution of a potentiometer must be taken into account at the design stage, and should not be greater than one-half of the permissible tolerance on potentiometer accuracy. For example, an instrument with an overall accuracy of 1 per cent should have a resolution of 0.5 per cent or less. To take full advantage of the resolution the slider must be correctly shaped to discriminate between adjacent turns on the winding and if this is not done it is most probable that the instrument accuracy will suffer.
When the impedance of the load on the output of a potentiometer is infinitely high, the output voltage will be accurately related to the displacement, that is, in the case of a linear potentiometer the output voltage will be directly proportional to the displacement. This condition is encountered when the output is applied direct into the grid of a valve, or in the case of a null balance circuit as in Fig. 2.
When the load impedance assumes finite proportions an error, known as the loading error, is introduced, as shown in Fig. 9. This is due to the fact that the load draws its current through one part of the potentiometer winding and so causes the current in one part of the winding to be higher than that in the other. In practice this error can be reduced by using end resistors in series with the potentiometer thereby limiting the working range of the potentiometer to the most linear part of the curve.
The maximum current through the resistance element is usually limited by the maximum permissible temperature rise in the winding. Heat dissipation tests carried out on the transducer element of Fig. 3 resulted in the temperature rise curve shown in Fig. 10. Results such as these are helpful in arriving at a figure for the maximum power that can be safely dissipated in a potentiometer winding, but such figures may have to be restricted if the instrument is required to function at high temperatures for long periods.