GB2083710A - Variable resistors for use in the sender units of vehicle liquid fuel gauges - Google Patents

Abstract

In a method of forming a variable resistor having a contact member movable to vary the effective resistance of the resistor, first and second layers (3, 4) of resistance material (e.g. resistance ink) are provided on a substrate (1) and are electrically connected in series. The layers (3, 4) are of different specific resistivities and cover different areas of the substrate (1). The profiles of the layers (3, 4) are so trimmed that movement of the contact member from the lowest effective resistance position to the highest effective resistance position over for example a grid of parallel conductor bars (2) on the substrate causes the effective resistance to be provided by a progressively increasing area of the first layer (3) and then by a progressively increasing area of the second layer (4) electrically in series with the whole area of the first layer (3). The final resistor may thus comprise the areas 14, 15 for movement of the contact member over a limited angle for one particular fuel tank, or the areas 16, 17 to give the correct resistor law and range for a different tank. <IMAGE>

Description

SPECIFICATION Variable resistors This invention relates to variable resistors for use in the sender units of liquid fuel gauges of vehicles. In such sender units the variable resistor has a movable contact whose position is varied, usually by a float-actuator, to vary the effective resistance value in dependence upon the liquid level in the vehicle fuel tank. An electric signal is transmitted from the sender unit to the fuel gauge meter to set the latter at a reading related to the effective resistance value.

In practice the change in position of the resistor contact member is not linearly related to the change in liquid level in the fuel tank. Also vehicle fuel gauge meters usually have a scale indicating liquid volume in the tank as a proportion of the "full" volume and in practice changes in liquid level are not linearly related to changes in liquid volume in the fuel tank. Consequently the variable resistor has to be designed so that as the contact member moves over the resistor the effective resistance varies non-linearly, in accordance with a predetermined resistance law, to compensate for the aforesaid non-linearities. For high accuracy the resistance law can also take account of other non-linearities in the fuel gauge set-up, for example inherent non-linearities in the fuel gauge meter and non-linearities in the scale of the fuel gauge meter.

The predetermined resistance law is therefore computed from the design parameters of the fuel gauge and is unique to one set of parameters. Consequently each different set of parameters requires a different variable resistor.

In view of space and other constraints placed upon a vehicle designer requiring for example a particular shape of fuel tank or a particular mounting position of the float arm in the fuel tank, even within the production series of one vehicle manufacturer the parameters of the fuel gauges vary from one model to another. To accommodate all models in the production series of a number of manufacturers therefore requires a large number of different variable resistors.

In the past wire wound resistors have been used but due to their complex winding configuration there are obvious production difficulties in providing a large number of different resistors. It has already been proposed therefore to use resistors comprising a layer of resistance material supported on a substrate since the profile of the resistance layer can be formed or trimmed to suit a predetermined resistance law relatively easily.

For reasons of economy it is advantageous to form a series of different resistors starting from a common base resistor having a resistance layer of predetermined profile which can be trimmed to accord with a series of predetermined resistance laws, and it is to the formation of resistors in this way with which the present invention is concerned. The object of the present invention is to provide a method of forming the resistors more economically and with more flexibility than hitherto.

According to the present invention, in a method of forming a variable resistor from a base resistor, the base resistor is provided with first and second layers of resistance material supported on a substrate and electrically connected in series, said first and second layers of resistance material being of different specific resistivities and covering different areas of substrate, and the profiles of the two layers are so trimmed that movement of the contact member from the lowest effective resistance position to the highest effective resistance position is progressively over the layer of lower specific resistivity material and then progressively over the layer of higher specific resistivity material.

The resistor may be designed so that the contact member moves over the layers of resistance material in direct contact with the layers or in contact with a grid of conductor bars provided on the substrate and which extend into contact with the layers of resistance material.

The invention will now be further described by way of example with reference to the accompanying drawings in which: Figure 1 shows a plan view of the base resistor, and Figure 2 serves to illustrate how the base resistor is trimmed to suit different resistance laws.

Referring to Figure 1, the base resistor comprises a substrate 1 on which are printed a grid of parallel conductor bars 2. Over the conductor bars are printed two elongated layers of resistance material 3 and 4 shown in outline profile. The material of the conductor bars 2 is chosen to have a high electrical conductivity with good wear properties. The resistance layers 3 and 4 are formed of resistance inks. The material of layer 3 has a lower specific resistivity than the layer 4. The left-hand end of the layer 3 is connected to one resistor terminal via a contact strip 5, a layer of resistance ink 6 and a contact strip 7 in series, the items 5, 6 and 7 also being printed on to the substrate 1.The float-controlled contact member (not shown) of the resistor is mounted on a pivoted contact arm and is arranged to track over the conductor bars 2 on an arc which extends adjacent the lower edge of layer 3 and between the two layers 3 and 4 as shown in Figure 2 by dotted line 20.

Also provided on the substrate 1 is a "low fuel" contact strip 8, which is used to indicate when the volume of fuel in the tank has reached a predetermined low level. A contact member (not-shown) separate from the resistor contact member is mounted on the pivoted contact arm and moves into contact with this strip 8 when the float reaches the "low fuel" level to complete a circuit to a warning light.

The resistance layers 3,4 and 6 are of a larger size than required as will be explained later so that they can be trimmed down to any one of a range of desired sizes. Also the contact strip 8 is of a longer length than required so that it can also be trimmed down. Contact pads 10 are provided at regular intervals along the grid of conductor bars 2. These pads 10 are used as tapping points for computerised laser trimming of the profiles of the resistor layers 3 and 4 and conductor bars 2 where necessary in accordance with known trimming techniques.

Figure 2 is a somewhat diagrammatic representation of the base resistor of Figure 1 and the same reference numerals have been used in both Figures to identify corresponding parts. Figure 2 illustrates how the base resistor can be trimmed in accordance with two different resistance laws. The first resistance law applies to an arc of movement of the resistor contact member that is considerably smaller than the arc of movement to which the second resistor law applies. Thus in the first case the resistor contact member moves between the lowest effective resistance position which is when it is in contact with the extreme left conductor bar 2 as seen in the drawing and the highest effective resistance position indicated by the line 12.

In the second case the resistor contact member moves between the lowest effective resistance position, i.e.

the extreme left conductor bar 2, and the highest effective resistance position indicated by the line 13.

As Figure 2 shows, the printed layers 3 and 4 of resistance material are co-extensive over a large annular range. The trimming technique is to initially trim the layer 3 of lower resistivity by laser scribing from the low resistance end i.e. the left hand end as seen in Figure 1, until it is no longer possible to increase the resistance without trimming below a chosen minimum width. Up to this point any higher resistivity material of the layer 4 is disconnected by a suitable cut. When this point is reached the remaining low resistivity material is disconnected and the trimming proceeds on the higher resistivity layer 4. The nett effect is that, whatever the angular extent of the movement of the resistor contact member is, the resistor track is made up of two consecutive sections of low and high specific resistivity material.

Thus in accordance with the first resistance law the layers of resistance material 3 and 4 are for example trimmed to leave as effective parts of the resistor only the areas 14 and 15 which have been shown shaded by vertical lines. The resistance layer 6 must also be trimmed since it provides the lowest effective resistance when the resistor contact arm is on the extreme left conductor bar 2 to indicate the "full" condition of the tank and must therefore be scaled to provide an incrementai value in accordance with the resistance law. The "low fuel" conductor strip 8 is also similarly trimmed so that its associated contact member contacts it at a predetermined position ahead of the line 12.

In accordance with the second resistance law the layers of resistance material 3 and 4 are trimmed to leave as effective parts of the resistor, areas 16 and 17 shaded by horizontal lines and the resistance layer 6 and the conductor strip 8 are correspondingly trimmed. It will be seen that the effective area 16 completely overlaps the area 14.

There are two components that make up a resistance law. First there is a fixed minimum amount of resistance at the low resistance end. In addition there is the angle dependent component which is made up by the profiling of the resistance layers. The relationship between angle and resistance covers a range represented by the ratio of the maximum and minimum slopes, this slope ratio being equal to the maximum and minimum widths of the layers of resistance material. The profiles of the layers of resistance material must therefore be capable of being trimmed to give the required change of resistance with angle whilst providing the required maximum and minimum resistances.

The resistor law requirements are complicated by the possible variations in angular range of movement of the contact member. This angular range may vary from a minimum of about 50 to a maximum of about 100 . As a result if only a single resistance layer is provided it is possible for almost haif of this to be unused in some cases. By providing two layers of different resistivity material with the higher resistivity material at the high resistance end it is possible to cope with the larger variations in slope with iess resistance material and in a compact form. The layer of higher resistivity material must of course be sufficiently wide and sufficiently long to allow trimming to provide areas at different positions for example at the two positions shown in Figure 2.

In order to minimise on the amount of resistance material used it may be more economical to print for example two substrates to cover all required resistance laws rather than one, if the requirements of the resistance laws polarise into two groups.

The profiles for the resistance layers 3 and 4 to be printed on the substrate must encompass the extreme resistance laws to be covered.

To simplify the analysis, the extreme laws can be assumed to be logarithmic and only one ink be considered. For two resistance layers of different specific resistivity material then any area of the lower resistivity material can be replaced by a similarly shaped area of the higher resistivity material scaled up in width by the ratio of their resistivities.

A logarithmic variation in resistance is produced by a profile whose width changes linearly with contact position along the length of the track. The equation for the law is: R = id LOgn ( d d L (d-x) where R = resistance.

x = contact postion along track # = resistivity L = maximum width d = track position at which the width would be zero.

An important parameter of the resistance law is the ratio of the slopes at each end of the track. This ratio as previously stated corresponds to the ratio of the maximum and minimum track widths.

If resistance slope ratio# L = Yo where Yo = minimum track width then d = X0# ......... 2 #- 1 where Xo is the track position at which the width equals Yo From 1 and 2 # R(Xo)Yo (6 -1 ) (-1) Xo ln5 3 where R (Xo) is the total track resistance AlsoYo = Xo (Log#G) R(Xo) (#-1) R (Xo) (5-1 ) .......... e Equation 3 is used to calculate the required ink resistivity. If the values of the variables are for the following extreme case: Smallest Xo Smallest Yo Largest R (Xo) Largest 5 then any other combination will require the profile to be trimmed narrower than the specified Yo.This can be seen by observing the affect of these variables on Yo as governed by Equation 4.

Given the value of resistivity calculated in Equation 3, the maximum profile widths required are defined by the following extreme cases: Largest Xo Smallest R(Xo) ) Maximum L Largest 5 Largest Xo Smallest R(Xo) ) MaximumYo Smallest 6 The printed profile must be the envelope of these two worst cases. To ensure that it can be trimmed in the worst case of printing tolerance, the print should be 25% wider than predicted by these Equations.

The actual print ink resistivity must be a predetermined factor higher than if it did not extend over the conductor bars but ony contacted their ends to allow for the area of shorting of its profiles by the conductors bars and another factor higher to compensate for the termination effect. This analysis allows the print profile to be calculated from worst case parameters.

An alternative approach to an analysis based on a logarithmic approximation may be necessary depending upon the particular parameters of the resistors required.

CLAIMS (Filed on 21/8/81) 1. A method of forming a variable resistor having a contact member movable to vary the effective resistance of the resistor, wherein first and second layers of resistance material are provided on a substrate and electrically connected in series, said first and second layers being different specific resistivities and covering different areas of the substrate, and the profiles of the first and second layers are so trimmed that movement of the contact member from the lowest effective resistance position to the highest effective resistance position causes the effective resistance to be provided by a progressively increasing area of said first layer and then by a progressively increasing area of said second layer electrically in series with the whole area of said first layer.

2. A method according to claim 1, wherein said first and second layers of resistance material are formed of resistance ink.

3. A method according to claim 1 or 2, wherein a grid of parallel conductor bars are laid on substrate for successive and direct contact by the contact member as it moves from the lowest to the highest effective resistance position and said first and second layers of resistance material are laid in contact with said conductor bars such that after trimming of said layers at least one of said bars connects the layers electrically in series.

4. A method according to claim 3, wherein said first and second layers are laid over said conductor bars such that after trimming of said layers a predetermined incremental resistance value is provided by said layers between each adjacent pair of conductor bars.

5. A method according to any preceding claim, wherein said layers are of elongate form and before trimming are co-extensive over at least part of their lengths in the direction of movement of the contact member between the lowest and highest effective resistance positions.

6. A method according to claim 5, wherein said second layer before trimming is shorter than said first layer, and said first layer before trimming is co-terminus with said second layer at the high effective resistance end of the resistor.

7. Avariable resistor made by the method of any preceding claim.

8. A variable resistor made by the method of claim 3, wherein said contact member is arranged to move along a path between said first and second layers.

9. A method of forming a variable resistor substantially as hereinbefore described with reference to the accompanying drawings.

10. A variable resistor substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. to be calculated from worst case parameters. An alternative approach to an analysis based on a logarithmic approximation may be necessary depending upon the particular parameters of the resistors required. CLAIMS (Filed on 21/8/81)

1. A method of forming a variable resistor having a contact member movable to vary the effective resistance of the resistor, wherein first and second layers of resistance material are provided on a substrate and electrically connected in series, said first and second layers being different specific resistivities and covering different areas of the substrate, and the profiles of the first and second layers are so trimmed that movement of the contact member from the lowest effective resistance position to the highest effective resistance position causes the effective resistance to be provided by a progressively increasing area of said first layer and then by a progressively increasing area of said second layer electrically in series with the whole area of said first layer.

2. A method according to claim 1, wherein said first and second layers of resistance material are formed of resistance ink.

3. A method according to claim 1 or 2, wherein a grid of parallel conductor bars are laid on substrate for successive and direct contact by the contact member as it moves from the lowest to the highest effective resistance position and said first and second layers of resistance material are laid in contact with said conductor bars such that after trimming of said layers at least one of said bars connects the layers electrically in series.

4. A method according to claim 3, wherein said first and second layers are laid over said conductor bars such that after trimming of said layers a predetermined incremental resistance value is provided by said layers between each adjacent pair of conductor bars.

5. A method according to any preceding claim, wherein said layers are of elongate form and before trimming are co-extensive over at least part of their lengths in the direction of movement of the contact member between the lowest and highest effective resistance positions.

6. A method according to claim 5, wherein said second layer before trimming is shorter than said first layer, and said first layer before trimming is co-terminus with said second layer at the high effective resistance end of the resistor.

7. Avariable resistor made by the method of any preceding claim.

8. A variable resistor made by the method of claim 3, wherein said contact member is arranged to move along a path between said first and second layers.

9. A method of forming a variable resistor substantially as hereinbefore described with reference to the accompanying drawings.

10. A variable resistor substantially as hereinbefore described with reference to the accompanying drawings.