GB2231728A - Trimming a variable resistor - Google Patents

Abstract

A variable resistor e.g. fuel tank sensor is produced by providing a plurality of electrical contact strips 22 on a ceramic substrate 13 which strips are interconnected by links 25 and upon which is located a resistive film 26 bridging said strips. The links between adjacent strips are then sequentially severed and the resistive film therebetween trimmed e.g. by laser in a similar sequential manner to produce a desired value for the electrical resistance of the resistor when measured across all the strips. <IMAGE>

Description

VARIABLE RESISTORS This invention relates to variable resistors of the kind which comprise an electrically insulating substrate upon which is formed a row of fixed electrical contacts for traverse by a moving electrical contact, the fixed electrical contacts being electrically connected to respective spaced electrically.conductive strips on the substrate, the strips being electrically interconnected by an electrically resistive film printed or otherwise deposited on the substrate.

Variable resistors of the kind mentioned in the preceding paragraph are known, and find use in, for example, the float operated level sensor of a vehicle fuel tank. Although some control over the electrical resistance of the film can be exercised during deposition of the film on the substrate, the accuracy is not sufficient for most applications, and thus it is usual to trim the film after manufacture of the substrate in order to calibrate the variable resistor. The relatively small size of the substrate ensures that the fixed contacts are in very close proximity to one another, and thus an extremely accurate moving probe assembly would be needed in any mechanism designed to measure the resistance of the film between adjacent strips, and to trim the film to achieve the desired resistance between strips.In an attempt to overcome this difficulty, and to automate calibration, it has been proposed to provide every fourth fixed contact of the row with an integral extension which defines a pad for receiving a metering probe. This proposal is disadvantageous in two fundamental respects.

Firstly the incorporation of extensions on every fourth fixed contact increases the size, and also the cost of the variable resistor since it necessitates the use of a larger substrate to accommodate the probe receiving pads, and the process of printing the fixed contacts and the parts electrically connected thereto is further complicated by the presence of the pads. Secondly, because the pads are associated with every fourth fixed contact then it is only possible to measure the electrical resistance across groups of four fixed contacts and thus the calibration from contact to contact cannot be guaranteed. It is an object of the present invention to provide a method of manufacturing a variable resistor wherein the aforementioned disadvantages are minimised.

In accordance with the present invention there is provided a method of manufacturing a variable resistor comprising the steps of providing on a ceramic substrate n spaced electrically conductive strips and a plurality of fixed electrical contacts arranged in a row to be swept by a moving contact in use, each of said strips being electrically connected to a respective fixed electrical contact and said strips being electrically interconnected by respective shorting links, providing an electrically resistive film bridging said strips to define the resistor element of the variable resistor, determining the electrical resistance R of the electrical path from the first strip to the nth strip through said shorting links, severing the shorting link between the first and second strips so that the only electrical interconnection between the first and second strips is through the relevant part of the resistive film, trimming the film between the first and second strips until the electrical resistance between the first strip and the nth strip is equal to the desired value R1 of the resistance between the first and second strips plus (n-1)R, severing n the shorting link between the second and third strips and trimming the film between the second and third strips until the resistance between the first strip and the nth strip is equal to the desired value R2 of the resistance between the second and third strips plus R2 plus (n-2)R, n severing the shorting link between the third and fourth strips and trimming the film between the third and fourth strip until the resistance between the first strip and the nth strip is equal to the desired value R3 of the resistance between the third and fourth strips plus R1 plus R2 plus (n-3)R, and so on up to and including n severing the shorting link between the (n-l)th strip and the nth and trimming the film therebetween until the resistance between the first strip and the nth strip is equal to the desired value R(n~l) of resistance between the (n-l)th strip and the nth strip plus the sum of R1, R2, R3, .... R(n-1).

It will be recognised that in accordance with the above method no moving probes for measuring resistance are needed, since the only resistance measurement which is made is the measurement of the resistance between the first strip and nth strip.

Preferably the trimming of the resistive film is computer controlled, the computer having access to the desired resistance values R1 R2, .... R(n-1), controlling the trimming apparatus to severe the appropriate shorting links, performing the appropriate calculation, and trimming the appropriate region of the resistive film in accordance with the result of the calculation.

Desirably the trimming device is a laser.

In some applications, notably where the variable resistor is used in the level sensor for a vehicle fuel tank, in addition to providing a variable resistor on the ceramic substrate it is desirable also to provide ON-OFF switching of a circuit which in use contains a low fuel level warning light. Conventionally this is achieved by providing an electrically conductive strip which is swept by a moving contact, the strip terminating at the point in the movement of the moving contact at which the warning lamp is to be energised. Leading to the end of the conductive strip is an insulating strip arranged to avoid wear to the moving contact which would otherwise arise from the contact sliding over the abrasive surface of the substrate and to avoid the presence of a step, over which the moving contact must slide to move onto the strip and so energise the warning light.Such an arrangement works well, but is inconvenient to manufacture since it requires a different strip configuration for each different type of level sensor being produced. Naturally different vehicles will require a warning at different points in the movement of.

the moving contact and thus each different sensor will require its own specific substrate and it is an object of a second aspect of the present invention to avoid this difficulty.

In accordance with a second aspect of the present invention there is provided a method of manufacturing a substrate for a sliding contact switching arrangement comprising providing on the substrate a row of fixed electrical contacts along which a moving contact can slide in use, and providing on said substrate, and integral with said fixed contacts, a shorting arrangement whereby the fixed contacts are electrically interconnected, and, severing said shorting arrangement at a predetermined point along its length to divide said fixed contacts into electrically separate sets at a point along the row of contacts at which switching is to occur as the moving contact, in use, slides along the row of fixed contacts, said shorting arrangement being outside the line of engagement of the sliding contact with the fixed contacts.

Preferably the substrate is provided with further contact areas in the path of movement of the moving contact, said further contact areas being electrically separate from said fixed electrical contacts and serving to ensure that the moving contact is held clear of the abrasive surface of the substrate throughout its whole range of movement.

One example of the invention is illustrated in the accompanying drawings wherein, Figure 1 is a plan view, to an enlarged scale, of a substrate for a fuel tank level sensor, prior to calibration thereof, Figure 2 is a view similar to Figure 1 illustrating the substrate after calibration, and Figure 3 is a diagrammatic representation of the substrate in use in a level sensor.

Referring to the drawings, as can be seen from Figure 3 the level sensor includes a moulded synthetic resin body 11 for attachment to the wall of a fuel tank so that the main portion of the body lies within the fuel tank and terminals for connection to the wiring harness of the vehicle are exposed at the exterior end 12 of the body. A ceramic substrate 13 is fixed within the body 11, and carries conductive areas to be described in more detail hereinafter. Pivotally mounted on the body 11 is a moving contact carrier supporting three integral leaf spring moving contacts 15 which sweep respective regions of the conductive areas of the substrate 13 as the carrier 14 pivots relative to the body 11. A float arm 16 is anchored at one end to the carrier 14 and at its other carries a float.The float moves in accordance with the liquid level in the fuel tank and so pivots the carrier 14 relative to the body 11 causing the moving contacts 15 to traverse their respective conductive areas on the substrate 13. Electrical connections are made between the conductive areas on the substrate and the terminals exposed at the end 12 of the body.

The substrate 13 is shown in more detail in Figures 1 and 2 and comprises a ceramic tile, conveniently alumina, upon which is deposited three conductor patterns 17, 18, and 19. The conductor patterns are metallic layers, of relatively low electrical resistance and can be provided on the tile in a number of different ways. For example, the conductor patterns could be provided on the tile by a plating operation, or by etching of a continuous metal layer.

However, it is preferred that the patterns are provided by printing a metal bearing ink onto the tile, the tile then being heated to fuse the metal particles in the ink to form continuous tracks.

The pattern 17 consists of an arcuate row of substantially equiangularly spaced, rectangular, fixed contacts, 21, the curvature of the row having its centre on the axis of movement of the contact carrier 14. Also forming part of the pattern 17, and extending parallel to the short sides of the tile are 53 spaced metal strips 22. Figures 1 and 2 illustrate strips numbers 1 to 15 and 42 to 53, the intervening strips being omitted for clarity. There are 58 fixed contacts, and with the exception of strips 1, 2, 3, 4 and 53 all of the remaining strips are integral with a respective single fixed contact. Strips 1, 2, 3, 4 and 53 are integral with respective pairs of adjacent fixed contacts, the outermost fixed contacts of strips 1 and 53 being extended to form respective contact pads 23, 24.

During creation of the pattern 17 (as seen in Figure 1) there is provided a shorting link 25 integral with all of the strips 22 so that a direct electrical connection exists between the pad 23 and the pad 24 through what is effectively a continuous shorting link extending between the strips 1 and the strip 53, and including each of the strips between strips 1 and 53.

Printed over the fingers 22 is an electrically resistive film 26. Thus a region of the film 26 extends between each strip 22 and its neighbouring strip and defines an electrical resistor bridging the strips. In use a moving contact 15 slides along the arcuate row of fixed contacts 21 and it is intended that the electrical resistance in series between the moving contact and a terminal pad 27 on the substrate shall vary in accordance with the position of the moving contact along the row of fixed contacts so that operation of the variable resistor so produced can be used to control a fuel level gauge.

However, although some control over the resistance of the film 26 can be effected during printing of the film 26 the accuracy thereof is insufficient for most purposes.

Thus it is necessary to calibrate the variable resistor before it can be used. In order to calibrate the variable resistor it may be necessary to trim the film 26 between adjacent pairs of strips 22 and in order to do this there is provided a computer controlled laser trimming device. The substrate 13 is positioned on a worktable associated with a laser trimming head the position of which, relative to the table, is controlled by a computer. A datum mark 20 is provided in one corner of the substrate and is utilized to establish the position of the substrate on the table.The computer controlling the laser trimming head is programmed with details of the patterns on the substrate and once the datum position of the substrate is achieved then the computer can move the laser trimming head to any chosen position on the patterns of the substrate by calculating the coordinates of the desired position with reference to the datum. In addition to data identifying the patterns, the computer also has access to data concerning the desired electrical resistance between adjacent strips 22.

The process of calibrating the variable resistor is as follows. First and second electrical probes associated with the trimming head are engaged with the pads 23 and 24 of the pattern 17. The computer then determines the electrical resistance between the pads 23 and 24, this value in effect being the resistance of the shorting link 25 extending between the strips 1 and 53.

The measured resistance value is stored. The computer now moves the laser trimming head to sever the shorting link 25 in its region between strips 1 and 2 thereby leaving strips 1 and 2 electrically interconnected only through the film 26 bridging the strips. The probes are maintained in contact with the pads 23, 24 throughout the whole operation, and it will be recognised that after the shorting link between the first and second strips has been severed then the electrical resistance between pads 23 and 24 comprises the resistance of the film bridging strips 1 and 2 and the resistance of the remainder of the shorting link 25. The computer has access to the desired resistance of the film between strips 1 and 2 which is to be achieved in a fully calibrated variable resistor, and next moves the trimming head to the region of the film bridging strips 1 and 2 and causes operation of the laser to trim the film. Trimming of the film continues until the resistance measured between pads 23 and 24 is the sum of the desired value of resistance between strips 1 and 2 plus 52, 53rds of the resistance of the originally measured resistance of the whole shorting link 25. The laser trimming head is then moved to the region between strips 2 and 3 and severs the shorting link between strips 2 and 3.The film bridging strips 2 and 3 is then trimmed under computer control until the resistance measured between the pads 23 and 24 is the sum of the desired (and now, after trimming, actual) resistance of the film between strips 1 and 2, plus the desired resistance of the film bridging strips 2 and 3, plus 51 53rds of the resistance of the shorting link 25. Next, the laser trimming head is moved to a position between strips 3 and 4 and severs the shorting link between strips 3 and 4.The laser trimming head is then controlled to trim the film between strips 3 and 4 so that the total resistance between the pads 23 and 24 is the sum of the desired (and now, after trimming, actual) value of the resistance of the film between strips 1 and 2 and between strips 2 and 3, plus the desired value of the resistance of the film bridging strips 3 and 4, plus 50 53rds of the resistance of the shorting link 25.

The sequence of steps is repeated, cutting the shorting link and trimming the film between adjacent strips as necessary until the region between strips 52 and 53 is reached. The last step therefore is severing the final part of the shorting link 25, that is to say between strips 52 and 53 and trimming of the film between strips 52 and 53 until the resistance measured between pads 23 and 24 is equal to the sum of the desired resistance of the film bridging strips 52 and 53 plus the desired (and now, after trimming, actual) value of the resistance of the regions of film bridging all of the other strips.

After calibration therefore the pattern 17 assumes the form seen in Figure 2 in which the shorting link 25 is effectively no longer present.

In practice a ballast resistor is provided in circuit with the variable resistor pattern by a printed resistive film 28 bridging terminal pad 27 and strip number 1. Before calibration of the variable resistor the resistor film 28 is trimmed to achieve a predetermined resistance value, the trimming being performed by the laser trimming head, and the resistance being determined by engagement of the two electrical probes of the head with the terminal pad 27 and the pad 23 respectively. Laser trimming of the film can be achieved by the laser reducing the thickness of the film, but more usually will be achieved by the laser actually burning away part of the film to reduce the length of the film bridging the strips 22.

The pattern 19 on the tile comprises an arcuate row of substantially equiangularly spaced fixed contacts 29 of rectangular form. In use a leaf spring contact 15 can slide along the row of contacts 29 as the liquid level in the fuel tank changes. The contacts 29 at the end of the row traversed by the moving contact when the liquid level in the tank is low include integral extensions 29a in turn integral with a shorting bar 31.

Thus the contacts 29 integral with the shorting bar 31 are electrically interconnected.

As the moving contact associated with the fixed contacts 29 is moved to a position corresponding to a low fuel level it is desired that an electrical circuit shall be completed between the moving contact and a terminal pad 32 integral with the shorting bar 31. The point along the row of contacts 29 at which the circuit is completed will differ dependent upon the nature of the sensor within which the substrate is to be utilized, and this in turn of course is determined by the nature of the fuel tank. In the form in which the pattern 19 is produced it is clear that the electrical connection would be established immediately the moving contact reaches the first of the fixed contacts having an extension 29a.

However, this position may not be suitable for all uses of the substrate, and thus after laser trimming of the film 26 the laser trimming head is moved to the region of the extensions 29a, and is used to sever a predetermined number of the extension 29a dependent upon the intended use of the substrate. Severing commences with the first (the left most in the drawings) extensions 29a and naturally the more extensions 29a that are severed, then the further along the row of fixed contacts 29 will be the point to which the moving contact must move before the circuit between the moving contact and the terminal pad 32 is completed. The computer will of course be programmed with the predetermined requirements, and it will be recognised that a variety of different substrates can be produced from the same basic printed pattern 19.

Moreover, the point at which the extensions 29a are severed lies outside the region actually swept by the moving contact and thus there is no risk of the moving contact depositing conductive material in the cut region and so causing inadvertent electrical bridging. The contacts 29 without extensions 29a without extensions 29a form no part of the electrical arrangement but are provided to ensure that the moving contact is supported out of contact with substrate throughout its whole travel, thus avoiding abrasion of the moving contact on the abrasive surface of the substrate.

The pattern 18 comprises a continuous arcuate conductor 33 lying between, and parallel to, the arcuate rows of fixed contacts 21 and 29. The track 33 has an integral terminal pad 34 and is swept by a third leaf spring moving contact 15, the track 33 constituting a common earth return track associated with both the variable resistor pattern 17 and the switching pattern 19. Thus in use the terminal pad 27 and the terminal 32 will be connected to the live supply, and all three leaf spring moving contacts are electrically interconnected, being integral parts of the same metal pressing.The variable resistor circuit will be from the pad 27 through the resistor 28, the first strip 22, an appropriate length of the film 26 determined by the position of the moving contact, the appropriate fixed contact 21, the moving contact associated with the fixed contacts 21, the moving contact associated with the track 33, and the track 33 itself to the terminal pad 34. Similarly the circuit through the switching pattern 19 will be from the terminal pad 32, to the bar 31, through the unsevered extensions 29a to the respective fixed contacts 29, thence through the appropriate moving contacts (assuming that it is engaged with the appropriate fixed contacts) to the moving contact of the track 33, and thence through the track 33 to the terminal pad 34.

A suitable conductive ink for producing the patterns 17, 18, 19 is DuPont 6120, and a suitable resistive ink for the resistor films 26 and 28 is DuPont 4811.

It should be recognised that although the devices described above are the thick film type utilizing ceramic substrates, other device types are possible, for example, thin film types and those using resin based substrates.

Although the invention has been described in relation to a substrate for use in a fuel tank level sensor it is to be understood that similar substrates produced in the same manner can be used in other environments. Moreover, the variable resistor pattern, and its calibration can be used separately from the switching pattern 19, and of course equally the switching pattern 19 and its manufacture could be used in a substrate which does not have the variable resistor.

Claims (9)

1. A method of manufacturing a variable resistor comprising the steps of providing on a ceramic substrate n spaced electrically conductive strips and a plurality of fixed electrical contacts arranged in a row to be swept by a moving contact in use, each of said strips being electrically connected to a respective fixed electrical contact and said strips being electrically interconnected by respective shorting links, providing an electrically resistive film bridging said strips to define the resistor element of the variable resistor, determining the electrical resistance R of the electrical path from the first strip to the nth strip through said shorting links, severing the shorting link between the first and second strips so that the only electrical interconnection between the first and second strips is through the relevant part of the resistive film, trimming the film between the first and second strips until the electrical resistance between the first strip and the nth strip is equal to the desired value R1 of the resistance between the first and second strips plus (n-1)R, severing n the shorting link between the second and third strips and trimming the film between the second and third strips until the resistance between the first strip and the nth strip is equal to the desired value R2 of the resistance between the second and third strips plus Rl plus (n-2)R, n severing the shorting link between the third and fourth strips and trimming the film between the third and fourth strip until the resistance between the first strip and the nth strip is equal to the desired value R3 of the resistance between the third and fourth strips plus R R1plus R2 plus (n-3)R, and so on up to and including n severing the shorting link between the (n-l)th strip and the nth and trimming the film therebetween until the resistance between the first strip and the nth strip is equal to the desired value R(n~1) of resistance between the (n-l)th strip and the nth strip plus the sum of R1, R2, R3, . . .

2. A method as claimed in claim 1 wherein the trimming of the resistive film is computer controlled, the computer having access to the desired resistance values R1, R2 R --- R(n controlling the trimming apparatus to severe the appropriate shorting links, performing the appropriate calculation, and trimming the appropriate region of the resistive film in accordance with the result of the calculation.

3. A method as claimed in claim 1 or claim 2 wherein the trimming device is a laser.

4. A method of manufacturing a variable resistor substantially as hereinbefore described.

5. A variable resistor manufactured in accordance with any one of claims 1 to 4.

6. A method of manufacturing a substrate for a sliding contact switching arrangement comprising providing on the substrate a row of fixed electrical contacts along which a moving contact can slide in use, and providing on said substrate, and integral with said fixed contacts, a shorting arrangement whereby the fixed contacts are electrically interconnected, and, severing said shorting arrangement at a predetermined point along its length to divide said fixed contacts into electrically separate sets at a point along the row of contacts at which switching is to occur as the moving contact, in use, slides along the row of fixed contacts, said shorting arrangement being outside the line of engagement of the sliding contact with the fixed contacts.

7. A method as claimed in claim 6 wherein the substrate is provided with further contact areas in the path of movement of the moving contact, said further contact areas being electrically separate from said fixed electrical contacts and serving to ensure that the moving contact is held clear of the abrasive surface of the substrate throughout its whole range of movement.

8. A method of manufacturing a substrate for a sliding contact switching arrangement substantially as hereinbefore described.

9. A sliding contact switching arrangement manufactured in accordance with any one of claims 6 to 8.