The invention relates to thick-film heating elements, and in particular to thick-film heating elements having embedded temperature sensors.
Thick-film heating elements are compact, offer high heat transfer performance, and can be produced in a variety of shapes. This renders them ideal for many applications, often where heat transfer into a flat contact surface or a fluid flow is required. Such applications range from commercial equipment such as medical and laboratory devices or manufacturing facilities, to consumer appliances such as washing machines, irons, personal care products and beverage dispensers.
The heating film 16 is defined by a heating track, or ‘trace’, namely a continuous, elongate track of conductive material, such as tungsten, formed using metallic ink. The track is formed in a suitable pattern, such as the serpentine shape shown in
The protective film 18 covers the heating film 16 to act as a mechanical shield to protect the heating track from damage and corrosion, in particular to guard against oxidation of the tungsten of the heating trace.
Although the substate 12 has an oblong cross-section in the
In this example, the substrate 12 is of stainless steel as is conventional. Hence, the insulating layer 14 is used to separate the heating film 16 from the substrate 12, to isolate the heating trace electrically from the substrate 12. However, if a non-conductive substrate is used the insulating layer 14 may not be required.
In this respect, ceramic-based substrates are beginning to find favour in certain applications, since they can offer higher power density than their metallic counterparts, in that a greater heat output can be achieved for a given surface area. Ceramic substrates can be fabricated as a sintered laminate structure formed from an initial stack of ceramic layers, allowing them to form unique shapes including curves, bends and irregular shapes, which are useful in certain applications and would be significantly more costly to machine from steel, for example. In such arrangements, the protective layer 18 can also be formed from a ceramic layer forming part of the initial stack and co-fired with the other ceramic layers, thereby forming a monolithic ceramic structure in which the heating film 16 is embedded, with only end connector terminals exposed.
Although not illustrated in
It is against this background that the present invention has been devised.
According to an aspect of the present invention there is provided a heater module comprising a thick-film heating element. The heating element comprises a heating conductor, a temperature sensor, and a substrate supporting the heating conductor and the temperature sensor. The heater module comprises a resistive conductor having at least one trimming cut formed by a trimming process.
The, or each, trimming cut adjusts the resistance of the resistive conductor in a predictable and accurate manner. Accordingly, if the resistive conductor is separate from the temperature sensor or the heating conductor, it can be connected to the temperature sensor or to the heating conductor to add a known resistance to that of the temperature sensor or heating conductor. Alternatively, if the resistive conductor is part of the temperature sensor, or if it is the heating conductor, the trimming cut directly influences the resistance of the temperature sensor or heating conductor. This in turn allows the performance of the temperature sensor or heating conductor to be controlled with more precision than manufacturing tolerances for the temperature sensor or heating conductor would ordinarily allow.
The temperature sensor may comprise a sensing conductor that is electrically coupled to the resistive conductor. Alternatively, the heating conductor may be electrically coupled to the resistive conductor.
The resistive conductor may be mounted on the heating element, and optionally within a recess in a surface of the heating element. The recess may be formed into the substrate of the heating element.
Alternatively, the resistive conductor may be mounted to a discrete resistor module that is configured to connect to the heating element. The resistor module may comprise an opening configured to receive a portion of the heating element.
The heater module may comprise vias configured to couple the resistive conductor electrically to the temperature sensor or the heating conductor.
The resistive conductor may be mounted by soldered joints.
The resistive conductor may comprise a trimmable resistor.
In an alternative approach, the temperature sensor or the heating conductor comprises the resistive conductor. For example, the resistive conductor may be the same feature as the heating conductor, or may represent a sensing conductor of the temperature sensor. In this case, trimming cuts may be applied to the sensing conductor or the heating conductor directly. To allow for this, the heating element may comprise a recess providing external access to a portion of the temperature sensor or the heating conductor including the or each trimming cut.
The substrate of the heating element may comprise a ceramic material.
The heating conductor optionally comprises a conductive trace, for example embodied as a film of the heating element. Similarly, the temperature sensor may comprise a conductive trace.
The temperature sensor may comprise a resistance temperature detector.
The at least one trimming cut may be formed by a laser trimming process.
The invention also extends to a personal care device comprising the heater module of the above aspect.
Another aspect of the invention provides a method of manufacturing a heater module comprising a thick-film heating element. The heating element comprises a heating conductor and a temperature sensor supported by a substrate. The method comprises removing material from a resistive conductor of the heater module to increase a total resistance of the temperature sensor and the resistive conductor, or a total resistance of the heating conductor and the resistive conductor, to a predetermined value. The resistive conductor may be the heating conductor, a sensing conductor of the temperature sensor, or a separate conductor to which the heating conductor or temperature sensor are connected, for example.
The method may comprise removing material from the resistive conductor using a trimming process.
In some embodiments, the temperature sensor comprises a sensing conductor that is electrically coupled to the resistive conductor, and the method comprises increasing a combined resistance of the sensing conductor and the resistive conductor to the predetermined value. Alternatively, the heating conductor may be electrically coupled to the resistive conductor, in which case the method comprises increasing a combined resistance of the heating conductor and the resistive conductor to the predetermined value.
The temperature sensor or the heating conductor may comprise the resistive conductor, in which case the resistive conductor may be embodied as a conductive trace of the heating element. In such embodiments, the method comprises trimming the conductive trace to increase the resistance of the trace to the predetermined value.
It will be appreciated that preferred and/or optional features of each aspect of the invention may be incorporated alone or in appropriate combination in the other aspects of the invention also.
In general terms, embodiments of the invention provide heater modules including thick-film heating elements, for example such as that illustrated in
For a temperature sensor to act as a thermal protection device, its performance must adhere to relevant regulatory standards. This typically entails achieving thermistor classification for the temperature range of interest, in particular the threshold trip temperature.
However, the mass manufacture of thick-film heating elements is highly prone to error and so manufacturing tolerances tend to be relatively large. For example, a resistance variability of plus or minus 15% is common for a heating element film, which is inadequate for the purposes of thermal protection and may also fail to satisfy EMC (electromagnetic compatibility) requirements. Whilst screen-printing processes can be optimised for each application to some extent, this entails a lengthy development process and with no guarantee of achieving the required outcome.
In view of these challenges, instead of attempting to improve the precision with which thick-film heating elements are manufactured, embodiments of the invention take an approach in which the usual manufacturing tolerances are accepted and adjustments are made to the element after it has been produced to achieve the required performance.
This entails adjusting the fabrication process to shift the relevant manufacturing tolerance bands relating to the resistance of the RTD trace, such that the upper limit of the manufacturing tolerance for the RTD trace resistance is below or coincides with the required resistance value. In other words, the RTD trace is deliberately manufactured with a resistance at, or usually below, its target value. Then, the resistance of the RTD trace is brought up to the required value using one of two approaches.
In the first approach, an additional tuning resistor is connected to the RTD trace such that the tuning resistor and the heating element together form a heater module. The tuning resistor is trimmable or otherwise adjustable such that its resistance can be modified to yield the required combined resistance from the RTD trace and the resistor at the relevant temperature, for example the threshold temperature at which a trip will occur. The tuning resistor may have a similar temperature-resistance relationship to the main RTD trace, but more likely has a resistance that is substantially insensitive to temperature. Accordingly, the temperature-resistance characteristics of the assembly of the resistor and the RTD trace are dominated by the temperature response of the RTD trace, but with a generally constant offset created by the tuning resistor.
The tuning resistor may be mounted directly onto the heating element, or alternatively the resistor may be integrated into a separate resistor module that is arranged to couple to the heating element in a manner that creates electrical continuity between the tuning resistor and the RTD trace, in which case the resistor module also forms part of the heater module.
In the second approach, part of the embedded RTD trace may be exposed, for example by creating a recess, or ‘window’, in the surrounding material, such that the trace can be trimmed or otherwise ablated directly to increase its resistance. In such arrangements, the heating element including the additional feature of trimming cuts to the RTD trace defines the ‘heater module’. The window is typically created by a punching machine that removes material from the ceramic stack to define the window in the ‘greenline’ stage of manufacture, prior to firing and sintering the stack.
The general principle is illustrated in
The plot shown to the left in
The plot shown to the right in
The bulk of the tuning resistor 20 is defined by a cuboidal substrate 22 of ceramic. An upper surface of the substrate 22 bears a resistive layer 24, typically of aluminium oxide, which in turn is covered by a protective overglaze 26, the resistive layer 24 and the overglaze 26 being formed by screen-printing in successive stages. The overglaze 26 is typically formed from a glass encapsulant composition such as DuPont QQ620, all registered trade marks being acknowledged.
U-shaped metallic end terminations 28 slide over each end of the substrate 22 and are in electrical contact with the resistive layer 24, such that the electrical resistance between the terminations 28 is defined by the properties of the resistive layer 24. Accordingly, the overall resistance of the tuning resistor 20, namely the resistance presented between the terminations 28, can be varied by modifying the resistive layer 24, specifically by removing material from the resistive layer 24 using an ablation process or similar, which is referred to as ‘trimming’ the resistor 20.
The trimming patterns shown in
Taking the example of laser trimming briefly, a continuous kerf 29 is formed progressively into the material of the resistive layer 24 by a concentrated beam of light of a few microns in diameter, the energy of which is absorbed by the resistive material, causing that material to vaporise. The changing resistance of the tuning resistor 20 can be monitored while the cut 29 is being made, with the trimming operation being terminated once the target value is reached. Feedback-controlled trimming equipment is available for this purpose. The accuracy of the final resistance of the tuning resistor 20 is therefore dependent on the speed at which the trimming process can be terminated.
The tuning resistor 20 can be incorporated either by mounting it directly onto a heating element or as part of a tuning module that is connected to a heating element 10. Examples of these different approaches are described below with reference to
The heating element 30 shown in
The heating element 30 of
The four terminals 36 associated with the heating trace 34 define heating terminals 36a, and include a common live terminal, which is that shown furthest to the right in
When electrical power is supplied to the heating terminals 36a, the heating trace 34 generates heat by the Joule effect. That heat is evenly spread across the surface of the heating element and so can be transferred effectively to air flowing across that surface.
The final pair of terminals, shown furthest to the left in
The tuning resistor 20 must be electrically coupled to the RTD trace 38 to impact the resistance of the RTD trace 38, and
As
The resistor 20 is aligned with the path of the RTD trace 38 such that the RTD trace 38 passes beneath a centreline of the resistor 20, with each end termination 28 of the resistor 20 resting directly on the RTD trace 38. Brazed connections couple the end terminations 28 electrically and permanently to respective spaced points of the RTD trace 38. Accordingly, the tuning resistor 20 extends parallel to and above a short section of the RTD trace 38 to define a sensing assembly comprising both the resistor 20 and the RTD trace 38. The sensing assembly has a greater overall resistance than the RTD trace 38 alone and, more importantly, can be tuned to a more precise resistance than could be achieved for the RTD trace 38 alone using ordinary thick-film processes.
In this respect, the tuning resistor 20 can be trimmed prior to mounting it on the heating element 30. This will entail taking separate measurements of the resistance of the RTD trace 38 and the tuning resistor 20 at the target temperature, and then trimming the tuning resistor 20 as necessary to increase its resistance at the target temperature such that, when combined with the measured resistance of the RTD trace 38, the overall resistance of the sensing assembly is equal to, or within a predefined tolerance band of, the target resistance.
Alternatively, the tuning resistor 20 can be trimmed in situ on the heating element 30 after it has been fitted, which conveniently allows the overall resistance of the sensing assembly, which is the variable of interest, to be measured directly during the trimming procedure. Trimming the tuning resistor 20 in situ also inherently accounts for any impact on the overall resistance of the sensing assembly caused by the soldered joints between the terminations 28 of the tuning resistor 20 and the RTD trace 38.
Whether the tuning resistor 20 is trimmed in advance or in situ, the trimming process follows the general principles outlined above with respect to
Significant heat may be generated in the tuning resistor 20 during the trimming process. This must be accounted for when the tuning resistor 20 is trimmed in situ, as the heat could impact the integrity of the soldered connections between the terminations 28 of the tuning resistor 20 and the RTD trace 38. For example, the speed of the trimming operation may be adjusted to avoid heating the tuning resistor 20 to an extent that could risk melting of the soldered joints. Also, the atmosphere in which the resistor 20 is trimmed may be modified to minimise the impact of heat generated in the trimming process.
Once the tuning resistor 20 is installed and the combined resistance of the RTD trace 38 and the tuning resistor 20 has been tuned to the relevant predetermined value, a protective covering may be applied over the tuning resistor 20 to shield the resistor 20 and any exposed portions of the RTD trace 38 from damage and corrosion thereafter, for example to guard against oxidation of the tungsten of the RTD trace 38. The protective covering may be created by plating the relevant area, for example using a nickel-boron (NiB) coating.
The skilled reader will appreciate that there are various ways in which such an arrangement might be implemented, and the example shown in
In this respect, the tuning module 42 comprises a cuboid tuning module body 44, an upper surface 46 of which supports the tuning resistor 20. It will be appreciated that the shape of the tuning module 42 can be adjusted to suit each application, however. A front surface of the tuning module body 44 extending in a plane orthogonal to that of the upper surface 46 supporting the tuning resistor 20 includes a recess (not shown) arranged to admit an end of the heating element 30, or a suitable protruding portion of the heating element 30. The recess is therefore of a size and shape corresponding to the cross-section of the corresponding part of the heating element 30, and is of a depth sufficient that the received part of the heating element 30, when fully inserted, lies directly beneath the tuning resistor 20.
A pair of spaced vias extend between the upper surface 46 of the tuning module body 44 and the interior of the recess, each via comprising an electrical contact point 48 at each end and a conductive path extending between the contact points 48 through the material of the tuning module body 44. Each end termination 28 of the tuning resistor 20 is fixed by a brazed connection to a contact point 48 of a respective one of the vias on the upper surface 46, whilst the contact points on the interior of the recess define respective inner terminals, each of which is therefore electrically connected to a respective one of the end terminals of the tuning resistor 20.
Correspondingly, the end of the heating element 30 is provided with a pair of windows similar to the window 40 of the arrangement shown in
In this way, the end terminations 28 of the tuning resistor 20 are electrically connected to the RTD trace 38, to create an equivalent sensing assembly to that of the arrangement shown in
The tuning module 42 further includes electrical contact points within the recess that are arranged to engage the RTD terminals 36b of the heating element 30 (not shown in
Using the tuning module 42 of
However, trimming in stages is possible if required, in that the tuning module 42 can be removed for an initial trimming operation, reconnected to the heating element 30 to test the combined resistance of the RTD trace 38 and tuning resistor 20, and then removed again to make adjustments by further trimming as may be necessary. This flexibility in terms of the different ways in which the tuning module 42 can be tuned, which derives from the mechanical and reconnectable nature of the connection between the tuning resistor 20 and the RTD trace 38 created using the tuning module 42, may be beneficial in certain contexts.
Turning finally to
A continuous trim influences both the effective width and the length of the RTD trace 38, and so the impact on resistance can be characterised as follows:
In addition, discrete trims create parallel conductive paths and so further impact resistance according to the following equation:
In general terms, trimming the RTD trace 38 directly follows the same principles as for trimming a tuning resistor, albeit potentially being more difficult to perform accurately in practice. There may also be some uncertainty regarding the compatibility of the resistive ink used for the RTD trace 38 and the substrate material with the trimming process, in contrast with using a separate tuning resistor having an aluminium oxide resistive layer whose properties are well characterised.
Another potential drawback of trimming the RTD trace 38 directly is that if the trimming process is unsuccessful, for example because the RTD trace 38 resistance increases too far, the entire heating element 30 may have to be discarded. In contrast, in approaches involving a tuning resistor, only the tuning resistor must be replaced in the event of an error in the trimming process.
It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.
For example, although the above description refers to adjusting the resistance of an RTD trace using a tuning resistor or by trimming the trace directly, the same principles can be applied in a corresponding manner to adjusting the resistance of a heating trace, for example to alter the power output of the heating trace and therefore refine performance. Accordingly, the performance of a heating trace can be modified as may be desired by trimming the heating trace directly, or by connecting the heating trace to a tuning resistor and trimming the resistor to achieve the required overall resistance.
Where trimming techniques are used with heating traces, it should be noted that power will be dumped around each trimming feature when the trace is in use, producing localised heating spikes. These hot zones must be accounted for, in particular to ensure that any brazed joints will not be compromised by such heating effects.
Trimming features may be added to both an RTD trace and a heating trace in a given heating element. For example, a tuning module to be connected to a heating element may include a respective tuning resistor for each trace.
Number | Date | Country | Kind |
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2019099.7 | Dec 2020 | GB | national |
Filing Document | Filing Date | Country | Kind |
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PCT/GB2021/052878 | 11/5/2021 | WO |