Comfort warming control system

Field of the invention

The present invention relates to a comfort warming control system according to the preamble of the independent claim.

Background to the invention

Most vehicles, both cars and trucks, have some form of warming of, for example, seats, steering wheel and perhaps gear lever. These warming systems are often called comfort warming systems. They often take the form of a simple system in which a constant voltage is applied to warm a series-resistive heating cable, the resistance of which depends on the temperature but also changes with the length of the cable. When the heat exceeds a certain level, a thermostat switches off until the heat decreases. This means that a thermostat is needed for each desired temperature level. It is also difficult to keep the temperature steady, since thermostats provide only relatively imprecise temperature regulation.

In a control system known from WO-2004/108446 for a comfort warming system in a vehicle, a desired temperature is compared with a current temperature determined by a temperature sensor, and the control system generates a pulsewidth-modulated signal which is supplied to the warming system and is therein adapted to achieve the desired

temperature. US-2005/0167418 likewise refers to a control system for a comfort warming system for a vehicle in which a pulsewidth-modulated signal is used to guide the temperature of heating elements towards a temperature which is measured by temperature sensors situated adjacent to them. The systems described above involve measuring the temperature to serve as a basis for controlling the energy supplied to the heating coil. This is a disadvantage in requiring further parts (e.g. temperature sensors) which make the system expensive as well as entailing more complicated fitting which also increases the costs.

The object of the present invention is to propose an improved comfort warming control system which eliminates the above disadvantages of today's systems.

Summary of the invention

The above object is achieved with the invention defined by the independent claim.

Preferred embodiments are defined by the attached dependent claims.

The present invention proposes a comfort warming control system which results in stepless and fully diagnosable electrical warming of heating coils, thereby generally contributing to improved regulation of the comfort warming system. A further advantage is that it is readily adaptable to the type and length of the heating coil used.

For example, the comfort warming control system according to the invention is usable for the same performance with heating coils which are series-resistive, parallel-resistive or self-regulating (resistance increasing with temperature).

As the comfort warming control system indirectly determines the temperature, no temperature measurement circuit is needed.

According to the invention, the heating coils are provided with a pulsewidth-modulated (PWM) warming signal.

According to a first embodiment, the temperature regulation is based on measuring the current which passes through the heating coils and applying a known relationship between the current through a heating coil (e.g. made of copper) and the temperature.

According to a second embodiment, the temperature regulation is based on measuring the current which passes through the heating coils, using the resulting measurement to calculate the resistance and then applying a known relationship between the resistance in a heating coil (e.g. made of copper) and the temperature.

The comfort warming control system according to the invention has the following advantages: • The same type of heating cable (W/m) can be used (adjusting only the length).

• All different types of heating cables are catered for.

• The number of heating cables can be minimised since the control unit can cater for different lengths.

• Stepless accurate heat regulation.

• More diagnostic scope.

• The technique can be used in all fields in which heating cables are used. Brief description of drawings

Figure 1 is a schematic block diagram of the comfort warming control system according to a first embodiment of the present invention.

Figure 2 is a schematic block diagram of the comfort warming control system according to a second embodiment of the present invention.

Figure 3 depicts graphs illustrating the warming signal according to the present invention. Figures 4 and 5 are schematic circuit diagrams of different heating coils.

Detailed description of preferred embodiments of the invention

The invention will now be described in detail with reference to the attached drawings, referring initially to Figures 1 and 2.

The present invention concerns a comfort warming control system 2 for a vehicle provided with a comfort warming system with one or more heating coils 4.

The comfort warming system 2 comprises a control unit 6 and a feed unit 8 adapted to conveying to the heating coil a pulsewidth-modulated warming signal with a cycle length P _H and a variable pulsewidth P _W .

The upper graph in Figure 3 shows how the voltage of the warming signal varies with respect to time. The pulsewidth P _W and the cycle length P _H , and a corresponding frequency F _H are indicated for the first pulse. In the example illustrated, the duty cycle is about 50%, i.e. the pulsewidth is about half of the cycle length. The duty cycle may be between 0 and 100% of the cycle length. The lower graph in Figure 3 shows how the current of the warming signal varies with respect to time. In addition, a broken line indicates the mean current IRMS- In this case it is level with half of the amplitude of the pulses, since the duty cycle is about 50%. The temperature of the heating coil depends directly on the strength of the mean current.

The frequency FR of the warming signal is preferably between 0.1 and 100 Hz, but the invention as defined in claim 1 is not limited to this range, since other frequency and pulsewidth values are possible if the particular application so requires. The comfort warming control system according to a first embodiment of the invention will now be described with reference to Figure 1. The system further comprises a measuring unit 12 adapted to measuring the current ½ in the heating coil when the warming signal is conveyed to the heating coil, and to generating a current signal 14 which depends on the measured current ½ and is for conveying to the control unit. The control unit is adapted to determining on the basis of the current a parameter related to the heating cable.

The comfort warming control system according to a second embodiment of the invention will now be described with reference to Figure 2. The measuring unit 12 is adapted not only to providing the current I _H but also to measuring the voltage UH across the heating coil when the warming signal is conveyed to the heating coil, to determining the resistance of the heating coil on the basis of the measured voltage UH and the measured current ½ and to generating on the basis thereof a resistance signal 14' for conveying to the control unit. The control unit is adapted to determining on the basis of the resistance a parameter related to the heating cable.

According to an embodiment, the measurement of the current according to the first embodiment, or the current and the voltage according to the second embodiment, takes place during a measuring period which comprises at least one pulse of the warming signal, which is advantageous in making it possible to calculate immediately the resistance and thereby determine the parameter. It is of course possible to perform measurements on a number of pulses of the warming signal and to calculate mean voltage and current values as a basis for calculating the resistance. According to an embodiment, the parameter represents the length of the heating coil. This means that the length of the heating coil can be determined by measuring the resistance.

According to another embodiment, the parameter represents the type of heating coil, e.g. series-resistive, parallel-resistive or self-regulating.

The values determined for the parameter, e.g. the length of the heating coil, are stored in the measuring unit in a memory unit (not depicted in Figures 1 and 2) and may be used in subsequent calculations. The measurements to determine the current or resistance of the heating coil may be preferably be done regularly to be able to detect any changes in the heating coil, e.g. changes caused by damage. The control unit is therefore adapted to determining the parameter related to the heating coil at a predetermined or settable first interval between consecutive measurements. This first measuring interval may be of the order of seconds, minutes, days or weeks, depending on the particular application.

According to a preferred embodiment, the comfort warming control system 2 comprises a temperature input unit 10 for input of desired temperatures for comfort warming system. This is represented by a box bounded by a broken line in Figures 1 and 2. A user indicates, e.g. via a knob or lever, the desired temperature, e.g. for seat warming. This may take the form of a desired number of degrees, e.g. 25 degrees, or a number on a scale graduated, for example, from 0 to 10, where 0 is zero warming and 10 is maximum warming.

The control unit is adapted to then determining a pulsewidth (Pw) for the warming signal on the basis of the resistance measured, the temperature desired and the parameter determined, so that the desired temperature is reached, and to delivering a control signal to the feed unit, which is adapted to conveying the warming signal to the heating coil with the pulsewidth determined.

According to a further embodiment, the control unit is adapted to determining the pulsewidth (Pw) for the warming signal at a predetermined or settable second interval between consecutive measurements. This second measuring interval depends on the previously mentioned first measuring interval in that a requirement for being able to determine the pulsewidth is that a current measurement and/or resistance measurement has been done, i.e. the second measuring interval may be shorter than the first measuring interval.

If quick temperature regulation is desired, the current measurement/resistance

measurement needs to be done relatively frequently, e.g. at an interval of the order of a number of seconds. According to the present invention, the present invention improves the regulation of the comfort warming system by current measurement and power regulation and enhances the possibility of using the same control technology with different types and lengths of heating cables. A series-resistive or parallel-resistive heating coil may be described electrically by the diagram in Figure 4.

A self-regulating heating coil may be described electrically by the diagram in Figure 5.

As the inductance is often low in these applications, and as the time constants for power regulation are relatively large, we may assume that the inductance is negligible, so the relationship of voltage, current and resistance may be indicated as U ~ R * I, as also depicted in Figures 4 and 5. The diagrams use symbols for the resistances R and the inductances L of the heating coils and shows how they depend on the length in metres (m). If we measure the current i _m through the circuit in the control unit, we can use Ohm's law (the voltage being known in the control unit) to calculate the total resistance of the heating coil. In a series-resistive or parallel-resistive heating coil the total resistance is proportional to the length, enabling us to determine the length of the heating cable.

Knowing which particular heating coil is concerned, we can apply a desired cycle length Pw (duty cycle) in order to arrive at correct current i _RM s and corresponding temperature, since these are proportional (open regulation is effected).

The same principle applies in the case of a self-regulating heating coil. The advantage of a self-regulating coil is that there is no need to know the length, as the control unit can instead be used to measure the temperature, since the resistance is proportional to the temperature (feedback regulation is effected).

The comfort warming control system according to the invention also makes it possible to diagnose problems with regard to the heating coil, e.g. whether it is too long or short, whether temperature differences have arisen and whether any short-circuit to the battery or short-circuit to earth has occurred.

The resistance is thus measured by the measuring unit by measuring the current conveyed to the heating coil from the feed unit and is then calculated by means of Ohm's law (resistance = voltage/current). The comfort warming control system is used, according to an embodiment, as a self- regulating heating coil, based on the fact that the temperature affects the resistance of a material differently depending on its resistivity.

According to a preferred embodiment, the control unit activates the warming of the heating coil if the measured resistance is below a preset threshold, in which case the resistance is supposed to increase with the temperature and the activation therefore takes place when the temperature is below a predetermined limit.

The control unit is adapted to determining a temperature for the heating coil on the basis of the resistance measured and to activating the warming of the heating coil if the temperature determined is below a preset threshold, e.g. 20°C, 22°C, 24°C or some other suitable temperature. According to a further embodiment, the control system is adapted to continuously regulating the provision of energy from the feed unit on the basis of the resistance measured or on the basis of appropriate temperature. For example, the feed unit may be activated at a preset resistance/temperature and may then continuously regulate the provision of energy, by varying the pulsewidth, so long as the temperature is below that level.

The electrically conductive material is preferably a metal, e.g. copper or an alloy containing copper, which has a given resistance at a certain temperature. Chrome, nickel, platinum and their alloys are other materials which may be used. There are of course a large number of other materials which are electrically conductive, have a temperature- dependent resistance and might be used in conjunction with the present invention.

The resistance of the heating coil is determined for all temperatures within the working range in controlled forms. This generates resistances and temperatures which are stored in a chart in a memory unit in the measuring unit or the control unit. The resistance is determined by measuring the current delivered to the heating coil from the voltage source.

The present invention is not restricted to the preferred embodiments described above. Sundry alternatives, modifications and equivalents may be used. The above embodiments are therefore not to be regarded as limiting the invention's protective scope which is defined by the attached claims.