Method and control device for operating an exterior mirror heating system of a motor vehicle

Abstract

Method for operating an exterior mirror heating system (14) of a motor vehicle (10) which is designed to remove a frozen and / or liquid water coating on at least one exterior mirror (12), in which, after activation of the exterior mirror heating system (14), the fulfilment of a criterion is monitored and, after the criterion is fulfilled, the exterior mirror heating system (14) is deactivated or a query is issued to a vehicle occupant (18) and, depending on a response to the query, the exterior mirror heating system (14) is further operated or deactivated, wherein - the criterion includes the condition that after activation of the exterior mirror heating (14) a required heating period has elapsed and / or a required heating energy has been expended; - the criterion alternatively or additionally comprises the condition that a reflected portion of a light beam (28) emitted in the direction of the exterior mirror (12) has a predetermined value; - at least in the event that the exterior mirror heating (14) is deactivated, a calculation rule is provided which determines the required heating time and / or required heating energy as a function of variables influencing the removal of the water coating, wherein after deactivation of the exterior mirror heating (14), the heating time elapsed up to that point and / or the heating energy expended is used to optimise the calculation rule; and / or - at least in the event that the exterior mirror heating (14) is deactivated, the criterion comprises the condition that a measured temperature development of the exterior mirror (12) follows at least one defined temperature profile (34, 36) for a predetermined duration.

Description

The present invention relates to a method and a control device for operating an exterior mirror heater of a motor vehicle. Furthermore, the invention also relates to a motor vehicle having a control device of this type.In particular in the cold months of the year, vehicle drivers must switch on an exterior mirror heating when the exterior mirrors are icy or even when the exterior mirrors are in a mist, that is to say side mirrors. It is usually necessary that after the switching on of such an exterior mirror heating, the vehicle driver has to switch off the exterior mirror heating itself again during the trip, in the event of a trip interruption or also, for example, after a restart of the motor vehicle, since this is otherwise continued to be operated. If the vehicle driver forgets, it may happen that even if, for example, both exterior mirrors are already longitudinally free from ice and / or moisture, the exterior mirror heating system continues to be operated. In such a case, a relevant on-board power supply system of the motor vehicle is loaded by an superfluous heating power of the exterior mirror heating system.It is known per se to counteract unnecessary consumption of heating devices in motor vehicles. For example, DE 101 20 098 A1 discloses a method for controlling heating of a functional unit of a motor vehicle, which can be an exterior mirror. During operation of the heating, characteristic features of the time profile of the heating temperature are evaluated for controlling the heating until a disturbance caused by a water surface on the exterior mirror is eliminated.DE 43 40 004 A1 describes an actuating device for a headlight of a motor vehicle. Depending on the ambient brightness, a driver of the vehicle is informed to the motor vehicle that a previously manually actuated activation of the headlight is not required.DE 10 2016 116 270 A1 describes a heating system for a front window of a motor vehicle. If a sensor attached to the front window detects that the front disk is free of snow, heating of the front window is deactivated.DE 20 2007 007 112 U1 describes an automatic mirror defrosting device which can determine the icing state and automatically performs the defrosting operation of the pane in response to the detection of the icing.JP S56-168 448 U describes a defrosting system for a vehicle window or the like, which is particularly suitable for defrosting a frozen part of a wiper rubber on a front window of a motor vehicle.DE 10 2013 215 470 A1 describes a device for removing moisture from and / or ice from a window of a motor vehicle by means of an infrared radiation device.JP S49-132 887 U describes a device for detecting the optical fogging of a glass pane surface of a window.It is the object of the present invention to provide a solution which enables a particularly effective and energy-economical removal of a frozen and / or liquid water surface on at least one exterior mirror.This object is achieved by a method and by a control device for operating an exterior mirror heating of a motor vehicle having the features of the independent claims. Further possible embodiments of the invention are specified in the dependent claims, the description and the figures.In the method according to the invention for operating an exterior mirror heating of a motor vehicle, which is designed to remove a frozen and / or liquid water surface on at least one exterior mirror, the fulfilment of a criterion is monitored after activation of the exterior mirror heating and the exterior mirror heating is deactivated after fulfilment of the criterion or a query is issued to a vehicle occupant and the exterior mirror heating continues to operate or deactivated as a function of a response to the query. The water covering can be, in particular, ice, snow, dew and any other moisture. At least in the case that the exterior mirror heating (14) is deactivated, a calculation rule is provided which determines the required heating duration and / or required heating energy as a function of variables influencing the removal of the water surface, wherein after deactivation of the exterior mirror heating (14) the heating duration and / or used heating energy that have elapsed up to then is used to optimize the calculation rule. Alternatively or additionally thereto, at least in the case that the exterior mirror heating (14) is deactivated, the criterion comprises the condition that a measured temperature development of the exterior mirror (12) follows at least one defined temperature profile (34, 36) for a predefined duration.According to the invention, the criterion comprises the condition that after activation of the exterior mirror heating a required heating duration has elapsed and / or a required heating energy has been used. Alternatively or additionally, the criterion comprises the condition that a reflected portion of a light beam emitted in the direction of the exterior mirror has a predefined value.Said exterior mirror can be a conventional exterior mirror. However, it is also possible for the external mirror to be an electronic rearview mirror with one or more integrated cameras. In the former case, therefore, the aim is that the exterior mirror heating can remove the water surface on an outer mirror surface of the exterior mirror, via which surface in particular a driver is intended to be able to see a rear surrounding region. In the second case mentioned, the aim is to remove, in particular, a transparent region of the electronic rearview mirror from the frozen and / or liquid water surface, by means of which the one or more cameras are directed into the rear vehicle environment. The said light beam can be, for example, a laser beam which is emitted in the direction of the external mirror. The query to the vehicle occupant can be, for example, a query for the result of the removal of the water surface.The invention is based, inter alia, on the finding that a decisive disadvantage in conventional approaches is that a heating power provided for operating the exterior mirror heating, i.e. a corresponding power consumption, can be provided which is not designed in a load-oriented manner. This means that during operation of the exterior mirror heating, there is no interaction between a requester, i.e. for example the driver, and the consumer in the form of the exterior mirror heating via the instantaneous result of the consumption of current for eliminating the visual restriction present on the relevant exterior mirror due to moisture and / or ice.If the viewing conditions on the outside mirror, whether it be a conventional outside mirror or else an electronic outside mirror or rear-view mirror, are such that the view to the rear is obstructed or restricted, for example by the external climatic conditions, the driver is usually required to take measures in order to be able to have a corresponding view via the outside mirror into the rear traffic space. In such a case, the vehicle driver will therefore usually activate said exterior mirror heating.In this context, it is provided according to the invention that after activation of the exterior mirror heating, it is monitored whether said required heating duration has elapsed and / or the required heating energy has been expended. As soon as one and / or the other should be fulfilled, the exterior mirror heating is either automatically deactivated or said query is output to the vehicle occupant, wherein the exterior mirror heating continues to be operated or deactivated depending on the response of the query.A load-oriented use of energy from the on-board power supply of the motor vehicle and at least optionally a human-machine communication between the driver and the vehicle thus take place in the form of a success check by the driver who requested the relevant heating power. In the case of the interrogation, it can be provided, for example, that the driver is visually and alternatively or additionally also audibly requested via a display, for example in the instrument cluster, to evaluate the result of the exterior mirror heating. For example, he can acknowledge this with "end" or "not yet end". If the result is "not yet finished", the question of "ending" or "not yet finished" is asked again after a certain period of time, for example, until the result of the heating power performance is sufficient, that is to say the driver can see the rear space to a sufficient extent by means of the outer mirror.Alternatively or additionally, said criterion can also comprise the condition that a reflected portion of a light beam emitted in the direction of the exterior mirror has a predefined value. Here, the interrogation or feedback of the vehicle driver or driver is not absolutely necessary, but can also be provided. For this purpose, for example, the reflectivity of the mirror or of the camera surface is used for assessing the heating power or the result with respect to the distance of the water coating via a laser installed in the outer mirror or the electronic outer mirror and an opposite sensor located in the reflection region of the laser beam.In this case, the fact is used according to the invention that as the moisture and / or ice layer decreases, the reflection behavior of the mirror or camera surface improves and approaches a defined optical optimum. If the reflected portion of the light beam emitted in the direction of the exterior mirror has the predefined value, it can be provided in particular that the exterior mirror heating is automatically deactivated. Alternatively, however, it is also possible for said query to be output to the vehicle occupant, wherein the exterior mirror heating system continues to operate or is automatically deactivated as a function of the response of the query.By means of the method according to the invention, it is therefore possible on the one hand to particularly effectively free an outer level from a frozen and / or liquid water surface. On the other hand, the method according to the invention also makes it possible to achieve this particularly economically in terms of energy. Thus, the energy requirement for mirror or camera release of climate-induced coating can be particularly energy-efficient. In addition, the safety can be increased by the feedback, which is possible directly in particular, between the driver and the result achieved via said query. In addition, the driver can be relieved by the interrogation, in particular if this is interrogated directly via a combination instrument, for example, and thus does not have to make active ideas about whether and when he should optionally deactivate the exterior mirror heating. In addition, the CO 2- requirement of the relevant motor vehicle can also be reduced by optimizing and reducing the heating energy and heating power.According to one embodiment of the invention, it is provided that a calculation rule is provided which determines the required heating duration and / or required heating energy as a function of variables influencing the removal of the water covering. The variables influencing the removal of the water surface can be the external temperature, the location of the motor vehicle, the solar radiation, the time, the air humidity and / or the elevation of the motor vehicle. In principle, these can be any desired variables which influence the required heating duration and / or required heating quantity which are required in order to free said exterior mirror heating of frozen and / or liquid water surface. The motor vehicle in question can have, for example, a corresponding sensor system for ascertaining the current climatic ambient conditions on the motor vehicle. Thus, measured values relating to environmental conditions can be present, which result in the exterior mirror permitting only a restricted view of the vehicle driver. Moreover, it is also possible to provide access of data such as the time of day, temperature, location, humidity, altitude, and the like via sensors, online, or the like, which may serve as a basis of a heating program for freeing ice, dew, and moisture.If the vehicle driver switches on the exterior mirror heating, the heating power and heating duration is calculated and called up in particular from the present current and also in particular from present historical data, the latter being called up, for example, via a conventional on-board power supply system or else, for example, via an HV power supply system in hybrid and electric vehicles. After a defined period of time has elapsed, the driver is requested visually and acoustically, for example via the combination display, to evaluate the result of the heating program, i.e. whether or not he is satisfied with the removal of the covering on the relevant exterior mirror. If the driver is satisfied with the result, he can express this, for example, by gestures, by statements or in any other way. In this case, the exterior mirror heating can be automatically deactivated. If, on the other hand, the driver responds that he is not yet satisfied with the result, i.e. he cannot still see sufficiently backwards via the exterior mirror, he can also clarify this by gestures, by speech or in any other way, as a result of which the exterior mirror heating is not yet deactivated. Due to the fact that said calculation rule takes into account said variables which influence the removal of the water surface, the heating duration and / or required heating energy which is expected to be required can be determined particularly exactly on the basis of the calculation rule. Thus, it can be determined or estimated very exactly based on current and also based on historical data which heating duration will be required at which heating energy or heating power will be required in order to remove the frozen and / or liquid water surface at all prospects. On the one hand, this makes it possible to eliminate the water surface particularly effectively and efficiently. On the other hand, it is thus also possible to do this particularly economically in terms of energy.According to one embodiment of the invention, it is provided that after deactivation of the exterior mirror heating, the heating duration that has elapsed up to then and / or the heating energy used is used to optimize the calculation rule. The data generated during the heating program are thus stored and supplied to an optimization. This is particularly useful when the automatic deactivation or the continued operation of the exterior mirror heating is linked to how the driver has answered the query. If, for example, it is determined that after the initially predefined heating duration and / or heating energy to be used, the driver was not yet satisfied with the removal of the water covering on the outer mirror, this information can be used to optimize the calculation rule to the effect that the heating duration and / or the heating energy to be used or else heating power is set to be greater than before. The calculation rule can thus be refined further and further. This makes it possible to operate the exterior mirror heating system more and more efficiently for removing the coating, so that the coating can be removed more and more quickly and in an energy-favorable manner.In a further possible embodiment of the invention, it is provided that the light beam is emitted in the direction of the exterior mirror even when the exterior mirror heating is deactivated and the reflected portion is measured, wherein the exterior mirror heating is automatically activated if it has been determined on the basis of the reflected portion that the exterior mirror is provided with frozen and / or liquid water surface. In this case, it is therefore not necessary at all for the driver to actively intervene in order to activate the exterior mirror heating. The emission of the light beam can take place, for example, at defined time intervals in order to thus assess or estimate, on the basis of the reflected component, whether the respective external mirror is free of deposits or not. If this is not the case, the exterior mirror heating can be activated automatically in order to remove the water covering without requiring a driver's action. In this case, the driver therefore does not have to be thought of activating the exterior mirror heating. Even with a small amount of coating formation, the outer mirror heating can be activated automatically, for example, in order to allow a particularly good view over the outer mirror throughout.According to an alternative or additional embodiment of the invention, it is provided that the criterion comprises the condition that a measured temperature development of the exterior mirror follows at least one defined temperature profile for a predefined duration. If this condition is fulfilled, the exterior mirror heating can again be automatically deactivated. Alternatively, it is also possible that said query is again output to the vehicle occupant in order to continue to operate or automatically deactivate the exterior mirror heating as a function of its answering of the query. In this context, it is provided in particular that a suitable sensor detects a heating behavior of a mirror pane of the exterior mirror, and in the case of an electronic rearview mirror, a corresponding pane is then provided, through which the at least one camera is directed into the rear space of the motor vehicle. For example, from a measured temperature behavior of an inside surface of the exterior mirror over time, it is possible to infer water coverings that impair the visual behavior and thus also to determine a degree of covering freeness. If, for example, an aqueous coating is caused to evaporate by a corresponding heating power at the outer mirror, a special temperature profile results depending on the aggregate state and thickness of the coating. These temperature profiles are specific to the coating and can be stored, for example, in an internal memory of the motor vehicle. In this context, the invention is thus based in particular on the finding that different types of water coverings have different evaporation profiles which are reflected in different temperature characteristics.In a further possible embodiment of the invention, it is provided that the defined temperature profile takes into account at least one type and / or aggregate state of water surface. Thus, the finding according to the invention can be taken into account that different water coverings have different evaporation profiles, depending on the type of water covering and aggregate state, which are reflected in different temperature characteristics. If, therefore, starting from a certain point in time, the actually measured temperature development of the exterior mirror follows the at least one defined temperature profile for the predefined duration, it can be established or estimated, for example, on the basis of this that the exterior mirror should be free of deposits. In this case, said query can be output to the driver again as to whether the exterior mirror heating can already be deactivated or should be operated even further. Alternatively, it is also possible that the exterior mirror heating is automatically deactivated in this case.In a further possible embodiment of the invention, it is provided that the measured temperature development of the exterior mirror is compared with the at least one defined temperature profile and the heating power is adjusted as a function of the comparison result. In particular, the heating power can thereby be adapted to the respective water covering. If the exterior mirror is covered only with liquid water, for example, the heating power can be selected to be lower than if the exterior mirror is covered with ice, for example. By comparing the temperature profile that is established or the temperature development of the exterior mirror with the at least one defined temperature profile, it is possible to infer the type of coating and thus to optimally adapt the heating power, such that the relevant water coating, whether frozen or liquid, can be removed particularly quickly and energy-efficientlyThe control device according to the invention for an exterior mirror heating of a motor vehicle, which is designed to remove a frozen and / or liquid water surface on at least one exterior mirror, is configured to carry out the method according to the invention or a possible embodiment of the method according to the invention. Possible embodiments of the method according to the invention are to be regarded as possible embodiments of the control device and vice versa, wherein the control device has in particular means for carrying out the method steps.The motor vehicle according to the invention comprises an exterior mirror heater which is designed to remove a frozen and / or liquid water surface on at least one exterior mirror of the motor vehicle, wherein the motor vehicle additionally has the control device according to the invention or a possible configuration of the control device according to the invention.Further features of the invention can be derived from the following description of the figures and from the drawing. The features and combinations of features mentioned above in the description and the features and combinations of features shown below in the description of the figures and / or in the figures alone can be used not only in the respectively specified combination but also in other combinations or alone without departing from the scope of the invention.The drawing shows in: FIG. 1 shows a schematic illustration of a motor vehicle having two exterior mirrors and an exterior mirror heating system and a control device for operating the exterior mirror heating system; FIG. 2 shows a schematic illustration, on the basis of which a first embodiment of a method for operating the exterior mirror heating is made clear; FIG. 3 shows a further schematic illustration for illustrating the first embodiment of the method for operating the exterior mirror heating system; FIG. 4 shows a sectional top view of an exterior mirror of the motor vehicle, wherein a second embodiment of a method for operating the exterior mirror heating is explained in more detail on the basis of this illustration; FIG. 5 shows a further sectional plan view of the exterior mirror for explaining the second embodiment of the method for operating the exterior mirror heating; FIG. 6 shows a further sectioned plan view of the exterior mirror and schematically indicated temperature profiles, wherein a third embodiment of the method for operating the exterior mirror heating is explained in more detail with reference to this figure.In the figures, identical or functionally identical elements are provided with the same reference numerals.FIG. 1 schematically illustrates a motor vehicle 10. The motor vehicle 10 comprises two exterior mirrors 12 serving as rear mirrors. However, it is also possible for the external mirrors 12 to be electronic rear-view mirrors with integrated cameras. Furthermore, the motor vehicle 10 has an exterior mirror heating 14, which is only schematically indicated and is designed to remove a frozen and / or liquid water covering, for example in the form of ice, snow, dew or the like, on the exterior mirrors 12. Furthermore, the motor vehicle 10 comprises a control device 16 which is configured to operate the exterior mirror heating 14. Furthermore, a driver 18 is schematically indicated. With reference to the following figures, different embodiments for operating the exterior mirror heating 14 are explained in more detail. The various embodiments can be carried out alone and also in combination with one another.In FIG. 2, one of the exterior mirrors 12, the exterior mirror heater 14, the driver 18, the control device 16 and a vehicle-external server device 20 are schematically indicated. In the present case, it should be assumed that the respective exterior mirror 12 has a frozen and / or liquid water surface, so that the driver 18 can only observe the rear traffic space to a limited extent or not at all by means of this exterior mirror 12. As a result, the driver 18 has activated the exterior mirror heater 14 itself. This can be visually indicated to him, for example, by means of a combination device 22 of the motor vehicle 10. Alternatively or additionally, it is also possible for the driver 18 to be provided with an acoustic signal in the vehicle interior, by means of which the driver is informed that the exterior mirror heating 14 is activated.The control device 16 can itself or via a correspondingly configured communication device of the motor vehicle 10 and not shown here exchange a wide variety of data with the external server device 20 in order to obtain information about an external temperature, a location of the motor vehicle 10, a time, an air humidity, an elevation and the like, for example. Furthermore, it is also possible to provide a wide variety of information via sensors of the motor vehicle 10 that are not shown here, which information influences the formation and removal of the water surface on the exterior mirror 12. Within the control device 16, in particular a calculation rule can be provided which can determine a required heating duration and / or required heating quantity or heating power depending on variables influencing the removal of the water covering. Thus, for example, it is possible to enable a heating power, heating duration and / or heating energy according to the present information regarding said variables via said calculation rule, according to which the exterior mirror heating 14 is then operated for removing the water surface. It is also possible that after activation of the exterior mirror heater 14, it is monitored whether a required heating duration has elapsed and / or a required heating energy has been used.In FIG. 3, the case is schematically indicated in which the outer mirror 12 has now been freed from the water coating. In particular, after activation of the exterior mirror heater 14, it can now be the case that a previously calculated required heating duration has elapsed and / or a required heating energy has been expended. In this case, it can be provided that a query is output to the driver 18, for example via the combination device 22 and / or also acoustically. The driver 18 can then be asked, for example, whether he can now see again well enough through the outside mirror 12. If the driver 18 does so in the affirmative, the control device 16 automatically deactivates the exterior mirror heating 14, and if the driver 18 does not do so, the control device 16 continues to operate the exterior mirror heating 14 and, for example after a defined period of time, asks once again whether the driver 18 is now satisfied with the result of the removal of the floor on the exterior mirror 12. If this is then the case, the control device 16 can deactivate the exterior mirror heater 14.In FIG. 4, one of the two outer mirrors 12 is schematically illustrated in a sectional plan view. With reference to FIG. 4 and the following figure, an alternative or additional method variant for operating the exterior mirror heating 14, not shown here, is explained in more detail. The arrow 24 schematically identifies the forward travel direction of the motor vehicle 10. Installed in or on the outer mirror 12 is a laser 26 with which a laser beam 28 is directed onto the outer mirror 12. Furthermore, a sensor 30 is provided, which can measure a reflected portion of the laser beam 28. The sensor 30 is connected signal-wise to the control device 16 in order to provide data on which portion of the emitted laser beam 28 has reached the sensor 30. Thus, in particular after the activation of the exterior mirror heater 14, it can be provided that it is monitored when the reflected portion of the laser beam 28 emitted in the direction of the exterior mirror 12 has a predefined value. If this predefined value is reached or exceeded, it can be provided that said query is again output to the driver 18. Alternatively, it is also possible for the control device 16 to automatically deactivate the exterior mirror heating 14 in the case.FIG. 5 again shows the outer mirror 12 in a cut plan view, wherein the water covering has already been removed here, i.e. has evaporated. In this case, a substantially greater proportion of the emitted laser beam 28 reaches the sensor 30 by reflection on the surface of the exterior mirror 12 which is now free of coating, and it can therefore be established on the basis of the now better reflection behavior that the exterior mirror 12 has been freed of its water coating in order then to output said interrogation to the driver 18 or else to automatically deactivate the exterior mirror heating 14.It is also possible that when the exterior mirror heater 14 is deactivated, the laser beam 28 is emitted in the direction of the exterior mirror 12 and the reflected portion is measured by means of the sensor 30. If it is determined on the basis of the reflected portion that the exterior mirror 12 is provided with frozen and / or liquid water surface, then it can be provided that the control device 16 automatically activates the exterior mirror heating 14. In this case, it is therefore possible, without involvement of the driver 18, to activate the exterior mirror heating 14 in good time even when there is formation of deposits. The exterior mirror heating 14 can thus be automatically activated as required in order to ensure good rearward view through the exterior mirror 12 as far as possible at any time.In FIG. 6, one of the two outer mirrors 12 is again shown in a sectional plan view. The procedure described below can also be combined with the preceding. In the case shown here, another sensor 32 monitors a temperature development of the exterior mirror 12, for example on the inside of the exterior mirror 12.After activation of the exterior mirror heating 14, it is monitored when a measured temperature development of the exterior mirror 12 follows at least one of the two defined temperature profiles 34, 36 schematically indicated here for a predefined duration. As soon as this should be the case, it can be provided that again said query is output to the driver 18 or the exterior mirror heating 14 is automatically deactivated. The sensor 32 thus detects the heating behavior of a mirror plate of the exterior mirror 12.The heating power causes the water coating to melt or evaporate. Depending on the aggregate state and thickness of the water surface, a special temperature profile results. These temperature profiles 34, 36 are deposit-specific and stored, for example, in the control device 16. In particular, different water coverings have different evaporation profiles, which can be reflected in these characteristic temperature profiles 34, 36. Accordingly, the driver 18 can be informed whether the exterior mirror heater 14 could be turned off. Depending on how the driver 18 responds to this, the exterior mirror heater 14 is then automatically deactivated or continues to operate, wherein it may also be that the exterior mirror heater 14 is automatically deactivated independently of a query from the driver 18 as soon as a measured temperature profile at the exterior mirror 12 follows one of the two characteristic or defined temperature profiles 34, 36 for a certain duration.List of reference characters10 Motor vehicle 12 exterior mirror 14 exterior mirror heating 16 control device 18 driver 20 vehicle-external server device 22 combination device 24 forward travel direction 26 laser 28 laser beam 30 sensor 32 sensor 34 defined temperature profile 36 defined temperature profile

Claims

Method for operating an exterior mirror heater (14) of a motor vehicle (10), which is designed to remove a frozen and / or liquid water covering on at least one exterior mirror (12), in which, after activation of the exterior mirror heater (14), the fulfilment of a criterion is monitored and, after fulfilment of the criterion, the exterior mirror heater (14) is deactivated or a query is issued to a vehicle occupant (18) and, as a function of an answer to the query, the exterior mirror heater (14) is operated or deactivated further, wherein - the criterion comprises the condition that, after activation of the exterior mirror heater (14), a required heating duration has elapsed and / or a required heating energy has been expended; - the criterion alternatively or additionally comprises the condition that a reflected portion of a light beam (28) emitted in the direction of the exterior mirror (12) has a predefined value; at least in the event that the exterior mirror heating (14) is deactivated, a calculation rule is provided which determines the required heating duration and / or required heating energy as a function of variables influencing the removal of the water surface, wherein after deactivation of the exterior mirror heating (14) the heating duration and / or heating energy used up until then is used to optimize the calculation rule; and / or at least in the event that the exterior mirror heating (14) is deactivated, the criterion comprises the condition that a measured temperature development of the exterior mirror (12) follows at least one defined temperature profile (34, 36) for a predefined duration.Method according to Claim 1, characterized in that the variables influencing the removal of the water covering comprise the external temperature, the location of the motor vehicle (10), the solar radiation, the time, the air humidity and / or the elevation of the motor vehicle (10).Method according to one of the preceding claims, characterized in that the light beam (28) is emitted in the direction of the exterior mirror (12) even when the exterior mirror heating (14) is deactivated and the reflected portion is measured, wherein the exterior mirror heating (14) is automatically activated if it has been determined on the basis of the reflected portion that the exterior mirror (12) is provided with frozen and / or liquid water coating.Method according to Claim 1, characterized in that the defined temperature profile (34, 36) takes into account at least one type and / or aggregate state of water surface.Method according to Claim 1 or 4, characterized in that the measured temperature development of the exterior mirror (12) is compared with the at least one defined temperature profile (34, 36), and the heating power is adapted as a function of the comparison result.Control device (16) for an exterior mirror heater (14) of a motor vehicle (10), which is designed for removing a frozen and / or liquid water covering on at least one exterior mirror (12), wherein the control device (16) is configured to carry out a method according to one of the preceding claims.Motor vehicle (10) having an exterior mirror heater (14) which is designed for removing a frozen and / or liquid water covering on at least one exterior mirror (12) of the motor vehicle (10), and having a control device (16) according to Claim 6.

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