TECHNICAL FIELD

The present invention lies within the field of traffic monitoring, in particular within the field of vehicle sensing devices. BACKGROUND OF THE INVENTION

Road traffic monitoring is an important tool for traffic management and for infrastructure development. Information regarding e.g. type, frequency, speed, etc. for vehicles is traditionally produced by people who sit by the road and manually collect the information. However, devices for collecting this information have started to replace these people, making the collection less labor intensive, cheaper and most importantly improving safety.

Traditionally, devices such as radar devices, inductive loops, and cameras are used for detecting vehicles. Another more recently introduced type of vehicle detecting device utilizes sensors such as magnetometers and accelerometers for detecting vehicles. These devices are in general driven by batteries.

A problem with these battery driven devices is that the batteries need to be replaced from time to time, which requires both systems for indicating when the battery needs to be replaced, and requires further the actual service of replacing the batteries. Moreover, the sensors in the devices are often very power-consuming and thus the batteries need to be replaced often.

A solution to this problem is to include a solar cell in the device.

Thereby, the device may be powered by the solar cell when there is sufficient light. Thus, battery life is extended.

Another solution is to utilize duty cycling in the device. By duty cycling, the device, or parts of the device, goes into a sleep mode frequently which utilizes less power. Thus, battery life is extended.

US 7,710,452 discloses a remote video monitoring unit (VMU). To reduce power consumption, the VMU can be operated in a low-power standby mode, which includes monitoring the ambient magnetic field in its

environment such that the VMU is switched to a high-power active mode when detecting a disturbance in the ambient magnetic field.

US 2002/0190856 A1 discloses a wireless vehicle detection system for which power conservation is improved by using a default low-power inactive mode which, when a vehicle is sensed by the detection system comprising e.g. a magnetic or vibrational sensor, is switched to a high-power active mode for monitoring of the vehicle.

However, there is still a need for improvement of these devices, in particular concerning their power consumption.

SUMMARY OF THE INVENTION

An object of the present invention is to alleviate the above mentioned drawbacks and problems. A further object is to provide a method and a sensor device with lowered power consumption in comparison to known techniques.

According to a first aspect of the invention, this and other objects are achieved by a method for operating a vehicle sensor device between a sleep mode and an active mode, the method comprising: generating, when the vehicle sensor device is in the sleep mode and when a photoelectric cell receives light, an input signal by the photoelectric cell; and activating, provided that the input signal lies within a predetermined interval set in relation to a predetermined strength level of the photoelectric cell and thereby assuming that at least a part of the received light is provided by a light source on a vehicle, a sensor in the vehicle sensor device by a controller, thereby setting the vehicle sensor device in the active mode.

The invention includes, and is based on, the realization that a photoelectric cell can be utilized for detecting a vehicle. This concept solves many of the problems mentioned above. The photoelectric cell does not consume any power for receiving light, thus a large reduction in power- consumption relative to conventional vehicle sensors such as magnetometers and accelerometers is provided. The capability of the vehicle sensor device to detect a vehicle in a power-efficient manner is further improved by utilizing the predetermined interval. This is achieved as the predetermined interval is set to correspond to the voltage interval generated in the photoelectric cell when receiving light from a light source, typically a head light, of a vehicle. Erroneous activation of the vehicle sensor device by other (artificial or natural) light sources resulting in an input signal having a voltage strength outside the predetermined interval may therefore be avoided. Thus, a reduced activation frequency and a reduced power-consumption of the vehicle sensor device are achieved.

The predetermined signal strength level may be set to correspond to the prevailing light conditions. The threshold for activating the vehicle device may therefore be set by adjusting the level of the predetermined strength level of the photoelectric cell in relation to the surrounding light conditions of the vehicle sensor device.

The predetermined signal strength may be arranged to be updated frequently in order to correspond to the prevailing light conditions. The predetermined interval follows the predetermined signal strength level if the predetermined signal strength is varied. Variations in the surrounding light of the vehicle sensor device may thereby be taken into account when detecting a vehicle. As a result, erroneous activation of the vehicle sensor device may be avoided which may result in reduced power-consumption.

The sensor is activated by the controller. The controller is preferably a micro controller, but could be other types of controllers as well, such as a field-programmable gate array (FPGA), a digital signal processor (DSP) or any other suitable controller.

During night, or during cloudy days, daylight is weak and not sufficient to power the sensor device by means of the photoelectric cell. However, the photoelectric cell still generates electric signals when receiving light. When the photoelectric cell receives light from the light source, such as a headlight on the vehicle, the photoelectric cell generates an input signal. If the light originates only from the vehicle's headlight, the input signal is of a voltage much smaller than from e.g. daylight on a sunny day. Even if the input signal is not sufficient to power to sensor device, it may still utilized for activating the sensor in the vehicle sensor device, thereby setting the vehicle sensor device in the active mode. It is assumed that the light originates from a light source such as a headlight of a vehicle, if the input signal lies within the

predetermined interval. Thus, the vehicle sensor device may be activated when a vehicle is driving nearby the sensor.

By that the input signal lies within a predetermined interval is meant that the strength of the input signal, such as the voltage value of the input signal, lies within a predetermined strength interval, such as a voltage interval.

Preceding the step of activating, the input signal may be amplified. The input signal may be amplified using an operational amplifier. Such an amplifier is cheap and has low power consumption.

By the amplification, the amplified signal may be measured on by a controller having a conventional A/D converter. Such an A/D converter has typical a resolution of 10-16 bits. Preferably, the input signal is amplified with an amplification factor in a range of 100-10 000, i.e. the input signal is amplified by 100-10 000 times. By using an amplifier and a controller having a conventional A/D converter, the method may be achieved in a cheap and simple manner.

Alternatively, the input signal may, without being amplified, be measured on by a controller with a high resolution A/D converter. Such an A/D converter may have a high resolution of 18-24 bits.

The vehicle sensor device may switch between the sleep mode and the active mode at least during periods in which there is not sufficient daylight in order to power the sensor device by means of the photoelectric cell or any other solar cell in the sensor device. For example, during the night and during cloudy days, there is typically not enough light for powering the device. The required power, e.g. for powering the sensor in an active mode, may then be provided by the battery.

There are several reasons for the great power-consumption savings provided by the inventive method.

Firstly, required components for the vehicle detection by the

photoelectric cell have a low power-consumption in comparison to the components commonly used, e.g. sensors such as a magnetometer or an accelerometer. Further, the photoelectric cell has a wider detection range than a magnetometer or an accelerometer. This allows for the utilization of the sleep mode and the active mode for the sensor device, in particular for sensors therein, since the sensors may be given time to start up and prepare for measuring on the vehicle before the vehicle has reached the sensing zones of the sensors. Thus, the sensors do not need to consume any power when they are not utilized, i.e. when no vehicles are passing.

Secondly, the sample rate may by the inventive method be below 10 Hz, thus the sample rate may be substantially reduced in comparison to typical sensors. Thus, the power consumption may be reduced further. The sample rate for e.g. a magnetic sensor is typically in the order of 50 Hz, and for an accelerometer in the order of several kHz. The sample rate of the inventive method is only limited to that at least one sample has to be taken during the time it takes for a vehicle to pass the detection area of the photoelectric cell. For example, if the detection area is 30 meters, and the sensor is arranged to detect vehicles having a speed of 1 10 kilometers per hour, i.e. approximately 30 meters per second, the photoelectric cell has to measure at least two times per second in order to detect the vehicle independent of when in the detection cycle the vehicle passes. In this example, the sample rate may thus be only 2 Hz. In order to cover for other scenarios requiring higher sample rate, or for possible sensing failures, the sample rate may need to be higher. Preferably, the sample rate lies within the range of 3-10 Hz.

Thirdly, the inventive method is based on comparing the input signal with a predetermined interval which is set to correspond to the input signal from a light source of a vehicle. Thus, the sensor device is not activated on any light input above a certain threshold. Unnecessary activations of the sensor device, due to other factors than a vehicle, may thus be counteracted.

Hence, the inventive method provides a great power consumption reduction. The method may provide advantages such as extending the lifetime of the sensor device and alleviating the need for frequent battery changes. A sensor device may be provided which may function continuously night and day with a low average power consumption.

The method may comprise that a plurality of sensors are activated by the controller. The sensors in the vehicle sensor device may for example be magnetic sensors, accelerometers, optical sensors, acoustic sensors, microphones, or a combination thereof. The vehicle sensor device may comprise a combination of different sensor types. The one or more sensors may be utilized for functions such as vehicle speed estimation, vehicle classification, and/or other measurements on the vehicle.

The method may further comprise activating a timer in connection to the activation of the sensor, wherein the sensor is deactivated by the controller when the timer reaches a predetermined timeout value, thereby setting the vehicle sensor device in the sleep mode.

Thus, the vehicle sensor device goes into its sleep mode when the one or more sensors therein are passive, i.e. when there are no vehicles present for the sensors to measure on. However, the photoelectric cell is always active for receiving any incoming light. When the photoelectric cell receives light assumed to be provided by a light source on a vehicle, the sensors are activated again. Very low power consumption may be achieved when the sensors are consuming power only during their measurements and not during the lapsed time between the measurements.

The vehicle sensor device is preferably arranged in or adjacent to a roadway.

The method may further comprise, provided that the strength of the input signal is above a predetermined charging threshold value, charging a battery by a solar cell, the battery being arranged to power the vehicle sensor device.

Thus, the battery may be charged during periods when there is sufficient daylight, and utilized during periods when there is not sufficient daylight for charging or powering the device by the solar cell. Since the vehicle sensor device may shift between the active mode and the sleep mode, the battery may power the device during a longer period without the need for charging in comparison with known techniques. The solar cell may also be utilized to power the vehicle sensor device when there is provided sufficient light. The vehicle sensor device may be arranged such that it shifts between the active mode and the sleep mode only when the there is not sufficient light provided to the solar cell in order to power the device, i.e. when the device must be powered by the battery.

The solar cell and the photoelectric cell may be the same cell, i.e. the photoelectric cell may be utilized both for charging the battery, powering components of the vehicle sensor device, and for detecting a vehicle.

The light source may be a headlight of a vehicle, in particular a headlight of a vehicle approaching the vehicle sensor device.

According to a second aspect of the invention, this and other objects are achieved by a vehicle sensor device operable between a sleep mode and an active mode, the vehicle sensor device comprising: a photoelectric cell; a controller; and at least one sensor; wherein the photoelectric cell is arranged to generate, when the vehicle sensor device is in the sleep mode and when the photoelectric cell receives light, an input signal by the photoelectric cell: whereby the at least one sensor in the vehicle sensor device is arranged to be activated by the controller, provided that the input signal lies within a predetermined interval set in relation to a predetermined strength level of the photoelectric cell, and thereby assuming that at least a part of the received light is provided by a light source on a vehicle, thereby setting the vehicle sensor device in the active mode.

At least one of the sensors may be one of, or a combination of, the following sensor types: a magnetic sensor, an accelerometer, an optical sensor, an acoustic sensor, or a microphone.

The vehicle sensor device may further comprise a timer which is arranged to be activated in connection to the activation of the sensor, wherein the at least one sensor is arranged to be deactivated by the controller when the timer reaches a predetermined timeout value, thereby setting the vehicle sensor device in the sleep mode.

The vehicle sensor device may further comprise an amplifier. The amplifier may be arranged to amplify the input signal. The amplifier may be an operational amplifier. The vehicle sensor device may be arranged in or adjacent to a roadway.

The vehicle sensor device may further comprise a battery, wherein the vehicle sensor device is arranged to, when the input signal is above a predetermined charging threshold value, charge the battery by a solar cell, the battery being arranged to power the vehicle sensor device.

The solar cell may be the photoelectric cell.

The vehicle sensor device may further comprise a wireless

communicator, such as a radio transmitter.

Thus, the above disclosed features and corresponding advantages of the first aspect is also applicable to this second aspect. To avoid undue repetition, reference is made to the discussion above.

It is noted that the invention relates to all possible combinations of features recited in the claims. BRIEF DESCRIPTION OF THE DRAWINGS

This and other aspects of the present invention will now be described in more detail, with reference to the enclosed drawings showing embodiments of the invention.

Figure 1 illustrates a vehicle sensor device.

Figure 2 illustrates the vehicle sensor device by a roadway on which a vehicle is travelling.

Figure 3 illustrates a method for operating the vehicle sensor device.

The figures are adapted for illustrative purposes and, thus, they are provided to illustrate the general concept of embodiments of the present invention. Like reference numerals refer to like elements throughout.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and for fully conveying the scope of the invention to the skilled person.

A vehicle sensor device 1 is illustrated in figure 1 . The vehicle sensor device 1 comprises a solar cell 10 which is connected to an amplifier 1 1 and a battery 16.

The vehicle sensor device 1 further comprises an amplifier 1 1 , which in this embodiment is an operational amplifier. The amplifier 1 1 is connected to an A/D converter 12 such that an amplified signal may be A/D converted.

The A/D converter 12 may be an A/D converter with a resolution of about 10-16 bits, or be a high resolution A/D converter, with a resolution of about 18-24 bits. In one embodiment, where the A/D converter 12 is a high resolution A/D converter, the vehicle sensor device 1 does not comprise an amplifier 1 1 , and thus the solar cell 10 is directly connected to the high resolution A/D converter. Thus, the present invention may be achieved without an amplifier and the measurement of the strength of the input signal may be performed directly on the non-amplified input signal.

A measurement on the amplified input signal is more easily

implemented and thus preferred. Therefore, in the embodiment disclosed hereafter in connection to the figures, the amplifier 1 1 is arranged between the solar cell 10 and the A/D converter 12. The output signal from the A/D converter 12 may be transmitted to a controller 13. The controller 13 is arranged to control the one or more sensors of the vehicle sensor device 1 . In this embodiment, the vehicle sensor device 1 has two sensors: a

magnetometer 14 and an accelerometer 15. The controller 13 controls the sensors by e.g. activating and deactivating them. In this embodiment, the controller 13 is a micro controller.

The A/D converter may be part of the controller 13.

The controller 13 comprises a processor 18. The processor 18 may be arranged to analyze the output signal from the A/D converter 12. The controller 13 further comprises a memory 19. The memory 19 may store data which is e.g. predetermined or generated during measurements.

The vehicle sensor device 1 further comprises a wireless

communicator 17 for communicating data from the vehicle sensor device 1 to other devices, such as another similar vehicle sensor device 1. The wireless communicator 17 may be controlled by the controller 13. The wireless communicator 17 is not a necessary component in order to achieve the present invention. However, the wireless communicator 17 is preferably included in the vehicle sensor device 1 for further reducing the need for time- consuming a costly service of the device 1. Instead of manually extracting data from the vehicle sensor device 1 , the data may be transmitted to a service center or another vehicle sensor device 1.

It is noted that the connections may be direct or indirect connections, and that there may also be provided other connections between the components.

The vehicle sensor device 1 is arranged such that its components may be powered either directly by the solar cell 10, by another solar cell of the vehicle sensor device 1 , or by the battery 16.

The solar cell 10 is a photoelectric cell arranged to receive light, thereby producing an electric signal with a voltage level depending on how much light the photoelectric cell receives. The vehicle sensor device 1 may comprise a plurality of photoelectric cells, i.e. solar cells, which may form one or more solar-cell panels. The solar cell 10 may be connected to a terminal box for a solar-cell panel which transmits the electric signal of the solar cell 10 to the amplifier 1 1. The electric signal of the solar cell 10 may be a part of a total electric signal comprising electric signals from a plurality of solar cells of the solar-cell panel.

Figure 2 illustrates the vehicle sensor device 1 arranged by a roadway 23. A vehicle 20 is travelling on the roadway 23 in a direction towards the vehicle sensor device 1 . The vehicle sensor device 1 is arranged to detect the vehicle 20 by means of the solar cell 10. Upon detection, the vehicle sensor device 1 is set in an active mode by activating the magnetometer 14 and accelerometer 15 therein. The detection and activation method is illustrated in figure 3, and will be described in detail in the following.

The headlight of the vehicle 20 is directed towards the vehicle sensor device 1 , and the solar cell 10 is arranged such that it may receive light from the headlight. When the light from the headlight of the vehicle is received by the solar cell 10, an input signal, being an electric signal, is generated by the solar cell 10, as denoted by 301 in figure 3. The electric signal of the solar cell 10 is hereafter referred to as input signal. The input signal is provided to the amplifier 1 1 , which amplifies the input signal, denoted by 302 in figure 3.

Thereafter, the strength of the input signal is determined.

The value of the input signal may be determined in many ways which are known in the art. In this embodiment, the input signal is amplified, by the amplifier 12, and A/D converted, by the A/D converter 12, in order to digitally process the converted input signal. Software is utilized to process the converted input signal and to determine its value. Based on the strength of the input signal, the software may initiate further action such as an activation of the sensors in the vehicle sensor device 1. The software may be part of the processor 18 and/or the controller 13.

In an alternative embodiment, the input signal, or the amplified input signal, is processed by a voltage detector or a voltage comparator. Thus, the determination may alternatively be hardware implemented. Based on the strength of the input signal, the voltage detector or voltage comparator may initiate further actions such as instructing the controller 13 to activate the sensors in the vehicle sensor device 1.

In one embodiment, the controller 13, and in particular the processor

18, may analyze the A/D converted input signal by e.g. applying processing algorithms. Thus, the input signal, or a processed signal thereof, may be evaluated, besides from being evaluated in respect to its strength, in order to more securely determine that the received light originates from the vehicle 20.

If the input signal lies within a predetermined interval, it is assumed that at least a part of light received by the solar cell 10 is provided by a light source on a vehicle which in this embodiment is the headlight of the vehicle 20. Thereupon, the vehicle sensor device is activated, as denoted by 303 in figure 3. Two examples of detection scenarios will be disclosed in the following.

In the first detection scenario, the surrounding light is substantially zero, as it may be for example by night when a single vehicle 20 passes by the vehicle sensor device 1 . Thus, the input signal originates substantially from the headlight of the vehicle 20. The predetermined interval may in this scenario be set to a fixed interval comprising the voltage values of the input signal, which is expected to be obtained by a typical light source on a single vehicle within a sensing range of the solar cell 10. For example, the input signal may have a voltage level in a range of 0.1-1 mV, if it substantially originates from the light source of the vehicle 20. This interval is thus the fixed predetermined interval for this scenario, in which it is to be determined if the input signal lies within.

The amplifier 1 1 is in this example arranged to amplify the input signal 1000 times. Thus, the amplified input signal will lie in a range of 0.1-1 V. If it is determined that the input signal lies within the predetermined interval, it is assumed that the input signal did originate from the light source of the vehicle 20. By measuring the amplified input signal and determining if it lies within the amplified predetermined interval it may be determined that the input signal lies within the predetermined interval.

In the second detection scenario, the surrounding daylight is not substantially zero, as it is for example by day or may be during the night. Thus, only a part of the input signal originates from the light source of the vehicle 20. The predetermined interval is in this scenario a flexible interval.

The flexible interval is set relative a predetermined strength level of a solar cell signal, which is generated before generating the input signal. The predetermined signal strength may be set to correspond to the prevailing light conditions. For example, the predetermined signal strength level may be a mean value over time for the strengths of signals generated by the solar cell 10, or be the strength of a signal provided by the solar cell 10 at a particular point of time. Other suitable predetermined signal strength levels may also be utilized, as realized by the skilled person.

The predetermined signal strength level may be updated frequently in order to correspond to the prevailing light conditions. The prevailing light conditions normally change during the day and may depend on different factors such as the position and phase of the moon as well as on the weather conditions. Changes in the artificial lighting and shadowing of the vehicle sensor device 1 may also change the light conditions for the vehicle sensor device 1 .

The predetermined signal strength level may be stored in the memory 19 of the vehicle sensor device 1.

The predetermined interval may thus be set as an interval added to the predetermined signal strength level. The predetermined interval follows the predetermined signal strength level if the predetermined signal strength is updated.

As disclosed in the previous example scenario, the input signal originating substantially from a typical headlight of a single vehicle lies in the range of 0.1-1 mV. If the predetermined signal strength level is for example 1 .6 mV, the predetermined interval may be set to 1.7-2.6 mV. If the strength of the input signal is determined to lie within this predetermined interval, it is assumed that a part of the input signal originates from the headlight of the vehicle 20.

The vehicle sensor device 1 may be arranged to shift between different predetermined intervals. If the input signal decreases towards 0 V, or decreases below a certain voltage level, the vehicle sensor device 1 may be arranged to shift from comparing against a flexible predetermined interval to comparing against a fixed predetermined interval. The vehicle sensor device 1 may alternatively apply the flexible predetermined interval also for when the surrounding light is substantial 0 V.

If the input signal is determined to be greater than a predetermined charging threshold, the input signal is utilized for charging the battery 16. The predetermined charging threshold depends on the electronics used. Typically, it lies within 1-5 V. If a Minimum Power Point Tracking (MPPT) device is utilized, 1 V may be sufficient for charging the battery 16. If a MPPT is not utilized, the strength of the input signal might have to be higher than the battery voltage, which may differ between different battery types. As an example, a lithium polymer battery typically requires a charge voltage of 4.2 V, while a nickel-metal hydride cell typically only requires 1.2 V.

Hence, how the vehicle sensor device 1 uses the input signal depends on the strength of the input signal. Typically, the input signal is utilized for charging the battery 16 and/or directly powering the components of the vehicle sensor device 1 during the day when the daylight is sufficient for these functions. When the daylight is sufficient for powering the vehicle sensor device 1 , there is no direct need for saving power. When the daylight is not sufficient for powering the vehicle sensor device 1 , the vehicle sensor device 1 goes into its power-saving mode. In the power-saving mode, the

magnetometer 14 and the accelerometer 15 are deactivated by default. When the vehicle 20 is detected by means of the solar cell 10, according to the inventive method, the magnetometer 14 and accelerometer 15 are

temporarily activated for performing further measurements on the vehicle 20. However, the shift between the active mode and the sleep mode may also be utilized when the daylight is sufficient for powering the vehicle sensor device 1.

It should be noted that the input signal may be utilized for a plurality of purposes. In one embodiment, the input signal may be utilized for charging a battery and, when it is detected that the input signal lies within a flexible predetermined interval, also utilized for detecting the vehicle and activating the vehicle sensor device 1 .

The different sensing components of the vehicle sensor device 1 have different sensing zones, as illustrated in figure 2.

The first sensing zone 24 is the sensing zone of the solar cell 10.

When the vehicle 20 enters the first sensing zone 24, the light received by the solar cell 10 from the headlight of the vehicle 20 is sufficiently strong in order to be received by the solar cell 10 and generate an input signal. The first sensing zone 24 reaches from the vehicle sensor device 1 to about 25-35 meters before and after the vehicle sensor device 1.

The second sensing zone 26 is the sensing zone of the magnetometer 14. Within the second sensing zone 26, the magnetometer 14 is able to sense the vehicle 20 by sensing its magnetic field or its magnetic impact on surrounding magnetic fields. The measurement of the magnetometer 14 may for example be utilized for confirming the assumption that the vehicle 20 is present. The second sensing zone 26 lies within the first sensing zone 24 and reaches from the vehicle sensor device 1 to about 5-7 meters before and after the vehicle sensor device 1.

The third sensing zone 28 is the sensing zone of the accelerometer 15. Within the third sensing zone 28, the accelerometer 15 is able to sense the vehicle 20 by sensing vibrations originating from the vehicle 20. The third sensing zone 28 lies within the second sensing zone 26 and reaches from the vehicle sensor device 1 to about 3-4.5 meters from the vehicle sensor device 1.

It is noted that the sensing zones are illustrated for the purpose of understanding the general concept. The sensing zones may in other embodiment have a different extension area or shape of extension area.

However, in preferred embodiments, the first sensing zone 24 of the solar cell 10 extends at least 20 meters further relative to a sensing zone of one of the sensors of the vehicle sensor device 1. In particular, it is preferred that the first sensing zone 24 extends at least 20 meters further relative to a sensing zone of any of the sensors of the vehicle sensor device 1.

Since the vehicle sensor device 1 is able to detect the vehicle 20, by means of the solar cell 10, already when the vehicle 20 enters the first sensing zone 24, the vehicle sensor device 1 is given time to activate the magnetometer 14 and accelerometer 15. Thus, the sensors have time to start up and may thus be ready to perform measurements when the vehicle 20 enters the second sensing zone 26 and the third sensing zone 28. In known techniques where the magnetometer 14 is utilized for detecting the vehicle, such activation/start-up time is shorter and may not be sufficient.

A timer is activated in connection to the activation of the sensors, as denoted by 304 in figure 3. The timer may be implemented in the processor 18. The timer may be activated at any time during the activation process, e.g. when it is determined that the input signal lies within the predetermined interval, when the controller activates a sensor, when a sensor performs a measurement, etc.

The timer runs until it reaches a predetermined timeout value, whereby the vehicle sensor device 1 is deactivated. The deactivation involves deactivating the sensors of the vehicle sensor device 1 , i.e. the magnetometer 14 and the accelerometer 15, such that they are put into a standby mode. The vehicle sensor device 1 is thereby set in its sleep mode, awaiting the next activation upon detecting a new vehicle by means of the solar cell 10. The timeout value may be stored in the memory 19.

The timeout value may be set to a fixed value such as three times the time it takes for a vehicle with an assumed speed to reach the vehicle sensor device 1 . For example, if it takes two seconds from the detection moment for the vehicle to reach the vehicle sensor device 1 , the timeout value is six seconds. The assumed speed may be e.g. the speed limit for the concerned roadway.

Alternatively, the timeout value may be a dynamic value. Depending on factors such as the allowed speed, roadway conditions, obstacles, dirt, and weather, the passing vehicles may travel with varying speed over time. The vehicle sensor device 1 may determine, by the controller 13 and/or the processor 18, the time it takes for a plurality of vehicles to e.g. first be detected by the solar cell 10 and then by the magnetometer 14. The timeout value may for example be set to three times the mean value for the passing time of the last twenty vehicles. Thus, the timeout value may change over time. Other ways for determining the value of the timeout value is also feasible, as understood by the skilled person.

In one embodiment, the vehicle sensor device 1 is arranged such that, when the input signal is below a predetermined deactivation threshold, the one or more sensors of the vehicle sensor device 1 are deactivated. Thus, the vehicle sensor device 1 goes from a mode where the vehicle sensor device 1 is activated by default to a mode where the vehicle sensor device 1 shifts between its active mode and its sleep mode. From the sleep mode, the one or more sensors are activated and deactivated as disclosed above. The predetermined deactivation threshold preferably coincides with the

predetermined charging threshold value, such that the device goes into the sleep mode when the input signal is too weak for charging the battery.

According to an embodiment the vehicle sensor device 1 may further comprise an arrangement for detecting obstacles on the photoelectric cell 10 of the vehicle sensor device 1 . The arrangement comprises a light source EP2013/062750

17 such as a light emitting diode (LED). The light source is arranged to send light from the vehicle sensor device 1. If an obstacle is present on or nearby the photoelectric cell 10 at least a portion of light emitted by the light source is reflected by the obstacle and back to the photoelectric cell 10. As a result the output voltage of the photoelectric cell 10 is increased. The increase indicates that an obstacle is present on or nearby the vehicle sensor device 1.

This obstacle detection allows for an improved monitoring of the efficiency of the photoelectric cell 10 to detect vehicles. Another advantage is that the system for detecting obstacles can be operated remotely, i.e. without the need of a person going out to the vehicle sensor device 1 and visually inspecting it.

The vehicle sensor device 1 may also be configured to at a certain time or at time intervals monitor if an obstacle is present or not. In an embodiment the vehicle sensor device 1 is further arranged such that it is possible to evaluate to what extent the presence of an obstacle may affect the vehicle detection capability. This can for instance be achieved by using a plurality of light sources that are arranged such that each is associated with a certain portion of the photoelectric cell 10. The extent to which the photoelectric cell 0 is covered by the obstacle may then be estimated by determining the number of portions of the photoelectric cell 10 that has received reflected light.

The vehicle sensor device 1 may similarly comprise an arrangement for detecting obstacles on a solar cell of the vehicle sensor device 1.

In summary, this application discloses a method for operating a vehicle sensor device 1 between a sleep mode and an active mode. The method comprises generating 301 an input signal by the photoelectric cell; and activating 303, provided that the input signal lies within a predetermined interval, a sensor 14, 15 in the vehicle sensor device, thereby setting the vehicle sensor device 1 in the active mode. The application further discloses a vehicle sensor device 1 comprising a photoelectric cell 10, a controller 13, and at least one sensor 14, 15. The components are arranged to perform the method for operating the vehicle sensor device 1 between the sleep mode and the active mode. It is understood that these embodiments may be combined or altered during different periods. For example, a fixed predetermined interval may be utilized when the input signal lies below a certain level, whereas a flexible predetermined interval is utilized when the input signal lies above a certain level.