AU2014100468A4 - Operator drowsiness detection in mines - Google Patents

Abstract

Abstract A system for determining a drowsiness state 5 of an operator of a movable object comprises means for determining a proximity of the movable object to another object, and a control unit for generating a collision warning dependent on the determined proximity or data de rived therefrom. The control unit is adapted to determine 10 the drowsiness state of the operator subject to the de termined proximity or the data derived therefrom, or sub ject to an input from a monitoring unit for monitoring a response of the operator to the collision warning.

Description

1 OPERATOR DROWSINESS DETECTION IN MINES Technical Field and Background Art 5 The invention relates to a system and a meth od for determining a drowsiness state of an operator of a movable object. Surface mines and similar sites or areas are io generally operated by means of a large number of vehi cles, some of which may be exceedingly large and diffi cult to control and have very limited visibility for the operator. In case an operator of such a vehicle or oth is er movable object becomes tired and possibly loses con trol over the vehicle, situations may occur in which the life of other persons working on site may be at risk or damages may occur at the vehicle or other objects on site. 20 Disclosure of the Invention The problem to be solved by the present in 25 vention is therefore to provide a system and a method for reducing the risk of collisions caused by drowsy or fa tigued operators. The present invention provides a system for 30 determining a drowsiness state of an operator of a mov able object, comprising: - means for determining a proximity of the movable object to another object, - a control unit for generating a collision 35 warning dependent on the determined proximity or data de rived therefrom, 2 - wherein the control unit is adapted to de termine the drowsiness state of the operator subject to an input from a monitoring unit for monitoring a response of the operator to the collision warning, and wherein: 5 the control unit is adapted to determine an interval between the generation of the collision warning and an event in the input from the monitoring unit iden tifiable as response of the operator to the collision warning, or 10 the control unit is adapted to identify an absence of an event in the input from the monitoring unit within a defined interval starting with the generation of the collision warning. The proximity may, for example, be understood is as distance between the two objects, or other measure that gives information about the position of the objects relative to each other. A "movable object" in this con text shall include any object that can change and is ex pected to change its position and/or orientation or con 20 figuration in space. It may e.g. be a truck or any other vehicle that moves from place to place and changes its orientation in respect to the general north-south direc tion, e.g. by steering, or it may be an object positioned at a fixed location but is able to rotate about its axis 25 or is able to change its physical configuration, e.g. by extending an arm, in such a manner that the volume of safety space attributed to it varies in significant man ner. In a preferred embodiment, the movable object is a mining vehicle, and especially is one of a vehicle, a 30 crane, a dragline, a haul truck, an excavator and a shovel. The other object may be a movable object, too, or may be a stationary object. At least part of the prox imity detection means is preferably mounted to the mov able object. 35 In addition, a control unit is provided for generating a collision warning dependent on the deter mined proximity or data derived therefrom. The control 3 unit preferably is mounted to the movable object, too, and may in one embodiment be arranged in a common housing together with the proximity determination means. Data that may be derived from the determined proximity and 5 specifically from proximity values determined over time may be a speed of the movable object or an acceleration of the movable object, wherein the speed and/or the ac celeration may in one embodiment be represented by a vec tor including a direction the movable object moves to. 10 Generally, the acceleration may either denote an increase in speed - e.g. as a result of pushing the throttle con trol - or a decrease in speed - e.g. as a result of pressing a brake. Any of speed or acceleration may be de termined with respect to ground or with respect to the is other object and as such may also represent a relative speed and/or a relative acceleration. A trigger of a col lision warning may depend on any of these proximity de termination based data. It was found by the applicant, that either 20 the proximity values themselves or data derived therefrom may represent an indicator of the drowsiness, or, con versely the alertness of the operator, i.e. the person operating the mobile device. For example, in a scenario of vehicles driving in a row, if a distance of the vehi 25 cle containing the driver drowsiness system to a preced ing vehicle is too low, i.e. the proximity is identified to be below a first threshold, a conclusion may be that this kind of handling of the vehicle may result from a drowsy or fatigued operator. The same may be true in case 30 the distance to the other object becomes too large, i.e. is above a second threshold, which may indicate that the operator is not alert enough either. Generally, the first threshold preferably is smaller than the second thresh old. It is emphasized that it is not necessarily required 35 to implement both criteria. Subject to the set up of the system, e.g. the distance may only be compared to a sin gle threshold.

4 Similar observations may be made with respect to speed and/or relative speed. The scenario of the speed of a movable object on a site exceeding a threshold, or exceeding a threshold for a certain time may point to a s drowsy operator. The same may be true for an operator go ing at a speed - e.g. for a certain time - that is too low and in particular lower than a threshold which threshold preferably denotes an unusual low speed for a certain situation or for a certain location. The situa 10 tion and/or the location the movable object is in may be taken into account in any of the drowsiness state deter mination, e.g. for determining the value of the thresh olds. The situation and/or the location may, for example, be derived from data received from a global positioning is system, or from the proximity data. The same may be true for an operator showing non-typical acceleration behaviour. For example, sudden braking may point to a drowsy driver, Hence, the accel eration derived from the proximity measurements may serve 20 as an indicator for a drowsy operator, especially when the positive acceleration is below a first threshold, or exceeds a second threshold. Or when the negative accel eration is below a first threshold or exceeds a second threshold which scenarios all may hint at unusual braking 25 activities. Subject to the proximity values determined or the proximity based data derived, the control unit may define a drowsiness state of the operator of the movable object. Available drowsiness states, for example, may in 30 clude at least two states, e.g. "drowsy", or "non drowsy", i.e. alert. In another embodiment, the proximity depend ent data may include points in time when these data ful fil a criterion such as exceeding a threshold for gener 35 ating a collision warning to the operator. In this em bodiment, a rate of collision warnings generated may be an indicator for a drowsiness state of the operator.

5 There more collision warnings are generated in a period of time the more drowsy the operator seems to be. In another preferred embodiment, which may be applied in addition to or alternatively to the above em s bodiments, a reaction of the operator in response to a collision warning resulting from proximity evaluations is preferably taken as an indicator as to the drowsiness state of the operator. It is assumed that the proximity determination means are mainly for detecting dangerous 10 situations possibly resulting in a collision. Hence, when such dangerous situation is identified, e.g. when the distance to the other object is or becomes too close, a collision warning is issued to alert the operator of the dangerous situation. Such collision warning preferably is includes a visual or an audible warning to the operator, wherein the output unit for such warning may be one or more of a display or a speaker, preferably arranged in the area of activity of the operator, which preferably may be an operators cab. It is preferred that a monitor 20 ing unit is provided, and specifically is provided in/at the movable object, which monitors and/or identifies a reaction of the operator in response to the collision warning. This may also include identifying that no event occurs that can be taken as a reaction of the operator. 25 In case the operator is drowsy or even asleep, he/she may completely ignore the collision warning such that the ab sence of a reaction may be taken as a strong indicator for a drowsy or fatigued operator. The control unit may automatically detect such reaction. 30 In a preferred embodiment, the monitoring unit may include sensors that are present anyway in the movable object for other primary purposes, however, an output of which sensors may also serve as an indicator for a reaction of the operator in response to the colli 35 sion warning. For example, the monitoring unit preferably comprises a sensor for identifying a deflection of a steering wheel of the movable object. The steering wheel 6 deflection sensor may anyway be present for serving a power steering system. However, any rapid deflection of the steering wheel in response to the collision warning may indicate a drowsy operator scared by the collision s warning, especially when additional information is brought into context herewith, e.g. when the abrupt steering wheel deflection takes the vehicle into a direc tion towards the other object and not away therefrom. No or a too small deflection of the steering wheel may also io indicate a drowsy operator in view of an upcoming colli sion wherein a steering into a certain direction could have reasonably been expected. In another embodiment, the monitoring unit comprises a sensor for identifying an acceleration of the is movable object. The sensor may be an acceleration sensor, or may be a position sensor or pressure sensor for deter mining a position / an operation of the gas and/or brake pedal which sensor may directly supply an analogue signal to the control unit, or may supply a digitized signal 20 that may be supplied directly to the control unit or via a bus system, for example. The output of the correspond ing monitoring unit may also serve as an indicator for the reaction of the operator in response to the collision warning. Any rapid braking in response to the collision 25 warning may indicate a drowsy operator being scared by the collision warning. Or, missing, too late or a too little braking may be interpreted as the operator being drowsy enough not to react to the collision warning suf ficiently, especially when in view of the collision sce 30 nario a sharp breaking could have reasonably been ex pected. In another embodiment, the monitoring unit comprises a sensor for monitoring at least part of an ar ea of activity of the operator, e.g. part or all of the 35 operators cab. The sensor may be an optical sensor, e.g. an IR sensor, or a camera. Any picture or a sequence of pictures taken by such optical sensor may be supplied to 7 the control unit which may then by means of an evaluation module interpret the picture/sequence of pictures for as sessing a reaction of the operator in response to the collision warning. The pictorial information is also re 5 ferred to as monitoring data. Information extracted from such data by means of the evaluation module may, for ex ample, that the operator has not changed position in re sponse to the collision warning. In another embodiment, the sensor may also be a runtime measurement sensor for io monitoring a certain space in the operator cab, which space is considered to be taken by the operator in case of a collision warning, e.g. the space in front of a monitor, the space in front of an emergency button, the space in front of control elements, or similar. In case 15 it is detected by the means of such sensor that someone enters this space in response to the collision warning, this event may be classified as an appropriate reaction of the operator which proves the alertness of the opera tor. 20 In an even more sophisticated approach, the optical sensor may be arranged and capable of identifying an operator face. In such embodiment, the monitoring unit aims at identifying if an operator looks - in response to a collision warning - into the right direction. Again, 25 the right direction may be a direction towards e.g. a display which is provided for transmitting pictures from a camera filming the outside of the movable object, and specifically filming a scene of the impeding collision. For example, one or more cameras may be mounted at the 30 outside of the movable object at different locations in order to scan the entire space around the movable object. In case of a collision warning, it may be preferred that the camera is selected for displaying its pictures on the display in the operator cabin which is directed at the 35 area the other object approaches from. In case of the monitoring unit, which preferably may again contain an optical sensor and specifically may contain a front cam- 8 era in the display, supporting the identification of the operator looking at the display in response to the colli sion warning, the operator may be classified as alert. Here, the monitoring unit or the control unit may com 5 prise a facial recognition module which in particular may be a piece of software for extracting facial information from a picture taken by a conventional camera or a camera operating in the infrared light spectrum, for example. In another embodiment, the display may not necessarily dis io play a scene from the outside of the movable object but may display, for example, rules of conducts for the op erator to handle the impeding collision, or any other relevant information for the operator, e.g. more informa tion about the impeding collision, more information on 15 resolving the impeding collision, etc.. In this respect, the display may contain one or more screens, or, in an other embodiment, contains one or more light signs, LEDs etc. In a preferred embodiment, the monitoring 20 unit contains a face recognition module that even is ca pable of determining a state of the eyes of the operator and hence may support a determination if the operator eyes are closed or wide open in response to the collision warning. 25 In a preferred embodiment, the fatigue moni toring may in turn also impact the collision warning op eration: In case, for example, the outside camera may transmit the relevant scene to the display in the opera tor cab even prior to generating a collision warning, and 30 provided the control unit detects an alert state of the operator, the issuance of a collision warning may be sup pressed given that the operators attention already is drawn to the relevant display. In a preferred embodiment, the timing of any 35 reaction of the operator in response to the collision warning may be taken into account. Specifically, the time lapsing from the generation/issuance of the collision 9 warning and an event identifiable as a reaction of the operator in response to the collision warning may be evaluated. If such time, denoted as interval in the fol lowing, exceeds a threshold the operator may be identi 5 fied as a drowsy operator given that the reaction time is considered as too long. In case a reaction may be moni tored within an acceptable time, i.e. within an interval below the threshold, but a magnitude of the reaction may be too small - e.g. the pressure of pushing the brake pe 10 dal is lower than a threshold - the operator may be iden tified as drowsy. Only in case the monitoring unit moni tors a reaction of sufficient magnitude in a sufficient interval of time the operator may be identified as alert operator. is In another embodiment, an interval is prede fined in which a reaction of the operator is to be ex pected. In case no event is identifiable as a reaction within this predefined interval, the operator may be identified as drowsy. 20 Preferably, the control unit is adapted to generate a drowsiness warning subject to the drowsiness state identified. The drowsiness warning may be an acous tic and/or a visual signal for alerting the operator. Ad ditionally, or instead, the drowsiness state as deter 25 mined may be displayed to the operator, e.g. by means of colour coded states, or as a number representing a level of drowsiness. A diligent operator may use this informa tion to plan a break. The monitoring unit and the control unit may 30 be set up to then monitor for a sufficient reaction of the operator in response to the drowsiness warning or an increased drowsiness state indicated to the operator as was previously done in response to the collision warning. In another embodiment, the drowsiness warning may be 35 taken as a trigger for requesting an active input of the operator. Hence, an input unit may be provided, e.g. in form of a switch which may also be referred to as a dead- 10 man switch, which the operator is supposed to press within a given time starting from the drowsiness warning. In case the control unit detects that the input unit is not operated within this interval in time, an emergency 5 signal may be issued which emergency signal is adapted to trigger one or more of the following activities: Reducing the speed of the vehicle; Stopping the vehicle; Generat ing an acoustic or a visual warning. In a preferred embodiment, the proximity de 10 termination means comprises a receiver for a radio based positioning system for identifying the position of the movable object or at least a movable part of the object, and a receiver for receiving positional information from the other object. The term "radio based positioning sys is tem" stands for a GNSS or for any other type of position ing system based on radio signals, such as a pseudolite system, a WiFi based Real Time Location System (RTLS), etc. The term GNSS stands for "Global Navigation Satel lite System" and encompasses all satellite based naviga 20 tion systems, including GPS and Galileo. Hence the mov able object is capable of determining its own position by means of the subject receiver preferably adapted to in teract with satellites of the corresponding GNSS. The po sition of other objects in the vicinity of the movable 25 object may be detected by means of receivers mounted to these other objects. It is preferred that the movable ob ject comprises a receiver for receiving positional infor mation from the other objects, and specifically for re ceiving their positions, such that the movable object is 30 in a position to determine the proximity to the other ob ject/s based on the own position and the position/s from the other object/s as received. Alternatively, or in ad dition, the proximity determination means of the movable object may comprise a runtime measurement device for de 35 termining a distance of the movable object from the other object, which runtime measurement device may be one of a radio detection and ranging device, a light detection and 11 ranging device, a sound detection and ranging device or an active RFID ranging device, based on measurement of the received signal strength or the roundtrip delay or both. One or more such devices may be arranged around the 5 movable object in order to identify objects approaching from either side. In another embodiment, the monitoring unit includes the very same proximity determination means or at least a part of that is used for determining if a col 10 lision warning is to be generated. In case the control unit has issued a collision warning in response to at least the determined proximity to the other object, the proximity data may be continue to be monitored after the issuance of the collision warning. In a typical reaction is of an alert operator, the distance to the other object at least should not decrease any longer, and preferably in creases again in case the alert operator either brakes or steers away from the other object. Hence, for example, the monitoring unit may make use of the position data of 20 the movable objects receiver and the position data re ceived from the other object. In another preferred embodiment, the drowsi ness determination system comprises an input unit at the operators area of activity on the movable object, such as 25 a switch or a touchscreen, for activating or deactivating the drowsiness detection system. For privacy protection purposes, the operator preferably is enabled to switch off the drowsiness detection system. In one embodiment, the input unit may not only electrically switch off the 30 drowsiness detection system but may also mechanically in hibit the drowsiness detection system to work. For exam ple, in case of a camera or another optical sensors pro vided for supporting the detection of the drowsiness state of the operator, in response to the operator oper 35 ating the input unit for switching off the drowsiness de tection system, the optical sensor, the camera, or any other sensor of the drowsiness detection system may be 12 covered e.g. by a cover in order to become inoperable. Such means may enhance the credibility of the site opera tor towards the operators. However, it is preferred that such deactivation is logged such that later on, it can be 5 proved that the drowsiness detection system was deacti vated for a certain period in time. For example, it is assumed that the operator may switch off the drowsiness detection system during a break. However, it is preferred that the drowsiness detection system is switched on again 10 automatically when a state of the movable object is de tected than indicates an upcoming operation and/or move ment of the object. In another embodiment, a change in the identity of the operator may cause a reactivation of the drowsiness detection system, too, which change may be is detected by sensing means at the movable object, such as RFID based means, for example. The present invention also provides a method for determining a drowsiness state of an operator of a movable object, comprising: 20 determining a proximity of the movable object to another object, generating a collision warning dependent on the determined proximity or data derived therefrom, determining the drowsiness state of the op 25 erator of the movable object subject to a response of the operator to the collision warning monitored by a monitor ing unit, whereby the control unit is adapted to deter mine an interval between the generation of the collision 30 warning and an event in the input from the monitoring unit identifiable as response of the operator to the col lision warning, or the control unit is adapted to identify an absence of an event in the input from the monitoring unit 3S within a defined interval starting with the generation of the collision warning.

13 According to a further aspect of the present invention, a computer program element is provided com prising computer program code means for performing a method according to the above embodiment when executed on 5 a processing unit. The described embodiments similarly pertain to the system, the method, and the computer program ele ment. Synergetic effects may arise from different combi nations of the embodiments although they might not be de 10 scribed in detail. Other advantageous embodiments are listed in the dependent claims as well as in the description below. 1s Brief Description of the Drawings Embodiments of the invention are described in the following detailed description. Such description makes reference to the annexed drawings, wherein: 20 Fig. 1 shows a schematic representation of a site with movable objects containing a system according to an embodiment of the present invention, and Fig. 2 is a block diagram of a system accord ing to an embodiment of the present invention. 25 Detailed Description of the Drawings Fig. 1 schematically depicts a site 1, such 30 as a surface mine, with movable objects to which embodi ments of the present invention may be applied. Typically, such a site covers a large area, in the case of a surface mine e.g. in the range of square kilometres, with a net work of roads 2 and other traffic ways, such as rails 3. 35 A plurality of objects is present in the mine, such as: - Large vehicles, such as haul trucks 4a, cranes 4b or excavators 4c. Vehicles of this type may 14 easily weigh several hundred tons, and they are generally difficult to control, have very large breaking distances, and a large number of blind spots that the driver is un able to visually monitor without monitoring aids, such as s e.g. cameras. - Medium sized vehicles 5, such as regular trucks. These vehicles are easier to control, but they still have several blind spots and require a skilled driver. 10 - Small vehicles 6. Typically, vehicles of this type weigh 3 tons or less. They comprise passenger vehicles and small lorries. - Trains 7. All the above objects may qualify as movable is object. A further type of object within the mine is com prised of stationary obstacles, such as temporary or per manent buildings 9, open pits, boulders, non-movable ex cavators, stationary cranes, deposits, etc. Those objects in combination with movable objects may qualify as "oth 20 er" object. The risk of accidents in such an environment is high. In particular, the large sized vehicles can eas ily collide with other vehicles, or obstacles. For this reason, one, more or all objects in the mine 1 are pro 25 vided with proximity determination means 12 that supports the generation of collision warnings for the personnel of the site, and in particular for operators of the movable objects, thereby reducing the risk of collisions and ac cidents. Specifically, a movable object may include means 30 12 comprising a receiver for a radio based positioning system interacting with satellites 16. This means 12 com municate in wireless manner, in particular by radio sig nals. Preferably, the means 12 comprises a GNSS receiver for identifying its position, i.e. the position of the 35 assigned movable object. Further, the means 12 comprises a radio transceiver or circuit for exchanging data with other radio transceivers belonging to other objects.

15 Hence, the means 12 preferably receive positional signals through the GNSS receiver and exchange data derived therefrom with via the transceiver with the transceivers of other objects in order to calculate relative positions 5 and probabilities for collisions. In short, each means 12 obtains positional data derived from a signal from the GNSS receiver. This positional data allows determining the objects own position and is stored in a "status data set". The status dataset also contains a unique identi io fier (i.e. an identifier unique to each of the means 12 used on the same site). The status dataset is emitted as a radio signal through the transceiver. At the same time, the transceiver receives status datasets from other ob jects, especially from neighbouring objects and therefore is can calculate a relative distance to the other object/s by subtracting its own position coordinates from those of the other object/s. An exemplary system for determining a drowsi ness state of an operator of a movable object according 20 to an embodiment of the present invention is shown in a block diagram in Fig. 2. The system 15 which is assumed to be mounted to a movable object comprises a control unit 14, such as a microprocessor, which controls the op erations of the system, and preferably not only of the 25 fatigue monitoring system but also of a collision warning system which systems share multiple resources. The con trol unit 14 accesses a memory 18 that comprises programs as well as various parameters. The system 15 further comprises means 12 for 30 determining a proximity between the movable object and another object including a receiver 121 for identifying the position of the movable object, and a transceiver 122 for receiving positional information from other objects. Although the receiver is called a GNSS receiver 122 in 35 the following, it can also be a receiver interoperating with any other radio based positioning system for deter mining its position.

16 An output unit 19 is connected to the control unit 14. The output unit 19 may in one embodiment com prise output elements such one or more of an optical dis play 20 using LED's, LCD's, etc. or an acoustic signal 5 source 21, such as a beeper, or a speaker. The output unit 19 preferably is located at the area of activity of the operator which may be the cabin of a vehicle. The control unit 14 may determine the proximity to one or more objects in the vicinity of the movable object, and 10 may generate a collision warning in form of a signal sup plied to the output device 19 for generating a visual or an audible alarm. Specifically such collision warning may be generated in case the proximity is too close, i.e. the distance of the two objects is less than a threshold. 15 This corresponds to the assumption that a circular volume in space is reserved for each object. The radius of the circular volume attributed to an object can e.g. be en coded in its device status dataset. However, a collision warning may be generated based on other algorithms based 20 on the determination of the proximity between the objects in question. In a preferred embodiment, the positional in formation of the movable object can also be compared to positional information of a preferably stationary object 25 electronically stored in a map 50 as shown in Figure 2. The map 50 may include information on stationary objects of a site, and the control unit 14 may be designed for determining a distance between the current position of the movable object from the one or more stationary ob 30 jects derived from such map 50. The control unit 14 may generate a collision when such distance is less than a threshold. The control unit 14 further is adapted to de termine a drowsiness state of the objects operator sub 35 ject to the determined proximity or data derived there from. In a very preferred embodiment the control unit 14 is adapted to determine a drowsiness state of the opera- 17 tor subject to an input received from a monitoring unit 40 for monitoring a reaction of the operator in response to the collision warning issued by the control unit 14 in case of a potential collision. The monitoring unit 40 s may, for example, be a camera mounted in the operators cab for monitoring the reaction of the operator. The mon itoring unit 40 or the control unit 14 may in this case include an image recognition module, which allows for better identifying persons such as the operator and their 10 reactions in response to a collision warning. For exam ple, if the monitoring unit 40 only shows a person in the operators area of activity who does not move/react at all in response to a collision warning issued by the output unit 19, the control unit 14 may determine that the op is erator is in a drowsiness state and may generate a drowsiness warning, which in one embodiment may also be issued by means of the output unit 19 as a visual and/or audible warning. It is emphasized, that functions of the sys 20 tem that are realized in software or firmware may be exe cuted in separate processors of the proximity determina tion means, the control unit and the monitoring unit. However, in another embodiment, these functions may be executed only in the processor of the control unit. For 25 example, the means for determining the proximity may only supply the positional raw data to the control unit while the final proximity values are extracted in the control unit. In another embodiment the proximity determination means may comprise its own processor which also realizes 30 the calculation of the proximity values from the posi tional raw data. On the other hand, the monitoring unit may in one embodiment comprise a processor which imple ments the identification of events in signals from the corresponding sensor while in another embodiment the mon 35 itoring unit only provides the sensor signals directly as input to the control unit while the identification of a 18 reaction event in the signals is implemented in the proc essor of the control unit. Throughout this specification and the claims which follow, unless the context requires otherwise, the 5 word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Claims (3)

1. 2. The system according to any one of the preceding claims, wherein the monitoring unit comprises a sen sor for identifying a deflection of a steering wheel of 30 the movable object, and wherein the monitoring unit is adapted to supply information related to the deflection of the steering wheel as input to the control unit. 35 3. The system according to any one of the preceding claims, 20 wherein the monitoring unit comprises a sen sor for identifying an acceleration of the movable ob ject, and wherein the monitoring unit is adapted to 5 supply information related to the acceleration as input to the control unit.
2. 4. The system according to any one of the preceding claims, io wherein the monitoring unit comprises a sen sor for monitoring at least part of an area of activity of the operator, and wherein the monitoring unit is adapted to supply data monitored or information retrieved therefrom is as input to the control unit.
3. 5. The system according to claim 4, comprising a display for displaying a scene in response to the collision warning, 20 wherein the sensor of the monitoring unit is an optical sensor for detecting if the operator looks at the display, and wherein the control unit is adapted to deter mine the drowsiness state dependent on the detection of 25 the operator looking at the display or information re lated thereto in response to the collision warning.