WO2016116146A1

WO2016116146A1 - Multi-car elevator system

Info

Publication number: WO2016116146A1
Application number: PCT/EP2015/051084
Authority: WO
Inventors: Mikko Puranen
Original assignee: Kone Corporation
Priority date: 2015-01-21
Filing date: 2015-01-21
Publication date: 2016-07-28

Abstract

Summary The invention concerns a multi-car elevator system with at least two cars (1, 101, 02), the cars being independently moveable on a moving track in an upward direction and a downward direction within a moving range, respectively, each car (1) comprising a first distance measurement radar (11) for capturing first distance signals or distances (D1) in an upward capturing direction, a second distance measurement radar (12) for capturing second distance signals or distances (D2) in a downward capturing direction relative to an adjacent second horizontal wall (22) or neighbouring car, a control unit (30), which calculates the real time velocity of the car in both the upward direction and downward direction from the measured distance signals (D1, D2) and calculating therefrom a minimum braking distance (db_min), and triggering an emergency brake in case the calculated braking distance (db_min) exceeds a predetermined threshold value.

Description

MULTI-CAR ELEVATOR SYSTEM

FIELD OF THE INVENTION

The present invention relates to a multi-car elevator system with at least two cars.

BACKGROUND OF THE INVENTION

From US 6,484,849 B2 a speed measurement system for an elevator car is known. The respective device for determining the speed in the elevator shaft of the car comprises a signal emitter and reflective device reflecting the signal to a signal receiver. When the car is moved in the elevator shaft, a signal is emitted from the emitter toward a structural member fixed in the elevator shaft being reflected thereon and received by the receiver. By analysing the reflected signal, information about the speed of the car can be determined and transmitted to a remote receiver where it is made available as speed information. The Chinese utility model CN 202848786 U discloses an elevator system with distance measurement. The system comprises an elevator car, a radar sensor and a controller. The radar sensor is mounted at a ceiling of the elevator shaft and comprises a radar transmitter unit and a radar receiver unit, wherein the radar sensor is connected to the controller. Using the signals provided by the radar sensor, the distance between the elevator cab and the top of the elevator shaft is determined.

In multi-car elevator systems there is a need for a distance measurement between each of the cars in order to avoid a car collision.

AIM OF THE INVENTION

The object of the invention is to provide a multi-car elevator system with at least two cars in which a car collision can be reliably excluded. SUMMARY OF THE INVENTION

The above object is ach ieved by subject matter of claim 1 . Advantageous embodiments are disclosed in the respective subclaims.

Accord ing to the invention, a multi-car elevator system comprises at least two cars wh ich are independently moveable on a moving track in an upward direction and downward d irection with in a moving range, respectively, each car comprising: a first d istance measurement radar for capturing first d istance signals in an upward capturing d irection relative to an adjacent obstacle, wh ich blocks a continuation of drive to the intended destination of the moving car, i.e. a horizontal wal l or neighbouring car,

a second distance measurement radar for capturing second d istance signals in a downward capturing d irection relative to an adjacent obstacle, wh ich blocks a continuation of drive to the intended destination of the moving car, i.e. a horizontal wal l or neighbouring car,

- a control un it, wh ich

calcu lates the real time velocity of the car in both the upward direction and downward direction from the measured d istance signals and calcu lating therefrom a min imum braking distance, and

triggering an emergency brake in case the calcu lated braking d istance exceeds a predeterm ined threshold val ue.

The calcu lated real time velocity thus means a relative velocity of the moving car referenced to a neighboured car. Th is calcu lation is carried out continuously at best, at least at those points of time, at wh ich detected signal information is transm itted. By means of th is a very comfort emergency brakage is enabled. Such comfortness means that braking even in emergency situations takes into account the velocity of a car, whether the car moves opposite or in direction of gravity, whether two cars approach each other in opposite direction, or whether two cars do move in the same d irection but with d ifferent velocity or whether one car approaches a fixed obstacle (stand ing car or bottom of the shaft), etc..

To th is end, the control un it determ ines from the first and second distance signals a pl ural ity of first and second d istance information each of wh ich corresponds to a predeterm ined point of time in a sequence of points of time. Then, the control un it can calculate the real time velocity of the car in both the upward direction and downward d irection for points of time in the sequence of points of time from the first and second d istance signals.

Further, the m inimum braking d istance can be calcu lated in dependency of a calculated real time velocity for a respective point of time and thus of a first car braking behaviour function wh ich defines a brake path length with regard to a predeterm ined braking mode of the car.

Further, in an embod iment of the mu lti-car elevator system an absol ute position of the car with in the moving range can be calcu lated by subtracting the determ ined first d istance information of the car, the distance information from one further car and a car length information of the at least further car from the moving range.

In a further embod iment of the mu lti-car elevator system, the radar is a frequency modulated continuous wave (FMCW) radar.

In another embod iment of the mu lti-car elevator system a reflector device for reflecting radio waves is positioned at least at one of the fol lowing positions: (a) a car-top, (b) a car-bottom, (c) the first horizontal wal l, (d) the second horizontal wal l .

In another embodiment, the mu lti-car elevator system comprises a central control un it which is functional ly coupled to the control unit of each car and/or a moving system for moving the cars.

A car for a mu lti-car elevator system with at least two cars is designed as fol lows:

The car comprises a car frame or housing which is designed such that the car is moveable along a predetermined moving track in a first moving direction and a second moving direction which is oriented opposite to the first moving direction, a first distance measurement radar which is mounted to the car frame or housing for capturing first distances in a first capturing direction along the moving track, wherein the first distance measurement radar generates first distance signals, a second distance measurement radar which is mounted to the car frame for capturing second distances in a second capturing direction along the moving track, wherein the second capturing direction is opposite to the first capturing direction, wherein the second distance measurement radar generates second distance signals, a control unit, a braking system, which is designed for braking the car, when the braking system is activated, wherein the braking system is activated when the braking system receives a braking command from a command function of the control unit, wherein with activating the braking system a braking mode is defined, wherein the control unit comprises: an input function which is functionally connected to the first distance measurement radar and to the second distance measurement radar for determining a first and a second distance information, wherein the input function determines a plurality of first and second distance information each of which corresponds to a predetermined point of time in a sequence of points of time, a collision avoidance function, comprising: - a velocity determination function which calculates first and second velocity information from the first distance information from the second distance information, respectively

- a braking distance calculation function which calculates - a first minimum braking distance in dependency of:

(a1 ) a calculated first velocity information for a respective point of time,

(b1 ) a first car braking behavior function which defines a first brake path length with regard to a predetermined braking mode of the car when the car is moved in the first moving direction,

- a second minimum braking distance in dependency of:

(a2) a calculated second velocity information for a respective point of time (b2) a second car braking behavior function which defines a second braking distance with regard to a predetermined braking mode of the car when the car is moved in the second moving direction,

- a comparison function which compares the calculated distance information with the calculated first minimum braking distance and the calculated second minimum braking distance,

- the command function which generates a braking command and transmits the braking command to the braking system in the case that

(i) first distance signal decreases to a value becoming equal to the calculated first minimum braking distance, or

(ii) second distance signal decreases to a value becoming equal to the calculated second minimum braking distance.

The first distances are distances between the first distance measurement radar and an external object in the upward direction. For the uppermost car in a shaft said external object is the top-wall of the shaft, i.e. the ceiling. For any other car beside the uppermost one, said external object is the bottom of a neighboured car above. The first d istance measurement radar can be mounted to a car-top, i.e. the upper surface of the car.

The second d istances are distances between the second d istance measurement radar and an external object or wal l . For the lowest car in a shaft said external object is the bottom-wal l of the shaft. For any other car beside the lowest one, said external object is the roof of a neighboured car below. The first d istance measurement radar can be mounted to a car-bottom, i.e. the lower surface of the car.

The first and second car braking behavior functions define a first and second braking distance, respectively, with regard to a predetermined braking mode of the car. The first car braking d istance depends on the first velocity, when the car is moved in the first moving direction. The second car braking d istance depends on the second velocity, when the car is moved in the second moving d irection. Optional ly, when the car is moved in the first moving d irection, i.e. up-direction, a first safety factor can be considered, and when the car is moved in the second moving direction, i.e. down-d irection, a second safety factor can be considered in the col l ision avoidance function. I n example, the safety factor can be added to the calculated brake path length. Alternatively, the calculated brake path length can be multipl ied with a safety factor. In an optional embod iment of the car, the input function of the control un it comprises a receiver for receiving a distance information from at least one further car wh ich is positioned in the first moving d irection, wherein the control un it comprises a position determination function wh ich calculates an absol ute position of the car with in the moving range by subtracting the determ ined first d istance information of the car, a d istance information from at least one further car and a car length information of the at least further car from the moving range.

Optional ly, the control un it of the car comprises a commun ication interface for transm itting and receiving distances from a moving system of an elevator system or a central control ler of an elevator system. In this embodiment, the receiver can be a radio wave receiver.

In a further embodiment of the car the input function of the control unit comprises a receiver for receiving a distance information from at least one further car which is positioned in the first moving direction. In a further embodiment of the car, the first and the second distance measurement second radar is a frequency modulated continuous wave (FMCW) radar.

In another embodiment of the car a reflector device for reflecting radio waves is positioned at least at one of the following positions: (a) the car-top, (b) the car- bottom. This is done to enhance the reflection of emitted radar signals.

SHORT DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described in the following in detail with reference to drawings which show:

Fig. 1 an embodiment of the elevator system according to the invention with two elevator cars,

Fig. 2 the elevator system of Fig. 1 wherein the elevator cars are arranged at different positions than in Fig. 1 ,

Fig. 3 a further embodiment of the elevator system according to the invention with three elevator cars, Fig. 4 a further embodiment of the elevator system according to the invention with two elevator shafts,

Fig. 5 an embodiment of the control unit for an elevator car of an elevator system according to the invention

Fig. 6 a further embodiment of the control unit for an elevator car of an elevator system according to the invention DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a multi-car elevator system according to one embodiment of the invention. Two elevator cars 1 are arranged in an elevator shaft 2. The elevator shaft 2 is i.a. defined by a first horizontal wall 21 (ceiling) and a second horizontal wall 22 (bottom). The cars 1 are movable in the elevator shaft independently from each other. Each of the cars 1 is movable on a moving track in an upward direction and in a downward direction defining a moving range for the car. As also shown in Fig. 1 , end points can be defined as points 23, 24 within the elevator shaft 2 lying in a certain distance from the ceiling 21 and the floor 22, respectively.

According to the invention, a car 1 of the elevator system comprises a first distance measurement radar 1 1 for capturing or generating first distance signals D1 in an upward or first capturing direction and a second distance measurement radar 12 for capturing or generating second distance signals D2 in a downward or second capturing direction.

The first distance measurement radar 1 1 can be mounted to a car frame or a car- top 1 3 of the car 1 or generally to a structural part of the car. The first distance measurement radar 1 1 captures first distances between the first distance measurement radar 1 1 and an external object or wall in the first capturing direction along the moving track. As shown in Figs. 1 and 2, when regarding the upper car the first distance signals D1 are referred to those distances dt1 between the first distance measurement radar 1 1 and the ceiling of the elevator shaft 2. As shown in Fig. 1 , for the lower one of the two cars the first distance signals D1 mean distances d12 between the first distance measurement radar 1 1 and another car 1 arranged above in the elevator shaft 2.

The second distance measurement radar 12 can be mounted to the car frame or a car-bottom 14 of the car 1 or generally to a structural part of the car. The second distance measurement radar 12 captures second distances between the second distance measurement radar 12 and an external object or wall in the second capturing direction. The second distance signals for the upper car mean distances relative to a roof of a neighbouring car arranged beneath, while for the lower car the second distance signals do refer to those distances to the bottom of the shaft.

As illustrated by the figures, an interspace extends between each of the cars 1 in the elevator shaft 2, between the floor 22 and the lowermost car 1 of the elevator system and between the ceiling 21 and the uppermost car 1 of the elevator system. In the case that an interspace is an interspace between two cars 1 , the same distance is measured from two different radars 1 1 , 12. This redundancy provides a high level of reliability and security. According to the invention, a car 1 of the elevator system further comprises a control unit 30 which calculates the real time velocity of the car in both the upward direction and downward direction from the measured distance signals D1 , D2. Alternatively this calculation is carried out by the central controller of the multi-car elevator system. From this information, the control unit 30 calculates a minimum braking distance db_min. The control unit 30 is functionally connected to the first and the second distance measurement radar 1 1 , 12 and receives the distance signals D1 , D2 generated by the first and the second distance measurement radars 1 1 , 12. From the first distance signals D1 , the control unit 30 calculates a first velocity or a velocity in the upward direction and from the second distance signals D2 the control unit 30 calculates a second velocity, i.e. a velocity in the downward direction. Accordingly, the first velocity can be a velocity of the upper car 1 relative to the ceiling 21 or a velocity of the lower car 1 relative to another car 1 , namely the upper one. The second velocity can be a velocity of the lower car 1 relative to the floor 22 or a velocity of the upper car 1 relative to another car 1 , namely the lower one. From the first and the second velocity, a minimum braking distance db_min is calculated, respectively. The minimum braking distance db_min is the minimum distance that may extend between a car 1 moving with a certain velocity relative to an external object, wherein during movement of the car the distance between the car and the external object can decrease, and then, the car 1 can be decelerated in such a way that a collision of the car 1 and the external object is excluded. The minimum braking distance db_min can be the minimum brake path length of the car 1 . Alternatively, the minimum braking distance db_min can take into account a security factor, e.g. to the minimum brake path length of the car 1 a security path length can be added, providing a certain reserve or buffer distance.

Further, from the first and the second velocity, a braking distance db_min or actual braking distance can be calculated, which is the path length needed for the car 1 to reduce its velocity to zero.

The car 1 comprises a car brake B which is coupled to the car 1 and which is functionally connected to the control unit 30 of the car 1 . The brake B is designed for braking the car 1 , when the brake B is activated. The brake is activated when the brake receives a braking command from the control unit 30. The control unit 30 can comprise a command function for generating command signals. With activating the brake, a braking mode is defined. The braking mode encompasses an emergency brake mode which generates a maximum brake force that reduces the velocity of the car to zero in minimal time and with a minimal brake path length of the car. Generally, the braking mode can generate a brake force which varies over the time such that a predefined braking behaviour of the car is achieved. This braking behaviour can comprise a constant deceleration of the car or a deceleration increasing or decreasing with the time. Further, the braking behaviour can comprise a maximum deceleration which may not be exceeded.

In case the calculated braking distance exceeds a predetermined threshold value, the emergency brake is triggered. The predetermined threshold value can be the calculated minimum braking distance db_min. Additionally or alternatively, the first and the second distances from the first and second distance signals D1 , D2 can be compared to the calculated minimum braking distance db_min. In case one of the distances becomes equal to the calculated minimum braking distance db_min, a brake event is automatically triggered. As shown in Fig. 5, the control unit 30 can comprise a velocity determination function 33. The velocity determination function 33 calculates first velocity information vl 1 for points of time from the first distance information DM and calculates a second velocity information vl2 for points of time from the second distance information DI2.

In an embodiment of the elevator system, the minimum braking distance db_min of a car 1 is calculated in dependency of the calculated real time velocity for a respective point of time in the sequence of points of time and of a car braking behaviour which defines a brake path length with regard to a predetermined braking mode of the car 1 . The calculated real time velocity can be the first or the second velocity for a point of time. The calculated real time velocity can also be a first or second velocity information vl 1 , vl2 determined by the velocity determination function 33. The car braking behaviour defines a brake path length with regard to a predetermined braking mode of the car 1 . Further, the brake path length of the car 1 varies in dependency of the moving direction of the car such that, when the car is moved opposite to the direction of gravity g, the brake path length achieved with a certain braking mode is smaller than the brake path length achieved with the same braking mode when the car is moved in the direction of gravity g. The car braking behaviour also depends on the velocity of the car. Accordingly, the car braking behaviour is as a function of the real time velocity of the car and the moving direction with respect to the direction of gravity g. Accordingly, the car braking behaviour can be provided as a factor of the velocity of the car 1 or as a function of the velocity itself.

As shown in Fig. 5, the control unit 30 can comprise a braking distance calculation function 36. When the car 1 is moved in the first moving direction, the braking distance calculation function 36 calculates a first minimum braking distance db_min_1 in dependency of a calculated first velocity information vl 1 for a respective point of time and in dependency of a first car braking behaviour. The first car braking behaviour defines the first brake path length with regard to a predetermined braking mode of the car 1 and in dependency of the first velocity information vl 1 . The car braking behaviour can general ly be formu lated as function of the real time velocity of the car. As shown in Fig. 5, the braking distance calcu lation function 36 can receive the car braking behaviour as an input signal b1 from a storage 34 where the first car braking behaviour is stored as a car braking behaviour function. When the car 1 is moved in the second moving direction, the braking distance calcu lation function 36 calcu lates a second min imum braking d istance db_min_2 in dependency of a calcu lated second velocity information vl2 for a specific point of time and in dependency of the second car braking behaviour. A second car braking behaviour defines the second brake path length with regard to a predeterm ined braking mode of the car 1 and in dependency of the second velocity information vl2. The second car braking behaviour can general ly be formulated as function of the real time velocity of the car 1 . As shown in Fig. 5, the braking distance calculation function 36 can receive the second car braking behaviour as an input signal b2 from a storage 35 where the second car braking behaviour is stored as a car braking behaviour function. For both, the first and the second m in imum braking distance, a respective safety factor can be taken into account for the m in imum braking distance in a way that the safety factor is added to the calcu lated min imum braking distance.

As shown in Fig. 5, the control un it 30 comprises a comparison function 37. The comparison function 37 compares the calcu lated d istance information DM with the calcu lated first m in imum braking d istance and the calcu lated d istance information DM with the calculated second min imum braking d istance.

Accord ing to Fig. 5, the control un it comprises a command function 38 for generating a braking command CB. The command function 38 is designed such that it generates a braking command in one or both of the fol lowing cases:

(i) the distance information DM comprising the distance between the first distance measurement radar 1 1 and an external object decreases between the respective point of time and a point of time lying adjacent the respective point of time, and the d istance between the first d istance measurement radar 1 1 and an external object becomes equal to the calculated first minimum braking distance,

(ii) the distance information DI2 comprising the distance between the second distance measurement radar 12 and an external object decreases between the respective point of time and a point of time lying adjacent the respective point of time, and the distance between the second distance measurement radar 12 and an external object becomes equal to the calculated second minimum braking distance.

In each of the cases (i) and (ii) the command function 38 generates a brake command and transmits the brake command CB to the car brake B or to a communication interface 70. The communication interface 70 is functionally connected to the moving system MS of the elevator system or the central controller 40.

In the embodiment of Fig. 5, the velocity determination function 33, the first and second car braking behaviour functions stored in the storages 34, 35, the braking distance calculation function, the comparison function 37 and the command function 38 form a collision avoidance function 32.

In the embodiment of Fig. 6, the control unit 30 additionally receives external distance information DE1 , being distance signals or distance information DM from at least one further car 1 which is positioned in the moving range between the respective car 1 and the ceiling 21 or between the respective car 1 and the floor 22. Generally, the control unit 30 can receive first and second distance signals D1 , D2 or distance information DM , DI2 from the at least one further car 1 .

In the elevator system in Fig. 1 , the cars 101 and 102 can comprise a control unit 30. When a car 101 is moving in the up-direction, the control unit 30 of the car 101 receives at least the second distance signals D2 generated with the distance measurement radar 12 of the car 102 or the distance information DI2 calculated from the signals D2. Optionally, the control unit 30 of the car 101 can additionally receive the first distance signals D1 generated with the first distance measurement radar 1 1 of the car 102 or the distance information DM calculated from the signals D1 . When car 102 is moving in the up-direction, the control unit 30 of the car 102 receives at least the first distance signal D1 generated with the distance measurement radar 1 1 of the car 101 or the distance information DM calculated from the signal D1 . Optionally, the control unit 30 of the car 101 can additionally receive the first distance signals D1 generated with the distance measurement radar 1 1 of the car 102 or the distance information DM calculated from the signals D1 .

Accordingly, in an elevator system according to Fig. 3, a car which is arranged between two other cars receives distance signals through its control unit 30 or distance information related to a specific interspace, wherein the control unit 30 receives at least one distance signal or distance information related to each interspace extending between the car 101 , 102 and the ceiling 21 or each interspace extending between the car 101 , 102 and the bottom 22. In an embodiment of the elevator system, the control unit 30 determines from the first and second distance signals D1 , D2 and from the at least one external distance information DE1 a plurality of first and second distance information DM , DI2 each of which corresponds to a predetermined point of time.

As shown in Fig. 6, the control unit 30 can also comprise an input function 310 which is functionally connected to the first distance measurement radar 1 1 and to the second distance measurement radar 12. The input function 310 additionally receives external distance information DE1 of at least one further car 1 as described above. From the distance signals of the first and second distance measurement radar 1 1 , 12 and from the external distance information, the input function 310 determines a plurality of first and second distance information DM , DI2 each of which corresponds to a predetermined point of time in a sequence of points of time.

The control unit 30 further comprises the collision avoidance function 32 comprising the functionality described above. The controller 30 further comprises a distance transmission interface 304, which transmits the distance information DM, DI2 to at least one further car 1.

Optionally, the control unit 30 calculates an absolute position of the car 1 within the moving range by subtracting the determined first distance information DI1 of the car 1, the distance information DE1 from the at least one further car 1 and a car length information of the at least one further car 1 from the moving range.

The control unit 30 can comprise a car length information storage 301, a moving range length storage 302 and a position detection function 303.

In the car length information storage 301 a car length information comprising a length L of the car 1 is stored. The car length L is the length of the moving range occupied by the car 1.

In the moving range length storage 302 the length of the moving range is stored.

The position detection function 303 calculates an absolute position hi, h2 of the car 1 within the moving range by subtracting the determined first distance information DM of the car, the distance information DE1 from the at least one further car and a car length information of the at least further car from the moving range. The position detection function 303 further transmits the car position hi, h2 to a distance transmission interface 304.

Each of the first and second distance measurement radars 11, 12 can be designed as a frequency modulated continuous wave (FMCW) radar.

As mentioned before, the multi-car elevator system can include reflector devices 41, 42 for reflecting radio waves which are emitted from the first and the second distance measurement radar 11, 12, respectively. The reflector devices 41, 42 are positioned accordingly, such that their reflecting surfaces face a corresponding distance measurement radar 11, 12. Therefore a reflecting device can be arranged at least at one of the following positions: (a) a car-top 13, as shown in Fig.3, (b) a car-bottom 14, as shown in Fig.3, (c) the first horizontal wall 21, as shown in Figs. 1, 2 and 4, (d) the second horizontal wall 22, as shown in Figs.1, 2 and 4. Fig. 4 shows a special embodiment of an elevator system according to the invention called "paternoster"-elevator. While a vertical shaft for the elevator is accepting upward moving elevator cars on an up-hoistway-track there is a separated down-hoistway-track assigned for and conveying the downward moving cars. At each end of the tracks, the cars have to be transferred from one track to the other, respectively, thus travelling at least partially a horizontal distance. Therewith, a closed infinite loop is formed on which all the cars travel around. This enables a more efficient traffic routing due to the fact that multiple cars can be stacked in a single shaft. As shown, the elevator cars 110, 111 and 112 are situated in a first hoistway-track 200a, while the elevator cars 113, 114 and 115 are situated in a second elevator hoistway-track 200b.

The cars 110, 111, 112, 113, 114, 115 are movable in the elevator shaft 200 independently from each other. As shown in Fig. 4, each of the cars 110, 111, 112, 113, 114 115 comprises a control unit 30, wherein the control units 30 of the cars 110, 111, 112, 113, 114 115 are functionally connected to each other, according to the above described functionality of the control unit 30. Optionally the control units 30 of the cars 110, 111, 112, 113, 114115 are also functionally connected to the central control unit 40. The functionality of the control unit 30 is defined according to one of the embodiments of the control unit 30 described herein.

Claims

1 . Multi-car elevator system with at least two cars (1 , 101 , 102), the cars being independently moveable on a moving track in an upward direction and a downward direction within a moving range of an elevator shaft having a bottom horizontal wall and an upper horizontal wall, respectively, each car (1 ) comprising:

a first distance measurement radar (1 1 ) for capturing first distance signals (D1 ) in an upward capturing direction relative to an adjacent first horizontal wall (21 ) or neighbouring car (1 , 101 , 102), a second distance measurement radar (12) for capturing second distance signals (D2) in a downward capturing direction relative to an adjacent second horizontal wall (22) or neighbouring car, a control unit (30), which

- calculates the real time velocity of the car in both the upward direction and downward direction from the measured distance signals (D1 , D2) and calculating therefrom a minimum braking distance (db_min), and

triggering an emergency brake in case the calculated braking distance (db_min) exceeds a predetermined threshold value.

2. Multi-car elevator system according to claim 1 , wherein the control unit (30) determines from the first and second distance signals (D1 , D2) a plurality of first and second distance information (DM , DI2) each of which corresponds to a predetermined point of time in a sequence of points of time.

3. Multi-car elevator system according to claim 2, wherein the control unit (30) calculates the real time velocity of the car (1 ) in both the upward direction and downward direction for points of time in the sequence of points of time from the first and second distance signals (D1 , D2).

Multi-car elevator system according to one of claims 1, 2 or 3, wherein the minimum braking distance (db_min) is calculated in dependency of a calculated real time velocity for a respective point of time in the sequence of points of time and of a first car braking behaviour function which defines a brake path length with regard to a predetermined braking mode of the car (1, 101, 102).

Multi-car elevator system according to one of the claims 1 to 4, wherein the control unit (30) receives an external distance information (DE1) from at least one further car which is positioned in the moving range between the respective car (1, 101, 102) and the first or second horizontal wall (21, 22), wherein the control unit (30) calculates an absolute position of the car within the moving range by subtracting the determined first distance information (DM ) of the car (DI1), the distance information (DE1) from the at least one further car and a car length information of the at least further car from the moving range.

Multi-car elevator system according to one of the preceding claims, wherein the first and the distance measurement second radar (11, 12) is a frequency modulated continuous wave (FMCW) radar.

Multi-car elevator system according to one of the preceding claims, wherein a reflector device (41, 42) for reflecting radio waves is positioned at least at one of the following positions: (a) a car-top (13), (b) a car- bottom (14), (c) the first horizontal wall (21), (d) the second horizontal wall (22).

Multi-car elevator system according to one of the preceding claims, comprising a central control unit (40) which is functionally coupled to at least one of the following: (a) the control unit (30) of each car, (b) a moving system (50) for moving the cars (1 , 101, 102).