Abstract:
An elevator control of an elevator installation having at least two elevator cars movable independently of one another in a common elevator includes a control device for maintaining a defined minimum distance between two successive ones of the elevator cars and a defined maximum distance between two successive ones of the elevator cars.

Description:
FIELD OF THE INVENTION 
     The invention relates to an elevator control of an elevator installation and to a method of controlling an elevator installation. 
     BACKGROUND OF THE INVENTION 
     An elevator installation with an elevator control, several elevator shafts and several, individually movable elevator cars is known from the specification EP 1 619 157 A1, wherein at least two of the elevator cars are loaded and unloaded simultaneously in at least two so-termed access regions lying directly one above the other and these at least two elevator cars subsequently serve individually allocated destination calls. 
     The specification US 2007/0089935 A1 discloses an elevator control for several elevator cars individually movable in a travel shaft, which elevator control for avoidance of a collision prevents travel of two elevator cars in direction towards one another in that one of the elevator cars is moved to a waiting position. 
     SUMMARY OF THE INVENTION 
     The invention is based on the task, in particular, of providing an elevator control of an elevator installation which with a high level of reliability and a high transport capacity can be of simple design. 
     The invention proceeds from an elevator control of an elevator installation with at least two elevator cars, which are provided for the purpose of being moved independently of one another in a common travel shaft, and with a control apparatus. 
     By “control apparatus” there is to be understood, in particular, apparatus with a computer unit, a memory unit and an operating program stored therein. By “control” there is to be understood in this connection a selective actuation in a pure control sequence and/or also in a regulating sequence. Through maintenance of the common travel direction for the elevator cars of a travel shaft it is advantageously possible to avoid priority conflicts which could lead to undesired stoppage times. An elevator control for an elevator installation can be provided which with a high level of reliability and a high transport capacity can be of simple design. 
     It is proposed that the control apparatus is provided for the purpose of maintaining a defined minimum spacing between each two elevator cars following one another and/or a defined maximum spacing between each two elevator cars following one another. 
     The minimum spacing to be maintained can advantageously be determined, by way of example, in the manner taught in the specification EP 0 769 469 B1. The maximum spacing to be maintained is preferably freely selectable. Through maintenance of the minimum spacing between two elevator cars following one another it is advantageously possible to avoid situations which due to the maintenance of necessary safety measures would lead to undesired stoppage times of the elevator cars. Through maintenance of the maximum spacing between the elevator cars a determined travel direction of the elevator cars can be kept over a longer period of time than without this maintenance, whereby with particular advantage priority conflicts can be avoided, which would lead to an increase in the transport capacity of the elevator installation. 
     The maximum spacing between each two elevator cars following one another is determinable on the basis of different criteria and depends on, for example, the length of the travel path, particularly the height of the elevator shaft, the number of elevator cars movable on a travel path and the speed at which the elevator cars are moved. 
     Moreover, it is proposed that the control apparatus is provided for the purpose, in the case of loading of a first elevator car of two elevator cars following one another, of influencing at least one travel parameter of the second elevator car for maintenance of the defined minimum spacing and for maintenance of the defined maximum spacing of the elevator cars. Advantageously, the maintenance of the defined minimum and/or maximum spacing of elevator cars following one another can thereby be achieved in particularly simple manner. 
     By “travel parameter” there is to be understood in this connection, in particular, a variable which has an influence on a total time required by the elevator car for processing a call, particularly a destination call, from the moment of boarding of a passenger until disembarkation of the passenger at a destination floor or from the moment of loading goods until unloading of goods at a destination floor. Travel parameters of this kind are a speed of an elevator car, an acceleration of the elevator car at the start of a journey and braking of the elevator car on reaching a destination floor, an opening time and a closing time of an elevator car door and a travel shaft door, and adoption of a waiting position of the elevator car (speed equal to zero). 
     By “loading” of an elevator car there shall be henceforth understood that an elevator car is loaded with passengers and/or goods. 
     In an advantageous embodiment the control apparatus is provided for the purpose, in the case of simultaneous loading of two elevator cars following one another, to influence at least one travel parameter of a trailing elevator car in order to maintain the defined minimum spacing and/or to maintain the defined maximum spacing of the elevator cars. Advantageously, the maintenance of the defined minimum and/or maximum spacing of elevator cars following one another can thereby be achieved in a particularly simple manner. 
     Moreover, it is proposed that the control apparatus is provided for the purpose, in the case of simultaneous loading of two elevator cars following one another, to influence at least one travel parameter of a leading elevator car in order to maintain the defined minimum spacing and/or in order to maintain the defined maximum spacing of the elevator cars. Advantageously, the maintenance of the defined minimum and/or maximum spacing of elevator cars following one another can thereby be achieved in particularly simple manner. 
     In addition, it is proposed that the control apparatus is provided for the purpose, in a case of simultaneous loading of two elevator cars following one another, to move the trailing elevator car into a waiting position in order to maintain the defined minimum spacing and/or to move the leading elevator car into a waiting position in order to maintain the defined maximum spacing of the elevator cars. Advantageously, the maintenance of the defined minimum and/or maximum spacing of elevator cars following one another can thereby be achieved in particularly simple manner. 
     The adoption of a waiting position of the elevator car can take place with advantage particularly at a floor, preferably with opened travel shaft door and/or car door, or also between two floors. In principle, further travel parameters familiar to the expert are conceivable, which can also be used in combination. Advantageously, maintenance of the defined minimum and/or maximum spacing of elevator cars following one another can thereby be achieved in particularly simple manner. 
     In a further advantageous embodiment the control apparatus is provided for the purpose, in the case of exclusive loading of the leading one of two elevator cars following one another, to influence at least one travel parameter of the trailing elevator car in order to maintain the defined minimum spacing and/or in order to maintain the defined maximum spacing of the elevator cars, wherein with advantage through an absence of passengers an impairment of a subjective perception of travel comfort can be excluded and thereby the maintenance of the defined minimum and/or maximum spacing of elevator cars following one another can be advantageously achieved in particularly simple manner. 
     Moreover, it is proposed that the control apparatus is provided for the purpose, in the case exclusive loading of the leading one of two elevator cars following one another, to move the trailing elevator car into a waiting position in order to maintain the defined minimum spacing and/or to maintain the defined maximum spacing of the elevator cars, wherein advantageously through the absence of passengers an impairment of a subjective perception of travel comfort can be excluded and thereby the maintenance of the defined minimum and/or maximum spacing of elevator cars following one another can be advantageously achieved in particularly simple manner. 
     With particular advantage the control apparatus is provided for the purpose, in the case of exclusive loading of the trailing one of two elevator cars following one another, to influence at least one travel parameter of the leading elevator car in order to maintain the defined minimum spacing and in order to maintain the defined maximum spacing of the elevator cars, wherein advantageously through the absence of passengers an impairment of a subjective perception of travel comfort can be excluded and thereby the maintenance of the defined minimum and/or maximum spacing of elevator cars following one another can advantageously be achieved in particularly simple manner. 
     In a further advantageous embodiment the control apparatus is provided for the purpose, in the case of exclusive loading of the trailing one of two elevator cars following one another, to move the leading elevator car to a waiting position in order to maintain the defined minimum spacing and in order to maintain the defined maximum spacing of the elevator cars, wherein advantageously through the absence of passengers an impairment of a subjective perception of travel comfort can be excluded and thereby the maintenance of the defined minimum and/or maximum spacing of elevator cars following one another can advantageously be achieved in particularly simple manner. 
     It is proposed that the control apparatus is provided for the purpose of determining a first common travel direction of the at least two elevator cars in the common travel shaft and of reversing this first travel direction for the at least two elevator cars due to at least one internal destination floor selection and/or external transport request and/or—particularly advantageously—destination call only when all internal destination floor selections and/or all external transport requests and/or all destination calls of the elevator cars in the first travel direction have been processed. 
     The control apparatus is, in particular, provided for the purpose of serving internal destination floor selections and/or external transport requests and/or destination calls for the elevator cars with priority when they lie in a travel direction currently determined for the elevator cars. By this there shall be understood in this connection, in particular, that the serving of an internal destination floor selection and/or an external transport request and/or a destination call has priority before a change of travel direction of the elevator cars. 
     By “internal destination floor selection” there is to be understood in this connection, in particular, a selection of a destination floor by a passenger in the elevator car. By “external transport request” there is to be understood in this connection, in particular, a request of an elevator car by actuation of a directionally coupled call button outside the elevator car. By “destination call” there is to be understood in this connection, in particular, a numerical selection of a destination floor, particularly by means of a numerical keyboard and/or by means of speech input, etc., outside the elevator car. By “provided” there is to be understood in this connection, in particular, specially equipped, designed and/or programmed. 
     The control apparatus is preferably in a situation in which all internal destination floor selections and/or all external transport requests and/or all destination calls of the elevator cars of a travel shaft were processed in a first travel direction, and a then newly arising internal destination floor selection and/or external transport demand and/or destination call, which can be served in the first travel direction, is given priority before an internal destination floor selection and/or external transport request and/or destination call which was already present beforehand and which obliges a reversal of the travel direction. Through serving of the internal destination floor selections and/or external transport requests and/or destination calls in travel direction this can be advantageously maintained, whereby it is with advantage possible to avoid priority conflicts which could lead to undesired stoppage times, which can lead to an improvement in the transport capacity of the elevator installation. 
     Moreover, it is proposed that the control apparatus is provided for the purpose, in times of increased travel requirement in a first travel direction, to move the elevator cars preferably directly to the starting point of the first travel direction after processing of all internal destination floor selections and/or all external transport requests and/or destination calls of the elevator cars in the first travel direction. 
     By “time of increased travel requirement in a direction” there is to be understood in this connection, in particular, a time in which a sum, which is formed over a time period of 30 minutes, of mathematical products of a number of the passengers boarding at a floor and a difference of the floor number between a boarding floor and a disembarkation floor differs from zero and, in particular, that an absolute amount of the thus-formed sum corresponds with a proportion of more than 20% of a sum which was formed from absolutely taken products of the number of the passengers boarding at a floor and the difference of the floor number between the boarding floor and the disembarkation floor in the same time period. If the proportion of the absolute amount of the sum is less than 20% of the sum of the absolutely taken products, then the travel requirement shall be termed “evenly distributed in both directions”. 
     By “starting point” of a travel direction there is to be understood, in particular, floors which are located in the uppermost or lowermost quarter of a travel shaft, so that an elevator car which starts from its starting point allocated by the control apparatus can serve at least three-quarters of the length of a travel shaft as a possible travel path. By “preferably directly” there is to be understood in this connection, in particular, that the elevator cars travel to their starting points allocated by the control apparatus without, in the case of at least a first transit, reaction to internal destination floor selections and/or external transport requests and/or destination calls. Through movement of the elevator car to the starting point of a travel direction with increased travel requirement it is advantageously possible to serve increased anticipated internal destination floor selections and/or external transport requests and/or destination calls in this travel direction, which leads to an increase in transport capacity of the elevator installation. 
     Moreover, it is proposed that the control apparatus is provided for the purpose of controlling the elevator cars arranged in at least one first travel shaft and second travel shaft, whereby advantageously expanded possibilities for co-ordination of travel directions in the at least two travel shafts and thus an increase in the transport capacity of the elevator installation are opened up. 
     Advantageously, the control apparatus is provided for the purpose, at times of uniformly distributed travel requirement for both travel directions, to determine a first travel direction for the first travel shaft and to determine a travel direction opposite to the first travel direction for the second travel shaft, whereby advantageously a uniform distribution of the elevator cars within the elevator installation can be achieved, which leads to a reduction in waiting times and thus to an increased transport capacity of the elevator installation. For determination of the times of evenly distributed travel requirement for the two travel directions the elevator installation can advantageously be equipped with means for ascertaining the incidence of traffic and with an evaluating unit for statistical evaluation thereof. Determination of times of evenly distributed travel requirement for both travel directions can, in principle, also be carried out by a manual input into the control apparatus. 
     In an advantageous embodiment the control apparatus is provided for the purpose of controlling elevator cars arranged in at least one first, second and third travel shaft and at times of increased travel requirement in a first travel direction to determine this first travel direction as travel direction for the elevator cars of a plurality of the travel shafts and to determine a travel direction opposite to the first travel direction for the elevator cars of the remaining elevator shafts, whereby it is advantageously possible to serve an increased number of destination calls anticipated in the direction of increased travel requirement, which leads to an increase in the transport capacity of the elevator installation. 
     In addition, it is proposed that the control apparatus is provided for the purpose of changing the travel directions for the elevator cars in the at least two elevator shafts at least substantially simultaneously. Through an oscillating operation, which arises in this manner, of the elevator cars in the various travel shafts it is advantageously possible to achieve a uniform distribution of the elevator cars within the elevator installation, which leads to a reduction in waiting times and thus to an increased transport capacity of the elevator installation. 
     By “substantially simultaneous” there is to be understood in this connection, in particular, that the subsequent changes of the travel directions take place within 10 seconds, preferably within 5 seconds and particularly preferably within 3 seconds, after the first change of travel direction of the elevator cars in a first travel shaft. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Further advantages are evident from the following description of drawings. Exemplifying embodiments of the invention are illustrated in the drawings. The description and the claims contain numerous features in combination. The expert will advantageously also consider the features individually and combine them into feasible further combinations. There: 
         FIG. 1  shows the schematic illustration of an elevator installation with three travel shafts each with three elevator cars; and 
         FIG. 2  shows the schematic illustration of an elevator installation with one travel shaft and three elevator cars. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A detail of an elevator installation  10  with an elevator control and with nine elevator cars  20 ,  22 ,  24 ,  26 ,  28 ,  30 ,  32 ,  34 ,  36  is illustrated in  FIG. 1 . In each instance three of the elevator cars are arranged in a common travel shaft  14 ,  16  or  18  to be movable independently of one another. The travel shafts  14 ,  16 ,  18  have travel shaft doors  56  at each illustrated floor. For reasons of clarity only one travel shaft door  56  is provided with a reference numeral; however, it is to be assumed that all illustrated floors are equipped identically with respect to the travel shaft door  56 . 
     In addition, the elevator control comprises a control apparatus  12  which has determined the upward direction as a first common travel direction  38  of the elevator cars  20 ,  22 ,  24  in the first travel shaft  14 , the downward direction as a second common travel direction  40  of the elevator cars  26 ,  28 ,  30  in the second travel shaft  16  and the upward direction as a third common travel direction  42  of the elevator cars  32 ,  34 ,  36  (symbolized in  FIG. 1  by corresponding arrows in the elevator cars). 
     An external transport request  44  in upward direction, two internal destination floor selections  46 ,  50  in the upward direction and an internal destination floor selection  48  in the downward direction are indicated in the elevator installation  10 . The internal destination floor selection  46  lies in the first common travel direction  38  of the elevator cars  20 ,  22 ,  24  and the external transport request  44  in upward direction lies oppositely to the travel direction  38  of the elevator cars  20 ,  22 ,  24 . The control apparatus  12  is provided for the purpose of serving the internal destination floor selection  46 , which lies in the first common travel direction  38 , with priority. For serving the external transport request  44  a reversal of the travel direction  38  determined by the control apparatus  12  is necessary. This reversal is determined by the control apparatus  12  only when all internal destination floor selections  46  and all external transport requests  44  of the elevator cars  20 ,  22 ,  24  and further internal destination floor selections which have been activated from the processing of the internal destination floor selection  46  by boarding passengers—insofar as those selections lie in the first common travel direction  38 —have been processed. 
     The control apparatus  12  is provided for the purpose of determining a defined minimum spacing between each two elevator cars  20 ,  22 ,  24 ,  26 ,  28 ,  30 ,  32 ,  34 ,  36  following one another in accordance with the manner taught in the specification EP 0 769 469 B1. A freely selectable maximum spacing was determined in the illustrated example for a magnitude of four floors. However, the determination of the maximum spacing is strongly situation-dependent and depends, for example, on the length of the travel path, particularly the height of the elevator shaft, the number of elevator cars movable on a travel path and the speed at which the elevator cars are moved. 
     In departure from the above-indicated example the maximum spacing can also be less or more than four floors. Thus, for example, in a building with less than fifteen floors a maximum spacing of two or three floors can be selected. In high buildings with thirty to one-hundred floors a larger maximum spacing of five to fifteen or more floors is also determinable. 
     The maximum spacing between a leading and a following elevator car is preferably at most 75% of the spacing between the leading elevator car and an obstacle lying behind the following elevator car. The lower end of an elevator shaft or a further, following elevator car, for example, represents such an obstacle. In a further preferred variant the maximum spacing is at most 50%, 30%, 25% or 10% of this spacing. 
     Moreover, in the case of a higher travel speed of an elevator car a greater maximum spacing can preferably also be determined, since, for example, a following empty elevator car catches up a leading elevator car, which stops at a floor, more quickly. 
     The control apparatus  12  is provided for the purpose of maintaining the defined minimum spacing and the defined maximum spacing between the elevator cars  20  and  22  following one another as well as the elevator cars  22  and  24  following one another in the first travel shaft  14 . The same applies to the elevator cars  26  and  28  following one another and the elevator cars  28  and  30  in the second travel shaft  16  and also to the elevator cars  32  and  34  following one another and the elevator cars  34  and  36  in the third travel shaft  18 . 
     The minimum spacing and the maximum spacing of the elevator cars  20 ,  22 ,  24  moving in the first travel shaft  14  is currently maintained for the elevator cars  20 ,  22 ,  24  so that no measures have to be undertaken by the control apparatus  12 . The elevator cars  20 ,  22 ,  24  are moved at the same speed in the determined travel direction  38 . 
     The minimum spacing is currently just achieved for the elevator cars  28  and  30  of the second travel shaft  16 . The control apparatus  12  has several possibilities of corrective action. Under the assumption that the two elevator cars  28 ,  30  transport passengers, the control apparatus  12  can, for maintenance of the defined minimum spacing, influence a travel parameter of the trailing elevator car  30  and reduce the speed of the elevator car  30  until the minimum spacing is achieved. Another possibility consists of letting the elevator car  30  after a stop at the floor with the internal destination floor selection  48 , which—lying in travel direction  40 —is preferentially served, depart at an acceleration which is slower than that filed in the control apparatus  12  for normal departure. Alternatively, the control apparatus  12  can, after a stop of the trailing elevator car  30  at the floor with the internal destination floor selection  48 , increase the opening and/or closing times for the elevator car door and/or the travel shaft door  56  of the floor relative to the times filed in the control apparatus  12  for a normal stop. Alternatively, the control apparatus  12  can move the trailing elevator car  30  to a waiting position until the minimum spacing for the elevator car  28  is maintained. 
     The spacing of the elevator cars  32  and  34 , which follow one another, of the third elevator shaft  18  is somewhat greater than the maximum spacing. The two elevator cars  32  and  34  transport passengers. The control apparatus  12  is provided for the purpose of influencing a travel parameter of the leading elevator car  34  in order to maintain the defined minimum spacing and reduces the speed of the leading elevator car  34 , or the control apparatus  12  lets the leading elevator car  34 , after a stop, depart at an acceleration which is smaller than that filed in the control apparatus  12  for a normal departure. Moreover, the control apparatus  12  can, during a stop of the elevator car  34 , increase the opening and/or closing times for the elevator car door and/or the travel shaft door  56  of the floor relative to the times filed in the control apparatus  12  for a normal stop. Alternatively, the control apparatus  12  can move the elevator car  34  to a waiting position until the maximum spacing for the elevator car  32  is maintained. 
     If in the elevator cars  28  and  30  following one another only the leading elevator car  28  transports passengers then the control apparatus  12  is provided for the purpose of influencing at least one travel parameter of the trailing elevator car  30  in order to maintain the defined minimum spacing between the elevator cars  28  and  30 . The control apparatus  12  moves the elevator car  30  into a waiting position, wherein a floor with a high probability of boarding is preferred. If the maximum spacing between the leading elevator car  28  and the trailing elevator car  30  is achieved, then the control apparatus  12  is provided for the purpose of letting the trailing elevator car  30  follow, travelling in an empty state, the leading elevator car  28  in order to maintain the maximum spacing. 
     In the third travel shaft  18  the trailing elevator car  34  with passengers and the leading elevator car  36  without passengers move in the upward direction determined as travel direction  42  by the control apparatus  12 . In this situation the control apparatus  12  is, for maintenance of the defined minimum spacing between the elevator cars  34  and  36 , provided for the purpose of influencing at least one travel parameter of the leading elevator car  36 . For that purpose the control apparatus  12  moves the elevator car  36 , travelling in an empty state, in front of the trailing elevator car  34  in the travel direction  42 . Alternatively, the control apparatus  12  can influence at least one travel parameter of the leading elevator car  36  and move the elevator car  36  to a waiting position, wherein a floor with a high probability of boarding is preferred. 
     In the exemplifying embodiment, which is illustrated in  FIG. 2 , of an elevator installation  10 ′ with an elevator control which is equipped in each floor with a numerical keyboard  60 ′ for the input of destination calls  58 ′ three elevator cars  26 ′,  28 ′,  30 ′ can be moved in a travel shaft  16 ′ independently of one another. The control apparatus  12 ′ contains manually input data with respect to times of evenly distributed travel requirement as well as with respect to times of increased travel requirement in the upward direction in the morning and in the downward direction in the evening. 
     There is currently a time of increased travel requirement in the upward direction for the elevator installation  10 ′. A destination call  58 ′ for a floor lying above the elevator cars  26 ′,  28 ′,  30 ′ is present. The control apparatus  12 ′ is provided for the purpose, in times of increased travel requirement in a first travel direction  40 ′, of moving the elevator cars  26 ′,  28 ′,  30 ′ preferably directly to the starting point of the first travel direction  40 ′ after processing of all destination calls  58 ′ in the first travel direction  40 ′. In the illustrated situation the elevator cars  26 ′,  28 ′,  30 ′ have already processed their destination calls in the upward direction. 
     The control apparatus  12 ′ now moves the elevator cars  26 ′,  28 ′,  30 ′, which are travelling in empty state, to their determined starting points for the upward direction without reacting to the destination call  58 ′ to one of the upper floors. This is served only when the elevator cars  26 ′,  28 ′,  30 ′, starting from the start points thereof, have processed the then present destination calls  58 ′. If the existing destination call  58 ′ were to be directed to a floor disposed below the elevator cars  26 ′,  28 ′,  30 ′, then the control apparatus  12 ′ is provided for the purpose of moving the elevator cars  26 ′,  28 ′,  30 ′ to the start points thereof for the upward direction and in that case going past the floor with the destination call  58 ′ at least once without stopping. The destination call  58 ′ could be served, for example, only on the occasion of a second travel of the elevator cars  26 ′,  28 ′,  30 ′ past in downward direction. 
     Manually input data with respect to the times of evenly distributed travel requirement as well as with respect to times of increased travel requirement in the upward direction in the morning and in the downward direction in the evening are similarly assumed for the control apparatus  12  of the exemplifying embodiment of  FIG. 1 . In the case of elevator installations  10  with several travel shafts  14 ,  16 ,  18  the control apparatus  12  is provided for execution of further measures of co-ordination of the travel directions in order to take into consideration times with increased travel requirement in a travel direction  38 ,  40 ,  42 . 
     The situation, which is illustrated in  FIG. 1 , of the elevator installation  10  with three travel shafts  14 ,  16 ,  18  corresponds with a control of the elevator cars  20 ,  22 ,  24 ,  26 ,  28 ,  30 ,  32 ,  34 ,  36  at a time of increased travel requirement in the upward direction. The control apparatus  12  has determined the upward direction as travel direction  38 ,  40  of the elevator cars  20 ,  22 ,  24 ,  32 ,  34 ,  36  for a plurality of the travel shafts  14 ,  16 ,  18 , namely for the travel shafts  14  and  18 . The elevator cars  26 ,  28 ,  30  in the travel shaft  16  are moved by the control apparatus  12  in the opposite travel direction  40 , i.e. the downward direction. 
     The control apparatus  12  determines, at times of evenly distributed travel requirement, a travel direction  38  for the elevator cars  20 ,  22 ,  24  of the first travel shaft  14  and an apposite travel direction  40  for the elevator cars  26 ,  28 ,  30  of the second travel shaft  16 . If all internal destination floor selections  46 ,  48 ,  50  and/or all external transport requests  44  of the elevator cars  20 ,  22 ,  24  in the travel direction  38  have been processed, but if in the travel direction  40  there are still internal destination floor selections  46 ,  48 ,  50  and/or external transport requests  44  of the elevator cars  26 ,  28 ,  30  for processing, then the control apparatus  12  moves the elevator cars  20 ,  22 ,  24  to a waiting position from which they can process further internal destination floor selections  46 ,  48 ,  50  and/or external transport requests  44  in the travel direction  38 . 
     When the internal destination floor selections  46 ,  48 ,  50  and/or external transport requests  44  of the elevator cars  26 ,  28 ,  30  in the travel direction  40  and the internal destination floor selections  46 ,  48 ,  50  and/or external transport requests  44  of the elevator cars  20 ,  22 ,  24  in the travel direction  38  have been processed, the control apparatus  12  reverses the travel direction  38  of the elevator cars  20 ,  22 ,  24  of the first travel shaft  14  and the travel direction  40  of the elevator cars  26 ,  28 ,  30  of the second travel shaft  16  within three seconds. Through this matching in terms of time of the elevator cars  20 ,  22 ,  24 ,  26 ,  28 ,  30  in the travel shafts  14 ,  16  an oscillating operation arises which increases the probability of the elevator cars  20 ,  22 ,  24 ,  26 ,  28 ,  30  being uniformly distributed over the floors. The travel direction  42  of the elevator cars  32 ,  34 ,  36  of the third travel shaft  18  is synchronized by the control apparatus  12  at times of evenly distributed travel requirement in accordance with the respective number of internal destination floor selections  46 ,  48 ,  50  which are present and/or external transport requests  44  in the described manner with one of the two travel directions  38 ,  40  of the two other travel shafts  14 ,  16 . 
     At times of increased travel requirement in a travel direction  38  the control apparatus  12  moves the elevator cars  32 ,  34 ,  36  of the third travel shaft  18  so that the common travel direction  42  thereof is also the travel direction  38  with increased travel requirement. If all internal destination floor selections  46 ,  48 ,  50  and/or external transport requests  44  of the elevator cars  20 ,  22 ,  24  in the travel direction  38  and all internal destination floor selections  46 ,  48 ,  50  and/or external transport requests  44  of the elevator cars  32 ,  34 ,  36  in the same travel direction  42  have been processed, but internal destination floor selections  46 ,  48 ,  50  and/or external transport requests  44  of the elevator cars  26 ,  28 ,  30  in the travel direction  40  are still to be processed, then the control apparatus  12  moves the elevator cars  20 ,  22 ,  24 ,  32 ,  34 ,  36  into a waiting position from which they can process further internal floor selections  46 ,  48 ,  50  and/or external transport requests  44  in the travel direction  38 ,  42 . When the internal destination floor selections  46 ,  48 ,  50  and/or external transport requests  44  of the elevator cars  26 ,  28 ,  30  in the travel direction  40  and the internal destination floor selections  46 ,  48 ,  50  and/or external transport requests  44  of the elevator cars  20 ,  22 ,  24 ,  32 ,  34 ,  36  in the travel direction  38 ,  42  have been processed, the control apparatus  12  reverses the travel direction  38  of the elevator cars  20 ,  22 ,  24  of the first travel shaft  14 , the travel direction  40  of the elevator cars  26 ,  28 ,  30  of the second travel shaft  16  and the travel direction  42  of the elevator cars  20 ,  22 ,  24  of the first travel shaft  14  within three seconds. 
     In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.