Abstract:
An elevator drive has a brake device with compression springs to actuate brake levers, and brake linings on a brake drum creating a braking force. A sensor is provided to detect the movement of a brake magnet armature tappet. A bracket is attached to the brake magnet tappet on one end and a distance piece carrying the sensor housing is arranged on the other end. A restoring lug is attached to the existing mechanical indicator. A monitor evaluates the sensor signal and turns off the elevator drive in the event of dangerous operational states via a safety circuit. The system allows the state of the brake device to be monitored. The more the brake linings wear off due to abrasion, the smaller the distance between the armature and the brake magnet housing. If the armature is in contact with the brake magnet housing, the braking ability of the brake linings is completely void.

Description:
[0001]    The present application is a continuation of PCT/EP2008/055303 filed Apr. 30, 2008. 
     
    
       [0002]    The invention relates to a device and a method for monitoring a brake device for an elevator drive consisting of brake levers with compression springs which exert on the latter a spring force, whereby brake linings cause a braking force on a brake drum and at least one brake magnet lifts the brake levers against the spring force. 
       BACKGROUND OF THE INVENTION 
       [0003]    From patent specification EP 1 156 008 B1 a brake device for a drive machine has become known. The brake device consists of a first brake lever and a second brake lever, arranged on each of which is a brake shoe that acts on a brake drum. At their lower ends the brake levers are supported in swivel bearings on a bearing pedestal and at their upper ends guided on a bar. For the purpose of actuating the brake shoes, a compression spring is provided for each brake lever. For the purpose of lifting the brake shoes, provided on each brake lever is a magnet which acts against the compression spring. The magnets are arranged on a frame which is joined to the bearing pedestal. Arranged on the inside of each magnet support is a microswitch. A tappet of the microswitch is actuated by means of a cam that is arranged on a plunger disk. The switching status of the microswitch indicates to the control of the elevator whether by means of the magnets the brake is released or lifted, or whether the brake is not released or not lifted. 
         [0004]    The present invention provides a device and a method for a safely acting brake device which prevents states that are dangerous for the users of the elevator. 
         [0005]    The main advantages derived from the invention are that not only, as hitherto, is the end-position of the brake levers in the released state as brought about by the brake magnet monitored, but also the position of the movable brake-magnet part, such as the brake-magnet tappet, and of the plunger of the brake magnet. By this means it is possible to avoid the movable brake-magnet part such as, for example, the brake-magnet tappet or brake magnet plunger coming into contact with the fixed brake-magnet part such as, for example, the brake-magnet housing, through gradual abrasion of the brake linings and thereby reducing, or in the extreme case eliminating, the braking capacity of the brake device. The elevator drive can hence be directly switched off before the brake fails, or before a state that is dangerous for the users of the elevator can occur. 
         [0006]    A further advantage is the simple construction of the device according to the invention, which can be realized by means of, for example, the use of a variety of sensors, such as a sensor proximity switch, linear emitter, etc. 
         [0007]    With the invention, an elevator drive can be advantageously constructed and also an existing elevator drive advantageously retrofitted. The sensor can be arranged in the fixed brake-magnet part as, for example, inside or outside the brake magnet housing, in either case the relative movement of the movable brake part as, for example, the brake-magnet tappet or the plunger, relative to the fixed brake-magnet part as, for example, the brake-magnet housing, being registered. 
         [0008]    With the simple construction of the position monitor, existing elevator systems can be retrofitted with the position monitor according to the invention without great outlay, for example by mounting the sensor on the brake-magnet tappet. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0009]    In accordance with the foregoing purposes and advantages, an elevator drive according to the invention has a brake device with brake levers to which a spring force is applied by means of compression springs. Brake linings cause a braking force on a brake drum, and at least one brake magnet lifts the brake levers against the spring force. At least one sensor is provided that monitors a movement or a distance between a plunger of the brake magnet and a brake magnet housing. With the sensor signal, not only a signal for the end-position of the brake magnet tappet or brake magnet plunger can be generated, but also further signals as, for example, a signal for brake travel, a signal for brake lining wear, or a signal for brake drum heating. The safety for the elevator users can thereby be additionally increased, since the operating states of the brake device that lead to dangerous situations are promptly recognizable. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The present invention is described in more detail by reference to the attached figures wherein 
           [0011]      FIG. 1  is a diagrammatic illustration of an elevator drive with a brake device with two compression springs and a brake magnet; 
           [0012]      FIG. 2  is a diagrammatic illustration of an elevator drive with a brake device with a double brake magnet; 
           [0013]      FIG. 3  is an illustration of variant embodiment of an elevator drive with a brake device with a compression spring and a brake magnet, 
           [0014]      FIG. 4  is a detail illustration of a connection of a brake-magnet tappet with a brake lever; 
           [0015]      FIG. 5  is a diagrammatic illustration of a sensor for registering a movement or a distance; 
           [0016]      FIGS. 6 and 6   a  are perspective detail views of alternative arrangements of the sensor on the brake-magnet tappet; 
           [0017]      FIGS. 6   b  and  6   c  are perspective detail views of a spring-returning vane; 
           [0018]      FIGS. 7 and 7   a  are graphs depicting an output signal of the sensor as it depends on the distance registered; and 
           [0019]      FIG. 8  is a block circuit diagram of a monitor for evaluating the sensor signal and for indicating the state of the brake device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      FIG. 1  shows diagrammatically a brake device  1  with a first compression spring  2 , a second compression spring  3 , a first brake lever  5 , a second brake lever  6 , and a brake magnet  4 . The first compression spring  2  exerts a spring force on the first brake lever  5 . The second compression spring  3  exerts a spring force on the second brake lever  6 . The first compression spring  2  is guided by means of a first bar  7  which at one end is joined to a machine housing  8  and at the other end has a first adjusting element  9 , for example nuts with locknuts mounted on threads of the bar  7 , the braking force and the opening of the first brake lever  5  being settable with the adjusting element  9 . This second compression spring  3  is guided by means of a second bar  10  which at one end is joined to the machine housing  8  and at the other end has a second adjusting element  11 , for example nuts with locknuts mounted on threads of the bar  10 , the braking force and the opening of the second brake lever  6  being settable with the adjusting element  11 . Arranged on the first brake lever  5  is a first brake shoe  12  that carries a first brake lining  13 , the first brake lining  13  creating a braking force on a brake drum  14 . Arranged on the second brake lever  6  is a second brake shoe  15  that carries a second brake lining  16 , the second brake lining  16  creating a braking force on the brake drum  14 . The first brake lever  5  is mounted in swiveling manner on a first lever axle  17  that is supported on the machine housing  8 . The second brake lever  6  is mounted in swiveling manner on a second lever axle  18  that is supported on the machine housing  8 . The brake drum  14  is usually joined to a motor axle that is not shown. 
         [0021]    The brake magnet  4  consists of a magnet coil  20  which is arranged in a fixed brake-magnet part as, for example, a brake-magnet housing  19  which, when carrying electric current, acts through its magnetic field on a movable brake-magnet part such as, for example, a plunger  21 . The brake-magnet housing  19  with the magnet coil  20  and the plunger  21  repel each other and act against the spring forces of the compression springs  2 ,  3 . The movable brake-magnet part executes a relative movement relative to the fixed brake-magnet part. At a first joint  22 , the brake-magnet housing  19  is connected to the first brake lever  5 . The plunger  21  is connected to a brake-magnet tappet  23  which in turn, at a second joint  24 , is connected with a third bar  25 . By means of third adjustment elements  26 , the third bar  25  is connected to the second brake lever  6 . 
         [0022]    The more the brake linings  13 ,  16  wear due to abrasion, the smaller the distance d of the plunger  21  from the brake-magnet housing  19  becomes. Should the plunger  21  rest against the brake magnet housing, the braking capacity of the brake linings  13 ,  16  is completely eliminated. So that this operating state that is dangerous for elevator users cannot occur, at least one sensor  27  is provided that detects the movement or the distance d. Sensor  27  can be, for example, a proximity switch, for example with analog output, or a linear emitter. The sensor  27  can be arranged on the plunger  21  and register the distance d from the brake-magnet housing  19 . The sensor  27  can also be arranged on the brake-magnet housing  19  and register the distance d from the plunger  21 . The sensor  27  can also be arranged on the brake-magnet tappet  23  and execute the relative movement of the brake-magnet tappet  23  relative to the brake-magnet housing  19 , the sensor  27  registering the relative position of the brake-magnet tappet  23  relative to the brake-magnet housing  19 . Details are explained more fully in  FIGS. 4 to 6 . The sensor arrangement according to  FIGS. 4 to 6  is preferred for retrofitting in existing elevator installations. For new installations, a sensor arrangement according to  FIGS. 4 to 6 , or a brake magnet  4  with a built-in sensor  27 , can be used. 
         [0023]      FIG. 2  shows diagrammatically a brake device  1  with a double brake magnet  4  consisting of a first magnet coil  20 . 1 , a second magnet coil  20 . 2 , a first plunger  21 . 1 , a second plunger  21 . 2 , a first brake-magnet tappet  23 . 1 , and a second brake-magnet tappet  23 . 2 . The first brake-magnet tappet  23 . 1  is connected in swiveling manner (joint  22 . 1 ) to the first brake lever  5 . The second brake-magnet tappet  23 . 2  is connected in swiveling manner (joint  22 . 2 ) to the second brake lever  6 . The brake-magnet housing  19  is joined to the machine housing  8 . A first sensor  27 . 1  monitors, or registers, the movement or the distance dl between the first plunger  21 . 1  and the brake-magnet housing  19 . A second sensor  27 , 2  monitors, or registers, the movement or the distance d 2  between the second plunger  21 . 2  and the brake-magnet housing  19 . The first sensor  27 . 1  can also be arranged on the swivel joint  22 . 1 . The second sensor  27 . 2  can also be arranged on the swivel joint  22 . 2 . 
         [0024]      FIG. 3  shows a variant embodiment of a brake device  1  with only one compression spring  2  and one brake magnet  4 . The compression spring  2  rests against the second brake lever  6  and against a fourth bar  28  which at its other end is connected to the first brake lever  5 . The compression spring  3  thus exerts a spring force on both brake linings  13 ,  16 . The brake magnet  4  functions as explained in  FIG. 1 , it being possible for at least one sensor  27  to be built into the brake magnet  4  or, as shown in  FIGS. 4 to 6 , mounted on the second swivel joint  24 . The brake magnet  4  acts against the spring force of the compression spring  3  and releases the brake linings  13 ,  16  from the brake drum  14 . The force of the brake magnet  4  can also be created manually by means of a brake-release lever  29 . A fifth bar  32  limits the displacement of the brake levers  5 ,  6  by the magnet  4  or by the brake-release lever  29 . Arranged on a gear output shaft  31 , and referenced with  30 , is a traction sheave over which suspension and traction means of the elevator car and of the counterweight are guided. 
         [0025]      FIG. 4  shows details of the connection of the brake-magnet tappet  23  with the second brake lever  6 . By means of a pin  33  that penetrates through the brake-magnet tappet  23 , the third bar  25  is connected in swiveling manner to the brake-magnet tappet  23 , spring rings  38  securing the pin at both ends. The end  37  of the brake-magnet tappet  23  may be fork-shaped. Provided at the free end of the third bar  25  is a thread  34  which, together with nuts  35 , serves as third adjusting element  26 . At least one brake lever switch  40  can be provided to monitor whether the brake levers  5 ,  6 , and thus the brake linings  13 ,  16 , have been released from the brake drum  14 . 
         [0026]    As shown in  FIG. 4 , the brake-lever switch  40  can monitor the position of the brake lever  6 , or be arranged in such manner that the former monitors the position of the pin  33  relative to the brake-magnet housing  19 . Normally arranged on the brake-magnet housing  19  is a sixth bar  41 , provided on which is a first vane  42 . With the relative position of the pin  33  relative to the first vane  42 , the distance of the plunger  21  from the brake-magnet housing  19  can be determined. The sixth bar  41  together with the first vane  42  is also referred to as a “mechanical indicator”. The more the brake linings  13 ,  16  wear through abrasion, the less the pin  33  is distant from the first vane  42 . As shown in  FIG. 6 , in the present exemplary embodiment the sixth bar  41  and the pin  33  are used as a mechanical reference point for the sensor  27 . 
         [0027]      FIG. 5  shows diagramatically the sensor  27  for registering the movement or the distance d of the plunger  21  from the brake-magnet housing  19 , or the relative movement of the brake-magnet tappet  23  relative to the brake-magnet housing  19 . In the present exemplary embodiment, provided as sensor  27  is an inductive proximity switch with an analog output which responds to ferromagnetic objects. The sensor  27  has a sensor housing  43  with a second thread  44  onto which a locking nut  45  can be screwed. The sensor housing  43  is screwed into a magnetically neutral distance piece  46  of, for example, plastic, and by means of the locking nut  45  secured against turning, the distance piece  46  on the end-face  47  of the sensor  27  having a defined wall thickness  48 , for example 1 mm. Referenced with  49  is an air gap between a second vane  50  and the distance piece  46 . Wall thickness  48  plus air gap  49  yields the sensor gap  51  from the second vane  50 . In  FIG. 7  and in  FIG. 7   a  the sensor gap  51  is referenced as “s.” With the distance piece  46 , setting work on site is avoided. As shown in  FIG. 6  and in  FIG. 6   a , the distance piece  46  serves also as a support for the sensor  27 . The sensor  27  can at the factory be completely screwed into the distance piece  46  and installed on site without settings in the axial direction. Power supply to the sensor  27 , and the signal output of the sensor  27 , take place via a connecting cable  52 . 
         [0028]      FIG. 6 ,  FIG. 6   a  show details of arrangements of the sensor  27  on the brake-magnet tappet  23 . A stirrup  53  is fastened onto one end of the existing pin  33 , and arranged on the other end is the distance piece  46  that bears the sensor housing  43 . In  FIG. 6 , with the aid of elongated slots  54  and screws  55 , the distance piece  46  can be aligned perpendicular to the direction of movement of the brake-magnet tappet  23  on the exisiting sixth bar  41 . The second vane  50  is fastened to the sixth bar  41 . A setting of the brake-magnet tappet  23  in the direction of movement is not necessary. 
         [0029]    In  FIG. 6   a , the stirrup  53  is fastened by means of screws  55   a  to the fork-shaped end  37  of the brake-magnet tappet  23 . As shown in  FIG. 6   a , the second vane  50  is arranged coaxial to the axis of the sixth bar  41  and of the sensor  27 . The second vane  50  is embodied in a spring-returning manner. 
         [0030]    By this means it is possible to avoid the suffering of damages by sensor  27  and/or the vane  50  should a collision occur between the sensor  27  and the second vane  50 , either because of incorrect mechanical settings or because of a travel of the brake magnet  4  that deviates from the norm. 
         [0031]      FIGS. 6   b  and  6   c  show details of the second vane  50 . A cylindrical base body  50 . 1  is connected with the sixth bar  41  and serves as a support for guides  50 . 2  which slide along pins  50 . 6  of a cap  50 . 3  with disk  50 . 4 . A spring  50 . 5  rests at one end on the base body  50 . 1  and at the other end on the disk  50 . 4  and holds the cap  50 . 3  with the disk  50 . 4  in the end-position shown in  FIGS. 6   a  and  6   c.  In the case of a collision of the sensor  27  with the vane  50 , the cap  50 . 3  with the disk  50 . 4  is moved against the spring force of the compression spring  50 . 5 . 
         [0032]      FIG. 7 ,  FIG. 7   a  show the output signal of the sensor  27  as it depends on the registered distance, sensor gap  51 , or relative movement of the brake magnet tappet  33  relative to the brake-magnet housing  19 . The varying distance of the end-face  47  of the sensor  27  from the second vane  50  is referenced in  FIG. 7 ,  FIG. 7   a  as “s.” The proximity switch with analog output that is used as sensor  27  may have a current output between 0 and 20 mA, which is robust against electromagnetic interference signals as shown in  FIG. 7 , or a current output signal between 0 and 5 mA as shown in  FIG. 7   a  at a sensor gap  51 , or s, of between 0 and 10 mm.  FIG. 7 ,  FIG. 7   a  show the characteristic of the current I as a function of the travel s or of the sensor gap  51 . Of interest is the linear area of the curve between 8 mA and 17 mA as shown in  FIG. 7 , or between 2 mA and 4.3 mA as shown in  FIG. 7   a , and a sensor gap  51 , or s, between 3 mm and 7.5 mm. The analog current output signal is fed to an analog/digital converter  64  of a monitor  60  that is shown in  FIG. 8  and by which it is evaluated. 
         [0033]    On elevators with many short trips and/or that stop at many floors, the brake linings  13 ,  16  can wear more quickly than usual. Elevators that are halted by the brake in the area of the story (so-called two-speed elevators), have higher wear of the brake linings. A defective state of the brake can be promptly deduced from the diminishing leveling accuracy of the elevator car on the story. With drives with releveling, the leveling accuracy is always the same, and a defective state of the brake does not manifest itself visibly. 
         [0034]    A further cause of excessive wear of the brake linings  13 ,  14  can be an at least partial failure of the magnet coil  20 , as a consequence of which the magnet coil  20  no longer produces the full force for releasing the brake lever  5 ,  6 , and the motor moves the traction sheave  30  with closed brake levers  5 ,  6 . As shown in  FIG. 4 , to avoid this state with the resultant excessive wear of the brake linings  13 ,  16 , a brake-lever switch  40  is provided which monitors the position of the brake levers  5 ,  6  when the brake is perceived by the elevator control to be lifted, and determines whether on a travel command the brake levers  5 ,  6 , and thus the brake linings  13 ,  16 , have been released from the brake drum  14 . Should the brake-lever switch  40  not be present, or not supported by the elevator control, travel without lifted brake cannot be avoided, but the monitor  60  nonetheless detects and prevents a total failure of the brake. 
         [0035]      FIG. 8  shows a block circuit diagram of the monitor  60  for analyzing the sensor signal of the sensor  27  and for indicating the state of the brake device  1 . A processor  61  of the monitor  60  operates according to a program that is stored in a program memory  62 , the processor placing data into a working memory  63  or fetching it from thence. The analog sensor signal of the sensor  27  is fed to the analog/digital converter  64  of the monitor  60 . Converter  64 , memory  63 , memory  62 , and the processor  61  communicate via a bus system  65 . By means of a diagnosis instrument  66 , the program or parameters can be modified or data read out. A first power-supply device  67  supplies the monitor  60  with electrical energy, for example with a voltage of 5 V. The first power-supply device  67  is supplied by a monitor-external second power-supply device  68 , for example with alternating voltage from the power supply network at 220 V or, for example, with direct voltage at 24 V from the not-shown elevator control. 
         [0036]    Depending on the sensor signal of the sensor  27 , a relay  69  is triggered. In the normal operating state of the brake device  1 , the relay  69  is activated and a potential-free contact  70  that belongs to the relay  69  is closed. To visualize the state of the closed contact, a first indicator  71 , for example a green-lit LED, can be provided. In the safety circuit  72  of the not-shown elevator control, the potential-free contact  70  is connected in series. A second potential-free contact can also be connected in series, and on failure of the relay  69  the safety circuit can be opened by means of the second potential-free relay-operable contact. The safety circuit of the elevator control is a series circuit of contacts that monitor important functions of the elevator operation as, for example, doors closed, brake lifted, normal speed of the elevator car, normal load, etc., and if at least one contact is open no car travel is executed. 
         [0037]    Depending on the sensor signal of the sensor  27 , different operating states are detected and indicated. The normal operating state is made visible by means of a second indicator  73 , for example by means of a green-lit LED. The operating state corresponding to excessively worn brake linings  13 ,  16  is made visible by means of a third indicator  74 , for example by means of a red-lit LED. A further operating state that corresponds to the stroke of the brake device  1  or of the brake-magnet tappet  23  is visualized by means of a fourth indicator  75 , for example by means of a red-lit LED. A further operating state corresponding to heating of the brake drum  14  is visualized by means of a fifth indicator  76 , for example by means of a red-lit LED. A further operating state corresponding to a failure detected by electronic tests is visualized by means of a sixth display  77 , for example by means of a red-lit LED. A further operating state of the brake corresponding to the closed position, or to the open position, of the brake levers  5 ,  6  is visualized by means of a seventh indicator  79 , for example by means of an orange-lit LED. The monitor  60  can be equipped with all, or with a selection of, the said indicators. 
         [0038]    With a push-button  78 , on electronic initialization of the monitor  60 , measurement values of the sensor  27  that were saved in the non-volatile working memory  63  (EEPROM) can be reset. After mechanical setting work on the brake device  1 , the push-button  78  must be pressed. For example, the processor  61  calculates the mean value of a plurality of measurement values of the sensor signal for the closed position of the brake levers  5 ,  6 , or for the brake that has been activated by means of compression springs  2 ,  3 , and the mean value of a plurality of measurement values of the sensor signal for the open position of the brake levers  5 ,  6 , or of the brake levers  5 ,  6  that have been lifted by means of the brake magnet  4 . After resetting of the mean values, mean values of new measurement values are calculated and saved. 
         [0039]    The sixth bar  41  with the first vane  42  and the tappet pin  33  are used as mechanical reference point for the sensor  27 , there being provided as sensor gap  51 , for example, 3 mm, or as air gap  49 , 2 mm. As shown in  FIG. 7 , at 3 mm sensor gap  51  the linear area of the sensor signal, or of the output current I, begins. With an air gap  49  of 2 mm, collision of the sensor  27  with the second vane  50  can normally also be avoided at maximum wear of the brake linings  13 ,  16 . 
         [0040]    In normal operation with closed position of the brake levers  5 ,  6  and with open position of the brake levers  5 ,  6 , the sensor gap  51  in the present exemplary embodiment is greater than 3 mm. In the closed position of the brake levers  5 ,  6 , the sensor gap  51  is given. Deviations are caused by wear of the brake linings  13 ,  16  or by heating of the brake drum  14 . The monitor  60  can differentiate between the deviations. As the brake linings  13 ,  14  wear, the brake-magnet tappet  23  moves relative to the brake-magnet housing  19 . On heating of the brake drum  14 , the brake magnet tappet  23  moves relatively away from the brake-magnet housing  19 . 
         [0041]    Based on the analog/digital transformed sensor signal, the processor  61  of the monitor  60  calculates the speed and the direction of the brake magnet tappet  23 . To determine the closed position and the open position of the brake levers  5 ,  6 , signal values or measurement values are allocated to the corresponding position if the brake-magnet tappet  23  does not, for example, move more than 0.01 mm in 100 ms. For the closed position, a sensor gap  51  of, for example, between 3 m and 5.5 mm is possible, and for the open position, for example, a sensor gap  51  of between 5 mm and 7.5 mm is possible. 
         [0042]    With each car travel the speed and the direction of the brake-magnet tappet  23  changes, whereby the number of trips is detected and saved in the working memory  63 . 
         [0043]    During the first, for example, 8 trips, the indicator  73  flashes at, for example, 10 Hz, with, for example, 8 signal values of the sensor  27 , or measurement values, being assigned to the corresponding closed position or open position and therefrom the corresponding mean values formed and saved in the working memory  63 . The closed position and open position learned on the brake device  1  serve as a starting point for the operating state of excessive wear of the brake linings  13 ,  16  or for the operating state of excessive heating of the brake drum  14 . Thereafter, the second indicator  73  flashes at, for example, 1 Hz and indicates a fully functional capability of the brake device  1 . Monitoring of the closed position and of the open position can be continued. Should the mean values of the measured signal values deviate by more than, for example, 0.5 mm, the saved mean values are overwritten with the current mean values. Alternatively, the aforementioned closed position and open position can be learned once only with a plurality of measurement values. 
         [0044]    With increasing wear of the brake linings  13 ,  16 , for example 0.5 mm before the critical point, the third indicator  74 , is switched on and switched off with, for example, a frequency of 10 Hz. On attaining the critical point (sensor gap  51 =3 mm), the relay  69  is switched off with a time delay and the potential-free contact  70  is opened. The time delay is of such magnitude that the elevator car can complete the current trip and the transported persons can leave the elevator car. The operating state of excessive wear of the brake linings  13 ,  16  is then signaled with the continuously switched-on third indicator  74 . 
         [0045]    Should the stroke or distance between the closed position and the open position be smaller than, for example, 2 mm during, for example, 3 seconds, the monitor  60  assumes a fault, for example an incorrect mechanical setting or a mechanical blocking. A further fault that can be detected from the stroke is the number of open positions in relation to the maximum travel time of the elevator car. On occurrence of a stroke fault, the relay  69  is switched off with a time delay and the potential-free contact  70  is opened. The time delay is of such magnitude that the elevator car can complete the current trip and the transported persons can leave the elevator car. The fourth indicator  75  is initially, for example, switched on and switched off with a frequency of 10 Hz and then continuously switched on. 
         [0046]    If, due to failure of the brake magnet, or due to software faults, or due to hardware faults in the electronic switching circuits, the brake device  1  is not lifted, or if the brake linings  13 ,  16  are not released from the brake drum  14 , the monitor  60  can also not detect a stroke fault. In the case of car travel with a closed brake, the brake drum  14  and the brake linings  13 ,  16  heat. Upon heating, brake drum  14  and brake linings  13 ,  16  expand and cause a movement of the brake-magnet tappet  23  relative to the brake-magnet housing  19  opposite in direction to the movement caused by wear. The deviation is evaluated in relation to the distance of the closed position from the critical point. The more advanced the wear of the brake linings  13 ,  16  is, or the thinner the brake linings  13 ,  14  are, the smaller is the deviation that causes switching-off. The deviation can lie in the range of, for example, 0.7 mm to 1.5 mm. On occurrence of an impermissible deviation, the relay  69  is switched off with time delay and the potential-free contact  70  is opened. The time delay is of such magnitude that the elevator car can complete the current trip and the transported persons can leave the elevator car. The fifth indicator  76  is initially switched on and off with a frequency of 10 Hz and then switched on continuously. 
         [0047]    The monitor  60  itself can also prevent switching-on of the relay  69  or effect switching-off of the relay  69  and an opening of the potential-free contact  70 . Reasons therefor are negative plausibility tests during electronic initiation or operation of the monitor  60 . Further reasons for switching-off are a missing sensor  27 , or a brake device  1  which has not been lifted for a long period of time, for example three months. This type of fault is visualized by means of the sixth indicator  77 .