Abstract:
The invention relates to a method for detecting wear in a frictionally locking clutch ( 50 ) or a frictionally locking brake ( 2, 40 ), the clutch ( 50 ) or the brake ( 2, 40 ) having at least one electrically excitable coil ( 4 ) and at least one element ( 20, 21, 23 ) which can move in rotation about an axis ( 3 ), to bring about frictional locking with at least one corresponding mating element ( 22, 28, 54 ), at least the element ( 20, 21, 23 ) which can move in rotation or the corresponding mating element ( 22, 54 ) being axially movable, and it being possible to form an airgap ( 26 ). The wear ( 46 ) can be inferred by virtue of the fact that the electric current is measured by the electrically excitable coil ( 4 ), evaluated and the evaluation compared with at least one reference value.

Description:
[0001]    The invention relates to a method for detecting wear in a frictionally locking clutch or a frictionally locking brake.  
         BACKGROUND OF THE INVENTION  
         [0002]    Clutches and brakes are known which use frictionally locking connections as a functional principle. These brakes and/or clutches have, for example, at least one electrically excitable coil, and at least one element which can move in rotation about an axis, to bring about frictional locking with a corresponding mating element. In this context, the element which can move in rotation or else the corresponding mating element can move axially. The clutch and brake respectively transmit forces and torques, the brake reducing the kinetic energy of at least one moving body by means of a frictionally locking connection to at least one nonmoving body, and the clutch coupling to one another, and decoupling, at least two movable bodies for their common movement.  
           [0003]    Brakes may, for example, be permanent magnet brakes. In this context, the force of a permanent magnetic field is used to generate the braking effect. Permanent magnets generate a permanent magnetic field and attract at least one rotationally movable element to a corresponding mating element for frictionally locking connection. To cancel the braking effect, an electromagnetic field with an at least partially opposed field direction is superimposed on the permanent magnetic field so that the fields can at least partially cancel one another out. The electromagnetic field can be generated by means of an electrically excitable coil. It is also possible to establish connections with this mechanism between the permanent magnetic field and the electromagnetic field.  
           [0004]    Brakes may also be spring-pressure brakes. In such brakes, the force of an electromagnetic field is used to cancel out the force, generated by a spring on an element which can move in rotation about an axis. The spring presses, an element which is moved at least in rotation toward a corresponding mating element to form a frictionally locking connection. In order to cancel the braking effect, an electromagnetic field which brings about a magnetic force, which is opposed to the force of the spring, is generated by means of an electrically excitable coil. Connections may also be established with this mechanism between the spring force and magnetic force.  
           [0005]    Permanent magnetic brakes, spring pressure brakes and spring pressure clutches have an electrically excitable coil in order to effect a quiescent current, a working current brake or clutch depending on the design of the brake or clutch. The electrically excited coil exerts a force on preferably ferromagnetic material, which is at least part of at least one of the elements used in the frictional locking, and which is located at a variable distance from the electrically excitable coil.  
           [0006]    The frictional locking between the element which moves in rotation about an axis and the corresponding mating element results in abrasion, i.e. wear on at least one of the elements involved in the frictional locking. The wear is a limiting factor on the service life of the brake or the clutch. Depending on the degree of wear, the function of the brake or of the clutch may no longer be sufficiently reliable and accordingly, at least a part of the brake or the clutch will have to be replaced.  
           [0007]    Hitherto, the user of the brake or of the clutch has determined the extent of the wear. Since abrasion is characteristic of wear, it is possible to measure the abrasion mechanically from time to time by means of a length meter. This procedure is both cumbersome, time-consuming and costly. To avoid the necessity of measuring the abrasion, it would be necessary to replace the brake or the clutch periodical. A further possibility is to count the number of braking operations, emergency stop operations and clutch activation operations. After a fixed number of operations, critical wear would be assumed and the brake or the clutch would be replaced. In this procedure, a large number of brakes and clutches would be unnecessarily replaced thus incurring costs, outage times, material and work time since the actual amount of wear would warrant replacement. Moreover, when wear occurs, depending on the type of brake or clutch, it is not absolutely necessary to replace the entire brake or the entire clutch, but rather only the one worn part. Also it may be possible to readjust the brake or the clutch in order to compensate for the wear.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention is intended to provide an improved, cost-effective method and device for detecting wear in a frictionally locking clutch, or in a frictionally locking brake. In the method for detecting wear in a frictionally locking clutch or in a frictionally locking brake, the electric current changes in the electrically excitable coil when the voltage across this coil is measured, and some of the measured values are evaluated by means of data processing equipment, and at least one result of the evaluation is compared with at least one stored value, with the wear being inferred from the comparison.  
           [0009]    The clutch or the brake has at least one electrically excitable coil and at least one element which can move in rotation about an axis, to bring about frictional locking with at least one corresponding mating element. The element which can move in rotation or the corresponding mating element is axially movable, and at least one airgap between the movable element and the corresponding mating element changes when the clutch or the brake opens or closes.  
           [0010]    The wear on friction surfaces, such as a brake lining, a brake disk and/or other elements involved in the frictional locking, is indicative of the wear of a frictionally locking clutch or a frictionally locking brake. This wear generally occurs in frictionally locking brakes or frictionally locking clutches with an electrically excitable coil in the region between an airgap of a magnetic circuit of the electrically excited coil, and influences it. As the wear increases, the airgap becomes larger and the inductance of the electrically excitable coil decreases. If the inductance decreases, the electrical time constant of the electrically excitable coil also decreases. The decrease in the inductance changes the profile of the electric current of the coil. Evaluations and/or criteria for this profile permit the wear to be inferred by means of a comparison with at least one stored set point value. The change in the time constant of the coil can also be used as a measurement variable for the detection of the wear. The smaller the inductance, the smaller the time constant.  
           [0011]    In a preferred method for detecting wear in a frictionally locking clutch or a frictionally locking brake, the time of at least one peak of the electric current of the electrically excitable coil during the excitation process and/or during the de-excitation process of the electrically excitable coil is compared with a stored reference time and the wear is inferred from the difference between the times.  
           [0012]    As the airgap becomes larger due to wear, for example the opening time or closing time of the brake becomes shorter, the inductance of the coil is reduced. This effect also occurs during clutch activation operations. In this way, the opening time or closing time can be used as a criterion for the wear. The reduction in the inductance can be detected when the voltage across the coil changes. Notches or peaks in the flow of current can be used as a criterion for the opening time or the closing time. These notches or peaks arise at the moment at which the elements which are provided for the frictional locking are released from one another and form an airgap, or else at the moment at which the elements which are provided for the frictional locking come into mutual contact and close the airgap. At these moments, the inductance of the coil changes to a very great extent owing to the airgap which is formed or which disappears, and has a decisive effect on the current flowing in the coil.  
           [0013]    A notch which occurs in an otherwise continuously rising current profile, or a peak which occurs in an otherwise continuously falling current profile, can occur, for example, as a result of the evaluation of the differences of adjacent measured values, or by means of the evaluation of time derivatives of the current. The time when the notch or peak is determined is compared with a stored reference time. This reference time specifies, for example, the time of the notch or the peak in the current profile in a clutch or brake without wear. If a value between the time of the notch or the peak of the reference value, and the current time of the notch or the peak is exceeded, it is possible, for example, to generate a warning message. When the times are compared, a comparable starting time is specified in order to determine the time. This starting time is advantageously the voltage jump when the exciter voltage is applied to the coil or the voltage jump when the exciter voltage is switched off.  
           [0014]    In a further preferred embodiment method for detecting wear in a frictionally locking clutch or a frictionally locking brake, a superimposed alternating voltage is applied to the electrically excitable coil, which alternating voltage leaves the operating state of the clutch or of the brake unchanged. The complex resistance and/or the virtual component of the complex resistance from this superimposed alternating voltage is measured together with the measured current, compared with a reference value, and the wear is inferred therefrom, with a smaller virtual component being set when the wear increases.  
           [0015]    The inductance of the electrically excitable coil, or of the impedance associated with it, also referred to as complex resistance, is measured by superimposing an alternating current on a supply voltage of the coil. The complex resistance is determined from the measured alternating current and from the measured or known impressed alternating voltage, given knowledge of the ohmic resistance of the coil. The ohmic resistance is determined, for example, from the d.c. component of the electrical current voltage. If both measurements, the measurement of the ohmic resistance, i.e. of the real part of the complex resistance, and the measurement of the virtual part of the complex resistance or of the impedance, are carried out approximately simultaneously, the temperature influence on the ohmic resistance is thus also taken into account. If the inductance, which can be determined from the measurements, or another comparable value such as the complex resistance, drops below a certain limiting value which is stored as a reference value, it means that the wear has progressed to such a point that a warning about the wear is triggered. The measurement is advantageously carried out when an airgap is present as the change in the inductance is marked in particular in the operating state with an airgap. Evaluation circuits for complex alternating variables are well known to those skilled in the art. If rectifiers are provided as a voltage source for exciting the coil, they often already have an alternating component. This alternating component of, for example, 100 Hz is advantageously used as a superimposed alternating voltage.  
           [0016]    If a binary signal is required for the detection of wear, a more precise detection can be implemented with the foregoing methods by comparing the measured values, or values from the evaluation of the measured values, for example the complex resistance or the time of the current notch, with different stored reference values. The reference values are each assigned a value which represents a degree of wear so that the wear may be determined. If the change in the wear is recorded, this can be used as a basis for further diagnostics.  
           [0017]    The following is an example of an equation for calculating an alternating component in the case of complex resistances:  
         Z   =       U   I     =         R   2     +       (     ω                 L     )     2             ,       where                 ω     =     2      π                 f                             
 
           [0018]    In principle, the calculation of alternating variables is concerned with complex variables, Z is an absolute value of a complex resistance, U represents a voltage, I is current, R is real resistance, f is frequency, and L is inductance.  
           [0019]    A device for carrying out the method for detecting wear in a frictionally locking clutch or a frictionally locking brake is also part of the present invention. The clutch or the brake has at least one coil which can be electrically excited by means of at least one voltage source, and at least one element which can move in rotation about an axis, to bring about frictional locking with a corresponding mating element, with the element which can move in rotation, or the corresponding mating element being axially movable, and a means for measuring the electric current of the electrically excitable coil, and a means for evaluating the measurement of the electric coil. In this device, the inclusion of the electric current of the excitable coil in the detection of wear is advantageous since the excitable coil is already a feature of the brake or of the clutch. An additional inductive means for generating an electrical magnetic field to determine the wear is unnecessary, which results in cost savings. The electrically excitable coil is preferably fixed with respect to a generating means, such as a voltage source. This allows sliding contacts to be avoided.  
           [0020]    While means for measuring the electric current include commercially available current measuring devices which are connected into the circuit, clip-on ammeters have the advantage that they can easily be used in already existing brakes or clutches without having to disconnect the circuit of the electrically excitable coil. A further possible way of measuring the current is to measure the voltage by means of a resistor which is part of the circuit of the electrically excitable coil.  
           [0021]    A means for evaluating the electric current is an electrical circuit or a data processing means which evaluates the measurement results of at least the measurement of the current. Already existing data processing means, for example a numerical controller, a computer for the numerical controller, an electronic part of a rectifier or a stored-program controller are advantageously and cost-effectively used for the data processing. The device for detecting wear can be used in spring pressure brakes, permanent magnetic brakes or multi-disk clutches having an electrically excitable coil. Frictionally locking clutches and brakes can synchronize two machine parts which rotate at different rotational speeds. They perform shift work in the process. If, for safety reasons, quiescent-current-actuated brakes or clutches which have to perform shifting work are required, it is possible, for example, to use multi-disk spring pressure brakes and spring pressure clutches.  
           [0022]    In a preferred embodiment of the device for detecting wear in a frictionally locking clutch or a frictionally locking brake, at least one element which is involved in the frictional locking comprises ferromagnetic material. Parts which are affected by the wear, such as a friction surface are preferably located in the region of the airgap of a magnetic circuit which is influenced by the excitable coil. This magnetic circuit is then influenced in the region of the airgap in particular if the parts which are subjected to wear have a ferromagnetic material. The greater the change in the inductance of the coil when wear occurs, the simpler the determination of the wear.  
           [0023]    In another preferred embodiment of the device for detecting wear in a frictionally locking clutch or a frictionally locking brake, the electrically excitable coil is excited at least by means of a voltage jump or by means of an alternating voltage. A changing electrical current through the coil is necessary to determine wear. A change in current occurs when there is a change in the voltage applied to the coil. Such a change in voltage is, for example, a voltage jump. In brakes or in clutches having coils excited by means of a d.c. voltage, a voltage jump occurs when d.c. voltage is applied, or is disconnected in order to change the operating state. Brakes are either opened or closed. Different braking forces are differentiated under certain circumstances. Clutches have a decoupled state and a coupled state as operating states. In brakes, the brake opens, for example when the d.c. voltage is applied to the coil or to the terminals of the coil. When the coil is disconnected from the d.c. voltage, the brake closes. When the brake is opened and closed, the element for frictional locking, which can move in rotation about an axis and which is usually a disk is advantageously moved axially toward or away from a mating element. An already existing voltage source for exciting the coil is advantageously used to generate the change in voltage. As a result, no additional means is necessary for this, which reduces the costs for the detection of wear.  
           [0024]    In another preferred embodiment of the device for detecting wear in a frictionally locking clutch or a frictionally locking brake, a measuring means determines information relating to the chronological axial movement profile of at least the element which can move in rotation about an axis, or of the corresponding mating element. The means for evaluating the chronological axial movement profile infers the wear. The inference of the wear is made by means of the comparison with reference values. During the movement process or during the coupling process, the inductance of the coil changes in a jump-like fashion when the elements just begin to touch and when the elements disconnect from one another. The sudden change in the inductance is manifested in a sudden change in the electrical current through the coil. The smaller the inductance, the sooner the jump-like change in the current occurs when there is a voltage jump in order to change the operating state of the brake or of the clutch. Thus, for example, the opening time of the brake becomes shorter as the airgap becomes bigger, and can thus be used as a criterion for the wear of the brake lining or elements which are involved in the frictional locking.  
           [0025]    In a further preferred embodiment of the device for detecting wear in a frictionally locking clutch or a frictionally locking brake, a superimposed alternating voltage is applied to the electrically excitable coil, a measuring means to determine information relating to the profile of the electrical current of the electrically excitable coil and at least one means for determining a value which is dependent on at least the virtual component of the complex resistance of the coil is provided. From the foregoing it is possible to compare this value with at least one reference value.  
           [0026]    Alternatively, or in addition, in order to determine the wear over the opening times and/or closing times, the change in the inductance of the coil itself can also be provided for determining the wear. An alternating voltage is superimposed on the coil voltage. This alternating voltage is generated, for example, by means of a separate alternating voltage source, or by using an alternating voltage component of an already existing d.c. voltage source, which is provided for exciting the coil. The superimposed alternating voltage leaves the operating state of the clutch or of the brake unchanged, as the amplitude and frequency are appropriately selected. Given knowledge of the fed-in voltage and/or after measurement of the voltage across the coil and after measurement of the electrical current through the coil, the wear can be determined using the evaluation means.  
           [0027]    In a coil which is excited by a d.c. voltage, the time constant of the inductance of the coil can be determined by means of a renewed voltage jump with the same polarity. When a coil is not excited, the time constant can also be determined by exciting the coil with the polarity which is opposed to that for operation.  
           [0028]    In a further preferred embodiment of the present invention, a means for detecting electrical voltage across the electrically excitable coil is provided. Such a means may be a volt meter, which is necessary in particular if the profile of the superimposed alternating voltage is unknown. The device for detecting wear is advantageously used in a holding brake of an electrical machine. Holding brakes are safety-related components of drives or of electric motors, for example in robot applications. The failure of a brake can put persons and objects at extreme risk. Brakes such as holding brakes have a limited service life, which is determined in particular by the wear of the brake lining and/or other friction surfaces. This depends, for example, on the number of emergency stops, on the rotational speed at which braking occurs, and with which overall moment of inertia braking is carried out. With the device according to the present invention the requirement, for example, of summing the number of emergency stops and issuing a warning and/or an alarm when a maximum permitted number is reached is no longer necessary. The detection of wear according to the invention is a direct method for monitoring the brake or the holding brake, and the shortcomings of an indirect method, such as the summing of the number of braking operations, can be eliminated.  
           [0029]    The device of the present invention for detecting wear is advantageously used in a holding brake of an electrical machine, the electrical machine having at least one data communication device. If the electrical machine has a data communication device, this is also used to transmit data and/or signals such as measurement variables to an evaluation means. If the electrical machine itself has the evaluation means, and is thus an intelligent electrical machine, at least one signal which indicates the wear, for example a warning signal, can be transmitted.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    The invention is further disclosed herein below in connection with the drawings in which examples of a brake and of a clutch as well as diagrams illustrating the method for detecting wear and a schematic illustration of the invention for detecting wear are shown. In the drawing:  
         [0031]    [0031]FIG. 1 illustrates a spring pressure brake;  
         [0032]    [0032]FIG. 2 illustrates a permanent magnet brake;  
         [0033]    [0033]FIG. 3 illustrates an electromagnet single-surface clutch;  
         [0034]    [0034]FIG. 4 illustrates an electrical current profile when a brake opens and closes;  
         [0035]    [0035]FIG. 5 illustrates a current profile of a brake which has been subjected to wear in comparison with a nonworn brake; and  
         [0036]    [0036]FIG. 6 illustrates a schematic view of a design of a device for detecting wear. 
     
    
       [0037]    Elements of the same type are provided with identical reference symbols in the following FIGS.  1  to  6 .  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0038]    [0038]FIG. 1 shows a spring pressure brake  2 . It has a fixed element such as an armature  22  and at least one element which can move in rotation about an axis  3 , here a driver  18  and a friction disk  20 . The armature  22  is connected via a spring  6  to a magnet housing  28 . Within the magnet housing there is an electrically excitable coil  4  in a coil housing  32 . The coil  4  can be excited by means of a coil terminal  8  so that a magnetic field is formed in the excited state. A magnet flux  5  is represented by lines. Between the armature  22  and the magnet housing  28  is an airgap  27 . This airgap  27  occurs when the spring  6  presses the armature  22  against the friction disk  20 , the friction disk  20  having a brake lining  10  and  12 . The brake lining  10  is then located in a frictionally locking connection with the armature  22 , whereas the brake lining  12  is in a frictionally locking connection with the friction plate  24 , which is also fixed like the armature  22 . If the brake is opened, an airgap  26  is thus also formed between the brake linings  10  and  12  and the corresponding friction surfaces of the armature  22  and of the friction plate  24 . As a result of wear, the brake linings  10  and  12  which are located on the friction disk  20  which is connected to the driver  18  become progressively narrower and the airgaps  26 ,  27  become progressively larger. The friction plate  24  is connected to the magnet housing  28  via a friction plate attachment  30 . When the coil is excited via the magnetic housing, the magnetic flux  5  extends to the airgap  26 , the armature  22 , the brake linings  10  and  12 , the airgaps  26  and the friction disk  20  and the friction plate  24 , and back to the magnet housing  28 . As a result of the excitation of the coil  4 , the armature  22  is attracted to the magnet housing  28  so that the friction disk  20  with the brake linings  10  and  12  is released for movement so that the driver  18  can move freely in rotation about the axis  3 . The electrically excited coil  4  therefore cancels the spring force of the spring  6  in the opened state of the brake, or counteracts it. As a result of wear, at least one airgap  26 ,  27  becomes larger so that the inductance of the coil  4  changes.  
         [0039]    [0039]FIG. 2 shows a permanent magnetic brake  40 . The function of the brake  6  in FIG. 1 is performed by a permanent magnet  42  in FIG. 2. Permanent magnet  42  is located on a magnet housing  28  which also holds an electrically excitable coil  4 . The electrically excitable coil  4  can be excited by means of the coil terminal  8 . The magnet housing  28  is bounded by an armature  21  which is connected to the driver  18  via at least one leaf spring  44  and at least one rivet  45 . In contrast to FIG. 1, the armature  21  in FIG. 2 can be entrained in rotation about the axis  3  together with the driver  18 . In such a permanent magnet brake, the force effect of the magnetic field which is generated by the permanent magnet  42  is used to generate a braking torque. In order to cancel the braking effect, i.e. to open the permanent magnet brake  40 , the magnetic field of the permanent magnet  42  is counteracted by the electromagnetic field when the coil  4  is excited. As a result of a braking process, wear occurs both at the magnet housing  28  and at the armature  21  in the wear region  46 . As a result of this wear, the airgap  26  becomes larger so that the inductance of the coil  4  changes, and thus the current profile also changes in the event of the coil  4  being fed with a changing voltage.  
         [0040]    [0040]FIG. 3 shows a clutch  50  in a coupled operating state in a view  59 , and a section  57 . Like the brakes  2 ,  40 , the clutch  50  also has a coil  4  within a magnet  0 housing  28 , the coil being locked within the magnet housing  28  by means of a locking means  52 . A magnet element  54  moves in rotation about the axis  3  and has a frictional lining  56 . When the clutch  50  is in the coupled state the frictional lining  56  is in frictionally locking contact with an armature  23 . In the uncoupled state, an airgap  26  is produced between the frictional lining  56  and the armature  23 . The fixed magnetic housing  28  is permanently connected to a fixed part, for example an electrical machine. The magnet element  54  is, for example, fixed in terms of rotation to the axis  3  by means of a shaft (not illustrated). The frictional lining  56  located at the end of the magnet element  54  closes as a result of the contact with the armature  23 . When the coil  4  is excited, the armature  23  is pressed against the friction lining  56  by the magnetic field which is generated, and is entrained in a frictionally locking fashion by the magnet element  54  as the armature  23  can also move in rotation about the axis  3 . Here too, as described above with the brakes  4 , a change in the size of the airgap  26  occurs as a result of wear so that it is possible to infer the wear from the profile of the current of the coil  4 .  
         [0041]    [0041]FIG. 4 shows a diagram  14  with a T axis on which a time t is to be entered, an I axis on which a current I is to be entered, and a U axis on which a voltage U is to be entered. The U axis and the I axis are perpendicular to the T axis. In the diagram  14 , both the current I through a coil  4  (known for example from FIG. 2) of a brake, and the voltage U across the coil  4  are plotted. At a time which is designated as the start of excitation  72 , a rated voltage UN is applied to the coil  4 . In the excited state of the coil  4  the brake is assumed to be open. The profile of the current U across the coil  4  is apparent from a voltage curve  74 . With the start of excitation  72 , the current I of the coil  4  rises. This is apparent from a current curve  76 . After the time period to, a voltage notch M 1  is apparent. At this time, the airgap  26  occurs. In the time period To, the brake is opened. At the time of the end of excitation  77 , the voltage U drops back from UN to zero. This results in a reduction in the coil current I. The current peak M 2  characterizes here the time at which the brake closes again and the airgap  26  disappears. The current peak indicates the time of contact between the elements which build up the frictional engagement. If the inductance of the coil  4  changes due to wear, the current notch M 1  and the current peak M 2  are shifted chronologically. It is possible to draw conclusions about the wear from a comparison between the actual time of the current notch M 1  or of the current peak M 2  with at least one reference time. By way of example it is possible to use both the voltage curve  74  and the current curve  76  with a threshold current value I 10%  (for example 10% of a maximum current I max ) as a starting point for counting time. The incrementation of the time during the measurement of current I and voltage U can be implemented by, for example, a timing counter or else by means of sampling frequency of the measurement. The sampling frequency should be selected with such a magnitude that the voltage notch M 1  or else the current peak M 2  are apparent.  
         [0042]    [0042]FIG. 5 shows a diagram with a time axis t and a U axis, with a channel CH 1  and CH 2  represented in a grid  60 . FIG. 5 is based on a representation on an oscillograph, the current profile of a brake without wear being based on a channel CH 1 , and the current profile of a brake with wear being shown on channel CH 2 . The current profile is assumed to be proportional to a voltage, and is thus plotted against the voltage U. A current notch M 11  characterizes, for channel CH 2 , the time at which an airgap occurs. It is apparent here that the current notch M 11  occurs chronologically before a current notch M 12  of the channel CH 1 . The wear of the brake which is represented in the current profile in channel CH 2  is greater than the wear of the brake, the current profile of which is represented in channel CH 1 . The signal profile in the channel CH 1  shows two current notches M 12  and M 32  which result from the fact that, for example during the formation of the airgap, initially a first region of an armature  21  is released from the magnet element  28  and only afterwards is the entire armature  21  released so that a sort of hinged effect is produced. If the airgap  26  occurs simultaneously in all the regions, only one current notch M 11  is produced, as illustrated in channel CH 2 .  
         [0043]    [0043]FIG. 6 shows a brake  66  for braking an electric motor  64 , a shaft  62  of which is also illustrated. The brake  66  is supplied with a feed voltage via the coil terminal  8 . A cable  65  leads from the coil terminal  8  to a means  67  for measuring the coil current. The coil current is evaluated using an evaluation means  68 . The brake  66  is supplied with a voltage by means of the exciter source  69  to which the cable  65  extends onward.