fig1 shows an electromechanically operable parking brake which basically includes a per se known drum brake 1 and an actuating unit 2 in a housing 3 . the actuating unit 2 is in a force - transmitting connection to an expanding lock 5 which mechanically actuates two brake shoes . only one of the brake shoes is shown in fig1 designated by reference numeral 6 . the brake shoes 6 are moved into engagement with a brake drum 7 when the brake is locked . the actuating unit 2 comprises an electric motor 8 , a reduction gear 9 , and a force - transmitting element 10 which is coupled to the expanding lock 5 by way of a steel cable line 14 . the steel cable line 14 has a ( non - illustrated ) drawbar eye at its end close to the expanding lock , while its other end is press - fitted or squeezed in the force - transmitting element 10 . the electric motor 8 shown in the embodiment of fig1 is realized in the form of an electronically commutated motor , yet other designs are also possible ( for example , a brush motor operated by direct current ). a stator 11 of the electric motor 8 is immovably arranged in the housing 3 of the actuating unit 2 , and its rotor 12 is favorably designed as a tubular sheet - metal part , onto the surface of which permanent magnet segments 13 are cemented . in this case , the end of the rotor 12 which faces the brake drum 7 is supported in a fixed bearing 18 , and its end which is averted from the brake drum 7 is supported in a movable bearing 19 . the reduction gear 9 is preferably arranged coaxially to the rotor 12 and radially encompassed by the rotor . in the embodiment shown , the reduction gear 9 is designed as a self - locking spindle drive , the spindle 16 of which forms the force - transmitting element 10 , and the spindle nut 17 of which is formed by a tubular part which is arranged coaxially to the rotor 12 and designed integrally with the rotor 12 in a preferred fashion . an axial tubular extension 20 of the housing 3 which extends into the interior of the rotor 12 serves as an antirotation mechanism of the spindle 16 . the extension 20 has a polygonal inner profile that cooperates with the matingly shaped end of the spindle 16 . a bearing cover 21 in which the movable bearing 20 is arranged defines in the housing 3 of the actuating unit 2 an annular hollow space 22 which can accommodate , for example , an electronic control unit 26 . also , a cable duct 23 is provided in the bearing cover 21 through which a preferably extrusion - coated connecting line 24 can extend which leads to the stator 11 of the electric motor 8 and / or to the electronic control unit . in an embodiment not shown in fig1 the reduction gear can be designed as a non - self - locking spindle drive or a ball screw , respectively . the ball screw can include a threaded spindle and a ball screw nut that is preferably pressed into the rotor . the rotational movement of the ball screw nut is converted into a translational movement of the threaded spindle by means of rows of balls . a locking mechanism which prevents loosening of the parking brake can be provided in addition . in another embodiment not shown , the electric motor can be a d . c . brush motor , the stator of which is formed by permanent magnet segments arranged in the housing of the actuating unit . the rotor that receives a current by way of a collector is formed by a tube which is supported at two points and carries an armature winding . the end of the tube close to the brake drum is realized in the form of a sun wheel with a planetary gear that is arranged between the electric motor and the reduction gear in terms of effect . the sun wheel drives planet wheels which revolve in a ring gear that is machined into the inner side of the housing and forms part of said housing . the electronic control unit 26 can e . g . include in a known fashion a pulse width modulator which generates a pulse - width modulated corrective signal for the d . c . brush motor wherein the information about the desired corrective signal is contained in the pulse width . the current variation against time supplied to the electric motor by the control unit will be explained hereinbelow with reference to fig2 . other embodiments for the actuator or electric motor and / or the reduction gear which are not illustrated or described are easily possible . instead of an electric motor , a magnet can be used as well which , for locking the brake , draws an armature plate blocked in its return movement , for example . fig2 shows a flow chart in which the nominal current i soll which is to be impressed on the electric motor by the control unit , the actual value of the current i ist and the locking force f are plotted on the abscissa , and the time is plotted on the ordinate . initially , the control unit impresses on the electric motor a current i soll which is limited in the rate of current ( 10 ampere , for example ). instead of a predetermined rate of current , a pulse - width modulated corrective signal can be furnished in the case of a d . c . motor having a pulse width which is limited accordingly towards the top . this current i soll is delivered until the load , that is the parking brake , is actuated up to a holding value at a defined locking force f 1 . the duration can be prestored in the control unit for the envisaged type of brake , for example , or it can be measured by a suitable sensing means ( not shown ). in the case of motor control , the duration may also be derived from the variation of the actual value of the current i ist supplied . among other things , this will be explained in the following . shortly after application of the current for starting the electric motor , the actual value i ist of the current is at its maximum until the electric motor has reached a defined acceleration or speed . after the drive torque has been overcome , the motor load as well as the actual current value i ist will drop . both values will subsequently rise again with increasing locking force f . finally , the actual current value i ist reaches its maximum value again which is fixed by a current limiter provided in the control unit . the locking force rises still further by continuous delivery of a current i soll at the maximum rate of current until finally the locking force f 1 at the holding value is reached . at this point of time ( for example , 500 msec after start of the electric motor ) the current supply is interrupted ( for example , for approximately 50 msec ). subsequently , a current i soll is delivered again for a short time ( for example , for approximately 20 msec ). during this time the electric motor is again accelerated from standstill and transmits a torque to the parking brake which makes the locking force rise further . thereafter , the sequence composed of interruption and repeated current supply is repeated several times . in general , it can be taken from fig2 that the actual value i ist of the current slightly lags behind the nominal value i soll . further , the number of current interruptions or reductions shown in fig2 is not final . depending on the case of requirement , a greater or smaller number of current interruptions may be necessary . as can be seen from fig2 the locking force continues to rise during each sequence even after the current delivery phase is interrupted until it adopts a constant value during the interruption period . this ‘ lagging ’ increase is due to the accelerated masses . the periods of time for the interruption and the renewed current delivery can be optimized with respect to the respectively prevailing locking force . thus , a bottom limit for the time of the renewed current delivery can be seen in the response time which it takes the electric motor to put its movable mass and that one of the transmission unit into motion . with the locking force rising , the times for a renewed current delivery can be shortened because the velocity amplitudes of the electric motor will also drop due to the decreasing clearances on parts of the drive . under certain circumstances , the locking force can be measured in this respect . the times for the interruptions can also be adapted accordingly , for example , with respect to the time that is necessary for tension relief of the material , which latter time can depend on the locking force . in the example shown in the embodiment of fig2 a final value of the locking force f 2 is reached by three times repeating the interruption sequence . compared to control units of the state of the art , the repeated interruption and renewed current delivery according to the present invention permits achieving locking forces in a magnitude which cannot be reached in prior art methods with an uninterrupted current delivery , not even with substantially higher currents . as has been indicated hereinabove , a possible explanation for this phenomenon can be seen in the clearances on mating parts of the drive ( for example , the planetary gear , etc .) in a combination with the starting torque of the electric motor so that a pulsed further rotation of the drive ( for example , the spindle drive ) permits great locking forces or cable forces ( e . g . in excess of 3000 n ). in locked brakes , the method of the present invention can also be used for the release of the parking brake in case an increased starting torque is necessary , e . g . after a long immobilization time . preferrably , inexpensive power transistors can be used in the control unit which do not have to generate high currents due to the current limitation . especially with respect to an emergency operation of the parking brake by way of a back - up battery , the control unit may have a current limiter which limits the current towards the top so that , different from the state of the art , back - up batteries can be employed which have a greatly reduced storage capacity . this permits drastically reducing the total costs for the control unit and the back - up battery ( for example , a lithium battery ).