The invention relates to an electric wear-compensating readjustment apparatus of a disk brake, in particular for a motor vehicle.  The invention also relates to a corresponding disk brake and to a method for measuring and setting an air gap and measuring wear of a disk brake of said type. 
Vehicles, in particular motor vehicles, are equipped with friction brakes for the purposes of converting kinetic energy.  Here, in particular in the passenger motor vehicle sector and in the utility vehicle sector, the disk brake is preferred.  In the typical structural form of a disk brake, this is composed of a brake caliper together with an internal mechanism, of generally two brake pads, and of the brake disk.  Using a pneumatically actuated cylinder, the cylinder forces are introduced into the internal mechanism, are boosted by an eccentric mechanism, and are transmitted as an application force via threaded spindles to brake pads and brake disk, wherein the wear of brake disk and brake pads is compensated by the threaded spindles. 
The application forces act via both brake pads on the brake disk.  Since the pads are, in terms of construction, designed as wearing parts, they are generally softer than the brake disk, that is to say the pads undergo a change in pad thickness over their service life; they become worn.  The brake disk can also become worn.  This wear gives rise to the need for compensation for the wear in order to maintain a constant air gap between the brake pads and the brake disk.  A constant air gap is required in order to keep the response times of the brake short, to ensure the free movement of the brake disk, and to maintain a stroke reserve for limit load situations. 
An example of a wear-compensating readjustment apparatus is described in document DE 10 2004 037 771 A1.  Here, a drive rotational movement is transmitted for example from a torque-limiting device, for example with a ball ramp, via a continuously acting clutch (slipping clutch) to an adjustment spindle of a pressure plunger.  Here, the air gap is adjusted in continuous fashion. 
An example of a readjustment device is described in document DE 10 2012 108 672 B3. 
Here, it is considered to be a disadvantage that, in the case of a mechanical readjuster, too small an air gap or too large an air gap cannot be kept in the optimum range, and cannot be adapted in accordance with the situation.  Here, in the case of too small an air gap, so-called grinding with a grinding torque can occur, which can result in premature wear of brake pads and brake disk and also increased fuel consumption. 
Geometrically, a so-called design air gap is determined by taking into account the manufacturing tolerances of the components and a worst-case combination of the component dimensions, thermal expansion of the components and elastic deformation during operation owing to the temperatures and loads. 
DE10252301B4 describes an example for illustrating a disk brake with an electrically driven readjustment apparatus. 
It is the object of the present invention to provide an improved readjustment device which avoids or at least considerably avoids the abovementioned disadvantages. 
It is a further object to provide an improved disk brake. 
It is a yet further object to create an improved method for air gap measurement, air gap setting and wear measurement. 
An electric wear-compensating readjustment apparatus is created which has an output element with a clamping sleeve.  The clamping sleeve may be used for directly driving a threaded plunger of a disk brake in a simple manner for readjustment for compensation of friction surface wear. 
An electric wear-compensating readjustment apparatus according to the invention provides for readjustment for compensation of friction surface wear on brake pads and brake disk of a disk brake of a motor vehicle.  The readjustment apparatus includes an electric motor; a gearing; and an output element.  The output element is connected rotationally conjointly to a clamping sleeve which has inwardly projecting longitudinal ribs on its inner side.  The motor vehicle may in particular be a utility vehicle. 
Alternatively, in one embodiment, in the case of a disk brake with two plungers, the gearing output can be placed directly in engagement with the sprocket of the synchronization unit.  The sprocket may be understood here to be a synchronizing wheel.  Here, the gearing output is connected in positively locking fashion in a circumferential direction to the sprocket.  Owing to the positively locking connection, the rotational movement introduced from the readjustment unit can be transmitted to the synchronization unit and the spindle.  The connection between the gearing output and sprocket may in this case be designed with radial play in order to compensate for movements of the bridge/crossmember of up to 1° during operation. 
In the alternative embodiment, an electric wear-compensating readjustment apparatus according to the invention provides for readjustment for compensation of friction surface wear on brake pads and brake disk of a disk brake.  The readjustment apparatus includes an electric motor, a gearing and an output element.  The output element is rotationally conjointly connected directly to a synchronizing wheel, wherein the synchronizing wheel is designed for rotationally conjoint engagement with a threaded spindle of the associated disk brake. 
A variant of this embodiment is distinguished by the fact that the output element is formed in one piece with the synchronizing wheel, whereby a number of parts is reduced. 
A disk brake according to the invention having the wear-compensating readjustment apparatus described above preferably is actuated by compressed air, in particular for a motor vehicle, having an application apparatus, preferably having a brake rotary lever having at least one spindle unit with in each case one threaded plunger, and has at least one electric wear-compensating readjustment apparatus with a control unit. 
The result is a space-saving, compact and lightweight wear-compensating readjustment apparatus, which can advantageously be installed on a threaded plunger in a simple manner.  The threaded plunger may have longitudinal grooves with which the longitudinal ribs of the synchronizing wheel can be placed in engagement in a simple manner.  The synchronizing wheel is furthermore in engagement, by a radial, positively locking connection, with the clamping sleeve, which permits longitudinal displaceability between synchronizing wheel and clamping sleeve.  This may be realized for example by virtue of the clamping sleeve being in engagement with the synchronizing wheel by balls in a ball cage, wherein the synchronizing wheel is in engagement with the threaded spindle. 
For this purpose, it is provided that the clamping sleeve is formed, on its free end, with an engagement section, which forms a constituent part of a rotationally conjoint coupling of the clamping sleeve to a first synchronizing wheel of a synchronizing unit of the associated disk brake. 
A method according to the invention for air gap measurement and air gap setting of the above-described disk brake includes the method steps of: (S20) providing a wear-compensating readjustment apparatus having an electric motor and a gearing; (S21) measuring the present air gap and setting a new air gap of the disk brake; and (S22) communicating with further wear-compensating readjustment apparatuses of an associated vehicle via data interfaces, and monitoring and performing plausibility checks on the measurements and settings. 
A further method according to the invention for measuring friction-induced wear of brake pads and of a brake disk of the above-described disk brake includes the method steps of: (S23) defining an initial position of the brake pad and storing the initial position in the control unit; (S24) measuring the air gap; (S26) calculating the friction-induced wear on the basis of the measured values of the air gap with the overall rotational angle αoverall, the gearing transmission ratio i, the specified air gap LS and spindle pitch p in accordance with the following calculation rule:Xwear=xinitial−αoverall*i*p+LS; (S27) deciding whether a pad change is necessary by comparing the calculated friction-induced wear with a predefined value, wherein, if a pad change is not yet necessary, the method returns to the method step (S25); and (S28) generating an item of information regarding the required pad change, and displaying and/or communicating said item of information. 
In one embodiment of the two described methods, the step (S21) of measuring the present air gap and setting a new air gap of the disk brake of the method for air gap measurement and air gap setting and the step (S24) of measuring the air gap of the method for measuring friction-induced wear of brake pads and of a brake disk correspond. 
Here, it is particularly advantageous that operational values of the electric motor can be taken into consideration as parameters for the calculation of the air gap and of a contact point. 
The air gap can be reduced to a necessary minimum.  Grinding torques can be ruled out to a considerable extent.  A service life of brake pads and brake disk can be advantageously lengthened. 
In one embodiment, a motor shaft of the electric motor, with a drive pinion, and the output element may be arranged coaxially.  In this way, only a minimal structural space is taken up. 
Furthermore, in another embodiment, it is provided that the gearing may be a two-stage planetary gearing.  In this way, a high transmission ratio is realized, whereby, at the same time, a high degree of self-locking against rotation of the threaded plunger of its own accord as a result of vibrations is possible. 
In another embodiment, a housing of the gearing may have a fastening section for positionally fixed fastening in a brake caliper of the associated disk brake, and may have a gearing section in which the two-stage planetary gearing is arranged.  This yields a compact construction. 
Furthermore, in a further embodiment, the fastening section and the gearing section of the gearing may be formed in one piece, wherein the gearing section may be formed with an internal toothing as an internal gear of the two-stage planetary gearing.  A high level of functionality is thus possible in a small structural space. 
In a yet further embodiment, it is provided that a first gearing stage of the gearing is in engagement with the drive pinion of the electric motor as sun gear, wherein a second planet carrier of the second gearing stage of the gearing may be a constituent part of the output element.  A compact design can thus be made possible. 
In a yet further embodiment, it is provided that the gearing may be a three-stage gearing.  In this way, a very high transmission ratio is realized, whereby the power demands on the electric motor can be reduced.  Expressed more generally, the gearing may be a multi-stage gearing.  Accordingly, the gearing can be designed correspondingly to the electric motor and to the power demand. 
If the electric motor has a high cogging torque, a resistance to undesired adjustment of the threaded plunger owing to vibrations can be increased. 
In another embodiment, the electric motor may be an electronically commutated (EC) electric motor.  In this way, rotational speed and torque of the electric motor can be adjustable over wide ranges, and adaptable in accordance with the situation, with high quality using commercially available, inexpensive controllers. 
In a further embodiment of the disk brake, the wear-compensating readjustment apparatus may be arranged above the threaded plunger, wherein the clamping sleeve is mounted onto the threaded plunger of the at least one spindle unit of the disk brake and at least partially encloses the threaded plunger.  In this way, an advantageously compact construction can be made possible. 
In one alternative, the disk brake furthermore has a first spindle unit with a first threaded plunger and a second spindle unit with a second threaded plunger.  The threaded plungers are screwed into a crossmember, the crossmember interacts with the application apparatus (preferably with the brake rotary lever) and a synchronizing device with synchronizing wheels arranged in the crossmember.  The wear-compensating readjustment apparatus may be arranged above the first threaded plunger of the first spindle unit, where the clamping sleeve is mounted onto the first threaded plunger of the first spindle unit and at least partially encloses the first threaded plunger, and a free end of the clamping sleeve is rotationally conjointly coupled to the first synchronizing wheel by a coupling section in a first synchronizing wheel of the synchronizing unit.  This yields a compact construction. 
In a further embodiment of the disk brake, it is provided that the inwardly projecting ribs of the clamping sleeve of the wear-compensating readjustment apparatus are in engagement with longitudinal grooves of the first threaded plunger.  In this way, a rotationally conjoint connection between clamping sleeve and threaded plunger is advantageously made possible, wherein the threaded plunger is guided so as to be displaceable relative to the clamping sleeve. 
In one alternative, a disk brake for a motor vehicle, actuated preferably by compressed air, includes an application apparatus (preferably having a brake rotary lever), at least one electric wear-compensating readjustment apparatus with a control unit, a first spindle unit with a first threaded plunger and a second spindle unit with a second threaded plunger.  The threaded plungers are screwed into a crossmember which interacts with the application apparatus and a synchronizing device with synchronizing wheels arranged in the crossmember.  The wear-compensating readjustment apparatus, which is described above as an alternative, is arranged above the first threaded plunger of the first spindle unit, where the output element at least partially encloses the first threaded plunger, and a coupling section of the output element is rotationally conjointly connected to a first synchronizing wheel of the synchronizing unit or is formed in one piece with said first synchronizing wheel. 
In the method for air gap measurement and air gap setting, it is provided that the method step (S21) has the following sub-steps: (S210) applying the brake pad by activation of the electric motor in a first direction of rotation using the control unit; (S211) applying the brake pad until the brake pad makes contact with the brake disk at a contact point; (S212) detecting said contact point or said contact by detecting an increase in current of the electric motor, by a force sensor, by a spacing sensor and/or by a temperature sensor for detecting a change in temperature on the surface of the brake disk and/or of the brake pad; (S213) stopping the rotational movement of the electric motor after detection of the contact point, and calculating the air gap using system parameters (rotational steps, rotational angle, rotational angle position, axial position, gearing transmission ratio); (S214) activating the electric motor in a second direction of rotation which is opposite to the first direction of rotation, and adjusting the brake pad away from the brake disk into a position that corresponds to the calculated air gap; (S215) storing the deviation or the rotation performed, both clockwise and counterclockwise (readjustment), in order to calculate the overall rotational angle for the purposes of sensing wear. 
In one embodiment of the method for wear measurement, it is provided that, in the method step (S24) of measuring the air gap, the method step of measuring the air gap is performed in accordance with the above-described method for air gap measurement and air gap setting with a defined frequency. 
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.