Abstract:
An electromechanical brake is configured to detect a change in motor characteristics through a parking mechanism added to an existing electromechanical brake. Particularly, the electromechanical brake controls braking force by predicting a change in motor characteristics even in the event of a breakdown of a force sensor or even in a case in which no force sensor is utilized.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims under 35 U.S.C. §119(a) the benefit of Korean Patent Application No. 10-2016-0056669 filed on May 9, 2016, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
     (a) Technical Field 
       [0002]    The present invention relates to an electromechanical brake, more particularly, to an electromechanical brake capable of detecting a change in characteristics of a motor for the electromechanical brake. 
       (b) Description of the Related Art 
       [0003]    In general, an electromechanical brake (EMB) has a higher response speed than a hydraulic brake, and may be precisely controlled, and thus the electromechanical brake may improve braking safety performance, and is essentially used for brake-by-wire. 
         [0004]    The electromechanical brake produces braking force by operating a motor and mechanically transmitting the rotational force of the motor. The electromechanical brake such as an electric parking brake (EPB) has been widely applied to a vehicle, and its use has been expanded because of the development of the electromechanical brake used for a main brake, which is substituted for a hydraulic brake in the related art. In particular, an electromechanical brake, which integrally implements functions of the electric parking brake, has been developed. 
         [0005]    The electromechanical brake is operated by various sensors and electrical devices without being mechanically connected with a driver, and it is necessarily required to ensure stability against a breakdown of such devices. 
         [0006]    For example, the electromechanical brake in the related art is provided with a force sensor in order to control braking force. However, the electromechanical brake including the force sensor has problems in that the force sensor is expensive, and in the event of a breakdown of the force sensor, it is impossible to control braking force because a change in motor characteristics cannot be predicted. 
         [0007]    The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
       SUMMARY 
       [0008]    The present invention provides an electromechanical brake capable of controlling braking force by predicting a change in motor characteristics even in the event of a breakdown of a force sensor or even in a case in which no force sensor is utilized. 
         [0009]    In one aspect, the present invention provides an electromechanical brake configured to detect a change in motor characteristics, the electromechanical brake including: a motor unit which rotates a driving shaft; a braking unit which produces braking force or cuts off the braking force in accordance with the rotation of the driving shaft; a parking drive unit which operates a driving rod; a parking ring which is freely and rotatably installed on the driving shaft and has catching portions formed radially; a rotatable member which is installed on the driving shaft so as to be rotatable alone or together with the parking ring in accordance with the rotation of the driving shaft; and an elastic body which is elastically deformed in accordance with a relative displacement difference between the rotatable member and the parking ring. 
         [0010]    In a preferred embodiment, the rotatable member may have a protruding portion that protrudes radially, a restricting portion may be formed on one surface of the parking ring along a movement trajectory of the protruding portion on the rotatable member, and the parking ring and the rotatable member may be rotated together when the protruding portion presses the restricting portion. 
         [0011]    In another preferred embodiment, the parking ring may be installed on the driving shaft below the rotatable member, and the restricting portion may have an arc shape protruding from an upper surface of the parking ring. 
         [0012]    In still another preferred embodiment, the protruding portion may be configured as a pair of protruding portions that face each other, the restricting portion may be configured as a pair of restricting portions that face each other, and the protruding portions may be configured to be moved between the pair of restricting portions in accordance with the rotation of the rotatable member. 
         [0013]    In yet another preferred embodiment, one end portion of the elastic body may be fixed to the parking ring, and another end portion of the elastic body may be fixed to the rotatable member, such that a relative displacement difference occurs between the parking ring and the rotatable member. 
         [0014]    In still yet another preferred embodiment, the elastic body may be a torsion spring. 
         [0015]    In a further preferred embodiment, one end portion of the elastic body may be fixed to the protruding portion, another end portion of the elastic body may be fixed to the restricting portion, and the elastic body may be elastically deformed due to a relative displacement difference that occurs when the protruding portion is moved between the pair of restricting portions. 
         [0016]    In another further preferred embodiment, the driving rod may move to restrict the rotation of the parking ring while coming into contact with the catching portions of the parking ring, and the rotatable member may elastically deform the elastic body while rotating alone in a state in which the rotation of the parking ring is restricted. 
         [0017]    In still another further preferred embodiment, the elastic body may be fixed to the rotatable member and the parking ring in a state in which the elastic body is in a no-load state or a compressed state. 
         [0018]    In yet another further preferred embodiment, the electromechanical brake may further include a sensor unit which detects a rotation angle and motor electric current of the motor unit. 
         [0019]    In still yet another further preferred embodiment, the electromechanical brake may further include a control unit which determines a motor torque constant corrected based on the rotation angle and the motor electric current detected by the sensor unit. 
         [0020]    In a still further preferred embodiment, the catching portions may be protrusions formed radially. 
         [0021]    In a yet still further preferred embodiment, the protrusion may have a serrated shape having a first surface and a second surface, and the first surface may have a smaller area than the second surface. 
         [0022]    In a yet still further preferred embodiment, the catching portions may have the same shape and size. 
         [0023]    In a yet still further preferred embodiment, the electromechanical brake may further include: a parking rod which is hingedly coupled to the driving rod and has a slot; and a guide pin which is inserted into the slot of the parking rod and fixed to a housing. 
         [0024]    In a yet still further preferred embodiment, the slot may have a track shape, the guide pin may have a circular cross section, and a width of the slot may be set such that the guide pin is not moved in a width direction. 
         [0025]    In a yet still further preferred embodiment, the electromechanical brake may further include: a driving gear which is installed on the driving shaft; and a reduction gear unit which is connected with the driving gear. 
         [0026]    In a yet still further preferred embodiment, the electromechanical brake may further include: a driving gear which is installed on the driving shaft; and a driven gear which is connected with the driving gear, in which the parking ring, the rotatable member, and the elastic body are installed on a shaft of the driven gear. 
         [0027]    According to the present invention, it is possible to omit an expensive force sensor, thereby reducing manufacturing costs of the electromechanical brake. 
         [0028]    It is also possible to check that clamping force of the electromechanical brake is erroneously detected, and as a result, it is possible to detect abnormality of the sensor, thereby improving system durability. 
         [0029]    In the case of the electromechanical brake including a force sensor, it is possible to control braking force even in the event of a breakdown of the force sensor, thereby improving stability against a breakdown. 
         [0030]    Other aspects and preferred embodiments of the invention are discussed infra. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated in the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein: 
           [0032]      FIG. 1  is a view schematically illustrating an entire configuration of an electromechanical brake according to an exemplary embodiment of the present invention; 
           [0033]      FIGS. 2A and 2B  are views illustrating internal components of an electromechanical brake according to a first exemplary embodiment of the present invention; 
           [0034]      FIGS. 3A to 3E  are views illustrating operations of the internal components of the electromechanical brake according to the first exemplary embodiment of the present invention; 
           [0035]      FIGS. 4A to 4D  are views sequentially illustrating states in which the electromechanical brake according to the first exemplary embodiment of the present invention is operated at the time of parking a vehicle; 
           [0036]      FIGS. 5A to 5C  are views sequentially illustrating states in which the electromechanical brake according to the first exemplary embodiment of the present invention is operated in accordance with a mode for estimating motor characteristics; 
           [0037]      FIGS. 6A and 6B  are graphs illustrating a relationship between a motor rotation angle and a motor electric current, in which  FIG. 6A  illustrates an initial state in which a spring is not compressed, and  FIG. 6B  illustrates an initial state in which a spring is compressed; 
           [0038]      FIGS. 7A and 7B  are views illustrating internal components of an electromechanical brake according to a second exemplary embodiment of the present invention; and 
           [0039]      FIGS. 8A to 8C  are views illustrating states in which the electromechanical brake according to the second exemplary embodiment of the present invention is operated in accordance with a mode for estimating motor characteristics. 
       
    
    
       [0040]    In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
       DETAILED DESCRIPTION 
       [0041]    It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles. 
         [0042]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Throughout the specification, unless explicitly described to the contrary, the word “comprise” and variations such as “comprises” or “comprising” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. In addition, the terms “unit”, “-er”, “-or”, and “module” described in the specification mean units for processing at least one function and operation, and can be implemented by hardware components or software components and combinations thereof. 
         [0043]    Further, the control logic of the present invention may be embodied as non-transitory computer readable media on a computer readable medium containing executable program instructions executed by a processor, controller or the like. Examples of computer readable media include, but are not limited to, ROM, RAM, compact disc (CD)-ROMs, magnetic tapes, floppy disks, flash drives, smart cards and optical data storage devices. The computer readable medium can also be distributed in network coupled computer systems so that the computer readable media is stored and executed in a distributed fashion, e.g., by a telematics server or a Controller Area Network (CAN). 
         [0044]    Hereinafter reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings and described below. While the invention will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention to those exemplary embodiments. On the contrary, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
         [0045]    The present invention provides an electromechanical brake which is provided with a parking mechanism and configured to estimate motor characteristics based on a change in electric current when a spring included in the parking mechanism is compressed. In the present invention, the parking mechanism means a mechanism which is added such that the electromechanical brake may function as an electric parking brake (EPB). Therefore, the parking mechanism according to the present invention excludes typical components of the electromechanical brake such as a motor, and a reduction gear unit, but includes a parking ring, a parking rod, and the like which will be described below. Further, the parking mechanism according to the present invention includes an elastic body such as a spring in order to estimate motor characteristics. 
         [0046]    Hereinafter, an electromechanical brake capable of detecting a change in motor characteristics according to an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
         [0047]      FIG. 1  is a view schematically illustrating an entire configuration of the electromechanical brake according to the exemplary embodiment of the present invention. 
         [0048]    As shown in  FIG. 1 , the electromechanical brake according to the exemplary embodiment of the present invention includes a motor unit  110 , a driving gear  120  which is connected to a driving shaft of the motor unit  110 , a reduction gear unit  130  which is connected to the driving gear  120 , and a braking unit  200  which is operated by a spindle  140  connected to the reduction gear unit  130 . All or at least some of these components may be mounted in a housing  300 . 
         [0049]    Specifically, the present exemplary embodiment includes the motor unit  110  for providing braking force for braking a vehicle, and a parking drive unit  160  which is operated to support braking force at the time of parking the vehicle. In addition, the motor unit  110  and the parking drive unit  160  are operated by an operation command from a control unit  191 . In the present invention, the motor unit  110  may be configured to be rotatable in two directions in order to provide braking force and eliminate braking force. Meanwhile, the parking drive unit  160  according to the present invention may be configured to provide unidirectional or bidirectional driving power. The driving power provided by the parking drive unit  160  moves a driving rod  161 . 
         [0050]    The motor unit  110  is configured to be operated by being supplied with electric power from a driving power source such as a battery in accordance with an operation command from the control unit  191 . The driving gear  120  is mounted on the driving shaft of the motor unit  110 , and the driving gear  120  is configured to transmit rotational force, which is provided by the motor unit  110 , to the spindle  140  for operating the braking unit  200 . In particular, the driving gear  120  is connected to the reduction gear unit  130 , and rotational force of the motor unit  110  is transmitted through the reduction gear unit  130 , such that the spindle  140  is rotated. The reduction gear unit  130  is a component for slowing down the rotation of the driving shaft by the drive motor to an appropriate level, and may be configured by combining a plurality of gears. 
         [0051]    The spindle  140  rotates to allow the braking unit  200  to produce braking force, and particularly, the spindle  140  may be configured in the form of a ball screw or a typical screw. The braking unit  200  includes brake pads  220  and a disc  210 , and is configured such that a movable member (not shown) thread-coupled to the spindle  140  presses the brake pads  220  against the disc  210  while moving. 
         [0052]    According to a braking operation of the electromechanical brake according to the present invention, at the time of braking the vehicle, when the control unit  191  receives a driver&#39;s intention of braking the vehicle, the control unit  191  allows the driving power source such as a battery to apply electric power to the motor unit  110 , such that the motor is operated, and the braking operation is started. When electric power is applied to the motor unit  110  and the driving shaft is rotated in a forward direction, the driving gear  120  connected to the motor unit  110  is operated, and rotates the spindle  140  via the reduction gear unit  130 . 
         [0053]    The rotational motion of the spindle  140  is converted into the rectilinear motion through a structure such as a ball screw, and thus the brake pads  220  press the disc  210  so as to perform the braking operation. 
         [0054]    Meanwhile, at the time of stopping the braking operation, the motor unit  110  is rotated in a reverse direction opposite to the aforementioned forward direction. For example, on the assumption that the forward direction is a clockwise direction, the driving shaft is rotated counterclockwise so as to stop the braking operation. 
         [0055]    At the time of stopping the braking operation, the same process of transmitting driving power of the motor to the braking unit  200  via the spindle  140  is carried out, but because the motor is rotated in the opposite direction, the brake pads  220  do not press the disc  210 , but move in a direction in which the braking force is released, that is, a direction in which the brake pads  220  move away from the disc  210 . 
         [0056]    The present invention may be configured to further include a sensor unit  192  for detecting a rotation angle and a motor electric current of the motor unit. In this case, the control unit is used to determine a motor torque constant which is corrected by detecting a change in motor characteristics. In particular, the control unit may be configured to determine a motor torque constant which is corrected based on the rotation angle and the motor electric current detected by the sensor unit  192 . 
         [0057]    Meanwhile, a configuration and an operation of an electromechanical brake according to a first exemplary embodiment of the present invention for detecting a change in motor characteristics will be described with reference to  FIGS. 2 to 4 . 
         [0058]      FIG. 2A  is a view illustrating internal components of the electromechanical brake according to the exemplary embodiment of the present invention, and  FIG. 2B  is a view illustrating the parking mechanism of which the portion illustrated in  FIG. 2A  is enlarged.  FIGS. 3A to 3E  illustrate states in which the parking mechanism according to the present exemplary embodiment is operated. 
         [0059]    The electromechanical brake according to the first exemplary embodiment of the present invention has a structure in which rotation of a parking ring  150  connected to the driving shaft of the motor may be inhibited by the parking drive unit  160  which is operated in two directions. 
         [0060]    As shown in  FIG. 2A , the driving gear  120  and the reduction gear unit  130  are accommodated in the housing  300 . The driving gear  120  is mounted on the driving shaft of the motor unit  110 , and the parking ring  150  and a rotatable member  170  are installed on the driving shaft. 
         [0061]    The parking ring  150  has a structure in which the rotation of the parking ring  150  may be restricted by the driving rod  161  of the parking drive unit  160 . 
         [0062]    The parking drive unit  160  is configured to perform the braking operation for parking the vehicle and stop the braking operation, and may be configured as a bidirectional solenoid that may operate the driving rod  161  in two directions. The solenoid is used to move the driving rod  161  so as to maintain the applied braking force and prevent the braking force from being eliminated. Therefore, the solenoid includes the driving rod  161  which moves in the two directions by being supplied with electric power. In particular, according to the first exemplary embodiment, the driving rod is moved in a direction as shown in  FIG. 4B  at the time of parking the vehicle, and the driving rod is moved in the opposite direction at the time of releasing a parking brake. 
         [0063]    The parking ring  150  is installed to be freely rotatable on the driving shaft of the motor. In contrast, the rotatable member  170  is configured to be rotated together with the driving shaft of the motor, and configured to be able to transmit driving power of the motor to the parking ring  150 . The position where the parking ring  150  is mounted in an example shown in  FIG. 2B  is one preferred example, and the position is not particularly limited as long as the parking ring  150  is rotatably mounted on the driving shaft of the motor unit. Therefore, the rotatable member  170  is installed to be rotatable alone or together with the parking ring  150  when the driving shaft is rotated. 
         [0064]    As shown in  FIG. 3A , according to the specific structure in the parking mechanism, protruding portions  171 , which protrude radially, are formed on the rotatable member  170 , and restricting portions  151  are formed on one surface of the parking ring  150  along a movement trajectory of the protruding portions  171  on the rotatable member  170 . 
         [0065]    Therefore, the parking ring  150  is not rotated by being synchronized with the rotation of the driving shaft of the motor, but may be selectively rotated only when the protruding portions  171  come into contact with the restricting portions  151  and the parking ring  150  may receive rotational force. Therefore, when the protruding portions  171  on the rotatable member  170  press the restricting portions  151  of the parking ring  150 , the parking ring  150  and the rotatable member  170  are rotated together. 
         [0066]    An elastic body is installed on the parking ring  150  and the rotatable member  170 . One end portion of the elastic body is fixed to the parking ring  150 , and another end portion of the elastic body is fixed to the rotatable member  170 . Therefore, the elastic body stores a difference in relative rotational displacement between the rotatable member  170  and the parking ring  150  as elastic force. In particular, the elastic body may be a torsion spring  180 , one end of the torsion spring  180  may be fixed to the restricting portion  151  of the parking ring  150 , and another end of the torsion spring  180  may be fixed to the protruding portion  171  of the rotatable member  170 . Thus, when relative displacement occurs between the rotatable member  170  and the parking ring  150 , for example, when only the rotatable member  170  is rotated while the parking ring  150  is not rotated, torsional displacement occurs as the torsion spring  180  is pressed. 
         [0067]    For example, the parking ring  150  is installed on the driving shaft below the rotatable member  170 , and the restricting portion  151  may have an arc shape that protrudes from an upper surface of the parking ring  150 . 
         [0068]    The protruding portions  171  may be configured as a pair of protruding portions  171  that face each other, and the restricting portions  151  may be configured as a pair of restricting portions  151  that face each other. In this case, the protruding portions  171  are moved between the pair of restricting portions  151  when the rotatable member  170  is rotated, and during this process, the elastic body is compressed. 
         [0069]    That is, in the present exemplary embodiment, when the driving rod restricts the rotation of the parking ring  150  while coming into contact with a catching portion of the parking ring  150 , the rotatable member  170  causes a rotational displacement difference from the parking ring  150  while rotating alone, thereby elastically deforming the elastic body. 
         [0070]    Meanwhile, in the present exemplary embodiment, the torsion spring  180  serves to transmit rotational force of the rotatable member  170  to the parking ring  150 . That is, in a case in which the torsion spring  180  is set in a no-load state (normal setting in which the spring is not compressed) or in a compressed state (setting in which the spring is compressed in an initial state), the parking ring  150  and the rotatable member  170  are rotated together because the parking ring  150  is connected with the rotatable member  170  through the torsion spring  180 . 
         [0071]    For example, when the driving shaft is rotated counterclockwise in an initial state as shown in  FIG. 3A , the parking ring  150  is also rotated together with the rotatable member  170  as shown in  FIG. 3B . 
         [0072]    In contrast, in a case in which the driving rod restricts the rotation of the parking ring  150  as shown in  FIG. 3C , the parking ring  150  is not rotated, but only the rotatable member  170  is rotated as shown in  FIG. 3D . As the rotatable member  170  is rotated, the parking ring  150  and the rotatable member  170  move relative to each other, and the torsion spring  180  is compressed. The rotatable member  170  is rotated until the protruding portion  171  comes into contact with the restricting portion  151  of the parking ring  150  ( FIG. 3E ). 
         [0073]    An initial state is set which includes a relative position relationship among the rotatable member  170 , the parking ring  150 , and the torsion spring  180 . In this initial state, the restricting portion  151  of the parking ring  150  and the protruding portion  171  of the rotatable member  170  may be set to be in contact with each other, and the torsion spring  180  may be set in the no-load state or the compressed state. 
         [0074]    Meanwhile, the parking ring  150 , which is operated in conjunction with the driving rod  161 , has a catching structure in which the driving rod  161  may come into contact with the parking ring  150  so as to restrict the rotation of the driving shaft. Therefore, in the present invention, the parking ring  150  may be variously implemented as long as the parking ring  150  has a structure that may be caught by the driving rod  161  so as to restrict the rotation. For example, the parking ring  150  may be structured to have a circular body with a groove formed therein, and particularly, may be structured to have outwardly protruding protrusions as shown in  FIG. 2 . 
         [0075]    In the present specification, the catching structure of the parking ring  150  is referred to collectively as a catching portion, and hereinafter, an example in which the catching portion is implemented in the form of a protrusion will be described. 
         [0076]    The parking ring  150  provided with a plurality of protrusions is mounted on the driving shaft. The parking ring  150  is rotated together with the driving shaft of the motor unit  110 . The protrusions of the parking ring  150  are radially disposed on a ring-shaped circular body, and particularly, configured as a plurality of protrusions having the same size and shape which covers regions equally divided, respectively. 
         [0077]    According to the exemplary embodiment of the present invention, the protrusion has a first surface A having a relatively small area, and a second surface B having a relatively large area. The first surface A is a surface approximately directed toward a center of the parking ring  150 , and the second surface B defines a surface that abuts on the circular body of the parking ring  150 . Therefore, as shown in  FIGS. 2B and 3A , the protrusions have a plurality of serrated structures radially disposed on the circular body. Meanwhile, in  FIG. 1 , the parking ring  150  is illustrated as being mounted at a lower side of the driving gear  120 , that is, at a side close to the motor, but a relative mounting position may be changed. 
         [0078]    According to the exemplary embodiment of the present invention, the parking ring, the rotatable member, and the elastic body are sequentially connected on the driving shaft connected to the motor as shown in  FIGS. 2A and 2B , but the present invention is not limited by the exemplary embodiment. For example, another exemplary embodiment of the present invention may be configured to further include a driving gear installed on the driving shaft, and a driven gear connected with the driving gear, and the parking ring, the rotatable member, and the elastic body may be installed on a shaft of the driven gear. 
         [0079]      FIGS. 4A to 4D  depict states in which the parking mechanism according to the first exemplary embodiment of the present invention is operated at the time of parking the vehicle. 
         [0080]    Although not shown in the drawings, since sufficient braking force needs to be produced between the disc  210  and the pads in order to perform the parking braking, it is based upon the premise that the motor unit  110  is being operated by the control unit  191 . 
         [0081]    As shown in  FIG. 4A , in a state in which the parking braking is not performed, the driving rod  161  is positioned so as not to restrict the parking ring  150 . 
         [0082]    Next, when the control unit  191  operates the parking drive unit  160  (i.e., solenoid), the driving rod  161  moves forward as shown in  FIG. 4B . 
         [0083]    As the driving rod  161  moves forward, the driving rod  161  moves to a position sufficient for restricting the parking ring  150  as shown in  FIG. 4B . 
         [0084]    In this case, as shown in  FIG. 4B , the parking ring  150  is required to be rotated in order to compensate for a clearance between the protrusion of the parking ring  150  and the driving rod  161 . 
         [0085]    Because braking reaction force is applied due to clamping force at the time of the parking braking, the rotatable member  170  and the parking ring  150  come into contact with the driving rod while rotating in a case in which motor driving power for producing braking force is cut off ( FIG. 4C ). However, on the contrary, the motor unit may be rotated in a direction for reducing pressure (i.e., counterclockwise in  FIG. 4C ). 
         [0086]    Therefore, the driving rod  161  restricts the rotation of the parking ring  150 , such that the parking braking is completed ( FIG. 4C ). 
         [0087]    Meanwhile, a process of estimating motor characteristics will be described with reference to  FIGS. 5A to 5C . The process of estimating motor characteristics is similar to the parking braking process. 
         [0088]    However, because the process of producing braking force at the time of parking braking is not required during the process of estimating motor characteristics, the process of estimating motor characteristics may be performed in a state in which the braking operation is stopped and braking reaction force is not formed. Therefore, an initial state shown in  FIG. 5A  is identical to the state shown in  FIG. 4A  except that there is no braking reaction force. However, whether to produce braking reaction force may not be essential. 
         [0089]    Next, as shown in  FIG. 5B , the driving rod  161  moves forward to a position sufficient for restricting the parking ring  150 . 
         [0090]    If the driving rod sufficiently moves forward, the driving rod and the parking ring  150  come into contact with each other by operating the motor, such that the rotation of the parking ring  150  is restricted ( FIG. 5C ). 
         [0091]    When the rotation of the parking ring  150  is restricted, only the rotatable member  170  is rotated by the motor unit, and as a result, relative displacement occurs between the rotatable member  170  and the parking ring  150 . Because of the relative displacement, the torsion spring  180  is compressed. 
         [0092]    During a process in which the torsion spring  180  is compressed, motor characteristics may be estimated by detecting a motor rotation angle and electric current applied to the motor. Here, the motor characteristics are determined by a relationship between the motor rotation angle and the motor electric current, which are monitored during the process in which the spring is compressed. 
         [0093]      FIGS. 6A and 6B  are graphs illustrating a relationship between the motor rotation angle and the motor electric current, in which  FIG. 6A  illustrates an initial state in which a spring is set not to be compressed, and  FIG. 6B  illustrates an initial state in which a spring is set to be compressed. 
         [0094]    Referring to  FIG. 6A , after the driving rod and the parking ring come into contact with each other, the electric current, which is applied to press the spring, is increased. Therefore, as shown in  FIG. 6A , the motor electric current is also increased as the motor rotation angle is increased, gradients of straight lines, which indicate the tendency of increase, are determined depending on a spring constant of the torsion spring  180 . 
         [0095]    In  FIG. 6A , the uppermost straight line means an initial normal state, the lowermost straight line means a state in which the motor is not normally operated and the motor breaks down. Meanwhile, the motor characteristics are changed as durability of the motor is changed, and a gradient of the straight line is changed as the motor characteristics are changed. 
         [0096]    Therefore, a state is changed as indicated by the intermediate straight line illustrated in  FIG. 6A , and a change in motor characteristics may be predicted in consideration of the state change, and the motor torque constant may be corrected. The correction to the motor torque constant may be implemented by presetting a correction value in accordance with a change in gradient, and applying the preset correction value. If the motor torque constant may be periodically corrected in accordance with a change in motor characteristics, accurate braking force may be calculated, and thus the force sensor may be removed or a breakdown of the force sensor may be addressed. 
         [0097]    Meanwhile,  FIG. 6B  illustrates a state in which the spring is set to be compressed, and the state illustrated in  FIG. 6B  is identical to a state illustrated in  FIG. 6A  except that because the spring is compressed in an initial state, additional electric current is required to compress the spring by overcoming the compressed state of the spring when the parking ring and the driving rod come into contact with each other (a region in  FIG. 6B  in which electric current is rapidly increased). The additional electric current for overcoming the compressed state is indicated by a portion in the graph of  FIG. 6B  in which the electric current is rapidly increased. 
         [0098]    Therefore, according to the present exemplary embodiment, a change in motor characteristics may be estimated by monitoring a change in motor rotation angle and a change in motor electric current while the spring is compressed from a point X at which the driving rod and the parking ring come into contact with each other (i.e., to a point in time at which the rotation of the rotatable member  170  is restricted by the restricting portion  151  of the parking ring), and the motor torque constant may be corrected by using the change in motor rotation angle and the change in motor electric current. 
         [0099]      FIGS. 7A and 7B  are views illustrating internal components of an electromechanical brake according to a second exemplary embodiment of the present invention, and  FIGS. 8A to 8C  are views illustrating states in which the electromechanical brake according to the second exemplary embodiment of the present invention is operated in accordance with a mode for estimating motor characteristics. 
         [0100]    The second exemplary embodiment is identical to the first exemplary embodiment except that the second exemplary embodiment further includes a parking rod  163  hingedly coupled to the driving rod  161 . 
         [0101]    In the present exemplary embodiment, the parking drive unit  160  may be configured as a unidirectional solenoid that may operate the driving rod  161  in a single direction. The solenoid includes the driving rod  161  which is moved in a single direction by being supplied with electric power, and particularly, further includes a return spring for providing restoring force for allowing the driving rod  161  to return to an initial position. In particular, according to the exemplary embodiment of the present invention, parking may be released just by the movement of the motor unit  110  for stopping the braking operation, that is, by rotating the motor unit  110  in a direction for reducing pressure without applying separate force for releasing the parking braking. Therefore, when the motor unit  110  is rotated in the direction for reducing pressure, the parking rod  163  and the driving rod  161  may be automatically moved to a position for releasing braking by using restoring force of the return spring. 
         [0102]    The parking rod  163  is hingedly coupled to the driving rod  161  by means of a hinge portion  162 , and has a slot  165  formed at a center thereof. Meanwhile, a guide pin  164  is formed on a bottom surface of the housing  300  or other members to which the guide pin  164  may be fixed, and the guide pin  164  is inserted into the slot  165 . When the parking drive unit  160  is operated, the parking rod  163  comes into contact with the parking ring  150  so as to restrict the rotation of the parking ring  150 , and the guide pin  164  is inserted into the slot  165  so as to guide the movement of the parking rod  163 . 
         [0103]    The slot  165  has a sufficiently larger size than the guide pin  164  so that the guide pin  164  may be moved in the slot  165 . In particular, the guide pin  164  has a circular cross section, and the slot  165  is configured to have a track shape having a width approximately identical to a diameter of the guide pin  164  (actually a width slightly larger than a diameter of the guide pin  164 ). Here, a width of the track shape means a portion having a relatively short width. The track-shaped slot  165  has a size sufficient to receive the movement of the driving rod  161 , and the driving rod  161  may be operated once as the guide pin  164  is positioned at both end portions in a longitudinal direction of the slot  165 . 
         [0104]    That is, as shown in  FIGS. 8A to 8C , when the driving rod  161  is at a parking braking releasing position (at a point where the driving rod  161  is extended farthest from the solenoid) ( FIG. 3A ), the guide pin  164  is positioned at a first end portion in the longitudinal direction of the slot  165 , and when the driving rod  161  is at a parking braking position (at a point where the driving rod  161  is retracted closest to the solenoid) ( FIG. 3C ), the guide pin  164  is positioned at a second end portion in the longitudinal direction of the slot  165 . 
         [0105]    The parking rod  163  needs to be formed to restrict the rotation of the parking ring  150 . To this end, the parking rod  163  has an upper end surface C which comes into contact with the protrusion of the parking ring  150  so as to support the parking ring  150 . The upper end surface C may be formed as a quadrangular flat structure. 
         [0106]    The parking rod  163  is configured to move to a releasing position by being pushed by the protrusion of the parking ring  150  at the time of releasing the parking. To this end, the parking rod  163  has a lateral surface D which extends from one end portion of the upper end surface C so that the parking rod  163  may be pushed away while coming into contact with the protrusion of the parking ring  150 . Like the upper end surface C, the lateral surface D may also include a quadrangular flat structure. 
         [0107]    Meanwhile, the parking ring  150 , which is operated together with the parking rod  163 , is identical to the parking ring described in the first exemplary embodiment. 
         [0108]    The operation of the parking mechanism for estimating motor characteristics is shown in  FIGS. 8A to 8C , and a principle of estimating motor characteristics is identical to that in the first exemplary embodiment except for a difference in movement of the driving rod. 
         [0109]    That is, in the initial state ( FIG. 8A ) in which the rotation of the parking ring is not restricted, the driving rod  161  moves rearward, and as the parking rod  163  is rotated, the driving rod  161  moves to a position sufficient for restricting the parking ring  150  ( FIG. 8B ). 
         [0110]    Next, the motor is operated, and the driving rod and the parking ring come into contact with each other such that the rotation of the parking ring is restricted ( FIG. 8C ). 
         [0111]    When the rotation of the parking ring is restricted, only the rotatable member  170  is rotated by the motor unit, and as a result, relative displacement occurs between the rotatable member  170  and the parking ring. The torsion spring  180  is compressed because of the relative displacement, and in this case, the motor characteristics may be estimated by detecting a motor rotation angle and electric current applied to the motor. 
         [0112]    As described above, an example including the motor, the reduction gear unit, the spindle and the like has been described through the exemplary embodiment of the present invention, but the present invention is not limited by this example, and it can be easily understood by those skilled in the art that the parking mechanism for estimating motor characteristics may be changed and implemented as other forms including a structure in which the elastic body, the parking ring, and the rotatable member are combined. 
         [0113]    The invention has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.