Abstract:
When a wiper is started, a wiper arm is moved at first in the backward path direction (S 1 ). If a motor is locked (S 2 ) before an original point position signal is outputted, a reciprocating operation for moving the wiper arm temporarily in the forward path direction and then moving the wiper arm again in the backward path direction is performed (S 5 -S 8 , S 1 ). When an absolute position signal is not outputted even after repeating the reciprocating operation a plurality of times (S 5 ) and the original point position cannot be recognized (S 2,  S 5 ), the motor is not forced furthermore but stopped (S 5→ END) On the other hand, if the motor is not locked when the wiper arm is moved in the backward path direction (S 1 ) and the original point position can be recognized (S 2 , S 3 ), wiping control is performed (S 4 ) by calculating the wiper arm position based on the original point position and the motor rotation pulse.

Description:
TECHNICAL FIELD 
     The present invention relates to a control technique for a wiper apparatus for vehicles such as a car and, more particularly, to a control method and a control system for a wiper apparatus using a forward/reverse drive motor as a drive source. 
     BACKGROUND ART 
     Conventionally, in a typical vehicle wiper apparatus, the rotation output of a motor rotating in a certain direction is converted into a reciprocating movement by means of a link unit so as to reciprocate a wiper arm (hereinafter, sometimes referred to merely as “arm”) on a surface to be wiped (e.g., windshield). In recent years, along with a requirement to narrow down the attachment space of the wire apparatus, a system that drives a wiper arm by a forward/reverse rotation of the motor has been developed in order to reduce the movement area of the link unit to less than half that of a conventional link unit and has been adopted in many cars. 
     In a wiper system that forwardly and reversely rotates a motor, the movement area (working limitation) in the up-down direction is limited by a mechanical stop. On the other hand, the reciprocating movement of the arm is realized by switching the rotation direction of the motor at the timing when the arm reaches the upper and lower turning positions. Therefore, in controlling the motor drive control, it is necessary to detect whether the wiper arm has reached the turning positions in the motor drive control. To this end, the position and movement speed of the wiper arm need to be detected. For example, in a system described in Patent Documents 1 and 2, the position and the movement speed of the wiper arm are detected by means of a motor rotation pulse generated in association with the rotation of the motor. 
       FIG. 7  is an explanatory view showing the basic configuration of a motor unit  50  used in the wiper system that forwardly and reversely rotates a motor. As shown in  FIGS. 7  ( a ) and  7  ( b ), in this wiper system, a multi-pole-magnetized magnet  53  having a plurality (e.g., six poles) of magnetic poles formed in the circumferential direction is fitted to a rotary shaft  52  of a motor  51 . Further, a magnetic sensor  54  such as a Hall IC is arranged opposite to the multi-pole-magnetized magnet  53 . When the motor is driven, the multi-pole-magnetized magnet  53  rotates with the rotary shaft  52  of the motor and the polarity of the magnetic pole located opposite to the magnetic sensor  54  changes accordingly. A sensor signal as shown in  FIG. 7  ( b ) is output from the magnetic sensor  54  each time when the polarity changes and the output signal is input to a control unit so as to be used as a motor rotation pulse. 
     There exists a correlation based on the reduction ratio or link operation ratio between the rotation angle of the rotary shaft  52  and movement angle of the wiper arm, so that it is possible to calculate the movement amount of the arm from the rotation angle of the rotary shaft  52 . Thus, the position of the wiper arm is detected by means of additions and subtractions of the number of motor rotation pulses. However, since there is a risk of pulse shift when relying only on motor rotation pulses, a magnetic sensor  55  serving as an absolute position detecting sensor is added to the wiper system and the pulse count is corrected by the output signal of the sensor. For example, a position detecting sensor is arranged near a storage position of the wiper arm and the pulse count is reset to a predetermined value when the output signal of the sensor is obtained in order to recognize the position of the wiper arm by the number of pulses counted from the absolute value. 
     As shown in  FIG. 7  ( a ), in the wiper system, a ring magnet  57  is fitted to a worm wheel  56  engaged with the rotary shaft  52 . The ring magnet  57  has two magnetic poles formed in the circumferential direction and rotates with the worm wheel  56 . As shown in  FIG. 7  ( c ), a sensor signal is output from the magnetic sensor  55  when the magnetic pole of the ring magnet  57  arranged opposite to the magnetic sensor  55  changes from N pole to S pole. Then, as shown in  FIG. 7  ( d ), by counting the number of motor rotation pulses with the sensor signal output position set as the original point position, it is possible to detect the movement of the wiper arm from the original point position. With the pulse count corresponding to the turning positions previously calculated and set, the motor is reversely rotated when the pulse count reaches a predetermined value, whereby the wiper arm is reciprocated between the upper and lower turning positions. 
     In the normal operation of such a wiper system, the original point position is recognized in the first wiping operation activated in response to turn-on of a wiper switch.  FIG. 8  is an explanatory view showing original point position recognition operation performed at the start-up time. In a switch-off state, the wiper arm is situated at the storage position, and the state of the ring magnet  57  at this time is as shown in  FIG. 8  ( a ). That is, N pole of the ring magnet  57  faces the magnetic sensor  55 , and the sensor signal thereof is in a High state. When the wiper switch is turned on in this state, the motor  51  drives in the backward path direction (direction from upper turning position toward lower turning position). Then, ring magnet  57  rotates leftward as shown in  FIGS. 8  ( a )→( b ) and the original point position (boundary between N pole and S pole) reaches the position corresponding to the magnetic sensor  55 . As a result, the sensor signal changes from High to Low state, whereby the original point position is recognized at the wiper start-up time. 
     However, when the wiper arm is stopped at a position below the original point at the wiper start-up time, the magnetic sensor  55  is situated in the S pole area. Thus, even though the motor is driven in the backward path direction, the N/S boundary does not face the magnetic sensor  55  during the abovementioned operation, with the result that it is impossible to recognize the original point position. Further, when the arm is situated at a position near the upper turning position of the wiping area, the magnetic sensor  55  is also situated in the S pole area. In this case, however, the original point position passes through a position corresponding to the magnetic sensor  55  by the abovementioned backward path movement at the start-up time, whereby the original point position can be recognized. 
     In order to cope with the above problem, when the arm is situated at a position below the original point position, the motor  51  is driven in the backward path direction and, then, the operation of the motor is controlled with the lower limit position at which the motor is locked set as a reference position. The control operation at the wiper start-up time is shown in a flowchart of  FIG. 9 . As shown in  FIG. 9 , when the wiper is turn on, the motor drives in the backward path direction as described above (step S 51 ). The flow advances to step S 52  where it is determined whether the motor  51  is in a locked state. In the case where the wiper arm is stopped at a position below the original point position (i.e., position between the original point position and lower limit position), the wiper arm reaches the lower limit position by the movement in the backward path direction and is then brought into contact with a mechanical stop. As a result, the motor  51  is in a locked state, and this locked state is detected in step S 52 . 
     The lower limit position is an absolute position that has previously been set. When the state where the wiper arm is situated at this position can be recognized, the current position of the wiper arm can be calculated by performing pulse count starting from the lower limit position as a reference. Thus, when it is determined in step S 52  that the motor  51  in a locked state, the flow advances to step S 54  where wiping control is performed based on the pulse count. However, also in this case, the pulse count is not performed starting from the proper original point position, so that it is necessary to correct a count value in course of the wiper arm operation.  FIG. 10  is an explanatory view showing pulse count correction control after the motor lock. 
     As shown in  FIG. 10  ( a ), when the motor  51  is in a locked state at the lower limit position by the movement in the backward path direction, the counter is once reset at the lower limit position as an original position. Afterward, as shown in  FIG. 10  ( b ), the motor  51  is driven in the forward path direction (direction from lower turning position toward upper turning position) and, at the same time, pulse count control is started. Along with the movement of the arm, another N/S boundary of the ring magnet  57  reaches the magnetic sensor  55 . This N/S boundary also faces the magnetic sensor  55  at the time when the wiper arm reaches a predetermined position. In this system, therefore, the predetermined position is used as a check position and, as shown in  FIG. 10  ( c ), when a signal indicating that the magnetic sensor  55  faces the check position is obtained, the pulse count value is corrected to a previously set value. Thus, even in the case where the arm position is recognized by a motor locked state, normal pulse count control can be restored by the correction based on the check position. 
     On the other hand, when it is determined in step S 52  that the motor  51  is not in a locked state, the flow advances to step S 53  where it is determined whether the original point position has passed the magnetic sensor  55  (i.e., whether the magnetic pole is changed from N pole to S pole). When the original point position has not passed, the flow returns to step S 51  where the movement in the backward path direction is continued to repeat the processing of steps S 52  and S 53 . When the wiper arm is stopped at a position above the original point position, the original point position reaches the magnetic sensor  55 , and it is detected in step S 53  that the original point position has passed the magnetic sensor  55 . In this case, it is determined that the wiper arm reaches the original point position, the backward movement is then stopped, and the flow advances to step S 54 . In step S 54 , wiping control is performed using the pulse count based on the original point position, whereby the wiper arm is reciprocated to perform wiping operation between the upper and lower turning positions. 
     Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 11-301409 
     Patent Document 2: Jpn. Pat. Appln. Laid-Open Publication No. 2004-274804 
     However, in such a wiper system, when an obstacle  62  such as snow exists on a windshield  61  as shown in  FIG. 11  ( a ) and a wiper arm  63  is stopped in the middle of the wiping operation, the following problem arises. When the wiper switch (or ignition switch) is turned off in the state shown in  FIG. 11  ( a ), the arm position information (pulse count value) obtained up to this time is reset. Therefore, at the wiper restart time, the processing shown in  FIG. 9  is executed so as to recognize the original point position and the like (original point position and arm current position detected based on the contact state to the lower limit position when the arm is stopped at a position below the original point) of the wiper arm  63 . 
     In this case, if there is no obstacle  62  and the wiper arm  63  can freely move in the backward path direction, the original position and the like of the arm is recognized by the control operation shown in  FIG. 9  at the start-up time and normal pulse count control can be restored without problems. However, as shown in  FIG. 11  ( b ), when the obstacle  62  interferes with the return of the wiper arm  63 , it is determined in the processing of  FIG. 9  that the motor is locked at this moment and, accordingly, the flow advances from step S 52  to step S 54 . In particular, in the case where the arm is stopped in the S pole area above the check position, there is no chance of detecting the arm position other than the arm lock state. Thus, the lock state caused due to the existence of the obstacle is determined as the arm lock state in step S 52 . That is, the stop of the wiper arm  63  due to existence of the obstacle  62  may falsely be recognized as the stop of the wiper arm  63  at its lower limit position and, in this case, the pulse count control is started at the falsely recognized stop position. 
     When the pulse count control is started based on the motor lock caused due to the obstacle  62 , the wiping control is executed in a state where the pulse count value and actual arm position do not coincide with each other. Thus, even though the wiper arm  63  actually reaches the upper turning position, it is recognized that the wiper arm  63  is in the middle of the forward path in the control, so that the wiper arm  63  is not stopped. Accordingly, the wiper arm  63  continues moving in the forward path direction with the result that, as shown in  FIG. 11  ( c ), a wiper blade  64  may overrun and collide with an end portion  61   a  (A-pillar) of the windshield  61 . When the wiper blade  64  overruns as described above, the wiper blade may be broken, or windshield  61  or A-pillar may be damaged. Thus, when the wiper is caused to operate in a state where the pulse count value and actual arm position do not coincide with each other as described above, the arm movement cannot appropriately be controlled. 
     An object of the present invention is to prevent a wiper apparatus from falsely operating in restart time even after occurrence of a problem in which snow or the like interfere with the movement of the wiper arm. 
     Summary of the Invention 
     According to the present invention, there is provided a wiper control method for driving a motor to rotate forwardly and reversely so as to reciprocate a wiper arm for a wiping operation and controlling the operation of the wiper arm according to an absolute position signal output when the wiper arm is located at a predetermined position and a relative position signal output in association with the rotation of the motor. The wiper arm is moved in a predefined first direction at the start-up time of the wiper arm, and in the case where the motor is in a locked state before the absolute position signal is output, reciprocation operation in which the wiper arm is once moved in a second direction opposite to the first direction and then the wiper arm is moved in the first direction once again is executed. 
     In the wiper control method according to the present invention, the wiper arm is first moved in the first direction. In the case where the motor is in a locked state before the absolute position signal is output, reciprocation operation is executed to move the wiper arm in the second direction first and then in the first direction. Thus, in the case where, for example, the original point position at which the absolute position signal is output and lower limit position at which the movement of the wiper arm is mechanically restricted are provided below the lower turning position of the wiper arm, when the wiper arm is started up in a state where the arm is stopped below the original point position, the wiper reaches the lower limit position before the absolute position signal is output to cause the motor to be in a locked state, whereby the reciprocation operation can be performed. In this case, by making a setting such that the arm passes through the original point position during the reciprocation operation, it is possible to reliably recognize the original point position at the start-up time even if the arm is located below the original point position. Also, in the case where there is an obstacle such as snow between the upper and lower turning positions of the wiper arm and the obstacle interferes with the movement of the wiper arm, the arm is stopped at the start-up time by the obstacle before the absolute position signal is output to cause the motor to be in a locked state and, then, the reciprocation operation is executed. In this case, in the case where the absolute position signal has not yet been output even after a plurality of number of times of the reciprocation operations and therefore the original point position cannot be recognized, the motor is not forced to operate furthermore but is stopped, thereby preventing occurrence of a malfunction of the arm due to false recognition. 
     In the wiper control method, in the case where the absolute position signal has not yet been output even after the execution of the reciprocation operation, the reciprocation operation may be executed once again. In the case where the absolute position signal has not yet been output even after the execution of the reciprocation operation, the reciprocation operation may be executed by a predetermined number of times. In the case where the absolute position signal has not yet been output even after a plurality of number of times of the reciprocation operations, the motor may be stopped. 
     In the wiper control method, the wiper apparatus may have: upper and lower turning positions at which movement direction of the wiper arm is reversed; upper and lower limit positions which are arranged beyond the upper and lower turning positions respectively and at which the movement of the wiper arm is mechanically restricted; and an original point position which is arranged between the lower limit position and lower turning position and at which the absolute position signal is output. Further, as the first direction, the direction in which the wiper arm approaches the lower limit position may be set. 
     Further, according to the present invention, there is provided a wiper control system characterized by comprising: a motor that can rotate forwardly and reversely; a wiper arm driven by the motor; a first sensor means for outputting an absolute position signal when the wiper arm is located at a predetermined position; a second sensor means for outputting a relative position signal in association with the rotation of the motor; a relative position signal detection means for detecting the relative position signal; a lock detection means for detecting presence or absence of occurrence of a locked state of the motor based on the relative position signal; a motor rotation direction determination means for setting the rotation direction of the motor; and a motor drive means for rotating the motor in a predetermined direction based on an instruction from the motor rotation direction determination means. The motor rotation direction determination means rotates the motor in a predefined first direction at the start-up time of the wiper arm. In the case where the motor is in a locked state before the absolute position signal is output, the motor rotation direction determination means executes reciprocation operation of rotating the motor in a second direction opposite to the first direction by a predetermined number of rotations or predetermined time period and then rotates the motor in the first direction once again. 
     In the wiper control system according to the present invention, the motor rotation direction determination means first moves the wiper arm in the first direction. In the case where the motor is in a locked state before the absolute position is output, the motor rotation direction determination means executes reciprocation operation to move the wiper arm in the second direction first and then in the first direction. Thus, in the case where, for example, the original point position at which the absolute position signal is output and lower limit position at which the movement of the wiper arm is mechanically restricted are provided below the lower turning position of the wiper arm, when the wiper arm is started up in a state where the arm is stopped below the original point position, the wiper reaches the lower limit position before the absolute position signal is output to cause the motor to be in a locked state, whereby the reciprocation operation can be performed. In this case, by making a setting such that the arm passes through the original point position during the reciprocation operation, it is possible to reliably recognize the original point position at the start-up time even if the arm is located below the original point position. Also, in the case where there is an obstacle such as snow between the upper and lower turning positions of the wiper arm and the obstacle interferes with the movement of the wiper arm, the arm is stopped at the start-up time by the obstacle before the absolute position signal is output to cause the motor to be in a locked state and, then, the reciprocation operation is executed. In this case, in the case where the absolute position signal has not yet been output even after a plurality of number of times of the reciprocation operations and therefore the original point position cannot be recognized, the motor is not forced to operate furthermore but is stopped, thereby preventing occurrence of a malfunction of the arm due to false recognition. 
     In the wiper control system, in the case where the absolute position signal has not yet been output even after the execution of the reciprocation operation, the motor rotation direction determination means may execute the reciprocation operation once again. In the case where the absolute position signal has not yet been output even after the execution of the reciprocation operation, the motor rotation direction determination means may execute the reciprocation operation by a predetermined number of times. In the case where the absolute position signal has not yet been output even after the reciprocation operation is executed by a predetermined number of times, the motor rotation direction determination means may stop the motor. Further, the wiper control system may include a counter for counting the number of executions of the reciprocation operation or a timer for controlling the reverse rotation time in the reciprocation operation. 
     According to the wiper control method of the present invention, in a wiper apparatus for driving a motor to rotate forwardly and reversely so as to reciprocate a wiper arm for a wiping operation and controlling the operation of the wiper arm according to an absolute position signal output when the wiper arm is located at a predetermined position and a relative position signal output in association with the rotation of the motor, the wiper arm is moved in a predefined first direction at the start-up time of the wiper arm. In the case where the motor is in a locked state before the absolute position signal is output, reciprocation operation in which the wiper arm is once moved in a second direction opposite to the first direction and then the wiper arm is moved in the first direction once again is executed. Thus, in the case where the control method according to the present invention is applied to a wiper apparatus in which, for example, the original point position at which the absolute position signal is output and lower limit position at which the movement of the wiper arm is mechanically restricted are provided below the lower turning position of the wiper arm, when the wiper arm is started up in a state where the arm is stopped below the original point position, the wiper reaches the lower limit position before the absolute position signal is output to cause the motor to be in a locked state, whereby the reciprocation operation can be performed. In this case, by making a setting such that the arm passes through the original point position during the reciprocation operation, it is possible to reliably recognize the original point position at the start-up time even if the arm is located below the original point position. 
     Further, according to the wiper control method of the present invention, also in the case where there is an obstacle such as snow between the upper and lower turning positions of the wiper arm and the obstacle interferes with the movement of the wiper arm, the arm is stopped at the start-up time by the obstacle before the absolute position signal is output to cause the motor to be in a locked state and, then, the reciprocation operation is executed. In this case, in the case where the absolute position signal has not yet been output even after a plurality of number of times of the reciprocation operations and therefore the original point position cannot be recognized, the motor is not forced to operate furthermore but is stopped. As a result, it is possible to prevent wiping control from being executed in a false recognition state where the pulse count value and actual arm position do not coincide with each other, thereby preventing overrun of the wiper arm at the upper turning position. Therefore, it is possible to prevent a wiper blade or car body from being damaged due to the overrun, thereby increasing reliability of the operation of the wiper apparatus. 
     The wiper control system of the present invention includes a motor that can rotate forwardly and reversely; a wiper arm driven by the motor; a first sensor means for outputting an absolute position signal when the wiper arm is located at a predetermined position; a second sensor means for outputting a relative position signal in association with the rotation of the motor; a relative position signal detection means for detecting the relative position signal; a lock detection means for detecting presence or absence of occurrence of a locked state of the motor based on the relative position signal; a motor rotation direction determination means for setting the rotation direction of the motor; and a motor drive means for rotating the motor in a predetermined direction based on an instruction from the motor rotation direction determination means. The motor rotation direction determination means first moves the wiper arm in the predefined first direction at the start-up time of the wiper arm. In the case where the motor is in a locked state before the absolute position signal is output, the motor rotation direction determination means executes reciprocation operation in which the wiper arm is once moved in the second direction opposite to the first direction and then the wiper arm is moved in the first direction. Thus, in the case of the wiper apparatus in which, for example, the original point position at which the absolute position signal is output and lower limit position at which the movement of the wiper arm is mechanically restricted are provided below the lower turning position of the wiper arm, when the wiper apparatus is started up in a state where the arm is stopped below the original point position, the arm reaches the lower limit position before the absolute position signal is output to cause the motor to be in a locked state, whereby the reciprocation operation can be performed. In this case, by making a setting such that the arm passes through the original point position during the reciprocation operation, it is possible to reliably recognize the original point position at the start-up time even if the arm is located below the original point position. 
     Further, according to the wiper control system of the present invention, also in the case where there is an obstacle such as snow between the upper and lower turning positions of the wiper arm and the obstacle interferes with the movement of the wiper arm, the arm is stopped at the start-up time by the obstacle before the absolute position signal is output to cause the motor to be in a locked state and, then, the reciprocation operation is executed. In this case, in the case where the absolute position signal has not yet been output even after a plurality of number of times of the reciprocation operations and therefore the original point position cannot be recognized, the motor is not forced to operate furthermore but is stopped. As a result, it is possible to prevent wiping control from being executed in a false recognition state where the pulse count value and actual arm position do not coincide with each other, thereby preventing overrun of the wiper arm at the upper turning position. Therefore, it is possible to prevent a wiper blade or car body from being damaged due to the overrun, thereby increasing reliability of the operation of the wiper apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory view showing a configuration of a motor unit used in a wiper control system which is an embodiment of the present invention; 
         FIG. 2  is an explanatory view showing the movement range of a wiper arm; 
         FIG. 3  is a block diagram showing a configuration of the wiper control system which is an embodiment of the present invention; 
         FIG. 4  is a flowchart showing a processing procedure of a wiper control method which is an embodiment of the present invention; 
         FIGS. 5   a - 5   d  are explanatory views showing the positional relationship between a ring magnet and Hall IC in a control scheme taken when the wiper arm is stopped at a position below the original position; 
         FIGS. 6   a - 6   d  are explanatory views showing the positional relationship between the ring magnet and Hall IC in a control scheme taken when the wiper arm is stopped by an obstacle; 
         FIGS. 7   a - 7   d  are explanatory views showing the basic configuration of the motor unit used in the wiper system that forwardly and reversely rotates a motor; 
         FIGS. 8   a - 8   b  are explanatory views showing original point position recognition operation performed at start-up time in a conventional wiper system; 
         FIG. 9  is a flowchart showing control processing performed at start-up time in a conventional wiper system; 
         FIGS. 10   a - 10   c  are explanatory views showing pulse count correction control after motor lock; and 
         FIGS. 11   a - 11   c  are explanatory views showing a control state at restart time after an obstacle existing on a windshield stops the movement of the wiper arm in the middle of its wiping operation. 
       EXPLANATION OF REFERENCE NUMERALS 
       
           
             1 : Motor unit 
             2 : Motor 
             3 : Gear box 
             4 : Rotary shaft 
             5 : Output shaft 
             6 : Yoke 
             7 : Armature core 
             8 : Commutator 
             9 : Permanent magnet 
             10 : Brush 
             11 : Case frame 
             12 : Worm 
             13 : Worm wheel 
             14 : Multi-pole-magnetized magnet 
             15 : Hall IC 
             16 : Ring magnet 
             17 : Printed circuit board 
             18 : Hall IC 
             19 : Link mechanism 
             21 : Wiper control apparatus 
             22 : Pulse detection means 
             23 : Pulse detection means 
             24 : Lock detection means 
             25 : Counter 
             26 : Timer 
             27 : Motor rotation direction determination means 
             28 : Motor drive circuit 
             50 : Motor unit 
             51 : Motor 
             52 : Rotary shaft 
             53 : Multi-pole-magnetized magnet 
             54 : Magnetic sensor 
             55 : Magnetic sensor 
             56 : Worm wheel 
             57 : Ring magnet 
             61 : Windshield 
             61   a : End portion 
             62 : Wiper arm 
             63 : Wiper arm 
             64 : Wiper blade 
         
      
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described in greater detail with reference to the accompanying drawings.  FIG. 1  is an explanatory view showing a configuration of a motor unit used in a wiper control system which is an embodiment of the present invention. Like the motor unit  50  shown in  FIG. 7 , a motor unit  1  of  FIG. 1  includes a motor  2  and a gear box  3 . The rotation of the rotary shaft  4  of the motor  2  is reduced in speed in the gear box  3  and is output to an output shaft  5 . The rotary shaft  4  is rotatably supported by a bottomed cylindrical yoke  6 , and an armature core  7 , around which a coil is wound, and a commutator  8  are fitted to the rotary shaft  4 . A plurality of permanent magnets  9  are fixed to the inner surface of the yoke  6 . A feeding brush  10  is slidably held in contact with the commutator  8 . The rotary speed (the number of rotations) of the motor  2  is controlled by the flow rate of the electric current supplied to the brush  10 . 
     The gear box  3  has a case frame  11  that is fitted to the edge of the open end of the yoke  6 . Note that  FIG. 1  shows a state where a cover of the case frame  11  is removed. The leading end portion of the rotary shaft  4  projects out from the yoke  6  and contained in the case frame  11 . A worm  12  is formed at the leading end portion of the rotary shaft  4  and engaged with a worm wheel  13 . The worm wheel  13  is fixed to the output shaft and rotatably supported by the case frame  11 . The drive power of the motor  2  is output to the output shaft  5  in a reduced velocity condition by way of the worm  12  and worm wheel  13 . 
     A multi-pole-magnetized magnet  14  (to be referred to simply as magnet  14  hereinafter) is fitted to the rotary shaft  4 . On the other hand, a Hall IC  15  is arranged opposite to the outer periphery of the magnet  14  in the case frame  11 . The Hall IC  15  corresponds to the magnetic sensor  54  of  FIG. 7  ( a ). When the rotary shaft  4  makes a full turn, a pulse signal corresponding to six periods is output from the Hall IC  15  as a motor rotation pulse. It is possible to detect the rotation speed of the rotary shaft  4  from the period of the motor rotation pulse. The number of rotations of the rotary shaft  4  and the moving speed of the arm show a correlation that is based on the reduction ratio and the link operation ratio, so that it is also possible to calculate the moving speed of the wiper arm from the detected number of rotations of the rotary shaft  4 . 
     A ring magnet  16  for detecting the absolute position of the arm is fitted to the worm wheel  13 . The ring magnet  16  has two poles magnetized in the circumferential direction. A printed circuit board  17  (denoted by the dot-and-dash line in  FIG. 1 ) is fitted to the case frame  11  and a Hall IC  18  is arranged opposite to the ring magnet  16  on the printed circuit board  17 . Like the Hall IC  18 , the abovementioned Hall IC  15  is also arranged on the printed circuit board  17 . The worm wheel  13  is configured to rotate by about 180° to reciprocate the wiper arm. As the worm wheel  13  rotates and the arm reaches a predefined original point position, the Hall IC  18  faces the magnetic pole boundary (between N pole and S pole) of the ring magnet  16  and then an absolute position signal is output to indicate the current position of the arm. 
     The wiper arm is driven by the motor unit  1  having the configuration described above to swing between the upper turning position and the lower turning position to remove the rain drops or the snow flakes adhering to the windshield of a vehicle.  FIG. 2  is an explanatory view showing the movement range of a wiper arm. During the wiping operation, the arm reciprocates in the wiping range between the upper turning position and the lower turning position indicated by hatching lines in  FIG. 2 . When the wiper is at rest, the arm is moved to a storage position located below the lower turning position of the arm and stored in a storage section. The storage section is arranged in the inside of the hood of the vehicle (not shown). An upper limit position and a lower limit position are provided for the arm respectively outside the upper and lower turning positions. The upper and lower limit positions are defined by mechanical restriction means arranged in the motor unit  1 . For example, a link member (not shown) fitted to the output shaft  5  is brought into contact with the restriction means to thereby define the upper and lower limit positions. 
     As in the case of the conventional system shown in  FIGS. 7 to 10 , an original point position, at which the absolute position signal is output from the Hall IC  18 , is set between the storage position and lower limit position. As described above, also in the system, the arm is caused to operate once in the backward path direction at the start-up time so as to detect the original point position, and the motor rotation pulse is counted based on the detected original point position. This allows the current position of the wiper arm to be detected, and by forwardly and reversely driving the motor  2  at the upper and lower turning position, the wiper arm can reciprocate in the wiping range. 
     In the conventional method for recognizing the original point position, when there is an obstacle on the windshield as shown in  FIG. 11  ( a ), a malfunction may occur at restart time after the stop. In order to cope with the problem, in a wiper apparatus control method according to the present invention, an obstacle detection function is imparted to the original point position recognition operation at the start-up time. That is, when there is an obstacle, the wiping operation is stopped to prevent the wiper blade from being damaged.  FIG. 3  is a block diagram showing an entire configuration of the wiper control system which is an embodiment of the present invention. A wiper control method according to the present invention is achieved by the system shown in  FIG. 3 . 
     As shown in  FIG. 3 , the drive of the motor  2  is controlled by a wiper control unit  21 , and the rotation of the motor  2  is output to the output shaft  5  by way of the worm  12  and worm wheel  13 . The output shaft  5  is connected to a link mechanism  19  of the wiper apparatus. When the motor  2  is driven, the link mechanism  19  is activated through the output shaft  5  to cause the wiper blade and wiper arm to operate. The Hall ICs  15  and  18  arranged in the motor unit  1  are connected respectively to pulse detection means  22  and  23 . The pulse detection means  22  (relative position signal detection means) detects the motor rotation pulse output from the Hall IC  15 , and the pulse detection means  23  detects an absolute position signal output from the Hall IC  18 . 
     A lock detection means  24  is provided in the rear stage of the pulse detection means  22 . The lock detection means  24  monitors the state of the motor  2  based on the period of the motor rotation pulse and determines that a motor lock has occurred when the pulse period becomes greater than a certain value. The lock detection means  24  is connected to a counter  25  and a timer  26  so as to detect the number of lock detections, elapsed time, and the like. The lock detection means  24  is further connected to the motor rotation direction determination means  27 . The motor rotation direction determination means  27  is also connected to the pulse detection means  23  so as to determine the rotation direction (wiper arm movement direction) of the motor  2  based on the absolute position signal and motor rotation pulse. 
     The motor rotation direction determination means  27  outputs a signal representing the motor rotation direction and rotation speed based on the motor rotation pulse or absolute position signal while considering the presence or absence of the motor lock or number of lock detections. This signal is sent from the motor rotation direction determination means  27  to a motor drive circuit (motor drive means)  28 . In the wiper system according to the present invention, a PWM control (Pulse Width Modulation) in which drive of the motor  2  is controlled by changing ON/OFF ratio of the pulse width of voltage to be applied to the motor  2  is executed. In performing the PWN control, the motor rotation direction determination means  27  sets the duty ratio of ON-period of a pulse voltage and transmits a control signal to the motor drive circuit  28 . Upon reception of the control signal, the motor drive circuit  28  applies a pulse voltage corresponding to the set duty ratio to the motor  2 . As a result, the motor  2  is feed-back controlled based on the motor rotation pulse or absolute position signal. 
     A wiper control method according to the present invention will next be described.  FIG. 4  is a flowchart showing its processing procedure. When the wiper switch is turned on, the motor  2  drives in the backward path direction (step S 1 ). The flow advances to step S 2  where the lock detection means  24  determines whether the motor  2  is in a locked state. When the motor  2  is not in a locked state in step S 2 , the flow advances to step S 3  where it is determined whether the original point position has passed (i.e., whether the magnetic pole is changed from N pole to S pole). When the original point position has not passed, the flow returns to step S 1  where the movement in the backward path direction is continued to repeat the processing of steps S 2  and S 3 . At this time, when the wiper arm is normally stopped at the storage position (above the original point position), the original point position (N/S boundary) will reach the Hall IC  18 . That is, in the case where the arm is stopped at a position above the original point position and there is no obstacle, the arm inevitably passes the original point position by the backward path direction, whereby the original point position can be recognized. 
     After the passing of the original point position (change of magnet pole from N pole to S pole) has been detected and the original point position has been recognized in step S 3 , the flow advances to step S 4  where wiping control is executed using the pulse count based on the original point position and, afterward, the flow exits the routine. In the wiping control in step S 4 , the motor rotation pulse is counted with the sensor signal output position of the Hall IC  18  as the original point and thereby the current position of the wiper arm is detected. The numbers of pulse counts corresponding to the upper and lower turning positions are previously calculated and set. When the pulse count reaches a predetermined value, the motor  2  is reversely rotated, whereby the wiper arm reciprocates between the upper and lower turning positions. 
     On the other hand, when the motor  2  is in a locked state in step S 2 , the flow advances to step S 5  where the number of motor lock detections is checked. In the case where the number of motor lock detections is less than n (e.g., 6), the flow advances to step S 6  where the number of motor lock detections of the counter  25  is incremented by 1, and the motor is then driven in the forward path direction in step S 7 . Subsequently, the motor  2  is driven in the forward path direction and it is confirmed whether the motor  2  has driven by a specified amount in step  8 , that is, the motor  2  is driven in the forward path direction until the operation amount reaches a specified operation amount (steps S 7 , S 8 ). At this time, the rotation direction of the motor  2  is detected by the motor rotation direction determination means  27 , and motor drive amount is determined based on the number of pulse counts of the motor rotation pulses. After the motor  2  has driven by a specified amount in the forward path direction in step S 8 , the flow returns to step S 1  where the motor  2  is driven in the backward path direction. 
     At this time, in the case where the wiper arm is stopped at a position below the original point position, the following operation is taken.  FIG. 5  is an explanatory view showing the positional relationship between the ring magnet  16  and Hall IC  18  in a control scheme taken when the wiper arm is stopped at a position below the original position. In this case, the processing sequence “S 1 →S 2 →S 3 →S 1 ” of  FIG. 4  is repeated until the wiper arm reaches the lower limit position ( FIG. 5  ( a )). When the arm reaches the lower limit position and is brought into contact with a mechanical stop, the motor  2  is locked ( FIG. 5  ( b )), and the flow advances from steps S 2  to S 5 . Since this is the first lock detection, the flow advances from step S 5  to step S 6  where 1 is incremented to 0 of the lock count (0+1=1), and then the flow advances to step S 7 . 
     In step S 7 , the motor  2  is slightly driven in the forward path direction and, accordingly, the ring magnet  16  passes through the original point position and reaches a position short of the storage position ( FIG. 5  ( c )). That is, as the specified amount in step S 8 , a value exceeding the distance between the lower limit position and original point position is set here. After the motor  2  has been driven in the forward path direction by the specified amount in step S 8 , the motor  2  is driven in the backward path direction once again in step S 1 . Then, as shown in  FIG. 5  ( d ), the original point position moves from N pole to S pole while passing through the Hall IC  18 , and then the sensor signal switches from High to Low. As a result, the original point position is recognized without occurrence of a motor lock. Then, the flow advances from step S 1 , S 2 , S 3 , up to S 4  where the wiping control is executed using the pulse count based on the original point position, and the flow exits the routine. 
     In the conventional control scheme shown in  FIG. 9 , it is believed that the absolute position of the wiper arm can be recognized by the stop at the lower limit position even in the case where the wiper arm is stopped at a position below the original point position. Therefore, in the conventional system, pulse count control is executed based on the lower limit position, and the pulse count value is corrected at the check position. On the other hand, in the control scheme shown in  FIG. 4 , the motor  2  is driven in the forward path direction after the motor lock by a mechanical stop to cause the Hall IC  18  to once pass through the original point position and move once again in the backward path direction for recognition of the original point position. This operation eliminates the need to perform correction of the pulse count value at the check position. Further, this eliminates a section (e.g., a section between the lower limit position and check position) in which the actual position and pulse count value may differ from each other, thereby increasing control accuracy. Furthermore, only the sensor signal output at the time of the change of the magnetic pole from N pole to S pole can be used as an absolute position signal while the sensor signal at the time of the change of the magnetic pole from S pole to N pole is not used particularly in the control, which simplifies the control scheme to thereby reduce a burden on the control unit. 
     In the case where the movement of the arm is stopped by an obstacle or the like as shown in  FIG. 11 , the following control scheme is taken.  FIG. 6  is an explanatory view showing the positional relationship between the ring magnet  16  and Hall IC  18  in a control scheme taken when the wiper arm is stopped by an obstacle. In the case of the conventional system, when the wiper arm is brought into contact with an obstacle and stopped to cause the switch to turn off, the position information of the arm is cleared with the result that a malfunction may occur at restart time as described above. In order to cope with this problem, in the system according to the present invention, the motor  2  is driven in the backward direction in step S 1  of  FIG. 4  ( FIG. 6  ( a )) and, when an obstacle interfere with the arm movement to cause the motor  2  to be in a locked state ( FIG. 5  ( b )), the flow advances from step S 2  to step S 5 . Since this is the first lock detection, the flow advances from step S 5  to step S 6  where 1 is incremented to 0 of the lock count (0+1=1), and then the flow advances to step S 7 . 
     When the motor  2  is slightly driven in the forward path direction in step S 7 , the wiper arm separates from the obstacle and moves in the forward path direction ( FIG. 6  ( c )). Thereafter, after the motor  2  is returned by a specified amount in step S 8 , it is once again driven in the backward path direction in step S 1 . Then, the wiper arm is brought into contact once again with the obstacle to cause the motor  2  to be in a locked state as in the case of  FIG. 6  ( b ). Thus, the flow advances from step S 2  to step S 5  where the number of motor lock detections is checked. At this time point, one motor lock has been detected. Assuming that n is 6, the number of motor lock detections does not reach the n (=6), so that the flow advances from step S 5  to step S 6  where 1 is incremented to 1 of the lock count (1+1=2), and then the flow advances to step S 7 . 
     In step S 7 , the motor  2  is slightly driven in the forward path direction once again as in the case of  FIG. 6  ( c ) to cause the wiper arm to be separated from the obstacle. After the motor  2  is returned by a specified amount, the flow returns to step S 1  (S 8 ) where the motor  2  is once again driven in the backward path direction. Also at this time, the wiper arm is brought into contact once again with the obstacle to cause the motor  2  to be in a locked state, and the flow advances from step S 2  to step S 5 . In step S 5 , the number of motor lock detections is checked. At this time point, n is 2, so that the flow advances to step S 6  where 1 is incremented to 2 of the lock count (2+1=3), and then the flow advances to step S 7 . 
     As described above, in the case where there is an obstacle and the obstacle is not removed, the processing sequence “S 2 →S 5 →S 6 →S 7 →S 8 →S 1 →S 2 ” is repeated to reciprocate the wiper arm in both the forward path direction and backward path direction. When the number of motor lock detections reaches 6 after the reciprocation is repeated five times, it is determined in step S 5  that the number of motor rock detections reaches the n (=6). Then, the flow exits the routine without performing the wiping control of step S 4 . That is, in the case where the number of motor lock detections exceeds five times without being able to recognize the original point position in spite of the repetitive arm reciprocation, it is determined that there is any obstacle, whereby the motor  2  is stopped without performing the wiping control. 
     Therefore, according to the control method of the present invention, it is possible to accurately stop the wiper operation without falsely recognizing a locked state due to an obstacle as a locked state due to the lower limit, thereby preventing a malfunction from occurring due to existence of an obstacle. This prevents the case shown in  FIG. 11  where the wiper blade overruns the upper turning position to collide with the A-pillar, thus preventing the wiper blade or car body from being damaged. In the control method according to the present invention, in the case where there is an obstacle such as snow or the like, the wiping operation is not performed and the wiper is in a rest state, as is the case with the conventional wiper system. Thus, the system is not damaged by a malfunction, so that normal wiping operation can be resumed after the obstacle has been removed. 
     The present invention is not limited to the above embodiments, and various modifications may be made without departing from the scope of the present invention. 
     For example, although in the above embodiment, the motor drive in the forward path direction by a specified amount in step S 8  is controlled based on the number of pulse counts of the motor rotation pulse, it may be controlled using the timer  26  provided in the motor control unit  21 . In this case, for example, the drive amount of the motor  2  in the forward path direction in step S 7  is set to 1 second so as to cause the flow to return from step S 8  to step S 1  after elapse of 1 second. Further, the number of rotation direction changes of the motor  2  detected by the counter  25  may be used in place of the number of lock detections in step S 5  to stop the motor  2  after it reaches a predetermined value. 
     Further, although the wiper apparatus according to the above embodiment has the storage position set below the lower turning position, the present invention is also applicable to a wiper apparatus having no wiper storage position. Further, the control method of the present invention is applicable not only to a wiper apparatus where the wiper arms at the driver seat and the passenger seat are driven to operate by a single motor and a single link mechanism but also to a wiper apparatus where the wiper arms at the driver seat and the passenger seat are driven to operate individually by separate motors. Additionally, the control method of the present invention is applicable not only to a wiper apparatus where the wiper arms are driven in parallel for wiping operation but also a wiper apparatus where the wiper arms are driven in opposite directions for a wiping operation. 
     Although the Hall IC is used as a means for detecting the rotation state and the rotation position of the motor in the above embodiments, the detection means of the present invention is not limited to the Hall IC and sensors using infrared rays or MR sensors (magnetoresistive effect element) may alternatively be used.