JPH0667715B2 - Seat belt retractor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a seat belt retractor for winding up a webbing in a seatbelt mounted on a vehicle and restraining an occupant in an emergency. The present invention relates to an improvement of a seat belt retractor that is wound by using a motor.

[Prior art and its problems]

The winding of the webbing in the seat belt retractor has conventionally been generally performed by a spring, but when winding the webbing by a spring, the webbing of the length required by the occupant is not affected by the spring force of the spring. When it is pulled out and worn, the tension of the webbing is strong and the wearer is expected to feel a sense of pressure. Further, if the spring force of the mainspring is weakened to eliminate the feeling of pressure on the wearer, there is a risk that the webbing may not be completely wound up and stored at the time of storage. For this reason, in recent years, a seat belt retractor has been developed in which the webbing is wound by a motor so that an optimum tension of the webbing is always obtained so as not to give a feeling of pressure when the webbing is worn. As described above, as an example of the seat belt retractor in which the webbing is wound by the motor, the one described in Japanese Patent Laid-Open No. 59-45243 is known.

The seat belt retractor disclosed in this publication controls a motor that rotationally drives a winding shaft that winds the webbing in a winding direction or a drawing-out direction of the webbing.
After mounting the webbing, excess webbing is wound up, and then a motor is driven in the direction of pulling out the webbing to impart a predetermined slack to the webbing so as not to give a feeling of pressure to the wearer.

However, in the seat belt retractor disclosed in this publication, when the webbing is wound by the motor while the webbing is being worn, when the webbing is pulled out from the winding shaft by the slack amount set value or more, the slack state of the webbing (tension state) The webbing is started by the motor when the wearer leans forward on the upper body and pulls out the webbing. It will give a great sense of oppression.

[Object of the Invention]

The present invention overcomes the problems with the conventional ones described above, and eliminates the feeling of pressure from the webbing during the wearing of the webbing regardless of the state of the occupant. An object of the present invention is to provide a seat belt retractor which is pulled out beyond a slack amount set value and is performed only when the tension of the webbing is not detected.

[Outline of Invention]

The present invention, a winding shaft for winding a webbing, a motor for driving the winding shaft to rotate in the webbing winding direction, a mounting detection device for detecting mounting of the webbing on a buckle, and a loosening amount of the webbing. A slack amount detector, a slack amount setting device that sets the allowable slack amount of the webbing according to the vehicle speed, a tension / slack detection device that detects the tension and slack of the webbing, the mounting detection device, a slack amount detector, and a slack A quantity setting device and a control device for controlling the motor by receiving each signal from the tension / slack detection device, wherein the control device receives a mounting signal from the mounting detection device before setting a reference position. When the slack signal from the tension / slack detection device is input, the motor is driven in the webbing winding direction, and the tension signal from the tension / slack detection device is input. The motor is stopped, the stop position is set to the reference position, and thereafter the slack amount signal detected by the slack amount detector by further withdrawal of the webbing from the reference position and the tolerance set by the slack amount setting device. The slack amount is compared with the slack amount signal, and the motor is driven in the webbing winding direction only when the slack amount exceeds the allowable slack amount and the slack signal from the tension / slack detection device is input. It is characterized by being.

[Action]

According to the present invention, when the webbing is attached to the buckle,
A mounting signal is input to the control device from the mounting detection device.When the webbing is loose at the time of mounting, a looseness signal is input from the webbing tension / slackness detection device, and the control device rotates the motor in the webbing winding direction. Take up the loose webbing. When the slack of the webbing is eliminated by winding the webbing by the motor, the tension signal from the tension / slack detecting device is input to the control device, and the control device stops the driving of the motor. Then, when the signal of the allowable slack amount is input to the control device from the slack amount setting device of the webbing, some slack is generated in the webbing, and even if the slack signal from the tension / slack detection device is input to the control device, Within the range of the allowable amount of slack, the control device keeps the motor stopped, so that the webbing does not press the wearer.

Further, the control device does not drive the motor unless the slack is generated in the webbing even if a signal indicating the amount of the webbing pulled out, which exceeds the allowable slack amount, is input from the slack amount detector due to the movement of the wearer. Leaning forward on the upper body does not give much pressure. When the wearer returns to the normal seating position, the tension / slack detector detects the slack of the webbing, so the controller drives the motor and the slack in the webbing disappears, and the tension / slack detector detects the webbing. Take up the webbing until the tension of the web is detected.

Example of Invention

 The present invention will be described below with reference to embodiments shown in the drawings.

FIGS. 1 to 3 show an embodiment of the mechanical structure of the present invention, in which a pair of side plates 2, 2 (only one of which is shown) having a substantially U-shaped cross section is used for winding the webbing 3. The shaft 4 is rotatably supported, and one end of the winding shaft 4 is provided with a lock device (not shown) for restraining the rotation of the winding shaft 4 in the pull-out direction in an emergency.

A motor 5 for rotationally driving the winding shaft 4 in the winding direction of the webbing 3 is mounted inside the corner of the side plate 2 shown in the figure. A pinion 7 is fitted on the rotary shaft 6. The motor 5 is configured such that the rotating shaft 6 is free to idle in the stopped state. An auxiliary plate 8 is stretched outside the main part of the side plate 2 so as not to affect the rotation of the winding shaft 4 and the rotating shaft 6, and the auxiliary plate 8 is provided with the winding plate. A support shaft 9 extending in a direction parallel to the shaft 4 and the rotary shaft 6 is provided in a protruding manner. This support shaft 9 has a reduction gear
10 is rotatably supported, and a large gear 11 of the reduction gear 10 meshes with the pinion 7. Further, a drive gear 13 is fitted to the end 4A of the winding shaft 4 so as to rotate synchronously with the winding shaft 4, and the driving gear 13 meshes with the small gear 12 of the reduction gear 10. ing. The drive gear 13 is integrally formed with a ratchet wheel 15 described later.

The auxiliary plate 8 includes an end portion 4A of the winding shaft 4, a rotation shaft 6,
A cover 16 for covering the support shaft 9, the pinion 7, the reduction gear 10, the drive gear 13 and the like is attached, and a guide pin 17 coaxial with the winding shaft 4 is attached to the cover 16. It is provided so as to project in the direction of the end 4A. A basin-shaped brake disc 18 is slidably fitted in the guide pin 17, and a bulge 19 at the tip of the guide pin 17 prevents the brake disc 18 from coming off. A plurality of long grooves 20 in the radial direction are formed on the outer peripheral edge of the brake disc 18, and a pin 21 protruding from the cover 16 is fitted into the long groove 20 to rotate around the brake disc 18. I have stopped. Further, a bamboo shoot spring 22 in a compressed state is interposed between the cover 16 and the brake disc 18, and the brake disc 18 is provided by the bamboo shoot spring 22.
Is urged so that the sliding contact portion 23 on the outer periphery thereof comes into pressure contact with the side surface 13A of the drive gear 13. Therefore, when the winding shaft 4 rotates, the drive gear 13 integral with the winding shaft 4 frictionally slides on the brake disc 18.

A tension detection lever 25 having a substantially U-shaped cross section for detecting the tension state, that is, the tension of the webbing 3 is rotatably supported by a support shaft 25 between the both side plates 2 on the upper part of the retractor body 1. A guide rod 27 is attached to the upper end portion of the tension detecting lever 24 so as to pass through the arcuate elongated holes 26 formed in the side plates 2 so as to center on the support shafts 25. The guide rod 27 includes the tension detection lever.
One end of a torsion spring 28 that urges 24 in the clockwise direction in FIG. 1 is engaged, and the webbing 3 pulled out from the winding shaft 4 is connected to the support shaft 25 and the guide rod 27.
Are crossed over. When the tension of the webbing 3 increases, the tension detecting lever 24 moves the torsion spring 28.
It is designed to be rotated counterclockwise against the spring force of. An arcuate slot 29 centered on the support 25 is formed on the side plate 2 at a position corresponding to the lower end of the tension detecting lever 24. The arcuate slot 29 is shown in FIG. As shown in detail, an actuator 30 projecting from the lower end of the tension detecting lever 24 is inserted. On the outside of the side plate 2, a switch 31 which constitutes a tension / slack detecting device and is used to detect the tension and slack of the webbing 3 is provided in a protruding manner. It has fixed contacts 32, 33 protruding from. Among them, the fixed contact 32 is a contact for detecting that the webbing 3 is loosened, and the fixed contact 33 is
It is a contact for detecting that the webbing 3 is in a tension state. On the other hand, movable contacts 34 and 35 for closing the fixed contacts 32 and 33 are arranged in parallel, and these movable contacts 34 and 35 are separated from the fixed contacts 32 and 33 in the free state, and the actuator 30 Pin 36 at the tip of is both movable contacts 34,35
It is projected so as to face in between. This pin 36 is a movable contact of either one of the pins according to the rotation of the tension detecting lever 24.
The movable contact 34 or 35 is brought into contact with the fixed contact 32 or 33 by pushing 34 or 35.

Returning to FIG. 1 and FIG. 2, a tumbler switch 37 for detecting the rotation angle of the winding shaft 4 due to the withdrawal or winding of the webbing 3 is disposed on the side plate 2 in the vicinity of the ratchet wheel 15. ing. This tumbler switch 37 is used for the ratchet wheel 15
Is provided with a tongue piece 39 that can come into contact with a tooth 38 that is continuously provided on the outer periphery of the tongue piece 39. The displacement is output and the displacement of the webbing 3 in the pull-out direction and the winding direction is detected. Therefore, the tumbler switch 37 can detect the amount of slack of the webbing 3 from the reference position when the certain state of the webbing 3 is set as the reference position, that is, the slack amount.
Reference numeral 37 constitutes a slack amount detector. Instead of detecting the displacement of the winding shaft 4 by the tumbler switch 37, detection by a photoelectric tube may be performed, or a variable resistor interlocked with the movement of the webbing 3 may be used.

The buckle body (not shown) is provided with a buckle switch (SW) which is a mounting detection device for detecting mounting of the tongue mounted on the tip of the webbing 3.

A tumbler switch 37 for detecting the output signal from the switch 31 for detecting the looseness or tension of the webbing 3 and the displacement of the winding shaft 4 for detecting the looseness of the webbing 3.
From the buckle switch 40 that detects the mounting of the webbing 3 on the buckle, and the output signal from the slack amount setter 41 that sets the allowable slack amount of the webbing 3 according to the vehicle speed. As shown in the figure, the data is input to a control unit (CPU) 42 which constitutes a control device. The control unit 42 is provided in a circuit provided with the motor 5 and the power source 43, and controls the driving of the motor 5.

That is, the control unit 42 basically wears the webbing 3 and outputs a mounting signal from the buckle switch 40.
Further, only when the switch 31 detects the loose state of the webbing 3, the motor 5 is driven to wind the webbing 3 so as to restrain the webbing 3 according to the body of an occupant without making the webbing cramped. It has become. The specific function of the control unit 42 will be described later.

Next, the operation of the above-described embodiment will be described with reference to the flowchart of FIG.

In the initial state shown in FIG. 5, the webbing 3 is completely wound around the winding shaft 4, and the tongue at the tip of the webbing 3 is disengaged from the buckle body.
40 is off.

In such a state, when the occupant seated on the seat pulls out the webbing 3 from the winding shaft 4 and winds it around the body, and attaches the tongue at the tip of the webbing 3 to the buckle body, the buckle switch 40 is pressed as shown in step ST _1. Is turned on, which turns on the power supply 43 (step ST ₂ ). In this state, since the webbing 3 is normally in a loosened state, the tension detecting lever 24 is set to the position shown in FIG. 1 by the action of the torsion spring 28, and the pin 36 of the actuator 30 brings the movable contact 34 into contact with the fixed contact 32. The switch 31 outputs a slack detection signal for the webbing 3 to the control unit 42 (step ST ₃ ). Then, the control unit 42 drives the motor 5 to wind the webbing 3 by the motor 5 (step ST ₄ ). The winding of the webbing 3 is performed by driving the motor 5 to rotate the winding shaft 4 as shown in step ST _5, and the tension of the webbing 3 causes the tension detecting lever 24 to rotate counterclockwise in FIG. The rotation is continued until the pin 36 of the actuator 30 brings the movable contact 35 of the switch 31 into contact with the fixed contact 33, the tension of the webbing 3 increases as shown in step ST ₆ , and the fixed contact 33 and the movable contact 35. When the closing signal is output to the control unit 42, the drive of the motor 5 is stopped by the control unit 42 and the winding operation of the webbing 3 is completed as shown in step ST ₇ . The position of the webbing 3 at this time is set to the webbing 3
And the reference position of the set (step ST _8), simultaneously activates a timer T ₁ of this setting (Step ST _9).

The timer start from for example, within 10 seconds the webbing from the switch 31 and the like occupant was stoop to (Step ST ₁₀₎ is or correct posture, or moving the body to the control unit 42 3
Slack detection signal is output (step ST _11), is likewise performed winding operation of the webbing 3 and step ST ₄ described above (step ST _12), the flow returns to step ST _10. Since the motor 5 can idle when the motor 5 is in the OFF state, if the webbing 3 is tensioned so that the body is kept in the webbing 3, the webbing 3 will be pulled out relatively lightly. Do not restrain the wearer more than necessary. Further, when the webbing 3 is pulled out, the brake disc is pressed against the drive gear 13 by the bamboo shoot spring 22.
Since the frictional resistance acts due to 18, there is no possibility that the tension of the webbing 3 is significantly reduced while the webbing 3 is being pulled out and the switch 31 outputs an erroneous looseness detection signal.

After 10 seconds have elapsed from the start of the timer in step ST ₁₀ , the webbing 3 set in step ST ₈ is set.
When the webbing 3 is pulled out from the reference position of (1) above the set value set by the slack amount setting device 41, the ratchet wheel 15
The tumbler switch 37 in which the tongue piece 39 is in contact with the tooth 38 of the webbing 3 detects the pulling out of the webbing 3 beyond the set value (step ST ₁₃ ), and as shown in step ST ₁₄ , the slack of the webbing 3 from the switch 31 is detected. When the detection signal is output, the winding operation of the webbing 3 is performed as in steps ST ₄ and ST ₁₂ described above (step ST ₁₅ ). When the winding state of the webbing 3 exceeds the reference position of the webbing 3 set in step ST _{8 as} shown in step ST ₁₆ , the new winding position is changed to the webbing 3
A reference position of the (Step ST _17). This step ST ₁₇
Setting of a new reference position of the webbing 3 due is specifically performed in step ST _16, first in step ST ₈
Set the reference position Na of the webbing 3 set in step (step ST ₁₈ ) and the webbing 3 wound in step ST ₁₅
The position Nb of is set by the signal from the tumbler switch 37 (step ST ₁₉ ). The positions Na and Nb are shown as distances from the winding shaft 4 in the pull-out direction. Then, by comparing the both positions Na and Nb in step ST _20, Nb-Na <0, i.e., when the position Nb is further wound position from the position Na is the new webbing 3 this position Nb a reference position (step ST _17). on the other hand,
Nb−Na ≧ 0, that is, position Nb is webbing 3 from position Na
When in or equal position in the drawer position, the reference position Na set in step ST ₈ is continued as it is a reference position.

By newly setting the reference position of the webbing 3 in this way, the state in which the webbing 3 is further in close contact with the wearer's body becomes the new reference position, and safety is increased.

When the reference position of the webbing 3 is newly set as described above, the buckle switch 40 is set in step ST ₂₁ .
Is detected and the buckle switch 40 is still ON.
If so, the process returns to step ST ₁₃ . Also, step ST ₁₅
In, the webbing 3 is returned from the step ST ₁₆ even when the motor 5 is stopped without being wound to the reference position of the webbing 3 set in step ST ₈ in step ST14. Furthermore, when the drawer has exceeded the set value of the webbing 3 is not performed in step ST _13, the process proceeds from step ST ₁₃ directly to step ST _21. Furthermore,
If the looseness of the webbing 3 is not detected in step ST ₁₄ , the detection operation is repeated until the looseness is detected.

Next, the wearer when removed from the buckle main body tongue so as to dissociate the webbing 3, a buckle switch 40 at step ST ₂₁ is activated OFF, the timer T ₂ (step ST _22). Then, in step ST ₂₃ , the switch 31
When the loosening detection signal of the webbing 3 is output, the winding operation of the webbing 3 is started in step ST ₂₄ , and this winding operation is continued until the switch 31 does not output the loosening detection signal of the webbing 3. This switch
When the loosening detection signal of the webbing 3 from 31 is no longer output, the time measured by the timer T ₂ is counted in step ST ₂₅ .
Until passage 30 seconds back to step ST _23, to monitor the slack of the webbing 3. And after 30 seconds, step
The power supply 43 is turned off by ST ₂₆ , and the webbing 3 returns to the initial state.

According to such an embodiment, when the webbing 3 is mounted and the webbing 3 is not being wound by the motor 5, the motor 5 can idle, so that a load acts on the webbing 3. For example, since the webbing 3 can be pulled out relatively lightly only by the frictional resistance force of the brake disc 18 pressed against the drive gear 13, there is almost no feeling of pressure on the body. It should be noted that, in an emergency, the drawer of the webbing 3 is, of course, restrained by the action of a lock mechanism (not shown).

Further, the winding of the webbing 3 during the mounting of the webbing 3 is premised on the detection of the slack of the webbing 3. Therefore, even if the webbing 3 is pulled out by tilting the body forward, the motor 5 may be pulled out only. The webbing 3 is not wound by the above, and the wearer is not unduly restrained.

Further, the reference position of the webbing 3 is further moved in the winding direction of the webbing 3 in order to set the further wound position as a new reference position, which is further safe.

In the above-described embodiment, the frictional force acts on the webbing 3 both when the webbing 3 is pulled out and when the webbing 3 is pulled out by the brake disc 18 which is pressed against the side surface 13A of the drive gear 13 by the bamboo shoot spring 22. However, the frictional force may be applied to the webbing 3 only when the webbing 3 is pulled out.

FIG. 6 and FIG. 7 show an embodiment of the frictional resistance means which works only in such a withdrawal, and this embodiment will be described below. It should be noted that the same components as those of the conventional one described above are denoted by the same reference numerals in the drawings, and description thereof is omitted.

A flat surface portion 4B is cut off on the peripheral surface of the end portion 4A of the winding shaft 4, and a sleeve 44 is formed on the outer periphery of the end portion 4A including the flat surface portion 4B.
The sleeve 44 is adapted to rotate synchronously with the winding shaft 4. The outer edge of this sleeve 44 has a flange
A drive gear 13 is fitted to the inner end of the sleeve 44 so as to rotate in synchronization with the sleeve 44. A boss 46 fitted to the outer circumference of the sleeve 44 is integrally provided on the drive gear 13 so as to project therefrom, and a brake disc 47 is fitted to the boss 46. A latch gear 48 is formed on the outer periphery of the brake disc 47 so as to face the direction in which the webbing 3 is pulled out. An annular slide contact portion 49 that can slide on the side surface 13A of the drive gear 13 is provided around the side surface 47A of the brake disc 47, and is interposed between the flange 45 and the brake disc 47. The disc spring 50 causes the sliding contact portion 49 to press against the drive gear 13. Therefore, when the rotation of the brake disc 47 is restricted, the brake disc 47 exerts a frictional resistance force on the rotation of the drive gear 13. Further, an annular groove such as a V-shaped groove and a circular groove, which is not shown, is provided around the outer periphery of the sliding contact portion 49, and an arc-shaped wire-like strip is provided in the annular groove.
51 is fitted. The strip 51 is fitted over substantially the entire circumference of the annular groove of the sliding contact portion 49, and one end of the condition 51 has a locking portion 52 that projects substantially outward in the radial direction of the brake disc 47. Are formed. Further, at the other end of the strip 51, a cam portion 53 protruding outward in the radial direction of the brake disc 47 in a curved shape and a cam portion 53 located at the tip end side of the cam portion 53 and having a substantially radius of the brake disc 47 are provided. A locking portion 54 protruding outward in the direction is formed.

On the other hand, a pin 55 is projectingly provided on the cover 16, and a latch gear 48 of the brake disc 47 is attached to the pin 55.
A ratchet 56 that can be engaged with is rotatably supported. Between the end of the ratchet 56 and another pin 57 projecting from the cover 16, a coil spring 58 in an expanded state is provided.
Are interposed, and the ratchet 56 is urged in the direction of engaging the latch gear 48. A guide pin 59, which is always located between the locking portions 52 and 54 of the strip 51 and is slidably in contact with the cam portion 53 of the strip 51, is provided at the tip of the ratchet 56.

Note that the ratchet 56 is the latch gear of the brake disc 47.
When the brake disc 47 engages with the rotation of the brake disc 47 to restrain the rotation of the brake disc 47, the friction resistance force acting on the winding shaft 4 by the brake disc 47 via the drive gear 13 is generated by the ratchet 56 of the ratchet 56 on the locking portion 54 of the strip 51. When the guide pin 59 engages and restrains the rotation of the strip 51, the brake disk 47 slides on the strip 51 and far more than the frictional resistance force acting on the winding shaft 4 via the drive gear 13. It is set large.

According to the configuration described above, when the webbing 3 is pulled out,
As shown in FIG. 8A, since the ratchet 56 is engaged with the latch gear 48 of the brake disc 47 by the spring force of the coil spring 58, the brake disc 47 can follow the rotation of the drive gear 13 in the arrow direction. Therefore, the drive gear 13 is brought into sliding contact with the sliding contact portion 49 of the brake disc 47 which is pressed against the drive gear 13 by the disc spring 50. Therefore, a frictional resistance force due to the spring force of the disc spring 50 acts on the drive gear 13 via the brake disc 47, the withdrawal of the webbing 3 is made relatively heavy, and erroneous looseness detection of the webbing 3 is prevented.

When the webbing 3 is wound up, as shown in FIG. 8B, the drive gear 13 is rotated in the direction of the arrow to drive the drive gear 13.
The brake disc 47 pressed against 13 is driven in the direction of the arrow, and along with this brake disc 47 the strip 51 rotates in the same direction by a small angle, and its cam 53 pushes the guide pin 59 to coil the ratchet 56. Rotate clockwise against the spring force of the spring 58, and latch the gear 48 of the brake disc 47.
To disengage from. After that, the locking portion 54 of the strip 51 engages with the guide pin 59 to restrain the rotation of the strip 51, but there is a small frictional resistance between the brake disc 47 driven by the drive gear 13 and the strip 51. Since only the force acts, the webbing 3 is lightly wound.

As described above, according to the present embodiment, only the withdrawal of the webbing 3 is performed with a relatively large friction resistance,
The winding of the webbing 3 by the motor 5 can be efficiently performed.

〔The invention's effect〕

As described above, according to the present invention, during wearing, the webbing is pulled out beyond the set value, and the webbing is wound when the tension does not act on the webbing. There is a practical effect that the wearer can move within a certain range without being restrained.

[Brief description of drawings]

FIG. 1 is a side view showing an embodiment of a seat belt retractor according to the present invention, and FIG. 2 is a longitudinal sectional view of an essential part of FIG.
FIG. 3 is an enlarged cross-sectional view taken along the line III-III in FIG.
1 is a block diagram of the control portion of the seat belt retractor shown in FIGS. 1 to 3, FIG. 5 is a flow chart showing the operation of the seat belt retractor shown in FIGS. 1 to 4, and FIG. FIG. 7 is a side view of a main portion showing another embodiment of the present invention, FIG. 7 is a sectional view taken along line VII-VII of FIG. 6, and FIG.
6A and 6B are schematic side views showing the operation of the embodiment of FIGS. 6 and 7. 1 ... Retractor body, 3 ... Webbing, 4 ... Winding shaft, 5 ... Motor, 6 ... Rotating shaft, 13 ... Drive gear,
15 …… Claw wheel, 18 …… Brake disc, 22 …… Bamboo shoot spring, 24 …… Tension detection lever, 31 …… Switch, 37 ……
Tumble switch, 40 …… Buckle switch, 41 …… Looseness setting device, 42 …… Control unit, 47 …… Brake disc,
48 …… Latch gear, 50 …… Disc spring, 51 …… Article, 52 ……
Locking part, 53 …… Cam part, 54 …… Locking part, 56 …… Ratchet, 58 …… Coil spring, 59 …… Guide pin.

Claims (2)

[Claims] 1. A winding shaft for winding a webbing, a motor for driving the winding shaft to rotate in a webbing winding direction, and a mounting detection device for detecting mounting of the webbing on a buckle.
A slack amount detector for detecting the slack amount of the webbing, a slack amount setting device for setting the allowable slack amount of the webbing according to the vehicle speed, a tension / slack detecting device for detecting the tension and slack of the webbing, and the mounting detecting device. , A slack amount detector, a slack amount setter, and a control device for controlling the motor by receiving each signal from the tension / slack detection device, the control device from the attachment detection device before setting the reference position. When the slack signal from the tension / slack detection device is input while the mounting signal is input, the motor is driven in the webbing winding direction, and the tension signal from the tension / slack detection device is input. The motor is stopped, the stop position is set to the reference position, and then the slack amount signal detected by the slack amount detector by further pulling out the webbing from the reference position. By comparing with the signal of the allowable slack amount set by the slack amount setting device, the motor is operated only when the slack amount exceeds the allowable slack amount and the slack signal from the tension / slack detecting device is input. A retractor for a seat belt, which is configured to be driven in a webbing winding direction. 2. The seat belt retractor according to claim 1, wherein the reference position of the webbing is such that the maximum winding position of the webbing while the webbing is being mounted is changed to a new reference position.