Patent Publication Number: US-11027867-B2

Title: Tying machine

Description:
CROSS-REFERENCE 
     This application claims priority to Japanese Patent Application No. 2017-252045, filed on Dec. 27, 2017, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The technique disclosed herein relates to a tying machine. 
     BACKGROUND 
     Japanese Patent Application Publication No. H10-46821 describes a tying machine provided with a twisting mechanism configured to twist a tying string. The tying mechanism is provided with a twisting motor. The tying machine obtains torque acting on the twisting motor as a twisting torque value, and stops the twisting motor when a predetermined tying completion condition is satisfied. The predetermined tying completion condition includes that the twisting torque value changes from an increase to a decrease. 
     SUMMARY 
     While the twisting mechanism is twisting the tying string, for example, if the tying string is displaced on a surface of an object to be tied, the twisting torque value may increase or decrease. In such a case, the technique of Japanese Patent Application Publication No. H10-46821 may determine in error that twisting of the tying string is completed although the twisting of the tying string is still insufficient, and may stop the twisting motor. The disclosure herein provides a technique capable of suppressing an error determination that twisting of a tying string is completed in a tying machine including a twisting mechanism. 
     A tying machine disclosed herein may comprise a twisting mechanism configured to twist a tying string. The twisting mechanism may include a twisting motor. The tying machine may be configured to obtain torque acting on the twisting motor as a twisting torque value, and stop the twisting motor when a predetermined tying completion condition is satisfied. The tying completion condition may include that an elapsed time since a rise in the twisting torque value was detected reaches a first predetermined time. 
     In the above tying machine, the twisting motor is stopped based on the elapsed time from the rise in the twisting torque value. Due to this, even if the twisting torque value increases and decreases due to the tying string being displaced on a surface of an object to be tied while the twisting mechanism is twisting the tying string, an error determination that twisting of the tying string is completed will not be made. 
     Another tying machine disclosed herein may comprise a twisting mechanism configured to twist a tying string. The twisting mechanism may include a twisting motor. The tying machine may be configured to obtain torque acting on the twisting motor as a twisting torque value, and stop the twisting motor when a predetermined tying completion condition is satisfied. The tying completion condition may include that a number of times the twisting motor rotated since a rise in the twisting torque value was detected reaches a first predetermined number of times of rotations. 
     According to the above tying machine, the twisting motor is stopped based on the number of times the twisting motor rotated from the rise in the twisting torque value. Due to this, even if the twisting torque value increases and decreases due to the tying string being displaced on the surface of the object to be tied while the twisting mechanism is twisting the tying string, the error determination that the twisting of the tying string is completed will not be made. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view seeing a rebar tying machine  2  according to an embodiment from an upper left rear side. 
         FIG. 2  is a perspective view seeing an internal structure of a tying machine body  4  of the rebar tying machine  2  according to the embodiment from an upper right rear side. 
         FIG. 3  is a cross-sectional view of a front part of the tying machine body  4  of the rebar tying machine  2  according to the embodiment. 
         FIG. 4  is a perspective view seeing internal structures of upper parts of the tying machine body  4  and a grip  6  of the rebar tying machine  2  according to the embodiment from an upper left front side. 
         FIG. 5  is a perspective view seeing a reel  10  and a braking mechanism  16  in the rebar tying machine  2  according to the embodiment from the upper right rear side in a case where a solenoid  46  is not electrically conducted. 
         FIG. 6  is a perspective view seeing the reel  10  and the braking mechanism  16  in the rebar tying machine  2  according to the embodiment from the upper right rear side in a case where the solenoid  46  is electrically conducted. 
         FIG. 7  is a block diagram showing an electric system of the rebar tying machine  2  according to the embodiment. 
         FIG. 8  is a flowchart explaining an example of processes which a main microcomputer  102  executes in the rebar tying machine  2  according to the embodiment. 
         FIG. 9  is a flowchart explaining an example of an initialization process which the main microcomputer  102  executes in the rebar tying machine  2  according to the embodiment 
         FIG. 10  is a flowchart explaining an example of an initial position returning process which the main microcomputer  102  executes in the rebar tying machine  2  according to the embodiment. 
         FIG. 11  is a flowchart explaining an example of a tying process which the main microcomputer  102  executes in the rebar tying machine  2  according to the embodiment. 
         FIG. 12  is a flowchart explaining an example of a wire feeding process which the main microcomputer  102  executes in the rebar tying machine  2  according to the embodiment. 
         FIGS. 13A and 13B  are graphs showing relationships of a voltage of a battery B, a current supplied from the battery B, and a rotation speed of a feeding motor  22  in the wire feeding process of  FIG. 12 . 
         FIGS. 14A and 14B  are graphs showing relationships of the rotation speed of the feeding motor  22  and a feed amount of a wire W in the wire feeding process of  FIG. 12 . 
         FIG. 15  is a flowchart explaining another example of the wire feeding process which the main microcomputer  102  executes in the rebar tying machine  2  according to the embodiment. 
         FIGS. 16A and 16B  are graphs showing relationships of the voltage of the battery B, the current supplied from the battery B, and the rotation speed of the feeding motor  22  in the wire feeding process of  FIG. 15 . 
         FIG. 17  is a flowchart explaining yet another example of the wire feeding process which the main microcomputer  102  executes in the rebar tying machine  2  according to the embodiment. 
         FIGS. 18A and 18B  are graphs showing relationships of the voltage of the battery B, the current supplied from the battery B, and the rotation speed of the feeding motor  22  in the wire feeding process of  FIG. 17 . 
         FIG. 19  is a flowchart explaining an example of a wire twisting process which the main microcomputer  102  executes in the rebar tying machine  2  according to the embodiment. 
         FIG. 20  is a block diagram showing an example of a feedback model  120  available for use in estimating load torque acting on a twisting motor  54  in the rebar tying machine  2  according to the embodiment. 
         FIG. 21  is a block diagram explaining a principle based on which the load torque of the twisting motor  54  is estimated by the feedback model  120  in the rebar tying machine  2  according to the embodiment. 
         FIG. 22  is a block diagram showing a control system equivalent to a control system of  FIG. 21 . 
         FIG. 23  is a block diagram showing an example of another feedback model  130  available for use in estimating the load torque acting on the twisting motor  54  in the rebar tying machine  2  according to the embodiment. 
         FIG. 24  is a block diagram showing an example of yet another feedback model  140  available for use in estimating the load torque acting on the twisting motor  54  in the rebar tying machine  2  according to the embodiment. 
         FIG. 25  is a block diagram showing an example of another feedback model  160  available for use in estimating the load torque acting on the twisting motor  54  in the rebar tying machine  2  according to the embodiment. 
         FIG. 26  is a flowchart explaining an example of a rate limiter value calculation process which the main microcomputer  102  executes in the rebar tying machine  2  according to the embodiment. 
         FIG. 27  is a graph showing a relationship between a chronological change in a twisting torque value and a chronological change in a rate limiter value in the rebar tying machine  2  according to the embodiment. 
         FIG. 28  is a graph explaining an example of a situation in which the twisting motor  54  is stopped in the rebar tying machine  2  according to the embodiment. 
         FIG. 29  is a graph explaining another example of the situation in which the twisting motor  54  is stopped in the rebar tying machine  2  according to the embodiment. 
         FIG. 30  is a graph explaining another example of the situation in which the twisting motor  54  is stopped in the rebar tying machine  2  according to the embodiment. 
         FIG. 31  is a graph explaining another example of the situation in which the twisting motor  54  is stopped in the rebar tying machine  2  according to the embodiment. 
         FIG. 32  is a graph explaining another example of the situation in which the twisting motor  54  is stopped in the rebar tying machine  2  according to the embodiment. 
         FIG. 33  is a flowchart explaining another example of the wire twisting process which the main microcomputer  102  executes in the rebar tying machine  2  according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Representative, non-limiting examples of the present invention will now be described in further detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Furthermore, each of the additional features and teachings disclosed below may be utilized separately or in conjunction with other features and teachings to provide improved tying machines, as well as methods for using and manufacturing the same. 
     Moreover, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described and below-described representative examples, as well as the various independent and dependent claims, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings. 
     All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter. 
     In one or more embodiments, a tying machine may comprise a twisting mechanism configured to twist a tying string. The twisting mechanism may include a twisting motor. The tying machine may be configured to obtain torque acting on the twisting motor as a twisting torque value, and stop the twisting motor when a predetermined tying completion condition is satisfied. The tying completion condition may include that an elapsed time since a rise in the twisting torque value was detected reaches a first predetermined time. 
     In the above tying machine, the twisting motor is stopped based on the elapsed time from the rise in the twisting torque value. Due to this, even if the twisting torque value increases and decreases due to the tying string being displaced on a surface of an object to be tied while the twisting mechanism is twisting the tying string, an error determination that twisting of the tying string is completed will not be made. 
     In one or more embodiments, a tying machine may comprise a twisting mechanism configured to twist a tying string. The twisting mechanism may include a twisting motor. The tying machine may be configured to obtain torque acting on the twisting motor as a twisting torque value, and stop the twisting motor when a predetermined tying completion condition is satisfied. The tying completion condition may include that a number of times the twisting motor rotated since a rise in the twisting torque value was detected reaches a first predetermined number of times of rotations. 
     In the above tying machine, the twisting motor is stopped based on the number of times the twisting motor rotated since the rise in the twisting torque value. Due to this, even if the twisting torque value increases and decreases due to the tying string being displaced on the surface of the object to be tied while the twisting mechanism is twisting the tying string, the error determination that twisting of the tying string is completed will not be made. 
     In one or more embodiments, the tying completion condition may further include that the twisting torque value reaches a predetermined torque threshold. 
     According to the above tying machine, the tying machine can be suppressed from receiving an excessive reaction force as a reaction to excessive twisting. 
     In one or more embodiments, the tying machine may be configured not to stop the twisting motor even when the tying completion condition is satisfied, in a case where a number of times the twisting motor rotated since the twisting motor started rotating has not reached a predetermined rotation number threshold. The tying machine may be configured to stop the twisting motor in a case where the tying completion condition is satisfied and the number of times the twisting motor rotated since the twisting motor started rotating reaches the predetermined rotation number threshold. 
     According to the above tying machine, the number of times of twisting that is required at minimum for tying the object to be tied can be applied to the tying string. 
     In one or more embodiments, when a predetermined cancellation condition is satisfied after the rise in the twisting torque value has been detected, the tying machine may be configured to cancel detection of the rise in the twisting torque value. 
     For example, in a case where the tying string is displaced greatly on the surface of the object to be tied while the twisting mechanism is twisting the tying string, it is preferable to redo the process to sufficiently twist the tying string again. According to the above tying machine, the detection of the rise in the twisting torque value can be cancelled to redo the process, and the tying string can sufficiently be twisted again. 
     In one or more embodiments, the detection of the rise in the twisting torque value may include detection of change from a state in which the twisting torque value is equal to a rate limiter value calculated based on the twisting torque value to a state in which the twisting torque value is higher than the rate limiter value. 
     The twisting torque value increases moderately until the tying string is brought into tight contact around the object to be tied, and increases rapidly once the tying string is in tight contact around the object to be tied. To detect the rise in the twisting torque value that changes as above, the above tying machine uses the rate limiter value. The rate limiter value moderately follows the twisting torque value in a range between a maximum increase value and a maximum decrease value. Due to this, the rate limiter value can follow the twisting torque value when the change in the twisting torque value is moderate, by which they become equal to each other. To the contrary, when the change in the twisting torque value is rapid, the rate limiter value cannot follow the twisting torque value, by which a difference between them increases. According to the above tying machine, the rise in the twisting torque value can be detected accurately by using the rate limiter value. 
     In one or more embodiments, the cancellation condition may include that the rate limiter value becomes equal to the twisting torque value again. 
     In a case where, after the rise in the twisting torque value has been detected due to a state switch from a state in which the rate limiter value is equal to the twisting torque value to a state in which the twisting torque value is higher than the rate limiter value, the twisting torque value continues to increase while the rate limiter value does not become equal to the twisting torque value again, this can be considered as that the tying string is not greatly displaced on the surface of the object to be tied, and the tying of the object to be tied is in progress under good condition. Contrary to this, in a case where the rate limiter value becomes equal to the twisting torque value again after the rise in the twisting torque value has been detected due to the state switch from the state in which the rate limiter value is equal to the twisting torque value to the state in which the twisting torque value is higher than the rate limiter value, that is, in a case where the twisting torque value decreases by a relatively large drop, the tying string is greatly displaced on the surface of the object to be tied, and it is necessary to redo the process to sufficiently twist the tying string again. According to the above tying machine, even in the case where the tying string is greatly displaced on the surface of the object to be tied while the twisting mechanism is twisting the tying string, the tying string can sufficiently be twisted again. 
     In one or more embodiments, in a case where the rise in the twisting torque value is not detected and a fall in the twisting torque value is detected, the tying machine may be configured to stop the twisting motor when an elapsed time since the fall in the twisting torque value was detected reaches a second predetermined time. 
     According to the above tying machine, the twisting motor can promptly be stopped in a case where the tying string breaks before stopping the twisting motor. 
     In one or more embodiments, in a case where the rise in the twisting torque value is not detected and a fall in the twisting torque value is detected, the tying machine may be configured to stop the twisting motor when a number of times the twisting motor rotated since the fall in the twisting torque value was detected reaches a second predetermined number of times of rotations. 
     According to the above tying machine, the twisting motor can promptly be stopped in the case where the tying string breaks before stopping the twisting motor. 
     In one or more embodiments, the detection of the fall in the twisting torque value may include detection of change from a state in which the twisting torque value is equal to a rate limiter value calculated based on the twisting torque value to a state in which the twisting torque value is lower than the rate limiter value. 
     The twisting torque value rapidly increases once the tying string is in tight contact around the object to be tied, however, it rapidly decreases when the tying string breaks. To detect the fall in the twisting torque value that changes as above, the above tying machine uses the rate limiter value. The rate limiter value moderately follows the twisting torque value in a range between a maximum increase value and a maximum decrease value. Due to this, the rate limiter value can follow the twisting torque value when the change in the twisting torque value is moderate, by which they become equal to each other. To the contrary, when the change in the twisting torque value is rapid, the rate limiter value cannot follow the twisting torque value, by which the difference between them increases. According to the above tying machine, the fall in the twisting torque value can accurately be detected by using the rate limiter value. 
     In one or more embodiments, a tying machine may comprise a feeding mechanism configured to feed a tying string, a battery, and a voltage detection circuit configured to detect a voltage of the battery. The feeding mechanism may include a feeding motor to which power is supplied from the battery. The tying machine may be configured to set a duty ratio for driving the feeding motor when feeding the tying string, in accordance with the voltage of the battery detected by the voltage detection circuit. 
     In the configuration in which the feeding motor has the power supplied from the battery, a rotation speed of the feeding motor changes according to the voltage of the battery. When there is a variation in the rotation speed of the feeding motor at a time point when the feeding motor is instructed to stop, an overshoot amount of the tying string caused until the feeding motor is actually stopped varies, and a total amount of the fed-out tying string also varies. According to the above tying machine, since the duty ratio for driving the feeding motor is set according to the voltage of the battery, the variation in the rotation speed of the feeding motor caused by the variation in the voltage of the battery can be suppressed. With this configuration, the amount of the tying string fed out from the feeding mechanism can be suppressed from varying. 
     In one or more embodiments, the tying machine may be configured to set the duty ratio for driving the feeding motor in accordance with the voltage of the battery detected by the voltage detection circuit before feeding the tying string, and maintain the duty ratio for driving the feeding motor constant while feeding the tying string. 
     According to the above configuration, the duty ratio set in accordance with the actual voltage of the battery is maintained constant while the tying string is fed out, so the variation in the rotation speed of the feeding motor caused by the variation in the voltage of the battery can be suppressed. The amount of the tying string fed out from the feeding mechanism can be prevented from varying. 
     In one or more embodiments, the tying machine may be configured to adjust the duty ratio for driving the feeding motor in accordance with the voltage of the battery detected by the voltage detection circuit so as to maintain an average applied voltage on the feeding motor constant while feeding the tying string. 
     According to the above configuration, the average applied voltage on the feeding motor is maintained constant while the tying string is fed out, so the variation in the rotation speed of the feeding motor caused by the variation in the voltage of the battery can be suppressed. The amount of the tying string fed out from the feeding mechanism can be prevented from varying. 
     In one or more embodiments, a tying machine may comprise a feeding mechanism configured to feed a tying string, and a battery. The feeding mechanism may include a feeding motor to which power is supplied from the battery, and a rotation speed sensor configured to detect a rotation speed of the feeding motor. The tying machine may be configured to adjust a duty ratio for driving the feeding motor in accordance with the rotation speed of the feeding motor detected by the rotation speed sensor so as to maintain the rotation speed of the feeding motor constant while feeding the tying string. 
     According to the above configuration, the rotation speed of the feeding motor is maintained constant while the tying string is fed out, so the variation in the rotation speed of the feeding motor caused by the variation in the voltage of the battery can be suppressed. The amount of the tying string fed out from the feeding mechanism can be prevented from varying. 
     Embodiment 
     A rebar tying machine  2  according to an embodiment will be described with reference to the drawings. The rebar tying machine  2  shown in  FIG. 1  is a power tool for tying a plurality of rebars R being an object to be tied by using a wire W being a tying string. 
     The rebar tying machine  2  includes a tying machine body  4 , a grip  6  provided at a lower part of the tying machine body  4 , and a battery receiving unit  8  provided at a lower part of the grip  6 . A battery B is detachably attached to a lower part of the battery receiving unit  8 . The tying machine body  4 , the grip  6 , and the battery receiving unit  8  are configured integrally. 
     As shown in  FIG. 2 , a reel  10  on which the wire W is wound is detachably housed in an upper rear part of the tying machine body  4 . As shown in  FIGS. 2 to 4 , the tying machine body  4  primarily includes a feeding mechanism  12 , a guide mechanism  14 , a braking mechanism  16 , a cutter mechanism  18 , and a twisting mechanism  20 . 
     As shown in  FIG. 2 , the feeding mechanism  12  is configured to feed out the wire W supplied from the reel  10  to the guide mechanism  14  at a front part of the tying machine body  4 . The feeding mechanism  12  is provided with a feeding motor  22 , a driving roller  24 , and a driven roller  26 . The wire W is held between the driving roller  24  and the driven roller  26 . The feeding motor  22  is a DC brush motor. The feeding motor  22  is configured to rotate the driving roller  24 . When the feeding motor  22  rotates the driving roller  24 , the driven roller  26  rotates in a reverse direction to a rotation direction of the driving roller  24 , the wire W held by the driving roller  24  and the driven roller  26  is fed out to the guide mechanism  14 , and the wire W is drawn out from the reel  10 . The feeding mechanism  12  includes an encoder  27  (see  FIG. 7 ) configured to detect a rotation angle of the driving roller  24 . The feeding mechanism  12  is configured to detect a feed amount of the wire W from the rotation angle of the driving roller  24  detected by the encoder  27 . 
     As shown in  FIG. 3 , the guide mechanism  14  is configured to guide the wire W fed from the feeding mechanism  12  around the rebars R in a loop. The guide mechanism  14  is provided with a guide pipe  28 , an upper curl guide  30 , and a lower curl guide  32 . A rear end of the guide pipe  28  is open toward a space between the driving roller  24  and the driven roller  26 . The wire W fed from the feeding mechanism  12  is fed into the guide pipe  28 . A front end of the guide pipe  28  is open toward an inside of the upper curl guide  30 . The upper curl guide  30  is provided with a first guide passage  34  for guiding the wire W fed from the guide pipe  28  and a second guide passage  36  (see  FIG. 4 ) for guiding the wire W fed from the lower curl guide  32 . 
     As shown in  FIG. 3 , the first guide passage  34  is provided with a plurality of guide pins  38  for guiding the wire W to give the wire W a downward curl, and a cutter  40  that constitutes a part of the cutter mechanism  18  to be described later. The wire W fed from the guide pipe  28  is guided by the guide pins  38  in the first guide passage  34 , passes through the cutter  40 , and is fed out toward the lower curl guide  32  from a front end of the upper curl guide  30 . 
     As shown in  FIG. 4 , the lower curl guide  32  is provided with a feed-back plate  42 . The feed-back plate  42  is configured to guide the wire W fed from the front end of the upper curl guide  30  and feed it back toward a rear end of the second guide passage  36  of the upper curl guide  30 . 
     The second guide passage  36  of the upper curl guide  30  is arranged adjacent to the first guide passage  34  thereof. The second guide passage  36  is configured to guide the wire W fed from the lower curl guide  32  and feed it out toward the lower curl guide  32  from the front end of the upper curl guide  30 . 
     The upper curl guide  30  and the lower curl guide  32  wrap the wire W fed from the feeding mechanism  12  around the rebars R in a loop. A number of windings of the wire W around the rebars R can be preset by a user. When the feeding mechanism  12  feeds out the wire W by a feed amount corresponding to the set number of windings, it stops the feeding motor  22  to stop feeding out of the wire W. 
     The braking mechanism  16  shown in  FIG. 2  is configured to stop rotation of the reel  10  in cooperation with the feeding mechanism  12  stopping feeding out the wire W. The braking mechanism  16  is provided with a solenoid  46 , a link  48 , and a brake arm  50 . The reel  10  is provided with engaging portions  10   a  at predetermined angle intervals in a circumferential direction, and the brake arm  50  engages with one of the engaging portions  10   a . As shown in  FIG. 5 , in a state where the solenoid  46  is not electrically conducted, the brake arm  50  is separated from the engaging portions  10   a  of the reel  10 . As shown in  FIG. 6 , in a state where the solenoid  46  is electrically conducted, the brake arm  50  is driven via the link  48  and the brake arm  50  engages with one of the engaging portions  10   a  of the reel  10 . When the feeding mechanism  12  feeds out the wire W, the braking mechanism  16  does not electrically conduct the solenoid  46  to keep the brake arm  50  separated from the engaging portions  10   a  of the reel  10  as shown in  FIG. 5 . Due to this, the reel  10  can rotate freely, and the feeding mechanism  12  can draw out the wire W from the reel  10 . Further, when the feeding mechanism  12  stops feeding out the wire W, the braking mechanism  16  electrically conducts the solenoid  46  to bring the brake arm  50  into engagement with one of the engaging portions  10   a  of the reel  10  as shown in  FIG. 6 . Due to this, rotation of the reel  10  is prohibited. Due to this, the wire W can be prevented from being loose between the reel  10  and the feeding mechanism  12  due to the reel  10  continuing to rotate by inertia even after the feeding mechanism  12  has stopped feeding out the wire W. 
     The cutter mechanism  18  shown in  FIGS. 3 and 4  cuts the wire W in a state where the wire W is wrapped around the rebars R. The cutter mechanism  18  is provided with the cutter  40  and a link  52 . The link  52  rotates the cutter  40  by cooperating with the twisting mechanism  20  to be described later. The wire W that passes within the cutter  40  is cut by rotation of the cutter  40 . 
     The twisting mechanism  20  shown in  FIG. 4  is configured to tie the rebars R with the wire W by twisting the wire W wrapped around the rebars R. The twisting mechanism  20  is provided with a twisting motor  54 , a reduction mechanism  56 , a screw shaft  58  (see  FIG. 3 ), a sleeve  60 , a push plate  61 , a pair of hooks  62 , and a magnetic sensor  63 . 
     The twisting motor  54  is a DC brushless motor. The twisting motor  54  is provided with a Hall sensor  55  (see  FIG. 7 ) configured to detect a rotation angle of a rotor (not shown). Rotation of the twisting motor  54  is transmitted to the screw shaft  58  via the reduction mechanism  56 . The twisting motor  54  is configured to rotate in both a forward direction and a reverse direction, and the screw shaft  58  is also configured to rotate in both the forward direction and the reverse direction accordingly. The sleeve  60  is disposed to cover a circumference of the screw shaft  58 . In a state where rotation of the sleeve  60  is prohibited, the sleeve  60  moves forward when the screw shaft  58  rotates in the forward direction, and the sleeve  60  moves backward when the screw shaft  58  rotates in the reverse direction. The push plate  61  is configured to move integrally with the sleeve  60  according to motion of the sleeve  60  in a front-and-rear direction. Further, when the screw shaft  58  rotates in a state where the rotation of the sleeve  60  is allowed, the sleeve  60  rotates together with the screw shaft  58 . 
     When the sleeve  60  moves forward from its initial position to a predetermined position, the push plate  61  drives the link  52  of the cutter mechanism  18  to rotate the cutter  40 . The pair of hooks  62  is provided at a front end of the sleeve  60 , and is configured to open and close according to the position of the sleeve  60  in the front-and-rear direction. When the sleeve  60  moves forward, the pair of hooks  62  closes to hold the wire W. After this, when the sleeve  60  moves backward, the pair of hooks  62  opens to release the wire W. 
     The twisting mechanism  20  rotates the twisting motor  54  in the state where the wire W is wrapped around the rebars R. In so doing, the rotation of the sleeve  60  is prohibited, and thus the sleeve  60  moves forward and the push plate  61  and the pair of hooks  62  also move forward by rotation of the screw shaft  58 , and the pair of hooks  62  close to hold the wire W. Then, when the rotation of the sleeve  60  is allowed, the sleeve  60  rotates and the pair of hooks  62  also rotates by the rotation of the screw shaft  58 . Due to this, the wire W is twisted and the rebars R are thereby tied. 
     When twisting of the wire W is finished, the twisting mechanism  20  rotates the twisting motor  54  in the reverse direction. In so doing, the rotation of the sleeve  60  is prohibited, and thus after the pair of hooks  62  opens to release the wire W, the sleeve  60  moves backward and the push plate  61  and the pair of hooks  62  also move backward by the rotation of the screw shaft  58 . By the sleeve  60  moving backward, the push plate  61  drives the link  52  of the cutter mechanism  18  to bring the cutter  40  back to its initial orientation. After this, when the sleeve  60  moves back to the initial position, the rotation of the sleeve  60  is allowed, by which the sleeve  60  and the pair of hooks  62  rotate by the rotation of the screw shaft  58  and return to their initial angle. The magnetic sensor  63  has its position in the front-and-rear direction fixed, and is configured to detect magnetism from a magnet  61   a  provided on the push plate  61  to defect whether or not the sleeve  60  is at its initial position. 
     As shown in  FIG. 1 , a first operation unit  64  is provided at an upper part of the tying machine body  4 . The first operation unit  64  is provided with a main switch  74  configured to switch on/off of a main power, and a main power LED  76  configured to display an on/off state of the main power. The main switch  74  is a momentary switch that is normally off and is turned on while it is being pressed by the user. 
     A second operation unit  90  is provided on an upper front surface of the battery receiving unit  8 . The user can set a number of windings of the wire W around the rebars R and a torque threshold for twisting the wire W via the second operation unit  90 . The second operation unit  90  is provided with setting switches  98  for setting the number of windings of the wire W around the rebars R and the torque threshold for twisting the wire W, display LEDs  96  for displaying current setting contents, and the like. The setting switches  98  and the display LEDs  96  are integrated in a sub-circuit board  92  (see  FIG. 7 ) housed inside the battery receiving unit  8 . 
     A trigger  84  which the user can operate to pull is provided at an upper front part of the grip  6 . As shown in  FIG. 4 , a trigger switch  86  configured to detect on/off of the trigger  84  is provided inside the grip  6 . When the user pulls the trigger  84  and the trigger switch  86  is turned on, the rebar tying machine  2  performs a series of operations to wrap the wire W around the rebars R by the feeding mechanism  12 , the guide mechanism  14 , and the braking mechanism  16 , cut the wire W and twist the wire W wrapped around the rebars R by the cutter mechanism  18  and the twisting mechanism  20 . 
     As shown in  FIG. 4 , a main circuit board casing  80  is housed at a lower part inside the tying machine body  4 . A main circuit board  82  is housed inside the main circuit board casing  80 . 
     As shown in  FIG. 7 , the main circuit board  82  is provided with a control power circuit  100 , a main microcomputer  102 , driver circuits  104 ,  106 ,  108 , failure detection circuits  105 ,  107 , a voltage detection circuit  110 , a current detection circuit  112 , an off-delay circuit  114 , and the like. Further, the sub-circuit board  92  is provided with a sub microcomputer  94 , the display LEDs  96 , the setting switches  98 , and the like. The main microcomputer  102  of the main circuit board  82  and the sub microcomputer  94  of the sub-circuit board  92  are configured to communicate with each other via a serial communication. The sub microcomputer  94  is configured to send contents inputted from the setting switches  98  to the main microcomputer  102 , and to control operations of the display LEDs  96  according to instructions from the main microcomputer  102 . 
     The control power circuit  100  adjusts power supplied from the battery B to a predetermined voltage and supplies power to the main microcomputer  102  and the sub microcomputer  94 . A passage through which the power is supplied from the battery B to the control power circuit  100  is provided with a main power FET  101 . When the main power FET  101  is turned on, power supply from the battery B to the control power circuit  100  is performed. When the main power FET  101  is turned off, the power supply from the battery B to the control power circuit  100  is cut off. In the disclosure herein, a state in which the power supply from the battery B to the control power circuit  100  is being performed is termed a state where the main power of the rebar tying machine  2  is on. Further, in the disclosure herein, a state in which the power supply from the battery B to the control power circuit  100  is not being performed is termed a state where the main power of the rebar tying machine  2  is off. A control input of the main power FET  101  is connected to a ground potential via a diode  103  and the main switch  74 . Further, the control input of the main power FET  101  is connected to a ground potential via a transistor  109 . Switching between on and off of the transistor  109  is executed by the main microcomputer  102 . The main switch  74  is connected to a power source potential via a resistor  111 . The main microcomputer  102  can identify the on/off state of the main switch  74  from a potential of a connection between the main switch  74  and the resistor  111 . Further, the trigger switch  86  has its one end connected to a ground potential and the other end connected to a power source potential via a resistor  118 . The main microcomputer  102  can identify the on/off state of the trigger switch  86  from a potential of a connection between the trigger switch  86  and the resistor  118 . 
     When the main switch  74  switches from off to on while the main power FET  101  is in the off state (that is, the main power of the rebar tying machine  2  is in the off state), the main power FET  101  switches to the on state. Due to this, the power supply from the battery B to the control power circuit  100  is performed, and the main power of the rebar tying machine  2  is turned on. When the power supply is performed from the control power circuit  100  to the main microcomputer  102 , the main microcomputer  102  starts up and the main microcomputer  102  identifies that the main switch  74  is being pressed. In this case, the main microcomputer  102  switches the transistor  109  to the on state. Even when the main switch  74  switches from on to off in this state, the main power FET  101  is maintained in the on state by the transistor  109 . 
     Further, when the main switch  74  switches from off to on while the main power FET  101  is in the on state (that is, the main power of the rebar tying machine  2  is in the on state), the main microcomputer  102  identifies that the main switch  74  is pressed. In this case, the main microcomputer  102  executes processes which should be executed before turning off the main power of the rebar tying machine  2 , and then switches the transistor  109  to the off state. After this, when the main switch  74  switches from on to off, the main power FET  101  switches to the off state, and the power supply from battery B to the control power circuit  100  is cut off. Due to this, the power supply to the main microcomputer  102  is cut off, and the main power of the rebar tying machine  2  is turned off. 
     The driver circuit  104  is configured to drive the solenoid  46  in accordance with an instruction from the main microcomputer  102 . Although not shown, the driver circuit  104  includes one FET as a switching element. The main microcomputer  102  can control operations of the solenoid  46  through the driver circuit  104 . 
     The failure detection circuit  105  is provided corresponding to the driver circuit  104 . The failure detection circuit  105  is configured to output a failure detection signal to the main microcomputer  102  in a case where the FET in the driver circuit  104  fails. 
     The driver circuit  106  is configured to drive the feeding motor  22  in accordance with an instruction from the main microcomputer  102 . Although not shown, the driver circuit  106  includes two FETs as switching elements. The main microcomputer  102  can control operations of the feeding motor  22  through the driver circuit  106 . 
     The failure detection circuit  107  is provided corresponding to the driver circuit  106 . The failure detection circuit  107  is configured to output a failure detection signal to the main microcomputer  102  in a case where at least one of the FETs in the driver circuit  106  fail. 
     The driver circuit  108  is configured to drive the twisting motor  54  in accordance with an instruction from the main microcomputer  102 . Although not shown, the driver circuit  108  includes an inverter circuit provided with six FETs as switching elements. The main microcomputer  102  can control operations of the twisting motor  54  by controlling operations of the inverter circuit in the driver circuit  108  based on a detection signal from the Hall sensor  55 . Unlike the driver circuits  104 ,  106 , the driver circuit  108  is not provided with a failure detection circuit for detecting failures of the FETs. This is because even when one or more of the FETs constituting the inverter circuit of the driver circuit  108  fail, the driver circuit  108  does not allow the twisting motor  54  to keep rotating. 
     The voltage detection circuit  110  is configured to detect the voltage of the battery B. The main microcomputer  102  can obtain the voltage of the battery B from a signal received from the voltage detection circuit  110 . 
     The current detection circuit  112  is configured to detect currents supplied from the battery B to the driver circuits  104 ,  106 ,  108 . The current detection circuit  112  is provided with a resistor  113  and an amplifier  115  configured to amplify a voltage drop in the resistor  113  and output the same to the main microcomputer  102 . The main microcomputer  102  can obtain the currents supplied to the driver circuits  104 ,  106 ,  108  from the battery B, that is, the currents supplied to the twisting motor  54 , the feeding motor  22 , the solenoid  46 , and the like from the battery B, based on signals received from the current detection circuit  112 . 
     A passage through which the power is supplied from the battery B to the driver circuits  104 ,  106 ,  108  is provided with a protective FET  116 . When the protective FET  116  is turned on, the power supply from the battery B to the driver circuits  104 ,  106 ,  108  is performed. When the protective FET  116  is turned off, the power supply from the battery B to the driver circuits  104 ,  106 ,  108  is cut off. An output of an AND circuit  119  is connected to a control input of the protective FET  116 . A control output from the main microcomputer  102  and an output from the off-delay circuit  114  are inputted to the AND circuit  119 . Due to this, the protective FET  116  shifts to an on state when an H signal is outputted from the main microcomputer  102  as the control output and an H signal is outputted from the off-delay circuit  114 . Further, the protective FET  116  shifts to an off state when an L signal is outputted from the main microcomputer  102  as the control output or an L signal is outputted from the off-delay circuit  114 . A control output from the sub microcomputer  94  may further be inputted to an input of the AND circuit  119 . In this case, the protective FET  116  shifts to the on state when the H signal is outputted from the main microcomputer  102  as the control output, an H signal is outputted from the sub microcomputer  94  as the control output, and the H signal is outputted from the off-delay circuit  114 , and shifts to the off state otherwise. 
     The off-delay circuit  114  is configured to normally output the H signal and output the L signal after a predetermined delay time has elapsed since the main switch  74  or the trigger switch  86  switched from on to off. When the off-delay circuit  114  outputs the L signal, the protective FET  116  switches to the off state regardless of contents of the control output from the main microcomputer  102 . The delay time of the off-delay circuit  114  is preset to a time that is longer than a required time for a tying process (wire feeding process, wire twisting process, and initial position returning process) to be described later. An output of a NAND circuit  117  is connected to an input of the off-delay circuit  114 . One input of the NAND circuit  117  is connected to the ground potential via the main switch  74 , and the other input of the NAND circuit  117  is connected to the ground potential via the trigger switch  86 . 
     In the rebar tying machine  2  of the present embodiment, presences and absences of the power supply to the driver circuits  104 ,  106 ,  108  can be controlled by the single protective FET  116 . With such a configuration, a number of components can be reduced as compared to a case where protective FETs individually corresponding to the driver circuits  104 ,  106 ,  108  are provided, and a space in the main circuit board  82  can be reduced. 
     In the rebar tying machine  2  of the present embodiment, the protective FET  116  is turned off by the output from the off-delay circuit  114  regardless of the contents of the control output from the main microcomputer  102  after the predetermined delay time has elapsed since the main switch  74  or the trigger switch  86  switched from on to off, by which the power supply to the driver circuits  104 ,  106 ,  108  is cut off. With such a configuration, the solenoid  46 , the feeding motor  22 , and the twisting motor  54  can be prevented from continuing to be driven if the main microcomputer  102  goes out of control. 
     In the rebar tying machine  2  of the present embodiment, the presence and absence of the power supply from the battery B to the driver circuits  104 ,  106 ,  108  is controlled by the protective FET  116  that operates according to the output control from the main microcomputer  102 , instead of by a mechanical switching mechanism. With such a configuration, even in a case where the main switch  74  is operated (that is, an operation to turn off the main power of the rebar tying machine  2  is performed) during the tying process (the wire feeding process, the wire twisting process, and the initial position returning process) to be described later, the power supply from the battery B to the driver circuits  104 ,  106 ,  108  is not cut off immediately at this time point, and the power supply from the battery B to the driver circuits  104 ,  106 ,  108  can be cut off after completion of necessary operations. 
     In the rebar tying machine  2  of the present embodiment, a momentary switch is used as the main switch  74 . With such a configuration, in a case where the main power of the rebar tying machine  2  is switched from on to off due to a cause other than the operation of the main switch  74  (for example, in a case where, as an automatic power-off function, the main power of the rebar tying machine  2  is turned off because the main microcomputer  102  switches the transistor  109  to an off state due to the main switch  74  and the trigger switch  86  not being operated over a predetermined time period), an operation for switching the main power of the rebar tying machine  2  to on again from off can be simplified. 
     Hereinbelow, processes which the main microcomputer  102  executes will be described with reference to  FIG. 8 . When the main power FET  101  is turned on according to the operation on the main switch  74  and the power is supplied from the control power circuit  100  to the main microcomputer  102 , the main microcomputer  102  executes the initialization process in step S 2 . After this, in step S 4 , the main microcomputer  102  waits until the trigger switch  86  is turned on. When the trigger switch  86  is turned on (YES in S 4 ), the process proceeds to step S 6 , and the main microcomputer  102  executes the tying process. After this, the process returns to step S 4 . 
       FIG. 9  shows a process which the main microcomputer  102  executes in the initialization process in step S 2  of  FIG. 8 . In step S 8 , the main microcomputer  102  turns on the protective FET  116 . Due to this, the power supply from the battery B to the driver circuits  104 ,  106 ,  108  is performed. 
     In step S 10 , the main microcomputer  102  determines whether or not an abnormality is detected. For example, the main microcomputer  102  may determine that an abnormality is detected in a case where a failure of one of the FETs in the driver circuits  104 ,  106  is detected by the failure detection circuit  105  or  107 . Alternatively, the main microcomputer  102  may determine that an abnormality is detected in a case where the voltage of the battery B detected by the voltage detection circuit  110  is below a predetermined lower limit. Alternatively, the main microcomputer  102  may determine that an abnormality is detected in a case where the voltage of the battery B detected by the voltage detection circuit  112  exceeds a predetermined upper limit. Alternatively, in a case where the rebar tying machine  2  is provided with a wire remaining amount detection mechanism (not shown) for detecting a remaining amount of the wire W wound on the reel  10 , the main microcomputer  102  may determine that an abnormality is detected in a case where the remaining amount of the wire W wound on the reel  10  is below a predetermined lower limit. 
     In a case where an abnormality is detected in step S 10  (in a case of YES), the process proceeds to step S 26 . In step S 26 , the main microcomputer  102  displays the occurrence of the abnormality on the display LEDs  96  via the sub microcomputer  94 . After step S 26 , the process proceeds to step S 24 . In step S 24 , the main microcomputer  102  turns off the protective FET  116 . Due to this, the power supply from the battery B to the driver circuits  104 ,  106 ,  108  is cut off. After step S 24 , the initialization process of  FIG. 9  is terminated. The process in step S 10  may be executed at any time while processes of steps S 12  to S 22  are being executed. 
     In a case where no abnormality is detected in step S 10  (in a case of NO), the process proceeds to step S 12 . In step S 12 , the main microcomputer  102  determines whether or not the sleeve  60  of the twisting mechanism  20  is at the initial position. Whether or not the sleeve  60  is at the initial position can be determined from the detection signal of the magnetic sensor  63 . In a case where the sleeve  60  is at the initial position (in a case of YES), the initial position returning process in step S 14  is skipped, and the process proceeds to step S 16 . In a case where the sleeve  60  is not at the initial position (in a case of NO), the process proceeds to step S 16  after the initial position returning process in step S 14  has been executed. 
       FIG. 10  shows processes which the main microcomputer  102  executes in the initial position returning process in step S 14  of  FIG. 9 . 
     In step S 32 , the main microcomputer  102  rotates the twisting motor  54  in the reverse direction. Due to this, the sleeve  60  located forward than the initial position moves backward. 
     In step S 34 , the main microcomputer  102  waits until the sleeve  60  moves back to the initial position. When the sleeve  60  moves back to the initial position (YES in S 34 ), the main microcomputer  102  stops the twisting motor  54  in step S 36 . 
     In step S 38 , the main microcomputer  102  further rotates the twisting motor  54  in the reverse direction. An instructed voltage to the twisting motor  54  at this timing is lower than an instructed voltage to the twisting motor  54  in step S 32 . As such, the twisting motor  54  rotates at a lower speed than its rotation in step S 32 . Due to this, the sleeve  60 , which moved backward to the initial position and is allowed to rotate, rotates toward its initial angle. 
     In step S 40 , the main microcomputer  102  determines whether or not the sleeve  60  has rotated to the initial angle and the twisting motor  54  is locked. For example, the main microcomputer  102  detects the current supplied from the battery B to the twisting motor  54  by the current detection circuit  112 , and determines that the twisting motor  54  is locked when the detected current is equal to or greater than a predetermined value. When it is determined that the twisting motor  54  is locked (YES in S 40 ), the main microcomputer  102  stops the twisting motor  54  in step S 42 , and terminates the initial position returning process of  FIG. 10 . 
     In a case where the operation on the main switch  74  is performed (that is, the operation to turn off the main power of the rebar tying machine  2  is performed) during when the initial position returning process shown in  FIG. 10  is being executed, the main microcomputer  102  stops the twisting motor  54  at that instant and switches the protective FET  116  to off, and further switches the transistor  109  to off to turn off the main power of the rebar tying machine  2 . 
     In step S 16  of  FIG. 9 , the main microcomputer  102  rotates the twisting motor  54  in the forward direction. Due to this, the sleeve  60  moves forward from the initial position. 
     In step S 18 , the main microcomputer  102  waits until a predetermined time period (such as 200 ms) elapses. When the predetermined time period elapses (YES in S 18 ), the process proceeds to step S 20 . 
     In step S 20 , the main microcomputer  102  stops the twisting motor  54 . 
     In step S 22 , the main microcomputer  102  executes the initial position returning process shown in  FIG. 10  again. 
     In step S 24 , the main microcomputer  102  turns off the protective FET  116 . Due to this, the power supply from the battery B to the driver circuits  104 ,  106 ,  108  is cut off. After step S 24 , the initialization process of  FIG. 9  is terminated. 
     Hereinbelow, the tying process in step S 6  of  FIG. 8  will be described.  FIG. 11  shows processes which the main microcomputer  102  executes in the tying process in step S 6  of  FIG. 8 . In step S 48 , the main microcomputer  102  turns on the protective FET  116 . Due to this, the power from the battery B is supplied to the driver circuits  104 ,  106 ,  108 . 
     In step S 50 , the main microcomputer  102  determines whether or not an abnormality is detected. For example, the main microcomputer  102  may determine that an abnormality is detected in the case where a failure of one of the FETs in the driver circuits  104 ,  106  is detected by the failure detection circuit  105  or  107 . Alternatively, the main microcomputer  102  may determine that an abnormality is detected in the case where the voltage of the battery B detected by the voltage detection circuit  110  is below the predetermined lower limit. Alternatively, the main microcomputer  102  may determine that an abnormality is detected in a case where the current of the battery B detected by the current detection circuit  112  exceeds a predetermined upper limit. Alternatively, in the case where the rebar tying machine  2  is provided with the wire remaining amount detection mechanism (not shown) for detecting the remaining amount of the wire W wound on the reel  10 , the main microcomputer  102  may determine that an abnormality is detected in the case where the remaining amount of the wire W wound on the reel  10  is below the predetermined lower limit. 
     In a case where an abnormality is detected in step S 50  (in a case of YES), the process proceeds to step S 60 . In step S 60 , the main microcomputer  102  displays the occurrence of the abnormality on the display LEDs  96  via the sub microcomputer  94 . After step S 60 , the process proceeds to step S 58 . In step S 58 , the main microcomputer  102  turns off the protective FET  116 . Due to this, the power supply from the battery B to the driver circuits  104 ,  106 ,  108  is cut off. After step S 58 , the tying process of  FIG. 11  is terminated. The process in step S 50  may be executed at any time while processes of steps S 52  to S 56  are being executed. 
     In a case where no abnormality is detected in step S 50  (in a case of NO), the process proceeds to step S 52 . In step S 52 , the main microcomputer  102  executes the wire feeding process. After this, in step S 54 , the main microcomputer  102  executes the wire twisting process. After this, in step S 56 , the main microcomputer  102  executes the initial position returning process shown in  FIG. 10 . In step S 58 , the main microcomputer  102  turns off the protective FET  116 . Due to this, the power supply from the battery B to the driver circuits  104 ,  106 ,  108  is cut off. After step S 58 , the tying process of  FIG. 11  is terminated. 
       FIG. 12  shows processes which the main microcomputer  102  executes in the wire feeding process in step S 52  of  FIG. 11 . 
     In step S 62 , the main microcomputer  102  detects the voltage of the battery B by the voltage detection circuit  110 . At this time point, since none of the twisting motor  54 , the feeding motor  22 , and the solenoid  46  is driven, the voltage obtained in step S 62  is an open voltage of the battery B. 
     In step S 64 , the main microcomputer  102  sets a feed amount threshold of the wire W based on the number of windings of the wire W set by the user and the voltage of the battery B obtained in step S 62 . In so doing, the main microcomputer  102  sets the feed amount threshold of the wire W to a small value when the voltage of the battery B is high, and sets the feed amount threshold of the wire W to a large value when the voltage of the battery B is low. 
     In step S 66 , the main microcomputer  102  sets a duty ratio for driving the feeding motor  22  based on the voltage of the battery B obtained in step S 62 . Specifically, the main microcomputer  102  sets the duty ratio according to the voltage of the battery B obtained in step S 62  so that an average applied voltage to the feeding motor  22  comes to be at a predetermined value. 
     In step S 68 , the main microcomputer  102  drives the feeding motor  22  at the duty ratio set in step S 66 . Due to this, the feeding motor  22  rotates and the wire W is thereby fed out. 
     In step S 70 , the main microcomputer  102  waits until the feed amount of the wire W reaches the feed amount threshold set in step S 64 . The feed amount of the wire W can be calculated based on a detection vale of the encoder  27  of the feeding mechanism  12 . When the feed amount of the wire W reaches the feed amount threshold (YES in S 70 ), the process proceeds to step S 72 . 
     In step S 72 , the main microcomputer  102  stops the feeding motor  22 . The feeding motor  22  stops after rotating for a while by inertia. 
     In step S 74 , the main microcomputer  102  electrically conducts the solenoid  46  of the braking mechanism  16 . Due to this, the brake arm  50  is driven through the link  48 . 
     In step S 76 , the main microcomputer  102  waits until a predetermined time elapses. During this time, the brake arm  50  of the braking mechanism  16  engages with one of the engaging portions  10   a  of the reel  10  and the rotation of the reel  10  stops. When the predetermined time elapses in step S 76  (YES in S 76 ), the process proceeds to step S 78 . 
     In step S 78 , the main microcomputer  102  cuts off electric conduction to the solenoid  46  of the braking mechanism  16 . Due to this, the brake arm  50  separates from the engaging portion  10   a  of the reel  10 . After step S 78 , the wire feeding process of  FIG. 12  is terminated. 
     As shown in  FIG. 13A , the voltage of the battery B and the current supplied from the battery B change over time upon driving the feeding motor  22 . When the rotation speed of the feeding motor  22  changes due to such changes in the voltage of the battery B, a degree of the rotation of the feeding motor  22  by inertia since the main microcomputer  102  outputted a stop instruction to the feeding motor  22  until the feeding motor  22  actually stops changes, by which a final feed amount of the wire W would thereby vary. According to the wire feeding process shown in  FIG. 12 , the duty ratio of the feeding motor  22  is set based on the open voltage of the battery B before the feeding motor  22  is driven and the feeding motor  22  is kept driven by the constant duty ratio, by which the variation in the rotation speed of the feeding motor  22  can be suppressed as shown in  FIG. 13B . With such a configuration, the variation in the feed amount of the wire W accompanying the variation in the voltage of the battery B can be suppressed. 
     Further, in the wire feeding process shown in  FIG. 12 , the feed amount threshold of the wire W is set based on the open voltage of the battery B before the feeding motor  22  is driven. In a case where the voltage of the battery B is high, as shown in  FIG. 14A , the applied voltage to the feeding motor  22  becomes high and the rotation speed of the feeding motor  22  becomes fast. In this case, the feeding motor  22  rotates for a while since the main microcomputer  102  outputted the stop instruction to the feeding motor  22  until the feeding motor  22  actually stops, so the final feed out amount of the wire W becomes large. On the other hand, in a case where the voltage of the battery B is low, as shown in  FIG. 14B , the applied voltage to the feeding motor  22  becomes low and the rotation speed of the feeding motor  22  becomes slow. In this case, the feeding motor  22  hardly rotates since the main microcomputer  102  outputted the stop instruction to the feeding motor  22  until the feeding motor  22  actually stops, so the final feed out amount of the wire W becomes small. In the wire feeding process shown in  FIG. 12 , the feed amount threshold of the wire W is set to a small value when the open voltage of the battery B before the feeding motor  22  is driven is high, and the feed amount threshold of the wire W is set to a large value when the open voltage of the battery B before the feeding motor  22  is driven is low. With such a configuration, the variation in the feed amount of the wire W caused by the variation in the voltage of the battery B can be suppressed. 
     The main microcomputer  102  may set the duty ratio to a constant value (such as 100%) for driving the feeding motor  22  in step S 66  of  FIG. 12 , regardless of the voltage of the battery B obtained in step S 62 . Even in this case, the variation in the feed amount of the wire W can be suppressed by setting the feed amount threshold of the wire W according to the open voltage of the battery B as aforementioned. 
     The main microcomputer  102  may execute a wire feeding process shown in  FIG. 15  instead of the wire feeding process shown in  FIG. 12 . Hereinbelow, the wire feeding process shown in  FIG. 15  will be described. 
     In step S 82 , the main microcomputer  102  sets the feed amount threshold based on the number of windings of the wire W set by the user, and sets the duty ratio to a predetermined value. 
     In step S 84 , the main microcomputer  102  drives the feeding motor  22  at the duty ratio set in step S 82 . Due to this, the feeding motor  22  rotates and the wire W is fed out. 
     In step S 86 , the main microcomputer  102  detects the voltage of the battery B by the voltage detection circuit  110 . 
     In step S 88 , the main microcomputer  102  sets a duty ratio for driving the feeding motor  22  based on the voltage of the battery B obtained in step S 86 . Specifically, the main microcomputer  102  sets the duty ratio according to the voltage of the battery B obtained in step S 86  so that the average applied voltage to the feeding motor  22  comes to be at a predetermined value. 
     In step S 90 , the main microcomputer  102  determines whether or not the feed amount of the wire W has reached the feed amount threshold set in step S 82 . In a case where the feed amount of the wire W has not reached the feed amount threshold (in a case of NO), the process returns to step S 86 . When the feed amount of the wire W reaches the feed amount threshold (YES in step S 90 ), the process proceeds to step S 72 . 
     Processes of steps S 72 , S 74 , S 76 , S 78  of  FIG. 15  are similar to the processes of steps S 72 , S 74 , S 76 , S 78  of  FIG. 12 . 
     In the wire feeding process shown in  FIG. 15 , the duty ratio for the feeding motor  22  is continuously updated based on the voltage of the battery B during when the feeding motor  22  is being driven so that the average applied voltage to the feeding motor  22  remains constant. Due to this, even in the case where the voltage of the battery B varies as shown in  FIG. 16A , the variation in the rotation speed of the feeding motor  22  can be suppressed as shown in  FIG. 16B . In the wire feeding process shown in  FIG. 15 , the duty ratio for the feeding motor  22  is continuously updated based on the voltage of the battery B during when the feeding motor  22  is being driven, so the rotation speed of the feeding motor  22  can further be stabilized as compared to the case where the duty ratio for the feeding motor  22  is set based on the open voltage of the battery B before the feeding motor  22  is driven and the feeding motor  22  is continuously driven at the constant duty ratio as in the wire feeding process shown in  FIG. 12 . With such a configuration as well, the variation in the feed amount of the wire W accompanying the variation in the voltage of the battery B can be suppressed. 
     Alternatively, the main microcomputer  102  may execute a wire feeding process shown in  FIG. 17  instead of the wire feeding processes shown in  FIGS. 12 and 15 . Hereinbelow, the wire feeding process shown in  FIG. 17  will be described. 
     In step S 92 , the main microcomputer  102  sets the feed amount threshold based on the number of windings of the wire W set by the user, and sets a duty ratio to a predetermined value. 
     In step S 94 , the main microcomputer  102  drives the feeding motor  22  at the duty ratio set in step S 92 . Due to this, the feeding motor  22  rotates and the wire W is fed out. 
     In step S 96 , the main microcomputer  102  calculates the rotation speed of the feeding motor  22  by using the detection signal from the encoder  27 . 
     In step S 98 , the main microcomputer  102  sets a duty ratio for the feeding motor  22  by PI control based on a difference between a targeted rotation speed of the feeding motor  22  and an actual rotation speed of the feeding motor  22  calculated in step S 96 . 
     In step S 100 , the main microcomputer  102  determines whether or not the feed amount of the wire W has reached the feed amount threshold set in step S 92 . In a case where the feed amount of the wire W has not reached the feed amount threshold (in a case of NO), the process returns to step S 96 . When the feed amount of the wire W reaches the feed amount threshold (YES in step S 100 ), the process proceeds to step S 72 . 
     Processes of steps S 72 , S 74 , S 76 , S 78  of  FIG. 17  are similar to the processes of steps S 72 , S 74 , S 76 , S 78  of  FIG. 12 . 
     In the wire feeding process shown in  FIG. 17 , the duty ratio for the feeding motor  22  is continuously updated by the PI control so that the rotation speed of the feeding motor  22  remains constant during when the feeding motor  22  is being driven. Due to this, even in the case where the voltage of the battery B varies as shown in  FIG. 18A , the rotation speed of the feeding motor  22  can be maintained constant as shown in  FIG. 18B . In the wire feeding process shown in  FIG. 17 , the rotation speed of the feeding motor  22  can further be stabilized as compared to the wire feeding process shown in  FIG. 12  and the wire feeding process shown in  FIG. 15 . With such a configuration as well, the variation in the feed amount of the wire W accompanying the variation, in the voltage of the battery B can be suppressed. 
     In a case where the operation on the main switch  74  is performed (that is, the operation to turn off the main power of the rebar tying machine  2  is performed) while one of the wire feeding processes shown in  FIGS. 12, 15, and 17  is being executed, the main microcomputer  102  does not immediately turn off the main power of the rebar tying machine  2  at that instant, but skips the processes preceding step S 72  and executes the processes from steps S 72  to S 78 , after which the main microcomputer  102  switches the protective FET  116  to off and switches the transistor  109  to off to turn off the main power of the rebar tying machine  2 . With such a configuration, the wire W can be prevented from being loosened due to the reel  10  rotating by inertia after the power supply to the feeding motor  22  has been cut off. 
     Hereinbelow, the wire twisting process in step S 54  of  FIG. 11  will be described.  FIG. 19  shows processes which the main microcomputer  102  executes in the wire twisting process in step S 54  of  FIG. 11 . 
     In step S 102 , the main microcomputer  102  clears both a first counter and a second counter. 
     In step S 104 , the main microcomputer  102  rotates the twisting motor  54  in the forward direction with 100% duty ratio. 
     In step S 105 , the main microcomputer  102  starts counting a number of times the twisting motor  54  rotates by using another counter that is different from the first and second counters. In the rebar tying machine  2  of the present embodiment, the main microcomputer  102  counts the number of times the twisting motor  54  rotates based on a detection signal of the Hall sensor  55 . 
     In step S 106 , the main microcomputer  102  obtains load torque that acts on the twisting motor  54  as a twisting torque value. In the rebar tying machine  2  of the present embodiment, the main microcomputer  102  estimates the load torque that acts on the twisting motor  54  according to the following calculation, based on the voltage detected by the voltage detection circuit  110  and the current detected by the current detection circuit  112 . 
       FIG. 20  shows an example of a feedback model  120  that the main microcomputer  102  uses to estimate the load torque that acts on the twisting motor  54 . The feedback model  120  outputs an estimated value τ e  of the load torque that acts on the twisting motor  54  based on a measured value i m  of the current flowing in the twisting motor  54  and a measured value V m  of an inter-terminal voltage of the twisting motor  54 . At a time point when the main microcomputer  102  executes the process of step S 106  of  FIG. 19 , the feeding motor  22  and the solenoid  46  are not driven. As such, the measured value i m  of the current flowing in the twisting motor  54  can be detected by the current detection circuit  112 . Further, the measured value V m  of an inter-terminal voltage of the twisting motor  54  can be detected by the voltage detection circuit  110 . The feedback model  120  is provided with a motor model  122 , a comparator  124 , and an amplifier  126 . 
     The motor model  122  is a model of characteristics of the twisting motor  54  which is configured as a two-input and two-output transfer system. In the motor model  122 , the inter-terminal voltage V of the twisting motor  54  and the load torque τ that acts on the twisting motor  54  are inputs, and the current i flowing in the twisting motor  54  and the rotation speed ω of the twisting motor  54  are outputs. 
     A characteristic of the motor model  122  can be specified based on an actual input-output characteristic of the twisting motor  54 . For example, in the case where the twisting motor  54  is a DC brushless motor as in the present embodiment, the characteristic of the motor model  122  can be determined as below. 
     In regard to an electrical system of the twisting motor  54 , a relational expression below is established, where L is an inductance, i is a current, V is an inter-terminal voltage, R is a resistance, KB is a power generation constant, and ω is a rotation speed: 
     
       
         
           
             
               
                 
                   
                     L 
                     ⁢ 
                     
                       di 
                       dt 
                     
                   
                   = 
                   
                     V 
                     - 
                     Ri 
                     - 
                     
                       KB 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       ω 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     On the other hand, in regard to a mechanical system of the twisting motor  54 , a relational expression below is established, where J is moment of inertia of a rotor, KT is a torque constant, B is a frictional constant, and τ is load torque: 
     
       
         
           
             
               
                 
                   
                     J 
                     ⁢ 
                     
                       
                         d 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ω 
                       
                       dt 
                     
                   
                   = 
                   
                     KTi 
                     - 
                     
                       B 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       ω 
                     
                     - 
                     τ 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In the disclosure herein, a left side of the above mathematical expression (2) is called inertial torque, a first term on a right side thereof is called output torque, a second term on the right side is called frictional torque, and a third term on the right side is called load torque. 
     When both sides of the above mathematical expressions (1) and (2) are integrated with respect to time, the following two relational expressions are obtained: 
     
       
         
           
             
               
                 
                   i 
                   = 
                   
                     ∫ 
                     
                       
                         ( 
                         
                           
                             
                               1 
                               L 
                             
                             ⁢ 
                             V 
                           
                           - 
                           
                             
                               R 
                               L 
                             
                             ⁢ 
                             i 
                           
                           - 
                           
                             
                               KB 
                               L 
                             
                             ⁢ 
                             ω 
                           
                         
                         ) 
                       
                       ⁢ 
                       dt 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   ω 
                   = 
                   
                     ∫ 
                     
                       
                         ( 
                         
                           
                             
                               KT 
                               J 
                             
                             ⁢ 
                             i 
                           
                           - 
                           
                             
                               B 
                               J 
                             
                             ⁢ 
                             ω 
                           
                           - 
                           
                             
                               1 
                               J 
                             
                             ⁢ 
                             τ 
                           
                         
                         ) 
                       
                       ⁢ 
                       dt 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     The two outputs i, ω for the two inputs V, τ can be calculated by performing numerical calculations based on the above mathematical expressions (3) and (4). As can be understood from the above, in the case where the motor model  122  is configured with the inter-terminal voltage V of the twisting motor  54  and the load torque τ that acts on the twisting motor  54  as the inputs and the current i flowing in the twisting motor  54  and the rotation speed ω of the twisting motor  54  as the outputs, the respective outputs can be obtained by integration calculations without performing differential calculations. Generally, in a case where the main microcomputer  102  is implemented with a single chip microcomputer or the like, it is difficult to accurately perform the differential calculations in an event where the inter-terminal voltage V of the twisting motor  54  and the current i flowing in the twisting motor  54  abruptly change. However, by constructing the motor model  122  to obtain the outputs by the integration calculations as above, behaviors of the twisting motor  54  can be simulated with high accuracy even in the event where the inter-terminal voltage V of the twisting motor  54  and the current i flowing in the twisting motor  54  abruptly change. 
     As shown in  FIG. 20 , the current output of the motor model  122 , that is, an estimated value i e  of the current in the twisting motor  54  is supplied to the comparator  124 . In the comparator  124 , a difference Δi between the measured value i m  of the current in the twisting motor  54  and the current output i e  of the motor model  122  is calculated. The calculated difference Δi is amplified by a predetermined gain G in the amplifier  126 , and is inputted to the torque input of the motor model  122  as the estimated load torque τ e  of the twisting motor  54 . The measured value V m  of the inter-terminal voltage of the twisting motor  54  is inputted to the voltage input of the motor model  122 . 
     In the above feedback model  120 , by setting the gain G in the amplifier  126  sufficiently large, a magnitude of the input torque of the motor model  122 , that is, a magnitude of the estimated value τ e  of the load torque that acts on the twisting motor  54  is adjusted so that the current output of the motor model  122 , that is, the estimated value i e  of the current in the twisting motor  54  converges to the measured value i m  of the current in the twisting motor  54 . With such a configuration, the load torque τ e  that acts on the twisting motor  54 , which would realize the current i m  flowing in the twisting motor  54  when the inter-terminal voltage V m  is applied to the twisting motor  54 , and the rotation speed ω e  of the twisting motor  54  at such timing can be calculated by using the motor model  122 . 
     A principle based on which the load torque τ of the twisting motor  54  is estimated by the feedback model  120  will be described with reference to  FIG. 21 . In  FIG. 21 , the actual twisting motor  54  is expressed by a transfer function M 1 , and the motor model  122  that is virtually implements the twisting motor  54  in the feedback model  120  is expressed by a transfer function M 2 . A relationship between an input τ 1  (a load torque value acting on the actual twisting motor  54 ) and an output τ 2  (a torque estimated value outputted from the feedback model  120 ) in a control system shown in  FIG. 21  is as follows: 
     
       
         
           
             
               
                 
                   
                     τ 
                     2 
                   
                   = 
                   
                     
                       
                         GM 
                         1 
                       
                       
                         1 
                         + 
                         
                           GM 
                           2 
                         
                       
                     
                     ⁢ 
                     
                       τ 
                       1 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     As such, by setting the motor model  122  in the feedback model  120  to have equivalent characteristics to those of the actual twisting motor  54 , replacement of M 1 =M 2 =M can be performed in the above expression, by which a relational expression as below is obtained: 
     
       
         
           
             
               
                 
                   
                     τ 
                     2 
                   
                   = 
                   
                     
                       GM 
                       
                         1 
                         + 
                         GM 
                       
                     
                     ⁢ 
                     
                       τ 
                       1 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     As can be understood from the above mathematical expression (6), the transfer function from the input τ 1  to the output τ 2  in the control system of  FIG. 21  is equivalent to a feedback control system as shown in  FIG. 22  in which a forward transfer function is GM and a backward transfer function is 1. As such, the output τ 2  changes to follow the input τ 1 . By setting the gain G in the amplifier  126  sufficiently large, the output τ 2  converges to the input τ 1 . Thus, the load torque τ 1  acting on the twisting motor  54  can be acknowledged from the torque estimated value τ 2  outputted from the feedback model  120 . 
     According to the feedback model  120  of the present embodiment, the load torque T that acts on the twisting motor  54  can accurately be estimated based on the inter-terminal voltage V of the twisting motor  54  and the current i flowing in the twisting motor  54  without providing a dedicated sensor for torque detection. 
     In the present embodiment, the feedback model  120  including the motor model  122  that uses the inter-terminal voltage V of the twisting motor  54  and the load torque τ that acts on the twisting motor  54  as the inputs and the current i flowing in the twisting motor  54  and the rotation speed ψ of the twisting motor  54  as the outputs is used to converge the current output i e  of the motor model  122  to the current i m  flowing in the actual twisting motor  54 . With such a configuration, the load torque τ that acts on the twisting motor  54  can accurately be estimated without using the differential calculations. 
     Alternatively, in a case where the twisting motor  54  is provided with a rotation speed sensor (not shown) configured to detect rotation speed, the load torque τ that acts on the twisting motor  54  may be estimated by using a feedback model  130  shown in  FIG. 23 . The feedback model  130  is configured to output the estimated value τ e  of the load torque that acts on the twisting motor  54  based on the measured value ω m  of the rotation speed of the twisting motor  54  detected by the rotation speed sensor and the measured value V m  of the inter-terminal voltage of the twisting motor  54  detected by the voltage detection circuit  110 . The feedback model  130  is provided with a motor model  132 , a comparator  134 , and an amplifier  136 . 
     The motor model  132  of the feedback model  130  of  FIG. 23  is same as the motor model  122  of the feedback model  120  of  FIG. 20 . In the feedback model  130  of  FIG. 23 , a rotation speed output of the motor model  132 , that is, an estimated value ω e  of the rotation speed of the twisting motor  54 , is supplied to the comparator  134 . In the comparator  134 , a difference Δω between the rotation speed output ω e  of the motor model  132  and a measured value ω m  of the rotation speed of the twisting motor  54  is calculated. The calculated difference Δω is amplified by a predetermined gain H in the amplifier  136 , and is inputted to a torque input of the motor model  132  as the estimated load torque τ e  of the twisting motor  54 . The measured value V m  of the inter-terminal voltage of the twisting motor  54  is inputted to a voltage input of the motor model  132 . 
     In the feedback model  130 , by setting the gain H in the amplifier  136  sufficiently large, a magnitude of the input torque of the motor model  132 , that is, a magnitude of the estimated value τ e  of the load torque that acts on the twisting motor  54  is adjusted so that the rotation speed output of the motor model  132 , that is, the estimated value ω e  of the rotation speed of the twisting motor  54  converges to the measured value ω m  of the rotation speed of the twisting motor  54 . With such a configuration, the load torque τ e  that acts on the twisting motor  54 , which would realize the rotation speed ω m  of the twisting motor  54  when the inter-terminal voltage V m  is applied to the twisting motor  54 , can be estimated by using the motor model  132   
     Alternatively, in a case where the twisting motor  54  is provided with a rotation speed sensor (not shown) configured to detect rotation speed, the load torque τ that acts on the twisting motor  54  may be estimated by using a feedback model  140  shown in  FIG. 24 . The feedback model  140  is configured to output the estimated value τ e  of the load torque that acts on the twisting motor  54  based on the measured value i m  of the current flowing in the twisting motor  54  detected by the current detection circuit  112 , the measured value ω m  of the rotation speed of the twisting motor  54  detected by the rotation speed sensor, and the measured value V m  of the inter-terminal voltage of the twisting motor  54  detected by the voltage detection circuit  110 . The feedback model  140  is provided with a motor model  142 , comparators  144 ,  146 , amplifiers  148 ,  150 , and an adder  152 . 
     The motor model  142  of the feedback model  140  of  FIG. 24  is same as the motor model  122  of the feedback model  120  of  FIG. 20 . In the feedback model  140  of  FIG. 24 , a rotation speed output of the motor model  142 , that is, an estimated value ω e  of the rotation speed of the twisting motor  54 , is supplied to the comparator  144 . In the comparator  144 , a difference Δω between the rotation speed output ω e  of the motor model  142  and the measured value ω m  of the rotation speed of the twisting motor  54  is calculated. The calculated difference Δω is amplified by a predetermined gain G ω  in the amplifier  148 , and is supplied to the adder  152 . Further, in the feedback model  140 , a current output of the motor model  142 , that is, an estimated value i e  of the current flowing in the twisting motor  54  is supplied to the comparator  146 . In the comparator  146 , a difference Δi between the measured value i m  of the current in the twisting motor  54  and the output value i e  of the motor model  142  is calculated. The calculated difference Δi is amplified by a predetermined gain G i  in the amplifier  150 , and is supplied to the adder  152 . The adder  152  adds the output from the amplifier  148  and the output from the amplifier  150 . An output of the adder  152  is inputted to a torque input of the motor model  142  as the estimated load torque τ e  of the twisting motor  54 . The measured value V m  of the inter-terminal voltage of the twisting motor  54  is inputted to a voltage input of the motor model  142 . 
     In the feedback model  140 , by setting the gain G ω  in the amplifier  148  and the gain G i  in the amplifier  150  sufficiently large, a magnitude of the input torque of the motor model  142 , that is, a magnitude of the estimated value τ e  of the load torque that acts on the twisting motor  54  is adjusted so that the rotation speed output of the motor model  142 , that is, the estimated value ω e  of the rotation speed of the twisting motor  54  converges to the measured value ω m  of the rotation speed of the twisting motor  54 , and the current output of the motor model  142 , that is, the estimated value i e  of the current in the twisting motor  54  converges to the measured value i m  of the current in the twisting motor  54 . With such a configuration, the load torque τ e  that acts on the twisting motor  54 , which would realize the current i m  flowing in the twisting motor  54  and the rotation speed ω m  of the twisting motor  54  when the inter-terminal voltage V m  is applied to the twisting motor  54 , can be estimated by using the motor model  142 . 
     Alternatively, in a case where the twisting motor  54  is provided a rotation speed sensor (not shown) configured to detect rotation speed, the load torque τ that acts on the twisting motor  54  may be estimated by using a feedback model  160  shown in  FIG. 25 . The feedback model  160  is configured to output the estimated value τ e  of the load torque that acts on the twisting motor  54  based on the measured value i m  of the current flowing in the twisting motor  54  detected by the current detection circuit  112  and the measured value ω m  of the rotation speed of the twisting motor  54  detected by the rotation speed sensor. The feedback model  160  is provided with the motor model  142 , the comparators  144 ,  146 , the amplifiers  148 ,  150 , the adder  152 , amplifiers  162 ,  164 , and an adder  166 . 
     The motor model  160  of  FIG. 25  is provided with a substantially same configuration as that of the feedback model  140  of  FIG. 24 . In the feedback model  160  of  FIG. 25 , instead of the measured value V m  of the inter-terminal voltage of the twisting motor  54 , an estimated value V e  of the inter-terminal voltage of the twisting motor  54  calculated from the measured value i m  of the current flowing in the twisting motor  54  and the measured value ω m  of the rotation speed of the twisting motor  54  is inputted to the voltage input of the motor model  142 . In the feedback model  160 , the estimated value V e  of the inter-terminal voltage of the twisting motor  54  is calculated by approximating Ldi/dt on the left side in the aforementioned mathematical expression (1) to zero. That is, in the feedback model  160 , the estimated value V e  of the inter-terminal voltage of the twisting motor  54  is calculated by adding a value obtained by multiplying the measured value i m  of the current flowing in the twisting motor  54  by the resistance R of the twisting motor  54  to a value obtained by multiplying the measured value ω m  of the rotation speed of the twisting motor  54  by the power generation coefficient KB of the twisting motor  54 . 
     Alternatively, the main microcomputer  102  may obtain the load torque that acts on the twisting motor  54  as the twisting torque value by using methods other than the ones described above. 
     When the twisting torque value is obtained in step S 106  of  FIG. 19 , the process proceeds to step S 108 . In step S 108 , the main microcomputer  102  executes a calculation process for a rate limiter value. 
       FIG. 26  shows processes which the main microcomputer  102  executes in the rate limiter value calculation process in step S 108  of  FIG. 19 . 
     In step S 132 , the main microcomputer  102  determines whether or not the twisting torque value obtained in step S 106  of  FIG. 19  exceeds a previous rate limiter value. In a case where the twisting torque value exceeds the previous rate limiter value (in a case of YES), the process proceeds to step S 134 . 
     In step S 134 , the main microcomputer  102  calculates a value obtained by subtracting the previous rate limiter value from the twisting torque value as a difference Δ. 
     In step S 136 , the main microcomputer  102  determines whether or not the difference Δ calculated in step S 134  exceeds a predetermined maximum increase value. In a case where the difference Δ does not exceed the maximum increase value (in a case of NO), the process proceeds to step S 138 . In step S 138 , the main microcomputer  102  sets the twisting torque value as a present rate limiter value. After step S 138 , the rate limiter calculation process of  FIG. 26  is terminated. 
     In a case where the difference Δ exceeds the maximum increase value in step S 136  (in a case of YES), the process proceeds to step S 140 . In step S 140 , the main microcomputer  102  sets a value obtained by adding the maximum increase value to the previous rate limiter value as the present rate limiter value. After step S 140 , the rate limiter calculation process of  FIG. 26  is terminated. 
     In a case where the twisting torque value does not exceed the previous rate limiter value (in a case of NO) in step S 132 , the process proceeds to step S 142 . 
     In step S 142 , the main microcomputer  102  calculates a value obtained by subtracting the twisting torque value from the previous rate limiter value as the difference Δ. 
     In step S 144 , the main microcomputer  102  determines whether or not the difference Δ calculated in step S 142  exceeds a predetermined maximum decrease value. In a case where the difference Δ does not exceed the maximum decrease value (in a case of NO), the process proceeds to step S 146 . In step S 146 , the main microcomputer  102  sets the twisting torque value as the present rate limiter value. After step S 146 , the rate limiter calculation process of  FIG. 26  is terminated. 
     In a case where the difference Δ exceeds the maximum decrease value in step S 144  (in a case of YES), the process proceeds to step S 148 . In step S 148 , the main microcomputer  102  sets a value obtained by subtracting the maximum decrease value from the previous rate limiter value as the present rate limiter value. After step S 148 , the rate limiter calculation process of  FIG. 26  is terminated. 
       FIG. 27  shows chronological changes in the twisting torque value and chronological changes in the rate limiter value calculated corresponding thereto. As shown in  FIG. 27 , the rate limiter value moderately follows the twisting torque value in a range between the maximum increase value and the maximum decrease value. Due to this, if the change in the twisting torque value is moderate, the rate limiter value can follow the twisting torque value, by which they can become equal to each other. To the contrary, if the change in the twisting torque value is rapid, the rate limiter value cannot follow the twisting torque value, and a difference between them increases. In the present embodiment, the rate limiter value calculated as above is used as a condition for stopping the twisting motor  54 . 
     When the rate limiter value is calculated in step S 108  of  FIG. 19 , the process proceeds to step S 110 . 
     In step S 110 , the main microcomputer  102  determines whether or not the twisting torque value obtained in step S 106  exceeds a torque threshold set by the user. In a case where the twisting torque value exceeds the torque threshold (in a case of YES), the process proceeds to step S 119 . In step S 119 , the main microcomputer  102  waits until the number of times the twisting motor  54  rotated since the twisting motor  54  started rotating exceeds a predetermined rotation number threshold. When the number of times the twisting motor  54  rotated exceeds the rotation number threshold in step S 119  (YES in S 119 ), the process proceeds to step S 128 . In step S 128 , the main microcomputer  102  stops the twisting motor  54 . After step S 128 , the wire twisting process of  FIG. 19  is terminated. 
     In a case where the twisting torque value does not exceed the torque threshold in step S 110  (in a case of NO), the process proceeds to step S 112 . In step S 112 , the main microcomputer  102  determines whether or not the twisting torque value obtained in step S 106  exceeds the rate limiter value calculated in step S 108 . In a case where the twisting torque value exceeds the rate limiter value (in a case of YES), the process proceeds to step S 114 . In step S 114 , the main microcomputer  102  increments the value of the first counter. After step S 114 , the process proceeds to step S 118 . In a case where the twisting torque value does not exceed the rate limiter value in step S 112  (in a case of NO), the process proceeds to step S 116 . In step S 116 , the main microcomputer  102  clears the value of the first counter. After step S 116 , the process proceeds to step S 118 . 
     In step S 118 , the main microcomputer  102  determines whether or not the value of the first counter exceeds a first predetermined value. The value of the first counter increases in the case where the twisting torque value exceeds the rate limiter value, that is, in a case where the twisting torque value increases rapidly and the rate limiter value cannot follow the twisting torque value. As such, the value of the first counter exceeding the first predetermined value means that a first predetermined time has elapsed from a rise in the twisting torque value without the rate limiter value reaching the twisting torque value. In a case where the value of the first counter exceeds the first predetermined value in step S 118  (in a case of YES), the main microcomputer  102  determines that the first predetermined time has elapsed since the rise in the twisting torque value was detected, and the process proceeds to step S 119 . In step S 119 , the main microcomputer  102  waits until the number of times the twisting motor  54  rotated since the twisting motor  54  started rotating exceeds the predetermined rotation number threshold. When the number of times the twisting motor  54  rotated exceeds the rotation number threshold in step S 119  (YES in S 119 ), the process proceeds to step S 128 . In step S 128 , the main microcomputer  102  stops the twisting motor  54 . After step S 128 , the wire twisting process of  FIG. 19  is terminated. 
     In a case where the value of the first counter does not exceed the first predetermined value in step S 118  (in a case of NO), the process proceeds to step S 120 . In step S 120 , the main microcomputer  102  determines whether or not the twisting torque value obtained in step S 106  is below the rate limiter value calculated in step S 108 . In a case where the twisting torque value is below the rate limiter value (in a case of YES), the process proceeds to step S 122 . In step S 122 , the main microcomputer  102  increments the value of the second counter. After step S 122 , the process proceeds to step S 126 . In a case where the twisting torque value is not below the rate limiter value in step S 120  (in a case of NO), the process proceeds to step S 124 . In step S 124 , the main microcomputer  102  clears the value of the second counter. After step S 124 , the process proceeds to step S 126 . 
     In step S 126 , the main microcomputer  102  determines whether or not the value of the second counter exceeds a second predetermined value. The second predetermined value is set to a value smaller than the first predetermined value. The value of the second counter increases in the case where the twisting torque value is below the rate limiter value, that is, in a case where the twisting torque value decreases rapidly and the rate limiter value cannot follow the twisting torque value. As such, the value of the second counter exceeding the second predetermined value means that a second predetermined time has elapsed from a fall in the twisting torque value without the rate limiter value reaching the twisting torque value. In a case where the value of the second counter exceeds the second predetermined value in step S 126  (in a case of YES), the main microcomputer  102  determines that the second predetermined time has elapsed since the fall in the twisting torque value was detected, and the process proceeds to step S 128 . In step S 128 , the main microcomputer  102  stops the twisting motor  54 . After step S 128 , the wire twisting process of  FIG. 19  is terminated. In a case where the value of the second counter does not exceed the second predetermined value in step S 126  (in a case of NO), the process returns to step S 106 . 
     As shown in  FIG. 28 , the twisting torque value increases moderately until the wire W comes into tight contact around the rebars R, and it rapidly increases once the wire W is in tight contact around the rebars R. After this, when the wire W breaks due to the twisting motor  54  being kept rotating without stopping, the twisting torque value thereafter rapidly decreases. 
     In the wire twisting process of  FIG. 19 , as shown in  FIG. 28 , the twisting motor  54  is stopped at a time point when the twisting torque value reaches the torque threshold set by the user. With such a configuration, the rebars R can be tied with the wire W with a twisting strength which the user desires. 
     Generally, the twisting torque value with which the wire W breaks varies largely, and as shown in  FIGS. 29 to 32 , the wire W may break before the twisting torque value reaches the torque threshold. If the wire W that ties the rebars R together breaks, the rebars R may not be tied firmly with the wire W. 
     In the wire twisting process of  FIG. 19 , as shown in  FIG. 29 , the twisting motor  54  is stopped at a time point when the first predetermined time ΔT 1  has elapsed from the rise in the twisting torque value, even before the twisting torque value reaches the torque threshold. As aforementioned, the twisting torque value starts to rapidly increase when the wire W comes into tight contact around the rebars R, and it is expected that the rebars R can be tied together firmly enough by the wire W by rotating the twisting motor  54  over the first predetermined time ΔT 1  after the tight contact has been achieved. According to the wire twisting process of  FIG. 19 , the rebars R can be tied together firmly with the wire W while the wire W is suppressed from breaking. 
     As shown in  FIGS. 30 and 31 , in the wire twisting process, there may be cases in which the twisting torque value increases and decreases due to the wire W being displaced on surfaces of the rebars R after the wire W came into tight contact around the rebars R and the twisting torque value started to rapidly increase. In the wire twisting process of  FIG. 19 , as shown in  FIG. 30 , in a case where the twisting torque value decreases significantly and the rate limiter value reaches the twisting torque value after the rise in the twisting torque value was detected, the first counter is cleared. Thereafter, the twisting motor  54  is stopped at a time point when the first predetermined time ΔT 1  has elapsed since the rise in the twisting torque value was detected again. With such a configuration, the rebars R can be tied firmly with the wire W even in the case where the wire W is displaced on the surfaces of the rebars R at a degree that would affect the tying of the rebars R with the wire W. Further, in the wire twisting process of  FIG. 19 , as shown in  FIG. 31 , in a case where the twisting torque value continues to increase without the rate limiter value reaching the twisting torque value despite the twisting torque value slightly decreasing after the rise in the twisting torque value was detected, the twisting motor  54  is stopped at a time point when the first predetermined time ΔT 1  has elapsed since the rise in the twisting torque value was initially detected. With such a configuration, breakage of the wire W can be suppressed and the rebars R can be tied firmly with the wire W even in a case where the wire W is displaced on the surfaces of the rebars R at a degree that would not affect the tying of the rebars R with the wire W. 
     Even with the wire twisting process of  FIG. 19 , as shown in  FIG. 32 , there is a case where the wire W breaks before the twisting motor  54  is stopped. In such a case, it is preferable to stop the twisting motor  54  as soon as possible. In the wire twisting process of  FIG. 19 , as shown in  FIG. 32 , after a rise in the twisting torque value is detected, the detection of the rise in the twisting torque value is cancelled (the first counter is cleared) at a time point when the rate limiter value reaches the twisting torque value due to significant decrease in the twisting torque value caused by the breakage of the wire W. Thereafter, the twisting motor  54  is stopped at a time point when the second predetermined time ΔT 2  has elapsed since a fall in the twisting torque value was detected. With such a configuration, the twisting motor  54  can be stopped promptly even when the wire W breaks before the twisting motor  54  is stopped. 
     The maximum increase value and the maximum decrease value of the rate limiter value used in the rate limiter value calculation process of  FIG. 26  may be preset based on a torque curve of twisting torque value with a minimum rebar diameter. Further, the maximum increase value and the maximum decrease value of the rate limiter value, as well as the first predetermined value and the second predetermined value in the wire twisting process of  FIG. 19  may be set by the user through the second operation unit  90 . 
     The main microcomputer  102  may execute a wire twisting process shown in  FIG. 33  instead of the wire twisting process shown in  FIG. 19 . 
     Processes in steps S 102 , S 104 , S 105 , S 106 , S 108 , S 110 , S 112 , S 116 , and S 118  of  FIG. 33  are same as the processes of steps S 102 , S 104 , S 105 , S 106 , S 108 , S 110 , S 112 , S 116 , and S 118  of  FIG. 19 . In the wire twisting process of  FIG. 33 , in the case where the twisting torque value exceeds the rate limiter value in step S 112  (in a case of YES), the first counter is incremented in step S 156  in cooperation with increase in the number of times the twisting motor  54  rotated. That is, in the wire twisting process of  FIG. 33 , the value of the first counter indicates the number of times the twisting motor  54  rotated since the time point when the twisting torque value exceeded the rate limiter value. In the case where the value of the first counter, that is, the number of times the twisting motor  54  rotated since the rise in the twisting torque value was detected, reaches the first predetermined value in step S 118 , the process proceeds to step S 119 . In step S 119 , the main microcomputer  102  waits until the number of times the twisting motor  54  rotated since the twisting motor  54  started rotating exceeds the predetermined rotation number threshold. When the number of times the twisting motor  54  rotated exceeds the rotation number threshold in step S 119  (YES in S 119 ), the process proceeds to step S 128 . In step S 128 , the main microcomputer  102  stops the twisting motor  54 . After step S 128 , the wire twisting process of  FIG. 33  is terminated. 
     Processes in steps S 120 , S 124 , and S 126  of  FIG. 33  are same as the processes in steps S 120 , S 124 , and S 126  of  FIG. 19 . In the wire twisting process of  FIG. 33 , in the case where the twisting torque value is below the rate limiter value in step S 120  (in case of YES), the second counter is incremented in step S 158  in cooperation with the increase in the number of times the twisting motor  54  rotated. That is, in the wire twisting process of  FIG. 33 , the value of the second counter indicates the number of times the twisting motor  54  rotated since the time point when the twisting torque value became lower than the rate limiter value. In the case where the value of the second counter, that is, the number of times the twisting motor  54  rotated since the fall in the twisting torque value was detected, reaches the second predetermined value in step S 126 , the process proceeds to step S 128 . In step S 128 , the main microcomputer  102  stops the twisting motor  54 . After step S 128 , the wire twisting process of  FIG. 33  is terminated. 
     In the case where the operation on the main switch  74  is performed (that is, the operation to turn off the main power of the rebar tying machine  2  is performed) while the wire twisting process shown in  FIG. 19 or 33  is being executed, the main microcomputer  102  stops the twisting motor  54  at that instant, after which it switches the protective FET  116  and the transistor  109  to off to turn off the main power of the rebar tying machine  2 . 
     In one or more embodiments, the rebar tying machine  2  (an example of a tying machine) includes the twisting mechanism  20  configured to twist the wire W (an example of a tying string). The twisting mechanism  20  includes the twisting motor  54 . The rebar tying machine  2  is configured to obtain the torque that acts on the twisting motor  54  as the twisting torque value (step S 106  of  FIG. 19 , etc.), and is configured to stop the twisting motor  54  when a predetermined tying completion condition is satisfied (step S 128  of  FIG. 19 , etc.). The predetermined tying completion condition includes that the elapsed time since the rise in the twisting torque value was detected reaches the first predetermined time (steps S 112 , S 114 , S 118  of  FIG. 19 , etc.). According to the above configuration, an error determination that the twisting of the wire W is completed will not be made even when the twisting torque value increases and decreases, for example, due to the wire W being displaced on the surfaces of the rebars R while the twisting mechanism  20  is twisting the wire W. 
     In one or more embodiments, the rebar tying machine  2  includes the twisting mechanism  20  configured to twist the wire W. The twisting mechanism  20  includes the twisting motor  54 . The rebar tying machine  2  is configured to obtain the torque that acts on the twisting motor  54  as the twisting torque value (step S 106  of  FIG. 33 , etc.), and is configured to stop the twisting motor  54  when a predetermined tying completion condition is satisfied (step S 128  of  FIG. 33 , etc.). The predetermined tying completion condition includes that the number of times the twisting motor  54  rotated since the rise in the twisting torque value was detected reaches the first predetermined number of times of rotations (steps S 112 , S 156 , S 118  of  FIG. 33 , etc.). According to the above configuration, the error determination that the twisting of the wire W is completed will not be made even when the twisting torque value increases and decreases, for example, due to the wire W being displaced on the surfaces of the rebars R while the twisting mechanism  20  is twisting the wire W. 
     In one or more embodiments, the tying completion condition further includes that the twisting torque value reaches the predetermined torque threshold (step S 110  of  FIG. 19 , step S 110  of  FIG. 33 , etc.). According to the above configuration, the rebar tying machine  2  can be suppressed from receiving a large reaction force as a reaction to excessive twisting. 
     In one or more embodiments, the rebar tying machine  2  is configured not stop the twisting motor  54  even when the tying completion condition is satisfied, in the case where the number of times the twisting motor  54  rotated since the twisting motor  54  started rotating has not reached the predetermined rotation number threshold (step S 119  of  FIG. 19 , step S 119  of  FIG. 33 , etc.), and is configured to stop the twisting motor  54  in the case where the tying completion condition is satisfied and the number of times the twisting motor  54  rotated since the twisting motor  54  started rotating reaches the predetermined rotation number threshold (steps S 119 , S 128  of  FIG. 19 , steps S 119 , S 128  of  FIG. 33 , etc.). According to the above configuration, the number of twisting which is required at minimum for tying the rebars R can be applied to the wire W. 
     In one or more embodiments, the rebar tying machine  2  is configured to cancel detection of the rise in the twisting torque value when the predetermined cancellation condition is satisfied after the rise in the twisting torque value has been detected (steps S 112 , S 116  of  FIG. 19 , steps S 112 , S 116  of  FIG. 33 , etc). When the wire W is displaced significantly on the surfaces of the rebars R while the twisting mechanism  20  is twisting the wire W, for example, it is preferable to redo the process to sufficiently twist the wire W. According to the above configuration, in such a case, the wire W can sufficiently be twisted again by the detection of the rise in the twisting torque value being cancelled. 
     In one or more embodiments, the detection of the rise in the twisting torque value includes detection of change from the state in which the twisting torque value is equal to the rate limiter value calculated based on the twisting torque value to the state in which the twisting torque value is higher than the rate limiter value (step S 112  of  FIG. 19 , step S 112  of  FIG. 33 , etc.). The twisting torque value increases moderately until the wire W comes into tight contact around the rebars R, and once the wire W is in tight contact around the rebars R, it rapidly increases. In order to detect the rise in the twisting torque value which changes as above, the rate limiter value is used in the above configuration. The rate limiter value moderately follows the twisting torque value in the range between the maximum increase value and the maximum decrease value. Due to this, the rate limiter value can follow the twisting torque value when the change in the twisting torque value is moderate, by which they become equal. To the contrary, when the change in the twisting torque value is rapid, the rate limiter value cannot follow the twisting torque value, and the difference between them increases. According to the above configuration, the rise in the twisting torque value can accurately be detected by using the rate limiter value. 
     In one or more embodiments, the cancellation condition includes that the rate limiter value becomes equal to the twisting torque value again after having deviated therefrom (step S 112  of  FIG. 19 , step S 112  of  FIG. 33 , etc.). In the case where the twisting toque value keeps increasing after the rise in the twisting torque value is detected by the state change from the state in which the rate limiter value is equal to the twisting torque value to the state in which the twisting torque value is higher than the rate limiter value, without the rate limiter value becoming equal to the twisting torque value again, it is expected as that the wire W is not displaced significantly on the surfaces of the rebars R and the tying operation for the rebars R is progressing under good condition. To the contrary, in the case where the rate limiter value becomes equal to the twisting torque value again after the rise in the twisting torque value is detected by the state change from the state in which the rate limiter value is equal to the twisting torque value to the state in which the twisting torque value is higher than the rate limiter value, that is, in the case where the twisting torque value decreases relatively significantly, it is expected that the wire W is displaced significantly on the surfaces of the rebars R, and the wire W needs to be twisted sufficiently again. According to the above configuration, even in the case where the wire W is displaced significantly on the surfaces of the rebars R while the twisting mechanism  20  is twisting the wire W, the wire W can be sufficiently twisted again. 
     In one or more embodiments, in the case where the rise in the twisting torque value is not detected and the fall in the twisting torque value is detected, the rebar tying machine  2  is configured to stop the twisting motor  54  when the elapsed time since the fall in the twisting torque value was detected reaches the second predetermined time (steps S 120 , S 122 , S 126 , S 128  of  FIG. 19 , etc.). According to the above configuration, the twisting motor  54  can be stopped promptly in the case where the wire W breaks before the twisting motor  54  is stopped. 
     In one or more embodiments, in the case where a rise in the twisting torque value is not detected and the fall in the twisting torque value is detected, the rebar tying machine  2  is configured to stop the twisting motor  54  when the number of times the twisting motor  54  rotated since the fall in the twisting torque value was detected reaches the second predetermined number of times of rotations (steps S 120 , S 158 , S 126 , S 128  of  FIG. 33 , etc.). According to the above configuration, the twisting motor  54  can be stopped promptly in the case where the wire W breaks before the twisting motor  54  is stopped. 
     In one or more embodiments, the detection of the fall in the twisting torque value may include detection of the change from the state in which the twisting torque value is equal to the rate limiter value calculated based on the twisting torque value to the state in which the twisting torque value is lower than the rate limiter value (step S 120  of  FIG. 19 , step S 120  of  FIG. 33 , etc.). The twisting torque value rapidly increases after the wire W is in tight contact around the rebars R, however, when the wire W breaks, it rapidly decreases thereafter. To detect the fall in the twisting torque value which changes as above, the rate limiter value is used in the above configuration. The rate limiter value moderately follows the twisting torque value in the range between the maximum increase value and the maximum decrease value. Due to this, the rate limiter value can follow the twisting torque value when the change in the twisting torque value is moderate, by which they become equal. To the contrary, when the change in the twisting torque value is rapid, the rate limiter value cannot follow the twisting torque value, and the difference between them increases. According to the above configuration, the fall in the twisting torque value can accurately be detected by using the rate limiter value. 
     In one or more embodiments, the rebar tying machine  2  (an example of a tying machine) includes the feeding mechanism  12  configured to feed out the wire W (an example of a tying string), the battery B, and the voltage detection circuit  110  configured to detect the voltage of the battery B. The feeding mechanism  12  includes the feeding motor  22  to which power is supplied from the battery B. The rebar tying machine  2  is configured to set the duty ratio for driving the feeding motor  22  when feeding the wire W in accordance with the voltage of the battery B detected by the voltage detection circuit  110  (steps S 62 , S 66  of  FIG. 12 , steps S 86 , S 88  of  FIG. 15 , etc.). In the configuration in which the feeding motor  22  is supplied with the power from the battery B, the rotation speed of the feeding motor  22  changes according to the voltage of the battery B. If there are variations in the rotation speed of the feeding motor  22  at the time point when the main microcomputer  102  instructs the feeding motor  22  to stop, the overshoot amount of the wire W caused until the feeding motor  22  actually stops would vary, by which the total amount of the wire W that is fed out varies as well. According to the above configuration, since the duty ratio for driving the feeding motor  22  is set according to the voltage of the battery B, the variation in the rotation speed of the feeding motor  22  caused by the variation in the voltage of the battery B can be suppressed. With such a configuration, the amount of the wire W fed out from the feeding mechanism  12  can be suppressed from varying. 
     In one or more embodiments, the rebar tying machine  2  is configured to set the duty ratio for driving the feeding motor  22  in accordance with the voltage of the battery B detected by the voltage detection circuit  110  before feeding the wire W (steps S 62 , S 66  of  FIG. 12 , etc.). The rebar tying machine  2  is configured to maintain the duty ratio for driving the feeding motor  22  constant while feeding the wire W (step S 68  of  FIG. 12 ). According to the above configuration, since the duty ratio set according to the actual voltage of the battery B is maintained constant while the wire W is being fed out, the variation in the rotation speed of the feeding motor  22  caused by the variation in the voltage of the battery B can be suppressed. The amount of the wire W fed out from the feeding mechanism  12  can be suppressed from varying. 
     In one or more embodiments, the rebar tying machine  2  is configured to adjust the duty ratio for driving the feeding motor  22  in accordance with the voltage of the battery B detected by the voltage detection circuit  110  so as to maintain the average applied voltage on the feeding motor  22  constant while feeding the wire W (steps S 84 , S 86 , S 88  of  FIG. 15 , etc.). According to the above configuration, since the average applied voltage on the feeding motor  22  is maintained constant while the wire W is fed out, the variation in the rotation speed of the feeding motor  22  caused by the variation in the voltage of the battery B can be suppressed. The amount of the wire W fed out from the feeding mechanism  12  can be suppressed from varying. 
     In one or more embodiments, the rebar tying machine  2  includes the feeding mechanism  12  configured to feed the wire W, and the battery B. The feeding mechanism  12  includes the feeding motor  22  to which power is supplied from the battery B, and the encoder  27  (an example of a rotation speed sensor) configured to detect the rotation speed of the feeding motor  22 . The rebar tying machine  2  is configured to adjust the duty ratio for driving the feeding motor  22  in accordance with the rotation speed of the feeding motor  22  detected by the encoder  27  so as to maintain the rotation speed of the feeding motor  22  constant while feeding the wire W (steps S 94 , S 96 , S 98  of  FIG. 17 , etc.). According to the above configuration, the rotation speed of the feeding motor  22  is maintained constant while the wire W is fed out, so the variation in the rotation speed of the feeding motor  22  caused by the variation in the voltage of the battery B can be suppressed. The amount of the wire W fed out from the feeding mechanism  12  can be suppressed from varying. 
     In the above embodiment, the rebar tying machine  2  configured to tie the plural rebars R with the wire W was described, however, the tying string may not be the wire W, and an object to be tied may not be the plurality of rebars R.