Patent Publication Number: US-7210417-B2

Title: Stitching method and apparatus employing thread payout detection

Description:
RELATED APPLICATIONS 
   This application is a continuation of PCT Application PCT/US2005/046830 filed on 21 Dec. 2005 which claims priority based on U.S. Provisional Application 60/638,959 filed on 24 Dec. 2004. This application claims priority based on both of said aforementioned applications which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates to a method and apparatus for producing uniform stitches in a stack of fabric layers while allowing a user to manually guide the stack across a planar surface beneath a stitch head. 
   BACKGROUND OF THE INVENTION 
   Applicant&#39;s prior U.S. application Ser. No. 10/776,355 (now U.S. Pat. No. 6,883,446) which is incorporated herein by reference, describes an apparatus which permits a user to manually move a stack of fabric layers across a planar bed beneath an actuatable stitch head. The apparatus includes a detector for detecting the movement of the stack for the purpose of synchronizing the delivery of stitch strokes to the stack movement. This approach enables the insertion of uniform length stitches while allowing the user to freely move the stack within a wide range of speeds, to start or stop the stack movement at will, and to guide the stack in any direction across the planar bed. 
   The preferred embodiments described in said U.S. Pat. No. 6,883,446 employ a detector configured to detect stack movement within the throat space of a quilting/sewing machine by measuring the movement of at least one surface of the stack as it moves across the planar bed. As described, a preferred detector responds to energy, e.g., light, reflected from a target area on the stack surface (top and/or bottom) within the machine&#39;s throat space. The detector preferably provides output pulses representative of incremental translational movement of the stack along perpendicular X and Y directions. The output pulses are processed to determine the distance the stack moves. When the stack movement exceeds a threshold magnitude, a “stitch stroke” command is issued to cause the stitch head to insert a stitch through the stacked layers. As the user continues to move the stack across the planar bed, additional stitch stroke commands are successively issued to produce successive stitches. 
   Applicant&#39;s U.S. Pat. No. 6,883,446 primarily contemplates that a user directly grasp, or touch, the stacked fabric layers to push and/or pull the stack across the planar bed. However, the application also recognizes that the user could, alternatively, mount the stack on a conventional quilt frame and then grasp the frame to move the stack across the planar bed to enable the detector to sense stack surface movement. 
   Applicant&#39;s U.S. Application 60/571,109 filed 14 May 2004, which is incorporated herein by reference, describes alternative embodiments for controlling stitch head actuation which involve using a frame for mounting the fabric layer stack to retain it in a substantially taut condition. The frame is supported for user guided movement beneath a fixedly located stitch head and a detector is provided to produce signals representing the magnitude of frame translation, and thus the magnitude of stack translation. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a further method and apparatus for controlling stitch head actuation as a function of stack movement. More particularly, the present invention is based on the recognition that inasmuch as thread is pulled, or paid out, from a bobbin in direct relationship to the movement of the stack, the length of thread payout can be detected and used to control stitch head actuation. In accordance with a preferred embodiment, control circuitry is provided to respond to the payout of a threshold length of bottom thread to actuate the stitch head, i.e., cause the stitch head needle to execute a cyclic movement. Alternatively, the control circuitry can respond to top thread payout. As a consequence, uniform length stitches can be produced as the stack is freely manually guided across the planar bed. 
   In accordance with a first preferred embodiment, the rotational motion of the bobbin which supplies the bottom thread is measured in order to determine thread payout length. The rotational bobbin motion is preferably measured by providing an encoder disc on the bobbin which rotates relative to a sensor, e.g., optical, magnetic, etc. 
   In accordance with a further preferred embodiment, the bottom thread payout is directly sensed by reading characteristics of the thread or markings formed on the thread. For example, the thread can be marked with invisible bands spaced along its length which fluoresce when illuminated by ultraviolet light. An optical sensor is provided to detect these fluorescent markings as they move away from the bobbin. 
   Various other techniques can also be employed to measure the length of thread payout. For example, the thread can engage and rotate an idler pulley as the thread is pulled from the bobbin and the incremental rotation of the pulley can be detected to determine the length of thread payout. Regardless of the particular means used to measure thread payout length, embodiments in accordance with the invention function to synchronize needle cycles to the rate of thread payout. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIGS. 1–7  herein correspond to figures in U.S. Pat. No. 6,883,446; 
       FIG. 1  is a generalized block diagram depicting a system for fastening stacked planar layers; 
       FIG. 2  is a diagrammatic illustration of an embodiment of the system of  FIG. 1  utilizing a motor/brake assembly to control a stitch head in response to movement of a stack of fabric layers; 
       FIG. 3  and is a diagrammatic illustration showing the stitch needle and hold-down plate of  FIG. 2  in their down position; 
       FIG. 4  is a diagrammatic illustration similar to  FIG. 3  but showing the needle and hold-down plate in their up position; 
       FIGS. 5 and 6  respectively show side and end views of an exemplary quilting/sewing machine housing; 
       FIG. 7  (presented as  7  A and  7  B) comprises a flow chart depicting dual mode operation, i.e., (1) impulse mode and (2) proportional mode; 
       FIG. 8  is a block/schematic diagram showing a first embodiment of the present invention including means for detecting bobbin thread payout for controlling stitch head actuation; 
       FIGS. 9A and 9B  comprise front and side views showing a bobbin and an encoder disc mounted on the bobbin for use in the embodiment of  FIG. 8 ; 
       FIGS. 10A and 10B  comprise front and side views showing the bobbin and encoder disc mounted in a case having a window so that the encoder disc can be viewed from outside of the case; 
       FIG. 11  is a block/schematic diagram showing a second embodiment of the present invention in which bobbin thread payout is measured by reading markings or characteristics of the thread; 
       FIG. 12  depicts an exemplary thread length marked with fluorescent bands for use in the embodiment of  FIG. 11  to enable the length of thread payout to be easily measured; 
       FIG. 13  is a circuit diagram illustrating control circuitry for responding to the output pulses produced by the embodiments of  FIGS. 8  or  10  for actuating the stitch head; and 
       FIGS. 14A ,  14 B, and  14 C comprise front, side, and bottom views of an alternative embodiment in which the thread rotates a toothed idler pulley as it pays out from the bobbin. 
   

   DETAILED DESCRIPTION 
   U.S. application Ser. No. 10/776,355 (now U.S. Pat. No. 6,883,446) is in its entirety incorporated herein by reference. However, for convenience sake, several of the figures and related text from the &#39;355 application are expressly reproduced in this application, e.g.,  FIGS. 1–6 , and  7 (A),  7 (B) herein respectively correspond to  FIGS. 1–6 , and  11 (A),  11 (B) of said &#39;355 application.  FIGS. 8–14  herein are first being introduced in this application 
   Attention is initially directed to  FIG. 1  which depicts a generalized system  10 , as shown in said &#39;355 application, for fastening together two or more flexible planar layers, e.g., fabric forming a stack  12 . The stack  12  is supported for guided free motion along a horizontally oriented X-Y planar surface  14  proximate to a fastening, or stitch, head  15 . The head  15  is actuatable to insert a stitch through the stacked layers  12 . A motion detector  16  is provided to sense the movement of stack  12  across surface  14 . Control circuitry  18  responds to increments of stack movement to actuate the head  15 . The detector  16  is preferably configured to measure stack translational motion proximate to the stitch head  15 . 
     FIG. 2  illustrates an exemplary embodiment  20  of the system of  FIG. 1  for stitching together fabric layers of a stack  22 . The embodiment  20  is generally comprised of a mechanical machine portion  26 , including an actuatable stitch head  28 , and an electronic control subsystem  30  for actuating the head  28  in response to movement of the stack  22 . The stack  22  is typically comprised of multiple fabric layers, e.g., a top layer  32 , an intermediate batting layer  34 , and a bottom backing layer  36 , which when stitched together will form a quilt. 
   The machine portion  26  of  FIG. 2  is depicted as including a machine frame  40  configured to support the stitch head  28  above a bed  44  providing a substantially horizontally oriented planar surface  45 . The stitch head  28  includes a needle arm  46  supporting a needle  48  for reciprocal or cyclic vertical movement essentially perpendicular the planar surface  45 . The bed surface  45  is configured for supporting the layered stack  22  so as to enable a user to directly grasp, the stack  22  for guiding it across the surface  45  by manual push-pull action. A hold-down plate, or presser foot,  50  is preferably provided to selectively press the stack  22  against the bed surface to assure proper stitch tension and to assist the needle to pull upwardly out of the stack after inserting a stitch. 
   A conventional hook and bobbin assembly  52  is mounted beneath the bed  44  in alignment with the needle  48 . The needle  48  operates in a conventional manner in conjunction with the hook and bobbin assembly  52  to insert a stitch through the stack  22  at a stitch site  54 , i.e., an opening  55  in bed  44 . When the needle  48  is lowered to its down position to pierce the stack layers ( FIG. 3 ), the hold-down plate  50  is also lowered to press the stack layers against the bed  44  to achieve proper stitch tension and assist the needle to pull up out of the stack. After completion of a stitch cycle, the needle  48  and hold-down plate  50  are raised ( FIG. 4 ). 
   The machine portion  26  of  FIG. 2  is further depicted as including a motor/brake assembly  56  which functions to selectively provide operating power and braking via a suitable transmission system  58  to an upper drive shaft  60  and a lower drive shaft  62 . The upper drive shaft  60  transfers power from the motor/brake assembly  56  to stitch head  28  for moving the needle  48 . The lower drive shaft  62  transfers power from the motor/brake assembly  56  to the hook and bobbin assembly  52 . 
   The stitch head  28  and hook and bobbin assembly  52  operate cooperatively in a conventional manner to insert stitches through stack  22  at stitch site  54 . That is, when the stitch head cycle is initiated, needle  48  is driven downwardly to pierce the stacked layers  32 ,  34 ,  36  and carry a top thread (not shown) paid out through the needle through the stitch site opening  55  in bed  44 . Beneath the bed  44 , the hook (not shown) of assembly  52  grabs a portion of the top thread before the needle  48  pulls it back up through the stack. The top thread portion grabbed by the hook is then looped around a portion of bottom thread pulled off the bobbin of assembly  52  to lock the top and bottom threads together at the stack to form a stitch. 
   The system of  FIG. 2  includes a transducer, or detector,  64  for detecting the movement, or more specifically, the translation of the stack  22  on bed  44  for the purpose of controlling the motor/brake assembly  56  via control circuitry  65 . In operation, a user is able to freely move the stack  22  on bed  44  relative to the stitch head  28  while the detector  64  produces electronic signals representative of the stack movement. Control circuitry  65  then responds to the detected stack movement for controlling the issuance of a stitch from head  28 . The control subsystem  30 , in addition to including motion detector  64  and control circuitry  65 , may also include a shaft position sensor  66 . The shaft position sensor  66  functions to sense the particular rotational position of the upper drive shaft  60  corresponding to the needle  48  being in its full up position. The control circuitry  65  preferably responds to the output of sensor  66  to park the needle  48  in its full up position between successive stitch cycles. This action prevents the needle from interfering with the free translational movement of the stack  22  on bed  44 . 
   In typical use of the apparatus of  FIG. 2 , an operator manually guides the fabric stack across the horizontally oriented bed  44  beneath the vertically oriented needle  48 . The motion detector  64  is mounted to monitor a target area coincident with a surface layer (top and/or bottom) of the stack  22  as the stack is moved across the bed  44 . 
   Although the motion detector  64  of  FIG. 2  can take many different forms, including both noncontacting devices (e.g., optical detector) and contacting devices (e.g., track ball), it is preferred that it detect stack movement without physically contacting the fabric layers. Accordingly, a preferred motion detector  64 , as discussed in said &#39;355 application, comprises an optical motion detector utilizing, for example, an optical chip ADNS2051 marketed by Agilent Technologies. 
   Suffice it to say that the accurate measurement of stack movement in  FIG. 2 , depends, in part, upon the stack target layer, e.g., backing layer  36 , being positioned near the focus of the motion detector window. The aforementioned hold-down plate or presser foot  50  assists in maintaining the stack layers at a certain distance from the detector window. The hold-down plate  50  preferably has a flat smooth bottom surface  51  for engaging the stack  22  and is fabricated of transparent material to avoid obstructing a user&#39;s view of the stack layers proximate to the needle  48 .  FIGS. 3 and 4  respectively illustrate the actuated and non actuated positions of the hold-down plate  50 . In  FIG. 3 , shaft  80  is moved down during the stitch cycle to cause the plate  50  to apply spring pressure, attributable to spring  82 , to the stack  22 . Between cycles ( FIG. 4 ), shaft  80  is moved up so the pressure of plate  50  against stack  22  is relieved to reduce motion-inhibiting friction of the plate against the stack. Nevertheless, during a non-stitch interval between cycles, the plate  50  is positioned closely enough to loosely hold the stack against the bed  44 . 
     FIGS. 5 and 6  schematically depict a typical quilting/sewing machine housing  84  for accommodating the physical components of the system of  FIG. 2 . The housing  84  comprises an upper arm  85  which contains the upper drive shaft  60  and a lower arm  86  containing the lower drive shaft  62 . The housing upper and lower arms  85  and  86  extend from a vertically oriented machine arm  87 . The upper and lower arms  85 ,  86  are vertically spaced from one another and together with the machine arm  87  define a space which is generally referred to as the throat space  88 . The needle  48  descends vertically from the upper arm into the throat space  88  for reciprocal movement toward and away from the lower arm  85 . The lower arm  85  carries the bed  44  which is sometimes referred to as the throat plate. The distance between the needle and the machine arm is generally referred to as the throat length. 
   Attention is now directed to  FIG. 7  (A, B) which comprises a flow diagram depicting an exemplary algorithmic operation of a microcontroller for controlling the motor/brake assembly  56  of  FIG. 2 . In  FIG. 7 , first note block  120  which functions to initialize a stitch cycle by acquiring a “stitch length” value which typically was previously entered via a user input. With the stitch length value set in block  120 , the algorithm proceeds to decision block  122  which tests for stack translation in the X direction, i.e., for an X pulse on lead  96  from the optical chip  95 . If a pulse is detected, then a store X count is incremented, as represented by block  124 . After execution of blocks  122 ,  124 , operation proceeds to decision block  126  which tests for Y translation, i.e., for a Y pulse out of the detector  64 . If a Y pulse is detected, then a stored Y count is incremented as represented by block  128 . Operation then proceeds from blocks  126  or  128  to block  130 . Blocks  130  and  132  essentially represent steps for determining the resultant stack movement magnitude attributable to the measured X and Y components of motion utilizing the Pythagorean theorem. That is, in block  130 , the X count value is squared and the Y count value is squared. Block  132  sums the squared values calculated in block  130  to produce a value representative of the resultant stack movement. 
   Block  134  compares the square of the preset switch length value with the magnitude derived from block  132 . If the magnitude of the resultant movement is less than the preset stitch length, then operation cycles back via loop  136  to the initial block  120 . If on the other hand, the resultant magnitude exceeds the preset stitch length, then operation proceeds to block  138  to initiate a stitch. In block  140 , the X and Y counts are cleared before returning to the initial block  120 . 
     FIG. 7  (A) as discussed thus far relates primarily to operation in the impulse, or single stitch, mode.  FIG. 7B  depicts dual mode operation, i.e., impulse mode at slow stack speeds and a continuous proportional mode at higher stack speeds. It is preferable to provide such a dual mode capability to be able to operate more smoothly at higher stack speeds. By way of explanation, it will be recalled that in order to accommodate slow stack speed operation, e.g., less than 20 inches per minute, it is desirable that each stitch command initiate a very rapid needle stroke to avoid the needle interfering with stack movement. As the stack translation speed and needle stroke rate increase, the needle&#39;s interference with stack movement diminishes. Thus, at fast stack speeds, e.g., greater than 20 inches per minute (or 200 stitches per minute assuming an exemplary 0.1 inch stitch length), it is appropriate to switch to a proportional mode in which the needle is continuously driven at a rate substantially proportional to the speed of stack translation. At a speed of 200 stitches per minute, each needle cycle consumes less than about 300 milliseconds. Accordingly, the algorithm depicted in  FIG. 7(B)  includes a step which tests for the time duration between successive stitch commands, i.e., a stitch time interval. If the duration of this interval is less than an exemplary 300 milliseconds, then operation proceeds in the proportional mode.  FIG. 7(B)  shows that block  138  is followed by block  152  which reads and resets a stitch interval timer (which can be readily implemented by a suitable microcontroller) which times the duration between successive stitch commands and records the angular position en of the needle drive shaft  60  (block  153 ). Decision block  154  then tests the interval timer duration previously read in block  152  to determine whether it is greater than the aforementioned exemplary 300 millisecond interval. If yes, operation proceeds to the impulse mode  155 . If no, operation proceeds to the proportional mode  156 . 
   Operation in the impulse mode  155  involves block  157  which is executed to assure deactivation of the proportional mode. Thereafter, block  148  is executed which involves waiting for a signal from the bobbin hook sensor. The motor (or clutch) is then actuated in block  142  and actuation terminates when a terminating pulse is recognized from the shaft position sensor (block  146 ). Block  158  then deactuates a motor/clutch relay and/or actuates a brake after a stitch recognized in block  146  to park the needle in its up position. 
   Operation in the proportional mode  156  includes step  159  which activates motor speed control operation. A motor speed control capability is a common feature of most modern sewing machines with motor speed being controlled by the user, e.g., via a foot pedal, and/or by built-in electronic control circuitry. 
   After block  159 , decision block  160  is executed. To understand the function of decision block  160 , it must first be recognized that as stack speed is increased, thus generating shorter duration stitch intervals, the shaft angle position ⊖ n  read in block  153  will decrease, in the absence of an adjustment of motor/needle shaft speed. In other words, a newly read shaft angle ⊖ n  will be smaller than a previously read shaft angle ⊖ p . Block  160  functions to compare ⊖ n  and ⊖ p  if stack speed increases. If ⊖ n  is smaller, the motor speed must be increased (block  161 ) to deliver stitches at an increased rate to maintain stitch length uniformity. 
   On the other hand, if stack speed is reduced so that ⊖ n  is greater than ⊖ p , motor speed is decreased (block  162 ) in order to produce uniform length stitches. If stack speed remains constant, then ⊖ n  equals ⊖ p  and no motor sped adjustment is called for (block  163 ). 
   The embodiments discussed thus far ( FIGS. 1–7 ) contemplate use of a motion detector  64  for observing energy reflected from the top and/or bottom stack surfaces to produce signals representing stack translation along X and Y axes. A microcontroller functions to resolve these X and Y components to determine the magnitude of stack translation for controlling stitch head actuation as explained by  FIG. 7 . The present invention, depicted in  FIGS. 8–14 , is based on the recognition that the magnitude of stack translation can be alternatively determined by detecting the length of thread payout e.g., from the hook and bobbin assembly  52 . 
   Attention is now directed to  FIG. 8  which illustrates a first embodiment in accordance with the present invention. As the fabric stack  200  is moved by the user along planar surface  201 , bottom thread  202  is pulled from the bobbin  204  and out of the bobbin case  206  via opening  208 . The thread pulled, i.e., paid out, from the bobbin  204  causes the bobbin to rotate about its axis. Thus, the angular rotation of the bobbin represents a measure of the length of bobbin thread payout. The bobbin angular rotation can be measured by a detector  210 , e.g., an optical sensor, operating in conjunction with an encoder disc  212  attached to a side face of the bobbin  204 . The detector  210  will produce electrical pulses  214  by detecting segment marks  215  of the encoder disc  212  as the bobbin rotates ( FIGS. 9 ,  10 ). Each pulse represents an increment of bobbin angular rotation and an increment of thread length pulled from the bobbin which in turn represents an increment of stack movement. These pulses  214  are applied to control circuitry ( FIG. 13 ) which issues a stitch command when the stack movement exceeds a certain threshold. 
     FIG. 9  illustrates the segmented encoding disc  212  attached to the side face of a conventional bobbin  204 . A typical bobbin has an inner thread circumference of about 25 mm and an outer thread circumference of about 50 mm. A practical encoding disc has a total of 100 segments. Thus, when the active thread layer on the bobbin has a circumference of 50 mm and the stitch length is selected as 2.5 mm, then five disc segments are equivalent to paying out thread for a single stitch. That is, it is desirable for the control circuitry ( FIG. 13 ) to produce a stitch command after every fifth output pulse  214 . 
     FIG. 10  illustrates bobbin case  206  having a window  216  through which segments  215  of the encoder disc  212  can be seen by the optical sensor  210  as the bobbin  204  rotates. 
   As thread  202  unwinds from the bobbin  204 , the circumference of the active thread layer decreases and the ratio of stack movement to pulse generation will increase. That is, with a constant rate of stack motion and using the aforementioned bobbin parameters, a near empty bobbin will rotate twice as fast as a full one. Thus, output pulses will be produced at twice the rate. If five output pulses generated when the bobbin is full produces a stitch length of about 2.5 mm, then five output produced by a near empty bobbin would produce a stitch length of about 1.25 mm. Deviation of stitch length over the capacity of the bobbin from a nominal 1.87 mm, is then approximately plus or minus 34%. Although this deviation may be tolerable in certain situations, it is preferable to provide some means for minimizing or correcting for this variation. 
   The aforediscussed deviation attributable to the diminishing thread circumference on the bobbin can be reduced by reducing the ratio of full-to-empty thread circumferences. For example, if the inner diameter of the bobbin is increased so that the near empty circumference becomes ¾ rather than ½ that of the full bobbin circumference, then the full-to-empty stitch length ratio improves to 1.37:1 rather than 2:1. With this arrangement the bobbin retains nearly 70% of the total thread capacity available on the referenced bobbin, but the deviation of stitch length from nominal is less than plus or minus 15% over the full range of bobbin thread payout. 
   It is noted that arbitrarily small deviations of stitch length can be achieved with further reduction of the ratio of outer to inner bobbin diameters. When the diameters are selected so that the thread available is half that of the aforediscussed bobbin, the deviation becomes less than plus or minus 10%. 
   The stitch length variation attributable to diminishing thread circumference can be further mitigated by introducing a correction factor that is developed by measuring the amount of thread remaining on the bobbin. To illustrate, the typical bobbin has a thread layer circumference of about 50 mm when full diminishing to about 25 mm when near empty. Assuming a nominal stitch length of 2.5 mm and a 100-segment encoder, when the bobbin is full, stitches are triggered on each fifth pulse. When the bobbin is near empty, a correction factor of 2.0 can be applied to cause stitches to be triggered on each tenth pulse. Similarly, interpolated correction factors can also be developed and applied as the bobbin thread diameter gradually decreases. Techniques for determining and reporting the amount of thread remaining on a bobbin are known in the art and can be readily employed to apply these correction factors. 
   In an alternate method of developing the correction factors, a counter can record the total number of stitches taken from the bobbin since it was last filled. That number bears an inverse mathematical relationship to the current thread circumference. Consequently, it can be used to develop the correction factor. 
   In a further alternative embodiment, represented in  FIGS. 11 and 12 , bobbin thread payout is measured by detecting uniformly spaced features, or marks, of the thread itself, or marks added to the thread to facilitate detection.  FIGS. 11 and 12  assume a bobbin thread  230  carrying “readable” marks  232  uniformly spaced along the length of the thread. As the fabric stack  234  is moved along planar surface  236 , thread  230  pays out from bobbin  238  emerging from case opening  240 . The marks  232  can be read by a detector  242 , e.g., an optical sensor, mounted near the opening  240  to produce the pulses  244 . In a preferred embodiment, the marks  232  comprise bands material normally invisible to the human eye but which become visible when illuminated by an appropriate source, e.g., ultraviolet light. As each band  232  emerges from the bobbin case through opening  240 , it is preferably illuminated with ultraviolet light from source  246 . Each band is thus energized to produce visible light which can be detected by the optical sensor  242 . The optical sensor  242 , produces the pulse train  244  used by the circuitry of  FIG. 13  to generate stitch commands. 
   Attention is now directed to  FIG. 13  which illustrates control circuitry  260  for responding to the pulse signals produced in  FIGS. 8  or  11  to actuate the stitch head via motor/brake assembly  56 . Note that  FIG. 13  depicts optical sensor  262  (which corresponds to optical sensors  210  and  242  of  FIGS. 8 and 11 , respectively) for providing output pulses at a rate related to the rate of bobbin thread payout. The output pulses produced by sensor  262  are coupled via line  264  to a data input  266  of a microcontroller  268  (e.g., microcontroller chip Microchip PIC 12C508). A source  270  of the previously discussed correction factors also supplies data via line  272  to microcontroller data input  274 . 
   In operation, the microcontroller  268  functions to count the pulses provided by sensor  262  in order to recognize when a preset, but variable, increment, i.e., threshold length, of the bobbin thread has paid out. The number of pulses required to reach the threshold increment is dependent on the correction factor data provided by source  270 , as has been previously described. When the microcontroller  268  recognizes that the threshold has been reached, it issues a signal via output  280  to timer circuit  282 . The timer circuit  282  then provides a stitch command signal on output  284  to load transistor  286 . Transistor  286  controls relay  288  which is shown as operating a single pole double, throw switch  290 . In the actuated lower position, switch  290  applies power to drive the motor of motor/brake assembly  56  of  FIG. 2 . The relay  288  is deactuated via the timer  282  and the transistor  286  by a pulse on line  292  from the aforementioned shaft position sensor  66  ( FIG. 2 ). In the deactuated upper position, switch  290  closes a shunt path around the motor  56  to thus brake the drive train and park the needle in its up position. 
   Attention is now directed to  FIGS. 14A ,  14 B,  14 C which illustrate a further embodiment  300  for detecting a threshold increment of thread as it pays out from the bobbin. The embodiment  300  employs an idler pulley  302 , in the form of a toothed gear  304 , mounted adjacent to a bobbin case  306  housing a bobbin  308 . The gear  304  engages a second toothed gear  310 . The thread  312  exiting from bobbin case  306  passes between and engages the teeth of the gears  304 ,  310  which are closely shaped and dimensioned to prevent thread slippage relative to the gears. After passing between gears  304 ,  310 , the thread  312  is directed by suitable guides  314  to the needle opening  316  in plate  318 . Thus, as the thread  312  pays off the bobbin  308 , to form stitches above plate  318 , it synchronously rotates meshed gears  304  and  310 . 
   Detector  320  is provided to detect the incremental angular rotation of the gears  304 ,  310  and produce an output pulse train  322  analogous to previously mentioned pulse trains  214  ( FIG. 8) and 244  ( FIG. 11 ). The detector  320  includes a sensing member  324  which responds to each tooth of gear  310  moving therepast. The sensing member  324  can most simply comprise a mechanical follower which physically contacts the teeth of gear  310  to generate an electrical signal and produce an output pulse as the high point of each gear tooth moves therepast. The sensing member  324  can alternatively comprise a contactless member capable of recognizing the proximity of a gear tooth by responding to its magnetic, capacitive, or optical characteristics to produce an output pulse. 
   The output pulse train  322  produced by the sensing member  324  in response to the thread  312  rotating gears  304 ,  310  is coupled to the data input (e.g.,  266 ) of the control circuitry of  FIG. 13 . Preferably, the pulse train  322  is wirelessly communicated to the control circuitry using a transmitter  326  located proximate to the sensing member  324  and a receiver  328  connected to the control circuitry of  FIG. 13 . 
   The transmitter  326  can be implemented in a variety of ways, for example, as a battery operated RF or IR transmitter. Preferably, however, the transmitter  326  comprises a passive device configured for remote powering by an inductive coil. 
   Although the embodiment  300  of  FIGS. 14A ,  14 B, and  14 C shows the thread  312  extending between and contacting both of the meshed gears  304 ,  310  to avoid thread/gear slippage, it is recognized that this result can be achieved with a variety of different mechanisms. For example only, a spring urged arcuate guide member can be used to hold the thread against the peripheral surface of a toothed drum (analogous to gear  304 ) so that as the thread is pulled by the fabric stack, it incrementally rotates the toothed drum. By counting the movement of teeth past a detector, the thread payout length can be determined to control needle speed. 
   From the foregoing, it should now be appreciated that a stitch head (e.g.,  28  in  FIG. 2 ) can be controlled in response to the detected length of thread payout to produce stitches of uniform length in a fabric stack manually guided beneath the stitch head. The control can be exercised in either an impulse mode or a proportional mode or a dual mode system. It should be understood from the prior discussion that operation in the impulse mode produces a single stitch for each threshold unit of stack movement, i.e., each unit of threshold length of thread payout. In the proportional mode, the needle cycles at a rate proportional to the rate of stack movement, i.e., the rate of thread payout. Dual mode operation contemplates use of the impulse mode at slow stack speeds and the proportional mode at higher stack speeds. Although the preferred embodiments specifically described herein show bottom thread payout detection for controlling needle cycle rate, or speed, it should be understood that equivalent embodiments could alternatively detect top thread payout. 
   It should also be understood that although it is preferable to incorporate thread payout detection as an integral part of a sewing/quilting machine, it is recognized that an existing conventional sewing machine can be modified, or retrofitted, to incorporate this function by exercising control of the stitch head via the normal foot pedal input. For example, the transistor  286  ( FIG. 13 ) can control motor speed via the foot pedal input in the manner shown in FIG. 16 of Applicant&#39;s aforementioned U.S. Pat. No. 6,883,446.