Abstract:
The present invention provides a method for rapidly switching selector feet ( 52 ) between activate and deactivate selection positions by friction coupling them to vibratory piezoelectric motors ( 58 ) and a fast selector ( 50 ) for a knitting machine comprising selector feet friction coupled to piezoelectric motors.

Description:
FIELD OF THE INVENTION 
     The present invention relates to knitting machines, and in particular to selectors which select which latch needles of a knitting machine are activated in the process of knitting a fabric. 
     BACKGROUND OF THE INVENTION 
     Automatic knitting machines use banks of large numbers of closely spaced latch needles to interlock threads in a series of connected loops to produce a knitted fabric. The latch needle is a flat needle generally with a long shaft having, at one end, a small hook with a latch, which latch swivels to open and close the hook. 
     Generally, in a modem knitting machine, many thousands of latch needles are accurately positioned and maintained in a closely packed parallel array. In the process of knitting a fabric, an activation station activates latch needles by moving them forwards and backwards, parallel to their lengths, so that the hook ends of the activated latch needles move towards and away from threads being woven into the fabric. As a latch needle is moved forwards and backwards, its latch swivels back and forth to alternately open and close the latch needle hook so that the latch needle can catch and hold one of the threads being woven into the fabric, pull it to create a loop of fabric, and then release the thread to repeat the cycle. 
     In rotary knitting machines the needles in an array are held in a cylindrical geometry and rapidly moved, in a rotary motion, into and out of the activation station. Depending upon the fabric being knitted, different ones of the needles moving through the activation station are activated. In linear knitting machines, latch needles are held in parallel slots in large flat needle beds. The activation station is a type of shuttle that moves rapidly back and forth over the needle bed, activating needles appropriate to the weave of the fabric being knitted. 
     In both rotary and linear knitting machines, a device called a “selector” determines (hereafter referred to as “selects”) whether a needle in the activation station of the knitting machine is to be activated or not. To prevent a needle from being activated, the selector presses on a small protuberance (hereafter referred to as an “activation fin” or “fin”) on the shaft of the needle. When pressure is applied to the activation fin by the selector, the needle moves away from an activating mechanism of the activator station and is “deactivated”. If the selector does not press on the activation fin, the needle is activated. 
     The selector presses on the fin of a needle, to deactivate the needle, with a “selector foot”. The selector foot has two operational selection positions. In a deactivate selection position, the selector foot presses on the fin of the needle thereby preventing the needle from being activated when the needle passes through the activation station. In an activate selection position, the selector foot does not press on the fin of the needle, thereby allowing the needle to be activated when the needle passes through the activation station. The selector foot is generally switched between the selection positions by displacing the selector foot by a small linear translation or by rotating the selector foot through a small angle. 
     When a knitting machine is operating, the selector of the knitting machine is set to an appropriate selection position for each latch needle that passes through the activation station of the knitting machine. If the selection positions for two needles that pass consecutively through the activation station are not the same the selector has to be switched from one selection position to the other. Prior art selectors generally use solenoids or piezoelectric bimorph actuators to effect the displacements necessary to switch a selector foot between selection positions. However, using these types of actuators, the time it takes to switch a selector foot between selection positions is too long to match the rate at which modern knitting machines move needles through activation stations. 
     In order to improve the speed with which prior art selectors operate, prior art selectors generally comprise a multiplicity of selector feet which are operated in parallel. In a selector operating with one selector foot, a decision to switch or not switch the selection position of the selector foot, hereafter referred to as “setting” the selector foot, has to be made and executed for every needle that moves through an activation station. In a selector with N activation feet on the other hand, each foot has to be set once for every N needles that move through the activation station. If the switching time needed to switch a selector foot between selection positions is τ secs, a selector with one foot can select 1/τ needles/sec, or equivalently, operate at a “decision” frequency of 1/τ Hz. A selector with N selector feet in parallel on the other hand, can select N/τ needles/sec, i.e. operate at a decision frequency of N/τ Hz. Switching times for prior art activation feet are on the order of 10 msecs. By operating approximately 10 activation feet in parallel, prior art selectors are able to operate at decision frequencies of up to about 1000 Hz. 
     The decision frequencies at which prior art selectors operate limit the rate at which needles can be moved through a knitting machine activation station and therefore limit the rate at which fabric can be produced. In order to increase the rate at which knitting machines produce fabric, it is desirable to have selectors that can operate at frequencies higher than 1000 Hz. 
     SUMMARY OF THE INVENTION 
     It is an object of some aspects of the present invention to provide a selector for knitting machines that can operate at decision frequencies substantially higher than 1000 Hz. 
     A selector, in accordance with a preferred embodiment of the present invention, achieves decision frequencies higher than those of conventional selectors by decreasing the switching time of selector feet comprised in the selector to less than the switching times of selector feet in conventional selectors. 
     When in operation, a selector foot constantly switches back and forth between selection positions at a rapid rate. When switching between selection positions, the selector foot generally moves a distance of about 2 mm in about 10 msecs. This change is resisted by friction, forces arising from part wear, machine design and tolerances, and random motional forces that occur during machine operation. The sum of these forces is on the order of between 0.2 and 0.5 Newton. Because of the close spacing within modem knitting machines and the small sizes of many of their components there is little room available for motors or actuators to provide the work required to accomplish the switching. An actuator or motor that can be used to improve the switching time of a selector foot in a selector must therefore be small, capable of switching direction rapidly and able to provide work at a greater rate than that available from motors or actuators in conventional selectors. 
     Piezoelectric motors can be produced that are small and powerful for their size and that can provide large accelerations of moveable elements in directions which can be reversed in time periods of microseconds. The switching time of a selector foot can be reduced to less than the switching times of selector feet in conventional selectors by using an appropriate piezoelectric motor to switch the selector foot between selection positions, in accordance with a preferred embodiment of the present invention. A selector foot, in accordance with a preferred embodiment of the present invention, is coupled to a piezoelectric motor that can displace a moveable element at a rate of about 400 mm/sec against a force opposing the motion which is on the order of from 0.2 to 0.5 Newton. Preferably, the selector foot comprises a friction coupling surface region suitable for friction coupling with the piezoelectric motor. Preferably, the selector foot is coupled to the piezoelectric motor by resiliently pressing a surface region of the piezoelectric motor, or an appropriate hard friction nub attached to the surface of the piezoelectric motor, to the friction coupling surface region of the selector foot. Preferably, the piezoelectric motor coupled to the selector foot is of a type described in U.S. Pat. No. 5,453,653, which is incorporated herein by reference. As a result, a selector foot in a selector, in accordance with a preferred embodiment of the present invention, can be switched between selection positions in a time on the order of 5 msec (2 mm/[400 mm/sec]=5 msec). 
     A selector, in accordance with a preferred embodiment of the present invention, comprises a multiplicity of activation feet, in accordance with a preferred embodiment of the present invention, operated in parallel. Preferably, the number of the multiplicity of selector feet is on the order of 10. With a switching time for the selector feet of 5 msec, in accordance with a preferred embodiment of the present invention, this results in a decision frequency for the selector in the range of 2000 Hz (the decision frequency=N/τ with N=10 and τ=5 msec). 
     There is therefore provided in accordance with a preferred embodiment of the present invention a selector for a knitting machine, which knitting machine comprises a plurality of latch needles and an activation station, such that when a latch needle of the plurality of latch needles is in the activation station, said selector determines whether said latch needle is activated or not activated, comprising: at least one selector foot selectively positionable to an activate or a deactivate selection position, wherein said selector foot has a friction coupling surface; and a piezoelectric motor coupled to said friction coupling surface of the at least one selector foot; wherein vibrations in said piezoelectric motor cause said at least one selector foot to switch between activate and deactivate selection positions. Preferably, the piezoelectric motor is coupled to the friction coupling surface by a resilient force which presses a contact surface of the piezoelectric motor to the friction coupling surface. Alternatively, the piezoelectric motor has a friction nub and the piezoelectric motor is coupled to the friction coupling surface by a resilient force which presses the friction nub to the friction coupling, surface. 
     In some preferred embodiments of the present invention the friction coupling surface is cylindrical. In other preferred embodiments of the present invention the friction coupling surface is planar. 
     In some preferred embodiments of the present invention vibrations in the piezoelectric motor cause the selector foot to switch between selection positions by rotating the selector foot through a given angle. Preferably, the vibrations in the piezoelectric motor rotate the selector foot through the given angle in a period of time less than  10  msec. More preferably, the vibrations in the piezoelectric motor rotate the selector foot through the given angle in a period of time less than 7 msec. Most preferably vibrations in the piezoelectric motor rotate the selector foot through the given angle in a period of time less than 5 msec. In some preferred embodiments of the present invention, the vibrations in the piezoelectric motor rotate the selector foot through the given angle in a period of time substantially equal to 5 msec. 
     In yet other preferred embodiments of the present invention vibrations in the piezoelectric motor cause the selector foot to switch between selection positions by causing a given linear displacement in the position of the selector foot. Preferably, the vibrations in the piezoelectric motor cause the given linear displacement in the position of the selector foot in a period of time less than 10 msec. More preferably, vibrations in the piezoelectric motor cause the given linear displacement in the position of the selector foot in a period of time less than 7 msec. Most preferably the vibrations in the piezoelectric motor cause the given linear displacement in the position of the selector foot in a period of time less than 5 msec. In some preferred embodiments of the present invention vibrations in the piezoelectric motor cause the given linear displacement in the position of the selector foot in a period of time substantially equal to 5 msec. 
     In some preferred embodiments of the present invention the at least one selector foot comprises a plurality of selector feet and each latch needle is associated with a particular one of the plurality of selector feet and when a latch needle is in the activation station the latch needle is activated or not activated according to the selection position of the particular selector foot of the plurality of selector feet with which the latch needle is associated. Preferably, each of the plurality of selector feet is coupled to a different piezoelectric motor. 
     There is further provided, in accordance with a preferred embodiment of the present invention, a method for switching a selector foot between an activate selection position and a deactivate selection position comprising: a) providing said selector foot with a friction coupling surface; b) coupling a piezoelectric motor to said friction coupling surface; and c) using vibrations of said piezoelectric motor to switch said selector foot between said activate selection position and said deactivate selection position. Preferably, coupling the piezoelectric motor to the friction coupling surface comprises pressing a contact surface of the piezoelectric motor to the friction coupling surface with a resilient force. Preferably, the piezoelectric motor has a friction nub and coupling the piezoelectric motor to the friction coupling surface comprises pressing the friction nub to the friction coupling surface with a resilient force. 
     Preferably, providing the selector foot with a friction coupling surface comprises forming a cylindrical friction surface. Alternatively, providing the selector foot with a friction coupling surface comprises forming a planar friction surface. 
     In some preferred embodiments of the present invention using vibrations of the piezoelectric motor to switch the selector foot between the activate selector position and the deactivate selector position, comprises using the vibrations to rotate the selector foot through a given angle. Preferably, using vibrations of the piezoelectric motor to switch the selector foot comprises using the vibrations to rotate the selector foot through the given angle in a period of time less than 10 msec. More preferably, using vibrations of the piezoelectric motor to switch the selector foot comprises using the vibrations to rotate the selector foot through the given angle in a period of time less than 7 msec. Most preferably, using vibrations of the piezoelectric motor to switch the selector foot comprises using the vibrations to rotate the selector foot through the given angle in a period of time less than 5 msec. In some preferred embodiments of the present invention using vibrations of the piezoelectric motor to switch the selector foot, comprises using the vibrations to rotate the selector foot through the angle in a period of time substantially equal to 5 msec. 
     In other preferred embodiments of the present invention, using vibrations of the piezoelectric motor to switch the selector foot, comprises using the vibrations to cause a given linear displacement in the position of the selector foot. Preferably, using vibrations of the piezoelectric motor to switch the selector foot, comprises using the vibrations to cause the given linear displacement in a period of time less than 10 msec. More preferably, using vibrations of the piezoelectric motor to switch the selector foot, comprises using the vibrations to cause the given linear displacement in a period of time less than 7 msec. Most preferably, using vibrations of the piezoelectric motor to switch the selector foot, comprises using the vibrations to cause the given linear displacement in a period of time less than 5 msec. In some preferred embodiments of the present invention using vibrations of the piezoelectric motor to switch the selector foot, comprises using the vibrations to cause the given linear displacement in a period of time substantially equal to 5 msec. 
    
    
     BRIEF DESCRIPTION OF FIGURES 
     The invention will be more clearly understood by reference to the following description of a preferred embodiment thereof read in conjunction with the attached figures listed below, wherein identical structures, elements or parts which appear in more than one of the figures are labeled with the same numeral in all the figures in which they appear, and in which: 
     FIG. 1 shows a schematic of parts of a conventional selector used with a shuttle type activation station in a linear knitting machine having a needle bed; 
     FIGS. 2A-2C show details of the construction and operation of a conventional selector foot; 
     FIG. 3 shows schematically parts of a selector, in accordance with a preferred embodiment of the present invention for use with the same shuttle type activation station and linear knitting machine shown in FIG. 1; and 
     FIGS. 4A-4C show the details of construction and operation of a selector foot in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a schematic of parts of a conventional selector  20  used with a shuttle type activation station in a linear knitting machine having latch needles held in a needle bed. In this and in the other figures only those parts of the knitting machine, selector and latch needles are shown that are needed for understanding the operation of the selector shown in the figure. The knitting machine needle bed holds a plurality (typically thousands) of latch needles with shafts  22  formed with activation fins  24  in a closely packed parallel array in which the needles are equally spaced from each other. Activation fins  24  are staggered at different positions along the lengths of shafts  22 , and shafts  22  are positioned in the needle bed in such a manner, that activation fins  24  are accurately aligned in N parallel equally spaced rows perpendicular to shafts  22 . In FIG. 1A, N=10. The rows are indicated by dashed lines  25  and the first two activation fins  24  of each row are shown. Between any two consecutive activation fins  24  in a row of activation fins  24  there are N−1 latch needles, i.e., the distance between two consecutive activation fins  24  in a same row of activation fins  24  is N times the distance between adjacent latch needles. 
     Selector  20  comprises a selector frame (not shown) and an array of N selector feet  26 . Selector feet  26  are mounted in a row in the selector frame so that they are parallel to each other and equally spaced one from the other. Each selector foot  26  is mounted to the frame by means of a pin  28  around which selector foot  26  is rotatable. A bimorph activator  30  is coupled to each selector foot  26  by a U coupler  32  having arms  34  and  36 . The space between adjacent selector feet  26  is equal to the space between adjacent rows of activation fins  24 . 
     Selector  20  moves together with the shuttle activation station of the knitting machine as the activation station shuttles back and forth over the knitting machine&#39;s needle bed. Selector  20  moves over and parallel to the needle bed and in a direction parallel to the rows, i.e., parallel to lines  25 , of activation fins  24 , with each selector foot  26  maintained accurately aligned over and close to a different row of activation fins  24 . Each selector foot  26  therefore moves over latch needle shafts  22  in the needle bed along a different row of activation fins  24  and encounters an activation fin  24  once for every N needle shafts  22  that the selector foot  26  passes. 
     FIGS. 2A-2C show details of the construction and operation of a selector foot  26 . FIG. 2A shows a bimorph activator  30  coupled to selector foot  26 . Bimorph activator  30  is a long thin rectangular strip of piezoelectric material having large face surfaces  38  and ends  40  and  42 . End  40  is situated between arms  34  and  36  of a U connector  32 . End  42  of bimorph  30  is fastened to the frame (not shown) of selector  20 . Bimorph activator  30  bends when a potential difference is applied between faces  38 , otherwise bimorph  30  is straight. In FIG. 2A there is no potential difference between face surfaces  38 , and bimorph  30  is straight. When a potential difference is applied between face surfaces  38 , bimorph  30  bends into an arc shape with one of face surfaces  38  concave and the other convex. The direction of the bend depends upon the polarity of the applied potential. The bend causes end  40  to displace and, depending on the polarity of the applied potential, push against and apply a force to arm  34  or arm  36  of U coupler  32 . 
     FIG. 2B shows bimorph  30  when a potential difference is applied between face surfaces  38  that causes bimorph  30  to bend so that end  40  presses on arm  36  of U connector  32 . The pressure exerted by end  40  on arm  36  causes selector foot  26  to rotate clockwise through a small angle. FIG. 2C shows bimorph  30  when a potential difference is applied between face surfaces  38  which is opposite in polarity to the potential difference applied to face surfaces  38  in FIG.  2 B. In this case bimorph  30  bends in a direction opposite to the bend direction shown in FIG. 2B, and end  40  of bimorph  30  presses on arm  34  of U connector  32 . The pressure exerted by end  40  on arm  34  causes selector foot  26  to rotate counterclockwise through a small angle. The size of the angle through which selector foot  26  rotates when end  40  presses on one of arms  34  or  36  depends upon the amplitude of the voltage applied between face surfaces  38 . 
     In FIG. 2C, the potential difference applied between face surfaces  38  of bimorph  30  is sufficient to cause selector foot  26  to rotate clockwise by an angle large enough so that selector foot  26  is in a deactivate selection position. In this selection position, as selector  20  moves along rows of activation fins  24 , selector foot  26  will “collide” with and depress any activation fin  24  that it encounters. This will deactivate the needle to which the activation fin  24  is connected. 
     In FIG. 2B, on the other hand, a potential difference is applied between face surfaces  38  of bimorph  30  which has a polarity opposite to the potential difference applied to face surfaces  38  in FIG.  2 C and which is large enough to cause selector foot  26  to rotate clockwise into an activate selection position. In this position selector foot  26  is out of the way of onrushing activation fins and when it encounters an activation fin  22 , it will not collide with the activation fin  24 . It will “miss” and pass by the activation fin  24  and not depress it. The latch needle to which the activation fin  24  is connected will therefore be activated by the activation station. 
     FIG. 3 shows schematically parts of a selector  50 , in accordance with a preferred embodiment of the present invention, for use with the same shuttle type activation station and linear knitting machine with which the prior art system shown in FIG. 1 is used. 
     Selector  50  preferably comprises a selector frame (not shown) and an array of N selector feet  52 . N is preferably on the order of 10 (FIG. 3 is shown with N=10). Selector feet  52  are preferably mounted in a row in the selector frame so that selector feet  52  are preferably parallel to each other and equally spaced one from the other. Each selector foot  52  is mounted to the frame by means of a pin  54  around which selector foot  52  is rotatable. Selector foot  52  is preferably formed with, or mounted with, a friction coupling surface  56 . Friction coupling surface  56  is preferably a circularly cylindrical surface with axis congruent with the axis of pin  54 . Friction coupling surface  56  couples selector foot  52  to a piezoelectric motor  58  which is mounted to the selector frame by methods known in the art. The space between adjacent selector feet  52  is equal to the space between adjacent rows of activation fins  24 . 
     FIGS. 4A-4C show the details of construction and operation of a selector foot  52 . Referring to FIG. 4A, selector foot  52  is shown between activate and deactivate selection positions, in the same orientation with respect to the needle bed of the knitting machine as the orientation of selector foot  26  in FIG.  2 A. 
     Piezoelectric motor  58  is preferably formed in the shape of a thin rectangular plate having two large planar faces  60 , and short edge surfaces  62  and  64 . For exciting vibrations in the body of piezoelectric motor  58  one planar face surface  60  preferably has at least two surface electrodes and the other planar face surface  60  preferably has at least one surface electrode. Preferably, piezoelectric motor  58  has four quadrant surface electrodes  66  on one face surface  60  and a ground surface electrode on the other, hidden face surface  60 . Piezoelectric motor  58  preferably has a friction nub  68  fixed to edge surface  62 , for coupling to friction coupling surface  56 . Preferably, piezoelectric motor  58  is of the type described in U.S. Pat. No. 5,453,653. 
     Friction nub  68  is preferably pressed to friction coupling surface  56  by a resilient force  70  applied between short edge  64  and a frame (not shown)of selector  50 . Quadrant electrodes  66  and the ground electrode of piezoelectric motor  58  are preferably connected to a control circuit (not shown) which electrifies them to produce vibrations in the body of piezoelectric motor  58  as described in U.S. Pat. No. 5,453,653. The vibrations preferably produce clockwise or counterclockwise elliptical motion in friction nub  68  which produce respectively clockwise or counterclockwise frictional forces tangent to friction coupling surface  56 . These frictional forces produce torques which rotate selector foot  52  clockwise and counterclockwise to switch selector foot  52  respectively into a deactivate selection position or an activate selection position. FIG. 4B shows selector foot  52  in a clockwise, activate selection position, and FIG. 4C shows selector foot  52  in a counterclockwise, deactivate selection position. 
     Preferably, piezoelectric motor  58  can displace a moveable element at a rate of about 400 mm/sec against a force opposing the motion on the order of from 0.2 to 0.5 Newton. At this rate of displacement, assuming the radius of friction coupling surface  56  is 10 mm, piezoelectric motor  58  can rotate selector foot  52  at an angular velocity of about 40 radians/sec or about 2350°/sec. Assuming selector foot  52  must be rotated about 15° to switch selector foot  52  from a deactivate to an activate selection position, selector foot  52  can be switched between selection positions, according to a preferred embodiment of the present invention, in a switching time of about 5 msec. 
     A selector  50 , in accordance with a preferred embodiment of the present invention, having 10 selector feet  52  operating with a switching time of 5 msec operates at a decision frequency of 2000 Hz. 
     The present invention has been described using a non limiting detailed description of a preferred embodiment thereof. Variations of the embodiment described will occur to persons of the art. For example, a selector foot can be constructed so that instead of being rotated to switch between selection positions, the selector foot is displaced linearly to switch between selection positions. In this case a friction coupling surface of the selector foot would be a planar surface and selector feet would be mounted to a selector frame so that they slide along appropriate linear guides in the selector. It should also be realized that switching time is a function of the way in which the piezoelectric motor is coupled to the selector foot, the dimensions of the selector foot and the amplitude of the motion needed to switch the selector foot between selection positions. For the cylindrical friction coupling surface described above and a piezoelectric motor of constant speed (and variable power output), for example, switching time is proportional to the radius of the friction coupling surface. Additionally, while the detailed description of a preferred embodiment of the present invention refers to a selector used with a linear knitting machine, a selector in accordance with a preferred embodiment of the present invention, is similarly constructed for selecting latch needles in rotary knitting machines.