Patent Abstract:
An optical position monitor for determining the position of a latch needle ( 104 ) in a knitting machine is provided that comprises: at least one fiducial ( 102 ) at a known fixed cation on the body of the latch needle; a fiducial imager that produces at least one optical image ( 262 ) of the at least one fiducial on at least one light sensitive surface ( 260 ) wherein the at least one optical image changes with changes in position of said at least one fiducial; and a controller that receives at least one signal responsive to the changes in the at least one image and uses the at least one signal to determine the position of the at least one fiducial ( 102 ) and thereby of the latch needle ( 104 ).

Full Description:
RELATED APPLICATIONS 
     The present application is a U.S. National application of PCT/IL98/00111, filed on Mar. 8, 1998, which is a continuation-in-part of PCT/IL97/00160 filed on May 15, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to knitting machines and in particular to means and methods for activating latch needles in knitting machines and monitoring latch needle positions. 
     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 long flat needle having, at one end, a small hook and a latch that swivels to open and close the hook. The hook ends of the latch needles are moved forwards and backwards towards and away from the threads being knitted into the fabric. As a latch needle is moved, its latch alternately opens and closes so that the hook catches a thread close to it, pulls it to create a loop of fabric, and then releases the thread to start the cycle over again and produce another loop of fabric. 
     Latch needles are arranged parallel to each other, in arrays of many hundreds to thousands of latch needles in modern knitting machines. The latch needles are placed into narrow latch needle slots that are machined into a planar surface, hereafter referred to as a “needle bed surface”, of a large rectangular metal plate, hereafter referred to as a “needle bed”. The latch needle slots hold the latch needles in position and confine their motion to linear displacements along the lengths of the latch needle slots. The latch needle slots are parallel to each other and equally spaced one from the other with spacing that varies depending upon the quality and type of fabric being produced. Spacing of two to three millimeters is typical, but spacing significantly less than and greater than two millimeters are also common. 
     The latch needle slots in a needle bed are sufficiently deep so that all or most of the body of a latch needle lies completely in the latch needle slot in which it is placed and below the needle bed surface into which the latch needle slots are machined. A small square fin that sticks out from one side of the shaft of the latch needle protrudes above the needle bed surface. The fins of all latch needles in a needle bed are accurately aligned in a single straight row perpendicular to the latch needle slots. 
     The latch needles are moved, hereafter referred to as “activated”, back and forth in their respective latch needle slots in order to form loops in a fabric being knitted, by a shuttle that travels back and forth along the length of the needle bed surface parallel to the row of aligned latch needle fins. The shuttle has a flat planar surface facing and parallel to the needle bed surface that extends the full length of the shuttle along the direction of travel of the shuttle. The surface has a channel extending the full length of the shuttle along the direction of travel of the shuttle. The channel is open at both of its two ends, and both ends are aligned with the row of aligned fins. As the shuttle moves along the row of latch needle fins, the fins of the latch needles sequentially enter the channel at one end of the channel, travel along the channel length and exit the channel at the other end of the channel. For most of its length the channel is parallel to the row of aligned fins, i.e. the direction of travel of the shuttle, however towards its middle it has a bend. A latch needle is activated when its fin encounters the bend and moves along the direction of the bend. In moving along the direction of the bend, the fin and its latch needle are moved back and forth along the direction of the latch needle slot in which the latch needle is placed, i.e. perpendicular to the row of aligned fins. 
     The conventional method for moving latch needles in a knitting machine as described above has a number of drawbacks. 
     For one, the sequential activation of latch needles by a shuttle as the shuttle moves along a needle bed limits the production rates of fabrics. Production rates of fabric produced by knitting machines could be increased if latch needles were individually activated and different combinations of latch needles could be moved simultaneously. Some shuttles in fact have more than one channel in order to simultaneously activate more than one latch needle and increase production rate. 
     In addition, in the process of knitting a fabric, dust and dirt accumulate in the slots in which latch needles of a knitting machine move. As the dust and dirt accumulate, more force is required to move the latch needles. At some point, dust and dirt accumulate to such an extent that a latch needle jams in its slot. The shuttle is too massive and moves too quickly for it to be practical for the shuttle to be sensitive to, or respond to, changes in the force needed to move a particular latch needle. As the shuttle rushes along the needle bed and encounters a jammed latch needle it breaks the fin or some other part of the jammed latch needle. When this happens physical damage to the knitting machine is often considerably more extensive than the damage to the single latch needle that jammed and knitting machine down time as a result of the damage is prolonged. 
     In order to prevent damage to knitting machines from jammed latch needles it would be advantageous to have a system for moving latch needles in a knitting machine that activates latch needles individually and is responsive to changes in the forces required to move individual latch needles. 
     Prior art direct needle drive systems exist that provide for individual activation of latch needles in a knitting machine. These systems, hereafter referred to as “DND” systems, generally provide an actuator for each latch needle and a system for monitoring the position of each latch needle. However, the prior art systems have not been completely satisfactory. The dimensions of actuators used in the prior art systems are large compared to the spacing between latch needles. Complicated spatial configurations are therefore required to pack large numbers of the actuators in a convenient volume of space near to the latch needles in order to couple the actuators to the latch needles. 
     Additionally, the response times of prior art DND systems are slow. This is the result of slow response times of actuators and of latch needle position monitoring systems used in these systems. The advantages in production rate and decreased knitting machine down time that should be provided by prior art DND systems are at least partly neutralized by the slow response times of these systems. 
     SUMMARY OF THE INVENTION 
     It is an object of one aspect of the present invention to provide a knitting machine comprising a fast response time DND system for activating latch needles in the knitting machine. 
     It is an object of another aspect of the present invention to provide a DND system in which each latch needle of a knitting machine is activated exclusively by at least one piezoelectric micromotor which activates only that latch needle. 
     An object of another aspect of the present invention is to provide a piezoelectric micromotor suitable for use in a fast response time DND system. 
     An additional aspect of the present invention is to provide a transmission for coupling each latch needle in a DND system, in accordance with a preferred embodiment of the present invention, to an at least one piezoelectric micromotor, which at least one piezoelectric micromotor, hereafter referred to as “at least one exclusive piezoelectric micromotor”, is not coupled to any other latch needle. 
     Piezoelectric micromotors can be made small and powerful and response times of piezoelectric micromotors can satisfy the fast response time requirements of modern knitting machines. The dynamic range of motion available from piezoelectric micromotors and the energy that can be transmitted in short periods of time from piezoelectric micromotors to moveable elements are also consistent with the requirements of modem knitting machines. A piezoelectric micromotor and transmission, in accordance with preferred embodiments of the present invention, can therefore be used to provide fast response time activation of individual latch needles in a knitting machine. 
     It is an object of yet another aspect of the present invention to provide a DND system comprising a fast response time system for monitoring the position of latch needles activated by the DND system. 
     It is a further object of another aspect of the present invention to provide an electro-optical latch needle position monitoring system, hereafter referred to as an “OPM”, that operates with a fast response time. 
     DND systems by their nature require fast response time position monitoring systems for monitoring the positions of latch needles that they activate. The positions of the latch needles are controlled in knitting machines to accuracy on the order of 25-50 micrometers (μm). A DND system that moves latch needles with a velocity “V” must therefore sample the position of each latch needle it activates with a frequency of between ˜2×(Vm/sec÷25 μm) to 2×(Vm/sec÷50 μm), in order to control the position the latch needle to an accuracy of 25 μm-50 μm. It therefore requires a position monitoring system with a response time on the order of (25 μm-50 μm)/2V. In many conventional knitting machines V is on the order of 1.5 m/sec. A DND system that moves latch needles with this velocity therefore requires a system that samples the position of latch needles with a frequency, or sampling rate, of between 50-100 kHz and a response time between 10 μsec and 20 μsec. 
     Electro-optical systems inherently operate at frequencies that are much faster than typical mechanical cycle frequencies of motion of knitting machine components. In particular an electro-optical OPM, in accordance with a preferred embodiment of the present invention, can provide the fast response time and accuracy of measurement required for monitoring latch needle positions in DND systems. 
     A piezoelectric micromotor for operating individual latch needles in a DND, in accordance with a preferred embodiment of the present invention, comprises a ceramic vibrator formed in the shape of a thin flat plate having two large planar surfaces and narrow edge surfaces. Piezoelectric vibrators of this type are described in U.S. Pat. No. 5,453,653, which is incorporated herein by reference. The thickness of the vibrator preferably ranges from one to a few millimeters. The thickness of the vibrator thus has dimensions on the order of the size of the spacing between latch needles in a needle bed. It is therefore possible to pack large numbers of these vibrators close to each other with their large planar surfaces parallel and with a thin edge of each vibrator aligned with a single latch needle in the needle bed. Each latch needle is activated (i.e. moved back and forth in its latch needle slot in order to form a loop in a fabric being knitted) by coupling to the latch needle vibratory motion of at least one exclusive piezoelectric micromotor having a thin edge aligned with the latch needle. Coupling of the latch needle and the vibratory motion of the at least one exclusive piezoelectric motor may be accomplished by means of a transmission, in accordance with a preferred embodiment of the present invention. 
     In a DND, in accordance with a preferred embodiment of the present invention, latch needles in a knitting machine needle bed and piezoelectric micromotors are coupled by a rotary transmission comprising a bearing shaft on which a plurality of annuli is stacked. The annuli rotate freely on the bearing shaft. Each latch needle in the knitting machine needle bed is coupled to vibratory motion of a different at least one exclusive piezoelectric motor via one of the plurality of annuli. 
     The bearing shaft is mounted over the needle bed, preferably close to the needle bed and with its axis parallel to the needle bed and perpendicular to the latch needle slots in the needle bed. The spacing between the annuli on the shaft is such that the fin of each latch needle in the needle bed is aligned with a different annulus on the bearing shaft. A preferably rigid connecting arm connects the fin of each latch needle in the needle bed to the annulus with which the latch needle fin is aligned. The connecting arm is attached to the fin, preferably by a slideable or flexible joint, formed using methods known in the art. 
     Each annulus on the bearing shaft is coupled to its own at least one exclusive piezoelectric micromotor, in accordance with a preferred embodiment of the present invention by resiliently pressing the at least one exclusive piezoelectric micromotor against the annulus. Activation of the piezoelectric micromotors coupled to an annulus causes the annulus to rotate. The rotation of the annulus is transmitted to the fin of the latch needle to which the annulus is connected, by the connecting arm. The joint connecting the fin and the connecting arm translates the rotational motion of the connecting arm to a linear motion of the latch needle forwards and backwards in its latch needle slot parallel to the length of the latch needle slot, thereby activating the needle. 
     In a DND system, in accordance with an alternative preferred embodiment of the present invention latch needles in a knitting machine needle bed and piezoelectric micromotors are coupled by a linear transmission. With the linear transmission each latch needle in a knitting machine needle bed has at least one exclusive piezoelectric micromotor pressed, preferably by resilient force, directly onto the shaft of the latch needle or onto a suitable extension of the shaft of the latch needle. The latch needle slots in which the latch needles are placed, and/or, the surfaces of the needles in contact with the latch needle slots are preferably provided with bearings or nonstick surfaces. This reduces the possibility of a latch needle jamming or sticking in its latch needle slot under the application of the resilient force pressing the at least one exclusive piezoelectric micromotor to the latch needle shaft or suitable extension thereof. Coupled in this way, vibratory motion of the at least one exclusive micromotor pressed to a latch needle shaft or extension thereof activates the latch needle by causing the latch needle to move back and forth in its latch needle slot. 
     In another form of linear transmission, in accordance with a preferred embodiment of the present invention, piezoelectric micromotors are coupled directly to a “coupling” fin of a latch needle in order to transmit motion to the latch needle. The coupling fin, except for its dimensions, is preferably similar in shape and construction to conventional latch needle fins. The coupling fin is a planar extension of the body of the latch needle having first and second parallel planar sides and thin edges. Preferably, the coupling fin is formed as an integral part of the latch needle and lies in the plane of the body of the latch needle (the latch needle is flat). A rectangular region of the first side and a rectangular region of the second side, hereafter referred to as first and second “coupling regions” respectively, are preferably clad in wear resistant material suitable for friction coupling with piezoelectric micromotors, such as for example, alumina. Preferably, the first and second coupling regions are congruent and directly opposite each other. 
     In one configuration for coupling piezoelectric micromotors to the coupling fin, in accordance with a preferred embodiment of the present invention, at least one micromotor is resiliently pressed to each of the first and second coupling regions so that a surface region of the micromotor used for transmitting motion from the micromotor to a moveable element, or a hard wear resistant friction nub on the surface region, contacts the coupling region. Preferably, the same number of piezoelectric micromotors is resiliently pressed to each of the first and second coupling regions. Preferably the at least one micromotor pressed to the first coupling region is identical to the at least one micromotor pressed to the second coupling region. Preferably, points at which the at least one micromotor pressed to the first coupling region contacts the first coupling region and points at which the at least one micromotor pressed to the second coupling region contacts the second coupling region are directly opposite each other. Preferably, the magnitude of the forces exerted on the coupling fin perpendicular to the plane of the coupling fin by the at least one micromotor pressed to the first and second coupling regions are equal. Preferably, the at least one piezoelectric micromotor pressed to each coupling region comprises one micromotor. 
     The latch needle is driven back and forth in its latch needle slot when the at least one piezoelectric micromotor pressed to the first and second coupling regions are activated so as to transmit linear motion in the same direction to the coupling fin. Preferably, the at least one piezoelectric micromotor pressed to the first and second coupling regions are activated in phase. This substantially prevents a torque that tends to twist the latch needle in its latch needle slot from developing. 
     In another configuration for coupling piezoelectric micromotors to the coupling fin, accordance with a preferred embodiment of the present invention, a piezoelectric micromotor coupled to a coupling fin is mounted in a transmission bracket. The transmission bracket comprises a bearing or a non-stick surface area against which a surface region of the micromotor used for transmitting motion to a moveable element, or preferably, a wear resistant friction nub on the surface region of the micromotor, is resiliently pressed. In order to couple the piezoelectric micromotor to the coupling fin, the coupling fin is inserted between the friction nub and the bearing or the non-stick surface. With this coupling configuration a single piezoelectric micromotor can be used to activate a latch needle without causing unwanted torque that twists the latch needle in its latch needle slot. Force exerted by the piezoelectric micromotor perpendicular to the plane of the coupling fin is opposed by an equal and opposite force exerted on the coupling fin by the bearing or the non-stick surface. 
     In order to couple adjacent latch needles in a needle bed to piezoelectric micromotors using coupling fins, in accordance with a preferred embodiment of the present invention, coupling fins of adjacent latch needles are preferably displaced with respect to each other in the direction of motion of the latch needles and/or protrude different distances above the latch needle bed. This provides sufficient space between piezoelectric micromotors coupled to coupling fins of adjacent latch needles so that the piezoelectric micromotors do not interfere with the motion of the latch needles 
     A DND system controls latch needle actuators responsive to the position of the particular latch needle to which the actuators are coupled. In a DND system, in accordance with a preferred embodiment of the present invention, latch needle positions are monitored by an OPM. 
     An OPM, in accordance with a preferred embodiment of the present invention, monitors the position of a latch needle by optically tracking the position of a small light reflecting region, or a region comprising areas of substantially different reflectivity, such as a light reflecting region with a black line, hereafter referred to as a “fiducial”, located at a known fixed position on the latch needle. The fiducial is illuminated by light from an appropriately located light source, hereafter referred to as a “fiducial illuminator”. The fiducial reflects a portion of the light from the fiducial illuminator with which it is illuminated into an optical device, hereafter referred to as a “fiducial imager”, comprising a detector having a light sensitive surface. The fiducial imager uses the reflected light to form an image of the fiducial on the light sensitive surface of its detector. A change in the position of the fiducial causes a change in the image of the fiducial on the light sensitive surface, which change is used to determine the change in position of the fiducial. 
     There are a number of other ways in which the latch needle can be provided with a fiducial, in accordance with preferred embodiments of the present invention. For example, a small retro-reflector can be fixed to a point on the body of the latch needle or an appropriate reflecting discontinuity, such as a scratch or dimple, can be formed on a region of the surface of the latch needle. Preferably, the fiducial reflects incident light diffusely within a cone of half energy angle on the order of 10°-20°. The detector and fiducial illuminator comprised in a fiducial imager, in accordance with a preferred embodiment of the present invention, are located so that at any position occupied by the latch needle in its operating range of motion, substantially all the light reflected by the latch needle fiducial into the half energy cone is incident on the detector. 
     In order to provide position measurements for a plurality of latch needles in a needle bed of a knitting machine, an OPM, in accordance with a preferred embodiment of the present invention, comprises a plurality of fiducial imagers arranged in an array. Preferably, the fiducial imagers are aligned collinearly in a line array defined by an axis that is a straight line. Preferably, the axis is parallel to the needle bed surface of the needle bed and perpendicular to the directions of the needle bed slots. 
     The number of the plurality of fiducial imagers in the array in a preferred embodiment of the present invention is preferably equal to the number of the plurality of latch needles. Each fiducial imager is aligned with a different one of the plurality of latch needles and provides position data for the latch needle with which it is aligned. The positions of all latch needles in the plurality of latch needles are thus, preferably, simultaneously measurable by the OPM. Preferably, the number of the plurality of latch needles is equal to the number of latch needles in the knitting machine. 
     In some preferred embodiments of the present invention, the number of the plurality of fiducial imagers in the array of fiducial imagers of an OPM is less than the number of the plurality of latch needles whose positions are to be determined using the OPM. In order to provide position measurements for all the latch needles of the plurality of latch needles, the array of fiducial imagers in the OPM is moved along the needle bed in which the latch needles are held. Preferably, the array of fiducial imagers is moved over the needle bed in a direction collinear with the axis of the array. 
     In one preferred embodiment of the present invention the fiducial imager comprises a lens and a detector having a light sensitive surface that is divided into first and second regions. The areas of the two regions are preferably equal and preferably abut each other along a straight line. The straight line is preferably oriented substantially perpendicular to the direction of motion of the latch needle. The detector sends first and second signals that are functions of the amounts of reflected light from the fiducial incident on the first and second regions respectively to a controller. The lens focuses reflected light from the fiducial to form an image of the fiducial on the light sensitive surface of the detector. The portions of the image, and thereby the amounts of reflected light, that fall on the first and second regions are different for different positions of the fiducial. The first and second signals, are therefore functions of the position of the fiducial and thereby of the position of the latch needle on which the fiducial is located. The controller uses the first and second signals to determine the position of the latch needle. 
     In another preferred embodiment of the present invention the fiducial imager comprises a lens, a detector and a light filter. The detector comprises a light sensitive surface sensitive to light in first and second non-overlapping wavelength bands of light. The light filter has first and second filter regions. Each of the filter regions transmits light in a different one of the wavelength bands and does not transmit light in the other wavelength band. The areas of the two filter regions are preferably equal and preferably abut each other along a straight dividing line. 
     The lens focuses light from the fiducial illuminator that is reflected from the fiducial to form an image of the fiducial on the light sensitive surface of the detector. The filter is positioned with respect to the detector and lens so that the dividing line of the filter and the optic axis of the lens intersect and so that all light from the fiducial focused on the light sensitive surface of the detector passes through the filter. (The filter can also be comprised in an appropriate coating on the lens.) As a result reflected light from the fiducial incident on a first one half of the lens is filtered by the first filter region and reflected light from the fiducial incident on the other half of the lens, a “second half”, is filtered by the second filter region. Therefore the amounts of light in the image of the fiducial in the first and second wavelength bands are proportional to the amounts of light incident on the first and second halves of the lens respectively. 
     Preferably, the fiducial illuminator illuminates the fiducial with substantially equal intensities of light in the first and second wavelength bands and the fiducial has substantially the same reflectivity for light in both wavelength bands. Preferably, the transmittance of the first filter region for light in the first wavelength band is substantially equal to the transmittance of the second filter region for light in the second wavelength band. Preferably, intensities registered by the light sensitive surface in the first and second wavelength bands are normalized to the intensities of light radiated by the fiducial illuminator in the first and second wavelength bands. The intensities are preferably corrected for differences in reflectivity of the fiducial in the two wavelength bands. Preferably, the intensities are corrected for differences between the transmittance of the first filter region for light in the first wavelength band and the transmittance of the second filter region for light in the second wavelength band. The intensities are preferably corrected for differences in sensitivity of the light sensitive surface to light in the two wavelength bands. 
     Hereinafter, when intensities, integrated intensities or amounts of light on light sensitive surfaces are compared, it is understood that they are appropriately normalized to the intensity of light radiated by the fiducial illuminator and corrected for biases introduced by various optical components. 
     The amounts of light incident on the first and second halves of the lens are functions of the position of the fiducial. When the fiducial is located on the optic axis of the lens the first and second halves of the lens receive the same amounts of reflected light. When the fiducial is displaced from the optic axis in the direction of one or the other halves of the lens, the half towards which the fiducial is displaced gets more light and the other half gets less light. Preferably, the dividing line of the filter is substantially perpendicular to the motion of the latch needle and thereby to the fiducial in order to maximize change in the amounts of light incident on the first and second halves of the lens with change of position of the fiducial. The first and second signals sent by the detector to the controller are therefore functions of the position of the fiducial. These signals are used by the controller to determine the position of the fiducial and the latch needle on which the fiducial is located. 
     In an alternate preferred embodiment of the present invention, the fiducial imager comprises two preferably identical light detectors, each having its own lens that focuses an image of the fiducial onto the detector&#39;s light sensitive surface. The two light detectors are displaced from each other by a short distance. The line between the two detectors is aligned parallel with and in the plane of the latch needle slot of the latch needle whose position the detectors are used to determine. The difference between the amounts of light from the fiducial illuminator that is reflected into each of the two detectors is different for different positions of the latch needle along the latch needles range of motion. For example, assume the fiducial illuminator is equidistant from both detectors. When the fiducial is equidistant from both detectors each detector receives the same amount of reflected light from the fiducial and the difference between the amounts of light received by the detectors is substantially zero. If the fiducial is displaced along the direction of motion of the latch needle towards one of the detectors, the detector towards which it is displaced receives an increased amount of reflected light and the other detector receives a decreased amount of light. The difference between the amounts of reflected light received by the detectors from the fiducial is a function of the displacement of the fiducial from the position of the fiducial at which both detectors receive the same amount of reflected light. This difference, and thereby the location of the fiducial and the latch needle, is determined by a Circuit that receives an input signal from each detector that is a function of the intensity of light incident on the detector. 
     In another preferred embodiment of the present invention the fiducial imager comprises one light detector and two lenses. The light sensitive surface of the light detector is sensitive to light in two non-overlapping wavelength bands of light. The fiducial illuminator illuminates the fiducial with preferably equal intensities of light from both wavelength bands. Each of the lenses transmits light in only one of the two different wavelength bands. Both lenses focus light reflected from the fiducial onto the light sensitive surface of the detector. The lenses are displaced a short distance from each other and the line connecting the centers of the lenses is aligned parallel with and in the plane of the latch needle slot of the latch needle whose position the fiducial imager is used to determine. As in the previous fiducial imager, when the fiducial is equidistant from both lenses the detector registers equal intensity (appropriately normalized as discussed above) of light in both of the wavelength bands for which it is sensitive. As the fiducial is displaced towards one or the other of the lenses, the difference between the intensities of light registered by the detector in the two wavelength bands changes as a function of the amount of the displacement. 
     In a yet another preferred embodiment of the present invention, the fiducial imager comprises one light detector and a lens. The light sensitive surface of the light detector is sensitive to light in two non-overlapping wavelength bands of light. The lens transmits light in both of the two wavelength bands. The latch needle whose position is measured using the fiducial imager is provided with two fiducials displaced from each other by a short distance along the length of the latch needle. Each of the fiducials reflects light in a different one of the wavelength bands to which the detector is sensitive and absorbs light in the other wavelength band. The lens focuses both fiducials on the light sensitive surface of the light detector. The difference between the light intensity registered by the detector in the two different wavelength bands is used to determine the position of the two fiducials and thereby of the latch needle. 
     In still yet another preferred embodiment of the present invention, the fiducial imager comprises a monochromatic light detector having a pixelated light sensitive surface, such as a CCD, and a lens that focuses an image of the fiducial on the pixelated surface. The location of the fiducial image on the pixelated surface is determined to be the center of gravity of the illumination pattern on the surface that is caused by the fiducial image. The location of the center of gravity is determined to sub-pixel resolution from the locations of pixels illuminated by the fiducial image and the intensities with which these pixels are illuminated using techniques known in the art. The position of the fiducial and its latch needle is determined from the location of the fiducial image on the pixelated surface by techniques that are well-known in the art. 
     It should be realized that an OPM, in accordance with a preferred embodiment of the present invention, is useable for any application requiring position monitoring of latch needles and its use is not restricted for use only in cooperation with a DND system. It should also be realized that an OPM, in accordance with a preferred embodiment of the present invention, is useable for providing latch needle position measurements for a DND system irrespective of the type of actuators used to activate latch needles in the DND system, and is not limited to use with DND systems that use piezoelectric micromotors or actuators. 
     There is therefore provided in accordance with a preferred embodiment of the present invention an optical position monitor for determining the position of a latch needle in a knitting machine comprising: at least one fiducial at a known fixed location on the body of the latch needle; a fiducial imager that produces at least one optical image of the at least one fiducial on at least one light sensitive surface, wherein the at least one optical image changes with changes in position of the at least one fiducial; and a controller that receives at least one signal responsive to the changes in the at least one image and uses the at least one signal to determine the position of the at least one fiducial and thereby of the latch needle. 
     Preferably, the optical position monitor comprises at least one fiducial illuminator that illuminates the at least one fiducial. Additionally or alternatively, the changes in the at least one image comprise changes in integrated intensity of the at least one image. Alternatively or additionally, the at least one fiducial comprises a single fiducial. 
     In some preferred embodiments of the present invention the at least one light sensitive surface comprises first and second light sensitive surfaces and the at least one signal comprises first and second signals responsive to the intensity of light reflected by the at least one fiducial imaged on the first and second light sensitive surfaces respectively. 
     Preferably, the first and second light sensitive surfaces comprise first and second contiguous light sensitive surfaces. The at least one image preferably comprises a single image having first and second portions on the first and second light sensitive surfaces respectively and the ratio between the first and second portions depends upon the position of the at least one fiducial. 
     Alternatively, the first and second light sensitive surfaces comprise first and second light sensitive surfaces that are preferably displaced from each other by a distance. Preferably, the optical position monitor comprises first and second lenses and the at least one image comprises first and second images, wherein the first and second light sensitive surfaces are optically aligned with the first and second lenses respectively, and the first lens produces the first image on the first light sensitive surface and the second lens produces the second image on the second light sensitive surface and wherein the ratio between the integrated intensities of the first and second images depends upon the position of the at least one fiducial. 
     In still other preferred embodiments of the present invention the at least one light sensitive surface comprises a single light sensitive surface sensitive to light in first and second non-overlapping wavelength bands of light and the at least one signal comprises first and second signals responsive to the integrated intensity of light incident on the single light sensitive surface in the first and second wavelength bands respectively. 
     Preferably, the optical position monitor comprises a light filter having first and second filter regions wherein the first region transmits light only in the first wavelength band and the second filter region transmits light only in the second wavelength band and light reflected from the single fiducial that is imaged on the light sensitive surface, passes through either the first filter region or the second filter region. 
     Preferably, the at least one image comprises a single image, wherein a first portion of light in the single image reflected from the fiducial passes through the first filter region and a second portion of light in the single image reflected from the fiducial passes through the second filter region, and wherein the ratio between first and second portions depends upon the position of the fiducial. 
     Alternatively, the optical position monitor comprises a first lens and a second lens displaced from each other by a distance, wherein the first lens transmits light only in the first wavelength band and the second lens transmits light only in the second wavelength band, wherein the first and second lenses produce first and second images of the fiducial on the light sensitive surface respectively, and the relative integrated intensity of light in the first and second images is a function of the position of the fiducial. 
     In some preferred embodiments of the present invention the at least one fiducial comprises at least a first and a second fiducial. Preferably, the at least one light sensitive surface comprises a single light sensitive surface sensitive to light in first and second non-overlapping wavelength bands of light and wherein the at least one signal comprises first and second signals responsive to the integrated intensity of light incident on the single light sensitive surface in the first and second wavelength bands respectively. Preferably, the first fiducial reflects light only in the first wavelength band and the second fiducial reflects light only in the second wavelength band, and the optical position monitor comprises: a lens that produces a first image of the first fiducial and a second image of the second fiducial on the light sensitive surface using light reflected from the first and second fiducials respectively; wherein the integrated intensity of light in the first and second images depends upon the position of the first and second fiducials. 
     In an optical position monitor in accordance with some preferred embodiments of the present invention, changes in the at least one image comprise changes in the location of the at least one image on the at least one light sensitive surface. Preferably, the at least one light sensitive surface comprises at least one pixelated surface. Preferably, the at least one signal comprises signals responsive to the intensity of light incident on each pixel of the at least one pixelated surface. The at least one image preferably comprises a single image on each of the at least one pixelated surface. In some preferred embodiments of the present invention the at least one pixelated surface comprises a single pixelated surface. 
     In some preferred embodiments of the present invention a location for each of the at least one image is defined as the location of an optical center of gravity of the at least one image, which location is determined from the signals responsive to the intensity of light incident on each pixel of the at least one pixelated surface, and wherein the location of the optical center of gravity is responsive to the position of the at least one fiducial. 
     In some preferred embodiments of the present invention wherein changes in the at least one image comprise changes in the location of the at least one image on the at least one light sensitive surface, the at least one fiducial comprises a single fiducial. 
     In some preferred embodiments of the present invention the single fiducial of a plurality of latch needles is imaged on different regions of the at least one pixelated surface, and the optical position monitor is used to determine the positions of a plurality of latch needles. Preferably, the number of the plurality of latch needles is greater than 5. Alternatively, the number of the plurality of latch needles is preferably greater than 10. Alternatively, the number of the plurality of latch needles is preferably greater than 20. 
     In some preferred embodiments of the present invention an optical position monitor comprises a means for selectively aligning the optical position monitor with different latch needles in the needle bed. 
     There is further provided an optical position monitor for simultaneously monitoring the position of a plurality of latch needles in a knitting machine needle bed, which needle bed has a plane surface having latch needle slots that are parallel to each other, comprising a plurality of optical position monitors in accordance with a preferred embodiment of the present invention. 
     Preferably, each of the plurality of the optical position monitors is aligned with a different latch needle and is used to determine the position of at least the latch needle with which it is aligned. 
     The optical position monitors in the plurality of optical position monitors are preferably aligned in a line array along a straight line. Preferably, the line array is parallel to the needle bed surface and perpendicular to the latch needle slots. Alternatively or additionally, the spacing between an optical position monitor in the line array and an adjacent optical position monitor is the same for any optical position monitor in the line array. Preferably, the spacing is equal to the spacing between adjacent latch needles of the plurality of latch needles. 
     In some preferred embodiments of the present invention, the number of the plurality of needles is equal to the number of needles in the needle bed. 
     In other preferred embodiments of the present invention the number of the plurality of latch needles is less than the number of needles in the needle bed and the optical position monitor includes a means for selectively aligning the optical position monitor with different groups of latch needles in the needle bed. Preferably the means for aligning the optical position monitor with different groups of latch needles comprises means for translating the optical position monitor in a direction parallel to the needle bed and perpendicular to the latch needle slots. 
     In some preferred embodiments of the present invention the optically reflective fiducial comprises at least two regions on the surface of the latch needle having different reflectivities. Preferably, at least one of the at least two regions comprises a retroreflector. Alternatively or additionally, at least one of the at least two regions comprises at least one discontinuity in the surface of the latch needle. Preferably, the at least one discontinuity comprises at least one straight line groove on the surface of the latch needle. Alternatively or additionally, the discontinuity preferably comprises at least one dimple depressed into the surface of the latch needle. Alternatively or additionally, at least one of the at least two regions is preferably substantially non-reflecting. 
     Additionally or alternatively, light reflected from the fiducial is substantially confined within a cone of half energy angle less than 20°. Additionally or alternatively light reflected from the fiducial is substantially confined within a cone of half energy angle less than 15°. 
     Additionally or alternatively, light reflected from the fiducial is substantially confined within a cone of half energy angle less than 10°. 
     There is further provided an actuator system for activating a latch needle, which latch needle has a shaft, comprising: a flat planar extension of the shaft having first and second parallel planar surfaces; at least one piezoelectric micromotor having a first surface region for transmitting motion to a moveable element, which first surface region is resiliently pressed to the first surface and at least one additional piezoelectric motor having a second surface region for transmitting motion to a moveable element which second surface region is resiliently pressed to the second surface; and wherein vibratory motions of the first and second surface regions apply forces to the flat extension that cause motion in the latch needle. 
     There is also provided an actuator system for activating a latch needle, which latch needle has a thin flat shaft comprising: a flat planar extension of the shaft having first and second planar surfaces; a piezoelectric micromotor having a surface region for transmitting motion to a moveable element; a transmission bracket for holding the piezoelectric micromotor, the transmission bracket comprising a bearing surface and a means for resiliently urging the surface region of the piezoelectric micromotor towards the bearing surface; and wherein the flat extension is inserted between the surface region of the piezoelectric micromotor and the bearing or the non-stick surface and wherein vibratory motion of the surface region applies force to the flat extension causing motion in the latch needle. 
     Preferably, the bearing surface is the surface of a rotatable roller or ball. Alternatively or additionally, the bearing surface is a surface having a low friction coating. 
     In an actuator system for activating a latch needle according to some preferred embodiments of the present invention, the surface region for transmitting motion to a moveable element comprises a wear resistant nub that makes contact with a surface of the moveable element towards which the surface region for transmitting motion is resiliently pressed in order to transmit motion to the moveable element. 
     In an actuator system for activating a latch needle according to some preferred embodiments of the present invention, points on surfaces of the flat extension at which said surface regions of the piezoelectric micromotors make contact are clad in wear resistant material. 
    
    
     BRIEF DESCRIPTION OF FIGURES 
     The invention will be more clearly understood by reference to the following description of preferred embodiments thereof read in conjunction with the attached figures listed below, wherein identical structures, elements or parts that 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 the basic structure of a latch needle; 
     FIG. 2 is a schematic illustration of a conventional system for activating latch needles in a knitting machine; 
     FIG. 3 is a schematic illustration of a system for coupling piezoelectric micromotors to latch needles in a needle bed by rotary transmission, in accordance with a preferred embodiment of the present invention; 
     FIG. 4 shows a schematic of a system for coupling piezoelectric micromotors to latch needles in a needle bed by linear transmission in accordance with an alternative preferred embodiment of the present invention; 
     FIG. 5 illustrates schematically the coupling of a latch needle with a coupling fin to two piezoelectric micromotors in accordance with a preferred embodiment of the present invention; 
     FIG. 6 illustrates schematically the coupling of a latch needle with a coupling fin to a single piezoelectric micromotor mounted to a transmission bracket in accordance with yet another preferred embodiment of the present invention; 
     FIGS. 7A-7C schematically illustrate an OPM comprising a single fiducial imager, imaging a latch needle fiducial, in accordance with a preferred embodiment of the present invention; 
     FIG. 8 schematically illustrates an OPM comprising a linear array of a plurality of imaging fiducials shown in FIGS. 7A-7C, imaging an equal plurality of latch needle fiducials in accordance with a preferred embodiment of the present invention; 
     FIGS. 9A-9C schematically illustrate an OPM comprising a single fiducial imager, imaging a latch needle fiducial, in accordance with an alternative preferred embodiment of the -present invention; 
     FIGS. 10A-10C schematically illustrate an OPM comprising a single fiducial imager, imaging a latch needle fiducial, in accordance with another preferred embodiment of the present invention; 
     FIGS. 11A-11C schematically illustrate an OPM comprising a single fiducial imager, imaging a latch needle fiducial, in accordance with yet another preferred embodiment of the present invention; 
     FIGS. 12A-12C schematically illustrate an OPM comprising a single fiducial imager, imaging a latch needle fiducial, in accordance with still another preferred embodiment of the present invention; and 
     FIGS. 13A-13C schematically illustrate an OPM comprising a single fiducial imager, imaging a latch needle fiducial, in accordance with another alternative preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a profile of a latch needle  20 . Latch needle  20  is a thin metallic structure with a long shaft  22  having a hook  24  and a tip  30  formed on one of its ends. A latch  26  is rotatable about a pivot  28  and is shown in the figure in the position where it caps tip  30  to close hook  24  and prevents hook  24  from hooking a thread. In an open position latch  26  is rotated clockwise almost to a position where it is parallel to shaft  22 . A fin  32  extends out from shaft  22 , generally on the same side of shaft  22  as hook  24 . 
     FIG. 2 is a schematic illustration of the arrangement of needle beds in a conventional knitting machine and a shuttle which transmits motion to latch needles in the needle beds. 
     Two needle beds  36  and  38  are rigidly joined at an angle to each other so that an edge  39  of needle bed  36  is close to and parallel to an edge  40  of needle bed  38 . A long narrow space  44  separates edge  39  and edge  40 . Needle beds  36  and  38  are identical or very similar and detailed discussion will be confined to needle bed  36  with the understanding that details and structures described for needle bed  36  apply equally to needle bed  38 . 
     Threads to be woven into fabric (not shown) are held under tension close to and parallel to edges  39  and  40 . Fabric (not shown), as it is produced moves downwardly from edges  39  and  40  into space  44 . As the fabric moves down it exits the knitting machine. 
     Needle bed  36  is provided with an array of equally spaced parallel latch needle slots  42  that are perpendicular to edge  39 . A latch needle  20  is placed in each latch needle slot  42 . The bodies of latch needles  20  are completely inside latch needle slots  42  and are not visible. Only fins  32  of latch needles  20  protrude above the surface of needle bed  36  and are visible. Fins  32  of all latch needles  20  that are at rest in slots  42  are aligned along a straight row which is perpendicular to latch needle slots  42 . Each needle  20  is moveable back and forth in its latch needle slot  42 . 
     A shuttle  46 , having ends  52  and  54 , moves back and forth parallel to edges  39  and  40  along the length of needle bed  36 . An interior face  48  of shuttle  46  is parallel to needle bed  36  and has a channel  50  formed in the face. Channel  50  is open on both ends  52  and  54  of shuttle  46 . The two open ends of channel  50  are in line with the row of fins  32 . A section  56  of channel  50  is not-collinear with the ends of channel  50 . Channel  50  is just wide enough and deep enough so that fins  32  can pass into and move through it. 
     As shuttle  46  moves back and forth with interior face  48  parallel to latch needle bed  36 , fins  32  of latch needles  20  enter channel  50  at one end and move along the length of channel  50 . When a fin  32  of a latch needle  20  encounters non-collinear section  56  of channel  50  the fin  32  and the latch needle  20  to which fin  32  is attached are displaced parallel to latch needle slot  42  in which the latch needle  20  is found. In FIG. 2, for clarity of presentation, only a few of latch needles  20  that are moving in channel  50  are shown. 
     FIG. 3 shows a system for exclusively coupling each of the latch needles in a needle bed to at least one exclusive piezoelectric micromotor using a rotary transmission, according to a preferred embodiment of the present invention. A long bearing shaft  58  is mounted over a needle bed  60  that is provided with slots  62  into which have been placed latch needles  63 . Bearing shaft  58  is mounted with a multiplicity of thin annuli  64 , one annulus for each latch needle (for clarity only three are shown). The annuli rotate freely on bearing shaft  58 . Each annulus is positioned opposite a fin  65  of a particular latch needle  63 . A connecting arm  66  connects each annulus  64  to a point  68  on fin  65 , to which annulus  64  is opposite. The connection at point  68  is a flexible or slideable connection produced by methods known in the art. One or more piezoelectric micromotors  70 ,  72 , and  74 , are resiliently pressed against each annulus  64  by methods known in the art. When piezoelectric micromotors  70 ,  72 , and  74 , are activated they cause annulus  64  and connecting arm  66  to rotate, which in turn moves latch needle  63  linearly in its slot  62 . The flexible connection at point  68  translates rotational motion of arm  66  to linear motion of latch needle  63 . It should be understood that this arrangement allows for a much higher speed of the latch needle than that available from the motor itself. 
     While three exclusive piezoelectric micromotors are shown coupled to annulus  64  in FIG. 3, a greater or lesser number of micromotors can be used depending on the speed or torque required for motion of the needle. Also, other types of piezoelectric micromotors constructed differently than the ones shown in FIG.  3  and described above may be used to rotate annulus  64  and are advantageous. U.S. Pat. No. 4,562,374 and the publication by Hiroshi et al., IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 42, No. 2, March 1995, incorporated herein by reference, describe rotary piezoelectric micromotors. These rotary piezoelectric micromotors comprise a cylindrical, annular or disc shaped rotor that is caused to rotate by coupling to a stator that is a cylindrical, annular or disc shaped vibrator. The rotor and stator are concentric. A vibrating surface of the stator is coupled to an inside edge surface or an outside edge surface of the rotor to impart a rotary motion to it Alternatively, a vibrating surface of the stator may be coupled to a face surface of the rotor to impart rotational motion to the rotor. Annulus  64  can be rotated by the use of stators similar to those described in the above references. Annulus  64  is coupled to the stators in similar fashion to the way that the rotors are coupled to the stators in the described rotary piezoelectric micromotors. 
     FIG. 4 shows another system for coupling each of the latch needles in a needle bed to at least one exclusive piezoelectric micromotor using a linear transmission, according to an alternative preferred embodiment of the present invention. 
     A latch needle bed  76  is provided with latch needle slots  78  in which are placed latch needles  80 . One or more thin piezoelectric micromotor  82  is resiliently pressed against the shaft  84  of each latch needle  80  (only one is shown for each latch needle for simplicity). Piezoelectric micromotors  82  on adjacent latch needles  80  are in line with each other so that they form a straight row. Alternatively, piezoelectric micromotors  82  may be staggered with respect to each other so that they are arrayed in two or more parallel rows. FIG. 4 shows an embodiment according to the present invention in which piezoelectric micromotors are aligned in two parallel rows. Staggered configurations allow for more space between closely packed vibrators  82  than would be available if vibrators  82  were arrayed in a single row and thus allow for thicker more powerful piezoelectric micromotors to be coupled to latch needles  63 . 
     Vibrations of piezoelectric micromotors  82  are directly translated into linear motion of latch needles  80 . Slots  78  are fitted with bearings (not shown) or with a non-stick surface so that the resilient force which presses a vibrator  82  to a shaft  84  of a needle  80  does not result in excessive friction between needle  80  and the bottom or sides of latch needle slot  78  in which needle  80  is placed. 
     Rotary piezoelectric micromotors similar to those described in U.S. Pat. No. 4,562,374 and the publication by Hiroshi et al. cited above may also be used to drive latch needles  80 . The edge surface of a rotor of a rotary piezoelectric micromotor is resiliently pressed against shaft  84  of each latch needle  80 . The axes of the rotors are perpendicular to latch needle slots  78  in which latch needles  80  are placed. Frictional forces at the area of contact between the edge surface of a rotor and the surface of shaft  84  of a needle  80  acts to prevent the edge surface of the rotor from slipping on the surface of shaft  84  when the rotor rotates. As the rotor rotates it therefore causes shaft  84  of latch needle  80  to displace linearly in latch needle slot  78  in which latch needle  80  is placed in the direction of motion of the mass points of the edge surface of the rotor which are in contact with the surface of shaft  84 . 
     FIG. 5 shows a latch needle  300  coupled to two identical piezoelectric micromotors  302  and  304 , in accordance with yet another preferred embodiment of the present invention. Latch needle  300  comprises a latch needle shaft  301  and a coupling fin  306 . Coupling, fin  306  has two parallel planar surfaces  308  and  310 . A coupling region  312  of each surface  308  and  310  (coupling region  312  of surface  308  is not seen in the perspective of FIG. 5) is preferably clad with a wear resistant material suitable for friction coupling with piezoelectric micromotors. 
     Piezoelectric micromotors  302  and  304  preferably comprise friction nubs  314  and  316  respectively. Piezoelectric micromotors  302  and  304  are resiliently pressed to coupling fin  306  so that friction nubs  314  and  316  contact coupling regions  312  of surfaces  308  and  310  respectively at points that are directly opposite each other. In order to move latch needle  300  back and forth in its latch needle slot (not shown) piezoelectric micromotors  302  and  304  are preferably simultaneously activated in phase to transmit motion to coupling fin  306 . 
     FIG. 6 shows latch needle  300  coupled to a single piezoelectric micromotor  320 , in accordance with still another preferred embodiment of the present invention. Piezoelectric micromotor  320  is mounted to a transmission bracket  322  preferably comprising a bearing  324  and a biasing means  326  such as a spring or resilient pad. Dashed lines indicate parts of piezoelectric micromotor  320  hidden by transmission bracket  322 . Piezoelectric micromotor  320  preferably comprises a friction nub  328  (shown in dashed lines). Biasing means  326  resiliently presses piezoelectric micromotor  320  in a direction so that friction nub  328  is urged towards bearing  324 . Transmission bracket  322  is held by an appropriate mechanical structure (not shown) so that coupling fin  306  is located between friction nub  328  and bearing  324 . 
     As a result of the action of biasing means  326  bearing  324  presses resiliently on coupling region  312  of surface  310  and friction nub  328  presses resiliently on coupling region  312  of surface  308 . Transmission bracket  322  is oriented so that the direction in which friction nub  328  is urged by biasing means  326  is substantially perpendicular to the plane of coupling fin  306 . Bearing  324  and friction nub  328  exert equal and opposite forces on coupling fin  306  perpendicular to the plane of coupling fin  306 . As a result piezoelectric micromotor  320  does not produce a torque on latch needle  300  that tends to rotate latch needle  300  in its latch needle slot (not shown). 
     Coupling fin  306  can be located at different positions along shaft  301  of different latch needles  300 . In addition coupling fin  306  can be formed so that it extends different distances from shaft  301  of different latch needles  300 . Adjacent latch needles in a needle bed can therefore preferably, have coupling fins that protrude different heights above the needle bed and/or are displaced with respect to each other in a direction parallel to their shafts in order to provide space for piezoelectric micromotors that are coupled to the coupling fins. 
     It is clear from the above discussion that piezoelectric micromotors in accordance with preferred embodiments of the present invention can be conveniently coupled to latch needles in a latch needle bed of a knitting machine so that each latch needle is exclusively coupled to at least one piezoelectric micromotor. 
     FIGS. 7A-7C schematically illustrate an OPM  98  comprising a fiducial imager  100  and a fiducial illuminator  101  imaging a latch needle fiducial  102  located on a latch needle  104 , in accordance with a preferred embodiment of the present invention. Fiducial imager  100  comprises a lens  106  and a detector  108 . Detector  108  has a light sensitive surface  10  (shown greatly exaggerated in thickness for convenience and clarity of presentation) that is divided into a first detector region  112  and a second detector region  114 . A region of Light sensitive region  110  is schematically shown from “underneath”, in a ventral view, as seen from fiducial  102 , in views  116 ,  118  and  120  to the left of detector  108  in each of FIGS. 7A-7C respectively. The areas of detector regions  112  and  114  preferably have the same shape, are equal and abut each other along a straight dividing line  122 . Detector  108  registers the intensity of light incident on first detector region  112  and second detector region  114  separately. Detector  108  sends a first signal to a controller (not shown) that is a function of the intensity of light registered on first detector region  112  and a second signal to the controller that is a function of the intensity of light registered by second detector region  114 . 
     Detector  108  is oriented with respect to latch needle  104  so that dividing line  122  is substantially perpendicular to the plane (the same as the plane of FIGS. 7A-7C) of the latch needle slot (not shown,) in which latch needle  104  is held, and perpendicular to the direction of the back and forth motion of latch needle  104  indicated by doubled headed arrow  124 . 
     Fiducial  102  is illuminated by light from fiducial illuminator  101  and reflects some of the light, indicated by dotted line  128 , onto lens  106 . Fiducial  102  preferably reflects light from fiducial illuminator  101  diffusely in a cone (not shown) of half energy angle on the order of 10°-15°. Fiducial illuminator  101  and fiducial imager  100  are located with respect to each other so that for any position of latch needle  104  in the operating range of motion of latch needle  104 , fiducial  102  reflects light from fiducial illuminator  101  into fiducial imager  100 . 
     Lens  106  forms an image  130  of fiducial  102  on light sensitive surface  110  from the light reflected by fiducial  102 . A first image portion  132  of image  130  falls on first detector region  112  and a second image portion  134  of image  130  falls on second detector region  114  (views  116 ,  118  and  120 ). First detector region  112  registers an intensity of light on its surface that is a function of the size of first image portion  130  and second detector region  114  registers an intensity of light that is a function of the size of second image portion  134 . Detector  108  therefore sends a first signal to the controller that is as function of the size of first image portion  130  and a second signal to the controller that is a function of the size of second image portion  134 . The relative sizes of first image portion  132  and second image portion  134  are a function of the position of fiducial  102  and first and second signals are used by the controller to determine the position of fiducial  102  and thereby of latch needle  104 . 
     The dependence of the sizes of first image portion  132  and second image portion  134  on the position of fiducial  102  is shown schematically in ventral views (seen from “beneath”, from the perspective of fiducial  102 ) 116 ,  118  and  120  in FIGS. 7A-7C respectively. In FIG. 7A fiducial  102  is located along the axis of fiducial imager  100 , which is coincident with the direction of line  128  that indicates the direction of reflected light from fiducial  102 . First image portion  132  and second image portion  134  are equal. In FIG. 7B fiducial  102  is shown displaced far to the right of the axis of fiducial imager  100  and first image portion  132  is much larger than second image portion  134 . In FIG. 7C fiducial  102  is shown displaced far to the left of the axis of fiducial imager  102  and second image portion  134  is much larger than first image portion  132 . 
     FIG. 8 shows an OPM  138 , in accordance with a preferred embodiment of the present invention, that comprises a plurality of fiducial imagers  100  shown in FIGS. 7A-7C. Fiducial imagers  100  are fixed with respect to each other by an appropriate mechanical structure (not shown) in a collinear line array  140  having an axis  142 . Line array  140  is mounted over a needle bed (not shown) of a knitting machine (not shown) in which a plurality of latch needles  104  are placed. Each latch needle  104  has a fiducial  102 . Axis  142  of line array  140  is preferably parallel to the surface of the needle bed and perpendicular to latch needles  104  (and thereby perpendicular to the directions of motion of latch needles  104 ). Dividing lines  122  (not shown) of light sensitive surfaces  110  of fiducial imagers  100  are preferably parallel to axis  142 . Each of fiducial imagers  100  in line array  140  is aligned over a different one of latch needles  104  and is used to measure the position of latch needle  104  over which it is aligned. 
     In OPM  138 , each fiducial  102  is illuminated with light from a fiducial illuminator  101  and reflects some of this light into the fiducial imager  100  that is aligned over and images the fiducial  102 . A central ray of light from each fiducial  102  reflected into the fiducial imager  100  that images the fiducial  102  is indicated by a dotted line  128 . Each dotted line  128  starts at a fiducial  102 , and ends on the image  130  of the fiducial  102  in the fiducial imager  100  that is used to measure the position of fiducial  102 . The positions of the first and second leftmost latch needles  104  and their fiducials  102  in FIG. 8 correspond to the positions of latch needles  104  and fiducials  102  shown in FIGS. 7C and 7A respectively. The positions of the rest of latch needles  104  shown in FIG. 8 correspond to the position of latch needle  104  shown in FIG.  7 B. 
     OPM  138  can be used to determine positions only for those latch needles  104  that are aligned with a fiducial imager  100  of line array  140 . At any one time therefore, the number of latch needles  104  in a knitting machine whose positions can be determined by OPM  138  is equal to the number of fiducial imagers in line array  140 . Preferably, the number of fiducial imagers  100  in line array  140  is equal to the number of latch needles in the knitting machine. If the number of the fiducial imagers in line array  140  is less than the number of latch needles in the knitting machine, OPM  138  must be moved in order to provide position measurements for all latch needles  104  in the knitting machine. Preferably, OPM  138  is moved parallel to axis  142  along the knitting machine needle bed in order to provide position measurements for all the latch needles  104  in the knitting machine. 
     In FIG. 8 each fiducial  102  is shown illuminated by its own fiducial illuminator  101 . This is not a necessity and some OPMs, in accordance with preferred embodiments of the present invention, comprise fiducial illuminators that illuminate groups of more than one fiducial  102 . Additionally, in some preferred embodiments of the present invention, lenses  106 , each of which is used to image one fiducial  102 , are replaced by lenses, such as extended cylindrical lenses, each of which is used to image more than one fiducial  102 . 
     FIGS. 9A-9C schematically illustrate an OPM  270  imaging fiducial  102  of latch needle  104 , in accordance with an alternate preferred embodiment of the present invention. OPM  270  comprises a fiducial imager  272  and a fiducial illuminator  274 . Fiducial imager  272  comprises a lens  276  having an optic axis indicated by line  278 , a detector  280  and a light filter  282 . Detector  280  comprises a light sensitive surface  282 , sensitive to light in first and second non-overlapping wavelength bands of light. Detector  280  sends a first signal to a controller (not shown) that is a function of the intensity of light registered on light sensitive surface  280  in the first wavelength band and a second signal to the controller that is a function of the intensity registered by light sensitive surface  282  in the second wavelength band. 
     Light filter  282  has a first filter region  284  and a second filter region  286 . First filter region  284  transmits light only in the first wavelength band and second filter region  286  transmits light only in the second wavelength band. First and second filter regions  284  and  286  are preferably equal and abut each other along a straight dividing line (not shown in fiducial imager  272 ). Filter  282  is oriented with respect to lens  276  so that reflected light from fiducial  102  incident on lens  276  passes through filter  282 . A central ray of reflected light from fiducial  102  is indicated by dotted line  288  in FIGS. 9B and 9C. In FIG. 9A the central ray is coincident with optic axis  278 . The dividing line of filter  282  and optic axis  278  of lens  276  intersect. Preferably, the dividing line is perpendicular to the direction of motion of latch needle  104  and the plane (the plane of the Fig.) of the latch needle slot (not shown) that holds latch needle  104 . As a result, light incident on a first half  290  of lens  276  is filtered by first filter region  284  and light incident on a second half  292  of lens  276  is filtered by second filter region  286 . Lens  276  focuses reflected light from fiducial  102  to form an image  130  of fiducial  102  on light sensitive surface  282  of detector  280 . A first portion of the intensity of image  130  results from light incident on first half  290  of lens  276  and a second portion of the intensity of image  130  results from light incident on second half  292  of lens  276 . Since first half  290  of lens  276  is filtered by first filter region  284 , the first portion of the intensity of image  130  results from light in the first wavelength band. Similarly, the second portion of the intensity of image  130  results from light in the second wavelength band. The first and second portions of the intensity of image  130  are proportional to the amounts of light from fiducial  102  that are incident on first and second halves  290  and  292  of lens  276  respectively. As a result, the intensities of light registered by light sensitive surface  282  in the first and second wavelength bands are proportional to the amounts of reflected light from fiducial  102  incident on first and second halves  290  and  292  of lens  276  respectively. 
     However, the amounts of light incident on first half  290  and second half  292  are functions of the location of fiducial  102  with respect to optic axis  278  of lens  276 . When fiducial  102  is on optic axis  278 , halves  290  and  292  of lens  276  receive the same amounts of reflected light. When fiducial  102  is displaced along the direction of motion of latch needle  104  (along the direction of double headed arrow  124  in FIGS. 9A-9C) towards one or the other of halves  290  and  292 , the half towards which fiducial  102  is displaced receives more light and the other half less light. This is because the distance from fiducial  102  to the half of lens  276  towards which fiducial  102  is displaced decreases and the distance towards the other half increases. The first and second signals that detector  280  sends to the controller are therefore functions of the position of fiducial  102 . These signals are used by the controller to determine the position of fiducial  102  and latch needle  104  on which fiducial  102  is located. 
     FIGS. 9A-9C show schematically the relationship between positions of fiducial  102  and the intensities of image  130  in the first and second wavelength bands A region of light sensitive surface  282  is shown schematically with image  130 , in ventral view, in a view  294  in each of FIGS. 9A-9C. The dividing line of filter  282  is shown as line  296  in view  294 . The relative intensities of image  130  in the first and second wavelength bands are represented schematically in greatly exaggerated scale and only qualitatively in proportion to the actual intensities of light in image  130  in the first and second wavelength bands by the size of arrows  298  and  300  respectively. 
     In FIG. 9A fiducial  102  is located on optic axis  278  and image  130  has the same (appropriately normalized and corrected) integrated intensity (i.e. integrated over the area of image  130 ) in both wavelength bands. Arrows  298  and  300  are shown the same size. In FIG. 9B fiducial  102  is displaced away from optic axis  278  towards first half  290  of lens  276 . Image  130  is displaced from optic axis  278  in the opposite direction and the integrated intensity of image  130  increases in the first wavelength band and decreases in the second wavelength band. Arrow  300  is shown much larger than arrow  298 . Similarly, in FIG. 9C, fiducial  102  is shown displaced away from optic axis  278  towards second half  292  of lens  276 . The integrated intensity of image  130  increases in the second wavelength band and decreases in the first wavelength band. 
     FIGS. 10A-10C schematically illustrate an OPM  150 , in accordance with another preferred embodiment of the present invention, imaging fiducial  102  of latch needle  104 . OPM  150  comprises a fiducial illuminator  152  and a fiducial imager  154  comprising two, preferably identical, detectors  156  and  158 . Fiducial illuminator  152  illuminates fiducial  102  of latch needle  104 . Fiducial  102  reflects some of the light incident on fiducial  102  towards each of detectors  156  and  158 . 
     Detectors  156  and  158  have light sensitive surfaces  160  and  162  (shown greatly exaggerated in thickness for convenience and clarity of presentation) and lenses  164  and  166  respectively. Lens  160  focuses reflected light from fiducial  102  to provide an image  168  of fiducial  102  on light sensitive surface  160 . Similarly, lens  166  provides an image  170  of fiducial  102  on light sensitive surface  162 . Light sensitive surface  160  with image  168 , and light sensitive surface  162  with image  170 , are shown schematically, in ventral view, in views  172  and  174  respectively in each of Figs. FIGS. 10A-10C. The intensities of images  168  and  170  are schematically represented in each of views  172  and  174  by the length of arrows  169  and  171  respectively. The relative sizes of arrows  169  and  171  are greatly exaggerated for clarity and ease of presentation in comparison to the actual relative intensities of images  168  and  170 . Each of detectors  156  and  158  provides a signal to a controller (not shown) that is a function of the intensity of reflected light imaged on its light sensitive surface. 
     Detectors  156  and  158  are displaced from each other a small distance, “d”, and both are located at a height, “r”, directly above latch needle  104 . OPM  150  is oriented with respect to latch needle  104  so that a line between the centers of lenses  164  and  166  is parallel to latch needle  104 . Dashed lines  176  and  178  represent central rays of light reflected from fiducial  102  into detectors  156  and  158  respectively. 
     In FIG. 10A fiducial  102  is located at a point  180  that is equidistant from detectors  156  and  158 . Both detectors receive substantially the same amounts of reflected light from fiducial  102 . Arrows  169  and  171  in views  172  and  174  respectively are therefore shown the same size. The difference between the intensities of light reaching detectors  156  and  158  is zero. 
     In FIG. 10B fiducial  102  is displaced from point  180  to the right. As a result of the displacement the distance from fiducial  102  to detector  158  decreases and the distance from fiducial  102  to detector  156  increases. This increases the amount of reflected light reaching detector  158  from fiducial  102  and decreases the amount of reflected light reaching detector  156  from fiducial  102 . The size of arrow  171  in view  174  is therefore shown much larger than the size of arrow  169  in view  172 . The difference between the intensities of light reaching detectors  156  and  158 , defined as the amount of light reaching detector  156  minus the amount of light reaching detector  156 , is negative. 
     In FIG. 10C fiducial  102  is displaced from point  180  to the left. This increases the amount of reflected light reaching detector  156  from fiducial  102  and decreases the amount of reflected light reaching detector  158  from fiducial  102 . In this case, the size of arrow  171  in view  174  is therefore shown much smaller than the size of image  169  in view  172 . The difference between the intensities of light reaching detectors  156  and  158 , as defined above, is positive. 
     From considerations of geometry it can readily be shown that when r&gt;&gt;d, if the displacement of fiducial  102  from point  180  is represented by “Δx”, the difference between the intensities of light reaching detectors  156  and  158  is proportional to Δxd/r 4 . The difference between the signals sent by detectors  156  and  158  to the controller, which are functions of the intensities of reflected light registered by detectors  156  and  158  respectively, can therefore be used to determine Ax and the position of fiducial  102 . 
     FIGS. 11A-11C schematically show an OPM  190 , in accordance with yet another preferred embodiment of the present invention, imaging fiducial  102  of latch needle  104 . OPM  190  comprises a fiducial illuminator  192  and a fiducial imager  194 . Fiducial imager  194  comprises a single detector  196  and two lenses  198  and  200 . Fiducial illuminator  192  illuminates fiducial  102  of latch needle  104 . Fiducial  102  reflects some of the light incident on it from fiducial illuminator  192  towards each of lenses  198  and  200 . A central ray of reflected light from fiducial  102  to lens  198  is represented by dashed line  202  and dashed line  204  represents a central ray from fiducial  102  to lens  200 . 
     Detector  196  comprises a light sensitive surface  206  (shown greatly exaggerated in thickness for convenience and clarity of presentation) that is sensitive to light in two non-overlapping wavelength bands of light. Fiducial illuminator  192  illuminates fiducial  102  with preferably equal intensities of light from both wavelength bands. Each of lenses  198  and  200  transmits light in only one of the two different wavelength bands. Lens  198  focuses reflected light in one of the two wavelength bands to form an image  214  on light sensitive surface  206 . Lens  200  focuses reflected light in the other of the two wavelength bands to form an image  216  on light sensitive surface  206 . Detector  196  sends a first signal to a controller (not shown) that is a function of the amount of light in image  214  and a second signal to the controller that is a function of the amount of light in image  216 . 
     Lenses  198  and  200  are displaced a short distance from each other and the line connecting the centers of lenses  198  and  200  is aligned parallel with and directly above latch needle  104 . Assume that fiducial illuminator  192  is either located equidistant from lenses  198  and  200 , or that any biases in the relative amounts of light reflected by fiducial  102  onto lenses  198  and  200  resulting from an asymmetric location of fiducial illuminator  192  with respect to lenses  198  and  200  are corrected for. Then, when fiducial  102  is equidistant from lenses  198  and  200 , detector  196  registers equal intensities of light for both images  214  and  216  (i.e. surface  206  registers the same intensity of light in both of the wavelength bands to which it is sensitive). As fiducial  102  is displaced towards one or the other of lenses  198  and  200 , the relative intensities of light registered for images  214  and  216  changes. 
     FIG. 11A shows fiducial light  102  located at a point  208  equidistant from lens  198  and  200 . FIGS. 11B and 11C show fiducial  102  displaced right and left respectively of point  208 . View  210  each of FIGS. 11A-11C is a ventral view of light sensitive surface  206 . View  210  shows schematically images  214  and  216  of fiducial  102  that are formed on light sensitive surface  206  by lenses  198  and  200  respectively. The sizes of arrows  215  and  217  in view  210  represent schematically with greatly exaggerated scale the relative amounts of light in images  214  and  216  respectively for the different positions of fiducial  102  shown in FIGS. 11A-11C. 
     From considerations of geometry it can readily be shown, as in the case of OPM  150  shown in FIGS. 10A-10C, that for a displacement Δx of fiducial  102  from point  208 , the difference between the intensities of light registered by detector  196  for images  214  and  216  is substantially proportional to Δx. The signals sent by detector  206  to the controller, which are functions of the intensities of light registered by detector  206  for images  214  and  216  can therefore be used to determine Δx and thereby the position of fiducial  102 . 
     FIGS. 12A-12C schematically show an OPM  220 , in accordance with yet another preferred embodiment of the present invention that is used to measure the position of a latch needle provided with two fiducials. In FIGS. 12A-12C, OPM,  220  is shown imaging a latch needle  222  provided with a fiducial  224  and a fiducial  226 . 
     OPM  220  comprises a fiducial illuminator  228  and a fiducial imager  230 . Fiducial imager  230  comprises a single detector  232  and a single lens  234  having a lens axis  235 . Detector  232  comprises a light sensitive surface  233  (shown greatly exaggerated in thickness for convenience and clarity of presentation) that is sensitive to light in two non-overlapping wavelength bands of light. Fiducial illuminator  228  illuminates fiducials  224  and  226  preferably with light having equal intensities in both wavelength bands. Fiducial  224  reflects light in only one of the two wavelength bands and fiducial  226  reflects light in only the other of the two wavelength bands. Lens  234  images the reflected light from fiducials  224  and  226  to form an image  236  of fiducial  224  on surface  233  in one of the two wavelength bands and an image  238  of fiducial  226  on surface  233  in the other of the two wavelength bands. Detector  232  sends a signal to a controller (not shown) for each of images  236  and  238  that is a function of the intensity of light in the image. 
     Images  236  and  238  have the same intensities, in their respective wavelength bands, only when fiducials  224  and  226  are substantially equidistant from axis  235  of lens  234 . For different positions of latch needle  222 , one or the other of fiducials  224  and  226  is closer to axis  235 . The image of the fiducial closer to axis  235  is more intense than the image of the fiducial farther from axis  235 . Differences in intensities of images  236  and  238  registered by detector  232  are used to determine the position of fiducials  224  and  226  and thereby of latch needle  222 . 
     FIG. 12A shows latch needle  222  in a position for which fiducials  224  and  226  are equidistant from axis  235 . FIG. 12B shows latch needle  222  in a position in which fiducials  224  and  226  are displaced to the right of their respective positions shown in FIG. 12A, and FIG. 12C shows latch needle  222  in a position in which fiducials  224  and  226  are displaced to the left of their respective positions shown in FIG.  12 A. In each of FIGS. 12A-12C, view  240  is a ventral view of light sensitive surface  234  schematically showing images  236  and  238 . The sizes of arrows  237  and  239  shown in ventral view  240  represent schematically and in greatly exaggerated scale, the relative intensities of images  236  and  238  for the position of latch needle  222  shown in the FIG. 
     FIGS. 13A-13C show an OPM  250  imaging fiducial  102 , in accordance with yet another preferred embodiment of the present invention. OPM  250  comprises a fiducial illuminator  252  and a fiducial imager  254 . Fiducial imager  254  comprises a lens  256  having an optic axis  257  and a detector  258 , such as a CCD, having a pixelated light sensitive surface  260  (shown greatly exaggerated in thickness for convenience and clarity of presentation). Lens  256  focuses reflected light from fiducial  102  to form an image  262  of fiducial  102  on pixelated surface  260 . 
     In OPM  250  the position of fiducial  102  is determined using the rules of basic optics from the location of image  262  on pixelated surface  260 . FIGS. 13A-13C show schematically the spatial relationship between the position of fiducial  102  and image  262  of fiducial  102  on pixelated surface  260 . Image  262  and pixels  264  of pixelated surface  260  are shown schematically in a ventral view  266  of pixelated surface  260  in each of FIGS. 13A-13C. In FIG. 13A fiducial  102  is located on optic axis  257  and image  262  is located at the center of pixelated surface  260  shown in view  264  (assuming lens  256  and detector  258  are aligned). In FIGS. 13B and 13C, fiducial  102  is displaced to the right and to the left of optic axis  257  respectively. Image  262  on pixelated surface  260  moves accordingly to the left and the right of the point at which image  262  is located when fiducial  102  is on optic axis  257 . 
     Image  262  is preferably focused by lens  256  so that it covers a plurality of pixels on light sensitive surface  260 . Using methods well known in the art, an optical center of gravity of image  262  can be defined and located on pixelated surface  260  to sub-pixel accuracy. Using the location of the optical center of gravity of image  262 , the position of fiducial  102  and latch needle  104  are determined by OPM  250  with an accuracy sufficient for controlling latch needle actuators in a DDM. 
     FIGS. 13A-13C show OPM  250  being used to determine the position of a single latch needle  104 , by imaging a fiducial  102  located on the latch needle  104 . However, a single OPM of the form of OPM  250 , in accordance with a preferred embodiment of the present invention, can be used to determine the position of a plurality of latch needles  104 . This is accomplished by providing the detector  258  of the OPM with a field of view that includes the fiducial  102  of each of the plurality of latch needles  104 . Each fiducial  102  of a latch needle of the plurality of latch needles is imaged on a different rectangular region of pixelated surface  260  of the OPM. As the latch needle  104  on which the fiducial  102  is located moves back and forth in its operational range of motion, (indicated schematically by double headed arrow  124 ) the image of its fiducial  102  moves back and forth along the length of the rectangular region of pixelated surface  260  on which it is imaged. 
     For example, in one preferred embodiment of the present invention, detector  258  is provided with a field of view that focuses an area of a needle bed having a dimension perpendicular to latch needles  104  that is on the order of 5 cm. The dimension of the field of view in the direction parallel to latch needles  104  is on the order of the operational range of motion of latch needles  104 . If the spacing between latch needles  104  in the needle bed is 2 mm the fiducials  102  of 25 latch needles  104  will be in the field of view of the OPM. Assuming that pixelated surface  260  of detector  258  comprises a square matrix, 5 mm on a side, comprising 512 rows and 512 columns of pixels fiducials  102  of the 25 latch needles  104  in the field of view of detector  258  are imaged on parallel rectangular regions of pixelated surface  260  that are approximately 20 pixels wide and 512 pixels long. If the operational range of motion of a latch needle  104  is on the order of 5 cm, and the optical center of gravity of the image of a fiducial is located with a resolution of 0.4 pixels, the position of fiducial  102  and its latch needle  104  are located with an accuracy of about 40 micrometers. 
     Variations of the above-described preferred embodiments will occur to persons of the art. The above detailed descriptions are provided by way of example and are not meant to limit the scope of the invention, which is limited only by the following claims.

Technology Classification (CPC): 3