Missing yarn detection apparatus

Abstract: Apparatus for the detection of a missing yarn in the weaving of a carpet of the gripper Axminster type comprises an infra-red scanner to detect yarn and a further scanner to detect grippers. The sensor outputs are processed in a logic circuit which produces a signal to indicate a missing yarn. Preferably the yarn sensor emits a broad infra-red beam and the grippers sensor emits a focussed beam.

Description: This invention relates to carpet looms especially but not exclusively of the type for weaving cut-pile carpets and provides means for the detection of a missing yarn during the weaving process. The detection of a missing yarn in a carpet is generally carried out by moving a comb-like element with individual extendible biassed fingers across the user surface of the carpet, whereby a tooth will extend into a gap left by a missing yarn to make contact with an underlying plate to generate an indicating signal. It is an object of the present invention to provide automatic means associated with the carpet loom for the detection of a missing yarn during the weaving process, especially where the carpet is of the cut-pile type having a gripper Axminster weave, produced by insertion of ends of yarn into the carpet substrate. In the manufacture of such carpets, yarn is stored on bobbins held in a creel frame and grippers remove yarn from a carrier mounted in front of the creel frame. The yarn is cut to form individual "ends", each held by a gripper, which are inserted between the cotton chains and weft of the carpet substrate and woven in by a reed box to form upstanding tufts. In order to detect missing yarns during the weaving process, GB-A 2096651 discloses a photoelectric device which traverses the loom ahead of the knife-blade and which directs sharply defined beams of infra-red radiation respectively on the tensioned yarns and the beaks of the grippers. The apparatus includes circuitry which notes each gripper and determines the presence or absence of an associated yarn. According to the invention, apparatus for the detection of a missing yarn and attachable to a carpet loom which includes yarn holding means and yarn control means comprises photoelectric sensors directable respectively at the yarn and the yarn control means, characterised in that the sensor to be directed at the yarn emits a broad beam which scans the yarns with at least portions of adjacent yarns, and the sensor to be directed at the yarn control means emits a focussed beam, the apparatus including processing circuitry which in use generates a signal, derived from the output from the sensors, when yarn is absent from a yarn control means. The signal is notified to the loom operator who can then investigate the cause, that is, either a broken yarn or the failure of the yarn control means to control the yarn, and take appropriate remedial action. Where the loom is of the type for weaving gripper Axminister carpets, the yarn holding means comprises a creel frame and the yarn control means comprises grippers. However, for other types of loom, the yarn control means is constituted by other elements, such as a comb, according to the particular type of loom. Throughout the remainder of this specification, the invention will be described with reference to a gripper Axminster type loom although it is to be understood that the invention may be applicable to other types of loom by suitable adaptation to the equivalent yarn control means in place of grippers, and yarn holding means in place of a creel frame. It is preferred that the sensors are mounted on a suitable movable frame for attachment to the loom, one sensor being positioned on the frame to be directed at the yarn and the other sensor being directed at the grippers. Conveniently, the sensors are mounted on the leading edge of the knife block to which is attached the blade which severs the ends of the yarn from the comb, a gripper sensor and a yarn sensor being associated with each knife so as to scan each yarn and associated gripper as the knife block traverses the loom on each stroke. The occurrence of a gripper is electronically noted and the occurrence of an associated yarn is also electronically noted; the presence of both is an acceptable result but the absence of a yarn from a gripper is unacceptable and a warning or other signal is generated. The output from each sensor is amplified and passed to a processing or logic circuit. One such circuit may include a suitable counter to which the sensor outputs are passed via a trigger circuit for converting the ramp edge of the amplified signal to an abrupt or fast edge signal to prevent false counts. The amplifiers may have variable gain, thereby enabling the gain to be set to account for, for example, yarn of different sensor-reactive properties due to colour or texture variations, and also to distinguish yarn or grippers, as the case may be, from background. A suitable trigger circuit is a Schmitt trigger. The sensors may operate on the basis of light reflectance from the yarn and grippers, conveniently of infra-red light so as to minimise the effect of visible or other ambient light and because suitably robust sensors are readily commercially available. In such a case, each sensor typically comprises an infra-red source and an infra-red detector mounted on a block and angled to each other such that the detector will "see" infra-red light transmitted from the source and reflected by a target. Such a sensor is termed a "diffuse" infra-red sensor, as opposed to one which operates by direct transmission from source to detector. In the above case, in order for all the various colours of yarns to be detected, a broad infra-red beam, that is, a beam that scans the yarns with at least portions of adjacent yarns, is used. The sensor, on traversing the loom, therefore sees the yarns as a continuum, and not as individual ends. The intensity of the emitted beam can therefore be increased sufficiently to detect even dark (i.e. low-reflective) yarns, while still noting the absence of reflectance from an individual missing yarn. In contrast, the infra-red beam to the grippers is a focussed beam to detect individual grippers in sequence. The sensor mounting blocks may conveniently be carried on a bracket attached to the knife block. Particularly for the gripper sensor operating with a focussed beam, the angle of the grippers in relation to the sensor is relatively critical for successful operation, in that a slight change in angle may cause the reflected beam to be "missed" by the detector. The sensor mounting blocks, particularly that for the gripper sensor, are therefore preferably angularly adjustable, thereby enabling the sensor to be accurately set for successful operation and also, if necessary, to be adjusted from time to time in use to accommodate operational variables, for example the angle of the knife block which gradually alters as the comb, against which the knife block traverses the loom, becomes worn. In the weaving of broad-loom carpets, two or more knife-blocks are generally employed, aligned across the loom and arranged to move in concert, so that the entire width of the loom is traversed not by a single knife-block but by the two or more knife-blocks, each traversing a portion of the overall width. Each knife-block is equipped with apparatus according to the invention. The apparatus according to the invention may include means for holding the system in reset until the grippers, having inserted their associated yarn ends in the carpet substrate in one weaving cycle or "pick", have returned through their arc of travel and have withdrawn yarn from the carrier ready for the next cycle. This is because, particularly in the weaving of broad-loom carpets with looms having a plurality of knife-blocks, the apparatus is likely to be falsely triggered by the gripper sensors (other than the sensor on the end knife block) "seeing" the grippers on their return travel and registering missing yarns. Such reset-holding means may comprise an additional sensor located on the trailing edge of the knife-block (or, in the case of a loom having a plurality of knife-blocks, on the trailing edge of the end knife block, that is, that block which is at a side of the loom at the beginning of the cutting travel) and arranged to hold the apparatus in the reset mode until it has detected the presence of a yarn. Therefore, at the start of a cutting stroke or traverse, the first few yarns (corresponding to those under the width of the knife block) will be actually cut and held by the grippers, before the circuitry is released from its reset mode to process the outputs from the yarn and gripper sensors. Although the reset-holding means is particularly useful for looms having a plurality of knife blocks, it is also a preferred feature on other looms since it facilitates and simplifies the installation and initial setting up of the apparatus according to the invention. Particularly for weaving patterned carpets, and where a plurality of knife-blocks are utilised, an automatic delay facility may be incorporated into the circuitry to account for the overlap at each end of the sensor travel, that is, to prevent false alarm signals being generated at each end, by virtue of the sensor detecting grippers associated with the adjacent knife-block but no yarn, the adjacent knife-block having already severed the yarns. For example, the electronics may be arranged to delay the generation of a signal until a predetermined number of grippers have been counted. A suitable predetermined number may be four, on the grounds that it is statistically highly improbable that four consecutive grippers will have yarns missing. Therefore, even if a single yarn is detected as missing, a signal will not be generated until the next successive three grippers have been counted and, if any of these has an associated yarn, a signal will be generated in respect of the missing yarn or yarns but, if all appear to have yarns missing, no signal will be generated as the knife block will be assumed to be at one end of its travel. The counter may be a repetitive or ring counter such as a Johnson counter, whereby the number of succeeding cascaded stages in the ring may be selected according to the said predetermined number of grippers without associated yarns. In the aforementioned circuit construction, the output from the gripper trigger circuit is fed to the input of a first counter and gated with the output of the yarn trigger circuit. The gate output is fed to a second counter which holds the first in a reset condition. The reset control circuitry of both counters is arranged not to latch up if more than the predetermined number, for example four, yarns are missing, but to latch and provide a suitable output if four or less yarns are absent. Embodiments of apparatus for the detection of a missing yarn in a carpet loom according to the present invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 shows side elevations of sensor mounting blocks for infra-red gripper and yarn sensors, and Figure 2 shows a circuit diagram of the apparatus. Referring to Figure 1, a mounting block for an infra-red gripper sensor is shown in Figure 1a, and for an infra-red yarn sensor in Figure 1b. In Figure 1a, the block consists of two parts 11 and 12, articulatably connected together at 13. Part 11 is for attachment to a suitable mounting bracket carried by the knife block, and part 12 is formed with housings 14, 15 respectively for infra-red source and detector. As shown, these housings subtend an angle of 30°. The part 12 is angularly adjustable about axis 13, to permit adjustment from time to time during use of the loom, to accommodate wear in the combs which in turn causes alteration of the angle of the knife block. In Figure 1b, a mounting block for a yarn sensor is shown. The bracket includes a proximal end 16 for attachment to a mounting bracket and a distal end 17 having housings 18, 19 respectively for an infra-red source and detector. Both the yarn and gripper sensors are vertically adjustable with reference to the mounting bracket by means of respective slots 20, 21. Referring to Figure 2, the circuit diagram for detecting a missing yarn can be seen to comprise essentially four sections: a gripper circuit with a gripper sensor 1A, an amplifier 1B and a signal conditioning device 1C; a yarn circuit with a yarn sensor array 2A, a sample and hold circuit 2B, an amplifier 2C and a signal conditioning device 2D; a logic circuit 3, and a system reset 4. The gripper sensor 1A consists of an infra-red emitter and photo-transistor operating at a wavelength of 940nm, directed at the yarns from a distance of up to approximately 12.5mm. The signals from the sensor 1A are fed to the gripper amplifier 1B which comprises an operational amplifier arranged in an inverting mode with fixed gain. The signal conditioning device 1C, which is necessary because the voltage from the gripper amplifier 1B may swing from zero to twelve volts, includes a current-limit resistor interposed between the amplifier output and a Schmitt inverter. This device 1C speeds up the edges of the signals and produces the correct level for interfacing with the logic circuit 3. The yarn sensor array 2A consists of an infra-red emitter and photo-transistor also operating at a wavelength of 940 nm. The photo-transistor and the emitter are focussed to detect the yarns at a distance of 3mm. Since the photo-transistor can react to ambient light, the yarn circuit 2 incorporates a sample and hold circuit 2B which negates this standing level and can adjust to changing ambient conditions on every reset cycle of the photo-transistor. The output from the photo-transistor is fed into the sample and hold circuit, which consists of a MOS-FET sampling element whose sampling rate is controlled from the system reset 4, and a hold element constituted by a polyester capacitor. A voltage follower provides a high-impedance interface stage between the sample and hold circuit 2B and one input of the yarn amplifier 2C. From the above, it can be seen that the output from the sample and hold circuit 2B is fed to one input of the yarn amplifier 2C, which is a differential amplifier, the other amplifier input being fed from the input of the sample and hold circuit. With the sampling FET in the ON state, in use, the output of the sample and hold circuit 2B and thus one input of the differential amplifier 2C will track the photo-transistor output, and will be identical to the input on the other input of the differential amplifier (as this is fed from the input of the sample and hold circuit). With both differential inputs balanced, the output of the amplifier 2C will be zero. With the sampling FET in the OFF state, in use, the hold capacitor will assume a charge equal to the instantaneous voltage present at the FET source at the time of the FET "switch off". This represents the standing DC conditions caused by the effect of ambient light on the yarn photo-transistor. This level is then fed to one input of the differential amplifier 2C, the other amplifier input being fed from the input of the sample and hold circuit 2B (that is, the photo-transistor output). This input will have the standing DC condition and variation caused by the yarn sensor 2A passing over the yarns. The yarn amplifier 2C will amplify only this variation and not the standing DC level. The variation will be amplified 3 times, producing a robust signal which can be supplied to the signal conditioning circuit 2D. The latter is necessary since the possible voltage swing from the yarn amplifier 2C is from zero to twelve volts, and includes a current-limit resistor between the amplifier output and a Schmitt inverter. This device 2D speeds up the edges of the signals and produces the correct level for interfacing with the logic circuit 3. The logic circuit 3 consists essentially of a dual D-type flip-flop (4013), and a dual monostable (4528). The logic circuit 3 accepts signals from three sources: the output of the yarn signal conditioning devices 2D, the output of the gripper signal conditioning devices 1C, and the output of the system reset 4. The output of the gripper signal conditioning device is fed to the clock input of one D-type flip-flop. This signal is then inverted and fed to the input of the other D-type flip-flop. The output of the yarn sensor conditioning circuit is fed to the CLEAR DIRECT input of the first D-type flip-flop. A HIGH will be supplied at the Q output of the first flip-flop. The falling edge of the gripper signal, that is, the movement of the gripper sensor 1A past the gripper, is applied to the second flip-flop. This signal, with the Q output of the first flip-flop connected to the DATA input of the second flip-flop, causes the second flip-flop to clock and latch with its Q output high. This high signal is fed via a transistor to the 'END OUT' light to illuminate it. The system reset 4 enables the apparatus to be reset during the "non detecting" phase of the detector. This is achieved by using an infra-red detector directed at the grippers. The outputs from the sensors are fed to an amplifier and the output signal therefrom is conditioned via a Schmitt inverter. The conditioned signal is fed to the various parts of the system via the monostables. The reset amplifier is of fixed gain. The invention thus affords a simple means for the detection of a missing yarn using no physical contact with the carpet and no mechanical linkages, which would be liable to be affected by dust, fluff and vibration. The invention also includes a method for the detection of a missing yarn during the weaving of a carpet, the method comprising scanning yarns and yarn control means by respective sensors to provide an output representing each control means and an output representing yarn, a signal being generated in the absence of a yarn from a yarn control means, preferably on the condition that consecutive absent yarn signals exceeding a predetermined number will cause the signal to cancel.

Claims: Apparatus for the detection of a missing yarn, the apparatus being attachable to a carpet loom which includes yarn holding means and yarn control means, the apparatus comprising photoelectric sensors (12-15, 18-19) directable respectively at the yarn and the yarn control means, characterised in that the sensor (18-19; 2A) to be directed at the yarn emits a broad beam which scans the yarns with at least portions of adjacent yarns, and the sensor (12-15; 1A) to be directed at the yarn control means emits a focussed beam, the apparatus including processing circuitry (3) which in use generates a signal, derived from the output from the sensors, when yarn is absent from a yarn control means. Apparatus according to Claim 1, in which the loom is of the type for weaving gripper Axminster carpets, the yarn holding means comprising a creel frame and the yarn control means comprising grippers. Apparatus according to Claim 1 or Claim 2, in which the processing circuitry includes a sample and hold circuit (2B) associated with the yarn sensor, to negate the effect of ambient light. Apparatus according to any preceding claim, in which the sensors are carried on the knife block of the loom. Apparatus according to any preceding claim, in which the sensors are connected to respective amplifier means (1B; 2C) and the processing circuitry includes a trigger circuit for converting the ramp edge of the amplified signal to an abrupt or fast edge signal. Apparatus according to Claim 5, in which the amplifier means comprises a variable gain amplifier to enable the gain to be set for yarn of different sensor-reactive properties. Apparatus according to any preceding claim, in which the sensors comprise infra-red sources and detectors. Apparatus according to any preceding claim, in which at least the sensor directed at the yarn control means is mounted for angular adjustment. Apparatus according to any preceding claim, further including means (4) for holding the system in reset until the grippers have returned from one weaving cycle to withdraw yarn ready for the next cycle. Apparatus according to any preceding claim, further including an automatic delay facility to account for the overlap at each end of the sensor travel. A method for the detection of a missing yarn during the weaving of a carpet, the method comprising scanning yarns and yarn control means by respective photoelectric sensors to provide an output representing each control means and an output representing yarn, a signal being generated from said outputs to indicate the absence of a yarn control means, characterised in that the yarn is scanned by a broad beam which senses the yarns with at least portions of adjacent yarns and the yarn control means is scanned by a focussed beam. A carpet loom for the weaving of broad-loom carpets and including two or more knife blocks aligned across the loom and arranged to move in concert, characterised in that each knife block includes apparatus according to any of Claims 1 to 10.

Inventors: MACDONALD JOHN STUART, WONG CHONG KWAN, MACDONALD, JOHN STUART, WONG, CHONG KWAN
Applicants: AXMINSTER CARPETS LTD, AXMINSTER CARPETS LIMITED
Application year: 1990
Main classification: D03D39
IPC codes: D03D39