Patent GB-1151989-A (GB)
Title: A Method and Apparatus for Measuring the Mass Per Unit Length of a Thread
IPC: G01N33/36
Filed: 1966-05-09
Abstract: 1,151,989. Measuring tension electrically: determining linear density photo-electrically. ALGEMENE KUNSTZIJDE UNIE N.V. May 9, 1966 [May 8, 1965], No.20494/66. Headings G1A and G1N. The mass m per unit length of a thread is measured by altering the tension s of a fixed length l of the thread until it vibrates at a set frequency f and measuring the final tension of the thread. The mass per unit length is then obtained from the formula. In the embodiment, the thread 1, Fig.1, is passed over a rope pulley 2 between two electrically operated clamps 3, 4 closed during measurements, and between fixed guides 5, 6, which define two modes of vibration of the thread. The latter guide 6 forms part of the thread vibrator 7, which is excited by the amplified output of a photoelectric cell 10 of a thread vibration detector including a light source 8 and a stop 9. During measurement, the amplitude of vibration of the thread is limited by means (not shown), such as a light source and photo-electric cell which, if the light beam is interrupted by the thread, emits a signal causing a reduction in the gain of amplifier 11. The output of the amplifier 11 is also connected to a squarewave generator 12 connected to an input of a gate circuit 13. Reference squarewave pulses from a pulse generator 15 of a higher frequency than those from generator 12 are also fed to the gate circuit 13. The gate circuit 13 is switched on, e.g. for the positive half cycle of a signal from the generator 12, and during this time allows pulses from the generator 15 to pass to a counter 14. When the number of counted pulses exceeds a pre-set number or the capacitance of the counter 14, it emits overflow pulses which are fed to a multi-vibrator 26, the first of which pulses causes the multi-vibrator to emit a marker pulse which is then fed to a device for tensioning the thread, via a commutator 59, amplifier 17 and reversing switch 18. During the negative half cycle of a signal from generator 12 the gate circuit 13 is switched off and the counter 14 is reset. The tensioning device, e.g. an electrical stepping motor, is movable step-wise in either direction, and is moved one step by each marker pulse. A drum 21, coupled to the shaft 20 of the motor, has a strap 22 attached to it which is also attached to a coupling device 23 connected directly or via a further strap 48 to the pulley 2. The coupling device consists of a rectangular frame 49, Fig.2, having two flexible sides 50, 51 and two rigid sides 52, 53. Supports 54, 55 to which the straps 22, 48 are attached are arranged at opposite points of the frame. Four strain gauges (not shown), arranged on either face of the flexible sides are connected into a bridge circuit, by which the tension in the thread can be measured. The output of the bridge circuit may be converted into a digital signal by means of an analogue-to-digital converter 24, which feeds a read-out device 25, e.g. a punching machine. The converter may take the form of a self-compensating bridge, whose moving part, e.g. the sliding contact of a voltage divider, is driven by a servomotor and coupled to an encoder disc. In operation, when sufficient thread has been fed through the device, during which time the clamps 3, 4 are kept open and the counting device 14 is blocked, the thread feeding motor 31 is stopped, clamps 3, 4 are closed and the blocking of the counter 14 is removed. The comparison between the frequency of vibration in the thread and the reference frequency is carried out as above causing the step-wise increase of tension in the thread. In order to control the measurement of the tension in the thread, the marker pulses are also transmitted to a central control 28, where they are used to reset a timing circuit each time the blocking of the counter 14 is removed. Thus if no marker pulse is produced, indicating that the desired frequency has been attained, the timing circuit emits a signal which causes the switching on of the transducer 23 and converter 24 and a second timing circuit. This latter circuit, after a delay which is sufficient for the tension to be measured to be registered in the memory of the converter, emits a signal which causes a pulse source 43 to be connected to the motor, to rotate it in the opposite direction, releasing the tension in the thread. Simultaneously a parallel serial converter 34 starts operating, which indicates which binary-coded decade of the converter shall be read out. This decade is, via the connection 39 and converter 34, fed to a device 37, where it is amplified and converted before being recorded by punching machine 25. When the stepping motor has been returned to its initial position, a limit switch 35 emits a start signal to the central control 28 which again blocks the device 14, opens clamps 3, 4 and restarts the thread transport. The measurement may also be controlled manually by push buttons 44, 45 which initiate the thread transport and count measurement respectively. In order that thick thread portions e.g. knots can pass the clamps, a guiding device, consisting of two pins (60, 61, Fig. 3, not shown) which project from the plane of the clamps and make an acute angle with the direction of travel of the thread, are provided on the clamps. The pins form two arms of a U-shaped member which is widened at the open upper end and through which the thread is guided, any knot or thickened portion in the thread being guided up the pin and past the clamps.