Patent GB-795808-A (GB)
Title: Improvements relating to methods of, and apparatus for frequency analysis of periodic variations of magnitudes, particularly in the substance cross-section of textiles
IPC: G01N33/36
Filed: 1954-09-06
Abstract: 795,808. Frequency measurement; measurement of cross-section of textile material. ZELLWEGER, Ltd. USTER FACTORIES FOR APPARATUS AND MACHINES. Sept. 6, 1954 [Sept. 4, 1953], No. 25835/54. Class 37. Periodic variations of a magnitude having a complex oscillatory spectrum, representing, for instance, the varying substance cross-section of a textile material, are frequency analysed by applying an electrical magnitude U(t) equivalent to the variable to plural channels individually corresponding to distinct component frequencies, in each of which the input is multiplied by the respective sinusoidal and cosinusoidal functions of time and of the particular channel frequency, the resultant signals being integrated over a small time interval by an appropriately proportioned resistance-capacitance circuit and applied to a multiplying modulator-phase quadrature-mixer circuit, whose output represents the square root of the sum of the squares of the input value and is averaged at prescribed time intervals over the whole or part of the duration of the function by an electrical filter to a measured signal magnitude representing the channel frequency amplitude. A voltage waveform representing variations with time of the quantity of cross-sectional substance of a textile yarn, roving, sliver or lap (Figs. 2, 3, not shown) is derived by drawing the material at constant speed through a measuring capacitance in known manner (Fig. 1, not shown) and the waveform U(t) is supplied to 12 distinct channels corresponding to 12 distinct fixed frequencies (Fig. 6). Each channel comprises a pair of multiplier circuits 26 wherein (Fig. 7) the input waveform U(t) is connected over terminals 31, 32 to equal resistances 33, 34 joined at terminal 35, and potentiometer 37 having tappings 38, 44 respectively variable cosinusoidally at the pulsatance w of the channel frequency (or alternatively cosinusoidally variable together with a spacing of 90 electrical degrees). The voltage waveforms U 1A (#)=U(t) cos #t and U 1A (#)=U(t) sin #t appearing between tappings 38, 44 and terminal 35 are respectively applied to a pair of integrators 27 comprising R.C. networks (Figs. 10, 11, not shown) having identical time constants, to develop output voltage waveforms, which are respectively applied to a pair of multipliers 28 comprising ring modulators 60, 61 (Fig. 14) supplied with voltage waveforms U A (#) and U B (#) together with voltage waveforms U A (#) and U B (#) together with modulation voltages U 3  of pulsatance # 3 , whose modulated outputs U 1A (#), U 1B (#) are paralleled over terminals 66, 67 and respective R.C. networks comprising series resistance 62 with parallel capacitance 63, and series capacitance 64 with parallel resistance 65, having identical time constants equivalent to the reciprocal of the modulation pulsatance; the two output voltage waveform components being therefore vectorially combined (Fig. 13, not shown) in quadrature at the terminals 66, 67 to a resultant value U k (#) proportional to This voltage waveform which is long by comparison with the period of the input voltage function U(t), and the resultant voltage waveform is rectified to a value U Km (#). It is shown that, for a Fourier analysis of a waveform U(t) into its frequency components over a time t, for a given frequency component of pulsatance #, so that the resultant voltage signal U Km (#) is proportional to the amplitude of the frequency component of the input signal U(t) corresponding to the particular channel at whose output it appears, so that the output signals of each frequency channel may be successively switched to an electric recording device (not shown) and permanently displayed as a spectrum comprising series of points 11 on a chart of which frequency represents the abscissµ and amplitude the ordinates (Fig. 5). The envelope curve 8 connecting the points is shown to represent also the amplitudes of Fourier components of the input signal U(t) for frequencies intermediate of the fixed channel frequencies to a close approximation. A mathematical analysis is given. The multiplier circuits 26 may comprise (Fig. 8) a modification wherein the input signal U(t) is applied across resistances 33, 34 as in Fig. 7, but the cosinusoidal potentiometer 37 is replaced by the opposed fixed contacts of a pair of changeover switches continuously vibratory at the required channel pulsatance #t, connected in parallel across the input terminals, and respectively operated in quadrature to develop across output terminals 35, 36 and 35, 43 respective trains of rectangular pulses (Fig. 9, not shown) having impulse lengths designated by α. It is shown that if the contacts are constructed so that α = 2#/3 and the input is preceded by a low-pass filter cutting off at the upper extremity of the range of frequency analysis of U(t), the output waveforms are approximately equivalent to U 1A (#) and U 1B (#) above; the error comprising only the 5th and higher order harmonics. In a further modification of the complete apparatus comprising four distinct frequency channels (Fig. 16), the input signal U(t) is applied over terminals 31, 32 to resistances 33, 34 and changeover switches 39, 80, 82, 84 and 49, 81, 83, 85; the pairs of switches 39, 49; 80, 81; 82, 83; 84, 85 operating each at the same frequency but with the component switches operative with a mutual phase displacement of 90 degrees. The resultant signals pass with reversed polarity over corresponding RC integrating circuits 41, 45; 86, 87; 90, 91; 94, 95; 42, 47; 88, 89; 92, 93; and 96, 97 to mechanically-operated switches 100, 101 sequentially selecting the integrated outputs for each frequency two at a time and connecting them to cathode follower triode amplifiers 102, 103 feeding two ring modulators respectively comprising diodes 113, 114, 115, 116 with resistances 117, 118, 119, 120 and diodes 123, 124, 125, 126, with resistances 127, 128, 129, 130, to which the modulating voltage is applied at terminals 111, 112, and 121, 122. The modulated outputs pass over RC phase-shift networks 62, 63 and 64, 65 and are combined in quadrature at the input of transformer 132 in the grid circuit of triode amplifier 133, coupled over tuned transformer 136 to half-wave rectifier 138. The rectified output appearing across load circuit 139, 140 is selectively and successively connected by switch 103 mechanically operated in synchronism with switches 100, 101 to averaging RC circuits 74, 75; 143, 144; 145, 146 and 147, 148 corresponding to the four distinct frequency channels to which the input signal U(t) is connected, and the outputs thereof are connected over a similarly synchronous switch 150 to a recording meter 5 (which may be linear or quadratic) displaying the spectrum of the voltage waveform U(t) referred to the four predetermined frequency channels. The recording meter may be given a square root characteristic and supplied with a signal proportional to the sum of the squares of the Fourier components U A (#) and U B (#), while in simplified forms of the apparatus either Fourier component may be recorded and displayed alone, or the series of fixed individual frequency channels may be replaced by a single channel whose frequency is variable over the range of the apparatus in synchronism with the traverse of the recording output instrument.