Patent GB-1439809-A (GB)
Title: Curvature measurement
IPC: G01B7/287
Filed: 1973-06-22
Abstract: 1439809 Measuring waveguide curvature electrically POST OFFICE 20 June 1974 [22 June 1973] 29791/73 Heading G1N A device 54 for determining deviation from straightness of an electromagnetic waveguide, by measuring lateral displacements of its internal wall at discrete points thereof, consists in a rigid element 12 bearing a first group of four displacement transducers 14, 16, 18, 28 mounted on the element and arranged to measure linear displacements relative thereto along lines lying in a first plane, these lines extending from the element in a first direction (as indicated by arrow 26) and in a second opposite direction, (as indicated by arrow 32), and being spaced apart by unequal distances a, b with the line 20 of the first transducer of the group intermediate the lines 22, 24 of the lines of the second and third transducers and the line 30 of the fourth transducer being coaxial with that 20 of the first. A second similar group of transducers arranged in a plane orthogonal to that of the first group includes transducers 34, 36, 38 and a fourth, opposed transducer not visible in Fig. 1. Thus to gauge a 600m. length of waveguide a device 54 in the form of a measuring carriage is inserted into the guide along with a signalling carriage 60 and is passed through it so that the carriage 60 provides analogue representing the displacements measured by the transducers on the carriage 54 to a receiver 68 preferably in the form of multiplexed microwave signals. Since the maximum significant frequency of waveguide deviation is ten cycles per metre the signals received are filtered in low pass filters 90, having a cut-off of 2 Hz. for a measuring carriage speed along the guide of 10cm/s. maximum. For reliable results the filtered signal must be sampled at twice the cut-off frequency i.e. 4Hz or forty times per metre. So for a 600m. guide there are 24,000 sample points at each of which five readings are taken i.e. two orthogonal diameters, two orthogonal radii of curvature and the attitude of the carriage in the guide. This last reading is determined by a gravitational sensor which establishes a datum orientation for the two orthogonal planes containing the transducers and the measurements of displacement therefrom since the attitude of the carriage cannot easily be controlled. In addition the distance between the measuring carriage 54 and the waveguide end is measured by a detector 70 and distancing means 72 which count the number of standing waves set up in the waveguide by the carrier signal from a Q-band oscillator 62 outside the waveguide. The oscillator transmits a carrier signal through the guide to a horn and modulating diode 66 on the signalling carriage 60 which modulates the carrier by signals representing the displacements measured by the transducers on the carriage 54. The displacement-representative signals from the two orthogonal groups of transducers i.e. eight in all, are passed simultaneously to processing units 78 giving voltages which are converted to pulse widths by converters 80 and multiplexed in unit 82. The unit 82 modulates the Q-band carrier and the modulated carrier is transmitted to the receiver 68 having a detector 84, regenerator 88 and de-multiplexer 86 which gives outputs representative of the transducer measurements which are fed through the filters 90 to a multiplexer 92. The outputs of the multiplexer and an output of the distancing means 72 are fed to an A-D converter 94 and the digital outputs are recorded on a tape unit 96. The recorded information is processed by computer 76 to derive the deviation from straightness of the guide. The steps in the computation are, first, in unit 98, calculation of the deviation from straightness in two orthogonal planes; second, in unit 100, digital filtering to remove curvatures of longitudinal spatial frequency greater than the highest significant frequency, ten cycles per metre for TE 0   1  guide; third, in unit 102, processing of the digitally filtered information to include the effect of variations in the attitude of the measuring carriage as it moves along the guide whereby the deviation is related to vertical and horizontal planes. In addition, allowance may be made for the bending of the guide due to the weight of the measuring and signalling carriages. The deviation relative to the horizontal and vertical planes is recorded on a tape unit 96a and, when the measuring carriage has passed along the whole length of the guide being measured, the attenuation caused by the deviation from straightness over the guide length is calculated at 104.