Patent GB-662177-A (GB)
Title: Dynamometric balance
IPC: G01M9/06
Filed: 1949-03-19
Abstract: 662,177. Dynamometers. OFFICE NATIONAL D'ETUDES ET DE RECHERCHES AERONAUTIQUES. March 19, 1949 [Sept. 3, 1948; Sept. 3, 1948], Nos. 7546/49 and 7547/49. Class 106 (ii). An aerodynamic balance for measuring the components of a system of forces has plain guides, derived from rectilinear guides, by substituting swivel joints for cylindrical joints between the tri-rectangular trihedron to which the forces are applied and the supporting frame. Referring to Fig. 1, the six-component dynamometer includes a trihedron T to which the forces to be measured are transmitted; this trihedron T may be of any description; however, for the sake of simplicity it -will be supposed to be constituted by an equilateral system of three orthogonal bars. The trihedron acted upon by the forces may be a virtual one as illustrated at T<SP>1</SP> in Fig. 3, only the operative portion of the trihedron being of interest as concerns its structure. Furthermore, a supporting frame B forming a rigid system is generally secured to the ground and produces a suitable reaction for the connecting means. This supporting frame is illustrated in Fig. 1 as formed by the edges of a cube. It is seen that, in reality, it is reduced to an irregular out-of-plane hexagon 11, 12, 13, 14, 15 and 16. In the arrangement illustrated, the apex of the trihedron T is located at the centre of the cube B and its bars have the same length as the edges of the cube. A connecting system or analyzing chain connects the trihedron to the frame. This connecting system is constituted by six links extending two-by-two in the same direction, i.e., 21-24, 22-25 and 23-26 that carry, at their ends 1, 4, 2, 5, 3, 6, spherical joints, which connect them with the ends of the bars forming the trihedron T, and, at their ends 11-14, 12-15 and 13-16, the spherical joints which connect them with the supporting frame and which are situated respectively at the' middle points of the edges E-H, G-F, A-B, E-F, C-G, B-F of the cubical frame. It is seen that the plane guidance of the trihedron T is in reality a spherical guidance that is similar to a plane guidance for displacements of very small amplitude of the said trihedron. In each of the six links 21, 22, 23, 24, 25, 26 operating under tractional or compressional conditions is inserted a measuring dynamometer that is not illustrated. In the balance of Fig. 1, the two analysing links co-operating in guiding a given bar of the trihedron acted upon are parallel and of the same direction; in plan view, the guidance is a circular one. In the balance shown in Fig. 2, the links 21 and 24, 22, and 25, 23 and 26 guiding the different bars of the trihedron are parallel and of opposite directions. In plan view, the guidance is carried out along a so-called long inflexion curve. It is seen that the supporting frame is reduced in practice to a regular hexagon lying in one plane and defined by the medial points of the edges of the cube 11, 12, 13, 14, 15, 16, as illustrated in Fig. 4. In this type, the flat guidance of the trihedron T is, in reality, a guidance over a so-called long inflexion surface that is similar to a flat guidance for displacements of the trihedron of very small amplitude. Fig. 5 illustrates a modification of the aerodynamic balance according to the invention wherein the measuring dynamometers are not inserted in the links (or flat guides) used for connection, but in the bars of the trihedron that operate under flexional conditions. The ends of the bars of the trihedron have the same degrees of freedom as in the foregoing systems. The guides are constituted by swivel-joint bearings 27, 28, 29, 30, 31, 32 rolling over double bearings forming a kinematic force closure. Strain gauges secured to the bars of the trihedron form very suitable detectors. It is also possible, instead of measuring the elongation of the stretched fibre, to measure the curvature of the sag of a bar of the trihedron. In Fig. 9, the aerodynamic balance that is of the type including a flat guide with parallel links and swivel joints is shown as incorporating a compensation system. It retains the hereinbefore described trihedron T connected to the masses or to the section to be tested, a supporting frame B forming a rigid system, generally anchored to ground and offering a suitable support for the connections, and an analyzing chain constituted by parallel links which are pivotally secured, by swivel joints, to the trihedron at 1, 2, 3, 4, 5 and 6 and to the supporting frame at 11, 12, 13, 14, 15 and 16 in which the dynamometric detectors are inserted. The trihedron T, which is acted upon by the forces and is constituted by the bars 1-2-3-6, 4-6 is thus an equilateral system of three orthogonal bars. The supporting frame is illustrated as constituted by the edges of a cube. The trihedron T<SP>1</SP> of the compensator, which is subjected only to inertia forces to be eliminated and which is consequently, in the case of an aerodynamic balance, connected with the compensating masses that are not acted upon by the stream line, comprises also an equilateral system of three orthogonal axes parallel to those of the trihedron T and the centres of both trihedra lie substantially at the centre of the cube. The trihedron T' is guided by plane guides constituted by parallel links pivotally secured by spherical joints at 1<SP>1</SP>, 21, 31, 41, 51 and 61 to the ends of the bars of the trihedron and at 11<SP>1</SP>, 121, 131, 141, 15<SP>1</SP>, 16<SP>1</SP>, to the supporting frame. The analyzing swivel-joint links 21, 22, 23, 24, 25 and 26 for aerodynamic balance and 21<SP>1</SP>, 22<SP>1</SP>, 231, 24<SP>1</SP>, 251 and 26<SP>1</SP>, for the compensating balance are illustrated respectively in alignment with one another and pivotally secured to the frame at the centres of the opposite edges of a face of the cube for reasons of ease of construction. They might be arranged coaxially as is the case for the bars of the trihedra T and T<SP>1</SP>. The former include the measuring dynamometers and the latter the compensating dynamometers. The inertia compensating balance of Fig. 10 differs from that of Fig. 9 only inasmuch as the plane guides formed by parallel links in Fig. 9 are of the three-bar type in Fig. 10. The trihedra for the aggregate measurement T and for compensation measurement T' is illustrated as real in Figs. 1 and 2 may also be virtual as in the case of Fig. 3. Similarly, the ratio of similarity equal to 1 in the embodiments disclosed may be made equal to any other desired value. It is possible to make the centres of gravity of the two systems coincide. Supposing p is the ratio between the masses and Ix, Iy, Iz are the ratios between the moments of inertia round the three axes and supposing X, Y, Z, L, M, N are the values of the six components provided by the six measuring dynamometers Xi, Yi, Zi, Li, Mi, Ni, the six values given out by the compensating dynamometers, the corrected values of the measurements will be: An improvement according to the invention consists in subtracting such terms as ( - p Xi) or ( - IxLi) by means of differential electric, electronic or pneumatic devices that combine the indications of the aggregate-measuring dynamometers and of the compensating dynamometers. It should be observed that it is always possible to distribute the masses over the compensating trihedron in such a manner that the coefficients p and Ix, Iy and Iz may all be equal to 1. If this is not the case the coefficient p or Ix, to which the terms X and L must be attached, may be obtained by a suitable selection of the sensitivity of the compensating detectors or by the incorporation of an amplifier having a suitable gain. The subtraction may be obtained by means of a differential fitting, such as the insertion, in the successive arms of a Wheatstone bridge, of a two-frame galvanometer, a differential pressure capsule, a negative feed-back electronic system or the like.