Abstract:
A device for measuring bending movements of an antenna support mast, wherein the device being comprises: at least two sets of sensors separated by a determined distance d and fixed to the mast in the vicinity of the free end of the mast; and a processor for processing the signals by the sensors; the processor including means for memorizing the signals. The invention is particularly applicable to radio beam links.

Description:
The present invention relates to a device for measuring the bending of an antenna-support mast and to the application thereof to controlling the pointing of a motor-driven antenna. 
     BACKGROUND OF THE INVENTION 
     A directional antenna may be mounted on a support such as a mast which suffers unacceptable bending when subjected to dynamic forces that are applied thereto by external fluids, such as the wind. 
     French patent No. 2 100 522 describes a device for stabilizing the pointing of a directional antenna mounted on a support. This device mainly comprises an antenna support, a power supply device, a transmitter/receiver assembly, and a directional antenna together with a spherically-shaped antenna cover for stabilizing antenna pointing, having the transmitter receiver assembly enclosed therein, and mounted at the top of the antenna support. This disposition cancels all twisting torque as a result of forces due to fluid engaging the outside surface of the cover. However it does not prevent either lateral displacement in the event of forces being applied to one side, thereby giving rise to parallel displacement of the radiated beam in the bearing plane (which has practically no effect on a radiocommunications link), or on elevation displacement in the event of forces being applied in the direction of said link or in the opposite direction, and this type of deflection must be corrected if the link is to be maintained. 
     Such a spherically-shaped radome protecting a directional antenna centered on the axis of the mast eliminates practically all twisting forces. However it does not compensate for movements due to the mast bending, and that is the object of the present invention. 
     SUMMARY OF THE INVENTION 
     To this end, the present invention provides a device for measuring bending movements of an antenna support mast, wherein the device comprises: 
     at least two sets of sensors separated by a determined distance d and fixed to a support which is in turn fixed to the mast in the vicinity of the free end of the mast; and 
     a processor for processing the signals delivered by the sensors; said processor including means for memorizing said signals. 
     Advantageously, such a device is capable of memorizing information relating to the movement of the mast. Such a device can therefore be used with any mast since it learns to &#34;know&#34; its mast. 
     Advantageously, such a device is usable for providing independent servo-control of a motor-driven antenna situated at the top of the mast. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention is described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagram of the device of the invention; and 
     FIG. 2 is a graph for explaining the operation of said device. 
    
    
     DETAILED DESCRIPTION 
     The device of the invention includes at least two sets of sensors Ci, Ci+1, disposed in pairs or triplets, . . . , in the vicinity of the free end of the mast 10 which supports a directional antenna 13, which sets of sensors at a distance d apart. The outputs from the sensors are connected to an information processor 11 which includes memory means. The processor 11 receives service commands CS for the device of the invention and it optionally delivers data relating to the pointing of the antenna 13 if the antenna is motor driven, which data is corrected to take account of earlier bending of the mast. 
     In FIG. 1, two pairs of sensors C1, C2 and C3, C4 are shown. These sensors may be constituted, for example, by clinometers for monitoring the bending of the mast 10 in two mutually perpendicular planes xoz and yoz, thereby providing complete monitoring of the bending. 
     In FIG. 1, these pairs of clinometers are mounted on a common support 12 which makes it easier to mount them at the top of a mast and ensures that their relative orientations remain stable. The fact that the support 12 is distinct from the mast 10 stiffens the relative positions of the clinometers which are not required, in this disposition, to accommodate the distortion that the bending per se of a length of mast would impart to this length. 
     By using pairs of clinometers, it is possible to obtain information concerning the orientation of the mast head which is independent of the bending mode of the mast. A single clinometer Ci provides information relating to the direction of the vertical for assemblies that are stationary or that are moving very slowly. 
     In a dynamic situation, e.g. when a mast head is subjected to the effects of the wind, the readings of a single clinometer Ci are greatly modified by the acceleration of the point Mi where the clinometer is fixed. The observed measurement is then the direction of a difference vector between the acceleration γ of the point Mi and the acceleration g due to gravity, and this depends both on the orientation and on the motion of the mast head. 
     The pseudo-periodic nature of the movements of a mast head nevertheless facilitates forecasting such movements at some instant T, providing these movements have been observed during preceding instants. 
     Parameters for amplitude, pseudo-period and damping of movement are initially unknown, but they are memorized successively by the processor which can then make use of this data. 
     Knowledge of the bending mode characteristics of the mast being used, relating the movement of the mast head to its orientation, then makes it possible to distinguish these two parameters and to provide orientation information as required for pointing an antenna. 
     By using a pair of clinometers in combination, observed measurements are no longer constrained in this way. 
     It is then no longer necessary to know the bending characteristics of the mast in advance, and this is a great advantage given that such characteristics are specific to each mast, and often to each occasion on which it is used (tension of stays, . . . ). 
     The information picked up by each clinometer is digitized and stored by the processor which performs the necessary extrapolations after determining the amplitude, the pseudo-period, and the damping coefficients of the observed movement. 
     When the antenna 13 situated at the mast head is motor driven, the device of the invention enables its orientation to be controlled so that it maintains a given direction relative to the vertical. 
     For a better understanding of the operation of the device of the invention, FIG. 2 shows the various movement parameters and the information given by the clinometers operating in the xoz plane, while the simplified calculation given below and suitable for use with small angles explains the principle whereby bending mode characteristics are eliminated. 
     For a sinusoidal movement of the mast head about a center of gyration C, with a mean position αO, such that: 
     
         α=α0+A sin wt, 
    
     then: 
     the position of sensor C1 at point M1 (x1, z1) is: 
     
         x1=R1 sin α 
    
     
         z1=R1 cos α 
    
     and the position of sensor C2 at point M2 (x2, z2) is: 
     
         x2=R2 sin α 
    
     
         z2=R2 cos α. 
    
     The measurements obtained from the two clinometers C1 and C2 are the following: 
     
         θ1=α+arc tan (-γ1x/(g-γ1z)) 
    
     
         θ2=α+arc tan (-γ2x/(g-γ2z)) 
    
     where: 
     γ1x, γ1z; γ2x, γ2z are components of the acceleration vectors at points M1 and M2; 
     g is acceleration due to gravity; 
     d=R1-R2=distance between M1 and M2; 
     MO1 and MO2 are the mean positions of M1 and M2. 
     Movements along the x-axis have an angular frequency w. However movements along the z-axis are small in amplitude for small angles and at an angular frequency of 2w. 
     The value of α0 is determined by the mean over time of θ1 or θ2. 
     γ2x and γ2z are negligible compared with g. 
     x1 is approximately R1α, giving γ1x=R1 A w 2  sin wt. 
     Similarly, taking the angle to be equal to its tangent: 
     
         θ1-θ2=(γ1x-γ2x)/-g 
    
     
         θ1-θ2=[(R1-R2)A w.sup.2 sin wt]/g=d A w.sup.2 sin wt/g. 
    
     The variable term of α(A sin wt) is thus directly deduced from θ1-θ2 once d, g and w 2  are known. 
     w may be measured by observing θ1 or θ2 over time. 
     The value of α used by the device of the invention is obtained by adding together α0 and A sin wt as determined in this way. 
     The bending characteristics of the mast under consideration relating the x deflection to the angle α at the mast head are not used, and the device of the invention is thus able to operate with any mast. 
     Naturally, the present invention is described and shown merely by way of preferred example and its component parts could be replaced by equivalent parts without thereby going beyond the scope of the invention. 
     The sensors used could be accelerometers. 
     The simplified calculation given above is merely by way of example. 
     Similarly, the damped nature of the oscillations of the mast head which describes movement of the form 
     
         α=α0+Ae.sup.-βt sin wt 
    
     does not impede evaluation of the angle α since the damping coefficient δ of the angle α is measured like w by observation over time of θ1 and θ2 which are stored by the processor.