If your missile is going to fly straight and true – and nobody wants a missile that can’t be accurately targeted – then you need to know several key pieces of information. Not least of these is the centre of gravity (c of g). Developing a system to obtaining these parameters was a challenge put to Matra BAe Dynamics.
The total mass and the c of g referenced to a datum on the missiles in the X, Y and Z axes were required to be known. A number of systems were considered upon which to build the measurement solution, with the designers eventually selecting the Nobel KIS shear force load cell with a E-1-WEI microprocessor controlled weight transmitter. The low combined error of the KIS (0.02% RO) was important as was the integration with E-1-WEI weight transmitter to produce a weighing system with extremely high resolution – 0.001%FS, or 1g in 100kg. A serial communications interface allowed its use with a host PC computer and software developed by Matra BAe to control measurements, compute mass and c of g, record results, and compile a statistical database.
The weighing jig is designed sing three load cells to support the missile on precision bearings at three exactly defined points in the horizontal plane – two at the front and one in the centre at the rear. The computed results from these three initial mass reaction measurements give the total mass and define the c of g in the X and Y planes for that orientation of the missile. The missile is then rotated 90 degrees in the jig and the mass at the three points is measured once more.
By looking at the change in mass at the three points for the 90 degrees rotation, the system can then compute the position of the c of g of the missile with respect to its datum in all of the three axes.
The rig achieves an accuracy of the order of 0.1mm.
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