## 2 Cage Armour Working Principle
### Cage Armour Design Principle  
From Section 2.2. it is possible to deduce the four main design principles for an optimized cage armour system. Firstly, the horizontal and/or vertical distance between the structural parts of the slat armour should not be larger than the maximum diameter of the projectile against which the cage armour is designed to offer protection. Failing to do so will offer a possibility for an impacting projectile to pass between the structural parts of the cage armour without any interference with the latter. This leads to a maximum allowable horizontal and/or vertical distance between the cage armour parts of \(\pm\)85 mm and \(\pm\)72 mm for, respectively, the PG-7 and PG-7M warheads. In reality, this distance has to be slightly smaller than the maximum diameter of the projectile to assure sufficient deformation of the outer cone to cause and maintain a short-circuit until the impact of the piezoelectric element on the hull of the target. In order to assure sufficient interaction between the cage armour components and the projectile, the cage armour must, of course, also offer sufficient resistance, either by strength or by inertia, or a combination of both. This leads to a second design principle being to determine minimum dimensions for the cage armour components to have sufficient strength, stiffness and/or inertia. Thirdly, the piezoelectric element may not be allowed to come into contact with the hull of the target before a short-circuit has been caused by the cage armour. For this, it is important to put the cage armour at a minimum standoff distance from the hull corresponding to the distance existing between the piezoelectric element in the nose of the projectile on the one side and the projectile nose section with the maximum diameter on the other hand. For the PG-7(M) projectile this corresponds with a minimum standoff distance of \(\pm\)190 mm. In practical applications, the selected standoff distance is generally considerably larger. This makes sure that there has been sufficient interaction between the cage armour components and the incoming projectile before the latter comes into contact with the hull of the vehicle. In certain cases, this might also lead to significant damage to the liner of the shaped charge, further reducing the risk of having a complete perforation of the hull. Additional advantages of this larger standoff are also that local variations in the geometry of the vehicle (e.g., the presence of boxes, connectors, handles, equipment, etc.) will not lead locally to a too-small standoff distance, next to reducing the blast effects on the hull of any explosive projectile impacting on the cage armour system. Last but not least, the probability that the piezoelectric element (with a diameter of \(\pm\)20 mm) impacts directly on the cage armour system instead of in-between its components must be minimized, as an impact of the piezoelectric element directly on the cage armour  
Figure 4: Working principle of the cage armour showing the short-circuiting of the inner and outer cone before the impact of the piezoelectric element on the hull of the target. Due to the short-circuit the main current will pass through the short-circuit instead of through the detonator. will generally lead to triggering of the detonator. The latter will initiate the shaped charge in a regular fashion. Although the standoff will be slightly increased compared to an impact directly onto the vehicle hull, it will, in almost all cases, not be sufficient to significantly lower the penetration capacity of the shaped charge (above 300 mm for even the most basic versions) to avoid a complete perforation of the armoured hull (see [4] for the residual penetration capacity of the PG-7 warhead as a function of standoff). This means that the horizontal and vertical distance between the different cage armour components must be maximized, but still taking into account the first principle that a projectile should not be able to pass between the cage armour components.