FIELD OF THE INVENTION

The present invention relates generally to shock-isolation systems used to support and protect sensitive equipment. More specifically, the invention is related to the protection of radar systems installed on a vessel against shock and vibration.

BACKGROUND OF THE INVENTION

In order to prevent delicate state-of-the-art electronic equipment onboard naval vessels from failure caused by adverse shock and vibration, shock and vibration isolation techniques have to be applied.

Specifically, modern electronically scanned radar systems (AESA) have very high requirements regarding platform stability in order to operate accurately. A relative movement between radar system and platform must be generally avoided and only a very small rotation deviation is tolerable.

A shock-isolation platform suitable for radar systems installed on vessels must therefore offer almost rigid behaviour when subjected to accelerations up to 5g but should act as a shock absorber when the load is exceeding 5g.

Typical and well-known damping means for such platforms are helical springs or wire rope isolators. However, this method does not ensure enough stability during normal naval vessel operation, resulting in rotatory motion of the system. As disclosed in US 2003/0075407A1 , a so called Stewart platform can be used for shock isolation of sensitive equipment on a vessel. The proposed shock-isolation platform is based on helical springs which do not have the needed damping properties outlined above for the protection of a radar system used on a vessel. The disclosed Stewart platform is not sufficiently stiff to ensure stability for the radar system to operate. Using this type of isolation structure requires a fixed predefinition of the spring rate. If a stiff spring is chosen, platform will be stable when loaded up to 5g, but in case of underwater detonation the acceleration will be transmitted and will damage electronic equipment. On the other hand, if one chooses soft springs, the platform will withstand an underwater detonation, but the radar will not be able to operate during ship motion because of insufficient stability and stiffness of the isolation structure.

Electrorheological or magnetorheological fluid (hereinafter designated as ERF and MRF, respectively) damping elements for a shock-isolation structure on a vessel are discussed in US 06752250 B2. However, the very simple mounting principle is not meeting the constructive requirements of a naval radar. The disclosed system is acting mainly in one axis, making it impossible to fix a complex radar system to it. As can be easily seen, the disclosed isolating structure can be exposed only to vertical shock. Due to the joints used the system does not have any stiffness in horizontal direction, making it unsuitable for use on naval vessels. A further disadvantage of the disclosed structure is the complex control of its damping properties. Usually MRF or ERF damping elements have soft damping properties and only when needed the stiffness is increased. According to the requirements associated with a naval radar system, however, exactly the opposite is needed, i.e. the damping elements are permanently under high voltage to ensure very high stiffness. Only when a specific event occurs (shock, detonation etc.) the stiffness is decreased. The mentioned disadvantages render the structure disclosed in US 06752250 B2 unsuitable for the protection of a naval radar contemplated by this invention. SUMMARY OF THE INVENTION

It is the object of the present invention to provide a shock isolation platform ensur- ing a high repositioning accuracy for naval radar systems.

According to the present invention the shock-isolation structure is designed according to the principles of a Stewart platform. This structure is able to absorb shocks in all earth directions. Its strut-like damping elements use ERF or MRF dampers. The damping properties of the ERF or MRF dampers can be electronically controlled. Adjustment can take place within a few milliseconds.

The structure according to the invention provides protection of the radar system against any shock or vibration forces and ensures that during vessel operation the radar system can operate without any restrictions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG 1 is a side view of a first embodiment of the shock-isolating structure

according to the invention;

FIG 2 shows a ball joint at the ends of each strut-like damping element;

FIG 3 is a top view of the shock-isolation structure, according to section view A-A of FIG 1.

FIG 4 shows an alternative embodiment of a damping element of the

shock-isolating structure; FIG 5 shows the application principle of the shock-isolation structure.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the shock-isolation structure according to the invention is shown in FIGs 1 and 3 installed on board of a vessel 6.

In the side view of FIG. 1 the radar system 1 with radar rotating unit 2 and radar interface plate 3 is mounted on a platform 14. The platform 14 and the deck 5 of the vessel 6 are connected by six strut-like damping elements 4 that operate in both tension and compression between deck 5 and platform 14. The six damping elements are arranged in a truss configuration according to the principles of a Stewart platform. As can be seen from FIG 3 the six damping elements 4 form three pairs with each pair oriented in a V-configuration.

Each end of damping element 4 is connected to platform 14 or deck 5 allowing universal movement in all directions. This can be achieved e.g. by a standard ball joint as shown in detail in Fig. 2 with a spherical end part 8 of a damping element 4 moving in a casing 9 that is mounted on an interface plate to the deck 5 or to the platform 14. In general, any connection that permits angular rotation about a central point in two orthogonal directions can be used.

Each damping element 4 comprises a MRF or ERF damper 12. In addition, each of the damping elements 4 includes a helical spring 13. Instead of a helical spring any other type of spring can be used, e.g. fluid or gas springs. The main purpose of the spring 13 is to dissipate the energy of the shock by transforming it into displacement. According to the type of springs used, the shock energy could also be transformed into friction, heat etc. The operative characteristics of MRF and ERF damper are known in the art. A MRF damper is a damper filled with magnetorheological fluid, which is controlled by a magnetic field, usually using an electromagnet. This allows the damping

characteristics of the shock absorber to be continuously controlled by varying the power of the electromagnet. Similarly, an ERF damper is a damper filled with electrorheological fluid, which is controlled by an electric field.

According to the embodiment shown in FIGs 1 and 3 the MRF or ERF damper 12 is arranged inside the helical spring 13. In an alternative embodiment shown in Fig. 4 the MRF or ERF damper 12 is arranged outside the helical spring 13 with their longitudinal axis ^' oriented in parallel relationship. In both embodiments spring 13 and damper 12 are arranged between two parallel plates 15 connected to the joints.

If the stiffness of the structure described above is not sufficient for shock isolation under certain operational circumstances one can use two or more of these structures for complex support of the radar system. For example in one specific embodiment one could attach two of these structures at opposite sides of the radar system. The control algorithm for the damping properties of the MRF and ERF dampers will now be described in further detail and with reference to FIG 5. This Fig. 5 shows the acceleration load over time during a shock. As indicated in FIG 5 the typical duration of such a shock is in the range of 10 to 20 ms. Also shown are the respective damping values of the structure. The duration of the shock can be subdivided into five distinctive periods A to E.

In order to monitor the acceleration load oh the ship, respectively on the radar system, an accelerometer 7 (Fig. 1 ) on the hull (or deck) of the naval vessel is arranged and connected to a damper controller. As long as no load exceeding 5g is detected, the ERF or MRF dampers are substantially stiff, i.e. the damping value is very high, and no relative motion between vessel and radar system is permitted. The very large damping values are preventing the damping elements from moving and the loads can be transmitted 1 :1. The radar system is able to operate properly (period A).

Typically, electronic components are capable of withstanding up to 15g without suffering any damage. If the accelerometers detect accelerations beyond 5g (period B), an electrical signal is sent to the ERF (MRF) damper 12 by the damper controller 20. As a result the damping value of the damper as well as the damping value of the overall structure decreases to achieve a minimum value before the acceleration reaches the 15g limit. The reaction time should be kept as short as possible. The typical time span until the minimum damping value is achieved is in the range of 1 to 3 ms but with existing ERF or MRF dampers even a reaction time of less than 0.3 ms is possible.

During the following period C the shock load exceeds 15g. System damping has already been adjusted to very soft properties and the damper element 4 is able to absorb the shock energy, transforming it in displacement or heat. Hence, the damage of the electronic equipment can be avoided.

Period D: After the maximum value of the acceleration has been reached, the structure is swinging with very low damping. The damping properties of the system are now increased again in order to accelerate the extinction of the swinging motion of the structure. The rate of change of the damping value (i.e. the slope of the graph of the damping value in period D) can be adjusted accordingly.

About 3-15 seconds after the defection of the shock the damping is automatically increased to its maximum value before the structure returns to its initial stiff position (period E). The energy dissipation of the springs ensures that a long-lasting swinging motion of the structure at very small amplitudes is avoided.