A rigid platform mounted on four supports is modeled with heave, pitch and roll motions as the three degrees of freedom. The stiffness of each support is varied independently depending upon the condition function. Two stiffness values are possible at each support. As long as the absolute value of the relative velocity across the support is greater than a design value, the actuator is switched off. Spring loaded friction pads are in contact and the two springs act in parallel to increase the stiffness of the support. When the relative velocity is close to zero, the actuator is switched on. The friction pads lose contact and only one spring of the support will be effective. The actuator develops the required force that overcomes the torque produced by the torsion spring. The torsion spring develops the necessary normal force at the friction pads that acts normal to the sliding surfaces of the spring and the base in order to produce the necessary coulomb friction for locking one of the springs with the base thus altering the stiffness. A mathematical model is developed to study the performance of the proposed model. The simulation results show that the proposed scheme is very effective in reducing the vibration levels on the platform for harmonic base inputs.
The proposed shock isolator is a single degree of freedom model where the payload mass is isolated from base disturbances by varying the stiffness of the spring. One spring always forms an interface between the mass and the base. The other spring makes or breaks contact with the base with the help of magnetorestrictive actuator depending upon the magnitude of the relative velocity. The actuator develops the required force that acts normal to the sliding surfaces of the spring and the base to produce the necessary coulomb friction force for locking the spring with the base thus altering the stiffness. The magnetorestrictive material considered here is Terfenol-D that responds to an external magnetic field proportional to the surrounding coil current. The actuator is switched on/off depending upon the magnitude of the relative velocity which in turn reduces or increases the stiffness of the isolator and thus provides the control on response. The performance of the proposed isolation and control scheme is evaluated in terms of the shock acceleration ratio and the relative displacement ratio across the isolator. A mathematical model for the proposed shock isolation system is developed and simulation results show that the performance of the proposed scheme is better when compared with that of a passive isolator for rounded pulse and rounded step displacement excitations at the base.