Passive Shock Isolation Utilising Dry Friction

Abstract

A novel shock isolation strategy for base excited system is presented by introducing a two-degree-of-freedom model with passive friction, where the friction is applied to an attached mass instead of directly to the primary isolated mass. The model is evaluated against the benchmark case of single-degree-of-freedom system with friction applied directly to the primary isolated mass. The performances of the models are compared in terms of the maximum displacement response and the acceleration during the application of the shock input for the case when the shock input duration is approximately equal to the natural period of the system (amplification region). From the results, the two-degree-of-freedom model can produce both maximum displacement reduction and smoother acceleration at the point of motion transition. An experimental rig was built to validate the theoretical results against the experimental results; it is found that the experimental results closely match the theoretical predictions.

A Friction Control Strategy for Shock Isolation

“…The control strategy is modified from the original concept widely published by including an additional parameter namely the mass velocity to perform the logic, as presented in Equation 1. Base input u (t) is a versed sine with pulse length Tp similar to the previous study by Ismail and Ferguson [1]. (1) is the value of the friction force when the friction is turned on.…”

“…Base input u (t) is a versed sine with pulse length Tp similar to the previous study by Ismail and Ferguson [1]. (1) is the value of the friction force when the friction is turned on. This constant value can be chosen to achieve the best isolated controlled response.…”

“…In a passive two degree of freedom system, previously introduced by Ismail and Ferguson [1], friction is applied to a secondary mass which is attached to the primary isolated mass by a secondary spring. Ismail and Ferguson [1] found that the instantaneous change during the start of the motion and every time friction changes its direction can be improved with a two degree of freedom configuration as friction is not applied directly to the primary mass. There is potential that applying the control strategy concept to reduce the net force acting on the primary mass could further reduce its displacement in a two degree of freedom configuration.…”

“…A linear vibration or shock isolator is often used, typically comprising an elastic element acting in parallel with a damping element. Ismail and Ferguson [1] utilised dry friction to isolate a simple system undergoing shock excitation and noted some advantages of dry friction over a viscous damping element. However, passive isolation systems have limitations because the choice of the constant system properties produce good characteristics and subsequent shock behaviour only for limited ranges.…”

“…Various control strategies were developed from the idea of reducing the net force acting on the isolated mass during the base input. The predicted response were then compared to the corresponding passive system response reported by Ismail and Ferguson [1]. Subsequently, an electromagnetic control system was designed and implemented.…”