| Helicopters are prone to crash accidents due to their structure configuration and working environment restrictions,which threatens the occupants’safety.The helicopter seat directly provides overload protection for the occupants,so its crashworthiness design is crucially important.The shock mitigation of the existing helicopter crashworthiness seat is mostly achieved by fixed load energy absorbers which are based on the 50th percentile male occupants and the predetermined probability of survival.It cannot be changed to achieve the best adjustment based on the occupants’weight and the impact strength.With the development of magnetorheological（MR）technology,the magnetorheological energy absorber（MREA）is a highly efficient semi-active impact control actuator.It has the advantages of simple and reliable structure,large adjustable range of damping force,and low energy consumption.The MREA provides the possibility to achieve an adaptive helicopter crashworthiness seat.This paper focuses on the research of MR shock mitigation system and its application in helicopter crashworthiness seats,designing the MREA suitable for high speed impact,and proposing appropriate semi-active control strategies to effectively protect the occupants in a helicopter crash.The existing MREA is limited by the high field-off damping force so that the dynamic range is low and it is difficult to be used in high-speed impact condition.This seriously restricts the application of MREA in shock mitigation.Based on this,a novel dual magnetic circuit MREA is proposed in this paper.The novel MREA can effectively break through the limitation of high viscous damping force and significantly improve dynamic damping range through the reasonable layout of electromagnetic coil and magnetic field design.The prototype of the novel MREA is designed and manufactured,and the characteristics of damping force under high-speed impact were tested by the drop hammer experiment.The results show that the novel MREA has a larger adjustable range of damping force under high-speed impact.The development of the novel dual magnetic circuit MREA can break through the applicable speed range of the shock mitigation system.Aiming at the nonlinear damping characteristics of the MREA,a generalized Bingham number control method for a single-degree-of-freedom shock mitigation system with MREA considering quadratic damping.To this end,the equation of motion of a single-degree-of-freedom（SDOF）shock mitigation system using an MREA with quadratic damping is analyzed.The payload acceleration,velocity and displacement are then obtained.To achieve a soft landing,the optimal Bingham number is derived.Through theoretical and experimental comparison with the soft landing control effect of the linear Bingham number control method of the MR shock mitigation system,it is verified that the soft landing control accuracy based on the optimal generalized Bingham number of quadratic damping is better than the existing linear Bingham number control method.The control method of generalized Bingham number based on quadratic damping is the basis of semi-active control of magnetorheological shock mitigation system.A minimum duration acceleration exposure（MDAE）control method is presented for drop-induced shock mitigation system using an MREA at high sink rates.The key MDAE control goal is that the payload should come to rest after fully using the available MREA stroke,i.e.,to accomplish a soft landing,without exceeding the maximum allowable acceleration and simultaneously minimizing the duration of exposure to the maximum allowable acceleration.An optimal criterion is developed to switch from the BN control method to MDAE control method.Finally,the optimal control method is applied to a helicopter seat suspension system by optimal selection criterion to automatically accommodate varying sink rates（impact speed）and occupant weights.A 5-degree-of-freedom occupant-seat model is established,and each part of the occupant is simplified into mass,spring and damping.It also considers the impact excitation of the helicopter cabin floor on the seat and the transient damping force characteristics of the MREA.The semi-active control method is proposed with considering the compliance of the occupant.The impact injury criteria of the human body are designed,and the semi-active control performances of the control strategies are evaluated.The proposed semi-active control method can effectively reduce the impact load to the occupants and ensure the safety of the occupants when the helicopter crashes.The rapid control experiment system of falling hammer impact protection is built,and the control methods described above are experimentally researched.For the generalized Bingham number control method and the minimum duration acceleration exposure control method,an open-loop controller and a force feedback PID controller are proposed respectively.The experiment further verifies that the MR shock mitigation system can achieve the goal of soft landing for different impact intensities. |
