Semi-Active Control of Vertical Stroking Helicopter Crew Seat for Enhanced Crashworthiness

Abstract

S HOCK load-induced injury minimization has become an important issue in helicopter seat design. Harsh vertical landings or crash landings of these aircraft tend to result in pilot or occupant spinal and pelvic injuries. The severity of injury, however, can be reduced if the vehicles are outfitted with crashworthy seat designs. Utilization of a seat suspension system to attenuate the vertical shock loads that are transmitted from the base frame of the aircraft of the vehicle and imparted into the human body is a prime factor in determining survivability [1]. Within the cockpit, energy-absorbing crew seats have greatly enhanced helicopter crash survivability. Energy absorbers (EAs) are a key component in these energy-absorbing seats. The first examples of crashworthy crew seat designs employed fixed-load energy absorbers (FLEAs) to limit an occupant’s spinal load. These FLEAs are not adjustable (i.e., passive) so that their stroke and load profile are fixed at a factory-established, constant load throughout their entire operating range. Variable load energy absorbers (VLEAs) were developed subsequently to permit the occupant to manually adjust the constant stroking load by setting a dial for occupant weight. The stroking load of the VLEA is selected a priori that is proportional to the occupant weight, so that each occupant will experience similar acceleration (typically 14.5 G) and use similar stroking space during a high sink rate event. VLEAs exploit the fact that the strength of an occupant’s spine is nearly proportional to occupant weight, so that the VLEA will deliver the same low injury risk regardless of occupant weight. This technology was applied in programs to retrofit new seats into platforms such as the U.S. Navy’s CH-53 Sea Stallion and SH-3 Sea King aircraft [2]. Both fixed and variable load energy absorbers, however, are passive, in that they cannot automatically adapt their energy absorption or stroking profiles as a function of occupant weight, or as a function of real-time environmental measurements such as vibration level, shock level, sink rate, etc. This motivates the development of a seat suspension that uses an electronically adjustable adaptive energy absorber that can respond to such changing environmental stimuli via commands from a real-time feedback control system. Magnetorheological energy absorbers (MREAs) offer an innovative way to achieve what is effectively a continuously adjustable profile EA [3]. Using feedback control, the MREA can smoothly adjust the load profile as the seat strokes during a crash. Thus, MREAs are expected to provide the optimum combination of short stroking distance and minimum spinal load, while automatically adjusting for the occupant weight and load level. Of the three potential seat suspension approaches (passive, semiactive, and active), the semi-active approach is very attractive. A major drawback of a passive seat suspension based on viscoelastic or hydraulic energy absorbers is that performance is limited because neither damping nor stiffness are controllable. Furthermore, compared to active approaches, semi-active systems tend to require less power and have no stability issues (because semi-active force is always dissipative). Many researchers have been inspired to develop novel seat suspensions showing improved shock and vibration attenuation performance by permitting stiffness or damping to be adaptable and controllable. Wu and Griffin [4] examined several semi-active control algorithms for reduction in the severity of seat suspension end-stop impacts. Choi et al. [5,6] evaluated the attenuation of seat vibration using skyhook and sliding mode control algorithms on both electrorheological (ER) andMR seat suspensions for commercial vehicles. Park and Jeon [7] developed a Lyapunovbased robust control algorithm which compensates for energy absorber time delay and evaluated vibration control performance of an MR seat suspension. McManus et al. [8] investigated the use of MR seat suspensions to reduce the incidence and severity of end-stop impacts, showing impressive end-stop impact attenuation performance and reduced vibration exposure levels. Recently, Choi and Wereley [1] analyzed the biodynamic response of the human body protected by a controlled MR rotorcraft seat suspension to both sinusoidal vibration and shock loads, and showed that the MR suspension had better performance than a passive hydraulic seat suspension. In the present study, a control algorithm is presented through which a magnetorheological energy absorber may be used in a crew seat suspension to automatically accommodate occupants of varying weight.