VIEWS: 11 PAGES: 10 POSTED ON: 6/12/2012
STABILIZATION OF HELICOPTER SLING LOADS VEFA NARLI 1 INTRODUCTION The focus of my proposed dissertation relates to a challenging application of nonlinear control theory: nonlinear control of under-actuated systems. This statement contains the objectives of the research to be undertaken in preparation of the dissertation the relation of the issues raised by this application to the engineering and science literature on the nonlinear control of under-actuated systems, anti-sway control, and active vibration suspension, and the rationale for the proposed methodology to be carried out in the dissertation research. Helicopters have vertical take-off and hovering capabilities. They can reach places where ground vehicles and other types of aerial vehicles can not. Therefore they are highly valuable in aerial transportation and as aerial cranes. Both of these modes of helicopters require carrying an external load suspended beneath the helicopter, called sling loading. However, use of these two modes of helicopters are limited due to the danger and difficulty of carrying a load that can freely swing beneath the helicopter. Even though helicopter automatic flight control systems (AFCS) are used widely on-board helicopters, , there aren’t any AFCSs that can stabilize slung loads to the best of the author’s knowledge. Hence the motivation of this thesis is to stabilize helicopter sling loads. This chapter of the thesis is dedicated to a brief overview of helicopter sling load transport and issues, importance of the stabilization of the helicopter sling loads, statement of the sling load stabilization problem, literature survey of relevant stabilization problems. 1.1 Helicopter Sling Load Transport and Issues Helicopters are being employed in rescue operations, emergency medical services, disaster relief, fire fighting, detection and assessment, construction, power line, heavy lifting, forestry, and petroleum support operations. In all of these operations and many other similar applications, an external load is lifted, transferred, and landed/positioned by helicopters (Figures 1-8). Helicopters offer the following advantages due to their vertical take-off, land, and hovering capabilities: heavy, oversized or bulky cargo can be handled, areas that are inaccessible by any other mode of transportation can be reached, and transfer time is faster than any other mode of transportation. They are used as aerial ambulances, aerial search and rescue units, aerial fire engines, and aerial cranes. They allow human beings to accomplish tasks on air. Therefore, helicopters are very critical in saving lives, and habitat, as well as saving many industries time and money by providing a way of aerial reach. This is especially true since we have skyscrapers all around and we don’t have a good means of rescuing people when a disaster such as a terrorist attack (September 11) or a fire on a high-rise building hits us. However, the helicopter sling loading is not fully utilized due to the free rotational motions of the sling load. Figure 1.1 (a) A Helicopter, filling its bucket to extinguish a fire nearby (left); (b) Helicopters are used in high rise rescue operation (right.) Figure 1. 2 (a) A helicopter, transporting cargo to a ship (right,) (b) Fire fighting helicopter (left.) Figure 1. 3 Helicopters in construction. Figure 1. 4 The picture depicts a helicopter that is being used in offshore cargo transportation or construction. Helicopter sling loading is a mode of helicopter transportation, where one or more loads are suspended beneath the helicopter from one or more provisions on the helicopter using one or more lines of cables (Figure 1. 5, and Figure 1. 6b.) This is the common way of carrying external loads with helicopters. Sikorsky’s CH-54A S64 Tarhe, Figure 1. 6a, is specifically designed to carry heavy loads and is an exception to that. Figure 1. 5 Helicopter sling loading with more than one cable/provision. Figure 1. 6 (a) Sikorsky’s CH-54A S64 Tarhe crrying a military vehicle, (b) Helicopter carrying a Heli-Basket through a cable (single line sling loading). The single line configuration causes the suspended load to have free rotations (3 rotational degrees of freedom, one being the twist of the cable) with respect to the helicopter. As can be seen from Figure 1. 6, the cable that carries the load may have a large angular deviation from the vertical. This motion of the suspended load arises when the helicopter accelerates, decelerates or maneuvers. Because the suspended load oscillates freely (lightly damped,) the pilot has to stabilize the suspended load most of the time, and compensate for the effects of the oscillations on the helicopter. The pilot, whose job is already challenging and vitally critical, now has to fly the helicopter in a way that would not disturb the suspended load oscillations. One element that helps the pilot keep his craft and the payload stable is the length of the cable. The cable length is usually kept fairly long (20-200 feet) to decrease the natural frequency of the suspended load system. This decreases the effectiveness of the operation, whatever it may be, and furthermore can be very dangerous. For example, the following excerpt from Arthur Negrette’s article issued on the February 1st of 1999’s Aviation Today is alarming: “…During the flight to reposition the helicopter, the 206 reached an altitude of 50 to 60 feet AGL when the aluminum mesh basket caught on a nearby fence…” Hence there are other tasks that are added to pilot’s list of survival when the helicopter is carrying a suspended load such as are clearing the obstacles that are on the load’s path, The pilot needs to watch for obstacles and maneuver the helicopter so that the load can clear the obstacles. He is supposed to perform that action while the load is far down beneath the helicopter. Generally, it is advantegous to carry the suspended load as close to the helicopter as possible. That way, the pilot is not looking hundreds of feet down to identify any obstacles there might be. Other tasks that the pilot may be required to perform is to position the load on a target location by estimating the relative position of the load with respect to the location. Moreover, there are other people that are affected by the way the load is carried by a helicopter. During the lifting of the external load, a ground crew hooks the load and guides or stabilizes it, mostly by means of ropes or sometimes even with their hands. During the landing or positioning of the external load, the crew maneuvers the load above on the target location. These tasks are dangerous and require clear communication between the pilot and the ground crew. Once the stabilization of the suspended load is accomplished, precise positioning of the load on a target location is inevitable. In conclusion, there are many issues related to the stability of the helicopter sling loading that makes the very critical and important uses of helicopters, such as rescue from high rise buildings, or aerial crane type operations cumbersome and even dangerous. Yet another issue that carrying a suspended load with a helicopter raises is the certification of the helicopter load system. When a load is suspended beneath the helicopter, the helicopter and suspended load system has to go through extensive flight tests before the helicopter is allowed to fly . The flight envelope of the helicopter usually gets smaller due to the added lightly damped suspended load system. There has been some effort to reduce the time and cost of flight tests that are carried to certify the helicopter load system . Cicolani et. al. analyzes the flight test to identify the key dynamic parameters such as aircraft stability margins, handling-qualities parameters and load pendulum stability. They also compare the helicopter suspended load simulation results with the flight test data to identify the weaknesses of the model. The goal of this work was to both revise the cumbersome flight tests and use validated simulation models. Cicolani et al.  emphasizes the reduced gain margin near the pendulum frequency (at about 1.5 rad/s) in the lateral axis (roll axis) for the helicopter and the suspended load system at hover. This results a loss of helicopter response at that frequency. They also mention the significant reduction of the attitude bandwidth, maneuver response, of the system with the addition of a suspended load. Being a lightly damped system, a suspended load can cause a helicopter to be unstable or to lose altitude if the flight test envelope of the helicopter is not revised. Therefore increasing the damping of the suspended load system, or in other words, stabilizing the motion of the suspended load would increase the stability and the flight envelope of the helicopter. Considering the need to carry suspended loads with helicopters, in a way that is more secure and efficient than the current means, and also considering the instability that a suspended load may add to the helicopter, we asked the following questions to ourselves: How can we stabilize a helicopter sling load? Should there be a stabilizing control mechanism that works independently from the control of the helicopter? What is the methodology to stabilizing the helicopter sling loads? How would the solution of this problem advance nonlinear control and control of under- actuated systems fields? 2 REFERENCES 1. Abdel-Rahman, E. M., Nayfeh, A. H., Masoud, Z. N., “Dynamics and Control of Cranes: A Review”, Journal of Vibration and Control, 9, pp. 863-908, 2003, cited by 15. 2. Alleyne, A., Hedrick, J. K., “Nonlinear Adaptive Control of Active Suspensions,” IEEE Transactions on Control Systems Technology, Vol. 3, No. 1, pp. 94-101, 1995, cited by 123. 3. Anonymous, “Handling Qualities Requirements for Military Rotorcraft, Aeronautical Design Standard,” ADS-33E-PRF, 2000. 4. Anonymous, “Requirements for the Certification of Sling Loaded Military Equipment for External Transportation by Department of Defense Helicopters”, MIL-STD-913A, 1997. 5. Balachandran B., Li, Y.-Y., Fang, C.-C., “A Mechanical Filter Concept For Control of Non-linear Crane-Load Oscillations”, Journal of Sound and Vibration, 228, pp. 651-682, 1999, cited by 12. 6. Bartolini, G., Pisano, A., Usai, E., “Second-order Sliding-Mode Control of Container Cranes”, Automatica 38 pp. 1783-1790, 2002, cited by 11. 7. Cicolani, L. S., Sahai, R., Tucker, G. E., McCoy, A. H., Tyson, P. H., Tischler, M. B., Rosen, A., “Flight Test Identification and Simulation of a UH-60A Helicopter and Slung Load”, NASA/TM-2001-209619, USAAMCOM-TR-01-A-001, 2001. 8. Corriga, G., Giua, A., Usai, G, “An Implicit Gain-Scheduling Controller for Cranes”, IEEE Transactions on Control Systems Technology, Vol. 6, No. 1, pp. 15-20, 1998, cited by 36. 9. Fang, Y, Dixon, W. E., Dawson, D. M., Zergeroglu, E., “Nonlinear Coupling Control Laws For An Underactuated Overhead Crane System”, IEEE/ASME Transactions on Mechatronics, Vol. 8, No. 3, pp. 418-423, 2003. 10. Fliess, M., Levine, J., Rouchon, P., “A Simplified Approach of Crane Control Via A Generalized State-Space Model”, Proceedings of the 30th Conference on Decision and Control, England, pp. 736-741, 1991, cted by 29. 11. Fusato, D., Guglieri, G., Celi, R., “Flight Dynamics of an Articulated Rotor Helicopter with an External Slung Load,” AHS International Annual Forum, 55th, Montreal, Canada, 1999, cited by 8. 12. Hrovat, D., “Survey of Advanced Suspension Developments and Related Optimal Control Applications,” Automatica, Vol. 33, No. 10, pp. 1781-1817, 1997, cited by 121. 13. Karnopp, D., “Active Damping in Road Vehicle Suspension Systems,” Vehicle System Dynamics, 12, pp. 291-316, 1983, cited by 50. 14. Karnopp, D., “Design Principles for Vibration Control Systems Using Semi-Active Dampers,” Journal of Dynamic Systems, Measurement and Control, Vol. 112, pp. 448-455, 1990, cited by 43. 15. Key, D. L., “Airworthiness Qualification Criteria for Rotorcraft With External Sling Loads”, NASA/TM-2002-211390, USAAMCOM AFDD/TR-02-A-002, 2002. 16. Kriikku, E., Singer, N., Singhose W., “An Input Shaping Controller Enabling Cranes to Move Without Sway”, American Nuclear Society 7th Topical Meeting on Robotics and Remote Systems, 1997, cited by 36. 17. Kureemun, R., Walker, D. J., Manimala, B., Voskuijl, M., “Helicopter Flight Control Law Design Using Hinf Techniques,” Proceedings of the 44th IEEE Conference on Decision and Control, and the European Control Conference, pp. 1307-1312, 2005. 18. Moustafa, K. A. F., Ebald, A. M., “Nonlinear Modeling and Control of Overhead Crane Load Sway”, Transactions of ASME, Vol. 110, pp. 266-271, 1988, cited by 38. 19. Omar, H. M., “Control of Gantry and Tower Cranes”, PhD. Thesis, Blacksburg, Virginia, 2003, cited by 5. 20. Postlethwaite I., Prempain, E., Turkoglu, E., Turner, M. C., Ellis, K., Gubbels, A. W., “Design and Flight Testing of Various Hinf Controllers for the Bell 205 Helicopter,” Control Engineering Practice, 13, pp. 383-398, 2005, cited by 1. 21. Singer, N. C., Seering W. P., “Preshaping Command Inputs to Reduce System Vibration”, Transactions of ASME, Vol. 112, pp. 76-82, 1990, cited by 331. 22. Stiles, T. R., Mayo, J., Freisner, A. L., Landis, K. H., Kothmann, B. D., “Impossible To Resist, The Development of Rotorcraft Fly-by-Wire Technology,” AHS 60th Annual Forum, Baltimore, MD, 2004. 23. Sunwoo, M., Cheok, K. C., Huang, N. J., “MRAC for Vehicle Active Suspension Systems”, IEEE Transactions on Industrial Electronics, Vol. 38, No. 3, pp. 217-222, 1991, cited by 15. 24. Tyson, P. H., Cicolani, L. S., Tischler, M. B., Rosen, A., Levine, D., Dearing, M., “ Simulation Prediction and Flight Validation of UH-60A Black Hawk Slung-Load Characteristics,” AHS 55th Annual Forum, Montreal, Canada, 1999. 25. Ulsoy, A. G., Hrovat, D., Tseng, T., “Stability Robustness of LQ and LQG Active Suspensions,” Journal of Dynamic Systems, Measurement, and Control, Vol. 116, pp. 123-131, 1994, cited by 28. 26. Walker, D. J., “Multivariable Control of the Longitudinal and Lateral Dyamics of a Fly-By-Wire Helicopter,” Control Engineering Practice 11, pp. 781-795, 2003, cited by 5. NOTES: The disturbance is in the form of acceleration or it may be taken as 2nd order nonholonomic constraint then go to the non-holonomic system control literature. Check the vibration control of acceleration input systems, like earthquakes… Might want to try shaping the control input, notch filter to see the effect of it.
Pages to are hidden for
"STATEMENT OF THE PROBLEM"Please download to view full document