Thomas Jones

Brief paper: Fast conflict detection using probability flow

Probabilistic conflict detection has recently attracted much attention from researchers due to its importance in safe automated transport systems; however, current methods struggle to accurately calculate in real-time the probability of conflict for complex vehicle maneuvers in cluttered environments. We present a formulation for the general probabilistic conflict detection problem using the flow rate of conflict probability at the boundary of a conflict zone. For Gaussian distributed vehicle states, we then derive a tight upper bound for the probability of conflict over a time period, which can be calculated in real-time using adaptive numerical integration techniques. We present two examples to illustrate the performance of this method: the first example shows that this method is very efficient for simple environments and the second example shows that this method can calculate the conflict probability upper bound in real-time even for complex vehicle maneuvers in cluttered environments.

A Piloted Orion Flight to a Near-Earth Object: A Feasibility Study

This viewgraph presentation examines flight hardware elements of the Constellation Program (CxP) and the utilization of the Crew Exploration Vehicle (CEV), Evolvable Expendable Launch Vehicles (EELVs) and Ares launch vehicles for NEO missions.

Large transport aircraft: Solving control challenges of the future ☆

Abstract The evolution of large transport aircraft is driven by safety, cost, passenger comfort and environmental impact, all within a highly competitive and regulated environment. As a result, creating the aircraft of the future presents various interesting control challenges. For the past 7 years, Stellenbosch University has partnered with Airbus and the National Aerospace Centre to investigate and solve some of these challenges. This paper focusses on key projects within this partnership, serving Airbus centres of competence in France, Germany and the UK. Goals range from improving efficiency (e.g. automated formation flight) to improvements in safety (e.g. automatic return to flight envelope) and general automation (e.g. automatic in-flight refuelling). Where appropriate, it is illustrated how detailed analysis and the application of advanced techniques may often lead to relatively simple answers and quite general conclusions.

Acceleration-based 3D Flight Control for UAVs: Strategy and Longitudinal Design

The design of autopilots for conventional flight of UAVs is a mature field of research. Most of the published design strategies involve linearization about a trim flight condition and the use of basic steady state kinematic relationships to simplify control law design (Blakelock, 1991);(Bryson, 1994). To ensure stability this class of controllers typically imposes significant limitations on the aircraft’s allowable attitude, velocity and altitude deviations. Although acceptable for many applications, these limitations do not allow the full potential of most UAVs to be harnessed. For more demanding UAV applications, it is thus desirable to develop control laws capable of guiding aircraft though the full 3D flight envelope. Such an autopilot will be referred to as a manoeuvre autopilot in this chapter. A number of manoeuvre autopilot design methods exist. Gain scheduling (Leith &. Leithead, 1999) is commonly employed to extend aircraft velocity and altitude flight envelopes (Blakelock, 1991), but does not tend to provide an elegant or effective solution for full 3D manoeuvre control. Dynamic inversion has recently become a popular design strategy for manoeuvre flight control of UAVs and manned aircraft (Bugajski & Enns, 1992); (Lane & Stengel, 1998);(Reiner et al., 1996);(Snell et al., 1992) but suffers from two major drawbacks. The first is controller robustness, a concern explicitly addressed in (Buffington et al., 1993) and (Reiner et al., 1996), and arises due to the open loop nature of the inversion and the inherent uncertainty of aircraft dynamics. The second drawback arises from the slightly Non Minimum Phase (NMP) nature of most aircraft dynamics, which after direct application of dynamic inversion control, results in not only an impractical controller with large counterintuitive control signals (Hauser et al., 1992) (Reiner et al., 1996), but also in undesired internal dynamics whose stability must be investigated explicitly (Slotine & Li, 1991). Although techniques to address the latter drawback have been developed (AlHiddabi & McClamroch, 2002);(Hauser et al., 1992), dynamic inversion is not expected to provide a very practical solution to the 3D flight control problem and should ideally only be used in the presence of relatively certain minimum phase dynamics. Receding Horizon Predictive Control (RHPC) has also been applied to the manoeuvre flight control problem (Bhattacharya et al., 2002);(Miller & Pachter, 1997);(Pachter et al., 1998), and similarly to missile control (Kim et al., 1997). Although this strategy is conceptually very promising the associated computational burden often makes it a practically infeasible solution for UAVs, particularly for lower cost UAVs with limited processing power.

Augmenting the helicopter-ship dynamic interface using an autonomous safe-landing prediction scheme

The operational interface between a helicopter and a ship deck is a complex, dynamic and hazardous environment that presents a unique set of challenges to both engineers and pilots alike. Unmanned helicopters require a specific set of conditions to be met and maintained in order to commit to a landing, where large amounts of energy or large angular deviations of the ship deck can cause undesirable and dangerous effects, such as dynamic rollover. An analysis of the prediction problem is given, based on datasets collected from a 70m ship in South African waters, as well as an illustration of a real-time technique based on standard ship measurements that can be used to indicate potential landing periods. This is intended for application as a subsystem that can be used to facilitate autonomous landings of Unmanned Aerial Vehicles (UAVs) while at sea. The system also has potential as a bridge landing aid to allow operators to select ideal landing periods for helicopter operations. Simulations, based on real datasets, illustrate adequate prediction quality ranging from 3 seconds to 20 seconds in the future, depending on the forward prediction coherency for the dataset.

Electronic Stabilization of Continuous-Wave and Pulsed Lasers Based on Macroscopic Rate-Equation Modelling

We present a macroscopic laser rate-equation model based on measurable laser parameters, allowing easy system identification. A numerical simulation based on the model is used in the design and testing of electronic laser feedback systems for intensity noise suppression and Q-switched pulse stabilization. A novel pulse energy control scheme is also presented, including experimental results.

An intuitive, aggressive control architecture for an unmanned helicopter in near-hover flight

Control system design for unmanned helicopters has become a widely studied topic, resulting in numerous design techniques with varying qualities. Many control designs rely on precise knowledge of the highly non-linear system dynamics inherent in a helicopter, and resulting techniques suffer from gain margin issues associated with the effect of the designed control laws on the variable system modes. A robust and heuristically defined control system structure is proposed that can be theoretically designed and empirically tuned in the field based on observed responses. The design is implemented as successively closed loops with four tiers to ensure stability and ease of design. The control architecture presented illustrates a theoretically robust system that, when used with analytic models and low control gains, provides implicit system controllability and stability open to optimization by means of various control techniques. The structure is intended and designed for outdoor operation in near-hover flight, and practical results are given for tests completed in an outdoor environment.

Laser pulse energy control using a high speed digital feedback controller

We present a novel electronic feedback scheme which allows laser pulse energy control and stabilization by using a custom high speed FPG A digital controller. This scheme was tested on a high-energy fiber-laser-pumped Ho:YLF ring laser.

Between the Moon and Mars: Piloted and surface operations at a NEO

In late 2006, NASAs Constellation Program (CxP) sponsored a study to examine the feasibility of sending a piloted Orion spacecraft to a near-Earth object (NEO — in the broadest definition these are small, primitive bodies that cross Earths orbit; the most likely and suitable targets for the Orion are those NEOs in heliocentric orbits similar to Earths). One of the significant advantages of this type of mission is that it strengthens and validates the foundational infrastructure of the United States Space Exploration Policy and is highly complementary to the already-planned lunar sorties and outpost build-up circa 2020. Sending a human expedition to a NEO not only underlines the broad utility of the CxPs Orion vehicle and Ares launch systems. Such a mission would also be the first human expedition to an interplanetary body beyond the Earth-Moon system. For the onboard crew and systems, as well as the mission control team, these deep space operations will present unique challenges not present in lunar missions. While our Phase 1 study focused solely on the practicality of using the lunar architecture and systems to mount NEO missions, it did not delve into potential radiation issues (and effective mitigation strategies) nor did it explore human operations in proximity to and on the surface of NEOs. Executing several such piloted NEO missions will enable NASA to gain crucial long-duration, deep space operational experience, a necessary prerequisite for future human missions to Mars, its moons, Phobos and Deimos, or even the Main Belt or Trojan asteroids.