Ola Härkegård

Resolving actuator redundancy-optimal control vs. control allocation

This paper considers actuator redundancy management for a class of overactuated nonlinear systems. Two tools for distributing the control effort among a redundant set of actuators are optimal control design and control allocation. In this paper, we investigate the relationship between these two design tools when the performance indexes are quadratic in the control input. We show that for a particular class of nonlinear systems, they give exactly the same design freedom in distributing the control effort among the actuators. Linear quadratic optimal control is contained as a special case. A benefit of using a separate control allocator is that actuator constraints can be considered, which is illustrated with a flight control example.

Efficient active set algorithms for solving constrained least squares problems in aircraft control allocation

In aircraft control, control allocation can be used to distribute the total control effort among the actuators when the number of actuators exceeds the number of controlled variables. The control allocation problem is often posed as a constrained least squares problem to incorporate the actuator position and rate limits. Most proposed methods for real-time implementation, like the redistributed pseudoinverse method, only deliver approximate, and sometimes unreliable solutions. We investigate the use of classical active set methods for control allocation. We develop active set algorithms that always find the optimal control distribution, and show by simulation that the timing requirements are in the same range as for two previously proposed solvers.In aircraft control, control allocation can be used to distribute the total control effort among the actuators when the number of actuators exceeds the number of controlled variables. The control allocation problem is often posed as a constrained least squares problem to incorporate the actuator position and rate limits. Most proposed methods for real-time implementation, like the redistributed pseudoinverse method, only deliver approximate, and sometimes unreliable solutions. We investigate the use of classical active set methods for control allocation. We develop active set algorithms that always find the optimal control distribution, and show by simulation that the timing requirements are in the same range as for two previously proposed solvers.

Dynamic Control Allocation Using Constrained Quadratic Programming

Control allocation deals with the problem of distributing a given control demand among an available set of actuators. Most existing methods are static in the sense that the resulting control distribution depends only on the current control demand. In this paper we propose a method for dynamic control allocation, in which the resulting control distribution also depends on the distribution in the previous sampling instant. The method extends the traditional generalized inverse method by also penalizing the individual actuator rates. Its main feature is that it allows for different control distributions during the transient phase of a maneuver and during trimmed flight. The control allocation problem is posed as a constrained quadratic programming problem which provides automatic redistribution of the control effort when one actuator saturates in position or in rate. When no saturations occur, the resulting control distribution coincides with the control demand fed through a linear filter which can be assigned different frequency characteristics for different actuators.

Flight Control Design using Backstepping

Todays prevailing nonlinear design method for aircraft flight control is feedback linearization. This paper presents a new method to deal with the nonlinear aerodynamic forces and moments acting on ...

A backstepping design for flight path angle control

A nonlinear approach to flight path angle control is presented. Using backstepping, a globally stabilizing control law is derived. Although the nonlinear nature of the lift force is considered, the pitching moment to be produced is only linear in the measured states. Thus, the resulting control law is much simpler than if feedback linearization had been used. The free parameters that spring from the backstepping design are used to achieve a desired linear behavior around the operating point.A nonlinear approach to flight path angle control is presented. Using backstepping, a globally stabilizing control law is derived. Although the nonlinear nature of the lift force is considered, the pitching moment to be produced is only linear in the measured states. Thus, the resulting control law is much simpler than if feedback linearization had been used. The free parameters that spring from the backstepping design are used to achieve a desired linear behavior around the operating point.

Backstepping control of a rigid body

A method for backstepping control of rigid body motion is proposed. The control variables are torques and the force along the axis of motion. The proposed control law and Lyapunov function guarantee asymptotic stability from all initial values except one singular point.

Vector backstepping design for flight control

A backstepping design for an aircraft described by rigid body dynamics is proposed. The system states are the unit velocity vector and the angular velocity in a body-fixed coordinate system. The res ...

Command Governor Approach to Maneuver Limiting in Fighter Aircraft

Modern fighter aircraft require maximum control performance in order to have the upper hand in a dogfight or when they have to outmaneuver an enemy missile. Therefore pilots must be able to maneuver the aircraft very close to the limit of what it is capable of while at the same time focus on the tactical tasks of the mission. To enable this, modern flight control systems have automatic systems for angle of attack and load factor limiting. These types of systems can utilize predictions of the aircraft response to pilot inputs and alter the properties of the closed loop system to minimize the predicted overshoot. Two such design techniques are model predictive control and reference and command governors. Model predictive controllers are most often used as inner loop feedback controllers which alter the control signal as function of the predicted output while reference and command governors are applied in an outer feedback loop around a nominal controller. There can be several benefits from using reference and command governors compared to model predictive controllers. First, the governors can be used as add-ons to existing legacy controllers so there is no need to redo the complete design. Furthermore the nominal inner loop controller can be tuned to achieve good performance in the nominal case, e.g., use nonlinear feedbacks to linearize the closed loop system, and the governor focus on the maneuver limiting task. It also gives a good modularity such that one can replace parts of the control system without the need to redo all of the design. Last but not least from a flight safety perspective it might be easier to certify optimization algorithms running in an outer loop which can be turned off in case of failures without affecting stability. While model predictive controllers have been extensively investigated for flight control applications [1–28] most of them consider reconfigurable flight control systems and only few focus on envelope protection and maneuver limiting [7, 13, 17, 21]. Even though reference governors have been subject to research for quite some time very little research has been performed on applying reference and command governors to flight control design and maneuver limiting [23, 29–33]. Most of these papers consider simplified conditions with only a single linear or nonlinear system and no complex simulation environments. In the papers by Petersen et al. [23] and Zinnecker et al. [29] the authors apply reference governors to the control of hypersonic vehicle. In the paper by Zinnecker the focus is mainly on input constraints. Kolmanovsky and Kahveci [30] uses a reference governor to handle control actuator limitations of a UAV glider and compare this to an adaptive anti-windup scheme and in the paper by Martino [31] the author investigates command governors for handling amplitude and rate constraints on a small commercial aircraft. The authors, Ye et al. [32], investigate reference governors for maneuver limiting in high angle of attack maneuvers. They investigate and compare static and dynamic reference governors with a reference governor structure based on a step response

Angle of Attack and Load Factor Limiting in Fighter Aircraft using Command Governors

Modern fighter aircraft require maximum control performance in order to have the upper hand in a dogfight or when they have to outmaneuver an enemy missile. Therefore pilots must be able to maneuve ...