George Piper

Complicated dynamics in spacecraft attitude control systems

It is shown that a commonly used, momentum exchange, spacecraft attitude control configuration gives rise to complex nonlinear behavior involving multiple limit cycles and strange attractors. However, if the momentum wheel assembly performance parameters are adequately matched to the spacecraft it is possible to achieve a globally stable equilibrium. The results are based on the analysis of a simple single axis problem.

/spl Hscr//sub /spl infin// active noise control of fan noise in an acoustic duct

In this paper, the problem of reducing fan related noise in an acoustic duct is considered. By installing magnetic bearings on the noise producing machinery, it is possible to collocate the anti-noise source with the disturbance noise source. This approach allows for global noise reduction through out the duct system. Using /spl Hscr//sub /spl infin// control theory, an active noise controller is designed that attains broadband as well as tonal noise reduction at all points along the duct. The controller design is based on a state space model identified from an infinite dimensional physical model. Simulation results demonstrate the global nature of this novel active noise control approach.

Control of time-delayed double integrator systems

The control of system with large time-delays is a very challenging problem. As Proportional-Integral-Derivative (PID) controllers and variants of these such as P, PI, or PD are widely used in industry, there has been extensive work to determine the range of their gains that will stabilize a linear time-invariant plant described by a rational transfer function. However, the extension of this work to systems with time-delays has been difficult. In this paper, the control of double integrators, a particular class of secondorder systems, with time-delays will be considered. Double integrator systems are a simple, but important, class of second order systems, as they model single-degree of freedom translational and rotational systems. The study of time-delayed double integrator systems in this paper is motivated by the formation flying challenge. In this paper, the ability for tight formation control in the presence of communication and measurement delays is investigated. The paper begins with a discussion of the cross-link requirements for formation control. A simple two degreeof-freedom model with communication delays is introduced for analysis. A typical approach for systems with small time-delays is to base the design on a nominal system model that does not contain the time-delays. The limitations of this approach on stability and closed-loop bandwidth are discussed. The time-delay is explicitly considered in the design of state-feedback (PD) controllers. The effects of the time-delay on stability, stability margins, and closed-loop bandwidth are investigated.

On the use of magnetic bearings for global control of fan noise

In this paper, a novel approach is presented to reduce fan related noise in an acoustic duct. By installing magnetic bearings on the noise producing machinery, it is possible to collocate the anti-noise source with the disturbance noise source. This approach allows for global noise reduction throughout the duct system.

Active Control of Axial-flow Fan Noise Using Magnetic Bearings

In this paper we present a novel approach to reducing blade-rate noise of axial-flow fans. By using magnetic bearings as a noise control actuator, it is possible to collocate the anti-noise source with the disturbance noise source. This approach allows for global noise reduction throughout the sound field. A DC motor connected to a fan by a short rigid shaft was used to demonstrate this approach. The shaft was supported in the radial and axial directions by magnetic bearings. The bearings provide position control of the shaft and fan; this position control can be used to vibrate the fan at a desired frequency and amplitude. Controlled vibration of the fan allows its use as a speaker in an active noise control scheme. Noise control was implemented on a dedicated digital signal processor using a least mean square algorithm. The output of the noise control algorithm supplies the position commands for the magnetic bearing controller. Experimental data showed that by actuating the axial thrust bearing, the noise output of a fan could be reduced by 4 dB at the error microphone, and 3 dB at points away from the error microphone.

ON THE IMPACT OF CROSS-LINK DELAYS ON SPACECRAFT FORMATION CONTROL

In this paper, the ability to perform tight formation control in the presence of communication and measurement delays is investigated. The paper discusses cross-link requirements for formation control and compares centralized and decentralized control strategies. A simple two degree-of-freedom model with communication delays is introduced for analysis. A typical approach for systems with small time delays is to base the design on a nominal system model that does not contain the time-delays. The limitations of this approach on stability and closed -loop bandwidth are discussed. Finally, the time delay is considered in the design of state-feedback with integral feedback controllers. The effects of the time -delay on stability, stability margins, and bandwidth are investigate d. An example illustrating these effects is presented.

An undergraduate course in active magnetic levitation: bridging the gap

This paper describes an undergraduate course in active magnetic levitation that was designed to engage the students interest while bridging the gap between the mathematical and the physical world. The objective of this course was to take the students through the complete design process of a nontrivial control system prior to their capstone design course. The course focused on a project where the students modeled, analyzed, simulated, designed, and implemented, an active magnetic levitation control system. The paper discusses the pedagogical objectives of the course, an overview of the course, and highlights of several of the labs in the course.

A systems engineering approach to teaching energy

In this paper we discuss the energy systems portion of an environmental engineering sequence for nonenvironmental engineering students. Energy is approached from the prospective of a systems engineer, who is concerned with the overall design of an energy system. Through novel laboratory exercises and projects, students become familiar with the various components of energy systems such as energy sources, conversion, storage, and utilization. They also get acquainted with sizing energy systems and the economics of energy.

ON THE CONTROL DESIGN OF THE DISTURBANCE REDUCTION SYSTEM FOR THE LISA MISSION

The Laser Interferometer Space Antenna (LISA) mission will consist of a three spacecraft formation acting as a large interferometer. The spacecraft formation will measure the distortion of space caused by passing gravitational waves. Each spacecraft will contain two shielded proof masses that define the optical arms of the interferometer. To mitigate the effects of external disturbances, the LISA Disturbance Reduction System will control the spacecraft to fly drag-free trajectories with reference to their respective proof masses. This paper investigates control design issues associated with the LISA Disturbance Reduction System (DRS). In particular, this paper presents a 2 degrees-of-freedom (DOF) model representing a spacecraft with a single proof mass. With this model, key control design issues are identified. A Linear Quadratic Gaussian (LQG) controller design is then presented to illustrate the feasibility of the DRS control concept. A 3 DOF model is used to explore the DRS implications of a spacecraft with two proof masses.

Systems Ball - a creative capstone design experience

This paper describes a design competition called Systems Ball that has been successfully used in the Systems Engineering design course sequence at the United States Naval Academy. In the Systems Ball competition, student teams build remotely operated vehicles that partake in a combative version of basketball. Systems Ball has been found to be an effective and fun way to guide multiple student teams through the design process while stimulating their creativity.