Anthony C. Zander

A multipole array magnetic spring

This paper presents research on a magnetic spring concept, which has application to the development of a vibration isolation table. Features of the design are scalable, noncontact load bearing and a single degree of instability.

A Review of Virtual Sensing Algorithms for Active Noise Control

Traditional local active noise control systems minimise the measured acoustic pres- sure to generate a zone of quiet at the physical error sensor location. The resulting zone of quiet is generally limited in size and this requires the physical error sensor be placed at the desired location of attenuation, which is often inconvenient. To overcome this, a number of virtual sensing algorithms have been developed for active noise control. Using the physical error signal, the control signal and knowledge of the system, these virtual sensing algorithms estimate the error signal at a location that is remote from the physical error sensor, referred to as the virtual location. Instead of minimising the physical error signal, the estimated error sig- nal is minimised with the active noise control system to generate a zone of quiet at the virtual location. This paper will review a number of virtual sensing algorithms developed for active noise control. Additionally, the performance of these virtual sensing algorithms in numerical simulations and in experiments is discussed and compared.

Active noise control in a pure tone diffuse sound field using virtual sensing

Local active noise control systems generate a zone of quiet at the physical error sensor using one or more secondary sources to cancel acoustic pressure and its spatial derivatives at the sensor location. The resulting zone of quiet is generally limited in size and as such, placement of the error sensor at the location of desired attenuation is required, which is often inconvenient. Virtual acoustic sensors overcome this by projecting the zone of quiet away from the physical sensor to a remote location. The work described here investigates the effectiveness of using virtual sensors in a pure tone diffuse sound field. Stochastically optimal virtual microphones and virtual energy density sensors are developed for use in diffuse sound fields. Analytical expressions for the controlled sound field generated with a number of control strategies are presented. These expressions allow the optimal control performance to be predicted. Results of numerical simulations and experimental measurements made in a reverberation chamber are also presented and compared.

A Simplified Force Equation for Coaxial Cylindrical Magnets and Thin Coils

A recently-published equation for calculating the force between coaxial cylindrical magnets is presented in simplified form. The revised equation is now very compact: it is defined with fewer parameters and contains fewer terms than the original equation. The new equation is purely real, unlike the original which contained imaginary components. As a result of the simplifications, the new equation is demonstrably faster to evaluate than the original, improving its utility for parametric optimization. A reference implementation is provided for Matlab and Mathematica.

A moving zone of quiet for narrowband noise in a one-dimensional duct using virtual sensing

A frequent problem in active noise control is that the zone of quiet created at the error sensor tends to be very small. This means that the error sensor generally needs to be located close to an observers ear, which might not always be a convenient or feasible solution. Virtual sensing is a method that can move the zone of quiet away from the error sensor to a desired location that is spatially fixed. This method has been investigated previously, and has shown potential to improve the performance of an active noise control system. However, it is very likely that the desired location of the zone of quiet is not spatially fixed. An active noise control system incorporating a virtual sensing method thus has to be able to create a moving zone of quiet that tracks the observers ears. This paper presents a method for creating a moving zone of quiet based on the LMS virtual microphone technique. To illustrate the proposed method, it is implemented in an acoustic duct and narrowband control results are presented. These results show that a moving zone of quiet was effectively created inside the duct for narrowband noise.

Nonlinear Dynamics of Magnetic Bearing Systems

Magnetic bearings use magnetic forces to support various machine components. Because of the non-contact nature of this type of suspension, magnetic bearing technology offers a number of significant advantages over conventional bearings, such as rolling element and fluid film bearings. An active magnetic bearing basically consists of an electromagnetic actuator, position sensors, power amplifiers, and a feedback controller. All of these components are characterized by nonlinear behavior and therefore the entire system is inherently nonlinear. However, in simulations of the dynamic behavior of magnetic bearing systems, the nonlinearities are usually neglected to simplify the analysis and only linear models are used. Moreover, many control techniques currently used in magnetic bearing systems are generally designed by ignoring nonlinear effects. The main reason for simplification is the intractability of the complexity of the actual model. In fact, the inherent nonlinear properties of magnetic bearing systems can lead to dynamic behavior of a magnetically suspended rotor that is distinctly different from that predicted using a simple linearized model. Therefore, the nonlinearities should be taken into account. This literature review is focused on the nonlinear dynamics of magnetic bearing systems and it provides background information on analytical methods, nonlinear vibrations resulting from a rotor contacting auxiliary bearings, and other active topics of research involving the nonlinear properties of magnetic bearing systems, such as nonlinear self-sensing magnetic bearings and nonlinear control of magnetic bearings. The review concludes with a brief discussion on current and possible future directions for research on the nonlinear dynamics of magnetic bearing systems.

A comparison of error sensor strategies for the active control of duct noise

A theoretical comparison of a number of error sensor strategies is undertaken for active control of periodic noise in a hard‐walled rectangular duct. Error sensor strategies investigated include the minimization of pressure at a point, the total real acoustic power output, and an estimate of the acoustic potential energy. A new error sensor strategy is also proposed and investigated. The strategy is based upon minimization of the power flow determined by a modal decomposition of the duct sound field. The error sensor strategies are analyzed for both plane‐wave and multimode sound fields, and for a range of duct termination conditions. The criterion used to assess the error sensor strategies is the minimization of the sound field downstream of the control sources.

Axial Force Between a Thick Coil and a Cylindrical Permanent Magnet: Optimizing the Geometry of an Electromagnetic Actuator

In this paper, a variety of analytical/integral methods are compared for calculating the axial force between a cylindrical magnet and a “thick” solenoid that consists of many turns both radially and axially. Two newly developed techniques are introduced: one being numerical integration-based and the other completely analytical. These are compared to two other techniques, each shown to have various advantages in different contexts. One method in particular is introduced that is shown to be the most computationally efficient in the majority of actuator designs. This method is then used to optimize a typical “sleeve-type” magnet-coil actuator based on the cost function of peak force, and it is shown that optimal values of wire thickness and magnet-coil geometry can be chosen based on desired coil impedance and magnet volume.

Quantification of the ultrasound induced sedimentation of Microcystis aeruginosa

It has been known for more than 40 years that vacuolate organisms can be induced to sediment with ultrasound. However, robust indicators are still needed to compare the efficacy of different treatments. A repeatable index is proposed that makes it possible to quantify the ultrasonic induced sedimentation. The procedure is used to monitor the long term sedimentation of Microcystis aeruginosa after sonication. Results reveal that the sedimentation process continues after gas vesicles have fully recovered, although at a slower rate.

Airfoil Trailing Edge Noise Source Location at Low to Moderate Reynolds Number

A study of the effects of Reynolds number and angle of attack on the dominant acoustic source location relative to the trailing edge for airfoils at low-to-moderate Reynolds number is presented. This study, which was performed using acoustic beamforming in an anechoic wind tunnel, helps assess the influence of each test parameter on the distance of the acoustic source from the trailing edge thus improving our knowledge of how airfoils produce tonal noise. The results show that the acoustic source location of a NACA0012 airfoil varies along the chord for the frequency range 500 Hz to 3000 Hz at low-to-moderate Reynolds number. Changing the angle of attack and Reynolds number resulted in small changes to the acoustic source location within this frequency range. A modified flat plate, which only radiated broadband noise under similar flow conditions, presented a scattered distribution of source location between the range 300 Hz to 1200 Hz.