Ilias Zazas

Harmonic control of a ‘smart spring’ machinery vibration isolation system

A major problem for isolating vibration from large marine machinery rafts is how best to deal with excited resonances. These generate large forces on the hull that create a significant vibration problem. The passive design of such mounts typically represents a compromise between providing good vibration isolation and good machinery alignment under seaway motion. The ‘Smart Spring’ isolation system, a new hybrid passive—active approach to solving this problem that is being developed by BAE Systems, has been described in a series of earlier papers. The fundamental concept utilizes digitally controlled actuators to apply forces that are independent of local displacement while controlling the response of the structures rigid body modes. The current paper describes recent work to extend the functionality of the isolation system to deal specifically with discrete-frequency vibration sources. Two novel harmonic control strategies are introduced and evaluated using a six-degrees-of-freedom experimental active mount. It is shown that a strategy that employs recursive least-squares estimation provides both exceptionally high isolation performance and rapid convergence.

Experimental validation of a geometric method for the design of stable and broadband vibration controllers using a propeller blade test rig

A systematic geometric design methodology to generate a stable controller for simultaneous local and remote attenuation that was previously proposed is experimentally validated on a structure. The local control path transfer function for this experimental system is non-minimum phase due to which the original broadband controller design would yield an unstable controller. Here a modified procedure for systems with local non-minimum phase dynamics is used to generate a stable controller. According to this method, reduction in vibration at local and remote points on a structure can be parameterised in terms of the available design freedom and a controller is realised in terms of the optimal selection of this using the minimum phase counterpart of the local control path transfer function. The modified method results in a controller that is both stable and stabilizing and which achieves the desired vibration attenuation at the local and remote points on the structure. An experimental facility that replicates the vibration transmission through the shaft of a propeller blade rig system is used to demonstrate the method. Vibration for excitation near the first bending mode frequency of the resonating part of this structure is attenuated at the non-resonating part of the system without deteriorating vibration at the resonating end.

Active control of a frequency varying tonal disturbance by a nonlinear optimal controller with frequency tracking

Abstract In this paper a method for suppressing frequency varying tonal disturbances is presented. In most of such processes additional components are required to be installed within the process to measure the disturbance frequency. The proposed nonlinear optimal controller is combined with an RLS-based frequency tracking algorithm. The resulting controller is adaptive in terms of the disturbance frequency and can act as a stand-alone solution for the suppression of tonal disturbances, requiring only measurements from the process output. The performance of the resulting controller is validated by implementation on a process emulating the problem of isolating vibration using flexible raft structures.

Stability and convergence analysis for different harmonic control algorithm implementations

In many engineering systems there is a common requirement to isolate the supporting foundation from low frequency periodic machinery vibration sources. In such cases the vibration is mainly transmitted at the fundamental excitation frequency and its multiple harmonics. It is well known that passive approaches have poor performance at low frequencies and for this reason a number of active control technologies have been developed. For discrete frequencies disturbance rejection Harmonic Control (HC) techniques provide excellent performance. In the general case of variable speed engines or motors, the disturbance frequency changes with time, following the rotational speed of the engine or motor. For such applications, an important requirement for the control system is to converge to the optimal solution as rapidly as possible for all variations without altering the systems stability. For a variety of applications this may be difficult to achieve, especially when the disturbance frequency is close to a resonance peak and a small value of convergence gain is usually preferred to ensure closed-loop stability. This can lead to poor vibration isolation performance and long convergence times. In this paper, the performance of two recently developed HC algorithms are compared (in terms of both closed-loop stability and speed of convergence) in a vibration control application and for the case when the disturbance frequency is close to a resonant frequency. In earlier work it has been shown that both frequency domain HC algorithms can be represented by Linear Time Invariant (LTI) feedback compensators each designed to operate at the disturbance frequency. As a result, the convergence and stability analysis can be performed using the LTI representations with any suitable method from the LTI framework. For the example mentioned above, the speed of convergence provided by each algorithm is compared by determining the locations of the dominant closed-loop poles and stability analysis is performed using the open-loop frequency responses and the Nyquist criterion. The theoretical findings are validated through simulations and experimental analysis.

Design of Stable and Broadband Remote Vibration Controllers for Systems With Local Nonminimum Phase Dynamics

A geometric-based methodology that was recently proposed provides a systematic controller design approach for controlling remote vibration at multiple points using only a restricted number of sensors and actuators. Valuable physical insight into the existence of control solutions for vibration attenuation at multiple locations is retained with this approach in contrast to alternatives, such as H2 and H∞ methods. A drawback of the existing geometric design approach is that the controller implementation for the broadband case incorporates an inverted local control path transfer function. When the sensor and actuator are noncollocated or when there is significant latency or phase lag in the system, the local control path model will have nonminimum phase characteristics. Therefore, the resulting controller for this situation will itself be unstable due to the inclusion of an inverted nonminimum phase transfer function. In this brief paper, a systematic procedure is presented, which extends the previous work and which yields both a stable and stabilizing controller without requiring a minimum phase control path assumption. Furthermore, robustness against control spillover at out-of-band frequencies is incorporated within this modified design procedure without deteriorating controller performance within the design bandwidth. The detailed control design procedure is illustrated using a simulated beam vibration problem. Finally, the design approach is experimentally validated using a test rig that replicates the problem of vibration transmission in rotary propulsion systems.

Noise suppression using local acceleration feedback control of an active absorber

A popular approach for active noise control problems has been the use of the adaptive filtered-X least mean square algorithm. A fundamental problem with feedforward design is that it requires both reference and error sensors. In order to reduce the size, cost and physical complexity of the control system, a feedback controller can be utilised. In contrast to filtered-X least mean square, a feedback controller utilises local acceleration measurements of a sound-absorbing surface instead of global pressure measurements. Most control problems, including active noise control, can be formulated in the general control configuration architecture. This type of architecture allows for the systematic representation of the process and simplifies the design of a vast number of controllers that include H ∞ and controllers. Such controllers are considered ideal candidates for active noise control problems as they can combine near-optimal performance with good robustness characteristics. This article investigates the problem of reflected noise suppression in acoustic ducts and the possibilities and trade-offs of applying H 2 control strategies. Hence, by controlling locally the reflecting boundary structure, a global cancellation of the undesired noise can be accomplished. In this article, the H 2 local feedback control strategy and performance are investigated using an experimental pulse tube. The H 2 design was chosen because it was able to provide consistently a stable response in contrast to the design.