Fluid-borne noise is one of the main components of hydraulic noise. Its attenuation may have a significant effect on the cost of hydraulic systems. Standard passive silencers and dampers can be useful in reducing it in certain frequency ranges; however, these tend to be heavy, bulky and expensive. Active control algorithms, which are a comparatively recent means of reducing fluid-borne noise, can be applied to overcome this compromise. The work presented in this thesis is the development of some active control algorithms utilized in a simple hydraulic system to cancel a number of harmonic orders of fluid-borne noise generated by a servo valve or a real pump. To realize cancellation the filtered reference least mean square (FXLMS) adaptive control method is mainly presented. Furthermore, a fast response servo valve is applied as an actuator to generate a proper anti-noise flow signal in real-time. For simplicity, an off-line identification method for the secondary path is applied in the time invariant working condition. Moreover, ripple reflection from both ends of the hydraulic circuit can produce different effects under different working conditions. In order to execute the cancellation without any prior information about the dynamics of hydraulic systems, the on-line secondary path identification method is discussed. However, in this algorithm an auxiliary white-noise signal applied to an on-line method may contribute to residual noise and an extra computation burden may be added to the whole control system. The performance of these control algorithms is firstly investigated via simulation in a hydraulic pipe model and the real-time application on a test rig using a servo valve as a noise source. Finally, these schemes are realized in a simple hydraulic system with a real pump noise source. The fluid-borne noise can be attenuated by about 20 dB in normal working conditions.

Excessive fluid-borne noise in hydraulic systems is a problem the fluid power industry has long struggled to address. Traditional noise control devices such as Helmholtz resonators, tuning coils, and Herschel-Quincke tubes are generally too large for fluid power systems unless the speed of sound in the device can be reduced. A compliant lining can achieve this effect, but compliance (and lossy compliance) has had little attention in noise control in general, and in fluid power in particular. One means to achieve compliance in these devices, especially at elevated pressures, is through a liner made of syntactic foam, which in this case is a urethane host matrix with embedded hollow, polymer microspheres. The material properties at elevated pressure are unknown by the liner manufacturer, but are known to be pressure- and temperature-dependent. Therefore, the effect of hydrostatic pressures from 2.1-21 MPa and temperatures from 20-45 C on the liner properties, thus the device performance, are studied. For a Helmholtz resonator, a theoretical model is fit to experimentally-measured transmission loss of the device using a least-squares routine, which solves the inverse problem for the complex bulk modulus of the liner. These material properties are used to compare a predictive model of a tuning coil to experimental data, and in a parameter study of a Herschel-Quincke tube. The compliance of the liner is found to lower the effective sound speed by an order of magnitude and decrease the volume of the cavity of a Helmholtz resonator by up to two orders of magnitude. This work is expected to result is more compact noise control devices for fluid power systems.

The aviation environment holds challenging application constraints for efficient hydraulic system noise reduction devices. Besides strong limits on component weight and size, high safety and reliability standards demand simple solutions. Hence, basic silencers like inline expansion chambers and Helmholtz-Resonators are state-of-the-art aboard commercial aircrafts. Unfortunately, they do not meet today’s noise attenuation aims regarding passenger comfort and equipment durability. Significant attenuation performance is expected from active concepts that generate anti-phase noise. However, such concepts remain a long term approach unless related costs, e.g. due to additional power allocation and real-time control equipment can be avoided. In this paper an active fluid borne noise attenuation concept is discussed that accounts for the mentioned constraints. An aircraft hydraulic pump is considered as main noise source. The active attenuator is an in-house rotary valve design. The basic feature is a known direct shaft coupling principle of pump and rotary valve, so no speed/ frequency control of the valve and no separate power supply are required. The common-shaft principle is further simplified here and proposed as integral feature of future “smart pumps”.

The aviation environment holds challenging application constraints for efficient hydraulic system noise reduction devices. Besides strong limits on component weight and size, high safety and reliability standards demand simple solutions. Hence, basic silencers like inline expansion chambers and Helmholtz-Resonators are state-of-the-art aboard commercial aircrafts. Unfortunately, they do not meet today’s noise attenuation aims regarding passenger comfort and equipment durability. Significant attenuation performance is expected from active concepts that generate anti-phase noise. However, such concepts remain a long term approach unless related costs, e.g. due to additional power allocation and real-time control equipment can be avoided. In this paper an active fluid borne noise attenuation concept is discussed that accounts for the mentioned constraints. An aircraft hydraulic pump is considered as main noise source. The active attenuator is an in-house rotary valve design. The basic feature is a known direct shaft coupling principle of pump and rotary valve, so no speed/ frequency control of the valve and no separate power supply are required. The common-shaft principle is further simplified here and proposed as integral feature of future “smart pumps”.