Nigel Johnston

Theoretical and experimental studies of a switched inertance hydraulic system

A switched inertance hydraulic system uses a fast switching valve to control flow or pressure and is potentially very efficient as it does not rely on dissipation of power by throttling. This article studies its performance using an analytical method which efficiently describes the system in the time domain and frequency domain. A lumped parameter model and a distributed parameter model have been used for investigation using different parameters and conditions. The analytical models have been validated in experiments and the results on a prototype device show a very promising performance. The proposed analytical models are effective for understanding, analysing and optimizing the characteristics and performance of a switched inertance hydraulic system.

An electro-hydrostatic actuator for hybrid active-passive vibration isolation

A novel electro-hydrostatic actuator (EHA) for active vibration isolation has been designed, modelled and tested. The EHA consists of a brushless DC motor running in oil and integrated with a bidirectional gear pump, driving a hydraulic cylinder. The actuator is designed to be integrated into a flexible strut connecting a helicopter rotor hub and fuselage, to provide isolation at the dominant rotor vibration frequency of around 20 Hz. The resonant frequency of the EHA is tuned to provide some passive vibration isolation. Active control increases the isolation performance by compensating for damping losses, and provides isolation over a broader range of frequencies. Tests on a prototype demonstrated a four-fold reduction of the root-mean-square transmitted force and a near elimination at the fundamental frequency. The advantages of the resonant EHA are a wider range of operating frequencies than a purely passive system, and lower power consumption than a purely active system.

Experimental Investigation of a Switched Inertance Hydraulic System With a High-Speed Rotary Valve

This paper reports on experimental investigations of a switched inertance hydraulic system (SIHS), which is designed to control the flow and pressure of a hydraulic supply. The switched system basically consists of a switching element, an inductance (inertance), and a capacitance. Two basic modes, a flow booster and a pressure booster, can be configured in a three-port SIHS. It is capable of boosting the pressure or flow with a corresponding drop in flow or pressure, respectively. This technique makes use of the inherent reactive behavior of hydraulic components. A high-speed rotary valve is used to provide sufficiently high switching frequency and to minimize the pressure and flow loss at the valve orifice, and a small diameter tube is used to provide an inductive effect. In this paper, a flow booster is introduced as the switched system for investigation. The measured steady-state and dynamic characteristics of the rotary valve are presented, and the dynamics characteristics of the flow booster are investigated in terms of pressure loss, flow loss, and system efficiency. The speed of sound is measured by analysis of the measured dynamic pressures in the inertance tube. A detailed analytical model of an SIHS is applied to analyze the experimental results. Experimental results on a flow booster rig show a very promising performance for the SIHS.

The transmission line method for modelling laminar flow of liquid in pipelines

The transmission line method is a very efficient technique for dynamic modelling of flow in pipelines, and uses delay elements to represent wave propagation. In this paper, an existing transmission line method is investigated and shown to have some deficiencies. Some adjustments are proposed to avoid these deficiencies and enhance the transient and steady-state accuracy. Very good agreement is obtained between this adjusted transmission line method and an analytical model. The method has been implemented in simulations of a number of highly dynamic systems, and has been found to be robust and reliable.

Active Control of Pressure Pulsation in a Switched Inertance Hydraulic System

The nature of digital hydraulic systems may cause severe pressure pulsation problems. For example, switched inertance hydraulic systems can be used to adjust or control flow and pressure by a means that does not rely on dissipation of power, but they have noise problems because of the pulsed nature of the flow. An effective method to reduce the noise is needed that does not impair the system performance and efficiency. This article reports on an initial investigation of an active attenuator for pressure pulsation cancellation in a switched inertance hydraulic system. Using the designed noise attenuator, the pressure pulsation can be decreased effectively by superimposing an anti-phase control signal. A high-performance piezoelectric valve was selected and used as the secondary path actuator in terms of its fast response and wide bandwidth. Adaptive notch filters with the filtered-X least mean square algorithm were applied for pressure pulsation attenuation, while a frequency-domain least mean square filter was used for secondary path identification. A ‘switched inertance hydraulic system’ in a flow booster configuration was used as the test rig. Experimental results show that excellent cancellation was achieved using the proposed method, which has several advantages over passive noise control systems, being effective for a wide range of frequencies without impairing the system’s dynamic response. The method is a very promising solution for pressure pulsation cancellation in hydraulic systems with severe noise or vibration problems.

An enhanced transmission line method for modelling laminar flow of liquid in pipelines

The transmission line method is a very efficient method for dynamic modelling of flow in pipelines and uses delay elements to represent wave propagation. In this article, an existing transmission line method model is investigated and shown to have some deficiencies. An alternative technique is introduced to enhance the transient and steady-state accuracy. Extremely good agreement is obtained between this new transmission line method and an analytical model. The model has been implemented in simulation of a number of highly dynamic systems and has been found to be robust and reliable.

Theoretical and experimental studies of a switched inertance hydraulic system including switching transition dynamics, non-linearity and leakage

This article reports on theoretical and experimental investigations of a switched inertance device, which is designed to control the flow and pressure of a hydraulic supply. The device basically consists of a switching element, an inductance and a capacitance. It is able to boost the pressure or flow with a corresponding drop in flow or pressure, respectively, analogous to a hydraulic transformer. In this article, an enhanced analytical distributed parameter model in the frequency domain, which includes the effect of switching transition, non-linearity and leakage of the valve, is proposed and validated by simulation and experiments. A flow booster test rig is studied as a typical system. Simulated and experimental results show good performance, and accurate estimation of system pressure and dynamic flowrate can be obtained using the enhanced analytical model. The model is very effective for understanding, analysing and optimising the characteristics and performance of a switched inertance device. It can also be used to aid in the design of a switched inertance hydraulic system.

Use of Pipeline Wave Propagation Model for Measuring Unsteady Flow Rate

A novel method for estimation of unsteady flowrate using pressure at two or three points along a pipeline is described in this paper. The pressure data are processed using a wave propagation model to determine the unsteady flow. Computer simulations show that the proposed method is effective for unsteady flowrate measurement to a high bandwidth. However, if the pressure values from two transducers are used, inaccuracies exist at certain frequencies when the transducer spacing coincides with multiples of half a wavelength. The accuracy can be improved by adding a third transducer with unequal spacing. The comparison and analysis of two-transducer and three-transducer techniques are investigated through simulation. This method may be applied to real time flowrate measurement, control systems or active noise cancellation systems.

Development Of A Position Controlled Digital Hydraulic Valve

Digital hydraulics is a key part of the continuing applicability of fluid power in the modern world. In order to realize the potential of digital hydraulic circuits, valves which are able to switch at high frequencies whilst retaining high flows are first required. This paper details the development of a valve which is capable of switching in 0.5ms whilst providing a flow rate of over 50L/min at a 10bar pressure drop. Unlike most of the other valves currently in development, position control is used as opposed to the more common bang-bang actuation. This has obvious benefits for valve robustness and offers the possibility of a hybrid control approach which utilizes both throttling and switching control. This paper will detail the design and empirical testing of the valve and benchmark it against published and commercial valves before proceeding to discuss the challenges present in developing the valve further.Copyright

Experimental investigation of a switched inertance hydraulic system

This article reports on experimental investigations of a switched inertance hydraulic system (SIHS), which is designed to control the flow and pressure of a hydraulic supply. The switched system basically consists of a switching element, an inductance and a capacitance. Two basic modes, a flow booster and a pressure booster, can be configured in a three-port SIHS. It is capable of boosting the pressure or flow with a corresponding drop in flow or pressure respectively. This technique makes use of the inherent reactive behaviour of hydraulic components. A high-speed rotary valve is used to provide sufficiently high switching frequency and minimise the pressure and flow loss at the valve orifice, and a small diameter tube is used to provide an inductive effect. In this article, a flow booster is introduced as the switched system for investigation. The measured steady state and dynamic characteristics of the rotary valve are presented, and the dynamics characteristics of the flow booster are investigated in terms of pressure loss, flow loss and system efficiency. The speed of sound is measured by analysis of the measured dynamic pressures in the inertance tube. A detailed analytical model of a SIHS is applied to analyse the experimental results. Experimental results on a flow booster rig show a very promising performance for the SIHS.Copyright