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.
Piezo pumps provide an attractive alternative for driving small actuators (e.g. less than 100W) compared to traditional valve controlled cylinders powered by a central hydraulic supply. This provides the ability to distribute power electrically rather than hydraulically, which can bring both weight and efficiency savings. Currently the use of piezo pumps is severely limited by the maximum power and flows that can be provided. This paper documents the simulation of a new pump which makes use of disc type reed valves to rectify the flow generate by a single piezostack-driven piston. The proposed valves have the potential to overcome frequency limitations of more conventional poppet or ball type check valves. This enables the pump to operate at higher frequencies and thereby produce larger flows. Simulation results suggest that a pump capable of producing a no load flow in excess of 1L/min would be possible using an off-the-shelf piezo stack.
Traditional valve-controlled hydraulic cylinders are usually very inefficient due to power loss through the control valve. An efficient alternative architecture is to distribute power electrically rather than hydraulically to a group of cylinders and drive each cylinder via individual servomotor-driven pumps. This arrangement is called electrohydrostatic actuation. Such actuators are currently available for power ratings of several hundred watts or greater, but not in the sub-100 W range. This paper details the design, simulation and testing of a piezopump which is intended to address this gap. The motivation is for aerospace applications, and in particular accessory actuators used in the landing gear system. The 10–100 W range is a high-power output for a piezopump, and to achieve this a novel design using disc-style reed valves was developed to allow pumping frequencies above 1 kHz. These high frequencies necessitated the development of custom power electronics capable of delivering 950 V peak-peak sine wave excitation to a largely capacitive load. Experimental results show that the piezopump is capable of delivering over 30 W of hydraulic power, and at no-load can deliver up to 2 L/min of flow at 1250 Hz. Future development includes a transition to multi-cylinder pumps, and improved reed-valve modelling to improve the accuracy of simulated performance.
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.
A switched inertance hydraulic system (SIHS) can provide an efficient step-up or step-down of pressure or flow rate by using a digital control technique. In this article, analytical models of a SIHS in a four-port high-speed switching valve configuration are proposed, and system characteristics and performance are studied. Using these models, the flow responses, system characteristics and efficiency can be estimated and investigated effectively and in detail. Numerical simulation models are used in validation of the analytical models. Results show that the models are accurate and reliable, and give a very promising way to understand the characteristics and trend of a four-port switched hydraulic system. A discussion and comparison is included of the three-port valve and four-port valve configurations, in terms of system power loss. It is found that the four-port configuration has higher losses, but provides greater control flexibility.
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