Abstract The fluid borne noise characteristics of hydraulic pumps and motors are commonly represented in the frequency domain using a source flow ripple spectrum in parallel with a source impedance spectrum. This is equivalent to Norton’s theorem for electrical sources in which the source is represented as a current source in parallel with a resistance or impedance. The validity of this representation is considered for two types of piston pump through simulation and for a third piston pump through experiment. It is shown that the Norton model of source flow ripple and source impedance is a reasonable approximation to the fluid-borne noise characteristics of positive displacement pumps. However for pumps in which fluid compression is a dominant feature in the source flow ripple, such as piston pumps, the source flow ripple varies somewhat with the system pressure ripple, and this causes significant problems with the Two Loads method, resulting in unrealistic source impedance measurements.
In 1984 a hydrostatic wind-turbine transmission with ‘secondary control’ was proposed by Stephen Salter using the, then only conceptual, Digital Displacement® (DD) principle for controlling the flow of the primary, rotor-driven, ring-cam pump. This transmission ‘could achieve the correct ratio of tip-speed to wind-speed in conjunction with true synchronous generation’.
In the following years DD machines were progressively developed. To start with they were relatively small in capacity but the power ratings were systematically increased, until it seemed that a high-power hydrostatic wind-turbine transmission might indeed be feasible. In 2006, Artemis Intelligent Power (Artemis), a company that had been formed from Salter's original university team, began working on a megawatt-scale, hydrostatic, wind-turbine transmission based on new pump and motor designs. In 2011 Artemis completed a 1.5 MW transmission and dynamometer test-rig. This was one of the largest hydraulic transmissions ever made and, with a shaft-to-shaft efficiency of 93%, one of the most efficient. Using secondary control to respond rapidly to gusting wind and to instantaneous grid faults, it was also the most controllable. This paper discusses the design of the transmission and test-rig and presents the results of steady-state efficiency tests. Subsequent papers will describe systematic experimental work to account for the various energy losses and to develop a comprehensive simulation model of the DD wind-transmission.
Abstract Digital Displacement pumps (DDP) are variable displacement radial piston pumps. Each cylinder comprises of a solenoid operated low pressure check valve and a passive high pressure check valve. Variable displacement is achieved by altering the number of active cylinders, and the proportion of the stroke for which the cylinder is pressurised. DDP has been demonstrated to be quieter and possess a significantly less tonal sound quality than traditional variable displacement axial piston swashplate pumps[1]. However, as the industry continues to trend towards battery electric driven excavators, the existing masking noise produced by conventional diesel combustion engines will cease to exist. Consequently, the hydraulic system will be the most significant noise source on the machine, driving the need to further refine pump Noise, Vibration & Harshness (NVH) performance. Passive check valves produce a characteristic in-cylinder pressure overshoot as upstream pressure has to be sufficient to overcome downstream pressure forces, plus resistive spring and damping forces responsible for holding the valve in place. Within a DDP, the transient nature of a pressure overshoot is very effective at exciting several different high frequency responses of the pump, most noticeably fluid-borne noise, and consequently radiated noise. This paper presents a method for incorporating a damping mechanism within a piston which reduces pressure overshoot, allowing an investigation into the relationship between pressure overshoot, fluid-borne noise and radiated noise. Modelling methodology is presented demonstrating a reduction in pressurisation rate and overshoot during the pumping cycle. Several key design variables are shown to give a large range in tuning scope. Initial test results are presented, demonstrating good model correlation and improvement in NVH performance.
Abstract Digital Displacement® pumps are a type of variable displacement, radial piston hydraulic pump currently being developed for use in efficient mobile hydraulic systems. The pump displacement is controlled by selective enabling of solenoid valves at the inlet of each cylinder and since the pump displacement can be controlled directly by software it is possible to use the pump as a flow source with any arbitrary demand. By enabling precise flow control and reducing leakage and throttling losses they can provide a significant improvement in efficiency over conventional pumps; however, this also leads to a decrease in the overall sources of damping in the system and may result in increased transmission of vibration and fluid-borne noise. This paper presents a method for characterising vibration sensitivity of a hydraulic system, using the pump as a frequency generator whose flow output follows a sinusoidal ‘chirp’ demand. Simulation results are presented of the pump open-loop frequency response, which show the control bandwidth and demonstrates that the pump can modulate its output flow at frequencies into the audible range. This enables the possibility of using the pump to identify potential sensitivities in a downstream hydraulic system up to 200 Hz. A method is described for characterising the noise and vibration of the connected system within this frequency range. Test data from a hydraulic excavator are presented and analysed to create a characteristic transfer function for the system, relating pump output flow to pressure ripple and vibration in the downstream system. These system transfer functions can be used to develop control methods to reduce the impact of vibration, either by active damping, filtering of the control signals or choice of cylinder enabling strategy. Test data are presented also showing the effect of some mitigation strategies in the same hydraulic excavator, leading to a reduction of overall vibration in the vehicle cabin.
Abstract Digital Displacement® pumps use solenoid operated on/off valves to control the pressurization of each individual cylinder on a stroke-by-stroke basis. The overall pump displacement is commonly varied by changing the proportion of cylinders which are not pressurized to those delivering their maximum volume. This strategy leads to high efficiency but can increase pulsation in the downstream hydraulic system. As an alternative, the timing of the valve actuation can be adjusted so that cylinders are only pressurized for a portion of their stroke. This paper presents measurements of overall power loss and efficiency of a 96cc/rev pump at a variety of speeds and pressures with variation of the cylinder stroke size from 10–100%. The efficiency of the pump decreases as cylinder stroke is reduced, but the change is relatively small, with efficiencies over 86% measured in all conditions above 100 bar and 50% stroke. The measured data is used to develop and validate a loss model for the Digital Displacement pump when operating using partial cylinder strokes. By combining this loss model with operating data from a 16-tonne excavator, the impact of different cylinder operating strategies on energy savings in an off-road system is evaluated. For the excavator performing digging, grading and travel it is predicted that changing from a full-stroke to a partial-stroke operating strategy has the effect of reducing the average pump efficiency by less than 2% in all duty cycles.
Abstract Digital Displacement® hydraulic pumps are a type of radial-piston machine with solenoid operated on/off valves used to individually control the pressurization of each cylinder on a stroke-by-stroke basis, thus adjusting the pump’s overall displacement. Previously developed cylinder enabling strategies based on using full strokes can lead to low frequency vibration, whereas partial stroke strategies pose challenges in audible noise and component lifetime. A new enabling algorithm is proposed, Quantized Part Stroke (QPS), which seeks to minimize the low frequency content in the pump output, as well as limiting noise due to actuating the valve near mid-stroke. The operating displacement fraction is quantized such that only integer fractions of displacement are permitted, the fraction’s denominator relating directly to a minimum allowable frequency in the pump output. This quantization is applied such that the chosen displacement fraction is higher than the demand and then part strokes are used to exactly achieve the desired flow. Simulation results are presented comparing this algorithm with well-known alternatives, as well as test data from a 12-cylinder pump showing a clear decrease in low frequency pulsation in the hydraulic system pressure, without a significant change in audible noise from the pump.
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