Typical FBN Measurements
Here are some typical flow ripple measurements performed on a variety of hydraulic components:
The figure below shows some typical measured flow ripple waveforms for an axial piston pump, at 40 bar (red) and 120 bar (blue). The most striking feature is the reverse flow spike which occurs repeatedly when the delivery port starts to open and the fluid in the cylinder is suddenly pressurised. This increases with pressure. Also apparent is a smaller ripple due to the sinusoidal motion of the pistons.
This pump contained no relief (silencing) grooves in the portplate and was consequently very noisy. The reverse flow spikes could be reduced by the use of relief grooves. Flow ripple measurements such as the above could be extremely useful in the design of relief grooves; the effect on the reverse flow spikes could be examined in detail.
The corresponding amplitude spectra are shown below. These measurements were taken up to 3.5 kHz and a large number of significant harmonics are apparent.
The picture below shows some measured flow ripple waveforms for a large reciprocating plunger pump, working with 95:5 water-oil emulsion, as used in mining applications. The red line is for a standard pump, and the blue line for a pump with modified valves. The flow ripple waveform is characteristic of such pumps, and the negative peaks occur when the valves open and close. The modified valves were found to have a faster response, resulting in advanced timing, as can be seen from the waveforms. The flow ripple is slightly smaller for the modified valves, and there was an associated increase in volumetric efficiency and reduction in noise. The measured flow ripple was of great value in determining the valve timing.
Here are some typical flow ripple measurements for a power steering vane pump, at low pressure (red) and high pressure (blue). Measurements such as these have proved extremely useful for the design of low noise pumps, and have also been used in the prediction of pressure ripple in power steering systems. Such predictions have been used to assist in the design of low noise systems.
The measured source impedance of this pump is shown below. These measurements are of very high quality, and are accurate over a very broad frequency range. A few 'outliers' are apparent in the data, but the software is capable of identifying and rejecting such points. The source impedance is needed before the pump's flow ripple can be determined, and is also needed for prediction of pressure ripple in circuits.
Hoses can have a very strong effect on fluid-borne noise levels. They generally exhibit very complex fluid-borne noise characteristics, because of wave behaviour in the hose wall as well as in the fluid. For system modelling, accurate flexible hose models are essential. However the dynamic parameters for such models are difficult if not impossible to measure accurately by traditional means.
The fluid-borne noise characteristics of a flexible hose can be described by its impedance matrix, which relates the pressure and flow ripples at both ends. This can be measured using the FBN package.
A typical result is shown below. This shows the amplitude spectrum of one term of the impedance matrix. The blue lines are measured data using the FBN package. The red lines are modelled characteristics. The model parameters were adjusted to get the best fit with the measurements, and the curve fitting was performed using the FBN package. Good agreement between model and measurements can be seen.