Two main drawbacks with hydraulic systems are noise and vibrations, mainly created from flow pulsations in positive displacement pumps. The flow ripple can be divided in two parts; kinematic and compressible. Kinematic flow ripple is created due to the limited number of pumping elements. Compressible flow ripple is created due to cornpressibility effects in the pumping chambers and is the dominating effect at high pressures. There are well proven methods to decrease noise in specific operation conditions, but there is an urgent need for new techniques in machines working under varying operation conditions. There is also a need for improved measurement methods to be used in product development and to judge new inventions to decrease noise.

Simulation techniques are useful in the early stages of the development process. There are useful and accurate simulation models capable of predicting source flow ripple in hydrostatic pumps and thereby useful for optimisation purposes. Simulation results are, however, of less practical use before they are experimentally verified . The experimental methods must be simple and straightforward if they are to be accepted in industry.

This paper describes a new method to measure source flow. It is called the source admittance method. Compared to earlier methods such as the two-microphone method, the main benefit is that there is no need for a model of the outlet channel - the source flow is measured through an additional pressure transducer inside the pump. The method works very well with a tested fixed bent-axis pump with a rather simple outlet channel. Experiments have also been performed with a variable in-line pump with added complexity to the out let channel. Although the overall impression is positive, these experiments show that the method needs to be further to be used for such machines. In both cases, the new method is compared with earlier verified simulation models and with measurements based on the two-microphone method. Apart from the mentioned discrepancies in a narrow frequency range, agreement is very good.

The source admittance method offers a new way to measure flow ripple. Compared to earlier methods such as the two-microphone method, the main benefit is that there is no need for a model of the outlet channel - the source flow is measured through an additional pressure transducer inside the pump. This makes the source admittance method easier to use and less modelling skill is needed. Furthermore, the method is more reliable because the somce flow is measured, i.e. pressure at the creation spot, directly. In earlier studies, the method proved to work very well for pumps with simple discharge channels but less well for pumps with complex discharge channels.

The experimental test object is a pump with a simple outlet channel. Its internal impedance can easily be changed in a controlled manner; complexity can thus be added step by step. The paper contributes to the understanding of how complexity of the pump's discharge channel influences the usefulness of the source admittance method.

This paper considers using the cross-angle in variable displacement hydraulic machines. The cross-angle is a fixed displacement angle around the axis perpendicular to the normal displacement direction. The cross-angle changes the angles to the pistons top and bottom dead centres as a function of the fraction of displacement in such a way that the valve plate timing is varied and different pre-compression and decompression angles are obtained. A non-gradient optimisation technique, the Complex method, is used together with a comprehensive simulation model in order to find the optimal cross-angle for a variable displacement hydraulic motor. The paper shows that the cross-angle can be used to reduce noise in variable displacement motors. One issue that makes the motor application more difficult is the increased dependence between outlet and inlet flow ripple which is not found in pump applications. Furthermore, the paper discusses how to use the cross-angle for machines which can work both as a motor and a pump.