Electronically commutated hydraulic machine and operating method to reduce generation of resonance effects

An apparatus comprising a prime mover and a plurality of hydraulic actuators, a hydraulic machine having a rotatable shaft in driven engagement with the prime mover and comprising a plurality of working chambers having a volume which varies cyclically with rotation of the rotatable shaft, a hydraulic circuit extending between a group of one or more working chambers of the hydraulic machine and one or more of the hydraulic actuators, each working chamber of the hydraulic machine comprising a low-pressure valve configured to regulate a flow of hydraulic fluid between the working chamber and a low-pressure manifold and a high-pressure valve configured to regulate the flow of hydraulic fluid between the working chamber and a high-pressure manifold,

• the hydraulic machine comprising a controller configured to actively control at least the low-pressure valves of the group of one or more working chambers to select the net displacement of hydraulic fluid by each working chamber on each cycle of working chamber volume, and thereby the net displacement of hydraulic fluid by the group of one or more working chambers, responsive to a demand signal,
• the controller configured to control the said valves to cause each working chamber to carry out either an active or an inactive cycle of working chamber volume during each cycle of working chamber volume, wherein the apparatus is configured such that fraction of working chambers which carry out active cycles is variable and is selected from a plurality of discrete fractions.