Apparatus for repetitively displacing an object , the apparatus comprising: an object engagement element , a hydraulic actuator connected to the object engagement element, a hydraulic machine having a low pressure manifold and a high pressure manifold , the high pressure manifold coupled to the hydraulic actuator, a rotatable body , comprising a rotatable shaft , a plurality of working chambers having a volume which varies cyclically with rotation of the rotatable shaft, a plurality of valves regulating the flow of working fluid between the respective working chambers and the low and high pressure manifold, at least one valve per working chamber being an electronically controlled valve, a controller which electronically controls the said valves to thereby regulate the net displacement of working fluid by each working chamber during each cycle of working chamber volume, including selecting between pumping cycles in which a working chamber makes a net displacement of working fluid from the low pressure manifold to the high pressure manifold, and motoring cycles in which a working chamber makes a net displacement of working fluid from the high pressure manifold to the low pressure manifold, a motor drivingly coupled to the rotatable body and configured to exert a torque on the rotatable body, characterised in that the speed of rotation of the rotatable body varies, and the controller is configured to alternately control the said valves to thereby regulate the pumping of working fluid to the hydraulic actuator to thereby displace an object which engages the object engagement element in use and then to control the said valves to thereby regulate the motoring of working fluid from the hydraulic actuator as a result of forces from the object bearing on the object engagement element, and wherein the control of the said valves by the controller takes into account the speed of rotation of the rotatable body. an object engagement element , a hydraulic actuator connected to the object engagement element, a hydraulic machine having a low pressure manifold and a high pressure manifold , the high pressure manifold coupled to the hydraulic actuator, a rotatable body , comprising a rotatable shaft , a plurality of working chambers having a volume which varies cyclically with rotation of the rotatable shaft, a plurality of valves regulating the flow of working fluid between the respective working chambers and the low and high pressure manifold, at least one valve per working chamber being an electronically controlled valve, a controller which electronically controls the said valves to thereby regulate the net displacement of working fluid by each working chamber during each cycle of working chamber volume, including selecting between pumping cycles in which a working chamber makes a net displacement of working fluid from the low pressure manifold to the high pressure manifold, and motoring cycles in which a working chamber makes a net displacement of working fluid from the high pressure manifold to the low pressure manifold, a motor drivingly coupled to the rotatable body and configured to exert a torque on the rotatable body, characterised in that the speed of rotation of the rotatable body varies, and the controller is configured to alternately control the said valves to thereby regulate the pumping of working fluid to the hydraulic actuator to thereby displace an object which engages the object engagement element in use and then to control the said valves to thereby regulate the motoring of working fluid from the hydraulic actuator as a result of forces from the object bearing on the object engagement element, and wherein the control of the said valves by the controller takes into account the speed of rotation of the rotatable body. <EOS>