University CrestEdinburgh Wave Power Group

Digital Displacement® - A New Generation of Hydraulic pumps & motors



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Since 1994, the development of advanced hydraulic machines has been led by our spin-out company Artemis Intelligent Power Ltd

Most hydraulic system loads need variable flow for their proper operation. Conventionally this can be achieved in three ways. One is through flow control valves which alter the flow at the expense of energy loss. Variable swash-plate axial-piston machines are frequently employed for hydrostatic drives where energy becomes a consideration. Less commonly, a fixed displacement pump can be driven by a variable speed prime-mover. A new technique, originally developed for the power-take-off for the Duck, provides yet another way of controllably transferring energy between mechanical and fluid power. This technology is now referred to as 'Digital Displacement®'.

Digital Displacement is a registered trademark of Artemis Intelligent Power Ltd

The basic structure of these new machines is similar to that of tthe conventional reciprocating machine, with poppet valves connecting to the low and high-pressure manifolds of each cylinder. But, instead of being self-acting, each of the poppet valves is equipped with an electro-magnetic actuator. The valves are operated by a micro-controller at precise times, near the ends of the stroke, in order to establish fluid connection between the moving piston and the appropriate manifold. This control allows cylinders to behave in any of the three ways, they can pump or motor - adding or subtracting fluid from the high pressure manifold - or they can be disabled. The function of each cylinder can be changed at each end of each stroke. As the valves are actuated at times in the cycle when there is almost no pressure difference across them, the actuators can be compact and use little power. Either permanent magnets or springs are used to maintain the disabled poppets at default positions. A micro-controller controls the valves from its output port via a bank of power semiconductors. The figure below shows the cylinder head arrangement of a single cylinder Digital Displacement® pump-motor which can be run under pressure-control, flow-control or ternary-mode of cylinder enabling.

Cylinder head of single-cylinder pump-motor
Poppet tester
Null-eccentric rig
Digital hydraulic power-supply
Win Rampen bending girders
Annular valve
First computer-controlled pump


Advantages of Digital-Displacement®

  • Fast response : These machines are capable of attaining either full or zero output from any starting condition, in less than a single shaft revolution.
  • Compatibility with micro-processor : The compatibility with micro-processors allow the use of advanced control logic. Also the same machine can be used as a pump, a motor or both.
  • Higher efficiency : As disabled cylinders are not pressurised, losses are reduced in comparison with swash-plate machines leading to higher efficiency, especially at part load.
  • Multi-banking of pump-motors : Unlike conventional machines banks of radial pump-motors can be combined along a common shaft and used as a summing junction of both torque and power whilst providing isolation between services. Accumulators may be used in conjunction with some of the banks to transfer power in or out of the system. The radial configuration provides good force balancing and gives optimal space for the mechanical components like valves and bearings.