Digital Displacement® (DD) technology is a fundamental advance in fluid power.
Conventionally, hydraulic pumps and motors are controlled varying the stroke of the pistons with an adjustable mechanism. They are robust, compact and low cost, but suffer from poor energy efficiency over typical duty cycles and suffer from the delay and inaccuracy of analog control. Despite these drawbacks, hydrostatic power transmission is used in applications where the benefits outweigh the drawbacks, such as mobile work vehicles and industrial machinery. However, large markets such as automotive transmissions and wind turbines are ruled out by the poor overall efficiency.
Digital Displacement is a fundamental advance. Instead of varying the stroke mechanically, the output is controlled by digitally enabling individual cylinders on a stroke-by-stroke basis. Each cylinder is provided with electronically-controlled digital valves, capable to change state in few milliseconds. Each cylinder may be switched from idle, motoring or pumping cycles once every shaft revolution, in a pattern determined by an embedded controller. This results in much faster and more accurate control response compared to variable-stroke machines.
The simplified animation on the left above shows all six (pink) cylinders of one bank of a Digital Displacement® pump idling. The low-pressure valves above the cylinders are continuously open and oil moves at low-pressure into and out of the cylinders. The animation on the right shows the low-pressure valve of each cylinder closing when its piston reaches bottom-dead-centre. The piston colour changes from green to red as pressure increases, the high-pressure valve to the side of each cylinder opens and high-pressure oil is delivered to the external circuit. In a typical application each cylinder is enabled only as individually required to satisfy the load requirements.
Digital Displacement® machines exhibit superior part-load efficiency, because cylinders which are not needed for the present demand will default to an “idle” state, in which they have very little parasitic energy loss. Conventional machines pressurise all cylinders on every stroke, suffering high fixed losses from bearing loads, leakage and oil compressibility, leading to poor part-load efficiency.
A comparison of the efficiency and energy losses of the E-dyn 96 Digital Displacement® pump with a conventional 70cc/rev swash plate pump is shown below. The measured loss results of the 70cc/rev swash plate pump are normalised to 96cc/rev to be directly comparable to E-dyn96.
Over a typical duty cycle, Digital Displacement® machines show a quarter of the energy losses of conventional machines. This translates into significant energy costs savings, as well as reducing oil heating.
For a fixed industrial application, the figure below shows the calculated electrical energy cost reduction of a 96cc/rev Digital Displacement® pump compared to a swashplate pump, on the basis of continuous operation.
At the 1.6MW scale for wind turbines, the machines are even more efficient: Artemis has demonstrated the most efficient hydraulic transmission ever built, with 93.5% peak efficiency from shaft-to-shaft, and a wide range of efficient operation even at part load. Working with parent company Mitsubishi Heavy Indistries, we have built 7 MW transmissions for offshore wind.
Making these game-changing machines has required us to achieve significant progress in a number of technology areas:
* Ultra-fast digitally-controlled valves, exhibiting low pressure drop and high reliability;
* Design of machines from the scale of a few kilowatts up to multiple megawatts;
* Embedded control electronics
* Simulation models predicting energy efficiency and dynamic response;
* Novel system architectures and control systems.
Artemis keeps all core development in-house, mixing mechanical, electronic, software and simulation engineers in a single creative environment including design, workshop, prototype build and test rigs up to 4MW capacity.
In the process, we have filed over 50 patent families, and dozens more with partner companies. Artemis’s innovation has been recognised with awards from the the Institution of Engineering and Technology (IET), Rushlight and Carbon Trust.
Contact us to find out how Digital Displacement® technology can enhance your product.