Artemis technology provides a new way to build variable-displacement hydraulic pumps and motors that have all of the strengths of traditional hydraulic machines but with huge gains in energy efficiency and controllability. Because the behaviour of our machines is directly controlled by fast-acting microprocessor-controlled solenoid-valves, we came up with the phrase 'Digital Displacement®' which we often abbreviate to 'DD'.
Please read on if you want to know more.
Rotating hydraulic pumps and motors usually contain multiple pistons and cylinders arranged in axial or radial configurations. At precise times, governed by the motion of its piston, each cylinder must be connected either to the low-pressure or the high-pressure side of the external system.
This critical process of flow-control, which is analogous to the ‘commutation’ of the rotors of simple electric machines, is traditionally performed by mechanically actuated valves or port-plates - whilst variable-displacement is commonly provided by using a ‘swash-plate’ to vary the length of each working stroke.
Traditional machines have unrivalled power to weight ratios, legendary toughness, reliability and good efficiency when working at or near full power. However, due to high standing losses associated with conventional valve and swash-plate operation, they generally have poor efficiency when working at part load. For years, this problem and the difficulties of fast control by computer, has locked hydraulics out of many applications.
A new approach
Whilst working on the development of an advanced variable-displacement hydraulic machine in the 1980s, the founders of Artemis Intelligent Power conceived of a new way to overcome these problems. (If you're interested in that part of our story, you can follow this external link to find out about the contribution of their colleague Robert Clerk to our practical knowledge and understanding of hydraulic machines).
To unlock the full potential of hydraulics, Artemis replaced the mechanical valves and swash-plates of conventional variable-displacement hydraulic machines with computer-controlled high-speed solenoid-valves. This opens up new markets for hydraulics because Digital Displacement® machines are efficient at all load levels and have super-fast response to computer control.
Please keep reading on for a simplified description of how Digital Displacement® works.
Idling - the default function
The animation below shows a single-bank from a Digital Displacement® Pump (DDP) that is ‘idling’ at low-pressure. There are six piston-cylinder pairs in a radial configuration that is suitable for high-speed machines. The outer ‘small end’ of each cylinder can move around within the spherical cup of an active low-pressure valve to accommodate changing angles as the single-eccentric shaft rotates. The bluish-coloured poppet of the low-pressure-valve at the outer end of each cylinder stays open all of the time. This means that, as they move inward toward the camshaft, the moving pistons suck oil in from the space around the machine – and as they move outward, they return the oil to the same low-pressure space. The pump is just ‘breathing’ and doing no useful work. However, this low-pressure idling consumes very little energy and is one reason why Digital Displacement® machines have unrivalled efficiency even when operating at fractions of rated power.
In contrast to the idling pump, the next animation shows the same bank of cylinders inside the same pump – but now all of the cylinders are ‘enabled’ once in every revolution. Watch the top piston whilst the animation runs. As it moves down to the centre of the machine it draws in oil from around the machine through the open poppet of its low-pressure valve – the green phase of the animation. However, as the piston passes bottom-dead-centre you can just see the solenoid of its low-pressure valve flash red - forcing the low-pressure valve poppet to close and to stay closed for the remaining half-cycle.
At this stage the piston colour changes to red, to show that the oil pressure increases from the low ‘boost’ value of a few bar up to the system pressure of perhaps 350 bar or more. Oil has low compressibility, so the cylinder pressure-rise is almost instantaneous. The high-pressure valve to the right of the low-pressure valve opens - you can just about see its purple poppet move to the right. In the case of a pump like this, this high-pressure valve is a simple spring-loaded check-valve. As the piston moves toward top-dead-centre, oil is forced at high-pressure out through the open poppet of the high-pressure valve and into the external hydraulic system.
The solenoids are activated by power FETs (Field Effect Transistors), which are in turn connected directly to the digital output of an embedded controller. This intimate marriage of mechanics, electronics and software is a defining characteristic of Digital Displacement®.
In the animations, the pump is shown either fully idling or fully enabled. Of course, it usually operates somewhere in between, with each individual cylinder being enabled by the controller as its contribution is required to satisfy the changing needs of the external system or task. The keys to Digital Displacement® are that these enabling decisions are made just-in-time to satisfy the instantaneous requirements of the external system and that idling cylinders consume very little parasitic energy. This makes the machines responsive and efficient.
This photograph from the early 1990s shows the first small prototype Digital Displacement® pump that was built by Artemis. It has a nominal power rating of around 15 kW. The animations on this page are based on this kind of machine. You can probably identify pistons and cylinders as well as low- and high-pressure valves and their component parts.
To the hydraulic machine designer, there are knock-on benefits from using active valves because mechanical layout is simplified. Valves can be placed around the perimeter where there is space for easier fluid breathing rather than around the centre of the machine – and this helps to reduce energy losses even more. Piston pads can be near to the centre of the machine where linear velocities are lower and shear losses therefore smaller.
At top-dead-centre of pumping strokes the high-pressure oil is slightly compressed compared with its volume at low-pressure. In conventional pumps most of this compression energy is lost when the low-pressure valve opens against residual cylinder pressure. This is the source of much of the high-frequency noise that is associated with hydraulic machines. In Digital Displacement® machines compression energy is naturally recovered because the low-pressure valves open only when the cylinders decompress naturally. The related and unprecedented reduction in noise made it possible to think of creating the Artemis Digital Displacement® series-hybrid car transmission.
Multiple banks of cylinders can be packed along a common crankshaft, to create either a single large-capacity machine or a multiple-service machine with a number of independent hydraulic outputs.
The Digital Displacement® pumps, discussed above, incorporate active, solenoid-driven, low-pressure valves and passive, spring-loaded, high-pressure check-valves. To create the equivalent Digital Displacement® motor, requires both valves to be actively controlled.
Generally, it’s straightforward to make a Digital Displacement® motor also function as a pump if required. Such combined-function machines are referred to as ‘DDPM’s (Digital Displacement® Pump Motor). A single or multi-bank DDPM can have some cylinders pumping into one function of an external task whilst others motor and recover energy from another task. In this way, complex control tasks, such as are found in off-road applications, can be addressed efficiently by a single physical machine without wasteful dumping of energy by pressure-relief valves.
This 1996 drawing by Stephen Salter, of his multi-bank or 'wedding cake' hydraulic machine, shows many of the features which Artemis has developed in its Digital Displacement® machines.