The simplest embodiment of digital hydraulic technology is the Variable Displacement Linear Pump (VDLP), which can displace a variable amount of fluid per unit length of stroke or allow a variable stroke per unit of volume displaced. Its functioning depends upon how it is plumbed and controlled, that is, whether a constant force on the piston or a constant fluid pressure is required.
In 2003, the inventor and managing member of DigitalHydraulic LLC, Elton Bishop, built a 4-bit proof of concept prototype, shown in Fig. 8, for demonstration of the technology. It successfully demonstrates linear variable displacement operability. As an example of possible application, the VDLP was configured to operate as if in place of an automotive shock absorber, creating a shock energy converter. Rather than dissipating shock energy as heat into air, this unit transforms dynamic mechanical linear input (the shock) into a constant pressure, variable displacement hydraulic flow. Theoretically, this flow of high pressure hydraulic fluid would be used to offset the amount of input power required of the prime mover of the vehicle (i.e. diesel engine).
The prototype VDLP was designed to displace a progressively larger amount of fluid per unit length of stroke as the velocity of the piston linearly increases. In other words, the faster it is compressed, the greater the resistance to motion and the more fluid it displaces. Illustrating the elegant simplicity of this technology, it is self-powered and controlled, and its performance curve, Fig. 9, is fully manually adjustable.
Another potential application of the VDLP is in ocean wave energy extraction. Attraction to ocean wave energy is clear. There is a massive known resource and huge market potential for a clean renewable energy source. Despite much interest in and research into these technologies, state-of-the-art designs are yet inadequate. The energy conversion efficiencies of these current devices are so poor they are precluded from commercialization. The International Energy Agency / Ocean Energy Systems (IEA/OES) had the following to say in its 2003 “Status and Research & Development Priorities / Wave & Marine Current Energy” publication regarding the need for improved hydraulic systems in ocean wave energy extraction systems:
“Many of the devices would benefit from greatly improved hydraulic systems. Current hydraulics are mostly too inefficient—particularly when used at part load—and too costly to be used for power conversion and few find application in this sector. They are also poor at combining power from different sources. However, hydraulic systems are much simpler to implement than electrical equivalents and so are attractive to many device developers. Research into methods for turning low-speed high-torque rotational and translational motion into electrical power efficiently and cost effectively would benefit many different device concept teams.” (emphasis added)
Most ocean wave energy extraction systems, whether moored or free floating, rely on some kind of hydraulic ram to convert the oscillatory motion of the waves into a flow of pressurized hydraulic fluid. This pressurized hydraulic fluid typically powers a rotary hydromotor, which spins a generator, ultimately providing electrical power.
A main problem with these current ocean energy extraction systems is that they are mass and buoyancy tuned for an “average” wave. Because sea state varies, these systems are only able to make use of a small percentage of a large wave’s energy, and if a small wave is encountered, there could be no energy conversion at all.
These systems are generally acknowledged to achieve an overall energy efficiency of just 12-15%. That is, 85-88% of the potential and kinetic energy in the waves acting on these systems is not extracted. New technologies are needed that go beyond the 12-15% efficiencies commonly found in current wave energy devices.
One potential option would be to develop a wave energy system that can constantly modify its energy extraction parameters in order to suit perfectly each unique wave geometry and velocity encountered. This system would have to take input from position, pressure and acceleration sensors to achieve a model-based control. Specifically, this system would utilize a linear power take-off, and it would be in the form of a variable displacement device capable of real-time modification of its functioning. The system would allow optimized dynamic ocean wave energy extraction from each unique wave geometry encountered.
DigitalHydraulic’s VDLP allows this system proposed, and is the focus of a separate project currently on hold.
By modifying the schematic, the operation of the VDLP can be reversed so that a single high-pressure hydraulic source may be transformed efficiently into a dynamic variable force output experienced at the rod. This more advanced embodiment of digital hydraulic technology is the Variable Displacement Linear Actuator (VDLA). It can be thought of as a conventional hydraulic cylinder, except that it can deliver a variable force output throughout its stroke with near instantaneous control response and near perfect efficiency.
Current linear actuator systems delivering a variable force output typically rely on throttling, a process of restricting fluid flow to artificially increase the load. This severely limits system efficiency, especially toward the lower end of the output range. Alternatively, digital hydraulic technology allows for energy transformation, where input energy (hydraulic) is equal to output energy (mechanical). This approach of ‘use only what is necessary’ results in perfect theoretical efficiency throughout the output range.
Additionally, if the output force is less than the opposing force, the VDLA acts as a VDLP. A double acting VDLA permits highly sought after functionality, called four-quadrant operation, in which operational transition is seamless throughout the entire range of motoring and pumping. In laymen’s terms, a four-quadrant linear actuator can produce a variable force in either direction while moving in either direction at, theoretically, any velocity (Fig. 10). If a control signal is sent for the VDLA to produce a specific force in a particular direction, and the opposition force of the load worked against is less, the opposition force is overpowered, and the rod and communicated load accelerate in the direction of actuator force. If, however, the opposing force of the load is greater than the output force of the VDLA, the rod and load begin to travel in the opposite direction; thus the VDLA operates as a VDLP.
In 2007, a 4-bit proof of concept prototype (Fig. 11) was built and evaluated by DigitalHydraulic LLC. Attached to a load cylinder to simulate working conditions, the prototype clearly demonstrates efficient four-quadrant operation. As illustrated by Fig. 12, with each successive mode of operation, the displacement increases incrementally.
Examples of potential applications of the VDLA include earthmoving machinery, active vehicle suspensions, flight control surface actuators, robotics, orthotics and prosthetics.
Management foresees the VDLA as having much potential in future hydraulic installations due to its energy efficient linear variable displacement in a single package. However, because its stroke is limited to approximately one third of a comparable conventional hydraulic cylinder, the VDLA is not a direct retrofit for current hydraulic systems. This only means that future machinery must be designed around this new technology.
The third embodiment, the subject of this proposal, is a device designed to be coupled with a fixed displacement hydraulic actuator (i.e. conventional hydraulic cylinder) in order to achieve variable displacement functionally. This device, the Digital Hydraulic Transformer (DHT), converts hydraulic energy by way of a proprietary new component, the Transtatic Bridge. An input flow at a certain given pressure is transformed to an output flow at another pressure level almost without loss. Except for small internal energy losses due to sliding friction and seal leakage, the conversion is also reversible, as the product of input pressure and flow is equal to the product of output pressure and flow.
Current designs allow for the transformation factor between input and output to be from .0667 to 15. For example, 1000 psi fluid could be transformed into fluid at a pressure between 66 and 15,000 psi. The DHT principle could easily be compared to an electric transformer where the product of voltage and amperage remains constant. Of the three primary embodiments of digital hydraulic technology, the DHT represents both the most advanced and the most immediately marketable form due to its compatibility with existing designs.
While current systems dissipate hydraulic energy by throttling flow, the DHT transforms hydraulic energy. In principle, throttling can yield efficiencies down to 0%. Conversely, transformation is a reversible process with, theoretically, 100% efficiency. In the case of throttling, input flow equals output flow. With transformation, input energy equals output energy. The transformation can even move in the opposite direction. That is, a low load pressure may be transformed back to a higher working level. At that point, this fluid is once again of use to the hydraulic system. This is energy redistribution, a necessary component of four-quadrant operation.
A proof of concept prototype, shown in Fig. 13, based around the preexisting VDLA prototype, was built in 2007 to demonstrate the DHT’s attainable energy savings as final control element of a hydraulic servo drive. DHT functionality was achieved through the use of the VDLA, an attached load cylinder, and appropriate directional valves. The prototype was powered by and performance quantified by a test rig, and was connected to the lift cylinder of a forklift. The lift cylinder served as end actuator, as shown in Fig. 14. By quantifying pressures and flows during the lifting and lowering of varying loads, it was proven that the DHT can be effectively implemented as final control element in energy saving four-quadrant hydraulic servo drives. The DHT-enabled forklift required over 70% less prime mover energy to complete standard work cycles.