Heavy off-road hydraulic machines, such as excavators, telehandlers and wheel loaders, are commonly used to perform work in the construction, agricultural, forestry and mining industries. These machines transmit large amounts of power to and from workloads through their hydraulic systems. They use hydraulic power transmission systems because hydraulics provides much greater power density and system flexibility than alternatives such as electrical or purely mechanical systems. However, traditional hydraulic systems suffer from relatively high energy losses due to throttling in the valves directing flows. In addition to throttling losses, overall efficiencies are further reduced by the need to cool the working hydraulic fluid to prevent excessive system-damaging heat. According to research which evaluated energy utilization of a hydraulic crane operating through a typical work cycle using different driving concepts, the overall efficiency of the system is limited to around 35%; that is, approximately 65% of the energy input into the conventional hydraulic system is wasted. This speaks to the need for higher efficiency hydraulic systems.
The main problem is that, in a conventional hydraulic cylinder, a fluid at a specific working pressure acts on a piston, with specific working area, resulting in a specific linear output force. If a different output force is required, the fluid pressure must change since the piston area is unchangeable. Hypothetically, if piston area could be changed, or if only a portion of the area could be acted upon, fluid pressure could remain constant and output force could vary. Unfortunately, there are no means by which fluid can act on only a portion of a piston, as a pressurized fluid acts equally on all area containing it.
Digital hydraulic technology represents a way around that limitation. By dividing the working area of a piston into axially offset binary weighted annular areas (Fig. 1), selectively pressurizing the individual annular chambers results in a cumulative output force that can be incrementally raised or lowered. This is the main concept of the Transtatic Bridge, which is central to the DHT’s novel functionality. Permutating the combinations of individually selectable areas results in a wide range of cumulative force output illustrated by Fig. 2.
In 1998, Elton Bishop recognized the lack of energy efficient hydraulic linear actuator systems in the world. He was unimpressed with the careless, wasteful utilization of valuable energy in modern systems, and believed that hydraulic systems should operate on principles similar to those of animal muscle, because natural systems are “perfectly designed.” A main performance characteristic of animal muscle that sets it apart from hydraulic actuators is that it operates like a variable tension rubber band, as opposed to the stiff, flow controlled nature of current linear drives. This characteristic served as the starting point for development of the new system.
An animal muscle is a parallel arrangement of individual muscle fibers controlled individually to function as a group. The control process is called orderly recruitment, and is, in some respects, a digital process. Each muscle fiber is capable of a discrete output force upon recruitment, and specific muscle fibers are recruited to cumulatively meet the force output requirement of the muscle.
If the analog of the muscle fiber, in the hydraulics realm, were a simple hydraulic cylinder with on/off control, muscle functionality would be approximated by connecting multiple hydraulic cylinders (bits) in parallel. Effectively, this would result in a Variable Displacement Linear Actuator (VDLA). Figure 3 illustrates a few possible bit arrangements for a VDLA.
This digital hydraulic actuator would allow an output force to be raised or lowered incrementally. However, as a muscle can be composed of several hundred muscle fibers, the direct hydraulic analog, with equivalent force output resolution, would not be realistically feasible, as hundreds of hydraulic cylinders would be required.
While there are different ways to weight each bit, such as equal weighting, binary weighting, and Fibonacci weighting, in order to economize this hydraulic muscle, binary weighting was applied to the pressure bearing surface areas. With this arrangement, a minimum number of components are required to allow a maximum force output resolution with equal step spacing. As N number of bits result in 2N-1 discrete force outputs (states), resolution increases exponentially as the number of bits is increased. Figure 4 illustrates the range of possible displacements of a four-bit binary weighted VDLA.
While the VDLA has some interesting characteristics that deserve attention, such as its ability to operate without intentional throttling losses on a constant pressure net as a secondary controlled unit, a thorough discussion of its characteristics is not included here. Rather, this paper focuses on the DHT, which shares many characteristics with the VDLA. However, one particular negative characteristic of the VDLA that should be mentioned is its inherent short stroke. According to a first order approximation, whereas a conventional hydraulic cylinder has a stroke of 100% of its installed length, a double acting VDLA, depending on its construction, is capable of a stroke that is limited to 25 - 33% of its installed length. This short stroke limits the potential application of the VDLA and precludes it from being a direct replacement for conventional cylinders. What would be quite useful to designers and users of hydraulic linear drives is a way to achieve the performance characteristics of the VDLA in a conventional cylinder. This would combine variable displacement and a long stroke.
If the cumulative output force of a VDLA is coupled with another piston, as depicted in Fig. 5, a variable ratio digital hydraulic transformer results. Here, the hydraulic power into and out of the transformer is equal, as the product of input flow and pressure equals the product of output flow and pressure. The transform ratio can be optimized for a specific load, and the transformed output can be used for actuation of a conventional hydraulic cylinder (or other fixed displacement hydromotor), allowing a smaller volume of the original high pressure working fluid to accomplish an amount of work.
For instance, if the load in Fig. 5 can be lifted by a pressure, pload, and the pressurization of areas 2A and 4A produce a cumulative linear force that, when divided upon the area of the coupled piston, 15A, results in transformed pressurization of fluid to a point above pload, the load is moved and a working fluid volume savings of 60% results, as only 40% of the ordinary volume of working fluid is used to complete the task. The amount of working fluid required to move a load a given distance is proportional to the magnitude of the load. That is to say, only the amount of working fluid necessary is used. As such, the working fluid volume savings are inversely proportional to the magnitude of the load.
In addition to working fluid volume savings while motoring, the DHT can operate as a pump, allowing redistribution of potential and kinetic energies of a load while it is being lowered or decelerated. Pumping, load pressure, pload, is greater than transform pressure, ptrans, and hydraulic energy is transformed back to the high working pressure and stored in an accumulator. Once redistributed, the energy, in the form of a volume of pressurized fluid, is again of use to the system. Upon a subsequent work cycle, the prime mover (i.e. diesel motor) can efficiently not supply that quantity of energy to the system, as it is supplied by the accumulator. Optimizing the transform ratio results in the maximum volume of redistributed fluid, which translates into maximum overall system efficiency.
Of course, the Transtatic Bridge depicted in Fig. 5 is only capable of displacing a finite volume of transformed fluid, which would be of limited use. That is why the DHT, as developed and claimed as intellectual property, is designed symmetrically, to reciprocate, with each chamber selectively connectable to high pressure, low pressure and transform pressure, thus allowing unlimited continuous flows. When the Transtatic Bridge reaches the end of its stoke, the ends of the DHT exchange functionality and transformation continues. With this arrangement, the DHT can transform fluid pressure to points both above (intensification) and below (deintensification) working pressure. A four-bit DHT is capable of 167 unique transformation ratios between 0.667 and 15, if all possible combinations are used.
Essentially, the DHT combines high pressure fluid, low pressure fluid and transform fluid in specific volume ratios to ultimately displace the fluid volume required by the actuator, at the optimized pressure. Figure 6 illustrates just one possible schematic for a four-bit DHT implemented as final control element of a double acting cylinder.
Whereas the simplified DHT in Fig. 5 is shown connected directly to a single acting cylinder, and is thus capable of an output force in just one direction, the DHT in Fig. 6 is shown connected to a double acting cylinder, via a 4/3 directional valve, which allows transformed fluid to be directed to either cylinder port. This results in four-quadrant operability, which means the DHT-controlled cylinder is capable of a variable force output in either direction while moving in either direction, at, theoretically, any velocity.
Because the DHT uses well established positive sealing technologies, the volumetric efficiency can be quite high. This is true even at partial loading and start/stop conditions, which is something rotary based hydraulic transformers cannot achieve. In fact, at load holding conditions the DHT can be essentially leak free. That is, except for port to port valve leakage. It should be noted that if poppet valves are used exclusively, this valve leakage can be minimized as low as a few drops per minute, according to manufacturers specifications.
Mechanical efficiency of the DHT is mainly dependent upon the pressure drops across the valves and sliding friction, which is dependent upon the sliding surface finish and the sealing technologies used. Along with sealing technologies, manufacturing methods are already well established. Thus, sliding friction can be minimized to the state-of-the-art.
Because the DHT uses parallel-connected on/off valves to control displacement, and can switch directly to and from any transform ratio, the main dynamics characteristic is delay. Additionally, DHT response time is equal to the response time of the slowest valve and is independent of amplitude. Because several schematics are possible for the DHT, and many different valves are applicable to a given schematic, it is difficult to determine response exactly. However, it is safe to conclude that, because applicable valves exist with response ˜10 ms, the potential exists for DHT response to be quite good.