Patent Application
20240309890
The present invention relates generally to electro-hydrostatic actuators, and more specifically to solutions for capturing and reusing potentially wasted energy during the operation of such actuators.
Energetic efficiency is imperative as it directly impacts the fuel consumptions and the environment. One of the areas where efficiency should be considered is in mobile hydraulic machines. In excavators, for example, we usually find a number of single rod hydraulic cylinders and hydraulic motors. Hydraulic cylinders are known for providing high power density [1], high force-to-weight ratios, compactness, and quick responses [2], [3] which makes them desirable when compared to their electric actuator counterparts. In our current energy-demanding world, it is accounted that more than 50% of the energy resources come from the coal and oil products (fossil fuels). The resulting greenhouse gas (CO2) emissions have increased by 1.5 times in 2020 when compared to 1990 and has been increasing till now [4]. Due to the associated greenhouse effect, reduction of fossil fuel consumption has been a major concern for manufacturers. For instance, in a typical excavator valve-controlled actuator require about 23% of the engine output to perform work such as digging and lifting while the remaining energy is dissipated within mechanical and hydraulic components [5]. Most of the energy dissipation occurs due to throttling loses at the control valves. Therefore, improving the energetic efficiency of hydraulic systems will, eventually, decrease emission rates and fuel demands in the long run.
A proven solution that increases efficiency by eliminating throttling loses, is the use of pump-controlled actuator systems where pumps are used to control cylinders instead of valves [6]. In these systems, flow can be controlled by either changing the pump displacement or the prime-mover rotational speed. In this latter case, it is the prime mover that ultimately controls the velocity of the hydraulic cylinder, and the resulting system is termed—“Electro-Hydrostatic Actuator” (EHA). EHAs, therefore, require a prime mover connected to a fixed-displacement, bidirectional pump-motor, whose ports are then connected to a hydraulic cylinder [7]. EHAs have many advantages when compared to valve-controlled actuators, such as easy maintainability, light weight and simple structure, high reliability and little heat loss [8]. However, due to the asymmetric nature of the single-rod cylinder, a challenge is posed when dealing with the uneven flows into and out of the pump. One solution, designed by Costa and Sepehri [9], is characterized by a particular four-quadrant division of the circuit operation, where pumping and motoring quadrants are precisely defined. It is thus understood that during motoring quadrants, the circuit receives mechanical power from the load. However, the pump-motor remains connected to the prime mover (AC servomotor) at motoring quadrants, so that power is still added to the system to control the cylinder speed, by providing a resistive torque at the pump-motor shaft that acts against the hydraulically-generated torque [9]. Therefore, the received mechanical power is simply wasted in the form of heat during motoring quadrants. To improve the EHA efficiency, it is necessary to minimize the potentially wasted energy at motoring quadrants. It is therefore desirable to develop techniques that can capture the otherwise wasted energy during motoring operations.
Several research studies have been conducted to capture and reuse the potentially wasted energy in excavators by employing energy storage components such as hydraulic accumulators, batteries, and supercapacitors. Combining one or more of these components with the actuator systems results in electric hybrids, hydraulic hybrids, or combined electric-hydraulic hybrids. Lin et al. developed a supercapacitor-based electric hybrid system for an excavator to store and reuse boom potential energy. The developed system consisted of an engine, an electric motor, a hydraulic pump, a directional valve, a proportional throttle valve, a controllable electric generator, and a hydraulic motor. During the energy storage process (boom lowering), the hydraulic motor runs the generator, which converts the potential energy into electrical energy, storing it in the supercapacitor. The stored energy can then be reused at the electric motor, adding to the engine power required by the system. Consequently, the power supplied by the engine can be lowered, improving system efficiency by 39%. The low energy density of supercapacitors, large sizes of generators and electric motors, and the presence of the directional valve, place some restrictions on this design. In another design, a battery-based electric hybrid that could increase efficiency up to 54% was proposed by Yoon et al. [11]. The system was actuated by a bidirectional fixed displacement pump-motor driven by an electric motor and a generator. A three-way/three-position directional valve was used along with a proportional pressure relief valve to distribute the uneven flow. During the energy storage process (boom lowering), the bidirectional pump-motor acts as a motor which drives the generator. The potential energy is thus converted into electric energy and is stored in a battery. The stored energy is employed, along with the main power, to lift the boom when necessary. Energy losses during the energy conversion process, low power density of the battery and the large generator and motor sizes limit this design.
A hydraulic hybrid system developed by Hu et al for an excavator arm, consisted of a displacement pump driven by an engine, a four-way directional control valve, a two-position/three-way valve and a two-position/two-way valve. During the energy storage process, the flow returning from the cylinder is directed into the accumulator, storing the gravitational potential energy in the form of hydraulic energy. The stored energy is then used to assist the hydraulic pump during actuation. This system is capable of increasing efficiency by 25.9%. However, the presence of the four-way directional control valve leads to undesirable energy dissipation. Ivantysynova et al. developed a hydraulic hybrid system for an excavator, operating with a 50% downsized engine. The design consists of two accumulators: a low-pressure accumulator and a high-pressure accumulator. The low-pressure accumulator is used for flow-compensation in the circuit. On the other hand, the high-pressure accumulator stores the braking energy of the swing motor, the unused energy from the engine and the potential energy entered through the cylinders. When the engine requires additional power, the high-pressure accumulator drives the variable displacement pump-motor. Energy is also reused by supplying power to the swing motor when necessary. The downside of this design is the high cost.
Hydraulic accumulators can only store a limited amount of energy besides occupying a considerable space in the hydraulic circuit. Based on this fact, studies have been conducted to combine electric and hydraulic hybrids. A combined electric-hydraulic hybrid system proposed by Chen and Zhao is capable of increasing the efficiency from 41.9% to 64.5%. This system uses two fixed displacement pump-motors, a DC motor and two on-off valves. During the energy storage process, valves are activated to direct part of the pressurized flow (potential energy) to the hydraulic accumulator via one of the two pump-motors. If the potential energy of the boom is higher than the storage capacity of the accumulator, the energy excess is branched off through the shafts of the pump-motors, to be stored in a supercapacitor. During the energy reutilization process (boom lifting), part of the cap-side flow is supplemented via the hydraulic accumulator through the second pump-motor. The stored energy in the supercapacitor is also used to drive the pumps. Ge et al. proposed a combined electric-hydraulic system using a novel asymmetric pump with three ports, driven by a servomotor. It has been reported that this design could recover about 82.7% of the total potential energy. Two of the asymmetric pump ports are connected to the cap-side and rod-side of the cylinder, respectively, while the third port is connected to an accumulator. During the energy storage process (boom lowering), the gravitational potential energy is stored as hydraulic energy and electric energy in the accumulator and the supercapacitor, respectively. In the energy recovery process (boom lifting), the stored energy drives the asymmetric pump.
Most of the research so far has focused on hydraulic actuators moving the excavator boom (down/up), and very little has been dedicated to hydraulic actuators working under more involved recoverable time periods. Accordingly, there remains a need for a more optimal solution, preferably one that is cost-effective, simple, easy to implement and maintain, and connectable to existing EHAs in a simple and safe manner. The proposed invention fulfills the aforementioned requirements.
According to a first aspect of the invention, there is provided an electro-hydrostatic actuator system comprising:
According to a second aspect of the invention, there is provided an energy storage and reutilization (ESR) system for cooperative use with an electro-hydrostatic actuator (EHA) having a hydraulic cylinder, a first bidirectional pump-motor rotationally coupled to an electric motor, and EHA valving and connections between the first bidirectional pump-motor and the hydraulic cylinder by which the EHA is operable in four distinct operating quadrants, including a first actuator-extending pumping quadrant, a second actuator-extending motoring quadrant, a third actuator-retracting pumping quadrant and a fourth actuator-retraction motoring quadrant, said system comprising:
According to a third aspect of the invention, there is provided a method for storing and reutilizing energy during operation of a hydrostatic actuator (EHA) having a hydraulic cylinder, a first bidirectional pump-motor rotationally coupled to an electric motor, and EHA valving and connections between the first pump-motor and the hydraulic cylinder by which the EHA is operable in four distinct operating quadrants, including a first actuator-extending pumping quadrant, a second actuator-extending motoring quadrant, a third actuator-retracting pumping quadrant and a fourth actuator-retraction motoring quadrant, said method comprising:
Preferred embodiments of the invention will now be described in conjunction with the accompanying drawings in which:
Reference is made initially to illustration of relevant prior art in
where Aa is the piston face area of the hydraulic cylinder on the cap side thereof, and Ab is the piston face area on the rod side.
To define the operation quadrants, a sign convention for the load pressure,
and cylinder rod velocity, {dot over (x)}p, is established. The cylinder rod velocity, {dot over (x)}p, is termed positive during cylinder extension and negative during retraction. Based on the signs of the cylinder rod velocity and load pressure, four quadrants of operation, I, II, III and IV are defined. Quadrant I is defined for PL>0 and {dot over (x)}p>0. Quadrant III is defined for PL<0 and {dot over (x)}p<0. These are pumping quadrants, where the energy flows from the circuit to the load. On the other hand, motoring quadrants (II and IV) are those where energy flows from the load to the circuit. Quadrant II is defined for PL<0 and {dot over (x)}p>0, while quadrant IV is defined for PL>0 and {dot over (x)}p<0. During quadrants I and III, the pump consumes energy from the prime mover to extend and retract the cylinder. In quadrants II and IV, the load assists the cylinder motion. As a result, energy coming from the load drives the pump, now operating as a motor.
The schematic of the arm linkage of the backhoe arm and the force balance on the actuator are shown in
Turning now to a preferred embodiment of the present invention, the inventive ESR system is shown in
Pump-motor PM1 of the EHA and pump-motor PM2 of the ESR are rotationally interconnected to one another, in the illustrated example, by a pair of respective belt transmissions 23A, 23B each coupled to the driveshaft of the AC servomotor 1. Other than electronic connection of sensors, solenoids and other electronic control componentry for controlled operation of the ESA and ESR system as a functional combination, this mechanical interconnection between the two pump-motors to enable rotation of either one thereof by the other is the sole physical connection between the ESA and the ESR system, the respective hydraulic circuits of which are fluidically isolated from one another. All energy transfer from the hydraulic circuit of one to the other occurs via intermediate conversion and transmission of mechanical energy via the belt transmissions, or other substitutable mechanical means of rotational energy transfer from one pump-motor to the other.
The ESR system has four operation modes: storage, reutilization, discharge and idle flow circulation. The shaft of the pump-motor PM2 can be rotated in two directions during storage and reutilization. Therefore, this design has two storage modes and two reutilization modes. Both storage and reutilization modes are operated using the float-centre directional valve V3, and the reutilization valve V4. The reutilization valve acts as an on-off valve for the hydraulic accumulator discharge. A built-in check valve guarantees that oil can only flow in one direction when storing energy in the hydraulic accumulator. When V4 is activated, oil flows in both directions and the accumulator can discharge into the pump-motor PM2, by activating either side of valve V3. The central position of V3, is used for flow recirculation.
Storage operation modes of the ESR are carried out during the motoring operations of the pump-motor PM1 of the EHA. As such, the gravitational potential energy of the load is stored in the hydraulic accumulator 16 of the ESR. When the assistive force rotates PM1 in counter-clockwise direction (quadrant II), PM2 is likewise rotated in this same counter-clockwise direction via belt transmission 23B, and the ESR circuit is operated to charge the hydraulic accumulator 16 through the pump PM2. To do this, valve V3 of the ESR circuit is shifted to the left (via activation of solenoid A) and while valve V4 is maintained at its rest position, which connects PM2 to the hydraulic accumulator 16. The flow pumped by PM2 passes through valve V3, reaching valve V4 through check valve 17, and continues onward from valve V4 to the hydraulic accumulator, where energy is stored, as shown in
Reutilization modes are initiated when the hydraulic accumulator 16 is fully loaded, and the main pump PM1 requires auxiliary power. It is known that the discharging pressure of hydraulic accumulator 16 is not constant, since the pressure depends on the pressurized gas volume. In order to have a constant-pressure discharge, the EPPRV 15 is used. The EPPRV 15 is, therefore, controlled by the pressures read at transducers 22A, 22B.
The auxiliary power stored in the hydraulic accumulator 16 is used to assist the servomotor 1 in operating the ESA during the energy reutilization process. This is shown in
The discharge and idle flow circulation operation modes are shown in
In order to combine the ESR system with the EHA and activate the ESR system during certain operation quadrants, a control algorithm is implemented. One exemplary embodiment of such control algorithm is shown in
where Pmax and Pmin are the maximum pressure of the accumulator and the pre charged gas pressure, and Pacc is the accumulator pressure) with the predefined storage limit, SOCSTH which limits the amount of energy stored in the accumulator. If SOCP<SOCSTH, then the load pressure PL and the accumulator pressure Pacc are compared. Energy can only be stored in the accumulator when the load pressure is greater than the accumulator pressure, given that, it is only during this period that the pump-motor PM1 is capable of driving the pump-motor PM2. So, the control algorithm checks if SOCP<SOCSTH, and PL>Pacc, and if so, then the control algorithm initiates the energy storage mode by activating the appropriate quadrant-dependent solenoid of valve V3. In addition, the servomotor torque, Te, can be automatically adjusted by the algorithm to an optimal value (by adjusting the servomotor input, U), in order to speed up the system when the load pressure reaches the accumulator pressure. If the load pressure PL is lower than the accumulator pressure Pacc, the motoring energy of PM1 is insufficient to drive PM2. In other words, if the gravitational potential energy of the load is lower than the energy stored in the accumulator 16, this would cause PM1 to stall at motoring quadrants. To overcome this, the servomotor torque, Te, is adjusted back to its original value by the control algorithm, which also switches the ESR system over to idle mode by deactivating the solenoids of valve V3.
If either pumping quadrant is identified as the current operating quadrant, this signals an opportunity for potential reutilization of stored energy from the accumulator. After such identification of either pumping quadrant as the current operating quadrant, the algorithm compares the state of charge SOCP, with the predefined reutilization limit, SOCRTH which determines the activation state of the energy reutilization process. If SOCP>SOCRTH, the solenoid of the reutilization valve V4, is activated by the algorithm to allow the accumulator 16 to discharge energy. Simultaneously, a proportional input is applied to the EPPRV 15 to provide for a predetermined output pressure, Pset. Next, the appropriate quadrant-dependent solenoid of valve V3 is activated to connect the accumulator 16 to PM2. The accumulator pressure Pacc is also compared with the EPPRV output pressure PPPRV to ensure that the accumulator pressure remains at least as high as the output pressure PPPRV of the EPPRV, in which case reutilization can continue, and the servomotor torque, Te, can be adjusted to an appropriate level to complement the power generated by the ESR system. On the other hand, if the accumulator pressure falls below the EPPRV output pressure PPPRV, the reutilization process is terminated by deactivating the solenoids of valve V3, valve V4 and the EPPRV 15, thereby switching the ESR system into idle mode.
The utility of the above described ESR system was experimentally confirmed with test rig employing a John Deere JD-48 backhoe arm, composed of a single rod cylinder that drives an attached inertial load. The load mass of the test rig is composed of a set of iron disks and can be adjusted by adding or removing disks. The cylinder is operated by the above cited and described EHA where the ports of the bidirectional pump-motor are directly connected to the cylinder ports. Flow compensation is, thus, necessary because of the differential cylinder areas and is carried out through a directional valve, controlled by a logic algorithm executed by a general-purpose computer connected to a data acquisition device, which thereby collectively embody an electronic controller. The selected data acquisition device (DAQ) of the test rig is a Quanser Q8, which receives input signals from the cap- and rod-side pressure transducers 12 at the cylinder 3, from a joystick controller and from the rod displacement sensor also provided at the cylinder 3. Based on the pressure and displacement readings, and evaluation thereof by the algorithm, the controller provides control signals to the solenoids of the EHA's directional valve V1. On the other hand, the joystick provides an external input for driving the prime mover (servomotor 1), which can also be commanded through a computer-generated signal, such as a step function.
ESR capability was added to the test rig by incorporating a hydraulic accumulator to store hydraulic energy, which was connected to the EHA using the inventive ESR circuit described above and schematically illustrated in
To validate the proposed design, a prototype of the ESR circuit was built. Before manufacturing the components, the minimum assistive torque that can be generated by the ESR system to the servomotor was determined. The servomotor in the prototyped example produces a maximum torque of 6 Nm to run the EHA, and based on this value, a 2 Nm assistive torque was chosen for the experimental model. Note this value was chosen based on the desired charging time and precharge pressure of the hydraulic accumulator. Next, the hydraulic accumulator was sized and selected. In the prototyped example, an existing gas loaded, piston-type accumulator, with an inner volume of 9.64 L was used. The commercial reference for the chosen accumulator is Parker-A4N0578D1K. The minimum and the maximum working pressures of the ESR system were determined using the maximum energy storage limit and the pre-charge pressure of the accumulator. Once determined, the remaining ESR components were selected.
Simulations and experiments were conducted to verify the efficacy and the applicability of the ESR system. The experimental evaluations suggest that the designed ESR system can decrease the input power of the main source by at least 44.2% in minimum and 70.2% in maximum. Thereby, enhancing the energetic efficiency. This proves the proposed ESR system is highly viable especially for heavy-duty hydraulic arms during load handling. Moreover, due to its easy implementation and simple configuration, it can even be used in any EHA applications for both energy storage and energy reutilization.
Since various modifications can be made in our invention as herein above described, and many apparently widely different embodiments of same made, it is intended that all matter contained in the accompanying specification shall be interpreted as illustrative only and not in a limiting sense.
This application claims benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 63/490,093, filed Mar. 14, 2023, the entirety of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63490093 | Mar 2023 | US |
