Variable Displacement Hydraulic Pump Research Articles

Design, Dynamic Modeling, and Experimental Validation of A Novel Alternating Flow Variable Displacement Hydraulic Pump

High power density, good controllability, and low cost are the most appealing characteristics that make hydraulic systems the best choice for many applications. Current state-of-the-art hydraulic variable displacement pumps show high efficiency at high displacement while they have poor efficiencies at low displacement. This paper proposes a novel alternating flow (AF) variable displacement hydraulic pump to: 1) eliminate metering losses by acting as a high-bandwidth pump for displacement control; 2) achieve high efficiency across a wide range of operating conditions and displacements; and 3) allow multiple units to be easily common-shaft mounted for a compact multi-actuator displacement control system from a single prime mover. A dynamic model using first principles describes the cylinder pressure, flows between pairs of cylinders, and net inlet and outlet flows as a function of the pumps phase-shift angle. The model captures hydraulic check valve dynamics, the effective bulk modulus, leakage flows, and viscous friction. Piston kinematics and dynamics are discussed and energy loss models are presented. A first prototype of the AF pump was constructed from two inline triplex pumps that were modified so that three piston pairs were created. Experiment tests were conducted and used to validate the model. After model validation, simulation results from the model can offer an initial evaluation of this novel pump concept and potential applications and can be used to guide the design for future prototype of the AF hydraulic pump.

Research on Response Characteristics and Energy Efficiency of Power Unit Used for Electric Driving Mobile Machine

Nowadays, the hydraulic pump driven by the diesel engine is widely used as the power unit for mobile machines, which consumes a large amount of fuel and cause high emissions. One solution is to use the electric driving mode. In this paper, the energy consumption characteristics of 4 electro-hydraulic power units, which are a fixed speed motor or a variable speed motor to drive a fixed displacement hydraulic pump or a variable displacement hydraulic pump, are compared and analyzed respectively. The results show that the power unit which a variable displacement hydraulic pump is driven by a variable speed asynchronous motor has the highest cost-performance ratio. However, due to the limitation of the battery peak current, the dynamic response of the power unit is slow. To solve this problem, a new power unit is proposed which add an accumulator to the hydraulic pump outlet to recover the power unit braking kinetic energy. The high-pressure oil stored in the accumulator can be absorbed into the hydraulic pump inlet to improve the power unit starting and braking performance and energy efficiency. The starting and braking characteristics of the power unit with and without the accumulator are compared experimentally. The results show that, if the power unit is driven by the motor alone, the power unit response is slow, and the load pressure has a significant influence on the response time. After adding the accumulator to the hydraulic pump, the power unit starting and braking time could be shortened significantly, and the starting power demand is also reduced. The proposed power unit is further used to drive a hydraulic excavator. Under the same working conditions and operating characteristics, energy consumption can be reduced by 28.5% compared with using the variable displacement hydraulic pump alone.

Thermodynamic Analysis of Hydraulic Braking Energy Recovery Systems for a Vehicle

In this study, a thermodynamic analysis of a hydraulic braking energy recovery system used in vehicles is performed for newly developed systems. The present system is related to the field of energy efficiency in vehicles. The energy recovery system comprises a first pump, a hydraulic accumulator, and a hydraulic motor. The first pump is a variable displacement hydraulic pump (VDP). The hydraulic accumulator is connected to the first pump which operates to store hydraulic fluid under pressure. The hydraulic motor is hydraulically connected to the accumulator to receive hydraulic fluid. The motor is adapted to drive a second hydraulic pump, which is hydraulically connected to the auxiliary system, using hydraulic energy stored in the accumulator. The overall charging and discharging efficiencies, and the overall system efficiency is calculated and presented in this paper. For the purpose of the analysis, EES (engineering equation solver) is used. In addition, parametric studies are performed to observe the effects of different substantial parameters, namely, the inlet pressure and temperature of the accumulator, and the reference environment temperature, in order to investigate the variations in the system performance in terms of the efficiencies. Two systems are developed and it is found that the charging and discharging efficiencies for one system are 83.81% and 87.73%, while for the other system the charging and discharging efficiencies are 81.84% and 85.67%, respectively.

Adaptive sliding mode back-stepping pitch angle control of a variable-displacement pump controlled pitch system for wind turbines

A variable-displacement pump controlled pitch system is proposed to mitigate generator power and flap-wise load fluctuations for wind turbines. The pitch system mainly consists of a variable-displacement hydraulic pump, a fixed-displacement hydraulic motor and a gear set. The hydraulic motor can be accurately regulated by controlling the pump displacement and fluid flows to change the pitch angle through the gear set. The detailed mathematical representation and dynamic characteristics of the proposed pitch system are thoroughly analyzed. An adaptive sliding mode pump displacement controller and a back-stepping stroke piston controller are designed for the proposed pitch system such that the resulting pitch angle tracks its desired value regardless of external disturbances and uncertainties. The effectiveness and control efficiency of the proposed pitch system and controllers have been verified by using realistic dataset of a 750 kW research wind turbine.

アキュムレータを用いたアイドリングストップ方式による油圧源の省エネルギー

As a result of a shortage of energy resources such as oil and the need to protect the earth’s environment, energy saving efforts have been made in a number of fields. In the case of hydraulic equipment, energy saving in pump units is key because this unit is the major consumer of energy.Up to the present, variable-displacement hydraulic pump units (VD pump units) and inverter-control hydraulic pump units (INV pump units) have been widely employed. Here the authors propose the use of an intermittently operated hydraulic pump unit coupled with an accumulator (ACC pump unit) . In a previous report, the superiority of the ACC pump unit was demonstrated through an experiment using a modeled machine tool and a simplified load model. In this report, we compare electric power consumption, pump output pressure, rotation speed and efficiency of the VD pump unit, INV pump unit and ACC pump unit when hydraulic fluid is supplied to the load at a fixed flow rate. In addition, the effects of the accumulator volume and the pressure ratio between the upper and lower limits on efficiency and the ON-OFF switching period of the ACC pump unit are determined.

Modeling and simulation of hydrostatic transmission system with energy regeneration using hydraulic accumulator

A new hydraulic closed-loop hydrostatic transmission (HST) energy-saving system is proposed in this paper. The system improves the efficiency of the primary power source. Furthermore, the system is energy regenerative, highly efficient even under partial load conditions. It can work in either a flow or pressure coupling configuration, allowing it to avoid the disadvantages of each configuration. A hydraulic accumulator, the key component of the energy regenerative modality, can be decoupled from or coupled to the HST circuit to improve the efficiency of the system in low-speed, high-torque situations. The accumulator is used in a novel way to recover the kinetic energy without reversion of fluid flow. Both variable displacement hydraulic pump /motors are used when the system operates in the flow coupling configuration so as to enable it to meet the difficult requirements of some industrial and mobile applications. Modeling and a simulation were undertaken with regard to testing the primary energy sources in the two configurations and recovering the energy potential of the system. The results indicated that the low efficiency of traditional HSTs under partial load conditions can be improved by utilizing the pressure coupling configuration. The round-trip efficiency of the system in the energy recovery testing varied from 32% to 66% when the losses of the load were taken into account.

Electric-hydrostatic drive—an innovative energy-saving power and motion control system

The paper describes investigations of an electric-hydrostatic drive system, its components and its application in the clamp of a plastics injection moulding machine. The aim of the research work was to reveal the potential advantages and challenges of the new direct pump control concept as well as the suitability of hydraulic pumps for variable-speed operation. Measurements of component efficiency have proven that hydraulic components have very good efficiency compared with variable-speed electric motors. Therefore, the total efficiency of the electric-hydrostatic drive at a high load (close to corner power) cannot be expected to be noticeably better than the conventional drive system comprising an asynchronous electric motor and variable-displacement hydraulic pump, although fixed-displacement pumps can have better efficiency than variable-displacement pumps have. Improvements may be seen at part-load. The major advantages of the new motor-pump concept are reduction in power losses and noise during part-load and during idling. This was demonstrated in a test cycle. The design of hydraulic pumps applied in an electric-hydrostatic drive has to be carefully evaluated for suitability. Some piston pumps and internal gear pumps without gap compensation seem to work well under the specific load conditions of variable-speed drives, but not all units worked sufficiently during endurance testing. Owing to the high overload capability of rare earth electric motors, the dynamic performance of the electric-hydrostatic drive can be compared with that of many standard variable-displacement pumps. This was evaluated with performance tests of the clamp unit of a plastics injection moulding machine. The dry cycle time (Euromap 6 standard) achieved (1.8 s for a machine with a 1600 kN clamping force) proved the excellent controllability of the electric-hydrostatic drive and the high dynamic performance capability. The combination of a speed-controlled a. c. servomotor and fixed-displacement pump increases the number of direct pump control concepts that a hydraulic project engineer can apply to design competitive hydraulic systems for customer applications.

ELECTRIC-HYDROSTATIC DRIVE SYSTEMS AND THEIR APPLICATION IN INJECTION MOULDING MACHINES

The aim of the research work was to reveal the potential advantages and challenges of the electric-hydrostatic drive system, a new direct pump control concept. Measurements of component efficiency have proved that hydraulic components have a very good efficiency compared with variable-speed electric motors. Therefore, the total efficiency of the electric-hydrostatic drive at high load cannot be expected to be noticeably better than that of the conventional drive system comprising an asynchronous electric motor and a variable displacement hydraulic pump. The major advantages of the new motor-pump concept are reduction of power losses and noise during part-load and during idling. Due to the high overload capability of rare-earth electric motors, the dynamic performance of the electric-hydrostatic drive can be compared to that of many standard variable displacement pumps. That was evaluated by performance tests of the clamp unit of a plastics injection moulding machine.The electric-hydrostatic drive system increases the number of direct pump control concepts which a hydraulic project engineer can apply to design competitive hydraulic systems for customer applications.

Quantitative feedback design for a variable-displacement hydraulic vane pump

In this paper the model development, problem specification, constraint formulation, and optimal feedback controller design for a variable-displacement hydraulic pump system are shown using the Quantitative Feedback Theory (QFT) technique. The use of variable-displacement pumps in hydraulic system applications has become widespread due to their efficiency advantages; however, this efficiency gain is often accompanied by a degradation of system stability. Here we develop a QFT controller for a variable-displacement pump based upon a linear, parametrically uncertain model in which some of this uncertainty reflects variation in operating point-dependent parameters. After presentation of a realistic non-linear differential equation model, the linearized model is developed and the effect of parametric uncertainty is reviewed. From this point, closed-loop performance specifications are formulated and the QFT design technique is carried out. An initial feasible controller is designed, and this design is optimized via a non-linear programming technique. In conclusion, a non-linear closed-loop system response is simulated. This paper is intended to have tutorial value, both in terms of the detailed hydraulic system model development, as well as in terms of the detailed exposition of the QFT controller design and optimal loop shaping processes. © 1998 John Wiley & Sons, Ltd.

Analogue and Digital Modelling in the Design of a Hydraulic Vehicular Transmission

This paper is concerned with the design and development of a hydraulic hybrid urban vehicle power plant using a small internal combustion engine prime mover, a hydraulic transmission, and a hydraulic accumulator as a small energy store. The driver controls the vehicle velocity by operating the swashplate of the axial-piston variable-displacement hydraulic motor. The remaining system inputs, i.e., the engine throttle and swashplate of the axial-piston variable-displacement hydraulic pump, are governed by a multiloop electronic controller via electro-hydraulic servo-actuators. The system aims to achieve a high overall efficiency by utilizing the hydraulic accumulator to maintain optimal loading of the prime mover and to provide regenerative braking. The design was aided by an analogue simulation implemented on an EAI 690 hybrid computer. For the final quantitative evaluation of the system, a digital simulation program was written using the MIMIC processor. Realistic modelling was attained in both cases by using manufacturers' data for commercially available components to define the dynamic response and efficiency of the individual elements of the overall system. The simulation model was subjected to accelerator/decelerator step input signals under various road conditions. ‘Drives’ through the standard LA-4 and EPA standard urban cycles were performed. The simulation results showed high system stability, satisfactory acceleration and deceleration performance and a good overall efficiency.