Performance of a Pump Controlled Asymmetric Actuator: A Comparison of Different Control Methods

This paper discusses different stabilizing pump control methods to improve and optimize the damping of a pump controlled asymmetric linear actuator response. The main purpose of the paper has been to study the redeeming features and drawbacks of influencing the actuator response by pump control without direct position, velocity or pressure feedback from the actuator. To the author’s knowledge, this type of stabilizing method is novel to pump controlled hydraulic drives. Pole placement techniques with both pump speed and displacement control are discussed and finally the system performance is compared by simulation with a system where dynamic pressure feedback has been used. Preliminary optimal tuning rules for the pump speed controller and pump volumetric displacement value have been derived and their applicability to high, varying inertial loading application has been assessed. Results indicate clear improvements of the actuator response damping level with the suggested rules but also raise the demands for pump torque and displacement range.

A Robust Multi-Input Multi-Output Control Strategy for the Secondary Controlled Hydraulic Hybrid Swing of a Compact Excavator With Variable Accumulator Pressure

Over the last decade, a number of hybrid architectures have been proposed with the main goal of minimizing energy consumption of excavator swing drives. One of the most notorious architectures is the secondary controlled hydraulic swing drive. One of the advantages of this system is that, through the installation of a hydraulic accumulator, energy which otherwise would be wasted can be stored and reutilized on demand. However, the fact that the hydraulic motor in this architecture operates under a constant high pressure at all times diminishes the overall system efficiency significantly. Therefore, to investigate machine power management strategies, it is imperative to formulate a controller that overcomes this weakness. In this paper, a robust multi-input multi-output controller is synthesized for the control of the hybrid swing velocity and for first time the control of the accumulator state of charge. The simplified plant is tested using a high fidelity nonlinear model developed in the Simulink-Matlab environment. The proposed controller is then tested and compared against a PI controller using the optimal accumulator pressure obtained from dynamic programming and the desired cab velocity. Results show satisfactory tracking of the swing drive velocity and pressure. In addition, a study of the nominal stability, robust stability and robust performance of the controlled system reveals the advantages of the H∞ controller.

Parallel Pump-Controlled Multi-Chamber Cylinder

This study focused on the use of fixed displacement pumps in parallel connection to control the velocity of a multi-chamber cylinder piston. The system’s basic principle was to combine the discrete flow supply control of parallel pumps with the discrete effective area control of a multi-chamber cylinder to produce a speed control resolution high enough for accurate velocity tracking and positioning. Some throttling was used in the return line to control the system with overrunning loads. The properties of the system were tested with a 1-DOF boom mockup mimicking a medium-sized mobile machine boom. The test system revealed a feature that caused load acceleration to drop when the effective cylinder area was reduced during movement. Additionally, some delay was observed in accelerating the piston against the load force. These two system properties along with the discrete control method resulted in mediocre speed and position tracking in the system when movement was directed against the load force. The system was able to control restricting and overrunning loads as well as a large inertia mass with a low load force. The system’s energy losses were low considering that no pressure accumulators were used, but the throttling losses in the return line and the lack of energy recuperation leave room for improvement.

Developed Measuring Methods for Hydraulic Fluid Dynamics and Viscosity at Extreme Pressures and Temperatures

Hydraulic fluid is one of the most important components in every fluid power system. Therefore, fluid properties have to be known with a good accuracy in an increasing number of applications, for example in system’s design, modelling and control. The fluid of interest may be a power transmission fluid as well as a fuel. In defining the needed fluid characteristics, the large variety of different fluid types sets many demands for a single measuring system. Moreover, known fluid properties, of fuels in particular, are needed at constantly higher pressures and temperatures, raising the bar for practical measuring concepts — user-friendliness, safety and equipment cost are also essential criteria. In this paper, two accurate, but rather simple and affordable measuring concepts are presented. The speed of sound in a fluid, hydraulic fluid density and adiabatic tangent fluid bulk modulus are all defined with a direct measurement of the pressure wave propagation. The dynamic and kinematic fluid viscosities are defined with a remotely operated, modified falling ball viscometer. Both the presented methods have been developed further from the previously published concepts of the same authors. With these improved systems, all the mentioned fluid parameters can reliably be measured at up to at least 2,500 bar and at up to at least +150°C. Moreover, the same equipment can be applied to any type of hydraulic fluid, a fuel or a power transmission fluid, regardless of the base fluid, additives or viscosity grade. In addition to presenting the measuring concepts and the equipment used in detail, a selected sample of experimental results will also be presented to demonstrate the performance characteristics of the methods.

Experimentally Validated Models of O-Ring Seals for Tiny Hydraulic Cylinders

To enable simulation of tiny hydraulic systems, including predicting system efficiency, it is necessary to determine the effect of the hydraulic cylinder piston seal. For tiny cylinders whose bore is less than 10 mm, O-ring seals are convenient. Simplified models for the O-ring were used to describe piston leakage and friction and based on the models, the force and volumetric efficiencies for tiny cylinders were predicted for a range of steady state operating conditions. To validate the models, a test stand was constructed to collect experimental data for 4, 6 and 9 mm bore cylinders, which were in the form of a vertical ram with a single O-ring seal. The ram was fully extended and put under load. A needle valve was then cracked to cause the ram to descend at different speeds. Pressure, load and velocity were recorded and the data used to calculate cylinder efficiencies, which were then compared to model predictions. The model and the experiment showed essentially zero leakage. The experimental force efficiency had good agreement with the model over a range of operating conditions. The study showed that simple O-ring models for tiny hydraulic cylinders suffice for building system level simulation models.

Research on Double-Axis Electro-Hydraulic Proportional Loading Control System With Intelligent Dual-PID for Membrane Materials

Membrane material is one kind of special material with nonlinear feature. Its mechanical characteristic is so complicated and anisotropic that the performance is quite different between the single-axis loading and the double-axis loading. So the experimental results with the single-axis loading will usually lead to unexpected results. With the development and widely application of membrane materials, the double-axis property experiment plays an increasingly important role all around the world. Considering the situation mentioned above, a double-axis electro-hydraulic proportional loading control system with intelligent dual-PID is proposed in this paper, which controls four cylinders respectively with the help of four electro-hydraulic proportional directional valves. The cylinders provide pulling force to membrane material directly under the control of electro-hydraulic proportional directional valves. And the opening of each electro-hydraulic proportional directional valve is adjusted by intelligent dual-PID according to the pulling force as well as the position of cylinder. The simulation model of double-axis electro-hydraulic proportional loading control system with intelligent dual-PID is built and an intelligent dual-PID algorithm is also designed in this paper. The simulation and experiments show that the double-axis electro-hydraulic proportional loading control system with intelligent dual-PID can fulfill the loading precision requirement of double-axis membrane material experiment.

Stability Analysis of a Pilot Operated Counterbalance Valve for a Big Flow Rate

Counterbalance valves are widely used in hydraulic deck machinery to balance the overrunning loads. However, as is well known, counterbalance circuit designed with poor choice of counterbalance valve tends to introduce instability to the system. This paper investigates the dynamic behavior of a pilot operated counterbalance valve which can operate at a flow rate about 2000L/min. A linearized stability analysis of such a hydraulic circuit which consists of a slip in cartridge, a pilot counterbalance valve and a hydraulic winch is presented. Pole-zero plots are employed to reveal the effect of the volume of control cavity, the hydraulic resistance on pilot line and counterbalance valve pilot area ratio on the stability of the system. The analysis results indicate that such a system will be unstable within the normal range of each parameter. An alternative approach that guarantees system stability by adding an accumulator on the pilot line is put forward. The approach stabilizes the pilot pressure by reducing the hydro-stiffness of pilot control cavity, thus the system can reach its stability condition. Finally, a numerical optimization method is putted forward, with the optimized parameters, the dynamic performance of considered system become better.

Comparison of Proportional Control and Displacement Control Using Digital Hydraulic Power Management System

The Digital Hydraulic Power Management System (DHPMS) is a solution based on the digital pump-motor technology and has shown to be a promising approach to improve the energy efficiency of hydraulic systems. The DHPMS is controlled by active on/off valves, but unlike the digital pump-motors the DHPMS has multiple independent outlets; hence, the DHPMS can operate also as a transformer. In this experimental study, a proportional control of a mobile boom is compared with a displacement control when a six-piston DHPMS is used. In the proportional control, the system pressure is controlled by the DHPMS and a lift cylinder with a proportional valve. In the displacement control, the cylinder fluid volumes are controlled directly using the DHPMS. Firstly, the systems under study are presented along with the control methods. Then the control performance of the DHPMS is studied and finally, the energy losses in the systems are analysed. The results show the versatility of the DHPMS; it is capable of fast and accurate pressure control but also handles the direct flow control. According to the measurements, the losses are significantly smaller in the displacement controlled system thanks to the minimised throttling losses and the energy recovery. Nevertheless, the energy losses in the prototype DHPMS are rather high due to the leakage in the control valves and their low flow capacity, and therefore improvements in the design are needed.

Fatigue Life Improvement of the Roller Swashplate Bearing of an Axial Swashplate Type Piston Pump

The roller swashplate bearing in hydraulic piston pumps is very unique in applications of roller element bearings. The main advantages of this type of bearings are low friction, very small difference between start torque and steady state torque, and no additional pressure source is required for their functioning. Therefore, better control performance and higher efficiency can be expected from piston pumps with roller swashplate bearings, compared with pumps with hydrostatic or plain ones. However, it is particularly challenging to design a roller swashplate bearing to meet the very demanding and widely varying load requirements for heavy duty off-highway machine operations, considering cost and pump size limitations. Quite different from most other roller bearing applications, a roller swashplate bearing for heavy duty hydraulic piston pumps is subject to (a) extremely high contact pressure between rollers and races due to high system pressure, (b) high frequency oscillating forces in three dimensions caused by the torque ripple and discharge pressure ripple inherent in pump rotating group, (c) very unfavorable local lubrication conditions when the pump is operated with no or little displacement change, and (d) sliding between rollers and races because of frequent sudden pump displacement changes. Based on the observed failure modes of bearings in this application, this paper discusses several measures to improve the fatigue life of the swashplate roller bearing for an axial swashplate type piston pump used in a heavy duty off-highway machine. Analyses, simulation results, and machine operation results are provided to show the effectiveness of the proposed measures.

Modeling and Simulation of the Swing System of Crawler-Type Medium Hydraulic Excavator Using Dynamic Friction Model

An analysis model is presented in this paper to simulate the dynamics of the swing system of crawler-type medium hydraulic excavator. It is found from experiments that the static friction model cannot simulate actual friction phenomena such as the Stribeck effect which can be observed in real swing system. A dynamic friction model, i.e., the simplified LuGre model, has been implemented for more accurate simulation in the theoretical analysis. The validity of the simulation model adopting the dynamic friction model has been verified by comparing in time domain the simulated swing angle with that from actual swing test. Cross-correlation between the simulated and measured swing angles turned out to be 0.92. It can be concluded, therefore, that the proposed dynamic friction model considerably improves the simulation accuracy.