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.

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.

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.

Feed-Forward Control of an Arbitrary Pressure Pulsation Generator and its Application to Testing an Orifice-Type Flow Meter

This paper presents a compact arbitrary pressure pulsation generator for assessing the characteristics of flow meters and sensors, particularly around their zero points. In this study, a compact arbitrary pressure pulsation generator was created using some pneumatic components, such as a high-precision quick-response pneumatic pressure regulator (HPR) and a spool-type servo-valve. The feed-forward compensation of the controller provided the desired pressure pulse to the tested flow meter by controlling the spool-type servo valve; 30 Hz was achieved. In order to validate the effectiveness of the developed generator, we evaluated the dynamic characteristics around the zero point of an orifice-type flow meter in the range from 1 Hz through 30 Hz.

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.

State Estimation and Fault Detection of an Electrohydrostatic Actuator

The electrohydrostatic actuator (EHA) is an efficient type of linear actuator commonly found in aerospace applications. It consists of an external gear pump (fluid), an electric motor, a closed hydraulic circuit, a number of control valves and ports, and a linear actuator. An EHA, built for experimentation, is studied in this paper. Two types of estimation strategies, the popular Kalman filter (KF) and the smooth variable structure filter (SVSF), are applied to the EHA for kinematic state and parameter estimation. The KF strategy yields the statistical optimal solution to linear estimation problems. However, the KF becomes unstable when strict assumptions are violated. The SVSF is an estimation strategy based on sliding mode concepts, which brings an inherent amount of stability to the estimation process. Recent advances in SVSF theory include a time-varying smoothing boundary layer. This method, known as the SVSF-VBL, offers an optimal formulation of the SVSF as well as a method for detecting changes or faults in a system. In addition to the application of the KF and SVSF for state estimation, the SVSF-VBL is applied to the EHA for the purposes of fault detection. The EHA is operated under various operating conditions (normal, friction fault, leakage fault, and so on), and the experimental results are presented and discussed.