Modeling and Simulation of Hydromechanically Constant Power Controlled Swash Plate Pumps

This paper presents a theoretical study of the performance of constant power operated swash plate pumps equipped with hydromechanical controllers incorporating either pivoted levers or two feedback springs. Mathematical models of these controllers are derived and used to simulate the static and dynamic characteristics of a small pump of 40 cc/rev geometric volume. Results show that the controller with the pivoted lever renders better static and dynamic characteristics compared to the controllers with feedback springs. Results also show that changing the power through varying the lever arm length is preferable than varying the valve spring initial compression, when the dynamic characteristics of the pumps with controllers of pivoted levers are considered. The effect of the valve spring initial compression on the static performance of a pump incorporating a controller with two feedback springs is also investigated.

Research on Electro-Hydraulic Control of Propellers of the Underwater Vehicles With Switch-Mode Hydraulic Power Supply

Switch-mode hydraulic power supply is a hydraulic pressure converting unit made of some distributed hydraulic components, which can boost or buck hydraulic pressure continuously with low power loss (about 20%)and continuous flow-rate. There are two types of switch-mode hydraulic power supply, pressure boost and pressure buck. (see "Switch-mode Hydraulic Power Supply Theory", 2005 ASME, IMECE-FPST No.79019)[1]. This paper introduces a new propeller driving system using the motor of the switch-mode hydraulic power supply for the underwater vehicle. And PFM (Pulse Frequency Modulation) control of high-speed switch-valves is applied to adjust the rotation speed of the propeller. The system has advantages over the widely used servo-valve valve-control system and pump-control system on the energy-weight ratio, anti-contamination performance and energy-saving capacity.

Deformation Effects on the Load Carrying Capacity of the Barrel Bearing in Axial Piston Pumps and Motors

Most hydrostatic pumps and motors apply mechanical face seals, often also acting as a thrust bearing. The load carrying capacity of these bearings is very much dependent on the pressure profile generated in the sealing gap. Previous research, outside pumps and motors, has already shown that the gap pressure profile is largely influenced by small radial deformations of the seal lands. This paper discusses the elastic deformation of pump components and the effects of these deformations on the load carrying capacity of a barrel in an axial piston machine.

Energy-Efficient Servo Control of Pneumatic Systems via Direct Cross Flow

This paper proposes a structure and control approach for the energy saving servo control of a pneumatic servo system. The energy saving approach is enabled by supplementing a standard four-way spool valve controlled pneumatic actuator with an additional two-way valve that enables flow between the cylinder chambers. The "crossflow" valve enables recirculation of pressurized air, and thus enables the extraction of stored energy that would otherwise be exhausted to atmosphere. A control approach is formulated that supplements, to the extent possible, the mass flow required by a sliding mode controller with the recirculated mass flow provided by the crossflow valve. Following the control formulation, experimental results are presented that indicate energy savings of 25% to 52%, with essentially no compromise in tracking performance relative to the standard sliding mode control approach (i.e., relative to control via a standard four-way spool valve, without the supplemental flow provided by the crossflow valve).

Software Enabled Variable Displacement Pumps: Experimental Studies

The majority of hydraulic systems are controlled using a metering valve or the use of variable displacement pumps. Metering valve control is compact and has a high control bandwidth, but it is energy inefficient due to throttling losses. Variable displacement pumps are far more efficient as the pump only produces the required flow, but comes with the cost of additional bulk, sluggish response, and added cost. In a previous paper [1], a hydromechanical analog of an electronic switch-mode power supply was proposed to create the functional equivalent of a variable displacement pump. This approach combines a fixed displacement pump with a pulse-width-modulated (PWM) on/off valve, a check valve, and an accumulator. The effective pump displacement can be varied by adjusting the PWM duty ratio. Since on/off valves exhibit low loss when fully open or fully closed, the proposed system is potentially more energy efficient than metering valve control, while achieving this efficiency without many of the shortcomings of traditional variable displacement pumps. The system also allows for a host of programmable features that can be implemented via control of the PWM duty ratio. This paper presents initial experimental validation of the concept as well as an investigation of the system efficiency. The experimental apparatus was built using available off-the-shelf components and uses a linear proportional spindle valve as the PWM valve. Experimental results confirm that the proposed approach can achieve variable control function more efficiently than a valve controlled system, and that by increasing the PWM frequency and adding closed-loop control can decrease system response times and of the output ripple magnitude. Sources of inefficiency and their contributions are also investigated via modeling, simulation and are validated by experiments. These indicate design parameters for improving inefficiency.

Bulk Modulus of Air Content Oil in a Hydraulic Cylinder

A model of oil with entrained air content is developed which considers fluid compression and the subsequent dissolving of mixed entrained air. According to the model the mixed entrained air affects the "gross" bulk modulus below some critical pressure, but has no effect above this value due to the complete dissolving of the entrained air into solution. The critical pressure is shown to be proportional to the square root of the amount of the initial mixed entrained air. The temporal pressure gradient has also a substantial effect on the critical pressure value and thus on the bulk modulus. The critical pressure value increases but tends towards an upper value with increasing temporal pressure gradient (a true dynamic condition); the opposite occurs when the pressure gradient decreases as the critical pressure converges to a lower value (essentially a static value). Thus regions of static and dynamic bulk modulus can be established. The model predicts that the upper critical pressure value is some 1.8 times that of the static one. Experiments have been designed to verify the feasibility of the model by measuring the temporal pressure gradient against the variation of compressed oil volume. It is demonstrated that the model is verified not only for the case of positive pressures (above atmospheric pressure) but also for pressures less than atmosphere. Finally a comparison of the proposed model is made with those proposed in the literature. The bulk modulus predicted by the proposed model is a little larger than these given in literature. The reason for such difference is attributed to the result of air being dissolved into oil.

Cascaded Robust Nonlinear Position Tracking Control of an Electrohydraulic Actuator: Sliding Mode and Feed Forward

An approach to position tracking control based on a cascade of a nonlinear force tracking controller derived from a near input-output linearization framework and a simple feedback plus feed forward position controller is presented. The method exploits the cascade structure to employ a sliding mode pressure force tracking controller as inner-loop and the position tracking controller as an outer-loop. Furthermore, it is highlighted that Lyapunov backstepping analysis can be used to drive performance bounds and reveal trade-offs between the size of uncertainty and measurement errors and the tracking accuracy. The performance of the proposed cascaded robust controller is demonstrated with experiments and simulations on a test system that doesn't necessarily satisfy all of the assumptions made for controller derivation. In particular, a typical comparison of the robust and nominal cascade controllers shows the robust version can recover the performance of the nominal near IO linearizing controller. In addition, model simulation results are included to show the performance of the controller in the presence of some combinations of perturbations or difficult to estimate parameters such as valve coefficient, supply pressure, piston friction, and inclusion of servovalve spool dynamics.

Application of Differential Evolution in System Identification of Servo-Hydraulic System With a Flexible Load

Electro Hydraulic Servo Systems (EHSS) with an asymmetrical cylinder are commonly used in industry. These kinds of systems are nonlinear in nature and their dynamic equations have several unknown parameters. System identification is a prerequisite to analysis of a dynamic system and design of an appropriate controller for improving its performance. In conventional identification methods, a model structure is selected and the parameters of that model are calculated by optimizing an objective function. This process usually requires a large set of input/output data from the system. In addition, the obtained parameters may be only locally optimal. One of the most promising novel evolutionary algorithms is the Differential Evolution (DE) algorithm for solving global optimization problems with continuous parameters. In this article, the DE algorithm is proposed for handling nonlinear constraint functions with boundary limits of variables to find the best parameters of a nonlinear servo-hydraulic system with flexible load. The DE guarantees Fast speed convergence and accurate solutions regardless the initial conditions of parameters. The results suggest that, DE is useful, reliable and easy to use tools in many aspects of control engineering and especially in system identification.

Robust Electric Load Sensing Applied to an Open Circuit Axial Piston Pump

Electronic load sensing (ELS) systems have long been expected to start replacing traditional (hydro-mechanical) load sensing (LS) systems, which today are the standard on most medium and high-end mobile hydraulic applications. The reason for this being the potentials that ELS brings relative to traditional LS-systems, in terms of better control and system utilization possibilities, combined with the increased acceptance and use of electronic sensors in mobile hydraulic machinery. The current work is to evaluate the suitability of an ELS concept applied to a Sauer Danfoss Series 45 H-frame open circuit axial piston pump. Emphasis is on performance robustness with respect to both variations in internal physical parameters of the pump as well as the type of application dependent load which the pump is expected to drive. The pressure control is established by means of cascade control utilizing four control loops with the outer being the pump pressure and then, in succession, the swash plate rotation, the spool position and the voice coil current. The proposed controller has been implemented experimentally and verified.

Energy-Based Control of a Pneumatic Oscillator With Application to Energy Efficient Hopping Robots

This paper presents an energetically derived control methodology to specify and regulate the frequency and amplitude of oscillatory motion of a pneumatic actuation system. A lossless horizontal pneumatic actuation system with an inertia is energetically shown to represent an oscillator with a stiffness, and hence frequency, related to the equilibrium pressures in the actuator. Following from an analysis of the conservative energy storage elements in the system, a control methodology is derived to sustain a specified frequency and amplitude of oscillation in the presence of energy dissipation. The control strategy takes advantage of the natural passive dynamics of the system to provide much of the required actuation forces, while the remaining forces needed to overcome the energy dissipation present in a non-ideal system with losses are provided by a nonlinear control law for the charging and discharging of the actuator. This control methodology is demonstrated experimentally to provide accurate and repeatable frequency and amplitude control of oscillation in the presence of dissipative forces. Finally, the energetic analysis of the horizontal pneumatic system is extended to the vertical case where gravity is present. This extension provides an energetically efficient hopping control methodology for pneumatically actuated robots.