Mathematical Modeling of Pulsation Dampeners in Fluid Power Systems

This paper presents a new method for computer-aided modeling and analyzing of pulsation dampeners used in fluid power systems for vibration reduction. The pulsation dampeners are widely used in various fluid power systems to reduce vibration induced by power pumps. The vibration induced by power pumps in fluid systems may be severe enough to cause the damage of components in pipelines if a pulsation dampener is not installed. However, the current methods used in industries for the design and analysis of the dampeners are manually experience-orientated procedures. They are not adaptable to new technologies. The new modeling method will efficiently automate and improve the current modeling and analysis procedure of various pulsation dampeners with a minimum user effort. The proposed method is a result of utilizing the analogy between electrical circuits and hydraulic circuits. In the new method, a spherical pulsation dampener can be equivalent to a lumped hydraulic circuit installed in a distributed fluid pipeline system. The new method has been developed from the authors’ previous work of an impedance-based model in which only the effect of capacitance and inductance was considered without fluid resistance. In reality, the influence of fluid resistance is significant. This paper will take fluid resistance into considerations and form a resistance-impedance-based model.

Design of Fluid Drive Spindle and Its Rotational Speed Control

Water drive spindle has been developed as a spindle for ultra-precision machine tool. Performances of the water drive spindle were evaluated by experiments and simulations. In addition, the spindle was applied to diamond cutting experiments, then, successfully the fine mirrored surfaces were finished. However, rotational direction of the water drive spindle is limited, which is due to the structure of the spindle. Thus, development of water drive spindle that is capable of rotating spindle rotor for both rotational directions is current our objective. In advance of developing the water drive spindle, fluid drive spindle that is similar structure with the water drive spindle, is designed and tested in the present paper. Furthermore, control performances of the fluid drive spindle are studied through simulations. Linearized mathematical models of the fluid drive spindle and servo valve are introduced, then, they are used for the simulations. It is verified that the developed fluid drive spindle is able to rotate for both rotational directions and the spindle speed can be controlled by designed feedback controller.

A Concept of a Knowledge-Based Assistance System for Servohydraulic Drive Design

Outsourcing of drive engineering is a tendency to be observed in many industrial enterprises today. As in-house expertise diminishes, competitiveness of hydraulic drive systems compared to electrical drives gains importance. Servohydraulic control systems, however, require very specific and complex approaches to circuit and control design. They are thus demanding on the designer’s knowledge and experience, which often leads to hydraulic solutions being implemented unsatisfactorily or not at all. The concept developed in this paper therefore aims at providing support in drive system design to inexperienced customers or sales people. In order to make the diversity of existing expertise accessible and utilizable, a knowledge-based approach is proposed. The objective is seen in an interactive software tool that guides the user through the iterative process of analyzing the problem, planning and designing the most appropriate drive solution. Requirements and selection criteria for a development environment are specified, and an expert system shell is selected as a means of implementation. A taxonomy of linear hydraulic servo drives is created that provides a frame for an object-oriented knowledge base. Knowledge extracted from text books and expert interviews is integrated into the system as a collection of rules and facts. Special attention is paid to the selection of a control strategy that delivers optimum performance beyond the capability of standard PID controllers. A comprehensive survey of hydraulic control technology is to ascertain that the developed expert system employs both approved and novel techniques to the benefit of overall system performance.

Investigation of a High Precision Hydrostatic Actuation System: How Nonlinearities Affect Its Performance

A prototype Electro-Hydraulic Actuator (EHA) system has demonstrated a positional accuracy in the order of 100 nanometer. Linearized models of the EHA have been formulated and have shown reasonable correlation to the performance of the physical EHA. However, these models predict zero steady state error (an impossible situation given the physical limitations of seals, friction etc.). Further, the prototype EHA indicates that the cut-off frequency decreases as the amplitude of the input signal decreases. This is not predicted by the linear models. In this paper the Bond-graph large scale modeling technique was used as the basis to formulate the describing equations of the EHA. The model was made increasingly more complex by introducing observable nonlinearities into the model. It was found that the introduction of nonlinear friction did show results whose trends were consistent with those observed experimentally. Assumed nonlinearities in the bulk modulus could not be substantiated. In addition, some of the observed experimental trends could not be predicted (such as order change) and pose additional challenges to be solved before a complete understanding of the true physics of the EHA can be realized.

Improved Loss Modeling of Hydrostatic Units: Requirement for Precise Simulation of Mobile Working Machine Drivelines

The aim of this paper is to analyze different loss modeling methods for hydrostatic pumps and motors as an input for dynamic system simulation. Using dynamic simulation in an early design phase allows for optimization of driveline, driveline control and working hydraulic within several operating conditions. Using precise loss modeling methods enables prediction of energy consumption and energy losses over various duty cycles. Rising energy costs, enhanced government guidelines and increased environmental awareness require more efficient drive concepts especially in mobile working machines. Because of the complex machine structure with numerous hydraulic power consumers, like driveline, working, steering and braking hydraulic, dynamic system simulation is a need in the early design phase. This paper focuses on partial and full hydrostatic drivelines of mobile working machines which operate mostly on basis of a closed circuit hydrostatic transmission. Hydrostatic power lines connect pump and motor directly without additional losses because of valves or system characteristics. Therefore losses in the hydrostatic units have major relevance for the loss behavior of the hydrostatic transmission. The use of precise loss models is essential for analyzing hydrostatic transmission by means of dynamic system simulation. In the scope of this paper, one physical-empiric and four mathematical loss modeling methods are introduced, investigated and compared. Finally, the most promising modeling method is used for the simulation of a state of the art hydrostatic transmission.

Design and Analysis of Hydraulic Pumping Units

The dynamic performance of hydraulic beam pumping units was analyzed in this paper by using the theory of mechanical vibrations. The house-head movement of the pumping unit is mainly uniform, except the alternation period of upper- and down-strokes. Under the action of the house-head movement, the vibration of the system, the sucker-rod and, furthermore, the dynamic stress will be induced. The results indicate that the movement of the downhole pump includes two parts. One is the movement following the horse-head. The other is the dynamic response excited by the support movement. When the parameters of the system are selected reasonably, over-stroke of the pump will appear. This is because the movement of the hydraulic piston obeys a particular law. The maximum displacement increases, and the maximum dynamic stress decreases with depth. The changing of maximum dynamic stress with depth obeys quadratic law.

Impedance Shaping for Improved Feel in Hydraulic Systems

Applying haptic control to mobile hydraulic equipment presents a practical yet challenging application. One criticism of newer electro-hydraulic system is a lack of “feel.” To a haptics researcher this sounds like a call for haptic feedback in the human-machine interface. However, for an operator the “feel” of the system likely has more to do with how the actual system responds to forces or higher work port pressures. At some point, the high pressures slow down the system or naturally redirect flow to lower pressure circuits in a hydro-mechanical system. How this is done plays a large part in the “feel” of the system. In this paper, a paradigm is presented that tries to merge these two concepts of “feel.” Instead of trying to make the system transparent, the goal is to make the system react to forces acting on the system then use haptic feedback to help alert the operator to these forces. This is done by shaping this impedance so that the system provides a response or “feel” that is closer to a typical excavator. A haptic interface is used to enhance the haptic feel. Performance is evaluated using data from human-in-the-loop testing.

Study on Mathematic Model of Air Valve Based on Real Gas Characteristics

Air valve is an important measure of water hammer protection in long water supply system. Accurate simulation of the air-inlet and air outlet process of air valve is directly relative to the safety of water supply engineering. Operational principle of air valve is analyzed and new mathematic model of air valve is built based on Van der Waals equation. Protective function of air valve in transient process caused by valve closing is analyzed with the characteristics method. The result shows the new mathematic model of air valve presents a series of new characteristics in the process of air-inlet and air-outlet comparing with the old mathematic model based on ideal-gas state equation.

Discrete-Time H2-Optimal Control for Hydraulic Position Control Systems

Hydraulic position control systems play an important role in industrial automation. This paper explores the application of discrete-time H2-optimal control for a hydraulic position control system (HPCS). By minimizing the H2-norm of the system, the discrete-time robust H2-optimal control both stabilizes the plant and minimizes the root-mean-square of the servo position error simultaneously. The intuitive nature of this advanced approach helps to manage the selection of design parameters, whereas, classical methods provide less insight into strategies for parameter selection and control design. Additionally, the powerful ability to address disturbances and uncertainty in the robust H2-optimal design offers a more direct alternative to the ad hoc and iterative nature of classical methods for the hydraulic servo position system. Computer simulations illustrate the design procedure and the effectiveness of the proposed method. Experimental studies which employ the H2-optimal control on a hydraulic positioning system are also conducted and the results show that the method is suitable for practical applications.

High Speed Rotary Pulse Width Modulated On/Off Valve

A key enabling technology to effective on/off valve based control of hydraulic systems is the high speed on/off valve. High speed valves improve system efficiency for a given PWM frequency, offer faster control bandwidth, and produce smaller output pressure ripples. Current valves rely on the linear translation of a spool or poppet to meter flow. The valve spool must reverse direction twice per PWM cycle. This constant acceleration and deceleration of the spool requires a power input proportional to the PWM frequency cubed. As a result, current linear valves are severely limited in their switching frequencies. In this paper, we present a novel fluid driven PWM on/off valve design that is based on a unidirectional rotary spool. The spool is rotated by capturing momentum from the fluid flow through the valve. The on/off functionality of our design is achieved via helical barriers that protrude from the surface of a cylindrical spool. As the spool rotates, the helical barriers selectively channel the flow to the application (on) or to tank (off). The duty ratio is controlled by altering the axial position of the spool. Since the spool no longer accelerates or decelerates during operation, the power input to drive the valve must only compensate for viscous friction, which is proportional to the PWM frequency squared. We predict that our current design, sized for a nominal flow rate of 40l/m, can achieve a PWM frequency of 84Hz. This paper presents our valve concept, design equations, and an analysis of predicted performance. A simulation of our design is also presented.