F. Gomes de Almeida

Reduced-order thermodynamic models for servo-pneumatic actuator chambers

Abstract This paper discusses thermodynamic models of air inside pneumatic actuator chambers. In servo-pneumatics common practice, these models are simplified by neglecting the temperature dynamics. Classical models in the literature assume the temperature inside the pneumatic chamber either to be constant or to follow a polytropic law. Furthermore, the mixing process of air entering the chamber and heat transfer between air and cylinder walls is often neglected or only implicitly taken into account. This work evaluates the impact of these simplifications and order reductions in the prediction of pressure inside the actuator chamber. Classical models are compared with several others not only taking into account the mixing process but also explicitly including the heat transfer between air and cylinder walls. Simulation studies show that the reduced-order models proposed in this paper can lead to a mean square error in pressure prediction of only 10 per cent of that obtained using classical models.

Semi-empirical model for a hydraulic servo-solenoid valve

Abstract High-performance proportional valves, also called servo-solenoid valves, can be used today in closed-loop applications that previously were only possible with servo-valves. The valve spool motion is controlled in a closed loop with a dedicated hardware controller that enhances the valve frequency response and minimizes some non-linear effects. Owing to their lower cost and maintenance requirements as well as increasing performance they can compete with servo-valves in a large number of applications. This paper describes a new semi-empirical modelling approach for hydraulic proportional spool valves to be used in hardware-in-the-loop simulation experiments. The developed models use either data sheet or experimental values to fit the model parameters in order to reproduce both static (pressure gain, leakage flowrate and flow gain) and dynamic (frequency response) valve characteristics. Valve behaviour is divided into two parts: the static behaviour and the dynamic behaviour. A parameter decoupled model, with a variable equation structure, and a flexible model, with a fixed equation structure, are proposed for the static part. Spool dynamics are modelled by a non-linear second-order system, with limited velocity and acceleration, the parameters being adjusted using optimization techniques.

A Neural Network Based Nonlinear Model of a Servopneumatic System

The use of pneumatic devices is widespread among different industrial fields, in tasks like handling or assembly. Pneumatic systems are low-cost, reliable, and compact solutions. However, its use is typically restricted to simple tasks due to the poor performance achieved in applications where accurate motion control is required. One of the key elements required to achieve a good control performance is the model of the servopneumatic system. An accurate model may be of vital importance not only in the simulation steps needed to test the control strategy but also as a part of the controller itself. This work presents a new servopneumatic system model primarily developed for control tasks, namely, to predict pneumatic and friction forces in dynamic tests. The model can also be used in simulation tasks to predict the piston position and velocity. The performance on both applications is validated experimentally.

Pneumatic servo valve models based on artificial neural networks

This paper presents a study where static models for the flow stage of three-port pneumatic servo valves are obtained using artificial neural networks. A new approach to build the models is introduced in detail. This approach considers two distinct models: for simulation purposes, the air mass flow is determined for a given working pressure and command input; for control purposes, the command input is determined given the working pressure and the desired air mass flow. This approach enables the use of air mass flow as the synthesized control action, thus rendering the overall pneumatic system affine. Therefore, control techniques requiring this condition on the system model can be directly used. The use of this approach in two different industrial pneumatic servo valves is presented in detail. A comparison between the different characteristics of each servo valve is also provided.This paper presents a study where static models for the flow stage of three-port pneumatic servo valves are obtained using artificial neural networks. A new approach to build the models is introduced in detail. This approach considers two distinct models: for simulation purposes, the air mass flow is determined for a given working pressure and command input; for control purposes, the command input is determined given the working pressure and the desired air mass flow. This approach enables the use of air mass flow as the synthesized control action, thus rendering the overall pneumatic system affine. Therefore, control techniques requiring this condition on the system model can be directly used. The use of this approach in two different industrial pneumatic servo valves is presented in detail. A comparison between the different characteristics of each servo valve is also provided.

Heat transfer evaluation of industrial pneumatic cylinders

Abstract Automatic positioning devices are used worldwide in tasks such as handling or assembly, making them key components of modern manufacturing systems. Pneumatic solutions are usually less expensive than their electrical counterparts, are more reliable, and require less maintenance. However, the complex non-linear nature and high model order of pneumatic systems lead to a very difficult control task. These problems make model order reductions and simplifications a common practice in servopneumatics. The heat transfer between air inside the cylinder and its environment is usually neglected or only indirectly accounted for, since it varies with the pressure, temperature, and speed of the actuator, which makes its experimental assessment difficult. In this work a simple and yet accurate procedure is presented on the basis of a thermal time constant, enabling its evaluation. The procedure is validated by simulation studies, and furthermore the heat conductance of three industrial actuators is experimentally determined.

Hybrid models for hardware-in-the-loop simulation of hydraulic systems Part 1: theory

Abstract Physical modelling of hydraulic systems results in systems of differential and algebraic equations (DAEs) that are normally stiff. This is because they often include multiple temporal scales and present complex and non-linear behaviour. The simulation of these stiff DAEs in real time demands small time steps, in order to achieve algorithm stability, because explicit fixed step solvers are usually applied. Modelling some fast dynamics as instantaneous changes in order to reduce the models stiffness has been an area of research in the last few years. The description of the dynamic behaviour as piecewise continuous modes, interspersed with discrete transitions, together with the reduction in the model complexity, facilitates the use of fixed time step integration methods and thus allows real-time simulation. The main goal of this work was to obtain not too complex models in order to allow the models to be used in hardware-in-the-loop experiments. This paper proposes the use of the statecharts formalism to describe hybrid behaviour in hydraulic systems and presents semiempirical models of a valve controlled hydraulic cylinder intended to test real controller performance on a hardware-in-the-loop setup. These semiempirical models require less computing power, and it is easier to adjust the model parameters.

Hybrid models of hydraulic systems for hardware-in-the-loop simulation. Part 1: Theory

The use of new control schemes for hydraulic systems has been the object of study during the last years. A simulated environment is the cheapest and fastest way of evaluating the relative merits of different control schemes for a given application. The real time simulation allows the parameterization and test of the performance of real controllers. This paper describes the setup of a real time simulation platform to perform hardware-in-the-loop simulation experiments with the hydraulic models proposed in a companion paper (Part 1). A set of parameterization techniques are proposed for the semiempirical models of a valve controlled hydraulic cylinder. Manufacturers data sheets and/or experimental measurements were used to adjust the model parameters. Some of them were directly calculated and others were estimated through the use of optimization techniques. Closed loop control experiments were then performed on the real time simulation platform, and on the real system, in order to evaluate the real time performance of the developed models.

VSC Approach Angle Based Boundary Layer Thickness: A New Variation Law and Its Stability Proof

A major drawback on the use of sliding mode controllers is their inherent intense control activity. A usual strategy to cope with this problem is to use a boundary layer around the switching surface. The boundary layer thickness choice is based on a compromise between smoothness in the control action and tracking error. Since this compromise may be difficult to achieve, several boundary layer thickness variation laws (BLTVL) have been proposed in literature. In a recent study [1] an interesting BLTVL was proposed, based on the approach angle of the state to the switching surface. Although innovative, that study does not provide a proof of the stability of the controller. Furthermore, the BLTVL definition varies according to the system order and sliding surface definition. This paper extends the work done in [1] by presenting a new BLTVL based on a generalised definition of the approach angle that can be applied to systems of any order using any sliding surface. Furthermore, an innovative stability proof of a variable structure controller (VSC) using the new BLTVL is provided. Experimental results obtained in a servopneumatic system validate the usefulness of this approach.Copyright

Hybrid models for hardware-in-the-loop simulation of hydraulic systems Part 2: Experiments

Abstract The use of new control schemes for hydraulic systems has been the object of study during the last few years. A simulated environment is the cheapest and fastest way of evaluating the relative merits of different control schemes for a given application. Real-time simulation allows parametrization and test of the performance of real controllers. This paper describes the set-up of a real-time simulation platform to perform hardware-in-the-loop simulation experiments with the hydraulic models proposed in the companion paper (Part 1). A set of parametrization techniques are proposed for the semi-empirical models of a valve-controlled hydraulic cylinder. Manufacturers data sheets and/or experimental measurements were used to adjust the model parameters. Some of these were directly calculated and others were estimated through the use of optimization techniques. Closed-loop control experiments were then performed on the real-time simulation platform and on the real system, in order to evaluate the real-time performance of the developed models.

Using a Conventional Servopneumatic System for Robust Motion Control in the Micrometer Range

Conventional pneumatic systems present several advantages over competitive technologies, as they do not exhibit significant heat or magnetic fields and present high force to volume ratios. Nevertheless, they are very difficult to control due to their inherent nonlinearities like friction forces or air compressibility. As a consequence, their use is limited to simple motion tasks. This paper uses a nonlinear control law, previously developed by the authors, to control a servopneumatic system in the micro meter range. Preliminary experimental results show that the control law leads to very accurate motion control in micro meter positioning tasks in any position of the piston stroke. The paper also shows, through experimental results, that the control law is robust to parametric variations (change of payloads) and to external constant disturbances (gravity force).Copyright