Influence of the process design on the control strategy: application in electropneumatic field
Introduction
The design of a modern process needs a good co-ordination between different engineers specialized in specific fields. More often, the new industrial products are born with advanced technologies in electronics, mechanics, materials knowledge, computing and with evolved sensors and actuators. Very often, after the product creation stage comes the need for a choice of control. For this, a good background and understanding of the modern automatic theories is required. It is then necessary to choose an appropriate control strategy to obtain good performances in terms of safety, stability, repeatability and accuracy. Even though during the last century the traditional solution was essentially a P.I.D. structure, nowadays, modern control laws are implemented.
The goal of this paper is to show the influence of the control strategy on the system design. Indeed, if the process design influences the choice of the control law, then it would be interesting to note that the people who choose the control strategy could influence the design. This idea will be developed in this paper in the context of an electropneumatic system.
In the first section, two different kinds of electropneumatic positioning systems will be presented and modelled. The following section elaborates on two kinds of control laws for each system. The choice of a nonlinear control strategy (Isidori, 1989; Fliess, Levine, Martin, & Rouchon, 1995) will be argued. The last section will be devoted to the experimental results. Both sets of results will be compared following an industrial benchmark. At the end, the aim is to conclude on the influence of the process design on the control strategy.
Section snippets
Two kinds of electropneumatic positioning systems
This part presents two different systems for electropneumatic position tracking. Both the systems use the following structure: servo-valve/actuator/mass in translation. In each case the cylinder and the moving mass are the same, only the first power stage is modified (see Fig. 1). The actuator under consideration is an in-line electropneumatic cylinder using a simple rod (32/20 mm) with a stroke equal to 500 mm. The rod is connected to one side of a carriage and drives an inertial load on guiding
Two kinds of nonlinear control strategy
Nowadays, electropneumatic or electrohydraulic systems are controlled in position tracking (Edge, 1997; Sohl & Bobrow, 1999; Brun, Belgharbi, Sesmat, Thomasset, & Scavarda, 1999a; Liu & Daley, 2000; Bouri & Thomasset, 2001) or in force tracking (Alleyne & Liu, 1999; Niksefat & Sepehri, 2000; Alleyne & Liu, 2000).
In the case of monocriteria, it seems more interesting to choose a system with one five-way servo-valve. In fact this structure is more attractive in terms of cost, regarding the
Experimental results
The experimental results are presented in terms of repeatability. So, in this section, each curves presents a mean value obtained after 30 tests. The standard deviation, mean and maximum values are calculated in each position and presented in Table 1, Table 2 for each control strategy. In order to show the performances of each control law under static and dynamic conditions, the standard deviations are calculated in each phase. The two tables show the better results obtained during static
Conclusions
This paper presents an example of electropneumatic system, where the control specialist chooses to act on the structure before working on an elaborate strategy of control. This study has proved the importance of including the control specification during the designing. Nowadays, more and more new products are developed including design, process architecture, choice of the structure actuators, place of sensors, sizing of component and control strategy. This approach permits a satisfactorily link
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