Since the behaviour of the fuzzy controller is unknown at this time, the definition range of each variable is pided into three segments, "positive", "null" and "negative"。 The set with the name "null" will be defined above the zero point and it extends over 1/3 of the entire definition range。 

4。  THE FUZZY – PLC CONTROL SYSTEM 

The general view of the designed system is shown in figure 8。 Figure 8 shows the servomechanism, a PLC system with digital/analog I/O, a connection module between the servomechanism and the PLC, a simulation board used to depict the status of sensors and switches of the PLC and a PC with the installed software for the programming of the PLC and the implementation of the fuzzy controller。 

Fig。 8: The designed system 

4。1  The simulation board 

The simulation board is operated with a stabilized DC voltage of 24 V, e。g。 supplied with the power supply unit of the PLC system。 The sockets are used to connect the sensors and switches on the simulation board to the PLC system。 The safety sockets on the system simulator are connected to the sensors and switches according to the assignment。 Each of these switches and sensors can physically act as make or break contact (Dumitru)。 

4。2  The PLC system 

Programmable logic controllers are modular, industrially hardened computers which perform control functions through modular input and output (I/O) modules。 The modularity of PLC allows the user to combine generic I/O modules with a suitable controller to form a control system specific for his most simply understood needs (Yasar Birbir)。 The operation of a controller by envisioning that repeatedly performs three steps: 

Reads inputs from input modules。

Solves pre-programmed control logic。

Generates outputs to output modules based on the control logic solutions。 Input devices and output devices of the process are connected to the PLC and the control program is entered into the PLC memory。 

In our application, the PLC system controls through analog and digital inputs and outputs the varying load-constant speed operation of the motor。 Also, the PLC continuously monitors the inputs and activates the outputs according to the control program。 

The PLC used in our application, is a SIMATIC S7-300 series, an industrial PLC。 

The standard software used for configuring and programming Simatic Programmable Logic Controllers is Step 7。 The programming languages which can be used for the S7-300 are Ladder Logic, Statement List, and Function Block Diagram: 

Statement List (or STL) is a textual representation, similar to machine code。 If a program is written in Statement List, the inpidual instructions correspond to the steps with which the CPU executes the program。 

Ladder Logic Diagram (or LAD) is a programming language primarily used to develop software for Programmable Logic Controllers (PLCs) used in industrial control applications。 LAD represents a program by a graphical diagram supported on the circuits’ diagrams of relay-based logic hardware。 Ladder allows you to track the power flow between power rails as it passes through various contacts, complex elements, and output coils (Ivanescu)。 

Function Block Diagram (FBD) is a graphic representation and uses the logic boxes familiar from Boolean algebra to represent the logic。 

The S7-300 is a modular control system for system solutions with a main emphasis on production engineering。 The wide ranges of assemblies are ideally suited to cope with all required demands and a flexible application。 The S7-300 contains the following types of modules (Siemens, 2004): 

Central processing units (CPU) 

                  Power supply modules (PS)