Design and development of a fuzzy-PLC for an earthquake simulator/shake table

This study presents the development of a fuzzy logic controller (FLC) for an earthquake simulator or shake table. This includes the use of a Programmable Logic Controller (PLC) as the main controller and the Variable Frequency Drive (VFD) as the motor´s inverter drive. Combination of ladder logic diagrams (LLD), structured texts (ST) and function block diagrams (FBD) were used to code the fuzzy program. The FLC was built with 36 rules from the two (2) inputs, Actual Error or Δ Speed and Actual Error Rate or Rate of Δ Speed, having six (6) membership functions each: Neg_Large, Neg_Medium, Neg_Small, Pos_Small, Pos_Medium and Pos_Large. Mini-fuzzy associative matrix (FAM) was also introduced in the study to reduce the number of computational rules by 75%. Center of gravity (COG) was the defuzzification technique used to convert the fuzzy outputs into crisp real data due to the simplicity of its computation in providing fast results. The proposed fuzzy controller was based from the data feedback value of the drive to attain the desired motor speed. The controller was able to consider the effect of different load variations in different loadings.