8.1. Electric drive conception
Presently, most of the wave makers worldwide are equipped with the hydraulic and electric driving mechanisms – 43.2% and 51.3%, respectively [1]. In line with the general trend of increasing the use of electric motors, the new implementations of the wave makers are based on the electric drives. Among the advantages of electric driven wave makers over hydraulic driven wave makers, the following can be mentioned:
immediate ability to work without the need for time-consuming heating of the hydraulic oil,
easier maintenance without the need for condition monitoring and periodic change of the hydraulic oil and elements of the hydraulic installation,
more advanced control of the drives themselves using modern methods and communication interfaces.
Due to development of the electric drive techniques in recent years [38] and mentioned advantages, it was purposeful to consider the implementation of an electric drive for the wave maker in CTO S.A. towing tank. For needs of the current conception and in accordance with the description in the following sections, the models of drive, transmission and actuator have been derived and the simulation of work has been carried out. Finally, the system with the electric drive has been compared with the system with the hydraulic drive under quality of regulation criterion.
8.2. Implementation of model
The model has been implemented in C to the Dev-C++ integrated development environment. It has been done in accordance with the diagram presented in Fig. 8.1. The PMSM model with FOC has been considered as follows. The PMSM has been considered as powered from PWM inverter (INV.) with an impulse period of 0.157 ms and a DC power source of voltage uDC=1.72 [p.u.].
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The parameters of the id PI and iq PI controllers have been experimentally tuned as:
Kp=1,Ti=10 and Kp=10,Ti=10 – respectively. The parameters were chosen to obtain the minimum torque oscillations on the motor shaft with satisfactory system dynamics.
The flap velocity and flap position fuzzy-logic control system FLS has been implemented in accordance with description in section 4.3. The PMSM rotor velocity ωr is taken instead of hitherto flap velocity signal. The PMSM rotor position θr is taken instead of hitherto flap position signal. The ωr and θr are measured using an encoder E.
The FLS output value had to be translated with a scaling factor SF into the reference current vector on q-axis. The SF has been experimentally chosen as equal to 4.0 to obtain satisfactory dynamics of the system with an electromagnetic torque not exceeding of 300% of the rated value at the peak. The simulation results of the implementation and tuning are presented in section 8.3.
Fig. 8.1. Structural diagram of the flap velocity and flap position fuzzy-logic control system with the electric drive with FOC type of control method and PMSM type of electric motor powered from the PWM inverter
8.3. Simulation of work
The work of the model implemented in subsection 8.2, has been simulated. The results are presented in Fig. 8.2. The steps of reference stroke AX2r are given in 0.1 s, 2.5 s and 6.0 s to test the response of the tuned system. The steps of the thrust torque TR are given in 5.4 s, 5.6 s and 5.8 s to test the system robustness for distortions that may originate from the reflected waves. The AX2 is the measured flap stroke. The TR is the flap thrust torque reduced to the PMSM shaft. The W is the shaft velocity of the PMSM. The Te is the electromagnetic torque of the PMSM.
The usd and usq are the stator voltages on d-axis and q-axis of the PMSM, respectively. The isd
and isq are the current vectors on d-axis and q-axis of the PMSM, respectively.
The simulation has confirmed that the system provides the required dynamics and robustness while the measured values do not exceed the permissible values.
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Fig. 8.2. Simulation of work of the synthesized model with electric drive
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The quality of regulation of the fuzzy-logic system with electric drive has been checked under the parameters of step response. It has been registered for the closed-loop system with AX2r given as input signal and AX2 given as output signal scaled to launch the step of 1 m stroke of the flap X2. Thus, the value of the steps are equivalent for two compared systems: the system with electric drive and the system with hydraulic drive. The step response is shown in Fig. 8.3.
Fig. 8.3. Step response of the closed-loop synthesized model with electric drive – stroke values: desired (black line) and measured (red line)
8.4. Conclusion
The fuzzy-logic control system of the wave maker flap with the electric drive has been modelled and simulated. In accordance with the simulation, the system is satisfactorily dynamic and robust. In accordance with the step response, the system with the electric drive in relation to system with the hydraulic drive, ensures shorter settling time tR with significantly less overshoot D and without oscillating d/D. The rise time tn and setting time tN are longer but satisfactory.
The satisfactory results of simulation and numerous advantages of electric drive, testify that the implementation of the simulated fuzzy-logic system with the electric drive, should be considered as future solution.