Table 3. Inverse dynamic analysis results for determining peak torque  From the inverse dynamic analyses, the torque and speed required to generate the sinusoidal motion are  used to design and size the actuator system. Table 3 indicates that a torque multiplier is needed to meet  the high torque and low speed requirements. High performance servo drive systems (for low and medium  inertia rotors) of interest are capable of about 3000 RPM at a rated constant torque. The required high  output torque and cyclic loadings lead toward a cycloid drive for long life and high momentary overload  margin. The reduction ratio of the cycloid torque multiplier is selected to match motor speed and rotor  inertia with expected total inertia load3. Typical high performance servo drives are tuned to operate most  stable when having a reflected inertia ratio of about 51. Optimization of design parameters including  motor torque, speed, reduction ratio, reflected inertias and spatial constraints for packaging led to the  system as designed.  Another consideration is the dynamic loading on the existing arc-sector model support system generated  by the new FOS. The system was designed for static loading only. The dynamic load induces unwanted  vibration and may excite natural modes in the system. Modal analyses were performed to determine the  natural frequencies and modes of the modified model support system. Figure 4 shows the first 5 modes  and their frequencies. As shown, the lowest natural frequency for mode 1 is near 4.55 Hz. This is near the  high frequency end of the theoretical oscillation curve shown on Figure 2. Care must be taken to operate  the FOS near this high frequency range. Other than that, the modified model support system can  accommodate the dynamic frequencies induced by forced oscillation testing.  System Description  Figure 5 shows the new FOS as installed in the 12-Foot LSWT. Figure 6 shows the schematic of the  overall system as implemented in the tunnel. As shown, the system may be controlled from the test  section or from the control room via a laptop computer. The LabVlEW based PXI real-time controller  (RTC) from National Instruments has custom developed software modules that manage the FOS  including system safety, communication, operations, and motion control according to user input and  system feedback.  The system safety module monitors and controls the 230VAC power to the servo drive system, safety  interlocks including control-keyed switches, Emergency Stops, end-of-travel limits (software and  hardware), and communications among user interface computers, and RTC controller. The system also  monitors safe operations from the existing arc-sector model support system. The communication module  connects the RTC with the user interfaces to process user inputs and provides system status. The motion  control module executes the positional closed-loop algorithm to track motion profiles as requested by a  user. The block diagram in Figure 7 shows how the motion tracking is achieved. As shown, the velocity  loop of the motor is internally managed and controlled by the BDS4 servo amplifier. The control algorithm  commands the velocity signal based on the error signal generated by the commanded motion profile and  the encoder feedback signal. For system verification and validation, a PID algorithm is used. For more  complex motion profiles, different control algorithms may be explored in the future.  391