Fft Analysis - Quanser 6 DOF Hexapod Laboratory Manual

Note: As explained in the Hexapod User Manual [2], if a limit switch is triggered the amplifier automatically stops
accepting any motor/joint commands in that direction. However, position commands in the opposite direction are still
accepted (e.g., joint can be moved back to home).

2.7 FFT ANALYSIS

This section shows how to evaluate the power spectrum of the commanded and measured Hexapod position and
acceleration signals using the fft_eval_pos.m and fft_eval_acc.m scripts. Given a time-based signal g(t), the power
spectrum shown on the plot generated is given by G(ω) , where G(ω) is the Fast-Fourier Transform (FFT). The FFT,
G(ω), is computed using the
1. Run an earthquake or sine wave using the Hexapod_Earthquake.mdl QUARC controller, as described in Sec-
tion 2.6. After Hexapod_Earthquake.mdl is ran, the commanded and measured position and acceleration data
are automatically saved to the Matlab data files data_xyz_pos.mat and data_xyz_acc.mat. Only one axis is
saved to these variables - either X, Y, or Z - depending on the selection in the Position Index block found in the
Performance Tracking subsystem, as shown in Section 2.8.5.
2. To produce the X(ω) position power spectrum similar to Figure 2.10, run fft_eval_pos.m. The desired or
commanded position is the blue trace and the measured position (from the encoder) is the red plot line.
Figure 2.10: FFT of desired and measured position along x-axis after running Northridge earthquake
3. To produce the acceleration power spectrum, A(ω), similar to Figure 2.11, run fft_eval_acc.m. The blue and
red traces are the desired and measured accelerations, respectively.
HEXAPOD Laboratory Guide
t
M command. See the power_spectrum.m script for further details.
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