Trans RINA, Vol 157, Part A3, Intl J Maritime Eng, Jul-Sep 2015
0.5 1.0 1.5 2.0 2.5 3.0
-15 -12 -9 -6
V=1.82 m/s V=2.30 m/s V=2.70 m/s
output angle is not exactly equal to the demand angle. This error is relatively more significant for low demand T-Foil incidence (α) range of ±5º as seen in Figure 12. In addition,
Figure 13 shows the observed phase lag
between measured angle of attack and demand angle of attack. As can be seen the observed phase lag increases with increase of frequency, the linear increase being indicative of a time delay in the control system.
-3 0369 12 T-Foil angle of attack, α (º)
Figure 10: Drag force measured on the T-Foil at fixed angles of attack for static tests undertaken at different water flow velocities.
V= 1.82 m/s ; f= 1.5 Hz ; Demand α = ±15º
10 15
-15 -10 -5 0 5
01 2 Time (s)
Figure 11: Lift L, and Drag D, forces measured at a flow velocity of 1.82 m/s, frequency of 1.5 Hz and demand T- Foil incidence (α) range of ±15º.
The local form of the peak and trough of the measured lift can be explained on the basis of the mechanical operation of the stepper motor used to drive the T-foil where there is a dead-space in the stepper-motor gearbox as well as slack in the connections between the motor and T-foil.
The results of the frequency response tests included the ratio of the measured angle of attack divided by the demand angle of attack. Also, the phase lag between demand and T-Foil movement was measured. Figures 12 and 13 show these results at the three different flow velocities of 1.82 m/s, 2.30 m/s and 2.70 m/s and also three different demand T-Foil
incidence (α) ranges of
±15º, ±10º and ±5º. As can be seen, the ratio of measured angle of attack to the demand angle of attack decreases with increase of frequency in particular above 4 Hz. Also, it can be seen that this ratio is not exactly equal to unity for the low frequencies. However it is close to unity. This phenomenon can be explained in terms of the mechanical operation of the T-Foil linkage set-up. As mentioned before, it was found that there is a dead-space in the stepper-motor gearbox which causes the error. When an angle is demanded for the stepper-motor, its
©2015: The Royals Institution of Naval Architects
α (º) L (N) D (N)
5 15
Referring to the previous studies on the INCAT Tasmania hydroelastic segmented model [18], it was found that peak motions and peak loads occurred at the frequencies between 1 Hz and 1.5 Hz. Considering this range of frequencies for a demand T-Foil incidence (α) range of ±10º, which is close to the maximum full-scale range, it can be seen that ratio of measured angle of attack to demand angle of attack is about 0.95 which is acceptable for such a small model test system. Also, the observed phase
lag in this range of frequencies is
between 10 and 20 degrees which is due to the stepper- motor limitation.
0 -5
0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
Demand α= ±15º ; V= 1.82 m/s Demand α= ±10º ; V= 1.82 m/s Demand α= ±5º ; V= 1.82 m/s Demand α= ±15º ; V= 2.30 m/s Demand α= ±10º ; V= 2.30 m/s Demand α= ±5º ; V= 2.30 m/s Demand α= ±15º ; V= 2.70 m/s Demand α= ±10º ; V= 2.70 m/s Demand α= ±5º ; V= 2.70 m/s
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 Frequency (Hz)
Figure 12: Ratio of measured angle of attack to demand angle of attack at three flow velocities of 1.82 m/s, 2.30 m/s and 2.70 m/s and also three demand T-Foil incidence (α) ranges of ±15º, ±10º and ±5º.
100 120 140
20 40 60 80
0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 Frequency (Hz)
Figure 13: Phase lag at three flow velocities of 1.82 m/s, 2.30 m/s and 2.70 m/s and also three demand T-Foil incidence (α) ranges of ±15º, ±10º and ±5º.
A-179
Demand α= ±15º ; V= 1.82 m/s Demand α= ±10º ; V= 1.82 m/s Demand α= ±5º ; V= 1.82 m/s Demand α= ±15º ; V= 2.30 m/s Demand α= ±10º ; V= 2.30 m/s Demand α= ±5º ; V= 2.30 m/s Demand α= ±15º ; V= 2.70 m/s Demand α= ±10º ; V= 2.70 m/s Demand α= ±5º ; V= 2.70 m/s
T-Foil angle of attack, α (º)
Drag force, D (N)
L (N) , D (N) Phase Lag (deg) α Out / α Demand
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