High Energy Laser Testbed for Accurate Beam Pointing Control

2.2.2 Narrow Field-of-View (NFOV) Video Tracker Control Loop
The NFOV control loop shown in Figure 6 includes an integrator type compensator (on each axis).
Figure 6. NFOV Control Loop
2.2.3 Alignment and Beam Jitter Control
The auto-alignment/jitter control FSM is controlled using an analog controller with an adjustable gain. The PSD signal is
used as a feedback signal to the analog controller.
2.3 System Identification
Several types of experiments were performed to determine characteristics of the HEL testbed. The results of the tests
were utilized for system modeling. The experiments include resonant frequency test, rate loop servo bandwidth and
stabilization test, FSM test, and NFOV bandwidth test. The target computer has an external terminal board which
interfaces all the signals between the beam control system and target computer. It also provides input/output test points.
Test equipment such as a dynamic signal analyzer, data acquisition system, and oscilloscope are used for signal
generation, data storage, and observation of test signals.
2.3.1 WFOV Resonance Frequency
Random signals were applied to the power amplifier of the azimuth and elevation gimbal stages and output signals were
received at the gyro and encoder respectively. With this open loop tests, resonance frequencies were calculated using
power spectral density analysis. Prior to performing the analysis, preprocessing was required to remove constant drift in
addition to outliers. The resonance frequencies are determined over a frequency range of 0-200Hz.
Table 1 – Summary of Resonance Frequency Test
Direction Output signal Resonance frequencies (Hz)
AZ
EL
Gyro 2.6, 8.3, 11.4, 15.9, 36.6, 69.5, 101, 134, 147.7
Encoder 2.6, 8.2, 11.4, 101
Gyro 3.8, 10, 14.9, 43.3, 61.4, 74.5, 101, 145, 195
Encoder 3.8, 11.6, 61.4, 101
2.3.2 WFOV Rate Loop Serve Bandwidth
Rate loop bandwidth was measured by applying a sweep sine signal to rate command and observing output at the gimbal
gyro. As with resonance testing, preprocessing was conducted for trend removal and outlier rejection. Data was
measured from 1-50Hz for both axis. The ultimate goal of this testing is to determine the transfer function of the rate
loop servos. Several intermediate calculations were required to make this determination, the first of which is the crosscorrelation
sequence as given by
R m = E x ⋅ y = E x ⋅ y
(1)
* *
xy ( ) { n+ m n } { n n−m} Proc. of SPIE Vol. 7587 75870G-5
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