Image Processing Reference
In-Depth Information
Chapter 5
5.4.3 Wavefront compensator
The wavefront compensator chosen for the image-stabilization system is a two-axis
tilt mirror driven by the voice coils of a pair of 2-inch speakers. A voice coil-based
corrector was chosen because the components are easy and inexpensive to obtain.
The voice coils support a bridge on which a mirror is placed so that it can be moved
in angle. This arrangement is shown in Fig. 5.6. One drawback to using a voice-
coil-based system is that voice coils require a large amount of current to move.
Each speaker in the tilt mirror is driven by a transistor amplifier connected to an
appropriate power supply. The transistor allows the current to be controlled by the
low voltage signal provided from a digital computer. It is important to recognize
that this approach is based on a force actuator system and so requires a significant
amount of power to operate and hold a position, so a good power supply is critical.
The speakers have a natural resonance frequency of approximately 170 Hz,
which can be easily excited in normal operation as part of the image-stabilization
mirror. The mass of the mirror and bridge also changes the resonance frequency. A
representative transfer function of the tilt compensator is shown in Fig. 5.7, illus-
trating the amplitude and phase plots. Notice that at the resonance points, the phase
Figure 5.6 Photograph of the tilt mirror compensator used in the image-stabilization
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