Image Processing Reference
In-Depth Information
44
Chapter 4
Figure 4.4
Photograph of a speaker, voice-coil-based, tip-tilt mirror corrector. (Image
courtesy of M. Colleen Gino.)
The two-DOF system has the most appeal from both a cost and simplicity
standpoint. Since it contains a centrally located pivot, only tip and tilt are available;
control of piston has been designed out. One drawback is that this arrangement of-
ten induces coupling between the axes. The three-axis configuration requires that
piston, and tip-tilt be controlled at the same time. Since the actuators are not on the
tip and tilt axis, mathematical decomposition of the
x
and
y
angles into the actuator
position is required. Also, the calculation must include a piston component so that
the mirror remains at a constant position.
One novel mirror-configuration system uses four commercial speakers ar-
ranged in a square to drive the mirror (Baker et al. 1996). Speakers are voice coil
actuators, basically a solenoid that moves in and out (an example is shown in
Fig. 4.4). The response of a speaker to a given voltage sets up harmonic oscillation
and makes the system difficult to control open loop. However, the response of the
speaker is well behaved and can be modeled (Baker et al. 1996). As such, a feed-
forward system that superimposes the reverse waveform on the speaker can be used
to cancel this out. This added complexity is more than compensated by the low cost
and high-speed operation of the system.
