Data processing: generic control systems or specific application – Generic control system – apparatus or process – Digital positioning
Reexamination Certificate
2001-04-25
2004-02-24
Khatri, Anil (Department: 2121)
Data processing: generic control systems or specific application
Generic control system, apparatus or process
Digital positioning
C700S057000, C700S058000, C700S059000, C700S060000, C700S061000, C700S062000, C700S063000, C700S064000, C356S614000, C356S003000, C356S022000, C356S004090, C356S004100, C356S138000, C356S155000, C356S399000, C342S132000, C342S134000, C372S009000, C372S020000
Reexamination Certificate
active
06697683
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
REFERENCE TO MICROFICHE APPENDIX
Not applicable.
1. Field of the Invention
The present invention relates generally to techniques for sensing and adjusting the position of a movable element, and more particularly of a pivoted armature reflector for steering optical beams at high speed and with high precision through sequences of positions including dwell intervals of constant position.
2. Description of Prior Art
There are many applications for steering an optical beam with high speed and precision between angular directions in either a random or a predetermined sequence. Agile beam steering in two dimensions permits successively illuminating targets in a number of locations to reflect the beam for subsequent detection by an optical receiver.
Agile tuning of lasers require sensors with a high speed-accuracy product. High accuracy is achievable with charge-coupled-devices (CCDs) and interpolation but readout is slow. On the other hand, position sensitive detectors (PSDs) and capacitive position sensors easily achieve the speeds required of the fastest pulse lasers but suffer long term accuracy and stability. Since tuning accuracy requirements are absolute ones, system configurations must be selected to achieve the tuning accuracy in spite of the slow speed sensors.
Tunable lasers typically include an intracavity diffraction grating and a rotatable mirror. The wavelength of such lasers is tuned by adjusting the angle of incidence of the laser cavity beam against the diffraction grating. Such intracavity tuning requires very high accuracy and stability. Tuned CO
2
lasers, for instance, require an angular range of typically 0.2 radians and an accuracy of 10 &mgr;radians. This represents a range to accuracy ratio of 20,000. In some cases, and at lower accuracy, an optical parametric oscillator (OPO) material (usually a crystal with highly non-linear optical properties) is used.
In the crudest implementations, grating reflection angles have conventionally been adjusted by grating alignment micrometer screws. Manual micrometer adjustment is slow and severely restricts beam steering agility. Automated micrometer adjustment by stepper motor is hindered by the angular momentum of the diffraction grating, backlash, screw friction, and other forces.
Laser radar (LIDAR) systems can be used to transmit different wavelengths of light into airborne suspensions (such as smog or poison gasses) which have differing reflectivities or absorption to different wavelengths. The reflected light intensity is then measured for remote spectrographic analysis of suspension samples. In remote spectroscopy LIDAR applications it is advantageous to maximize the stability and repeatability of the output at each different wavelength. On the other hand, it is very advantageous to minimize intervals between transmitting wavelengths in order to reduce measurement interference by relative motion between the LIDAR unit, the intervening atmosphere and the suspension sample. Maximum accuracy is achieved by successively transmitting different wavelengths at the laser's maximum cyclic rate.
LIDAR system laser formats can consist of single wavelength high power pulses in a sequentially repeating pattern of wavelengths. Another class of applications require a pulse burst of multiple pulses at a given wavelength, periodically repositioning to the next wavelength for another pulse burst. Alternately the tuning/positioning requirement is for a dwell time of a CW (continuous wave) laser periodically repositioning for repeating dwell times of CW operation at other wavelengths. Thus, several types of laser formats have been used to sequentially transmit multiple wavelengths; single pulse, single wavelength pulse burst and CW. The latter two require a dwell or hold time at each wavelength.
FIG. 1
shows typical timing of these formats. Waveform
40
a
shows single pulse timing with a pulse at wavelength &lgr;
0
followed by one at wavelengths &lgr;
1
and &lgr;
2
finally repeating &lgr;
0
for a continuous pattern of three wavelengths. Dwell time for this case can be quite short. Waveform
40
b
is pulse burst laser timing where a burst of pulses at wavelength &lgr;
0
is output, separated by a retuning time for the next pulse burst at wavelength &lgr;
1
. Finally waveform
40
c
is a CW laser output at wavelengths &lgr;
0
, &lgr;
1
and &lgr;
2
separated by the retuning intervals. Tuning stability for the pulse burst and CW lasers must be maintained during the dwell times, typically shown as the interval between times
41
and
42
in FIG.
1
. The present invention is aimed at systems requiring a significant dwell but has applications for the single pulse type as well.
FIG.
2
and
FIG. 1
, positional waveform
40
d
, illustrate an example of prior art which can provide limited multiwavelengths with dwell times. Here a separate laser is used for each wavelength. In this example, laser
27
at wavelength &lgr;
0
, laser
29
at wavelength &lgr;
1
and laser
31
at wavelength &lgr;
2
are selected to the output beam
25
by flip mirror
33
driven by actuator
35
and conventional servo or stepper
37
. Actuator
35
can be a low accuracy device since it simply controls the pointing of the output beam. The individual laser output power and stability are set by the accurate internal optics of lasers
27
,
29
and
31
. This approach suffers from the high complexity and poor flexibility of having an entire laser for each wavelength, since there are upwards of 100 possible wavelengths in the CO
2
lasing spectrum. Chemicals may not be detected very efficiently with the limited repertoire of wavelengths that practicality dictates. In order to reliably detect a single chemical, 3 to 10 wavelengths may be required and other chemicals use their own unique set of wavelengths.
In
FIG. 3
of the prior art, an intracavity conventional servo implementation is shown. Intracavity beam
74
is tuned by rotating mirror
76
which responsively changes the incidence angle of grating
72
thereby selecting lasing wavelength. Mirror
76
is rotated by actuator
78
in response to control by servo control
82
. Mirror position is sensed by position sensor
79
then combined with desired position data
81
. Servo control
82
provides output drive to actuator
78
responsive to the desired position data
81
and the actual position information from position sensor
79
.
FIG. 1
waveform
40
e
shows the required angular function. For CW or pulse burst tuning of CO
2
lasers, mirror
76
must have sufficient accuracy from time
41
to time
42
to satisfy the power stability requirements of the laser or approximately 10 &mgr;radians. The prior art is replete with high performance servo techniques for servo control
82
and sensors for position sensor
79
. In order to enhance speed and stability, the servo techniques aim to keep the actuator drive high as it approaches its final position and to compensate for the delays inherent in sensors and actuator/load inertias. These techniques, to name a few, involve lead-lag networks, phase compensation, error integration, gain switching, open loop/closed loop switching, dither, observer models, velocity and acceleration feedback and mode switching. Conventional servos, to date, have been unable to satisfy either the single pulse applications or the CW/pulse burst applications for CO
2
LIDAR systems. Although such arrangements as
FIG. 3
can easily satisfy either the speed or accuracy required by the typical laser tuner, limitations prevent satisfying both. Sensor
79
implemented with PSDs or other high speed position sensing techniques are adequately fast but lack the required accuracy for tuning lasers.
FIG. 1
waveform
40
i
illustrates the positional drift. When sensor
79
is implemented using interpolated CCD technology for adequate accuracy, none of the servo speed enhancements satisfy the 200 Hz and up tuning speed requirements.
FIG. 4
of the prior art represents related U.S. Pat. No. 5,450,202 which disc
Burns Doane , Swecker, Mathis LLP
Khatri Anil
Pham Thomas
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