Optics: measuring and testing – Angle measuring or angular axial alignment
Reexamination Certificate
2003-09-26
2004-11-30
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
Angle measuring or angular axial alignment
C356S601000
Reexamination Certificate
active
06825922
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of scanning devices. In particular, the present invention relates to the profiling of a component which includes scanning a spot of light through a range of angles. More specifically, the present invention relates to the measuring of the external surface profile of a component using a non-contact optical technique. Even more specifically, the present invention relates to the measuring of the external surface profile of a component using a non-contact optical technique which scans the field of view by utilizing a rotating mirror and which precisely determines the angular position of the spot of light during scanning.
BACKGROUND OF THE INVENTION
The present invention describes several highly accurate methods for determining the pointing angle of a laser beam. The theory and use of these inventions are introduced by examining how these inventions aid the construction and use of a non-contact laser scanning system. A body of useful prior art for this work is described in U.S. Pat. No. 6,441,908, issued to Johnston et al.
We have developed an instrument where some form of light illuminates a spot on the surface of an object to be measured. The light source is usually a laser beam and the beam is usually first reflected off a mirror, as shown in FIG.
1
. By repositioning the mirror, usually using a rotational motion, we can reposition the light spot to measure a series of locations along the surface of the object. The challenge is to determine, very accurately, what the position of the mirror is so we can accurately determine the position of the laser (ideally to within 50 &mgr;rad) and ultimately determine the position of the laser on the object.
Mechanical Position Method
One class of previous techniques for determining the pointing angle of a laser have required measuring the mechanical deflection of the mirror using techniques that include the use of rotary encoders and specialized potentiometers. The laser deflection can then be inferred using the fact that
&thgr;
Optical
=2&thgr;
Mechanical
. (0)
Due to noise, jitter and limited resolution, these mechanical deflection methods typically do not suffice for measuring beam position down to 50 &mgr;rad.
Constant Velocity Method
A potentially more useful technique for determining the pointing angle of the laser is to make optical measurements of the laser beam as it is swept over a range of angles and then infer the position using angular velocity. As shown in
FIG. 2
, the laser first sweeps across an initial position sensor (Tick). This event latches a high precision counter, resulting in an initial timer value (T
Tick
). The timer is also latched for each i
th
desired measurement of the laser beam position (T
i
). Finally, as the laser crosses the end of scan position sensor (Tock), the timer is again latched (T
Tock
). These sensors can be any photo-detector but it is assumed in this disclosure that the high resolution bi-cell optical trigger configuration is used.
If the angular distance between Tick and Tock (&Dgr;&thgr;) are known (i.e. fixed or predetermined) and we assume a constant angular velocity, then it is possible to calculate a high precision angular position value for the i
th
position measurement (&thgr;
i
) using
θ
i
=
θ
0
+
ω
0
t
i
+
1
2
α
t
i
2
(
1
)
where &ohgr;
0
is the initial angular velocity and &agr; is the (constant) angular acceleration. First, we arbitrarily set &thgr;
0
=&thgr;
Tick
=0. This reduces (1) to
θ
i
=
ω
0
t
i
+
1
2
α
t
i
2
.
(
2
)
The average value of &ohgr; can be found using
⟨
ω
⟩
=
Δθ
Δ
T
=
θ
Tock
-
θ
Tick
T
Tock
-
T
Tick
.
(
3
)
Assuming no angular acceleration, e.g. &agr;=0, the i
th
angular position (&thgr;
i
) relative to &thgr;
Tick
can be determined after the sweep is complete by substituting <&ohgr;> for &ohgr;
0
and using
θ
i
=
⟨
ω
⟩
T
i
=
θ
Tock
-
θ
Tick
T
Tock
-
T
Tick
T
i
.
(
4
)
This optical angular velocity technique can suffice for measuring beam position down to 50 &mgr;rad as long as there is no angular acceleration and the counter is of sufficiently high resolution with low jitter in the triggering/latching circuits. This technique has the advantage of not requiring a specific value of &ohgr; as long &ohgr; remains constant during the sweep so that at any time the instantaneous angular velocity ((&ohgr;
i
) approximates the average velocity,
&ohgr;
i
≈<&ohgr;>. (5)
Constant Acceleration Method
The constant velocity method can fail when there is sufficient angular acceleration present between Tick and Tock to invalidate (5). In the special case where &agr; is slowly varying, such as a sine wave, then the value of &agr; can be approximated as constant between Tick and Tock. In such a case, knowledge of the initial angular velocity (&ohgr;
Tick
) and final angular velocity (&ohgr;tock) suffice to determine the laser position with sufficient accuracy. These new angular velocity values can be measured using a variation of the configuration as shown in
FIG. 3. A
pair of Tick trigger sensors separated a known distance (&Dgr;&thgr;
Tick
) is provided, as well as a corresponding pair of Tock sensors.
The i
th
angular position can now be found using
θ
i
=
ω
0
T
i
+
1
2
⟨
α
⟩
T
i
2
where
(
6
)
⟨
α
⟩
=
ω
Tock
-
ω
Tick
Δ
T
Tock
=
ω
Tock
-
ω
Tick
T
Tock
-
T
Tick
T
i
and
(
7
)
ω
0
=
ω
Tick
=
θ
Tick2
-
θ
Tick1
T
Tick2
-
T
Tick1
,
ω
Tock
=
θ
Tock2
-
θ
Tock1
T
Tock2
-
T
Tock1
.
(
8
)
Challenge of Instantaneous Acceleration
The constant velocity and constant acceleration methods of determining the angular position of the beam using optical measurements suffice for “well behaved” motion profiles of the rotating deflection mirror, e.g. where there is no appreciable instantaneous acceleration. However, when &agr; is not constant between Tick and Tock, large errors in the determined value of &thgr;
i
can result. Such a case can result if a resonant device such as a torsional pendulum structure rotates the mirror. This is a case where both &agr; and &ohgr; are described by sine waves with periods of approximately twice the time between Tick and Tock. If the structure was tuned so that the motion was just sufficient for the beam to cross both Tick and Tock, then &ohgr;
Tick
≈&ohgr;Tock=0 and (6) clearly breaks down. A similar extreme case is simulated in
FIGS. 4
a
and
4
b
where a mirror rotating with a base velocity of &ohgr;
0
=1.0 RPS has a 6 Hz sinusoidal velocity variation with amplitude of 0.5 RPS. The solid line in
FIG. 4
a
shows the actual velocity and the dashed line shows the velocity calculated using (3). The dashed line in
FIG. 4
b
shows the expected position of the mirror using (4), which is significantly different than the actual position shown as a solid line, up to 10 degrees in the center of the sweep.
FIG. 4
b
would be identical if (6) were used because the initial and final velocities are the same.
Another case where (6) breaks down is when an instantaneous acceleration “event” perturbs an otherwise constant motion profile. This can occur many ways, such as when an impulse from nearby equipment causes a vibration event or when the whole system is in motion in a fashion that couples with the angular momentum of the mirror or, even more subtle, when the bearings bind or drop into a “groove” in their races.
Acquiring and Using Angular Motion Data
The preceding discussion illustrates that a more refined method is needed to profile the acceleration, velocity or position profile between the Tick and Tock sensor
Bass Charles M.
Franklin Joseph A.
Johnston Kyle S.
Nelson Spencer G.
Esserman Matthew J.
McNichol, Jr. William J.
Metron Systems, Inc.
Reed Smith LLP
Stafira Michael P.
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