U.S. patent application number 09/317654 was filed with the patent office on 2002-02-14 for apparatus and method for scanning a surface.
Invention is credited to DORSEL, ANDREAS N., SADLER, JOHN W., STATON, KENNETH L..
Application Number | 20020017562 09/317654 |
Document ID | / |
Family ID | 23234674 |
Filed Date | 2002-02-14 |
United States Patent
Application |
20020017562 |
Kind Code |
A1 |
SADLER, JOHN W. ; et
al. |
February 14, 2002 |
APPARATUS AND METHOD FOR SCANNING A SURFACE
Abstract
Apparatus and method for scanning a surface. An optical system
generates a light beam to illuminate a surface. A carrier supports
the surface for reciprocating motion with respect to the light beam
to form one axis of a raster. A propulsion system moves the carrier
at a substantially constant speed and a position sensor provides an
output signal representing the surface position with respect to the
light beam. A control system responsive to the output signal
modulates a sample period reciprocally to carrier speed to achieve
substantially constant scan length per sample and to control data
acquisition timing.
Inventors: |
SADLER, JOHN W.; (BELMONT,
CA) ; DORSEL, ANDREAS N.; (MENLO PARK, CA) ;
STATON, KENNETH L.; (SAN CARLOS, CA) |
Correspondence
Address: |
AGILENT TECHNOLOGIES, INC.
INTELLECTUAL PROPERTY ADMINISTRATION, LEGAL DEPT.
P.O. BOX 7599
M/S DL429
LOVELAND
CO
80537-0599
US
|
Family ID: |
23234674 |
Appl. No.: |
09/317654 |
Filed: |
May 24, 1999 |
Current U.S.
Class: |
235/454 |
Current CPC
Class: |
G01N 21/6452 20130101;
G01N 2021/6463 20130101 |
Class at
Publication: |
235/454 |
International
Class: |
G06K 007/10 |
Claims
What is claimed is:
1. Apparatus for scanning a surface comprising: an optical system
to generate a light beam and to deliver the beam to a surface; a
carrier supporting the surface for reciprocating motion with
respect to the light beam to form one axis of a raster; a
propulsion system for moving the carrier; a position sensor
providing an output signal representing the surface position with
respect to the light beam; a servo system responsive to the output
signal for commanding the propulsion system to move the carrier at
a substantially constant speed; and a control system responsive to
the output signal to modulate a sample period reciprocally to
carrier speed to achieve substantially constant scan length per
sample and to control data acquisition timing.
2. The apparatus of claim 1 wherein the position sensor is selected
from the group comprising a counting position encoder, optical
encoder, magnetic encoder, capacitive encoder, laser
interferometer, LVDT, or reflective optical triangulation
device.
3. The apparatus of claim 1 wherein the propulsion system comprises
a voice coil.
4. The apparatus of claim 1 wherein the propulsion system comprises
a galvanometer.
5. The apparatus of claim 1 wherein the propulsion system comprises
a gas jet.
6. The apparatus of claim 1 wherein the propulsion system comprises
a graphite piston in a glass cylinder powered by a gas or
liquid.
7. The apparatus of claim 1 wherein the propulsion system comprises
a rotary servo motor.
8. The apparatus of claim 1 further including a state observer
responsive to the output signal to generate an estimate of carrier
speed.
9. The apparatus of claim 1 further including means for
compensating for variable integral illumination per sample.
10. The apparatus of claim 9 wherein the means for compensating
comprises scaling amplitude of a measured signal by function of the
ratio of an actual sample period to a nominal sample period.
11. Method for scanning a surface comprising: measuring relative
speed between a surface being scanned and a light beam; determining
variation in the relative speed; and using the variation in
relative speed to modulate a sample period reciprocally to the
variation to achieve substantially constant scan length per
sample.
12. The apparatus of claim 1 wherein data acquisition timing is
controlled by a state machine that triggers data acquisition when a
selected number of new counts in a correct direction has
occurred.
13. The apparatus of claim 12 wherein inputs to the state machine
are quadrature decoded direction and quadrature decoded count
information.
14. The apparatus of claim 12 wherein the state machine is
programmed so that new counts in the correct direction are detected
by counting backwards quadrature state changes and incrementing a
trigger counter on quadrature state changes only when a backwards
counter is zero.
15. The apparatus of claim 1 wherein data acquisition timing is
triggered by the first equivalence of an actual position value and
a selected trigger position.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to scanning a surface and more
particularly to such a system which can achieve constant scan
length per data sample.
[0002] Scanning imaging systems form a map of some characteristic
of a surface of interest by exposing the surface to light and
measuring a response from the surface. A focused beam of light is
moved in a deliberate and repeatable pattern with respect to the
surface. The response is generally time-correlated to the position
of the scanning beam in order to form a final map of the property
with respect to a location on the surface. In most situations, it
is important to be able to infer the position of the surface
represented by each pixel and to be able to guarantee that the
pixel spacing is uniform within some tolerance dictated by the size
of the features being scanned.
[0003] Relative motion between the light beam and the surface can
be achieved by maintaining the surface stationary and moving the
beam or, alternatively, keeping the beam stationary while moving
the surface. A high performance (high numerical aperture) optical
system of reasonable cost, often has a scanner in which a surface
moves while the illuminating beam stays in a constant position
relative to the beam optics. Such an arrangement results in the
desirable property of high numerical aperture. Alternatively, the
surface may remain fixed with the light beam being moved. The
components that position the surface relative to the beam generally
exhibit some systematic position errors that are a function of
position or time. These errors degrade the quality of the final map
of the desired property with respect to a location on the
surface.
[0004] The present invention has particular application to gene
chips which contain arrays of short DNA chains in an array of
sequences bound to a substrate (usually glass). The chip is indexed
so that the particular DNA sequence bound in any area is known. A
region having a homogeneous composition is referred to as a
"feature." The DNA chips can be incubated with a solution
containing RNA or DNA bound to a fluorescent tag, allowing the
binding of RNA to individual features. Such systems can be used for
the determination of both genotype and gene expression levels.
[0005] If fluorescence is observed in a particular region, binding
has occurred and a DNA sequence is identified by consulting the
index of DNA positions on the chip. The present invention is
particularly useful in this context. As will be discussed below,
the present invention sets forth a methodology for measuring and
compensating position errors which may be a function of position or
time to an arbitrary level of linearity.
SUMMARY OF THE INVENTION
[0006] In one aspect, the apparatus for scanning a surface includes
an optical system to generate a light beam and to deliver the beam
to a surface. A carrier supports the surface and is mounted for
reciprocating motion with respect to the light beam to form one
axis of a raster. Of course, the surface may be fixed with the
optics arranged to sweep the light beam. A propulsion system
generates forces for moving the carrier and a position sensor
generates an output signal representing the surface position with
respect to the light beam. A servo system responsive to the output
signal is provided for commanding the propulsion system to move the
carrier at a substantially constant speed in a scanning region. A
control system responsive to the output signal is provided to
modulate a sample period reciprocally to carrier speed to achieve
substantially constant scan length per sample and to control data
acquisition timing.
[0007] In one embodiment, the position sensor, which is monotonic
and repeatable, is selected from a group including a counting
position encoder, optical encoder, magnetic encoder, capacitive
encoder, laser interferometer, an LVDT or reflected optical
triangulation device. The propulsion system may include a motor,
voice coil, a galvanometer, a gas jet or a graphite piston in a
glass cylinder powered by a gas or liquid. A state observer may be
provided to generate an estimate of carrier speed for use by the
sample period modulation system. A system may also be provided for
compensating for variable integral illumination per sample. One
such technique includes scaling the amplitude of a measured signal
by a function of a ratio of an actual sample period to a nominal
sample period.
[0008] In yet another embodiment of the invention, data acquisition
timing is controlled by a state machine that triggers data
acquisition when a selected number of new counts in a correct
direction has occurred. The inputs to the state machine may be
quadrature decoded direction and quadrature decoded count
information. The state machine is programmed so that new counts in
the correct direction are detected by counting backwards quadrature
state changes and incrementing a trigger counter on quadrature
state changes only when a backwards counter is zero.
[0009] In yet another embodiment, data acquisition timing is
triggered by the first equivalence of an actual position value and
a selected trigger position in which a counter is rapidly
incremented, for example, ten times.
[0010] The approach of the present invention permits a highly
linear scan (in the sense that the servo system actively regulates
the sample period to give constant pixel size) even in the presence
of disturbances such as friction and vibration.
BRIEF DESCRIPTION OF THE DRAWING
[0011] FIG. 1 is a schematic view of the system of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0012] With reference to FIG. 1 a "gene chip" or plate 10 is
mounted on a carrier 12. The plate 10 includes an array 14 of
biochemical specimens or test spots on its surface. The biochemical
specimens may be DNA chains bound to the plate 10 which may be
glass. Of course, the scanning system of the invention has
application to scanning systems apart from scanning gene chips.
[0013] The carrier 12 preferably has a low mass and is mounted for
reciprocating movement with respect to a support 16. The carrier 12
and the support 16 may form an air bearing allowing low friction
motion of the carrier 12 with respect to the support 16. In the
embodiment illustrated in FIG. 1, the carrier 12 is moved by a
motor 18 such as a linear motor having one portion fixed on the
support 16 and the other portion on the carrier 12. Alternatively,
the carrier 12 may be moved relative to the support 16 with voice
coils, galvanometers, gas jets or graphite pistons in glass
cylinders. Obviously one could move the lens while holding the
substrate carrier fixed.
[0014] The position of the carrier 12 with respect to the
underlying support 16 (and also with respect to a collimated light
beam described below) is measured by an encoder 20. It is preferred
that the encoder 20 be a non-contact position sensing device (in
order to reduce friction) to measure position of the carrier 12
with respect to the support 16 as the carrier 12 moves. Example
non-contact sensors include optical, magnetic, or capacitive
encoders, laser interferometers, LVDT's or reflective optical
triangulation. Counting position encoders such as optical and
non-optical grating encoders allow a particularly simple
implementation. As another example, one could measure the position
of a laser spot on the chip carrier with a CCD wherein the laser
projects a line at an angle to the axis of motion.
[0015] An output of the encoder 20 becomes an input to a servo
system 22 whose output controls the motor 18. The servo system 22
commands the motor 18 to drive the carrier 12 in a reciprocating
motion and to maintain a substantially constant speed in a scanning
region to be described hereinbelow.
[0016] A laser 24 and lens 26 form an exemplary optical system for
delivering a focused beam of light onto the gene chip or plate 10.
A test spot within the array 14 may fluoresce upon illumination by
the focused beam and the fluorescence is detected by a detector 28.
A sample period modulation system 30 responds to the speed of the
carrier 12 to alter the scanner sample period reciprocally to the
speed of the carrier 12 to achieve a constant scan length per
sample and to control data acquisition timing. The output of the
detector 20 is then stored in storage element 32 which may be part
of a digital computer system (not shown). The sample period
modulation system 30, such as a digital integrator, responds to
position information from the encoder 20. The system 30, in some
circumstances, may derive carrier 12 speed by measuring the time
interval between sensed position increments from the encoder 20.
Alternatively, a state observer 34 may be provided which includes a
dynamic model of the carrier-motor-encoder system. The state
observer 34 responds to an output from the encoder 20 and to
commands to the motor 18 from the servo system 22 to provide an
estimate of a state variable such as speed of the carrier 12. This
speed is used by the sample period modulation system 30 to modulate
the scanner sample period to achieve substantially constant scan
length per sample.
[0017] In operation, the carrier 12 reciprocates rapidly under the
focused light beam from the laser 24. The servo system 22 attempts
to maintain a highly constant speed of the carrier 12 in the
scanning region, but speed can vary because of disturbances such as
friction and vibration. Because speed may vary, actual speed is
measured, inferred or estimated and any speed variation serves as
an error signal for modulating the sample time period in the system
30. That is, if the speed is too low then the sample period will be
increased to assure a constant scan length per sample. Similarly,
if the speed of the carrier 12 is too high, the sample period will
be reduced to achieve constant scan length per sample. Speed errors
can thereby be compensated to an arbitrary level of linearity. Said
another way, T.sub.SN.congruent.T.sub.S(N-l)- k.epsilon. where
T.sub.SN is the sample period for sample N, T.sub.S(N-l) is the
sample period for the sample N-1, and .epsilon. is the variation or
error in speed which, of course, may be a positive or negative
value. It is recognized that there is a trade-off in the uniformity
of illumination per unit distance as more nearly constant scan
length is achieved.
[0018] In a particularly simple implementation of the system of the
invention, the encoder system 20 is used to control scan speed only
to a level at which spatial variations of saturation and/or
bleaching of dye molecules in the array 14 are kept below an
acceptable limit. Pixel acquisition is then simply synchronized to
(triggered by) the encoder 22 output directly and the number of
samples averaged within a pixel is kept fixed at a number that can
be completed in the shortest expected pixel time.
[0019] For realistic conditions, the small gaps in data acquisition
will be sufficiently averaged over by the reading spot size being
wider than the gaps. A synchronization signal can be derived
directly from transitions of a grating encoder (in which case it
can be set to one value out of a fixed (infinite) set of ratios
relative to the grating period) or it can be derived from the
digitized encoder outputs, often allowing further interpolation.
For example, with a nominal linear scan speed of 1 m/s, a pixel
size of 10 .mu.m and a gap of 1 .mu.m (which does hardly degrade
S/N for a 5 .mu.m FWHM spot) a speed variation of 10% would be
acceptable if other sources are neglected. Spending half of this
margin on, for example, clock frequency drift of the oscillator
controlling sample acquisition still allows for a 5% P-V speed
variation. (In a typical system, 226 samples would be acquired in 9
.mu.s at a fixed rate of one sample/40 ns). Alternately, one can
accumulate as many pixels as possible during the pixel time and
then normalize by either dividing by the number samples (times a
scale factor) or by using a look-up table for speeding up this
division.
[0020] As stated above, modulating the sample period according to
the invention to deliver constant pixel size results in variable
integral illumination per sample. This effect can be compensated
for by scaling the amplitude of the measured signal by a function
of the ratio of the actual sample period to a nominal value. In the
case of laser excited fluorescent scanning, the function will be
proportional to the reciprocal of the sample period only if the
fluorescent dye is in its linear region, that is, not saturated or
bleaching appreciably. Alternatively, variable integral
illumination per sample can be compensated by controlling intensity
of the light source. Note that any repeatable and invertible
relationship between integral illumination and emission intensity
can be used for the basis for a compensating function in the
non-linear case. The scaling operation changes noise proportionally
to amplitude rather than to the square root of amplitude so that
artifacts of the compensation may be noticeable.
[0021] In a particularly preferred embodiment, the position sensor
or encoder 20 triggers data acquisition at specified positions or
position intervals. The sensor output signal is integrated using a
digital integrator which may be included in the sample period
modulation unit 30 that accumulates signal samples at, for example,
a 40 ns rate. The number of samples varies with the velocity of the
carrier 12. The data read includes the integrated signal and number
of samples (integration time). It is appropriate to use 32 bits, 10
bits for the number of samples and 22 bits for the signal. The
signal value is divided by a quantity which is the number of
samples divided by a scale factor. The sensed position is compared
to a sequence of trigger positions. This comparison is based on a
threshold and requires hysteresis (where the output depends on the
input and its recent history) to prevent multiple triggers for the
same pixel. Defining the amount of hysteresis to use is an issue.
If the position sensor were ideal and the motion were always in the
direction of scan travel, then no hysteresis is needed. Real
systems, however, have vibration and sensor jitter (position that
alternates between two values due to the least significant bit's
threshold).
[0022] Hysteresis defines separate thresholds for high to low and
low to high input transitions. We want the output to ignore the
input after the threshold is crossed the first time. We also need
to be able to change the direction and change the threshold
(trigger position).
[0023] The position sensor jitter will result in extra pixels being
acquired. One solution to this situation is to implement a state
machine that triggers data acquisition when the desired number of
new counts in the correct direction has occurred. The servo system
defines the correct direction for this state machine. Other state
machine inputs are the quadrature decoded direction and the
quadrature decoded count (that indicates when a quadrature state
change occurs). With a 1 .mu.m encoder resolution and a 10 .mu.m
pixel size, data acquisition is triggered every 10 new encoder
counts. New counts in the correct direction are detected by
counting backwards quadrature state changes and incrementing a
trigger counter on quadrature state changes only when the backwards
counter is zero (no backwards state changes have been
recorded).
[0024] Another way of implementing this scheme is to compare the
actual position value to a trigger position and on the first
equivalence trigger data acquisition and add 10 to the trigger
position. The trigger position can be implemented as a counter that
is rapidly incremented 10 times. Ideally, this counter increment
occurs before the next encoder (actual position) state change,
which is about one microsecond in our system, so the counter must
be incremented at a 100 ns (10 mhz) rate.
[0025] Note that the encoder triggered data acquisition is
registered by an index pulse at each end of a scan line. This index
signal is also derived from the sensed position and is also
susceptible to multiple transitions unless hysteresis is used. The
same position qualification scheme used for the data trigger can be
used for the two indices.
[0026] Velocity controlled sampling avoids this position
comparison. It requires predicting the sample time. The digital
integrator must now be told how many samples to take by the servo
because the servo and digital integrator are asynchronous and the
servo is slower. The prediction is still limited by position sensor
jitter if the velocity is derived from the position rather than
directly measured. The servo loop runs at 100 microseconds but the
data acquisition must run at 10 microseconds (for 10 .mu.m
pixels).
[0027] It is intended that all modification and variations of the
invention disclosed herein be included within the scope of the
appended claims.
* * * * *