Motion tracking using applied thermal gradients

An object tracking and motion control system includes a thermal marking unit such as a laser for inducing localized thermal indicia on objects. A thermal tracking unit, typically a two dimensional thermal sensing array, is positioned to measure movement of objects marked with localized thermal indicia. A motion control unit is connected to the thermal tracking unit to permit adjustment of motion of objects marked with induced localized thermal indicia based on their measured movement.

FIELD OF THE INVENTION 
The present invention relates to thermal tracking of moving objects. More 
specifically, the present invention relates to determination of position 
and velocity of an object through selective induction of a thermal 
gradient on the object. 
BACKGROUND AND SUMMARY OF THE INVENTION 
A material processing system must often precisely control position and 
velocity of objects moving through the system. Commonly, material 
processing systems control object movement by physically engaging the 
object with a separate object drive mechanism moving at a predetermined 
velocity along a predetermined path. For example, gear driven ratchets, 
rollers, hooks, or conveyors are widely employed to move objects as 
diverse as paper, semiconductors, plastics, or steel by mechanically 
engaging the objects, and moving the engaged objects along a desired path 
at a fixed velocity. While commonplace, mechanical or frictional 
engagement of objects does have a disadvantage of requiring direct 
physical contact with an object. For certain applications, including 
processing of high purity or delicate materials, contamination or damage 
to the object may result from mechanical grasping or contact. This is 
particularly true for high speed processing systems, which may damage 
objects simply by engaging them. For example, high speed rollers may 
damage paper through differential engagement of misaligned paper with the 
roller, resulting in ripping or tearing of the paper. As another example, 
the possibility of contaminating high purity silicon wafers moving along a 
processing line is greatly increased by the use of high speed mechanical 
arms or grippers. 
Fortunately, mechanical or frictional engagement is only one possible means 
for moving an object. Object drive mechanisms based on fluid support, 
electrostatic, or electromagnetic systems have all been employed to move 
objects with requiring solid mechanical contact. For example, 
electromagnetic flotation systems can be used to move ferroelectric 
materials without physically contacting a ferroelectric object. More 
commonly, material processing systems that rely on some form of fluid 
support are used, with object entrainment in liquids, bubble flotation 
methods, support on a laminar air flows, or support by directed air jets 
all being used to lift and propel objects through a materials processing 
system. 
In contrast to mechanical engagement systems that hold objects in spaced 
apart relationship at predefined and discrete distances, exact 
determination of object position for non-mechanically engaged object drive 
mechanisms is much more difficult. Typically, a separate position and 
velocity sensor system is required. This can be a mechanical sensor, such 
as a lightweight ball roller that is situated in revolving contact with a 
moving object, or a non-contacting sensor such as an object edge detecting 
laser or video tracking camera. Unfortunately, available mechanical sensor 
systems are difficult to use on levitated objects, still increase the risk 
of contamination, and are often fragile and difficult to calibrate. In 
addition, even lightly contacting roller bearings, lever arms, or other 
mechanical devices can unfavorably alter the dynamic behavior of the 
object. Finally, such mechanical sensors are often overly sensitive to 
changes in object topography and surface properties, making consistent 
measurements difficult. 
Optical position/velocity measurement systems do not have the limitations 
of mechanical systems, but do have their own distinct disadvantages. For 
example, a laser emitter/light detector combination that measures object 
position as a function of light blockage as an object's edge passes 
between the user and its corresponding light detector may greatly reduce 
the risk of contamination as compared to mechanical sensors, but can be 
expensive and require an inordinate number of separate sensor/detector 
pairs to track three dimensional movements. Video tracking camera systems 
are similarly expensive, and may require substantial image processing to 
detect object features and reliably determine position and velocity. 
The disadvantages of commonly available optical detector systems is most 
apparent when position and velocity of a fast moving, delicate, and 
visually featureless material is required. Examples of such materials 
include continuous rolls of paper, extruded plastics, metallic foils, 
wires, or optical fibers. Absent externally applied marking indicia (inks, 
dyes, or physical perforations such as holes), edge tracking or optical 
systems will only be able to determine lateral position and velocity 
transverse to the direction of motion, and not velocity in the direction 
of process motion. This problem does not completely disappear even if the 
featureless material is not continuous, but instead consists of discrete 
units such as sheets of paper or disks of semiconductor wafer material. 
Although the gross velocity of featureless material can generally be 
determined with edge detection methods, rotations, slight misalignments, 
or other orientation problems can still be difficult to quickly detect and 
provide suitable movement compensation without application of undesirable 
markings to an otherwise featureless object. 
Accordingly, the present invention provides an apparatus and method for 
tracking velocity and position of objects that does not require physically 
contacting or permanently marking the tracked object. Furthermore, the 
present invention does not rely on edge or feature detection to determine 
object velocity, position, or orientation, and can easily work with either 
continuous or discrete objects moving through a materials processing 
system. In addition, the present invention does not require permanent 
physical alteration of the object, such as by deposition of patterned 
inks, cut grooves or lines in the object, or punched holes through the 
object. The present invention is an object tracking and motion control 
system that includes a thermal marking unit for inducing localized thermal 
indicia on objects, an adjacent thermal tracking unit for measuring 
movement of objects marked with localized thermal indicia, and a motion 
control unit connected to the thermal tracking unit for adjusting motion 
of objects marked with induced localized thermal indicia based on their 
measured movement. 
In preferred embodiments, the thermal marking unit includes a directable 
heat source configured to elevate the temperature of a localized region on 
an object. This directable heat source can be a laser emitting coherent 
optical or infrared radiation, or may alternatively be a non-coherent 
radiative heat source such as provided by electrical heating of metals. 
For those applications permitting transfer of small amounts of force to an 
object, direct jets of either heated or cooled gas may be used, depending 
upon whether a positive or negative induced temperature gradient is 
needed. In certain applications it is even possible to lightly contact an 
object with a heated or cooled probe to provide transient temperature 
modifications. Touching an object with a probe is particularly useful for 
conductive cooling of a localized spot on an object, and may be enabled 
with a "cold finger" in contact with an open or closed circuit evaporative 
cooler or Peltier effect device. 
Operation of thermal marking can be continuous or intermittent, depending 
on the desired shape of the induced temperature gradient. Typically, 
stable continuous operation provides a temperature gradient centered on a 
line directed in the direction of object movement. When only temperature 
sensors having a coarse temporal resolution (i.e. unable to quickly 
measure changing temperatures) are available, the linear temperature 
gradient induced by a continuous thermal marker is easily detectable and 
provides useful information concerning the travel path and two dimensional 
rotational orientation of an object. 
For those situations where high temporal resolution temperature sensors are 
available, intermittent, pulsed, or periodic operation of the thermal 
marking unit is possible. Pulsed or periodic operation may include both 
discrete thermal marking (e.g. short heat bursts applied every tenth of a 
second) or amplitude modulated heat application (e.g. a continuous heat 
source having its available directed thermal energy sinusoidally varied). 
Advantageously, multiple intermittent, pulsed or periodic thermal marking 
allows accurate determination of object position and velocity, and in 
appropriate circumstances can allow determination of the two or three 
dimensional orientation of the object. As will be appreciated, such 
information generally requires high speed thermal scans, or better yet, 
two dimensional temperature sensor arrays for best operation. 
For example, a laser can be used to heat a plurality of localized regions 
of an object. The time required for the movement of the heated regions to 
adjacent temperature sensors of the thermal tracking unit is inversely 
proportional to the speed of the object, assuming of course that the 
object's velocity is substantially constant. Instantaneous velocity can be 
determined of two dimensional temperature gradient information is 
available. Using temperature information from each sensor (which 
individually detects a temperature in a subregion of each heated region of 
the object), a temperature gradient can be calculated. Since the shape of 
the gradient varies according to object velocity, the speed and direction 
of movement of the object can be determined. Further, since a temperature 
centroid based on a two dimensional temperature gradient information 
derived from subregion temperatures can be derived, a highly accurate 
object position can be calculated. Further, if the object is multiply 
marked with thermal indicia, orientation information derived from the 
temperature centroid is determinable, and even rotational speed of the 
object can be calculated. 
Upon determination of position, velocity, and orientation information, the 
motion control unit for adjusting motion of objects can use this 
information to correct for object misalignments, incorrect speed or travel 
path, or even object pitch, roll, and yaw (if three dimensional 
orientation information is available). In a most preferred embodiment of 
the present invention, paper or other graphically markable material is 
among the objects capable of being thermally marked and tracked in 
accordance with the present invention. High speed movement of paper can be 
enabled by use of independently adjustable mechanical movers such as 
differential rollers, or more advantageously, with air jets that support 
and move paper through a paper processing system such as a xerographic 
apparatus, laser printer, or electrostatic ink jet printer. The paper 
handling system includes a thermal marking unit (typically an infrared 
laser) for inducing a localized temperature gradient on a region of paper 
moving through the paper handling system. A thermal sensing unit 
(typically a two dimensional infrared sensing array, constructed using 
conventional micro electrical mechanical systems (MEMS) technology) is 
positioned adjacent to the thermal marking unit, with the thermal sensing 
unit being configured to measure a plurality of localized subregion object 
temperatures over the region of paper having the induced localized 
temperature gradient. A paper movement calculating module is connected to 
the thermal tracking unit, with the paper movement calculating module 
determining paper movement relative to the MEMS type thermal sensing unit 
based on determination of a centroid of the induced localized temperature 
gradient. In response to the calculated position and velocity, a paper 
motion control unit connected to the paper movement calculating module is 
Used to modify paper movement (for example, by selectively increasing or 
decreasing velocity of air jets impacting defined regions of the paper) to 
nearly instantaneously correct for paper misalignments. 
Additional functions, objects, advantages, and features of the present 
invention will become apparent from consideration of the following 
description and drawings of preferred embodiments.

DETAILED DESCRIPTION OF THE INVENTION 
A processing system 10 optimized for handling objects without requiring 
direct physical contact, including sheets of paper 12, is partially 
illustrated in FIG. 1. The processing system 10 has a conveyor that 
includes a plurality Of air jets 26 for supporting, moving, and guiding 
paper 12 through the system 10. Active object guidance is enhanced by 
provision of a thermal marking unit 30, typically an infrared laser 31. 
The laser 31 is capable of directing a laser beam 32 to induce a localized 
temperature gradient 34 on a region of paper 12. A thermal sensing unit 
40, typically positioned adjacent to the thermal marking unit 30 (within a 
few meters, or often within a few centimeters) is used to measure object 
temperatures over the region of paper 12 having the induced localized 
temperature gradient 34, and pass this temperature information to a 
temperature analysis unit 50 capable of calculating movement of paper 12, 
including its position and velocity, relative to the thermal sensing unit 
40. Using this calculated movement information, a motion control unit 52 
connected to the temperature analysis unit sends control signals to modify 
movement of paper 12. 
In operation, use of a thermal sensing unit 40 for feedback control of 
object movement allows for precise micromanipulation of object position. 
For example, in FIG. 1 paper 12 is sequentially illustrated in three 
distinct positions along conveyor 20, respectively labelled as paper 
position 14, paper position 16, and paper position 18. In position 14, the 
paper 12 is thermally marked by laser 31. As paper 12 is moved along 
conveyor 20 toward position 16 by air jets 26, it becomes slightly 
misaligned (note, the severity of misalignment is greatly exaggerated in 
the Figure). The sensor 40 provides a spatial measurement of the 
temperature of the region of paper 12 passing beneath it, and passes the 
information to the temperature analysis unit 50. The temperature analysis 
unit 50 uses the temperature information (i.e. the sensor measured 
temperature gradient in one, two, or three dimensions) to accurately 
determine position of the thermal marking (and consequently the paper 12). 
This positional information is passed to the motion control unit 52, which 
sends signals to selected air jets 26 to correct the misalignment, 
bringing the paper 12 back into an aligned position 18, ready for further 
processing by system 10. 
Advantageously, the present invention allows for thermal tracking, 
manipulation and control of a wide variety of objects and processes. Note 
that the description of the present invention in conjunction with air jet 
conveyer 20 is for illustrative purposes only, and in suitable 
circumstances the conveyor 20 can be replaced by belts, friction drives, 
slides, chutes, mechanical grippers, vacuum attachment mechanisms, or any 
other conventional conveyor or drive mechanism. In addition to paper 
handling, other articles of manufacture, including those composed of 
plastics, ceramics, metals, wood, or any other conventional material can 
be thermally tracked according to the present invention. Thermal tracking 
can also be employed to control movement of processing machinery. For 
example, belts or rollers of xerographic copiers or other machinery having 
moving parts can be transiently marked with thermal indicia, the thermal 
indicia being thermally tracked to ensure proper speed, position, or 
rotational velocity of the moving parts. 
As will be appreciated by those skilled in the art, although the present 
invention can be used with ordinary irregular articles capable of being 
visually or mechanically distinguished by appropriate 
imaging/identification systems, the present invention is of particular 
utility in conjunction with processes that require precise high speed 
movement of delicate and visually featureless material. To maintain 
purity, quality, or consistency, such materials are often unsuited for 
marking with conventional marking indicia such as inks, dyes, or physical 
cuts, notches or perforations. Examples of such materials include rolls or 
sheets of paper, extruded plastics, metallic foils, wires, silicon wafers, 
high quality ceramics or machined parts, or even optical fibers. The 
present invention permits ready detection and correction of rotations, 
slight misalignments, or other orientation problems that can be difficult 
to quickly detect and provide suitable movement compensation for without 
application of undesirable markings to an otherwise featureless object. 
In order to ascertain object position properly, the thermal sensing unit 40 
must be reliable and accurate, having a spatial, thermal, and temporal 
resolution sufficient for thermal tracking of a relatively small area 
(typically less than one square centimeter) at less than about 1/10 of a 
degree Celsius temperature gradient intervals. To prevent thermal damage 
to materials, relatively low temperature elevations of a region of an 
object, typically in the range of 10 to 100 degrees Celsius over ambient 
(ambient taken as about 20 degrees Celsius), are used. For particularly 
delicate materials or for high precision applications, even smaller 
temperature gradients may be employed. For example, if sinusoidal heat is 
applied to an object, temperature elevations as small as 1/100 of a degree 
Celsius may be detected after conventional signal processing techniques 
are used to filter out low frequency temperature changes. 
In many processes the object is moving quickly, allowing less than a 100 
milliseconds for thermal measurements. Fortunately, infrared sensors such 
as a micro electro mechanical thermal sensors (MEMS-type sensors), 
thermocouples, temperature sensitive diodes, pyroelectric devices, or 
certain other conventional thermal detectors are capable of providing 
suitable spatial, thermal, and temporal resolutions. For best results, two 
dimensional sensor arrays or scanned one dimensional temperature arrays 
are utilized, however, fixed one dimensional sensor arrays can also be 
used, especially if only coarse resolution in either the thermal or 
temporal domain is required. 
In a preferred embodiment of the present invention, illustrated with 
reference to FIGS. 2 and 3, the thermal sensing unit 40 is a two 
dimensional MEMS-type sensor array 41. The sensor array 41 includes a 
substrate 43, typically silicon, that may be coated with single or 
multiple layers of doped silicon, polysilicon, silicon nitride, silicon, 
silicon oxide, oxynitride, or aluminum. A plurality of upwardly extending 
arms 42 that provide thermal isolation and enhance spatial resolution of 
the array 41 are attached to the substrate 43, with each arm 42 
terminating in a thermally sensitive diode 44. Typically, a sensor array 
41 will have overall dimensions between about one millimeter square to 
about one centimeter square, with anywhere from about 100 to 100,000 
separate arms being used to support thermally sensitive diodes. In a most 
preferred embodiment, overall dimensions of between about 1 millimeter and 
1 centimeter, with between about 100 and 100,000 thermally sensitive 
diodes being sufficient for measurement of temperature gradients with a 
desired spatial resolution 
Various conventional construction techniques can be used to build the 
MEMS-type sensor array 41, including chemical etching, electron beam 
lithography, photolithography, or other standard integrated circuit batch 
processing technologies. As will be appreciated, various MEMS-type thermal 
sensor designs can be used to practice the present invention. For example, 
material surrounding each arm 42 can be selectively etched away, except 
for a defined pivot attachment, and the arms electrostatically maneuvered 
(by rotation upward) and locked into the shown position. Alternatively, 
the material surrounding an arm already oriented in an upright position 
can simply be etched away. In other MEMS-type thermal sensor embodiments, 
not illustrated, thermal isolation of thermally sensitive diodes can be 
achieved by emplacement of diodes in wells, or surrounding the diodes with 
partitions. 
No matter which MEMS-type sensor is utilized, for best results in thermal 
tracking each diode 44 must be attached by provided data lines (not 
illustrated) to the temperature analysis unit 50 to allow calculation of 
thermal gradients. To better appreciate operation of the present 
invention, FIG. 3 illustrates temperature isogradient lines 45 for sensor 
array 41. Once isogradient lines 45 are determined, a temperature centroid 
46 can be calculated. Presumably, the apparent position of the calculated 
centroid 46 closely approximates the initial position of a region of an 
object (such as paper 12) that is thermally marked at a spot with, for 
example, laser 31. By tracking position of the centroid 45, the relative 
movement of the object between the initial thermal marking and the present 
position of the object can be estimated. In practice, even relatively 
crude experimental sensor systems (with consequent crude centroid 
determinations) have been found to allow determination of object position 
with sub-centimeter precision. Higher density sensor arrays will 
accordingly allow even better tracking of objects with submillimeter 
accuracy. 
To better appreciate various aspects of possible thermal tracking schemes 
using the present invention, FIGS. 4-7 are provided to illustrate 
isotemperature gradients on object 100. FIG. 4 illustrates an 
isotemperature gradient 145 in response to continuous application of heat 
as object 100 linearly passes in direction 70 under a heat source (not 
shown). As can be seen by inspection of FIG. 4, lateral or two dimensional 
rotational orientation of object 100 can readily be determined by 
calculation of the center line 155 of the temperature gradient. If the 
isotemperature gradient is sufficiently detailed, as shown in FIG. 4, it 
may even be possible to determine speed of object 100, based on the extent 
of broadening of the temperature gradient 145 as the object 100 moves away 
from the heat source and normally cools. 
Similarly, FIG. 5 illustrates the isotemperature gradient 146 in response 
to intermittent application of a heat spot (for example by a laser, not 
shown) as object 100 linearly passes in direction 71. As can be seen by 
inspection of FIG. 5, the two dimensional position of object 100 can 
readily be determined by calculation of centroid 156 of the temperature 
gradient. Again, it is possible to determine velocity of object 100, based 
on the direction and extent of broadening of the temperature gradient 146 
as the object 100 moves away from the heat source and cools. 
FIGS. 6 and 7 illustrate possible thermal tracking schemes using multiple 
spaced apart thermal markings 147 and 148. The use of multiple thermal 
markings, in conjunction with multiple thermal sensors (not shown), 
improves accuracy of calculating object position, velocity, and 
orientation. FIG. 7 illustrates application of periodic temperature pulses 
at discrete intervals to an object 100 to allow for accurate velocity 
determination. 
As will be appreciated, many possible devices can be used to induce 
measurable temperature gradients in an object. For example, FIGS. 8-11 
illustrate a few possible methods for heating paper 12 in addition to 
laser heating (heating via coherent radiation) as previously discussed in 
relation to FIG. 1. For continuous heating, a resistance element 82 
connected to an electrical power source 80 can be used to provide a low 
cost and relatively steady source of radiative thermal energy to paper 12 
moving in direction 73. Use of this resistance element 82 for heating an 
object would be expected to yield a isotemperature gradient qualitatively 
similar to that indicated in FIG. 4. 
Another contemplated thermal heating device is illustrated by FIG. 9. 
Relying on heated jets of air 85 released by operation of valve 86 and 
heated air reservoir 84, instead of radiative heating, permits quick 
response and intermittent operation. Similarly quick intermittent response 
is also possible with the device illustrated in FIG. 11. As shown, FIG. 11 
uses cooling jets of air 93 released by operation of valve 92 and cooled 
air reservoir 90 to induce a negative thermal gradient on paper 12 moving 
in direction 76. 
For certain applications, direct contact with an object for conductive 
heating or cooling may be used. FIG. 10 illustrates use of a ceramic probe 
89 containing resistive elements that can be electrically heated and 
placed into thermal contact with paper 12. If intermittent, rather than 
continuous, heating operation is desired the probe 89 can be mechanically 
withdrawn from contact with paper. 
While the present invention has been described in conjunction with specific 
embodiments thereof, it is evident that many alternatives, modifications, 
and variations will be apparent to those skilled in the art. Accordingly, 
the various embodiments described herein should be considered 
illustrative, and not limiting the scope of the present invention as 
defined in the following claims.