Note: Descriptions are shown in the official language in which they were submitted.
CA 02292372 1999-12-17
ROBOT FEATURE TRACKING DEVICES AND METHODS
FIELD OF THE INVENTION
The present invention relates to robot feature tracking
devices and methods, and more particularly to an assembly, a
system and a method for providing additional positioning
ability to a tool at an end of a robot arm, and improving the
positioning accuracy of a robot tool over a feature to be
processed. The invention applies for example to laser
processing, such as laser welding, and to arc welding. It
also applies to other types of processing that involve the
guidance of a tool over a joint or feature to be processed.
BACKGROUND
It is well known that process robot tasks are often
programmed using the method of play back of a taught path. If
the work piece to be processed by the robot is not accurately
positioned and oriented to correspond with this taught path,
the robot will not position its tool accurately over the work
piece and flaws will result.
The current solution to this problem is to install a
sensor in front of the robot tool and to link this sensor
with the robot through a special interface. In a welding
operation, for example, the sensor measures the position and
orientation of the joint, and communicates this information
to the robot to correct its trajectory and tool orientation
at the right time and place.
One problem is that many robots are not equipped with
this type of interface. They cannot be linked with a sensor
for joint or feature tracking.
Another problem is the calibration that is required in
order to define the physical relation between the tool center
point and the sensor. This relation must be well defined to
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allow the control unit of the sensor to accurately control
the position and orientation of the tool while getting
position information about the joint some distance in front
of the tool. This calibration is usually performed by
accurately positioning the tool center point over a reference
object. If the operator does not accurately position the tool
center point during this operation, the calibration will not
be accurate. The robots usually have a very good positioning
repeatability but poor absolute positioning accuracy. This
means that the tool center point can be repeatedly sent back
to the same position with a good accuracy, but the
coordinates of this position in space will not be known
accurately. The robot also makes an error when it informs the
sensor about its current position during joint or feature
tracking because of the response time of the robot arm and
because of its mechanical elasticity. In the case of arc
welding with a filler wire, the calibration problem is
further complicated by the fact that the filler wire is not
always straight when it gets out of the tool tip. It often
gets out with a variable curve so that the tip of the wire
does not correspond to the position of the tool center point.
In the case of laser welding, the focal point of the laser
beam moves relative to the theoretical position of the tool
center point because of imperfections in the optical path.
US patents Nos. 4,952,772 (Zana), 4,954,762 (Miyake et
al.), 4,969,108 (Webb et al.), 5,006,999 (Kuno et al.),
5,014,183 (Carpenter et al.), 5,015,821 (Sartorio et al.),
5,066,847 (Kishi et al.), 5,463,201 (Hedengren et al.),
5,465,037 (Huissoon et al.), 5,582,750 (Hamura et al.) and
5,624,588 (Terawaki et al.) provide examples of welding
control systems and methods of the prior art, some of which
including error correction algorithms. Yet, none of them
provides easy robot path correction for joint and feature
tracking by an industrial process robot, which would be
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applied even at very high speed and without directly
intruding into the robot control itself. Likewise, none of
them satisfactorily solves the problem of accurate computing
of the sensor to robot tool center point geometric relation,
in static and dynamic operating modes, which is so critical
to high speed joint tracking due to the use of the delayed
shift method usually applied when a laser vision system is
used in front of the robot tool.
SUMMARY
An object of the invention is to provide easy robot path
correction for joint and feature tracking by an industrial
process robot, which can be applied even at very high speed
and without directly intruding into the robot control itself.
Another object of the invention is to provide additional
positioning ability to a tool at an end of a robot arm.
A subsidiary object of the invention is to allow a robot
to perform joint and feature tracking even if the robot is
not equipped with the proper interface.
Another object of the invention is to provide a solution
to the problem of accurate computing of the sensor to robot
tool center point geometric relation, in static and dynamic
operating modes, which is so critical to high speed joint
tracking due to the use of the delayed shift method usually
applied when a laser vision system is used in front of the
robot tool.
According to the present invention, there is provided a
motorized slide assembly for providing additional positioning
ability to a tool at an end of a robot arm. The assembly
comprises a slide arrangement having a base and a sliding
element movable along a predetermined course relative to the
base. A motor is mounted onto the slide arrangement. A drive
device is connected to the motor for moving the sliding
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element along the course upon operation of the motor.
Fasteners are provided for fastening the base of the slide
arrangement to the end of the robot arm, and for fastening
the tool onto the sliding element.
According to the present invention, there is also
provided a motorized slide system for providing additional
positioning ability to a tool at an end of a robot arm. The
system comprises a motorized slide assembly including a slide
arrangement having a base and at least one sliding element
movable along a predetermined course relative to the base. A
motor is mounted onto the slide arrangement. A drive device
is connected to the motor for moving the sliding element
along the course upon operation of the motor. Fasteners are
provided for fastening the base of the slide arrangement to
the end of the robot arm and for fastening the tool onto the
sliding element. An encoder is operatively coupled to the
motor to provide motor positional information. A control unit
is provided for the motorized slide assembly. The control
unit includes a communication interface for receiving sensor
related data, a I/0 interface for receiving and transmitting
synchronization signals, a CPU for controlling positions of
the sliding element, a memory, a servo-amplifier circuit for
powering the motor, a slides control for controlling the
servo-amplifier circuit in response to the CPU and the motor
positional information provided by the encoder, and a bus
circuit interconnecting the communication interface, the I/0
interface, the CPU, the memory and the slides control
together.
According to the present invention, there is provided a
compensation method for compensating errors made by a control
unit of a robot sensor when evaluating a relation between a
position of a robot guided tool behind the sensor and a
position of a feature to be followed by the guided tool. The
method comprises the steps of recording position data
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generated by the sensor during a dry pass of the guided tool
over the feature, the position data representing consecutive
positions of the feature detected by the sensor, and
subtracting the recorded position data from joint position
errors computed by the control unit during a feature tracking
operation where the guided tool is operated to process the
feature.
According to the present invention, there is provided a
control unit for a robot sensor tracking a feature to be
processed with a robot tool positioned behind the robot
sensor. The control unit comprises a sensor interface having
a sensor control output and a video input. A memory is
connected to the sensor interface. A CPU is connected to the
sensor interface and the memory. A communication interface is
connected to the CPU, the memory and the sensor interface,
and has a communication port. The memory includes a look-
ahead buffer that stores a number of successive feature
position data computed by the CPU from signals received at
the video input, as a function of tracked successive
positions reached by the robot sensor during displacement
over the feature. An additional buffer is connected to the
look-ahead buffer, and stores a number of the successive
feature position data as a function of tracked successive
positions reached by the robot tool. The CPU has an operating
mode causing a computation of a corrected position value
required to maintain the robot tool correctly positioned over
the feature by subtracting a current position of the robot
tool and one of the position data stored in the additional
buffer related to the current position of the robot tool from
one of the position data stored in the look-ahead buffer
related to the current position of the robot tool, and a
transmission of the corrected position value through the
communication port of the communication interface.
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According to the present invention, there is also
provided a robot sensor assembly for simultaneously detecting
a position of a feature at a given look-ahead distance in
front of a tool and a position of a tip of the tool. The
robot sensor assembly comprises a sensor body, a bracket for
side attachment of the sensor body to the tool, a first probe
device attached to the sensor body and directed toward the
feature in front of the tool, for providing surface range
data along the feature whereby the position of the feature at
the look-ahead distance in front of the tool is determinable,
and a second probe device attached to the sensor body and
directed toward a target region including the tip of the tool
and the feature under the tip of the tool, for providing an
image of the target region whereby the position of the tip of
the tool is determinable.
According to the present invention, there is also
provided a sensor control unit for a robot sensor assembly as
hereinabove described. The sensor control unit comprises a
range processing circuit having an input for receiving a
video signal produced by the robot sensor, and an output for
producing surface range data extracted from the video signal.
A frame grabber has an input for receiving the video signal
produced by the robot sensor, and an output for providing
image frames stored in the frame grabber. A main CPU has an
input connected to the output of the range processing
circuit, and a communication port. A secondary CPU has an
input connected to the output of the frame grabber, and a
communication port. A communication link interconnects the
communication ports of the main and the secondary CPUs. A
communication interface is connected to the communication
link. The secondary CPU has an operating mode causing a
processing of the image frames stored in the frame grabber, a
determination of the position of the tip of the tool from the
image frames, and a transmission of the position of the tip
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of the tool to the main CPU via the communication link. The
main CPU has a sensor-tool calibration mode causing a storage
of the position of the tip of the tool received from the
secondary CPU as calibration data, and a subsequent
processing mode causing a comparison of the position of the
tip of the tool received from the secondary CPU with a
corresponding position in the calibration data, a computation
of tool positioning correction values, and a transmission of
the correction values through the communication interface.
To sum up, the addition of motorized slides at the end
of a robot arm and the installation of the tool and the
sensor on the motorized slides allow for joint and feature
tracking to be performed even if the robot is not equipped
with the proper interface, and thereby provide additional
positioning ability to the tool at the end of the robot arm
as the orientation of the slides can be set as needed and
desired.
In one preferred embodiment of the invention, motorized
slides are added at the end of a robot arm in order to enable
real-time seam tracking while the control unit of the robot
is not necessarily equipped with a sensor interface. A tool
and a sensor are installed on the motorized slides so that a
control unit, provided with a vision system to process the
data from the sensor and a slides driver to control the
position of each slide, maintains the tool correctly
positioned over a joint or feature of an object by moving the
motorized slides according to the position information
computed by the vision system, while the robot arm moves
along the joint or feature by following a programmed path.
The compensation method compensates for robot teaching
inaccuracies, for calibration errors in the robot arm and for
errors caused by the response time of the robot arm.
In another preferred embodiment of the invention, the
compensation method, based on data recorded while the robot
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follows a programmed path, is used to modify the position
correction information computed by the control unit of the
sensor. This method compensates for errors made by the
control unit of the sensor when it evaluates the relation
between the position of the tool and the position of the
joint or feature to be tracked, these errors being caused by
incorrect programming of the robot path or by inaccuracies in
the robot.
Accuracy can be improved also with the use of a sensor
that gets information from the joint or feature in front of
the tool and from the real position of the tip of the tool.
In another preferred embodiment of the present
invention, a sensor with two distinct vision zones is used to
get information about the position of the tip of the tool, as
well as the position of the joint or feature some look-ahead
distance in front of the tool, in order to help in
calibrating the sensor/tool relation.
BRIEF DESCRIPTION OF THE DRAWINGS
A detailed description of preferred embodiments will be
given herein below with reference to the following drawings,
in which like numbers refer to like elements:
Figure 1 is a perspective view of a motorized slide
assembly according to the present invention, installed at an
end of a robot arm.
Figure 2 is an enlarged view of the motorized slide
assembly shown in Figure 1, with a tool and a sensor.
Figure 3 is a schematic block diagram representing
control units for the sensor, the motorized slides, and the
robot, according to the invention.
Figure 4 is a schematic diagram representing a control
system for the motorized slide assembly according to the
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invention, and the I/0 interface with the control unit of the
robot.
Figures 5A, SB and 5C are complementary flow charts
representing the method for the feature tracking with a
motorized slides assembly according to the invention.
Figure 6 is a schematic diagram representing a
processing made by a CUS for the trajectory control of a
tool, including a look-ahead buffer.
Figures 7A and 7B are schematic diagrams illustrating an
error that a sensor makes during a feature tracking if the
operator did not correctly position it while teaching the
path of the robot.
Figures 8A and 8B are schematic diagrams illustrating an
error that a sensor makes during a feature tracking if the
taught path is not straight but follows a deviation of the
feature to be tracked.
Figures 9A, 9B and 9C are complementary flow charts
representing the compensation method used to compensate for
the errors illustrated in Figures 7 and 8, according to the
invention.
Figure 10 is a schematic diagram representing a
processing made by a slides control unit for the trajectory
control with compensation according to the invention,
including a look-ahead buffer and an additional buffer.
Figure 11 is a side elevation view of a sensor that
integrates two distinct vision zones, according to the
invention.
Figure 12 is a cross section view of a possible optical
arrangement for the sensor shown in Figure 8.
Figure 13 is a block diagram illustrating the data
acquisition and processing system for a sensor with two
distinct vision zones, according to the invention.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figures 1 and 2, there is shown a motorized
slide assembly for providing additional positioning ability
to a tool 2 at an end of a robot arm 4, according to the
invention.
In the illustrated embodiment, the slide assembly
combines two motorized slide arrangements 6, 8 assembled in a
block 12. Each slide assembly 6, 8 has a base 14, 16 and a
sliding element 16, 18 (the slicing element 16 of the slide
assembly 6 being provided by the back of the base 16) movable
along a predetermined course relative to the base 14, 16. The
assembly is installed at the wrist of the robot arm 4. For
this purpose, any kind of suitable fastener and fastening
arrangement can be used, like a mounting bracket 10 which
fastens the base 14 of the slide arrangement 6 to the end of
the robot arm 4 as best shown in Figure 2. The base 14
provides a mounting surface adapted to receive the end of the
robot arm 4.
The processing tool 2 is mounted on the motorized slide
assembly, with the tool center point 20 being preferably as
close as possible to the position where it used to be without
the motorized slide assembly, so that the robot can be
programmed to weld a piece 22 as usual. For this purpose, any
suitable fastener and fastening arrangement can be used, like
a clamp 24 projecting from the sliding element 18 opposite
the base 16 thereof.
A sensor 26 can be affixed to the tool 2 or the
motorized slide assembly to detect the joint feature 28 to be
tracked in front of the tool 2 as best shown in Figure 1. For
this purpose, the clamp 24 has preferably a mounting surface
opposite the sliding element 18, adapted to receive the
sensor 26.
Only one motorized slide arrangement 6 or 8 can be
installed if the trajectory corrections must be made in only
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one direction, for example laterally or vertically,
perpendicularly to the programmed trajectory. A second
motorized slide arrangement 6, 8 can be added perpendicularly
on the first one if the trajectory corrections must be
applied both laterally and vertically. If necessary, other
motorized slide arrangements including linear and rotational
slide arrangements can be used to support more degrees of
freedom for the movement of the tool 2.
Referring in particular to Figure 2, the sliding element
18 of the slide arrangement 8 is in the form of a plate and
the base 16 has spaced apart, opposite lateral surfaces 44
slideably receiving the plate. In the case of the slide
assembly 6, the equivalent of the plate is provided simply by
the back of the base 16 of the slide arrangement 8. Each base
14, 16 may take the form of an elongated frame having spaced
apart, opposite end faces 38, 40, extending between the
lateral surfaces 42, 44.
Each slide arrangement 6, 8 has a motor 30, 32 mounted
onto the slide arrangement 6, 8 and preferably one of the end
faces 42, 44. A worm screw 34, 36 extends between the end
faces 42, 44 and is coupled to the motor 30, 32. A toothed
member (hidden by the base 16 and the sliding element 18)
projects from the plate 18 or the back of the base 16 and is
meshed with the worm screw 34, 36. The worm screw 34, 36 and
the toothed member form a drive mechanism for moving the
sliding element 16, 18 along the corresponding course upon
operation of the motor 30, 32. Any other suitable drive
configurations can be used. The toothed member can be made
for example by a nut screwed about the worm screw 34, 36,
which has the advantage of holding the sliding element 16, 18
against the base 14, 16 without requiring additional guiding
members.
The motors 30, 32 are preferably provided with encoders
46, 48 for control of the motors' positions.
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Referring to Figures 1 and 3, in use, the robot can be
first programmed off-line or by a "teach and play back"
method as usual. During this robot teaching phase, the
motorized slides 6, 8 are maintained in their central
reference position, in order to provide the maximum
trajectory correction range on either side of the programmed
trajectory. The relation in the 3D space between the tool
center point 20 and a given reference position in the sensing
range of the sensor 26 must also be determined. This relation
is used to calibrate the position of the tool center point 20
in the coordinate system of the field of view of the sensor
26. This calibration data is programmed in the control unit
50 of the sensor (CUS). This allows the control unit 50 of
the sensor (CUS) to calculate the position of the tool center
point 20 relative to the position of the joint 28, knowing
the position of the joint 28 in the sensing range of the
sensor 26.
Referring to Figures 3 and 4, the CUS 50 is interfaced
through a communication link 52 with the control unit 54 that
drives the motorized slides 6, 8 (CUMS). The CUMS 54 is
interfaced with the control unit 56 of the robot (CUR)
through a I/0 line 58 for synchronization. The I/O signals
can be sent through electrical wires 60, 62, 64, 66, 68, 70
and can consist of voltage variations, a high voltage
representing the activated state and a low voltage
representing the deactivated state. The six signals required
for the synchronization between the CUMS 54 and the CUR 56
are illustrated in Figure 3. This synchronization can also be
accomplished by sending messages through a communication
device, such as a serial communication link or a parallel
bus. The CUS 50 has a sensor communication interface 51 for
communicating with the sensor 26 through a communication link
53 over which control and video signals are transmitted. A
bus 55 interconnects the sensor interface 51 with a memory
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57, a CPU 59 and a communication interface 61 forming a
processing circuitry of the CUS 50. The CUMS has a
communication interface 63 for receiving sensor related data
from the CUS 50 through the communication link 52.
Referring to Figures 5A-5C, there is shown a flowchart
illustrating the steps that can be carried out by the system
for feature tracking with motorized slides installed on a
robot, according to the invention. As depicted by block 72,
the CUR 56 activates a home signal to inform the CUMS 54,
through the I/0 link 58, that it is time to bring the
motorized slides to their central position, operation which
is depicted by block 78.
Referring to Figure 3, the CUMS 54 is interfaced with
the motorized slides 6, 8 through a slides controller 74 and
servo amplifiers 76. The motors 30, 32 of the slide
arrangements 6, 8 are powered by the servo amplifiers 76 and
the slides controller 74 senses their position through the
position encoders 46, 48 that are coupled to the motors 30,
32. By sending the successive positions to be reached to the
slides controller 74, the CPU 80 of the CUMS 54 controls the
position of the motorized slides 6, 8. A memory 65, an I/0
interface 67 and an interconnecting bus 69 complete the
processing circuit of the CUMS 54.
Referring to Figure 5A, once the central or home
position of the motorized slides is reached, the CUMS 54
activates a signal to inform the CUR 56, through the I/0 link
58, that the home position of the motorized axes is reached
and that the process can start, as depicted by block 82.
Once the home position is reached, the tool 2 is brought
to the beginning of the path, where the sensor 26 will start
looking for the joint or feature 28 to be tracked, as
depicted by block 84. The CUMS 54 waits for a search start
signal from the CUR 56. When this signal comes as depicted by
block 86, the tool 2 starts moving forward along the
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programmed path and the CUS 50 starts looking for the feature
28.
Referring to Figure 6, there is shown computations
related to the trajectory control and a look-ahead buffer 88
implemented in the CUS 50. Once the CUS 50 has found the YZ~
coordinate of the feature 28 (in the reference system of the
tool 2) at the current XS sensing position along the feature
28, it adds this coordinate YZ~ to the current YZTC coordinate
of the tool to get the YZF coordinate of the feature 28. The
CUS 50 stores this YZF value in the look-ahead buffer 88,
associated with the current XS position of the sensor 26.
Since the motorized slides 6, 8 are still at their home
position, the YZTC coordinate is considered to be (0,0) at
this moment. The look-ahead buffer 88 is a circular buffer
that contains the data sampled along the feature 28 between
the observation zone 90 of the sensor 26 and the position of
the tool center point 20 as shown in Figure 1. The CUS 50
carries on this process until the tool 2 reaches the X
position where the feature 28 was first found, as depicted by
blocks 92, 94 in Figure 5A.
When the tool 2 reaches the X position where the feature
28 was first found, the CUS 50 extracts from the look-ahead
buffer 88 the YZF coordinate of the feature 28 at the current
XT position of the tool 2 along the feature 28. It computes
the position correction YZcORR by subtracting the YZTc
coordinate from the YZF coordinate. The new YZTC coordinate of
the tool 2 after this movement is computed by adding the
YZcoaR correction to the previous YZTC coordinate. The CUS 50
informs the CUMS 54, through the communication link 52, that
the start position is reached as depicted by block 96 in
Figure 5B, and sends the YZcORR position correction required
by the CUMS 54 to move the motorized slides 6, 8 to bring the
tool 2 above the feature 28 as depicted by block 98. Once
this operation is achieved, the CUMS 54 activates a start
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position signal to inform the CUR 56, through the I/O link
58, that the tool 2 reached the start position, as depicted
by block 100. When the CUR 56 receives this signal, it stops
the movement of the tool 2. It then starts the welding
operation as depicted by block 102, starts moving the robot
arm 4 along the programmed path and activates a signal to
inform the GUMS 54 that the tracking operation can start as
depicted by block 104.
The CUMS 54 informs the CUS 50 that the tracking
operation started. The CUS 50 computes a new YZ~ feature
coordinate in the tool reference system, adds this coordinate
to the current YZT~ coordinate of the tool 2 and stores the
resulting information YZF in the look-ahead buffer 88,
associated with the current XS position of the sensor 26. The
CUS 50 extracts from the look-ahead buffer 88 the YZF
position data corresponding to the current XT position of the
tool 2. It subtracts the current YZTC position of the tool 2
from the YZF position to obtain the YZ~o~ position correction
required to maintain the tool center point 20 correctly
positioned over the feature 28. It sends this correction to
the CUMS 54 that moves the motorized slides 6, 8 to apply the
correction. This tracking cycle continues until the tool 2
reaches the end of the feature 28, as depicted by blocks 106,
108. The CUS 50 recognizes that the end of the feature 28 is
reached when the look-ahead buffer 88 does not contain valid
position information at X positions that are beyond the
current X position of the tool 2. The CUS 50 informs the CUMS
54 that the end position is reached, as depicted by block 109
in Figure SC. The CUMS activates an end of feature signal to
inform the CUR 56, through the I/0 link 58, that the tool 2
reached the end of the feature 28, as depicted by block 110.
The CUR 50 stops the movement of the robot arm 4 and
terminates the welding process, as depicted by block 112.
CA 02292372 1999-12-17
This joint or feature tracking process assumes that the
path of the tool 2 was perfectly programmed in the CUR 56.
However, because the operator cannot maintain the sensor 26
at a constant position over the joint or feature 28 during
the robot teaching phase, the sensor 26 will detect that the
feature 28 moves while the robot executes its program, even
if the tool center point 20 maintains its position over the
feature 28, as illustrated in Figures 7A and 7B. Figure 7A
shows an example of the possible path 114 of the sensor 26
and the path 116 of the tool 2 and the position of the
feature 28 during a robot teaching phase, with the sensor 26
and tool 2 moving in the direction of the arrow 118. Figure
7B shows the position 120 of the feature 28 detected by the
sensor 26 for the case of Figure 7A. The same problem happens
if the robot is programmed to follow a deviation in the path
of the feature 28, as illustrated in Figures 8A and 8B. In
these cases, the CUS 50 will try to correct the error that
the sensor 26 detects and will bring the tool center point 20
out of the joint 28. To eliminate this error, a compensation
method is added according to the invention, to record the
error during a dry pass over the joint 28 after the robot
teaching phase.
Referring to Figures 9A-C, a dry pass is added for the
memorization of the position of the joint or feature 28 while
the tool 2 moves along the programmed path., according to the
invention. During this dry pass, the same general sequence is
followed (as hereinabove described and illustrated in Figures
5A-C) and the same signals are activated through the I/0 link
58 between the CUMS 54 and the CUR 56. However, the CUMS 54
does not move the motorized slides 6, 8 after being informed
by the CUS 50 that the start position is reached, and it does
not move the motorized slides 6, 8 to track the joint or
feature 28.
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Referring also to Figure 10, the CUS 5.0 memorizes the
consecutive positions of the feature 28 associated with the
XTM position of the tool 2 in a second or additional buffer
122, where XTM means X position of the tool 2 during the
memorization pass, as depicted by blocks 124, 126, 128, 130,
with the CUR 56 setting the robot in motion as depicted by
blocks 132, 134.
During a normal processing pass, when the CUS 50
computes the corrections that are sent to the CUMS 54, it
extracts from the look-ahead buffer 88 the YZF position data
corresponding to the current XT position of the tool 2. It
also extracts from the second buffer 122 the YZFB position
data corresponding to the current XTT position of the tool 2,
where XTT means X position of the tool 2 during the tracking.
It first compensates the YZF position data extracted from the
look-ahead buffer 88 by subtracting the YZEB position data
extracted from the second buffer 122. Knowing the current
YZT~ position of the tool 2, it computes the YZ~o~ correction
required to maintain the tool center point 20 correctly
positioned over the joint or feature 28. Because the position
data extracted from the look-ahead buffer 88 is compensated
for the teaching errors, the CUS 50 will compute corrections
that will not track the errors illustrated in the Figures 7A-
B and 8A-B. This compensation method applies to the feature
tracking performed with the motorized slides 6, 8 installed
on the robot arm 4 as well as to the feature tracking
performed directly by the robot without motorized slides.
When the feature tracking is performed on a robot
without additional slides, this compensation method is used
to compensate for the calibration errors of the robot arm 4
that cause its absolute position inaccuracy and for the
dynamic errors that are caused by its response time and its
mechanical elasticity. To compensate for these errors, the
CUR 56 is programmed to maintain the tool center point 20
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correctly positioned over the feature while moving at the
desired production speed. A dry pass is then performed while
the compensation process in the CUS 50 records, at
consecutive X positions of the sensor 26, the feature
position data and the tool center point position information
received from the CUR 56. During the processing operation,
the compensation process subtracts the recorded data from the
position error calculated by the CUS 50, at a given tool
center point position received by the robot, to compensate
for the positioning errors of the robot and to send the tool
center point 20 accurately over the real position of the
feature 28.
Referring to Figure 11, the problem of sensor/tool
calibration can also be solved by using a special sensor 136
that simultaneously detects the position of the joint 28 in
front of the tool 2 and the position of the tip of the tool
20. This special sensor 136 can be a vision sensor that
contains two detectors or probes, a first one looking at the
joint 28' at a look-ahead distance in front of the tool 2 for
providing surface range data along the feature 28 such that
the position of the feature 28 at the look-ahead distance is
determinable, and a second one looking at the tip 20 of the
tool 2 for providing an image of a target region including
the tool tip 20 and the feature 28 under the tool tip 20 such
that the position of the tip 20 of the tool 2 is
determinable. In the illustrated embodiment, the sensor 136
has a body 138 and a bracket 140 for side attachment of the
sensor body 138 to the tool 2.
Referring to Figure 12, the special sensor 136 can also
be embodied by a vision sensor that contains only one
detector 142, a section of this detector 142 receiving the
signal from the feature 28' in front of the tool 2 and
another section receiving the signal from the area 144 of the
tool center point 20, by using a special arrangement of
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CA 02292372 1999-12-17
optical components. It must be understood that other optical
arrangements are also possible as long as the simultaneous
recording of the joint or feature 28 and of the tool center
point 20 is made possible. The area 144 of the tool center
point 20 preferably includes the tool tip and the joint under
the tool tip. A laser source 146 produces a laser beam which
is focused and expanded by a line generator 148 in a
crosswise direction relative to the general direction of the
feature 28. The expanded laser beam is reflected by a double
mirror arrangement 150 and projected on the work piece 152.
The light line derived from the line generator 148 is thus
directed at a tilt angle relative to a plane in which the
feature 28 extends and substantially crosswise to the feature
28 in the measuring field in front of the tool 2. The
scattering of the laser beam from each intersection point
between the spread laser and the surface of the work piece
152 is collected by an imaging lens 154 and focused on a CCD
array sensor 156. The CCD sensor 156 is properly positioned
and oriented so that every point within the measuring field
is preferably in exact focus. A diaphragm 158 with two
apertures separates two optical input channels. The left
aperture of the diaphragm 158 faces a filter 160 and limits
the collection of radiance from the scattering of the laser
beam. The filter 160 lets only the laser light pass and
blocks the background lighting, which is considered noisy
light for profile measurement.
One part of the sensitive area of the CCD 156 is
reserved for profile data acquisition and the other part is
used for passive 2D imaging. The range measurement profile
data acquisition of the sensor 136 is based on an active
optical triangulation principle. The position of the peak of
a focused point on one horizontal CCD scan line is related to
the range information of the corresponding point on the
surface of the work piece 152. The second vision module
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integrated in the same sensor 136 is a passive 2D imaging
module. The passive imaging module has an orientation-
adjustable mirror 162 directed toward the target region 144,
a fixed mirror 164 facing the mirror 162, a group of optical
filters mounted on an adjustable disk 166, a wedge prism 168,
the right side aperture of the diaphragm 154 and a second
part of the CCD sensor 156. The mirror 162 is oriented to
capture the desired observation scene. The observation scene
is then reflected by the mirror 164. The light rays are
filtered by one of the optical filters mounted on the filter
disk 166. One of these filters with a particular spectral
transmission is selected to emphasize the observation of a
specific process. The selection of one filter is realized by
simply turning the filter disk 166. The wedge prism 168
deviates the incident light rays from the mirror 164 into the
right side aperture of the diaphragm 158. This wedge prism
168 physically separates two optical channels so that the
mirror 164 can be used without blocking the incident beam of
the first vision module. The light rays from the right side
aperture of the diaphragm 158 is focused by the imaging lens
154 on the sensitive area of the second part of the CCD
sensor 156.
Referring to Figure 13, in order to support the added
function of the special sensor 136, a frame grabber 170 and
its associated CPU 172 are added in the CUS 50. The video
signal from the sensor 136 is transmitted to the range
processing hardware 174 and to the frame grabber 170. The
range processing hardware 174 extracts from the video signal
the range information that is used by the main CPU 59 of the
CUS 50 to compute the position of the feature 28. The frame
grabber 170 stores consecutive image frames from the sensor
136. Its associated CPU 172 processes the stored image frames
and extracts the position of the tool tip 20 and sends this
information to the main CPU 59.
CA 02292372 1999-12-17
During a sensor/tool calibration procedure, the position
of the tool tip 20 is detected by the secondary CPU 172 and
sent to the main CPU 59 to be recorded in the calibration
data (e.g. in the memory 57 as shown in Figure 3). During the
subsequent processing operations, the main CPU 59 compares
the position of the tool tip 20, received from the secondary
CPU 172, to its position recorded in the sensor/tool
calibration data. If there is a difference, it is included in
the computation of the corrections sent through the
communication link 52 by the CUS 50 to the CUMS 54 or the CUR
56 when motorized slides are not used.
While embodiments of this invention have been
illustrated in the accompanying drawings and described above,
it will be evident to those skilled in the art that changes
and modifications may be made therein without departing from
the essence of this invention. All such modifications or
variations are believed to be within the scope of the
invention as defined by the claims appended hereto.
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