 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates generally to robot devices, and, more particularly,
to apparatus for positioning a movable part of a robot in relation to a
surface.
Robots are mechanical devices which can perform tasks in a manner that
simulates human activity. One type of robot, the industrial robot, is
finding widespread acceptance in manufacturing and other industrial
operations, and promises many benefits in the automation of repetitive
industrial operations. A typical industrial robot includes a stationary
portion, usually referred to as the body, and a movable portion, usually
referred to as the arm, wrist or hand, with the movable portion adapted
for performing an operation on a workpiece. By way of example, robot arms
can assemble components, join components as by welding, and finish
components as by cleaning and painting.
In most industrial operations, a critical aspect of the operation of robot
devices is the precise positioning of the movable robot arm with respect
to the workpiece. In the simplest robots, each workpiece must be
positioned at an exact location and orientation with respect to the robot
arm, so that no separate sensing device is necessary to orient the robot
arm with respect to each successive workpiece. For the robot arm to
perform operations which require it to move over the surface of a
workpiece, usually referred to as a continuous path operation, as in the
painting of a part, a coordinate map of the surface of the workpiece must
be coded into the memory of a computer which controls the trajectory of
the moving robot. In such simple robots, failure to orient a workpiece in
its exactly proper position can result in a failure of the operation to be
performed by the robot. Further, such robots must be dedicated in the
sense that a large amount of information concerning each particular type
of workpiece must be coded into the control computer, and changes in the
workpiece require reprogramming.
A more complex type of control utilizes some form of sensor to gather
information about the workpiece and transmit this information to the
robot, thereby providing a control input to the robot. As an example, a
tactile or proximity sensor incorporated in the hand of the robot may be
used to indicate the presence of a workpiece, and may also give some basic
information about its orientation. Multiple tactile sensors can also be
used to advantage. In a somewhat similar approach, light sources and photo
cells may be used in combination to provide light beams which are broken
when a workpiece is moved into position. However, both these approaches
have not proved sufficiently versatile for use in many applications,
especially where the robot arm is not in contact with, or in the close
proximity of, the workpiece.
More recently, solid state video imaging systems have been developed for
controlling robot devices. Such video imaging systems typically operate in
a manner similar to television, wherein a visual field is scanned by a
solid state camera to produce a sequential electronic signal having the
visual image encoded thereupon. The digital signal is used to reconstruct
an image on a television viewer or, for the purposes of controlling a
robot, may be analyzed by existing pattern recognition techniques to
provide information to the robot about the position, shape, and
orientation of the workpiece, and the spacing of the robot arm from the
workpiece. While robots having electronic video imaging systems represent
an advance over the more primitive robots, such systems have severe
disadvantages that limit their utilization in many applications. In many
adverse working environments it is impossible to provide enough light to
the camera. Image enhancement techniques are known, but in adverse
environments the image may be insufficient for their use. More
significantly, however, in all working environments such video imaging
systems require a complex system utilizing extensive hardware components,
including solid state cameras, a monitor and a computer, and complex
programming and algorithms to recognize the patterns. The information from
such video imaging systems is provided to a controlling computer which
follows the encoded coordinate maps to guide the robot to take each
successive step. Once the robot moves to its next step, the entire process
of detecting the robot position and guiding it further must be repeated.
In addition, the information transmission between interfaced devices is
inherently slow, so that the system can communicate at a rate no greater
than about 10-50 functions per second, thus limiting the speed and
performance of the robot.
Robots equipped with video imaging systems must be controlled and their
movement integrated by a central controller computer. This computer must
necessarily be large and complex to provide the robot controller with a
high degree of versatility, since it is often necessary to perform major
computer reprogramming if the design of the workpiece is changed. For
example, the computer may be programmed with a mathematical model of the
surface of the workpiece for use in the pattern recognition function, and
this mathematical model must be changed when the robot is to operate upon
a different or modified workpiece. To some extent, such computers are
therefore dedicated to use with a single type of workpiece, although the
dedication may be changed by reprogramming.
There has been a need for a more versatile, non-dedicated apparatus to
enable robots to sense the positioning of their movable arms with respect
to the surface of a workpiece. Desirably, such an apparatus would be
operable in adverse environments and would permit more rapid signal
processing with less complex, less costly hardware and software. Such
apparatus should be operable to allow the movable part of the robot to be
positioned in a controllable manner adjacent the workpiece, with little or
no preprogramming required for adapting the robot to operation in a
continuous path on different workpieces. The present invention fulfills
this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention resides in apparatus for sensing the presence and
orientation of a surface, and then positioning a movable part of a robot,
such as a robot arm, wrist, or hand, with respect to the surface, as on a
workpiece to be processed by the robot. The apparatus utilizes parallel
processing of signals to provide very high analysis cycle rates, typically
on the order of 40,000-200,000 functions per second. The apparatus is
operable in a wide range of liquid and gaseous environments, and its
operation is largely unaffected by transient environments, such as those
having vibrations, humidity or sparks, and environments having low or high
light levels. The apparatus requires relatively simple hardware and no
software to perform the basic positioning functions, so that the robot
command control functions may be separated, with the control function
remotely positioned in the movable arm. The control circuitry may be
significantly reduced and simplified, thereby reducing the cost and
complexity of the robot and eliminating most programming costs, while at
the same time greatly increasing its versatility for operating upon a wide
variety of types of workpieces.
In accordance with the invention, the apparatus for positioning a movable
portion of a robot device with respect to a surface includes a precision
positioning subsystem, an approximate positioning subsystem, or,
preferably, both subsystems. The precision positioning subsystem comprises
means for emitting an emitted signal toward the surface; means for
receiving a response signal originating at the surface as a result of the
emitting signal striking the surface, the means for receiving being
mounted on the movable portion of the robot device and including at least
two signal receivers; means for cmparing the time of flight of the signals
received by the signal receivers on a pair-wise basis to create a
comparison signal; and means for adjusting the movable portion of the
robot device to maintain a predetermined value of the comparison signal on
a pair-wise basis. The precision positioning subsystem may also include
means for calculating the distance of the movable portion of the robot
from the surface, from the time of flight of the signal received by at
least one of the signal receivers, and means for comparing this calculated
distance with a distance command signal so that the spacing of the movable
portion of the robot may be adjusted to maintain the calculated distance
equal to the distance command signal. Thus, the orientation of the movable
part of the robot with respect to the surface may be determined and
controlled through comparison of the time of flight of the signals
received by the receivers, and the distance may be determined and
controlled by the absolute value of the time of flight.
The approximate positioning subsystem comprises means for emitting at least
two non-colinear emitted signals; means for receiving the respective
response signals, if any, resulting from the striking of the respective
emitted signals on a surface, said means for receiving being mounted on
the moveable portion of the robot device; means for detecting the presence
of a received signal; and means for adjusting the moveable portion of the
robot device into an approximate facing relationship to the detected
received signal. The approximate positioning subsystem is used to locate
the surface and to approximately orient the precision positioning
subsystem for precise robot control, but does not itself adjust the
ranging of the moveable portion.
In a presently preferred embodiment, four pairs of emitting and receiving
transducers of the approximate positioning subsystem are mounted
peripherally on side lips of a sensor head and angularly directed
outwardly and upwardly so as to sense the presence of a surface over a
broad viewing area. The precision positioning subsystem transducers are
mounted centrally among the approximate-positioning transmitters and
receivers and have a relatively narrow field of view. The apparatus can
thus seek out and locate a surface with the approximate positioning
subsystem, rotating the moveable portion so that the precision positioning
subsystem transducers approximately face the surface. Precise positioning
and distance control are achieved by the precision positioning subsystem.
In this preferred embodiment the apparatus is responsive to one or more of
four location incrementing command signals, three angular positioning
command signals and one distance (range) positioning command signal.
Mounted on the robot arm, the transmitters and receivers are arranged in a
manner to provide a spatial field of view of about
135.degree.-140.degree.. The four pairs of wide beam transmitters and
receivers of the approximate positioning subsystem view in four orthogonal
directions within the field of view, while one narrow beam transmitter and
three receivers of the precision positioning subsystem are mounted to view
in the direction along the common axis of the four orthogonal directions.
The electronic circuitry includes a signal generator to trigger the
transmitters; a switching logic to provide the orientation control signal;
two comparators, each of which receives the signal from one pair of the
acoustic receivers and compares the signals to produce an output
positioning control signal proportional to the difference in the time of
flight of the signals received by each respective pair of acoustic
receivers; a distance calculator which calculates a distance based on the
time between emission of the acoustic signal and its receipt; and a
controller for adjusting the position of the robot arm so that the two
orientation command signals are respectively equal to the two orientation
control signals, and the distance command signal to equal to the distance
control signal.
The use of acoustic frequencies is preferred, since acoustic transmitters
and receivers are readily available, and the signal propagation times are
sufficiently rapid to allow high functional repetition rates, but
sufficiently slow to allow the use of conventional comparators and time of
flight measurement circuitry. The acoustic transmitters and receivers are
preferably mounted on a square, flat sensor head having angled side lips
at preferably 60.degree. upward inclination on each of the four sides of
the head, which in turn is mounted on the hand of the robot arm. One
narrow beam acoustic transmitter is located centrally within the pattern
formed by the three narrow beam acoustic receivers on the flat face of the
sensor head, preferably spaced 120.degree. apart on the square. One pair
each of the broad band acoustic transmitter and receiver are mounted to
the lip face such that the field of view is about 135.degree.-140.degree..
Orientation control is achieved when the switching logic identifies which
pair of the broad beam acoustic receivers mounted on the side lips is
receiving a response signal after reflection from an obliquely oriented
reference surface in its field of view. The switching logic sends an
orientation control signal to the robot motor controls and moves the robot
hand in the direction from which the response signal was first received.
The movement is continued until the response signal is no longer received
by any of the side mounted broad beam acoustic receivers. At this time the
robot hand is approximately in a facing relation with the flat face of the
sensor head parallel to the reference surface. Precise position control is
then achieved by measuring the difference between the time of flight of
the acoustic signal to the different precision positioning subsystem
acoustic receivers. For example, the sensor head is oriented parallel to
the surface when the three time of flight signals are equal. Total
distance from the surface of the workpiece to the robot hand is
proportional to the time of flight of the acoustic signal. The controller
drives positioning motors which can reorient the hand of the robot arm and
also change its distance from the surface, thereby achieving complete
positioning control of the arm in relation to the workpiece, without the
need for a complex pattern recognition function or pre-mapping of the
surface of the workpiece.
It will be appreciated from the foregoing that the present apparatus
represents an important and significant advance in the field of
controlling robot devices. The apparatus allows functional separation of
the command and control operations so that the control function is
accomplished remotely at the movable portion of the robot rather than
requiring transmission of signals to a central command computer. Control
is accomplished by parallel rather than serial processing to enable use of
a high functional control rate, and the control function is achieved
utilizing relatively simple hardware rather than a combination of complex
hardware and complex software. Reprogramming costs are therefore
significantly reduced as compared with prior devices. The apparatus is
operable in a very wide range of operating environments without the need
for readjustment or recalibration, including all light levels, dirty
environments, and transient environments, such as clouds of opaque
particles. Other features and advantages of the present invention will
become apparent from the more detailed description, taken in conjunction
with the accompanying drawings, which illustrate, by way of example, the
principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a robot arm employing an apparatus in
accordance with a preferred embodiment of the invention, for orienting the
arm with respect to a surface;
FIG. 2 is a block functional diagram of a prior approach to positioning of
a robot arm using a video camera;
FIG. 3 is a block functional diagram of one embodiment of the present
approach to orienting a robot arm;
FIG. 4 is a perspective view of a multiple-sensor array used in approximate
position control;
FIG. 5 is a perspective view of a presently preferred sensor head;
FIG. 6 is a side view of the sensor head of FIG. 5;
FIG. 7 is a block circuit diagram of the approximate positioning subsystem
used in orienting a robot arm;
FIG. 8 is a circuit diagram of the precision positioning subsystem used in
orienting a robotm arm; and
FIG. 9 is an examplary schematic signal timing diagram for the precision
positioning subsystem.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
As is shown in the drawings with reference to a preferred embodiment, the
present invention is concerned with apparatus 10 for controlling the
movable portion on arm 12 of a robot device 14. The robot 14 comprises a
stationary base 16 and the movable arm 12. Robots may have a variety of
physical arrangements for attaining movement in three dimensions, and the
robot 14 of FIG. 1 illustrates just one such possibility. In the robot 14,
the movable arm 12 achieves vertical movement by a vertical movement motor
18. Rotational movement about two axes in a horizontal plane is
accomplished by a first rotational motor 20 and a second rotational motor
22. The first rotational motor 20 causes rotation of an outer arm 24 in
relation to an inner arm 26, about a first rotational axis 28. The second
rotational motor 22 accomplishes rotation of a sensor head 30 in a yoke 32
of the outer arm 24, about a second rotational axis 34 which is
perpendicular to the first rotational axis 28. Complete three-dimensional
movement of the sensor head 30 is accomplished by coordinated operation of
the motors 18, 20, and 22.
The present invention relates to apparatus 10 for controlling the movement
of the motors 18, 20, and 22. By way of contrasting the operation of the
apparatus 10 with prior apparatus which also achieves control of the
motors, FIGS. 2 and 3 present block functional diagrams of the prior
approach and the present approach, respectively. As illustrated in FIG. 2,
in the prior approach, a camera 36 is focused on a surface 38 to create a
scanned image of the surface 38. The scanned image is fed in a serial
manner to a digitizer 40, whose output is presented to a pattern
recognizer 42. The pattern recognizer 42 analyzes key features of the
digitized serial representation of the image and compares it with a
digital map provided in the memory of a computer 44 controlling the robot
14. An analysis of the position of the movable arm 12 with respect to the
surface 38 is made by the computer 44, which compares the present position
with a command signal. A repositioning signal is then provided to a
controller 46 by the computer 44. The controller 46 generates a control
signal which is provided to the motors 18, 20, and 22 which in turn drive
the movable arm 12 to a relative position whereat the computer 44
recognizes the position relative to the surface 38 to be the same as that
commanded. The speed of such recognition, as well as the overall system
speed, is limited by the complex pattern recognition algorithms utilized
in the pattern recognizer 42, and the electronic interfaces used for
communication between the pattern recognizer 42, the computer 44, and the
controller 46. Because of this complexity, the position of the movable arm
12 with respect to the surface 38 can typically be updated at a rate of
only about 10-50 times per second. For many applications, this update rate
for the adjustment of the position is far too slow.
The slowness and complexity of the prior approaches based on video scanning
are believed to stem essentially from the fact that such approaches
provide far more information and analysis than required for the operation
and control of typical industrial robots by requiring complex pattern
matching and analysis. In a typical solution, such detail is not required.
Instead, it is desired to maintain the sensor head 30 of the robot 14 in a
specified angular orientation with respect to the surface of a workpiece,
and at a specified distance from the surface of the workpiece. For many
operations, it is not necessary to obtain a complete analysis of the
viewable portion of the workpiece, but rather it is necessary only to
maintain the specified orientation and distance, and then to move to
another location on the surface. That is, the control of orientation and
distance is desirably accomplished automatically, independently, and
separately, apart from the incremental movement of the sensor head 30 to
another location. For example, in many applications it is desirable to
retain the same relative orientation and distance of the sensor head 30
from the workpiece surface 38 at all relative positionings as where the
robot is operating to weld two workpieces together, or to pain, clean,
spray or treat a curved workpiece surface. In such situations and in many
others, it is far more efficient to place the relative angular and
distance positioning function remotely in the movable arm 12, while
providing a separate location incrementing command to the motors 18, 20
and 22, which increments the location of the robot arm with respect to the
workpiece surface.
FIG. 3 illustrates the functional approach embodied in the present
invention. The movement of the sensor head 30 in relation to a surface 38
is viewed as comprising three components, an orientation control
component, a location incrementing control component and a positioning
control component. The orientation component is the approximate relation
of the sensor head 30 to the surface 38. It is determined by the
approxiamte positioning subsystem in a manner to be described below in
relation to FIG. 8. The positioning component includes the precise angular
orientation of the sensor head 30 to the surface 38, and the precise
vertical distance of the sensor head 30 from the surface 38. The location
incrementing control component reflects the coordinate position of the
sensor head 30 with respect to the surface 38, in the sense that the
surface 38 may be viewed as having a set of coordinates defining each
point or location on the surface 38. Its value is set externally in a
manner to be described in relation to FIG. 8. As indicated, in many
operations the positioning component remains constant or follows some
simply defined pattern, as for example maintaining the sensor head 30
parallel to the surface 38 and at some fixed distance from the surface 38.
The angular and distance commands are provided to a controller 50, which
compares the commands to the actual relationship between the sensor head
30 and the surface 38, as determined by the electronic circuitry of the
precision positioning subsystem. Utilizing the approach to be described,
this comparison can proceed very rapidly and in a parallel processing
manner, without the need for pattern recognition of the surface 38. Any
deviation from a desired positioning command can be corrected with a
positioning control signal provided to the motors 52, and originating in
the precision positioning subsystem included in electronic circuitry 48.
With the relative positioning of the sensor head 30 thus determined and
controlled, the coordinate location may be directly controlled by
providing a location control signal to the same motors 18, 20 and 22. It
is not necessary to reevaluate location in order to maintain control of
distance and angular position, nor need distance and angular position be
considered explicitly when incrementing location.
In accordance with the invention, precision positioning of the sensor head
with respect to the surface is accomplished by means of absolute and
relative time of flight measurements for signals propagated from an
emitter toward the surface 38, and return signals received from the
surface 38 for at least two signal receivers located on the movable arm
12. The use of two signal receivers allows the determination of distance
and relative orientation along a single axis, but the use of three
receivers mounted in a non-colinear fashion is preferred, as this approach
allows relative determination of angular orientations in two axes, and
also the determination of distance. The following description is directed
to the use of three receivers, although the principles are equally
applicable to the use of two receivers, or more than three receivers.
As illustrated in FIGS. 4-6, the apparatus 10 includes the sensor head 30
and associated electronic signal processing circuitry. The signal head 30
comprises a flat square mounting plate 55 having attached thereto a
downwardly facing narrow beam transmitter 56 and three non-colinear narrow
beam receivers, a first receiver 58, a second receiver 60, and a third
receiver 62, which may be mounted in radial slots to allow radial
adjustment. In the illustrated preferred embodiment, the receivers 58, 60,
and 62 are located in a triangular arrangement, with the three receivers
regularly spaced 120.degree. apart from each other. The transmitter 56 is
located generally in the center of the triangle formed by the three
receivers 58, 60, and 62, so that the transmitter 56 is located at
approximately the average height when the receivers 58, 60, and 62 are
positioned at different heights from the surface of the workpiece.
Also in accordance with the invention, sensing of the approximate
positioning of the sensor head 30 with respect to the surface 38 is
accomplished by the use of four pairs of transmitters and receivers
mounted on four lips 100 angularly attached to the mounting plate 55. As
illustrated in FIGS. 5 and 6, each of the lips 100 is a rectangular flat
plate rigidly joined to the mounting plate 55 at an upward and outard
inclination, preferably oriented at about 60.degree. from the plane of the
mounting plate 55. One lip is so joined to each side of the square
mounting plate 55. A broad beam transmitter 102 and a broad beam receiver
104 are attached flush to the surface of each lip 100. This arrangement
provides a forward spherical field of view for the approximate positioning
subsystem of about 135.degree.-140.degree. when the transducers 102 and
the receivers 104 have a beam angle of about 25.degree..
Another preferred arrangement of the transducers and receivers in the
approximate positioning subsystem is illustrated as a sensor array 106 in
FIG. 4. This sensor array 106 comprises an array of transceivers 108, each
of which is capable of sending and receiving signals. The transceivers 108
are angularly arrayed so that the entire forward field of view is included
within the beams of the transceivers 108 taken collectively. In fact, by
extending the array, a greater spherical viewing area is possible,
extending to a 360.degree. view if necessary. A multiplexer 110
individually addresses the transceivers either sequentially or randomly,
so that a single set of signal generator and analysis logic, to be
described subsequently, may be utilized.
All of the transmitters and the receivers described herein are adapted for
use of the same general type and frequency of energy, preferably from
about 20,000 to about 200,000 cycles per second. Although in theory, any
frequency of energy may be utilized, in practicing the invention with the
presently available electronic components, it is preferably to utilize
energy transmitted in waves travelling at a relatively low velocity, such
as acoustic radiation. In the most preferred embodiment, a narrow beam
ultrasonic transducer may be utilized for both the transmitter and
receiver functions for the forward facing precision positioning subsystem
(i.e., the transmitter 56 and the receivers 58, 60, and 62). One
acceptable and preferred transducer is the model E-188 transducer
available from Massa Products Corporation, Hingham, Mass. This transducer
may be driven by a signal generator 64, such as model MP215, available
from Metrotech, Inc., Richland, Wash. to emit acoustic waves, or can
operate as a receiver to receive acoustic waves. The most preferred
transducer for use as the side facing transducers 102 and 104 in the
approximate positioning subsystem is a broad beam transducer having a beam
angle of about 25.degree.. Two acceptable and preferred transducers are
the models V189 and A189R, available from Panametrics, Waltham, Mass.
In the following description of the apparatus and its operation, the
components of the approximate positioning subsystem and the precision
positioning subsystem are presently separately to maintain clarity. The
approximate positioning subsystem provides the orientation component to
the drive motors 18, 20, and 22, to bring the sensor head 30 into an
approximate facing relation to the surface 38. The approximate positioning
subsystem includes the side facing transmitters 102 and receivers 104, and
electronic processing components such as illustrated in FIG. 7. The
precision positioning subsystem provides the positioning component to the
drive motors 18, 20, and 22, to establish precise positioning once the
approximate facing relationship has been reached. The pecision positioning
subsystem includes the forward facing transmitter 56 and receivers 58, 60
and 62, and electronic processing components such as illustrated in FIG.
8.
In a typical operation to be performed by a robot 14, it is known that a
surface 38 will be presented to the robot 14 at some time, but the
position and orientation of the surface 38 at the time of presentation are
not known with certainty. In particular, it cannot be known whether the
surface 38 will be presented so as to be within the field of view of the
precision positioning subsystem transducers 56, 58, 60 and 62. The
approximate positioning subsystem is therefore provided to sense the
presence and general or approximate position of the presented surface 38,
and to move the sensor head 30 into an approximate facing relationship
with the surface 38, a "facing relationship" being an orientation of the
sensor head 30 wherein the surface 38 is within the field of view of the
transmitter 56 and the receivers 58, 60 and 62. FIG. 5 illustrates the
operation of the transducers 102 and 104 in locating a surface 38 that is
outside the field of view of the transducers 56, 58, 60 and 62 of the
precision positioning subsystem.
Turning first to the approximate positioning subsystem illustrated in FIG.
7, the orientation of the surface 38 is sensed by determining the side
direction form which a response signal is first received. This
determination is achieved by using the four broad beam acoustic
transmitters 102 mounted on the side lips 100 of the sensor head 30, which
continuously transmit acoustic signals under excitation of the signal
generator 64, in all directions within their collective spherical
135.degree.-140.degree. field of view. The broad beam acoustic receivers
are mounted on the lips 100 in a pairwise fashion, with the receivers 104
used to sense the direction of a responsive signal, if any is found. As
illustrated in FIG. 7, a switching logic 120 identifies which receiver is
sensing the response signal, if any, thus determining the approximtae
orientation of the surface 38 in respect to the sensor head 30. The
switching logic 120 then commands a controller 122 to send orientation
control signals to motors 18, 20, or 22 to rotate the sensor head 30 in
the direction toward which the responsive signal was received The rotation
of the sensor head 30 is continued by operating motors 20 or 22 until the
signal from the receivers 104 disappears. In this position the sensor head
30, and the movable arm upon which it is mounted, is approximately in a
facing relationship to the surface 38, and none of the four receivers 104
on the side lips 100 of the sensor head 30 receive any response signal.
Operation of the precision positioning subsystem follows this initial step
of bringing the sensor head 30 approximately to a facing relation to the
surface 38.
Referring to the precision positioning subsystem illustrated in FIG. 8, the
output signals from the receivers 58, 60, and 62 are provided in pair-wise
fashion to two comparators 66 and 68. The first comparator 66 determines
the difference in the time of flight between the signals produced by the
first receiver 58 and the third receiver 62, while the second comparator
68 determines the difference in the time of flight between the signals of
the first receiver 58 and the second receiver 60. The output signal of the
first comparator 66 is therefore an indication of the difference in the
distance from the surface of the workpiece of the first receiver 58 and
the third receiver 62, which in turn is an indication of the angular
orientation of the axis defined by the first receiver 58 and the third
receiver 62.
FIG. 9 represents an exemplary schematic illustration of the
interrelationship of some acoustic pulses transmitted to a surface and
received by the embodiment illustrated in FIG. 8. It is hypothesized, for
the purposes of the illustration presented in FIG. 9, that the emitted
signal is received back by the first receiver 58 and the second receiver
60 at the same time, while there is a delay in receipt of the return
signal by the third receiver 62. The output signal of the first comparator
66 is proportional to the difference in the time of receipt by the first
receiver 58 and the third receiver 62, a value indicated in FIG. 9 as
DEL13. While DEL13 is a time value, the corresponding distance may be
determined by multiplying this value by the velocity of the acoustic wave
in the medium, about 1100 feet per second for acoustic waves in air. From
the known length of the baseline between the first receiver 58 and the
third receiver 62, and the difference in distance from the surface of the
first receiver 58 and the third receiver 62, the angular orientation of
the axis defined by the first receiver 58 and the third receiver 62 may be
readily calculated. Of course, in most cases an actual calculation is niot
required, as the desired time difference values, if any, may be used
directly to control the motors.
In a similar fashion, the signals from the first receiver 58 and the second
receiver 60 are provided to the second comparator 68, wherein the
difference in propagation time of the signal, the difference in distance
between the surface and the two receivers 58 and 60, and the angular
orientation of the axis defined by the first receiver 58 and the second
receiver 60 may be determined in the fashion described above. In the
illustration of FIG. 9, it has been postulated that the first receiver 58
and the second receiver 60 are at the same distance from the surface, and
therefore there is no difference in the time of receipt of the signals. It
is not necessary that a third comparator be provided, inasmuch as the
orientation of the sensor head 30 in respect to the surface 38 may be
defined fully by the angular displacement of two axes. However, if
desired, a third comparator (not illustrated) may be provided as a check
against the results determined by the comparators 66 and 68. In other
embodiments, only two receivers and a single comparator could be provided
if it were desired only to know the angular misorientation along a single
axis, such as, for example, where the workpiece is highly elongated and
essentially one-dimensional, and the robot has no freedom of movement
| | |
