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Description  |
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TECHNICAL FIELD
This invention generally relates to a safety system for an industrial robot
which assures a safe environment for the robot during operation and more
particularly to a robotic safety system whereby any aberrant motion of the
robot as well as abnormal obstructions of personnel or equipment within
the robot's path of travel are sensed so that the robot is put on hold
upon such sensing to avoid injury to personnel or damage to equipment.
BACKGROUND ART
Prior art safety systems regarding industrial robots have involved alarm
and restraint systems of a fixed or static nature over substantially the
entire area of the robot's capability of operational movement regardless
of the specific area or precise pattern of movements to be made during the
operational assignment of the robot. Examples of fixed nature alarm
systems are pressure sensitive floor and light beam arrays established
around the perimeter of the total capable traval area of the robot, while
examples of static restraint systems include a chain or guard rail
structure surrounding such overall area of operational capability of the
robot.
These prior art safety systems are an inefficient use of floor space or
area when the robot is to be operated over only a portion of the area of
its capability. Due to this lack of specific correlation between the
movement of the robot and the area sensed by the safety system, a false
alarm will result should an individual or piece of equipment enter into
the controlled area or to a location that would not interfere in any
manner with the operational mode of the robot. Thus, the prior art safety
systems provide an inefficient means of protection when viewed in terms of
operational productivity.
Additionlly, these prior art safety systems are poorly adapted for handling
a situation where the robot exhibits aberrant motion, which oftentimes,
occurs when the robot is unattended. Aberrant motion usually results when
the robot is put on hold, for example, to permit the operator to leave the
immediate area, and despite the hold condition, the robot exhibits
unprogammed motion. With the prior art safety systems, this aberrant
motion will remain undetected unless the robot blocks a light beam or the
like of the safety system.
DISCLOSURE OF THE INVENTION
It is, therefore, an object of the present invention to provide a safety
system for a robot, the safety system being adapted to monitor the
presence of unexpected people or objects in a work area, or detect
aberrant motion behavior of a robot and subsequently modify the robot's
behavior and/or warn any intruder or operator of the exception condition.
This invention teaches the use of a plurality of individual ultrasonic
ranging transducers arranged in a horizontal array, and mounted at the end
of the arm of an industrial robot, but independent of any end manipulator
or hand installed on the robot arm. The mounting of the array on the robot
arm is such that the array is arcuately pivotable in a horizontal sense so
as to be located horizontally in the intended direction of movement of the
robot arm, as well as being arcuately pivotable in a vertical sense. The
pivotal movements of the array are effected by a pair of stepping motors
which in turn are controlled by a microcomputer or controller which
monitors the sensing array and provides the necessary interaction with the
robot and its directions of movements whereby the detection of an
unprogrammed object (personnel or equipment) in the intended path of
travel of the robot will interrupt such travel thereof before any physical
contact takes place.
The major components of the ultrasonic safety system comprise azimuth and
elevation stepping motors, stepping motor controllers, a microcomputer, an
ultrasonic ranging transducer or sensor array, position encoders and robot
interface logic. Physically, the safety system or device is contained in
three packages. The transducer array, motors and encoders sit on the end
of the robot on top of, for example, the link between the fifth and sixth
axes (yaw and roll, respectively). The ultrasonic electronics are likewise
mounted on the robot arm just behind the transducer array. The stepping
motor controllers and the microcomputer and robot interface logic reside
in a cabinet sitting on a support behind the robot.
The motors, their controllers and encoders are used to move the ultrasonic
sensor array in two directions. Azimuth movement is used to point the
sensor in the direction of the robot's intended motion. The elevation
motor steps the array through an up/down sweep in the vertical plane. The
sensor array comprises, for example, five Polaroid sensors arranged in an
arc in a single substantially horizontal plane. The robot interface logic
is employed to interface the signals of the robot with the microcomputer.
The microcomputer controls the signals necessary for obtaining motion for
the two stepping motors, calculates ranging information based on the
ultrasonic array, and activates the hold signal of the robot.
The ultrasonic sensor array scans in an up/down fashion the entire time the
robot is active. The sensor array direction can be one of sixteen
horizontal positions. Outputs from the robot are used to select one of
these positions which faces the sensor array in the direction of the
robot's intended movement. When any object or person moves within a
predetermined threshold distance of the robot, a signal is sent which
stops the robot, starts a warning signal and results in a message being
started from the computer overseeing the application. Once the obstacle is
removed and the stop or hold condition eliminated the robot continues in
its programmed path. Since simple thresholding is used with no
discriminatory capabilities, the hold circuit is disabled once the robot
approaches a work jig and enabled once it is at the work jig where the
sensor array is looking away from the robot to the side. In the case of
aberrant motion, the system is programmed to detect motion during a hold
condition. Thus, any unexpected motion of the robot during a hold
condition is sensed by the system and the hold condition re-established.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is an overall perspective view of one exemplary industrial robot
with the movable ultrasonic sensing array of one embodiment of this
invention shown mounted proximate the robot arm end;
FIG. 2 is a perspective view showing the portion of the ultrasonic
transducer array assembly to be mounted to the end of a robot arm;
FIG. 3 is a block diagram of the preferred embodiment of the invention;
FIG. 4 is an exemplary pattern array of the plurality of sonar or
ultrasonic transducers utilized in the depicted embodiment of the
invention; and,
FIG. 5 is a flow diagram representative of the preferred embodiment of the
invention showing the signal processing by the microcomputer to effect
operation of the safety system.
BEST MODE OF CARRYING OUT THE INVENTION
Referring to FIGS. 1 and 2, the invention comprises an array 10 of
ultrasonic ranging transducers 11 mounted on the end 12 of the arm 13 of
an industrial robot 14. In the preferred embodiment, the array 10
comprises a plurality of ultrasonic transducers 11 which each emit an
individual beam of acoustical energy. The transducers 11 are mounted on
the array in an angular manner so as to emulate a single transducer with a
wide transmit/receive pattern in the horizontal plane as shown in FIG. 4.
The individual transducers 11 are arranged, for example, arcuately about a
horizontal plane approximately 20 degrees apart whereby with a quantity of
five such transducers a horizontal beam width of approximately 100 degrees
can be attained.
The transducers 11 utilized in the preferred embodiment of the invention
are commercially available from the Polaroid Corporation of Cambridge,
Mass. These items are marketed as a Polaroid Ultrasonic Ranging Unit,
which comprises two primary components, an acoustical transducer, which is
identified in the drawings by the numeral 11, and an ultrasonic transducer
circuit board or circuit means. The ultrasonic transducer circuit boards
are contained in a circuit board magazine 15 that is mounted near array 10
with each individual transducer 11 connected to its respective ultrasonic
transducer circuit board in magazine 15 by appropriate wiring.
The base supporting the vertically pivotable array 10 is located, for
example, atop a horizontal pivoting bidirectional stepping motor 17 that,
in turn, is located on the top or upper surface of the end 12 of robot 14.
The array 10 and drive motor 20 are adapted to be positioned or swung in
the directions of arrows A about a vertical axis relative to the end 12 by
bi-directional stepping motor 17.
The array 10 is also pivotally mounted in a horizontal sense to the shaft
of a second bi-directional stepping motor 20 to drive the array 10 in an
up and down vertical sweep about a horizontal axis in the directions of
arrows B. Thus, the transducer array 10 provides wide coverage in the
vertical plane by physically sweeping the array 10 with stepping motor 20.
The motion is, however, not continuous, but rather the array 10 stops,
transmits, listens and moves.
In the preferred embodiment, the extent of the rotational movement of array
10 and its drive motor 20, such movement being represented by arrows A, is
approximately 270.degree. or approximately 135.degree. in each direction
from the horizontal centerline defined by arm 13 of robot 14. Likewise,
the extent of vertical rotation or up and down sweep of array 10
represented by arrows B is approximately 150.degree. or approximately
75.degree. in each direction from a position of array 10 parallel to the
end of robot arm 12.
Referring to FIG. 3, it will be seen that certain components of the
disclosed embodiment are depicted within a cabinet 21; it being
understood, however, that the disclosed placement of a particular
component or components in a designated or particular cabinet is a matter
of design selection and is not critical.
Operational control of this invention is effected by a controller or
microcomputer 22 comprising a microprocessor chip or circuit board, a
program storage memory including read only memory (ROM), and random access
memory (RAM); each of which may be appropriately selected from a number of
such microcomputer elements which are currently available and familiar to
those skilled in the art.
Microcomputer 22 is connected by bi-directional buses or lines 23, 24, and
25, respectively, to the transducer electronics of circuit board magazine
15, the azimuth motor electronics 26, and the elevation motor electronics
27. Data and information are also received by microcomputer or controller
22 from limit and position sensors 28 associated with both the azimuth and
elevational stepping motors 17 and 20, and from the main control 29 of
robot 14 over lines 30 and 31. Line 32 interconnects the microcomputer 22
with the robot control 29 to transmit a "hold" or "stop" signal to the
robot 14 should an unprogrammed or abnormal obstruction in the path of
travel of the end 12 of robot 14 be sensed by operation of the array 10.
In the embodiment shown, line 32 is also connected to an audio warning
circuit 33 to energize an appropriate audio warning device upon the
occurrence of transmittal of a "hold" signal to robot 14; it being
understood this feature of audio warning being merely operational and not
constituting a critical feature or element of the invention.
Also contained within cabinet 21 is an appropriate power supply 34, which
while not shown in FIG. 3, is to be understood as supplying electrical
power to all of the various components of the system through well-known
cabling and bussing techniques.
Referring to FIG. 4, there is depicted a typical transmitting beam or
acoustical lobe composite of the five ultrasonic transducers 11 that form
array 10. Each acoustical transducer 11, controlled by its individual
ultrasonic circuit board contained in magazine 15, is capable of detecting
the presence and distance of objects within a range of approximately 0.9
to 35 feet that is within the individual acoustical lobe pattern.
Preferably, the sensitivity of the transducers 11 is set at approximately
10 feet. It should be noted that up to the maximum achievable limit of 35
feet, the operating range of the system is totally programmable. This is
due to the fact that the threshold limits are determined by the software.
Each transducer 11, (heretofore identified as a component in a Polaroid
Ultrasonic Ranging Unit from Polaroid Corporation), serves as both an
emitter to transmit an outgoing signal and an electrostatic sensor to
receive a reflected signal or echo. The diameter of the transducer 11
determines the individual acoustical lobe pattern, or acceptance angle,
during the transmitting and receiving operations, with each lobe pattern
comprising a main or central peak 35 and reduced side lobe patterns 36 on
each side of the main peak 35. With the array 10 consisting of five
transducers in the preferred embodiment, the side lobe patterns 36 of each
transducer 11 will overlap with each of their adjacent transducers 11 to
produce an overall total pattern of five peaks 35 and four valleys 37 as
seen in FIG. 4.
Should the presence of the pattern valleys 37 be objectionable or
undesirable in any application or practice of this invention, such valleys
37 may be reduced, if not completely eliminated, by a modification of the
signals to the azimuth and elevation stepping control motors (17 and 20,
respectively) by microcomputer 22 as will be explained in more detail
hereinafter.
When the transducers 11 of array 10 are activated, each transducer 11 emits
a sound pulse, then waits to receive the echo returning from whatever
object the sound pulse has struck. The emitted pulse is a high-frequency,
inaudible "chirp" lasting for approximately one millisecond and comprising
fifty-six pulses at four separate ultrasonic frequencies: e.g., 60 kHz, 57
kHz, 53 KHz and 50 kHz. Occasionally, a single frequency could be
cancelled because of certain target topographical characteristics, and no
echo would be reflected. Thus, by the use of four frequencies, such
possibility is minimized if not overcome.
The elapsed time between transmissions and echo detections is converted to
distance with respect to the speed of sound. For example, for a
transmitted pulse to leave a transducer 11, strike an object ten feet
(3.05 meters) away, and to return to the transducer 11, requires an
average time lapse of 17.75 milliseconds or 1.78 milliseconds per foot
(0.3 meters) of round trip distance. Thus, utilizing the elapsed time
between transmission and detection, it is possible to determine the
distance of a detected object.
The utilization of the invention will be explained with reference to FIG. 5
which reflects the flow chart of the control and operation effected by
microcomputer 22 (FIG. 3) when the arm 13 of robot 14 is in operation.
Upon actuation of the robot 14, initialize flags indicate the boundary of
the field of movement of the transducer array 10. Thereafter, the array 10
begins a vertical sweep which comprises a reciprocating up and down motion
which is continued during the operation of the device.
As the array 10 is scanned in the vertical direction, information is
acquired concerning the range or distance of the nearest object. If the
distance sensed is less than a preset threshold then a hold enable
evaluation is conducted. If the object detected is not expected then a
hold signal is sent to robot control 29 and the operation of the robot 14
is stopped. However, should the object be an expected object then
operation of the device continues uninterrupted. Once the upper or lower
limit of vertical pivot of the array 10 is sensed, the operation of
stepping motor 20 is reversed and the array is moved in the opposite
direction.
As described previously, the output signals from robot control 29 are fed
via input 31 to microcomputer 22 for horizontal plane positioning of array
10 under the control of stepping motor 17. Essentially, array 10 is
positioned so as to look in the quadrant that the robot arm 13 is moving
toward. If the input information from the robot control 29 is the same as
the current position, then the position of the array 10 is maintained.
However, should horizontal reposition be required, the new position is
calculated and the array 10 moved accordingly prior to the movement of the
robot arm 13. This operation is continued and repeated during the
operation of the device.
Thus, to position the array 10 in the desired direction, which is not
necessarily the direction that the robot 14 is moving, the controller 22
reads simple binary signals from the robot's control console. Based on
these signals, the controller 22 instructs the stepping motor 17 to
position the array 10 in the desired horizontal quadrant of intended robot
movement. No special interface is required from the robot 14 other than
the ability to provide simple binary (on-off) outputs at any desired point
in the robot's program.
The ultrasonic sensor array 10 continuously scans in an up/down fashion
during the entire time the robot is active; however, the positioning of
the array 10 in the horizontal plane comprises one of 16 predetermined
positions. The stepping motor 17, thus, has the ability to precisely
attain fixed and repeatable positions. Single step error is noncumulative
and is generally specified at tolerances of less than about 5% of one
step. Therefore, the motor 17 is intended to move the array 10 to a
specific position and hold if necessary. The electronic drive circuitry is
basically digital to simplify interface requirements to the microcomputer
22. Because of their positional accuracy, the stepping motor can be
employed without feedback in an "open-loop" control system. However, to
preclude the possibility of a missing step, preferably, optical encoders
(not shown) are mounted on the shafts of both axes to provide both
feedback and positional reference points.
The microcomputer 22 comprises, for example, an INTEL 8085 microprocessor
having 2K of RAM. Port assignments are used for controlling the direction
and pulsing of both motors 17 and 20, reading the limit and position
sensors 28, resetting and monitoring the ultrasonic sensor array 10 and
interfacing with the robot control 29. From a programming point of view,
the system provides the robot 14 with a primitive sensing capability in
the direction of movement of the robot 14 utilizing the system of the
present invention.
The control of the safety system comprises two different activities. First,
it is necessary to activate and monitor the ultrasonic sensor array 10 and
second, it is necessary to control the positioning of the array 10
utilizing the stepping motors 17 and 20. In activating and monitoring the
array 11, all five sensors 11 are treated as one. All are fired
simultaneously and an echo is received by the system when any one of the
sensors 11 receives an echo. Once an echo is received, the value of a
counter included in the ultrasonic transducer electronics 15 is compared
with a predetermined threshold counter value programmed into the
microcomputer 22. When the counter is below the predetermined threshold,
this indicates that an object is closer to the robot 14 than desired and
therefore a flag is set putting the robot in a hold mode. The system
continues with this cycle regard1ess of whether the robot is in a hold
mode or not.
Another predetermined counter value is stored in the microcomputer 22 which
indicates the maximum distance of concern. This prevents the problem of a
"no echo" situation from putting the microcomputer 22 in an impasse
situation, waiting for a nonexistant echo.
If there are areas in the robot's path where it needs to operate despite
the detection of an object, the robot hold circuit is disabled. Either the
program running on the ultrasonic ranging unit can inhibit the halting of
the robot or a simple on-off signal derived from the robot program itself
can disable the halt function. This allows the robot to maneuver and
operate in areas when detection of an object would normally halt the
robot.
In the case of aberrant motion, the system is programmed to detect motion
of the robot arm 13 during a hold condition. This is accomplished by
providing the system with the sensitivity to detect changes in ranging
information which results when the robot arm unexpectedly moves during a
hold conditon. Because the array 10 constantly scans in an up/down fashion
the entire time the robot is activated, including a hold condition; any
change in position of the array 10 due to the movement of the robot arm 13
will produce a change in the ranging information. By programming the
system to sense such changes in ranging information during a hold
condition, it is possible for the system to instruct the robot 14 to
reestablish the hold condition when aberrant motion is detected or shut
down the robot all together.
While the invention has been described in its preferred embodiments, it is
to be understood that the words which have been used are words of
description rather than limitation and that changes may be made within the
purview of the appended claims without departing from the true scope and
spirit of the invention in its broader aspects.
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