Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: CONTROL OF ROBOTIC GRIPPING BY DBTBCTION OF
ACOUBTIC ENIBBIONB
Field of the Invention
This invention relates to sensors in the field
of robotics. More particularly it relates to robotic
gripping mechanisms that control their grip through the
detection of acoustic emissions that are a precursor or
incipient indicator of slippage.
Background to the Invention
Acoustic emissions are vibratory stress waves
that occur during the deformation of solids. Such waves
arise when solids fracture, deform or undergo phase
transitions, and particularly occur at the interface
between objects in contact when slippage develops between
such objects.
The detection of slippage, or its anticipation,
is important in the field of robotics because it relates
to the gripping function. When a robotic manipulator
grasps an object control must be exercised over the
applied grasping force. Too great an applied force can
crush an object. Too little an applied force will result
in the premature release of the object.
In the prior art, the emission of acoustic
energy in conjunction with the onset of slippage between
two objects that are frictionally engaged has been the
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subject of several studies. References relevant to this
field include the following:
(1) D. Dornfeld and C. Handy, "Slip Detection Using
Acoustic Emission Analysis," Raleigh, North
Carolina Proc 1987, IEEEE International Conference
Robotics Automation, pp. 1868-75;
(2) S. Rangwala, F. Forouhar, and D. Dornfeld,
"Application of Acoustic Emission Sensing to Slip
Detection in Robotic Grippers". Int J. Mach Tools
Manufacturing Vol 28 No. 3 pp 207-215, 1988,
Pergamon Press.
(3) Robert D. Howe; Nicholas Popp; Prasad Akella; Imin
Kao and Mark R. Cutkosky, "Grasping, Manipulations,
and Control with Tactile Sensing". Proc 1990, IEEE
Conf. Cincinnati, Ohio, May 13 - 18, 1990 at pp
1258 - 1263.
(4) R.D. Howe and M. R. Cutkosky, "Sensing Skin
Acceleration for Slip and Texture Perception",
Proc. 1989 IEEE International Conference of
Robotics and Automation, Scottsdale Arizona, May
1989, pp 145-150.
(5) J. L. Cuttino; C. O. Huey; T. D. Taylor "Tactile
Sensing of Incipient Slip", Proceedings of the USA-
Japan Symposium on Flexible Automation, 1988 pp.
547-555.
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(6) M.R. Tremblay; W. J. Packhard; and M.R. Cutkosky,
"Utilizing Sensed Incipient Slip Signals for Grasp
Force Control", Japan/USA Symposium on Flexible
Automation, Vol. 2 ASME 1992 pp 1237-1243.
The detection of acoustic emissions has been
used in the past to control the grasping force applied to
an object. The emissions that have arisen upon the
occurrence of minute amounts of slippage have been
detected and used to increase the grasping force to
arrest the occurrence of further slippage: vis Ref's (1)
and (2). See also U.S. patent 4,605,354 to Daly where a
slip-grip mechanism is not based on detection of acoustic
emissions described.
Piezoelectric sensors have been employed to
detect acoustic emissions for use in control of the grip
of a robotic manipulator. In the above cited Reference
(2) an acoustic emission sensor was attached to an
aluminium block pair which were subjected to increasing
differential applied forces to induce slippage. At slip
initiations, a distinct spike of acoustic emissions was
detected. In this report, the acoustic emission detector
is depicted as being placed in contact with the side of
a grasping end effector, outside the path of the applied
force.
The micro-AE signals that arise when slippage
is incipient must be detected against a background of
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noise. The placement of the AE sensor therefore becomes
of importance if this phenomena is to be exploited.
It is known to incorporate a piezoelectric
sensor beneath the rubber skin on a robotic fingertip
located at the point of contact to detect local contact
stresses vis, Ref (3). Such a sensor is thus employed to
measure applied forces. This same reference describes
the use of an accelerometer positioned behind the skin
layer of the finger, but not in the path of the gripping
force, to measure vibrations (p 1259):
"generated by a number of manipulation events,
including the making or breaking of contact,
incipient slip and collisions between the
grasped object and other objects in the
environment".
Such an arrangement is also described in a further Ref.
(4).
The problem of detection of true incipient slip
Per se is addressed in Reference (5). This paper reports
that:
"Acoustic signals were detected during the
transition from static friction to slipping,
but, the conditions that yielded signals were
difficult to reproduce and no signals at all
were obtained if the normal force was high
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enough to ensure firm, intimate contact over
the entire friction surface."
To create useable signals, the authors
introduced hard, abrasive particles carried on a rubbery
object that is being held in contact with the other,
hard, surface at the friction interface, to serve as
secondary emitters. In all cases the pick-up point for
acoustic emissions is reported as being located
collaterally to the force path being applied to the
articles being subjected to slip testing.
The use of an incipient slip signal to control
a grasping force is described in Reference (6). In this
reference an accelerometer mounted behind a rubber nib-
covered "skin" is used as a sensor for AE. This detector
is reported as having a dynamic range of 1 Hz to 25 kHz.
A further accelerometer was mounted in the object. This
paper expressly recommends mounting the finger-tip
accelerometer so as to isolate it from the surrounding
foam substrate that lies in the force path backing-up the
finger-tip skin.
The described experiment in Reference (6)
allowed the grasping force to decay until an incipient
slip signal was detected, whereupon the grasping force
was increased in order to repeat the cycle. The
incipient slip signal that was detected arose from the
release of elastically deformed nibs formed on the
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surface of the contacting skin and not from acoustic
emissions generated from generally featureless contacting
surfaces.
It is also known to provide robotic
manipulators with an array of sensors that provide
pressure information over a spatial field. A typical grid
of 10 x 10 elements with 2mm spacing is described in Ref
[6]. The graphic data shows the presence of pressure
sensor signals that commence co-incidently with the
initiation of displacements on the order of 1/2
millimetre.
Notwithstanding all of the foregoing
disclosures, the inventors of the invention hereafter
described have developed new arrangements based on the
critical placement of piezoelectric sensors to detect
acoustic emissions at the moment when slippage of a
grasped object is about to occur. Such signals can
usefully be applied to control the gripping force to be
applied to a grasped object by a robotic manipulator.
The invention in its general form will first be
described, and then its implementation in terms of
specific embodiments will be detailed with reference to
the drawings following hereafter. These embodiments are
intended to demonstrate the principle of the invention,
and the manner of its implementation. The invention in
its broadest and more specific forms will then be further
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described, and defined, in each of the individual claims
which conclude this Specification.
SummarY of the Invention
According to the invention in one of its
broader aspects a robotic gripper having gripping
surfaces has located beneath at least a portion of such
surfaces a piezoelectric acoustic emission sensor
material, which sensor material is so positioned as to be
subject to stress arising from the gripping of a grasped
object by the robotic gripper.
By a further feature of the invention the
piezoelectric sensors are closely coupled acoustically to
the object being grasped by the presence of an interface
layer having an interface surface that is in contact with
the grasped object and overlies the piezoelectric
material, being efficiently acoustically coupled to such
piezoelectric material.
By a further feature of the invention all of
the grasping force applied to the grasped object is
transmitted through piezoelectric material that is acting
as a sensor to detect acoustic emissions. Such force may
be directed through multiple piezoelectric sensors that
are separately used to detect acoustic emissions.
According to one feature of the invention, the
gripping force of the robotic gripper is controlled by a
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controller which responds to acoustic emissions detected
by the sensor and, in particular, cyclically loosens and
re-asserts the gripping force based on the detection of
acoustic emissions characterized by a trigger signal
S level that is above the level of the background noise
which occur when slippage between the grasped object and
the gripper is imminent. This trigger signal may be set
at twice the level of the background noise. More
preferably, the trigger signal level may be between 5 and
15. Even more preferably it may be set at substantially
ten times the level of the background noise.
The foregoing summarizes the principal features
of the invention and some of its optional aspects. The
invention may be further understood by the description of
the preferred embodiments, in conjunction with the
drawings, which now follow.
SummarY of the Figures
Figure 1 is a graphic display of the output of
the piezoelectric ceramic sensor as a function of time
over the period from before to after the commencement of
slippage.
Figure 2 is a pictorial depiction of a robotic
gripper of the invention supporting an object.
Figure 3 is a cross-section of the gripping
interface of the gripper fingers of Figure 2.
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Figure 4 is a schematic of the control system
for Figure 2.
DescriPtion of Preferred Embodiments
A piezoelectric ceramic disk 49.5 mm in
diameter and 2.5 mm thick was used as an acoustic sensor.
This sensor is of a type that may be acquired from any
commercial source that meets United States Military
Standard 2376. It was bonded directly to the base of
aluminum plate for the test set-up. The material used as
the "gripper" interface surface, namely respective layers
of alumina, steel and rubber in separate tests, was then
bonded to the piezoelectric ceramic disk to provide good
m~ch~n;cal and acoustically coupled contact. The test
object, consecutively having differing materials at the
slip interface, was placed with its bottom surface on the
gripper surface with a pivoting beam resting on its top
surface. Weights could then be placed on the upper
surface of the beam to control the contact force between
the test object and the gripper surface. A constant
tangential force was applied to the test object using a
known weight hanging from a string attached to the test
object. When the force was applied, the test object
would slide along the gripper surface.
A sensitive linear variable differential
transformer (I.VDT) was used to measure the displacement
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of the test object as the constant tangential force was
applied and sliding began. The acoustic emission from a
slipping object was correlated with its displacement.
The piezoceramic sensor output was amplified by a multi-
stage band-pass amplifier wherein it was filtered to
limit the measured signal to within a frequency band from
about 100 to 2800 kHz. The amplified output of both the
piezoceramic sensor and the LVDT were digitized by a
high-speed 16-bit analog-to-digital converter installed
in a personal computer. The digitized signals were
plotted on the computer screen for interpretation. An
example of the appearance of the screen after a test run
is shown in Figure 1.
Two values were calculated during each test
run:
(a) warning time; and
(b) displacement at a preselected trigger level of
detected acoustic emissions that is an
indicator of incipient slip.
The warning time is the delay (in milliseconds)
between when the level of signals from the piezoceramic
sensor reaches a trigger threshold value and the moment
where the test object has moved a total of 0.1 mm from
its initial location. The larger this value, the more
time is available to apply corrective action to prevent
significant slippage from occurring. A preferred trigger
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level is ten times the level of background noise although
values as low as two times or between 5 and lS times may
be used. Tests were done at the ten times level.
The displacement at trigger level is the
5distance that the test object has moved from its position
at the beginning of the test at the moment when the
trigger threshold signal occurs in the output of the
piezoceramic sensor. In other words, this is the
distance that the test object has moved ~t the point when
10the sensor has determined that slip is occurring or is
about to occur.
Both the net displacement of the test object at
the trigger point and the warning time are displayed in
the upper right corner of the display shown in Figure 1.
15It can be seen from this example that a slip signal could
be detected before any noticeable motion of the test
object had occurred. That is, the acoustic emissions due
to the existence of an incipient slip condition were
detected.
20Before the data found in the tables below was
collected, the noise characteristics of the test set-up
were investigated. The noise level in the output of the
piezoceramic was identified to be less than approximately
+ 10 mV or 10% of the voltage trigger level used in all
25of the experiments. The noise in the displacement signal
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from the LVDT corresponded to an error of approximately
+3 um.
Experiments were performed using this test set-
up to investigate the effects of the following factors on
the generation of acoustic emissions prior to and during
slip:
(a) the combination of materials used for the
gripper surface and the test object;
(b) the degree of surface roughness of the gripper
surface and test object; and
(c) the normal force applied to the test object.
For each set of test conditions, 20 test runs were
performed. The statistics for each set of test
conditions are based on the results of each of the 20
tests.
In Table 1, the results for five different test
object surfaces sliding on alumina (Al203) as the
interface surface are presented.
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Table 1: Various Test Objects on
an Alumina Gripper Surface
Slip Warning (ms) Displacement at
Mean (Std.Dev.) Trigger (um)
Test Obiect Mean (Std.Dev.)
Alumina 78 (20) o (2)
Steel 70 (15) 1 (2)
Aluminum
(rough) 51 (13) 8 (8)
Aluminum
(smooth) 60 (17) 7 (7)
Rubber 29 (10) 14 (11)
In Table 2, the warning time results for all
combinations of test object and gripper interface surface
using alumina, steel and rubber are presented.
Table 2: Warning Times for all Combinations
of Alumina, Steel and Rubber
Alumina Steel Rubber
Test Ob~ect Interface Interface Interface
Alumina 78 (20) 66 (14) 36 (14)
Steel 70 (15) 63 (13) 25 (11)
Rubber 29 (10) 29 (9) 26 (8)
Table 3 presents the corresponding values of
the net displacement at the trigger point.
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Table 3: Net Displacement at Trigger for all
Combinations of Alumina. Steel and Rubber
Displacement Of Test Object At Trigger
Threshold (um), Mean (Std.Dev.)
Alumina Steel Rubber
Test Obiect Interface Interface Interface
Alumina o (2) 3 (2) 27 (20)
Steel 1 (2) 1 (2) 32 (24)
Rubber 14 (11) 8 (10) 7 (6)
In Table 4, the warning times for two different
normal forces applied to the test object are presented
for steel, alumina and rubber sliding on an alumina
gripper.
Table 4: Warning times for Two Applied Normal Forces
Slip Warning (ms) For Test Object;
Means (Std.Dev.)
Test Obiect Normal Force = 1 N Normal Force = 4.5N
Steel 70 (27) 62 (15)
Aluminum 60 (17) 53 (9)
Rubber 29 (10) 24 (9)
From the above data the following observations
can be made.
With all combinations of materials tested in
the course of these experiments, the time delay between
25the slip signal trigger and the test object moving 0.1 mm
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was consistently greater than 24 milliseconds as
indicated in Table 2. Thus if corrective action, such as
increasing the grip force, could be applied in less than
24 ms, this slip could be limited to less than 0.1 mm.
Best results are obtained when neither material
is compliant. This can be seen in Table 2 where the test
object is rubber or the gripper interface surface is
rubber. This is most likely due to the less severe local
relaxation of stress prior to gross slippage when one of
the materials is compliant.
Of the non-complaint materials, steel and
alumina appear to provide more notification of impending
slip than does aluminum (see Table 2).
As can also be seen from Table 2, slightly more
incipient slip warning signal time seemed to be generated
when alumina rather than steel was used as the gripper
surface. This may be due to the fine surface roughness
of the alumina sample as compared to the smoother surface
of the steel sample.
With all combinations of hard materials the net
displacement of the test object at the time of trigger on
the piezoceramic sensor is negligible. From Table 3, the
net displacements at the trigger vary from O um to 3 um
with a standard deviation of 2 um, which is not
statistically significant. Therefore, it is possible to
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consistently detect the acoustic emissions resulting from
incipient slip, prior to any measurable displacement.
Comparing the results shown in Table l for the
aluminum test object with rough and smooth surfaces, it
can be seen that there is not a statistically significant
difference in the warning times. It, therefore, appears
that the surface roughness of the test object is not a
principle factor, provided there is some degree of rough
gripper surface.
Increasing the normal force as shown in Table
4, and therefore the frictional force applied to the test
object, does not have a great effect on the slip warning
time. Presumably, this would indicate that increasing
the frictional force does not have a major effect on the
stress relaxation process prior to gross slip, since
incipient slip signals are already detected.
As can be seen in Table 2, all combinations of
hard materials provide good warning times (consistently
in excess of 50 ms). Also, the displacements at the time
of the trigger on the piezoceramic sensor output as shown
in Tables 1 and 3 are negligible for hard materials. In
other words, it is possible to consistently detect the
acoustic emissions resulting from incipient slip, prior
to any measurable displacement of the test object if the
piezoelectric sensor carrying the applied force is
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closely acoustically coupled to the grasped object
through acoustically efficient interface surfaces.
For any material sliding on rubber, the level
of acoustic vibrations is lower than for the case of a
hard sliding surface. The low level of the acoustic
vibrations, particularly in the early stages of slip is
probably a combination of two causes: the impedance
mismatch at the rubber/piezoceramic interface and the
plastic behaviour of the rubber at the interface as the
tangential force is first applied.
For rubber sliding on rubber, the transition to
sliding is quite sudden, implying a large difference
between the static and dynamic coefficients of friction
and the build-up of the tangential force prior to any
significant slippage. The result of the sudden
transition is that the warning time available from the
slip trigger to the object have moved 0.1 mm is quite
small even though the test object has only moved a few
microns when slip threshold is triggered.
For hard materials (steel, alumina) sliding on
a rubber gripper surface, the transition to sliding
appears to be a more gradual process which implies that
the difference between the static and dynamic
coefficients of friction is much smaller than for rubber
sliding on rubber. The object, therefore, slips further
before acoustic emissions are detected resulting in
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larger values for the displacement when the slip
threshold is triggered. Also, since the transition to
sliding is slower, the warning time available in this
case is actually longer than for rubber sliding on
rubber.
It should be noted that the warning times
calculated from these experiments are based on a fairly
narrow range of test object accelerations. In a typical
test run, the acceleration of the test object is
loapproximately 50 mm/s2. This value tends to vary from
run to run, but usually falls within the range 10 mm/sZ
to 200 mm/s2. This implies that the test object velocity
typically falls within the range 0.5 mm/s to 10 mm/s for
a measured warning time of 50 milliseconds.
15Based on the results of these experiments, the
optimum material for the gripper surface should have the
following characteristics:
(a) fine surface roughness for the generation of
strong incipient slip signals;
20(b) hard wearing to maintain the surface roughness
over the life of the gripper; and
(c) good impedance match to piezoceramic to
maximize the coupling of the acoustic
emissions generated at the slip interface into
25the piezoceramic.
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Figure 2 depicts a robot gripper 1 carrying
piezoelectric ceramic sensors 2 within its grippers 3,
directly beneath the gripping interface 4, so that each
sensor 2 is in the force path, and experiences stress,
from the application of a gripping force to an object 5.
When activated, a controller 8 governed by software,
drives the gripper motor 9 that activates the grippers 3
through mechanical links 6 (that may have a pliable outer
sheath 10 for aesthetics) to close the gripper fingers 3
slowly. During this time, the gripper position sensor 7,
which senses the position of the fingers 3, is monitored
by the controller 8. When the fingers 3 come into
contact with the object 5 to be held, the increased load
on the gripper motor 9 allows the controller 8 to sense
the resistance and the gripper motor 9, under direction
from the controller 8, slows to a stop. The software
detects this lack of motion from the position sensor 7
and enables the slip detection routine to be implemented.
If the object 5 begins to slip, the software
causes the controller 8 to increase the motor control
voltage upon detection of a sufficient threshold
amplitude of acoustic slip signal. When the object 5
stops slipping, the acoustic slip signal drops below the
threshold level, and the software causes the controller
8 to slowly decrease the motor control voltage towards a
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minimum force level until slip is detected again. This
process can be repeated cyclically, or intermittently.
While this control scheme is easy to implement,
it relies upon there being a close relationship between
the gripping force and the control voltage. Thus, a
large, high-torque motor 9 is preferred to drive the
gripper in order to produce a sufficient gripping force,
without backlash being present as would occur if high
reduction gearing is employed. However other options for
force-control to improve the performance of the system
and decrease the size may be employed.
The detection of the acoustic emissions at low
levels is critically dependant on the efficient coupling
acoustically of the piezoelectric sensor to the gripping
interface.
In Figure 6 the piezoelectric sensor 2 is shown
supported by the mechanical link 6 present within the
gripper 3. Bonded to the sensor 2 is the interface
surface material 4a. This interface material 4 should be
an efficient conductor of acoustic emissions and should
be efficiently acoustically coupled to the sensor 2.
In use, it will be seen that all of the applied
gripping force is transmitted from the link 6, through
the sensor 2 and gripping interface material 4a to the
gripping interface 4. This produces an efficient means
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for detecting low level acoustic emissions, sufficient to
provide an early warning signal to detect incipient slip.
Conclusion
The foregoing has constituted a description of
specific embodiments showing how the invention may be
applied and put into use. These embodiments are only
exemplary. The invention in its broadest, and more
specific aspects, is further described and defined in the
claims which now follow.
These claims, and the language used therein,
are to be understood in terms of the variants of the
invention which have been described. They are not to be
restr~icted to such variants, but are to be read as
covering the full scope of the invention as is implicit
within the invention and the disclosure that has been
provided herein.
