U.S. patent application number 14/063602 was filed with the patent office on 2016-05-05 for force limiting device and method.
This patent application is currently assigned to Universite Laval. The applicant listed for this patent is GM Global Technology Operations LLC, Universite Laval. Invention is credited to Dalong Gao, Clement Gosselin, Martin Grenier, Nicolas Lauzier, Robin Stevenson.
Application Number | 20160121491 14/063602 |
Document ID | / |
Family ID | 44067847 |
Filed Date | 2016-05-05 |
United States Patent
Application |
20160121491 |
Kind Code |
A9 |
Lauzier; Nicolas ; et
al. |
May 5, 2016 |
FORCE LIMITING DEVICE AND METHOD
Abstract
The present invention relates to a method and apparatus for
limiting the contact force between a moving device and another
object, using a parallel mechanism and torque limiters where the
threshold force to activate the force limiting mechanism is not
related to the configuration of the moving device or the location
of the contact force relative to the activation point of the force
limiting mechanism, and where the mechanism may be configured for
one, two or three degrees of freedom. A counterbalance mechanism is
also provided to counteract gravity load when the force limiting
mechanism is configured for three degrees of freedom and responsive
to contact forces including a vertical element. In particular, the
invention relates to a method and apparatus for limiting the
contact force between a moving robotic device and a contactable
object.
Inventors: |
Lauzier; Nicolas; (Quebec,
CA) ; Gosselin; Clement; (Quebec, CA) ; Gao;
Dalong; (Troy, MI) ; Grenier; Martin; (Quebec,
CA) ; Stevenson; Robin; (Bloomfield, MI) |
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Applicant: |
Name |
City |
State |
Country |
Type |
Universite Laval
GM Global Technology Operations LLC |
Quebec
Detroit |
MI |
CA
US |
|
|
Assignee: |
Universite Laval
Quebec
MI
GM Global Technology Operations LLC
Detroit
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20150114165 A1 |
April 30, 2015 |
|
|
Family ID: |
44067847 |
Appl. No.: |
14/063602 |
Filed: |
October 25, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12627407 |
Nov 30, 2009 |
8601897 |
|
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14063602 |
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Current U.S.
Class: |
74/490.05 ;
901/28; 901/46 |
Current CPC
Class: |
B25J 9/0048 20130101;
B25J 17/00 20130101; B25J 19/0091 20130101; B25J 17/0208 20130101;
Y10S 901/28 20130101; Y10T 74/20329 20150115; B25J 19/063 20130101;
B25J 17/0266 20130101; Y10S 901/46 20130101; Y10T 74/20305
20150115 |
International
Class: |
B25J 17/00 20060101
B25J017/00; B25J 19/06 20060101 B25J019/06; B25J 19/00 20060101
B25J019/00 |
Claims
1. A method for limiting a contact force of a robot with an object,
the method comprising: operatively attaching a force limiting
device between a robot and a suspended portion; wherein the force
limiting device: includes at least one parallelogram linkage
including a joint; and is activated by an activation force;
transmitting a contact force to the force limiting device when the
suspended portion contacts an object; activating the force limiting
device when the contact force exceeds the activation force, such
that the force limiting device becomes compliant; and decreasing
the contact force inputted to the object by the suspended portion
when the force limiting device becomes compliant.
2. The method of claim 1, wherein the joint of the at least one
parallelogram linkage is a rotatable joint including a torque
limiting mechanism for activating the force limiting device when an
input torque generated by the contact force and transmitted to the
torque limiting mechanism exceeds a torque limit equivalent to the
activation force.
3. The method of claim 1, further comprising: wherein the at least
one parallelogram linkage is configured to be operatively connected
to the robot through one or more rotatable joints; wherein each of
the at least one parallelogram linkage is configured to be
operatively connected to the suspended portion through the one or
more rotatable joints; wherein one of the rotatable joints of each
of the at least one parallelogram linkage includes a torque
limiting mechanism for activating the force limiting device when
the input torque transmitted to the torque limiting mechanism
exceeds a torque limit equivalent to the activation force.
4. The method of claim 1, further comprising: wherein the at least
one parallelogram linkage includes a first parallelogram linkage
and a second parallelogram linkage; and wherein the first
parallelogram linkage is perpendicular to the second parallelogram
linkage.
5. The method of claim 1, further comprising: wherein the at least
one parallelogram linkage includes at least three parallelogram
linkages arranged in a Delta configuration.
6. The method of claim 5, further comprising: wherein the force
limiting device includes a gravity compensating mechanism.
7. The method of claim 6, further comprising: wherein the gravity
compensating mechanism includes at least one of a spring and a
counterbalance.
8. The method of claim 6, further comprising: wherein the gravity
compensating mechanism counteracts the weight of at least one of
the suspended portion and the payload.
9. The method of claim 1, further comprising: wherein the
parallelogram linkage is in a non-compliant state when the contact
force is less than the activation force such that the suspended
portion is rigidly fixed in a first position relative to the
robot.
10. The method of claim 9, wherein activating the force limiting
device further comprises: passively moving the parallelogram
linkage of the activated force limiting device from the first
position to a second position such that in the second position the
suspended portion is compliantly attached to the robot by the force
limiting device.
11. The method of claim 9, further comprising: preventing movement
of the parallelogram linkage from the first position when the
contact force is less than the activation force.
12. The method of claim 1, further comprising: wherein the robot is
configured to be movably attached to an overhead suspension; and
the suspended portion is movable by the robot toward the
object.
13. The method of claim 12, further comprising: braking the robot
relative to the overhead suspension when the contact force exceeds
the activation force.
14. A method for limiting a contact force of a robot with an
object, the method comprising: maintaining a force limiting device
of a robot in a non-activated state when a contact force of the
robot contacting an object generates an input torque transmitted
through the force limiting device which is less than an activation
torque; the force limiting device comprising: a first attachable
portion defining a first end of the force limiting device and
including an interface configured to fixedly attach the first
attachable portion to the robot; a second attachable portion
defining a second end of the force limiting device and including an
interface connectable to a suspended portion; a parallelogram
linkage including two legs; wherein each leg is rotatably connected
to the first attachable portion and to the second attachable
portion; and a torque limiter including an input shaft in
communication with the parallelogram linkage and activatable from a
non-activated state to an activated state when the input torque
transmitted through the parallelogram linkage to the input shaft
exceeds the activation torque; the method further comprising:
restraining rotation of the legs relative to the first attachable
portion and the second attachable portion to define a fixed
orientation between the first attachable portion, the second
attachable portion and the legs when the torque limiter is in the
non-activated state; preventing movement of the second attachable
portion relative to the first attachable portion when the input
torque transmitted to the input shaft is less than the activation
torque; and releasing the legs to passively rotate relative to the
first and second attachable portions to decrease the contact force
between the moving object and the contacted object when the torque
limiter is in the activated state.
15. The method of claim 14, further comprising: wherein the robot
has a moving portion and a suspended portion; wherein the first
attachable portion is fixedly attached to the moving portion and
the second attachable portion is attached to the suspended portion;
wherein the moving portion is configured to move relative to the
object; and wherein the orientation between the moving portion and
the suspended portion of the robot is defined by a fixed
orientation between the first attachable portion, the second
attachable portion, and the legs when the input torque defined by
the contact force transmitted through one of the suspended portion
and the force limiting device is less than the activation
torque.
16. The method of claim 15, further comprising: wherein the moving
portion is movable by a support system and includes a braking
mechanism configured for selectively stopping movement of the
moving portion by the support system; the method further
comprising: stopping movement of the moving portion by the support
system when the input torque exceeds the activation torque.
17. The method of claim 15, further comprising: a contact point
defined by the location of contact between the contacted object and
one of the suspended portion and the force limiting device at a
distance from the input shaft; wherein the contact force is
transmitted between the one of the suspended portion and the force
limiting device and the contacted object through the contact point
and defines the input torque inputted to the input shaft; the
method further comprising: transmitting the input torque through
the parallelogram linkage to the input shaft of the torque limiter
such that the magnitude of the input torque defined by the contact
force is independent of the distance between the input shaft and
the contact point.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method and apparatus for
limiting the force between a moving device and another object,
using a parallel mechanism and torque limiters where the threshold
force is not related to the device configuration, and in particular
to limiting the collision or impact force between a moving robotic
device and a contactable object.
CROSS-REFERENCE TO RELATED APPLICATIONS
[0002] This application claims priority to and the benefit of U.S.
patent application Ser. No. 12/627,407 filed Nov. 30, 2009, which
is hereby incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
[0003] The use of industrial robots is well established in
applications where the robots are in controlled environments where
they are separated by fences or cages. A collision between a robot
and a contacted object is a complex situation, where one of the
most severe situations is when a collision with clamping occurs,
entrapping the contacted object between the contacting portion of
the robot and a static fixture, such as a wall. In this case, the
severity of the collision may be indexed using the maximum contact
force, which can then be used as a reference for unexpected
collisions during the design process.
[0004] Control and dependability are characteristics needed from a
robot to allow it to interact in a minimally controlled, e.g.,
unfenced and uncaged, environment with other objects, which may be
moving or stationary. Roboticists may typically use three different
strategies to develop these characteristics. First, roboticists may
develop algorithms that use vision systems, proximity sensors or
the like to anticipate and avoid potentially harmful contacts
between robots and objects. Secondly, methods may be developed to
detect a collision by monitoring joint torques or a robot skin and
to quickly react to manage the contact forces under a certain
level. Thirdly, roboticists pursue robot designs that will
intrinsically prevent damaging contact.
[0005] Avoidance, reaction and design strategies can be combined
together to improve robot design. However, the first two options
alone may not fully guarantee the desired result. Consider that a
robot intended to interact physically with a contactable object
will require the ability to distinguish desirable and undesirable
contacts, e.g., good and bad contacts. This can be done either by
disabling sensors on the robot parts intended to interact or by
running an algorithm that will decide if the upcoming contacts are
desirable or not. In either case, control is compromised either by
unprotecting certain parts of the manipulator or by giving the
robot some sort of "judgment capability" which may in some
situations be wrong. Furthermore, avoidance and reaction strategies
rely on electronic components that can fail. Finally, one could
argue that an operator may feel insecure working with a machine
protected only by an algorithm. Thus, a third strategy may be
employed to obtain compliant and dependable robots, which is to use
a design strategy, e.g., to design robots that intrinsically
prevent damaging contact.
[0006] To design robots to intrinsically prevent damaging contact,
a typical approach is to make the robot compliant, to reduce the
peak contact force attained during a collision. Compliance may also
extend the duration of the contact, allowing the controller to
sense a collision and react to reduce potential damages, within
certain constraints, i.e., reaction time. However, adding
compliance may limit the precision and stiffness of the robot,
compromising performance and precision.
[0007] Some robots designed to avoid damaging contact incorporate a
flexible flange with breakaway function that links the tool to the
manipulator. This device triggers an emergency stop when the
contact force at the tool control point exceeds a certain
threshold, which may be a breakaway torque measured at the flange.
This device therefore limits the moment, not the force that can be
transmitted by the manipulator to the end-effector, which means
that the threshold depends on the location of the collision point.
Therefore, for a breakaway system, the design of the system must be
sub-optimized for the worst case moment arm, which may result in a
system which is overly sensitive and prone to false triggering in
high inertia non-collision situations, which may require
limitations on robot velocity.
[0008] Active compliance systems are in some aspects derived from
admittance control techniques, e.g., efforts are measured at the
effector and processed to command a displacement equal to the
contact force divided by a virtual spring stiffness. Thus, the
robot behaves like a spring around its trajectory. However, the
response time of traditional actuators is larger than what is
required to accommodate high frequency forces applied during
collisions. Consequently, during a collision, the robot may not
achieve a compliant behavior and thus this technique is not optimal
as a design strategy.
[0009] Techniques may be used to provide passive compliance, at
each joint of the robot, which may be programmable or non-linear.
Programmable passive compliance consists of using a compliant joint
for each axis of the robot and a supplemental set of actuators to
allow the adjustment of the stiffness of each joint. Either two
antagonistic actuators or a second actuator that adjusts the
stiffness via a mechanism may be used, to allow high stiffness and
precision at low velocity and low stiffness at high velocity, i.e.
when contact with the manipulator may be more severe. This gives
the controller the ability to continuously adjust the compromise
between control and performance. However, using this type of
passive compliance system adds weight and complexity to the
manipulator. Also, for many mechanisms, the ratio between the
largest stiffness and lowest stiffness is not sufficient to obtain
high precision at low velocity, when collisions are less severe and
high precision is required for acceptable robot performance.
SUMMARY OF THE INVENTION
[0010] Nonlinear passive compliance uses a method which places on
each joint a mechanism whose compliance varies by purely mechanical
means. By placing a mechanical device, such as a torque limiter, in
series with each joint actuator, the resulting manipulator will be
rigid unless external forces applied on it exceed a certain
threshold, in which case the joint will become compliant. This
technique allows the design of robots that are stiff and accurate
in normal operating conditions, but compliant when collisions
occur. Moreover, this principle is realized mechanically, which
means that the reliability of this system does not depend on
electronic components. However, this method is not optimized. By
adding a torque limiter on each joint of a serial robot, the force
threshold will depend on the configuration of the manipulator,
because the relation between external forces and articular torques
is determined by the Jacobian matrix of the manipulator, which is
generally a function of the manipulator's pose. The threshold will
also depend on the contact location and on the force orientation,
which is not optimal since it means that the compliance level will
vary throughout the robot's external surface.
[0011] Therefore, a force limiting device that comprises torque
limiters placed in a Cartesian architecture provides numerous
advantages over its articular counterpart. Provided herein is a
force limiting device, which in a preferred embodiment improves the
compliance level of suspended robots with constant end-effector
orientation relative to the gravity direction, such as robots
performing the Schonflies motions, as related to physical
object-robot interfaces. The force limiting device includes a
parallel mechanism with torque limiters which provide a rigid
connection between the robot and its end-effector during normal
operation, e.g., during non-collision conditions. When the robot
end-effector contacts, as in a collision, a resisting object, the
transmitted contact force activates the torque limiters of the
force limiting device to yield a compliant connection between the
robot and its end-effector, reducing the contact force and the
severity of the impact. The force limiting device presented herein
may be configured for one, two and three degrees of freedom (DOF)
(respectively, 1-DOF, 2-DOF, 3-DOF).
[0012] In a preferred embodiment, the device provided herein is
configured for use with an overhead robot to limit the collision
force resulting from contact with the manipulator, end-effector
and/or parts other than the robot's end-effector, such as tooling
and/or payload, suspended from the overhead robot. Suspended robots
of the type discussed herein are often found in manufacturing
plants, and may also be found in other applications, such as
hospital and medical applications and warehousing applications, for
example.
[0013] As provided herein, if an excessive force in a direction
corresponding to the degrees of freedom (DOFs) of the force
limiting device is applied during a collision, the force limiting
mechanism is activated and thus the end-effector is free to move
relative to the robot and typically opposite to the direction of
contact or collision. The activation of the mechanism is detected
and brakes are applied to stop further motion of the robot in the
direction of contact. The inertia of the parts located
kinematically upstream of the force limiting device, e.g., the
inertia of the robot from which the force limiting device is
attached and the end effector is suspended, is thus removed from
the collision. Also, for a quasi-static collision in which an
object is clamped between the robot and a wall or other static
fixture, the maximum contact force is the activation force of the
force limiting device for that orientation, as determined by the
configuration of the torque limiters in the mechanism. Hence, the
contact force is reduced to improve damage control for all types of
blunt collisions.
[0014] Architectures are provided herein for force limiting devices
configured for one, two and three degrees of freedom (respectively,
1-DOF, 2-DOF, 3-DOF). The 1-DOF force limiting device provided
herein includes a single parallelogram linkage that could be used
when the robot's motion in one direction is more prone to contact
than in other directions. The 2-DOF force limiting device provided
herein includes four legs that form two parallelograms and thus
behaves similarly to the 1-DOF mechanism. The 1-DOF and 2-DOF force
limiting devices, as configured, are not sensitive to the weight of
the suspended manipulator or end-effector and thus do not require
gravity force compensation. A 2-DOF force limiting device may be
especially appropriate for applications in industry requiring large
and fast horizontal motion and small and slow vertical
displacements.
[0015] The 3-DOF force limiting device presented herein is
configured based on a Delta architecture. The 3-DOF force limiting
device can be applied more generally than the 1-DOF and 2-DOF
mechanisms because it may react to collisions occurring in any
direction on the end-effector, e.g., it may react to contact forces
independent of orientation of the contact force to the
end-effector. Methods to compensate the effect of gravity on the
3-DOF delta configuration, where required when the end-effector
weight or payload weight combined with the end-effector weight is
large compared to the maximum static force limit that is imposed,
are provided herein. Other possible configurations for a 3-DOF
force limiting device are also provided within the scope of the
claimed invention.
[0016] The force limiting mechanisms provided herein have a force
threshold that is independent from the contact point on the
end-effector, in contrast to known "moment limiting" devices. The
force limiting devices also allow a larger displacement of the
end-effector, which increases the distance and time available to
mechanically stop a heavy overhead mounted manipulator or robot
located above the force limiting device and suspended end-effector
after a threshold contact force is detected. Because the nonlinear
Cartesian compliance mechanism of the force limiting device
described herein will react to Cartesian efforts, the polytope of
the achievable forces will not be dependent on the pose of the
contacting or colliding mechanism, e.g., the end-effector, and the
force limiting device may be optimized by appropriately selecting a
mechanism architecture (1-DOF, 2-DOF, 3-DOF) and by appropriately
selecting limit torques for the torque limiters incorporated
therein. To optimize the effectiveness of the force limiting
device, the mechanism should preferably be isotropic, such that the
achievable forces polytope will be a square in 2D or a cube in
3D.
[0017] The Cartesian mechanism (force limiting device) provided
herein may be constructed using a parallelogram mechanism
architecture incorporating torque limiters. A parallelogram
architecture has the advantage of being stiffer than serial
mechanisms. In a preferred embodiment, the force limiting Cartesian
mechanism is placed between the robot and its end-effector. Thus,
contact force is reduced to reduce damage and improve compliance
for collisions between a contactable object and any portion of the
end-effector or manipulator that is located upstream from the force
limiting mechanism in the robot or manipulator's kinematic chain,
e.g., between the contacted object and the force limiting device.
For a robot or manipulator suspended on an overhead rail-bridge
system, this configuration provides comprehensive protection
against collisions of the manipulator and its payload with a
contactable object, which may be an operator, by operatively
disconnecting the end effector or colliding portion from the robot
or overhead manipulator upstream from the force limiting device, so
as to release the rigid connection between the end effector and
overhead robot from which the end effector is suspended through the
force limiting device.
[0018] Further, because the 1-DOF and 2-DOF mechanisms presented
herein are not affected by gravity forces, the force limiting
device may be effective without limiting the payload to be carried
by the robot. This is also the case for the 3-DOF architectures
when a gravity compensating mechanism is included, as provided
herein. However, accelerations of the robot may induce inertial
forces that may activate the torque limiters of the force limiting
mechanism. Thus, for a given load, accelerations must be limited to
a certain level to prevent the force limiting device from
activating such that the end-effector becomes compliant, e.g.,
constructively disconnected, during movement in the absence of a
collision. The maximum velocity that can be typically imposed on a
robot is the maximum velocity that corresponds to blunt,
unconstrained collisions which may be qualified as compliant. This
"compliant" velocity is usually very low for heavy robots. However,
if during a collision the end-effector is disconnected from the
robot, e.g., the rigid connection between the robot and end
effector is released so as to become compliant, the effective
inertia to which the contacted object is subjected is then greatly
reduced. Therefore, it can be assumed that using a force limiting
mechanism as provided herein will allow an increase in the maximum
velocity of a robot moving in an environment with potential for
contactable object-robot interaction or physical object-robot
interaction. This maximum velocity should typically be evaluated
using a collision model that considers a broad spectrum of
collision parameters, including, typically, the way the robot
reacts when a collision is detected (braking force, delay before
the brakes are applied, etc.).
[0019] As noted previously, collisions in which a contactable
object is clamped to a wall or against another fixed object by a
robot can be most severe. The force limiting mechanism described
herein effectively reduces the maximum clamping force that the
robot can apply in quasi-static condition to a force level
determined by the limit torque levels set as limits for the torque
limiters incorporated in the force limiting device. As the velocity
of the robot system increases beyond a quasi-static, or very low
velocity condition, compliance is improved because the inertia
impacting the contactable object against the wall in a clamping
condition is reduced. Because the force limiting mechanism is
unable to store elastic potential energy, the robot will not
continue to push on the contacted object after the collision has
taken place and the force limiting device has been activated. This
is an advantage since it will help the movement of the robot away
from the contacted body after the collision.
[0020] As discussed previously, some robots incorporate a flexible
flange with breakaway function to limit the moment, not the force,
transmitted by the manipulator to the end-effector during a
collision. These systems are often sub optimized for the worst case
moment arm, resulting in a limited velocity, overly sensitive
system prone to false triggering of the breakaway mechanism and
deteriorated performance. In contrast, the collision force required
to activate the Cartesian force limiting device provided herein is
constant across the entire end-effector collision space, e.g., the
activation force does not vary with a moment arm, as described
further herein. This activation behavior is preferable since a
collision generally occurring anywhere on the end-effector will be
reacted to at a relatively constant activation force, optimizing
the robot design by allowing the minimum activation force to be
maximized while reducing the sensitivity of the force limiting
mechanism to non-collision inertia during robot movement. As an
additional advantage, the force limiting mechanism provided herein
has a large achievable displacement compared to the breakaway
device, which yields the space, and therefore reaction time,
required by a heavy overhead mounted manipulator to stop prior to
non-compliant contact with the object involved in the
collision.
[0021] A force limiting device configured to limit the contact
force of a moving object with a contacted object is provided
herein. The force limiting device includes a first attachment,
which may be an upper platform or interface, a second attachment,
which may be a lower platform or interface and one or more
parallelogram linkages. The force limiting device may be connected
at a first end to the first attachment and may be connected at a
second end to the second attachment, using one or more connection
points. The connection points may include rotatable joints. The
connection of the parallelogram linkage to the attachments may be
through a rotatable joint or through an intermediate segment, such
as a leg connected to the parallelogram linkage at one end and to
the attachment at the other end. The one or more parallelogram
linkages establish the orientation of the first attachment to the
second attachment; e.g., the first attachment may be oriented to be
parallel to the second attachment with the same orientation
relative to their common normal axis.
[0022] The force limiting device may include two parallelogram
linkages which are arranged to be perpendicular to each other, such
that the axes of the planes of the parallelogram linkages are
coincident. The parallelogram linkages are configured to transmit
an input torque, where the input torque is, for example, a couple
resultant from a force against the suspended portion attached to
the second attachment opposing the movement of the robot attached
to the first attachment.
[0023] The force limiting device may be attached to the moving
portion and the suspended portion of a robot, to operatively
connect the moving portion and the suspended portion of the moving
object. The force limiting device is configured to maintain a rigid
orientation between the first attachment and the second attachment
when the input torque is less than the activation torque; and to be
activated when the input torque transmitted through one or more of
the parallelogram linkages exceeds an activation torque. When the
force limiting device becomes activated, it is configured to become
compliant and thereby cause the contact force between the moving
object and the contacted object to be decreased. The force limiting
device may be compliant by compliance of the parallelogram
linkage.
[0024] Alternatively, the joints of each of the one or more
parallelogram linkages may be rotatable. One of the rotatable
joints of each parallelogram linkage may include a torque limiting
mechanism configured to activate the force limiting device when the
input torque exceeds a torque limit. Alternatively, a torque
limiter may be substituted for one of the rotatable joints of each
parallelogram linkage, or may be operatively included at an
attachment point between the force limiting device and the robot.
The torque limit of the torque limiting mechanism is typically set
equivalent to the activation torque.
[0025] The force limiting device may include three or more
parallelogram linkages arranged in a Delta configuration, e.g., a
parallel arm arrangement. In a preferred embodiment, the
parallelogram linkages are configured so as to be spaced 120
degrees equidistant from each other; however spacing with angles
different than 120 degrees is understood to be within the scope of
the claimed invention. The Delta configured force limiting device
may further include a gravity compensating mechanism configured to
include a spring, an actuator, a counterbalance system which may
include counterweights and pulleys, or a combination of these. The
gravity compensating mechanism may be configured to compensate for
the weight of the suspended portion, the weight of a payload, which
may be variable, or the combined weight of the suspended portion
and a payload.
[0026] The force limiting device may be included in a robot system
adapted for overhead suspension, the system including a robot
capable of moving, and a suspended portion which is operatively
connected to and suspended from the robot. The force limiting
device may be attached between the robot and the suspended portion,
to operatively connect the robot and the suspended portion. When
the suspended portion, which is manipulated and moved by the robot,
exerts a contact force against an object, an activation force is
inputted to the suspended portion opposing the contact force. The
force limiting device is configured to become activated and when
activated, become compliant when the activation force exceeds an
activation level. When the force limiting device becomes compliant,
the contact force exerted by the suspended portion against the
contacted object is immediately and substantially decreased, to
prevent or minimize damage to the contacted object. In a workspace
including robots, the contacted object may be another piece of
equipment or stationary fixture.
[0027] The force limiting device provided herein increases the
compliance level of physical object-robot interactions. As would be
understood by those skilled in the art to be within the scope of
the claimed invention, the force limiting device may be used in
other environments, for example, controlled (fenced or gated) work
cells, to minimize damage to the robot system, robot, end-effector,
manipulator, payload, tooling, other objects and equipment in the
workspace, etc. by minimizing the force of non-intended moving
robot-to-moving object or moving robot-to-stationary object
collisions. The force limiting device may be used in any scenario
where a release of a rigid connection in response to an activation
force is desirable to alleviate or minimize damage resulting from a
collision with one or move moving or static objects incorporating
the force limiting device.
[0028] The above features and advantages and other features and
advantages of the present invention are readily apparent from the
following detailed description of the best modes for carrying out
the invention when taken in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1A is a schematic perspective view of a robot system
including a force limiting device, showing the robot system in
motion toward an object adjacent a stationary object such as a
wall;
[0030] FIG. 1B is a schematic perspective view of the robot system
of FIG. 1A with the robot system end effector contacting an object
in a clamping collision;
[0031] FIG. 1C is a schematic perspective view of the robot system
of FIG. 1A with the force limiting device activated;
[0032] FIG. 2A is a schematic perspective view of a force limiting
device in a 1-DOF parallelogram configuration;
[0033] FIG. 2B is a schematic perspective view of the force
limiting device of FIG. 2A in an activated condition;
[0034] FIG. 3 is a schematic perspective view of a force limiting
device in a 2-DOF parallelogram configuration;
[0035] FIG. 4A is a schematic perspective view of a force limiting
device in a 3-DOF delta configuration;
[0036] FIG. 4B is a schematic perspective view of the force
limiting device of FIG. 4A configured for gravity compensation;
[0037] FIG. 5 is a schematic plan view of a gravity compensating
device for use with the force limiting device of FIG. 4A; and
[0038] FIG. 6 is a schematic plan view of a counterbalancing system
for use with the force limiting device of FIG. 4A.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0039] Provided herein is a force limiting device to increase the
compliance level of suspended robots, as related to physical
object-robot interfaces. The force limiting device described herein
is a parallelogram mechanism with torque limiters which provide,
during non-collision operation, a rigid connection between the
robot and its end-effector. If an excessive force in a direction
corresponding to the degrees of freedom (DOFs) for which the force
limiting device is configured is applied during a collision, the
force limiting mechanism is activated and the end-effector becomes
compliant, e.g., is free to move relative to the robot and
typically opposite to the direction of the collision. Brakes or
like functioning devices are applied to stop further motion of the
robot in the direction of contact when the force limiting device is
triggered or activated. The inertia of the moving robot located
kinematically upstream of the force limiting device is thus removed
from the collision, and for a quasi-static collision, the maximum
contact force is the activation force of the force limiting device
for that orientation, as determined by the configuration of the
torque limiters in the mechanism.
[0040] In a preferred embodiment, the device provided herein is
configured for use with an overhead mounted robot to limit the
collision force resulting from contact with the end-effector, robot
arm, tooling and/or payload suspended from the robot. Different
architectures are provided for force limiting devices configured
for one, two and three degrees of freedom (respectively, 1-DOF,
2-DOF, 3-DOF).
[0041] Referring to the drawings, wherein like reference numbers
refer to like components, shown in FIGS. 1A, 1B and 1C is a
schematic perspective view of a robot system 100 including a force
limiting device 110. The robot system 100 includes a robot 105,
which is suspended from an overhead support system, which may be a
ceiling or an overhead rail system. Robot system 100 further
includes a force limiting device 110 operatively connected to robot
105 through an interface 150, and a suspended portion 115, which
may be a robot arm, end-effector or similar mechanism. The
suspended portion or end-effector 115, which is operatively
connected to the lower portion of force limiting device 110, may
include additional tooling and/or include a payload. Robot system
100 may also include, as would be understood by those skilled in
the art, a control system, which may include a controller,
controls, sensors and other mechanisms commonly included in a robot
system. Robot system 100 further includes a braking mechanism 113
to stop motion of robot 105. The braking mechanism 113 of robot 105
may be initiated by the robot controls or by triggering the force
limiting device 110.
[0042] Referring to FIG. 1A, shown is the robot system 100 moving
in a direction 125 toward an object 130. In the arrangement shown,
object 130 is positioned between a stationary object, such as a
wall 117, and the robot system 100 such that continued movement of
robot system 100 in direction 125 will result in contact of the
robot arm or end-effector 115 with the object 130. As described
previously, end-effector 115 is suspended from force limiting
device 110, which is operatively attached through interface 150 to
robot 105. Force limiting device 110 includes a jointed
parallelogram linkage 111 in which one revolute joint is replaced
with a torque limiter 120, which will be described further. Under
the conditions shown in FIG. 1A, there is no torque input into
torque limiter 120 with respect to robot 105 and end-effector 115,
therefore torque limiter 120 maintains the parallelogram linkage
111 of force limiting mechanism 110 in a static or rigid state,
thus maintaining a rigid connection between end-effector 115 and
robot 105.
[0043] As shown in FIG. 1B, robot 105 has continued in direction
125 such that end effector 115 has made contact with object 130 at
contact point 135, in a clamping collision, whereby object 130 has
become clamped against the stationary fixture or wall 117. As robot
105 continues movement in direction 125, end effector 115 exerts an
increasing contact force on object 130 at contact point 135. Object
130 exerts an opposing force at contact point 135 on end effector
115 which is transmitted to force limiting device 110, including
the joint replaced by torque limiter 120. When the opposing force
exerted against end effector 115 at the point of contact 135 in
combination with the continued movement of robot 105 in direction
125 results in a torque input to torque limiter 120 exceeding its
torque limit or trigger point, the force limiting device 110 is
activated.
[0044] Upon activation of the force limiting device 110, and
referring now to FIG. 1C, two events occur. First, the torque
limiter 120 releases, resulting in the compliant movement of the
parallelogram linkage 111 of force limiting device 110 in a
direction 140 which immediately and substantially decreases and/or
relieves the contact force exerted by end effector 115 on object
130. Secondly, the activation of force limiting device 110 triggers
the braking mechanism 113 of robot 105 to stop any movement of
robot 105 in direction 125. Thus, by activating force limiting
device 110, damage to object 130 from contact with end effector 115
may be minimized or avoided.
[0045] The nonlinear Cartesian compliance mechanism 110, or force
limiting device 110 can therefore be said to operatively
"disconnect" the end-effector 115 from the robot 105 over a certain
distance when a collision occurs, that distance being determined by
the geometry of force limiting device 110 as it complies, releasing
and/or relieving the contact force of end-effector 115 from the
contacted object, which may be a object 130. The operative
"disconnection" of end-effector 115 over a certain distance or
space provides the distance and time required to stop the movement
of the robot 105 and robot system 100, thus substantially or fully
relieving the collision force at contact point 135. As shown in
FIGS. 1A-1C, the force limiting device 110 as provided herein is
well suited for applications such as robots suspended on
rail-bridge systems, however may be applicable to other systems and
configurations within the scope of the claimed invention.
[0046] Returning to FIGS. 1A-1C, the contact force over time can be
described in phases. The contact force or collision force between
the end effector 115 and contacted object 130, is zero in FIG. 1A,
during the pre-collision phase. As robot system 105 continues
movement in direction 125, and when end-effector 115 makes contact
with object 130, which is entrapped between end-effector 115 and
the stationary wall 117 in an initial collision phase, the contact
force at contact point 135 increases until the activation torque of
torque limiter 120 is attained and force limiting device 110 is
triggered. In a third phase, force limiting device 110 becomes
compliant, resulting in an immediate and substantial decrease in
the contact force at contact point 135, due to the activation of
the torque limiter 120. The force required to overcome the
continued movement of robot system 100 is still present, but is
eliminated in a final phase when the movement of robot 105 is
stopped by triggering brake 113 concurrently with activation of
force limiting device 110 during the initial collision phase. So
long as robot 105 can be stopped within the geometric compliance
limits of force limiting device 110, the inertia from robot 105
does not contribute further to the contact force on object 130 at
contact point 135. The trigger value or torque limit established
and set for torque limiter 120 should consider the geometric
compliance limits of the force limiting device 110 and the braking
dynamics of robot 105, which may be different depending on the
orientation of the collision angle of the end effector 115 with
respect to the parallelogram configuration of torque limiting
device 110.
[0047] Referring now to FIGS. 2A and 2B, shown is a schematic
perspective view of a force limiting device 110 in a 1-DOF
parallelogram configuration. FIG. 2A shows force limiting device
110 in a non-activated state, where torque limiter 120 acts to
maintain the parallelogram linkage 111 of device 110 in a rigid
configuration. FIG. 2B shows force limiting device 110 in an
activated state, where the threshold torque has been met to release
torque limiter 120 such that the parallelogram linkage 111 reacts
compliantly to force 140, the contact force at contact point 135
during a collision of the robot system 100 with a fixed or
constrained object, e.g., the object 130 in the clamping collision
illustrated in FIGS. 1A-1C. The force limiting device 110 includes
an upper platform or interface as a first attachable portion 150
which is operatively attachable to robot 105. FIGS. 2A and 2B
provide holes 155 as an attachment interface through which device
110 may be bolted, pinned or riveted, for example, to robot 105. As
would be understood, any suitable means known to those skilled in
the art may be used to fixedly attach the upper interface 150 of
force limiting device 110 to robot 105. The force limiting device
110 includes a lower platform or interface as a second attachable
portion 160 which is operatively attachable to robot arm or
end-effector 115. FIGS. 2A and 2B provide holes 165 as an
attachment interface through which device 110 may be bolted, pinned
or riveted, for example, to robot arm or end-effector 115, however
any suitable means known to those skilled in the art may be used to
fixedly attach the lower interface 160 of force limiting device 110
to robot arm or end-effector 115.
[0048] Referring now to FIG. 2A, the 1-DOF force limiting device
110 shown includes a single parallelogram linkage 111 that may
optimally be used when the motion of robot system 100 in one
direction is much more undesirable than in other directions. FIG.
2A shows a simple 1-DOF nonlinear Cartesian compliance mechanism
110, also referred to as a 1-DOF force limiting device 110 mounted
between a suspended robot 105 and its end-effector 115. Mechanism
110 includes a parallelogram linkage 111 in which one revolute
joint 185 is replaced with a torque limiter 120. The parallelogram
linkage consists of two legs 170 attached through revolute joints
185, 195 and shafts 180, 190 to platforms 150, 160, and generally
configured as illustrated by FIG. 2A. Referring to FIG. 2A, the
three joints 195 are passive revolute joints which are rotatable
about shafts 190 where shafts 190 are rotatively or fixedly
attached to platforms 150, 160. Alternatively, the joints 195 may
be fixedly attached to shafts 190, where shafts 190 are rotatively
attached to platforms 150, 160. The fourth joint 185 is fixedly
attached to input shaft 180, such that as the parallelogram linkage
111 is subject to a generally horizontal force (as oriented in FIG.
1A), such as a force 140 (see FIG. 2B), the input shaft 180
provides an input torque, such as input torque 175, to torque
limiter 120.
[0049] Under normal conditions, the torque limiter 120 restrains
the rotation of shaft 180 and thus prevents the parallelogram
linkage of force limiting device 110 from moving, maintaining a
rigid connection between robot 105 and end-effector 115. However,
if a contacting collision occurs, for example, a collision of the
type illustrated in FIG. 1B, the couple 175 passing through torque
limiter 120 exceeds set limits and force limiting mechanism 110 is
activated and moves responsively to force 140, as shown in FIG. 2B.
This compliant movement functionally "disconnects" end-effector 115
from robot 105 with respect to a generally horizontal plane and
thus immediately and substantially relieves the contact force of
end effector 115 from the object involved in the collision, e.g.,
the object 130 shown in FIGS. 1A-1C. Following activation and
responsive compliant movement of force limiting device 110, the
object 130 is only subjected to the inertia of end-effector 115,
which can be significantly lower than the inertia of the entire
robot system 100. For the force limiting mechanism 110 to be
effective in improving compliance by reducing the contact force on
the object, the collision must be detected and robot 105 must be
stopped or braked before the parallelogram linkage 111 of mechanism
110 reaches the end of its travel, e.g., its geometric limit. The
collision can be detected with a limit switch (not shown) placed on
the mechanism 110, e.g., in contact with one of the parallelogram
links and a signal can be sent to the controller of robot 105 to
brake the system, or alternatively, an emergency stop signal can be
sent directly to the brake system 113 of robot 105 without passing
through the robot's controller, thus improving the reliability of
the system by reducing the risks of electronic component failure.
Once robot 105 is stopped, the gravity force of the suspended robot
arm 115 tends to naturally return force limiting mechanism 110 to
its original position. One important advantage of the parallelogram
architecture of force limiting device 110 is that the couple
passing through the torque limiter 120 only depends on the
magnitude of the horizontal force 140 (referring to FIG. 2B)
applied on end-effector 115 and is not affected by the height of
the point of application of the force, e.g., the distance between
contact point 135 and torque limiter 120 (see FIG. 1B). This
implies that the same force level will cause the activation of the
force limiting mechanism 110 whether the collision occurs at the
top, middle or bottom portion of the object 130, or at an end or
middle portion of end-effector 115. This provides a significant
advantage over breakaway systems which are sensitive to length of
the moment arm of the contact force, e.g., the distance of the
collision contact point 135 from the actuation point of the force
limiting mechanism.
[0050] FIG. 3 is a schematic perspective view of a force limiting
device 210 in a 2-DOF parallelogram configuration. 2-DOF force
limiting device 210 has four legs 270 that form two parallelogram
linkages 211 and thus behaves similarly to 1-DOF force limiting
device 110. 1-DOF force limiting device 110 and 2-DOF force
limiting device 210, as configured and provided herein, are not
sensitive to the gravity force of the suspended weight of
end-effector 115, 215 and thus do not require a gravity
compensating mechanism. A 2-DOF force limiting device 210 is
especially appropriate for applications requiring large and fast
horizontal motion of a robot system 100 and small and slow vertical
displacements.
[0051] Referring to FIG. 3, 2-DOF force limiting device 210 has a
parallelepipedic architecture making it potentially suitable as a
mechanism to reduce the collision force for collisions occurring
across the horizontal plane, e.g., the X-Y plane of FIG. 3. This is
accomplished using a parallel architecture as generally shown in
FIG. 3, which is composed of four identical legs 270 which each
include a pivoting joint 297 and a pivoting joint 295, where each
pair of pivots 295, 297 has their axes in two perpendicular,
horizontal directions. The legs 270 are placed in such a way that
the axes of the first set of pivots 297 intersect at a single
point, e.g., the X-Y origin generally at the center of upper
platform 250, two of them sharing the same axis, perpendicular to
the axis of the other two. The axes of the second set of pivots 295
are oriented similarly to the first set of pivots 297, in a plane
parallel to the X-Y plane defined by the axes of the second set of
pivots 297. The parallelogram linkages 211 are operatively attached
to the attachable portion or lower platform 260 through legs 290.
Additionally, brackets 222 may be included to operatively attach
parallelogram linkages 211 and/or torque limiters 220 to the
attachable portion or upper platform 250. Upper platform 250 is
fixedly attachable to robot 105 and lower platform 260 is fixedly
attachable to end-effector 215. Similar to the attachment
configuration discussed for device 110, and as shown for the
embodiment of device 210 in FIG. 3, upper platform 250 is provided
with holes 255 as an attachment interface through which device 210
may be bolted, pinned or riveted, for example, to robot 105, or any
suitable means known to those skilled in the art may be used to
fixedly attach the upper interface 250 of force limiting device 210
to robot 105. The force limiting device 210 includes a lower
platform or interface 260 which is operatively attachable to robot
arm or end-effector 215. FIG. 3 provides holes 265 as an attachment
interface through which device 210 may be bolted, pinned or
riveted, for example, to robot arm or end-effector 215, however any
suitable means known to those skilled in the art may be used to
fixedly attach the lower interface 260 of force limiting device 210
to robot arm or end-effector 215.
[0052] The workspace of force limiting mechanism 210 is a sphere
centered on upper platform 250 and the orientation of lower
platform 260 remains the same relative to upper platform 250. Two
of the first sets of pivots 297 are connected to input attachments
285, to provide input to torque limiters 220, providing similar
behavior and function as 1-DOF mechanism 110 described earlier. For
the 2-DOF architecture of device 210, only three of the four legs
270 are required to kinematically constrain mechanism 210 in a
non-collision situation. The fourth leg 270 over-constrains
mechanism 210, providing the advantage of adding stiffness and
reducing the effect of backlash if, for example, the length of one
of the leg 270 is adjusted to provide an internal pre-load to
mechanism 210. Force limiting device 210 presents similar
advantages as force limiting device 110, e.g., the magnitude of
contact or collision force that will activate force limiting
mechanism 210 is only dependent on the orientation and not the
height of contact point 135 or the distance between contact point
135 and torque limiters 220 (see FIG. 1B), and after a collision
triggering force limiting device 210, gravity tends to return
mechanism 210 to its original configuration.
[0053] Referring to FIG. 3, the activation sequence of force
limiting device 210 is similar to that described for force limiting
device 110 in FIGS. 1A through 2B. Under normal conditions, torque
limiters 220 restrain the rotation of pivots 295, 297 and shafts
270 and thus prevents the parallelogram linkages 211 of force
limiting device 210 from moving, maintaining a rigid connection
between robot 105 attached to attachment plate 250 and end-effector
215 attached to attachment plate 260. However, if a contacting
collision occurs, for example, a collision of the type illustrated
in FIG. 1B, torque is transmitted though the parallelogram linkages
of force limiting device 210 to torque limiters 220. When this
transmitted torque exceeds a set limit for either of the torque
limiters 220, force limiting mechanism 210 is activated and moves
responsively in opposition to the contact force in direction 140
(see FIG. 1C). When at least one of the torque limiters 220
releases its respective input attachment 285 when the input torque
from the transmitted contact force exceed the set limit of the
torque limiter 220, the force limiting device is released from its
rigid configuration and becomes compliant. Legs 270 rotate and
pivot using pivots 295, 297 responsive to and opposing the contact
force. This compliant movement functionally "disconnects"
end-effector 215 from robot 105 with respect to a generally
horizontal plane and thus immediately and substantially relieves
the contact force of end effector 215 from the object involved in
the collision, e.g., object 130 shown in FIGS. 1A-1C. Following
activation and responsive compliant movement of force limiting
device 210, the object 130 is only subjected to the inertia of
end-effector 215, which can be significantly lower than the inertia
of the entire robot system 100. For the force limiting mechanism
210 to be effective in improving compliance by reducing the contact
force on the object, the collision must be detected and robot 105
must be stopped before the parallelogram linkage 211 of mechanism
210 reaches the end of its travel, e.g., its geometric limit. The
collision can be detected with a limit switch (not shown) placed on
the mechanism 210, e.g., in contact with one of the parallelogram
links 211 and a signal can be sent to the controller of robot 105
to brake the system, or alternatively, an emergency stop signal can
be sent directly to the brake system 113 of robot 105 without
passing through the robot's controller, thus improving the
reliability of the system by reducing the risks of electronic
component failure. Once robot 105 is stopped, the gravity force of
the suspended robot arm 215 tends to naturally return force
limiting mechanism 210 to its original position.
[0054] Referring now to FIGS. 4A and 4B, shown in FIG. 4A is a
schematic perspective view of a force limiting device 310 in a
3-DOF delta configuration; and FIG. 4B provides a schematic
perspective view of the force limiting device 310 of FIG. 4A
configured for gravity compensation. Referring to FIG. 4A, the
3-DOF force limiting device 310 presented herein is configured
based on a Delta architecture. A 3-DOF force limiting device 310
can be applied more generally than a 1-DOF force limiting device
110 or a 2-DOF force limiting device 210 because a 3-DOF device 310
can react to collisions occurring in any direction with
end-effector 315 including collisions with a vertical force
component. Methods to compensate the effect of gravity on a 3-DOF
delta configuration, where required when the weight of the
end-effector or combined weight of the payload with the
end-effector is large compared to the maximum static force limit
that is imposed, are provided herein. Other possible configurations
for 3-DOF force limiting devices are also provided within the scope
of the claimed invention.
[0055] As discussed previously for force limiting devices 110, 210,
force limiting device 310 includes an upper platform 350 which is
configured to be fixedly attachable to robot 105 and a lower
platform 360 is fixedly attachable to end-effector 315. Similar to
the attachment configuration discussed for devices 110, 220 and as
shown for the embodiment of device 310 in FIG. 4A, upper platform
350 is provided with holes 355 as an attachment interface through
which device 310 may be bolted, pinned or riveted, for example, to
robot 105. As is understood, any suitable means known to those
skilled in the art may be used to fixedly attach the upper
interface 350 of force limiting device 310 to robot 105. The force
limiting device 310 includes a lower platform or interface 360
which is operatively attachable to robot arm or end-effector 315.
FIG. 4A provides holes 365 as an attachment interface through which
device 310 may be bolted, pinned or riveted, for example, to robot
arm or end-effector 315, however any suitable means known to those
skilled in the art may be used to fixedly attach the lower
interface 360 of force limiting device 310 to robot arm or
end-effector 315.
[0056] Referring to FIG. 4A, shown is a preferred embodiment of a
3-DOF force limiting device 310 with a Delta architecture
comprising three legs 370, each operatively connected to a torque
limiter 320 that positions the upper link 395 of a parallelogram
linkage 311 whose lower link 395 is operatively attached to lower
platform 360. Each of the four corner joints of each parallelogram
linkage 311 are schematically represented in FIG. 4A by a spherical
joint, although other types of joints, e.g., universal joints, may
be used in the parallelogram linkage within the claimed scope of
the invention. The parallelogram linkages 311 constrain the
orientation of lower platform 360 in a way such that upper platform
350 and lower platform 360 maintain a constant orientation relative
to each other. Force limiting device 310, when activated by a force
above the preset threshold of any of the torque limiters 320, can
perform translation in the X, Y and Z directions (see FIG. 4A). For
the configuration of device 310 shown in FIG. 4A, it is assumed
that the optimal design of a force limiting device 310 using the
Delta architecture will comprise identical legs 370 that are
equally spaced, i.e., placed 120.degree. relative to one another
with equal radii for attachment points on the platforms 350, 360.
However, it is understood that the Delta architecture is not
restricted to three legs. For example, four or more legs could be
used with the same general behavior, although the mechanism would
be over constrained. It is further understood that the legs may be
spaced with angles different than 120 degrees and yield the same
general behavior.
[0057] Two kinematic properties of the 3-DOF configuration should
be optimized for efficiency of force limiting device 310. The first
property is the workspace of force limiting device 310. Since robot
105 must be capable of braking prior to travel a distance exceeding
the geometric motion limit of force limiting device 310, the
optimal workspace for device 310 will be a sphere centered at its
reference point. The radius of that sphere needs to be equal to the
maximum braking distance of robot 105 considering collisions
occurring in any direction. Secondly, an isotropic Jacobian matrix
will give the maximal ratio of the minimum over the maximum forces
needed to activate force limiting device 310. The isotropy of the
achievable force space is more difficult to obtain for a 3-DOF
mechanism 310, however, it is obtainable for the reference point of
the mechanism 310 by choosing design parameters assuming the
optimal achievable force polyhedron is a cube.
[0058] Referring to FIG. 4A, the activation sequence of force
limiting device 310 is similar to that described for force limiting
devices 110 and 210. Under normal conditions, torque limiters 320
restrain the rotation of pivots 395 and legs or shafts 370 and thus
prevents the parallelogram linkages 311 of force limiting device
310 from moving, maintaining a rigid connection between robot 105
attached to attachment plate 350 and end-effector 315 attached to
attachment plate 360. However, if a contacting collision occurs,
for example, a collision of the type illustrated in FIG. 1B, torque
is transmitted though the parallelogram linkages 311 of force
limiting device 310 to torque limiters 320. When this transmitted
torque exceeds a set limit of any one of the torque limiters 320,
force limiting mechanism 310 is activated and moves responsively in
opposition to the contact force in direction 140 (see FIG. 1C).
When at least one of the torque limiters 320 releases its
respective pivot point 385 when the input torque from the
transmitted contact force exceed the set limit of the torque
limiter 320, the force limiting device 310 is released from its
rigid configuration and becomes compliant. Legs 370 become
rotatable and pivot about pivots 385, 395, and parallelogram
linkages 311 move responsively to and opposing the contact force.
This compliant movement functionally "disconnects" end-effector 315
from robot 105 with respect to a generally horizontal plane and
thus immediately and substantially relieves the contact force of
end effector 315 from the object involved in the collision, e.g.,
the object 130 shown in FIGS. 1A-1C. Following activation and
responsive compliant movement of force limiting device 310, the
object 130 is only subjected to the inertia of end-effector 315,
which can be significantly lower than the inertia of the entire
robot system 100. For the force limiting mechanism 310 to be
effective in improving compliance by reducing the contact force on
the object, the collision must be detected and robot 105 must be
stopped before the rotating legs 370 and parallelogram linkages 311
of mechanism 310 reach the end of travel, e.g., the geometric
limits of the collective linkage and structure of force limiting
device 310. The collision can be detected with a limit switch (not
shown) placed on the mechanism 310, e.g., in contact with one of
the parallelogram links 311 and a signal can be sent to the
controller of robot 105 to brake the system, or alternatively, an
emergency stop signal can be sent directly to the brake system 113
of robot 105 without passing through the robot's controller, thus
improving the reliability of the system by reducing the risks of
electronic component failure. Once robot 105 is stopped, the
gravity force of the suspended robot arm 315 tends to return force
limiting mechanism 310 to its original position.
[0059] The 3-DOF force limiting device 310 as configured in FIG.
4A, which is sensitive to collisions from all directions, including
collision directions with a vertical component, may also be
sensitive to the gravity force representing the weight of
end-effector 315 and its payload. The gravity force of end-effector
315 or the combined gravity force of end-effector 315 with a
payload may create a load on torque limiters 320 that will
eventually limit the force that the end-effector 315 can apply to
accomplish a certain task. Also, the gravity force from the weight
of the payload and/or end-effector 315 might exceed the activation
limits of force limiting device 310, potentially rendering device
310 ineffective in a collision situation. The potential effect of
the gravity force from the weight of the end-effector 315 can be
counteracted using gravity balancing when the combined weight of
the payload and the end-effector 315 is large relative to the
activating contact force limit for the force limiting device 310,
to maintain the effectiveness of force limiting device 310 in a
collision event.
[0060] Referring to FIG. 4B, counterbalancing a parallel mechanism
such as force limiting device 310 is usually complex because of the
nonlinear and coupled relation between Cartesian and articular
displacements. In the present situation, however, the force
limiting device need only be balanced for one configuration or
condition of use, e.g., suspending the end-effector arm 315 during
normal operation. In a collision situation, when the force limited
device 310 is activated to respond to the contact force of the
collision from any direction, it is unnecessary to counterbalance
the gravity load of end-effector 315, because the prioritized
response in a collision situation is relief of the contact force,
not robot performance. Therefore, it is acceptable to incorporate a
counterbalancing mechanism that balances the gravity load from the
weight of the end-effector 315 in only the neutral configuration,
e.g., when the arm 315 is suspended and in use during normal
non-collision robot operating conditions. In this case, and when
the gravity force from the weight of arm 315 and any payload is
relatively constant, the counterbalancing mechanism may be simply a
pre-loaded spring 398, as shown in FIG. 4B, which provides
advantages of mechanical simplicity and low weight. Notably, this
counterbalancing method is valid only if the spring 398 does not
limit the workspace of device 310, which might only be possible for
smaller leg length ratios.
[0061] Alternatively, and referring now to FIG. 5, when the gravity
force from the weight of arm 315 and any payload may be variable,
balancing the variable gravity load with a spring 415 may require
an actuator 405 to modify the position of one of the spring's
anchor points 410, where the actuator 405 must provide a force
equal to the gravity force. Shown in FIG. 5 and generally indicated
at 400, a counterbalance system may be provided where an actuator
405 could adjust the counterbalance when the gravity force changes,
for example, when the robot arm 315 is picking up a new payload. To
do so, the upper anchor point 410 of counterbalance spring 415
could be mounted on a linear guide 420 with a locking system 410
(included in upper anchor point 410). In normal (unloaded) mode,
the anchor point 410 is locked and thus the force limiting device
310 is displacing the gravity force of the end-effector 315.
However, when robot arm 315 picks up or releases a payload, the
anchor point 410 of the spring is unlocked and the actuator 405
counterbalances the change in gravity force via spring 415, while
the force limiting device 310 counterbalances the weight of
end-effector 315. If, for example, the gravity force increases, the
heavier load will pull on spring 415 and anchor point 410 will be
adjusted by actuator 405 until the elastic force in spring 415
reaches a force equal to load's weight. Then, anchor point 410 is
locked again and the robot system 100 goes back to normal operating
mode where end-effector 315 and its payload can be displaced
vertically in the event of a collision with sufficient contact
force to activate force limiting device 310.
[0062] Referring now to FIG. 6, a counterbalancing system generally
indicated at 450 is provided, which uses a passive system of
counterweights and/or springs to accomplish the required
counterbalancing. The system 450 may be configured using remote
counterweights 470, 475, where the balancer and the counterweights
are placed away from robot 105, to avoid adding horizontal inertia,
and the counterbalancing force is transmitted with cables 460, 465
via routing pulleys 490, 485, respectively, designed in a way that
horizontal displacements of robot 105 do not move counterweights
470, 475. This allows the balancing of load 455, which may, for
example, be comprised of robot arm 315 and a payload, without
adding inertia for displacements in axes other than the vertical
one. To configure the counterbalancing system 450 in this manner,
the load that needs to be balanced must be separated into two
parts, a first load, and a second load 455, by the force limiting
device 310. The first load, which is configured to be relatively
constant, may include the load represented by robot 105 and device
310. The first load may additionally include the load of an
end-effector or robot arm 315 without a payload, in which event
pulley 460 would be operatively connected to end-effector 315,
also. The second load 455 may include the load of a payload, which
may be variable, and may additionally include the load of
end-effector or robot arm 315, if this load is not included in the
first load. The effectiveness of the force limiting device 310 in
an activated condition, e.g., responsive as a force limiting device
in the event of a collision, requires that the first and second
loads be allowed to move relative to one another when an activating
contact force threshold is met. This implies that the second load,
which is also the portion of the load which may be variable, must
be moved with the same actuator as the first load under normal
(non-collision) conditions. It should be noted that in the case
where the second load is small compared to the level of force
required to accomplish the task, for example, where there is no
incremental payload, balancing of the second load is not required
and the system can be counterbalanced with a single pulley system,
shown in FIG. 6 as including cable 460 and pulleys 490,
counterweight 470 and an actuator 480. However, in the general
case, a second balancing or cable and pulley system, as shown in
FIG. 6 including cable 465 and pulleys 485, will be required to
counterbalance the second part of the load 455. This second
balancing system adds minimal additional parts and complexity to
the overall counterbalance system and robot system 100. The first
counterweight 470, used to balance the first load, which is the
constant load of the system, is not required to be variable since
it always balances substantially the same load, and only one
actuator 480 is needed. Therefore, balancing a first and second
load separately represents only a limited increase in system
complexity.
[0063] The particular balancing method employed depends on the
particular application and anticipated variability of the loads,
and whether the robot system 100 uses a balancing system with
counterweights for other purposes, where, for example, the
additional complexity of a pulley and counterweight
counterbalancing system is minimal and limited to incorporating a
second routing-pulley system. As discussed previously, using
counterbalancing springs provides a simpler mechanical design that
is likely less expensive than using remote counterweights. This is
especially true when the load is constant, since for that situation
there is no need to add a mechanism or actuator to adjust the
balancing force or to change the spring after a certain number of
cycles to avoid fatigue. Therefore, determining the most suitable
balancing system is dependent on numerous factors that need to be
evaluated for each application.
[0064] A 3-DOF force limiting device based on Delta architecture,
such as mechanism 310 shown in FIGS. 4A and 4B, may be preferably
suited for the exemplar scenario provided herein, e.g., as a force
limiting device operatively connecting a suspended robot arm to an
overhead moving robot, as shown in FIGS. 1A to 1C, to relieve
contact force between the robot arm or end-effector 115 and a
substantially stationary object 130 in a clamping collision, for a
number of reasons. First, the workspace of a force limiting device
configured with Delta architecture such as mechanism 310 is the
intersection of three toruses and can be optimized to center a
large spherical volume on the neutral position, maximizing the
available motion in all directions to allow robot 105 to be stopped
before reaching the geometric limits of mechanism 310. Second, a
force limiting device 310 incorporating Delta architecture can be
designed such that the mechanism will be isotropic in its neutral
configuration, optimizing the available force that the robot 100
can apply in any direction to accomplish a task while limiting the
overall maximum static force to a certain level, which may be a
compliant level, to minimize impact on performance. Third, a force
limiting device 310 incorporating Delta architecture is sensitive
only to forces, not moments, which allows the response of the force
limiting device to be independent from the location of the
collision contact point on the end-effector. Further, the device
configuration based on Delta architecture is geometrically compact
and simple, making it potentially less costly and more reliable
while limiting its footprint. Notwithstanding the advantages of a
Delta based 3-DOF configuration for the force limiting device 310
provided herein, it is understood that other configurations of
3-DOF force limiting devices with may also be provided within the
scope of the claimed invention.
[0065] While the best modes for carrying out the invention have
been described in detail, those familiar with the art to which this
invention relates will recognize various alternative designs and
embodiments for practicing the invention within the scope of the
appended claims.
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