U.S. patent application number 14/716487 was filed with the patent office on 2015-12-31 for dynamic physical constraint for hard surface emulation.
This patent application is currently assigned to PERCEPTION RAISONNEMENT ACTION EN MEDECINE. The applicant listed for this patent is PERCEPTION RAISONNEMENT ACTION EN MEDECINE. Invention is credited to Antony Hodgson, Nikolai Hungr, Christopher Plaskos.
Application Number | 20150375394 14/716487 |
Document ID | / |
Family ID | 41054483 |
Filed Date | 2015-12-31 |
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United States Patent
Application |
20150375394 |
Kind Code |
A1 |
Hodgson; Antony ; et
al. |
December 31, 2015 |
DYNAMIC PHYSICAL CONSTRAINT FOR HARD SURFACE EMULATION
Abstract
A method and apparatus for haptic hard surface emulation using a
dynamic physical constraint are provided. The movement and position
of the dynamic physical constraint is actively controlled in order
to emulate a hard surface. The dynamic physical constraint may be
controlled by a computer. In another aspect of the invention, the
dynamic physical constraint limits the motion of a manipulator
joint in space. The position at any time of the dynamic physical
constraint is dependent on the position in space of the
manipulator's end effector.
Inventors: |
Hodgson; Antony; (Vancouver,
CA) ; Plaskos; Christopher; (Plymouth, MA) ;
Hungr; Nikolai; (Grenoble, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PERCEPTION RAISONNEMENT ACTION EN MEDECINE |
La Tronche |
|
FR |
|
|
Assignee: |
PERCEPTION RAISONNEMENT ACTION EN
MEDECINE
La Tronche
FR
|
Family ID: |
41054483 |
Appl. No.: |
14/716487 |
Filed: |
May 19, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12399714 |
Mar 6, 2009 |
9037295 |
|
|
14716487 |
|
|
|
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61064486 |
Mar 7, 2008 |
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Current U.S.
Class: |
700/160 ;
700/255; 901/11; 901/41 |
Current CPC
Class: |
A61B 34/20 20160201;
A61B 2090/033 20160201; A61B 17/1764 20130101; B25J 9/1676
20130101; B25J 13/025 20130101; Y10S 901/41 20130101; A61B 2090/034
20160201; A61B 90/03 20160201; A61B 17/1757 20130101; Y10S 901/11
20130101; A61B 34/76 20160201; A61B 34/30 20160201; A61B 17/1742
20130101 |
International
Class: |
B25J 9/16 20060101
B25J009/16 |
Claims
1. An apparatus for guiding a user comprising: a robot having an
end-effector which is adapted to be manipulated by an application
of external forces, wherein the robot has at least one actuated
joint and zero or more passive joints; wherein the at least one
actuated joint is configured to position a physical constraint that
limits the motion of at least one other joint in at least one
direction.
2. The apparatus of claim 1, further comprising a tool is attached
to an end of the end-effector.
3. The apparatus of claim 1, further comprising a milling tool is
attached to an end of the end-effector.
4. The apparatus of claim 3, wherein a base of the apparatus is
configured to be mounted to a bone.
5. The apparatus of claim 1, wherein the at least one actuated
joint includes an active linkage that is controlled by relating a
constrained position of the physical constraint to a function of
the current position of the end-effector.
6. The apparatus of claim 5, wherein the current position of the
end-effector is determined using an external position measurement
system.
7. A method for controlling movements of an end-effector comprising
the steps of: applying a force on the end-effector of a manipulator
causing motion at joints of the manipulator; determining a current
position of the end-effector; comparing the current position of the
end-effector to a predetermined position; determining a required
position of a physical constraint based on the comparison of the
current position of the end-effector to the predetermined position
in order to prevent incursion of the end-effector beyond the
predetermined position; and positioning the physical constraint at
the required position.
8. The method of claim 7, wherein the manipulator comprises a
robot.
9. The method of claim 7, wherein the current position of the
end-effector is determined using measurements of positions of the
joints of the manipulator.
10. The method of claim 7, wherein the current position of the
end-effector is determined using an external position measurement
system.
11. An apparatus for guiding a user's control of an end-effector
comprising: a robot having: at least one actuated joint, at least
one other joint connected to the at least one actuated joint, and a
physical constraint assembly movable by actuation of the at least
one actuated joint, wherein the physical constraint assembly
consists of a single physical constraint that limits the motion of
the at least one other joint in only one direction; and an
end-effector configured to be manipulated by an application of
external forces and connected to the at least one other joint.
12. The apparatus of claim 11, further comprising at least one
passive joint connected to the at least one actuated joint.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims the benefit of U.S. patent
application Ser. No. 61/064,486 filed Mar. 7, 2008, which is hereby
incorporated by reference in its entirety.
TECHNICAL FIELD
[0002] The present invention relates to the field of haptic hard
surface emulation and in particular, to an improved haptic method
and system for emulating a hard surface which overcomes the
disadvantages associated with conventional methods and systems and
can advantageously be applied in a number of different
applications, including surgical procedures.
BACKGROUND
[0003] Hard surface emulation is the ability of a manipulator to
simulate a rigid virtual surface of relatively arbitrary shape. The
use of such a surface implies projecting a virtual interface into a
separate environment or workspace. As such, a user is able to move
freely within this workspace until they come into contact with the
interface. The most basic application of this concept is the use of
a physical template, such as a ruler on a piece of paper.
[0004] Hard surface emulation in the context of assisting users to
perform precision motion control tasks has a wide array of
potential applications, ranging from large industrial part handling
tasks to surgical procedures. Humans are not endowed with the high
repeatability, precision or stability of robots. They are, however
much better adapted to decision making and strategic planning in
variable environments and in controlling physical interactions,
such as those involved in using various tools. Haptic interfaces
can be used to merge these distinct abilities. To date, virtually
all haptic research on stiff wall emulation has focused on
impedance or admittance-generating algorithms. These algorithms are
used to determine the forces or displacements required by the
haptic architecture to emulate the virtual environment. Hard
surfaces are typically approximated as a spring of given stiffness
and hence require actual penetration of the virtual surface to
activate the restoring forces. Due to hardware and software
limitations, such as response lag time, joint backlash, structural
flex, sensor noise, etc, systems based on this concept are not
capable of rendering truly hard surfaces or of handling large or
sustained user forces without causing instability or lack of
precision. These requirements are, however, essential if one wishes
to use haptic force feedback in guidance and region-restriction
tasks.
[0005] A number of new concepts for emulating hard surfaces based
both on modified control algorithms as well as on new mechanical
concepts have been developed in order to resolve these challenges.
One such concept is described in US patent application publication
No. 2004/0128026A1, which is hereby incorporated by reference in
its entirety. It has been applied in the form of a haptic robot,
named Acrobot, and used in cutting tool guidance during total and
unicompartmental knee arthroplasties. The concept introduces a
region of increasing robot stiffness at the boundary of the
free-motion and restricted regions. Within this region, the robot
impedance increases and the admittance decreases based on the
current location of the tool with respect to the restricted
boundary. The purpose of Region ii is to provide a smoother
transition between the free motion region and the restricted
region, thus preventing instability and decreasing the possibility
of surface penetration due to delays in the control loop. The
drawbacks are springiness at the boundary, vibrating motion at an
inclined boundary and restricted motion along the boundary due to
the increased impedance in this region. Some practical drawbacks
are that a force transducer is required on the interface between
the user and the device. Additionally, the structural architecture
and motors must be able to provide sufficient impedance to the
user, requiring large parts. This also creates significant friction
in the system, requiring motion assistance from the robot to
emulate uninhibited motion in the free region. All of this results
in a relatively costly robot.
[0006] Another concept is described in U.S. Pat. No. 5,952,795,
which is hereby incorporated by reference in its entirety, and is
based on a continually variable transmission (CVT) concept. A CVT
device is strictly defined as one having a continuous range of
transmission ratios, independent of the amount of torque being
applied to it. The drawbacks of this concept are that it
necessarily requires force sensors to keep track of user
intentions. Depending on the task, it can also result in rather
bulky architectures with large amounts of Inertia. More
importantly, the inherent characteristic of the design in which the
wheel steers continuously rather than in discrete steps, causes a
sense of hesitation when the user rapidly pushes the device from
rest and surface penetration when the device approaches a boundary
at a high angle.
[0007] A third concept uses a double freewheel and motor
combination that allows passive motion within a set of dynamic
constraints, as described in U.S. Pat. No. 5,529,159, which is
hereby incorporated by reference in its entirety. A freely rotating
shaft is constrained by a freewheel to the rotational speed of a
second parallel shaft driven by a motor. The concept allows the
control of relative motion between two serial manipulator arms,
each connected to one of the shafts. Two drawbacks with the design
include low stiffness of the system and jagged motion in certain
regions during path or surface following.
[0008] A fourth concept, called PTER and described in U.S. Pat. No.
5,704,253, which is hereby incorporated by reference in its
entirety, is based on the use of clutches and brakes to regulate
the relative rotational velocity between two manipulator links. The
primary disadvantages of this system are penetration of the hard
surface and smoothness during path-following tasks.
SUMMARY
[0009] The present invention provides a haptic method for emulating
a hard surface including a hard, curvilinear surface. Unlike
conventional methods, the present method provides a realistic feel
of a hard surface to a user operating a manipulator. The present
method can be used in any number of different applications. For
example, the haptic method to emulate a hard surface according to
one embodiment of the present invention can be used in surgery,
such as but not limited to, robot-assisted surgery.
[0010] The present invention thus provides a means for hard surface
emulation in haptic processes that creates a realistic feel in the
virtual space for the user. It is another object of the invention
to provide a means to prevent incursion within a virtual surface.
The invention represents a improved haptic concept that aims to
fulfill a number of objectives not readily met by presently
available technologies. These objectives include but are not
limited to: (1) realistic surface collision: collision with the
virtual surface is as realistic as possible. Hence, minimal or no
penetration of the surface occurs upon contact, regardless of the
approach velocity and applied force; (2) realistic surface
rigidity: a constant applied force by the user on the surface does
not allow any detectable penetration of the surface or any motion
of the surface (i.e. no springiness); (3) unimpeded surface
departure: the action of pulling away from the surface does not
result in any feeling of stickiness or impulse, regardless of the
departure acceleration, initial velocity, or initial applied force;
(4) smooth and precise surface tracing: intents by the user to
trace the surface in any direction, result in unimpeded motion, as
described in the next objective (no over- or under-penetration of
the surface should occur, regardless of the speed of motion and
applied force (i.e. no hysteresis or instability)); and (5)
unimpeded motion freedom: when not in contact with the virtual
surface, user motion is completely unimpeded with minimal apparent
friction from the device. Gravity effects are situation-dependent
and should be considered separately. Ideally, the haptic system is
transparent to the user, as though it did not exist. By strictly
adhering to this, gravity effects should not be compensated for, as
is the case with any freely-held tool. However, depending on the
situation, it may be useful to consider such compensation, where it
may be necessary to prevent collapse of the system.
[0011] Several advantages of the present invention, in some of its
forms, over existing methods and apparatus for the emulation of
hard surfaces include but are not limited to: (1) provides the
effect of a true collision with a hard surface; (2) does not
necessarily require sensing of user's applied force; (3) allows
smooth, unrestricted tracing of a virtual hard surface; (4) no
stickiness when pulling away from a surface; (5) can successfully
emulate planar, multi-planar and curvilinear surfaces; and (6)
low-friction system with no backdriving of motors and gears.
[0012] In one embodiment, an apparatus for guiding the hand of a
user includes a robot having an end-effector which is adapted to be
manipulated by an application of external forces. The robot has at
least one actuated joint and zero or more passive joints. The at
least one actuated joint is configured to position a physical
constraint that limits the motion of at least one other joint in at
least one direction.
[0013] In another embodiment, a system for performing hard surface
emulation in haptic processes includes a manipulator, such as a
robot, that is adapted to be mounted to an object. The manipulator
includes a first link that is movable about a first axis and a
second link that is coupled to the first link and is movable about
a second axis. The manipulator also includes an end-effector
coupled to the second link and adaptable to be manipulated by an
application of external forces. A physical constraint is provided
and is selectively driven about the second axis to limit motion of
the second link in at least one direction to control a user's
movements relative to a virtual surface such that the end-effector
is prevented from entering the virtual surface.
[0014] In another embodiment, a method for controlling a user's
movements relative to a virtual surface includes the steps of: (a)
applying a force on an end-effector of a manipulator causing motion
at joints of the manipulator; (b) determining a current position of
the end-effector; (c) comparing a position of the end-effector to a
shape and location of the virtual surface; (d) determining a
required position of a physical constraint in order to prevent
incursion of the end-effector into the virtual surface; and (e)
actively positioning the physical constraint to prevent the
end-effector from entering into the surface.
[0015] These and other aspects, features and advantages of the
present invention can be appreciated further from the description
of certain embodiments and the accompanying drawing figures.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0016] FIGS. 1A and 18 are perspective views illustrating general
concepts of certain elements of the present invention;
[0017] FIG. 2A is a perspective view of one embodiment of the
present invention;
[0018] FIG. 2B is another perspective view of the embodiment of
FIG. 2A;
[0019] FIG. 2C is a side view of embodiment of FIG. 2A;
[0020] FIG. 3 is a side view showing different possible positions
of the embodiment of FIG. 2A throughout a range of motion;
[0021] FIG. 4 shows an exemplary control scheme for controlling the
embodiment of FIG. 2A;
[0022] FIG. 5A is a side view of an exemplary configuration for
peripheral bone milling;
[0023] FIG. 5B is a side view of an exemplary configuration for
top-milling of a bone;
[0024] FIG. 6 shows an exemplary control program block diagram for
the embodiment of FIG. 2A;
[0025] FIG. 7 illustrates an additional method of determining the
required physical constraint location for the embodiment of FIG.
2A;
[0026] FIG. 8 shows an exemplary control scheme block diagram for a
manipulator based on one embodiment of the present invention;
and
[0027] FIG. 9 shows an additional embodiment of the present
invention based on a piston-cylinder configuration.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0028] A haptic method for emulating a hard surface according to at
least one embodiment of the present invention has wide scale
applicability to a number of different fields and it will be
appreciated that the method can, therefore be used, for one, to
provide directional guidance to a human user. Examples of such
applications, that show the broad nature of the present invention,
include, and are not limited to, the installation of automobile
doors on an assembly line, needle guidance during pericardial
puncture procedures to precisely guide a needle along a
predetermined trajectory or drill guidance in pedicle screw
placement in the spine.
[0029] Similarly, the method of the present invention can also be
used to limit the user's motion to a virtual three-dimensional
region. The system, therefore, allows the user to move freely
within the region until he or she comes in contact with the
boundaries. Some illustrational examples include: the avoidance of
collisions when a human is handling fragile parts within a
constrained environment, the prevention of a surgical tool from
damaging soft tissue surrounding the operating environment, or a
haptic device for bone sculpting used to provide cutting precision
in a manner analogous to using a physical template, while still
allowing the surgeon freedom of motion along the surface of the
cut.
[0030] The present invention thus provides an apparatus and method
for the emulation of a hard surface boundary in virtual space to
replicate a physical space. The invention uses a dynamic physical
constraint as a moving physical template that adjusts its position
depending on the user's position in space. The invention provides a
means for the replication of a hard surface boundary, and to
prevent penetration of this surface, in virtual space, in order to
recreate a physical space. The invention is applicable in haptic
processes. In particular, the invention is applicable to haptic
processes for surgery. The invention is especially applicable in
bone sculpting surgery, including joint resurfacing arthroplasty
surgery, such as knee or hip resurfacing. The present invention
provides an apparatus and method for hard surface emulation by use
of a physical constraint to limit the motion of a manipulator
joint, where the position of the physical constraint is automated
and controlled, and the position of the physical constraint is
based on the location of the manipulator's end effector. The method
and apparatus allow for the mechanisms which generate the dynamic
physical constraints to be mechanically decoupled from the
mechanisms that provide the passive guidance.
[0031] In yet another aspect, the present invention uses a physical
constraint to limit the amount of motion of a manipulator joint.
These manipulator joints may be of a variety of types, including,
but not limited to, revolute (R) and/or prismatic (P). The position
of the physical constraint can be controlled and automated using a
computer. The positioning of the physical constraint at any given
time is based on the location of the manipulator's end effector at
that time. The present invention also allows for the mechanical
decoupling of the mechanisms which generate the dynamic constraints
from those that provide the passive guidance.
[0032] As previously mentioned, the present invention has
application in a very wide array of uses. Such uses include, but
are not limited to: any passive, semi-active or active manipulator
or robot that requires joint constraints of the form of true hard
collisions; the replacement of existing haptic joints so as to
provide hard constraints; and the superposition over existing
haptic joints to provide hard constraints. Further examples include
a haptic manipulator used to constrain motion within a specified
region; a haptic manipulator used to cast a virtual solid template
into a working environment so as to improve motion precision; and
superposition on existing haptic force-feedback joints to apply a
final, hard limit.
[0033] FIG. 1(A) illustrates a simple form of a physical restraint
concept in order to assist the reader in understanding the present
invention. FIG. 1(A) is an example of a two-dimensional semi-active
manipulator 100 having a rotational joint 102 at a base and a
parallel prismatic joint 104 at an arm (RP configuration). The
prismatic joint 104 is an active joint wherein its position along a
direction 106 is controlled by an actuator (not shown). A second
passive linkage (link) 110 has an end-effector 112 at the end of
the link 110 which is intended to be the interface with the user
(ie the part the user holds onto). The passive link 110 slides
along the same direction as the active prismatic joint 104. At some
point in the course of motion of the passive link 110, the passive
link 110 physically collides with the active link 104 and cannot
slide any further (i.e., it is `blocked`). Both the active link 104
and passive link 110 rotate together about the rotational joint
102.
[0034] Given any convex virtual surface shape, the prismatic joint
104 adjusts its length radially (in direction 106), based on the
manipulator's rotational position about the centre, to ensure the
end effector is always on the surface 108. In this case, the
manipulator is semi-active since the rotational degree of freedom
102 is passive, while the translational degree of freedom 104 is
active.
[0035] In order to control the position of the user along the
surface 108, the rotational degree of freedom may also be `blocked`
in a similar manner to how the passive linear link 104 is blocked,
by using an angular restraint.
[0036] Another example of a two-dimensional implementation of the
physical constraint concept is shown in FIG. 1B. Once again, a
first joint 122 is revolute about a fixed centre point. A second
joint 121 is also revolute, resulting in a common two-link
rotational manipulator (RR configuration). The user holds on to an
end-effector 123. The dynamic physical constraint, in this case, is
applied at the manipulator's elbow, allowing free rotation of link
120 away from it, but obstructing motion towards it at a given
angle 125. In other words, the user can apply scissor-like motion
to the manipulator until contact with the angular (physical)
constraint is achieved. This configuration is semi-active because
all degrees of freedom are passive, while the revolute physical
constraint actively adjusts itself depending on the current
position of link 119 in order to prevent incursion into the surface
126. A particular advantage of this configuration is that a
rotational constraint is very easily implemented using an electric
motor. Its workspace is also large and adjustable.
[0037] The above described configurations are by no means
exhaustive. PP, PR, and three or more degree of freedom
configurations are possible. The invention relates to the dynamic
physical constraint method of joint motion control. It should be
noted that the architecture of the manipulator is
situation-dependent, and the invention can thus be applied to
numerous architectures for various applications. Multiple dynamic
physical constraints can be used within the same device or system.
A plurality of dynamic physical constraints could be used to
provide joint constraint in multiple directions.
[0038] An aspect of the present invention is the use of a physical
constraint that has its position dynamically controlled in order to
create a physical barrier to motion of a manipulator in certain
directions such that a predetermined curvilinear surface is
emulated. The position of the dynamic physical constraint at any
given time is determined by the position of the manipulator and/or
end effector at that time. The position of the manipulator and/or
end effector can be monitored by various means that perform the
intended function. The steps of sensing the location of the
manipulator and/or end effector, determining the correct position
of the dynamic physical constraint, and controlling the movement of
the dynamic physical constraint to the desired position can each be
automated and controlled using various means, including the use of
a computer(s).
[0039] In one embodiment, the apparatus and method of the present
invention are particularly suited for application to robot-assisted
surgery procedures, including, but not limited to, bone sculpting.
Bone sculpting is the surgical procedure of shaping bone surfaces
in preparation for the placement of orthopaedic implants. The
typical tools used in such a procedure are an oscillating saw or a
mill. The benefit of the former is its ability to do rapid planar
cuts, and that of the latter is to do more complex cuts. The vast
majority of commercial implants have a bone-mating surface whose
profile is based on a single standard geometric shape, or a set of
these shapes. A current trend in surgery, however, is placing
increased importance on bone-conserving implants with more complex,
anatomically similar bone-mating surfaces that require new tools
capable of sculpting curvilinear surfaces in bone. Bone sculpting
is a broad topic in orthopaedic surgery and finds its use in many
procedures. Hip and knee arthroplasty are of primary importance
because of their frequency.
[0040] An example of one architecture amongst many, which could be
used for a semi-active femoral bone-mounted sculpting apparatus
(tool) 200 is shown in FIGS. 2A-C. The apparatus 200 consists of a
two-link RR manipulator and, as described with reference to FIG.
1B, the apparatus 200 is rigidly fixed to the lateral side of a
femur bone 10. Fixation to the femur 10 can be done in a variety of
methods, including those described in the following references,
which are hereby incorporated by reference in their entirety: 1)
PCT patent application publication No. WO2006106419, entitled
Robotic guide assembly for use in computer-aided surgery; and 2) C.
Plaskos `Modeling and Design of Robotized Tools and Milling
Techniques for Total Knee Arthroplasty.` PhD Thesis, Universite
Joseph Fourier, Grenoble, France, 2005.
[0041] The apparatus 200 includes a number of components that are
constructed and configured to be coupled to and engage one another
to permit controlled and precise sculpting of the femur 10. The
apparatus 200 includes a first link 210 that can rotate freely
about a first axis 212, and is equipped with a first sensor 500
(FIG. 6) to track its angular position about the first axis 212. A
motor unit (not shown) is intended to be attached at a second axis
220 to provide controlled movement to the apparatus 200 as
described in detail below. The illustrated first link 210 is an
elongated structure with the first axis 212 being located proximate
one end of the first link 210 and the second axis 220 being located
proximate the opposite end of the first link 210. The position
sensors disclosed herein can be any number of suitable conventional
sensors, e.g., a Hall sensor. Similarly, any number of different
motor units can be used in the present invention as described in
the incorporated material. For example, the motor unit can be a
brushless DC servomotor (Faulhaber BL 20368).
[0042] The apparatus 200 also includes a second link 230 that can
rotate freely about the second axis 220, as shown in FIG. 3, until
it comes in contact with a physical constraint 240 which is
actively driven about the second axis 220 by the motor unit. In one
embodiment, the second link 230 is an elongated structure that is
coupled to the first link 210 at or proximate one end thereof and
along the second axis 220. The physical constraint 240 can take any
number of different forms so long as it operatively coupled to the
motor unit and includes a portion that creates an interference or
obstruction with the second link 230 when the two are in contact
with one another. For example, in the illustrated embodiment, the
physical constraint 240 includes a base 242 that is disposed about
and rotatable about the second axis 220 and includes a protruding
portion 244 (e.g., a pin) that represents the portion of the
constraint 240 that is selectively placed in contact with the
second link 230. The protruding portion 244 extends in a direction
away from the first link 210.
[0043] An extension 250 to the second link 230 allows for the
positioning of a bone mill 260 in the sagittal and coronal planes.
Similar to the links, the extension 250 can be in the form of an
elongated structure that is coupled to the second link 230 and
includes a slot 255 formed therein. As shown, the extension 250 can
have an L-shape in that a first leg 252 is coupled to the second
link 230 and a second leg 254 represents the portion that receives
the bone mill 260. The bone mil 260 extends through the slot 255
and can be moved longitudinally along the second leg 254 by moving
within the slot 255. The first leg 252 can be coupled to the second
link 230 by being coupled to a slot 233 that is formed in the
second link 230, thereby permitting the extension 250 to be coupled
to the second link 230 at different locations along the length of
the second link 230 due to the ability of the extension 250 to move
longitudinally within the slot 233.
[0044] The physical constraint 240 can be a backdrivable or a
non-backdrivable positionable constraint. A non-backdrivable or
non-reversible constraint 240 can be achieved by, for example,
using a non-backdrivable gear such as a worm and worm-wheel gear
coupled to the motor unit, and integrated into the motor unit.
These gears absorb the forces at the output shaft using friction
between their teeth, and not by the motor itself. Therefore, a
relatively small and low power motor can be used.
[0045] A control box 300, illustrated by the block diagram in FIG.
4, consists of a power supply 310 and two motion controllers 320,
330 connected in series (one for each axis). In particular, the
first motion controller 320 is for the first axis 212 and the
second motion controller 330 is for the second axis 220. It will be
appreciated that the first and second motion controllers 320, 330
communicate with a computer 400 through a communication port,
generally shown at 402, and control the motor and sensor. The
motion controllers 320, 330 can be any number of different motion
controllers that are suitable for use in the intended applications
described herein and others.
[0046] The user moves the bone mill 260 (end effector) located on
the second link 230. The second link 230, in turn, interacts with
the first link 210 continuously and with the physical constraint
240 only when the second link 230 and the physical constraint 240
are in contact. During these interactions, a first sensor 500
tracks the position of the first link 210 about the first axis 212
and sends the information continuously through the first motion
controller 320 to the computer 400. The computer 400 then uses this
information to decide where the physical constraint 240 should be
positioned and sends this through the second motion controller 330
to the motor. When the second link 230 is not in contact with the
physical constraint 240, the two robot links 210, 230 act passively
and independently from the active part of the system 200. The
active part of the system 200 includes the computer 400, the two
motion controllers, the motor, the sensors and the dynamic physical
constraint. A second sensor 510 tracks the position of the physical
constraint 240 about the second axis 220.
[0047] In an alternative configuration, the motion controllers 320,
330 can be directly integrated into the motor unit, requiring only
a short cable connection between the first and second motion
controllers 320, 330 and the motors. This simplifies the cable
connection between the robot (apparatus 200) and control box 300
allowing a cable with fewer wires to be used, and reduces the
susceptibility of the system to interference and noise arising in
the sensor signals.
[0048] Two distinct milling configurations are considered for
purposes of illustration, and are graphically represented in FIGS.
5A and 58. FIG. 5A shows a peripheral milling configuration in
which a cylindrical milling bit 600 is oriented normal to the
sagittal plane. FIG. 5B shows a top-milling configuration in which
a ball-mill 610 is oriented in the sagittal plane. The former has
the advantage of allowing a cylindrical surface to be cut in a
single pass. Unless a shaped cutter is used, it is, however,
limited to two-dimensionally varying surfaces. Since access is from
the side, the peripheral milling configuration is less invasive
than the top-milling configuration which requires full bone
exposure from the top. The major advantage of the top-milling
configuration is that it allows for multi-pass contoured cutting of
three-dimensionally varying surfaces. It also decreases the
interference of the robot architecture with the surgeon's milling
motions, since the mill is further away from the robot
architecture.
[0049] The active part of the system 200 is controlled by the
computer 400. FIG. 6 shows a sample block diagram of a control
program that can be used to control the active components of the
apparatus 200 of the present invention. The control program relies
on continuous readings of the position of the first axis 212. Based
on this position, the program determines the respective position in
which the physical constraint 240 must be placed to prevent
incursion of the end effector into the virtual surface. This is
done using a modified inverse kinematics calculation where the
known variables are the position of the first joint, the equation
of the virtual surface and the length of the robot links (e.g.,
links 210, 230), and the unknowns are the position of the second
joint and the end effector. The actual implementation of these
calculations can be done in various ways. Two possible ways
include: A) the modified inverse kinematics can be calculated
online using numerical methods to solve the non-linear problem, or
B) basic inverse kinematics can be used beforehand to create a
lookup table of the two link positions spanning the entire surface.
The latter method then uses a generic binary search algorithm and
interpolation to find the matching physical constraint position.
Forward kinematics is computation of the position and orientation
of robot's end effector as a function of its joint angles. It is
widely used in robotics, computer games, and animation. The reverse
process is known as inverse kinematics (i.e. the process of
determining the jointed angles of a robot (or a kinematic chain) in
order to achieve the desired pose). Inverse kinematics is a type of
motion planning. Further details about these solutions to inverse
kinematics can be found in `HAPTIC EMULATION OF HARD SURFACES WITH
APPLICATIONS TO ORTHOPAEDIC SURGERY` UBC Master's thesis, March
2008, by Nikolai Hungr, which is hereby incorporated by reference
in its entirety.
[0050] The exemplary control method of FIG. 6 is now described in
more detail. First, at a step 610, the robot (apparatus 200) is
calibrated in its zero position. The program is run at step 620.
The robot 200 is moved to an initial position in step 630 and in
step 640, the motor is actuated. At step 650, the current positions
of the first link 210 and the physical constraint 240 are obtained
from the first and second sensors 500, 510. At step 660, the
controllers 320, 330 obtain the positions from the first and second
sensors 500, 510 and these positions are communicated back to the
computer 400 at step 670. At step 680, the computer 400 determines
the required position of the physical constraint 240 to stay on the
surface. If it is determined that the position is out of range at
step 685, then the program loops back to step 650. After the
required position is determined at step 680, the motor is moved to
a new position at step 690. At step 700, the controller sends the
position to the motor and at step 710, the controller obtains the
positions from the sensors 500, 510. At step 720, the positions are
sent back to the computer 400. The process continues to loop back
to the step 650; however, at step 730, if a stop button or the like
is pressed, then the motor is turned off at step 735. Another
control method can be seen in FIG. 7. As opposed to the previously
described control method of FIG. 6, the computer 400 reads the
current position of both joints. Applying forward kinematics, the
current position of the end effector (whether in contact with the
virtual surface or not) can be calculated online. Then, using
geometry, the radial distance R between the robot base and the
virtual surface can be calculated and used to determine the
physical constraint's required position to prevent penetration into
the surface. Alternatively, the closest position on the surface to
the current end-effector position can be calculated. The current
direction of the end-effector velocity could also be used to
predict the required position of the physical constraint as it
approaches the surface.
[0051] These control methods can be summarized by the block diagram
shown in FIG. 8. At step 800, the user applies a force on the
end-effector (e.g., bone mill 260). These forces act on the robot
200 causing motion at the joints (between the parts (links) of the
robot 200). The kinematics of the manipulator allow one to
determine the current position of the end-effector which is
compared to the shape and location of the virtual surface. The
required position of the physical constraint 240 in order to
prevent incursion into the surface is determined and applied to the
robot 200, affecting the robot dynamics and hence closing the
loop.
[0052] An additional embodiment of this invention would be the
extension of the dynamic physical constraint concept shown in FIGS.
2A-C to three or more degrees of freedom. For example, a linear
encoder could be placed on the slot 255 in the extension piece 250.
This would be used to track the location of the mill 260 in the
medio-lateral direction. The three-dimensional milling surface
could be modeled by a set of planar contours in the sagittal plane.
Depending on the medio-lateral Location of the mill 260, the
physical constraint 240 would be positioned to the appropriate
contour.
[0053] An additional embodiment includes an architecture based on a
ball joint (or double revolute joint) and a linear physical
constraint or piston-cylinder arrangement (RRP configuration), as
illustrated in FIG. 9. In such a system, a ball joint 850 would be
freely mobile and its position would be monitored. Based on this
position, the linear physical constraint 860 would adjust its
radial length preventing incursion into the virtual surface 870.
This could be applied for 2D as well as 3D surfaces.
[0054] In another embodiment of the present invention, one or more
of the passive links can be configured so that they can be actively
controlled by an actuator, motor or the like. This would allow an
additional level of control in regions inside the workspace of the
manipulator, and could be used for a variety of purposes (for
example, for providing gravity compensation or variable stiffness
near the boundaries of the virtual surface). Any known control
strategy can be used, including impedance control.
[0055] The application of the dynamic physical constraint concept
of the present invention is very broad. The design provides a
realistic feeling of a hard surface. The invention provides a true
sense of touch through a manipulator. The concept could be used in
the design of haptic interfaces in a large variety of applications,
and in a large variety of fields. Some examples include
robot-assisted surgery procedures, desktop virtual environments
(such as a three dimensional spatial mouse or joystick), industrial
applications, and telemanipulation tasks. Other examples include
use to prevent collision of a manipulator with other parts in a
constricted industrial environment, such as in manufacturing
processes.
[0056] More specifically in the surgical field, the invention could
also be used for a large variety of applications other than that
stated in the preferred embodiment. For example, it could be used
for implant-bone surface preparation in joints other than the knee,
including for example hip, elbow, and ankle joints. It could also
be used in osteotomies, in general, wherever there is a need for
accurate curvilinear bone milling. An example is a Ganz osteotomy,
in which the acetabulum is deepened to restore the patient's
anatomical alignment.
[0057] Depending on the intended use of the invention, certain
parameters of various embodiments can be adjusted and/or controlled
as necessary to provide an optimal response. Such parameters
include, but are not limited to: the response time of the
controlling mechanism, the attachment point of an axis of the robot
with respect to the surface being emulated, the length(s) of the
link(s), the motor and sensor sizes, etc.
[0058] Furthermore, the invention could be incorporated in a
computer-assisted orthopaedic surgery (CAOS) system, such as the
one described in the abovementioned patent WO2006106419, or any
other commercially available system. These systems typically use 3D
position measuring localizers to track the positions of bones and
tools in space. Such a positioning measurement system could be used
to track the position of the milling tool during bone sculpting,
and if precise enough, could replace the sensor used to track the
orientation of the passive link or links.
[0059] Although the present invention has been described as a
method and apparatus for haptic hard surface emulation, it should
be noted that the system is very flexible and could also be used to
emulate non-rigid, soft or springy surfaces. This can be
accomplished by adjusting the stiffness and motor torque parameters
of the active system accordingly, as is normally done in haptic
robotics and previously described in the abovementioned patent
application US 2004/0128026A1. The present invention is therefore a
very flexible and useful one as it can more realistically emulate
hard surfaces while still being capable of emulating soft ones.
[0060] It will be appreciated that the dynamic physical constraint
240 can be used to emulate a variety of virtual surfaces. For
example, an ellipse is an example of relatively simple surface
requiring minimal motion of the dynamic physical constraint 240
along the length of the surface. A horizontal line is a more
complex example as it requires more motion of the dynamic physical
constraint as the extremities of the line are approached. A hybrid
circle/ellipse surface is a modified version of the ellipse and has
a tighter radius of curvature. A sine wave is a much more complex
surface, as the wave sits on the already complex horizontal line. A
tri-planar surface is a simplified version of the horizontal line
that keeps the surface more convex about the center of the
robot.
[0061] While the invention has been described in connection with a
certain embodiment thereof, the invention is not limited to the
described embodiments but rather is more broadly defined by the
recitations in the claims below and equivalents thereof.
* * * * *