Patent Application
20060241539
This invention relates to a method and associated apparatus for transferring the weight of a plurality of weight bearing articulated members from a first supporting point to a second supporting point and more particularly to a method and passive orthosis apparatus for rehabilitating patients suffering from muscle weakness.
A vast number of people are affected by conditions that result in profound muscle weakness or impaired motor control. People with severe muscle weakness from neurological injury, such as hemiparesis from stroke, often have substantial movement limitations. One of the aims of rehabilitation after stroke is to improve the walking function. However, equipment available to facilitate this is severely limited.
Several lower extremity rehabilitation machines have been developed recently to help retrain gait during walking. Lokomat® is an actively powered exoskeleton, designed for patients with spinal cord injury. The patients use this machine while walking on a treadmill. Mechanized Gait Trainer® (MGT) is a single degree of freedom powered machine that drives the leg to move in a prescribed gait pattern. The machine consists of a foot plate connected to a crank and rocker system. The device simulates the phases of gait, supports the subjects according to their abilities, and controls the center of mass in the vertical and horizontal directions.
Auto-Ambulator® is a rehabilitation machine for assisting individuals, with stroke and spinal cord injuries, in leg motion impairments. This machine is designed to replicate the pattern of normal gait.
A limiting feature of these machines is that they move patients through predetermined movement patterns rather than allowing them to move under their own control. The failure to allow patients to experience and practice appropriate movement prevents necessary changes in the nervous system to promote relearning of typical patterns.
There is, therefore, still a need for a rehabilitation device that simulates the phases of gait, provides passive assistance, support a patient according to his/her abilities and allows the patient to move using muscle power.
There is provided according to this invention a method and associated equipment that allows patients to use their impaired muscles to move their limbs under their own power by balancing the effects of gravity on the afflicted limbs thereby reducing the effort needed to use such limb(s). Such balancing is achieved by transferring the weight of the afflicted limbs to a support external to the limbs, such as, for example a harness worn by the patient or a supporting structure forming part of a complete training system.
In its broader aspects the invention is a method for transferring the weight of articulated members of a system comprising a first supporting structure, and at least two interconnected articulated members pivotally attached to the support from the pivoting point to a new point on the supporting structure. According to the present invention, such method comprises:
(1) identifying a center of mass for each of the articulated members, together with any weight that may be supported by such articulated members;
(2) identifying a system center of mass representing the combined center of mass for all the articulated members;
(3) calculating a scale length for each of the articulated members and a parallelogram arm attachment point at one end of said scale length; and
(4) connecting the system center of mass:
to the parallelogram arm attachment point on each of the articulated members with an articulated lever structure forming a parallelogram;
(i) to the external support with a primary connecting structure comprising at least one spring; and
(iii) to at least one of the articulated members with a secondary connecting structure also comprising at least one spring.
In accordance with the present invention there is also provided an apparatus for transferring the weight of at least two interconnected articulated members from a first support to which they are pivotally attached to a second support. In addition to the interconnected articulated members, the apparatus further comprises:
a parallelogram lever structure comprising articulated lever arms connecting the system center of mass to the articulated members at the end of a scale length measured from the point of attachment of the articulated arm;
a primary connecting structure comprising at least one spring connecting the system center of mass to the second support; and
at least one secondary connecting structure also comprising at least one spring, connecting the system center of mass to at least one of the articulated members.
In a preferred embodiment, at least one spring in the primary connecting structure is a zero free length spring.
Still according to this invention, the articulated members are designed to attach to and support the weight of human limbs such as a leg. In addition, both the pivotal attachment and the second support are located on a harness adapted to be worn by a human and the articulated members form an exoskeleton adapted for attachment to a human limb.
Alternatively, the second attachment point may be on a fixed structure external to the harness supporting the articulated members, and the fixed structure may incorporate a treadmill or a chair.
FIGS. 2(a) and 2(b) show the significant elements and dimensions used in calculating the parallelogram design and spring attachment points for the apparatus of
a and 3b are a schematic representation of a simple practical implementation of a zero free length spring in accordance with this invention.
Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention. The figures and drawings are not to scale and only include those elements that are necessary in describing and explaining the invention. Such figures are not intended to replace complete engineering drawings.
The principle involved in removing the weight of the leg, for example, is to support the weight of the leg using articulated members attached to the thigh and calf and place springs at suitable mathematically calculated positions on the articulated members such that they completely balance the effect of gravity, of both the leg and members. The weight of the leg is then transferred to a support which may be a harness worn by the patient or an external structure such as a fixed support (i.e. a wall) or which may be a part of a training device such as a treadmill.
Gravity balancing, according to this invention, is achieved by fixing a center of mass (COM) of the combined articulated equipment members and supported weight of the limb in space using a parallelogram mechanism and once so identified making the total potential energy for any configuration of the articulated members of the system constant using springs. The method used to make a gravity-balanced assistive device for the human body comprises the following steps:
(i) Determining the combined COM of the articulated supporting members and attached parts of the human body using auxiliary parallelograms; and
(ii) Selecting springs to connect to the COM such that the total potential energy of the system is invariant with configuration.
The invention will next be described with reference to certain specific embodiments such as an apparatus having two degrees of freedom, and one having more than two, specifically three. The examples given are illustrative and are used herein for the purpose of explaining rather than limiting this invention.
The two degrees of freedom apparatus described is for use in full or partial support of a paretic limb i.e. a leg or a trunk, to help reduce the effect of gravity on the patient's motion. The present invention overcomes the problem of supporting the weight of the afflicted member during a dynamic activity like walking here the weight of the leg continuously shifts by balancing the weight of the leg in all configurations, thereby putting the leg in, so to speak, a state of neutral equilibrium.
We will use
Referring to
Preferably, the members are constructed of lightweight material such as aluminum, and their length is adjustable to fit patient's having different length limbs.
A parallelogram formed by the members and arms 34 and 36 connects the COM 38 to points 40 and 42 on the articulated members 24 and 30. A primary spring connection 44 connects the COM to the primary support 20 on the harness 16. A secondary spring connection 46 connects the COM to either or both members 24 and 30. The two spring loaded mechanisms serve to transfer the weight of the leg from pivot points 28 and 22 to support 20.
In FIGS. 2(a) and (b), line OH represents the harness, or any external structure on which the primary support point H is located. Link OA represents the length of articulated member 24 with O corresponding to the pivotal attachment 22 and A to the pivotal attachment 28. Link AB represents the member 30, where B is an end point to which is also transferred the weight of the foot of the patient. The joints or pivots usually contain bearings or similar heavy objects, which are approximated as point masses mp1, mp2, and mp3. mp3 includes the weight of the foot.
Let:
li=length of the ith link;
l*i=distance of COM of the ith primary link from the joint of the previous link;
l*ai=distance of COM of the ith auxiliary link from the joint of the previous link;
mi=mass of the ith primary link (mass of the leg segments included),
mai=mass of the ith auxiliary link,
mpi=mass of the ith point mass,
ûi=unit vector along the ith primary link,
ui=position vector from the point O to the center of mass of ith primary link,
uai=position vector from the point O to the center of mass of ith auxiliary link,
upi=position vector from the point O to the center of mass of ith point mass,
s1=distance OD,
s2=distance AE
s1 and s2 determine the two legs of the parallelogram that identifies the COM. We will refer to s1 and s2 as the scale length. The scale length further identifies the point of connection of the parallelogram arms to the articulated members.
In the preferred case where the links are made with an adjustable length, such as for example by using aluminum telescopic members, their mass remains constant, independent of their length. l*ai is then a linear function of the length of the ith auxiliary link.
Let:
l*1=α1l1
l*2=α2l2
l*a1=β1(l1−s1)
l*a2=β2s2
Where αi and βi are the ratios between 0 and 1. The COM is given by:
Rcom=Σmiui/ΣmI
Where:
ΣmiuI=m1u1+m2u2+ma1ua1+ma2ua2+mp1up1+mp2up2+mp3up3
Σmi=m1+m2+ma1+ma2+mp1+mp2+mp3
Rewriting the vectors ui in terms of unit vectors along the primary links ûi as follows:
u1=l*1û1;
u2=l1û1+l*2û2;
ua1=s1û1+s2û2+l*a1û1;
ua2=s1û1+l*a2û2;
up1=0
up2=l1û1:
up3=l1û1+l2û2;
Since point C is the center of mass of the entire system, ucom=s1û1+s2û2 and therefore:
s1=l1(ma1+m2+mp3+ma1β1+mp2)/(m1+m2+mp1+mp2+mp3+ma1β1) and
s2=l2(m2α2+mp3)/(m1+m2+ma2+mp1+mp2+mp3−ma2β2)
Having thus obtained the COM the remaining step needed to obtain gravity balancing is to determine the springs. Balancing is achieved using springs as shown in
Let x1 and x2 be the extended lengths of the springs with corresponding stiffness k1 and k2 respectively. The springs are attached to the COM making the potential energy V constant for all configurations of the articulated members. That means:
V=(k1x12/2)+(k2x22/2)+Mgh.
x12=∥CH∥2=(d1+s1 cos θ1+s2 cos(θ1−θ2))2+(s1 sin θ+s2 sin(θ1−θ2))2
x22=∥CS∥2=d22+s22−2d2s2 cos θ2 and
h=d1+s1 cos θ1+s2 cos(θ1−θ2)
Substituting and simplifying yields V=C0+C1 cos θ1+C2 cos θ2+C3 cos(θ1−θ2), Where:
C0=(k1d12/2)+(k2d22/2)+(k1s12/2)+(k1s22/2)+(k2s22/2)−Mgd
C1=k1s1d1−Mgd
C2=k1s1s2−k2d2s2
C3=k1s2d1−Mgs2.
All the Ci are constants if the coefficients containing trigonometric variables vanish, i.e. C1=C2=C3=0, in which case the total potential energy is given by V=C0 which is a constant. The potential energy thus becomes configuration invariant and gravity balancing is achieved. These conditions yield two independent equations:
k1=Mg/d1 and k2=Mgs1/d1d2
and thus provide the spring constant characteristic.
Preferably, the springs used are zero free length springs, which means that the tension in the spring is proportional to the distance between the two connection points. If the distance between the two connection points is zero, the tension in the spring is also zero.
Implementation of a zero free length springs was done using springs, cable and pulleys as illustrated in
More than two articulated members are also within the scope of this invention. The calculation of the COM and the selection of spring constants will next be illustrated in connection with the design of a 3 degree of freedom system for supporting and training device impaired subjects in sitting and getting up from a chair, with reference to
The term three degrees-of-freedom is used to refer to the motion of the body at the hip, knee, and ankle. In the present design, the device is an orthosis device with straps or other convenient attachments between the corresponding moving segments of the machine and the patient's leg. In this example, the following assumptions are made:
(i) the motion of the body is in sagittal plane;
(ii) both legs have the same motion during the sit to stand cycle (STS) motion; and
(iii) the device links are lightweight and do not add significant mass to the moving.
The human body can be modeled during sit to stand (STS) motion as having 3 degrees-of-freedom (DOF), as shown in
To form the parallelograms one again needs to find the scaled lengths in each of the articulated members. Scaled lengths dj are factors of geometry and mass distribution. The three scaled lengths used to form two parallelograms and to identify location of the COM C (rOC=dsbs+dtbt+dHbH) are shown in
ds=(1/M)(mtls+mHls+mslcs)
dt=(1/M)(mHlt+mtlct)
dH=(1/M)mHlcH
and where:
M=ms+mt+mH.
mj=mass of a length j of the combined supporting member and attached weight;
lj=length of supporting member j; and
lcj=is the distance to a supporting member j center of mass.
Having determined the COM of the system, the spring constants are next determined. The human body and the device is next gravity-balanced by attaching four springs to the system as shown in
V=Vs+Vg=(½)kx2+(½)k1x12+(½)k2x22+(½)k3x32−Mg*roc
upon substitution of
x2=∥PC∥·∥PC∥,
x12=∥O1S1∥·∥O1S1∥,
x22=∥CS3∥·∥CS3∥ and
x32=∥O2S2∥·∥O2S2∥
and expanding the results thus obtained in terms of joint angles, one obtains:
Mg·roc=Mg(dssa+dtsak+dHSakh)
x2=(dsca+dtcak+dHcakh)2+(dssa+dtsak+dHsakh−d)2
x12=dt+(ls−ds)2−2(ls−ds)dtck
x22=dH2+(ls−ds)2−2dH(ls−ds)ckh
x32=(lt—dt)2+dH2−2dH(lt−dt)ch.
Here, ci, si, cij, Sij, cijk and sijk stand for cos θi, sin θi, cos (θi+θj), sin (θi+θj), cos (θi+θj+θk) and sin (θi+θj+θk), respectively. Also d=∥OP∥ is the distance along the gravity as shown in
Setting next the coefficients of the configuration variables in the potential energy to zero, the desired stiffness of the springs for gravity balancing of the system are derived as:
k=Mg/d
k1=kds/(ls−ds)
k2=kds/(ls−ds)
k3=kdt(lt−dt).
Again in the preferred physical implementation a zero free length spring is used behind a pulley as previously explained, where the spring force can be transmitted through a cable.
Examples 1 and 2 illustrate the invention and explain the different calculations needed in order to practice the invention in two specific applications. Example 1 shows the invention as an exoskeleton worn by a patient, while example 2 discusses in general terms an arrangement for assisting a patient in sitting down and getting up.
The devices shown may be part of another device. For example, as shown in
Since the springs compensate for the weight of whole body and have large extensions, the resulting calculated stiffness of the springs is high and it is hard to select springs with large extensions and high stiffness. Additionally, the size of the auxiliary parallelograms is sometimes small and it is difficult to physically fabricate them.
To alleviate these problems, it is preferred to modify the design to have smaller stiffness of the springs and larger size for auxiliary linkages by: (a) Using an added ankle weight me leading to larger size for the parallelograms; (b) Using a body weight support (harness) to partially reduce the weight of the body as shown in
While preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. These examples are not limiting the method and apparatus to the two specific embodiments shown and described but the method and principles taught herein may also be applied to an arm assisting device and more broadly to robotic devices involving articulated members motion as well as other variations, changes and substitutions that will occur to those skilled in the art without departing from the spirit of the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.
The work leading to this invention was financed in part by the National Institute of Health (NIH) under a grant or contract No. 1 RO1 HD38582-01A2.
