Patent Application
20160143514
Untethered magnetic devices, such as magnetic microrobots and magnetically actuated capsule endoscopes, have become an active area of research because of the potential impact to minimally invasive medicine. These devices typically consist of some form of mechatronic or micro electromechanical systems (MEMS) device with a rigidly attached magnetic body on which magnetic forces and torques are applied by an external field. Some approaches to actuation utilize magnetic forces for pulling while others apply torque generated by rotating magnetic fields to roll on a surface, swim through a fluid, crawl through a lumen via helical propulsion, or screw through soft tissue. Because these devices can be viewed as simple end-effectors of a larger robotic system, and they may range in size from the microscale to the mesoscale, they are referred to herein as magnetically actuated tools (MATs) without any implied size.
The ability to control untethered MATs using a single rotating permanent magnet (RPM) has previously been shown where a rotating MAT can be propelled by a single RPM with the RPM placed in any position relative to the MAT, provided a specific position-dependent RPM rotation axis is established. In some experiments, an RPM can be rotated by a DC motor that is rigidly mounted to the tool-frame of an industrial six-degree-of-freedom (6-DOF) robotic manipulator. In this setup, the rotation axis of the RPM is fixed with respect to the tool-frame of the robotic manipulator. Such methods are capable of placing the RPM with a correct rotation axis to guide a MAT through relatively simple trajectories. However, when tasked with navigating a MAT through tortuous paths, the physical constraints of the robotic manipulator (i.e., joint limits and singularities) limit how the MAT can be actuated. To propel a MAT through a tortuous path, the RPM's rotation axis may undergo large continuous changes in direction, which may require the robotic manipulator to move into unfavorable configurations, known as singularities. Such singularities can result in temporary loss of control of the MAT position while the manipulator readjusts to a more favorable position.
The present technology provides a mechatronic device that rotates a spherical magnetic body to act as an RPM for the control of MATs. Accordingly, a magnetic manipulation device can comprise a housing and a spherical magnetic body contained within the housing. The spherical magnetic body can be rotatable about a sphere axis of rotation which is omnidirectionally variable. Rotators can be attached to the housing and are in contact with the spherical magnetic body to rotate the spherical magnetic body about the sphere axis of rotation.
The present technology can include a system for manipulation of a magnetic capsule endoscope which includes a magnetic manipulation device. The system can further include a robotic arm movable along at least two axes and supporting the magnetic manipulation device. A magnetic field sensor can be proximal to the spherical magnetic body to measure a magnetic dipole direction of the spherical magnetic body. A processor can be used to cause movements of the robotic arm and the plurality of rotators based on a location of the magnetic capsule endoscope and the magnetic dipole direction of the spherical magnetic body.
There has thus been outlined, rather broadly, the more important features of the invention so that the detailed description thereof that follows may be better understood, and so that the present contribution to the art may be better appreciated. Additional variations and aspects of the invention can be appreciated from the following detailed description.
While these exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be realized and that various changes to the invention may be made without departing from the spirit and scope of the present invention. Thus, the following more detailed description of the embodiments of the present invention is not intended to limit the scope of the invention, as claimed, but is presented for purposes of illustration only and not limitation to describe the features and characteristics of the present invention, to set forth the best mode of operation of the invention, and to sufficiently enable one skilled in the art to practice the invention. Accordingly, the scope of the present invention is to be defined solely by the appended claims.
In describing and claiming the present invention, the following terminology will be used.
The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a housing” includes reference to one or more of such features and reference to “orienting” refers to one or more such steps.
As used herein with respect to an identified property or circumstance, “substantially” refers to a degree of deviation that is sufficient so as to measurably detract from the identified property or circumstance. The exact degree of deviation allowable may in some cases depend on the specific context.
As used herein, “adjacent” refers to the proximity of two structures or elements. Particularly, elements that are identified as being “adjacent” may be either abutting or connected. Such elements may also be near or close to each other without necessarily contacting each other. The exact degree of proximity may in some cases depend on the specific context.
As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary.
Concentrations, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a numerical range of about 1 to about 4.5 should be interpreted to include not only the explicitly recited limits of 1 to about 4.5, but also to include individual numerals such as 2, 3, 4, and sub-ranges such as 1 to 3, 2 to 4, etc. The same principle applies to ranges reciting only one numerical value, such as “less than about 4.5,” which should be interpreted to include all of the above-recited values and ranges. Further, such an interpretation should apply regardless of the breadth of the range or the characteristic being described.
Any steps recited in any method or process claims may be executed in any order and are not limited to the order presented in the claims. Means-plus-function or step-plus-function limitations will only be employed where for a specific claim limitation all of the following conditions are present in that limitation: a) “means for” or “step for” is expressly recited; and b) a corresponding function is expressly recited. The structure, material or acts that support the means-plus function are expressly recited in the description herein. Accordingly, the scope of the invention should be determined solely by the appended claims and their legal equivalents, rather than by the descriptions and examples given herein.
The present technology provides a mechatronic device that rotates a spherical magnetic body to act as an RPM for the control of other MATs. The device can generally utilize three rotators or omniwheels to enable holonomic control of an orientation of the spherical magnetic body, such that an instantaneous axis of rotation of the spherical body can be set arbitrarily. In one exemplary application, the device can be mounted as an end-effector of a simplified robotic manipulator that controls a Cartesian position of the device, and thus the RPM, without requiring a singularity-prone robotic wrist. The dipole moment of the RPM (i.e., the vector pointing from the south to the north magnetic poles, whose magnitude is equal to the strength of the magnet) can be estimated on-line, utilizing three or more sensors (e.g., Hall effect) which can be mounted to the device or otherwise located proximal to the device. The sensed RPM dipole is used for closed-loop control of the RPM's dipole vector axis of rotation. These and other aspects of the invention are described in more detail through the following exemplary description.
In some aspects, physical constraints 26 are positioned around the spherical magnetic body 22, and rotatably contact the body 22, to prevent it from translation in space within the housing 20. The constraints 26 may be positioned through a plurality of spaced threaded apertures 28 of the housing 20. In some aspects, the constraints 26 are a set of four ball-roller tipped precision set screws. The tip of each constraint 26 may contain a freely rotating ball supported by many subrollers to reduce friction between the body 22 and the constraints 26. The constraints 26 can be adjustable via matching threaded apertures 28 (or inserts) in the body of the housing 20, and can be finely tuned in such a way as to allow spherical magnetic body 22 to freely rotate while allowing minimally perceptible translation.
The housing 20 includes openings 30 to permit the rotators 24 to at least partially extend through the housing 20 to rotatably contact the spherical magnetic body 22 within the housing. Controllable motors (not shown) can be mounted adjacent the magnetic manipulation device 16 (e.g., within a robotic end-effector) to drive each of the rotators 24. For example, the motors can be coupled to driveshafts 32a, 32b, and 32c. Driveshaft 32b extends through mounting brackets 34 of the housing 20 and drive omniwheel 24b. In the present example, each driveshaft 32a, 32b, and 32c is parallel to one another to eliminate the need of flexible driveshafts or other complicated forms of power transmission, although non-parallel driveshafts can be used. Driveshafts 32a and 32c can be connected to 90-degree gearboxes 35. Each gearbox 35 can optionally have a 1:1 gear ratio so as to match torque/velocity of the driveshaft 32b. The output of each of driveshaft can be optionally supported by integrated ball bearing pillow-block assemblies in order to reduce torque loss in the transmission.
As discussed with reference to
A magnetic field sensor 42 can be positioned proximate the magnetic spherical body 22 and measures the dipole moment of the body 22 for closed-loop control of the angular velocity and dipole orientation of the magnetic spherical body 22. Magnetic field sensor(s) can be coupled to the housing. In one example, two, three or more magnetic field sensors can be used to measure the dipole orientation of the spherical magnetic body. The sensor 42 can be used with an end-effector of a robotic manipulator performing remote magnetic-manipulation tasks, such as described with reference to
The magnetic manipulation device 16 (3. Because an omniwheel is a mechanism that enables free rotation about two axes and controlled rotation about a third axis, designing the three omniwheel axes to be linearly independent enables any instantaneous sphere rotation axis to be achieved. By making the axis of rotation of the spherical magnetic body 22 continuously variable (irrespective of the robotic manipulator that positions the device), the workspace constraints of a robotic manipulator are avoided, which also avoids undesirable singularities. This allows the robotic manipulator free to position itself for robust control of MAT locomotion, while leaving the axis of rotation of the RPM unrestricted and in the control of the device. This also enables simpler robotic manipulators to be considered (e.g., 3-DOF (degree of freedom) or 4-DOF gantry and SCARA (Selective Compliance Assembly Robot Arm) robots) with the same level of MAT manipulatability.
Other spherical manipulation devices have been previously demonstrated in “ballbot” systems in which a robot balances itself atop a sphere (e.g., a soccer ball) to achieve holonomic control, and inverted-pendulum based controllers are used to stabilize the robot. With ballbots, only the instantaneous angular velocity of the sphere is important for control (i.e., there is no preferred “north pole” of a soccer ball), so only open-loop control is required. However, for the control of a magnetic spherical body, knowledge of the magnet's dipole orientation is important, both for the establishment of the sphere's angular velocity (in which the dipole rotates such that it is perpendicular to the axis of rotation), as well as the orientation-control of the dipole itself for other non-rotating tasks.
Because knowledge of the dipole's orientation is important for the magnetic manipulation device 16 to function, the system includes the magnetic field sensor 42, which in some aspects is comprised of three or more Hall-effect sensors to estimate the dipole orientation of the rotating magnetic sphere (see e.g.,
For some desired angular velocity Ω of the spherical magnetic body 22, the omniwheel rotation speeds are determined. As an illustration of this approach, let the unit-length vectors {circumflex over (d)}o1, {circumflex over (d)}o2, and {circumflex over (d)}o3 point from the spherical magnetic body 22 center to the contact point where each of the three omniwheels 24 touches the spherical magnetic body 22. The omniwheel axes âo1, âo2, and âo3 are assumed perpendicular to {circumflex over (d)}o1, {circumflex over (d)}o2, and {circumflex over (d)}o3, respectively, and that there is no slip between each omniwheel 24 and the spherical magnetic body 22. Given a magnet angular velocity Ω, the surface velocity of the magnet at the ith omniwheel-magnet contact point is
u
oi
=r
s
Ω×{circumflex over (d)}
oi,
where rs is the radius of the spherical magnetic body 22.
The components of uo1, uo2, and uo3 parallel to each omniwheel axis are transferred directly into rotation of the omniwheel rollers 29, and cause no rotation of the omniwheels themselves. All other components of uo1, uo2, and uo3 cause each omniwheel 24 to rotate with scalar rotation speeds ωo1, ωo2, and ωo3, respectively. The component direction of uoi that cause causes the ith omniwheel to rotate is
{circumflex over (q)}
oi
={circumflex over (d)}
oi
×â
oi.
Under the assumption of no-slip, the projection of uo1, uo2, and uo3, onto the directions {circumflex over (q)}o1, {circumflex over (q)}o2, and {circumflex over (q)}o3, respectively, are mapped to the scalar rotation speeds of each omniwheel 24 by the reciprocal of each omniwheel's radius (denoted by roi) as
where (d) is the skew-symmetric matrix defined as
All three omniwheel scalar rotation speeds can be packed into the vector ω and related to the spherical magnet angular velocity Ω, in matrix form as
The omniwheel axes and positioning are designed such that has full rank, otherwise there will exist a direction of Ω that cannot be achieved with any selection of omniwheel rotation speeds. Although linear independence of the rows of
is a sufficient condition mathematically, in practice the rows can be designed to be as close as possible to being mutually orthogonal. Otherwise, some desired Ω will result in an unnecessarily, and possibly unachievably, large omniwheel rotation speed.
It is possible to violate the assumption that each vector {circumflex over (d)}oi from the spherical magnet center to the contact point of the ith omniwheel is not perpendicular to the ith omniwheel axis âoi. However, this would require a different style of omniwheel to ensure that contact is continuously maintained through each omniwheel revolution in both the omniwheel's passive and active degrees-of-freedom.
Another feature that contributes to maximized or absolute surface-to-surface contact between the omniwheels 24 and the spherical magnetic body 22 is a compliance unit 48 (also shown on
Although a solid homogeneous spherical magnetic body can be used, non-spherical magnetic portions can optionally be embedded or encapsulated in a spherical structure.
Three or more sensors can be used to calculate dipole moment M of the spherical magnetic body.
The dipole moment of the magnetic body 22 (denoted by the vector M) is the vector from the south to north poles of the magnetic body 22. Methods of magnetic manipulation using a single permanent magnet typically require the magnet's dipole moment to be specifically directed and the moment to be known. The dipole moment M of the present the magnetic body 22 can be determined by measuring its magnetic field H.
Hall effect sensors measure the component of the field in the direction perpendicular to the sensor's face. The general case of n Hall-effect sensors can be assumed. In such case, each sensor of
s
i
={circumflex over (v)}
i
T
H
i
The magnetic field Hi, at each sensor position pi, can be predicted with the point-dipole model described by equation (5):
where I is the 3×3 identity matrix. Equation (5) nearly exactly predicts the field produced by an ideal spherical permanent magnet, although imperfections in the magnet can cause minor variations. For all other geometries, it is an approximation that becomes more accurate with increasing distance. Substituting equation (5) into equation (4) produces an expression relating the magnet's dipole moment M to each of the n sensor measurements, which can be aggregated into the matrix equation (6):
where S=[S1 . . . sn]T. The n×3 constant matrix S encapsulates the complete geometric description of the sensor arrangement, as it pertains to the estimation problem. If the matrix S has full column rank, then a solution for the dipole moment M can be found as equation (7):
M=S
†
S
where S†=VΣ†UT is the Moore-Penrose pseudo-inverse of S, using the singular-value decomposition S=UΣVT, where the columns of U and V are the output and input singular vectors of S, respectively, Σ contains the singular values of S on the main diagonal and zeros elsewhere, and Σ† is the transpose of Σ in which the positive singular values have been replaced by their reciprocals. The matrix S should be made to have full column rank by using at least three Hall-effect sensors and appropriately selecting the positions (pi) and directions ({right arrow over (v)}i) of each sensor. When n>3, equation (7) provides the best estimate of M in a least-squares sense. The constant matrix S† can be calculated off-line. When the sensors are rigidly attached to the device housing device and never move after the device has been built, then the matrix S can be determined and fixed based on the device onfiguration. However, when the sensors move in relation to the magnet center (e.g., Hall sensors are mounted to the robot arm) then the sensor matrix S may change over time. In this case, the matrix S can be constructed using equations 5 and 6 (or determined using a lookup table).
The vector of sensor measurements S can be modeled as a normal multivariate random process S˜N(μ, C) with mean vector μ and covariance matrix C. The sensor measurement distribution S is propagated through equation (7) to a normal multivariate random process of the measured dipole moment as in equation (8):
M˜N(S†μ,S†CS†T).
Under the assumption that the sensor measurements are independent with the same variance ρ2, the covariance matrix can be expressed as C=ρ2 I, which simplifies the distribution of the measured dipole moment to equation (9):
M˜N(S†μ,S†CS†T).
where A is the 3×3 diagonal submatrix of Σ with the singular values of S on its diagonal. Along with making S full column rank, the sensors can also be ideally arranged to minimize the variance of the measured dipole moment by decreasing the singular values of the dipole moment covariance (stored on the diagonal of Λ−1), which is equivalent to maximizing the singular values of S.
While the forgoing examples are illustrative of the principles of the present technology in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the technology. Accordingly, it is not intended that the technology be limited, except as by the claims set forth below.
This application claims priority to U.S. Provisional Patent Application No. 61/834,387, filed on Jun. 12, 2013, which is incorporated herein by reference.
This invention was made with government support under Grant #0952718 awarded by the National Science Foundation. The Government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2014/042143 | 6/12/2014 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61834387 | Jun 2013 | US |
