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Unicycle Two-Link Arm (UTLA) Cobot
Background
The Unicycle Two-Link Arm (UTLA) is a one-wheeled, two degree-of-freedom (DOF) cobot
that consists of two links and two rotational joints connected to a fixed reference frame. Located at the end
of the second link is a handle where a user interacts with the cobot, and a wheel that supports the cobot.
There is a force sensor located beneath the handle.
The UTLA is
spring-loaded at the elbow to apply a constant preload to the wheel. Friction between the table and the wheel prevents motion
in directions perpendicular to the wheel's rolling direction. The wheel, which is actuated via
a traction drive by a motor, acts as a translational continuously variable transmission (CVT).
The UTLA has three encoders - one at each of the two rotational joints, and an additional one above the wheel
to monitor its position.
By controlling the steering angle of the CVT (or its time-derivative) the UTLA can constrain and allow certain motions.
This provides the UTLA with a number of potential modes of operation:
- Virtual Caster
In virtual caster mode, the UTLA allows a user to move the handle in any direction. The user's desired direction of motion
is detected by the force sensor, and the wheel is oriented to allow motion in that direction. In this mode, the wheel, which
naturally provides only one degree of freedom, is constantly being oriented to allow two degrees-of-freedom motions. This provides
a user with the sensation of moving around on a ball caster, overriding the natural constraints of the rolling wheel.
This mode is useful for allowing a user to move freely through unconstrained regions of the workspace.
- Virtual Path
In virtual path mode, the wheel is oriented to allow motions only along a software-defined path. This establishes a bilateral
constraint on a user's motion. Forces applied by a user are ignored
by the UTLA.
This mode is useful for studying how users interact with bilateral constraints.
- Virtual Wall
In virtual wall mode, the UTLA allows free motion in some regions of space but provides unilateral constraints in others. Virtual
wall mode is a combination of virtual caster and virtual path modes. When a user is against a wall, and the user is directing a
force into the wall, the UTLA operates in virtual caster mode. Otherwise the UTLA operates as a virtual caster.
This mode is useful for studying how users interact with unilateral constraints.
Videos
- Enforcing Virtual Paths (10.6 MB)
The UTLA can constrain a user to almost any path. This video illustrates a user constrained to the path of a figure eight.
As the user is moving along the figure eight, the path is changed to a circle.
- Dynamic Effects (13.8 MB)
As a passive device, the dynamics of the UTLA are apparent to the user.
This video demonstrates the effects of the dynamics on a user's motion.
The user is constrained to a circular path. When the user is at the top and bottom of the path, only the second link
of the UTLA must be moved. When the user is at the left and right side of the circle, however, both links must be moved
in order for the end-effector to move. This means that for a specified power input at the top and bottom,
the end-effector will move faster than for the same power input at the left and right sides. Thus as the user moves around the
circle, the end-effector will speed up and slow down.
- The Steered Wheel (21.3 MB)
The UTLA, operating in Virtual Caster mode, steers its wheel to allow motion in the direction indicated
by a user's force vector. A close-up of the wheel is shown, including a view of the traction drive
transmission, the motor, and the force sensor.
- Reaching Demo (2.7 MB)
One of the possible applications for the UTLA is as a tool for stroke rehabilitation.
A patient can be asked to perform a repetitive reaching task through the UTLA's workspace.
While reaching, if the UTLA detects that the user is veering off-course, it can actively correct the user's
direction of motion.
This video illustrates a healthy user reaching along various paths through the UTLA's workspace. The paths are altered after each
reaching motion is complete.
Research History
The UTLA has been the focus of four different research projects. Its history is outlined below.
- Present - Peng Pan Uses the UTLA to Study Human-Arm Motion and Constraint Interactions
Peng Pan is currently using the UTLA
for human-arm motion studies. Read more about his research here.
- 2003-2004 - Tom Worsnopp Studied the Dynamics of the UTLA
Tom Worsnopp studied the dynamics of the UTLA in
order to gain an understanding of how the cobot moves through task space.
When a user interacts with the end-effector of a typical two-link arm robot, the dynamics of the robot are not noticeable because the
robot's powerful motors control the dynamics at the end-effector. A passive two-link arm cobot, on the other
hand, is unable to dictate how its dynamics along its direction of motion will be felt by a user. This inability
to control the dynamics encouraged the exploration of passive robot arm behavior and the study of other
methods for providing a user with a dynamics-insensitive interaction.
Worsnopp's research included a study of paths along which the apparent inertia of the cobot does not
vary. Such paths hide the dynamics of the device from the user. Other techniques must be used to hide
dynamics of the cobot on paths along which the apparent inertia is not constant. There are a number of
ways to compensate for the cobot's dynamics along such paths. For more information about apparent inertia
and masking of robot dynamics, view "Controlling the Apparent Inertia of Passive Human-Interactive Robots",
ASME Dynamic Systems, Measurement and Control, 2005.
(pdf)
- 2001-2003 - Tom Worsnopp Designed and Implemented a Controller for the UTLA
Between 2001 and 2003, Tom Worsnopp designed and implemented a virtual caster
and a virtual path controller for the UTLA. In his work to develop a controller for the UTLA, Worsnopp also examined
some of the design decisions made, and analyzed how these affect performance of the UTLA.
To find out more about the controller, view Worsnopp's Master's Thesis (pdf).
- 2001 - Marcelo Yambay Designed and Built the UTLA
In 2001, Marcelo Yambay designed and built the UTLA for use in stroke rehabilitation and human-arm motion studies. As a tool in such studies,
Yambay identified a number of criteria that the UTLA should satisfy. The UTLA must have a large workspace, a low inertia, as little friction
in the joints as possible, a low physical profile, and a very responsive CVT.
To read more about the design of the UTLA, view Yambay's Master's Thesis (pdf).
Publications
- P. Pan, K.M. Lynch, M.A. Peshkin, J.E. Colgate,
"Human Interaction with Passive Assistive Robots",
IEEE 9th International Conference on Rehabilitation Robotics (ICORR), June 2005.
- P. Pan, K.M. Lynch, M.A. Peshkin, J.E. Colgate,
"Static Single-Arm Force Generation with Kinematic Constraints",
Journal of Neurophysiology, Vol. 93, p. 2752-2765, May 2005.
- T. Worsnopp, M.A. Peshkin, J.E. Colgate, K.M. Lynch,
"Controlling the Apparent Inertia of Passive Human-Interactive Robots",
ASME Dynamic Systems, Measurement and Control, 2005.
(pdf)
- T. Worsnopp, M.A. Peshkin, J.E. Colgate, K.M. Lynch,
"Controlling the Apparent Inertia of Passive Human-Interactive Robots",
International Conference on Robotics and Automation (ICRA), 2004.
(pdf)
- T. Worsnopp, "Design of a Unicycle Cobot Controller", M.S. Thesis, Northwestern University, 2003.
(pdf)
- M.Y. Yambay Valiente, "Design of a Unicycle Cobot", M.S. Thesis, Northwestern University, 2001.
(pdf)
Students
Definitions
- Cobot
A cobot is robot that is intended for safe collaboration with humans.
- CVT
A continuously variable transmission (CVT) is a device that is able to vary its input-to-output velocity ratio over a continuous range.
The CVTs used in LIMS are infinite CVTs, meaning they are capable of varying the ratio continuously from positive infinity,
through zero, to negative infinity. There are two types of CVTs currently used in LIMS research: translational CVT and
rotational CVT.
The translational CVT relates translational velocity input to translational velocity output. For example, a wheel rolling in the x-y plane
relates its velocity along the x-axis to its velocity along the y-axis. The relationship between the two velocities
depends on the angle, φ, between the rolling direction and the x-axis. The figure below illustrates a wheel in a plane and its
angle relative to the x-axis.
The rotational CVT, not surprisingly, relates rotational input to rotational output. There are a number of different configurations
for rotational CVTs, but they all share the same fundamental components: a transmission sphere in contact with both a pair of drive rollers and a
pair of steering rollers. These components are illustrated below in a tetrahedral configuration. The drive rollers are connected to
the input and output shafts. The angle of the steering rollers, θ, dictates the sphere's axis of rotation. In turn, the sphere's axis
of rotation dictates the ratio of the input velocity to output velocity.
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Last updated by Tom Worsnopp on April 05, 2007.
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