This document describes a device called the Gravity Compensator that was designed to support the arm of a stroke patient undergoing rehabilitation without requiring a therapist. The device uses a counterweight system of pulleys and adjustable weights to balance the weight of the patient's arm and allow for free range of motion. Sensors on the arm brace can provide feedback on the patient's movements as they do exercises interacting with a Baxter robot. The final prototype was constructed of an adjustable steel frame, counterweight stacks, and an arm brace that can accommodate patients of different sizes and fully support each joint of the arm.

1. Gravity Compensator for the
Rehabilitation of Paralyzed Stroke Patients
| Sean McHale | Spenser Jenkins | Mike Calabro | Arch Wilson | Sponsor: Dr. Nitin Sharma
ABSTRACT
Currently, a therapist must assist a stroke patient who is
undergoing rehabilitation of a paralyzed arm. The therapist must
support a patient’s arm while they rehabilitate their motor skills
by doing simple tasks involving the movement of their arm. The
goal of our project was to create a device that could compensate
for the effects of gravity by supporting someone’s paralyzed arm,
thereby eliminating the need for a therapist to support the arm
and work towards making the rehabilitation process autonomous.
The patient undergoing rehab will work with a Baxter robot, as
seen in Figure 1, by interacting with one of Baxter’s arms in
order to relearn diminished motor skills. Our design must
support the patient’s arm without infringing on the patient’s
interaction with Baxter, and also allow the patient to move their
arm with no resistance to their exercise. Our solution is a
counterweight mechanism that balances the weight of a person’s
arm using weights and pulleys, similar to those used in fitness
centers. During operation, the patient will be seated in front of
the prototype with their arm supported, so they can interact face
to face with the Baxter robot. Their arm is supported in a brace
that attaches to counterweight system. The brace will have
sensors attached to it so it can give feedback to the therapist
regarding the patient’s movement during the rehabilitation.Figure 1: Baxter Robot
DESIGN CONCEPT
Our design was inspired by the Armeo® Boom pictured below in Figure 2. We melded this telescoping suspension
design with the idea of a counterweight lifting mechanism, similar to those in fitness centers. The result was our final
design which is seen below in Figure 3. This system achieves the functional requirements we established. It is a
relatively simple design to create and understand, and can be summarized as using 2 independent counterweight systems
to balance the weight of the arm. The design is adjustable and it can accommodate a range of people’s weight, from
100-250 pounds. The design can fit any arm as the brace is able to extend to accommodate arm length and has Velcro
straps to adjust for any arm width. Our system allows the patient to undergo rehabilitation with a free range of motion.
Finally, our system is mobile and can move easily from place to place as well as stand rigidly due to its casters with
brakes. The design concept we settled on was approved by our sponsor before we proceeded to prototyping.
Figure 2: Armeo® Boom Figure 3: SolidWorks Model of Design Concept
FRAME
Our design required a material that was readily available, could easily
be put together, and was easy-to-machine if needed. We found a few
types of pre-fabricate framing pieces and chose the perforated steel tube
commonly used in street signs. This tubing was used because it offered
fabricated brackets and pre-assembled telescoping pieces. The holes
that run along the sides of the tube were also needed because we were
going to be bolting pieces like brackets, pulleys, and casters to the
frame, and these holes made it so we would not have to machine our
frame to accommodate for those pieces. We designed the frame so the 4
main lengths of perforated tubing would easily bolt together with the
base plate and tube anchor. The V shape of the two legs will ensure the
product will be stable when in operation, even as the patient moves
their arm left to right. The overhanging horizontal tube is 4.5 feet long,
and the vertical tube is made up of two 4-foot telescoping tubes
allowing the Gravity Compensator to extend to roughly 8 feet tall when
in use and collapse to about 5 feet when stored or being moved. We
needed to get the device to be as tall as possible when in use, while still
being able to fit in most rooms, to eliminate as much resistance as
possible when the patient is moving their arm side to side.
Figure 4: SolidWorks Model of Frame
COUNTERWEIGHT SYSTEM
The counterweight system was designed based on the amount of
weight it would support. We designed with the intent of
supporting 100% of the weight of a patient's arm. Research found
that an average arm weighs about 5% of a person's total body
weight. To satisfy our functional requirements, the weight system
had to accommodate a minimum of 5 pounds and a maximum of
12 pounds. We chose to model our weight system after a
weightlifting machine, as opposed to the rotating bar used by the
Armeo® Boom. We made this decision based on the simplicity
and adjustability of the design. The stacked weight system shown
in Figure 5 allowed us to add or take away plates once we had
tested our device, alternative to fabricating a new rotation bar if
we determined our initial bar to be too heavy or too light. We
chose to make the plates out of wood, specifically pine, so we
could create light weights that allow for small changes within the
spectrum of weight the system can support. This allowed for
adjustability between the elbow and wrist joint. Each joint can be
counterbalanced by sliding a pin into the desired weight blocks,
just like a weightlifting machine. The two stacks of weights will
move independently of each other letting the patient move his or
her arm in all ways they are capable of. The weight stacks are
lifted by a light weight plastic rod and guided by a pair of thin
steel tubes.
Figure 5: SolidWorks Model of
Counterweight System
BRACE AND FEEDBACK
We chose to use the brace shown in Figure 6 to act as the exoskeleton for the
patients arm. The brace will be connected to the counterweight system through the
pulley system that is bolted to the frame. This particular brace is comfortable and
allows for a full range of motion, which will be very useful as patients progress in
their rehabilitation and are able to cover more area with their arm. The
adjustability of the brace was also appealing, as our gravity compensator can
accommodate any shape or size of person. The elbow joint of the brace also
provides arm angle or angle rate, as per request of our sponsor. These values can
be extremely helpful in the rehabilitation of current patients as well as improving
the process for future patients. The graduate students in Dr. Sharma’s lab will
include IMU (inertial measurement unit) sensors which can be easily be attached
to the brace.
Figure 6: Brace Selected for Prototype
DESIGN FAILURES
FINAL DESIGN
We were concerned with three types of failure in our system. First, we wanted to know how much force could be applied at
the end of the horizontal top beam that would cause our vertical beam to buckle. While at rest, our system will not buckle,
but we wanted to make sure that if someone were to lean or hang on our top beam, our design would be able to resist
buckling. After applying the material buckling equation, it was found that our design would not buckle until 167 pounds of
force was applied at rest. The same result was found when calculating for the force that would cause the top horizontal beam
to yield. Finally we tested for the amount of force that would cause the bolts to fail, and after simple shear stress calculations
it was found that 2,200 pounds of force would be needed to cause them to fail.
Figure 7 features a picture of our final design. Our machine performed
just as we had designed for, compensating for the effect of gravity on
a person’s arm. The order of operation for a person to use our machine
starts with the user standing up and strapping into the brace at a height
that is comfortable to them. Then, they can sit down and the machine
will compensate for the effect of gravity and the patient may begin
their rehabilitation by interacting with the Baxter robot.
Figure 7: Photo of the
Gravity Compensator
We would like to give a thank you to the following people for their
help on this project:
•Dr. Nitin Sharma and the graduate students in his lab
•Andy, Thorin, and all the students who helped us in the machine
shop
•Dr. David Schmidt for his assistance throughout the class
Acknowledgements
MEMS SENIOR DESIGN PROJECT