Gravity Compensator for the Rehabilitation of Paralyzed Stroke Patients

| Sean McHale | Spenser Jenkins | Mike Calabro | Arch Wilson | Sponsor: Dr. Nitin Sharma

ABSTRACTCurrently, 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 CONCEPTOur 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

FRAMEOur 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 SYSTEMThe 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 FEEDBACKWe 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