Ergonomic Handle for a 2DoF Robotic Hand Rehabilitation Device

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Harold Cox
5 years ago
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Ergonomic Handle for a 2DoF Robotic Hand Rehabilitation Device Design Team Steven Adams, Kyle Hackmeister Lucas Johnson, Daniel Lau, Nicholas Pappas Design Advisor Prof.

1 Ergonomic Handle for a 2DoF Robotic Hand Rehabilitation Device Design Team Steven Adams, Kyle Hackmeister Lucas Johnson, Daniel Lau, Nicholas Pappas Design Advisor Prof. Constantinos Mavroidis Co-Advisors Dr. Paolo Bonato, Brian Weinberg Ozer Unluhisarcikli Abstract A previously developed two degree of freedom robotic hand rehabilitation device lacks a handle that adequately caters to the needs of stroke patients. A new ergonomic handle design that mimics the natural motion of the hand is needed to allow neurological patients to comfortably grasp, release, and rotate the device. The rehabilitation device has a clear place in the market as it is unique in combining both rotary and linear degrees of freedom into the same device. This project is limited to the handle and must leave intact the existing robotic system. The redesigned unit supports movement of the fingertips in a more natural way than the previous handle and many other devices currently on the market. Rather than a strictly linear grasp and release movement, the fingertips move in a circular trajectory centered on the first knuckle with respect to palm. The robotic system uses a linear actuator with a small travel distance to control the grasping function; this handle uses a mechanical linkage to interface with the actuator. Rotational pronation and supination movements are also supported. The handle uses light weight, simple components for ease of use and low cost.

2 The Need for Project No commercially available rehabilitation devices combine pronation / supination of the hand with grasp and release functionality. In the United States alone 795,000 strokes are predicted this year (Ref. 1); approximately 50% of victims will lose some form of motor control (Ref. 2). There are currently no hand rehabilitation devices on the market that combine the pronation/supination and grasp/release functions of the hand into a single unit (Rep. 2.5). These movements are often used in combination, for example, when picking an object up off the ground. Combining these motions during therapy will provide practical rehabilitation exercises to stroke and neurological patients. The Design Project Objectives and Requirements The proposed design must add Design Objectives natural grasp/release capability, A robotic hand rehabilitation device that allows for grasp, release, maintain rotary capability, pronation, and supination of the hand is under development at increase comfort, and interface Northeastern University. The device did not have a comfortable with the existing robotic system. handle, nor did it mimic the natural trajectory of the fingers during grasp and release. This project adds increased comfort and natural motion to the robotic device. The original objectives stated that the fingers should follow a natural trajectory during grasp and release (Rep. 2.4), the handle must accommodate pronation and supination of the hand, and it must interface with the existing robotic device (Rep. 1.3). The existing device included special linear and rotary actuators that could not be changed. A major design challenge was to convert the linear motion of the actuator into the rotational motion needed to accommodate the natural trajectory of the grasp and release motion. Design Requirements For compatibility with the existing device it was critical to consider the actuator specifications. The rotary actuator, for pronation and supination, is capable of 5.5 N-m of torque. The linear actuator, which drives the grasp and release function, is capable of 350 N of force, with a travel of 2.5 inches. A successful design adds the new functionality, interfaces with the existing system, has low mechanical impedance and play, is lightweight, and made from washable and sterilizable materials. The occupational therapists at Spaulding Rehabilitation Hospital will rank the device on its usability and compare it to the old handle.

Design Concepts considered Many concepts were considered, Several design concepts were generated and a decision matrix components of each of them (Rep. 4.

recommended design concept: Four-Bar Concept The four bar mechanism concept (Rep 4.1.

3 Design Concepts considered Many concepts were considered, Several design concepts were generated and a decision matrix components of each of them (Rep. 4.2) was used to decide which concepts to develop further. The influenced to the recommended following concepts were explored in detail before arriving at a concept. recommended design concept: Four-Bar Concept The four bar mechanism concept (Rep 4.1.2) is based on traditional four bar mechanisms found in a variety of mechanical applications: backhoes, for example. Using computer software called FOURBAR by R.L. Norton, trajectory synthesis and analysis of the mechanism was performed. The simple linkage is the same on the top and bottom of the handle, and geometry of the links results in the ideal trajectory of the fingertips. The image shows the fingertip handle with a simple strap, the handle for the rest of the hand would be attached to the yellow rails to the left of the fingertip handle. Low part count and simple pin connections allowed the four-bar to score higher on the decision matrix, but the design was dropped in favor of a simpler, slider/crank concept. Slider/Crank Concept The Slider/Crank concept (Rep , 4.3.2) is a simplified version of the four bar concept. The linkage analysis also utilized computer software, this time SLIDER by R.L. Norton. The strengths were similar to the Four Bar, but there were fewer connections and parts making it a stronger design. The problem with this design was that it had greater than the maximum 2.5 inches of travel of the linear actuator. Recommended Design Concept The design concept utilizes a Design Description mechanical linkage to translate The recommended design includes a handle butt similar to the one the motion of a linear actuator on the existing rehabilitation robot for the hand to rest on. The linear into a natural grasp and release actuator connects to a sliding link which transfers the force to small motion, and support pronation and link bars on the top and bottom of the handle plate assembly. The supination by connection to the fingers strap to this handle plate assembly which pivots in a circular rotary actuator. fashion. To alternate hands, the link can be unpinned and installed in a hole on the opposite side of the handle plate assembly to reverse the direction of motion. Thumbscrews on the upright plate that the hand

4 attaches to can be removed to flip the orientation of the plate to accommodate the opposite hand. The sliding link uses linear bearings and all rotational connections use bearings and precision shoulder bolts or pins for minimum mechanical play. This concept is designed for a patient with average hand size (Rep. 5.2), but includes some adjustability. The vertical hand plate is capable of moving laterally to adjust for differences in hand thickness, and the straps hold fingers of varying lengths. Since the robotic device and the patient can apply forces in both the grasping and releasing motions, the hand must be securely mounted. This concept addresses this issue. Analytical Investigations After a detailed analysis (Rep. 5.4), it was determined that a circular trajectory was acceptable for the fingertips, so the recommended concept uses this trajectory to simplify the linkage. The resulting linkage geometry uses 2.4 inches of linear travel, which is less than the 2.5 inch maximum travel of the actuator. The geometry also determines stress distributions and forces in the device during the grasp and release function. Key components such as the link between the sliding link and the handle plate (Rep ), the sliding link, and the handle plate were examined using geometry analysis and Finite Element Analysis (FEA) (Rep. 5.7) to ensure the components were of adequate strength. Key variables such as torque around the MCP joint, forces in the link, forces at the actuator, and forces on the sliding bearings both with and perpendicular to the direction of motion were calculated as a function of the link angle to describe the system and ensure that all parts will perform correctly. Care was taken to minimize the weight and complexity of the device. Weight reduction was performed on the top and bottom plates of the handle plate assembly to remove material that was not necessary for the integrity of the device. The handle was designed to avoid the use of CNC, be simple to machine, and use standard parts that are easily assembled. Key Advantages of Recommended Concept The recommended concept adequately addresses the need to provide a handle that follows a more natural grasp and release motion.

5 Financial Issues Cost for the device was kept to a minimum by using available materials and resources. Purchased items for the prototype cost $ Recommended Improvements By providing a circular pivoting motion around the MCP joint, this motion is attained. The device is also allowed to pronate and supinate. Most importantly, the concept provides this added functionality while interfacing with the existing rehabilitation robot s linear and rotary actuators. The cost of the prototype was $ This cost is based on hardware, bearings, and a linear potentiometer. Aluminum for the parts was sourced from existing stock in the shop, the handle butt was made in the Mechatronics lab s rapid prototyping machine, and the machining was done by group members. Assembly of a device without existing stock and resources would result in higher cost. There is currently a technological entrepreneurship group designing a marketing plan for the robotic rehabilitation device as a whole, which includes the handle addressed by this project. The marketing group proposed a sale price of $75,000. The handle is a small, low cost component relative to the entire device, and at larger volume, cost may be reduced. Some improvements to the device Potential future improvements include improved adjustability to could add thumb and individual accommodate different hands, independent finger movement, thumb finger movements, greater actuation, and medial/lateral adjustment. There are different ways of adjustability, and increased accommodating different hands and individual finger movement; comfort. some solutions may involve creating more of an exoskeleton design for the hand. Thumb actuation would add a button or trigger for the thumb to interact with to give it a part in the grasp and release function. Medial/lateral adjustment would change the handle to allow for the hand to rest and approach the device from different angles. All of the above improvements would result in added comfort and functionality, but also greater complexity.

CHAPTER 13 SAN DIEGO STATE UNIVERSITY

CHAPTER 13 SAN DIEGO STATE UNIVERSITY College of Engineering Department of Mechanical Engineering San Diego, CA 92182-1323 Principal Investigator: Karen D. May-Newman (619) 594-5652 kmaynewm@mail.sdsu.edu