Proceedings of the KGCOE Multi-Disciplinary Engineering Design Conferencepage 1

Proceedings of the KGCOE Multi-Disciplinary Engineering Design ConferencePage 1

Project Number: P08023

Proceedings of the KGCOE Multi-Disciplinary Engineering Design ConferencePage 1

Air Muscle Artificial Limb

Proceedings of the KGCOE Multi-Disciplinary Engineering Design ConferencePage 1

Jonathan Kasper / Project Manager

Matthew Lewis / Design Lead

Josa Hanzlik / Air Muscle Team

Ellen Cretekos / Air Muscle Team

Abstract

The primary goal of the Air Muscle Artificial Limb project is to design, build, and control a robotic hand with realistic finger motions; all gesticulations are made possible via forces produced by pneumatic muscles. Dr. Kathleen Lamkin-Kennard, of the Mechanical Engineering Dept. at Rochester Institute of Technology, facilitated the project with specific product requirements and team guidance. In order to achieve the project objective, a team of engineers was divided into Design/Build, Controls, and Air Muscles sub-teams. During the initial stage of the project, three fingers were prototyped, control algorithms were created, and air muscles were characterized in order to produce a consistently and accurately controlled hand. The final product is an aluminum hand with index, middle, and ring phalanges that are capable of achieving four degrees of freedom: flexion, extension, abduction, and adduction. In this paper, the design, fabrication, control, and testing processes and results will be described in detail.

introduction

Biomedical engineeringis a newly developing and vastly growing field which utilizes problem solving and technology to help improve the quality of lives and overall patient healthcare. Innovative research and design focuses on a wide-range of disciplines, including artificial limbs which can be used for numerous purposes. Biocompatible prosthesesare utilized to replace limbs that were lost

Jenna Fike / Lead Engineer

Nick Rappa / Controls Team

Mark McKann / Controls Team

Eric Giang / Controls Team

from disease, injury, or congenital defects; while macro- to microscopic mechanical “hands” are being designed to perform intricate surgical procedures. In this particular project, an artificial hand prototype that can imitate the actuations of a real human hand was developed. The primary purpose of the design is to attract students’ interest to the biomedical engineering field at RIT. Another goal is to produce a hand platform for future senior design projects or graduate students to build from and improve upon.

In this first phase of the project, a robotic hand was designed to mimic the functionality of a human index,middle, and ring finger. Each of the fingers includes distal interphalangeal (DIP), proximal interphalangeal (PIP), and metacarpal phalangeal joints (MCP). The DIP and PIP joints are capable of flexion and extension, while the MCP joints are capable of flexion, extension, abduction, and adduction. Thus, all three fingers are devised to have the degrees of freedom (DOF) that are accomplished by real human hand phalanges.

The actuation of each joint is made possible through the usage of pneumatic air muscles. As pressurized air is supplied to a muscle bladder, the shape is distorted so that it contracts and thickens. This contraction produces a force that is great enough to succumb the opposing load forces and actuate a finger. Control algorithms are employed in LabVIEW software to consistently and accurately control the compressed air that is infused into the air muscles, thus allowing precise and realistic finger motions. Unlike a human hand, all of the muscles in this robotic limb are extrinsic and located on the forearm. Each finger utilizes a single air muscle for the purpose of flexion, one for abduction, and a third for adduction. Extension is accomplished via “ligaments” that bring the fingers to their extended resting state.

Nomenclature

abduction / adduction – the physical action of bringing an anatomical feature away from the center (ab-) or toward the center (ad-) of a defined medial point

ABS – acrylonitrile butadiene styrene; thermoplastic used to make light, rigid, molded products

air muscle – a pneumatic device that functions very similarly to a human muscle; high-pressured air is supplied to the bladder causing it to contract and thicken

CAD - computer aided design; use of computer technology to aid in the design and production of a product. Typical CAD packages are 3D solid surface modelers.

CNC –computer numerical control; computer “controller” that reads G-code instructions to drive a machine-powered cutting tool to selectively remove material

DOF – degrees of freedom

flexion / extension – the physical action of decreasing a joint’s angle (flexion) or increasing a joint’s angle (extension)

FDM – fused deposition modeling; works on an additive principle by laying down material in layers. During this process, a plastic filament or metal wire is unwound from a coil and supplies material to an extrusion nozzle, which can turn on and off the flow.

LEGO© – a line of building toys manufactured by the LEGO group, a privately owned company based in Denmark

PET –PolyEthylene Terephthalate; expandable braided sleeving

rapid prototyping – a process that takes virtual designs from CAD and transforms them into thin, virtual, horizontal cross-sections and uses each layer to make a model in physical space

overview

In order to produce a functional hand that meets the project’s needs and requirements, three distinct groups were formed and functions were assigned. The Design/Build Team was responsible for configuring and producing a robotic hand that was capable of the requisite hand motions. Tasks performed by the Design/Build Team included the production of CAD drawings, prototypes, and a final functioning hand. The Controls Team was responsible for implementing control mechanisms and algorithms for management of the solenoid valves that were used to manipulate air flow. The Air Muscle Team focused on the development and implementation of air muscles for the project. The Air Muscle Team determined the method for constructing reproducible muscles, evaluated optimal sizes and materials, and characterized the bladders so that they were capable of consistently producing the necessary forces. At the conclusion of the project, all three groups were merged to produce a single functioning product.

Hand Design Methodology

The primary objective of the Design/Build Team was to design and build a robotic limb capable of mimicking human hand motions. The hand contains 3 fingers; the index, middle, and ring each having 4 degrees of freedom. The desired motion of the fingers was to be smooth and lifelike, and as a result, the fingers were designed to produce the same motion patterns as human fingers. Specific requirements for the design were that coordinated, independent, and repeated motion must be achieved by the fingers.

CAD Design Conception:

Original construction of the finger, palm and forearm design was completed using SolidWorks© (SolidWorks Corporation, MA). This software package allowed the team to quickly and accurately model many different proposed ideas in order to assessdesign feasibility.

The team’s DOF requirement provided certain constraints on the design. Attachment points were needed on the finger to fasten tendon cables.These points allowed the finger to flex and abduct or adduct sequentially when cables at the base of the finger were displaced. Figure 1 displays the design for an individual finger in SolidWorks.

Fig. 1 CAD representation of an individual finger

The fitting manifold at the base of the forearm accommodatesthe required air muscles. The cable guide at the top of the forearm contains slots in order to allow for quick and easy service of the air muscles. Figure 2 shows a magnified view of the SolidWorks air muscle manifold. Figure 3 displays the entire design, including the three fingers, palm, and forearm.

Fig. 2 CAD representation of the base of the forearm where the air muscles are attached

Fig. 3 CAD representation of the entire forearm, hand, and fingers

Prototyping:

Feasibility assessment of the proposed design concept was completed using Rapid Prototyping Technology to modelthe fully functioning fingers. These prototypes were assembled into a custom constructed LEGO forearm and palm so that all aspects of the design could be tested and verified. The LEGO forearm contained air muscle attachment points for finger flexion. For extension, return ligaments were attached between the LEGO palm and fingers. Figure4 below displays a picture of the prototype. One main difference between the prototype and the actual product was that the prototype’s cabling could not run through the palm in the same manner as the final product. In addition, the prototype was very difficult to service, however it proved to be a vital aspect of the project’s success.

Fig. 4 Initial model with Rapid Prototype fingers, LEGOs, air muscles, and tendons.

Building and testing conducted with the prototype air muscle artificial limb showed that serviceability must be a main focus of the final design. Increased serviceability in the final design allowed for adjustments to be made as necessary throughout the final assembly process. The team also decided to use both rubber bands and elastic cord for the return ligaments in order to help return the fingers to there rest positions. The stiffer elastic cord was used to begin returning the finger to its initial position while the shorter rubber bands finished the fingers extension.

Manufacturing:

Rapid Prototyping

Initial concept generation exercises determined that it would be beneficial to construct a prototype of the finger design by using a FDM. This rapid prototyping machine used the existing CAD model to create the fingers out of ABS plastic. The prototype fingers were used to make sure that all motion clearances were appropriate between finger sections and that cable routing would produce the required overall motions.

CNC Machining

Due to the dimensional constraints and commonality of the parts, the team chose to have the final fingers and other parts CNC machined as opposed to using manual machining. Once the team had verified all design clearances using SolidWorks the stock material, technical drawings and computer files were given to members of RIT's Brinkman Lab for CNC machining on an Okuma ACE Center MB-46VAE.

Manual Machining

The RIT machine shop’s band saw and 3-axis Bridgeport mill were used to cut and machine the stock aluminum to size and to make all surfaces perpendicular. The palm plates were cut to size using a sheet metal break and drilled holes according to the technical prints using the 3-axis mills. A standard band saw was used to make the simple cuts to the aluminum rod that is the forearm of the new design.

Tolerances

Geometric tolerances were used to insure that all parts would assemble correctly and that any components that contained motion constraints would move freely. Interference or force fits were used between all components other than the fingers and the palm. There is a 0.030 in (0.076 cm)tolerance in all finger joints in order to achieve smooth motion.

COntrol System

The Controls Team’s specific goals for this project included the design and production of a system (including hardware and software) that accurately and consistently manipulates an individual finger or multiple fingers on command. The flexion and extension motions move in defined increments (i.e. 20% of the total flexion), while the abduction and adduction motions move in an all-or-nothing fashion.

The controls system utilized linear potentiometers to obtain real-time analysis of achieved finger displacements during flexion. The LabVIEW© (National Instruments, TX) interface was designed to be user-friendly so thatthe user caneasily define the desired motion anddisplacement for a given finger.

Demonstrate Feasibility and Design Verification:

Stage I

Initial controls’ testing was performed to verify that an individual air muscle could be actuated incrementally via the selected valves. A 4-way 3-position closed-center double solenoid valve was set-up to allow for flexion movements. One side of the valve was used to apply compressed pressure into the air muscle, and the second was connected to the exhaust of the first. The valve was then connected to the compressed air from the wall source, and an air muscle was hooked-up to the valve. When the button on the first side of the valve was manually pushed (in small steps), the air muscle was able to incrementally fill with air. In the reverse situation, when the button on the exhaust side of the valve was manually pushed, the air muscle incrementally released air from the muscle through the exhaust. Although this initial testing was performed manually, it demonstrated that the valves were capable of adequately applying and removing compressed air from an air muscle.

Stage II

In the next stage of feasibility testing, a relay board was utilized for an interface between the valves and LabVIEW; thus, automatic control of the valves was attempted. The relay board was first tested in LabVIEW to verify that each relay was correctly functioning. The relay board was able to communicate with the computer through USB. Proper functionality was confirmed by closing each relay switch via LabVIEW, which then caused the corresponding LED for each relay to light-up.

Thethree Rapid Prototyped fingers that were previously discussed were utilized in this stage of testing. The fingers were each attached to three air muscles via tendons that would allow for flexion, abduction, and adduction motions. Flexion was attempted in this stage of testing because this motion requires incremental gesticulation. Abduction and adduction was tested to determine approximate air muscles size to accommodate these motions. Three 4-way 3-position closed-center double solenoid valves (for flexion) and three 4-way 3-position exhaust center double solenoid valve (for abduction and adduction) were connected to the relay board for control and a wall source for compressed air.

A short program was written in LabVIEW as shown in Figure 5. The program consisted of a simple loop that considered the percentage the finger moved due to one valve cycle (a single valve being turned on and off once) for flexion. The number of cycles needed to reach the target percentage was then determined.

Fig. 5 LabVIEW Program Logic for Feasibility Testing

When this program was tested, each of the fingers was capable of incrementally flexing. Furthermore, a finger was shown to be able to flex and remain in flexion while another finger was being flexed simultaneously. Next, abduction and adduction were tested using the same sized air muscles as used for flexion. When compressed air was applied to the abduction air muscle via the LabVIEW interface, the force was too great and the tendon snapped in half.

From this initial testing, it was evident that the valves were correctly assembled, the relay board and valves were properly wired, and the interfacing with LabVIEW was feasible. More importantly, incremental application of air was permitted via LabVIEW control and multiple air muscles were able to be filled simultaneously. It was apparent that the abduction and adduction air muscles necessitated smaller air muscles than were used for flexion.

Development Procedures

There are two primary elements of the controls system: hardware and software. The hardware portion includes the valves, relay board, DAQ, and connections between each other and the computer. The software portion of the control system is performed in LabVIEW; a code has been produced to control the relays’ switches in order to allow the flow of compressed air into and out of the air muscles. A high-level overview of the entire system is shown in Figure 6.

Fig. 6 Overview of Controls System

Hardware Selection

The relay board is responsible for receiving control signals from a computer and activating the solenoid values. The USB SSR24 relay board from Measurement Computing is able to accommodate up to twenty-four connections. Because only nine muscles are currently being used, more valves can be included as needed without an additional relay board.

The data acquisition unit (DAQ) is used to read input from linear potentiometers, which provide feedback for the displacement of the digits. The chosen DAQ, PMD-1208LS, is also made by Measurement Computing. The DAQ connects to the relay board via USB and can provide up to 5 V to power the potentiometers.

Two types of valves are needed to perform the desired action, as shown in Figure 7a. The abduction and adduction motions are implemented using 4-way 3-position exhaust-center double solenoid valves, as displayed in Figure 7b. This valve model eliminates the need for two separate valves, and the exhaust center ensures that only one ab/adduction air muscle can be fired at one time. The ab/adduction air muscle inflates as long as the solenoid valve is activated, and is deflated when the valve is deactivated. A closed center version of the 4-way 3-position double solenoid valve, as pictured in Figure 7c, is used to implement the flexion and extension motion. The closed center keeps constant air pressure in the air muscle when the valve is deactivated, making incremental motion possible.