TB6: Human Assist Devices (Fluid Powered Ankle-Foot-Orthoses)

Research Team

Project Leader: / Prof. Elizabeth Hsiao-Wecksler, MechSE UIUC;
Other Faculty: / Prof. Will Durfee, ME Minnesota;
Dr. Geza Kogler, Applied Physiology Georgia Tech
Post Doc(s):
Graduate Students: / David Li, UIUC; Morgan Boes, UIUC; Mazharul Islam, UIUC;
Kathy (Braun) Houle, UMN; Jicheng Xia, UMN
Undergraduate Students: / UIUC: Lee Ann Monaghan (undergrad diversity supplement), Megan Hodgson (REU – Johns Hopkins); UMN: Ellen Weburg (REU), Kali Johnson, Connor Mulcahy
Industrial Partner(s): / Parker Hannifin (Richard McDonnell)

1. Statement of Testbed Goals

The goal of this testbed is to drive the development of enabling fluid power technologies to:

(1) Miniaturize fluid power systems for use in novel, human-scale, untethered devices that operate in the 10 to 100 W range.

(2) Determine whether the energy/weight and power/weight advantages of fluid power continue to hold for very small systems operating in the low power range, with the added constraint that the system must be acceptable for use near the body.

Human assist devices developed in TB6 provide functional assistance while meeting these additional requirements: (1) operate in the 10 to 100 W target power range, (2) add less than 1 kg of weight to a given segment of the body, excluding the power supply, and be designed to minimize physical interference during use, and (3) provide assistance from 1 to 8 hours. The focus of this testbed is the development of novel ankle-foot-orthoses (AFOs) to assist gait. An AFO with its stringent packaging constraints was selected because the ankle joint undergoes cyclic motion with known dynamic profiles, and requires angle, torque, and power ranges that fit within the testbed goals.

2. Project Role in Support of Strategic Plan

This testbed facilitates the creation of miniature fluid power systems by pushing the practical limits of weight, power and duration for compact, untethered, wearable fluid power systems. This testbed benefits society by creating human-scaled fluid power devices to assist people with daily activities and is creating new market opportunities for fluid power, including opportunities in medical devices.

3. Test Bed Description

A. Description and explanation of research approach

Problem Statement: In the US alone, individuals who suffer from or have been affected by stroke (4.7M), polio (1M), multiple sclerosis (400K), cerebral palsy (100K) or acute trauma could benefit from a portable, powered, daily wear lower limb orthoses [1]. For individuals with impaired ankle function, current solutions are passive braces that provide only motion control and joint stability. These designs often fail to restore normal ankle function because they lack the ability to actively modulate motion control during gait and cannot produce propulsion torque and power.

Challenges: The ideal AFO should be adaptable to accommodate a variety of functional deficits created by injury or pathology, while simultaneously being compact and light weight to minimize energetic impact to the wearer. These requirements illustrate the great technological challenges facing the development of non-tethered, powered AFOs. The core challenges that must be met to realize such a device are: (A) a compact power source capable of day scale operation, (B) compact and efficient actuators and transmission lines capable of providing desired assistive force, (C) component integration for reduced size and weight, and (D) control schemes that accomplish functional tasks during gait and effectively manage the human machine interface (HMI). Therefore, the development of light, compact, efficient, powered, un-tethered AFO systems has the potential to yield significant advancements in orthotic control mechanisms and clinical treatment strategies.

State-of-the-Art: Passive AFO designs are successfully used as daily wear devices because of the simplicity, compactness, and durability of the designs, but lack adaptability due to limited functionality. To date, powered AFOs have not been commercialized and exist as research laboratory devices constructed from mostly off-the-shelf components [2, 3].The size and power requirements of these components have resulted in systems that require tethered power supplies, control electronics, or both [4, 5].

Research Approach: We are following a roadmap for developing portable fluid powered AFO devices with increasing complexity and performance requirements. In 2008, the design and construction of an energy-harvesting AFO that selectively restricted joint motion using a pneumatically-driven locking mechanism was completed [6, 7]. The lessons learned during this design process were used to accelerate the design of a portable fluid powered AFO. Using a systems engineering approach, the fluid powered AFO system has been divided into four subsystems that align with our core system challenges: power supply, actuator/valving, structural shell, and control system (electronics, sensors, and HMI). The subsystems have target specifications that must be met to realize a fully functional device. The power supply must weigh < 500 g, produce at least 20 W of power, run continuously for ~ 1 hour, and be acceptable for use near the human body. The actuator and valving must weigh < 400g and provide a minimum of 10 Nm of assistive torque at a reasonable efficiency. The structural shell must weigh < 500 g, be wearable within a standard pair of slacks (fit inside a cylinder with 18 cm OD), and operate in direct contact with the body. The control system must control the deceleration of the foot at the start of stance, permit free ankle plantarflexion up to mid stance, generate a propulsive torque at terminal stance, and block plantarflexion during swing to prevent foot drop; all in a robust and user friendly manner. In 2008, University of Minnesota students were added to the testbed team to examine opportunities to increase propulsion torque and power through high pressure hydraulics. Over subsequent years, Illinois and Minnesota teams have been using the portable fluid powered AFO platform to explore lower pressure pneumatics and higher pressure hydraulics, respectively, as promising technology paths for tiny fluid power systems suitable for untethered human assist devices.

B. Achievements

Portable Pneumatic AFO (PPAFO) UIUC

In 2010, we constructed our first generation portable powered ankle-foot orthosis (PPAFO) using off-the-shelf (OTS) components to demonstrate device feasibility [8-10]. The Gen1.0 PPAFO is an improvement over state-of-the-art passive and active systems [4, 5] because it provides subject-specific motion control and torque assistance without tethered power supply or electronics. The device can provide modest dorsiflexor (toes-up) and plantarflexor (toes-down) torque actuation at the ankle. A U.S. patent on the technology embodied by the PPAFO has been filed; co-inventors are CCEFP students and faculty from the U Illinois, U Minnesota, Georgia Tech, and MSOE [11]. While the Gen 1.0 device demonstrates the feasibility of utilizing low pressure pneumatics to provide torque assistance at the ankle, this testbed platform highlights the need for advancements in miniaturized fluid power systems.

Over the years, we have been working on improving the efficiency, compactness, control, usability, and possible applications of the PPAFO. Using an OTS pneumatic rotary actuator located lateral to the ankle joint (originally from SMC, currently from Parker Hannifin) and a canister of compressed CO2 at the waist to serve as a placeholder for a more compact power source, the Gen 1.0 PPAFO can generate up to 12 Nm at 100 psig with run times less than 30 minutes.

To address efficiency improvements, we have performed efficiency studies, explored regenerating exhaust gas with an accumulator, and investigated thermal regulation of the CO2 power source. Preliminary theoretical component and system efficiencies of the Gen1.0 PPAFO system suggest an overall efficiency of 19% based on calculations from the product of component (50%) and system (39%) efficiencies [12]. That analysis also suggested that the exhaust gas from the higher pressure plantarflexor actuation (100 psig) could be captured into an accumulator and then recycled to power the lower pressure dorsiflexor actuation (30 psig). In 2012, bench-top tests, conducted by REUs during the spring and summer, found 11 J of total work loss across all components, and expected fuel savings of up to 30% with a fixed volume accumulator. Recent testing on the effect of two actuation control schemes on net work and fuel consumption during walking tests found that the regenerative scheme improved fuel consumption by 17% [13]. Working with students at Vanderbilt on Project 2C.2 (strain energy accumulator), we constructed a pneumatic elastomeric accumulator for use with the recycling scheme that was tested during the walking tests (Figure 1). In 2013, we are investigating the bench-top and walking test differences in fuel savings, issues with losses and actuation timings, and additional design changes to the accumulator. We will also explore the implementation of a thermal regulation scheme on the CO2 power source. Our previous REU and E&O sponsored senior design team studies in 2011 suggested that the thermal cooling nature of liquid CO2 and subsequent pressure decrease over time could be mitigated by maintaining an isothermal condition for the canister or hoses.

To address compactness, we have been pushing the development of pneumatic ankle actuation systems with higher torque output (target: 25Nm @ 120 psig) than commercially available, modular and integrated shell structures, and promote the need for miniature pneumatic valves and power sources. We realized that the compact integrated rotary actuator developed by MSOE in 2010 would not be a viable design (max capable 50psig for 6 Nm). Therefore, in effort to continue to drive a technology pull for a compact pneumatic actuator, we pursued three avenues. (1) In 2011, MSOE tried to improve their original design and also tried proposing a new design based on bellows technology (35Nm @ 115psig). However in 2012, no further work on either design was pursued due to funding cuts to MSOE and project 2D. (2) In 2011, we began collaborating with CCEFP industry partner Parker Hannifin to utilize their expertise in pneumatic rotary actuators to design a custom product. This work has substantially slowed in 2012 due to Parker work priorities and manpower issues. The design is nearly complete, but there are issues with finding cost-effective fabrication methods. (3) During AY11-12 and AY12-13, the CCEFP sponsored a Mechanical Engineering capstone design team each year at Bradley University in Peoria, IL. The first team developed a prototype using additive manufacturing to create a novel rotary actuator with integrated planetary gear train; unfortunately the design had leakage problems and could not be tested. The prototype was displayed at the Y6 NSF site visit May 2012 and CCEFP Annual Meeting Sept 2012. The current team decided to design a completely different actuation system using a linear actuator and modified rack & pinion configuration. In 2013 and beyond, we continue to seek solutions for compact and higher torque actuation systems. In 2012, we developed a lighter and less complex structural shell design for the Gen 2.0 PPAFO, which will allow for swapping of modular components. The new shell no longer requires metal vertical struts and has no medial support (Figure 2). We are currently awaiting multiple sized (S, M, L, XL) foot and shank bilateral shells to support testing on a variety of sized test subjects. We continue to work with Project 2F (MEMS proportional valves), Project 2B.2 (HCCI engine) and Project 2B.4 (Stirling engine) to address compact pneumatic valves and power sources. The thermal shroud for the HCCI engine (Project 2D) was halted in 2012 due to funding cut of Project 2D at MSOE.

To address control of the PPAFO for appropriate gait function across a variety of user populations (able-bodied and impaired) and walking environments (level ground, stairs, ramps), we have examined different actuation-timing control strategies, solenoid vs. proportional control, and recognition and control for different gait modes. Our initial controller for level ground walking was a simple direct event threshold-based control using just the heel and toe sensors [9]. To better accommodate impaired gait, we developed a model-based state estimator controller that also added the angle sensor [8]. In 2012 and 2013, we are examining how the pneumatic system (work and fuel use) are affected by these two controllers [13]. A simulation and bench-top study highlighted that proportional valve control has better tracking and efficiency performance compared to solenoid valves [10]; however due to low torque generation ability of current actuator systems, inclusion of proportional control on the PPAFO has not yet been implemented. These results again highlight technological barriers to compact fluid-powered orthoses. In 2011, we began work in recognition and control for different gait modes using a 6DOF inertial measurement unit (IMU) [14]. Progress in 2012 resulted in success rates of identifying level ground, stairs, or ramps of 97-99% on average. It was determined that only stair or ramp descent require a different control scheme than level ground or stair/ramp ascent, and differential gait mode control has been implemented [15]. Control issues will continue to be addressed based on applications for the device.

We have targeted the PPAFO to be a portable gait assistance and rehabilitation device. Starting in 2011, CCEFP faculty and students at NCAT began development of two user interfaces: (1) a computerized clinician user interface for tracking patient medical history and therapy progression, and (2) an interactive game interface (using a serious gaming approach) to be used by the patient while using the PPAFO as a joy stick as part of a seated rehab therapy. In 2012, project 1 was halted and progress on project 2 was slowed due to funding cuts to NCAT. In 2012 and beyond, an associated project at UIUC is determining if the PPAFO can be used as a gait initiation cueing device for people with Parkinson’s disease [16]. A highlight story on this associated project is included in the Y7 report. We are also working with clinical researchers about possible applications in stroke and partial foot amputee rehabilitation.

Hydraulic AFO (HAFO) activity at Minnesota:

In 2009, we identified high pressure hydraulics as a promising technology path for tiny fluid power systems suitable for applications such as the untethered AFO. In 2010, theoretical analysis of tiny hydraulic systems was conducted to understand their limits [17]. Additionally, a compact fluid power EHA system was assembled with LiPoly battery, Maxxon motor, Oildyne cartridge pump and Bimba hydraulic cylinder, to demonstrate the capabilities and limits of using off-the-shelf components.

During 2011 and 2012, continuing theoretical analysis of tiny hydraulic systems resulted in identifying the design guidelines of the HAFO. The analysis showed that a piston pump and a gear head should be used to minimize the weight of the system. Further analysis showed that the power unit of the HAFO must be separated from the actuator unit to capitalize on the weight advantage of hydraulic actuation over the equivalent electromechanical system.