Human Assist Devices - Fluid Powered Ankle-Foot Orthosis (Test Bed 6)
Prof. Elizabeth Hsiao-Wecksler (UIUC)
Statement of Test Bed 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 five-year initial 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.
Test Bed’s Role in Support of the 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.
Generation 1 Portable Powered Ankle Foot Orthosis (PPAFO). Portable power
source and the onboard electronics result in an untethered system.
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 AFO . 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.
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 . 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 2010, we continued to advance our first generation portable, powered, ankle-foot orthosis (PPAFO). The Gen1 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. A U.S. patent application covering the technology embodied by the Gen1 PPAFO was filed . In the current reporting year, a description of the PPAFO system hardware, a characterization of system performance and preliminary results from both healthy and impaired walkers were formally detailed . Subject testing with two impaired individuals demonstrated the PPAFO's ability to provide functional assistance. These subjects were examined because their deficits span the space of impairments that the PPAFO is capable of assisting.
Figure 1: The Gen1 portable powered ankle foot orthosis (PPAFO) shown assisting an impaired walker (Left). The rotary actuator (A) is powered using a compressed CO2 bottle (B) worn by the subject on the waist. Onboard electronics (C), force sensors (D), and an angle sensor (E) are used to control the solenoid valves (F). A second pressure regulator (G) is used to modulate the magnitude of the dorsiflexor assistance.
We analyzed the performance and the efficiency of the Gen1 PPAFO system. The initial low energy efficiency limited the performance of the Gen1 system. Currently, the can run continuously for about 40 min at 30 psig for both plantarflexor and dorsiflexor assistance, falling short of the more than 1 hr of use requirement. To analyze system efficiency, the problem was divided into two parts: component efficiency and operational efficiency . Component efficiency analysis identified energy loss due to the pressure drop across different components (e.g., line loss, valve loss) as well as backpressure. The operational efficiency analysis identified the energy loss due to how the system was used (e.g., currently energy is wasted when compressed gas is exhausted after an actuation cycle). An overall system efficiency of 19% was calculated from the product of the two efficiencies (component: 50% and operational: 39%). Solutions to improve the overall system efficiency have been proposed and will be investigated in 2011. These include recycling the compressed exhaust gas, eliminating system backpressure and improving the efficiency of the valving. Preliminary analysis indicates that the proposed solutions could raise overall system efficiency to 45%.
We also improved the control of the system. The control problem was divided into two parts: (1) the detection of the gait events during the cycle that determine AFO control objectives, and (2) the implementation of the control. To address the first part of the control problem, we proposed a new cross-correlation based algorithm to accurately estimate events during gait . Gait event detection is essential to the control of the PPAFO because the timing of gait events (heel strike, foot flat and toe off) is used to determine the assistance required by the user. The Gen1 PPAFO uses embedded force sensors with thresholds to identify gait events, but this method lacks the desired accuracy and robustness for the system due to the use of pneumatic power. Experimental results from five healthy subjects walking with the PPAFO were used to verify the performance and highlight the advantages of the cross-correlation algorithm.
We addressed the second part of the control problem through model-based system analysis to facilitate improved control design . The model included the associated pneumatic components of the PPAFO (rotary actuator and valves), and a simplified rigid body model of the human leg (shank and foot). The model was used to evaluate the simulated performance of control schemes and hardware. The results from this work led to adding a proportional valve to the Gen1 system to addresses performance and efficiency limitations of the original binary valves.
Work continued on developing the Gen2 PPAFO. Several CCEFP projects are contributing to the testbed to improve subsystem performance given target specifications. For the Gen2 design, work at MSOE (Project 2D) resulted in significant compactness and performance gains in the actuator and valve subsystem. The MSOE actuator was bench tested and integrated into the Gen2 PPAFO structure (Figure 2). The compactness of the new actuator was enhanced by integrating the valves, silencers, and sensors directly into the actuator housing and including the actuator directly into the structural subassembly. Additionally, center technologies are being used to address other subsystem limitations, including an integrated shell with vibration and noise abatement (Project 2D), a miniature HCCI air compressor power supply (Project 2B2), passive noise control (Project 3B1), improved human-machine interface (Project 3A3), and a new class of pneumatic MEMS valves to improve compactness (Project 2F).
Last year we identified high pressure hydraulics as a promising technology path for tiny fluid power systems suitable for applications such as the untethered AFO. During the past year theoretical analysis of tiny hydraulic systems was conducted to understand their limits. For example, to understand small-scale hydraulic cylinder efficiency, four configurations of including or omitting seals were analyzed . The key result is shown in Figure 3, which indicates that removing the piston seal improves cylinder efficiency if the clearance between piston and cylinder wall is small. During 2010, the TB6 team held a 2-day workshop to discuss systems engineering ideas and the SysML tool (MagicDraw). The workshop was led by Prof. Chris Paredis and his students from Georgia Tech. Participants were students and faculty from UIUC, UMN, and MSOE who work on projects affiliated with TB6. The workshop output included modeling the requirements and some system architectures for the PPAFO designs, which will be used to guide further PPAFO development. The improvement becomes significant as cylinder bore becomes smaller.
Figure 3: Cylinder efficiency versus bore size
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. The Oildyne pump is the smallest commercially available pump and can output more than 300 Watts of power, more than required for the orthosis. For the custom system we are developing, the vane pump was selected because it is the most compact among all pump types for a given displacement. Preliminary analysis of vane pumps showed that a smaller rotor results in higher pump efficiency. The results also showed that high efficiency is theoretically achievable for small-scale pumps. We will continue this path with further analysis and prototype hardware next year.
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