Test Bed 6: Human Assist Devices
Fluid Power Assisted Orthosis
Leader
Elizabeth Hsiao-Wecksler (UIUC)
Statement of Project Goals
The key goal of this test bed is to drive the development of enabling fluid power technologies to:
- Miniaturize fluid power systems so that they can be used in novel, human-scale, untethered devices that operate in the 10 to 100 W range.
- 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.
The initial (five-year) focus of this test bed is the development of novel Ankle-Foot-Orthoses (AFOs) for gait stabilization. An AFO with its stringent packaging constraints was selected since the ankle joint undergoes cyclic motion with known dynamic profiles, and requires angle, torque, and power ranges that fit within the project goals.
Project’s Role in Support of the Strategic Plan
This test bed will drive the transformation of fluid power by pushing the practical limits of weight, power and duration for compact, untethered, wearable fluid power systems. Through this test bed, we will bring to the world human-scaled fluid power devices that provide useful work and provide new fluid power opportunities to industry including the medical device industry.
Fundamental Research Barriers and Methodologies for Addressing Them
Normal unimpaired ankle joint dynamics experience a cyclic motion with ranges of motion from -5° (dorsiflexion/toes-up) to +15° (plantarflexion/toes-down). During push-off (~50% of the gait cycle), propulsion torque peaks at 75 Nm for a healthy unimpaired 85 kg person. To achieve this peak torque, a power surge of ~200 W is required; while average power requirement is ~13 W @ 100% efficiency (Fig 2). Current health guidelines suggest a daily regiment of “10,000 steps per day” (or 5,000 per foot), which equates to walking 5 miles per day. {Sedentary individuals average 1,000 to 3,000 steps.} 5,000 steps results in an energy requirement of about 70 kJ per day, based on 14 J per step.
Fluid power is a superior technology for achieving these requirements. For example, to accommodate the propulsion power spike, a fluid power system with an accumulator is an excellent match. Electrical-battery systems do not handle power surges well.
For individuals with impaired ankle function, current solutions for daily-wear assistance are passive devices that provide only joint stability. To fit inside of a patient’s footwear, these current designs have become extremely compact, weighing less than 1 kg with a total volume of approximately 10 cm3. Due to their passive nature, these designs fail to restore normal ankle function due to a lack of ability to actively produce the necessary propulsion torque and power. Current work on actively actuated ankle orthoses has focused on using these devices for clinical rehabilitation or research laboratory investigations where compactness is not an issue. As a consequence, these devices use tethered power sources with bulky actuators. A major opportunity is available to develop untethered, daily-wear active devices.
To achieve such untethered, compact assistive devices, this test bed is pushing fluid power technology to produce new capabilities. The following design requirements and specifications have been identified for a successful design from this test bed: (a) small and quiet power sources capable of producing 20 W with 140 kJ of run time @ 50% efficiency, (b) miniature bidirectional actuators capable of producing a unidirectional peak torque of 75 Nm, and (c) integrated housing systems that contain conduits, accumulator and valves to achieve an overall design that weighs no more than 1 kg and total added volume of 10 cm3.
Since the needed technologies are currently not available, we will take a progressive approach toward demonstrating that fluid power can be used effectively to develop assistive human devices. Our initial proof-of-concept designs focus on using existing capabilities, off-the-shelf components, and external fluid power sources. Advanced computer simulations that integrate fluid power systems, control systems and walking dynamics are being developed to understand the operation of future active systems.
Fig 2. Ankle power for unimpaired 85 kg person
Achievements to Date
A wearable prototype for a self-contained, self-controlled passive device has been developed (Fig 3). The design uses pneumatic power to control abnormal foot drop behavior when the leg is swinging forward. Pneumatic power is harvested while walking via a novel design based on a collapsible air bellow embedded in the sole. The pneumatic system is used to actuate a cam-follower locking mechanism that supports the foot during swing while also allowing free ankle motion during stance. An invention disclosure and two manuscripts have been submitted for this design. The design improves upon passive AFOs, but does not provide active assistance for restored propulsion function.
Work has begun on active devices that can provide propulsion torque and power. University of Minnesota students and faculty have been added to the team. Preliminary computer simulations were completed on healthy and pathological gait to understand design requirements. Work has begun to integrate human movement dynamic simulation software (OpenSim) and various hydraulic system simulation software packages (SimHydraulics, Dymola, Automation Studio, and AEMSim) to better understand the operation of fluid power assistive devices on a human. Preliminary designs for a miniature hydraulic rotary actuator revealed gaps in currently available sealing, structural material and fabrication options that must be addressed to allow for the construction of very small fluid power actuators.
Fig 3. Foot drop prototype