Actuation Timing Strategies for a Portable Powered Ankle Foot Orthosis

Ankle-foot orthoses (AFOs) are used to assist persons with lower-limb neuromuscular impairments. We have developed the portable powered AFO (PPAFO). This device uses a bidirectional pneumatic actuator powered by a CO2 bottle to provide dorsiflexor and plantarflexor torque assistance. The PPAFO operates tether-free, allowing for use outside of the laboratory. This system has been tested on one impaired and multiple healthy subjects. Timing of the assistance provided by the PPAFO has been determined by: 1) direct event detection using sensor feedback with threshold triggers, and 2) state estimation in which gait events are estimated using a crosscorrelation based algorithm. Direct event detection, while simple to implement, can be unreliable for subjects with certain gait impairments. State estimation, while more complicated to implement, provides access to state information that cannot be directly measured by the AFO, which allows for greater flexibility in assistance timing. Current hardware limitations and future work are also discussed. NOMENCLATURE AFO Ankle-foot orthosis. PPAFO Portable powered ankle-foot orthosis. DE Direct event detection. CC Cross-correlation state estimator. INTRODUCTION THE NEED FOR POWERED ORTHOSES Walking is a fundamental part of everyday life for most individuals, and greatly contributes to overall quality of life. Gait itself is a cyclic task, with one cycle defined as the initial ground contact of the foot to the following contact by the same foot. During a gait cycle the ankle joint performs four main functional tasks: deceleration of the foot during loading response, support for stability during early to mid-stance, propulsion during late stance, and motion control of the foot during swing [1-3]. The ability of the ankle joint to perform these functional tasks can be impaired by injury or neuromuscular pathologies. Sizeable populations exist in the United States alone with these types of impairments: stroke (8M), spinal cord injuries (1.3M), multiple sclerosis (1M), cerebral palsy (412K), and polio (272K) [4, 5]. Pathological gait can manifest in a variety of ways, but two common lower-leg symptoms are weakness in the plantarflexors (calf muscles) or in the dorsiflexors (shin muscles). Weakness in the dorsiflexor muscles affects both the loading response and swing phases of gait. This affect could present as an audible foot slap during weight acceptance or foot-drop during swing. Weak plantarflexor muscles, on the other hand, primarily affect limb stability and propulsion [1]. Proceedings of the ASME 2011 Dynamic Systems and Control Conference DSCC2011 October 31 November 2, 2011, Arlington, VA, USA 1 Copyright © 2011 by ASME DSCC2011-6170 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 09/19/2015 Terms of Use: http://www.asme.org/about-asme/terms-of-use Ankle-foot orthoses (AFOs) are prescribed to help correct these types of lower limb muscle impairments [6]. AFOs can be broadly grouped into three categories: passive, semi-active, and active devices. The three main types of orthoses have benefits and limitations. For example, passive AFOs provide assistance by preventing unwanted foot motion with direct resistance and are simple and commercially available. However, by their very nature, passive AFOs may inhibit desirable motion of the shank and foot, and do not provide assistive torque. Semi-active orthoses control the impendence at the ankle joint during gait for motion control, but do not provide propulsive torque (putting energy into the system). Active orthoses use feedback from electronics and sensors to control ankle joint motion and provide assistive torque [7, 8]. Unfortunately, the size and weight of the actuators and tethers, required by active systems for power or control, limits the use of active AFOs as daily-wear devices. These limitations are the primary reasons why active AFOs are used in rehabilitation or for diagnostic purposes [9]. To address these limitations the portable powered ankle foot orthosis (PPAFO) was developed. The PPAFO utilizes pneumatic power, is lightweight, compact, and tether free, which means the device can be used outside of the lab. The performance of the controller used with the PPAFO depends critically on the ability to detect gait events based on measurements from onboard sensors (e.g., accelerometers, potentiometers, and force sensors), and to use those events to determine proper assistance for the user. In this work, two event detection methods, direct event detection and cross-correlation based state estimation are introduced, and experimental results from healthy and impaired subject trials are used to compare the performance of the two techniques. A PORTABLE POWERED ANKLE FOOT ORTHOSIS There are particular benefits and limitations of the different methods of actuation used with the state-of-the-art powered AFOs. Electric motors come in a range of powers, are easy to control, and are clean running, but are generally high velocity, low torque actuators. Human motion, on the other hand, is typically low in velocity, but requires high torque. As a result, electric motors require a transmission for use in applications like powered orthotics. The transmissions must be co-located by the motor, and are typically at least as heavy as the motor. Fluid power systems, on the other hand, have actuators with high force to weight and force to volume ratios, and do not require a transmission to drive the system. The pressurized fluid can be transported to the actuator through flexible tubing, which allows for flexibility in component placement. Further, unlike some high-ratio mechanical transmissions, fluid powered systems are backdrivable. Of the two types of fluid powered systems, pneumatic and hydraulic, pneumatic systems are cleaner, and the drag through the lines and orifices is smaller. System Hardware The pneumatic PPAFO system uses an off-the-shelf portable, compressed CO2 bottle and regulator (JacPac J-690191, 9oz capacity; Pipeline Inc., Waterloo, ON, Canada) to power a dual-vane bidirectional rotary actuator at the ankle joint (Fig. 1). The rotary, dual-vane actuator (CRB2BW4090D-DIM00653; SMC Corp of America, Noblesville, IN, USA) is rated for a maximum pressure of 150 psig. The bottle regulator controls the supply pressure for the PPAFO system. Plantarflexor pressure comes directly from the bottle, while dorsiflexor pressure is further reduced by an additional regulator (LRMA-QS-4; Festo Corp-US, Hauppauge, NY). Excessive dorsiflexor torque is not necessary to support the weight of the foot during early stance and swing, and can actually result in subject discomfort if the pressure is not reduced. Two solenoid valves (VOVG 5V; Festo Corp-US, Hauppauge, NY) control actuation timing, based on feedback from two resistive sensors (406, 2” square Interlink Electronics Inc., Camarillo, CA, USA) placed at the heel and toe of the foot plate and a potentiometer (RV4NAYSD502A 5kΩ; HoneywellClarostat, Morristown, NJ, USA) located at the ankle joint, which senses angular position. Onboard electronics (eZ430F2013 microcontroller; Texas Instruments, Dallas, TX, USA) and the CO2 bottle worn at the waist allow the PPAFO to provide untethered powered assistance. FIGURE 1. THE FIRST GENERATION PORTABLE POWERED ANKLE FOOT ORTHOSIS (PPAFO). THE ACTUATOR IS DRIVEN BY A BOTTLE OF COMPRESSED CO2 (NOT PICTURED), WHICH IS WORN ON THE SUBJECT’S WAIST. The output torque of the PPAFO is dependent on the pressures delivered to the actuator. When the plantarflexor valve opens, the pressure delivered to the actuator is at 110 psig (758 kPa, gauge), while the dorsiflexor valve is typically regulated to 30 psig (207 kPa, gauge). The maximum torque Dorsiflexor