Control of a Powered Ankle-Foot Prosthesis Based
The control system for the powered prosthesis is structured in two levels, the lower of which controls torque at the ankle joint, providing emulation of a desired impedance. The torque reference is generated by the top level controller, which is implemented as a finite-state machine where each state is defined by passive joint impedance characteristics, modified slightly from that developed by Sup et al. . Specifically, the required joint torque in each state is determined according to the following model
Evaluation of a Powered Ankle-Foot Prosthetic System During ..
The top row of shows the ankle angle versus stride for the powered prosthesis, passive prosthesis, and healthy norm, for the three respective walking speeds. Note that the powered prosthesis falls largely within the healthy data band for all three speeds. Regarding the passive prosthesis, no powered push-off can be achieved, resulting in the absence of a plantarflexive peak in late stance/early swing. Additionally, notice that for all three cadences the peak dorsiflexion in stance occurred at least 10% later in the stride than for the powered prosthesis.
contains kinematic and kinetic data corresponding to published healthy subject data from , the amputee subject walking with the powered prosthesis, and (for kinematics) the amputee subject walking with his passive prosthesis, at each of the three speeds evaluated in this work. In each plot, data characterizing plus and minus one standard deviation around the corresponding mean are shaded in gray, providing a standard for comparison which incorporates inter-subject gait variability for healthy individuals. The blue (dark) line is mean powered prosthesis data, and the red (light) line, shown in ankle angle plots only, represents mean passive prosthesis data. Note that the three treadmill speeds used in the previously described experiments corresponded to respective cadences of 91, 101, and 112 steps/min with the powered prosthesis, and 93, 104, and 110 steps/min, respectively, for the passive prosthesis. Note also that the corresponding healthy subject data shown in the plots corresponds to average cadences of 85, 105, and 125 steps/min, respectively.
Control of a powered ankle-foot prosthesis based on ..
The ankle behavior is segmented into four basic functions within one cycle of walking gait: damping during late swing and heel strike, (stiffening) spring-like support during middle stance, power delivery during push-off, and finally a return to a neutral ankle angle during swing. This control structure is presented in schematic form in the state chart shown in . Since this device does not utilize a load cell, heel strike is detected by a negative (plantarflexive) ankle angular velocity during late swing/early stance (mode 3) that occurs when the ankle angular position is near or greater than (i.e., more dorsiflexed than) the equilibrium position for that mode.
Au 2009 Powered Ankle Foot Prosthesis | Prosthesis | …
The walking controller impedance parameters and transition conditions were determined experimentally in over-ground walking during a series of training trials, and subsequently on a treadmill. The controller parameters were iteratively tuned based on a combination of quantitative (ankle joint kinematic and kinetic data) and qualitative (user feedback, external observation) information, in order to provide appropriate kinematics and kinetics as well as reliable and natural gait mode transitions. This tuning process is similar to the approach implemented by a prosthetist as he or she selects passive components with appropriate stiffnesses according to the quality of gait demonstrated by the subject. A distinct set of parameters was found for each walking cadence (slow, normal, and fast), where the parameters which varied with cadence were chiefly related to the timing and strength of push-off as well as the middle stance stiffening spring component.
Control of a Powered Ankle-Foot Prosthesis ..
There are several notable differences from the control strategies previously implemented for level ground walking in powered ankle prostheses. This controller does not employ EMG signals or associated electrodes and instrumentation. Additionally, the controller does not utilize high gain position control (i.e., does not enforce a trajectory), facilitating more natural interaction between the user, the device, and the environment. Further, this device does not utilize a sensor which directly measures ground contact or load but instead infers such conditions based upon other sensor information, allowing minimization of both the quantity of sensors and device build height without jeopardizing functionality.