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Assistive Joint Constraint Device Hip Knee Ankle FNS IRGO Only IRGO Locked Locked None FNS Only None None AFO Open-Loop HNPIRGO IRGO SCKM Locked Open-Loop HNP open VCHM SCKM Locked Open-Loop HNP VCHM SCKM Locked Rule-Based Feedback AFO = ankle-foot orthosis, FNS = functional neuromuscular stimulation, HNP = hybrid neuroprosthesis, IRGO = isocentric reciprocating gait orthosis, SCKM = stance-control knee mechanism, VCHM = variable-constraint hip mechanism.
The exoskeleton weighing 22.2 kg was a modified IRGO where the reciprocating bar was replaced with the VCHM and the drop-lock knee joints were replaced with a pair of prototype hydraulic stance-control knee mechanisms (SCKMs) [19-20].
The SCKM attached to the KAFO, as shown in Figure 3, weighs approximately 3.3 kg (7.3 lb).
A closed-loop controller was developed to unlock the SCKM during the swing phase of gait and lock the mechanism in extension during stance.
The finite state machine uses four rules that can act independently or mutually to unlock the SCKM for flexion.
The knee flexion is detected by two thresholds--the input knee angle must have extended below a first threshold (=3[degrees]) for the knee to be considered fully extended and flexed beyond a second threshold (=13[degrees]) for the knee to be consider flexed--to accommodate the mechanical compliance of the SCKM when locked and loaded in flexion.
Rule 2 states that the SCKM unlocks if all three conditions are met: (1) the contralateral knee valve is closed unless the contralateral knee is extending and the valve is open to meet conditions of Rule 1; (2) the ipsilateral heel is off the ground (i.e., FSR is low), indicating either terminal stance or preswing; and (3) either the contralateral forefoot or heel is in contact with the ground (i.e., FSR is high), indicating stance.
Rule 3 states that the SCKM must be unlocked during the swing phase of gait initiated by the timing signal from the preprogrammed stimulation pattern of the ipsilateral knee flexors and extensors muscle during swing.
Rule 4 states that once the stimulation pattern signal is low, the SCKM is unlocked until the knee has returned to a fully extended position (as determined by knee angle thresholds of Rule 1) or if both Rule 1 and Rule 2 dictate that locking should occur.
The performance of the prototype SCKM was first characterized in bench testing to verify the design criteria.
Bench testing was conducted on the SCKM to (1) quantify the passive resistance with respect to angular velocity, (2) verify that the mechanism can re sist at least 50 N-m of flexion torque at/near full extension, (3) test whether the knee mechanism can reliably unlock under relatively high loads, and (4) quantify the mechanical compliance of knee flexion when the knee mechanism is locked.
The flexion torque contributed by gravity (12 N-m), from the mass of the dynamometer attachment and SCKM, was added to the measured torque applied by the dynamometer to determine the total applied torque to the locked SCKM.
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- SCK OR
- SCL SEL
- Sclater, Philip Lutley