Abstract
In assistive robotics applications, the human limb is attached intimately to the robotic exoskeleton. The coupled dynamics of the human-exoskeleton system are highly nonlinear and uncertain, and effectively appear as uncertain load-torques at the joint actuators of the exoskeleton. This uncertainty makes the application of standard computed torque techniques quite challenging. Furthermore, the need for safe human interaction severely limits the gear ratio of the actuators. With small gear ratios, the uncertain joint load-torques cannot be ignored and need to be effectively compensated. A novel disturbance observer based dynamic load-torque compensator is hereby proposed and analysed for the current controlled DC-drive actuators of the exoskeleton, to effectively compensate the said uncertain load-torques at the joint level. The feedforward dynamic load-torque compensator is proposed based on the higher order dynamic model of the current controlled DC-drive. The dynamic load-torque compensator based current controlled DC-drive is then combined with a tailored feedback disturbance observer to further improve the compensation performance in the presence of drive parametric uncertainty. The proposed compensator structure is shown both theoretically and practically to give significantly improved performance w.r.t disturbance observer compensator alone and classical static load-torque compensator, for rated load-torque frequencies up to 1.6 Hz, which is a typical joint frequency bound for normal daily activities for elderly. It is also shown theoretically that the proposed compensator achieves the improved performance with comparable reference current requirement for the current controlled DC-drive.
Highlights
Increase in elderly population in the society has driven active research in field of assistive exoskeletons so to enable the elderly to stay active and independently perform their daily activities [1,2]
Even though Dynamic Load-Torque Compensator (DLTC) is expected to give a good performance in compensating the load-torque in medium to high frequency range, its low frequency performance is seen to be effected by the drive's parametric uncertainties
The xPCeHost computer hosts the SimulinkTM section of the code for different compensators and is linked to the xPC-Target computer through a dedicated Ethernet link. xPC-Host computer generates and downloads the real-time code to the xPC-Target and is used to control the x-PC target properties, display and datalogging in real time. xPC-Target computer on the other hand implements the code for different torque compensator schemes for the current controlled DC (CCDC)-drive in real time and generates a rated sinusoidal load-torque at 1.6 Hz, by controlling a separate DC motor acting as load-torque generator
Summary
Increase in elderly population in the society has driven active research in field of assistive exoskeletons so to enable the elderly to stay active and independently perform their daily activities [1,2]. To compensate for the load-torque disturbance in this case, each joint controller should exhibit good tracking performance which in turn requires high loop gain of the joint level system [20]. It is shown here that when the CCDC-drive is modelled using the first order dynamics, DLTC simplifies to a classical static torque constant, referred here as the Static Load-Torque Compensator (SLTC). Since a typical human joint frequency bound for normal daily activities of elderly (walking, sit to stand, picking and placing an object) is 1.6Hz [38,39], performance is compared theoretically under a rated load-torque disturbance of 2.5 Nm at a higher frequency of 5 Hz (see Section 7.2) with 10% parametric uncertainty in CCDC-drive parameters. The proposed compensator structure is practically shown to give more than 5-dB mean improvement w.r.t DOB-alone and a 12-dB mean improvement w.r.t SLTC-alone in rejecting the load-torque disturbance up to 1.6 Hz
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