Impedance in Human-Machine interaction

The physical interaction between a human being and a technological artefact implies a power flow from or toward the human body. Power can be defined as the product between a generalized effort and a generalized flow. For example, in Mechanics the effort variable is a force/torque and the flow is a velocity. In the electric domain, the current represents the flow variable and the voltage the effort. In the frequency domain these quantities can be combined to obtain a dynamic property of the compound human-machine named impedance. Depending on what kind of interaction is established, the impedance to be addressed can be mechanical (either related to the robotic system or to the human body) or electrical. In this thesis three biomedical scenarios in which it is crucial to take into proper consideration the effects of impedance on the human-machine interaction have been explored. The first application case regarded the interaction of the human locomotor apparatus with an exoskeleton assisting cyclic motions. In the interaction with a human being the robot has to synchronously adapt to the intended motion of the user, who in turn should be allowed to exploit the robotic physical support and to reduce the effort needed to perform a task. In the design of wearable robots that physically interact with a human, mechanical impedance has to be properly considered in order to implement control strategies capable of achieving a smooth, natural and non-constraining interaction. In order to minimize the perturbation induced by the robot on the natural efficient pendular nature of legs, a Switching Controller has been proposed that intermittently injects energy parcels into the human-robot system feeding the natural intrinsic oscillatory dynamics of the system, with the minimum required amount of energy. Rather than rigidly imposing a pre-defined trajectory, the presented controller delivers intermittent assistive torques to produce functional motion and to minimize unwanted perturbations to user's desired kinematic status. The proposed Switching Controller was experimentally validated on 8 healthy subjects performing knee flexion-extension motions in unassisted and assisted conditions. Electromyographic activity of main flexor-extensor knee muscles showed that the proposed controller favours extensor muscles during extension, with a statistically significant reduction in muscular activity. In the second scenario, the interaction between bone tissues and a surgical drilling tool has been analyzed. In particular it is reported the design of the end-effector of a surgical robot for the treatment of intervertebral disc degeneration through the injection of drugs in the disc following a transpedicular route. This surgical approach implies that the intervertebral disc is reached by means of a perforation of the vertebral peduncle. The robot was conceived to support and not to substitute the surgeon in performing the procedure. Therefore, it acts as a passive holder that guides the orientation of the drilling trajectory based on preoperative planning, the surgeon maintain the full control of the procedure advancing the driller along a guided path. Such approach preserves the haptic feedback that the surgeon receives from the interaction with the tissues. The impedance of bone tissues influences the pushing force and the feed rate during drilling. In order to provide the surgeon with supporting information for the identification of the tissues that are being crossed, a driller embedded with force and position sensors has been designed. The analyses of force and feed rate in the frequency domain have led to the definition of a parameter related to the mechanical properties of the bone layers encountered during drilling (i.e. cortical bone, cancellous bone, bone marrow). Taking inspiration from mechanical impedance of viscoelastic bodies, such parameter has been defined as the ratio between the Fourier transform of the force and the feed rate, averaged over a moving time window. An algorithm has been developed that allows the implementation of a real-time system to provide the surgeon with visual and audio information while performing bone drilling along guided trajectories. Finally, a third application scenario regarded the interaction with the nervous system by means of neural interfaces for prosthetic applications. Invasive interfaces with the peripheral nervous system currently rely on electric means for both nerves stimulation and signals recording. Recent studies showed that the quality of the signal-to-noise ratio of the afferent channel might be negatively affected by physiological reactions, including fibrosis. The formation of a fibrotic capsule around implanted electrodes leads to an increase in the electric impedance of the nervous tissue that impairs the long term efficacy of the implant. The possibility to stimulate the peripheral nervous tissue by means of electromagnetic (EM) waves has been investigated. EM stimulation does not require a direct contact with the tissue to be stimulated, therefore is capable of overcoming fibrotic capsules. A versatile calculation framework has been developed to investigate the properties of the electric field generated by a plurality of miniature coils with arbitrary shape and spatial orientation, arranged in cuff configuration. The capability of the miniature coils to elicit a neuronal response in specific portions of the peripheral nerve has been investigated.